Multiwavelength fiber lasers, especially those operating at optical communication wavebands such as 1.3 μm and 1.5 μm wavebands, have huge demands in wavelength division multiplexing communications. In the past decade, multiwavelength fiber lasers operating at 1.5 μm waveband have been widely reported. Nevertheless, 1.3 μm waveband multiwavelength fiber laser is rarely studied due to the lack of proper gain mechanism. Random fiber laser (RFL), owing to its good temporal stability and flexible wavelength tunability, is a great candidate for multiwavelength generation. Here, we reported high power multiwavelength generation at 1.3 μm waveband in RFL for the first time. At first, we employed a section of 10 km G655C fiber to provide Raman gains, as a result of which, 1.07 W multiwavelength generation at 1.3 μm waveband with an optical to signal noise ratio of ∼33 dB is demonstrated. By tuning the pump wavelength from 1055 nm to 1070 nm, tunable multiwavelength output covering the range of 1300-1330 nm can be achieved. Furtherly, we realized 4.67 W multiwavelength generation at 1.3 μm waveband by shortening the fiber length to 4 km. To the best of our knowledge, this is the highest output power ever reported for multiwavelength fiber lasers.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Multiwavelength fiber lasers have been widely investigated owing to their wide applications in wavelength division multiplexing communications, optical sensors, precise spectroscopy, and microwave generation [1–5]. In terms of the lasing process, multiwavelength fiber laser can be classified into ‘passive’ ones and ‘active’ ones. The ‘passive’ ones employ various mechanisms, including Fabry–Perot filter [6,7], Mach–Zehnder interferometer [8,9], Sagnac fiber loop filter (SFLF) [10,11], Lyot filter [12,13], fiber Bragg gratings [14,15], nonlinear polarization rotation , single-mode-multimode-single-mode fiber filter , and micro-air cavity filter  to provide multiwavelength feedback. While the ‘active’ ones adopt nonlinear optical process such as stimulated Brillouin scattering [19–22] and four-wave-mixing effects [23–25] to achieve multiwavelength lasing directly. Conventional multiwavelength fiber lasers are generally based on rare-earth-doped fibers thanks to the easily available wideband flat gains. By adopting different kinds of rare-earth-doped fibers, multiwavelength fiber lasers operating at various wavebands have been demonstrated [26–31]. In recent decades, random fiber laser (RFL) based on the passive fiber is becoming an important approach for multiwavelength generation.
Proposed by Turitsyn et al. in 2010 , random fiber laser (RFL) has attracted great attention for its great advantages, such as simple configuration, modeless emission and flexible wavelength tunability. As it operates via week Rayleigh scattering and Raman amplification in long passive fiber, RFL demonstrates high stability, making it a good choice for multiwavelength generation [10,12-14, 33–35]. Besides, the flexible wavelength tunability makes it possible for multiwavelength RFL to operate at any wavelength within the transparent band of the fiber. To date, the multiwavelength RFLs have covered 1∼1.2 μm [36,37] and 1.45-1.56 μm  wavebands. For the important optical communication waveband at 1.3 μm, lots of single wavelength RFLs have been demonstrated [38–39], but multiwavelength RFL has not been reported.
In this paper, through four orders cascaded stimulated Raman scattering, we realized multiwavelength generation at 1.3 μm waveband in RFL for the first time. By adopting a tunable amplified spontaneous emission (ASE) source as pump and a SFLF to provide multiwavelength feedback, flexible wavelength and channel spacing tunability are demonstrated. Thanks to the great temporal stability of the ASE pump source, this multiwavelength RFL demonstrates good temporal stability and high spectral purity. Moreover, multiwavelength RFLs with different fiber lengths are investigated for power scaling. As a result, up to 4.67 W multiwavelength laser output at 1.3 μm waveband is achieved. To the best of our knowledge, this is the highest output power ever reported for multiwavelength fiber lasers. The proposed tunable multiwavelength RFL at 1.3 μm waveband has huge demands in wavelength division multiplexing communications.
2. Experimental setup
The configuration of the proposed tunable multiwavelength RFL at 1.3 μm waveband is shown in Fig. 1. A homemade high power ASE source with a tuning range of 1055-1075 nm is adopted as pump source . A section of G655C telecom fiber is adopted to provide Raman gain as well as random distributed feedback. As a comparison, two different lengths 10 km and 4 km are employed in the experiment. The core and cladding diameters of the G655C telecom fiber are 9 μm and 125 μm, respectively. Between the ASE source and the G655C telecom fiber are two wavelength division multiplexers (WDM), 1070/1120 nm WDM (WDM1) and 1090/1280 nm WDM (WDM2), which are applied to separate different orders backward scattering Stokes light components from the pump light. The 1120 nm port of WDM1 is connected with a broadband fiber mirror, which is meant to provide forward feedback for the low order Stokes light components. While the 1280 nm port of WDM2 is spliced with a self-built SFLF to provide multiwavelength feedback for the 4th order Stokes light component at 1.3 μm waveband. The self-built SFLF, which operates through birefringence and interference effects , are composed of a 3 dB coupler operates at 1.31 μm, a polarization controller and a section of polarization maintaining fiber (PMF). The core and cladding diameters of this PMF are 10 μm and 125 μm, respectively. Moreover, the output port of the passive fiber was cleaved at an angel of 8° to minimize the backward reflection. After the output, an optical spectrum analyzer with a resolution of 0.02 nm and a power monitor are adopted to measure the output spectrum and power.
The transmission spectra of the two adopted WDMs are measured by connecting the common port with the homemade supercontinuum source , and the measured results are shown in Fig. 2. It can be seen that the backward scattered 4th order Stokes light at 1.3 μm waveband can be guided into the SFLF through the 1280 nm port of WDM2.
The mechanism of the SFLF can be described as follow: The input signal is equally split into two counter propagating waves by the 3 dB coupler. Before passing through the PMF, the polarization of one wave has been controlled by the polarization controller while the other is not. Thus, the polarization state of these two waves are different. Due to the birefringence effect, these two waves with different polarization state travel at different velocities in the PMF. After a round trip inside the loop, the two waves recombine at the coupler with a phase difference, which is a periodic function of the wavelength. The interference of these two waves results in a period transmission spectrum at one port of the coupler, which can be expressed as:2), it can be seen that the channel spacing is proportional to the length of the PMF in the SFLF. At 1.3 μm waveband, when a section of 2 m PMF is adopted, the channel spacing is ∼2.8 nm.
3. Result and discussion
3.1 Low power operation
At first, the output characteristics of this multiwavelength cascaded RFL is investigated under a fixed pump wavelength of 1055 nm. While the lengths of the adopted G655C fiber and PMF in the self-built SFLF are 10 km and 2 m, respectively. Figure 3(a) shows the power evolution of this multiwavelength RFL. It can be seen that once the pump power exceeds 7.91 W, the power of the 4th Stokes lines (1.3 μm waveband) increases rapidly. And when the pump power reaches 9.46 W, the power of the 4th Stokes light reaches a maximum of 1.07 W, corresponding to a conversion efficiency of 11.2%. The high transmission loss induced by the long fibers is responsible for the low conversion efficiency. With the further increasing of pump power, the power of the 4th order Stokes light component will be converted into the next order. Figure 3(b) shows the output spectrum when the power of 4th order Stokes light component reaches its maximum (1.07 W), while the inset figure shows the detailed spectrum of the 4th order Stokes light component at 1.3 μm waveband. Six lines ranging from 1297.6 nm to 1310.2 nm with an optical to signal noise ratio (OSNR) of ∼33 dB can be observed. Here, the OSNR of the multiwavelength output is defined as the average OSNR each line [13,20]. Besides, the measured channel spacing is 2.52 nm, which is slightly smaller than the theoretical result (∼2.8 nm). This difference could be the result of the deviation in birefringence, the actual birefringence could be larger than the nominal birefringence.
3.2 Flexible wavelength and channel spacing tunabilities
Then, by varying the central wavelength of the ASE pump source, tunable multiwavelength output are demonstrated. Figure 4 shows the tunable output spectra of this multiwavelength RFL under the same pump power of 9.46 W. It can be seen that by increasing the pump wavelength from 1055 nm to 1070 nm, tunable multiwavelength output covering the range of 1300-1330 nm is achieved. However, compared to the multiwavelength output at 1.3 μm (1055 nm pump), the OSNR decreases from ∼33 dB to <20 dB while the power variation increased a lot at ∼1.33 μm (1070 nm pump). The reason of this phenomenon is as follow: Limited by the operating waveband of the adopted 3 dB coupler (centered at 1.31 μm), the self-built SFLF only work well within certain wavelength region near 1.31 μm. The more the wavelength deviates from 1.31 μm, the worse the multiwavelength filtering effect is, which account for the relatively poor multiwavelength output spectra under longer pump wavelengths. Moreover, different degrees of stimulated Brillouin scattering induced instability can be observed under these four pump wavelengths .
Besides, flexible channel spacing tunability is demonstrated through adjusting the length of PMF in the self-built SFLF. Figure 5 shows the output spectra with different lengths of PMF. When 2 m PMF fiber is adopted, as shown in Fig. 5(a), 6 lines with channel spacing of 2.52 nm is observed. And when the length of PMF is increased to 4 m, 12 lines with a nearly halved channel spacing of 1.24 nm is achieved. These results are in good agreement with Eq. (2). Nevertheless, with the narrowing of the channel spacing, the OSNR of the multiwavelength output is decreased from ∼33 dB to ∼14 dB, which could be the result of severer nonlinear effects induced spectral broadening under narrower channel spacing [42,43].
3.3 High power operation
To increase the output power, the adopted G655C fiber is shortened to 4 km to reduce the transmission loss. The pump wavelength is tuned to 1060 nm while the length of the adopted PMF fiber in the self-built SFLF is 2 m. The output spectra of this high power multiwavelength RFL under different pump powers are presented in Fig. 6. With the increasing of the pump power, the Raman Stokes light of different orders are generated subsequently.
Figure 7(a) shows the power evolution of this multiwavelength RFL. Compared to the low power ones, the threshold power of the 4th order Stokes light component increases from 7.91 W to 12.1 W. And under a pump power of 18 W, the power of the 4th Stokes light (1.3 μm waveband) reaches a maximum of 4.67 W, corresponding to a conversion efficiency of 25.94%. The related output spectrum is presented in Fig. 7(b). Thanks to the great temporal stability of the ASE pump source [44–46], the Raman conversion in this RFL is nearly complete. The power intensity of the 4th order Stokes light component is 19.39 dB higher than residual power of other orders Stokes light components and its spectral purity is up to 99.29%. Figure 7(c) shows the detailed spectrum of the 4th order Stokes light component at 1.3 μm waveband, six lines with a channel spacing of ∼2.57 nm and power variation within 3 dB are generated. Compared to the low power ones, the OSNR is reduced to 13.74 dB, which could be the result of severer nonlinear effects induced spectral broadening under higher power [10,37], but no stimulated Brillouin scattering induced instability is observed. The temporal stability of this 4.67 W multiwavelength output at 1.3 μm waveband is tested with an interval of 1 minute in 7 minutes duration. As shown in Fig. 7(d), this multiwavelength RFL shows good temporal stability.
In this paper, we report multiwavelength generation at 1.3 μm waveband in RFL for the first time. By adopting a section of 10 km G655C fiber to provide Rayleigh distributed feedback as well as Raman gains, 1.07 W multiwavelength output at 1.3 μm waveband with an OSNR of ∼33 dB is demonstrated. By varying the pump wavelength of the ASE source, tunable multiwavelength output covering 1.3-1.33 μm is realized. With a broadband 3 dB coupler, the operating wavelength range of this multiwavelength random fiber laser is expected to be furtherly extended. Flexible channel spacing tunability is also demonstrated by adjusting the length of the PMF in the SFLF. Furtherly, by shortening the length of G655C fiber to 4 km, up to 4.67 W multiwavelength output with a spectral purity of 99.29% at 1.3 μm waveband is achieved. To the best of our knowledge, this is the highest output power ever reported for multiwavelength fiber lasers. The proposed tunable multiwavelength RFL at 1.3 μm waveband has great potential in wavelength division multiplexing communications.
National Natural Science Foundation of China (61635005, 61905284, 61911530134); National Postdoctoral Program for Innovative Talents (BX20190063); Hunan Innovative Province Construction Project (2019RS3017).
We are grateful to Sen Guo for his help on this work.
The authors declare that there are no conflicts of interest related to this article.
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