A large aperture fused silica tapered fiber phase conjugate mirror is presented with a maximum 70% stimulated Brillouin scattering (SBS) reflectivity, which is obtained with 1 kHz repetition rate, 15 ns pulse width and 38 mJ input pulse energy. To the best of our knowledge, this is the highest SBS reflectivity ever reported by using optical fiber as a phase conjugate mirror for such high pulse repetition rate (1 kHz) and several tens of millijoule (mJ) input pulse energy. The influences of fiber end surface quality and pump pulse widths on SBS reflectivity are investigated experimentally. The results show that finer fiber end surface quality and longer input pulse widths are preferred for obtaining higher SBS reflectivity with higher input pulse energy. Double passing amplification experiments are also performed. 52 mJ pulse energy is achieved at 1 kHz repetition rate, with a reflected SBS pulse width of 1.5 ns and a M2 factor of 2.3. The corresponding peak power reaches 34.6 MW. Obvious beam quality improvement is observed.
©2012 Optical Society of America
Phase conjugate mirror based on stimulated Brillouin scattering (SBS) is an attractive device for dynamical compensation of the thermal aberration by providing wavefront reversal when master oscillator power amplifier (MOPA) laser system is referred to . Till now, various kinds of SBS phase conjugate mirror (SBS-PCM), including gaseous, liquid and solid materials, have been developed .
Nowadays the diodes pumped solid-state lasers are widely used. When it comes to developing a laser system with a SBS-PCM, some solid materials are preferred. In addition, there are several other advantages for solid materials compared with their gaseous or liquid counterparts, such as innocuity, free of high pressure and easily manufactured .
Optical fiber and bulk fused silica are two kinds of promising candidates for solid SBS-PCMs  and there have been many reports on them [5–8]. For optical fiber SBS-PCM, though its SBS reflectivity reaches more than 80%, its damage threshold is typically restricted to a few millijoules [5,6]. For bulk fused silica, more than 90% reflectivity could be obtained with input pulse energy of hundreds of millijoules to a few joules [7,8]. However, most of them are operated with very low repetition rate, typically from 1 Hz to 10 Hz. Recently, another remarkable solid PCM, BaTiO3 crystal self-pumped photorefractive PCM, is reported [9–13]. However, its typical phase conjugate reflectivity is limited to ~50% [9–13].
Then, what is the problem when laser pulse repetition rate increases? It is the drop of SBS reflectivity. In 2006, Lixin Tong et al. reported a fused silica rod SBS-PCM, which has a SBS reflectivity of 42% at 50 Hz repetition rate but only 22% with 400 Hz repetition rate with input pulse energy of 48 mJ . In 2007, S. Wang et al. demonstrated a SBS-PCM combining a fused silica rod and a fiber, which showed a 42% SBS reflectivity with maximum input pulse energy of 42 mJ at 100 Hz repetition rate, but only 11.6% with input pulse energy of 55 mJ at 400 Hz repetition rate . In fact, Yoshida et al. has ever pointed out that for high repetition rate operation of a PCM using fused silica, a careful operation condition should be required because the macroscopic damage of solids under repeated laser irradiation increases the probability of damage with increasing numbers of pulses, and the multiple-pulse damage strongly depends on spot size it occurred in many pulses at larger spot sizes . Even till now, only Kiriyama et al. ever reported a MOPA laser system operated at 1 kHz repetition rate, with 362 W average power, and 30 ns pulse width, which means a peak power of 12 MW . They used liquid material FC-75 as a SBS-PCM medium . To the best of our knowledge, there have been no reports on solid state SBS-PCM used for kilohertz regime.
Tapered fiber SBS-PCM was first demonstrated by A. Heuer , with 92% SBS reflectivity at 10 Hz repetition rate and 2 mJ input pulse energy. Afterward, C. Liu et al. demonstrated some experimental investigations on this kind of PCM [17–19] and improve the repetition rate to 100 Hz and pulse energy to 4 mJ . Recently, using a large aperture tapered fiber, we further improve the repetition rate to 400 Hz and input pulse energy to near 40 mJ with a SBS reflectivity of 50% .
In this paper, we report our latest results on the large aperture tapered fiber SBS-PCM. With 1 kHz repetition rate and 38 mJ input pulse energy, we get a maximum 70% SBS reflectivity. This is, to the best of our knowledge, the highest SBS reflectivity ever obtained with solid state SBS-PCM under such high repetition rate (1 kHz) and input pulse energy (38 mJ). We investigate the influence of different fiber end surface qualities and input pulse widths on SBS reflectivity experimentally. The results show that finer fiber end surface quality and longer input pulse widths are preferred for obtaining higher SBS reflectivity. Double passing amplification experiments are also performed. A pulse energy of 52 mJ is achieved after amplification at 1 kHz repetition rate, with a pulse width of 1.5 ns and a M2 factor of 2.3. The corresponding peak power reaches 34.6 MW. Obvious beam quality improvement is observed.
2. Experimental setup
The experimental setup of a Nd:YAG MOPA laser system with a fiber SBS-PCM is shown in Fig. 1 . The 1 kHz repetition rate Nd:YAG MOPA laser system consists of a low power master oscillator, two single pass pre-amplification stages and two single pass power amplification stages. The low power oscillator is an EO Q-switched single longitudinal mode (SLM) Nd:YAG laser side pumped by pulsed laser diodes. Its average output power is 1.2 W at 1 kHz repetition rate. After passing though the beam expander, the laser beam from the master oscillator incidents into the next two-stage pre-amplifiers. Between master oscillator stage and pre-amplifier stage, an optical isolator is used to prevent backward traveling beam or other detrimental feedback from subsequent stages. The isolator is composed of two pieces of symmetrically-placed thin film polarizer (TFP), a Faraday rotator and a 22.5° oriented λ/2 plate. The pre-amplifiers are two diodes-side-pumped Nd:YAG laser heads with peak pump power of 700 W and pump pulse width of 200 μs. An ultra-fast photodiode (Alphalas UPD-40-UVIR-P) with bandwidth of 7.5 GHz is used to receive the very weak laser beam reflected by TFP-1. The pulse shape is shown by an oscilloscope (Tektronix DPO7104) with bandwidth of 1 GHz. Then, we can monitor the pulse shape in real time to know whether the laser is operated in single longitudinal mode or not by observing if the pulse shape is smooth. Modulation of the pulse shape means there is beat effect between multi longitudinal modes.
The laser beam from pre-amplifiers is expanded, isolated again and enters the two-stage power amplifiers for further amplification. We can adjust the incident power by rotating the λ/2 plate (HWP) before TFP-2. Similarly, the power amplifiers contain two diodes-side-pumped Nd:YAG laser heads with pump pulse width of 200 μs, but peak pump power of 2400 W. A 90° quartz rotator and a set of 4f imaging system are used between the two laser heads for depolarization compensation of the strongly pumped Nd:YAG rods. In addition, a vacuum tube is adopted to avoid the air breakdown. Output laser beam from the power amplifiers is reflected by TFP-4 and TFP-5, and then coupled into the tapered fiber by a positive lens with near 90% efficiency. The λ/4 plate (QWP) before the lens is used to change the polarization state when laser beam pass through it two times. In this way, the returned SBS laser could be coupled out by TFP-5 and measured by power meter I expediently. The transmitted pump laser could be measured by power meter II. If the QWP is removed, returned SBS laser beam backtracks due to the characteristics of phase conjugation and the thermal induced aberrations are dynamically and effectively compensated by double-pass amplification. The final amplified laser beam is coupled out by TFP-2 and measured by power meter III. The depolarized component is coupled out by TFP-3.
The geometry of the tapered fiber is also shown in Fig. 1. The fiber has a total length of 3 m with a 0.8 m long tapered region. The core diameters of two end surfaces are 1 mm and 400 μm, respectively. One can consider that the tapered fiber consists of three parts, i.e., a thin part, a thicker part and a tapered region. Because the SBS threshold in fibers is proportional to the fiber cross-section area, this thin part of the fiber acts as the SBS generator. The thicker part could bear higher power, which acts as the SBS amplifier. The main portion of incident power is reflected in the amplifier part of the fiber. In this way, one can achieve low SBS threshold and large dynamic range. A relative long tapered region is also necessary, which effectively decreases the power coupling loss of incident beam between the thicker part and the thin part.
3. Experimental results and discussions
3.1 Influence of fiber end surface quality
In order to investigate the influence of fiber end surface quality, we cleave the fiber using a ruby cutter (TTK-176, from TAIWAN) and an automated fiber cleaver (LDC-200, from VYTRAN company), respectively. Figure 2 shows their reflected SBS pulse energy and SBS reflectivity with an input pulse width of 24 ns. For the ruby cutter cleaved fiber, the maximum SBS reflectivity is 40%. However, for the LDC-200 cleaved fiber, a maximum SBS reflectivity of 70% is obtained with input pulse energy of 38 mJ.
Both of the fiber ends were damaged by the laser pulses with the incident energy of ~40 mJ. We checked the damage site of the fiber end surface with a microscope. Figure 3(a) shows the damaged fiber end surface cleaved by ruby cutter. Figure 3(b) shows the LDC-200 cleaved fiber end surface and Fig. 3(c) shows the damaged fiber end surface cleaved by LDC-200. Unfortunately, we did not capture the picture of fiber end surface cleaved by ruby cutter which is not damaged. In Fig. 3 we observe that the end surface quality cleaved by LDC-200 cleaver is much better than that cleaved by the ruby cutter. This is the reason why it shows higher damage threshold and higher SBS reflectivity. However, it is also destroyed with 40 mJ input pulse energy, which corresponds to a peak power of 1.67 MW for input pulse and nearly 6 MW for reflected SBS pulse.
In fact, there are two key factors that limit the maximum SBS reflectivity we can get. One is the optical breakdown of air in front of fiber input end surface. The other is the optical quality of fiber input end surface. Generally, optical breakdown of air starts at the focus of the laser beam or the input surface of fiber if there are many flaw and micro-granulation at the end surface of the fiber. We observed the damage of fiber end surface after the air breakdown with dazzling spark when the input pulse energy exceeds the damage threshold. At present, for high-energy-transport fibers, several methods have been developed to avoid the optical breakdown of air, such as using beam homogenizers , setting the coupling unit inside a vacuum cell , using hollow core fibers  and bundled hollow core fibers . On the other hand, surface damage threshold of silica is smaller than its bulk counterpart. Polishing methods can affect the surface damage threshold . Based on the abovementioned two aspects and our actual situation, we will focus on adding vacuum cell and polishing the fiber end surface using better methods for further advancing the SBS reflectivity and damage threshold of fiber in next step experiments.
3.2 Influence of input pulse widths
Two kinds of input laser pulses (pulse widths of 24 ns and 15 ns in FWHM respectively) are used to investigate the influence of input pulse width on SBS reflectivity with the fiber cleaved by LDC-200. Figure 4 shows the measured reflected SBS power and corresponding SBS reflectivity. Maximum SBS reflectivity of 70% and 50% are obtained for 24 ns and 15 ns input pulse width respectively. In addition, we simulate the SBS reflectivity curve for these two pulse widths using the theoretical model developed in our group . They have good agreement with the experimental results. It seems that longer input pulse width leads to a higher SBS reflectivity, which is consistent with our previous experimental results on ordinary silica fiber. For this phenomenon, it can be understood as follows. Steady state SBS process is formed if the input pulse width is much longer than the SBS medium’s phonon lifetime. Also, the existing theory has proved that under steady state condition, theoretical SBS reflectivity can approach 100% as long as the fiber is long enough and input pulse energy is high enough. Therefore, we can expect a higher SBS reflectivity with longer input pulse width, better fiber end surface quality and appropriate methods for avoiding the optical breakdown of air.
3.3 Doubling passing amplification
To verify the efficacy of the large aperture tapered fiber PCM on improving the beam quality, we also carry on double passing amplification experiments with input pulse width of 15 ns and good fiber end surface quality. In experiments, we first keep the QWP inserted and measure the reflected SBS power and transmitted input power. Then we move out the QWP and the reflected SBS pulse backtracks due to the phase conjugation characteristic. We can measure the double passing amplification output power at TFP-2 with power meter III. The results are shown in Fig. 5 . With input pulse energy of 30 mJ, the double passing amplification pulse energy is boosted to 52 mJ with a compressed SBS pulse width of 1.5 ns, which means a corresponding peak power of 34.6 MW. The inserted two figures show the typical spots shape captured with photographic paper. Considering the safety of other optical elements under such high peak power, we did not make the fiber SBS-PCM at full load operation. The beam quality factor M2 is also measured at 1 kHz repetition rate using a Spiricon M2-200 device. The laser beam from pre-amplifiers has a M2 factor of 2.68 in x direction and 2.50 in y direction. After double-pass amplification using the fiber SBS-PCM, the laser beam has a M2 factor of Mx2 = 2.30 and My2 = 2.24 as shown in Fig. 6 . If we replace the fiber SBS-PCM with a 0° HR mirror, the M2 factor of the laser beam is Mx2 = 4.05 and My2 = 4.12 after double passing amplification. Evidently, when fiber SBS-PCM used, the beam quality is improved instead of further deteriorated. Furthermore, a more uniform spatial distribution is obtained, shown in Fig. 5. Similar double passing amplification experiment was not performed for 24 ns input pulse width because of some other causes. However, relying on the fact that more than 26 mJ reflected SBS pulse energy is obtained for 24 ns input pulse width shown in Fig. 4, we believe that 1 kHz 100 mJ near diffraction limited laser is possible with this fiber SBS-PCM by employing much longer pump pulse width and optimizing the laser beam quality emitted from the pre-amplifiers.
In conclusion, we presented a large aperture tapered fiber SBS-PCM with 70% SBS reflectivity at 1 kHz repetition rate and 38 mJ input pulse energy for 24 ns input pulse width and 50% SBS reflectivity at 1 kHz repetition rate and 38 mJ input pulse energy for 15 ns input pulse width. By double passing the amplifier using the SBS-PCM, pulse energy of 52 mJ is achieved at 1 kHz repetition rate with input pulse width of 15 ns and reflected SBS pulse width of 1.5 ns. The corresponding peak power reaches 34.6 MW. According to the experimental results with this fiber PCM, 1 kHz 100 mJ near diffraction limited laser is possible and is expected to achieve by employing much longer input pulse width and optimizing the laser beam quality emitted from pre-amplifiers. The good performance of tapered fiber PCM in MOPA laser system opens up the opportunity for fiber SBS-PCMs to show their talents. In addition, multi-segment type large aperture fiber PCM with specific function such as coupling, SBS amplifier, SBS generator is under consideration.
Project supported by the National Natural Science Foundation of China (Grant No. 60908013).
References and links
1. D. A. Rockwell, “A review of phase-conjugate solid-state lasers,” IEEE J. Quantum Electron. 24(6), 1124–1140 (1988). [CrossRef]
2. A. Brignon and J. P. Huignard, Phase Conjugate Laser Optics (John Wiley & Sons 2003), Chap. 2.
4. A. Brignon and J. P. Huignard, Phase Conjugate Laser Optics (John Wiley & Sons 2003), Chap. 2.
5. H. J. Eichler, J. Kunde, and B. Liu, “Quartz fibre phase conjugators with high fidelity and reflectivity,” Opt. Commun. 139(4-6), 327–334 (1997). [CrossRef]
6. H. J. Eichler, A. Mocofanescu, T. Riesbeck, E. Risse, and D. Bedau, “Stimulated Brillouin scattering in multimode fibers for optical phase conjugation,” Opt. Commun. 208(4-6), 427–431 (2002). [CrossRef]
7. 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. 36(9), 2557–2562 (1997). [CrossRef]
8. H. Yoshidaa, H. Fujitaa, M. Nakatsukaa, A. Fujinokib, and K. Yoshida, “Fused quartz glass with low optical quality as a high damage-resistant stimulated Brillouin scattering phase-conjugation mirror,” Opt. Commun. 202(1-6), 257–267 (2003). [CrossRef]
9. Y. Ojima, K. Nawata, and T. Omatsu, “Over 10-watt pico-second diffraction-limited output from a Nd:YVO4 slab amplifier with a phase conjugate mirror,” Opt. Express 13(22), 8993–8998 (2005). [CrossRef] [PubMed]
10. K. Nawata, Y. Ojima, M. Okida, T. Ogawa, and T. Omatsu, “Power scaling of a pico-second Nd:YVO(4) master-oscillator power amplifier with a phase-conjugate mirror,” Opt. Express 14(22), 10657–10662 (2006). [CrossRef] [PubMed]
11. T. Omatsu, K. Nawata, M. Okida, and K. Furuki, “MW ps pulse generation at sub-MHz repetition rates from a phase conjugate Nd:YVO(4) bounce amplifier,” Opt. Express 15(15), 9123–9128 (2007). [CrossRef] [PubMed]
12. N. Shiba, Y. Morimoto, K. Furuki, Y. Tanaka, K. Nawata, M. Okida, and T. Omatsu, “Picosecond master-oscillator, power-amplifier system based on a mixed vanadate phase conjugate bounce amplifier,” Opt. Express 16(21), 16382–16389 (2008). [CrossRef] [PubMed]
13. K. Nawata, M. Okida, K. Furuki, K. Miyamoto, and T. Omatsu, “Sub-100 W picosecond output from a phase-conjugate Nd:YVO4 bounce amplifier,” Opt. Express 17(23), 20816–20823 (2009). [CrossRef] [PubMed]
14. L. Tong, Q. Gao, X. Chen, J. Chen, C. Tang, and F. Liu, “Experimental study on high repetition rate quartz glass rod stimulated brillouin scattering phase conjugation,” Chin. J. Lasers 33, 144–146 (2006).
15. H. Kiriyama, K. Yamakawa, T. Nagai, N. Kageyama, H. Miyajima, H. Kan, H. Yoshida, and M. Nakatsuka, “360-W average power operation with a single-stage diode-pumped Nd:YAG amplifier at a 1-kHz repetition rate,” Opt. Lett. 28(18), 1671–1673 (2003). [CrossRef] [PubMed]
16. A. Heuer and R. Menzel, “Phase-conjugating stimulated Brillouin scattering mirror for low powers and reflectivities above 90% in an internally tapered optical fiber,” Opt. Lett. 23(11), 834–836 (1998). [CrossRef] [PubMed]
17. C. Liu, J. Chen, T. Zhou, and J. Ge, “Analysis of transient stimulated Brillouin scattering in a combined fiber with different core diameter,” Laser Phys. 15, 1576–1580 (2005).
18. C. Liu, J. Chen, T. Zhou, and J. Ge, “Tapered-fiber phase conjugator with high stability and high reflectivity used for master oscillator power amplifier systems,” Opt. Eng. 46(1), 014201 (2007). [CrossRef]
19. C. Liu, Z. Zhao, J. Chen, L. Tong, L. Cui, Q. Gao, and C. Tang, “Large aperture tapered fiber phase conjugate mirror in MOPA laser systems with high repetition rate and high pulse energy,” Opt. Commun. 284(4), 1029–1033 (2011). [CrossRef]
20. T. S. Uhlig, P. Karlitschek, G. Marowsky, and Y. Sano, “New simplified coupling scheme for the delivery of 20 MW Nd:YAG laser pulses by large core optical fibers,” Appl. Phys. B 72, 183–186 (2001).
21. B. Richou, I. Schertz, I. Gobin, and J. Richou, “Delivery of 10-MW Nd:YAG laser pulses by large-core optical fibers: dependence of the laser-intensity profile on beam propagation,” Appl. Opt. 36(7), 1610–1614 (1997). [CrossRef] [PubMed]