Effect of simulated space radiation on solidstate Brillouin phase conjugate mirrors

Transcription

1 Effect of simulated space radiation on solidstate Brillouin phase conjugate mirrors A.Gusarov, F.Berghmans, Member IEEE, A.Brignon, S.Richard, M.Georges, T.Thibert, and Y.Lien Abstract We investigated radiation hardness of phase conjugate mirrors (PCM) based on stimulated Brillouin scattering. Proton- and gamma-irradiated PCMs were tested in a single-frequency flash-lamp pumped Nd:YAG system. PCMs made of Lithosil show a good performance. Index Terms LIDAR, phase conjugate mirrors, Brillouin scattering, space radiation environment H I. INTRODUCTION igh-power laser systems are attracting a growing interest for space applications especially for LIght Detection And Ranging (LIDAR) systems. An important issue for LIDARs is the need for a high optical output power combined with a high quality very good output beam. These features are traditionally in conflict with each other since thermally induced phase distortions corrupt the beam of high-power solid-state lasers. It is well known that such aberration can be corrected using optical phase conjugation (OPC). OPC can be generated exploiting several different mechanisms of nonlinear light-matter interaction. Among those Stimulated Brillouin Scattering (SBS) appears to be most attractive because of its simplicity, efficiency, and intrinsically short response time. However SBS mirrors commonly used in terrestrial application are based on liquid (such as heavy fluorocarbons and tetrachlorides) or gas (CH 4, SF 6 ) high pressures cells, which are not suited to space applications. The solid-state alternative is much more appropriate because of its ruggedness and ease of handling. Quartz was the material used in the first practical demonstration of SBS in 1964 [1]. In subsequent works various glasses were also tested [2, 3]. Nevertheless, the use of solid-state SBS materials was limited due to the relatively Manuscript received Spetember 7, 27. This work was supported in part by ESA-ESTEC contract 18284/4/NL/CP. A. Gusarov is with the SCK CEN, Boeretang 2, 24 Mol, Belgium (phone: ; agusarov@ sckcen.be)). F. Berghmans is with SCK CEN, Boeretang 2, 24 Mol, Belgium and Vrije Universiteit Brussel, Pleinlaan 2, 1 Brussels, Belgium. A.Brignon and S.Richard are with the Thales Research & Technology, RD 128, Palaiseau, France M.Georges and T.Thibert are with the Centre Spatial de Liège, B-431 Angleur-Liège, Belgium. Y. Lien was with European Space Agency, ESTEC, 22 AG Noordwijk, the Netherlands. high intensity threshold required to provide phase conjugation and due to related optical damage issues. Only recently a group from Osaka University has obtained very promising results with bulk glasses [4-6] and highly resistant organic crystals such as LAP and DLAP [7]. They reported an SBS gain coefficient for fused silica of 3. and 3.8 cm/gw from 18 ns and 38 ns pulse width measurements, respectively. SBS gain of about 3 cm/gw and 6 cm/gw respectively, were obtained in optical glasses such as BK7 in the crown group and LF1 or LF in the flint group. The SBS gain of SF6 flint glass reached the highest value (9.3 cm/gw) for the measured glass samples. An SBS gain coefficient of 1 cm/gw was obtained from measurements of SBS threshold energy at 164 nm for LAP. The for DLAP the SBS gain was as high as 18 cm/gw. The last values are already comparable with typical values of SBS-liquids and with an optimized focusing geometry and a narrow bandwidth single longitudinal mode laser they allow for a very high (> 92 %) phase conjugate reflectivity. Recently a very high Brillouin gain of ~1 cm/gw was demonstrated in TeO 2 [8]. In a 4-cm long crystal the phase conjugate reflectivity of % was observed for 7.8 ns pulses and an input energy as low as 1.2 mj. The optical beam was parallel to the crystal c axis to use the largest elasto-optic coefficient and hence to obtain the largest Brillouin gain coefficient. The authors found that surface damage occurs at 4.4 J/cm 2 and pointed out a poor polishing quality. Those results show that SBS in bulk materials could be used for space applications. To exploit the method in space, one has to assess the influence of the space environment, and in particular of space radiation, on the properties of the materials that will be used as phase-conjugating mirrors (PCMs). The dose levels that PCMs should withstand can be at a Mrad level. To the best of our knowledge this issue was never addressed so far. In the present work PCM materials and actual PCMs intended for use in space-born Nd:YAG laser system (1.64 µm) were studied. Radiation-induced transmission degradation measurement were preformed on spectroscopic samples prepared of five types of fused silica with the OH-concentration in a range from several up to 12 pmm and of TeO 2. The samples exposed to gamma and to proton radiation with a maximal dose up to 1 Mrad for

2 gamma-radiation and 6 Mrad equivalent ionization dose for protons. Proton- and gamma-irradiated PCMs in the form of 3-cm long Suprasil-1 and Lithosil rods and -cm long TeO 2 crystals were tested in a single frequency flash-lamp pumped Nd:YAG system. The phase-conjugated reflectivity and the laser-induced damage threshold (LIDT) were measured and compared with those for non-irradiated samples. II. EXPERIMENT The effect of ionizing radiation on the transmission degradation was studied in a spectral range from 2 to 3 nm using a commercial double-beam spectrum analyzer (Varian Cary-). Samples for spectroscopic measurements were prepared as - and 1-mm thick 2-cm diameter discs for types of silica and as x1x1 mm 3 parallelepipeds for TeO 2 with two polished sides. The gamma irradiation was performed in the 6 Co irradiation facility RITA (Radio Isotope Test Arrangement) at SCK CEN (Belgium) at a dose-rate of 3 krad/h. The samples were first irradiated up to a fraction of the total dose. The transmission spectra were then recorded with at least 1 hour delay to avoid the effect of fast postradiation relaxation. This sequence "irradiationmeasurements" was repeated until the accumulated dose reached 1 and 3. Mrad for silicas and TeO 2, respectively. The proton irradiation of the spectroscopic samples was performed using a similar dose-step accumulation procedure with 37-MeV protons at the VUB cyclotron. The beam current was at a na level, corresponding to a Mrad/min dose-rate. The variation of the beam intensity was less that 2% over the 2 cm apperture. For 37 MeV protons the penetration depth in silica is about 7 mm, which results in a 7% variation of the dose distribution along the beam propagation in a mm thick sample. To reduce the non-uniformity the irradiation was carried out from both sides of the samples, with each side receiving approximately the same dose. That allowed a dose non-uniformity of less than 1% inside the irradiated sample. The equivalent ionization dose reached 6 Mrad. The spectroscopic measurements showed that in silicas absorption bands are induced in the UV range. The intensity of those bands is strongly dependent on the OH-concentration, Fig.1. The shape and the amplitude of the bands is also radiation type dependent. However, when the optical transmission at 1 µm is of concern, for all types of silica and TeO 2 crystals even for the highest radiation doses of 1 and 3. Mrad the induced absorption was below the detection limit estimated as.2 cm -1. Extrapolation based on the UV-data shows that the induced absorption at a 1 Mrad dose should be below.2 cm -1. This is a very small value but it does not allow us to rule out the possibility for the OPC performance to be influenced by ionizing radiation due to thermally-induced self-focusing effect, which may have a threshold of.1 cm - 1. We repeated transmission measurements at 1.64 µm wavelength with an improved accuracy using longer samples. High-OH Suprasil-1 and Lithosil UV were selected for a more detailed study because they show a higher LIDT and lower Induced abs, 1/cm Induced abs, 1/cm Wavelength, nm 3 4 (a) Wavelength, nm (b) Fig.1 UV-range induced absorption of different types of silica as a result of proton- (left) and gamma- (right) irradiation with the doses of.8 and 6.9 Mrad, respectively. 1 HOQ; 2 Infrasil; 3 - SU1; 4 - U311; Lithosil. Curves 1, 2 and 3,4, correspond to the lhs and rhs scales, respectively. radiation-induced attenuation in the UV as compared with dry silica types. Two 3-cm long silica rods and a -cm long TeO 2 bar were exposed in RITA at a dose-rate of 2 krad/h up to the qualification dose of 1 Mrad. Another set of two silica rods and one TeO 2 bar was exposed to 2 MeV protons at the Proton Irradiation Facility (PIF) of the Paul Scherrer Institute, Switzerland. The irradiation was performed in air at ambient temperature with a nominal proton flux of 1 8 p/cm 2 /s up to a total fluence corresponding to a 2 krad dose, i.e and p/cm 2 for silica and TeO 2, respectively. Two silica rods were

3 irradiated simultaneously with the proton beam direction perpendicular to the rod axis. The penetration depth of 2 MeV protons in fused silica is significantly longer than the silica rod diameter and the whole volume was quasi-uniformly exposed. A collimator placed at the exit of the proton beam make it rectangular with a size of 7x7 mm 2. In order to expose the rods over their entire length they had to be translated across the beam. This translation was done manually using marks on the pack and a laser pointer built in the irradiation system. This translation was not very accurate and several areas with a width of several mm received a double exposure. Such a nonuniform irradiation should not considered as a problem because in space the dose-distribution in a PCM will be also non-uniform due to the shielding effect of the spacecraft elements combined with a limiting penetration range of space radiation. To perform transmission measurements with 3 cm long components we realized a dedicated measurement bench, based on a CW single-frequency Non-Planar Ring Oscillator Nd:YAG laser delivering up to 2 mw. The use of the laser was motivated by its excellent power stability. The samples to test were placed on a V-groove forming a small angle (about 2 ) with respect to the laser beam to avoid the superposition of the multiple reflections on the sample faces and subsequent intensity instabilities due to the interferences. Two different calibrated power meters (Coherent Labmaster LM-1 and Ophir Power Meter) were used for cross-checking the results. The transmission of the irradiated samples was compared with a reference non-irradiated sample. To measure the SBS response of the silica rods we used the setup shown in Fig. 2. The injection seeded flash lamp pumped Nd:YAG system delivers up to 22 mj pulses with the linear polarization and a 2 ns duration at a Hz repetition rate. A Faraday isolator was used to protect the laser against the optical feedback. A half-wave plate and a Brewster angle thin film polarizer were used to control the energy incident on the tested samples. A quarter wave plate was used to flip the polarization after the double pass to reflect most of the energy of the phase conjugated beam on the Brewster angle thin film polarizer and to provide an extra protection against the optical feedback in the laser. The polarization of the incident beam in the PCM material was thus circular. The PCM was formed by focusing the laser beam inside the sample. The focusing geometry has been optimized to avoid LID at the entrance and the exit face of the sample as well as in the bulk of the material. The focal length of the lens was mm (see. Fig. 2) and the beam waist a diameter of ~2 µm. The beam waist was placed inside the 3-cm long silica rod at 23 cm from the entrance face. It is also very important that the laser operates in a perfect single frequency single-mode regime. A multimodal operations results in the optical intensity fluctuations due to mode-beating. We have observed that in many cases laser breakdown in silica is correlated with appearance of the of side-band modes in the injection-seeding laser. Injection seeding Nd:YAG laser system single mode 2 ns, Hz HR Mirror Faraday Isolator /2 TFP Detector 2 (Phase conjugate pulse energy) Glass Plate /4 Detector 1 (Incident pulse energy) f = mm Silica rod Waist diameter = 2 µm Fig.2 Experimental setup used to measure the SBS reflection coefficient of a silica rod. /2 - half-wave plate, /4 - quarter-wave plate, HR - High Reflectivity mirror, TFP - Brewster angle Thin Film Polarizer III. RESULTS AND DISCUSSION No difference in transmission at the working wavelength 1.64 µm was detected for the gamma-irradiated and nonirradiated silica rods. The detection limit for the induced absorption was ~8x1 - cm -1. Similarly, no radiation-induced transmission changes have been detected on irradiated TeO 2 crystals. The detection limit in this case was slightly higher ( 1-4 cm -1 ) because of the shorter length of the samples. These results indicate that OPC in silica and TeO 2 is not expected to be influenced by radiation via radiation-induced absorption. Phase conjugate reflectivity as a function of the incident pulse energy is shown in Fig. 3a, for non-irradiated and irradiated Lithosil rods. The Brillouin threshold (and hence the Brillouin gain coefficient) is not affected by irradiation. The phase conjugate reflectivity is very similar for irradiated and non-irradiated samples. We found a SBS threshold at a ~1 mj pulse energy. For perfect single frequency operation of the laser, we obtained an OPC reflectivity up to 6 % at 9 mj incident pulse energy. In other set of experiments, an OPC reflectivity as high as 8 % was obtained at 22 mj incident energy. It is important to note that the mirrors, i.e. the silica rods and the TeO 2 crystals, had polished surfaces without any coating. The use of an anti-reflecting coating would allow to improve the results. A stable and reliable operation of the PCM for more than 2 1 pulses was observed. We have also investigated a possible influence of radiation on the LIDT. No detectable difference between irradiated and non-irradiated rods has been observed. To measure the SBS response of the TeO 2 crystal, we use a setup similar to the one used for fused silica except that the focusing lens has a focal length of 2 mm given a beam waist diameter of 7 µm inside the crystal. In these conditions the phase conjugate reflectivity obtained in non-irradiated and in irradiated TeO 2 crystals are given in Fig. 3b. The results show an attractively low SBS threshold (~μj) but with a

4 low LIDT of a few mj only. It is relevant to note that the optical quality of the crystals was not very high. Nonuniformities could be observed with a naked eye. Unfortunately, a better quality crystals were not available at the moment when the experiments were performed. Reflectivity Reflectivity Proton irradiated Gamma irradiated Non irradiated Incident pulse energy (mj) (a) Proton irradiated Gamma irradiated Non irradiated Incident pulse energy (mj) (b) Fig.3 Phase conjugate reflectivity as a function of the incident pulse energy for irradiated and non-irradiated Lithosil silica rods (a) and TeO 2 crystals (b). Fig. 3 shows that the SBS response is not affected by irradiation. Instead, an effect on the LIDT was found. LID was frequently observed in the gamma-irradiated sample even when the laser was stable in frequency. On a non-irradiated sample the damage was observed only for pulse energies above 1.9 mj, while on the gamma-irradiated samples damage systematically happened already at 1.1 mj. It was therefore not possible to plot the phase conjugate reflectivity curve above 1.1 mj with the gamma irradiated TeO 2 crystal. For the TeO 2 bar irradiated with high energy protons no detectable difference with non-irradiated bars could be observed. The difference between the gamma and proton irradiated samples should be attributed to the difference in the radiation doses. It was already mentioned that the equivalent proton ionization dose was about times smaller than that of gamma radiation. Intrinsic volume breakdown in glasses is often related to the thermal ionization of absorbing impurities, like metallic nanoparticles [9]. Such impurities can be heated by laser light. The local temperature increase make surrounding glass opaque, increasing the effective size of the impurity. The process continues until the stresses related with the presence of the heated domain reach a critical value, at which mechanical failure occurs. However, it is unlikely that such absorbing impurities could be influenced by ionizing radiation and we can rule out the possibility that absorbing impurities play dominating role in the LID of TeO 2. If that were the case, ionizing radiation would not influence the LIDT. The situation is different if the damage occurs due to linear ionization under the condition of band gap shrinkage via the Franz-Keldysh effect [9]. In the latter case gamma or proton radiation induced defects can influence the damage threshold because near such defects optically induced ionization can happen with a smaller forbidden gap shrinkage, i.e. at lower power densities. A sufficient concentration of defects is required in order the electron avalanche could develop. This may explain why LID is not observed at low radiation doses. The linear ionization mechanism of the LID also implies that the threshold will not improve significantly with the crystal quality improvement, because the breakdown mechanism is not directly linked to the presence of imperfections. IV. CONCLUSION The goal of the present work was to assess space radiation effects on the solid-state Brillouin PCMs, which could be used to dynamically correct aberrations in space borne laser systems. First, we theoretically assessed the impact of radiation on the parameters defining the SBS-process, which allowed us to select materials to be tested under radiation. We demonstrated that the radiation induced absorption can be the most deteriorating effect. Therefore, we chose fused silica based on the relation between the Laser-Induced Damage Threshold (LIDT), the Brillouin gain, and the sensitivity to ionizing radiation. We tested five types of silica with an OHconcentration ranging from (Infrasil-31) to 12 (Lithosil) ppm for a more detailed study. We also opted for TeO 2 crystals based on their very high Brillouin gain coefficient. We then first studied the effect of ionizing radiation on the optical transmission in a spectral range from 2 to 3 nm. We found the induced attenuation at 1.64 µm to be below the detection accuracy, but that accuracy was not sufficient to guarantee that a weak induced absorption would not deteriorate the Brillouin amplification. Then, we irradiated with both protons and 6 Co gamma-rays 3-cm long silica rods and -cm long TeO 2 crystals and subsequently used them in a laser system as PCMs. Our measurements relied on a single frequency flash-lamp pumped Nd:YAG system delivering up to 22 mj pulses with a 2 ns duration at a Hz repetition rate. OPC was generated by focusing the laser beam inside the sample. We optimized the focusing geometry to avoid LID at the entrance and exit faces of the sample as well as in the bulk of the material. The results with TeO 2 show an attractively low SBS threshold (~µj) but a low LIDT as well of a few mj only. The LIDT of the irradiated TeO 2 was also slightly lower than the non-irradiated

5 sample. For fused silica, we found a SBS threshold at a ~1 mj pulse energy. For perfect single frequency operation of the laser, we obtained an OPC reflectivity up to 8 % at 22 mj incident pulse energy in a Lithosil rod. We witnessed a stable operation of the PCM for more than 2 1 pulses. The difference in the reflectivity between irradiated and nonirradiated components was within the measurements errors. Our results show that fused silica, and Lithosil in particular, can be used for generation the OPC in laser systems operating in a space radiation environment. Following these results, a Lithosil rod has been used as PCM material incorporating a demonstrator which has been designed and manufactured in order to cope with space-borne LIDAR working conditions [1]. Finally it has been evaluated at the level of conjugation performances in space simulated environmental vacuum-thermal chambers. This part of the study shall be presented elsewhere.. REFERENCES [1] R.Y. Chiao, C.H. Townes, and B.P. Stoicheff, "Stimulated Brillouin scattering and coherent generation of intense hypersonic waves," Phys.Rev.Lett., Vol. 12, no. 21, pp. 92-9, [2] B.N. Borisov, Y.I. Krizhilin, S.A. Nashchekin, V.K. Orlov, and S.V. Shklyarik, "Wavefront inversion in induced Mandel'shtam-Brillouin scattering in a glass without failure," Sov. Phys. Tech. Phys., Vol. 2, pp. 64-6, 198. [3] A.F. Vasil'ev and V.E. Yashin, "Stimulated Brillouin scattering at high values of the excess of the pump energy above the threshold," Sov. J. Quantum Electron., Vol. 17, no., pp , [4] H. Yoshida, H. Fujita, M. Nakatsuka, and K. Yoshida, "Stimulated Brillouin scattering phase-conjugated wave reflection from fused-silica glass without laser-induced damage," Opt.Eng., Vol. 69, no. 9, pp , [] H. Yoshida, H. Fujita, M. Nakatsuka, and K. Yoshida, "Generation of Stimulated Brillouin scattering phase-conjugated wave using optical glasses," Rev. Laser Eng., Vol. 27, no. 7, pp. 49-, [6] H. Yoshida, M. Nakatsuka, H. Fujita, T. Sasaki, and K. Yoshida, "High-energy operation of a stimulated Brillouin scattering mirror in an L-Arginine phosphate monohydrate crystal," Appl.Opt., Vol. 36, no. 3, pp , [7] M. Yoshimura, Y. Mori, T. Sasaki, H. Yoshida, and M. Nakatsuka, "Efficient stimulated Brillouin scattering in the organic crystal deuterated L-arginine phosphate monohydrate," J. Opt. Soc. Am., B, Vol. 1, no. 1, pp. 446-, [8] M.A. Dubinskii and L.D. Merkle, "Ultrahigh-gain bulk solid-state stimulated Brillouin scattering phase-conjugation material," Opt.Lett., Vol. 29, no. 9, pp , 24. [9] L.B. Glebov, "Intrinsic laser-induced breakdown of silicate glasses," in: Laser-Induced Damage in Optical Materials: 21, Boulder, CO, 22, SPIE Proc., Vol. 4679, pp [1] A. Brignon, S. Richard, M. Georges, J.-Y. Plesseria, T. Thibert, P- A.Blanche, A. Gusarov, F. Berghmans, and Y. Lien, "A spacequalifiable solid-state phase conjugate mirror for correction of laser aberrations," in: Sixth Int. Conf. on Space Optics, Noordwijk, The Netherlands, 26, ESA-ESTEC, Vol. SP-621.