1 Introduction

Recently, a novel kind of fiber lasers based on random distributed feedback (RDFB) through Rayleigh scattering in a ultra-long fibers has been reported [1]. RDFB fiber lasers have several attractive advantages, such as low cost and simple fabrication. Although the pump threshold of such lasers is usually high due to both the extremely low scattering fraction and high transmission loss of the long-distance fiber, slope efficiency may be comparable to normal fiber lasers because the feedback is accumulated over such a long distance, normally over tens of km [1, 2]. RDFB fiber lasers with different kinds of gain mechanisms such as stimulated Raman scattering (SRS) [3–10] and stimulated Brillouin scattering (SBS) [11] were reported in various laser configurations. Multi-wavelength RDFB fiber lasers have also been demonstrated by utilization of various comb filters such as fiber Sagnac interferometers [8], fiber Bragg gratings (FBGs) [5–7], fiber Lyot filter [9, 12], waveshaper programmer filter [13] and Mach–Zehnder interferometer combined with fiber Fabry–Perot (FFP) [14]. However, in optical fibers, the gain efficiency of SRS is low while the gain bandwidth of SBS is too narrow. In our previous studies, erbium-doped fibers (EDFs) were used as gain media in RDFB lasers to achieve single-/dual-wavelength and tunable laser output [15, 16]. The demonstrated EDF-based RDFB lasers have high slope efficiencies up to 54 % and low pump thresholds down to 10 mW.

In this paper, we present a multi-wavelength EDF laser based on RDFB with an optical fiber Sagnac loop mirror and a 20-km single-mode fiber (SMF). Multi-wavelength generation is limited by the homogenous gain broadening in EDF lasers at room temperature [17, 18]. Wavelength competition due to homogenous gain broadening can be mitigated through RDFB. The fiber laser demonstrated in this paper can generate multi-wavelength output with the number of channels from 2 to 8 by adjusting the pump power. The threshold pump power can be as low as 28.6 mW.

2 Laser design

Figure 1 shows the experimental setup of the proposed multi-wavelength EDF laser based on RDFB. A 980-nm laser diode source with the maximum power of 180 mW is used to pump a 8-m-long EDF in the backward direction through a 980-/1550-nm wavelength-division multiplexer (WDM). The EDF has the peak absorption coefficient of 6.7 dB/m at 1532 nm. Another port of the WDM was spliced to a 20-km-long SMF, which provides RDFB via Rayleigh backscattering. Unwanted Fresnel reflection was eliminated by using angled polished connectors. The Sagnac loop mirror is consist of a 2.5-m-long polarization-maintaining fiber (PMF), a 3-dB coupler and an optical polarization controller (PC). Its transmission spectrum, as shown in Fig. 2, indicates that the free spectrum range is 1.5 nm and the extinction ratio is about 15 dB. An optical power meter and an optical spectrum analyzer with resolution of 0.05 nm were applied to measuring the laser output in this experiment.

Fig. 1
figure 1

Schematic diagram of the multi-wavelength erbium-doped fiber laser based on RDFB. WDM wavelength-division multiplexer, EDF erbium-doped fiber, SMF single-mode fiber, PMF polarization-maintaining fiber, PC polarization controller

Full size image
Fig. 2
figure 2

Measured output spectrum of the Sagnac filter by a broadband source

Full size image

3 Results and discussion

When EDF is pumped and the pump power is below the threshold, the output spectrum just consists of the amplified spontaneous emission (ASE) spectrum modulated by the Sagnac filter. When pump power slightly excessed the lasing threshold, 28.6 mW, multi-wavelength laser emission was obtained. Figure 3 shows the measured spectra when the pump power was 30, 60, 120 and 180 mW, respectively. It can be seen that the number of channels increases from 2 to 8 with pump power, and each channel contains several emission lines or spikes, which, by further detailed measurement, are separated by 11 GHz and unstable with time. So the emission lines must be the Stokes lines of SBS generated in the long SMF. These Stokes generated components are amplified twice by the EDF via reflection of the Sagnac loop mirror and are fed into the SMF again. Random distributed feedback through Rayleigh backscattering in the long SMF makes them resonant in the laser cavity. With increasing pump power, powers of the lower-order Stokes components exceed threshold and produce higher-order SBS lines so that emission lines in each laser channel are increased. In this way, more and more SBS components are achieved until this positive effect of increasing pump power is canceled by the negative effect of increased insertion loss with spectrum extending in the Sagnac loop mirror. As a result, overlapping of multi-order SBS lines broadens the radiation spectrum and makes it stronger and relatively stable. Therefore, cascaded SBS and distributed Rayleigh backscattering contributes in this laser generation process and the unstable laser emission, as a common characteristic of such kind of laser, is mainly caused by the RDFB in the long SMF through Rayleigh backscattering.

Fig. 3
figure 3

Laser spectra with different pump powers of 30, 60, 120 and 180 mW, respectively

Full size image

In our experiment, we found it is quite easy to achieve multi-wavelength laser output from this fiber laser at room temperature, meaning that the mode competition induced by the homogeneous gain broadening of EDF is significantly suppressed by RDFB. We recorded the output spectra under 180 mW pump power every 3 min for 6 times at room temperature, and the results are shown in Fig. 4. Multi-wavelength operation is maintained well even though the maximum variations in intensity for individual channel are up to 20 dB. We believe more lasing channels can be achieved by further increasing the pump power.

Fig. 4
figure 4

Laser output spectra scanned every 3 min under 180 mW pump power

Full size image

The measured result of total laser output power against pump power is shown in Fig. 5. It increases about linearly with pump power, and the slope efficiency is 2.5 %. The laser threshold is 28.6 mW, which it is relatively higher than our previously reported RDFB-based erbium-doped fiber lasers with only single or dual wavelengths [15, 16]. However, it is much lower than that of previously reported multi-wavelength RDFB fiber lasers amplified by stimulated Raman scattering effect [6, 14, 19].

Fig. 5
figure 5

Total output power as a function of pump power

Full size image

4 Conclusion

A low-cost and low-threshold multi-wavelength EDF laser based on RDFB from a 20-km SMF, and a Sagnac loop mirror has been experimentally demonstrated. Laser emission with up to 8 channels has been obtained at room temperature with a pump threshold of 28.6 mW and slope efficiency of 2.5 %. Although the multi-wavelength laser emission is not quite stable due to the stochastic character of RDFB, we found that wavelength competition arising from homogenous gain broadening of the erbium-doped fiber is significantly suppressed. The demonstrated fiber laser also possesses advantages such as low cost, low-threshold pump power and simple fabrication.