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Active mode-locking of Raman fiber laser is experimentally investigated for the first time. An all fiber connected and polarization maintaining loop cavity of ~500 m long is pumped by a linearly polarized 1120 nm Yb fiber laser and modulated by an acousto-optic modulator. Stable 2 ns width pulse train at 1178 nm is obtained with modulator opening time of > 50 ns. At higher power, pulses become longer, and second order Raman Stokes could take place, which however can be suppressed by adjusting the open time and modulation frequency. Transient pulse evolution measurement confirms the absence of relaxation oscillation in Raman fiber laser. Tuning of repetition rate from 392 kHz to 31.37 MHz is obtained with harmonic mode locking.
© 2015 Optical Society of America
1. Introduction
Mode-locked fiber lasers have widespread applications in spectroscopy, biomedical research, telecommunication, optical frequency comb, etc [1]. Compared to rare-earth-doped fiber lasers, Raman fiber lasers have more flexible output wavelength and ultrafast gain dynamics, and use gain fibers without rare earth doping that is easier to fabricate [2]. Therefore, mode-locked Raman fiber lasers have received attention for the potential in generating pulsed laser at wavelength not achievable with rare earth doped fiber lasers. So far, mode-locked Raman fiber lasers have been demonstrated with passive methods, for example, nonlinear optical loop mirror [3, 4], dissipative four-wave mixing [5], nonlinear polarization evolution [6, 7], semiconductor saturable absorber mirror [8], nanotubes [9], graphene [10, 11], and intermodal beating technique [12, 13]. However, the overall performance of those passively mode locked Raman fiber lasers is yet worse than the rare earth doped counterparts. Therefore, further investigation on mode locked Raman fiber lasers is highly desirable. There is a renewed interest in synchronously pumped Raman fiber lasers very recently [14–16], rather good performance in terms of power and pulse width has been reported. However, it needs a mode locked laser as pump source in the first place.
In this paper, we investigate active mode-locking of Raman fiber laser for the first time. Active mode-locking is preferred in some applications, where synchronization between multiple instruments or to a master clock is necessary. Active mode-locking operation also avoids the self-starting difficulty and multiple peak problems associated with passive mode locking [17]. An all-fiber polarization maintaining Raman fiber laser is constructed with a fiber-pigtailed acousto-optic modulator (AOM) as mode-locker. The AOM introduces amplitude modulation within the ring fiber cavity. Mode locked pulses with approximately 2 ns width is generated with modulator open time of > 50 ns. The transient pulse evolution dynamics, the influence of 2nd Raman Stokes, and harmonic mode locking are experimental investigated.
2. Experimental configuration
The experimental configuration is shown in Fig. 1. The ring resonator consists of ~500 m long polarization maintaining (PM) single-mode fiber (PM980, Raman gain coefficient is 1.8 W−1km−1 at 1120 nm) acting as the Raman gain medium, an optical isolator ensuring unidirectional laser propagation, three PM 1120/1178 nm wavelength division multiplexers (WDM), an AOM with pigtail fiber (center frequency 150 MHz, rise/fall time < 10 ns), and a PM 1178 nm 50% output fiber coupler. The pump source is a linearly polarized Yb-doped 1120 nm fiber laser, and is coupled into the ring resonator through WDM1. The pump and signal light propagate in opposite direction in the cavity. WDM2 and WDM3 are used as residual pump extractors. The laser cavity is all polarization-maintaining such that it is more environmentally stable.
The output signal is recorded and analyzed by an optical spectrum analyzer with a resolution of 0.02 nm, a digital phosphor oscilloscope with 1 GHz bandwidth, and a radio-frequency (RF) spectrum analyzer (9 kHz-13.2 GHz).
3. Results and discussion
When the 1120 nm pump power increases, stimulated Raman scattering (SRS) would take place in both forward and backward direction. The forward light is stopped by the isolator. Only the backward Raman light is allowed to oscillate in the ring-cavity. Counter propagation of the pump and Raman light is essential for maximizing the interaction between them in the fiber and achieving short pulses with peak power higher than the pump power. Active mode locking can be achieved by driving the AOM at a repetition rate matching the cavity round-trip frequency, f = c/nL, where c is light speed, n is the effective refractive index, and L is the total cavity length. The initial guess of the repetition rate is determined by analyzing the longitudinal mode beating of the Raman fiber laser in continuous wave operation. The AOM is driven with rectangular pulses, and open time of the AOM is set to 50 ns in the experiments if not specified.
The threshold of the Raman fiber laser is about 2.4 W. Stable mode locking can be achieved from 2.5 W to 4 W of the pump power. For instance, at a pump power of 2.8 W, stable mode-locked pulse train with a pulse width of 2.2 ns is obtained, as shown in Fig. 2(a). Figure 2(b) depicts the RF spectrum at the fundamental repetition frequency 392.075 kHz, which matches the Raman-cavity round-trip time. The inset is an RF spectrum of the output from 0 to 20 MHz. The RF trace had a contrast of approximately 62 dB against the noise floor, which indicates good mode-locking stability and low pulse timing jitter. Figure 2(c) shows the output spectrum, which has a peak centered at 1178 nm. Figure 2(d) shows pulse profiles with longer AOM open time of 80 ns and 150 ns, but in the same pump power and modulation frequency. One can see that the actively mode-locked Raman fiber laser is not sensitive to the open time of the AOM at this condition.
Fig. 2 Laser output characteristics at a pump power of 2.8 W: (a) typical pulse profile and pulse train (inset). (b) RF spectrum around the fundamental repetition frequency and 0-20 MHz RF trace (inset). (c) output spectrum. (d) pulse profiles with AOM open time of 80 ns and 150 ns.
The pulse repetition rate and average output power, respectively, as a function of pump power is shown in Fig. 3(a). When the pump power increases from 2.5 W to 3.9 W, the average output power increases from 6 mW to 110 mW. At each power, the AOM modulation frequency is adjusted to achieve stable mode locking. The optimum pulse repetition rate is found to decrease slightly from 392.1289 kHz to 391.7494 kHz.
Fig. 3 (a) The repetition frequency and average output power, respectively, as functions of pump power. (b) Pulse duration and pulse peak power versus the pump power.
The pulse duration and peak power versus the pump power are also examined and the results are shown in Fig. 3(b). As the pump power increases from 2.5 W to 3.9 W, the pulse duration has a significant broadening from 2 ns to 10 ns. With the increasing of pump power, the pulse peak power increases to as high as 35 W at a pump power of 3.2 W. Notably, it has a steep drop when the pump power is higher than 3.2 W. We find this transition is due to the onset of second Stokes Raman light, which reduces the first-order Raman laser and broadens the pulse width.
Figure 4(a) depicts the pulse profile at a pump power of 3.4 W, where the pulse width is about 7.6 ns. The repetition rate is adjusted to 391.925 kHz to achieve stable single pulse mode locking with 50 ns AOM open time. Figures 4(b) and 4(c) illustrate the pulse train and the RF spectrum at the fundamental repetition frequency. The SNR of the RF spectrum is 66 dB, which shows that the mode locking is stable at this power level. However, when one broadens the open time of the AOM from 50 ns to 80 ns and 150 ns at the same condition, the mode-locked pulses are seriously distorted, as shown in Fig. 4(a). This observation is completely different from what is seen at lower power. The output spectra of the mode locked laser at this power level but with different AOM open time are measured and shown in Fig. 4(d). The second Stokes Raman emission at 1250 nm is observed, and is much stronger with AOM open time of 150 ns than 50 ns. It is clear that the pulse distortion with larger modulator opening time is due to the 2nd Stokes Raman emission. Due to chromatic dispersion, the higher-order Stokes Raman light has a slightly higher speed and needs less time (~2 ns) for a round trip in the fiber cavity. Therefore, by finely tuning the repetition rate and narrow open time, the high order Stokes Raman light can be suppressed at the AOM. That is why stable mode locking is achievable even above the 2nd Stokes Raman threshold. With a fixed repetition rate and increased open time, as in the above experiments, the suppression of 2nd Stokes light is relaxed, which results in pulse distortion.
Fig. 4 Laser output characteristics at a pump power of 3.4 W: (a) typical pulse profile with a AOM repetition frequency of 391.925 kHz and open time of 50 ns, 80 ns and 150 ns; (b) pulse train of the mode-locked laser; (c) the RF spectrum at the fundamental repetition frequency; (d) the output optical spectrum with AOM open time 50 ns and 150 ns, respectively.
The active mode locking nature of the laser allows us to investigate the transient behavior of the laser output conveniently. Figure 5 is a typical measurement result of pulse evolution dynamics from switch-on to steady-state, when the pump power is 2.8 W and the AOM repetition frequency is 392.075 kHz and open time is 50 ns. The pulse grows exponentially without relaxation oscillations, which is expected for Raman fiber laser because of the ultrafast response time of Raman gain. After the pulse evolves to highest power, it decreases a little bit and reaches a steady state. This process is slow in the time scale of 0.1 ms, which suggests it is a thermal induced process. Further investigation will be carried out to understand the observation.
Fig. 5 Transient pulse evolution of the mode locked Raman fiber laser from establishment to steady state.
Actively mode-locking also has an advantage that harmonic mode-locking can be easily achieved and the harmonic order can be directly changed by adjusting the modulation frequency. In the experiments, the performance of mode locked Raman fiber laser versus harmonic order is also investigated. At lower order, the performance is similar to fundamental mode locking. When reaching the limit of AOM, the performance degrades. Figure 6 shows the pulse train and corresponding RF spectrum at 80th order harmonics at a pump power of 3.2 W. The repetition rate is 31.37 MHz, and the pulse width is still around 2 ns. However, obvious side lobes are seen in the RF spectrum. We believe that much higher order harmonic mode locking with much higher repetition rate can be readily achieved with commercially available high speed electro-optic modulator.
4. Conclusion
An all fiber connected and polarization maintaining loop cavity of ~500 m long is pumped by a linearly polarized 1120 nm Yb fiber laser and modulated by an acousto-optic modulator. Stable 2 ns pulse train at 1178 nm is obtained with modulator opening time of > 50 ns. At higher power, pulses become longer and second order Raman Stokes could take place, which however can be suppressed by adjusting the open time and modulation frequency. The measurement on transient pulse evolution confirms that relaxation oscillation is absent in Raman fiber laser due to ultrafast gain response. Harmonic mode locking is also investigated and up to 80th order (31.37 MHz) is demonstrated, limited by the response time of AOM. To the best of our knowledge, it is the first report of study on active mode locking of Raman fiber laser. The technique can be used to generate pulsed fiber laser at a variety of wavelength range for diverse applications.
Acknowledgment
The work is supported by the National Natural Science Foundation of China (No. 61378026).
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