Pulsed solid-state laser with cascaded frequency conversion of the radiation at higher harmonics

 

The invention relates to laser engineering, in particular to solid-state pulsed lasers operating in the nanosecond range of pulse durations. The device contains a deaf and partially transparent mirror, the optical wedge or a dispersive prism, nonlinear optical elements for cascaded frequency conversion of the radiation, electro-optical modulator, an active element. The active element is symmetrically located with respect to the axes of the resonator. The resonator trehseriynyy. The technical result of the invention is to increase the limit pulse repetition rate of the radiation, the expansion of the spectral band of radiation at the expense of minority laser transitions. 2 Il.

The invention relates to laser technology, in particular to solid-state pulsed lasers.

Pulsed lasers with electro-optical q-switched resonator as generators of powerful radiation pulses in the nanosecond range of durations of the pulses with repetition frequencies up to hundreds of Hertz in the near IR, visible and UV spectral ranges widely used in applied scientific research, medicine, environmental monitoring systems environment.

In Kim and UV ranges used cascaded frequency conversion of the radiation of the infrared lasers in higher harmonics in nonlinear crystals, among them we should mention the relatively new crystals KTP, BBO and LBO, with high nonlinearity and high radial strength.

To further expand the spectral range of the laser radiation is applied, the generation of non-core or weak laser transitions, i.e. transitions of the inverted ions on other levels with smaller sections of stimulated emission in comparison with the cross section of a major transition.

For example, it is known multimode pulsed lasers YAG garnet with neodymium (YAG:Nd), capable of generating radiation with a wavelength of 1064 nm (primary transfer) and 1320 nm (the first "weak" transition) followed by frequency conversion in the second and third harmonics in the red-green-blue region of the spectrum in the nonlinear elements of the KTR [1]. However, in the UV range of the spectrum elements from KTR possess strong absorption and do not have the directions of synchronism to generate third and fourth harmonics.

Transparent in the UV range and with appropriate direction matching the elements of the input have a significant drawback - the angular width of synchronism in the critical plane of ~0.5 mrad. Multimode laser radiation typically has a beam divergence of 3-5 mrad. not enough high power density in the cross section of the beam of infrared lasers. The latter circumstance occurs when the IR laser generates a "weak" transition and efficiency it is therefore considerably reduced. The second common reason is a big pulse repetition frequency of the laser radiation. Increasing the pulse repetition rate is necessary, for example, to reach the required level of average power UV radiation by putting it into a fiber optic cable, which is often used in medicine for transmission of radiation from the laser to the patient. The increase of the average radiation power by increasing the energy of the radiation pulses is limited because of the low beam strength of a fiber.

One solution to the above problems is the creation of an effective single-mode lasers or lasers with astigmatism beams, single-mode in the same plane, and with a low divergence, and higher power density in the beam cross-section.

The closest in technical essence of the present invention is a pulsed solid-state laser with cascaded frequency conversion of the radiation at higher harmonics in the nonlinear elements with resonator, educated deaf and partially transparent mirrors containing active and electro-optical elements, obespecheniya special "wedge" aperture, forming a slit-like beam, which is output from the resonator is extended to the square shape, changes the linear polarization to the orthogonal, again through the input polarizer is put in the active element of the laser, where it is additionally amplified, displayed on the output polarizer and is sent to the amplifier, and then in the non-linear elements for the generation of higher harmonics.

This laser has the following disadvantages: - the active element should be made of optically isotropic crystals (YAG: Nd, GSGG: CR, Nd, and so on), because the active elements of uniaxial crystals AU: Nd, ILF:Nd because of the polarized luminescence radiation will not increase orthogonal polarized radiation; - by increasing the pulse repetition rate above 50 Hz induced birefringence in the active element leads to lower efficiency of the laser and to the deterioration of the uniformity of the spatial structure of radiation; even for the case of a major transition (=1064 nm) must be submitted to the lamp pulses of relatively high energy to get the high overall efficiency.

Thus, the known laser is designed to produce powerful impulses at the frequencies of higher harmonics (532, 355, 266, 213 nm) with frequencies povtorenii with large repetition frequency of pulses (up to hundreds of Hertz) or with radiation at other wavelengths, the corresponding weak transitions and their harmonics (for example, the second power transition in yttrium aluminate with neodymium AI:Nd corresponds to the wavelength 1341,4 nm and higher harmonics: 670,7 nm, 447,1 nm, 335,3 nm).

The present invention is to increase the limit of the pulse repetition rate of the radiation solid-state lasers with cascaded frequency conversion of the radiation at higher harmonics, the extension of spectral range at the expense of minority laser transitions, and improving the efficiency of lasers.

To solve the problem in a pulsed solid-state laser with cascaded frequency conversion of the radiation at higher harmonics in the nonlinear elements with resonator, educated deaf and partially transparent mirrors containing active and electro-optical elements, deaf and partially transparent mirror mounted on one side of the active element, and on the other side of the active element installed additional deaf mirror, while the orientation of the mirrors is such that the optical axis of the resonator, broken on additional remote mirror on two axis, forming the Roman numeral five, the active element is located symmetrically with respect to these axes, electro-optical leiceste wedge with the angle at the vertex, equal arcctg are nkor set a dispersive prism with an angle at the apex equal to 2arcctg npso that the top of the wedge or prism located on the geometric axis of the active element, and their faces next to the active item to make with the axis of the resonator the Brewster angle, while the optical wedge is set to generate radiation on the main laser transition and has a working side, nearest to the active element, a polarizing coating, on the other working face - ar coating, and a dispersive prism installed for generation of radiation in non-core laser transition, where nkis the refractive index of the material of the optical wedge at a wavelength of major transition, npis the refractive index of the material dispersion of the prism at a wavelength of minority laser transition.

Salient features of the proposed laser from the known (the prototype) are: the use of 3-mirror resonator and the symmetrical arrangement of the active element with respect to the axes of the resonator. This creates qualitatively new properties of laser: - you can use active elements of uniaxial crystals (AI: Nd, ILF:Nd and so on), in which the effect of the nave is th pulse repetition rate and to increase the efficiency of generation of radiation at other wavelengths; - when using active elements of isotropic crystals (YAG:Nd, GSGG: Nd, Cr, and so on) effect induced birefringence affects the efficiency of the laser and the spatial structure of radiation to a lesser extent due to the increase of the gain and passive losses in the resonator, which also allows to increase the maximum pulse repetition frequency; - the length of the active element equivalent increases in 2 times, and the cross section decreases in 2 times, which ultimately leads to increasing the efficiency of conversion of radiation at higher harmonics; in the resonator occurs is equivalent to "soft" aperture due to the gradual decline of the gain of the lateral surface of the active element, which increases the volume generated by the resonator mode, and thereby the efficiency of the laser; - the presence of the wedge leads to decrease of 1.5 times the radial load on the electro-optical element that increases the marginal output energy of monopulse radiation; - the laser resonator can be easily transformed into the resonator with a stronger dispersion by changing the geometrical shape of the element, diluent axis (replacement wedge prism), which allows you to suppress the generation on the main transition.

In prti using the equivalent of "soft" aperture, resulting in the divergence of the output radiation is reduced to ~2 times.

In Fig.1 and 2 shows an optical schematic of the proposed device for the generation of radiation on the main laser transition and non-core laser transition, respectively.

The laser resonator is formed partially transparent 1 and deaf 2 mirrors and additional mirror 3. On the secondary mirror 3 axis resonator broken in two axis. The active element has a cylindrical shape 4 of the crystal, activated Nd ions having a cubic crystal lattice (YAG: Nd, GSGG:Cr, Nd, YSGG:Cr, Nd, and so on) and grown in the [001] direction, is oriented so that the crystallographic axis X and Y are the angles45owith the drawing plane.

If the laser is designed to operate with a high pulse repetition rate, preferably as a crystal for the active element to choose uniaxial crystals AU:Nd, ILF:Nd, etc., In this case, the element 4 azimuth is oriented so that the plane of polarization of the luminescence coincides with the plane of the drawing. In both these cases, located symmetrically with respect to the axes of the active element is placed in a luminaire comprising a reflector and Lama through the geometric axis of the element 4 and the lamp, perpendicular to the plane of the drawing.

On the top axis of the resonator is an optical wedge 5 (Fig.1), the top of which lies on the line, which is the geometric axis of the active element 4. Closest to the element 4, the face of the wedge 5 has a dielectric polarizing coating and makes with the axis of the Brewster angleto= arctan ntowhere ntois the refractive index of the material of the optical wedge at a wavelength of major transition.

The second face of the wedge 5 is from the first angle equal to arcctg are ntoand with axis angle 90oand has an ar coating at the wavelength of major transition. The wedge 5 plays the role of the optical element, diluent axis far enough to accommodate the electro-optical element 6 and the mirror 2. Moreover, the wedge 5 increases in ntotimes the linear dimension of the cross section of the radiation in the plane of the drawing on the faces of the element 6, which is the most unstable radiation strength element of the resonator. Thereby limit the output energy of laser pulses increases by ~1.5 times.

Electro-optical element 6 in the form of a parallelepiped can be made of crystal LiNbO3with the working faces, skoshennym angle Brustna of LiNbO3has a small passive losses at the wavelengths of both primary and weak transitions in ions Nd.

From a partially transparent output mirror 1 radiation prism 7 is sent to the element 8, which converts linear polarized light into circular or elliptical.

The element 8 may be made in the form of plates/4 or rotator of the plane of polarization 90o. And in both cases the angular orientation of the element 8 is selected according to the maximum output power of the second harmonic generated in the nonlinear element 9, or third harmonics generated in nonlinear element 10.

To generate the fourth harmonic of the nonlinear element 10 is selected orientation corresponding to the interaction of the I-type second harmonic radiation. For the generation of the second harmonic is preferable to use a non-linear element of the CTD (type interaction (II), for generating third and fourth elements of the LBO and BBO.

For the generation of radiation on weak transitions of the proposed device can be easily transformed into a device, scheme of which is shown in Fig.2.

The main differences between the schemes in Fig.2 from the circuit of Fig.1 the following:
mirrors 2 and 3 deaf at a wavelength of "weak" per the Torah are from the upper radius angle Brewsterp= arctan npwhere npis the refractive index of the material dispersion of the prism at a wavelength of minority laser transition, and the vertex lies on the line, which is the geometric axis of the active element 4, the angle at the vertex of the prism is equal to 2arcctg npand so the second working face of the prism makes with the axis angle of Brewster;
elements 9 and 10 is replaced by the nonlinear elements of the slice corresponding to the new wavelength (can even be changed and the material of the crystal; for example, the third harmonic radiation "weak" transition 1320 nm, it is preferable to replace the element of the input element of the CTD).

The proposed laser operates in the following manner. In the pulse-periodic mode during each pumping pulse when closed, the electro-optical shutter, which form the electro-optical element 6, deaf mirror 2 and the polarizing element 5, the accumulation of inverted population in the active element 4.

When applying an enabling pulse of high voltage to the electrodes of the electro-optical element 6, the shutter opens and the resonator is generated monoenoic radiation.

The spatial structure of the radiation is determined by the size allowed for the gene of the ti drawing is reduced in 2 times due to the selection of transverse modes in comparison with the case when the cross-section of the active element in a conventional two-mirror cavity completely filled with radiation. In the plane perpendicular to the plane of the drawing, the divergence of the radiation is practically unchanged.

As shown by the energy calculations of the laser, the transition from conventional two-mirror resonator to offer trehserijnom the resonator in case of optimization of the transparency coefficient of the output mirror at the fixed energy of the pump-pulse energy E and the duration0,5monopulse radiation does not change.

Since the cross-sectional area of the generated radiation has decreased to ~2 times, the energy density (capacity) in the cross-section of the beam has increased also in ~2 times. This increases the conversion gain of the frequency of radiation at higher harmonics. In the approximation of the specified field (at low energy densities) growth is expected to be linear (in ~2 times). In the case of generation of higher harmonics in the UV region, where applicable elements of BBO with a narrow angular width of synchronism, the increase of the conversion efficiency is expected due to the reduction of divergence.

Unlike the prototype [2] proposed a laser makes it possible to increase the repetition rate toty radiation at higher harmonics.

The test results of the two lasers confirm the effectiveness of the proposed device.

The first laser YAG:Nd (=1064 nm) with an active element size5100 mm, a wedge angle at the apex of the 34o, electrooptical element of LiNb3with faces at the Brewster angle, the nonlinear element of the CTD (second harmonic) and the nonlinear element of the BBO (fourth harmonic) at a repetition rate of 200 Hz, with a pulse energy of the pump 4 j at the output of fiber-optic cable was generated pulses with the following parameters:=266 nm, E=0.5 MJ,0,5= 30 NS.

The second laser AU:Nd (1= 1341 nm) with an active element6100 mm, the dispersion prism with an angle at the apex 68o, electrooptical element of LiNbO3with faces at the Brewster angle, the nonlinear element of the CTD (second harmonic) and the nonlinear element of the CTD (third harmonic) at the repetition rate up to 50 Hz when the energy of the pump pulses 32 j generated pulses with the following parameters:
2rc="https://img.russianpatents.com/chr/964.gif">0,5= 35 NS.
Thus, the proposed laser can work effectively in a pulse-periodic mode with a q-switched resonator with frequency conversion of the radiation at higher harmonics at high repetition frequency of the pulses or the wavelength corresponding to the non-core laser transition of the active medium.

Sources of information
1. U.S. patent 5144630, H 01 S 3/10, 1991

2. RF patent 2162265, H 01 S 3/10, 1999


Claims

Pulsed solid-state laser with cascaded frequency conversion of the radiation at higher harmonics in the nonlinear elements with resonator, educated deaf and partially transparent mirrors containing active and electro-optical elements, wherein the deaf and partially transparent mirror mounted on one side of the active element, and on the other side of the active element installed additional deaf mirror, while the orientation of the mirrors is such that the optical axis of the resonator, broken on additional remote mirror on two axis, forming the Roman numeral five, the active element is located symmetrically with respect to these axes, the electro-optical element is located on the m at the top, equal arcctg are ntoor set a dispersive prism with an angle at the apex equal to 2 arcctg are npso that the top of the wedge or prism located on the geometric axis of the active element, and their faces next to the active item to make with the axis of the resonator the Brewster angle, while the optical wedge is set to generate radiation on the main laser transition and has a working side, nearest to the active element, a polarizing coating, on the other working face - ar coating, and a dispersive prism installed for generation of radiation in non-core laser transition, where ntois the refractive index of the material of the optical wedge at the wavelength of the primary transition npis the refractive index of the material dispersion of the prism at a wavelength of minority laser transition.

 

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