We present a high power all-fiberized master oscillator power amplifier (MOPA) structured superfluorescent source based on dual-cladding ytterbium-doped fiber. The seed source is a 0.814 W homemade amplified spontaneous emission (ASE) source. Two-stage high power fiber amplifier is utilized to boost the seed power to 1.01 kW with a beam quality of Mx2 = 1.688 and My2 = 1.728. The central wavelength of the output light is 1074.4 nm, and the full width at half maximum (FWHM) linewidth is about 8.1 nm. No self pulsing or relaxation oscillation effect is observed and the power fluctuation is less than 2% in 100 seconds continuous operating. In additional, spectral evolution effects of central-wavelength-drifting and linewidth-narrowing of broadband amplification in high power superfluorescent source system are investigated. To the best of our knowledge, this is the first demonstration of an all-fiberized superfluorescent source with output power exceeding kilowatt.
© 2015 Optical Society of America
Superfluorescent sources (also referred to as ASE sources) are of great importance in many applications, such as optical coherence tomography (OCT), low coherence interferometry, and fiber optical sensor for their high temporal and thermal stability, short coherence length and good beam quality [1–3]. Meanwhile, fiber lasers have been widely applied in scientific research, industry production due to their high stability, high beam quality, and maintenance-free operation characteristics [4–6]. Especially, high power superfluorescent sources from rare-earth doped fiber is an ideal pump source for Raman laser, and it maybe an alternative solution to obtain a novel high brightness light source for materials processing and metrology .
There are many reports on high-power superfluorescent source in the last decade. In the 1 μm wavelength ranges, P. Wang et al. obtained 110 W  (with M2 = 1.6) and 107 W  (with M2 = 2.8) double-ended superfluorescent source by utilizing multimode-offset-core-fiber and helical-core-fiber to suppress lasing, respectively. In 2007, they reported a 106 W superfluorescent source with M2<1.1 in two-stage MOPA configuration . In 2011, O. Schmidt et al.  demonstrated a 697 W narrow-band three-stage MOPA structured ASE source with M2≤1.34. In 2012 and 2008, W. T. Chen et al.  and D. Shen et al.  demonstrated 16.1 W and 11 W superfluorescent fiber source in 1.5 μm and 2 μm wavelength ranges, respectively. However, most of these high-power fiber-based superfluorescent sources typically employ bulk optics configuration. The use of all-fiber components can significantly make the laser more compact and reliable. In 2012, Q. Xiao et al.  presented a 68.3 W all-fiber superfluorescent source operating at ~1 μm with fused angle-polished side-pumping configuration. In 2012, Y. Cao et al.  reported 102 W-level ytterbium-doped fiber superfluorescent source based on all-fiber MOPA structure. In 2014, J. Liu et al. constructed thulium-doped all-fiberized MOPA structured superfluorescent sources with maximal output power of 122 W .
In this paper, we demonstrate a kW-class high power all-fiberized MOPA structured superfluorescent source which employs a homemade backward pumped ASE source. Two stage amplification configuration is adopted in the amplification chain. The maximal output power of the main amplifier is 1.01 kW with an optical-to-optical conversion efficiency of 80.3%. Meanwhile, linewidth-narrowing and central-wavelength-drifting effects of broadband amplification in high power superfluorescent source system are investigated. To the best of our knowledge, this is the first demonstration of kW-level superfluorescent fiber source in all-fiberized configuration ever reported.
2. Experimental setup
Schematic diagram of the high power superfluorescent source is depicted in Fig. 1. Seed laser we employed is a homemade backward pumped ASE source pumped by fiber-pigtailed multimode laser diode (LD) at 976 nm via a (2 + 1) × 1 pump combiner. The pump delivery fibers of the combiner and pump LD are both 105µm /125 µm with 0.22 NA. The output port of the pump combiner exhibits 10 µm /125 µm core/inner cladding diameter with 0.08/0.46 NA. A segment of 13 m Yb-doped dual-cladding fiber is employed as gain fiber, which matches well with the output port fiber of pump combiner. The average cladding absorption coefficient for 976 nm pump light is about 3.4 dB/ m. An 8° angle is cleaved to decrease back reflection of backward port of gain fiber and suppress parasitic oscillation. The output of backward port is utilized for the monitoring of the operation state of seed ASE source. Two fiber isolators (ISO) are utilized to protect the components of the seed source. LDs, pump combiner, gain fiber and ISO in pre-amplification stage are the same types as employed in seed source. A tapper is inserted between the pre-amplifier and the main amplifier, and the1‰ port is employed to pick out a small proportion of the backward light from the fiber core. Then the backward power will be monitored and damage of pre-amplifier and seed source will be avoided. The input port of mode-filed-adapter (MFA) is 10 µm /125 µm, and the output port of MFA is 20 µm /400 µm, which matches well with the signal fiber of pump combiner in main amplifier.
The main amplification stage of the MOPA laser system is a double-clad ytterbium-doped fiber amplifier (YDFA). The pre-amplified seed light and the pump light are injected into the power scaling stage via a (6 + 1) × 1 fiber combiner, whose signal input fiber exhibit 20 µm /400 µm core/inner clad diameter. Six 200 W-class pump LDs are utilized and the pump delivery fibers of fiber-pig-tailed LDs and combiner are both 200 µm /220 µm. The active fiber in this stage is a piece of 22 m long double-clad YDF exhibiting 20 µm /400 µm core/inner clad diameter with 0.06/0.46 NA (provided by China Electronics Technology Corporation). The average absorption coefficient of the inner clad for 976 nm pump light is about 1.4 dB /m. A section of 0.5 m long Ge-doped double-clad passive fiber with the same core/inner clad diameter and NA as YDF is spliced to the active fiber for power delivery. The spliced region is covered with high-index gel to strip the residual pump laser and leaky laser in the inner clad of the active fiber. An 8° angle is cleaved at the output port of the power delivery fiber to suppress back reflection and prevent the damage of pre-amplifier due to backward power. All the components of the power amplifier and pump LDs are heat-sunk to aluminum baseplates with cold water circulating inside for stable high power operation.
3. Experimental results and discussion
For the seed ASE source, backward and forward port obtains maximal output powers of 0.275 W and 1.278 W, respectively. This power ratio is determined by the effective feedback reflectivities of forward and backward end . With more pump light injected, relaxation oscillation will be observed. The spectrum of forward output port for power amplification at different output power is charted in Fig. 2. The central wavelength of output spectrum of forward output light is 1066 nm, and the FWHM linewidth is 21 nm. As the seed source we employed is a broadband ASE source, two fiber isolators are utilized to provide sufficient isolation. The maximal seed power after ISO2 is 0.814 W, and this power is boosted to 30 W after pre-amplification stage.
Figure 3 shows the output characteristics of the main amplifier. Output power and optical-to-optical conversion efficiency as a function of pump power are depicted in Fig. 3(a). The ultimate output power of main amplifier is 1.01 kW for given 1.22 kW pump power and 30 W seed power, corresponding to an optical-to-optical conversion efficiency of 80.3%. The absorption coefficients of gain fiber for pump light at different wavelengths are not equivalent. As the driven currents of the six pump LDs are enhanced, the central wavelengths of LDs shift slightly from 972 nm to 976 nm due to temperature increment, and the optical-to-optical conversion efficiency changes from 61.1% to 80.3%. Figure 3(b) shows the optical spectrum of the main amplification stage at maximal output power. The central wavelength of main amplifier driftes to 1074.4 nm. Meanwhile, the FWHM linewidth of amplified light is about 8.1 nm, which is narrower than the 21 nm of seed source. The causes of these central-wavelength-drifting and linewidth-narrowing effects will be analyzed later. The residual pump laser has been almost totally absorbed or dumped, and no stimulated Raman scattering (SRS) effect has been observed. A beam quality of Mx2 = 1.688 and My2 = 1.728 at central wavelength is measured with 4σ method by Ophir-Spiricon M2-200s beam propagation analyzer, as depicted in Fig. 3(c). Figure 3(d) shows the temporal stability of high power operation. The power fluctuation under maximal output power is less than 2% in 100 seconds continuous operating. The measurement in μs-class domain, as depicted in the insertion graph of Fig. 3(d), can illustrate that no self pulsing or relaxation oscillation effect is observed even under maximum power. As the output power enhanced monotonously with the increasing of pump power and no power roll-over was observed, we believe that the output power can still be increased with more powerful pump source.
In additional, we investigate the central-wavelength-drifting and linewidth-narrowing effects of broadband superfluorescent source system. As the length of YDF utilized in pre-amplifier is only 3.5 m and the maximal output power is as low as 30W, the central wavelength and FWHM linewidth of seed source can be maintained after pre-amplification. With the injection of 1 W pre-amplified signal light into the main stage, the central wavelength drifts from 1066 nm to 1071.2 nm, and the FWHM linewidth decreases from 21 nm to 11.1 nm, respectively. Limited by the power level of pre-amplified light, the alteration of spectrum at different pre-amplified power can be neglected. Figure 4(a) shows the spectrums of pre-amplified signal light at maximum power measured before and after the main amplifier. As seed source we applied is a broadband ASE source, the 22 m-long YDF applied in the main amplifier can absorb the short wavelength light and the long wavelength light can be amplified by tandem-pumping. So the central wavelength drifts to longer wavelength and FWHM linewidth narrows for the injection of pre-amplified light into the main stage. For the main amplifier, spectral details, FWHM linewidth and central wavelength of the main stage at different power levels are shown in Fig. 4(b) and Fig. 4(c). The central wavelength drifts from 1071.2 nm to 1074.4 nm, and FWHM linewidth decreases from 11.1 nm to 8.1 nm, respectively, as a function of output power. Theoretically speaking, the short and long wavelength segments of broadband seed can be amplified simultaneously, but the re-absorption of short wavelength piece is sufficient with the increasing of operation power and temperature of YDF. The net-gain spectrum will be alternated, and the long wavelength section of broadband seed light is more predominant in high power amplification. So central-wavelength-drifting and linewidth-narrowing effects can be observed. These analyses indicate that short gain fiber should be utilized to diminish re-absorption effect to maintain the broadband characteristic of seed source in high power amplification. Inconsistently, the pump absorption rate and optical-to-optical conversion efficiency will be limited at low level by the application of short gain fiber. So it is important to select an appropriate length of gain fiber for broadband high power amplification.
In summary, we demonstrate an all-fiberized MOPA structured superfluorescent source with record kilowatt output power. The seed laser is a homemade backward pumped ASE source, and the maximum output power of backward and forward output power is 0.275 W and 1.278 W, respectively. The maximal output power of the main amplifier is 1.01 kW, corresponding to an optical-to-optical conversion efficiency of 80.3%. The central wavelength is 1074.4 nm and the FWHM linewidth is about 8.1 nm at full power. Spectral evolution effects of central-wavelength-drifting and linewidth-narrowing of broadband amplification in high power superfluorescent source system are investigated. A beam quality of Mx2 = 1.688 and My2 = 1.728 is measured. No self pulsing or relaxation oscillation effect is observed and the power fluctuation is less than 2% in 100 seconds continuous operating. The output power is limited only by the pump power, and further power scaling of this MOPA source is available.
This work is supported by the National Natural Science Foundation of China (NSFC, Grant No. 61322505), and the Program of China for the New Century Excellent Talents in University. We are particularly grateful to Xiaolin Wang, Wei Liu, Jinwei Liu, Lingchao Kong and Hanwei Zhang for their supports on this work.
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