We report on a diode-pumped master-oscillator/fiber-amplifier (MOFA) system consisting of a passively Q-switched, single-frequency Nd:LSB microchip laser that seeds a dual-stage Yb-doped fiber amplifier. A large-core, single-mode photonic crystal fiber was used for the final amplifier. The MOFA generated 1062nm wavelength, 1-ns long, ~10kHz repetition-rate, diffraction-limited pulses of energy >1mJ, peak power >1MW, average power >10W, and spectral linewidth ~9 GHz.
©2005 Optical Society of America
Scientific and military applications of airborne LIDAR and remote sensing/imaging rely on compact, robust, and efficient high-pulse-energy optical sources. Pulsed fiber lasers and amplifiers are promising candidates for replacing bulk solid-state lasers in such applications. Of special interest are fiber sources producing multi-kHz repetition-rate, ns pulses of narrow spectral linewidth and high signal-to-noise ratio, which afford, for example, rapid and high-resolution terrain mapping. For a given pulse format, pulse energies from fibers have been traditionally lower than from bulk lasers, which translates into shorter operating ranges. The lower performance is due to detrimental nonlinear effects resulting from tight in-fiber optical confinement and long silica/optical-field interaction path, which limit the achievable peak power .
Recently, a 450ps-pulse, 0.5mJ pulse-energy, 1.1MW peak-power fiber source has been reported, which exhibited also diffraction-limited beam quality and narrow linewidth . The main component in that source was a large-core (40μm diameter), Yb-doped photonic-crystal fiber (PCF) amplifier. In these fibers, an internal air/silica microstructure affords fine control on the refractive-index profile, thus permitting intrinsic single-mode operation in very large cores, with modest bend loss.
In this paper, we report on a master-oscillator/fiber-amplifier (MOFA) system featuring an Yb-doped, large-core PCF similar to that described in Ref. 2, as the power amplifier. The MOFA generated 9.6-kHz repetition-rate, diffraction-limited, 1062-nm wavelength, 1-ns pulses of energy >1mJ, peak power ~1.1MW, average power ~10.2W, and spectral linewidth ~9GHz. These values represent a unique set of simultaneous performance parameters in a fiber amplifier operated in the “Q-switch” regime (ns pulses at multi-kHz repetition rates) and provide further evidence for power-scalability and flexibility of pulsed fiber-based sources.
2. Experimental setup
The MOFA architecture is shown in Fig. 1. The master oscillator is a Nd:LSB, single-frequency, passively Q-switched microchip laser (1ns pulse duration, 9.6kHz repetition-rate, 1062nm wavelength, ~6μJ pulse energy). The microchip laser output was pre-amplified in a 2m-long solid-silica, double-clad Yb-doped fiber (core diameter ~30μm, cladding diameter ~250μm) backward-pumped by a fiber-coupled diode laser (976nm, 3nm linewidth). The pre-amplified output was filtered in a 600pm-bandwidth optical band-pass filter to remove most of the amplified spontaneous emission (ASE) generated in the preamplifier and spectrally concentrated near the Yb gain peak at ~1035nm. The filtered pulses were coupled into a large-core, single-mode, Yb-doped PCF similar to the one used in Ref. 2 (~2m long, 40-μm core diameter, 170-μm cladding diameter). The coupled pulse energy was ~100μJ. The PCF output facet was equipped with beam-expanding endcaps to prevent optical damages and backward-pumped by a second fiber-coupled diode laser (976nm, 3nm linewidth).
3. Results and discussion
Figure 2 shows the output pulse energy (measured with a high-repetition-rate pyroelectric joulemeter) and corresponding average power of the PCF amplifier output as a function of launched pump power. The maximum pulse energy was 1.05 mJ, corresponding to an average power of 10.2 W, all in the pulse. A linear fit provides slope efficiency ~66%. The total output power (pulses and ASE) measured with a thermopile was ~10.5 W, which means that the ASE power was 300mW. This value includes ASE spectrally distributed both within and outside the inter-stage filter pass band and corresponds to a pulse-to-ASE contrast ratio in excess of 15dB. The inset in Fig. 2 shows the pulse temporal profile, recorded at maximum pulse energy by a digitally sampled oscilloscope featuring a 30ps-resolution optical head. The vertical scale in the inset has been calibrated in units of power by equating the area under the pulse to the measured pulse energy. The resulting peak power is ~1.1 MW. At maximum energy, the amplified pulses began to exhibit a more irregular profile compared to the Gaussian-like pulses emitted by the master oscillator. This distortion is due to gain saturation and nonlinear phase modulation .
We evaluated the output beam quality at the highest energy by focusing the beam to a waist and measuring (with a knife-edge detection apparatus) the beam radii, Rx and Ry, in directions perpendicular and parallel to the table top as a function of distance, z, from the waist location. As shown in Fig. 3, the corresponding best-fit values of M2 were 1.04 ± 0.02 (perpendicular) and 1.06 ± 0.04 (parallel), which confirms that the beam was single-mode diffraction-limited.
Figure 4 shows the direct (unfiltered) spectrum of the PCF-amplifier output pulses. Besides the 1062nm signal, the spectrum exhibits two sidebands caused by four-wave mixing (one centered at ~1090nm, the other partly overlapped with ASE at ~1030nm) and the first-order Stokes peak of stimulated Raman scattering at ~1125nm. These spectral features lie more than 55dB below the signal level, which represents an excellent spectral signal-to-noise ratio. Note that stimulated Brillouin scattering, the lowest-threshold nonlinearity in fibers, is suppressed here by the short pulse duration . The inset of Fig. 3 shows a high-resolution scan of the signal spectrum. The amplified pulses were not transform-limited and their full width at half maximum was ~ 9 GHz (~ 33 pm) i.e. approximately 5 times broader than the near-transform-limited seed pulses produced by the master oscillator. Despite this broadening, which is due to self-phase modulation , the overall amplifier spectral fidelity is very good given the high peak power attained and demonstrates the benefits of large-core PCFs in minimizing nonlinear effects. To obtain transform-limited pulses and/or improve spectral purity at these or higher peak powers, shorter and larger-core fibers are desirable for suppression of residual optical nonlinearities.
In conclusion, we demonstrated a MOFA producing 1ns pulses of pulse energy >1mJ, peak power >1MW, average power >10W, spectral-linewidth ~9GHz, spectral signal-to- noise ratio >55dB, pulse-to-ASE contrast ratio >15dB, and slope efficiency ~66%. To our knowledge, these values represent the best combined performance from a pulse fiber amplifier reported to date. Compared to the PCF amplifier in Ref. 2, for example, the MOFA reported in this paper provides 100% increase in pulse energy, 45% increase in average power, 35dB increase in spectral signal-to-noise ratio, and 5dB increase in pulse contrast ratio, while retaining the same peak power, spatial beam quality, and record peak spectral brightness of ~10kW/(cm2 sr Hz). This work has also demonstrated improved pulse energy, slope efficiency and increased spectral signal-to-noise ratio of ~30dB over other recent results . Finally, Cheng et al.  demonstrated a pulse fiber amplifier generating record peak power (2.4MW) in 4ns, 9.6mJ pulses, although the output beam was multimode (M2~6.5), the repetition rate was ~100Hz (<1W average power), and the spectral linewidth > 1nm.
Although the results reported in this paper are directly relevant for many applications that currently rely on bulk lasers, the MOFA performance described in this letter does by no means represent a limit for pulsed fiber sources. Further scaling of pulse energy and peak power will be the subject of future work.
References and links
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2 . F. Di Teodoro and C. D. Brooks , “ 1.1-MW peak-power, 7-W average-power, high-spectral-brightness, diffraction-limited pulses from a photonic crystal fiber amplifier ,” Opt. Lett. , in press. [PubMed]
3 . A. E. Siegman , Lasers ( University Science Books, Sausalito, Calif. , 1986 ). See Ch. 10.2, pp. 384 – 386 .
4 . W. Torruellas , Y. Chen , B. McIntosh , J. Chen , J. Farroni , K. Tankala , S. Webster , D. Hagan , and M. J. Soileau , “ High peak power Ytterbium doped fiber amplifiers ,” Tech. Digest Solid State and Diode Laser Technology Review (SSDLTR), Los Angeles, USA, June 7–9, 2005 , Fiber-7.
5 . M.-Y. Cheng , Y.-C. Chang , A. Galvanauskas , P. Mamidipudi , R. Changkakoti , and P. Gatchell , “ High-energy and high-peak-power nanosecond pulse generation with beam quality control in 200-μm core highly multimode Yb-doped fiber amplifiers ,” Opt. Lett. 30 , 358 ( 2005 ). [CrossRef] [PubMed]