Cascaded Random Raman Fiber Laser With Low RIN and Wide Wavelength Tunability

Cascaded random Raman fiber lasers (CRRFLs) have been used as a new platform for designing high power and wavelength-agile laser sources. Recently, CRRFL pumped by ytterbium-doped random fiber laser (YRFL) has shown both high power output and low relative intensity noise (RIN). Here, by using a wavelength- and bandwidth-tunable point reflector in YRFL, we experimentally investigate the impacts of YRFL on the spectral and RIN properties of the CRRFL. We verify that the bandwidth of the point reflector in YRFL determines the bandwidth and temporal stability of YRFL. It is found that with an increase in the bandwidth of the point reflector in YRFL from 0.2nm to 1.4nm, CRRFL with higher spectral purity and lower RIN can be achieved due to better temporal stability of YRFL pump. By broadening the point reflector’s bandwidth to 1.4nm, the lasing power, spectral purity, and RIN of the 4th-order random lasing at 1349nm can reach 3.03W, 96.34%, and −115.19 dB/Hz, respectively. For comparison, the spectral purity and RIN of the 4th-order random lasing with the point reflector’s bandwidth of 0.2 nm are only 91.20% and −107.99dB/Hz, respectively. Also, we realize a wavelength widely tunable CRRFL pumped by a wavelength-tunable YRFL. This work provides a new platform for the development of ideal distributed Raman amplification pump sources based on CRRFLs with both good temporal stability and wide wavelength tunability, which is of great importance in applications of optical fiber communication and distributed sensing.

Particularly, cascaded random Raman fiber lasers (CRRFLs) can generate high power and wavelength agile lasing beyond rare earth emission bands [2,8,[29][30][31][32][33][34][35][36][37], which can find important applications in distributed Raman amplification of optical fiber communication and sensing systems. To improve the long-distance optical fiber communication and sensing systems performances, the low-noise distributed Raman pump sources are preferred. Pumped by tunable conventional ytterbium-doped fiber laser, continuously wavelength tuning covering 1 µm to 1.9 µm of CRRFL can be obtained [32]. However, with longitudinal mode beating, temporal intensity fluctuations of such a CRRFL are severe. With the help of the high power and low-noise amplified spontaneous emission (ASE) of ytterbium-doped fiber (YDF) as the pump source, the spectral purity and temporal stability of the CRRFL can be significantly enhanced [29][30][31][32][33]. It is also found that with the broader filtering bandwidth of the ASE source, higher spectral purity and temporal stability of the CRRFL can be realized [34]. However, wavelength tuning range of filtered ASE pump is limited due to low filtered power of ASE source with filtering wavelength beyond 1 080 nm, which further limits wavelength tuning range of CRRFL [35]. Recently, CRRFLs pumped by modeless ytterbium-doped random fiber lasers (YRFLs) have been proposed with high power and low relative intensity noise (RIN) output in a simple structure [36,37]. Compared to conventional ytterbium-doped fiber laser and ytterbium-doped filtered ASE source, YRFL can act as a pump source for the CRRFL featuring both low-noise and wide wavelength tunability [38]. However, impacts of the YRFL pump characteristics on spectral and RIN properties of CRRFLs are still under investigation.
In this paper, we experimentally demonstrate a CRRFL with wavelength-and bandwidth-tunable YRFL pump. The influences of the point reflector's bandwidth on the bandwidth and temporal stability of the YRFL pump are first investigated, which plays an important role in the spectral and RIN properties of the CRRFL. The experimental results show that with an increase in the bandwidth of the point reflector in the YRFL from 0.2 nm to 1.4 nm, the CRRFL can have higher spectral purity and lower RIN. Furthermore, a wavelength continuously tunable CRRFL with a tunable YRFL pump is demonstrated. This work shows that the CRRFL with both good wavelength tunability and temporal stability can be achieved with the tunable broadband random lasing pump, which provides a new kind of distributed Raman amplification pump source for long-distance optical fiber sensing.

Experimental setup
The schematic diagram of the proposed tunable YRFL pumped CRRFL is shown in Fig. 1. For the tunable YRFL seed, as shown in the dash box of The power of the YRFL is further boosted in the master oscillator power amplifier (MOPA) stage which is consisted of another LD and a 10-m-long YDF. The amplified YRFL is used as the pump source for the CRRFL and is injected into a wavelength division multiplexer (WDM) (pass port: 1 040 nm-1 095 nm, reflection port: 1 105 nm-1 700 nm) through another isolator (ISO). The reflection port of the WDM is connected to another 1:1 coupler based wideband fiber loop mirror, providing point feedback for cascaded random Raman lasing. The common port of the WDM is spliced with a 4-km-long dispersion shifted fiber (DSF) to provide the Raman gain and random distributed Rayleigh backscattering. To generate stable 1.3 μm 4th-order random Raman lasing and avoid the unwanted nonlinear effect, the DSF with zero dispersion wavelength of 1 499 nm is used here rather than the standard single-mode fiber with zero dispersion wavelength of about 1 310 nm. In this way, the CRRFL is realized in a forward-pumped scheme, and the spectral and RIN properties of the CRRFL can be tailored by tuning the wavelength and bandwidth of the point reflector in the YRFL.

Lasing properties of the YRFL seed
First, the influence of the point reflector's bandwidth on the spectral property of the YRFL seed is investigated by tuning the bandwidth of the filter. Figure 2(a) displays the bandwidth-tunable spectra of the YRFL seeds according to the point reflector's bandwidths. The power of the YRFL seed is 0.63 W with the LD power of 4 W. The reflection bandwidth of the point reflector, which is determined by the bandwidth tunable filter, in the forward-pumped structure allows the selective gain only for the YRFL radiation [39,40]. Thus, the bandwidth of the ytterbium-doped random lasing could be changed according to the bandwidth of the point reflector. It can be observed that the -3 dB bandwidths of the 1 085.5 nm YRFL seeds are broadened with an increase in the point reflector's bandwidth. As shown in Fig. 2(b), the -3 dB bandwidths of the YRFL seeds with 0.2 nm, 0.4 nm, 0.6 nm, 0.8 nm, 1.0 nm, and 1.4 nm of point reflector's bandwidths are 0.23 nm, 0.28 nm, 0.35 nm, 0.50 nm, 0.53 nm, and 0.65 nm, respectively. Further increasing the point reflector's bandwidth to 2 nm would result in an unstable random lasing spectrum with multi-peaks structure. This is mainly due to the existence of significant ripples in the transmission spectrum of the filter when the filtering bandwidth is beyond 2 nm. Further work can be done to realize a broader YRFL by using a ripple-free bandpass filter with the broader filtering bandwidth. The temporal characteristics of the YRFL seeds versus different point reflector's bandwidths are measured by a photodetector with a 400 MHz bandwidth and an oscilloscope with a 2 GHz bandwidth. It can be seen in Fig. 3(a) [41,42], which will be carried out in the future.

Temporal stability evolution of the amplified YRFL
We further study the temporal stability evolution of the amplified YRFL after the isolator in the MOPA stage. The measured optical conversion efficiency from the LD power to the amplified YRFL power after the isolator is 70.9%. The bandwidth of the point reflector in the YRFL seed is fixed as 1.4 nm. As presented in Figs. 4(a) and 4(b), the STD/mean and RIN of the amplified YRFL at 18 W of the LD power are only increased by 0.81% and 0.79 dB, respectively, which confirms that the temporal stability of the amplified YRFL only deteriorates slightly in the MOPA stage.

Characterization of CRRFL pumped by random lasing
The amplified YRFL is used as the pump source for the CRRFL. By fixing the point reflector's bandwidth as 1 nm in the YRFL, the evolution of cascaded random Raman lasing is studied in this section. The spectra of the 1st-to 4th-order random lasings are illustrated in Fig. 5(a), with the amplified YRFL pump power, which is measured after the WDM with 0.80 dB insertion loss of 2.62 W, 4.10 W, 6.07 W, and 9.03 W, respectively. With an increase in the YRFL pump power, the 1 141 nm 1st-order random lasing generates at first with an unstable behavior. Plenty of random spikes can be observed in the lasing spectrum, which could be attributed to the generation of narrow-band spectral components and the cascaded stimulated Brillouin scattering [1,2]. By further increasing the pump power, the 2ndto 4th-order random lasings with smooth and stable spectra can be stimulated successively.   5(b), corresponding to the left column of the spectra. It can be seen that, for the unstable 1st-order random lasing, it shows a pulsed scenario in time domain as well. The time domain traces of the 2ndto 4th-order random lasings show quasi-continuous wave behaviors, and the STD/mean value increases with the order of Raman lasing.
The output power evolution of the 1st-to 4th-order random lasings is depicted in Fig. 5(c). The threshold lasing powers of the 1st-to 4th-order random lasings are 1.14 W, 2.62 W, 4.10 W, and 6.07 W, respectively. As a result, the maximum output power of the 4th-order random lasing reaches 3.03 W at the pump power of 9.03 W, corresponding to an optical conversion efficiency of 33.6%.

Influence of YRFL pump characteristics on 4th-order CRRFL
The influences of the YRFL pump on the 4th-order CRRFL are also investigated. Firstly, the spectral purities of the 4th-order random lasing, which is defined as the ratio of the output of the 4th-order random lasing to the total output, as a function of point reflector's bandwidths in the YRFL are measured and plotted in Fig. 6(a). The spectral purity of the 4th-order random lasing pumped by the YRFL with 0.2 nm of point reflector's bandwidth is only 91.20%. While, by employing a relatively broadband YRFL pump with 1.4 nm of point reflector's bandwidth, the spectral purity of the 4th-order random lasing can be improved to 96.34%. The higher spectral purity of random lasing could be associated with the lower RIN of the YRFL pump [35]. The influence of the YRFL pump on the temporal stability of the 4th-order random lasing is also measured and presented in Fig. 6(b). It can be seen that with the broader YRFL pump, the temporal stability of the 4th-order random lasing can be enhanced. This is due to the temporal intensity fluctuation of the pump is transferred directly to the CRRFLs by the ultra-fast responding process of the stimulated Raman scattering effect [43]. As a result, the time domain STD/mean value of the 4th-order random lasing decreased from 20.74% to 10.81% by increasing the bandwidth of the point reflector in the YRFL from 0.2 nm to 1.4 nm. Meanwhile, the RIN spectra of the 4th-order random lasing in Fig. 6(c) also verify that the RIN of the 4th-order random lasing can be decreased by 7.2 dB with the broader YRFL pump. The results illustrated in Fig. 6 show that the YRFL pump bandwidth has a significant effect on the lasing performances of the CRRFL, due to the different temporal stability behaviors of the YRFLs under different bandwidths.

Tunable wavelength emissions of the CRRFL
The tunability of the CRRFL is tested as well, by adjusting the central wavelength of the filter used in the YRFL. By fixing the bandwidth as 1.4 nm and changing the central wavelengths from 1 055 nm to 1 095 nm of the point reflector, the lasing wavelength of the YRFL is tuned accordingly and the normalized spectra are shown in Fig. 7(a). Compared to the tunable filtered ASE pump [35], the wavelength tuning range of the proposed YRFL is wider. Thus, the wavelength tuning ranges of each order random lasing in the CRRFL can also be broadened. Since the 1st-order random lasing is unstable, we record the normalized tunable spectra of the 2nd-to 4th-order random lasings in Figs. 7(b), 7(c), and 7(d), indicating the tuning ranges of 1 169 nm -1 219 nm, 1 233 nm -1 287 nm, and 1 304 nm-1 364 nm, respectively. The left peaks of several random lasing spectrum in the Fig. 7(b) is caused by another Raman gain maxima. Further decreasing the YRFL wavelength below 1 055 nm, the wavelength of the 1st-order random lasing falls outside the wavelength range of the reflection port of the WDM (1 105 nm -1 700 nm). It is thus possible to further broaden the wavelength tuning range of the CRRFL and realize gap-free wavelength tuning by using the more suitable WDM [32]. Therefore, cascaded random Raman lasing with the good temporal stability and specific wavelength can be simultaneously obtained by tuning the bandwidth and the wavelength of the YRFL pump.

Conclusions
In summary, we experimentally investigate the impact of the YRFL on the spectral purity and temporal stability of the CRRFL by using a wavelength-and bandwidth-tunable point reflector, for the first time. It is shown that the broader bandwidth of the point reflector in the YRFL leads to the better temporal stability of the YRFL, thus resulting in higher spectral purity and lower RIN of the CRRFL. As a result, with the point reflector's bandwidth of 1.4 nm, a 4th-order CRRFL at 1 349 nm with the output power of 3.03 W, spectral purity of 96.34%, and RIN of -115.19 dB/Hz is realized, which is superior to that with 0.2 nm point reflector's bandwidth. Moreover, the wide tunability of the proposed CRRFL is also verified experimentally by continuously tuning the YRFL pump wavelength from 1 055 nm to 1 095 nm. This work indicates that both the wide wavelength tunability and good temporal stability of the CRRFL can be simultaneously achieved with the broadband random lasing pump, providing a way to further improve distributed Raman amplification pump sources performances by adopting CRRFLs based on the YRFL pump with the broader bandwidth.