1. Introduction

Multi-wavelength fiber lasers have attracted much interest in recent years because of their potential applications [in e.g. wavelength division multiplexing (WDM) systems, optical fiber sensors, optical component testing, and spectroscopy] and their various advantages (such as multi-wavelength operation, low cost, compact structure, and compatibility to fibers) [1–3]. Multi-wavelength lasing has been achieved with various gain mechanisms including Erbium-doped fiber (EDF) amplification, semiconductor optical amplification (SOA) [1, 2], and fiber Raman amplification (FRA) [3–5]. Multi-wavelength fiber lasers using EDFs have been investigated widely due to their advantages such as the high power conversion efficiency and lower threshold. However, it is an intrinsic disadvantage that an EDF has a strong homogenous line broadening and cross-saturation gain at room temperature [6]. In other words, the serious gain competition among different lasing wavelengths (within the large homogenous line broadening region) will make the multi-wavelength laser unstable at room temperature. To make a multi-wavelength laser stable at room temperature, various techniques have been utilized such as cooling the EDF to cryogenic temperature with liquid nitrogen, using some special fibers [7], utilizing a frequency-shifted feedback technique in a laser cavity [8], and employing a highly nonlinear fiber [6].

Recently, a multi-wavelength fiber laser based on a hybrid gain medium of EDF and SOA has been introduced to suppress the homogenous line broadening of Erbium ions [9, 10]. However, SOA processes a relatively large insertion loss and is highly sensitive to polarization [10]. Comparing with SOA, FRA has the advantages of compatibility with fiber structure, higher saturation power (giving higher output power of the fiber laser) [11] and larger gain bandwidth (FRA also has a dominant inhomogeneous line broadening property at room temperature) [3, 4]. Utilizing the complementary advantages of FRA and EDF lasers, we introduce in this letter a stable and uniform multi-wavelength fiber laser of good performance. To the best our knowledge, this is the first time that a multi-wavelength fiber laser based on the hybrid gain of Raman and Erbium-doped fibers is reported.

2. Experimental setup, results and analysis

The experimental setup of the proposed multi-wavelength fiber laser is shown in Fig. 1. A section of dispersion compensating fiber (DCF) (with a length of about 4 km, which is originally used for compensating the dispersion of 20 km SMF) is employed as the Raman gain fiber. Three laser diodes (LD2, LD3 and LD4) are used as the Raman pumps, and their wavelengths (maximum output powers) are 1430 nm (185 mW), 1440 nm (183 mW), and 1467 nm (117 mW), respectively. A section of EDF with a length of 6.3 m and a laser diode (LD1, whose wavelength and maximum output power are 1480 nm and 240 mW, respectively) form the EDF part of amplification. A Sagnac loop reflector (SLR) [12] and a Sagnac loop filter (SLF) form a resonant cavity. The 2 % arm of an optical coupler (OC) is used as the output port. The SLF, which determines the lasing wavelengths, is composed of a section of PMF (with a birefringence value of 3.2×10^-4), two polarization controllers (PCs) and a 3-dB optical coupler. The wavelength spacing is given by Δλ = λ ²/(Δn · L) [13, 14], where Δλ, Δn, L and λ are the channel spacing, the fiber birefringence, the effective fiber length and the operational wavelength, respectively. In our experiment, L = 15.2 m and thus the calculated channel spacing of the SLF is 0.5 nm around λ = 1560 nm.

 

Fig. 1. Schematic diagram of the present multi-wavelength fiber laser based on a line structure of EDF A and FRA.

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Fig. 2. Variation of the gain for signal #A influenced by signal #B at a neighboring channel (with wavelength difference Δλ.) for the RFA and EDFA.

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Table 1. Input Parameters for Both Types of Amplification(a) FRA

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The instability of multi-wavelength lasing is mainly due to the gain competition effect. First we experimentally study and compare the gain competition (between two signal channels) for the FRA and EDFA parts of the proposed fiber laser. The setups of the FRA and EDFA are shown in Box (I) and Box (II) of Fig. 1, respectively. The parameters of the signals and pumps are listed in Table 1(a) (for FRA) and (b) (for EDFA). Signal #A (whose wavelength is fixed to λ_A) is injected into port (a) for both FRA and EDFA. We measured the gain of signal #A from port (b). Another signal (#B) at wavelength λ_B (which can be tuned from 1525 nm to 1580 nm) is used to study the gain competition. The gains for signal #A when signal #B is off ( G_A,B-off ) and on ( G_A,B-on ) are measured. Figure 2 shows the gain difference ΔG_A(Δλ) =G_A,B-off(λ_A;Δλ)-G_A,B-on(λ_A;Δλ) for signal #A in EDFA (squares) and in FRA (circles) as wavelength spacing Δλ=λ_B-λ_A varies. In our experiment, the minimal values of Δλ are 0.43 nm and 0.1 nm in EDFA and FRA, respectively. The minimal Δλ of 0.43 nm is sufficient to show how significant the EDFA gain of signal #A can be influenced when neighboring channel #B wavelength is approaching the wavelength of signal #A. From this figure one can see that the gain competition effect is very strong in the EDFA (it occurs even for two wavelengths separated by more than 20 nm) whereas the gain competition is hardly observed in the FRA (even for two wavelengths separated by less than 0.1 nm). Therefore, we can expect stable multi-wavelength lasing of good performance by incorporating the FRA gain mechanism (small gain competition) into the EDFA gain mechanism (to obtain e.g. large gain).

 

Fig. 3. Experimental results of the present multi-wavelength fiber laser. (a) Output spectrum; (b) Repeated scanning spectrum per minute; and (c) Fluctuation of each peak power within 15 minutes.

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Table 2. Wavelengths and Powers of the Four Pump LDs Injected into the Line Cavity

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Next we demonstrate multi-wavelength lasing of the proposed hybrid gain fiber laser. By carefully adjusting the PCs in the SLF (see Fig. 1) and the four pump powers of the laser, we obtain the optimal output shown in Fig. 3(a). The corresponding pump powers are listed in Table 2. The output spectrum of span 7.5 nm is shown in Fig. 3(a), from which one can see that the maximum fluctuation of the output powers at these 12 wavelengths is less than 2.2 dB and the extinction ratios are over 37 dB. To show the good stability of the multi-wavelength lasing at room temperature, we show the repeated scanning spectrum per minute within 15 minutes in Fig. 3(b). The fluctuation of each peak power is less than 0.37 dB and the minimum fluctuation is only 0.07 dB within 15 minutes [see Fig. 3(c)]. Thus, one sees that the present multi-wavelength laser is uniform and very stable in power and wavelengths. The total measured output power is more than 0.72 dBm. Note that all the outputs in our experiments are measured by an Optical Spectrum Analyzer (OSA) (Agilent 86142B), and the resolution and sensitivity of the OSA used in the measurement are 0.06 nm and -50 dBm, respectively.

Note that the small signal gains (measured separately for the two amplifiers in our experiments) of the EDFA (FRA) under the pump condition of Table 2 are about 13.5 dB (5.1 dB) at 1564 nm, 11.0 dB (5.1 dB) at 1566.5 nm, and 10.5 dB (5.2 dB) at 1569 nm. This indicates that the EDFA is the dominant gain medium in the proposed fiber laser (i.e., our proposed laser is based on FRA-assisted EDFA) and FRA can improve the stability of the laser.

 

Fig. 4. Repeated scanning spectrum of the laser per minute after removing the RFA part (i.e., DCF, LD2, LD3 and LD4).

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To verify the contribution of the FRA in improving the stability of the multi-wavelength laser, we made similar measurement after removing the RFA part (i.e., Box (I) in Fig. 1) from the structure shown in Fig. 1. As expected, the multi-wavelength laser (even after optimization) becomes very unstable (see Fig. 4). The pump power of LD1 is 180 mW (note that we set this value of the pump power because the laser seems “more stable” at this pump level during our experiment; i.e., we choose the best result of the laser with only EDFA to compare with our proposed stable laser with both FRA and EDFA). The minimal fluctuation of the peak powers is about 2.2 dB, and the fluctuation of some peak power can be more than 10 dB within 8 minutes. If we remove only the EDF (keeping the FRA part), the total output power would decrease significantly. This indicates the EDFA part can improve the output power of the multi-wavelength laser effectively (note that the gain of an EDFA is typically much larger than that of an FRA).

In fact, the undepleted 1480-nm EDFA pump (LD1) light passing through the EDF can also act on the DCF and contribute some Raman gain. Similarly, the 1467-nm FRA pump (LD4) light can also act on the EDF and contribute some EDF gain. Higher powers of 1480-nm and 1467-nm pump can improve the power conversion efficiency of the multi-wavelength laser with the present hybrid gain of the DCF and EDF. Moreover, good power uniformity of the multi-wavelength lasing is achieved due to the flat gain provided by this hybrid gain mechanism and the optimization of the pump powers.

3. Conclusion

In summary, a stable and uniform multi-wavelength fiber laser based on the hybrid gain of a Raman gain fiber and an EDF has been introduced. The gain competition which causes the instability of the multi-wavelength lasing has been analyzed and compared for the EDFA and FRA. By incorporating the FRA gain mechanism (small gain competition) into the EDFA gain mechanism (to obtain a large gain), we have obtained an effective hybrid gain mechanism for multi-wavelength lasing stable at room temperature. The novel gain mechanism can also increase the lasing bandwidth and output power as compared with a pure EDF laser, and improve the power conversion efficiency as compared with a pure fiber Raman laser. Together with a line cavity structure and a PMF SLF, we have achieved stable and uniform 12-wavelength lasing, with the fluctuation of the peak powers less than 0.37 dB in 15 minutes, a power nonuniformity of less than 2.2 dB, extinction ratio of over 37 dB, and a total output power of 0.72 dBm.