This work demonstrates a simple dual-wavelength random distributed feedback fiber laser via excitation of the Raman-erbium hybrid gain by a single pump source. Lasing wavelengths at 1568 nm and 1595 nm with 48.48 dBm maximum OSNR were generated without the need for physical reflectors. Enhancements were performed using pump power distribution and a seeded feedback to reduce the peak disparity to only 0.16 dB. The long cavity hybrid random laser with its balanced and broadly spaced dual-wavelength output offers immense potential for long distance dual laser applications.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Multiwavelength fiber lasers (MWL) have always been a popular research interest in the last few decades due to its low cost, compact structure and coupling simplicity. Apart from the more obvious deployment in wavelength division multiplexing (WDM) systems, MWL also found its niche in optical characterization and sensing. Within this category, dual-wavelength fiber lasers has its own merit, with applications in signal generation, instrument testing, and optical sensing via self-mixing interferometry [1–5].
In recent times where research on random fiber lasers has gained prominence, dual-wavelength fiber lasers based on random distributed feedback can offer a simpler design scheme. This is because random distributed feedback fiber lasers (RDB-FL) operate based on Rayleigh scattering mechanism where propagating photons inside the cavity are scattered repeatedly in an inhomogeneous amplified gain medium [6,7]. This process creates small virtual mirrors along the cavity, which assists light oscillation and increase gain of the photons simultaneously. Consequently, this eliminates the dependency of RDB-FLs on precise cavity structure and point-action reflectors. Nevertheless, reported studies on dual-wavelength generation based on random distributed feedback fiber lasers are limited. As of now, dual-wavelength generation has been demonstrated through the use of two fiber Bragg gratings of different wavelengths [1,8] with fiber loop mirrors .
One of the techniques initially proposed for multiwavelength generation random fiber lasers was to use a hybrid gain; a combination of different gains such as Raman gain and Erbium gain [9–13]. It was inferred that pairing of these two gains is suitable towards multiwavelength generation as the broad, high gain efficiency, and homogenous gain broadening of erbium-doped fiber (EDF) [14,15] supports the inherent interference process of large number of random modes instigated in the random fiber laser , while the nonlinear gain by Raman suppresses mode-hopping and mode-competition influenced by the EDF [16–19]. However, pairing of the two gains, often required multiple pumps, point action reflectors, and comb filters which led to intricate designs [9–13]. On top of that, the addition of multiple pumps and components is costly and reduces the practicality of random fiber lasers.
In this paper, an open-ended cavity random fiber laser exploiting the combined amplification of EDF and Raman is demonstrated. The proposed configuration achieves dual-wavelength generation via fiber mixing of SMF and EDF to form the cavity. Using only a single pump and without the assistance of any reflector-based components, dual-lasing peaks are obtained in two different bands; the C-band and the L-band (1568 nm and 1595 nm). To the best of our knowledge, the proposed configuration is the simplest dual-wavelength random fiber laser configuration yet. A seed signal is later added to the configuration to further enhance the performance of the dual random fiber laser allowing the initial peak power disparity to be improved from 10.94 dB to 0.16 dB.
2. Open-ended cavity hybrid erbium random fiber laser
Figure 1 shows the experimental setup of the HRFL. The setup has a symmetrical design and utilizes a 1455 nm Raman pump unit (RPU). The RPU is connected to a 3-dB coupler with each output leg of the coupler spliced to a 1480/1550 wavelength division multiplexer (WDM). Both WDMs are connected at the extreme ends of the 80 km SMF-28e fiber interposed by a spool of 50 m EDF in the middle. The EDF used in the setup is OFS RightWave LSL EDF with peak absorption of 3 dB/m at the 1455 nm pumping wavelength and peak absorption of 16 dB/m at 1530 nm wavelength . In this configuration, the launched pump power is divided equally between two WDMs, forcing the pump waves to propagate in counter-propagating directions from both ends of the fiber cavity. C-band isolators (ISO) are placed prior to the cavity output to mitigate the impact of Fresnel reflection. Spectral measurements are taken at a variation of pump powers, ranging from 0.25 W mW up to 5.5 W. The total output power is taken from the summation of Output 1 and Output 2 while the spectral results will only be shown from one side of the output as the spectra on both sides will be the same due to the symmetrical design of the cavity.
The HRFL operates based on hybrid amplification of SRS and EDF gain together with the Rayleigh scattering-contributed resonance in the SMF. The 1455 nm Raman pump is launched into the SMF-28e fiber which instigates the SRS effect. This effect shifts the pump radiation by 13.2 THz and yields a Raman Stokes emission centered around 1555-1570 nm wavelength. Residual pump power unconverted to Raman Stokes wave then excites the EDF to achieve population inversion. The generated Raman Stokes wave also becomes the incident signal to the EDF and triggers stimulated emission. This provides co-operative amplification to enhance the lasing performance. As the generated radiation propagates through the fiber, it is randomly backscattered by the Rayleigh scattering. The random scattering process advocates oscillation in the cavity, which is an essential mechanism for lasing in RDB-FL. Intracavity losses are overcome by the increasing gain as the pump is increased and lasing threshold condition is achieved.
Figure 2(a) depicts the progression of the laser from its pre-lasing condition until the eventual dual-wavelength outcome. The spectral development can be categorized into four distinct regions as indicated in Fig. 2(b). At the initial pump power of 0.25 W (region R1), the spectrum is similar to the gain profile of EDF. The low pump power is sufficient to provide inversion in the EDF but is incapable of instigating SRS, which explains the lack of Raman influence in the spectrum. As the pump power in increased to 0.60 W, a slight peak appears at 1595 nm wavelength, indicating photons at that wavelength has reached resonance and that the system is approaching threshold condition. This spectral component later develops into a gain peak composed of multiple stochastic peaks at pump power of 0.80 W (region R2). At this pump power, SRS has been initiated; hence, hybrid amplification commences.
At 0.95 W pump power, the spectral profile is loaded with stochastic peaks. The formation of the high peak gain centering at 1595 nm is quite peculiar since the ASE at the 2nd Raman gain peak (1565 nm) is considerably higher in comparison. However, this peculiarity is relevant when the absorption  and emission coefficients of the EDF are considered. With the long length of EDF employed, the ratio of emission to absorption coefficient is the highest around the longer wavelength region. This causes the initiation of random modes at 1595 nm as the emission at that location requires the least energy to kick-start the lasing process.
This situation changes with increasing pump power as the gain of erbium and Raman is pushed towards the shorter wavelength. This culminates in the formation of a peak at 1568 nm that eventually at 1.9 W, surpasses the 1595 nm peak. The ever-present stochastic emission is an effect from the combination of distributed Rayleigh scattering, stimulated Raman scattering and cascaded stimulated Brillouin scattering. The presence of the amplified narrow stochastic peaks induced the exponential growth of Brillouin cascaded effect which also causes instability in the system . Simultaneously, sufficiently high powered narrow stochastic emission in the 1st order Raman Stokes region provokes the emergence of stochastic spectral components in the 1670 nm 2nd order Raman Stokes due to the instantaneous scattering of Raman effect.
Both peaks at 1568 nm and 1595 nm wavelengths develop with increasing pump power. and eventually reaches stable operation at 4.2 W. The stable dual-wavelength laser characteristics is displayed in Table 1, where the peak power difference and 3 dB linewidth difference between the dual peaks are 10.94 dB and 0.26 nm, respectively. In the optical spectrum, this is indicated by the transformation of the stochastic profile to a ‘smooth’ profile with distinguished laser peak(s) as exhibited in Fig. 2(a). The stability of the random fiber laser is acquired by the suppression of cascaded Brillouin process that produced the chaotic-like emission displayed in Region R3 of Fig. 2(b). The stable emission in the optical spectrum is always accompanied with quasi-CW emission in the temporal domain . One of the key factors in the suppression of chaotic emission in RDB-FL is the nonlinear spectral broadening of the stochastic spikes due to cross phase modulation and four wave mixing, annihilating SBS effect [21,23]. After stability has been attained in the HRFL, the spectral broadening effect of the dual peaks is manifested from 4.20 W to 5.25 W (see Region R4 Fig. 2(b)). The nonlinear power broadening in HRFL is similar to other RDB-FLs  and also ultra-long Raman fiber lasers [24,25] whereby spectral broadening is attributed to four wave mixing caused by turbulent behavior of a large group of interacting cavity modes.
The power development of this laser is presented in Fig. 3, which exhibits threshold condition at 1.5 W. Linear power growth is observed up to the maximum output power of 900 mW with calculated slope efficiency of 25%. Interestingly, a closer look at the graph (as shown in the inset) depicts a threshold-like increase noted around 0.25 W. The slight growth is achieved as the given pump power is sufficient to produce population inversion in the EDF but is unable to generate the hybrid gain necessary for random lasing to occur. This observation matches the spectral development in Fig. 2(a).
3. Hybrid erbium random fiber laser with seeded feedback
Taking into account the high peak power disparity between the laser peaks, steps must be taken to amplify the 1595 nm peak to balance the peaks. An L-band tunable optical filter (TOF) spliced to a 3-port circulator is incorporated in the HRFL, as illustrated in Fig. 4. The structure functions as a selective wavelength reflector (SWR) almost similar to a fiber Bragg grating (FBG). The TOF used is a product from Lightwaves2020 and is characterized using Amonics C + L band ASE Light Source as shown in Fig. 5, where the suppression at 1568 nm is 34 dB, the insertion loss and 3 dB linewidth at 1595 nm are 0.89 dB and 1.24 nm, respectively. Any light that travels to the end of the cavity will go through the filter, suppressing and permitting radiation according to the wavelength profile tuned by the TOF. The filtered radiation will enter the cavity again via the circulator’s mechanism, becoming a seeded feedback to the hybrid gain.
It is assumed that by routing a 1595 nm seed back into the HRFL cavity, a higher 1595 nm peak will be produced at the output end, which will potentially provide peak power parity to the 1565 nm laser. The amount of pump power that goes into each side of the cavity should be unbalanced to minimize the differences between both peaks. The pump power distribution variation can be achieved via assorted ratio of power couplers or the employment of another pump of the same wavelength. We have chosen to use a 1455 nm pump with fixed pump power (1.5 W) employed on one side of the cavity while varying power from another 1455 nm pump on the other side of the cavity. The pump with maximum power of 1.5 W is power fixed and labeled as RPU 1 while the pump with maximum power of 5.5 W is labeled as RPU 2. The RPU of lesser power should be situated near the SWR component so that the higher power RPU can be used to form the hybrid gain spectrum purely via random distributed feedback.
The spectral results of the pumping scheme are shown in Fig. 6. When RPU 2 is set at 0 W, the sole source powering the cavity is from RPU 1 at 1.5 W. Hence, the setup is simply a forward pumped laser scheme. At this point, the spectral output shows a narrow peak at 1595 nm at approximately 0 dBm and a broad peak at 1565 nm of less than −25 dBm (see Fig. 6(a) or region R1 of Fig. 6(d)). The absence of a pump on the right end of the cavity forces the EDF to be dependent on the radiation that comes from the left-side cavity. As a result of this condition, EDF can only reach strong population inversion after the dynamics of Raman Stokes is well established in the left cavity region. This process also treats the EDF akin to an amplifier, as it will be well inverted along the length only after a strong Raman Stokes signal is available. This causes the forward ASE from EDF to be weak. The combination of this event with fiber attenuation and the distance of the pump has dissipated the ASE EDF at the output.
Subsequently, increasing RPU 2 which is near to the output side, alters the initial spectrum to adhere to the typical profile of Raman gain spectrum. This is shown by the growth of 1565 nm peak and sideband at 1555 nm which can be observed in Fig. 6(a) or region R2 of Fig. 6(d). From 1.30 W up to 2.65 W of pump power, the 1565 nm peak grows to almost the same power as the 1595 nm peak. At 2.65 W pump power from RPU 2, a stable HRFL of balanced dual wavelength laser with the least discrepancy is exhibited. Increasing the pump power beyond 2.65 W contributed to unstable laser operation which could potentially be the onset of the 2nd order Raman Stokes. This is manifested as stochastic peaks formation in the 1st and 2nd order Raman Stokes region, shown in Fig. 6(b) or region R3 and R4 of Fig. 6(d).
The power evolution of the enhanced HRFL with respect to the total pump power is exhibited in Fig. 7. The graph shows a similar linear trend with a slope efficiency of 23.7%. SWR-influenced feedback contributes to the lower slope efficiency and higher lasing threshold observed here. Nevertheless, the dual wavelength lasing operation is devoid of Brillouin-induced instabilities from the onset which is in contrast to the stochastic regime observed with the earlier HRFL configuration. It is presumed that suppression of light at the SWR minimizes resonance for the stochastic modes and elevated the SBS threshold , assisting in the formation of stable dual-wavelength laser generation at lower pump powers.
The actual wavelength, OSNR, and 3 dB linewidth of the best performance dual-laser are listed in Table 2. The best discrepancy between the peaks is calculated to be 0.16 dB and OSNR of at least 48.32 dB as measured from the peak power level to the noise level at 1568 nm wavelength. This was obtained with RPU 2 at 2.65 W and total pump power of 4.15 W.
Comparison of the performance between the open-ended HRFL and the enhanced HRFL is tabulated in Table 3. The enhanced HRFL shows better OSNR by 0.75 dB and peak discrepancy improvement of 10.8 dB compared to the open-ended HRFL. The 3-dB linewidth difference of the dual peak wavelengths is also decreased immensely. The lower pump power dedicated towards the development of 1565 nm peak and the SWR-influenced resonance of 1595 nm lasing line have limited the nonlinear broadening effect present in the laser cavity.
We have demonstrated a dual-wavelength hybrid random fiber laser at 1568 and 1595 nm which operates based on the hybrid amplification of EDF gain and stimulated Raman scattering that is powered by a single wavelength pump source. The open-ended symmetrical configuration employed balanced inward pumping scheme with 4.2 W pump power divided equally and did not integrate any reflective optical components to achieve stable lasing. The high peak power disparity of the dual-laser was then reduced from 10.94 dB to 0.16 dB using a combination of seeded feedback and asymmetric inward pumping scheme with 4.15 W total pump power at 34/64 ratio. The dual-wavelength random fiber laser is useful for typical dual wavelength source applications and could open up new opportunities for long distance self-mixing interferometry-based remote sensing applications.
UPM Research University Grant Scheme (RUGS) (05-02-12-2307RU); Academic Training Scheme for Institutions of Higher Education (SLAI); Ministry of Higher Education (MOHE) Malaysia; Royal Society Newton-Ungku Omar Advanced Fellowship (NA150463).
References and links
1. A. E. El-Taher, M. Alcon-Camas, S. A. Babin, P. Harper, J. D. Ania-Castañón, and S. K. Turitsyn, “Dual-wavelength, ultralong Raman laser with Rayleigh-scattering feedback,” Opt. Lett. 35(7), 1100–1102 (2010). [CrossRef] [PubMed]
2. S. Mo, Z. Feng, S. Xu, W. Zhang, D. Chen, T. Yang, W. Fan, C. Li, C. Yang, and Z. Yang, “Microwave Signal Generation From a Dual-Wavelength Single-Frequency Highly Er3+/Yb3+ Co-Doped Phosphate Fiber Laser,” IEEE Photonics J. 5(6), 5502306 (2013). [CrossRef]
3. D. Liu, N. Q. Ngo, S. C. Tjin, and X. Dong, “A Dual-Wavelength Fiber Laser Sensor System for Measurement of Temperature and Strain,” IEEE Photonics Technol. Lett. 19(15), 1148–1150 (2007). [CrossRef]
4. F. M. Danson, R. Gaulton, R. P. Armitage, M. Disney, O. Gunawan, P. Lewis, G. Pearson, and A. F. Ramirez, “Developing a dual-wavelength full-waveform terrestrial laser scanner to characterize forest canopy structure,” Agric. For. Meteorol. 198–199, 7–14 (2014). [CrossRef]
5. S. Ma, F. Xie, L. Chen, Y. Z. Wang, L. L. Dong, and K. Q. Zhao, “Development of dual-wavelength fiber ring laser and its application to step-height measurement using self-mixing interferometry,” Opt. Express 24(6), 5693–5698 (2016). [CrossRef] [PubMed]
6. S. K. Turitsyn, S. A. Babin, A. E. El-Taher, P. Harper, D. V. Churkin, S. I. Kablukov, J. D. Ania-Castañón, V. Karalekas, and E. V. Podivilov, “Random distributed feedback fibre laser,” Nat. Photonics 4(4), 231–235 (2010). [CrossRef]
7. S. K. Turitsyn, S. A. Babin, D. V. Churkin, I. D. Vatnik, M. Nikulin, and E. V. Podivilov, “Random distributed feedback fibre lasers,” Phys. Rep. 542(2), 133–193 (2014). [CrossRef]
9. I. Aporta Litago, R. A. Perez-Herrera, M. A. Quintela, M. Lopez-Amo, and J. M. Lopez-Higuera, “Tunable Dual-Wavelength Random Distributed Feedback Fiber Laser With Bidirectional Pumping Source,” J. Lightwave Technol. 34(17), 4148–4153 (2016). [CrossRef]
10. S. Sugavanam, M. Z. Zulkifli, and D. V. Churkin, “Multi-wavelength erbium/Raman gain based random distributed feedback fiber laser,” Laser Phys. 26(1), 015101 (2016). [CrossRef]
11. I. A. Litago, M. Á. Quintela, H. S. Roufael, and J.-M. Lopez-Higuera, “Stability study of ultra-long Random distributed feedback fiber laser based on Erbium fiber,” in Workshop on Specialty Optical Fibers and Their Applications (OSA, 2015), p. WT4A.18. [CrossRef]
12. S. Qin, Y. Tang, and D. Chen, “Multi-wavelength hybrid gain fiber ring laser based on Raman and erbium-doped fiber,” Chin. Opt. Lett. 4, 50–52 (2006).
13. S. Wang, W. Lin, W. Chen, C. Li, C. Yang, T. Qiao, and Z. Yang, “Low-threshold and multi-wavelength Q-switched random erbium-doped fiber laser,” Appl. Phys. Express 9(3), 032701 (2016). [CrossRef]
14. Z. Chun-Liu, Y. Xiufeng, L. Chao, N. J. Hong, G. Xin, P. R. Chaudhuri, and D. Xinyong, “Switchable multi-wavelength erbium-doped fiber lasers by using cascaded fiber Bragg gratings written in high birefringence fiber,” Opt. Commun. 230(4–6), 313–317 (2004). [CrossRef]
15. Y. Liu, X. Dong, M. Jiang, X. Yu, and P. Shum, “Multi-wavelength erbium-doped fiber laser based on random distributed feedback,” Appl. Phys. B 122(9), 240 (2016). [CrossRef]
16. S. Pan, C. Lou, and Y. Gao, “Multiwavelength erbium-doped fiber laser based on inhomogeneous loss mechanism by use of a highly nonlinear fiber and a Fabry-Perot filter,” Opt. Express 14(3), 1113–1118 (2006). [CrossRef] [PubMed]
17. A. Bellemare, M. Karásek, M. Rochette, S. LaRochelle, and M. Têtu, “Room temperature multifrequency erbium-doped fiber lasers anchored on the ITU frequency grid,” J. Lightwave Technol. 18(6), 825–831 (2000). [CrossRef]
18. P. Wang, D. Weng, K. Li, Y. Liu, X. Yu, and X. Zhou, “Multi-wavelength Erbium-doped fiber laser based on four-wave-mixing effect in single mode fiber and high nonlinear fiber,” Opt. Express 21(10), 12570–12578 (2013). [CrossRef] [PubMed]
19. M. Bravo, V. de Miguel Soto, A. Ortigosa, and M. Lopez-Amo, “Fully Switchable Multi-Wavelength Fiber Lasers Based on Random Distributed Feedback for Sensors Interrogation,” J. Lightwave Technol. 33(12), 2598–2604 (2015). [CrossRef]
20. M. H. Abu Bakar, F. R. Mahamd Adikan, and M. Mahdi, “Rayleigh-Based Raman Fiber Laser With Passive Erbium-Doped Fiber for Secondary Pumping Effect in Remote L-Band Erbium-Doped Fiber Amplifier,” IEEE Photonics J. 4(3), 1042–1050 (2012). [CrossRef]
21. A. A. Fotiadi, “An incoherent fibre laser,” Nat. Photonics 4(4), 204–205 (2010). [CrossRef]
22. S. A. Babin, A. E. El-Taher, P. Harper, E. V. Podivilov, and S. K. Turitsyn, “Tunable random fiber laser,” Phys. Rev. A – At. Mol. Opt. Phys. 84(2), 1–4 (2011).
23. L. Wang, X. Dong, P. P. Shum, C. Huang, and H. Su, “Erbium-doped fiber laser with distributed Rayleigh output mirror,” Laser Phys. 24(11), 115101 (2014). [CrossRef]
25. S. A. Babin, V. Karalekas, E. V. Podivilov, V. K. Mezentsev, P. Harper, J. D. Ania-Castañón, and S. K. Turitsyn, “Turbulent broadening of optical spectra in ultralong Raman fiber lasers,” Phys. Rev. A – At. Mol. Opt. Phys. 77(3), 1–5 (2008).