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Abstract
We propose and demonstrate a 10 GHz spacing multi-wavelength Brillouin-Raman fiber laser (MBRFL) with wide bandwidth and an outstanding optical signal-to-noise ratio (OSNR). This is achieved by utilizing loop mirror at one end of the laser cavity through a symmetrical bi-directional Raman pumping scheme. The setup is arranged in a double pass configuration by employing different lengths of dispersion compensating fibers (DCF). The attainment of MBRFL with outstanding performance necessitates the optimization of Raman pump power, Brillouin pump wavelength, and DCF length. By employing an 11 km DCF, when setting the Brillouin pump wavelength at 1531.4 nm, 504 Stoke lines are produced with a channel spacing of 0.08 nm. All the counted laser lines have less than a 1-dB peak amplitude variation and are spread across a 40.4 nm bandwidth that covers from 1531.4 to 1571.8 nm wavelength. In this case, the Raman pump power is fixed at 900 mW which results in average OSNR and Stokes peak amplitude level of 28 dB and -7 dBm respectively. To date, this is the simplest cavity design with the widest bandwidth and flattest spectrum together with outstanding OSNR attained in 10 GHz spacing multi-wavelength fiber lasers that incorporate a single low-power Raman pump unit.
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1. Introduction
Multi-wavelength fiber lasers have made remarkable innovations in optical communication, sensor, and component characterization [1–3]. Among different types of fiber lasers [4–8], the Brillouin-Raman fiber laser (BRFL) has been considered as one of the best solutions with several important benefits. These include a broad gain bandwidth, stable multi-wavelength operation at room temperature. There have been numerous successful examples of MBRFL generation with single (10 GHz) frequency spacing [9–13] . In these hybrid BRFLs, Rayleigh scattering (RS) is 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) [11,12]. In [12], an output RPP higher than 10 W was required to achieve 798 Brillouin Stoke lines (BSL) at 3-dB flatness with poor OSNR over 61.65 nm wavelength range. However, this method complicates the fiber laser design and increases the cost. Numerous challenges have been addressed to optimize the performances of the 10 GHz spacing MBRFL, especially the number of channels [14,15], flat-amplitude bandwidth [9] and other important characteristics such as OSNR [16,17] and channel spacing [17–24]. Nevertheless, one of the major challenges is to achieve uniform 10 GHz spacing Stoke lines over a wider bandwidth domain with outstanding OSNR [9,23,24]. Interestingly, half-open or fully linear cavities using highly reflective mirrors at one or both ends of the laser cavity have also been explored. In these approaches, 210–500 Stoke lines with OSNRs ranging from 12.5 to 18 dB [15,17,22] are generated. Single spacing multi-wavelength outputs can also be attained by properly adjusting the coupling ratio in a nonlinear amplifying loop mirror (NALM)-based cavity [18]. In their setup, a 50/50 NALM configuration generates 443 channels with a 16.5 dB OSNR. Concurrently with the development of BRFLs, various techniques for obtaining switchable BRFLs have been reported using different structures and methods. The frequency spacing can also be switched by manipulating the Brillouin pump (BP) average power as the laser cavity optical pump [23]. In these methods, a significant power variation between the generated Stoke lines can be easily observed in the laser output spectrum. In a more recent work, the feedback power adjustment technique was employed, resulting in the achievement of 468 and 242 lines with 10 GHz and 20 GHz frequency spacing, respectively [22]. Nonetheless, this approach requires a variable optical attenuator to modify the feedback power, introducing further complexity and cost. 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, besides complicated structures, output characteristics do not satisfy all requirements for generating 10 GHz spacing MBRFL with desirable performances. In this regard, to address these problems, based on benefits of our earlier works, a 10 GHz wavelength spacing MBRFL is established by utilizing a symmetrical bi-directional pumping structure along with the incorporation of a mirror loop at one side of the cavity. The concept is similar to implementing an artificial optical mirror with high reflection. The discussion is initiated by explaining the Stokes light propagation behaviors as a function of this reflection through a bi-directional pumping laser arrangement. With respect to other earlier reports [9–24], the main accomplishment of this research work is to introduce novel improvements to Stokes OSNR (28 dB), simplicity, wide bandwidth (40.4 nm), flat spectrum (less than 1-dB variation), and high Stokes peak amplitude (SPA) level (-7 dBm). This is the best described by the selection of a symmetrical coupler while different length of DCFs is configured. The high-quality Stoke lines are achieved at optimum values of DCF, BP wavelength, and RPP.
2. Experimental setup
The double-pass structure of MBRFL that incorporates a bi-directional pumping scheme is outlined in Fig. 1. The linear cavity consists of an external cavity tunable laser source (TLS) that is utilized as a BP source. Its wavelength tunability ranges from 1520 nm to 1620 nm and output power can vary from around -3 dBm to a maximum value of 12 dBm. The BP signal is sent through a 3-port optical circulator (OC1) to the nonlinear 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. In addition, different length of DCFs are used as a low-threshold hybrid highly nonlinear Raman/Brillouin gain medium which can provide Raman amplification in the range of 1510 nm–1570 nm. Their Nonlinear coefficient is 7.3 (Wkm)−1 and the effective area is 20 µm2 which justify its intrinsic properties of strong nonlinear effects in comparison with other gain medium. In the laser structure, a bi-directional Raman pumping scheme is realized by dividing the RPU through a 50/50 coupler and splicing to the DCF through two 1480/1550 nm wavelength selective couplers (WSC1 & WSC2). This allowed the symmetrical propagation of forward and backward Raman input power along the two fiber entry point. Double pass configuration is established using a fiber loop mirror constructed by looping back the second optical circulator (OC2). This ensures the full contribution of Rayleigh scattering to the laser action. The generated BSL, after improvement through double-pass scheme amplification, is extracted from port 3 of the first circulator (OC1). During experiment, an optical spectrum analyzer (OSA) with a 0.02 nm resolution bandwidth is employed to carry out data analysis.
3. Results and discussions
As the bi-directional pumping concept in a linear cavity has already been reported in [17], we only mention the principle of multi-wavelength lasing lines generation in a double-pass transmission of the linear cavity. In our previous work in [17], the effects of Raman pump power distributions on BSLs with 10 and 20 GHz spacing and different characteristics were investigated. From [17], it is understood that employing 50/50 coupler leads to reduction in gain competition and can result in an improvement on Stokes OSNR (S-OSNR). Therefore, with reference to the previous results, a 50/50 coupler is fixed for the scheme presented in this work. To understand the physical procedure underlying this pattern, the generation process of Stoke lines is sketched and shown in Fig. 2.
Fig. 2. Microscopic propagation of Brillouin photons in the DCF. BPS: Brillouin pump signal, 1st BSL and 2nd BSL: first and second Brillouin Stoke lines respectively. RBP: Rayleigh scattering Brillouin pump.
In this report, the double-pass configuration allows the transmitted BP signal (black color) injected to the fiber to be amplified by bi-directional Raman pumping. The 1st BSL is produced in the opposite way (red color) once the threshold level is satisfied. The OC2 initiates a loop mirror mechanism by reflecting back some fractions of the remaining BP photons to the DCF. As the remaining BP signal passes through the DCF, the RS of the remaining BP, 1st BSL, and anti-Stoke lines (ASL) with green color is amplified through bi-directional Raman pumping. However, the 1st BSL acquires significantly higher gain than the other two lines and is saturated quickly owing to low nonlinear threshold power. Additional energy that assists in the further amplification of SBS is provided via the loop mirror, supporting the reduction in the threshold of 1st BSL. Similar to the process accomplished by BP, when the threshold condition is satisfied, the 1st BSL also acts as a new 2nd BP seed. This results in the generation of the 2nd BSL (red arrow) in the same BP photon propagation direction and un-shifted frequency Rayleigh scattering. The residual 1st BSL photon is backscattered in the same direction of 2nd BSL. Most of these light beams are consumed to sustain the additional amplification of 2nd BSL and facilitate its decrease in threshold. Subsequently, the 1st BSL and 2nd BSL are created and amplified through high gain bi-directional Raman pumping. As the 2nd BSL grows, it acts as a new 3rd BP and backscatters through Rayleigh and Brillouin scattering. Afterward the 2nd and 3rd BSLs experience distributed Raman amplification. The 3rd BSL can experience saturation due to the lower threshold power of Brillouin scattering. The same process is repeated to generate higher-order Stoke lines. However, the 2nd BSL and subsequent even-order Stoke lines, which are backscattered through the weak virtual mirror Rayleigh scattering effect, cannot saturate due to higher threshold power of Rayleigh scattering. In fact, a small part of Rayleigh scattered light is recaptured, some of which propagates in the forward direction (FW), and the remaining propagates in the backward direction (BW). Some part of the Rayleigh scattered light in the forward direction is recycled back into gain medium by the loop mirror at the cavity end and is added to the part that propagates in backward direction. This causes the incremental stimulation emission, allowing laser modes to overcome gain competition. Consequently, the Rayleigh components get higher gain and experience absolute saturation in this double-pass BRFL configuration. After the 3rd BSL grows and saturates, it acts as the new 4th BP source and is backscattered through Brillouin and Rayleigh scattering once again. The same process of generation of higher-order BSLs will continue until the end of Raman gain profile, where the Raman gain is not sufficient to compensate for cavity losses. Considering λn ≈ λBP + n (0.08), where n is an odd value, the Stoke lines are termed as Brillouin components. If n is an even value, the Stoke lines are termed as Rayleigh components. As a result, the pump recycling behavior for BP and Brillouin Stokes (BS) even-orders, strengthened by the loop mirror, is responsible for achieving a lower threshold for subsequent higher odd-orders. Therefore, odd-order channels are realized at the left fiber arm together with the reflected components of even-orders, thus producing 10 GHz wavelength operation.
In order to better understand the double-pass MBRFL generation procedure and the effect of the loop mirror at the end of the laser cavity, the output spectra with and without the loop mirror are illustrated in Fig. 3. During measurements, all pumping characteristics including RPP, Brillouin pump power (BPP), and BP wavelength are fixed at 1000 mW, 12 dBm, and 1555 nm respectively. As an example, 11 km DCF is utilized for this assessment. From the spectra illustrated in Fig. 3, it can be understood that the loop mirror at one end of the laser cavity plays a critical role in the creation of flat amplitude and uniform lasing lines with 10 GHz spacing in the entire spectrum bandwidth. Since the Rayleigh scattering effect acts as a weak virtual mirror, all the Rayleigh components (even-orders) have lower amplitude in comparison with the amplitude power of Brillouin Stokes components (odd-orders), which are formed by the stronger Brillouin scattering. In addition, from the spectra in Fig. 3, it is obtained that the employment of a loop mirror can produce BSL with similar amplitude power. Moreover, the similar spectrum BSLs in the case with a loop mirror acquires higher gain than Brillouin-Rayleigh components in the case without a loop mirror. The main factor that influences the decline in threshold for this wave type is determined by the backscattered odd-orders. Most of these latter components experience attenuation that happens in proportion to the amplification growth of the higher even-orders as shown in Fig. 3. This justifies the formation of 10 GHz (0. 08 nm) cascaded SBS shifts.
Fig. 3. Effect of loop mirror at one end of the cavity on the evolution of back scattering lines by Raman gain at bi-directional pumping scheme (RPP = 1000 mW, BP wavelength = 1555 nm, BPP = 12 dBm).
After understanding the underlying physics of the laser cavity, which is feasible for generation of high uniform amplitude MBRFL with 10 GHz spacing, solving the other previous problems in 10 GHz spacing and achieving the highest number of channels (wide bandwidth) with outstanding OSNR is also preferable. To realize these goals, the optimization of injected BP wavelength and RPP is carried out when different lengths of DCF are configured. For clarification, estimating the average S-OSNR and number of channels in the spectral envelope is defined by including all lasing lines that have appropriate signals with a peak amplitude difference of less than 1-dB within entire spectrum bandwidth and with the absence of any turbulence wave. Figure 4 depicts an average S-OSNR and Stoke lines count (SLC) as a function of the RPP increment. In this assessment, the BPP is set at 12 dBm, and its BP wavelength is fixed at 1555 nm, while the RPP was varied gradually from 700 to its maximum value of 1200 mW. Taking into account that the higher RPP leads to higher SLC, so, analysis on larger values of RPP is only carried out. It can be inferred from this figure that, with the increment of RPP from 1000 mW to 1200 mW, the S-OSNR gradually reduces for different DCF lengths due to the spectral broadening phenomenon on each lasing line. From this figure, when employing the shorter DCF (3 km), a maximum of 200 Stoke lines are attained. These are produced with 25 dB OSNR when the RPP is set at 1100 mW. In contrast, for the longest DCF (14 km), the maximum Stoke lines of 190 are achieved with a 27 dB S-OSNR. This occurs when the RPP is set at 900 mW. Various balances between gain and cavity loss in these two fiber lengths are found to be the main reasons for this discrepancy. Additionally, according to this figure, the optimal DCF length is found to be 11 km, resulting in the highest S-OSNR of 29 dB at an RPP of 900 mW, surpassing the S-OSNR values for other DCF lengths. It is observed that employing a longer DCF (14 km) increases the gain competition among the self-lasing modes. Consequently, a significant portion of the energy is directed towards suppressing free-running modes, leaving insufficient remaining energy to enhance SPA. The obtained results demonstrate an improvement in the number of Stoke lines with increasing RPP. However, for longer DCFs, the channel count deteriorated as the RPP exceeded 1000 mW. This can be attributed to the stronger feedback effects of distributed Rayleigh scattering at the peak Raman gain, as demonstrated in random distributed feedback fiber lasers. These oscillations compete with the BSL generation in the same cavity due to the pump depletion. The SLC of MBRFL is 180, 185, and 190 at 900 mW RPP for DCF lengths of 3 km, 11 km, and 14 km, respectively. It is clear that, for higher RPP, the difference in SLC for different DCFs is not substantial. Therefore, due to the achievement of an excellent OSNR at 900 mW for the 11 km DCF, it is selected as the optimum RPP value, resulting in the high Stoke lines generation with an excellent S-OSNR. As a result, the optimum RPP is chosen based on performance of different length of DCFs at different values of RPP on SLC and S-OSNR together. Since S-OSNR was decreased for all DCF lengths at higher values of RPP (more than 900 mW), and achievement of high SLC with excellent OSNR is achieved at 900 mW for 11 and 14 km DCF, 900 mW was chosen as an optimum value.
Fig. 4. Evolutions of S-OSNR and SLC against RPP increment, different DCF length labels are arranged from top to bottom levels in descending-order of S-OSNR values (BP wavelength = 1555 nm, BP power =12 dBm).
Subsequently, the relationship between BP wavelengths with number of Stoke channels is also investigated when the RPP is set to 900 mW and BPP is maintained at previous value as depicted in Fig. 5. Notably, for this assessment, only the effect of 11 km and 14 km DCFs are investigated since the 3 km DCF does not meet our criteria for excellent S-OSNR. From this figure, a maximum of 504 Stoke lines is attained by utilizing an 11 km DCF. These are produced over 40.4 nm bandwidths when the BP wavelength is set at 1531.4 nm. In contrast, for the longer DCF, a maximum of 375 Stoke lines channels are achieved at the longer BP wavelength. At a shorter BP wavelength (1531 nm), the Raman gain is not sufficient to generate first Stokes line. In other words, at a shorter wavelength span, higher energy is required for the attainment of a wider multi-channel lasing bandwidth. This latter attribute is influenced by the wavelength dependencies on BP and accessible Stimulated Raman scattering bandwidths. A higher Raman gain might broaden the SRS bandwidths. Also, it is noted that at the similar BP wavelength, the SLC for 11 km is also higher than 14 km DCF. The decline in the number of Stoke lines for the 14 km DCF occurs since longer DCF introduce more cavity loss, necessitating higher Raman gain to overcome the gain competition. From Fig. 5, it is clear that at 1550 nm, there is a sharp increase in number of Stokes for 11 km DCF; this can be attributed to the effect of Raman peak gain area.
Fig. 5. . Number of channels versus BP wavelength for different DCF lengths . The RPP is set at 900 mW and the BPP is fixed at 12 dBm.
As a consequence, from all assessments, it can be inferred that 11 km DCF provides the optimum value that leads to generation of a wide multi-wavelength bandwidth with outstanding OSNR. In order to prove this, its spectral feature and corresponding magnified profile are illustrated in Fig. 6. Moreover, it can be seen that a total of 504 Stoke lines, with an average of -7 dBm SPA and 28 dB OSNR at 1-dB flatness range, is achieved. These achievements are realized with a specifically chosen BP wavelength at 1531.4 nm and power of 12 dBm, while RPP is maintained at 900 mW. Thus, our results introduce excellent improvements to Brillouin Stokes features with respect to outstanding S-OSNR (28 dB), wide bandwidth (1531.4 nm), and high number of channels (504) by employing a simple design. Therefore, to the best of our knowledge, our scheme resolves all previous problems especially with respect to number of lasing lines, Stokes power level, complexity, and S-OSNR. In addition, it is mentioned that the stability of the fiber laser can be attributed to the effective reduction of self-lasing modes, which is achieved by equal Raman power propagation in both forward and backward directions. This balanced power transfer into the cavity and the gain saturation levels of BSLs contribute to power stabilities throughout the Raman amplification bandwidth.
Fig. 6. Representation of optimized spectral bandwidth with 10 GHz spacing and its enlarged view of the Stokes spectrum (BP wavelength = 1531.4 nm, RPP = 900 mW, DCF = 11 km, and BPP = 12 dBm).
In summary, it is better to noted that from Fig. 3 and Fig. 6, we can conclude that a single spectral line is similar, but has the shape that is composed of multiple peaks. This event is happen since the Brillouin frequency shift (BFS) in optical fibers is subject to variations caused by changes in temperature and mechanical strain [25–27]. Additionally, the phenomenon can be influenced by factors like pump-depletion [28,29]. 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.
4. Conclusion
A wide-bandwidth MBRFL with 10 GHz spacing is demonstrated, achieved with excellent OSNR without compromising performances in amplitude flatness and the number of Stoke lines. The key element is determined by the loop mirror, initiating the reflectivity back to the cavity and controlling relatively weaker Rayleigh scattering effects. Together with the bi-directional Raman pumping scheme with featuring symmetrical power distribution, a lower threshold power of SBS for the next Stokes line is provided, continuously validating the generation of successive higher-order components with a wider bandwidth. This also consume most of the energy to initiate self-flattening of the overall signal envelope. Achieving MBRFL with excellent performances requires the optimization of RPP, BP wavelength, and DCF length. 504 Stoke lines are achieved with a 1-dB amplitude level difference between adjacent channels. Other pumping characteristics that assist this operation include a 900 mW RPP and a 1531.4 nm BP wavelength when an 11 km DCF is utilized. Besides excellent results, the average S-OSNR and Stokes amplitude are 28 dB and -7 dBm respectively, covering the entire bandwidth of 40.4 nm. In summary, the simple proposed configuration is capable of generating high flat stoke lines with a wide bandwidth and excellent OSNR, surpassing others in performance.
Acknowledgments
G. M. generated the initial ideas of this work and developed the design approach; also she wrote the first draft of manuscript; M. R. K. S. discussed the results and contributed to the final manuscript. Two authors have read and agreed to the published version of the manuscript.
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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