A passively -switched fiber laser near 2 μm is achieved with a semiconductor saturable absorber mirror (SESAM) as a saturable absorber. Stable -switched pulses are generated from an extremely compact setup with a central wavelength of 1958.2 nm. Under the bidirectional pump configuration, the repetition rate of the fiber laser can be widely tuned from 20 to 80 kHz by increasing the pump power at the same time the pulse width decreases from 1 μs to 490 ns. When the incident pump power is 1.3 W, the average output power, the pulse repetition rate, the pulse width, and the highest single pulse energy are 91 mW, 80 kHz, 490 ns, and 1.14 μJ, respectively. To further optimize the system configuration, the pulse width can be reduced to 362 ns when the cavity length is reduced.
© 2012 Optical Society of America
Recently, Tm-doped fiber lasers operating at an eye-safe wavelength range around 2 μm have attracted significant attention. Tm-doped fibers have a wide gain spectrum from 1.8 to 2.1 μm and have been demonstrated to be efficient gain media for high-power laser sources near 2 μm [1–3]. 2 μm fiber lasers have a variety of attractive applications such as medical treatment, spectroscopy, and gas remote sensing. 2 μm lasers can also be used as the ideal pump sources to generate a mid-IR supercontinuum [4–5].
-switching and mode locking are two general methods for pulsed laser generation. Mode-locked fiber lasers routinely operate at a high repetition rate with a short pulse width in the range of tens of femtoseconds to tens of picoseconds [6,7]. On the other hand, -switching is an effective approach to generate a long pulse width from tens of nanoseconds to several microseconds with a low repetition rate, usually in the range of 10–200 kHz [8–13]. Both active and passive -switched fiber lasers have been studied at 2 μm wavelength range. Typically, active -switching is generated by using an acoustic-optic modulator (AOM) or electro-optic modulator (EOM) [8–10]. Myslinski et al. reported the first -switching of a Tm-doped fiber laser in 1993 using an AOM . 130 ns pulses at a 4 kHz repetition rate with average output power of was generated. Recently, an all-fiber -switched single-frequency laser based on the stress-induced polarization modulation in a Tm-doped distributed Bragg reflector fiber laser was reported . The laser emitted -switched single-frequency laser pulses with a pulse repetition rate ranging from tens of hertz to hundreds of kilohertz and an average power of several milliwatts. Compared to active -switching, passive -switching is an attractive approach for a simple, robust, and cost-efficient pulse laser. Passive -switching is initiated by saturable absorbers made of various semiconductor materials or crystals doped with ions such as , , , or . Zhang et al. reported a passively -switched Tm, Ho:YLF laser at 2053 nm with a Cr:ZnS crystal as a saturable absorber . The pulse width was around 1.25 μs, the pulsed energy was about 4 μJ, and the pulse repetition was between 1.3 and 2.6 kHz. However, reports of an around 2 μm wavelength range using a passively -switched method with SESAM as a saturable absorber are few. Kivistö et al. reported a passively -switched fiber laser operating at 2 μm based on a semiconductor saturable absorber mirror (SESAM) and boosted in a Tm/Ho amplifier up to 2 W of average power . The study presented the first demonstration of a 2 μm -switched fiber laser using antimonide semiconductor technology.
In this paper, we demonstrate a SESAM passively -switched fiber laser near 2 μm with a compact setup. Under the bidirectional pump configuration, the repetition rate of the fiber laser can be widely tuned from 20 to 80 kHz along with an increase of pump power. The highest single pulse energy of 1.14 μJ with a 490 ns pulse width is achieved when the incident pump power is 1.3 W. To further optimize the system configuration, the pulse width can be reduced to 360 ns when the cavity length is reduced.
2. Experimental Setup
The experimental setup of the SESAM passively -switched fiber laser is shown in Fig. 1. The master oscillator is comprised of a 1 m length of thulium/holmium codoped single mode fiber, two filter wavelength division multiplexers (FWDMs), a chirp fiber Bragg grating (CFBG), and a SESAM. The total length of the fiber laser cavity is . The thulium/holmium codoped fiber (THDF) has a core/cladding diameter of and an effective numerical aperture of 0.18, and the absorption coefficient is at 1570 nm. The CFBG is used as an output coupler with 50% reflectivity at 1958 nm and a bandwidth of 2 nm, and the chromatic dispersion of the CFBG is . The SESAM (SAM-1960-54-25.4S-500fs, BATOP) is coupled with fiber through two lenses. The central working wavelength of the SESAM ( at 1900–2050 nm) is 1960 nm. The modulation depth, saturation fluence, and relaxation time constant of the SESAM are 30%, , and 500 fs, respectively. The FWDM is based on environmentally stable thin film filter technology, and the pump laser is injected through the reflect port of the FWDM.
The pump source is a single-mode fiber laser with a central wavelength of 1570 nm and a maximum output power of 1.46 W. The FWDM is suggested to function under a pump power below 500 mW, so a bidirectional pump is needed for long term stability under high pump power. The pump laser is divided into two parts by a optical coupler (OC), and the FWDMs combine the two pump lasers into the THDF. An isolator (ISO) is used to prevent the possibility of remnant pump laser due to the bidirectional pump configuration. The output signal is monitored by a high-speed photodetector with a rising time (DET10D, THORLABS) and a 20 GHz sampling rate oscilloscope (Tektronix, TDS7154).
3. Results and Discussion
When the incident pump power is increased to higher than 165 mW, continuum wave laser operation can be obtained. Then unstable Q-switched pulses can be observed when the incident pump power is increased to 183 mW. It should be noted that incident pump power means the total power of the two lasers after the OC. Stable SESAM passively -switched pulses of the fiber laser occurred at a pump power of 278 mW; the repetition rate, pulse width and average output power were 20 kHz, 1 μs and 9.5 mW, respectively.
The average output power increased almost linearly from 9.5 mW to 91.4 mW when the incident pump power changed from 278 mW to 1.3 W (max pump power), and we achieved stable passively -switched operation in this dynamic range. The low optical efficiency obtained here is mainly caused by two factors: the insertion loss of the FWDM and the low coupling efficiency of the coupling system. Each FWDM has an insertion loss of , and the coupling efficiency is only . The large loss in the laser cavity and the big dispersion introduced by CFBG are not propitious to the SESAM mode locking, but they are conducive to the -switching formation, which we think results in the stable -switching. Figure 2(a) shows the stable pulse train of the SESAM passively -switched fiber laser with a 59 kHz repetition rate when the incident pump power is 788 mW. In this case, the pulse width is 570 ns, just as Fig. 2(b) shows, and the output power is 55.5 mW. The optical spectrum of the -switched pulses is shown in Fig. 3, with a central wavelength of 1958.2 nm.
Figure 4 shows the pulse repetition rate and pulse width with the increase of incident pump power. The pulse repetition rate increases with the augmentation of the incident pump power, but the change of the pulse width is the opposite. As the pump power varied from 278 mW to 1.3 W, the pulse repetition rate almost linearly increased from 20 to 80 kHz, whereas the pulse width decreased from 1 μs to 490 ns. With an increase of the pump power, the relationship between the pulse duration and pump power becomes weak, which results from the saturation of the SESAM . When the SESAM is fully saturated, the pulse duration will be independent of the pump power. For the maximum incident pump power of 1.3 W, the average output power, the pulse repetition rate, the pulse width, and the highest single pulse energy are 91 mW, 80 kHz, 490 ns, and 1.14 μJ, respectively.
For -switched lasers, the pulse width is a function of the cavity round-trip time and the cavity gain. The decrease of the pulse width could be achieved by reducing the laser cavity length . In order to reduce the cavity length, we put the FWDM outside the cavity, and the pump power is incident through the CFBG. The experimental setup of the fiber laser with a shortened cavity length is shown in Fig. 5. The components in Fig. 5 are the same as those in Fig. 1. The total length of the laser cavity is only , which is much shorter than that in Fig. 1. However, unstable -switched pulses can be observed when the pump power is increased from 180 to 380 mW. Figure 6 shows the unstable pulse trains (solid curve and dashed curve) of the laser when the pump power is 272 mW, which are measured at different times under the same pump power. The amplitude variation and timing jitter are noticeable in the pulse trains. When the pump power is greater than 380 mW, the SESAM can be easily damaged because of -switching instabilities.
We think the unstable -switched pulse output is caused by the unabsorbed pump light, considering the fact that the unabsorbed pump light is directly injected into the SESAM in the experiment, as Fig. 5 shows, where the length of the THDF is only 1 m. When we use a 1.5 m length of THDF instead of the 1 m length of THDF (the total length of laser cavity adds up to ), stable -switched pulses can be observed. In this case, the pulse repetition rate and pulse width with the increase of the pump power are shown in Fig. 7. The shortest pulse width and pulse repletion rate are 362 ns and 54 kHz, respectively, when the pump power is 400 mW, and at the same time the average output power is 36 mW. With further increase of the pump power beyond 400 mW, the SESAM can be easily damaged because of the low damage threshold. In another experiment, we keep the total length of the laser cavity of 2.5 m unchanged and reduce the length of the THDF to 1 m; the result is the same as that in Fig. 6. This experiment result eliminates the effect of the laser cavity length and further demonstrates our explanation that the unstable pulses are caused by the unabsorbed pump light. Compared with the results in Fig. 4, where the shortest pulse width is 490 ns, a pulse width decrease can be realized by reducing the laser cavity length.
In conclusion, we have demonstrated stable pulse generation near 2 μm in a passively -switched fiber laser based on a SESAM. Under the bidirectional pump configuration, the repetition rate of the fiber laser can be widely tuned from 20 to 80 kHz with increases in the pump power, and at the same time the pulse width is decreased from 1 μs to 490 ns. The highest single pulse energy is 1.14 μJ with a 490 ns pulse width and average output power of 91 mW when the incident pump power is 1.3 W. We expect the pulse width to decrease with the reduction of the laser cavity length. However, unstable -switched pulses are observed, and the amplitude variation and timing jitter are noticeable in the pulse train. We think the unstable -switched pulse output is caused by the unabsorbed pump light. When the length of the gain fiber is increased, the shortest pulse width of 362 ns is produced, and at the same time the pulse repetition rate and average output power are 54 kHz and 36 mW, respectively. For shorter -switched pulse operation, our future work will optimize the structure of the laser cavity and the parameters of the experimental components. Mode-locked fiber laser can also be achieved in the same structure with the appropriate components, which is also our further work.
This work was supported by projects of the National Natural Science Foundation of China under grant no. 61077076.
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