1. Introduction

Fiber lasers feature a high single pass gain, excellent beam quality, very good heat dissipation, efficient diode-pumped operation, broad gain bandwidth, and the potential of all-fiber integrity. Ultra fast fiber lasers have many practical advantages over conventional Ti:sapphire and Nd:glass lasers, particularly in industrial environments, due to their robustness and compactness [1–6]. Fiber-based high-power picosecond laser is of great interest in many application areas such as remote sensing, light detection, micro-machining, laser radar, medical treatment, space telecommunication, underwater optical-communication, laser projection display, and fundamental science so on [1–12]. Many remarkable results have been obtained with such high-power fiber lasers in recent years. An average power as much as 97 W at a repetition rate of 47 MHz with 10 ps pulse duration, corresponding to a peak power as high as 200 KW was achieved by J. Limpert [7]. The ytterbium-doped-fiber–based chirped pulse amplification system delivering 175 W of average power of picosecond pulses before compression at a repetition rate of 73 MHz was demonstrated by F. Röser [8]. By using a Q-switched microchip laser generating 1064 nm wavelength, subnanosecond pulses at a 13.4 KHz repetition rate as seed, Yb-doped photonic-crystal fiber as the power amplifier, and an average power of 9.5 W with peak power at 1.5 MW was obtained by Fabio Di Teodoro [9]. A 321 W average power with a repetition rate of 1 GHz, a wavelength at 1064 nm, and pulse duration at 20 ps was demonstrated by P. Dupriez [10]. Picosecond pulses at 1.5 µm generated by a passively mode-locked fiber oscillator at a repetition rate of 70 MHz have been amplified in a 15-cm-long heavily Er-Yb codoped fiber amplifier to the average output power of 1.425 W [11]. An all-fiber picosecond watt-level master-oscillator-power-amplifier (MOPA) system at 1.5 µm based on rapid amplification of mode-locked pulses in heavily Er:Yb codoped phosphate fiber was realized by Pavel Polynkin [12].

In particular, fiber-based wavelength tunable ultrafast lasers have been the subject of intensive researches for its interest as laser sources for probing fast phenomena in physics, chemistry and biology, remote sensing applications, and underwater optical-communication. The 74 fs bandwidth-limited pulses with an average power of 0.4 W, pulse energy of 8 nJ, and central wavelength-tunable from 1.00 µm to 1.070 µm based on a tunable Raman-shifted and frequency-doubled Er-fiber soliton laser were obtained by M. E. Fermann [13]. The first picosecond mode-locked operation of an Yb-doped fiber laser with wavelength-tuning over a 90nm range from 980 nm to 1070 nm, delivering pulses of 1.6-2 ps in duration was reported by O. G. Okhotnikov [14].

In this paper, we demonstrate for the first time to our knowledge the generation of tunable high-power picosecond laser based on Yb-doped fiber amplification of gain switch semiconductor laser diode, which is an attractive technology for compact, robust, and reliable picosecond pulse source with non-mode-locked operation [10, 15]. Using this method, the 3.5 W average power 90 ps pulses at 1 MHz repetition rate were generated with high stability and good beam quality, and the central wavelength with the line width of 1.8 nm was tunable from 1053-1073 nm by self-made tunable filter, which was placed between the two stage preamplifiers. Compared with mode-locked tunable fiber lasers, this kind of laser source has various advantages such as operational stability and flexibilities of the repetition rate and the easy tunability of the operating wavelength.

2. Experimental setup

The setup of our fiber-based tunable high power picosecond laser system is depicted in Fig. 1. Generally, high-power fiber-based picosecond sources are typically based on master oscillator power amplifier (MOPA) designs that use mode-locked lasers and gain switch semiconductor laser diodes as seed lasers. Compared with mode-locked lasers, gain switch semiconductor laser diodes are particularly more appropriate to generate stable picosecond pulses. Despite of the former features such a short pulse output (less than 100 fs), higher output average power and excellent pulse quality, it is weak in stability, and easily influenced by environment. However, pulses generated by gain switch semiconductor laser diodes have adjustable pulse duration and repetition rate, and good adaptability to environment, and can be used to develop highly stable laser source. Therefore, a broadband picosecond gain switch semiconductor laser diode (Pilas, A.L.S. GmbH), which generates 70 ps pulses with adjustable repetition rate among 1 MHz, 500 KHz, 200 KHz, 100 KHz… 10 Hz, and single shot, 15 nm bandwidth, 1064 nm central wavelength, and 15 µW average power put out by single mode fiber coupling, was used as the seed source to ensure the laser system high stability and compactness. The source signal was amplified to produce sufficient signal to seed the main amplifier by the two stage single-mode Yb-doped fiber pre-amplifiers. The first stage pre-amplifier was constructed by three-stage amplifier cascade connections, which efficiently reduced amplification of spontaneous emission in signal and improved gain at the same time. In order to prevent signal spectral gain narrowing in the first stage pre-amplifier and to ensure an output of flat broadband spectrum for wavelength-tunability, we have optimized amplifier’s parameters to compensate the signal spectrum gain and obtain “flat top” broadband spectrum output. For achieving wavelength-tunable output, a self-made tunable filter with single mode fiber coupling was used to filter broadband picosecond laser pulse from the first stage preamplifier by spectrum gain compensation. The second stage pre-amplifier was constructed by the two-stage amplifier cascade connections. Finally, the main amplifier pumped by 976 nm laser was constructed by a Yb-doped double-clad fiber with a core diameter of 20 µm and a numerical aperture (N.A) of 0.08, which resulted in a single transverse mode laser output. Fiber-coupling isolators were among the three amplifiers. The output power and wavelength tuning were measured from the main amplifier of the set-up via power meter and a conventional optical spectrum analyzer.

 

Fig. 1. The setup of fiber-based tunable high power picosecond source.

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3. Experimental results

The spectrum of the seed pulse is illustrated in Fig. 2. It was found that the power in the shorter waveband of the seed pulse is very low but the spectrum is wider, i.e. the rising edge in the spectrum profile is gently, but the falling edge is sharp. Since a “flat top” broadband spectrum is the necessary condition for wavelength-tunability, the first stage pre-amplifier’s parameters including fiber length and pump power have been optimized to compensate the seed spectrum gain. Thus, the Yb-doped fiber, whose gain spectrum profile is reversed to signal spectrum as shown in Fig. 2, was used as gain medium. As a result, the amplification in the rising edge of the spectrum profile was several times than the falling edge, this leads to the generation of “flat top” spectrum as shown in Fig. 3. The seed pulses were amplified to a 40 mW average power by the first stage pre-amplifier with spectrum gain compensation.

For realizing wavelength-tunable output, a self-made tunable filter with single mode fiber coupling was used to filter the “flat top” broadband spectrum from the first stage preamplifier. Consequently, 88 ps pulses with 1.8 nm line bandwidth and a central wavelength tunned from 1053 nm to 1073 nm were achieved by adjusting the filter, the continuously tunable spectral bandwidth reached 20 nm. Although the wavelength-tunability is an important feature of a light source, the tunable range depends on the spectral bandwidth of seed source. Especially, the variable spectral line width was also tunned from 0.8 nm to 2 nm by adjusting the line width of the filter.

 

Fig. 2. The spectrum of seed source.

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Fig. 3. The output spectrum of the first stage pre-amplifier.

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After the tunable filter, the second stage pre-amplifier was constructed by the two-stage amplifier cascade connections. The generation of high peak powers is usually restricted by the high nonlinearity of such a doped fiber, which has its origin in the tight confinement of laser radiation over considerably long fiber lengths. In picosecond pulses self-phase modulation (SPM) is the limiting nonlinear effect. The spectral shape is typical for the regime where SPM dominates over dispersion due to the relatively long pulses. Pulses mainly undergo the effects of SPM led to a large pedestal. There is still a small increase in pulse duration as a result of dispersion of the spectrally broadened pulse. A simple way to overcome this limiting factor is to use high doped gain fiber, which decreases the fiber length and leads to increase the nonlinear effect threshold. When the pulse duration (T₀) is about 100 ps, the peak power (P₀) is 1.2 KW, and the central wavelength is 1064 nm, we can obtain the dispersive length (L_D) and nonlinear length (L_NL) from equations (1) and (2):

Where β₂=20 ps²/km, denoting the group velocity dispersive parameter, and γ=3 W^-1km^-1, representing nonlinear coefficient of the fiber. From the equations (1) and (2), we can achieve L_D=500 km and L_NL=0.3 m. Obviously, the gain fiber length L is far less than the dispersive length L_D, while it is almost the same as the nonlinear length. Therefore, we believed that the spectrum was broadened by SPM, which usually causes spectral broadening of an ultrashort optical pulse owing to the time dependence of the nonlinear phase shift, which is a consequence of the intensity dependence of the refractive index [7]. As the power increases the level of amplified spontaneous emission (ASE) rises and spectral broadening caused by SPM is observed with line width up to 3.2 nm (Fig. 4(a)). Picosecond fiber amplifiers have been demonstrated to high average and peak powers but with exorbitant spectral broadening, which limits the usability of these pulses for many applications such as frequency conversion to the visible spectral range owing to the limited spectral acceptance of nonlinear crystals. In order to compress the spectrum broadening, a self-made spectrum shaping filter, which was formed by inserting a fiber polarization controller into a Sagnac loop, was used in the experimental setup. The Sagnac loop consists of a fiber coupler whose output ports are connected together by a piece of long fiber to form the loop. A single input was split into two counter propagating fields, which returned in coincidence to recombine at the coupler. Then some part of the light with high power transmitted out of the loop, and the other part of the light with low power reflected back. By the use of the spectrum shaping filter, the spectrum broadening from SPM was efficiently suppressed. Figure 4 illustrated the comparisons of spectrum broadening and compression at 1060 nm, 1063 nm, 1066 nm and 1069 nm wavelengths. The spectrum line width of the seed pulses, which were amplified by the second stage amplifier, was broadened from 1.8 nm to 3.2 nm as shown in the Fig. 4(a), and was again compressed to 1.8 nm by the spectrum shaping filter as shown in Fig. 4(b). The average output power was 70 mW from the spectrum shaping filter.

 

Fig. 4. The spectrums at 1060 nm, 1063 nm, 1066 nm and 1069 nm: (a) before spectrum shaping filter; (b) after spectrum shaping filter.

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Finally, the main amplifier was pumped by a high-power multimode semiconductor laser, which consisted of two pigtailed laser diodes operating at 976 nm that were combined through a pump combiner. When the main amplifier was pumped with a power of 8 W launched pump power, the amplified pulses with average power of 3.5 W in the whole tunable spectral range were produced. We were able to continuously tune the laser wavelength from 1053 nm to 1073 nm as shown in Fig. 5. The temporal profile of the amplified output pulse with 90 ps pulse duration is shown in Fig. 6. The main amplifier produced a near diffraction limited output with an M²≤1.1. The output power of this laser system was limited by available pump power and not by degradation in pulse quality owing to nonlinearity or even by fiber damage. Thus, further scaling of the output power with the use of additional amplifiers consisting of large mode area fibers is possible with the present system.

 

Fig. 5. The output tunable spectrum after the main amplifier.

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Fig. 6. The amplified signal pulse shape after the main amplifier.

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4. Conclusion

In summary, we have demonstrated a fiber-based tunable high-power picosecond laser source, which was constructed by two-stage Yb-doped fiber preamplifiers and one-stage Yb-doped double-clad fiber main amplifier. A gain switch semiconductor laser diode was chosen as seed source to ensure stable picosecond pulse output. Single-mode Yb-doped fiber was introduced as gain medium to obtain “flat top” broadband spectrum output. Pre-amplified broadband signal pulses were filtered by a self-made tunable filter to obtain line-width-variable and wavelength-tunable picosecond laser pulses. To compress the spectrum broadened by SPM, a spectrum shaping filter formed by Sagnac loop was used to optimize the spectrum shape. This laser source produced the average power of 3.5 W with 1 MHz repetition rate and 90 ps pulse duration within the central wavelength tunable range from 1053 nm to 1073 nm with excellent beam quality. Higher output power could be achieved with the use of additional power amplifiers constructed by the short fiber with a large core and a relatively small inner cladding ensured minimum pulse distortion by fiber nonlinearities and dispersion. This source is very useful for remote sensing applications and underwater optical-communication.