Phase-conjugation of high power molecular C02 and CO lasers radiation inside their active medium

Transcription

1 Phase-conjugation of high power molecular C02 and CO lasers radiation inside their active medium L.Afanas'ev, A.Ionin, Yu.Klimachev, A.Kotkov, D.Sinitsyn P.N.Lebedev Physics Institute ofrussian Academy of Sciences 53 Leninsky pr., Moscow, Russia. ABSTRACT The results of experiments on four-wave interaction of electron-beam-controlled discharge (EBCD) pulsed C02- and CO- lasers' radiation inside their active medium are presented. Linearly polarized probe C02 (COW) laser beam intersects strong electromagnetic waves counterpropagatmg through an inverted medium inside the laser resonator. The laser beam reflected from the active medium has been registered both in near-field zone and in far field one. The experiments on recovering of optical images in near-field zone and recovering of angular divergency of laser radiation in far-field zone demonstrated that reflected beam was phase-conjugated one. The reflectivity of phase-conjugated beam was up to 2% for C02- laser and up to 0.2% for CO- laser. The time-history of phaseconjugation reflectivity and comparison with theoretical results are discussed. 1. INTRODUCTION High power pulsed and repetitively - pulsed molecular C02- and CO- long pulse lasers (pulse duration ms > time of relaxation) can be useful for different applications (for instance, laser material processing, optical pumping, laser chemistry etc.). An optical quality ofthese lasers' radiation (disturbed by aberrations) can be improved by non-linear optical method of phase conjugation (PC) of laser radiation by means of degenerate four-wave mixing (DFWM) technique inside a non-linear medium 1. Because of high thermal input into a non-linear medium and potential possibility of its destruction ( especially for solid state and liquid media ), an application of active (or inverted) gas medium itself as non-linear one may be more attractive for CO2- and CO- lasers characterized both high energy in a single pulse and high average laser power. First experiments have already demonstrated a possibility of using inverted CO2 laser active medium as a PC mirror for pulsed TEA C02- laser with high peak power and short pulse duration (t<200 ns) 2.The PC of CO2- laser radiation by DFWM inside own active medium was observed also for long pulses (t l0 is and 20 is) in 34. Though characteristics ofboth those laser installations were quite similar to each other, the obtained results on PC reflectivity (PCR) measuring were contradictory (50% in 3 and 0.2% in 4 ). Unfortunately it is impossible to compare the results of researches mentioned above because of information lack in 3about some important parameters of active medium (gas pressure, laser gain, etc.) and to compare results of the research with theoretical ones (for instance, 4,5). The PC of CO- laser radiation was observed for gas flow pulsed CO- laser with optical loop feedback (t-0.2 ms, PCR l0%) 6. However, as was mentioned in 7, a mechanism of PC in the experiments was unclear itself. Besides using of the loop feedback makes it difficult to interpret the experimental results. In this paper the results of experimental investigation of PC process by DFWM inside the own inverted medium oflong pulse CO2- and CO lasers with EBCD pumping are discussed. The experiments were carried out by means of classical optical scheme with two counterpropagating intracavity strong waves and probe wave. The timehistory of PCR for DFWM is also discussed, and diffraction grating formation mechanisms inside active medium and their contribution to PCR are taken into consideration. The maximum energetic PCR was up to 2% for CO2- laser radiation (t20 ps) and up to 0.2% for CO- laser radiation (t-0.2 ms) for the interaction length of 1.0 m. 2. EXPERIMENTAL INSTALLATION The optical scheme of the experiments is presented in Fig. 1. The experiments were carried out with an universal (CO-/C02-) pulsed EBCD laser installation with active medium length of 1.2 m. The gas temperature for CO-laser (mixture CO:N2:He=l:4:5, pressure 110 mbar) was 110 K, and for CO2-laser (mixture C02:N2:He=l:2:4, pressure 280 mbar) it was 300 K. The CO-laser gas mixture density expressed by units of density at normal conditions, corresponded to 0.3 Amagat. A specific input energy was J/l.Amagat for both lasers. The pumping pulse duration was 30 p.s. At the conditions mentioned above the active medium gain was approximately 230 ISPIE Vol /94/$6.00

2 identical for both C02- and CO-lasers (-4% cm-' ). The optical windows of laser chamber (NaCl for C02-laser and CaF2 for CO-laser) were placed under Brewster angle to optical axis. Nonselective 1 2 m length optical resonator consisted of plane and concave (radius of curvature 24 m) mirrors was used in our experiments. Laser radiation pulse durations and laser beam diameters were 20 jts and 20 mm respectively (for C02- laser) and 0.2 ms and 15 mm respectively (for CO- laser). The laser pulse energy amounted to 4 Joules. The spectral structure of radiation was as follows: C02-laser operated on a single line P(20), CO-laser operated on about 25 spectral lines within spectral range of 5,0-5,5 tm. A linearly polarized output laser beam (probe wave E3) was directed into the active medium under the angle of 15 mrad to the optical axis of the laser resonator and intersected it a center of the active medium. The length of the DFWM interaction was 1 m. For carrying out synchronism condition and temporary coherence between probe wave E3 and one of the basic waves (the wave Ei) the optical delay between them was reduced to zero. ACTIVE MEDIUM a b Figure 1. Optical scheme of experiments on phase conjugation of CO and CO2 laser radiation inside own active medium. (a) - near field measurements, (b) - far field measurements. The optical scheme included a registration system, which used JR video camera for radiation energy distributions study of probe wave E3 and reflected wave E4 in near-field (Fig. la) and far-field (Fig. 1.b) zones. The JR video camera was supplied by two JR lenses with non-reflection coating on 5 im and 10.tm wavelengths. To measure the distribution of the laser radiation the beams were directed on a diffused scattering screen. The image of scattered radiation was transformed to a video-signal by the JR video camera and digitized for subsequent computer SPIEVo!. 2206/231

3 processing. To compare distributions of energy radiation probe and reflected signals were approximately leveled by calibrated optical attenuators and were registered simultaneously. 3. EXPERIMENTAL RESULTS An IR optical signal reflected from active medium of C02 (CO) laser by PC process was reliably observed in the experiments. A maximal background radiation from the laser resonator registered by the IR video camera in the absence of DFWM did not exceed 1% and 5% of reflected signal for C02-laser for CO-laser, accordingly. The distribution of background radiation was represented by the chaotic set of spots within the limits of aperture of rotating mirrors. On the base of observed signal-to-background ratio a study of reflected signal characteristics was carried out. To examine an angular distribution of reflected radiation in far-field zone the distribution of the energy radiation in focal plane of a spherical mirror with radius of curvature of 18 m was registered. The comparison of farfield patterns of probe beam and reflected one allowed us to conclude that the reflected signal has an angular divergency which differs from an angular divergency of probe laser radiation not more than times. In some experiments the angular divergency of reflected radiation (Fig.2b) was less than that of probe beam (Fig.2 a). It can be explained by larger diameter of the probe beam in comparison with cross size of DFWM-area in the experiments. Intencity, A.U. 1. a Intencity,A.U. b 0. o.o Distance, A.U Distance, A.U. Figure 2. Far field patterns for C02 laser radiation (a) and phase conjugated signals (b). To confirm observation of PC-effect at DFWM in own active medium of C02 (CO) laser the demonstration experiments on recovering of optical image of an object by reflected signal in near-field zone were carried out. An optical L-mask with slit width of 2 mm was used as the object. The image of the mask in a thermal radiation is shown in Fig.3a. The energy distributions of radiation passed through the optical mask are presented in Fig.3b (for C02-laser at a distance of 4.5 m from the mask )and in Fig.3d (for CO- laser at a distance of 2.5 m). The computer analysis of 232 ISPIE Vol. 2206

4 L. b C e Figure 3. Near field patterns for C02 (b,c) and CO (d,e) laser radiations profiled with optical masks. (a) -just behind the mask, (b) - at the distance of 4.5 m behind the mask, (d) - at the distance of 2.5 m behind the mask, (c,e) - phase conjugated signal at the place ofmask location. mentioned above distributions has shown that the given optical patterns appear to be complicated diffraction ones created by diffraction on the edges of the optical L-mask. The energy distributions of reflected radiation in plane of the mask (the optical splitter 1 in Fig. 1 is at equal distance from optical mask and scattering screen) are in Fig.3c and 3e for C02- and CO- lasers, accordingly. The analysis of distributions has shown that the reflected signal restores the image of the object at DFWM in own active medium of EBCD C02 (CO) laser. A slight discrepancy of observable image of the optical mask between ideal one is connected with a quality of a PC-mirror and with a loss of part of optical information about the object due to diffraction on the edges of the L-mask (the L-mask was situated at a distance of 5 m from DFWM-area). As a result, a part of the probe beam propagates beyond of aperture of interaction zone. The calorimeter with the sensitivity of 0.01 inj was used for measurement of PC reflectivity. The energetic PCR for C02- laser radiation was of 2 % (the energy of E3 wave was up to 3J) and of 0.2% for CO- laser (the energy of E3 wave was up to 4J). To measure a time-history of C02 (CO) laser radiation Ge:Au photo detector with response time of 0. 1 Ls and drag-detector with response time 1 ns were used. The time-history of C02-laser pulse is compared with that of the PC-signal in Fig.4 (a-c). Unlike the form of C02- laser pulse generation (Fig.4a, high frequency component connected with multimode structure is in Fig.4a*),which might be named triangular, the time-history of the reflected signal (Fig.4b) is characterized by "bump". Fig.4c demonstrates a dependence of reflected-to-probe-signal intensity (power PCR) ratio on time. After the first peak the sharp recession of PCR is observed. Then the slow PCR increasing and subsequent sharp reduction at the end of laser pulse are observed. The great difference of PCR buildup time (about ten times) for various parts of the dependence observed (Fig.4c) indicates on possibility, at least, two physical PC mechanisms, which give the appreciable contribution to energetic PCR: resonant or amplitude mechanism (grating of local gain coefficient) and thennal or phase one (grating of refraction index). The observed value of PCR is agreed with theoretical results ( under our experimental conditions) of researches 4,5 dealt with physical mechanisms of PC at DFWM mentioned above. SPIE Vol / 233

5 Co2 las 1 A.U. Co rell t -6 do s e PCI 1 t t o106s Figure 4. Oscillograrns of laser pulses (a,a*,d), phase conjugated signals (b,e) temporal behavior of reflectivity (c,f) for C02 (a,b,c) and CO (d,e,f) lasers. The time-history of CO-laser pulse is compared with that of PC-signal in Fig.4(e-g). Unlike the CO- laser pulse (Fig.4e) the time-history of reflected signal (Fig.4f) had a distinguishing peak in the beginning of laser pulse. Fig.4g shows a time-history of reflected-to-probe-signal intensity ratio. After the first peak a slow reduction of PCR down to the ending of laser pulse is observed. The response time of the coefficient PCR 1.ts on front of laser pulse indicates to the resonant nature of PC at DFWM in own active medium of EBCD CO-laser during the beginning of laser pulse. The duration of considered peak approximately corresponded to the duration of input energy pulse. In the part of slow recession of PCR the thermal mechanism of PC plays a significant role that has ensured the smoothing of reflected signal at sharp change of laser power (under an influence of shock waves in active medium) at the end of pulse. 4. DISCUSSION The lasing mechanisms of CO2- and CO- lasers considerably differ from each other. However, during experiments on PC at DFWM inside own active medium of EBCD long pulse lasers (with durations of pulse more than 10 ts) it was found out, that the time-history of PCR has the similar nature. Therefore it was interesting to compare data on characteristics of PC-signals received with the same experimental laser installation. The comparison of time-history of PCR for considered lasers demonstrates, that the time of increase of PCR ( PC-response time) is defined by resonant mechanism of PC at DFWM inside an inverted medium. Then the thermal mechanism with the PC-response time of dozens of microseconds is prevailed. But the value of PCR for CO2-laser medium was ten times 234 / SPIE Vol. 2206

6 higher than the appropriate value for CO-laser. It may be explained by the great difference of C02- and CO- laser spectral structure and also by difference ofheat generated processes taking place in CO2- and CO- lasers. 5. CONCLUSIONS The PC at DFWM inside own inverted medium of EBCD CO2- and CO-lasers was observed. The timehistory of reflected signal at DFWM inside the active medium of EBCD molecular lasers with pulse duration more than 10 j.ts indicates on two main grating-formation mechanisms delivering the main contributions to PCR: thermal mechanism and resonant one. The PCR observed for C02-laser under conditions of our experiments (energetic PCR up to 2% for interaction length 1 m and pulse duration of 20 ps) is agreed with theoretical results 4$. The first results of PC process at DFWM carried out with classical scheme inside own active medium EBCD pulsed CO-laser with the laser pulse duration of 0.2 ms and with PCR of 0.2% are presented. 6. ACKNOWLEDGMENTS Authors wish to acknowledge Dr. M.Bunkina, Dr. V.Kovalev and Dr. V.Morozov for discussions of the results and Mr. E.Kiselev for technical assistance. 7. REFERENCES 1. B.Y.Zel'dovich, N.F.Pilipetsky, V.V.Shkunov "Phase Conjugation Process", (in russian), Moscow, "Nauka", R.A.Fisher, and B.J.Feldman Opt. Leits., v.4, p.140, S.A.Dimakov, and M.A.Robachevskaya Proc.Conf on Phase Conjugation processes, OVF-89 (in russian), Minsk, p.75, E.K.Gorton, A.M.Richmond Opt. Corniiz., v.86, p.34!, 199!. 5. R.C.Lind, D.G.Steel, G.J.Dunning Optical Engineering, v.21, p.190, D.V.Belousov, A.M.Borodin et a! Proc. XIV Int.Conf on coherent and nonlinear optics (in russian), Leningrad, v.1, p.l'77, A.V.Berdyshev, A.K.Kurnosov, A.P.Napartovich Kvantovaya Electronica (in russian), v.20, No6, p.529, SPIE Vol / 235