DE19732759C2 - Device for improving the blocking behavior of an acousto-optical modulator - Google Patents

Description

Lasers can with the help of active switches in the optical Re sonator can be modulated in their quality. This allows in active medium through the optical pumping process a high Inversion can be built up by the resonator through the active switch is initially blocked. After opening the switch turns the stored inversion into one short pulse emitted. Solid state lasers in particular (e.g. Nd: YAG laser, ruby laser) can be used very efficiently Ent quality switch, because the life of the upper Laserni veaus is very large compared to the resonator orbit time.

For active quality control of continuously pumped Nd: YAG lasers is the use of acousto-optical Mo Dulators (AOM) widely used as an active switching element tet.

A suitable solid material (e.g. Quartz) an acoustic vibration with a frequency of some 10 megahertz (typically 27 MHz) a corresponding refractive index grating is generated. By Diffraction of the laser beam at this grating can lead to laser radiation are diffracted into the higher diffraction orders, and thus the laser beam in the 0th order can be lost be stamped. These losses can be caused by mod tion of acoustic power with frequencies in kilos hertz range can be switched on and off, with which the desired emission of short pulses can be achieved can.

From an essay by M. G. Cohen, in: "IEEE Journal of Quantum Electronics, Vol. QE-9 (1973), 633/4 ", it is known that the diffraction efficiency of AOMs differs from that of Polarization of radiation with respect to the direction of sound spread depends.  

One problem, however, is that the diffraction efficiency of AOM's is not always sufficient to start the Prevent lasers before the actual switching process. This is especially the case when the small signal amplification of the active medium, e.g. B. by use of several serially arranged active media, very much is large and, in addition, low quenzen (pulse to pulse distance <lifespan of the upper Laser levels) a high initial gain for the pulse builds up.

An increase in the diffraction efficiency of an AOM higher driver performance limits are imposed as there are too thermal effects in the AOM. The usage several AOM's in the resonator to improve the barrier behavior is possible, but increases the technical upside wall for the laser system.

The invention relates to a device according to the preamble of Claim as it is known from JP 3-30380 A.

The invention is that Problem underlying that in conventional arrangements Number of passes of the laser beam through the acoustical optical modulator (two passes per resonator cycle) not sufficient to allow efficient diffraction chen. This has a reduced efficiency of the laser or an unstable emission of laser pulses at certain Combinations of pump power (or gain) and Re frequency of petitions.

This problem is solved by the claim led characteristics solved. By using additional Chen laser mirrors and polarization influencing optical components (polarizer, delay plate) is the number of passes through the AOM per resona number of goals increased from two to at least four. The Beu efficiency increases accordingly. The Ver  use of passive optical components to increase diffraction efficiency is considerably cheaper and less complex as the use of several AOMs in the resonator.

The diffraction efficiency increases the Working range of the laser regarding repetition rate and Pumping capacity.

Two possible embodiments of the device (the first of which is not a device according to claim 1) are shown in FIG. 1 and FIG. 2 and are explained in more detail below.

Show it

Fig. 1: A non-collinear arrangement of the AOM's for realizing four passes through the AOM per resonator circulation.

Fig. 2: An embodiment of a device according to the invention with a collinear arrangement of the AOM for realizing four passes through the AOM per resonator revolution with the aid of polarizing elements.

In Fig. 1, which corresponds to the modified coupling of the state of the art according to JP 3-30380 A, 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 resona tor end mirror, mirror 1 b serves to fold the resona tor. The acousto-optic modulator 3 is so positio ned that he caused, ges due to the folding of the Strahlengan by the mirror 1 b at a full resonators torumlauf, 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.

Such an arrangement is basically from JP 3-30380 A. known.

In acousto-optical modulators, the diffraction efficiency of which 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 polarization direction ( FIG. 2), in which the angle of incidence on the AOM is the same for both beams.

For this purpose, a polarization prism 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 polarization prism 4 and the mirror 1 b. The optical axis of the retardation plate 5 is oriented at an angle of 45 ° relative to the polarization direction of the polarization prism 4 . A left to right lau fender, polarized parallel to the plane of beam first passes through the AOM 3 by the return reflection on the mirror 1 b twofold. Due to the 90 ° rotation of the polarization through the delay plate 5 , the beam on the polarization prism 4 is coupled out. With the help of the mirror 1 c, the now vertically polarized beam is reflected back into itself, passes through the AOM 3 two more times and transmits the polarization prism 4 by rotating the polarization back into the plane of the paper. With a complete resonator circulation, the AOM 3 will therefore rotate twice as often as with a conventional arrangement.

Claims (1)

1. Device for improving the blocking behavior of an acousto-optical modulator,

• 1. which is contained in a resonator, the resonator being formed by two resonator mirrors ( 1 a, 1 b) and by an additional resonator mirror ( 1 c) and one ( 1 b) of the two resonator mirrors ( 1 a, 1 b) and the additional che resonator mirror ( 1 c) are arranged so that the acousto-optic modulator ( 3 ) of the laser beam generated in the resonator is passed through several times, thereby increasing the diffraction efficiency of the acousto-optic modulator ( 3 ),

characterized in that

• 1. the resonator between the two resonator mirrors ( 1 a, 1 b) contains a polarization-dependent beam splitter ( 4 ), the acousto-optical modulator ( 3 ) and a quarter-wave delay plate ( 5 ), the acousto-optical modulator ( 3 ) and the quarter-wave delay plate ( 5 ) are arranged between the polarization-dependent beam splitter ( 4 ) and one of the resonator mirrors ( 1 a, 1 b),
• 2. the laser beam, whose polarization plane is rotated by 90 ° degrees after the reflection at one ( 1 b) of the two resonator mirrors ( 1 a, 1 b) and again through the quarter-wave delay plate ( 5 ), through the polarization-dependent beam splitter ( 4 ) coupled out of the resonator and after reflection at the additional resonator mirror ( 1 c) through the polarization-dependent beam splitter ( 4 ) is fed back into the resonator.