DE2223945A1 - LASER OSCILLATOR WITH DECOUPLING MODULATOR - Google Patents

Description

Laser oscillator with decoupling modulator The invention relates to a laser oscillator with a decoupling modulator.

A laser oscillator, usually referred to as a laser for short, consists of an amplifying medium with an optical resonator.

The amplifying medium can e.g. a suitable gas mixture (gas laser), be a doped crystal (solid-state laser) or a semiconductor material. the The optical resonators used are mostly open cavity resonators, i. Arrangements in which on two opposite sides of the reinforcing medium Mirrors are provided, but the other four sides are open. Further Details are e.g. in the book by R. Dändliker "Laser-Kurzlehrgang", Fachschriftenverlag Aargauer Tagblatt AG, Aarau, 1971.

For pulse-like modulation of the electromagnetic generated in the laser oscillator Radiation it is known (e.g. Sci. Am.

June 1969, p. 17 ff.), In the laser beam outside the laser oscillator a controlled electro-optic crystal, e.g.

a Pockels cell or a magneto-optical crystal. It is also known (e.g. R. Dändliker loc. Cit., P. 16), within the optical Resonator to arrange a Pockels cell, by means of which the quality factor Q of the optical Resonator can be changed quickly (so-called giant pulse laser or Q-switch laser). Finally, it is also known (J. Appl. Phys. 41 (1970) No. 4 p. 1552 ff.), Within of the resonator to provide an acousto-optic quartz modulator.

The known arrangements with an external modulator have the disadvantage that the laser oscillator has to work in continuous wave mode and therefore only one provides limited output power, which is also necessary for the application in accordance with the Duty cycle of the pulse modulation is still reduced.

Giant pulse lasers have the advantage that the yarn is available Energy delivered in a single pulse of high peak power and short duration can be, since outside the momentum the energy through the occupation of the upper Laser liveaus are saved, however, have the disadvantage that their linschalt time is relatively large, since when the quality factor Q is obtained, the laser oscillation must first build.

Finally, internal acousto-optical quartz modulators have the disadvantage that that they are constantly exposed to the high internal laser radiation intensity, which is much greater than the external radiation intensity, which is considerable Material problems can arise and that they often have additional optical resonances It is the object of the invention to remedy these disadvantages and a stable Create a laser oscillator with a decoupling modulator, with which laser pulses high peak performance, controllable repetition frequency, good monochrome, high local and temporal coherence and a short rise time can be generated, without special material problems arising, This task is solved by that according to the invention the optical resonator at least on one side of the reinforcing Medium has a tunable interferometer instead of a single mirror, preferably a Fabry-Perot interferometer. Tunable interferometers are available in different Forms known, so mechanically tunable Fabry-Perot interferometers are known e.g. from A.F. Harvey, "Coherent Light", Wiley-Interscience, 1970, P. 283 ff., Or F. Kohlrausch, Practical Physics, 20th Edition, Volume 1 P. 568. You consist of two opposite sides on the sides facing each other partially transparent mirrored plates, between which one of the Plates a plane-parallel air layer of variable thickness can be set. Their transmission T and their reflection R show sharpness for light of a frequency v Maxima or minima as a function of the mirror distance.

An electrically tunable Fabry-Perot interferometer with between The Pockels cell arranged in the mirrors is known, for example, from sample. IEEE 51 (1963) 1258.

A tunable interferometer with spherical mirrors is known z, B. from Applied Optics 3 (1964) 1471-1484.

With the laser oscillator according to the invention, powerful Generate laser pulses which have the above-mentioned disadvantages of the known laser oscillators do not have, and their coherence is sufficient for holographic recordings.

Further details of the invention emerge from the following Exemplary embodiments explained with reference to figures.

1 shows a laser oscillator according to the invention, at which the interferometer is tunable by means of an electro-optical crystal Fabry-Perot interferometer, Fig. La is a special form of the Fabry-Perot interferometer, 2 shows the transmission of the Fabry-Perot interferometer.

Fig. 1 or la as a function of the optical path length between the two Mirror, Fig. 3, two pulses of a pulse train, as they are with a laser oscillator can be generated according to the invention, FIG. 4 shows the transmission T of the Fabry-Perot interferometer and the power density N B inside the laser as a function of the voltage U the electro-optic crystal in the Fabry-Perot interferometer, and Fig. 5 denotes Block diagram of a regulation for the voltage on the electro-optical crystal.

In Fig. 1 the gas discharge tube of an argon ion laser is indicated, in which the reinforcing medium 1 is located.

On one side of the reinforcing medium 1 is a concave, spherical mirror 2, on the other side a tunable Fabry-Perot interferometer 3 provided. The Fabry-Perot interferometer 3 comprises two slightly wedge-shaped, juxtaposed for that of the laser oscillator too emitting Radiation-permeable plates 4 and 5, which between them a plane-parallel layer of air and mirror the sides facing each other by more than 99% are. An electro-optical element 6 is arranged between the plates 4 and 5, e.g. in the form of a Pockels cell, i.e. in the form of one with electrodes KDP crystal, its optical refractive index by means of the electrical voltage U is controllable.

Instead of providing mirrored plates 4 and 5 separately, you can also the end faces of the electro-optical element 6 are mirrored. Is in Fig. La shown.

The optical resonator of the laser oscillator shown is through the mirror 2 and the Fabry-Perot interferometer 3 formed. With the known laser oscillators a second mirror is provided in place of the Fabry-Perot interferometer 3. Since the Fabry-Perot interferometer 3 shown in Fig. 1 acts like a plane mirror, can the optical resonator of the laser oscillator according to the invention in this case be hemi-confocal or hemi-concentric. Using others known forms of tunable interferometers, e.g. with spherical mirrors (Appl. Opt. 3, loc. Cit.), Any other resonator configuration can be implemented.

To operate the laser oscillator according to the invention as a single frequency laser to be able to, is still a Fabry-P3rot etalon 7 between the mirror 2 and the ver ;; ¼ the medium 1 arranged.

With this etalon 7, a single longitudinal mode is made up of several vibratory selected.

In Fig. 2, the transmission is T (e) of the Fabry-Perot interferometer 3 as a function of the optical path length e, i.e. the product of the distance e of the inner surfaces of the plates 4 and 5 and the optical refractive index n des therebetween located medium, shown. T (e) is the fraction of the weight indicated by the arrow S radiation impinging on the Fabry-Perot interferometer 3, which at arrow T exit. R is the reflection coefficient of the measured in the direction of the arrow R. Fabry-Perot interferometer 3. The following applies: l-R = T + const.

As can be seen from FIG. 2, T (Q) has for values l = lk = k. c k sharp maxima, where k = 1,2,, c = speed of light and v = frequency of radiation selected by etalon 7.

e can be adjusted by changing the distance between the plates 4 and 5 however, it becomes more advantageous by changing the optical refractive index of the Pockels cell 6 adjusted by means of the voltage U. The electrical change from e can obviously be done much faster than the mechanical one.

The properties of the Fabry-Perot interferometer 3 are essentially determined by its so-called finesse where V is the attenuation due to losses between plates 4 and 5 and R1 and R2 are their reflection coefficients. The half width ae of a maximum is Ag = λ / 2Fn, with, L = = c / #, and the ratio of the maximum transmission Tmax to the minimum transmission Tmin: Tmax / Tmin # 4F² / #². in the case shown in FIG. 4, Tmax / Tmin is approximately 104.

To operate the laser oscillator according to the invention, B becomes, for example periodically between a value ek at the maximum k of the transmission and a value l # outside of it changed. Since the difference e - ek # 4 # l = 2 = 2 # / nF # 0.02 # / n is only a fraction of the wavelength # of the laser oscillator is that for the Pockels cell 6, the control voltage required for this is relatively small (approx. 100 V). For values T = Tmin, a standing wave is formed in the laser resonator in a known manner because the resonator then, like a normal one, consists of a concave mirror and a Plan mirror existing cavity resonator acts. The Etalon 7 limits the frequency ç of the radiation field. But if T is now controlled to Tmax, the in the optical resonator decoupled energy stored and suddenly in the direction S emitted through the Fabry-Perot interferometer 3.

The intensity I (t) of the coupled-out laser radiation, which is dependent on the time t is, as shown in Fig. 3, modulated in a pulse-like manner. The # 1 rise time of a If the change in e is sufficiently rapid, the impulse is only due to the settling time of the Fabry-Perot interferometer 3 limited, # 1 = 2 e / Fc. His duration g2 = 2L / c when L is the distance between the mirror 2 and the plate 4. if the transmission T (l) is controlled to Tmin again after the pulse, can change build up a new radiation field of frequency v again. The time it takes to do this is typically about # 3 = 200 # 2. After this time can be any time a new intensity pulse can be generated again.

The peak intensity 1max is much greater than the continuous one Output power of a corresponding ordinary laser, since between the individual Pulses store at least part of the energy not required in the resonator will. The average output power at the optimal pulse frequency is only slightly smaller than the output power of a corresponding continuous laser and decreases by no means according to the duty cycle.

With a continuously operated argon laser with 500 mW power With the decoupling modulation described, pulses of about t2 qY 6 / nsec duration with 20 W peak power and a maximum repetition frequency of approx. 800 kHz can be generated. The rise time is about # 1 # 4 / nsec, the temporal one Pulse to Pulse Distance? 3 1.2 psec. The wavelength is just as constant as in the normal argon laser with an etalon as mode selector.

In an advantageous embodiment of the invention, for Control of the Fabry-Perot interferometer 3 generates a DC voltage U ", whose Value is constantly regulated so that the interferometer 3 is set to maximum transmission controls, in Fig. 2 so to Q = eks if there is no further voltage at the Pockels cell 6 is present. This direct voltage U then becomes a pulse-like modulation voltage U superimposed, which is zero for the time of coupling out, so that the interferometer 3 is tuned, and otherwise has finite values, so that the interferometer 3 is out of tune is. This ensures that any externally caused upsets of the Interferometer 3, for example due to a temperature drift or the like., Compensated will.

The embodiment outlined above is illustrated with reference to FIGS and -5 explained as follows: As already mentioned above, the laser resonator has the left in Fig. 1 a mirror 2 with about 0.1% transmission. That through Light of power N B exiting this mirror 2 is the light power density proportional in the laser resonator. In the steady state, the following applies: When the situation is out of tune Fabry-Perot interferometer 3, the power density is maximum, with a matched interferometer 3 it is zero. This is shown in Fig. 4, from which it can be seen that at maximum transmission T (U) of the interferometer 3 <tuning) the Power NB (U) of the light emerging from mirror 2 is zero, and at a minimum Transmission T (U) (detuning) the power is maximum, i.e. equal to NB.

Search cycles are now used to generate the regulated DC voltage U * the interferometer 3 by monotonically changing the voltage on the Pockels cell 6 Quasi-stationary tuned across a resonance. Meanwhile, the laser power is NB measured, which runs from the maximum NB to zero and then back to NB. The voltage Um, in which the interferometer 3 is controlled for maximum transmission, results is then the algebraic mean of the voltage values for which NB = NBS2. "Quasi-stationary" in this context means that all changes are slow are against the above-mentioned time g3, which is a few psec, which is required, to build up the radiation field in the laser The practical implementation of the above The thought described can take place as shown in FIG. 5: Accordingly, a Switch 8 is provided, which is from its operating position B for the search cycle in the Position A can be brought. At the Pockels cell 6 is then the voltage of a Sd .-. Tooth generator 9 with a period of more than F .qr3 (F = the above defined finesse) and an amplitude that is greater than the voltage V die for tuning the interferometer from one response to another, i.e. over / 2 (Fig. 2), is required. The sawtooth generator 9 is by a respect.

its generation still to be explained start impulse together with the Switches switch 8 from B to A started.

The laser light output emerging from mirror 2 during tuning NB (U) is measured by a photodetector 10, which is in a switch position I of switch 11 sends a proportional electrical signal to both a peak detector 12 than outputs to one input of a comparator 13. The peak detector 12 stores the value NB, which is then halved by the voltage divider R-R the other input of the comparator 13 is passed on. The comparator 13 sends a positive pulse if NB (U), coming from larger values, through NB / 2 runs, and a negative pulse if NB (U), coming from smaller values, through NB / 2 is running.

The positive pulse then controls the sample and hold circuit 14, the negative Pulse, inverted via the inverter 16, the sample-and-hold circuit 15 on hold. the Sample-and-hold circles then store the one that is located on the Pockels cell 6 Voltage at which NB (U) = NB / 2. From the stored in the sample-and-hold circles 14,15 Voltage values are then summed up in a first adder 17, and its half value formed by the voltage divider R'-R '- the algebraic mean the voltages at which NB (U) = NB / 2 - the input a second Adder 18 supplied. The said voltage value fed to the adder 18 is then the regulated voltage value U mentioned above.

After the described search cycle, i.e. after both the The sample-and-hold circuit 14 and the sample-and-hold circuit 15 are each controlled once to hold have been, the switch 8 is from its search position A back to its operating position B switched. At the Pockels cell 6 there is then a voltage that is the sum of the regulated DC voltage U * and that fed to the second input of the adder 18 Modulation voltage U is the same. The modulation voltage U applies the second input of the adder 18 for the time at which the light pulses are coupled out from the laser on earth potential. In the meantime the voltage U has a value of V / 2 150, e.g. 100V, where V1 / 2 is to be understood as defined above.

The tuning process takes less in the example of the argon-ion laser than 1 msec. If the light pulses have a repetition frequency of more than 1 kHz, the operation must then be interrupted at regular intervals of e.g. a few seconds and a search or tuning cycle are switched on during which the pulsed Laser oscillator according to the invention cannot be used. The time between Search and tuning cycles depend on the stability of the interferometer 3. at t Repetition frequencies of less than 1 kHz can be used by the search or tuning cycle can be carried out without loss of pulse. The activation of the search or tuning cycle is effected by clock pulses from a clock generator 19, wherein the switch 20, like the switch 11, assumes the position I.

When the regular loss of light pulses from the laser during of the search tuning cycle cannot be tolerated at high repetition frequencies the regulation of the voltage u: also takes place in that the fall time Ta of the light output NB is measured after the pulse voltage U has changed from the maximum value to zero, and the search tuning cycle is only initiated when Ta is greater than a reference value is.

A circuit suitable for this is obtained when the switches in FIG 11 and 20 are brought into position II. The operating state of this circuit is given when both switch 8 and switch 21 are in position B are. The signals from the photodetector 10 then run to an integrator after being inverted 22, which measures the fall time T and a voltage value corresponding to the one The input of a nomarator 23 supplies, at the other input of which a reference voltage from the source 24 lies. If the interferometer 3 is well matched, this is Fall time Ta approx. 2/2 = half the round-trip time of the light in the laser resonator. If those Time is essential is exceeded, or the initial value accordingly of the integrator 22 is greater than that of the source 24, this indicates a detuning of the interferometer 3, and a search or tuning cycle is initiated., that a pulse at the output of the comparator 23 switches 8 and 21 out of the operating position B brings into the search position A and the sawtooth generator 9 starts. The search tuning cycle then proceeds exactly as described above.

The RC element 25 limits the storage time of the integrator 22 a time that is less than the time defined above A realistic implementation of the integrator 22 could for example be done so that the current from a Konstafltstromquelle from the time of the voltage change of the pulse voltage U to the value Nult of the light output NB is integrated.

The Fabry-Perot interferometer 3 is expediently manufactured in such a way that that a finesse F> 50 results. Smaller values of F result in too small a value Ratio Tmax / Tmin and an ae that is too large. For this purpose R1 = R2 = 99% are selected, with Vz 0.98.

The distance between the plates 4, 5 of the Fabry-Perot interferometer 3 is about 10 mm or less.

The laser oscillator according to the invention can also be used as a multi-frequency laser used if the bandwidth of the laser Radiation smaller is as the resonance width 2 ze of the iransmissionsspitze T of the Fabry-Perot interferometer 3. The max Etalon 7 is then omitted.

Stable, single-frequency, pulsed and synchronizable lasers of high medium power, such as laser oscillators according to the invention, are of Meaning for stroboscopic holography or generally stroboscopic coherent Lighting, e.g. a vibration analysis according to DT-OS 2 019 617 for rotating Objects.

With pulse followers with peak powers of 100 W and more are such Laser oscillators are also useful for material processing.

The electrical power for controlling laser oscillators according to the invention is low.

The outcoupling modulation according to the invention is also the other Lasers can be used as the argon-ion laser described, e.g. for continuous operated C02 laser or solid-state and dye lasers.

Instead of the Pockels cell for tuning the Fabry-Perot interferometer could also be a Kerr cell or another in Function of a electrical or magnetic quantity the optical refractive index changing element be used.

Claims (20)

Claims 1. Laser oscillator with decoupling modulator, characterized in that that the optical resonator on at least one side of the amplifying medium (1) has a tunable interferometer (3) instead of a single mirror. 2. Laser oscillator according to claim 1, characterized in that the interferometer (3) is a Fabry-Perot interferometer. 3. Laser oscillator according to claim 2, characterized in that the tunable Fabry-Perot interferometer (3) from two a plane-parallel space delimiting, passages mirrored surfaces (4,5; 4 ', 5'), between which an electro-optical element (6) is arranged. 4. Laser oscillator according to claim 3, characterized gekennzeichr.et that the electro-optical element (6) an electrically controlled KDP crystal with permeable mirrored end pieces (4 ', 5'). 5. Laser oscillator according to claim 2, characterized in that the mirrored surfaces (4,5; 4 ', 5') of the interferometer (3) reflection coefficient of more than 99%. 6. Laser oscillator according to claim 1, characterized in that for the purpose of selecting a single longitudinal mode within the optical resonator a Fabry-Perot etalon (7) is provided. 7. The method for operating a laser oscillator according to claim 1, characterized in that for the purpose of changing the transmission of the interferometer (3) the optical distance (e) between its two transparent reflective surfaces (4.5; 4 ', 5') is changed, 8. A method for operating a laser oscillator according to claim 3, characterized in that for the purpose of changing the transmission of the Fabry-Perot interferometer the optical refractive index of the between the two mirrored surfaces (4.5; 4 ', 5') located electro-optical element (6) by applying a control voltage U is changed to the same thing. 9. The method according to claim 8, characterized in that the control voltage U has a constant component U ″, which is regulated to a value UR, which is equal to the algebraic mean of the two voltage values at which the Light power density in the laser resonator is equal to half of the value that is adjusts between the coupling out of light pulses. 10. The method according to claim 9, characterized in that the control voltage U from the sum of the regulated voltage U * and a pulse-like modulation voltage U exists, which alternates between a finite value and the value zero, where the finite value is smaller than the voltage V> / 2 by means of which the interferometer (3) is tuned from one resonance to the next (l k + 1). 11. The method according to claim 11, characterized in that the finite value of the modulation voltage U is equal to V / / 50. 12. The method according to claim 11, characterized in that the Modulation voltage; U between 100 V and zero with a repetition frequency of 200 kHz changes. 13. The method according to claim 9, characterized in that for the purpose Determination of the light power density in the laser resonator that of the Fabry-Perot interferometer (3) On the opposite side of the laser resonator, emitting light output - NB detected will. 14. The method according to claim 13, characterized in that the Fabry-Perot interferometer (3) by changing the control voltage U from one state outside of a resonance via a subsequent resonance up to the sixth State outside the resonance is tuned quasi-stationary and the two voltage values are stored at which the detected light power density Ng is half of the initial value, and the two stored voltage values can be added and halved and the result then as a DC component U * for the control voltage U of the interferometer (3) is used. 15. The method according to claim 14, characterized in that the tuning occurs periodically. 16. The method according to claim 14, characterized in that the light output NB is continuously detected and the tuning takes place when the in a Integrator (22) determined fall time Ta of the light power NB from the maximum value Zero when coupling out a light pulse is greater than half the round-trip time t2 of the light in the laser resonator, 17. Method of manufacturing a laser oscillator according to claim 1, characterized in that in the Fabry-Perot interferometer (3) a finesse F greater than 50 is produced. 18. The method according to claim 17, characterized in that the distance between the two mirrored surfaces (4, 5; 4 ', 5') of the Fabry-Perot interferometer (3) is smaller than 10 mm. 19. Use of a laser oscillator according to claim 1 for the stroboscopic Holography. 20. Use of a laser oscillator according to claim 1 for material processing. Blank page