Autocorrelator light pulses

 

(57) Abstract:

Autocorrelator light pulses contains a light beam splitter, line variable optical delay, the node combining beams, optically coupled to the node registration. The light beam splitter, line variable optical delay and the node combining beams is made in the form of a beamsplitter cube with a beam-splitting layer, made on its diagonal, and the input and output faces is planar and orthogonal to each other, and the other two are made so that the difference of various parts of the wave front has changed by a certain precisely known to the law, and also applied reflective coating, while multi-element photodetector aligned and optically coupled with the output side of the cube. The technical result - the creation of stable autocorrelator light pulses. 4 C.p. f-crystals, 6 ill.

The invention relates to a device for measuring one of the main characteristics of optical radiation is the autocorrelation function of the light wave in time.

A device for implementing the method of measuring the optical thickness of the plane-parallel transparent objects, prozrachnoe flat mirror, covering the plane-parallel transparent object, biprism Fresnel (in one embodiment of the invention) and spatial-sensitive multi-element photodetector.

The disadvantages of this device is the lack of stability for use in portable devices, field conditions, moving objects, in systems designed for operation in conditions of high vibration. Two reasons cause a decrease in its resistance to vibration and sensitivity. The first is due to the large distance between the lens and the photodetector. The second is associated with insufficient quantity of the light flux, which is caused by the requirement of high spatial coherence of the beam and, accordingly, the use of a narrow slit in the collimator beam. In addition, the device has a small range of change of the difference of the stroke, due to conflicting requirements between the angle of the prism at the top and the dimensions of the prism, or the resolution of the spatial-sensitive photodetector.

Known autocorrelator light pulses containing the light beam splitter, line variable optical delay, the node combining direct and delayed the ohms specified autocorrelator is its low stability and difficulty settings due to the presence of a large number of degrees of freedom structural elements.

The basis of the invention is the creation of stable autocorrelator light pulses by executing divider light pulses, a delay line pulse, the node combining beams and registration system in a single beam-splitting cube and the placement of a sensor directly on the output surface of the cube.

Achieving the above technical result is ensured by the fact that in the known autocorrelator light pulses containing the light beam splitter, line variable optical delay, the node combining beams, optically coupled to a node in the Desk, and a radiation receiver, the recording power of the waves, the light beam splitter, line variable optical delay and the node combining beams is made in the form of a beamsplitter cube with a beam-splitting layer, placed on the diagonal of the cube, and the input and output faces of the cube is planar and orthogonal to each other, while the other two made so to the difference of various parts of the wave front has changed by a certain precisely known to the law, and n is n and optically coupled with the output side of the cube.

The implementation of all parts of autocorrelator into a single beam-splitting cube allows you to maximize the stability of the device.

The invention is illustrated in Fig.1. In Fig.1 light source 1 having a low degree of coherence in time, lights using achromatic condenser 2, which may be a lens, mirror, small diaphragm 3, the dimensions of which are selected from conditions provide sufficient spatial coherence for radiation of all measured frequencies. Aperture 3 is placed in the front focal plane of the collimator 4, which can be combined with the input surface of the cube, creating a parallel light beam 5. The parallel beam is directed to a beam splitter 6, inside the beam-splitting cube 7, made of a substance that is transparent throughout the required range of wavelengths. The cube has two faces - the input 8 and output 9 is flat and orthogonal to each other, and the other two are 10, 11 is made so that the difference of various parts of the wave front parallel beams is changed by a specific and precisely known to the law, in particular, Fig.1 the difference varies linearly along one of the coordinates, the PE the spine). In the latter case, the surface of the cube are planes normal to which is inclined relative to the axis of the beam. On the verge 10, 11 is applied reflective coating with high refractive index throughout the working spectral region, guiding the reflected beams back to the beam splitter. Passed through the beam splitter, the beam reflected from the faces 10, and the beam reflected from the faces 11 and then reflected from the beam splitter 6, adding to the space behind the beam splitter, form in space arrangements beams, shown in Fig.1 hatch, an interference pattern which is recorded using the spatial-sensitive sensor 12 directly behind the beam-splitting cube. It can also be projected on the photodetector using the additional optical system. The operation of the photodetector is controlled by the electronic system 13. The signal of the photodetector is processed either the same electronic system, or a specialized 14.

The device can be used beamsplitter cubes made with polished at a given angle edges, and faces orthogonal, having deposited on the surfaces of wedge-shaped layers of transparent videolina, placed on the flat surfaces of the cube, is made orthogonal to the optical axis. The number, size and thickness of the layers are selected in accordance with the desired set of values of the differences of the stroke and with consideration of the influence of diffraction. You can also use wavefronts with a surface curved in exactly the known law (e.g., spherical).

For approval of resolution multi-element photodetector with the structure of the interference pattern between the photodetector and the output face of the cube can accommodate optical system for forming a real image of the interference pattern on the surface of the photodetector, including a variable focal length (zoom).

The work of autocorrelator can be explained as follows.

In this invention, when receiving the interference pattern to measure the autocorrelation function instead of dividing the wave front method of dividing the light wave amplitude. With this purpose, create a collimated light beam and directing it to the translucent mirror, forming two coherent beam, fully match the distribution of the amplitude volnyichya orthogonal polarization States (two plane-polarized waves, or two waves oppositely directed circular polarization). Impose a delay between different areas of the wavefronts, and various known accurately for different sections of the front. Then using the same, or the second special semi-transparent mirror or a polarizing prism keep these two beams so that a respective different region of wave fronts were kept in the same order as they were in the original beam. Due to the interference amplitude plots of the wave front, which experienced a different delay will be different. Measure the spatial distribution of light intensity of the interference field using spatial-sensitive photodetector and share the light on constant and variable field components. Allocate a variable component, and find its dependence on well-known difference of course between the beams. The obtained dependence is a function of the autocorrelation of the light flux, with accuracy up to a constant factor, which depends on the integral brightness of the light source and the geometry of the optical system.

To study the autocorrelation function of the light flux from the light source with a different geometry of the light beam in oostergo image source and homogeneous filling of the collimator light.

In the correlator shown in Fig.1, the interference pattern has a one-dimensional character shown in Fig.2: the illumination of the photodetector standing on one of the coordinates (y) changes with orthogonal axes (x). The use of two-dimensional spatial-sensitive photodetector in this case leads to unnecessary overestimation of requirements, that is accompanied by an increase in the noise power. The contradiction is resolved in the apparatus shown in Fig.3, which shows the diagram of a device operating on the above principle, supplemented by a cylindrical lens 15, is installed on the output cube. The lens can be combined with the output surface of the cube, instead of lenses can be used cylindrical or toroidal mirror. Focusing is performed in the direction (y) orthogonal to the direction of measurement of the difference of the stroke of the beams. Instead of a two-dimensional matrix of a sensor in this case is linear, oriented along the direction (x), which leads to reduction of the area of the photodetector and reduce noise level. Due to this increase the accuracy of the measurement of the autocorrelation function and increase the sensitivity, but also extends the range of possible fotopro the fronts on the amplitude and on the front. With this aim, the inclined outer faces of the cube 10, 11 is applied to a system of strips of transparent substances of the same width, oriented in the direction of the change in the difference between progress and manufactured so that each further has an optical thickness less than the previous by an amount equal to the difference between the optical thickness of the cube in the extreme points of the interference pattern. A simplified form of the beam-splitting cube shown in Fig.4. As a result of this interference field is consisting of a system of parallel strips. In each of the bands of the difference varies linearly, and the difference in one of the bands differs from the next by a constant amount equal to the full change in the difference of the operation in this band. Processing the light distribution in the interference pattern through the device 14, the end of the distribution for one of the bands mathematically "sew" with the beginning of another. A complete change of the difference of the operation in the interference pattern is equal to the sum of the changes in the difference of course all the bands.

Between the output face of the described complex beam-splitting cube, shown in Fig. 4, and space-sensitive photodetector can be installed additional optyka. Such a system may consist of a linear raster formed by parallel cylindrical lenses oriented its generatrix direction parallel to the sweep direction of the interference pattern. The narrowing of the images of the illuminated strips allows the use of a photodetector smaller effective area, thereby increasing the signal to noise in registered autocorrelation function. A raster is made in the form of a system of parallel PLANO-convex cylindrical lens, may be its flat surface is mechanically combined with the output surface of the cube.

The use of multi-element photodetectors with a discrete set of elements (pixels) leads to the sampling of the interferogram to change the difference of course. This fact allows the use of the interference of the cube, creating a discrete set of difference values of a course consisting of N values. The number and magnitude of the difference of a course chosen from the same conditions as used for the FTIR with selectable scan. To this end, the cube mentally divided into N independent "elementary blocks", each of which corresponds to the specified value times the RMS-parallel transparent layers of material, the area of each of which corresponds to one or more elements (pixels) of the photodetector, and the optical thickness corresponds to the set for this "elementary cube" the difference of course. A simplified structure of such a cube is shown in Fig.5 for N = 4. The image element of the interference pattern is recorded by using one element of a sensor, or some combination of elements, you can use an additional optical system for forming a real image of the interference pattern on the surface of the photodetector, including raster.

Much of the information about the nature of the wave process regular type if known, the average frequency spectrum can be obtained from the shape of the envelope of the autocorrelation function. As an example of the device for measuring film thickness sufficient to determine the position of the maxima of the envelope system of lateral interference fringes. In this case, the resolution of the multi-element photodetector can be much smaller than the period of oscillations in the autocorrelation function. A simple reduction in resolution leads to a smoothing of interferents. In order to ensure the ability to measure low-frequency characteristics while neglecting high frequency, you can enter into the gap between the output side of the cube and the photodetector environment with non-linear characteristics of the transmission or use of nonlinear photodetector. The following variants are possible nonlinear registration device interference pattern.

In any area between the photodetector and the output face of the cube is placed Wednesday, with the threshold characteristics of the transmission, such as high transmittance for radiation of high power have filters made of glass type NA or a substance having the property of nonlinear luminescence in the multiphoton absorption, or used when solving problems of up-conversion of radiation (e.g., nonlinear glass and crystals). Under the action of a sufficiently powerful illumination of the investigated radiation, or a combination of auxiliary and studied radiation, there is a partial enlightenment nonlinear filter in the areas of the surface where the field strength of the light wave in the interference pattern maximum, or in the same places occurs induced by multiphoton absorption luminescente nature of the interference pattern illustrated in Fig.6. Her example shows the autocorrelation function of radiation "almost white" source. Fig.2,and shows the response of a linear system, and Fig. 2, b-threshold, and the threshold value shown in Fig.2,and the dotted line. The deterioration of the resolution of the photodetector to the width of one strip in the first case will lead to the fact that it will show a constant illumination, equal to the average signal level and the second will reproduce approximately the shape of the envelope.

To reduce the threshold is preferably non-linear optical elements near the plane of focus coming out of the cube radiation, applying the use of luminescence additional optical system projecting the field of luminescence on the surface of the photodetector.

The use of nonlinear photodetector, in particular, with adjustable sensitivity threshold is preferred because it provides greater sensitivity, efficiency, and ease of device management.

1. Autocorrelator light pulses containing the light beam splitter, line variable optical delay, the node combining beams, optically coupled to the node registration, characterized in that the divider snogo cube with beam-splitting layer, executed on its diagonal, and the input and output faces is planar and orthogonal to each other, and the other two are made so that the difference of various parts of the wave front has changed by a certain precisely known to the law, and also applied reflective coating, while multi-element photodetector aligned and optically coupled with the output side of the cube.

2. Autocorrelator under item 1, characterized in that the two other faces are tilted with respect to the axis of the beam.

3. Autocorrelator under item 1, characterized in that the inclined faces of the cube put a system of strips of transparent substances of the same width, oriented in the direction of the change in the difference between progress and manufactured so that each further has an optical thickness less than the previous by an amount equal to the difference between the optical thickness of the cube in the extreme points of the interference pattern.

4. Autocorrelator under item 1, characterized in that between the output face of the described complex beam-splitting cube and a photodetector installed additional optical system including a linear raster formed by parallel cylindrical lenses, orientation is">

5. Autocorrelator under item 1, characterized in that between the output face of the cube and the photodetector entered Wednesday with a non-linear or nonlinear characteristics of the photodetector.

 

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