Open Access
Add to BibSonomy
Abstract
A random fiber laser with flexible wavelength interval switching is proposed and demonstrated through two switching methods. One is to change the effective structure of the laser cavity by controlling the switches of 980 nm pump laser diodes (LDs) for erbium-doped fibers (EDFs), which can achieve the switching of the wavelength interval from a single Brillouin frequency shift (BFS) of 0.088 nm to a double BFS of 0.176 nm. Another method is to manipulate the gain provided by the two EDF amplifiers by controlling the power of the three 980 nm LDs, thereby realizing the optical switching of the wavelength interval. This kind of wavelength interval switchable random fiber laser increases the flexibility and functionality of multi-wavelength light sources, and further expands the application range of the random fiber lasers. Furthermore, the alternative wavelength interval switching mechanisms with simple structure enable it to meet the application requirements of various occasions.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Recently, a kind of laser which uses Rayleigh scattering (RS) caused by the inherent refractive index inhomogeneity in optical fiber to provide feedback for laser oscillation has attracted much attention, which is called random distributed feedback fiber laser (RDFBFL) by Turitsyn [1]. Benefiting from both distributed Raman gain and RS in ultra-long single-mode fiber (SMF), the random backward RS light can be amplified and maintain oscillation. Therefore, the RDFBFL has the unique advantages of simple configuration, easy maintenance and flexible design. Various research motivations have been successively carried out, such as high power and high efficiency [2–5], multi-wavelength generation [6–8], wavelength tunability [9], narrow linewidth [10–13] and different operation wavebands [14–16].
Stimulated Brillouin scattering (SBS) is an important nonlinear effect in SMF. Its low threshold and frequency shift characteristics may contribute to the generation of multi-wavelength emission. For most Brillouin fiber lasers (BFLs), the wavelength interval is fixed at a single Brillouin frequency shift (BFS) of about 0.088 nm, which greatly limits the application of the laser in optical communication systems and lacks flexibility. In order to overcome this limitation, several methods have been proposed to adjust the optical frequency interval. One is to slightly adjust the frequency interval of the output Stokes light based on the coordination between the comb interval of the additional filter and the Stokes comb interval [17,18]. In this method, the comb spacing of the used filter must be close to that of the Stokes light comb, so the adjustable range of the frequency interval of the output light is limited to 0.08 nm∼0.088 nm. The second method is to use the first- and third-order Stokes light initially generated by SBS to generate higher odd order Stokes light through four-wave mixing effect [17,19]. This method can generate Stokes light sequence with a double BFS interval, but the intensity of Stokes light decreases sharply with the increase of order. The third method is to use the bidirectionality of the laser to output odd Stokes light and even Stokes light in clockwise and counterclockwi sedirections, respectively [17,20]. The cavity can be a single ring structure or a double ring structure, and the frequency interval of output light is twice that of the BFS. However, all of the above lasers only produce a fixed output with a single or multiple BFS intervals. For some applications that require adjustable wavelength intervals, multi-wavelength light sources with fixed output wavelength intervals need to be replaced repeatedly, which increase the overall cost. Random fiber lasers with switchable wavelength intervals embody the flexibility and functionality of the multi-wavelength light sources.
As a new type of BFL, RDFBFLs based on Brillouin gain not only have the advantages of SBS lasers, but also can combine the advantages of random fiber lasers, especially in multi-wavelength generation [21]. Multi-wavelength fiber lasers have been widely used in wavelength division multiplexing systems, optical fiber sensing systems and spectroscopy [22–24]. In this paper, a multi-wavelength Brillouin erbium-doped random fiber laser with switchable wavelength interval is proposed by controlling and optimizing the gains of two erbium-doped fiber amplifiers (EDFAs).
2. Experiment setup and principle
The proposed multi-wavelength Brillouin-erbium RDFBFL with switchable wavelength interval has a half-open-cavity configuration, as shown in Fig. 1. The laser configuration is composed of two parts: the left ring structure and the right linear structure. A tunable laser source (TLS) with a tuning range of 710 nm (970 nm to 1680 nm) and an output power range of 7.4 dBm to 12.4 dBm is used as the Brillouin pump (BP). The ring cavity can not only provide unidirectional feedback, but also be used for Brillouin pump injection and laser output. The linear structure is mainly composed of two rolls of 10 km SMF, two EDFAs pumped through three LDs (P1, P2 and P3) and an isolator. Long SMF can provide both Brillouin gain and random distributed feedback for SBS. The isolator on the far right avoids Fresnel reflection and ensures stable random laser output. The EDFAs are used to compensate for low Brillouin gain. An optical spectrum analyzer (OSA, AQ-6370D) monitors the spectral characteristics at the output of the left ring structure.
Fig. 1. Experimental setup of our proposed multiwavelength Brillouin-erbium RDFBFL with switchable wavelength interval.
When the pump power of P1 is kept at 0 mW and the total power of P2 and P3 is over the threshold, the working principle of the laser is as follows: firstly, the BP light enters the linear cavity through the 3 dB coupler and the port 2 of the circulator. As P1 power is 0 mW, the BP light is not amplified by the 1.1 m erbium-doped fiber (EDF). When the BP light passes through SMF1 (SBS may occurs), the 1.3 m EDF provides linear gain for the BP light. Once the amplified BP light reaches the SBS threshold in SMF2, the first-order Stokes light will be excited. The backward propagating first-order Stokes light will pass through 1.3 m EDF with amplification. Once the threshold value of the second-order Stokes light is reached, the second-order Stokes light will be generated in SMF1. The remaining first-order Stokes light will enter the ring cavity through the port 3 of the circulator and output from the 3 dB coupler. At the same time, SMF2 may provide random distributed feedback for the first-order Stokes light. The second-order Stokes light will undergo the same process as the BP light in the linear cavity, and the third-order Stokes light will be output at the output port of the ring cavity. Therefore, as long as the low-order Stokes light amplified by the 1.3 m EDF meets the threshold value of the high-order Stokes light, a cascaded Brillouin process can be formed in SMF1 and SMF2. As a result, the spectrum with a double BFS interval can be observed at the output port.
On the other hand, when the total power of P2 and P3 is kept at 0 mW and the power of P1 is over the threshold, the working principle of the laser is as follows: the BP light is amplified by the 1.1 m EDF after passing through the 3 dB coupler and the port 2 of the circulator. As the total pump power of P2 and P3 is 0 mW, the 1.3 m EDF does not amplify the BP light. At this time, the SBS in SMF1 and SMF2 together provides nonlinear gain, and RS functions as random distributed feedback. When the SBS threshold is reached, the first-order Stokes light will be excited in SMF1 and SMF2. After the first-order Stokes light passes through the left ring, 50% of the power is output, and the rest is used as the new pump light. When the linear cavity part meets the second-order Stokes light threshold, the second-order Stokes light will be excited. This cascaded process will continue until the threshold of some high order Stokes is not satisfied. In this way, the spectrum with a single BFS interval can be observed at the output port of the laser. In more complicated cases, through the investigation of the influence of two EDFAs working together on the laser output, it is found that in addition to choosing whether one EDFA is in working state to control the wavelength interval switching, the function of wavelength interval switching can also be realized by reasonably controlling the power of P1, P2 and P3.
3. Laser characteristics
3.1 Multiwavelength output with wavelength interval of single BFS
When the total power of P2 and P3 is 0 mW, we systematically study the influences of P1 power, BP power and BP wavelength on the output spectrum. Figure 2 shows the output spectra at different P1 power when the BP wavelength and pump power are fixed at 1562.83 nm and 7.4 dBm, respectively. When the power of P1 is 130 mW, it just reaches the threshold of the first-order Stokes light, as shown in the black line in Fig. 2(a). When the power of P1 is 230 mW, 430 mW, 530 mW and 630 mW, the corresponding highest order Stokes light is three, six, seven and nine, respectively. Therefore, increasing the power of P1 not only increases the power of each order Stokes light in the output spectrum, but also increases the number of Stokes lines. Figure 2(b) is an enlarged view of the blue part of Fig. 2(a). It can be clearly seen that the wavelength intervals between the first-order Stokes light and the BP light, the second-order Stokes light and the first-order Stokes light are all 0.088 nm of a single BFS.
Fig. 2. (a) Spectra at different P1 powers when the total pump power of P2 and P3 is 0 mW; (b) local enlargement of the marked portion in Fig. 2(a).
Figure 3 shows the spectra of different BP powers when the total pump power of P2 and P3 is 0 mW. Here the BP wavelength is set to 1562.80 nm, and the power of P1 is set to 670 mW. When the BP power is 7.4 dBm, 9.4 dBm and 11.4 dBm, there are seventeen, fifteen and eight order Stokes lines in the spectra, respectively. It can be seen that as the BP power increases, the power of each order in the spectrum increases slightly, but the number of Stokes lines decreases. Under the optimized conditions, i.e. the P1 power of 670 mW and BP power of 7.4 dBm, the number of Stokes lines is as high as seventeen, and the interval between neighboring Stokes lines is about 0.088 nm. The tuning range is also measured under this condition.
Figure 4 is the output spectra obtained by changing the BP wavelength. When the BP wavelength is set at 1540.88 nm, seven Stokes lines appear (Fig. 4(b)). When the BP wavelength is set at 1580 nm, there are nine Stokes lines in the spectrum (Fig. 4(c)). The spectra with the BP wavelengths of 1550 nm, 1560 nm, and 1570 nm have also be presented. In the process of adjusting the BP wavelength, the spectra always keep stable multi-wavelength lasing, and there is no self-excited cavity mode. Therefore, when the wavelength interval is a single BFS, the laser tuning range can achieve ∼40 nm. In addition, when the pump wavelength is 1562.828 nm, we select 9 Stokes lines to monitor the stability of the laser within one hour, as shown in Fig. 5. Figure 5(a) and Fig. 5(b) shows the wavelength stability and peak power stability, respectively.
Fig. 4. (a) Output spectra with BP wavelength tuned from 1540.88 nm to 1580.852 nm. Spectra at BP wavelength of (b) 1540.88 nm and (c) 1580.852 nm.
Fig. 5. Stability measurement when the total pump power of P2 and P3 is 0 mW: (a) wavelength shift and (b) Stokes peak power fluctuation.
3.2 Multiwavelength output with wavelength interval of double BFS
When the power of P1 is 0 mW, the effects of the total power of P2 and P3, BP power and BP wavelength on the output spectrum are then studied. Firstly, the BP wavelength and power are fixed at 1562.82 nm and 7.4 dBm, respectively. When the total pump power of P2 and P3 is 200 mW, the threshold of the first-order Stokes light is reached. The first-order Stokes light appears in the spectrum (Fig. 6(a)), and the wavelength interval between the first-order Stokes light and the BP is the single BFS of about 0.088 nm (Fig. 6(b)). When the total pump power of P2 and P3 is 600 mW, there are seven Stokes lines in the spectrum, and the interval between neighboring lines becomes a double BFS of 0.176 nm (Fig. 6(b)). This is because when the P1 power is 0 mW, the even order Stokes light and the odd order Stokes light are separated, and thus the output interval of the random fiber laser is doubled. When the pump power is 950 mW, there are still seven Stokes lines in the spectrum, but with higher intensity.
Fig. 6. (a) Spectra at different P2 and P3 powers when the power of P1 is 0 mW; (b) local enlargement of the marked portion in Fig. 5(a).
Figure 7 shows the effect of increasing the BP power on the output spectrum when the output wavelength interval is about 0.176 nm. The BP wavelength is kept unchanged and the total power of P2 and P3 is set to 950 mW. When the BP power is 7.4 dBm, 9.4 dBm and 12.4 dBm, eight odd Stokes lines can be always observed in the spectra, which indicates that the EDF gain for BP is not saturated and has little effect on the number of output Stokes lines.
Fig. 8. (a) Output spectra with BP wavelength tuned from 1550.088 nm to 1600.808 nm. Spectra at BP wavelength of (b) 1550.088 nm and (c) 1600.808 nm.
Figure 8 shows the spectra with a double BFS under different BP wavelengths. The total power of P2 and P3 is set to 950 mW, and the BP power is set to 12.4 dBm. When the BP wavelength is 1550.088 nm, nine Stokes lines can be observed in the spectrum (Fig. 8(b)), and there is no self-excited cavity mode in the range from 1545 nm to 1610 nm. When the pump wavelength is 1600.808 nm, eleven Stokes lines appear in the spectrum (Fig. 8(c)). In addition, the spectra at 1560 nm, 1570 nm, 1580 nm and 1590 nm are also given, as shown in Fig. 8. Therefore, when the output wavelength interval is a double BFS, the tunable range of the laser is increased to 50 nm. Similarly, when the pump wavelength is 1562.828 nm, 13 Stokes lines are chosen to monitor the stability of the wavelength and peak power within 1 hour, as shown in Fig. 9.
Fig. 9. Stability measurement when the total pump power of P1 is 0 mW: (a) wavelength shift and (b) Stokes peak power fluctuation. Fig. 10. Spectra under different P2 and P3 powers when the power of P1 is 630 mW.
3.3 Comprehensive effect of P1, P2 and P3 on output characteristics
As described above, the switching of the wavelength interval is realized by selectively turn on the EDFAs used in the laser cavity. However, it is also possible to develop a switching method by controlling the gain of the two EDFAs. The next main work is to investigate the output characteristics of the random laser when the three pumps work together. In Fig. 10, the spectral wavelength interval is a single BFS when the P1 pump power, the BP wavelength and BP power are fixed at 630 mW, 1562.816 nm and 12.4 dBm, respectively. When the total power of P2 and P3 is 150 mW, the 11th order Stokes light can be observed in the spectrum, and the power of higher order Stokes light is gradually lower than that of lower order Stokes light, as shown by the black line in Fig. 10. When the total power of P2 and P3 is 550 mW, there is no significant increase in the number of Stokes lines in the spectrum, and the wavelength interval is still a single BFS, but the intensity of each wavelength is slightly increased, and the four-wave mixing effect is also enhanced. When the pump power is 950 mW, the intensity of each wavelength is significantly increased, and the stronger four-wave mixing effect increases the number of Stokes light and anti-Stokes light on both sides of the spectrum, as shown by the blue line in Fig. 10. To sum up, when P1 is 630 mW, even though the total power of P2 and P3 increases a lot, the switching of wavelength interval will not happen, and the wavelength interval remains a single BFS. The whole spectrum is only partially changed in terms of the output optical intensity of the Stokes lines and the enhancement of the four-wave mixing effect. It can be concluded that the high P1 power plays a dominant role in the laser cavity, resulting in the unchanged wavelength interval of the single BFS, which is independent of the total power of P2 and P3.
Figure 11 shows the spectra at different powers of P1, and the total power of P2 and P3. As shown in Fig. 11(a), the BP wavelength and BP power are fixed at 220 mW, 1562.44 nm and 12.4 dBm, respectively. When the P1 power is 0 mW, the first-order Stokes light threshold is reached, but the generated Stokes light power is significantly lower than the pump light. When the P1 power is 73 mW, the first and third order Stokes lines appear in the spectrum, and the power difference between the first and second order Stokes light is 22.256 dB, so the wavelength interval is obviously a double BFS of about 0.176 nm. When the P1 power is 94 mW, the first-order and third-order Stokes lines are still in the spectrum, but the power difference between the first and second order is reduced to 17.676 dB. When the P1 power is 135 mW, the second-order Stokes light power increases obviously, and the third-order Stokes light has not reached the threshold, so the third-order Stokes light is not completely stable. However, the power difference between the first and second order has been reduced to 6.505 dB, so the wavelength interval has been switched from 0.176 nm to 0.088 nm. It can be concluded that when the total power of P2 and P3 is set to 220 mW, the gradual increase of the P1 power can achieve the purpose of wavelength interval switching. Figure 11(b) and Fig. 11(c) correspond to the total power of P2 and P3 is 350 mW and 950 mW, respectively.
Fig. 11. Spectra at different powers of P1 when the total power of P2 and P3 is (a) 220 mW, (b)350 mW and (c) 950 mW.
Comparing the performance of the above three cases, one can clearly see that different pump power can realize wavelength interval switching. Table 1 shows the spectral performance under different pump power arrangements. When keeping the total power of P2 and P3 unchanged, if the P1 power is very low and only increases within a certain power range, the structure of double Brillouin frequency shift interval dominated by P2 and P3 will still dominate, and the energy provided by P1 will continue to increase the power difference between the first and second order. When the P1 power is increased beyond a certain range, the structure of the single Brillouin frequency shift interval dominated by P1 will dominate, so the power difference between the first order and the second order will decrease with the P1 power increasing, and the wavelength interval will switch. These results provide a reference for the design of an efficient wavelength switchable Brillouin erbium-doped random fiber laser.
Table 1. Comparison of spectral performance with different pump power arrangements
4. Conclusion
In conclusion, a multi-wavelength random fiber laser with switchable wavelength interval has been proposed and demonstrated with two switching methods. By controlling which EDFA is in working state, the switching of the wavelength interval can be realized, and the characteristics of the laser under the two wavelength intervals have been discussed and analyzed in detail. It is found that the switching of the wavelength interval can also be realized by controlling the gain of the first EDFA when both EDFAs are in working state. The proposed random fiber laser with alternative wavelength interval switching mechanisms and simple structure has the potential to meet the application requirements of various occasions.
Funding
National Key Research and Development Program of China (2018YFE0117400); National Natural Science Foundation of China (61775074, 91950105).
Disclosures
The authors declare no conflicts of interest.
References
1. S. K. Turitsyn, S. A. Babin, A. E. El-Taher, P. Harper, D. V. Churkin, S. I. Kablukov, J. D. Ania-Castanon, V. Karalekas, and E. V. Podivilov, “Random distributed feedback fibre laser,” Nat. Photonics 4(4), 231–235 (2010). [CrossRef]
2. H. Zhang, P. Zhou, H. Xiao, and X. Xu, “Efficient Raman fiber laser based on random Rayleigh distributed feedback with record high power,” Laser Phys. Lett. 11(7), 075104 (2014). [CrossRef]
3. Z. Wang, H. Wu, M. Fan, and L. Zhang, “High power random fiber laser with short cavity length: theoretical and experimental investigations,” IEEE J. Sel. Top. Quantum Electron. 21(1), 10–15 (2015). [CrossRef]
4. I. D. Vatnik, D. V. Churkin, E. V. Podivilov, and S. A. Babin, “High-efficiency generation in a short random fiber laser,” Laser Phys. Lett. 11(7), 075101 (2014). [CrossRef]
5. Z. Wang, H. Wu, M. Fan, and Y. Rao, “Random fiber laser: simpler and brighter. Optics and Photonics News,” Optics and Photonics News 25(12), 30 (2014).
6. A. E. El-Taher, P. Harper, S. A. Babin, and D. V. Churkin, “Effect of Rayleigh-scattering distributed feedback on multiwavelength Raman fiber laser generation,” Opt. Lett. 36(2), 130–132 (2011). [CrossRef]
7. H. Zhang, X. Shu, and Z. Xu, “Tunable multiwavelength random fiber laser with odd and even order stokes separated,” IEEE Photonics Technol. Lett. 30(5), 455–458 (2018). [CrossRef]
8. L. Zhang, Y. Xu, P. Lu, and S. Mihailov, “Multi-wavelength Brillouin random fiber laser via distributed feedback from a random fiber grating,” J. Lightwave Technol. 36(11), 2122–2128 (2018). [CrossRef]
9. Y. Li, P. Lu, X. Bao, and Z. Ou, “Random spaced index modulation for a narrow linewidth tunable fiber laser with low intensity noise,” Opt. Lett. 39(8), 2294–2297 (2014). [CrossRef]
10. S. Sugavanam, N. Tarasov, X. Shu, and D. V. Churkin, “Narrow-band generation in random distributed feedback fiber laser,” Opt. Express 21(14), 16466–16472 (2013). [CrossRef]
11. M. Pang, X. Bao, and L. Chen, “Observation of narrow linewidth spikes in the coherent Brillouin random fiber laser,” Opt. Lett. 38(11), 1866–1868 (2013). [CrossRef]
12. M. Pang, S. Xie, X. Bao, D. Zhou, Y. Lu, and L. Chen, “Rayleigh scattering-assisted narrow linewidth Brillouin lasing in cascaded fiber,” Opt. Lett. 37(15), 3129–3131 (2012). [CrossRef]
13. T. Zhu, X. Bao, and L. Chen, “A self-gain random distributed feedback fiber laser based on stimulated Rayleigh scattering,” Opt. Commun. 285(6), 1371–1374 (2012). [CrossRef]
14. R. Teng, Y. Ding, and L. Chen, “Random fiber laser operating at 1115 nm,” Applied Physics B 111(2), 169–172 (2013). [CrossRef]
15. L. Chen and Y. Ding, “Random distributed feedback fiber laser pumped by an ytterbium doped fiber laser,” Optik 125(14), 3663–3665 (2014). [CrossRef]
16. H. Zhang, H. Xiao, P. Zhou, and X. Wang, “High-power random distributed feedback Raman fiber laser operating at 1.2-μm,” Chin. Opt. Lett. 12(7), 073501 (2014). [CrossRef]
17. X. Wang, Y. Yang, M. Liu, Y. Yuan, Y. Sun, Y. Gu, and Y. Yao, “Frequency spacing switchable multiwavelength Brillouin erbium fiber laser utilizing cascaded Brillouin gain fibers,” Appl. Opt. 55(23), 6475–6479 (2016). [CrossRef]
18. M. P. Fok and C. Shu, “Spacing-adjustable multi-wavelength source from a stimulated Brillouin scattering assisted erbium-doped fiber laser,” Opt. Express 14(7), 2618–2624 (2006). [CrossRef]
19. J. Tang, J. Sun, T. Chen, and Y. Zhou, “A stable optical comb with double-Brillouin-frequency spacing assisted by multiple four-wave mixing processes,” Opt. Fiber Technol. 17(6), 608–611 (2011). [CrossRef]
20. H. Ahmad, M. Z. Zulkifli, N. A. Hassan, and S. W. Harun, “S-band multiwavelength ring Brillouin/Raman fiber laser with 20 GHz channel spacing,” Appl. Opt. 51(11), 1811–1815 (2012). [CrossRef]
21. D. Leandro, S. Rota-Rodrigo, D. Ardanaz, and M. Lopez-Amo, “Narrow-Linewidth Multi-Wavelength Random Distributed Feedback Laser,” J. Lightwave Technol. 33(17), 3591–3596 (2015). [CrossRef]
22. L. Han, S. Liang, and H. Wang, “Fabrication of low-cost multiwavelength laser arrays for OLTs in WDM-PONs by combining the SAG and BIG techniques,” IE EE Photon. J. 7(4), 1502807 (2015). [CrossRef]
23. S. Diaz, D. Leandro, and M. Lopez-Amo, “Stable multiwavelength erbium fiber ring laser with optical feedback for remote sensing,” J. Lightw. Technol. 33(12), 2439–2444 (2015). [CrossRef]
24. R. Xu and X. Zhang, “Multiwavelength Brillouin–erbium fiber laser temperature sensor with tunable and high sensitivity,” IEEE Photon. J. 7(3), 1501708 (2015). [CrossRef]
