RU2728491C1 - Method of tuning laser emission frequency converter to third harmonic - Google Patents

Abstract

FIELD: physics.SUBSTANCE: invention relates to non-linear frequency converters of laser radiation. Method of adjusting frequency converters (FC) of laser emission (LE) into a third harmonic provides FC tuning in two steps. At the first stage, the crystal-frequency converter is tuned along the azimuthal angle to obtain the required angle between the main plane of the crystal and the direction of polarization of the working LE. At the second stage, the precision absolute synchronism angle is pre-determined and the obtained angle is fixed after the given crystal-converter, after that, FC is adjusted to this synchronism angle relative to direction of working LE falling on FC. Synchronism angle is measured using a geodetic method.EFFECT: technical result consists in providing possibility of conversion of LE to the third harmonic not optimized for this FC, while maintaining high accuracy of setting both small and large synchronism setting angles.1 cl, 2 dwg

Description

The invention relates to nonlinear frequency converters (IF) of laser radiation (LI) and concerns issues of tuning the IF for converting LI to the third harmonic.

There is a method of tuning the frequency converter on a Nova laser without obtaining the value of the absolute tuning angles [M.A. Summers, L.G. Seppala, F. Rienecker, D. Eilmerl, B.C. Johnson, "Nova Frequency Conversion System", Lawrence Livermore National Laboratory, Livermore, UCRL-86009, October 28. Prepared for submittal to 9th Symposium on Engineering Problems of Fusion Research, Chicago, Illinois]. The disadvantages of this method of tuning crystals for converting LR to the third harmonic include the lack of a master oscillator on many other laser installations capable of operating in a repetitively pulsed mode, which allows such tuning.

Closest to the claimed is the method of tuning the converter (IF) of laser radiation in the third harmonic, which is used for the American multichannel laser installation NIF [Wegner P., Auerbach J., Biesiada T., Dixit S., Lawson J., et all - NIF final optics system: frequency conversion and beam conditioning. - SPIE Photonics West, San Jose, California, January 24-29, 2004]. The crystals of the frequency converter and the final lens on the NIF installation are fixed into a single monolithic aluminum structure (final optics cell). All elements of the cell and their fastening elements are made with great precision (the angular error of each element does not exceed 10 mrad). The IF is adjusted to the phase matching angle relative to the direction of the laser radiation incident on the IF. When tuning, each cell is rotated with the help of three activators so that the beam reflected from the doubler crystal makes an angle of 1.16 mrad with respect to the incident one. The adjustment is made according to the image of the CCD camera installed in the focus of the output lens of the transport spatial filter (TPF) at a short distance from the center of the TPF diaphragm. The described tuning method presupposes practically ideal production of frequency converter crystals from the point of view of the orientation of the optical axis relative to their surface. To solve this problem, a special CATS system - Crystal Alignment Test System [Hunt J.T. - National Ignition Facility Performance Review 1999. - LLNL Report UCRL-ID-138120-99, 2000, 313 p]. It allows precise measurement of the phase matching angle of each new crystal grown in relation to a reference crystal with precisely known axes. Further, the difference between the crystals is eliminated by diamond milling of the new crystal. Crystals optimized in this way are expensive, and the tuning method is not universal and is suitable with satisfactory accuracy for inverters with small tuning angles, while tuning for large angles is impossible.

The technical problem solved by the invention is to enable tuning of non-optimized frequency converters.

The technical result consists in the technological simplification of the process of tuning the inverter with the provision of satisfactory tuning accuracy for both small and large angles.

This technical result is achieved due to the fact that, in contrast to the known method of tuning the frequency converter (IF) of laser radiation to the third harmonic, which consists in the fact that the IF is tuned to the phase matching angle relative to the direction of the laser radiation incident on the IF, the proposed method uses the non-optimized IF. geometry, provide the IF adjustment in two stages, for which, at first, at the first stage, the IF is set at an angle to the direction of polarization of the radiation incident on the IF, after which, at the second stage, the IF is adjusted to the phase matching angle relative to the direction of the laser radiation incident on the IF, and at the first stage for setting the IF at an angle to the direction of polarization, azimuthal adjustment is carried out, for which the position of the reference strips on the screen is preset, and then the setting of the IF at an angle to the direction of polarization of the laser radiation is provided optically by passing the laser radiation through the IF by obtaining interference on the screen on the screen, the azimuth adjustment is judged by the parallelism of the interference and reference stripes on the screen, after ensuring the parallelism of the interference and reference stripes on the screen at the first stage, at the second stage, the exact absolute angle of synchronism is preliminarily determined , after which the IF tuning to the phase matching angle relative to the direction of the laser radiation incident on the IF is performed geodesically, the accuracy of the IF laser radiation tuning in the third harmonic is judged by the coincidence of the angular position of the harmonic generation maximum with the direction of the laser radiation incident on the IF incident on the phase matching angle.

That is, the physical basis of the method lies in the fact that an approach has been found that makes it possible to provide conditions for tuning non-optimized frequency converters. The first stage of tuning (by the azimuthal angle) makes it possible to determine and set the position of the main plane of the crystal relative to the polarization of the LR without using the working radiation of the laser setup, which technologically simplifies the tuning process, excluding the operation of the setup itself at this stage. This tuning step is necessary when working with non-optimized crystals, because in this case, the position of the principal plane of the crystal is not oriented and requires adjustment.

The novelty of the second stage in the use of the geodetic adjustment method. The essence of the approach is the use of an autocollimation theodolite that allows you to measure and adjust the absolute angles of synchronism of a large value (crystals can have synchronism angles of 10 ° -15 °), providing the same accuracy as for small angles of adjustment (angles ~ units of minutes). That is, the tuning process adjusts to the geometry of the existing crystal, allows you to use this crystal for various tasks, for example, for converting to both the second and third harmonics without its modification. In the prototype, a separate crystal is manufactured with high precision for each task, which requires expensive optimization. Thus, the proposed approach provided the possibility of using unoptimized converter crystals and technological simplification of the IF tuning process, providing satisfactory tuning accuracy for small and large angles.

FIG. 1 schematically shows a stand for adjusting crystals in azimuthal angle ϕ, where 1 is a collecting lens, 2 is a frequency converter crystal, 3 is a Glan prism, 4 is a collecting lens, 5 is a screen with stripes rotated by an angle ϕ relative to the vertical.

FIG. 2 shows the optical scheme of setting the angle for the maximum synchronism on the channel of the installation, where 6 is the amplifier of the laser installation, 2 is the crystal of the frequency converter, 7 is the autocollimation theodolite.

To solve the problem, the proposed method is implemented as follows. An experimental non-optimized crystal sample (FC) is set relative to the LR polarization and its synchronism tuning angles are determined. The absolute angles of synchronism tuning are controlled by the geodetic method, are fixed behind the crystal sample and are used for its further tuning in experiments on LR conversion to the third harmonic.

Thus, complete crystal tuning takes place in two stages.

To tune the crystals by the azimuthal angle ϕ (the first stage of full tuning), we used the scheme shown in Fig. 1.

A parallel beam of a tuning laser (with a wavelength in the visible range) falls on lens 1, then the converging beam passes through the crystal of frequency converter 2 and analyzer 3, then after lens 4 the parallel beam falls onto a screen with reference strips 5, which is installed in such a way that the lines drawn on it made the required angle ϕ with the vertical (since in this example, the working laser radiation is vertically polarized).

Radiation passing through the crystal changes its polarization state; after the analyzer, a system of interference fringes is formed on the screen, perpendicular to the main plane of the crystal. By rotating the crystal in the azimuthal direction along the angle ϕ, the bands are matched. The setting is judged by the degree of parallelism of the interference and reference stripes on the screen.

The direction of synchronism in the IF during implementation was determined according to the generally accepted technique, irradiating it with a cone of converging beams of the fundamental frequency with the registration of the synchronism band of the generated optical harmonic and the reference mark on the CCD camera [S.E. Barker, D. Milan, R. Boyd "High Fluence Third Harmonic Generation", LLNL UCRL-LR-105821-93-2, Volume 3, Number 2, 1993, 55-62]. The angular position of the nonlinear element was monitored relative to the alignment laser beam using a geodetic method. When placing the crystals in the angle of phase matching in the experiment for the generation of the third harmonic, the method used to measure and control the angles of incidence of radiation on the surface of the frequency converter in experiments to determine the direction of phase matching is also used.

The optical scheme of the geodetic method of setting the angle for the direction of phase matching using an autocollimation theodolite (the second stage of full tuning) is shown in Fig. 2.

The autocollimation theodolite 7 is installed behind the frequency converter crystal 2 and is exposed to the alignment beam of the laser installation. The direction of the beam is measured directly from the theodolite by observing the quotation beam through the eyepiece. To reduce the brightness of the beam observed in the theodolite to a safe level, a specially made eyepiece attachment is used, consisting of light filters that attenuate the radiation of the alignment laser. The theodolite is installed on levels in a horizontal working position. Next, we find the image of the alignment beam, precisely align with it the theodolite thread mesh and take the readings (with a vertical and horizontal circle) of the angular position of the alignment beam. These counts are zero for us. With respect to this position, the previously experimentally measured angle of the crystal position is plotted for the direction of the phase matching of the converter taking into account the temperature correction. We correct the position of the crystal with the help of micrometric shifts until the lines of our own filament mesh and collimation reflection match. The crystal is now considered exposed. The total crystal alignment error using the described method does not exceed ± 17 arc seconds (82 μrad). And this error remains both for tuning angles less than 1 °, and for large angles ~ 10 ° -15 °. As a result, the proposed approach provided the possibility of using non-optimized converter crystals and technological simplification of the IF tuning process, providing satisfactory tuning accuracy for small and large angles.

Claims (6)

The method of tuning the frequency converter of laser radiation to the third harmonic, which consists in the fact that the frequency converter (FC) is tuned to the phase-matching angle relative to the direction of the laser radiation incident on the IF, characterized in that - use the inverter of non-optimized geometry, - provide the IF tuning in two stages, for which, at the first stage, the IF is set at an angle to the direction of polarization of the radiation incident on the IF, after which, in the second stage, the IF is adjusted to the phase matching angle relative to the direction of the laser radiation incident on the IF, - moreover, at the first stage, to set the IF at an angle to the direction of polarization, azimuthal adjustment is carried out, for which the position of the reference strips on the screen is pre-set, and then the position of the IF at an angle to the direction of polarization of the laser radiation is provided optically by passing the laser radiation through the IF by receiving interference fringes and by rotating the IF of setting their position relative to the reference stripes on the screen, the azimuth adjustment is judged by the parallelism of the interference and reference stripes on the screen, - after ensuring the parallelism of the interference and reference stripes on the screen at the first stage, at the second stage, the exact absolute angle of synchronism is preliminarily determined, after which the IF adjustment to the synchronism angle relative to the direction of the laser radiation incident on the IF is performed geodesically, - the accuracy of tuning the IF of laser radiation to the third harmonic is judged by the coincidence of the angular position of the maximum of the harmonic generation with the direction of the laser radiation incident on the IF, tuned to the phase matching angle.

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