1. Introduction

Multi-wavelength fiber lasers are largely exploited in optical communication, sensor, and component characterization [1–3]. Among different types of fiber lasers [4–9], the Brillouin-Raman fiber lasers (BRFLs) have attracted much attention due to their promising features, such as a broad gain bandwidth, and stable multi-wavelength operation at room temperature. There have been numerous successful examples of MBRFL generation with a single (10 GHz) frequency spacing [5,10–16]. In these hybrid BRFLs, Rayleigh scattering (RS) is a crucial for inducing stimulated Brillouin scattering (SBS) and the generation of Stoke lines is a result of the combination of SBS, RS, and stimulated Raman scattering (SRS) in the presence of high Raman pump power (RPP) [10,11,12]. In [12], output RPP of higher than 10W was required to achieve 798 Brillouin Stoke lines (BSLs) at 3-dB flatness over 61.65 nm wavelength range. However, this method complicates the fiber laser design and increases the cost. Several challenges have been carried out to optimize the performances of 10 GHz spacing MBRFL especially number of channels [14,15] and flat bandwidth [12–16]. Other important characteristics of concern are OSNR [16–18] and channel spacing [19–26]. Nevertheless, one of the major challenges is to achieve uniform 10 GHz spacing Stoke lines over a wider bandwidth domain with outstanding OSNR [16–18,27]. Interestingly, half-open or full linear cavities using highly reflective mirrors at one or both ends of the laser cavity have also been revealed. In these approaches, 210–500 Stoke lines with OSNRs that ranges from 12.5 to 18 dB [9,11,14,15,19] are generated. Single spacing multi-wavelength outputs can also be attained by proper adjustment of the coupling ratio in a nonlinear amplifying loop mirror (NALM) based on cavity [19]. In their setup, a 50/50 NALM configuration generates 443 channels with 16.5 dB OSNR. Concurrently, there have been numerous successful methods for MBRFL with 20 GHz spacing, such as optimizing Raman pumping ratios using various couplers have also been studied [25]. In this scheme, the 50/50 coupler offers 212 channels with around 28 dB OSNR. A mirrorless open cavity was experimentally demonstrated in which the reflectivity of virtual mirrors at both sides of the laser configuration results in the generation of a number of Stoke lines. In our earlier work, 195 lines at 20 GHz spacing were obtained by combining a forward Raman pumping together with 11 km long dispersion compensating fiber (DCF) in a cavity with a 30 cm long Bismuth-oxide Erbium-doped fibre [22]. Recently, some other investigations have been developed to improve the performance of MBRFLs with switchable wavelength spacing [19,27–30]. However, the aforementioned methods for 10 GHz, 20 GHz, and switchable wavelength spacing require separate and different cavity configurations, making the overall cavity complex. In addition, apart from complicated structures, the output characteristics were also does not satisfy all requirements for generating 10/20 GHz spacing MBRFL with desirable performances. In this standpoint, and with a focus on the development of a cost and energy-efficient Raman pump source, we experimentally demonstrate in this paper, the generation of the MBRFL with switchable wavelength spacing through tremendous simple design. This is achieved by employing a bi-directional Raman pumping structure together with incorporation a physical mirror at one side of the cavity. The discussion is initiated by explaining the light propagation behaviors at different sides of the cavity arrangement. With respects to other earlier reports [5–30], the main achievement of this research work is to introduce novel improvements to simplicity, flatness, Stokes-OSNR (S-OSNR), spectrum bandwidth, and SPP for both wavelengths spacing.

2. Experimental setup

The structure of the switchable MBRFL is presented in Fig. 1. The Brillouin pump (BP) source is provided by a tunable laser source with maximum output power of 12 dBm. The laser cavity is comprised of one 4-port optical circulator (OC1) and a 3-port optical circulator (OC2). The BP signal is sent through port 1 of 4-port OC1 to 12 km DCF as the nonlinear gain medium. Furthermore, a Raman pump unit (RPU) that has a maximum power of 1200 mW and operates at 1455 nm wavelength regime is also employed. Additionally, dissimilar length of DCFs are used as a low-threshold hybrid highly nonlinear Brillouin/Raman gain medium which can provide Raman amplification in the range of 1530 nm–1570 nm at different sides of the cavity. In the laser structure, a bi-directional Raman pumping scheme is realized by dividing the RPU through a variable optical coupler and splicing it to the 12 km DCF through two 1480/1550 nm wavelength selective couplers (WSC1 & WSC2). According to the results attained in our previous report [25], couplers with splitting ratios of 50/50, 90/10, and 95/5 are only connected individually between these components since generation of 20 GHz spacing with high maximum power discrepancy between odd- and even-order Stoke lines through these couplers was achieved. The second DCF coil with a length of 2.7 km at the other side of the cavity together with one physical mirror provides a significant Rayleigh scattering effect which is necessary for generation MBRFL with 10 GHz spacing. A physical mirror is formed by using a fiber loop mirror constructed by splicing port 3 to port 1 of the OC2. This ascertains the full contribution of Rayleigh scattering to the laser action for the purpose of generation of 10 GHz spacing. An optical isolator (ISO) is used to prevent any back-reflection light that can interrupt the laser stability. The output spectra of the MBRFL with different wavelength spacing are observed using an optical spectrum analyzer with a 0.02 nm resolution bandwidth through an output port of the ISO and port-4 of the OC1 which is represented by OSA1 and OSA2 respectively.

 

Fig. 1. Configuration setup of MBRFL with switchable wavelength spacing in (a) Single-pass arrangement for 20 GHz frequency spacing, and (b) Double-pass configuration for 10 GHz frequency spacing.

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3. Results and discussions

Since the bi-directional pumping concept in a linear cavity has already been reported in [25], we only mention about the principle operation of switchable wavelength spacing through generation of forward and backward Stoke lines in a single-pass and double-pass transmission of the laser cavity. In our previous work [25], the effects of Raman pump power distributions on BSLs with 10 and 20 GHz spacing and different characteristics were investigated. From [25], it was understood that employing 50/50 coupler leads to reduction in gain competition and can result in an improvement on Stokes behavior especially on S-OSNR and wavelength tunability. However, by employing 90/10 and 95/5 couplers, higher number of channels was achieved due to higher Raman gain. Moreover, in [16], by utilizing a bi-directional equal RPP distribution (50/50 coupler), the highest flatness uniform BSLs with an average 28 dB OSNR was achieved. Hence, with refereeing to the previous results, these aforementioned couplers are chosen for the scheme presented in this work. As the BP signal passes throughout the 12 km DCF and experiences bi-directional Raman gain, is backscattered through the non-shifted frequency of Rayleigh and up-shifted frequency of Brillouin scatterings. Rayleigh backscattering of BP and 1^st BSL are amplified through bi-directional Raman pumping. However, the 1^st BSL experiences higher gain than its Rayleigh line and is saturated faster owing to lower nonlinear threshold power. When the threshold condition is satisfied for both Rayleigh and Brillouin scatterings, the 1^st BSL can generate 2^nd BSL in forward direction. Simultaneously, the 1^st BSL is backscattered through non-shifted frequency of Rayleigh scattering. It is noted that Rayleigh scattering of the 1^st BSL and subsequent odd-order Rayleigh components, for all coupling ratios, undergo slight amplification due to the lower Raman gain at the end of the fiber entry point. This result in the observation of multiple BSLs with 20 GHz spacing that is observed in OSA1 as realized in Figs. 2 and 3. Furthermore, the higher Raman gain for coupling ratios of 90/10 and 95/5, as shown in the inset graph in Fig. 2, leads to greater Raman amplification. This is occurred for the odd-order Stoke lines and the Rayleigh scattering of even-order Stoke lines, which propagate in the same direction as the odd-order lines. This higher Raman gain surpasses that of the generated Brillouin Stoke signals (BSSs) achieved through symmetrical bi-directional Raman pumping. Consequently, the odd-order BSLs in the backward direction passes through ports-2 and 3 of OC1 and undergoes a double-pass scheme. In this scenario, OC2 initiates a loop mirror mechanism by reflecting back fractions of the Rayleigh scattering of BP, and 1^st BSL to the 2.7 km DCF and amplified. However, the 1^st BSL acquires significantly higher gain than the Rayleigh scattering lines and quickly saturates due to the low nonlinear threshold power. Utilizing loop mirror provides additional energy that assists in the further amplification of SBS and supports the reduction in the threshold of the 1^st BSL. Once the threshold condition is satisfied, the 1^st BSL also acts as a new 2^nd BP seed. As a result, the 2^nd BSL is generated in a different direction from the 1^st BSL in the double-pass arrangement. The residual 1^st BSL photons are backscattered in the same direction of 2^nd BSL. Most of these light beams are utilized to sustain the additional amplification of the 2^nd BSL and contribute to its decrease in threshold. Then the 1^st BSL and 2^nd BSL undergo amplification through the loop mirror. At the same time, when the second odd-order BSL in the single-pass configuration is generated and travels into the double-pass arrangement, it acts as a new 3^rd BP source and is backscattered through Rayleigh and Brillouin scattering. The 3^rd BSL can experience saturation due to the lower threshold power of Brillouin scattering. Same process is repeated to generate higher-order Stoke lines. It should be noted that in the case of single-pass, the 2^nd BSL and subsequent even-order Stoke lines, which are backscattered through weak virtual mirror Rayleigh scattering effect, cannot reach saturation. This is happen due to the higher threshold power of Rayleigh scattering in the single-pass configuration. In fact, small portion of the Rayleigh scattered light is recaptured, with some propagating in forward direction and the remainder in the backward direction. The backward-propagating portion of the Rayleigh scattered light is recycled back into the 2.7 km DCF through the loop mirror on the other side of the cavity, adding to the portion that propagates in the backward direction. This leads to an increase in stimulated emission, allowing the Rayleigh components to gain higher amplification and reach absolute saturation in this double-pass BRFL configuration. The same process of generating higher-order BSLs continuous until the end of the Raman gain profile, where the Raman gain is not sufficient to compensate the cavity losses. As a result, the pump recycling behavior for BS odd-orders, strengthened by the loop mirror, contributes to the lower threshold achievement of the subsequent higher-order BSLs. Consequently, channels are generated from port-4 of OC1 at the left side of the cavity, together with the reflected components of even-order BSLs, resulting in 10 GHz wavelength operation as depicted in Figs. 2 and 3. Ultimately, when the light oscillations in both sides of the cavity are completed, the respective odd- and even-order BSSs with different power levels and frequency spacing are generated. In the case of the single-pass arrangement, the even-order Stoke lines with 20 GHz spacing are produced. However, it is the odd-order BSLs in the backward direction, which undergo the double-pass structure, that are responsible for the generation of multiple BSLs with a wavelength spacing of 10 GHz.

 

Fig. 2. Maximum power level difference between adjacent BSLs for 50/50, 90/10, and 95/5 couplers at different sides of the laser cavity (RPP = 1200 mW, BP wavelength = 1555 nm, BPP = 12 dBm).

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Fig. 3. The spectral outputs at OSA2 for couplers (a) 50/50, (b) 90/10 and at (c) OSA1 for both couplers (RPP = 1200 mW, BP wavelength = 1555 nm, BPP = 12 dBm).

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To achieve multiple BSLs with switchable wavelength spacing together with excellent performances, evaluations are conducted on different parameters. These are include the average maximum power discrepancy between adjacent channels across the entire laser lines bandwidth for different RPP distribution ratios of 50/50, 90/10, and 95/5 as shown in Fig. 2. During the measurements, all pumping characteristics including RPP, Brillouin pump power (BPP), and BP wavelength are fixed at 1200 mW, 12 dBm, and 1555 nm respectively. The amplified spontaneous emission (ASE) assessments for all couplers are taken from OSA1 for clarity. From Fig. 2, at OSA2, it can be observed that coupler 90/10 exhibits a lower maximum power difference between channels due to the higher growth of the Stokes peak amplitude (SPA) of odd-order lines. Moreover, the Rayleigh scattering of even-order lines in backward direction is higher compared to the generated Rayleigh scattering in forward direction. Additionally, odd-order BSLs in backward direction experience significant amplification due to the higher Raman gain at the input of the fiber entry point. As a result, the odd-order lasing lines with maximum power amplitude, along with the significant SPA of Rayleigh backscattered even-order lines and the even-order Stoke lines in the 2.7 km DCF are generated. This leads to a reduced power level discrepancy between adjacent channels.

From Fig. 2, we can conclude that the 90/10 coupler provides the most desirable structure, resulting in only 0.5-dB discrepancies between odd- and even-order lines. This leads to an improvement in BSSs characteristics compared with the results completed in previous study [11–15,17–29]. However, the 50/50 coupler is suitable for generating 20 GHz spacing at OSA1, with a 21 dB difference between adjacent channels. Based on these findings, the laser structure utilizing both 50/50 and 90/10 couplers is employed for the entire evaluations, as the 95/5 does not meet our requirements. It should be noted that variation in BPP and RPP do not have an effect on the peak power difference between odd- and even-order BSLs. This is because the power levels of odd- and even-order Stoke lines increase or decrease simultaneously with different pumping powers. Thus, resulting in no dissimilarity in values for maximum power discrepancy among odd- and even-order BSLs.

To gain a clear understanding of how the BSSs are generated on different sides of the laser cavity, magnified views of the spectral bandwidth for couplers 50/50 and 90/10 are monitored at OSA1 and OSA2, as shown in Fig. 3(a) to Fig. 3(c). These observations are made with pumping characteristics set at RPP of 1200 mW and BPP of 12 dBm. It is evident from the figures that in the single–pass arrangement, the Rayleigh scattering effect acts as a weak virtual mirror. As a result, all the Rayleigh–assisted Stoke lines shown in Fig. 3(c) have much lower power level compared to the power level of SBS-assisted Stoke lines (even-orders), which are formed by the stronger Brillouin scattering. In addition, from the spectra shown in Figs. 3(a) and 3(b), taken from OSA2, it is observed that the inclusion of the physical mirror plays a vital role in reflecting the generated signals back to 2.7 km DCF. This reduces the threshold for Rayleigh components and enhances the growth of Rayleigh scattering components, providing the preference for 10 GHz spacing with equal linewidth and high flat amplitude. However, the observed even-order BSLs at OSA1 for both couplers, as depicted in Fig. 3(c), result from insufficient transmission of Raman gain to the Rayleigh components, which leads to the generation of 20 GHz spacing MBRFL. It is noted that the shape of single spectral line have the slight fluctuation on peak power at a multi-wavelength channel spacing of specially 10 GHz. This is occurred since that the Brillouin frequency shift (BFS) in optical fibers is subject to variations caused by changes in temperature and mechanical strain [31–33]. Additionally, the phenomenon can be influenced by factors like pump-depletion [34,35]. In polarization maintaining fibers (PMF), characterized by significant birefringence, an inherent difference in BFS exists between the two principal axes, spanning several Megahertz [36]. Consequently, in standard single-mode fibers (SMFs) with lower birefringence levels typically ranging from 10⁻⁸ to 10⁻⁶, it is expected that the inherent BFS difference falls within the range of several tens of hertz to several kilohertz. Therefore, to mitigate the noise generated by spontaneous guided acoustic-wave Brillouin scattering and reduce the BFS fluctuations, it becomes necessary to cool the fiber down to the extremely low temperatures of liquid helium [37].

After understanding the physics behavior of the laser cavity, the next step is to address other previous challenges in MBRFL with switchable frequency spacing, such as achieving the highest number of channels (wide bandwidth) with outstanding OSNR.

To achieve this, the optimization of the injected BP wavelength and RPP for 50/50 and 90/10 couplers is conducted. Figure 4 illustrates the average S-OSNR as a function of the RPP increment. In this analysis, the BPP is fixed at 12 dBm with a BP wavelength of 1555 nm, while the RPP is gradually varied from 700 to its maximum value of 1200 mW. It should be noted that the average OSNR calculation of the generated 10 and 20 GHz Stoke lines at different BP wavelengths is also measured by comparing the maximum power level of each Stokes line with respect to its noise floor. By considering that the higher RPP leads to higher Stoke lines count (SLC), analysis on larger values of RPP is only carried out. From Fig. 4, it can be inferred that as the RPP increases from 900 mW to 1200 mW, the S-OSNR gradually decreases for both couplers. This can be attributed to the spectral broadening phenomenon on each lasing line and the presence of a physical mirror on the left side of the cavity for 10 GHz spacing BSLs. When employing the 50/50 coupler, S-OSNR of 26 dB and 30 dB are achieved for MBRFL with 10 GHz and 20 GHz spacing respectively. Conversely, when utilizing the 90/10 coupler, an S-OSNR of 28 dB and 29 dB is attained for 10 GHz and 20 GHz spacing, respectively. These values are obtained when the RPP is set at 900 mW.

 

Fig. 4. Measurement of S-OSNR against RPP increment (BP wavelength = 1555 nm, BP power =12 dBm).

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In the following experiment, the effect of BP wavelength is investigated to determine the optimal selection of the coupling ratio, which is responsible for generation switchable frequency spacing with excellent performances. In Fig. 5, the relationship between BP wavelengths and the number of Stokes channels and S-OSNR is examined. The measurements are conducted with fixed pumping values of 900 mW and 12 dBm for both 10 GHz and 20 GHz wavelength spacing individually.

 

Fig. 5. Evolution of SLC and the corresponding average S-OSNR as a function of BP wavelength for (a) 10 and (b) 20 GHz frequency spacing (RPP = 900 mW, BPP = 12 dBm).

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From Fig. 5, it is observed that by utilizing the 90/10 coupler, a maximum of 460 and 170 Stoke lines is achieved for 10 GHz and 20 GHz spacing, respectively. These are generated over bandwidths of 37 nm and 27 nm when the BP wavelengths are set at 1535 and 1543 nm, respectively. On the other hand, with the 50/50 coupler, maximum 425 channels are achieved at longer BP wavelength for the case of 10 GHz spacing. These findings indicate that the choice of coupling ratio and BP wavelength significantly influences the number of Stoke channels generated and the achievable performance of the switchable frequency spacing in the MBRFL setup. From Fig. 5, it is evident that there is a decrease in the number of BSLs as the BP wavelength is tuned from 1535 to 1572 nm. This decrease can be attributed to the reduced Raman gain profile, as shown in inset graph in Fig. 2. The tuning range of both schemes is limited due to the gain bandwidth restriction of the Raman amplifier. Furthermore, Fig. 5 demonstrates that the OSNR is directly proportional to the BP wavelength for both cases. This relationship can be related to the distribution of the available energy among the generated Stoke lines. In other words, Stoke lines at longer BP wavelengths tend to have higher OSNR. It is also obvious that the utilization of the 90/10 coupler results in a better quality of OSNR, with an improvement of 2 dB compared to the 50/50 coupler for the case of 10 GHz spacing. This improvement is due to the higher contribution of signal power reflected into the cavity, thanks to the higher Raman gain. As a result, to achieve high flat MBRFL with a wide bandwidth and excellent S-OSNR, the utilization of the 90/10 coupler is more favorable. This configuration satisfies the expectations and solves the previous problems effectively. These results represent the best achievements thus far in multi-wavelength fiber lasers arranged in a simple configuration that employs only a single low-power RPU.

Based on these findings, the laser structure demonstrates the capability to generate high number of channels while maintaining a wide tuning range. To verify this, the output spectra of the laser arrangement utilizing the 90/10 coupler, along with the corresponding magnified view of BSLs, at different sides of the cavity are presented as shown in Fig. 6. These measurements were conducted with all pumping characteristics maintained at 900 mW, 12 dBm, 1535 nm and 1543 nm for 10 GHz and 20 GHz wavelength spacing, respectively.

 

Fig. 6. (a) Representation of output spectra of switchable wavelength spacing utilizing 90/10 bi-directional Raman power distribution (RPP = 900 mW, BPP = 12 dBm). The magnified view of the Stoke lines spectrum at (b) for 10 GHz and 20 GHz spacing.

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4. Conclusion

In conclusion, a switchable MBRFL is demonstrated experimentally. The laser setup is arranged with bi-directional Raman pumping scheme with one physical mirror at one end of the cavity. The maximum power level difference among odd- and even–order Stoke lines is adjusted by controlling Raman power distribution in the bi-directional pumping scheme. This is implemented by optimizing the coupling ratio of the optical coupler and also the initial Raman pump power. The optimized coupling ratio is 90/10 which satisfies the condition required for switching wavelength interval behavior between laser lines. The widest bandwidth of 37 nm corresponding to 460 high flat lines (10-GHz spacing) with only 0.5-dB flatness range, 26 dB OSNR and −5 dBm SPP is achieved at RPP and BPPs of 900 mW and 12 dBm, respectively. Concurrently, 170 Brillouin Stokes with 20 GHz spacing is realized with around −2 dBm SPP and 28 dB OSNR in 27 nm bandwidth. We believe that the proposed fiber laser system is the simplest, and its optimization approach is practical for obtaining high flatness, high output power, and wide-bandwidth multi-wavelength laser lines with functionality of frequency switching simultaneously. This opens-up a new standpoint towards exploring the advantages of this structure for improving the performances of multi-wavelength lasing generation.