In this report, we demonstrate a wide multiwavelength Brillouin-erbium fiber laser (MBEFL) with improved flatness that integrates a micro-air cavity. This air-gap introduces a cavity loss to overcome the gain saturation as well as providing efficient pump recycling scheme through Fresnel back-reflection. In addition, the efficient four-wave mixing in the highly nonlinear fiber contributes to the self-flattening of the output spectra. During operation, the optimized pumping values are set at 13 dBm Brillouin power and 600 mW erbium-ytterbium doped fiber amplifier when the air-gap length is fixed at 10 µm. A total of 180 Stokes lines are produced with a channel spacing of 0.08 nm. The flat lasing bandwith is 14 nm that consists of 1557 to 1571 nm wavelengths within 3-dB span. The average optical signal-to-noise ratio is 18 dB, having high peak power of −8 dBm. To our knowledge, this is the best result attained in MBEFLs with respect to the spectral flatness. In fact, the power stability of 0.76 dB order over 45 minute durations merits it applications in optical fiber sensing and communications.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Stimulated Brillouin scattering (SBS) is one of the alternative efficient techniques for initiating multiwavelength fiber lasers. The scheme involves a linear gain medium such as erbium-doped fiber (EDF) or nonlinear gain such as Raman medium to compensate the laser cavity loss and enhance channels generation. The exploitation of this technique is relatively simple and cost effective, and it has been regarded as a promising solution for cascaded multiwavelength generation . Amongst its several significant advantages are such as narrow linewidth, low intensity noise and stable operation at room temperature. In these laser types [1–4], achieving Stokes lines over a wide range is the main challenge, there are several means available where the most common one is by inducing four-wave mixing (FWM). Recently, a multiwavelength Brillouin-erbium fiber laser (MBEFL) with the assistance of this process has been proposed, producing up to 150 Stokes lines successfully . In the similar class of laser, almost 160 lines are developed when the input signal is self-seeded within the cavity without any external assistance . Up to 437 Stokes lines band, covering a 33.67 nm wavelength with an optical signal-to-noise ratio (OSNR) fluctuation above 10 dB is also achieved by suppression of the Brillouin pump (BP) noise floor . Despite of the broadening success in these ordinary cavities [2–4], their use for practical applications is limited. The power variation among the comb lines from maximum to minimum peak is relatively high. This differs from more than 21 dB  to higher than 40 dB range [2–4] where in a certain case, very low OSNR of only 4 dB estimation is evaluated .
The key point to resolve this issue emerges from better exploitation of multiple Rayleigh scattering that initiates random distributed feedback [5–8]. This principle is physically combined cooperatively with SBS in the layouts that incorporate fully-open [9,10] and half-open cavities [11–15]. Two key elements are required in this technology where the first is a gain medium that provides amplification. This determines the types of lasing transitions that are based on SBS-stimulated Raman scattering (SRS) [10–12] and SBS-erbium ion doping [13–15]. The second factor is a virtual optical cavity that is contributed elastically by multiple Rayleigh scattering. This provides primary SBS amplification and results in the Stokes lines broadening. For instance, 210 to 500 Stokes lines are generated successfully in multiwavelength Brillouin-Raman fiber lasers (MBRFLs) with an OSNR that ranges from 12.5 to 18 dB [10–12,16,17]. All of this preceding progress indicates 2- to 3-dB flatness in a configuration that simply utilizing a single Raman pumping source.
Regardless of the maturity in MBRFL developments associated to their favorable properties of inhomogeneous broadening, not much advancement is reported in MBEFLs. The homogenous broadening in Brillouin-erbium lasing transition prefers strong gain competition which justifies the difficulty in maximizing Stokes channels over a wide bandwidth domain. However the prospect is promising with the implementation of hybrid-scattering technique owing to the important role of Rayleigh scattering as explained above. In this latter scheme that exploits random lasing fundamentals, the reduction in peak power discrepancy below 8.0 dB scope is compromised by the lower number of Stokes combs just below 15 lines [1,15]. One of the examples is the FWM-induced self-flattening mechanism contributed by the 11 km DCF . In this case, the peak power difference between the first and the fifth output channel is 4.59 dB. This denotes a 3.73 dB enhancement compared to the result without the assistance of this response. However by using the same concept of FWM, a big change is fulfilled in 2017. Over 200 Stokes channels are initiated at 0.08 nm spacing. The measurements are assessed by counting all lasing lines within 4.65 dB peak power flatness . The lasing bandwidth of 16 nm begins from 1534 to 1550 nm wavelengths and the average OSNR is 15 dB. These are achieved for the erbium-ytterbium doped fiber amplifier (EYDFA) power of 350 mW and the BP power at −12 dBm. To compete with those attained in MBRFLs, a more drastic approach should be taken namely with the integration of micro-air cavity (MAC).
A few reports have demonstrated the benefits of this structure for more Stokes lines formation  and switchable frequency spacing merely with the adjustment of the air-gap length, . The former case indicates that the additional Fresnel reflectivity can facilitate in reducing multiple higher-order Stokes lines threshold that results in the attainment of wider wavelength segment. Therefore, to further improve the flatness in multiwavelength bandwidth (MBW) with outstanding OSNR, the MAC is introduced in our proposed single-spacing MBEFL system. The discussion of this report is started by explaining the principles of MAC, pump-recycling technique  and FWM. This is the main idea for understanding the key improvements behind the results obtained. The high degree of tolerance for cavity loss and air-gap length, as confirmed in the experiment eases the optimization of Stoke line counts and flatness in the spectral envelope. For comparison to justify the aid of a MAC loop, a setup with zero µm air gap is also investigated under the same pumping conditions.
2. Experimental setup
The experimental layout of a MBEFL system that employs two ring cavities is shown schematically in Fig. 1. The first represents the main cavity that is illustrated in the dashed-box and the second comprises a Brillouin gain medium (BGM). The nonlinear BGM is provided by a strand of 4 km length DCF that is efficient to induce 10 GHz cascaded SBS effects. The DCF is spliced to the 3-dB coupler in a circulating loop that allows double directions of light propagation as represented by the clockwise (CW) and anti-clockwise (ACW) beams in Fig. 2. In this loop, the embedded MAC as represented by its length, is formed by aligning coaxially two separate fiber interfaces, A and B by utilizing Fujikura splicer model FSM-100P. This machine allows the precise control of the air gap within tens of micrometer distance. The two fiber end facets are smooth and terminated with flat-angles within the acceptable tolerance of less than 2°. This induces cumulative Fresnel reflection back coupling to the cavity due to changes in refractive indices between fiber, n1 and air space, n0 as discussed in detail before . An external cavity tunable laser source (TLS) that can be tuned over a range of 31 nm from 1535 to 1566 nm serves as a BP signal. The maximum power is 13 dBm and this BP signal is injected into the MBEFL through an optical circulator (C1). Besides this, the single pass amplification scheme in the main cavity consists of an EYDFA that has maximum of 30 dBm power. The EYDFA module amplifies the BP signal and the successive generated BS lines that are recycled back to the secondary cavity. A piece of 500 m highly nonlinear fiber (HNLF) that has an effective area of 11 μm2 supports stronger MBW coverage through FWM owing to its beneficial intrinsic parameters. These include nonlinear coefficient of 11.5 (W.km)−1 and dispersion slope of 0.016 ps/(nm2.km). As FWM is inversely proportional to fiber dispersion, the zero dispersion wavelength of HNLF at 1564 nm is preferable for the optimization of this process. During experiment, the output spectrum is monitored by the optical spectrum analyzer (OSA) that is set at bandwidth resolution of 0.02 nm.
3. Role of micro-air cavity
The main objective of performing this assessment is to introduce more flatness to the multiwavelength output spectrum with outstanding qualities in its respective bandwidth, number of Stokes lines [2–4,10,16,18], Stokes peak power and OSNR. To realize this, a few cavity designs are implemented namely the MAC  that represents the main focus of interest and the pump recycling technique . In the first aspect, by exerting this type of fiber to air interface that has adjustable domain, the appropriate amount of cavity loss can be modified. This introduces the efficient mean to control the gain saturation that results in the improvements of the lasing performances. In addition to this, the signal Fresnel reflection is also enhanced between these interfaces (A & B) to support bidirectional wave propagation in the DCF that leads to efficient BS amplification. Both fulfillments satisfy multiple-orders SBS threshold decrement which is favorable for more generation of BS waves. The role of this pump recycling technique is discussed where the theoretical model is elaborated in . Additional contributions by FWM in the HNLF support the aforementioned objective where the fundamental laser operation needs to be understood first before initiating any discussion on the results obtained. In general, the operation principle of the MBEFL is depicted in Fig. 2.
The BP signal from TLS with the frequency propagates through C1 and passing through the 3-dB coupler before being diverted into the single pass amplification box. Any backward signal from the HNLF induced by distributed Rayleigh scattering is blocked by C2 and only the forward direction beam undergoes amplification. The input amplified BP signal (ABP), enters port 2 of the coupler where a fraction of this beam is reflected at fiber interfaces A and B which is labeled as . The transmitted part of circulates in the ACW direction before interacting with the DCF. Once enough power to exceed the SBS threshold and MAC loss integrated in the secondary cavity is attained, the first backward Stokes line emerges in the CW direction with the frequency of . As a result of the existence of , a portion of this beam is reflected back in the opposite direction (ACW) and is identified as . In the same way to that happens to , the transmitted exits port 3 of the coupler to have amplification in the main cavity. This new frequency of also functions as a new amplified BP signal that pumps back the DCF to produce the second-order backward Stokes signals, when the threshold condition is satisfied. The process continues to carry-on consecutively until the next higher orders threshold condition cannot be surpassed anymore. In this case, the multiwavelength spacing between the lasing lines is 0.08 nm.
For detail description when concentrating on the secondary cavity, the input beams comprise amplified waves of , , up to . The last beam component faces Fresnel reflection at fiber interfaces that is defined as and also reaches the DCF to produce the final backward BS frequency of in the CW direction. The beam is also subjected to amplification in the main cavity with a new frequency of that no longer can overcome the SBS threshold in the DCF. The remaining amplified BS and BP beams, ABS and ABP as well as those of reflected BS waves, RBS that represents the unamplified components keep circulating in the ACW direction and act as additional recycle pumping sources for the BGM. The same behavior also takes place simultaneously in the CW direction for another part of the BS signals together with the fraction of reflected amplified BS and BP beams, RABS and RABP. These CW beams also provide additional minor amplification to the corresponding generated BS signals. Although multiple Rayleigh scattering still happens in both directions, the influence of Fresnel reflection is more significant which outrivals the former effect. In fact, it is very critical to emphasize that the recycle pumping technique of the BGM contributed by ACW beams is more dominant than that for CW beams. This is because the input sources (ABP and ABS) already have major amplification in the main cavity. Furthermore, the main feedback to the laser cavity is primarily determined by Fresnel reflection at fiber interfaces, A & B. At optimized EYDFA and BP power, this is responsible for more supplies of interacting beams to the HNLF. Inside this fiber, all these collective BP and CW beams (BS, RABS, RABP) perform as the seeds for FWM to initiate extra BS lines . Self-flattening is initiated as the multiwavelength output exchanges their power transfer from the higher laser level to that of the lower one. The process is further strengthened by the EYDFA amplification mechanism and its presence is proven by the anti-Stokes and other higher-order Stokes waves linewidths. These justify the wider coverage of MBW output with more uniform amplitude observed in the OSA as explained in this section.
4. Results and discussions
This experiment is begun by evaluating the amplified spontaneous emission (ASE) of the laser scheme at three different cases as explained in Fig. 3(a). The unstable lasing runs in the free running condition that is defined as stochastic modes. This is realized when the TLS is switched-off and the for the last two cases is fixed at 10 µm (black and lime profiles). For reference, the ASE of EYDFA measured directly to OSA indicates two peak wavelength gains at 1535 and 1544 nm (see Fig. 6(a) in ). In the first case that represents a half-open cavity design that excludes a MAC, the EYDFA power is arranged at 100 mW (blue profile). A peak lasing transition at 1544 nm is observed that coincides well with one of the EYDFA peak gain profile. Without any fiber Bragg grating insertion, the feedback that contributes to the lasing oscillation growth in the main cavity is provided by multiple Rayleigh scattering from the DCF in the CW direction. The output characteristic in the saturation regime indicates the influence of self-lasing cavity mode that is contributed by the continuous light oscillation in the ACW direction. In contrast for the second case of a design of conventional MBEFL that includes a MAC, the EYDFA is fixed at the same power level (black profile). As more CW beams are distributed to the HNLF through Fresnel reflection, the lasing transition shifts to longer wavelengths. The first appearance of FWM-induced beam effect is monitored at 1.7 nm wavelength range from 1564.5 to 1566.2 nm as shown in Fig. 3(b). As the saturation regime is relieved for the ACW beam owing to the introduction of cavity loss, the noise floor level of stochastic modes is decreased and the increment of peak power is realized. This facilitates the improvement in the OSNR that becomes one of the subjects of interest in this assessment. By maintaining the same setup in the second case but with further increase in EYDFA power to 500 mW (lime profile), stronger nonlinearities are initiated. The lasing power spreads to longer wavelengths from 1563 to 1571 nm approximately through FWM in the HNLF. At the same time, the noise floor is slightly higher and the peak power is slightly lower to those observed in the second case. Although the deterioration in the OSNR of the Brillouin lines occurs at this higher power, the overall cavity gain can be enhanced in a wider wavelength range. This is preferable for the optimization of multiwavelength lasing behaviors under the precondition that feasible counter balance between EYDFA power and BP attributes are fulfilled.
Once the ASE properties are understood, the spectral behaviors at different EYDFA power from 0 to 1 W are analyzed as depicted in Fig. 4. The BP wavelength and power are set at 1565 nm and 10 dBm respectively where the is chosen at 10 µm distance. In theory, a much higher threshold is expected in shorter fibers [22–25] compared to those of longer fibers [5,26]. When the EYDFA power reaches 10 mW, the first BS line threshold is attained. In another attempt that incorporates a longer strand of DCF around 11 km , 12 mW SBS threshold is developed. By taking into account a few times shorter DCF utilized in this assessment, this reverse achievement ascertains the remarkable role of MAC that accumulates more Fresnel reflection as the feedback to satisfy the low threshold purpose. Other works that utilize a 5 km spool single mode fiber (SMF) produces 8 mW threshold in a simpler setup with lesser cavity losses , where all of these results relate to the efficient pump recycling technique. Once the threshold level is addressed, the number of Stokes lines increases linearly with the increment of EYDFA power. The self-flattening between the neighboring channels [13,18,19] is first recognized when adjusting the EYDFA power from 200 to 250 mW as shown in Fig. 4(a). As a result, the flat MBW grows to 8.0 nm for the OSNR of 20.4 dB (lime profile). At higher power additions, the bandwidth is broadened (blue and red profiles) as more amplification is provided for generating more BS lines within the accessible EYDFA gain bandwidth (see Fig. 6(a) in ). This is followed simultaneously by the increase in peak power and noise floor that signify the initiation of gain saturation. The gain competition between free-running cavity modes that coexist together with BS components is also identified. As a consequence, the degradation in OSNR quality arises owing to self-lasing around 1565 nm wavelength range as demonstrated in the spectral regime especially at 1000 mW (red profile). This indicates that the right selection of EYDFA power is needed for achieving wide flat channels with uniform OSNR. For better clarification, we standardize the OSNR evaluation in the entire assessment where one of the examples is shown in Fig. 4(b). This parameter is defined from the highest noise level of the signal to the average peak power level in the coverage of 3-dB bandwidth.
Next to further satisfy this objective, the BP wavelength is changed to 1560 nm which is close to the peak ASE transition (Fig. 3). The BP power is varied from 7 dBm to 13 dBm to investigate the right power option that is sufficient to suppress the associate noise generation. The experimental results are presented in Fig. 5 where other lasing criteria such as EYDFA power is chosen at 600 mW and is maintained at the previous distance. When the BP power is arranged at 7 dBm, the growth of self-lasing is more prevailed than that of SBS lines. This happens particularly at the center wavelength of 1565 nm as represented by the appearance of the noise floor in Fig. 5(a). The inability to suppress this response is owing to the lack of adequate Brillouin energy provision which results in distortion of OSNR uniformity. Nevertheless to avoid this, the upgrade in BP power is implemented to ease more efficient energy transference from BP signal to Stokes waves. This justifies the improvement in OSNR qualities that is realized in the clean-cut output spectra as illustrated in Figs. 5(c) and 5(d). As a result, the MBW are preserved at 10.1 to 11.0 nm approximation where the uniform OSNR develops from 17.7 to 19.9 dB. From this figure also, the number of Stokes lines decreases slightly when the BP power is scaled-up to 13 dBm. This is easily understandable because more pump energy is consumed to amplify higher number of incoming photons that leads to the gain saturation. The stronger amplification at higher power of signal wavelength implies less noise build-up from ASE which justifies better increase in the OSNR. At the same time, the excitation of anti-Stokes lines via FWM in the optical fiber is also more prominence as indicated by the band-edge in both features [see Figs. 5(c) and 5(d)].
The optimized BP power of 13 dBm has been determined and by retaining the same BP and lasing properties, the is modified from 0 to 50 µm. The results obtained are plotted in Fig. 6 together with the corresponding spectra in Fig. 7. With the absence of MAC ( = 0), only 22 Stokes lines are produced that relate to 1.7 nm bandwidth [Figs. 6 and 7(a)]. A very drastic advancement is introduced when the is modified from 2 to 20 µm as the MBW indicates minor fluctuations from 11.1 ± 0.3 nm. This implies that the Stokes waves number can be controlled from 144 ± 4 lasing lines. Some of the spectral profiles within this length that have MBW of 10.8 and 11.3 nm are depicted in Figs. 7(b) and 7(c). In both figures, the Stokes line numbers are 140 and 146 respectively with a high quality uniform OSNR that can reach a maximum of 19.2 dB. However a different tendency is realized at a longer air-gap segment from 25 to 50 µm. The MBW falls from 10.2 nm to only 1.2 nm that signifies the Stokes line counts decrease from 132 to 16 lines. This happens as the photons are inherently susceptible to attenuation during transmission in the extended free space arrangement. Some of these examples at relevant are exhibited in Figs. 7(d) to 7(f). The images characterize the deficiency in spectral flatness at above 40 µm distance.
At no separation, the setup is almost similar to Configuration C (see Fig. 1 in ). The reason behind its attainment of very low number of Stokes lines and poor in flatness as illustrated in Fig. 7(a) relates to the gain saturation. The gain is reduced especially when high input power is utilized in this experiment. This is a result of the elastic build-up of unnecessarily extra waves initiated by multiple Rayleigh scattering. For reference in , only 350 mW EYDFA power and −12 dBm BP power is used for the development of 200 Stokes lines within 4.65 dB span. By mitigating this disadvantageous nature with the introduction of air-gap adjustment, the number of BS lines is effectively improved with flat amplitude as manifested in Figs. 7(b) and 7(c). Although the BP wavelength is still not being optimized yet, this positive feature is owing to a few factors. The first pertains to the introduction of cavity loss that relax the gain saturation. This is counter-balanced by the FWM initiation which is in commensurate to accumulative interacting beams in the HNLF initiated by the Fresnel reflection feedback. To understand this issue, the reflectivity and insertion losses influenced by the changes in are analyzed. This is carried-out in a fiber coupling connection by using the ASE from EYDFA that is set at 600 mW pump power as shown in Fig. 8(a). From this figure, an isolator is utilized to allow a single directional light propagation. The first optical power meter (OPM 1) assesses the transmitted power where the reflected power is evaluated at the second optical power meter (OPM 2). The data obtained at OPM 1 is converted into the insertion loss as shown in Fig. 8(b). Based on the earlier findings (Figs. 6 and 7), a relatively broad range of from 2 to 20 μm is recommended without any necessity for critical cavity length alignment. Within this gap, the Fresnel reflectivity contribution is found to decrease from 9.4 to 7.4% range as illustrated in Fig. 8(b). The plots give the right estimation for the insertion of cavity losses from 7.4 to 27.4% that relate to the decrease of gain saturation feasible for initiating lower SBS threshold and higher-orders Stokes lines formation.
The relation between cavity loss tolerance and Fresnel reflectivity have been elucidated, next the TLS is tuned with a step of 1 nm from 1555 nm to 1566 nm in order to investigate the impact of BP wavelengths on this subject. All other lasing criteria such as EYDFA power and BP power are maintained in the entire assessments when is fixed at 10 μm. In this study, the flat amplitude Stokes combs plotted in Fig. 9(a) are measured by considering only channels located within 3-dB power discrepancy. From this figure, the maximum MBW of 14 nm is attained at the BP wavelength of 1557 nm. Tuning at lower wavelengths indicates the decline in this trait as the shifting away from the peak gain implies insufficient BP power availability to overcome the competing self-lasing modes. One of the examples is manifested in Fig. 9(b) at 1556.5 nm that shows the side effect of poor OSNR quality. Figure 9(a) also depicts the lasing combs tunability that can cover a range of 9 nm from 1557 to 1566 nm with the absence of self lasing appearance. Tuning at this longer wavelength segment also leads to bandwidth reduction due to the limitation in the accessible EYDFA gain bandwidth. The representation of these spectra at 1561 nm, 1564 nm and 1566 nm are shown in Figs. 9(c), 9(d) and 9(e), respectively. From all these images, it is clearly seen that the Stokes lines extend from the BP wavelength region and nearly terminated at the edge gain of 1572 nm. This behavior is similar to that observed in the conventional MBEFL. The signals also experience more peak envelopes at both sides of their uniform bandwidth which is contributed by phase mismatch at the probe signal [Figs. 9(c) to 9(e)]. These side bands represent strong cascaded FWM especially by taking into account that the zero dispersion wavelength of HNLF is situated at 1564 nm. At this wavelength regime, the proposed configuration acts as an optical parametric amplifier/oscillator. Accordingly, the number of Stokes lines is greatly reduced due to the dominant parametric effect in this region as shown in Fig. 9(d). The role of MAC that supports stronger Fresnel reflection feedback to assist high nonlinearity responses such as cascaded FWM and parametric amplification in the HNLF is proven. This light reflection scheme as explained in Fig. 2 improves the performance of MBEFL and confirms the finding reported in .
Figure 10(a) illustrates the best result that is attained when the BP wavelength is set at 1557 nm. By including the signals within 3-dB span yields 180 Stokes lines that cover 14 nm flat bandwidth segment from 1557 to 1571 nm wavelengths. The spectral envelope is also clean of spurious cavity mode noises at the bottom of its level. A closer view of the output laser spectrum measured by an OSA with resolution bandwidth of 0.02 nm is shown in Fig. 10(b). From this magnification, the lasing lines have an average OSNR of 18 dB and peak power of −8 dBm with a channel spacing of 0.08 nm. Other progresses in the same area for ordinary and half-open cavities demonstrate only 4 to 150 Stokes lines. Below 15 lasing combs, the flatness differs from 2.8 dB up to 7.8 dB [1,13–15] and the worst case of above 40 dB peak power discrepancy is defined for 150 lasing combs . In addition 437 lasing combs are developed in a Brillouin fiber laser, but the main problem is also due to more than 40 dB power discrepancy . When looking towards MBRFL, 210 to more than 500 lasing combs are initiated within 3-dB power span when incorporating only one pump source [10–12,16,17]. This shows that a lot of works need to be done in the future attempts to ensure the competency of this device. Nevertheless, the significant function of multiple Rayleigh scattering feedback in  is for initiating wider BS lines attainment. In comparison to this, the main contribution of Fresnel reflection based feedback in this research work is mainly to assist stronger FWM and self-flattening effect in the HNLF. Both theories are right under the prerequisite that the input signals are set at the specific optimized values depending on the cavity arrangement.
To confirm the reliability for robust operation, the stability is also examined at room temperature with the same setting as discussed in Fig. 10. By connecting an optical power meter to the laser device, the total output power in dBm is recorded every minute over 45 minute duration as presented in Fig. 11. It can be seen from this figure that the total fluctuations of the generated Stokes lines are about 0.76 dB by taking into account the maximum value of 14.80 dBm and the minimum value of 14.04 dBm. This indicates that our proposed MBEFL design satisfies the challenging demand for practical and industrial applications, thus favorable for many applications that include optical communications and fiber sensing.
We have proposed and demonstrated an architecture of single-spacing MBEFL based on the adjustable length of a MAC. The main strength of this attainment is the advancements to the flatness of the spectral envelope with better OSNR and Stokes peak power. These are accomplished without compromising the MBW and number of Stokes lines, which satisfy the main objectives set for this research work. This role is dictated by Fresnel reflection between fiber-to-air interfaces that dominates over relatively weaker multiple Rayleigh scattering scheme. As a result, the stronger feedback enhances cascaded FWM and self-flattening of the output spectra in the HNLF. Besides this the gain saturation is alleviated by introducing a high margin of cavity loss from 7% to 27% that relate to the modification of from 2 to 20 µm distance. This ensures the flexibility of the device as any critical cavity alignment is removed. Another striking element is the contribution of efficient pump recycling technique of the BGM in both CW and ACW directions. All these specifications result in the lower SBS threshold, thus yield improvements in Stokes line numbers with wide amplitude uniformity. When fixing the to 10 µm, a maximum of 180 Stokes lasing combs are developed with a clean-cut feature, extending from 1557 to 1571 nm wavelengths. In addition to these are the excellent OSNR of 18 dB and the exceptionally high peak power of −8 dBm. The high stability performance around 0.76 dB fluctuation over 45 minute ascertains the reliability and robustness of the device which is not restricted only to the lab-bound applications. In the next attempt, the lasing channels count can be further improved if a linear gain medium with a longer gain bandwidth is employed. This trait together with excellent bandwidth flatness and stability offer unprecedented competitiveness that previously unattainable by the restrictive competency of its predecessors.
Royal Society-Newton-Ungku Omar Advanced Fellowship (NA150463).
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