Raman dissipative soliton is generated in a mode locked polarization maintaining fiber laser with a nonlinear optical loop mirror. Ultrafast Raman laser with a repetition rate of 1.23 MHz is obtained. Signal to noise ratio of the radio frequency spectrum of the Raman dissipative soliton is as high as 85 dB. As the pump power increasing, the pulse energy and the spectral width increase, while the pulse width decreases. The highest pulse energy and lowest pulse width is 1.23 nJ and 63 ps, respectively. It is the first report of Raman dissipative soliton generation from an all polarization maintaining mode locked fiber laser to the best of our knowledge. This configuration provides a method to obtain linearly-polarized ultrafast laser at flexible wavelengths.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
In the past decades, ultrafast fiber lasers have attracted considerable attention due to their applications in the fields of fundamental research, biomedicine, and industry [1–3]. In the majority of ultrafast fiber lasers, rare earth doped fibers are used as gain medium, which can provide high gain. Nevertheless, the working wavelength regime is limited. By contrast, another type of ultrafast fiber lasers that use the nonlinear effect of stimulated Raman scattering (SRS) to provide gain, has the advantage of wavelength agility [4–6]. Ultrashort Raman pulses at flexible wavelengths can be obtained with appropriate pump laser and fiber components, which may extend the application fields of ultrafast fiber lasers.
Ultrafast Raman fiber lasers can be obtained with continuous wave (CW) pump. A variety of mode locking techniques for Raman fiber laser have been investigated, including passive methods utilizing intrinsic saturable absorbers like semiconductor saturable absorption mirror (SESAM) , graphene [8,9] and carbon nanotube [10,11], and equivalent saturable absorber like nonlinear optical loop mirror (NOLM) [4,12] and nonlinear polarization rotation (NPR) [13–15] etc., active one with an intra-cavity modulator  and hybrid mode locking . Among these techniques, mode locking with intrinsic saturable absorbers fails to have a desirable performance due to the long recovery time and the instantaneous response of the SRS effect. The NPR mode locking depends on polarization evolution in optical fiber, which is usually sensitive to environmental disturbance. Some recently studies show that NPR mode locking with an environmentally-stable all polarization maintaining (PM) fiber configuration is possible, but with precisely controlling fiber length and spliced angle [18,19]. NOLM mode locking, however, is compatible with environmentally-stable all-PM fiber configuration. Nevertheless, all-PM NOLM mode locking had not been reported previously for Raman fiber lasers. Soliton, stretched-pulse, self-similar and dissipative soliton (DS) regimes are major pulse-shaping mechanisms in ultrafast fiber laser. Among them, the DS regime that bases on dynamical balance in dispersion, nonlinearity, gain and loss is a suitable choice for mode locked Raman fiber laser for performance improvement. In a dissipative system, the solution of DS is a unique and fixed one that leads to a more stable state in the cavity . Therefore, the overall performance of ultrafast Raman fiber lasers may be improved by utilizing the pulse-shaping mechanism of DS. Raman dissipative soliton (RDS) was firstly reported in an ultrafast Raman laser mode-locked by nanotubes in 2011 . After that, RDS generation from mode locked Raman fiber lasers and synchronously pumped Raman fiber lasers were extensively studied [5,15,20,21].
Recently, we had proposed the use of amplified spontaneous emission (ASE) as pump source for performance improvement of mode locked Raman fiber lasers. Due to high power stability of the ASE source, highly stable RDS was obtained with a radio frequency (RF) signal to noise ratio (SNR) of 85 dB . However, NPR mode locking was applied in the study which was sensitive to environmental disturbance. Nonlinear optical loop mirror (NOLM) mode locking with all-PM Raman fiber lasers was investigated as well. Rectangular Raman pulse generation has been observed, in which the length of the NOLM was set to introduce a peak power clamping effect .
In this letter, we report a RDS fiber laser with an all PM cavity. The Raman laser is also mode locked by a NOLM and pumped by a CW ASE source. Here in order to bring in the DS mechanism, an all-fiber Lyot filter is inserted into the cavity. RDS pulses with a repetition rate of 1.23 MHz are obtained. RF SNR of the linear-polarization RDS pulses is also as high as 85 dB. In addition, the signal has a better performance than that in  without chaotic pedestal. As the pump power increasing, the pulse energy and the spectral width increase, while the pulse width decreases. The highest pulse energy and lowest pulse width is 1.23 nJ and 63 ps, respectively. It is the first report of environmentally-stable RDS generation from a mode locked fiber laser to the best of our knowledge. The configuration provides a new method to obtain linearly-polarized ultrafast laser at flexible wavelengths.
2. Experimental setup
The experimental setup of the RDS fiber laser is illustrated in Fig. 1. The Raman laser has a figure-8 cavity, which is formed by a unidirectional ring and a NOLM ring. In the unidirectional ring, an ASE source at 1064 nm with high temporal stability is used as the pump source to generate stable Raman laser. WDM1, WDM2 and WDM3 are all 1064/1120 nm wavelength division multiplexers (WDM), which are used to couple the pump laser into the cavity and remove the residual pump after the Raman gain medium. The Raman medium is a piece of PM Raman fiber (OFS Optics Inc.) in a length of 80 m, its Raman gain coefficient is estimated to be about five times larger than normal PM single-mode fiber like PM980 at the 1 μm band. A polarization dependent (PD) isolator is used to ensure unidirectional light propagation and also works as a polarizer. A PM coupler is used to extract the Raman laser from the cavity and the coupler is inserted between the WDM1 and the isolator to achieve the highest energy output from the cavity. The splitting ratio of the output coupler is 75: 25 and the 25% port is the output. Between the isolator and the output coupler, a short piece of PM980 fiber is inserted with a spliced angle of 45° at both ends, it works together with the PD isolator to act as an all-fiber Lyot filter. Another PM fiber coupler with a splitting ratio of 2:8 is adopted to connect the NOLM ring and the unidirectional ring. The length of the NOLM ring is about 77 m. Apart from the Raman fiber, the remaining cavity is linked by PM980 fiber. The length of the cavity is about 170 m. The total group velocity dispersion (GVD) in the figure-8 setup is about 3.5 ps2. The whole laser setup is in an all PM configuration.
3. Experimental results and discussion
In our previous demonstration of the RDS fiber laser mode locked by NPR , ASE source was for the first time used in mode locked Raman fiber laser for the improvement of pulse stability. The ASE source used here is an optimized one, in which a narrowband spectral filter (Bandwidth: 2nm) with a higher extinction ratio is used to improve the spectral SNR and then improve the temporal stability of output laser. Spectrum and intensity dynamic of the ASE source are shown in Fig. 2(a) and 2(b), respectively. Full wave half maximum (FWHM) bandwidth of the spectrum is about 2.5 nm. The spectrum has a shape of a flat-topped peak on a wide pedestal, which is resulted from the two stage spectral filtering. SNR of the flat-topped peak is about 35 dB, which is much larger than the previous value of 20 dB. Temporal measurement of the ASE source used in the NPR RDS fiber laser is shown in Fig. 2(c). It is obvious that the optimized ASE source has a better temporal stability. The intensity fluctuation of the ASE source is 1.2% in root-mean-square (RMS), while the RMS fluctuation of the previous one is 2.3%.
In the DS mode locking, spectral filtering is necessary for spectral and pulse shaping [23,24]. Due to the fact that mode locked Raman fiber laser has the greatest advantage of wavelength agility, a spectral filter that can be obtained at various wavelengths with easy implementation is the key to achieve high performance RDS ultrafast laser at flexible wavelengths. Here all-fiber Lyot filter that bases on the birefringence and dispersion of PM fiber is the best choice. In the laser setup, a short PM fiber segment is inserted before the isolator with a splicing angle of 45° at both ends. The overall transmission of the Lyot filter, T, can be expressed as cos2(πLΔn/λ) . In this equation, L is length of the PM fiber segment, Δn is the birefringence of the PM fiber and λ is the wavelength of the light. It can be found that the transmission spectrum is quasi-periodic with a free spectral range (FSR) given by Δλ~λ2/(LΔn) . Thus, the bandwidth of the Lyot filter is determined by the length of the PM fiber segment. In the Raman laser cavity, length of the angle-spliced PM fiber segment is 25 cm. Taking the birefringence Δn = 6.5 × 10−4 into consideration, the FSR of the Lyot filter is about 8 nm and corresponding bandwidth of the periodic passbands is about 4 nm.
The threshold of lasing and mode locking is reached at a pump power of 2 W. Once mode locking is achieved, the Raman laser works in a fundamental DS mode. The Raman laser has a strong pump hysteresis effect. After mode locking is activated, the pump power can be reduced to a low level while the laser still maintains DS mode locking. When the pump power reaches RDS laser with a pulse energy of 1.03 nJ can be obtained at a pump power of 1.83 W. Figure 3(a) plots the spectrum of the RDS pulses at this point, it was recorded by an optical spectrum analyzer (Yokogawa, AQ6370D) at a resolution of 0.02 nm. The RDS pulse has a spectral full wave half maximum (FWHM) bandwidth of about 2 nm. The central wavelength is located at 1114.8 nm, matching the peak of the Raman shift of 13.2 THz in silica well because the wavelength of pump laser is 1064 nm. 10 dB bandwidth of the RDS spectrum is about 6.56 nm, which means that majority energy of the RDS pulses is concentrated in this spectral range. The steep edge and tip on both sides like cat ear in spectrum are typical features for DS pulses. A spectrum with a wide range is shown in the inset of Fig. 3(a). It can be observed that the residual pump and the second stokes light are 30 dB less than the RDS laser.
Figure 3(b) shows a temporal measurement of the RDS laser. The pulse train was measured by an oscilloscope of 2.5 GHz bandwidth (Keysight, DSO-S 254A) and a Si-based detector with a bandwidth of 1.2 GHz (Thorlabs, DET02AFC). It is a typical mode locked laser pulse train with a pulse spacing of 810 ns corresponding to a repetition rate of 1.23 MHz, matching the cavity length of 170 m well. Due to the limited bandwidth of the oscilloscope and detector, the acquisition of pulse width requires further measurements, which is known as nonlinear autocorrelation measurement. Autocorrelation trace of the RDS pulses in measurement range of 150 ps and 300 ps are shown in Fig. 3(c) and 3(d), respectively. The signals are measured by a commercial autocorrelator (APE PulseCheck, SM1200). With the range of 150 ps, only the middle part of the signal was recorded in the autocorrelation measurement. But the trace indicates that the RDS pulse is a pure pulse without fluctuating and stochastic sub pulses. In the 300 ps scan, due to the scanning characteristic of the autocorrelator, only the right part of the signal can be recorded. However, because autocorrelation trace is symmetry in principle, the shape of the left part can be inferred from the right part. In this way, pulse width of the RDS laser can be calculated. Gaussian fits of the autocorrelation traces are also presented in Fig. 3(c) and 3(d). Both fits are perfect, indicates that the RDS pulse has a typical Gaussian shape in time domain. Pulse width of the RDS laser is estimated to be about 68.9 ps by dividing the width of autocorrelation signal with a Gaussian conversion factor of 1.41. It is a longish pulse, which should be ascribed to the long Raman laser cavity and all normal dispersion structure. DS pulses are usually compressible because they are considered to have linear chirp. According to the FWHM spectral width of the RDS pulse, the estimated spectrally-limited pulse duration is about 900 fs.
Radio frequency (RF) characteristics of the RDS pulses were analyzed with a 20 GHz RF spectrum analyzer (Keysight, N9020A) and the Si-based detector mentioned above. A RF spectrum of the RDS pulses around the pulse repetition rate at a resolution of 10 Hz is presented in Fig. 4(a). The SNR of the narrow spectral peak is as high as 85 dB, which is same with the RDS fiber laser with NPR . However, in the RF spectrum, no chaotic pedestal is located at the bottom of the peak and the baseline is flat, which is better than that of the NPR mode locked one. The improvement should be attributed to the optimized ASE source with higher temporal stability and all-PM configuration. Figure 4(b) presents a RF spectrum up to 14 MHz at a resolution of 100 Hz, it contains 10 combs which includes the fundamental and harmonics of the repetition rate. At high harmonics, the high SNR is maintained. Between any two adjacent comb teeth, a small tip can be observed, this may be caused by limited isolation of the isolator. A tiny fraction of the Raman laser could propagate in the unidirectional ring in a reverse direction and be outputted once every two round-trips. Thus, a fundamental repetition rate that is half of the main output can be observed.
Stable RDS pulse can be obtained in a pump power range from 1.8 W to 2.1 W. Beyond the value of 2.1 W, the Raman pulses become unstable and keep bouncing around different states. Pulse energy of the RDS laser is plotted against pump power in Fig. 5(a), which increases almost linearly with the pump power. The maximum pulse energy is 1.23 nJ at the pump power of 2.1 W. Output spectrum of the RDS laser at the pulse energy of 1.23 nJ is shown in Fig. 5(b). Compared with the spectrum of RDS pulse at low energy, typical features of DS are maintained at high energy apart from an obvious broadening at bottom, it should be caused by the limited modulation depth of the Lyot filter. Spectral width of the RDS pulses also increases together with the pump power, which is due to the strong nonlinear effect under high peak power. 10 dB bandwidth of the output spectrum is plotted against pump power in Fig. 5(c). The width also increases near linearly with the pump power and the maximum 10 dB width is about 6.85 nm. The increasing of spectral width also brings in temporal evolution. Pulse width of the RDS laser is plotted as a function of the pump power in Fig. 5(d). As the pump power increases, the pulse width keeps decreasing. This phenomenon conforms to the basic principle of mode locking, which is known as the wider the spectrum, the narrower the pulse width. The minimum pulse width is about 63 ps at the pump power of 2.1 W.
In conclusion, we have demonstrated an RDS mode locked fiber laser with an all PM cavity. The Raman laser is mode locked by a NOLM ring and pumped by a CW ASE source. An all-fiber Lyot filter is formed in the cavity to bring in the DS mechanism. Stable RDS pulses with typical features can be obtained at a repetition rate of 1.23 MHz. RF SNR of the RDS pulse is as high as 85 dB, which indicates an excellent performance of the RDS pulse train in temporal stability. The pulse energy and spectral width increase with the pump power while the pulse width is the opposite. The highest pulse energy is 1.23 nJ and the lowest pulse width is 63 ps. It is the first report of RDS generation from a NOLM mode locked fiber laser and an all PM mode locked fiber laser to the best of our knowledge. The present work not only offers a new method to obtain linearly-polarized ultrafast laser at flexible wavelengths, but also reveals how to obtain ultrashort Raman pulse train with high temporal stability.
National Natural Science Foundation of China (No. 61575210 and 61805262), and China Postdoctoral Science Foundation No. 2018M630474.
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