We demonstrated a hybrid ceramic master-oscillator high-power fiber amplifier with a diode-pumped Yb:YAG ceramic laser as the seeding oscillator, which was passively mode-locked at 103.29 MHz repetition rate around 1031 nm by using a semiconductor saturable absorption mirror, and a two-stage double-clad photonic crystal fiber amplifier, which power-scaled the ceramic laser oscillator up to an average power of 303 W. The amplified pulses were further compressed to 237 and 418 fs at 50 and 150 W output powers, respectively. The compressed pulses exhibited about 0.05% deviation from the Gaussian fit, implying that the high-power fiber amplification induced neither observable temporal and spectral distortion nor significant nonlinear de-chirping of the chirped pulses.
© 2012 OSA
In recent years, rare-earth ion-doped materials have been rapidly developed and widely used for the realization of high-power ultrashort laser pulses [1–4]. Ytterbium ion (Yb3+) has been considered to be a very attractive dopant for efficient laser operations around 1 µm [5–8], exhibiting merits of broad absorption and emission bandwidths, as well as absence of excited-state absorption, upconversion or concentration quenching . Various ytterbium doped materials have been progressively investigated in the demonstration of ultrashort-pulsed lasers, such as tungstates Yb:KGW , oxyorthosilicates Yb:GSO , and garnet Yb:YAG . Among those materials, Yb:YAG ceramic has been proved to be promising for high-power ceramic lasers with remarkable advantages [13, 14], such as easy fabrication of various shapes and sizes, high hardness and fracture toughness, and high resistance to thermal shock. So far, mode locking of Yb:YAG crystal lasers has been widely discussed [12,15,16]. An average output power up to 76 W with a pulse energy up to 25.9 μJ was realized using a mode-locked thin-disk Yb:YAG laser , and 100 fs pulses at a center wavelength of 1051 nm were obtained with a Kerr-lens mode-locked Yb:YAG crystal laser . However, the performance of high-power thin-disk Yb:YAG lasers was restricted by their configuration and thermal load, it is still a challenge to realize high-power ultrashort lasers at high repetition rates. The first diode-pumped mode-locked Yb:YAG ceramic laser was demonstrated in Ref. 17, generating 286 fs pulses at a wavelength of 1033.5 nm and 233 fs pulses at 1048.3 nm with the average output power of tens of milliwatts . Further efforts are desired to amplify ultrashort ceramic lasers to high powers at high repetition rates. Recent development of cladding-pumped fiber lasers has dramatically increased output power of ultrashort mode-locked lasers. The master-oscillator power amplifier (MOPA) meets the demand to realize high-efficiency ultrashort ceramic-fiber hybrid lasers with high peak powers at high repetition rates and excellent beam qualities [18–21].
Since fiber lasers possess an enormous power scaling potential owing to their excellent thermal-optical properties, rare-earth-doped fiber amplifiers also offer a promising alternative to conventional solid-state lasers. However, power scaling of pulses in single-mode fiber amplifiers suffers from nonlinear pulse distortions due to the detrimental nonlinear effects caused by the large product of intensity and interaction length inside the fiber core. During the past decade, considerable progress has been made in alleviating these disadvantageous nonlinear effects by using large-mode-area (LMA) fiber chirped-pulse-amplification (CPA) , which usually employs rare-earth-doped double-clad LMA fibers as the gain media with large surface-to volume ratios for diffraction-limited stretched pulse amplification with excellent heat-dissipation and long signal-pump interaction distances to ensure high optical-to-optical efficiencies . Moreover, double-clad LMA photonic crystal fibers (PCFs) provide a robust single-mode operation during high-power amplification, mitigating undesired nonlinear effects such as stimulated Raman and Brillouin scattering. Thus, MOPA with LMA double-clad fiber constitutes an attractive technological approach for the development of versatile, robust, and compact high-power short-pulse sources. Up to now, pulses with energy of µJ-scale at tens of MHz repetition rate have been realized by MOPA with LMA Yb-doped double-clad fiber (YDCF) amplifiers [24, 25]. Another key issue to attain ultrashort pulses is to minimize the high-order dispersion in the MOPA system. Mode-locked fiber oscillators are unavoidably accompanied with pulse distortion from large positive third-order dispersion, which could not be compensated in transmission grating-pair pulse compressors typically used in standard high-power MOPA system. A hybrid MOPA with a ceramic oscillator and high-power fiber CPA may provide a good solution to the high-order dispersion problem. A mode-locked ceramic oscillator may output sufficient high-power seed pulses for LMA amplifiers without a requisite fiber preamplifier.
In this paper, we reported on a MOPA system with a diode-pumped passively mode-locked Yb:YAG ceramic laser as the seeding oscillator. The picosecond ceramic pulses were power-scaled up to about 303 W by using a two-stage LMA Yb-doped PCF amplifier. This is to our knowledge the highest average power ever reported for an Yb:YAG ceramic solid-state laser system.
2. Tunable range of Yb:YAG ceramic laser
As for the properties of the home-made ceramic samples used in our experiments, we made a measurement of the laser tunability before being used in the ceramic-fiber MOPA system. The tunability of 5 at.% Yb3+ doped Yb:YAG with 970-nm diode laser pump at 9.5W was investigated in a three-mirror laser cavity with an intracavity SF10 dispersion prism, as exhibited in the inset of Fig. 1(a) . Figure 1(a) shows the dependence of the output power versus the laser oscillation wavelength with an OC of T = 2%. Under 7 W incident pump power, two narrow-tunable bands were observed for this 5% Yb:YAG ceramic sample, the full-width at half-maximum was 2.2 nm around 1031 nm and 2 nm around 1049 nm, respectively. No other wavelength was observed between the two bands. This tunability could be useful in the near infrared absorption spectrometry, such as the blood glucose inspection in medical science, and remote sensing of atmosphere water vapor properties in meteorological science. The energy levels of Yb3+ ions were shown in Fig. 1(b), here we constructed mode-locking at the transition from the lowest level of 2F5/2 manifold to another sublevel of 2F7/2 with the emission band around 1031 nm.
3. Hybrid ceramic master-oscillator high-power fiber amplifier
The experimental setup of the MOPA system is schematically shown in Fig. 2 . The MOPA consisted of a Yb:YAG ceramic laser oscillator, single-mode fiber pulse stretcher, two-stage ytterbium-doped double-clad PCF amplifier, and pulse compressor with transmission grating pairs and prism pairs.
A diode-pumped passively mode-locked Yb:YAG ceramic laser with a z-fold cavity configuration was built as the seed of the MOPA system. We employed a 976-nm high-brightness laser diode as the pumping source with a fiber core diameter and numerical aperture of 400 μm and 0.22, respectively. The pumping beam was imaged with a series of lenses resulting in a relay to the ceramic sample with a pump spot of 400 μm in diameter. The ceramic laser resonator consisted of a semiconductor saturable absorber mirror (SESAM) and four mirrors: an input ñat mirror M1 with high transmission (HT) at 976 nm and high reflection (HR) in a broad band from 1030 nm to 1130 nm, a 2% OC flat mirror, two folded concave mirrors M2 and M3 both anti-reflection (AR)-coated at 976 nm and having high reflectivity from 1030 nm to 1130 nm with 500 and 300 mm radius of curvature (ROC), respectively. The distance between M1 and M2 was about 252 mm, while M2 and OC were separated by 500 mm, and the length between M3 and SESAM was 160 mm. The total cavity length was about 1450 mm. The passive mode-locking was achieved by a SESAM (BATOP, Germany) with 2% saturable absorption at 1040 nm, 70 µJ/cm2 saturation fluence and 500 fs relaxation time constant. The laser beam was focused onto the SESAM by M3 with the fold angle of 5°. The Yb:YAG ceramic used in this experiment was 5 at.% Yb3+ doped and cut as 4-mm-thick, 8*8 mm2 in aperture with antireñection-coatings from 940 nm to 1130 nm on both surfaces. In order to efficiently remove the generated heat under diode-pumping, the Yb:YAG ceramic was wrapped with indium foil and mounted in a copper heat sink block which was cooled by flowing water at 14°C.
The laser cavity was designed to have dual outputs, and one was used to monitor the pulse stability of the oscillator with a photo-detector, the other output with about 120 mW average power was used as the seed for the two-stage fiber amplifier. Before being coupled into the first-stage amplifier, the seed pulses were stretched in a 55-meter-long single-mode fiber (SMF).
The first and second stage power ampliðers respectively used 1.5- and 2-meter-long LMA ytterbium-doped double-clad PCFs (DC-200/40-PZ-Yb-01, CRYSTAL FIBER) with a diameter of 40 μm (NA = 0.03) for the active core and a diameter of 200 µm (NA = 0.55) for the inner clad. This Yb-doped PCF had a pump-absorption of 10 dB/m at 976 nm. The fiber were sealed at both ends to protect the capillaries from environmental influences and polished at an angle of 8° in order to suppress parasitic lasing. Optical isolators (>30 dB) were inserted between amplifiers to avoid possible detrimental effects caused by the reflection of the cascade amplifiers. Two high-power 400-µm-fiber-core pigtailed diode lasers at 976 nm were used as the pumping sources for the two-stage amplifier. A pump light coupling efðciency of 75% was reached in our experiment. Water-chillers were introduced in the fiber amplifier system to remove the thermal-optical heat and protect the polymer coating of the PCF from thermal damaging.
4. Ultrashort ceramic-fiber hybrid MOPA
The beam waists on the ceramic and SESAM, estimated by the so-called ABCD analysis, were about 146 and 74 μm respectively in our experiment. The proper beam waist in the cavity guaranteed a robust mode-locking of the laser since the gain medium had a sufficient mode area and the SESAM was also operated in the strong saturation regime within its damage threshold.
The output pulse train exhibited in Fig. 3(a) was detected by a high resolution pin-diode detector down to 300 ps and recorded with a digital oscilloscope with 1 GHz bandwidth (Agilent infiniium 54833A DSO). The pulse power spectrum recorded by a spectral analyzer (Agilent N9010A) was demonstrated in Fig. 3(b), which clearly showed that the mode-locked laser was stably operated at 103.29 MHz without any observable sidebands. The shot-to-shot pulse energy fluctuation of the output pulse train was monitored by the oscilloscope. In addition, long-term mode-locking stability of the laser was measured by checking the output power of the pulse train. Through precise alignment, a slight pulse-to-pulse intensity fluctuation was obtained, with a standard deviation of 0.54% as shown in Fig. 3(c). There was no dispersion compensation in the cavity since we had to stretch the pulse for the following CPA, resulting in an output pulse duration of 2.2 ps as shown in Fig. 3(d).
The seed pulses were stretched to 20 ps after the 55-meter-long single-mode fiber and the spectrum was also broadened from 4.1 nm (dotted line) to 9.0 nm (solid line) as shown in Fig. 4(a) which could be understood from the self-phase modulation during pulse propagation in the fiber pulse stretcher.
In the first stage of the cascade amplifier, the injected 120 mW signal was amplified to 5 W as the seed for the second-stage main power amplifier. The output of the main amplifier chain was illustrated in Fig. 4. Up to 303 W average power was obtained with a slope efficiency of 77% with respect to the launched pump power, as shown in Fig. 4(b). The spectrum was narrowed to about 7.1 nm due to the gain narrowing effect (dashed line) in the first stage fiber amplifier, as shown in Fig. 4(a), and it was unchanged during the main fiber amplifier. Additionally, the central wavelength was slightly shifted owing to gain and reabsorption in the fiber ampliðers. No spectral splitting was observed, which implied that no pulse distortion occurred within such a range of pulse energy.
About 98% of the amplified pulses were output for further applications while 2% were dechirped with the compressor consisted of an SF14 Brewster prism pair and a pair of transmission gratings of 1250 lines/mm which were designed to have a maximum diffraction efficiency at 1064 nm with 41.7° Littrow degree. The autocorrelations of the compressed pulses were shown in Fig. 4(c) for low and higher output powers, measured with a commercial Pulse Check from APE. With only grating pair, 2% of the 50-W output pulses were dechirped to 237 fs, which results in a time-bandwidth product of 0.474, slightly above the Fourier limit for a Gaussian pulse (0.441). When the output power rose up, a prism pair should be introduced in the compressor as nonlinear phase modulation in the fiber amplifier caused some additional spectral distortion at high powers, which could not be easily uncompensated by grating pair. As shown in Fig. 4(c), the 2% of the 150-W output pulses were compressed to 418 fs. The autocorrelation width increase could be understood from nonlinear phase shifts induced during the amplification.
In conclusion, we demonstrated an ytterbium-doped fiber MOPA with 303 W of average output power seeded by a diode-pumped passively mode-locked Yb:YAG ceramic laser at 1031 nm with a repetition frequency of 103.29 MHz. To our knowledge this is the highest average power ever reported for a hybrid Yb:YAG ceramic laser system. Since the obtained output power was only limited by available pump power, further power-scaling is possible with the current configuration. Such a high-power MOPA system with high efficiency, compactness, and robustness, as well as versatility and flexibility represents an attractive source for the XUV comb generation, atomic and molecular spectral measurements and femtochemistry.
This work was supported by National Natural Science Fund (11004061&51102257), National Key Project for Basic Research (2011CB808105), Special Research Fund for the Doctoral Program of Higher Education (20090076120004), Shanghai Educational Development Foundation (09CG18), projects from Shanghai Science and Technology Commission (10ZR1409000&11JC1412400).
References and links
1. S. Ye, B. Zhu, J. Chen, J. Luo, and J. Qiu, “Infrared quantum cutting in Tb3+,Yb3+ codoped transparent glass ceramics containing CaF2 nanocrystals,” Appl. Phys. Lett. 92(14), 141112 (2008). [CrossRef]
2. X. G. Meng, J. R. Qiu, M. Y. Peng, D. P. Chen, Q. Z. Zhao, X. W. Jiang, and C. S. Zhu, “Near infrared broadband emission of bismuth-doped aluminophosphate glass,” Opt. Express 13(5), 1628–1634 (2005). [CrossRef] [PubMed]
3. A. Shirakawa, K. Takaichi, H. Yagi, J.-F. Bisson, J. Lu, M. Musha, K. Ueda, T. Yanagitani, T. S. Petrov, and A. A. Kaminskii, “Diode-pumped mode-locked Yb3+:Y2O3 ceramic laser,” Opt. Express 11(22), 2911–2916 (2003). [CrossRef] [PubMed]
4. A. Mori, Y. Ohishi, and S. Sudo, “Erbium-doped tellurite glass fibre laser and amplifier,” Electron. Lett. 33(10), 863–864 (1997). [CrossRef]
5. J. Kong, D. Y. Tang, C. C. Chan, J. Lu, K. Ueda, H. Yagi, and T. Yanagitani, “High-efficiency 1040 and 1078 nm laser emission of a Yb:Y2O3 ceramic laser with 976 nm diode pumping,” Opt. Lett. 32(3), 247–249 (2007). [CrossRef] [PubMed]
6. Q. Hao, W. Li, H. Zeng, Q. Yang, C. Dou, H. Zhou, and W. Lu, “Low-threshold and broadly tunable lasers of Yb3+-doped yttrium lanthanum oxide ceramic,” Appl. Phys. Lett. 92(21), 211106 (2008). [CrossRef]
8. F. Druon, S. Chénais, P. Raybaut, F. Balembois, P. Georges, R. Gaumé, P. H. Haumesser, B. Viana, D. Vivien, S. Dhellemmes, V. Ortiz, and C. Larat, “Apatite-structure crystal, Yb(3+):SrY4(SiO4)3O, for the development of diode-pumped femtosecond lasers,” Opt. Lett. 27(21), 1914–1916 (2002). [CrossRef] [PubMed]
10. G. Paunescu, J. Hein, and R. Sauerbrey, “100-fs diode-pumped Yb:KGW mode-locked laser,” Appl. Phys. B 79(5), 555–558 (2004). [CrossRef]
12. J. Neuhaus, D. Bauer, J. Zhang, A. Killi, J. Kleinbauer, M. Kumkar, S. Weiler, M. Guina, D. H. Sutter, and T. Dekorsy, “Subpicosecond thin-disk laser oscillator with pulse energies of up to 25.9 microjoules by use of an active multipass geometry,” Opt. Express 16(25), 20530–20539 (2008). [CrossRef] [PubMed]
13. S. Nakamura, H. Yoshioka, Y. Matsubara, T. Ogawa, and S. Wada, “Efficient tunable Yb:YAG ceramic laser,” Opt. Commun. 281(17), 4411–4414 (2008). [CrossRef]
15. S. Uemura and K. Torizuka, “Kerr-Lens mode-locked diode-pumped Yb:YAG laser with the transverse mode passively stabilized,” Appl. Phys. Express 1(1), 012007 (2008). [CrossRef]
16. E. Innerhofer, T. Südmeyer, F. Brunner, R. Häring, A. Aschwanden, R. Paschotta, C. Hönninger, M. Kumkar, and U. Keller, “60-W average power in 810-fs pulses from a thin-disk Yb:YAG laser,” Opt. Lett. 28(5), 367–369 (2003). [CrossRef] [PubMed]
19. A. Malinowski, K. T. Vu, K. K. Chen, J. Nilsson, Y. Jeong, S. Alam, D. Lin, and D. J. Richardson, “High power pulsed fiber MOPA system incorporating electro-optic modulator based adaptive pulse shaping,” Opt. Express 17(23), 20927–20937 (2009). [CrossRef] [PubMed]
22. D. Strickland and G. Mourou, “Compression of amplified chirped optical pulses,” Opt. Commun. 55(6), 447–449 (1985). [CrossRef]
24. T. Eidam, S. Hanf, E. Seise, T. V. Andersen, T. Gabler, C. Wirth, T. Schreiber, J. Limpert, and A. Tünnermann, “Femtosecond fiber CPA system emitting 830 W average output power,” Opt. Lett. 35(2), 94–96 (2010). [CrossRef] [PubMed]
25. Y. Zaouter, D. N. Papadopoulos, M. Hanna, J. Boullet, L. Huang, C. Aguergaray, F. Druon, E. Mottay, P. Georges, and E. Cormier, “Stretcher-free high energy nonlinear amplification of femtosecond pulses in rod-type fibers,” Opt. Lett. 33(2), 107–109 (2008). [CrossRef] [PubMed]