1. Introduction

Multiwavelength fiber lasers, as cost-effective and efficient light sources, have attracted considerable interest due to their applications in test and measurement, optical sensing, and high-capacity wavelength-division-multiplexing (WDM) optical communication systems [1]. Many different methods have been proposed to implement stable room-temperature multiwavelength emission of fiber lasers, such as providing frequency-shifted feedback in the cavity to prevent the single mode operation [2], introducing four-wave mixing in nonlinear fiber to self-stabilize multiwavelength operation [3], incorporating a semiconductor optical amplifier or a Raman amplifier to make use of their inhomogeneous gain characteristics [4, 5], and using the cascaded stimulated Brillouin scattering (SBS) to introduce nonlinear gain [6–13]. Among these, the multiwavelength Brillouin-Erbium fiber laser (BEFL) seems the most attractive for generating multiwavelength because of its simple configuration and its intrinsic properties of rigid frequency spacing and extremely narrow linewidth.

Since Cowle and Stepanov firstly reported the BEFL [6], different kinds of BEFLs have been developed including ring, linear and figure-of-eight cavity configurations [9]. As for the tunability of multiwavelength BEFLs, a 12-wavelength comb with 14.5 nm tunable range was obtained by incorporating a Sagnac loop filter into the fiber ring [11]. A widely tunable multiwavelength BEFL with up to 60 nm tuning range was achieved utilizing a linear cavity through carefully optimizing the Brillouin pump power and the 980 nm pump power [12]. Their vital inconvenience for practical application is the need of external Brillouin pump, and furthermore the wavelength of the Brillouin pump should been adjusted accordingly in the process of wavelength tuning.

In this paper, a tunable self-seeded multiwavelength BEFL has been demonstrated using a simple cavity configuration. The Brillouin pump is self-excited in the cavity instead of an external one. As a result, the generation of more than 70-wavelength combs with about 11-nm tuning range has been demonstrated through adjusting the reflection profile of the birefringence loop filter incorporated into the laser cavity. Meanwhile, the laser output power efficiency is enhanced by 13 dB through utilizing the spare port of the 3 dB coupler as the output port, and the laser configuration is further simplified.

2. Experimental setup and operation principle

 

Fig. 1. Schematic diagram of the tunable self-seeded multiwavelength BEFL. The section connected with broken lines is to measure the reflection spectrum of the Sagnac loop mirror.

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The configuration of the tunable self-seeded multiwavelength BEFL is shown in Fig. 1. The laser consists of an Erbium-doped fiber amplifier (EDFA), an optical circulator (OC), a length of single mode fiber (SMF) and a birefringence loop mirror filter. The high-birefringence fiber Sagnac loop mirror is composed of a 3-dB coupler, a section of polarization maintaining fiber (PMF) and two polarization controllers (PCs), and its reflection profile can be modified through adjusting the PCs [14, 15]. The port 3 of the OC is connected to the port 1 via an EDFA. The EDFA provides the linear gain. The nonlinear Brillouin gain is generated in the 12.5-km long SMF. The laser output is directly from the spare port of the 3-dB coupler, and measured using an optical spectrum analyzer (OSA) with a spectral resolution of 0.065 nm. To simultaneously measure the reflection spectra of the Sagnac loop mirror in the process of wavelength tuning, a 1×2 optical switch is inserted in the point A in right of the SMF. Broadband light from the amplified spontaneous emission (ASE) source is injected from the port 1 of OC2, and then enter the Sagnac loop mirror through the port 2. The reflected part transits again the port 2 of OC2, finally detected with the OSA from the port 3 of OC2.

The operation principle of the tunable self-seeded BEFL is described as follows. The lasing oscillation is formed in the laser cavity built by the two-port connected OC and the Sagnac loop mirror. The wavelength that experiences the largest net gain acts as the strongest Brillouin pump in the SMF. Due to the bidirectional propagation, once more Rayleigh-backscattered radiation provides feedback, an interferometer consisting of two distributed Rayleigh mirrors is formed in SMF. Although the reflection coefficient of the distributed Rayleigh mirror is very small, the SBS amplification in SMF is great enough to compensate for these low reflections. The feedback caused by the Rayleigh interferometer demonstrates strong spectral selectivity, and can provide primary SBS amplification and lasing for some spectral components inside the SBS line. Thus, the cooperative SBS and Rayleigh processes lead to unusually narrow spectra of the Stokes wave [16, 18]. Here, the Brillouin pump ω_P passes the SMF in both directions. The Brillouin Stokes lines ω_B=ω_P±Δω (Δω is the Stokes frequency shift) with narrowed linewidth are generated simultaneously. The total gain, which combines the linear gain from EDF and the nonlinear gain from SMF, exhibits an inhomogeneous broadening mechanism. Therefore, the multiwavelength comb can be generated when the Brillouin gain is high enough. The balance between the loss and the gain determines the self-excited Brillouin pump wavelength. It is usually at the peak of the gain. Though adjusting the PCs, we can change the reflection profile of the Sagnac loop mirror. Thus the tunable self-seeded multiwavelength BEFL can be obtained just by adjusting the PCs.

3. Results and discussions

In the experiment, the output power of the EDFA was set at 19 dBm, and a length of 17.2 cm PMF is incorporated into the Sagnac loop mirror. The stable Brillouin multiwavelength comb can be generated under the arbitrary polarization states of two PCs. It is observed that the wavelength of the combs can be tuned in the wavelength range of ~30 nm from ~1546 nm to ~1576 nm. This agrees with the bandwidth of Sagnac loop mirror, which is measured to be 30.1 nm. But in the whole tuning range, the wavelength number is not always unchanged. Figure 2 shows the tuning multiwavelength spectra from 1551.3 nm to 1568.2 nm in our experiment. The combs have the same spacing of 0.088 nm (i.e. 11 GHz). The numbers of the generated Brillouin wavelengths within 6-dB bandwidth are 74, 70, 72, and 70 in Figs. 2(a), 2(b), 2(c), and 2(d), respectively. The tunable range is ~11 nm. The tuning process is continuous and the lasing bandwidth almost keeps a constant. In this range, the generated lines are almost unchanged and the output power of combs is uniform distribution. In order to further understand the role of Sagnac loop mirror, its reflection spectra have been simultaneously measured in the process of wavelength tuning, as shown in Fig. 3. During the measurements, the polarization state of the PCs is fixed corresponding to the same conditions as Figs. 2(a), 2(b), 2(c), and 2(d) respectively, the only need to do is to switch the optical circuit to the side of broadband light source. The results show that the reflection spectra of the Sagnac loop mirror have been tuned ~11.3 nm, which is in agreement with the wavelength tuning range of the multiwavelength comb in Fig. 2.

 

Fig. 2. Tunable multiwavelength spectra with more than 70 Brillonin wavelengths.

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Fig. 3. Reflection spectra of the Sagnac loop corresponding to multiwavelength combs in Fig. 2.

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It should be noted that, although the wavelength can been tuned within the bandwidth of the Sagnac loop mirror, the number of wavelengths is not always the same. Figures 4(a) and 4(b) show the multiwavelength spectra with wavelengths centered at 1546.3 nm and 1575.7 nm, respectively. 18 Brillouin wavelengths are generated within 6-dB bandwidth. The distinct difference between Fig. 2 and Fig. 4 is attributed to the gain profile of the EDFA. The self-seeded multiwavelength BEFL is also demonstrated when a fiber wide-band mirror replaces the Sagnac loop mirror. If so, the lasing wavelength band cannot be tuned, whose bandwidth is ~2.8 nm, fixed at the range from 1557.8 nm to 1560.6 nm. It is indicated that the linear gain provided by the EDFA in this band is larger than that within the other tuning range. As known that the number of wavelengths generated from BEFL is related to the EDFA gain, higher EDFA gain favors more Brillouin lines [16].

 

Fig. 4. Multiwavelength spectra with 18 Brillouin wavelengths. (a) Centered at 1546.3 nm, (b) centered at 1575.7 nm.

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Fig. 5. Contrast between multiwavelength optical spectra (a) from the 10-dB coupler and (b) from the spare port of the 3-dB coupler.

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The previous reported similar multiwavelength BEFLs [13, 16–17] all had an additional output coupler in the laser cavity to output signal, which not only increased the complexity but also degraded the output power efficiency of the lasers. The half laser power is wasted through the spare port of the 3-dB coupler, and the usable output power is 10 percent of the intracavity power if a 10-dB coupler is used as the output coupler. Directly utilizing the spare port of the 3-dB coupler as the output port, the laser output power efficiency is largely enhanced. To prove the feasibility, a 10-dB coupler is inserted into the point A in Fig. 1 with replacement of the switch. The Brillouin multiwavelength laser outputs from the spare port of the 3-dB coupler and the 10-dB coupler at the same time are shown in Fig. 5. Figure 5(a) is the output from the spare port of the 3-dB coupler, and Fig. 5(b) is from the 10-dB coupler. Here, we can see than both have the same spectral shape including wavelength range and number, but have power difference of 13.13 dB, which is in agreement with the above estimation.

To verify the operation stability of the tunable self-seeded multiwavelength BEFL, at the same polarization state of the PCs we repeatedly scanned the multiwavelength lasing spectra with 74 Brillouin lines in half-hour span with five-minute spacing. The results, shown in Fig. 5, indicate that the tunable self-seeded multiwavelength BEFL can operate stably on long time scale if no external perturbation is applied to the laser system.

 

Fig. 6. Repeatedly scanned multiwavelength spectra with 74 Brillouin lines in half-hour span with five-minute spacing.

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4. Conclusion

We have demonstrated a tunable self-seeded multiwavelength BEFL. The laser output wavelengths can be tuned through modifying the filtering profile of the birefringence filter. The wavelength of multiwavelength combs can be tuned in the bandwidth of the Sagnac loop filter. Although the number of wavelengths is variable in the whole tunable range due to EDFA gain profile, the generation of more than 70-wavelength combs with uniform output power has been obtained in about 11-nm tunable range. Also, the output power efficiency has been enhanced by 13 dB utilizing the spare port of the 3-dB coupler as the output port.