DE19732759A1 - Laser acousto-optical modulator device for solid state laser - Google Patents

Abstract

The device allows the number of times the laser beam is passed through the acousto-optical modulator (3) to be doubled using mirrors (1a-1c) and/or optical elements controlling the polarisation of the laser beam.

Description

Lasers can be modulated in their quality with the help of active switches in the optical resonator become. This can result in a high level in the active medium due to the optical pumping process Inversion can be built up by the resonator through the active switch first is blocked. After opening the switch, the saved inversion is in one short pulse emitted. Solid state lasers in particular (e.g. Nd: YAG laser, Ruby laser) Q-switches can be switched very efficiently because the lifespan of the upper laser level in the Compared to the resonator orbit time is very large.

For the active quality control of continuously pumped Nd: YAG lasers is the Wide use of acousto-optical modulators (AOM) as an active switching element spread. An acoustic is used in a suitable solid material (e.g. quartz) Vibration coupled in with a frequency of a few 10 megahertz (typically 27 MHz), which generates a corresponding refractive index grating. By diffraction of the laser beam laser grating can be diffracted into the higher diffraction orders and thus losses are imposed on the laser beam in the 0th order. These losses can by modulating acoustic power with frequencies in the kilohertz range be switched on and off, whereby the desired emission of short pulses is achieved can be.

The problem with this is that the diffraction efficiency of AOM's is not always sufficient to prevent the laser from vibrating before the actual switching process. This is particularly the case when the small signal amplification of the active medium, for. B. by using several active media arranged in series, is very large and furthermore at low repetition frequencies (pulse-to-pulse distance <lifespan of the upper laser level) builds up a high initial gain for the pulse.  

An increase in the diffraction efficiency of an AOM is due to higher driver powers Set limits as there can be thermal effects in the AOM. The usage several AOMs in the resonator to improve the blocking behavior is possible, but increases the technical effort for the laser system.

The invention specified in claim 1 is based on the problem that conventional arrangements the number of passes of the laser beam through the acousto-optic modulator (two passes per resonator cycle) is not sufficient to to allow efficient diffraction. This has a reduced efficiency of the laser or an unstable emission of laser pulses with certain combinations of pump power (or amplification) and repetition frequency.

This problem is solved by the features listed in claim 1. By Use of additional laser mirrors and / or polarization influencing optical components (polarizer, delay plate) is the number of passes increased by the AOM per resonator revolution from two to at least four. The Diffraction efficiency increases accordingly. The use of passive optical components to increase diffraction efficiency is considerably cheaper and less expensive than the use of several AOMs in the resonator.

Due to the increased diffraction efficiency, the working range of the laser is expanded Repetition rate and pump power.

Two possible embodiments of the device are shown in FIG. 1 and FIG. 2 and are explained in more detail below.

Show it

Fig. 1: Non-collinear arrangement of the AOM's for realizing four passes through the AOM per resonator cycle.

Fig. 2: Collinear arrangement of the AOM for realizing four passes through the AOM per resonator cycle with the aid of polarizing elements.

In Fig. 1, the active medium ( 2 ) is in an optical resonator, which is formed by the mirror ( 1 a- 1 c). Of these, the mirrors 1 a and 1 c are the resonator end mirrors, mirror 1 b serves to fold the resonator. The acousto-optic modulator (3) is positioned so that, due to the folding of the optical path by the mirror 1 b at a complete resonator round is irradiated four times (instead of twice without folding). It is important to ensure that the folding angle must be kept as small as possible so that the so-called Bragg condition for maximum diffraction efficiency is fulfilled as well as possible for both beams.

In acousto-optical modulators, whose diffraction efficiency depends critically on the Bragg condition, the non-collinear arrangement according to FIG. 1 does not lead to the desired increase in the effective diffraction efficiency. In this case, a collinear arrangement can be selected by using the direction of polarization ( FIG. 2), in which the angle of incidence on the AOM is the same for both beams.

For this purpose, a polarizer ( 4 ) and a quarter-wave delay plate ( 5 ) are introduced into the laser resonator, which is again formed by the mirrors ( 1 a- 1 c) and by the active medium ( 2 ). The AOM ( 3 ) is positioned between the polarizer and the mirror ( 1 b). The optical axis of the delay plate ( 5 ) is oriented at an angle of 45 ° relative to the polarization direction of the polarizer ( 4 ). A beam polarized from left to right, parallel to the plane of the paper, first passes through the AOM ( 3 ) twice as a result of the back reflection at the mirror ( 1 b). Due to the 90 ° rotation of the polarization through the delay plate ( 5 ), the beam is coupled out at the polarizer ( 4 ). With the help of the mirror ( 1 c), the now vertically polarized beam is reflected back in itself, passes through the AOM ( 3 ) two more times and transmits the polarizer ( 4 ) by turning the polarization back into the plane of the paper. With a complete resonator circulation, the AOM ( 3 ) is therefore run through twice as often as with a conventional arrangement.

Claims (4)

1. A device for improving the blocking behavior of acousto-optical modulators (AOM), characterized in that with the aid of mirrors and / or the polarization of the laser beam influencing optical components of the AOM is passed through several times by the laser beam and thus the diffraction efficiency of the AOM is increased. 2. Arrangement according to claim 1, characterized in that with the aid of a deflecting mirror Laser beam is folded at an acute angle by the AOM and only then by the actual resonator mirror is reflected back in itself. 3. Arrangement according to claim 2, characterized in that the folding described Laser beam in the same AOM is performed several times to improve diffraction efficiency further increase. 4. Arrangement according to claim 1, characterized in that in a linearly polarized laser the AOM along with a quarter wave delay plate (VWV) between one internal resonator polarizer and one of the resonator mirrors is positioned and by 90 ° rotation the polarization by the VWV the laser beam only after decoupling on the polarizer by a additional mirror is fed back into the resonator.