Device for laser imaging
(57) Abstract:The device includes a first structure, responsible for individual photons of a laser beam having a coherence L, the phase shifts in accordance with a specified distribution. The resulting phase shift root-mean-square value of the difference in optical path length greater than the length of coherence L, multiplied by a factor 1/(12)1/2. The first structure may be made of a transparent material containing particles having a higher refractive index. The device may include a second structure containing the elements that help to reduce the coherence length of the laser beam. Ensures the reduction of speckles. 19 C.p. f-crystals, 10 ill. The present invention relates to a device for laser imaging, which sent the laser beam has a certain coherence length L with a given wavelength and in which on the way of propagation of the laser beam is the first structure that allows for individual photons of the laser beam phase shifts in accordance with a specified distribution.The best known and most common on the s in accordance with the printed information using the laser beam of the charged surface of the photosensitive drum, and then on the drum in the illuminated laser beam areas is applied adherent thereto a coating of toner, which is then in turn transferred when the printing paper.Other devices are known, for example, from DE 19501525 C1, the laser is used for the sequential illumination of the pixels of the television image on the screen. Because of persistence of vision, the individual points are merged, allowing the observer perceives visual information in the form of a video.In both types of devices, lasers are used primarily to achieve a high point-by-point resolution, which is possible mainly due to the high parallelism of the laser beams. Another advantage of the laser compared with other light sources is the high energy density radiation, the preferred primarily for video of the specified type that allows you with enough brightness to reproduce an image when a very large area projection screen with a diagonal of more than 1.50 m or even on the silver screen.Such advantages of the laser based on stimulated, or stimulated, the emission of photons, which, obviously, I have STV coherence interference during image playback, because it can cause interference structures appear on the reproduced image in the form of brilliant points. Such interference, called speckles, distort the displayed image and is not valid for optimal playback.In a review article, "Speckle Reduction in Coherent Information Processing" authors Toshiaki Iwai and Toshimitsu Asakura, Proceedings of the IEEE, I. 84, No. 5, may 1996, describes the various possibilities for reducing speckles. Particular interest is a graph showing a constant increase in the number of publications between 1970 and 1990, clearly pointing to the fact that a satisfactory solution to the problem of reducing speckles still not found.In this review article contains theoretical calculations to reduce speckles. In addition, there are many ways to breach the spatial or temporal coherence of laser beams. In particular, under the assumption the assumption that the speckles are smoothed due to local or spatial changes of the laser beam, reducing the contrast of the speckles.Try local violations of coherence was taken in the already mentioned patent DE 19501525 C1 using phase p the major part of the light beam of the laser beam different phases within order-of-magnitude, comparable with the wavelength. In particular, certain areas on the phase plate stochastically distributed order to obtain different phase difference, therefore one would assume that the phases of the individual parts of the light beam is distributed similarly to the light of conventional light sources.It has been experimentally confirmed that by using such a phase plate may substantially reduce the speckles. However, it was noted that a separate structure on the phase plate providing an appropriate amount of phase shift for part of the light beam, determine new manifestations of diffraction. Therefore, light beams from the diffraction peak of any order necessary to colliergate pass through the lens, which, obviously, as a result of this diffraction worsens a little indicator laser light beam (metric, which is the product value of the mean deviation of the rays in the beam of the average axis of the beam by the sine of the average angle of divergence, i.e. the average value of the angle that the rays in the beam form with the average axis of the beam of rays (radiation product)). In addition, according to the observations of the raster of the phase plate was visible on the projected image, the eye, the contrast of the speckles.However, the disadvantage of a reduced rate of a light beam could be eliminated by using instead of a separate phase plate of the screen with the scattering elements, which with the help of statistical dispersion of different phase shift is provided by differences in optical path length. However, the experiments showed that such screens, in which the difference in optical path length for different photons of the laser beam is of the order of several wavelengths, do not lead to the desired result of removing speckles.Thus, one would assume that the laser beam, which arise speckles, and other physical properties significantly different from the other rays of light sources, which have still not seen the speckles. Another physical quantity characterizing the light source, is the coherence length. Conventional visible radiation values of the coherence length, typically significantly less in comparison with laser radiation.In the application WO 96/08116 says that when using a pulsed laser with a pulse duration of 1 PS, i.e., the coherence length of 0.3 mm, was much smaller contractory the effect observed with smaller coherence length or with the special design of the laser. In the rest, although the coherence length and changes ripple, in principle, to obtain sufficient brightness to each pulse has a significantly higher density of photons than the radiation in continuous mode, resulting in interference due to the large number of photons would even intensify. The only effect that could reduce the speckles, is based on the greater width of the spectrum . But, as follows from the known equation =2/L, where L is the coherence length, and taking into account the fact that the width of the interference peak is basically proportional to the wavelength , this spectral broadening can not explain the observed reduction of speckles at pulsation on the basis of all hitherto existing understanding of the causes of speckles.In particular, the measurement data described in the application WO 96/08116, still show the presence of small speckle patterns. If the correct interpretation, according to which the speckle structure mainly depends on the coherence length, then the other light sources such as gas discharge lamp with the same coherence length (1 PS corresponds to L0,3 mm) could create similar speckle structure. However, nothing about this Auctionsi only a few moments, consequently any method of reducing the speckle is mainly based only on experimental data.From a technical point of view, the disadvantage of the above is that described in the literature method of reducing speckles may not necessarily be applicable not only different, but even to the same devices. Due to the lack of a General theory of the appearance of speckles on the basis of which it would be possible to make a conclusion about the appropriate mechanisms for reducing speckles may even be the case when a method of reducing speckles, who happened to be effective on the prototype during production will encounter insurmountable difficulties. Thus, in relation to any of the known methods cannot with absolute certainty that he will provide a sufficiently high reproducibility of the results.The present invention was based on the task to improve the known from the prior art device in such a way as to ensure effective, generally applicable and reproducible reduction of speckles.This task is solved through the distribution of the phase shift of the laser beam, which is received in reasoon shift more coherence length L, multiplied by a factor of
This decision is unexpected. According to DE 19501525 C1 would be expected that the average phase shift would have to be comparable with the wavelength. To infer any connection with the coherence length on the basis of the specified publication impossible.Known from WO 96/08116 results reduce the speckles could, as mentioned above, lead to the conclusion that the reason for the reduction of speckles is to design a special type of laser. Above in this regard gives reason to doubt the effectiveness of such reduction of speckles, because proceeding from a high density of photons and matches the phase itself would be expected that in the same time interval are still a sufficient number of photons to be coherent.The invention is distinguished, in particular, the fact that owing to the phase shift difference in the path length should be not less than that is not obvious from the technical solutions proposed in the application WO 96/08116.The proposed solution is based on new considerations necessary for appropriate interpretation of the results of our experiments, Davitaia follows, for its implementation it is possible to refuse required under DE 19501525 C1 small structures used for out-of-phase parts of the light beam, and therefore the invention can be used without recognized adverse arising due to the small structures of the phenomena of diffraction, resulting in deterioration of the light beam.Because this technical solution has arisen on the basis of General theoretical reasoning regarding occurrence of speckles that match all conducted in this regard, experiments, there is no reason to doubt the possibility of transferring this decision on variety of devices such as laser printers, video equipment or other devices for playback of images. Therefore, the reproducibility of reducing speckles according to the invention is provided with the greatest possible reliability.As mentioned above, necessary according to the invention the phase shifts can not only stochastically distributed, but follow some functional patterns that clearly distinguishes this invention primarily from occurring usually in the literature, methods of reducing speckles using stochastic is al, due to the appropriate location which different photons will pass the way of various lengths.However, the device can also implement a very simple way using the preferred option where the first structure is made of transparent material with stochastically included particles having compared with the surrounding material of higher refractive index, and the resulting difference in refractive index leads to phase shifts.This way you can, for example, to obtain a structure with a small transparent particles having a higher refractive index and inside linking these particles. In particular, for making it possible to use a commercially available conventional materials, providing, for example, the placement of the first structure on the screen, and the thickness of the screen according to the invention should be chosen so that the mean value of the phase shift was possible to obtain the optical path length, the greater the value specified
Therefore, the thickness of the structure, i.e. the path that the laser beam passes through this structure, the mod is of a technical solution. However, the thickness of, for example, of the screen, substantially defines a selected index of refraction. For practical use, in particular for laser vision system with lasers, it was found that the thickness of the first structures can be implemented within reasonable limits, the components up to a few millimeters, if the difference in the refractive index of the particles in accordance with the preferred implementation of the invention in comparison with the refractive index of the surrounding material exceeds 0.1.From the above it follows that the particles, leading to stochastic phase shift, also should not be too large to install adequate phase difference on the basis of the refractive index or differences in optical path length. In this regard, in accordance with the preferred execution of the invention it is assumed that the particle size in the propagation direction of light should be less than 0.5 mm, in particular at least some of the particles should be less than 0.1 mmWe have already mentioned that the first structure should have only a small length in the propagation direction of the laser beam. For Scala to increase the phase shift due to the increase of the wavelength of the laser beam.In another preferred embodiment of the invention for image playback screen, and the first structure is at least partially located in the screen or in the same layer of the screen.Thanks to this option when designing the first structure is not necessary to store the index of the laser beam. This provides great freedom of choice when creating the first structure. The area of the screen when it does not play a significant role, as the screen, such as a video, himself, had to scatter light, so that the audience could see the video from various directions, and away from the screen even require reduced the indicator beam.You can use special low first structure, if the coherence length due to proposed according to the invention the solution itself is very small. Therefore, according to a preferred variant implementation is envisaged to use as the laser is a pulse laser with a pulse duration of less than 10 PS. The coherence length is calculated usually by multiplying the duration of the pulse at the speed of light. This means that the coherence length in such a laser is 3 mm, Thus, the duration of them="ptx2">With such a small coherence lengths for tangible reduction of speckles, it is advisable to use lasers with a bandwidth gain of more than 100 GHz, primarily over 300 GHz.In particular, it has been found that video can be significantly improved if a greater degree to reduce the width of the pulse, and thus the coherence length. In accordance with this preferred embodiment, the invention provides for the use of the device at least one laser, emitting in the red, green, or blue regions of the spectrum with pulse duration less than 4 PS and especially less than 2 PS, if this laser emits in the red region of the spectrum, less than 3 PS and especially less than 1.5 PS, if this laser emits in the green region of the spectrum, and less than 2 PS and especially less than 1 PS, if this laser emits in the blue region of the spectrum.This execution allows to obtain the coherence length of the order of 0.3 mm When it is a particular advantage that the structure can be performed in very small sizes.However, in particular at the video hosts on the path of the laser beam, the optical element is to give they will also lead to slight differences of the phases of the photons in various places that, therefore, when the respective design data of the optical elements as the first phase-shifting patterns allows you to refuse the application of the relevant individual structural elements to obtain the phase difference according to the invention.As the optical system in this respect may serve, for example, an extender optical system or Fresnel lens installed in front of the screen along the beam and originally used for other purposes, such as to increase the deflection angle deployed in accordance with a particular video standard laser beam.Essential for carrying out the invention is the choice of an appropriate length of coherence. As follows from the Fourier transform of the finite wave train of length L in a vacuum, the final wavelength always reflects the width of the spectrum associated with the coherence length L ratio L =2/. However, a priori it is not necessary that each width of the spectrum certainly led to a decrease in the coherence length. As it has been unexpectedly found that, however, is understandable given its large, in order to implement according to the invention is associated with the width of the coherence length.To this end the device in accordance with the preferred execution of the invention there is a second structure, which has a source of local quantum-mechanical perturbation crystallites or elements and which allows for such local quantum-mechanical perturbation of photons to reduce the laser beam, primarily by increasing the width of the wavelength spectrum of the laser coherence length. In accordance with that obtained from the relationship L =2/ the coherence length is reduced by increasing the width of the spectrum .
It is known that the spectrum of the photons can be extended due to the recoil energy of molecules or atoms in the passage of light through the material. Related effects, such as the Raman effect is usually negligible, and therefore they only can be used to increase the width of the spectrum.However, more detailed studies show that local quantum-mechanical perturbation can be obtained, and by a special suitably designed structures. But for understanding emerging affectional the execution of the invention.Used the basic principle of such structures is that the photons in the laser beam at the time localized in a narrow range, resulting on the basis of the uncertainty principle there is a slight broadening of spectral lines. At the corresponding number expected according to the principles of quantum mechanics perturbation occurs, the corresponding spectral width, which due to the broadening of spectral lines can provide effective reduction of the coherence length.As in the case of pulse duration, in relation to a given width of the spectrum you can also specify the optimal values for certain devices. Based on calculations similar to the above relative to the pulse duration, in accordance with the preferred implementation of the invention defined by the coherence length of the laser beam or its reduction by using the second structure, the width of the distribution of spectral lines with wavelengths greater than 0.5 nm.However, under this option perform the required width of the distribution of spectral lines is achieved not only through the second structure. You can find lasers treat fiber lasers.Because on the basis of differences in refractive index can be implemented as first and second patterns, in accordance with another of the preferred embodiment of the invention, the first and second patterns can also be combined into one common structure. Thereby reducing the speckle patterns can first run very small, which will allow you to find a place for them, for example, even in a laser printer.In particular, for the second structure in accordance with the preferred implementation of the invention, it was found that it is expedient to provide for the variety along the beam propagation of the phase-shifting elements having a small length in the direction of the beam, which does not exceed twenty-fold and above all two-wavelength. Due to this broadening of spectral lines in the quantum local perturbations becomes so large that to set the width you want only a small number of such elements.Due mainly Gaussian broadening as a result of perturbations in a separate element, it can be expected that the total broadening with a large number of such is more effective broadening of spectral lines in the second structure can be achieved in accordance with the preferred implementation due to the streamlined forms and even the location of the phase-shifting elements at this distance, where local quantum-mechanical perturbation in the formation of the scattering matrix describing perturbations from several elements, common-mode stack.As described in more detail below in the description of examples of implementation, the width of spectral lines obtained in this case using the second patterns, proportional to the amount of phase-shifting elements, and not the root of their number. Thus, to establish the required width should be significantly less items, so the second structure may have a simpler construction and can be cheaper to manufacture.As in the case of determination of the length of coherence of the pulsed lasers with pulse width, the width of spectral lines can also be specified optimum value. Accordingly, if preferred embodiment of the invention provides at least one laser emitting in the red, green, or blue regions of the spectrum, the spectrum width for the individual colors is:
> 1.3 nm for the red region of the spectrum,
> 0,9 nm for the green region of the spectrum and
> 0,75 nm for the blue region of the spectrum.The first structure may izgotovlen the positive option of carrying out the invention has been set, first of all, it is advisable to use a grain size less than 0.5 mm, in particular less than 0.1 mmIt was found that for combining the first and second structures in one particularly preferable to use particles consisting of at least two phases with different refractive index. This is because using these two phases can be implemented and the second structure, while the grains themselves will take on a significant function of the first structure. Thus can be a simple way to get above joint structure for the first and second structures.Forming a second structure consisting of particles of the material of the screen, as already detailed above, in accordance with the preferred implementation is optimal only in the case when the length of the at least one zone between the two boundaries of the phases in the direction of propagation of the laser beam is less than 20 wavelengths and especially less than 2 wavelengths.To comply with the specified requirements of the joint structure for the first and second structures in a preferred embodiment, the most suitable polytetrafluoroethylene (Teflon). Teflon comes in chastest the grain size, component of less than 0.5 mm. in Addition, grains contain so-called crystallites with a diameter of about 1 micrometer, in which the inclusions are present in the amorphous phase. Diameter of about 1 micron also provides the optimum of the second structure with the desired order of magnitude, slightly higher than twice the wavelength. Thus, the Teflon is a material that allows you to comply with requirements for structures requirements.In relation to the properties of Teflon in this context, one should refer to the following publications: C. J. Speerschneider and C. H. Li, "A Correlation of Mechanical Properties and Microstructure of Polytetrafluoroethylene at Various Temperatures", Journal of Applied Physics, I. 34, No. 10, October 1963, page 3004-3007, and Solomon Fischer and Norman Brown, "Deformation of polytetrafluoroethylene from 78 to 298 K and the effects of environmental grazing", Journal of Applied Physics, I. 44, No. 10, October 1973, pages 4322-4327.Teflon is also a good surround diffuser, so it is most suitable for use as a material for the manufacture of screens in a preferred embodiment of the invention, where the first structure is the screen or the screen layer, achieves several advantages. One of the advantages of Teflon is that inspires fear the deterioration of the laser PI.In particular, from the screen requires a certain distribution of scattering angles of radiation, so that the audience could see from various directions, the video is played, for example, with a laser.In the construction of the screen it should be noted that it is also possible following options perform, based on other considerations, in particular, small coherence lengths of the order of several millimeters or less.In one of such options is envisaged that the thickness d of the screen or layer defined in the propagation direction of the emitted laser to form the image beam is higher than the critical thickness dkritcalculated on the basis of the average distance b between the generated laser beam on the surface layer or screen and is known as a speckle interference maxima according to the following formula:
< / BR>where denotes the average value of the cosine of the scattering angle for the distribution of scattering angles characterizing the deviation of the laser beam particles, respectively structures.Thus, compared to the prior art the above solution is surprisingly simple and is based on the selection sootvetstvuyuschih on a fairly large depth of speckles scattered in this place structures, and therefore, the interference minima on the surface of the screen from its surface also comes light. Thereby effectively decreasing the contrast of the speckles. Given the equation forkrita simple consequence of this view, as described in more detail below. At a much greater thickness of the screen or the applied layer in relation to the dkritother layers help to reduce the contrast of the speckles, which consequently must be completely blurred.This solution is very simple and inexpensive to implement, which is primarily due to the following variants of execution of the invention.The speckle size is determined mainly by the diameter of the beam. As can be calculated from a simple consideration on the basis of the interference optics (see M. I. Yoder, D. G. Youmans: "Laser radar wavelength selection and trade-offs", SPIE, so 999, Laser Radar III (1988), pages 72-83) at a distance S from the laser to the screen, and when the diameter D and an effective wavelength in the spectrum of the laser speckle size is Stan/D. However, since the speckle size can not be larger than the diameter similar to the estimated fair only under the condition that D2>s
In such cases, in accordance with a preference for the main wavelength of the outgoing laser beam equitable dependence of D2>S, and the thickness d of the screen or more layers
This thickness may be approximately twice the dkritthanks to what is expected and much better erosion of speckles and, therefore, less contrast.In accordance with another preferred implementation of the invention, the thickness d has a value whose functional characteristic values of contrast K(d) depending on the thickness d takes a value less than 0,20, in particular less than or equal to 0.05, and the specified functional characteristic values of the contrast is measured by the formula of dependent arising, the maximum light intensity Imaxand the minimum light intensity Iminin uniformly irradiated by the laser beam area at the corresponding layer thickness d on the surface of the screen.This thickness, as experience shows, also significantly exceeds the dkritbut it is expected to significantly lower the contrast of the speckles. In addition to the above physical characteristics of the contrast of the speckles depending on thickness in this embodiment of the invention takes into account the physiology of the eye. This is because, as has been established, the speckles below the boundary contrast to enemalta, and when it is, in particular, 0.05 is generally not perceived by the eye. Thus, the thickness of the layer in accordance with this option run selected so that the speckles effectively extinguished, because they cease to be perceived by the eye.For the practical implementation of the above option, you can measure the functional characteristics for the evaluation of various materials. However, to reduce the cost of measurement, it is possible to estimate the required thickness in accordance with another preferred implementation of the invention.According to this variant, the thickness of the screen should be where f denotes a number less than 0,20, in particular less than or equal to 0.05, where Imaxmean maximum, and Iminmeans the minimum luminous intensity within a uniformly illuminated by the laser field on the screen thickness dkrit.All the above-mentioned embodiments of the invention can be implemented in a simple way, if you run the screen of the sintered polytetrafluoroethylene (Teflon material).However, in accordance with the preferred implementation of the invention, the thickness d should be larger than 0.1 mm, in particular greater than 1 mm and pricedata significant contrast of speckles. However, unexpectedly it was found that the thickness required for complete disappearance of the contrast of the speckles depends on the coherence length. The optimum thickness is determined by the coherence length of the used laser beam in accordance with the preferred implementation of the invention, according to which the laser light has a coherence length L and the thickness d of the screen or layer is d(0,04 L)1/225%. In particular, the display of material Teflonmade from powder source material with grain size ranging from 20 to 800 μm in the form of a screen thickness of 4 mm, when using a laser with a coherence length of 4 cm (according to the manufacturer's laser) more speckles were observed.Below the invention is explained in more detail with examples of its implementation with reference to the accompanying drawings on which is shown:
in Fig. 1 device with a laser to display the image shown in the example video,
in Fig. 2 is a schematic diagram explaining the origin and elimination of speckles,
in Fig. 3 is a variant of the first patterns of the correct form to reduce speckles intended for Ostrikov, intended for the device of Fig. 1,
in Fig. 5 - the first structure, which can, for example, be provided in the screen provides playback of stereo images
in Fig. 6 is a schematic depiction of structures made of a material Teflonthe screen for the device of Fig. 1,
in Fig. 7 - second structure of regular form to change the spectrum width of the laser beam,
in Fig. 8 is a flow chart of manufacturing the structure according to Fig. 7,
in Fig. 9 is a diagram illustrating the contrast of the speckles of the screen at a given layer thickness d, and
in Fig. 10 - diagram of the contrast as a function of the ratio of intensity of light in an interference maximum light intensity in the interference minimum.In Fig. 1 shows an example of the device with the laser and screen for displaying images. In this example, the generated color video image, therefore there is not one, but three laser 10, 20, 30, emitting light with wavelengths of the primary colors to generate pixels of a video image. However, coming from lasers 10, 20, 30 laser beams 12, 22, 32 in this example is not yet modulated to adjust the brightness and color correction points videosurgery is a part to perform directly using videocasette. When using laser diodes instead of gas lasers 10, 20, 30, the intensity of the laser beams 12, 22, 32 is changed directly by modulation of lasers information intended for reproduction of image points.In addition, it was found that to not be associated with high costs reduce the speckles due to their wide range is preferably used, in particular, fiber lasers, as described in more detail below.However, for modulation gas lasers 10, 20, 30 on the path of propagation of the laser beams 12, 22, 32 have special modulators 14, 24, 34. The modulators are made of DKDP crystals that change the polarization direction of the laser beams 12, 22, 32, so they installed them the polarizing filter is modulated in intensity depending on the control voltage. Next, the laser beams 12, 22, 32 using system 38 mirrors are in full light beam 40, which in this form passes through the subsequent elements of the system.Full light beam 40 with deflector consisting of a mirror drum 41 and oscillating mirror 42, is deployed in rows and frames on the screen 43, the serial is 4, 24, 34 is provided corresponding to the brightness and chromaticity of each pixel in the image.Used in laser TV technology scan beam is known from conventional television, which uses tubes. However, used in this case, the technology is different from the usual fact that instead of an electron beam is used full light beam 40, and a conventional magnetic deflection of the beam in the picture tube replaced by a manual scan using the mirror drum 41 and the tilting of the mirror 42.However, the scan is not limited to the use of mechanical means. It can be done, for example, and using acousto-optics.In addition, in Fig. 1 presents an extender optical system 44 and the Fresnel lens 45, used in this example to increase the size of the image even at small angles of deflection of the beam. However, these optical elements can be constructed in such a way as to create a Fresnel lens 45 or lens extender optical system 44 different local phase shifts, consistent with the coherence length of the laser, and the data elements are the same as more sweat beam 40 it is the best fit for interference. This property of laser beams, which in other cases may be regarded as positive, for example in areas such as interference optics or holography, in the projector according to Fig. 1 is an extreme unwanted. This is because any small deviation in the trajectory of the light beam causes interference, manifested in the form of brilliant points within each pixel in a video image, the so-called speckles. Speckles in General interfere with the viewer, and therefore, to obtain an acceptable video image in any case it is necessary to eliminate or suppress.Usually the appearance of speckles described in the literature as single-photon interference, arising, as it is customary to consider in optics, due to the spread of individual waves in the state of a photon in the simple addition of waves. However, this simplified view is contrary to the following experimental results:
1. a diffuse screen speckles are visible, but on a smooth reflective screen is not visible;
2. the scattering screen with stochastic phase shifts of the order of one or more wavelengths in any case gives a high contrast speckles;
surface of stimulated emission, reproduces the image without speckles.What these observations contradict the simple addition of waves in the form in which it is known by the single-photon interference, more substantiated below. When this value beginning with capital letters x, y, and z, denote the corresponding vectors.To p. 1:
If we proceed from the fact that due to the high coherence of the laser photon, coming from a point x1 of the laser and the photon emerging from the point x2 laser, emitted each respectively with wave number k = 2/ and that these photons have a constant ratio of the phases due to stimulated emission, when a simple superposition of waves locally would be the next interference member:
regardless, does the screen rays or scatters. Such independence was not observed. Such a representation, on the basis of which one could conclude that reflective screen visible speckles, would be contrary to and describing light emission Maxwell equations.To p. 2:
As follows from the above interference member, stochastic phase difference of the order of one wavelength would lead to the disappearance of n completely eliminate speckles.To p. 3:
The difference between the generation of incoherent optical radiation and generating coherent optical radiation indicates the presence of the effect of optical density. The flux density of photons in the generation of coherent radiation is much higher.The conclusion that can be drawn from paragraph 3 suggests the existence of phenomena multiphoton interference, which in the literature about speckles, not mentioned. This circumstance may have led to the fact that the problem of speckles when playing back images to date have not found a satisfactory solution.Multiphoton interference is discussed below on the example of the most simple model of the interference of two photons. Neglecting the insignificant values of the normalizing coefficients, essentially no effect on the outcome. In the following discusses the two-photon wave function module of the square which shows the probability of detecting two photons at one point or at different spaced apart locations. The probability other than zero, is an important prerequisite to ensure that the interference of two photons could take place.
< / BR>The second member is the result of zimmerservice first based on the statistics of Bose, which are subject to the photons according to the principles of quantum mechanics.The probability of the presence of both photons at the point z is calculated on the basis of the following formula:
If we compare this expression with the above, based on unrealistic interpretation of phenomena as single-photon interference, which is a sum of the wave function, we can see that the possible interference depends on the width of the spectrum (k1-k2), and not only on the wave vector k. This means that the laser radiation of high coherence, i.e., at a sufficiently small value (k1-k2), the cosine is equal to 1, and the interference is absent, i.e. there are no speckles on reflective surfaces. Conversely, if the path of propagation of beam scattering structures may occur interference, as illustrated below by the example in Fig. 2.In Fig. 2 shows two points x1 and x2 and two points y1 and y2, in which, for example, two points on the screen 43 are scattering the VA photon to interfere. This point z can be, for example, another center of scattering on the screen 43, or it can serve as the retina of an observer.In principle, for the preparation of two-photon wave function would have to add all the components generated in the different photons emerging from the points x1, x2 in the output section of the laser and passes through the points y1 and y2 to the point z. However, for understanding the mechanism of occurrence of speckles interest in this case are only those members that have the status of photons on the intervals of z-y1, z-y2 uniquely describes the wave numbers k1 and k2. To visualize this condition separate segments of the trajectory of the beam in Fig. 2 the corresponding wave numbers k1 and k2.The corresponding square of the amplitude gives the following interference member of the two-photon wave function:
This member is constantly equal to one if x1=x2, i.e., if you are able to interfere at the point z photons is largely emanate from the same point of the output section of the laser and from points spaced from each other at distances at which the phase difference in the expression 1.0 multiple of 2. Thus, in the case of two-photon interference already VCE.For further consideration, we should consider the approximation of the above expression describing the functional behavior of the interference member in respect of the points y1 and y2. Here, it is assumed that the points x1 and x2 lie in the same plane located perpendicularly to the plane of the drawing, and y1 and y2 are any two points lying in a plane parallel to the first and also perpendicular to the plane of the drawing, both of these planes are separated from each other by a distance A. in Addition, it is assumed that the vectors of the points x1, y1, x2 and y2 lie in the plane of the drawing, and the values X1, X2, Y1, Y2 are the corresponding projected components of these vectors in the plane in which lie the points x1, y1, x2, y2. At a very great distance A with respect to the components X1, Y1, X2, Y2 for any arbitrarily taken pairs of components of X and Y in the respective planes you can use the following approximation:
< / BR>Then for an argument after obtaining the average value of x1 and x2, excluding continuous phase, based on this averaging in the argument of the cosine receive an expression of the form:
< / BR>where <X1-X2> as the average values for the points x1 and x2 of the output section of the laser is approximately equal to the diameter is at such distances on the screen, which give in-phase integer multiple of 2, i.e., they mostly separated from each other by a distance of order A/<x2-x1>. Thus consisting of pairs of photons of light coming from the y plane, according to their ability to interfere can generally be compared with the lattice. However, this ability is partially suppressed possible uneven scattering from the scattering elements, which in this case leads to the appearance of speckles.In contrast to the cases described in the reflective surface such interference maxima far again are in full beam, so this example explains why the speckles can be observed only from scattering surfaces.It should be noted that in the above expression 1.0 for the cosine of the phase difference do not affect the differences in the path length of the rays y1-z and y2-z. It follows that shifts phases on the screen with values of the order of the wavelength to the output photons as a factor affecting the member, can be neglected. Thus, resulting from multiphoton interference speckles by using the phase shift of the order of the wavelength according to this observation cannot completely eliminate.Next, based on the above interpretation can be the clov: the above observations for x1 and x2 in one plane only apply laser, working in the mode of generation of coherent optical radiation, because only in this case, the phase state in the output section of the laser unambiguously defined. In contrast, in the mode of generation of incoherent optical radiation from the fiber laser points x1 and x2 occur photons are shifted relative to each other in the direction of the optical fiber, resulting in the propagation direction of the laser beam data points x1 and x2 is considerably removed from each other, and therefore, when integrating over x1 and x2 averaging the expression cosine disappears.These examples are considered in the models can explain the occurrence of speckles in accordance with experimental results, but not lead a priori to finding solutions to problems of speckles.According to the invention for removing speckles offers various ways to provide for photons corresponding phase shifts. For clarification should be considered are highlighted in Fig. 2 local zone 50. If this place near the x1 could change the path length of the photons on the value of A that goes from x1 photons regardless of the situation with wave numbers k1 and k2 were more a long way in variabledata with A specific width > (2/k1-k2) vanishes interference member 1.0, and the speckles should not appear anymore.This means that in order to effectively eliminate speckles must comply with the following condition:
The more A, the more effective the suppression of speckles. The recommended lower limit, in addition to the above-mentioned equation 1.1, is defined, in particular, the inequality
However, on closer examination it appears that the exact lower bound for the effective suppression of speckles depends substantially on distributions, effective to change the path length of photons. The following examples help to illustrate it more clearly.Suppression of speckles can be made at other points other than the point 50. For example, a screen as described in the following examples, can be characterized by a stochastic distribution of optical path length with a Gaussian of width A. In this case, the statistical broadening of the points y1 and y2 occurs twice, i.e., since the width of the Gaussian function is the square of the ratio of 1.0 implies the following inequality:
< / BR>Other boundary value for the coherence length, which comes effective reduction of speckles, get next item is Uchenie, as is known, equal to the total width divided by and therefore, for effective reduction of speckles at all possible distributions as the lower border of the specified width k of the laser radiation we have the following inequality:
< / BR>However significant in all of these equations is that the necessary increase in path length is always inversely proportional to k. Therefore, the spectral width of the laser beam should be as large as possible if the effective reduction of speckles need to get at small values of the difference in the length of the path A.
To this end, for example, you can choose a laser with a very wide spectrum. In particular, fiber lasers have such a wide spectrum, and therefore needed to reduce speckles the difference in the path length A can be maintained within reasonable bounds in the millimeter range and below.In addition, the pulsed laser depending on the pulse duration also obtained a sufficiently large width. The corresponding broadening of the Fourier transform result of the final Zug light waves with a length L defined by the following formula:
k = 2/L 1.2
However, it is necessary to take into account that the Fourier transformed.The Gaussian width in this case cannot be specified in General terms. However, it should be assumed that the laser pulse is not strictly rectangular shape, which is obtained by using Fourier transform, the width does not match exactly the actual spectrum. Therefore, the dependence according to equation 1.2, however, corresponds to the practical conditions. However, in General the calculation of the coherence length given by equation 1.2, caution should be exercised, because less than 1/10 of the maximum intensity, as shown in the example in Fig. 5-7, can be used to effectively reduce the speckles.Therefore, to formulate a condition for the optical path length depending on the length of coherence with the aim of reducing speckles can only take the lowest possible limit for standard A. This means that, in General, can be expected to effectively reduce the speckles. However, in principle we have the following rule: the higher the selected value of A, the more effectively suppressed speckles.If provided, the difference in optical path length must be within a few millimeters, in accordance with the above equation.E. 3 mm optical path length for momentum.In particular, for example according to Fig. 1 depending on the color of the lasers 10, 20, 30 were set to the following values:
- 4 PS and especially less than 2 PS for laser, emitting in the red region of the spectrum,
- 3 PS and especially less than 1.5 PS for laser, emitting in the green spectral region, and
- 2 PS and especially less than 1 PS for laser, emitting in the blue region of the spectrum.Since the spectral width is usually measured not in wave numbers, and depending on the wavelength, especially important following dependence arising from equations 1.2 and related to the known ratio of the wave number and wavelength:
< / BR>With this expression it can be easily determined and the spectrum width of the laser, whereby when a given difference in optical path length is expected to reduce speckles. Only with a slight difference in optical path length, so that it is possible, for example, to use a thin screen 43, the optimal values for the example in Fig. 1 is > 1.3 nm for laser, emitting in the red region of the spectrum, > 0,9 nm for laser, emitting in the green spectral region, and > to 0.75 nm for laser, emitting in the blue region of the spectrum. This hotel is no effective reduction of speckles in "pearl" screen for diapouli.Similar to the width of the lines can easily be obtained also by using fiber lasers, so they are preferred for use in the example according to Fig. 1.However in the future it is also planned projection of stereoscopic images using polarized light. This can be achieved by using special glasses and a separate play one image for each eye of the viewer using light with different polarization. However, to preserve the state of polarization thus, you cannot use these as examples of "pearl screens. Therefore, the elongation of the beam path on the value of A in this case will have to implement otherwise.As explained above in the example of Fig. 2, at point 50 in accordance with Fig. 2 can also include the change in optical path length to reduce speckles.An example of implementation of this variant is schematically shown in Fig. 3. The diagram shows a prism 52, mounted according to Fig. 1, respectively, between the modulator 14, 24 or 34 and the system 38 convergence. The placement of the prism 52 before system 38 information is most appropriate, because in this case not in Knuth when placed over the system 38 information due to differences in the deviation of the laser beams 12, 22, 23 in the full light beam 40.Depending on the location x1, respectively x2 photon in the laser beam according to Fig. 2 in the prism have varying phase difference, which can annihilate each other in obtaining the average value in accordance with equation 1.0, if these different phase difference caused by the prism 52 is sufficiently large.In the example of Fig. 3 maximum phase difference in diameter of D <x1-x2> laser beam 32 is, as shown, 2w(n-1), where n denotes the refractive index of the material. When using a glass prism with an angle of 90oand the approximate diameter of the laser beam D = 2 mm is possible, therefore, the phase shift due to the refractive index from one edge of the pulse laser beam 32 to another to obtain an average effective difference in optical path length of about 2 mm In larger coherence length of the laser beam emerging speckles need to purchase an elliptical shape, and with significantly lower values in this case should be expected bands.The fact that the speckles in the result is not completely extinguished, and are manifested only in the form of strips, is due to the fact that shifts phases when using prism 52 action is at least three different prisms 52.In contrast, in the example according to Fig. 4 requires only one element 54. It is made in the form of an almost rotationally symmetrical dome with the outer surface 56 and the cavity 57. The outer surface 56 and the inner, which is in the cavity surface 57 and 58 are of such form that the prisms in the material of the element 54, the beam runs parallel to the Equatorial plane 60. This ensures the preservation of the laser beam 32 its shape as it passes through the element 54. However, in this case, as in the example according to Fig. 3, the individual photons depending on their entry point into the optical element 54 are way different lengths, which leads to an effective phase shifts. According to estimates by using this optical element with the same ratio of dimensions, as shown in Fig. 4, and when the beam diameter of 2 mm can be obtained for the difference in optical path length of the order of a few tenths of a millimeter, which is sufficient to effectively reduce the speckles in the above-mentioned width of the spectrum.Instead used in the example according to Fig. 4 element complex in production form for the parallel passage of all light beams in the cavity 52, it is possible, if we allow for some expanded is checked against if necessary, the expected expansion of the laser beam through an additional optical system.The examples in Fig. 3 and 4 show that achieved in the refractive material of the phase difference sufficient to produce a corresponding change in optical path length. Such changes phases can be expected from the Fresnel lens 45, extending from the optical system 44, and under certain conditions even from system 38 information. These changes may be on the order of several tenths of a millimeter, which when appropriately matched to the coherence length of the laser is sufficient to use these systems as the first structure to reduce speckles.In Fig. 5 schematically shows a screen 43, which can also be used to play without speckles stereoscopic images. Thus in order to simplify the explanation of the principle of reducing speckles not shown scattering structure for expansion of the solid angle for different observers.Screen 43 of Fig. 5 mainly comprises beam splitter or partially transmissive mirrors 62 and another mirror 64. Between the mirrors 64 and 62 is sitopladi transparent material 66, which, firstly, gives the screen 43 sustainability, and secondly, allows for a given length coherentness the possible phases.When you hit full light beam 40 to the beam splitting mirror 62 of its rays are reflected. The other part is reflected, getting on the mirror 64, which thereby causes a difference in optical path length. In particular, is also a distribution of optical path length, because a certain part of the laser beams, as shown in Fig. 5, is reflected many times. Thanks to this emerging stream of light rays occurs desired difference in optical length of their way to avoid speckles.The screen of a desired type may be performed, for example, from a sheet of glass with double-sided rough surface, on which napylyaetsya mirrors 62 and 64. For more thin and light screen 43 may be used commercially available Mylar film with a mirror coating on one side to the other side of which may be coated with a partial reflection. The roughness of the surface for scattering of rays in different directions can be achieved by inhomogeneous heat treatment of different places Mylar film, in which she jarred on small plots.Several options, which is an alternative to the screen shown in Fig. is anati the polarization of the light and to avoid therefore the ability to display stereoscopic images.Such screens may be molded from sintered granulate or of the phase-shifting particles, respectively changing the optical path length. The grains with the diameter determined by grain differences in optical path length is approximately from 0 to , so when the difference in refractive index n relative to the surrounding grain space beam when passing through the grain, you can expect the maximum difference in optical path length of the (n-1), which roughly corresponds to the mean change in optical path length under the condition of uniform distribution is approximately (n-1)/121/2. When the thickness d of the screen, the laser beam passes approximately through the d/ grains. The standard deviation along the length of the waves add a few grains in a quadratic dependence, resulting in this case, when the thickness d of the screen 43 expected standard deviation, approximately amounting to:
< / BR>The experiments showed that the most suitable for damping speckles material is Teflonthat is sintered granules and from which the molding is made screens. The expected refractive index material Teflonranges from 1.2 to 1.4.In Visina the rum grain 0,4 mm
Thus, in accordance with the above equation is made of granular material Teflonscreen with a grain size of 0.4 mm, a refractive index of 1.4 and a thickness of 4 mm would have to give the difference in optical path length in accordance with the mean value a of 0.15 mm, This means that on the basis of theoretical calculations made above in the description of Fig. 2, such a screen could suppress speckles of the laser radiation with the coherence length
In fact, it was found that the speckles on the screen was not perceived visually already when specified by the manufacturer of the laser coherence length in 4 see This contradicts the expected result. Although the above evaluation of the quadratic difference in optical path length is very rough, however, more accurate calculations, in which consideration was taken into account and differences in wavelengths due to scattering rays grains showed that the assessed value is too small, a maximum of 2-3 times. This means that there is still a large discrepancy between theoretically calculated coherence length, which allows the observed reduction of speckles, and the coherence length specified fir is the principal effect, due to which is a more effective damping of speckles. To examine this assumption, we studied the dependence of the coherence length. For fiber laser with a wavelength of approximately 500 nm, a spectral width of 2 nm, i.e., the coherence length of approximately 0.2 mm, it was found that the speckles are no longer visually perceived when the thickness of a layer of Teflon1 mm, However, the laser with a coherence length of 4 cm was given when this thickness is clearly visible speckles. This implies that a reliable damping of speckles occur depending on the length of coherence, and therefore must be fair described above with model options.The resolution of this discrepancy must be sought in the material Teflon. This is because Teflonhas a very complex structure, schematically shown in Fig. 6.In Fig. 6 schematically shows the grain 67 Teflon screen, which should correspond to the above-mentioned refractive index of 1.2 to 1.4. However, as already known from specified at the beginning of the description of the literature, grains themselves 67 also have some structure. In other words, they contained the so-called crystallites 68, present is the observations, had a length of about 100 μm by a very small thickness of a few microns or less prior to 1 μm. Between crystallites 68 there are inclusions of air (n = 1).In between crystallites 68 is an amorphous material 69, the difference in refractive index compared with crystallites 68 should be on its own measurements of the applicant of the order of 0.1. However, due to the small refractive index from that consisting of crystallites 68 and amorphous material 69 of the second structure should not be expected to change the optical path length.However, this second structure formed by crystallites and amorphous material, can affect the coherence length on the basis of the following considerations.About the wave having a certain length, it is possible, as underscored by the Fourier transform, to speak only if the wave extends from negative infinity to positive. When there is any interference, for example, when the wave train is limited to pulsed radiation, or when directed by the wave quantum in a locally limited region of space goes into another state, should, on the contrary, expect the spectral broadening.Yes is role related to its momentum.For example, the wave propagating only in a limited region of space, i.e., the coherence length, always spectral broadening, as is evident from the following calculations.For a single photon with wave train having a coherence length L, and with the wave vector k0, wave whose length L normalized by the number of photons equal to 1, get the following dependency:
for-L/2xL/2, otherwise null.In a known manner by Fourier transform using operator
< / BR>in k-space get the following expression:
< / BR>Thus a range whose width is determined by the coherence length L. the effect of spectral broadening, as described above, can be used with pulsed lasers.For further consideration, you must enter a distribution P:
P(k,k0) = |(k|k0)2|2,
which usually describes the probability of detection occurred with wave vector k0a photon with wave vector k.Hereinafter for a better understanding of the coherence length should highlight the following points.In the local space on faviroute with each other, because the interference involves the superposition of the amplitudes of both photons in the same place at the same time. A similar explanation follows for k-space: a constant ratio of the phases can be maintained only when the two wave train have approximately the same wavenumber. If too large broadening of the spectrum due to too time-varying ratio of any phase dependent on the phase of the superposition is lost.This explanation clearly shows that violations of coherence is not necessarily to get a very short wave train, and the only important thing is how extended range. In the case of two waves of infinite length with various wave number of the relative phase differences in optical path length is lost when the phase difference of 2 or a multiple of it, and so for any width of the spectrum can also receive an amount equivalent to the concept of coherence length, and referred to below effective coherence length.In accordance with this approach, the effective coherence length L' for any distribution is calculated using the following formula:
A significant quantity is the average value of different is each time quickly decreases in the direction of infinity the distribution of the Gaussian width can be determined from the following formula:
< / BR>where k0means the average value of the distribution of k. It is obvious that the Gaussian width cannot be set in calculated above distribution, because painterly function due to member sin2(k-k0)L/2 for large wave numbers k still gives large components.The cause of high wave numbers are sharp edges at L/2 in the local space. However, this is adopted for calculating a steep increase from the physical point of view is unrealistic. It is therefore advisable to carry out the integration, taking the integral for calculating the Gaussian width only to a limited number of oscillations. Due to this uncertainty about the path of integration can be eliminated as follows.As noted above, the value of the Gaussian integral to a significant extent depends on the slew rate, respectively recession fronts that can be taken into account due to the limited length of integration. However, it is also expected that accounted for the length of the coherence width will vary depending on the form.Therefore, you should enter the coefficient of form, or the form factor F to account for various boundary parametera P receive the following:
< / BR>The expression in square brackets does not contain any more physical quantities and can therefore only be considered as dependent on the cutoff frequency is constant, at least for infinitely large areas of integration.To calculate the value in square brackets in this case we need only one value for the form factor f. However, it is easy to obtain, making the logical assumption that the effective coherence length L' at the condition of the above wave train is equal to the actual coherence length L. thus come to the following result: if the specified functional relationships, the value of the expression in square brackets should be equal (2)2that L' = L.The approach described above allows to resolve divergent in other cases, the integrals occurring in subsequent calculations, and get a finite, rational from the point of view of physics results.As follows from the above calculations, the end of the wave train and/or on the final width of the spectrum is obtained, the effective coherence length, i.e. the length over which the difference of two arbitrary ways photon interference more to come can't.From the point of view of the Oia indicates the broadening of the pulse, affecting limited local uncertainty in the detection of a photon.On the other hand, the uncertainty principle due to the fact that spatial limitation of the measurement process should be expected broadening of the pulse due to its absorption measuring instrument. As for the fundamental validity of the uncertainty principle type of measurement process does not matter, the essence of the physical process caused not by the measuring device, and the interference caused by them in relation to the measured particle. Similarly, the measurement process from any interference in this case we should expect a similar broadening of the pulse.The following calculations can be completely made without the introduction of quantum mechanical momentum. Instead, in the present case, all using the wave vector k. In spite of this these calculations coincide with the quantum-mechanical interpretation, since k according to the Planck formula is proportional to the pulse wave.A particle with wave vector k0with an amplitude of
< / BR>it is found in the form of particles with wave vector k. It is calculated using a transformation of the local space in the form of:
However, from these equations we can conclude that the broadening of the spectrum with small effective coherence length should be expected when the photon locally is in a state other than the state of the radiation, but may in this case be observed in the initial state. For example, in this case, one would expect also the broadening of the spectrum with the passage of a photon through a locally constrained element, in which the photon will have a slightly different wave vector due to other than 1 of the refractive index.On the basis of these calculations it is possible to calculate the effective length for Toora k0, which is located at the point x = b is the element thickness and the refractive index n has a wave vector nk0. The condition for such a wave train in the local space in this case can be represented as follows:
< / BR>In this phase equation written in the form of multipliers so that immediately make it apparent that the absolute magnitude and phase at each boundary surface take the required values, ensuring the continuity of the wave. If you compile the indicators, the following picture is obtained, greatly simplifies further calculations:
< / BR>For the amplitude except for trivial factors, such as AA, we get the following expression for k = k0-k:
Using the above equations for the effective coherence length L' in the approximation of a << L, based on the above expression to determine the effective coherence length, get a simple function that resembles the following:
< / BR>Thus, the effective coherence length due to interference significantly shorter than the actual length of coherence. However, for very large coherence lengths, this effect is very minor. Spatial interference from the element thickness of Dolce follows from consideration of several elements. If to denote the quantum-mechanical scattering matrix for a single element, with which quantum state with wave vector k2is translated into a state with wave vector k1then for m elements of the matrix has the following form:
< / BR>This integral is easy to calculate when the following conditions. Length L in this case again is very large compared to the thickness of the element a, so the member sinkL in the expression 1.0 can be approximated using the function . In addition, all S-matrices are omitted constant phase, because these calculations are of interest, only the absolute values of the squares of the matrices. In addition, the different elements located respectively at a distance of bjfrom the zero point, the amplitude in accordance with the expression 2.0 can be taken to correct for integrating the requirements in the following form:
< / BR>When this function Dirac corresponds containing sin(kL) the first term, and T corresponds to the term with a member of the sin(ka) in equation 2.0.In this case, after integration for the amplitude obtain the following expression:
< / BR>Integrating upromiseremindu is only in the composition of the phases on the basis of different locations bjelements of thickness a.To calculate the width of the spectrum of wave vectors need to re-calculate the following expression:
< / BR>Components-functions thus make no contribution, and involved only the squares of the sums of phases. When determining the numerical value of the integral is the integration sum of the phases gives the m elements provided statistical distribution of the location b of the individual elements. On the other hand, for a special occasion get the value of m2when the difference of path bjmultiplied by the wave vector, differ from individual elements in an integer multiple of 2.
Thus, for the length of coherence receive the following General equation for m items:
< / BR>where meffmean effective number of elements in the stochastic case is equal to the square root of the number of elements, but in the case of selecting the spacing between elements in accordance with the magnitude of a phase angle equal to a multiple of 2, can be increased up to m values.Thereby it becomes easier to understand the behavior of light in Teflon, acting as an effective coherence length in accordance with the considered patterns must be under the value n in a vacuum. In this case, however, the only important difference in the refractive index of the crystallites 68 from amorphous material. When the measured value of a difference of refractive indexes equal to 0.1, the expression n/(n-1) should in this case be approximately equal to 10. The value for a in podsolennom the expression must be averaged out based on different locations of the crystallites 68 relative to the trajectory of the light beam and thickness. On the basis of the S-matrix dependence of 1/a is the average value thus it is necessary to find through 1/a, i.e., a smaller thickness is given by averaging the largest component. The corresponding estimate for schematically shown in Fig. 6 patterns of Teflon leads to average
Obviously, if the length of the crystallites 68 an average of 100 μm on the path of propagation of the laser beam are only about sixty crystallites 68, therefore it should be assumed meffapproximately equal to 8. With these values for the coherence length 4 cm falling on the screen 43 of the laser beam 40, the effective coherence length is approximately equal to 0.4 mm Teflon that is most consistent with the above estimate of the average path length equal to approximately 0.5 mm, for experimentally found tological second structure, the average phase difference depends on the square root of the thickness. On the other hand, changes in the length of coherence in terms of meffdepends on the reciprocal of the square root of the thickness, and therefore, from the condition L<
above which could effectively reduce speckles.This generally means a material constant the size of the same length, and this constant includes the dependence of the refractive index, the refractive index difference between the grain size and so on, Instead of carrying out complex calculations on the mentioned parameters of this formula for materials having the first structure to obtain the trajectory of a certain length, and the second structure to reduce the length of coherence allows to measure the constant of the material in one coherence length and to assess the appropriate thickness for other lengths of coherence.If the screen 43 was not the second structure, for the mean-square deviation would be fair power dependence with fractional exponent, due to what could be dkrit~ L2.For the specified material is Teflon with a grain size of 0.4 mm was determined constant of the material K = 0.4 mm 25%. Pointing to a large error mainly related to the fact that the thickness of the x above calculations, the coherence length can be further reduced much more effectively if you use regular second patterns, the distance between the individual elements have mostly maintained so that it is an integer multiple of the phase difference 2.
Similar to the second structure 70, which in the projector according to Fig. 1 is preferably positioned between the modulators 14, 24, 34 and the block 38 information, so you don't have to take into account the chromatic aberration shown in Fig. 7. Its manufacturing process is explained in more detail below on the example of Fig. 8.The structure consists of a silicon substrate 72 on which the etching received several elements 74. Then the silica was allowed to oxidize, making the resulting elements 74 consisted of a transparent silicon oxide. The oxidation time was selected so that the substrate is also formed kremmidiotis layer 76 with a thickness at which he acts as a dielectric mirror for the incident laser beam 32.On the structure 70 is another mirror 78, which in combination with a dielectric mirror 76 repeatedly reflects the laser beam 32 from one surface to the other. In the mirror 78 is provided with two openings 80 and 82, the Xia to the above mathematical calculations.It should be noted that shown in Fig. 7 sizes reproduced without scale. While the laser beam 32 may have a diameter of several millimeters, the thickness of the elements 74 should preferably be maintained in the range of 2 to 20 wavelengths, so that the coherence length in accordance with the above was as minimal as possible. In addition, the height of the elements 76 should be the maximum, so that the laser beam 32 as evenly as possible, passed through a second structure formed by elements 74.In the journal Physikalische Blatter" 52, 1966, N 7 and 8, pages 661-664, in the article "Photonische Bandstruktur in makro-porosem Silizium" described method to produce the above elements of a thickness of 2.3 μm and a height of about 0.1 mm, This method is explained with the example of Fig. 8.First surface alloyed donor impurity silicon plate 86 is structured, for example a lithographic method. In the present example, for the formation of the second structure, altering the coherence length, must obtain strip drawing surface in contrast to the above article, which describes the obtaining of micropores.Then the elements 74 etched by electrochemical method to dissolve the separation carried out only in areas with a high concentration of electrons in silicon. To obtain the free electrons of the substrate is irradiated from the rear ultraviolet radiation 92'. The highest concentration of electrons is achieved in those places, where in the beginning, for example a lithographic method, has been deepening. The larger the hole, the greater the effect of locally selective etching in the manufacture of the elements 74.Avoid electropolishing silicon plate 86 use small currents with a voltage of from 1 to 2, which can be regulated with stabilizer 94 voltage and controlled by the measuring device 96. Description of the required parameters of current and voltage is given in the above article.Quoted below also provide the following features on the constructive design of the screen.From the interference optics it is known that the speckle size mainly depends on the size of the interfering structures. Thus, the dimensions of the smallest structure in the interference pattern are most conducive to the emergence of the interference area. In terms of image projection in the example according to Fig. 1, this area is defined by the diameter D of the laser beam. According to estimates based on privatenet distance from lasers 10, 20, 30 to the screen 43, and D is a mean diameter falling on the screen of the laser beam. In Fig. 9 the appearance of speckle 92 are marked in each case by three arrows originating from different points called speckle 92 interference.Because of the scattering elements 67 are distributed in the material of the screen 43 evenly, it can be assumed that in the cross section perpendicular to the normal to the screen surface 43 of each layer will appear similar pattern of speckles. This situation is reflected in Fig. 9 as located at a greater depth of speckles 92. The diameter of these speckles almost will not differ from those on the surface, because it's during normal thickness d of the screen in the order of several millimeters, and the distance of the order of several meters, based on the formulas above, will change only slightly. However, the position of the speckles on the different depths can be different. In Fig. 9 shows the most unfavourable case, in which are located on the depth and lying on the surface of the screen speckles 92, when viewed from the side of the screen, superimposed on each other.Thus, to reduce speckles depth d must be large enough, and there is an all-time low on the surface, blurring the contrast. This means that the light from the speckles appearing in the screen 43 at a greater depth, should receive approximately half of the average distance d between speckles.Distribution recorded from a greater depth d to the screen surface 43 of the light with full intensity I for any scattering angle normalized to 1 distribution of scattering angles f() to determine for each scattering element 67 by the following formula:
< / BR>where x indicates the length of the path of the light beam from the scattering element 67 to the surface. At an angle and very small thickness d, in which other types of scattering reflected to the surface of the light can be neglected, x can be represented as d/cos.
Effective reduction of speckles due to getting more light in the interference minima at b/2 between the surface speckles 92 is achieved if, according to Fig. 9 the distribution of scattering angles in the middle is in accordance with i.e. if the approximation one can assume that:
< / BR>moreover, the average value for tan is calculated from the following relations:
< / BR>and integrating gives the integral solid angle of the hemisphere for scattering in the direction of the surface.So is it the thickness of the screen speckles are reduced even more, since in this case coming from the deeper layers of the light, which also cannot be neglected, provides an even greater reduction of contrast of the interference between the interference maxima and minima.In the case when D2>S, speckles resulting from the finite diameter D of the laser beam, is smaller than the diameter D of the beam. Therefore, it is expected that the diameter D of the beam essentially determines the size of speckle, therefore, for values of b can be evaluated, based on the fact that b is approximately equal to twice the diameter of the speckle. This is due to the fact that certain structures interference is very uncertain, resulting in the characteristic function can be approximated using simple functions sine and cosine. In this case, the distance between two peaks is approximately equal to double the width of the maximum. From this it follows that in order to effectively suppress the speckles should choose:
< / BR>Above mentioned notion of contrast. The contrast K speckle when the thickness d below is determined by the following formula:
< / BR>where Imaxmean light intensity in a single speckle, and Iminmeans intensivnostyakh. The graph shows that with increasing Iminthat is , when a greater thickness d of the interference phenomena due to this thickness more "lubricated" by the light coming from a greater depth, and contrast sharply reduced.It was found that the values of K(d) function of contrast is less than 0.2, in particular below 0.05, the contrast of speckles physiologically ceases to be perceived.Thus, if you want not only to reduce the speckles, but to completely eliminate them from a physiological point of view, in this case, it is possible to measure the amount of contrast K(d) depending on thickness, then pick up the thickness d of the screen 43 so that K(d) was less than 0.2, in particular less than 0.05.In a less time-consuming, we explain below how to approximate the optimal layer thickness is used, the contrast at the critical thickness of dkrit. To determine the contrast K(dkrithe or mathematically estimated on the basis of the above calculations in the critical layer thickness of dkritwith additional consideration is not considered until now, reflection and multiple scattering coming from the scattering elements of the light beam 50, or simply measured the contrast K(dkrit) when critise contrast is:
< / BR>In this way it is easy to estimate the thickness at which contrasts physiologically no longer perceived, namely:
< / BR>While the number of f describes the extent to which it is necessary to suppress the contrast of the speckles on the screen 43. In accordance with the above f, thus, indicates a number less than 0.2, in particular less than or equal to 0.05, if the contrast of the speckles should be below the limits of perception.From the above examples it follows that according to the invention there are many opportunities to reduce speckles. It is important that the optical path length of the individual photons is consistent with the coherence length. To obtain the length of coherence with the magnitude, the most suitable for practical conditions, it can be calculated in accordance with the required conditions by the selection of the lasers 10, 20, 30, or pulse, and using the second patterns. 1. The device with the laser (10, 20, 30) to play back images, which are sent by the laser (10, 20, 30) beam (12, 22, 32) has a certain coherence length L with a given wavelength and in which on the way of propagation of laser meadows (12, 22, 32) is the first structure(44; 45; 52; 43; 67), allowing production is trichomania fact, that the resulting distribution of the RMS value of the difference in optical path length at the specified phase shift is greater than the length of coherence L, multiplied by a factor 1/(12)1/2.2. The device under item 1, characterized in that the first structure(44; 45; 52; 43; 67) made of transparent material with stochastically included particles (67), having compared with the surrounding material of higher refractive index, and the resulting difference in refractive index leads to phase shifts.3. The device according to p. 2, characterized in that the difference in the refractive index of the particles (67) compared with the surrounding material exceeds 0.1.4. The device under item 2 or 3, characterized in that the particles (67) in the direction of light propagation are smaller than 0.5 mm, in particular at least some of the particles (67) have a size less than 0.1 mm5. The device under item 3 or 4, characterized in that the particles (67) granulate composed of at least two phases (67, 62) with different refractive index.6. The device under item 5, wherein the length of at least one zone between the two boundaries of the phases in naprosny.7. Device according to any one of paragraphs. 1 - 6, characterized in that the first structure(44; 45; 52; 43; 67) has mirror (62; 64; 78) to increase the phase shift due to the increase in path length of the laser beam (12, 22, 32).8. Device according to any one of paragraphs.1 to 7, characterized in that the reproduction of the received image includes a screen (43), and the first structure(44; 45; 52; 43; 67) represents the screen (43) or made in the form of a layer of the screen (43).9. Device according to any one of paragraphs.1 to 8, characterized in that the laser (10, 20, 30) has a band amplification of a width exceeding 100 GHz, primarily over 300 GHz.10. Device according to any one of paragraphs.1 to 9, characterized in that the laser (10, 20, 30) is a pulsed laser operating with a pulse duration of less than 10 PS.11. The device according to p. 10, wherein the presence of at least one laser (10, 20, 30), emitting in the red, green, or blue regions of the spectrum with pulse duration less than 4 PS and especially less than 2 PS, if this laser (10, 20, 30) emits in the red region of the spectrum, less than 3 PS and especially less than 1.5 PS, if this laser (10, 20, 30) emits in the green region of the spectrum, and less than 2 PS and especially less than 1 PS, if this laser (10, 20, 30) emits in the blue region of the spectrum.13. Device according to any one of paragraphs.1 - 12, characterized in that provided at least one laser (10, 20, 30), emitting in the red, green, or blue regions of the spectrum, this spectrum width for the individual colors is > 1.3 nm for the red region of the spectrum, > 0,9 nm for the green region of the spectrum and > to 0.75 nm for the blue region of the spectrum.14. Device according to any one of paragraphs.1 - 13, characterized in that the first structure(44; 45; 52; 43; 67) at least partially made of polytetrafluoroethylene (Teflon material).15. The device under item 8, characterized in that the screen (43) or the layer has multiple scattering particles (67), the thickness d of the screen (43) or layer defined in the propagation direction of the emitted laser (10, 20, 30) for forming an image of the laser beam (40), greater than a critical thickness dkritcalculated on the basis of the average distance b between the generated laser beam on the surface layer or screen and is known as spectra (92) of the interference maxima according to the following formula:
< / BR>where <tan scattering for characterizing the deviation l is m, when the distance S from the screen (43) to the laser (10, 20, 30), and when the diameter D and the effective wavelength of the outgoing laser beam equitable dependence of D2> S, and the thickness d of the screen or more layers
17. The device under item 15 or 16, characterized in that the thickness d has a value at which the characteristic function of the magnitude of the contrast K(d) depending on the thickness d takes a value less than 0.20, the first is less than or equal to 0.05, with specified functional characteristic values of the contrast is measured by the formula
< / BR>depending on the resulting maximum luminous intensity Imaxand the minimum light intensity Iminin uniformly irradiated by the laser beam (40) area with appropriate thickness d of the layer on the screen surface (43).18. Device according to any one of paragraphs.15 to 17, characterized in that the thickness is equal to where f denotes the number of the lesser of 0.20 and above all less than or equal to 0.05, where Imaxindicates the maximum, and Imindenotes the minimum luminous intensity within a uniformly illuminated by the laser (10, 20, 30) area on the screen (43) of thickness dkrit.19. Device according to any one of paragraphs.15 to 18, characterized in that the screen is made of Politiche 3 mm20. Device according to any one of paragraphs. 15 to 19, characterized in that the laser radiation (10, 20, 30) has a coherence length L and the thickness d of the screen (43) or layer is
d (0.04 cm L)1/225%.
FIELD: optical instrument engineering.
SUBSTANCE: device can be used for designing optical circuits of high-resolution laser printers. System has laser, focusing element, collimating element, polygon, and astigmatic singlet. Range of possible numerical meanings of all free parameters of the circuit is determined preliminary from the developed system of algebraic equalities and inequalities while taking properties of laser radiation into account.
EFFECT: reduced time and cost for designing of optical system; widened range of tolerance when making a singlet.
1 cl, 2 dwg
FIELD: mechanical engineering.
SUBSTANCE: system has external charging module, made in form of semiconductor diodes block with fiber-optic emission output, output of which is connected detachably to input opening, made in body, with serially set and optically mated focusing lens, laser resonator, collimator, two-coordinate scanning and output optical device. In input opening of body a hermetic window is set.
EFFECT: higher durability, higher efficiency.
SUBSTANCE: method involves spatially scanning object under study with probing laser bundle, detecting scattered optical speckle-field and determining optical heterogeneity. Laser radiation frequency is restructured in each point of probed space with given frequency deviation being inversely proportional to time delay value in scattering medium, Transverse scattered optical speckle-field intensity distribution is fixed for each frequency value, its two-dimensional laser radiation frequency deviation correlation factor is calculated and optical heterogeneity is determined.
EFFECT: wide range of functional applications.
2 cl, 7 dwg