A wide bandwidth and flat multiwavelength Brillouin-erbium fiber laser is demonstrated experimentally. In the proposed laser setup, the combination of a Brillouin mirror with feedback and a ring cavity with four-wave mixing assistance is realized. The efficiency of Brillouin Stokes lines generation is enhanced by the feedback-based Brillouin mirror structure. The effect of four-wave mixing in highly nonlinear fiber increases the generation of Brillouin Stokes lines in a wider bandwidth. The laser lines over 16 nm bandwidth (i.e 200 channels) within 4.65 dB power difference are obtained. The generated laser lines span from 1534 to 1550 nm with wavelength spacing of 0.08 nm and optical signal-to-noise ratio of at least 15 dB. The laser can also be freely tuned over 32 nm and is stable with power fluctuations of 0.7 dB over 1 hour duration.
© 2017 Optical Society of America
Fiber lasers have drawn interest of researchers in recent decades as a result of their potential applications in optical communication, sensing, spectroscopy and imaging to name a few . Consequently, various types of fiber lasers have been proposed and demonstrated to date in order to fulfill the needs and improve the laser performances. Among mechanisms that have been adopted to realize fiber lasers is the manipulation of nonlinear phenomena in optical fibers such as stimulated Brillouin scattering (SBS), stimulated Raman scattering and four-wave mixing (FWM). Examples of fiber laser technology that manipulate fiber nonlinearities are multiwavelength erbium-doped fiber laser (EDFL) , fiber optical parametric oscillator (FOPO) [3, 4], Brillouin-Raman fiber laser (BRFL) [5, 6] and Brillouin-erbium fiber lasers (BEFL) [7–20]. Multiwavelength BEFLs have the edge over multiwavelength EDFLs and FOPOs owing to their narrow linewidth, single longitudinal mode frequency and narrow wavelength spacing of 0.08 nm . For BRFLs, the fiber section in the cavity is utilized as a gain medium and SBS generation simultaneously. In order to create adequate gain for lasing, BRFLs are injected with extreme pump power for the SRS generation. On the contrary, BEFLs require moderate pump powers to generate multiwavelength outputs. Nevertheless, the main challenge of BEFL is to produce more channels that are comparable to the ones generated by BRFLs.
The large number of channels is of significant interest as it can increase the performance of a system to support larger bandwidths in optical communication systems. The first experimental demonstration of multiwavelength BEFLs was reported in  where only as many as six channels were generated. To enhance the number of channels further, a Sagnac loop filter was incorporated in the laser cavity, resulting in generation of 12 channels . In addition, applying single and double pass Brillouin pump (BP) pre-amplification technique  meanwhile were reported to be effective in increasing the number of channels up to 18 and 28 respectively . Motivated for higher numbers of channels, self-seeded multiwavelength BEFLs were proposed in which the external BP was avoided in the design. Based on the proposed laser structures, as many as 120 , 160  and 200  channels were experimentally demonstrated as a result of self-seeded operation. Despite the enhancement of many channels number, the multiwavelength lasers suffer from low optical signal-to-noise ratio (OSNR) and output power. In addition, the operation requires adjustment of the polarization controllers, thus posing extra complexity into the design. In more recent developments, the self-seeded technique was revisited in multiwavelength BEFLs with enhanced cavity designs [13, 14]. Although the channel OSNRs are improved in those designs, the number of channels are limited to just 16  and 67  only. Meanwhile, in order to have more functionality, BEFLs with switchable frequency spacing of 11 and 22 GHz were proposed [15, 16]. However, these proposed techniques produce only 5 lines  and 16 lines . This is owing to the complexity of wavelength tuning mechanism that works at the expense of cavity loss. Therefore, the generation of larger number of laser lines becomes more challenging.
Another method to increase the number of channel is to incorporate the FWM effect in BEFL structures [17, 18]. Based on the published works, both designs can produce output channels up to 150 lines as a result of the assistance of FWM processes in highly nonlinear optical fibers. The obtained OSNR are reasonable but the designs have a disadvantage of flatness due to the emergence of anti-Stokes lines as a result of FWM effect. The peak power difference between the highest and lowest channel is found to be in excess of 40 dB and this could be detrimental for some applications especially in optical communication systems. In essence, previous multiwavelength BEFLs are deficient of a large number of channels with reasonable flatness and OSNR.
In this work, a wide bandwidth (i.e large number of channels) and flat multiwavelength BEFL is proposed. The architecture is based on the combination of a ring cavity with FWM assistance and a Brillouin mirror with feedback. The combination of these two elements presents uniqueness in generating multiwavelength output in a wider bandwidth. Based on this enhanced design, a multiwavelength BEFL with a wide bandwidth of 16 nm which translates to 200 channels with OSNR exceeding 15 dB and flatness of 4.65 dB is experimentally demonstrated. A tuning range of 32 nm is also realized for this laser configuration.
Three configurations of experimental setup are proposed in this work as illustrated in Figs. 1(a)-1(c). The first configuration (Configuration A), see Fig. 1(a), consists of a ring cavity multiwavelength BEFL with a Brillouin mirror. The internal reflection is formed through the SBS effect in 11 km long dispersion compensating fiber (DCF) with specification of 5 dB total insertion loss, 7.31 (W.km)−1 nonlinear coefficient and 20 μm2 effective area. In this Brillouin mirror structure, the other end of DCF is properly terminated with an angle-cleaved to minimize the Fresnel reflection. The ring cavity contains a circulator C2 and a commercial erbium/ytterbium-doped fiber amplifier (EYDFA), IPG Photonics model EAU-1LT, as the gain medium. The insertion of EYDFA in the ring cavity is also essential to ensure the light propagation in unidirectional only (anti-clockwise direction). The Brillouin pump (BP) for this laser is catered by a tunable laser source and is coupled into the cavity through a circulator C1 and a 3-dB coupler. Part of light oscillating in the cavity is coupled out through the 3 dB coupler and C1 to become the laser output in which the spectrum is recorded by an optical spectrum analyzer with the resolution bandwidth of 0.015 nm. The operating principle of the proposed laser in the Configuration A can be described as follows; at first, the injected BP signal from the TLS propagates to the ring cavity through port 1 and 2 of coupler in which light amplification by the EYDFA takes place. Afterwards, it is guided to the DCF through port 2 and 4 of coupler that acts as the Brillouin mirror. When the amplified BP signal power exceeds the SBS threshold of the DCF, the first order Brillouin Stokes (BS1) signal is created and propagates in the opposite direction to the amplified BP signal. The BS1 has a red-shifted wave of 0.08 nm (10.8 GHz frequency shift) from its pump (BP) central wavelength. In this case, the BS1 propagates from the port 4 to the port 2 of coupler and no power is diverted to the port 3 of coupler (open path). From the port 2 of coupler, the BS1 and BP signals are subsequently guided into the ring cavity via the circulator C2. For the BP signal, the round trip condition is satisfied and for the BS1 signal, it is amplified by EYDFA and its round trip is completed when it reaches the DCF in the Brillouin mirror structure. Reaching the DCF, the second order Brillouin Stokes (BS2) signal is created when the SBS threshold condition is satisfied. In the same way, the BS2 creates the third order Brillouin Stokes (BS3) signal. The process continues until the power of the Stokes signal is not high enough to trigger the SBS effect, hence ceasing the creation of subsequent higher order Stokes signals.
To enhance the bandwidth of the multiwavelength BEFL, a feedback is introduced in the Brillouin mirror as shown in Configuration B [see Fig. 1(b)]. The feedback is formed by connecting the port 3 of the 3-dB coupler to the DCF, thus establishing a secondary cavity into the setup. The operating principle is as follows; at first, the BP propagates into the ring cavity (primary cavity) for amplification before heading towards the DCF in the secondary cavity. When the BP goes beyond the SBS threshold power of the DCF, BS1 is triggered in the opposite direction. However, not all the incident BP power is transferred to the backward BS1 . There is some remaining BP power that propagates in the forward direction and it ends up oscillating in the secondary cavity (clockwise direction). Due to Rayleigh feedbacks, this oscillation contributes to higher power for the oscillating BP in the primary cavity, resulting in a reduction of BS1 threshold power in the DCF. The same mechanism applies for BS2 threshold power. The Rayleigh feedback of the oscillating BS1 in the secondary cavity reinforces the oscillating BS1 in the primary cavity. This causes a reduction of BS2 threshold power in the DCF. In summary, the reduction of Stokes lines (BS1, BS2 etc) threshold power in the DCF will accommodate many more laser lines for an EYDFA output power and this will enhance the number of spectral lines in the proposed multiwavelength BEFL.
In order to further enhance the bandwidth of the multiwavelength BEFL, a highly nonlinear fiber (HNLF) is inserted into the ring cavity, called Configuration C as depicted in Fig. 1(c). The ring cavity now contains the HNLF which has nonlinear coefficient of 11.5 (W•km)−1, dispersion slope of 0.016 ps/(nm2•km), zero dispersion wavelength of 1557.6 nm, length of 500 m and effective area of 11 μm2. The operating principle is as follows; when the BP reaches the HNLF, part of the power is backward reflected and the remaining is forward transmitted to the EYDFA . The backward light is blocked by circulator C2, so the forward transmitted BP is the one that oscillates in the cavity. The amplified remaining BP then propagates to the DCF and creates BS1 based on the operating principle aforementioned above. As the backward signal is blocked by the C2, the signals that oscillate are the forward transmitted BP and BS1. Along the HNLF, the remaining BP and BS1 become the seeds for FWM processes to generate many more spectral lines. The BP, BS1 and FWM products then get amplified before they are guided to the DCF. The mechanism afterwards is the same as the previously explained. All in all, the FWM processes in the HNLF and the Rayleigh feedback of the oscillating signals in the secondary cavity enhance the power of oscillating lasers, leading to improvement in the number of spectral lines generated.
3. Results and discussions
Firstly, the effect of laser configuration on the BEFL performance is investigated in the absence of the BP signal as illustrated in Fig. 2. In this experiment, the output spectrum is measured at the EYDFA output power of 350 mW. The initial oscillation signal is generated from the amplified spontaneous emission (ASE) of the EYDFA itself. In this case, all laser configurations operate under free running condition that generate unstable lasing modes which are termed as stochastic modes. For the Configuration A, the feedback mechanism is solely based on the Rayleigh scattering from the DCF. Thus, the highest peak at 1544 nm range is observed which corresponds to the peak of erbium/ytterbium gain profile. The stochastic modes fluctuate in terms of intensity within the wavelength range of 1530 – 1565 nm. The same observation is also recorded for the Configuration B. However, the out-of-band noise floor of stochastic modes is raised. This increment is owing to the effect of the continuous oscillation of ASE in the feedback ring (clockwise direction). Therefore, the Rayleigh scattering in the feedback ring (anti clockwise direction) plays major role for the signal oscillation build-up in the laser cavity. Since the stochastic modes operate in the saturation region, therefore no significant increment of peak power is observed. In general, the feedback is stronger in the case of Configuration B as compared to Configuration A. On the other hand, when the HNLF is introduced in the ring cavity (Configuration C), the output spectrum changes dramatically. The bandwidth of stochastic modes is greatly widened (1542 – 1555 nm) at the expense of peak powers. Under this condition, the lasing powers are transferred to the longer wavelengths through the FWM process in the HNLF. These findings give an indication that the overall cavity gain can be enhanced in a wider wavelength range which is beneficial for multiwavelength laser generation.
The bandwidth of the laser configurations in the presence of 1540 nm BP wavelength is then investigated. We define the bandwidth as a region over which the peak power of the spectral lines must be at least −20 dBm. In this investigation, the BP and EYDFA powers are fixed at −8 dBm and 350 mW, respectively. Figure 3 shows the bandwidths of those three laser configurations. In case of employing Brillouin mirror only (Configuration A), the Stokes lines spread over wavelength range of 0.6 nm. The low bandwidth is attributed by insufficient power of higher order Stokes lines to overcome the SBS threshold. This is closely related to the wavelength range of free running modes as shown in Fi.e 2. In the presence of the feedback though (Configuration B), the spectrum bandwidth is improved to 1.3 nm. This results from the stronger feedback mechanism that contributes to higher power for the Stokes lines. Thus, more higher order Stokes lines are generated due to more Stokes lines having sufficient power to overcome the SBS threshold. When the HNLF is inserted into the setup (Configuration C), the bandwidth is notably increased to 7.7 nm. The enhancement is due to the Rayleigh scattering in the feedback ring and FWM processes in the HNLF. The proposed fiber laser setup (Configuration C) promotes these co-operative processes that lead to the enhancement of Brillouin Stokes (BS) lines generation.
The BEFL output spectrum with different BP powers is then investigated at the EYDFA power of 350 mW. In this study, the BP wavelength is fixed at 1540 nm and its power is varied. Figure 4 shows the BEFL spectrum with respect to the variation of the BP power. It is found that the number of spectral lines reduces with the increment of BP power. In other words, the BEFL bandwidth reduces as BP power increases. This is due to the fact that EYDFA is pushed to its saturation regime and less gain is available to amplify the higher order BS lines . Therefore, the number of Stokes lines is reduced or smaller BEFL bandwidth is obtained in this experiment.
The behavior of the BEFL against the EYDFA output power is studied afterwards as illustrated in Fig. 5. The BP wavelength is fixed at 1540 nm with the power of −10 dBm, while the power of EYDFA is varied from 0 mW to 350 mW. In this experiment, the first Stokes line appears at the EYDFA output power of 12 mW which marks the pump power threshold at the set BP power. As the EYDFA output power increases, the number of the generated Stokes lines increases as well. The enhancement is attributed to the higher efficiency of FWM processes in the HNLF. An increase of EYDFA power means more power is transferred to higher order Stokes lines, resulting in an increase of the BEFL bandwidth. At the EYDFA output power of 340 mW, a large number of lasing wavelengths is obtained with the spectral bandwidth is maximized to 9 nm, ranging from 1540 nm to 1549 nm. The definition of the bandwidth is stated earlier.
Figure 6(a) depicts measurements for the total output power of the BEFL as a function of BP wavelength ranging from 1534 nm to 1565 nm at the EYDFA output power of 350 mW and BP power of −12 dBm. The ASE output from the EYDFA is also plotted as a reference. From this figure, the data shows the effect of the EYDFA gain profile on the total output power of the BEFL, where the highest output power is obtained at two wavelength regions centered at 1535 nm and 1544 nm. When the BP wavelength is tuned to longer wavelengths, a gradual decrease in the total output power is obtained. This corresponds to the lower EYDFA output power (lower gain) that cannot generate higher order Stokes lines in a wider bandwidth. For other BP wavelengths not in the stated range, no appearance of BS lines is observed, see Fig. 6(b). From these findings, strong free-running modes are formed from 1543 nm to 1555 nm that consume most of the pump energy. Therefore, the generation of BS line for BP wavelengths in low gain region cannot be realized owing to this scenario.
Tunability is of importance as it can provide flexibility for an optical communication system. In this setup, the tunability of the laser is achieved by varying the BP wavelength from 1534 nm to 1566 nm. The EYDFA and BP powers are fixed at 350 mW and −12 dBm respectively. Under these conditions, the output spectrum at different BP wavelengths is recorded as illustrated in Fig. 7(a) and subsequently the spectral bandwidth of the BEFL as a function of the BP wavelength is plotted, see Fig. 7(b). The magnified view of the generated Stokes lines for the BP wavelength of 1534 nm is illustrated in Fig. 7(c). From this experiment, up to 16 nm of BEFL spectral bandwidth with the peak power difference of 4.65 dB is obtained from 1534.2 nm to 1550.9 nm. This output spectrum consists of BS lines with 0.08 nm spacing, see Fig. 7(d).
It can be seen in Figs. 7(a) and 7(b) that the spectral bandwidth of the BEFL becomes narrower as the BP wavelength is tuned to longer wavelengths. This is related to the limitation of the erbium/ytterbium gain profile, see Fig. 6(a). Apparently, the gain profile of erbium/ytterbium starts from 1530nm and ends at 1565 nm. Therefore, when the BP wavelength is set at longer wavelengths towards the edge of gain profile, the availability of gain bandwidth becomes narrower. In addition, the gain is also decreased at longer wavelength, thus leading to the fewer number of generated BS lines as a result of insufficient signal power to exceed the SBS threshold .
The stability of the multiwavelength BEFL, which is an important characteristic for optical communication systems, is also measured. For the evaluation, the EYDFA and BP signal at 1540 nm are fixed at 350 mW and −12 dBm respectively. The total output power of the BEFL is measured for every one minute over one hour by utilizing an optical power meter. Figure 8 depicts the total output power fluctuations of the BEFL spectrum. It is observed that the BEFL is stable with output power fluctuations of less than 0.7 dB over one hour. This shows that the proposed BEFL is suitable to be utilized over a long period of time.
Experimental demonstration of a wide bandwidth (i.e high number of channels) and flat multiwavelength BEFL is reported in this paper. The combination of the ring cavity with FWM assistance (primary cavity) and the Brillouin mirror with feedback (secondary cavity) in the design leads to the generation of 16 nm spectral lines (i.e 200 channels) with flatness of 4.65 dB. In this configuration, the 500 m HNLF placed in the primary cavity serves as the seeds provider via FWM processes and the introduction of the secondary cavity provides Rayleigh components for the oscillating signals. Besides wide bandwidth, the proposed laser has wide tunability of 32 nm and is also stable with output power fluctuations of less than 0.7 dB over one hour.
King Saud University, Saudi Arabia (International Scientific Partnership Program, ISPP#0106).
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