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Abstract
Multiwavelength Brillouin-Raman fiber laser (MBRFL) features broadband multiwavelength generation with flat-amplitude and high optical signal to noise ratio (OSNR), which has great potential in optical fiber communication applications. Till now, the spectral regions of MBRFLs are mostly concentrated at conventional C- and L-band and the tunability of MBRFL is limited by using the Raman pump with fixed wavelength. Here, by utilizing wavelength-agile random fiber laser which can emit tunable lasing at 1.2 µm band as the Raman pump, we experimentally demonstrate the tunable MBRFL in the O-band for the first time, to the best of our knowledge. At Raman and Brillouin pump powers of 920 mW and -3 dBm, respectively, up to 90 Stokes lines with 0.13 nm wavelength spacing and >13 dB OSNR can be obtained when the Raman and Brillouin pump wavelength are set at 1231 nm and 1300 nm, respectively. Moreover, by tuning the wavelength of Brillouin pump from 1295 nm to 1330 nm, tunable MBRFL can be achieved with similar multiwavelength generation bandwidth by simultaneously tuning the Raman pump wavelength, and the number of Stokes lines are beyond 85 across the tuning range. The bandwidth of the demonstrated O-band MBRFL is also the widest wavelength span ever reported for multiwavelength Brillouin fiber lasers at 1.3 µm band. Our work indicates that the use of wavelength-agile random fiber laser as Raman pump in MBRFL can provide an effective way to extend the spectral regions of MBRFL and also improve the tunability performance of MBRFL.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Multiwavelength fiber lasers have made tremendous advances over the past decades due to their important applications in wavelength division multiplexing (WDM) systems, microwave generation, gas spectroscopy and optical fiber sensors [1–3]. The “passive” ways to realize multiwavelength fiber lasers are incorporating various comb filters inside the laser cavity, such as Fabry–Perot filter, Mach–Zehnder interferometer, Sagnac fiber loop filter and Lyot filter [4–7]. Another “active” way to generate multiwavelength fiber lasing is utilizing stimulated Brillouin scattering (SBS) as the gain mechanism, and the wavelength spacing is determined by the Brillouin frequency shift and cavity configuration [8–11]. To increase the number of generated Brillouin combs, the combinations of rare-earth ions doped gain or Raman gain with Brillouin gain are commonly used [12–16]. Among these, multiwavelength Brillouin-Raman fiber laser (MBRFL) show significant advantages in terms of ultrabroad multiwavelength bandwidth and excellent flatness [17–20].
During the past few decades, MBRFLs operating in the conventional C-band fiber transmission window have concentrated the most substantial interest and now well-developed [17–20]. In principle, by adopting the hybrid Brillouin-Raman gain, the MBRFLs can be realized at arbitrary wavelengths in the transparent window of the used passive fibers. MBRFLs have been reported to work in the S-band [21], L-band [22], and U-band [23] by using different Raman pump sources. However, the Raman pump wavelengths are fixed in the demonstrated MBRFLs, which set the limitations on the spectral coverage and tunability of MBRFLs. Although tunable MBRFLs can be realized by tuning the Brillouin pump wavelength at the fixed Raman pump wavelength, the tuning ranges are limited and the multiwavelength generation bandwidth would experience significant deterioration during wavelength tuning [24,25]. On the other hand, the multiwavelength fiber lasers in the O-band (1260-1360 nm) are also highly demanded to further extend the capacity of current communication and sensing networks [26]. Multiwavelength Brillouin fiber lasers in the O-band have been reported by using the combination of semiconductor optical amplifier (SOA) [27], bismuth-doped gain [14] or praseodymium fiber amplifier [28] with Brillouin gain. However, the multiwavelength spans of multiwavelength Brillouin fiber lasers in O-band are still below 2 nm, and MBRFL has not been experimentally demonstrated in O-band yet. Therefore, the utilization of wavelength-agile Raman pump in MBRFL could be essential to further extend the wavelength coverage and tunability performance of MBRFL.
Recently, random fiber lasers (RFLs) utilizing stimulated Raman scattering (SRS) as the gain mechanism and Rayleigh scattering along long single mode fiber (SMF) as random distributed feedback become promising wavelength versatile laser sources with high power output [29,30]. With tunable pump and cascaded SRS process, nearly-octave wavelength tunable RFLs covering 1.1-2 µm region have been realized [31–33]. Such wavelength-agile RFLs have found various applications in nonlinear frequency conversions [34], mid-infrared fiber laser pump [35] and optical sensing [36]. Compared to the conventional tunable fiber lasers, tunable RFLs not only have the superior performances in terms of ultra-wide tuning range, high output power and excellent temporal intensity stability [37,38], but also can be constructed in much simpler cavity configurations. Therefore, the wavelength-agile RFLs have great potential as universal Raman pumps in MBRFLs to further extend the spectral regions of MBRFL and improve the tunability performance of MBRFLs.
In this work, a tunable MBRFL in the O-band with broad flat-amplitude bandwidth is proposed and experimentally demonstrated for the first time, based on a wavelength-agile random fiber laser which can emit tunable lasing at 1.2 µm band as the Raman pump. Moreover, by tuning the wavelength of Brillouin pump from 1295 nm to 1330 nm, tunable MBRFL can be achieved with similar multiwavelength generation bandwidth by simultaneously tuning the Raman pump wavelength, and the number of Stokes lines are beyond 80 across the tuning range. To the best of our knowledge, it is the widest wavelength span ever reported for multiwavelength Brillouin fiber lasers at 1.3 µm band.
2. Experimental setup and principle
The schematic experimental setup for proposed tunable MBRFL is depicted in Fig. 1. The tunable MBRFL consists of a tunable Raman pump (RP), a Brillouin pump (BP), and a 10 km-long dispersion compensation fiber (DCF) as the hybrid Brillouin-Raman gain medium. A narrow width laser (Santec TSL-550, 500 kHz of linewidth) with a tunable wavelength range of 1260 nm-1360 nm serves as the BP. The BP is injected into the DCF through an optical circulator and a 1240/1310 nm WDM (Pass port:1290-1350 nm, Reflection port:1220-1260 nm). The tunable RP source is a home-made Raman random fiber laser (RRFL) whose detailed configuration is presented in the dashed box of Fig. 1. The RRFL consists of an ytterbium-doped random fiber laser (YRFL) seed and a cascaded random Raman fiber lasing (RRFL) generation section. In the section of YRFL, A wavelength tunable filter with 0.1 nm -3 dB bandwidth is integrated into a 1:1 coupler-based fiber loop mirror to provide the wavelength-selectable point feedback for the ytterbium-doped random fiber lasing. By combing with a 5 km-long single mode fiber (SMF) and 5 m-long ytterbium-doped fiber (YDF), an ytterbium-doped random fiber laser (YRFL) seed with a continuous wavelength tuning range of 1040 nm-1090 nm can be realized. Subsequently, the YRFL seed with 500 mW optical power is injected into the cascaded RRFL through an optical isolator. In the section of cascaded RRFL, the YRFL seed is injected into another 6 m-long YDF pumped by another 976 nm laser diode through a 1:1 coupler 2 and the signal port of the pump combiner to boost its optical power. Then the amplified YRFL is used as the direct pump to stimulate the cascaded random Raman lasing in a 4 km-long dispersion shifted fiber (DSF). To provide the broadband point feedback for the cascaded random Raman lasing, a coupler-based fiber loop mirror is connected at the other input port of the 1:1 coupler 2. In this way, by increasing the power of amplified YRFL seed, cascaded random Raman fiber lasing can be generated successively. As a result, by tuning the wavelength of YRFL seed, a tunable 3rd-order RRFL of 1210 nm-1270 nm can be realized and a filtered WDM with the cut-off wavelength of 1200 nm (Pass port:1000-1180 nm, Reflection port:1210-1600 nm) is used to extract the 3rd-order random Raman lasing as RP for MBRFL. The RP is injected into the 10 km-long DCF through an isolator and the 1240/1310 nm WDM, and the end of the DCF serves as the output port of MBRFL, the spectrum of the MBRFL is monitored by an optical spectrum analyzer (OSA) with a resolution of 0.02 nm.
Fig. 1. Schematic experimental setup for MBRFL. FBG, fiber Bragg grating; WDM, wavelength division multiplexer; DCF, dispersion compensation fiber; LD, laser diode; Com, combiner; YDF, ytterbium-doped fiber; SMF, single-mode fiber; ISO, isolator; DSF, dispersion shifted fiber; DCF, dispersion compensation fiber.
The working principle of the MBRFL is as follows: The RP can provide distributed Raman amplification along the DCF. The BP source is amplified through the distributed Raman amplification, and once the amplified BP power reaches the SBS threshold, the 1st-order Brillouin Stokes light is initiated with the opposite propagation direction of the BP source. Similarly, the 1st-order Brillouin Stokes light can also be amplified by distributed Raman amplification and acts as a new BP to stimulate the 2nd-order Brillouin Stokes light, which propagates in the opposite direction of the 1st-order Stokes light. In this way, with the assistance of distributed Raman amplification, cascaded SBS process in DCF can result in the generation of multiwavelength Brillouin-Raman fiber lasing, and the spectrum at the output would consist of BP source and the even order of the Brillouin Stokes lines with the channel spacing corresponding to the double Brillouin frequency shift.
3. Experimental results and discussions
By increasing the power of LD 2 and tuning the wavelength of YRFL seed, tunable 1st-order random lasing in the range of 1095-1150 nm, 2nd-order random lasing in the range of 1150-1210 nm and 3rd-order random lasing in the range of 1210-1270 nm can be successively generated. To realize O-band MBRFL, 3rd-order random lasing from the proposed RRFL is used as the Raman pump, and Fig. 2 presents the output characteristics of the proposed tunable Raman pump. The power of LD 2 is set as 14W. The wavelength tunability of 3rd-order random Raman lasing is shown in Fig. 2(a), which verifies the central wavelength of 3rd-order random Raman lasing can be continuously tuned from 1210 nm to 1270 nm, and the -3 dB bandwidth are in the range of 1.5 nm-2.2 nm. Figure 2(b) shows the output powers as a function of 3rd-order random Raman lasing wavelengths. The output power gradually increases from 2.25 W to 2.5 W when the central wavelength increases from 1210 nm to 1270 nm, which is mainly attributed to the decrease in the fiber loss. It is worth noting that the proposed Raman pump based on the random fiber laser can produce continuous tunable random laser from 1 µm to 1.5 µm. Specifically, the YRFL seed with a spectral tuning range of 1040 -1090 nm can be realized by adjusting the central wavelength of the tunable filter. By increasing the power of amplified YRFL, and simultaneously changing wavelength of YRFL seed, the cascaded random Raman fiber lasing with tunable wavelength can be generated successively. The spectral tuning range of 1st to 5th order Raman laser are 1100 nm-1150 nm, 1150 nm-1210 nm, 1210 nm-1270 nm, 1270 nm-1350 nm, 1350 nm-1430 nm, respectively. The output power of specific order of random Raman fiber lasing can be designed by adjusting DSF length [30–32]. Therefore, the proposed RRFL features wavelength agility and watt-level output power can be flexibly designed as the universal Raman pump for MBRFL, and as long as there is a matched BP wavelength, the MBRFL based on the proposed Raman pump can be generated from 1.1µm to C-band.
Fig. 2. (a) The tunable spectra of 3rd-order random Raman lasing; (b) The output powers as a function of 3rd-order random Raman lasing wavelengths.
The spectrum evolution of the MBRFL at different RP powers is shown in Fig. 3. The BP wavelength is fixed at 1300 nm and the BP power is -3 dBm. As shown in Fig. 3, the power of Raman pump needs to be sufficient so that efficient Raman gain can be provided to amplify the Brillouin pump and the cascaded Stokes combs. Without RP, only the BP component can be observed at the output. By gradually increasing the RP power, the power of BP is increased and cascaded SBS process can be stimulated. When the RP power is set to 478 mW, the first even-order Stokes line appears at the output of MBRFL. Further boosting the power of RP, the number of even-order Stokes lines continues to raise, and when the RP power increases to 920 mW, the peak powers of the higher-order Stokes lines reach almost the same saturation level of approximately -12 dBm. However, it should be noted that when the RP power is too high, the existence of mode competition between the Brillouin comb and the Raman pump excited free-running lasing modes at the Raman peak gain could deteriorate the OSNR of Brillouin comb and finally the self-excited Raman lasing oscillation would dominate the spectrum [23,39].
Figure 4 shows the spectrum of MBRFL with the widest flat-amplitude bandwidth when the BP wavelength is fixed at 1300 nm. The optimized BP power is -3dBm, while power and wavelength of RP are optimized as 920 mW and 1231 nm according to the experimental results. The wavelength of RP should be optimized to make sure the BP wavelength is close to but shorter than the peak of Raman gain profile, so that efficient Raman gain can be provided for the Brillouin pump and the cascaded Brillouin Stokes lines at the longer wavelength range [13,23]. As shown in Fig. 4(a), up to 90 Stokes lines with 0.13 nm wavelength spacing are generated. The channel spacing of the MBRFL is determined by the value of Brillouin frequency shift, since the value of Brillouin frequency shift is depended on the pump wavelength and the refractive index of the fiber at pump wavelength, the Stokes spacing is not equidistant by nature, however, the differences of the channel spacings in the MBRFL with 10 nm bandwidth are quite small (at the level of 0.001 nm).The flat-amplitude bandwidth is more than 11 nm containing a total number of 85 Stokes lines within 3 dB spectral flatness, and the average individual line peak power is about -12 dBm. Figure 4(b) and (c) provide the corresponding magnified views of the output spectrum that were taken at different wavelength regions of MBRFL. In both two cases, the Brillouin combs exhibit excellent peak power flatness and the >13 dB OSNR. Additionally, the components of odd-order Stokes lines are well suppressed at the output of MBRFL. Compared to the previously reported multiwavelength Brillouin fiber laser in the O-band assisted by SOA or rare-earth ions doped gain [14,27,28], the demonstrated O-band MBRFL shows great enhancement on the multiwavelength bandwidth and the spectral flatness.
Fig. 4. Optimized flat-amplitude MBRFL spectrum when BP wavelength is 1300 nm; (a) The full spectrum; (b) The magnified span of the Stokes lines in 1303-1304 nm; (c) The magnified span of the Stokes lines in 1306-1307 nm.
Compared to the MBRFL at C-, L- and U-band [17–19,23], the spectral performances of the demonstrated O-band MBRFL in terms of flat-amplitude bandwidth and OSNR are still poorer. Since the spectral performance of MBRFL is highly depended on the complex interactions among SBS, Raman gain and fiber loss, the variations of Brillouin/Raman gain coefficients and fiber loss coefficients at different wavelength bands can result in the different spectral performances of MBRFLs. We have measured the fiber loss coefficients of DCF at 1550 nm and 1350 nm are 0.56 dB/km and 1.21 dB/km, respectively, which may contribute to the reduction of MBRFL bandwidth at O-band. The influences of fiber parameters on the spectral performance of MBRFL need to be thoroughly studied in the future. It should also be noted that by using multiple wavelength Raman pump [17], more sophisticated cavity designs [18,24] or pump schemes [19], the flat bandwidth of the O-band MBRFL can be further broadened.
The stability measurement of the proposed MBRFL is recorded and depicted in Fig. 5. The spectra of Stokes lines in the span of 1305 nm to 1306 nm are recorded at every 2 minutes interval for a period of 20 minutes. Figure 5(a) shows the stable spectra with no observable signs of wavelength shifts and power fluctuations. The detailed analysis of peak power variations for the corresponding Stokes lines are show in Fig. 5(b), which confirms that the maximum power fluctuation of the Stokes lines is about 0.48 dB. As various kinds of physical processes including stimulated Brillouin scattering, stimulated Raman scattering and Rayleigh scattering are interacted in MBRFL, the temporal dynamics and noise properties of MBRFL could be complex and important to be explored. Although the temporal dynamics and noise properties of multiwavelength Brillouin fiber laser and multi-wavelength Brillouin–erbium fiber laser have been reported [8,40], such characteristics are rarely reported for MBRFL. Fully understanding the temporal dynamics and noise properties of MBRFL could be an important future direction which needs both the thorough theoretical and experimental investigations.
Fig. 5. Stability measurement of the MBRFL: (a) optical spectra of MBRFL over 20 mins and (b) power fluctuations of different Stokes lines over 20 mins.
Since the BP power is a crucial factor that affects the generated number of Stokes lines, the influence of BP power on the performance of MBRFL is studied and the results are depicted in Fig. 6. The RP wavelength is kept at 1231 nm with 920 mW output power, and the BP wavelength is 1330 nm. It reveals that the optimal BP power is in the range of -4 dBm to 3 dBm, and up to 90 even Stokes lines with 0.13 nm wavelength spacing are obtained. Further increasing the BP power from 3 dBm to 7 dBm, the number of obtained Brillouin Stokes lines decreases from 90 to 80. The decrease of the number of Brillouin combs when BP power is too high may be the results of the interaction between SBS and SRS inside the cavity, in the case of too high BP power, the power of generated Brillouin comb with decreased number of Stokes light can exceeds the linear region of the Raman amplifier and the Raman gain reaches saturation. On the other hand, when the BP power is too low (-11 dBm), only a few of Brillouin Stokes lines can be generated.
We further investigate the tunability performance of MBRFL with different BP wavelength, while fixing the RP wavelength as 1231 nm, and the power of BP and RP are -3 dBm and 920 mW, respectively. The spectra of tunable MBRFL with fixed wavelength RP are shown in Fig. 7. The results indicate that the number of Brillouin Stokes channels of MBRFL dramatically changes with BP wavelength. When the BP wavelength is 1295 nm or 1308 nm, only a few Stokes lines can be generated due to a large deviation of the BP wavelength and Raman gain peak region. At the BP wavelength of 1297 nm, MBRFL with a number of 81 Stokes lines is produced. The optimum BP wavelength is found at 1300 nm with 90 Stokes lines covering 1300.13 nm to 1311.83 nm. The number of Stokes lines decrease gradually when the BP wavelength is increased from 1300 nm to1306 nm. Therefore, we can conclude that although tunable MBRFL can be realized at the fixed Raman pump wavelength, the multiwavelength generation bandwidth would experience significant deterioration during BP wavelength tuning, and the tuning range is limited in this case (1297-1306 nm).
Fig. 7. Spectra of tunable MBRFL with different Brillouin Pump wavelengths while fixing the Raman pump wavelength.
Finally, we demonstrate the remarkable improvement on the tunability performance of MBRFL by simultaneously tuning the wavelengths of Brillouin and Raman pump. In contrary to the existence of MBRFL bandwidth deterioration shown in Fig. 7, the results in Fig. 8 show that by tuning the wavelength of BP from 1295 nm to 1330 nm, tunable MBRFL can be achieved with similar multiwavelength generation bandwidth by simultaneously tuning the RP wavelength. Table 1 lists the optimized RP wavelengths and the wavelength spans of the generated Brillouin combs at different BP wavelengths. We can see that the number of Stokes lines are beyond 85 across the whole tuning range, and the maximum multiwavelength spectral range is 14.82 nm with 114 Stokes lines when the BP wavelength is 1295 nm. It should be noted that the demonstrated tuning range of MBRFL is mainly limited by the passport spectral range of the WDM (1290-1350 nm) and can be further extended in principle.
Fig. 8. Spectra of tunable MBRFL by simultaneously tuning the wavelengths of Brillouin and Raman pump.
Table 1. Wavelength tunable performance of the MBRFL.
4. Conclusions
In summary, we experimentally demonstrate the generation of tunable flat-amplitude MBRFL operating in the O-band by utilizing wavelength-agile random fiber laser as Raman pump for the first time, to the best of our knowledge. At Raman and Brillouin pump powers of 920 mW and -3 dBm, respectively, up to 90 Stokes lines with 0.13 nm wavelength spacing and >13 dB OSNR can be obtained when the Raman and Brillouin pump wavelength are set at 1231 nm and 1300 nm, respectively. The effects of wavelength and output power of BP on the performance of MBRFL are also investigated. Furthermore, we experimentally demonstrated the remarkable improvement on the tunability performance of MBRFL by simultaneously tuning the wavelengths of Brillouin and Raman pump. By varying the wavelength of BP from 1295 nm to 1330 nm and simultaneously tuning the RP wavelength, tunable MBRFL covering 1295.13 nm to 1341.18 nm with similar multiwavelength generation bandwidth is achieved, and the number of Stokes lines are beyond 85 across this range. The bandwidth of the demonstrated O-band MBRFL is also the widest wavelength span ever reported for multiwavelength Brillouin fiber lasers at 1.3 µm region. The proposed tunable flat-amplitude MBRFL by utilizing the wavelength-agile random fiber laser as Raman pump can provide an effective way of extending the operation wavelength region of MBRFL with excellent wavelength tuning performance.
Funding
National Natural Science Foundation of China (62005186, 62375189).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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