YB再生放大器

High Power thin disk Yb:YAG regenerative amplifier Daniel Miiller, Steffen Erhard, and Adolf Giesen Institut fur Strahlwerkzeuge, Universitdt Stuttgart Pfaffenwaldring 43, 70569 Stuttgart, Germany phone: +49 (0)711 685 6858, fax: +49 (0)711 685 6842 email: Daniel.Mueller@ifsw.uni-stuttgart.de www. ifsw. uni-stuttgart. de Abstract: The amplification of ultra short pulses to an energy of 1 mJ at a repetition rate of 10 kHz and a pulse length of 5 ps has been demonstrated with a diode-pumped thin disk Yb:YAG regenerative amplifier without pulse stretching and compression. At a lower repetition rate of 1 kHz, pulse energies of 4.5 mJ have been obtained. A similar system based on Nd:YVO4 with slightly longer pulses has also been investigated. © 2001 Optical Society of America OCIS codes: (140.3070) Infrared and far-infrared lasers; (140.3480) Lasers, diode-pumped; (140.3280) Laser amplifiers; (140.3580) Lasers, solid-state; (999.9999) Thin disk lasers Introduction Reliable sources of laser pulses in the picosecond domain with repetition rates of several kilo- hertz, millijoule energy levels and good beam quality are of great interest for micromachining, dental surgery and other applications. The best design to meet all these specifications consists of a mode-locked oscillator generating the ultra short pulses followed by a regenerative amplifier, which provides the necessary amplification of several orders of magnitude [1, 2]. As thermal effects at high average power deteriorate the beam quality in rod based solid state lasers, a compromise between repetition rate and pulse energy has to be found in these systems. The thin disk laser approach overcomes this limitation with an efficient cooling principle and a good scalability to high powers [3]. Mode-locked operation [4] as well as regenerative amplification [5] have already been realized. This paper presents an improved thin disk regenerative amplifier based on Yb:YAG and compares the performance to a similar Nd:YVO4 system. Experimental setup Fig. 1 shows the setup of the regenerative amplifier. The essential parts of the system are the seed laser, a pulse picker system, the separation of in- and output beam and the amplifier resonator. A diode pumped Yb:YAG oscillator (HighQLaser, Austria) which is passively mode-locked with a semiconductor saturable absorber serves as seed laser. The pulse length has been measured to be 1.6 ps and the repetition rate is 60 MHz. Protection against back reflected pulses is provided by an optical isolator. To minimize the background of the amplified pulses, a pulse picking system has been employed. It lowers the repetition rate of the oscillator to a suitable rate for amplification of 1 to 10 kHz. The Pockels cell (Quantum Technology, USA) is synchronized with the seed laser using a fast photo diode. The separation of input and output pulses of the amplifier resonator is performed by a thin film polarizer and a Faraday rotator in combination with a half-wave plate. While the input pulses with OSA TOPS Vol. 50, Advanced Solid-State Lasers Christopher Marshall, ed. 319 ©2001 Optical Society of America 黄文发 矩形 黄文发 线条 黄文发 线条 黄文发 线条 黄文发 线条 Advanced Solid-State Lasers Fig. 1. Layout of the regenerative amplifier. horizontal polarization pass the separation device with unchanged polarization, the output pulses are reflected at the thin film polarizer. The resonator of the regenerative amplifier has been designed for fundamental mode operation. Special care has been taken to generate a large mode diameter of 2.2 mm on the Pockels cell and the thin film polarizer in order to minimize the risk of laser induced damage. A thin disk laser head [6] with a 200 //m thick 9%-doped Yb:YAG crystal is used as active medium. In this laser head, the crystal is mounted and cooled at the HR-coated backside and pumped through the AR- coated front. A 65 W fiber-coupled diode laser (Optopower, USA) operating at a wavelength of 940 nm is used as pump source. The pump spot diameter is 1.2 mm, matching the fundamental mode of the cavity. The BBO Pockels cell in the resonator (Quantum Technology, USA) has an aperture of 6 mm and is synchronized with the pulse picker system. When no voltage is applied to the Pockels cell inside the resonator the polarization of an incoming seed pulse changes from horizontal to vertical during its double pass through the quarter-wave- plate and it is reflected at the thin film polarizer. Then the Pockels cell is switched to the quarter- wave-voltage thus compensating the effect of the quarter-wave-plate. The pulse is trapped in the resonator, until the Pockels cell is switched off again. Then the polarization of the pulse is turned back to horizontal polarization and it leaves the resonator through the thin film polarizer. Yb:YAG regenerative amplifier results The experiment has been conducted in four steps. First, the laser head and the crystal have been aligned and tested in a short multi-mode resonator with 3 % output coupling yielding an output power of 27.5 W. The optical efficiency of 43 % is lower than in similar setups. This is attributed to the broad spectrum of the pump diode used in this experiment. In a second step, the resonator of the regenerative amplifier has been aligned without Pockels cell with the help of the seed laser. In this mode of operation, the rotatable quarter-wave plate in combination with the thin film polarizer acts as variable output coupler. The maximum output is 320 黄文发 线条 黄文发 线条 黄文发 线条 Advanced Solid-State Lasers 12 11 10 g 9 CD 8 5 6 Q_ B 5 o CD 4 56 round trips 73 round trips pump power 65 W 2 4 6 8 10 12 14 16 repetition rate [kHz] Fig. 2. Average output power of the Yb:YAG regenerative amplifier as function of the repetition rate. 19.2 W (29 % optical efficiency). Third, the Pockels cell has been inserted into the resonator and has been aligned. The maximum output power dropped to 13.3 W (21 % optical efficiency) because of the high single pass loss of 2 % in the Pockels cell. The beam propagation ratio has been measured with a Coherent Modemaster to M2 < 1.2. In the last step, the laser has been transformed into a regenerative amplifier by turning the quarter wave plate to the appropriate position. The energy of the input seed pulses has been measured to be 2 nJ. The round trip time of the resonator is 13 ns and the Pockels cell has been operated to permit 56 or 73 round trips of the seed pulse. A fast photo diode behind one of the dielectric HR mirrors allows to monitor the build-up of the pulse in the resonator. At a repetition rate of 10 kHz the average output power has been measured to be 10.2 W (16 % optical efficiency). Since a pulse picker system is used for the seed pulses, it can be assumed that the output contains no significant background and therefore this value corresponds to a pulse energy of 1 mJ. We have also demonstrated 4.5 mJ at 1 kHz and 3 mJ at 2 kHz. Fig. 2 shows the average output power as function of the repetition rate. Above a repetition rate of 1.5 kHz, the average output power remains nearly constant, whereas it decreases rapidly at lower repetition rates. This power loss is mainly caused by spontaneous emission which becomes important when the time between two successive pulses exceeds the lifetime of the upper laser level of Yb:YAG (0.95 ms [9]). The pulse length has been measured to be 4.5 ps using an autocorrelator. Fig. 3 shows the autocorrelation function of the seed pulses as well as of the amplified pulses. Gain narrowing in the amplifier material is mainly responsible for the increase in pulse length during amplification. First results with the Nd:YVO4 regenerative amplifier Additional experiments have been performed with a similar system based on Nd:YVO4. A passive mode-locked oscillator with a saturable absorber (GWU, Germany) has been used as seed laser. The thin disk in the regenerative amplifier is a Nd:YVO4 crystal with a thickness of 230 /um and a doping level of 0.5 at.%. The disk has been pumped by 61 W of pump power at a wavelength of 808 nm. The pump spot diameter is 1.8 mm, yielding a beam propagation ratio of M2 < 1.3 with 321 黄文发 椭圆形 黄文发 椭圆形 黄文发 椭圆形 黄文发 线条 黄文发 线条 黄文发 线条 Advanced Solid-State Lasers 1600 1400 1200 .9 1000 jd CD t 8 CD t 800 60 ° ——— seed pulse -----amplified pulse rep. rate 10 kHz pump power 65 W delay time [ps] Fig. 3. Autocorrelation function of the seed pulses and the amplified pulses in the Yb:YAG system. an adapted resonator. Fig. 4 shows the average output power as a function of the repetition rate. The pulse energy is 0.17 mJ and does not depend on the repetition rate because the gain recovers completely between two amplification cycles due to the short fluorescence lifetime of Nd:YVO4 (95 ps [7]). The pulse length of the seed pulses is increased during amplification from 9 ps to 14 ps, which is again due to gain narrowing. About 40 round trips are necessary for an efficient energy extraction. Fig. 5 shows the pulse buildup in the resonator. The optimum time to switch the Pockels cell is reached, when the round trip gain equals the round trip loss. At this point, the best pulse to pulse stability is obtained and the energy extraction is at its maximum. In this system the optical efficiency is rather poor due to several reasons: first, the energy storage time of Nd:YVO4 is short compared to the time interval between to amplification cycles. More effi- cient operation in the range of 1-20 kHz is possible only with pulsed pump diodes. For cw pumping, the repetition rate must be larger then the inverse of the storage time, i.e. above 10 kHz. Second, the energy difference between pump and laser photons is 24 %, which results in a comparatively high heat generation and therefore thermal phase distortions. The third reason is technological: the anti-reflection coating did not meet the design specification, introducing an additional round trip loss of about 4%. •—35 round trips pump power 61 W 10 15 20 repetition rate [kHz] Fig. 4. Average output power of the Nd:YVC>4 regenerative amplifier as a function of the repetition rate. 322 黄文发 椭圆形 Advanced Solid-State Lasers O O _C Q_ 400 450 500 550 600 650 700 750 800 t[ns] Fig. 5. Pulse build up in the resonator of Nd:YVC>4 system measured with a fast photo diode behind a dielectric HR mirror. Comparison of Yb:YAG and Nd:YVO4 Which material is more suitable for ultra short pulse generation with the thin disk regenerative amplifier system? Since it is very important for the intended application that pulses are as short as possible, the Yb:YAG system, which produces approximately three times shorter pulses then the Nd:YVO4 system, has a clear advantage. The advantage of Nd YVC^is the higher small signal gain compared to Yb YAG. This allows pulse amplification in less round trips and at least in principle a better extraction of the energy stored in the crystal [8]. The shorter build up time of the Nd:YVO4 system reduces the requirements for the Pockels cells driver to keep the applied voltage constant over the amplification time. But for cw pumping and repetition rates below 10 kHz, the fluorescence loss of Nd:YVO4 leads to a significant drop in overall efficiency. Additionally, when scaling to higher output powers is considered, the thermal problems and the brittle nature of Nd:YVC>4 limits its usage in the thin disk laser design. Summary We have demonstrated a millijoule level Yb YAG regenerative amplifier with multi-kilohertz repeti- tion rate and a pulse length of a few picoseconds. No pulse stretching and compression was required. We expect to obtain an improved efficiency with a low loss Pockels cell and a better pump diode. Scaling to higher average power is feasible by enlarging the pump spot size on the thin disk at a constant pump power density. Future experiments will be carried out with the YbYAG system, that produces shorter pulses than the Nd:YVO4 system and is suitable for high power operation. Other ytterbium doped hosts providing a larger amplification bandwidth like Yb:KGW will also be analyzed. Acknowledgements Parts of this research have been funded by the German Ministry of Education and Research under contract number 13N7709. 323 黄文发 线条 黄文发 矩形 Advanced Solid-State Lasers References 1. L. Turi, and T. Juhasz, "High-power longitudinally end-diode-pumped Nd:YLF regenerative amplifier," Optics Letters 20 (2) , pp. 154-6 (1995). 2. M. GifTord, and K. J. Weingarten, "Diode-pumped Nd:YLF regenerative amplifier," Optics Letters 17 (24) , pp. 1788-90 (1992). 3. C. Stewen, K. Contag, M. Larionov, A. Giesen, and H. 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