1. Introduction

The output power of continuous wave fiber lasers increased rapidly in the last decade to almost ten thousand watt with diffraction limited beam quality [1]. However, the average power of ultrafast pulsed fiber lasers with picosecond or femto-second pulse duration has been limited to a relatively lower power level, i.e. several hundred watts [2–4]. Even by using combining technology, the average power of pulsed fiber laser system is still much lower than continuous wave ones [5]. The main limitation of pulsed fiber lasers to high average power is the onset of nonlinear effects such as stimulated Raman scattering and self-phase modulation due to the small core and long gain medium of fiber lasers. A direct method to suppress nonlinear effects in fiber lasers is to increase the fiber core diameter and/or reduce the fiber length, so as to decrease the power density and/or the interaction length. This method can increase the nonlinear threshold to a certain extent, however is ultimately restricted by the fiber fabrication technology. Further more, increase the fiber core diameter may degrade the laser beam quality. An alternative way to suppress nonlinear effect is to upgrade the repetition rate so as to decrease the peak power while maintaining the same average power level. By using a gain switched picosecond diode laser with up to 1 GHz repetition rate as the seed source, over 300 W average power was obtained from a 43 μm fiber core with an output beam quality M² of 2.4 in a master oscillator power amplification (MOPA) configuration [3]. Although this represents an impressive achievement, it should be noted that this system incorporated free space pump and signal coupling which greatly compromising the practicality of the system. After this, a similar system with 100 W diffraction limited output from a 25 μm fiber core was demonstrated with an improved all fiber signal coupling system [6]. However, the last stage still remains lens pump coupling with a dielectric mirror to separate the pumping and signal lights. To date, the average output power of strictly all fiber picosecond lasers still remains at a very low power level [7]. In this paper, an all fiber picosecond laser with 96 W average power is demonstrated in a four stage MOPA configuration.

2. Experimental setup

The experimental setup of the proposed high power picosecond laser is schematically shown in Fig. 1 . It is constructed in a four stage MOPA configuration. The seed source stage comprises a passively mode-locked ytterbium-doped single mode fiber laser, an optical isolator, a bandpass filter and a repetition rate increasing system. A semiconductor saturable absorber mirror (SESAM) is utilized to mode lock the seed laser and also acts as the total reflection cavity mirror. A fiber Bragg grating (FBG) with 10% peak reflectivity at 1064 nm acts as the output cavity mirror. The bandpass filter is utilized to eliminate deleterious amplified spontaneous emission (ASE). The repetition rate increasing system is an all fiber device that can octuple the repetition rate of the seed laser. The second stage is a conventional single mode ytterbium-doped fiber amplifier (YDFA) followed by a bandpass filter and an optical isolator. The third stage is a double clad YDFA pumped by two 20 W fiber pigtailed 976 nm laser diodes via a (2 + 1) × 1 fiber combiner. The gain fiber in this stage exhibits a 15 μm/130 μm core/cladding diameter with 0.08/0.46 NA. The last stage is another double clad YDFA pumped by six 20 W fiber pigtailed 976 nm laser diodes via a (6 + 1) × 1 fiber combiner. The ytterbium-doped fiber in this stage exhibits 30 μm/250 μm core/cladding diameter with 0.06/0.46 NA. An angle polished fiber end cap is arranged at the output port of the system to eliminate back reflection and prevent end facet damage. All the amplifiers in this MOPA laser are forward pumped to avoid laser diodes destruction by insufficient isolation of high peak power signal lasers.

 

Fig. 1 Schematic diagram of the high power picosecond pulsed laser.

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3. Results and discussion

The seed laser exhibits about 20 mW average power, 13 ps pulse width and 59.8 MHz repetition rate. Figure 2(a) is the spectrum of the seed laser measured after the filter. A direct amplification of this seed source encounters obvious nonlinear effects hence serious spectral broadening at only ten watt power level. Figure 2(b) is the output spectrum when the seed laser is directly amplified to 16 W average power by a double clad amplifier chain with 30 μm core diameter at the last stage. As can be seen from the figure, strong spectral broadening caused by nonlinear effects occurs. To avoid these nonlinear effects, we octupled the repetition rate of the seed laser to about 478 MHz though a self-made all fiber device before amplification to high average power.

 

Fig. 2 Spectrum of the (a) Mode locked seed laser after the filter, (b) 16 W laser directly amplified from the seed source.

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As Fig. 1 shows, a two-stage intermediate amplifier chain is utilized at first to amplify the low power seed laser with 478 MHz repetition rate, so as to provide enough power for the last stage power amplifier. The intermediate amplifier chain consists of a conventional core pumped single mode YDFA and a cladding pumped double clad YDFA with 15 μm core diameter. The spectrum and power properties of the 15 μm core double clad YDFA is shown in Fig. 3 . As Fig. 3(a) shows, the spectrum only broadens slightly as pump power increases. Serious nonlinear spectral broadening like Fig. 2(b) does not appear until the output power reaches 20 W, although the fiber core of this amplifier is only 15 μm. This should be attributed to the repetition rate increasing system, which effectively decreases the peak power of the laser and consequently suppresses the nonlinear effects. The output power of the 15 μm core YDFA increases monotonously with an increasing pump power till the maximum power level [Fig. 3(b)]. However, obvious saturation effect occurs when the pump power is larger than 25 W. This saturation effect is caused by the relatively lower seed power (about 60 mW from the conventional single mode YDFA). The insufficient seed power also causes a large ASE hump around 1038 nm [Fig. 3(a)], especially at high pump powers. A more powerful pump for the single mode YDFA or another more amplifier stage might be used to avoid this power saturation effect and the insufficient ASE suppressing. This is one of the reasons why high power MOPA fiber lasers usually contain lots of amplifier stages.

 

Fig. 3 Output properties of the 15 μm core double clad amplifier. (a) Spectrum at different output power levels. (b) Output power versus pump power.

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Outputs from the 15 μm core YDFA is then amplified by the 30 μm core YDFA with results shown in Fig. 4 . “P_seed” and “P_pump” in the figure respectively mean the power from the 15 μm core YDFA and that from the six pumping diodes of the last stage YDFA. As pump power increases, the spectrum only broadens slightly [Fig. 4(a)]. However, great spectrum broadening occurs with an increasing seed power [Fig. 4(b)]. This should be attributed to the spectral broadening of the seed source itself [Fig. 3(a)]. Even so, the difference of the laser peak to the ASE peak remains above 25 dB at the highest power level, indicating that most part of the output power concentrates on the lasing wavelength [see the inset in Fig. 4(b)]. The output power of the 30 μm core YDFA increases monotonously with an increasing pump power [Fig. 4(c)]. No sign of power roll over can be seen till the maximum value, indicating that this picosecond laser system can be further power scaled as long as larger pump power is available. A maximum output power of 96 W was obtained under total 156 W pump power, giving an optical conversion efficiency of 61.5%. The pulse width of the ultimate output laser was measured to be 16 ps by using a commercial autocorrelator. The beam quality M² was roughly measured to be less than 1.5.

 

Fig. 4 Properties of the last stage power amplifier. (a) Spectrum at various pump powers with 4 W seed power. (b) Spectrum at various seed powers with 116 W pump power. (c) Output power versus pump power under various seed powers. Inset in (b) is the linear scaled spectrum at the maximum power level.

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4. Conclusion

An all fiber picosecond laser with 96 W average power in MOPA configuration is demonstrated. An ytterbium-doped fiber laser passively mode locked by an SESAM is utilized as the seed source. It produces an average output power of ~20 mW, a pulse width of 13 ps, and a repetition rate of 59.8 MHz. This seed exhibits a relatively lower repetition rate as compared to those in the previously reported high average power picosecond laser systems (59.8 MHz versus 1 GHz). A direct amplification of this seed source encounters obvious nonlinear effects hence serious spectral broadening at only ten watt power level. To avoid these nonlinear effects, we octupled the repetition rate to about 478 MHz though a self-made all fiber device before amplification. The new high repetition rate seed is then amplified by a tri-stage all fiber amplifier chain to an average power of 96 W. The ultimate output laser exhibits a pulse width of 16 ps, a repetition rate of 478 MHz, a beam quality M² of less than 1.5, and an optical conversion efficiency of 61.5%.