1. Introduction

Multi-wavelength fiber lasers are of interest for applications in photonic component characterization, optical instrumentation, and optical sensing. In addition, narrow linewidth, dual-wavelength lasers can be used to generate microwave or millimeter wave radiation [1]. For operation at 1550 nm, fiber lasers are usually implemented using semiconductor optical amplifiers (SOAs) or erbium-doped fiber amplifiers (EDFAs) as the gain medium. Due to the inhomogeneous broadening of semiconductor gain media, it is possible to obtain stable, room-temperature operation in multi-wavelength SOA-based fiber lasers (a wavelength spacing of 6.25 GHz was reported in Ref. [2]). On the other hand, the homogeneous broadened nature of EDF gain media normally prohibits multi-wavelength operation in EDF lasers, especially for narrow wavelength spacings, unless techniques such as careful gain equalization, spatial or polarization hole burning (PHB), frequency-shifted feedback, composite/cascaded cavities, inhomogeneous loss mechanisms [3], or four-wave mixing (FWM) in a length of highly nonlinear fiber (HNLF) [4, 5], are used.

For operation in the wavelength range 1000 nm – 1100 nm, Yb-doped fiber lasers (YDFLs) are of interest, especially for high power operation [6]. Indeed, impressive results have been achieved with YDFLs, including kW output powers, efficiencies as high as 80%, and very good M² numbers. There have also been recent demonstrations of dual- or few-wavelength YDFLs. In Ref. [7], Chi et al. used a spatial mode-beating filter for wavelength selection. Stable multi-wavelength operation was obtained; however, the narrowest wavelength spacing was 1.3 nm and the output power was only a few mW. The linewidths were 0.16 nm and the signal-to-noise ratios (SNRs) were about 40 dB. In Ref. [8], Feng et al. used a few-mode fiber grating to enhance PHB and realize dual-wavelength operation with a wavelength spacing of 0.9 nm. Again, the output power was limited to a few mW and the SNR was only 30 dB. In Ref. [9], Guan and Marciante reported a short-cavity, dual-wavelength YDFL based on a fiber Bragg grating (FBG) written in polarization maintaining fiber to define the two output lasing wavelengths. A wavelength spacing of 0.3 nm was obtained and stability was due in large part to PHB. The output power was 43 mW and the SNR was 60 dB.

In this paper, we report the observation of FWM in a high-power dual-wavelength YDFL. In contrast to the results reported previously in Ref. [7–9], stable dual-wavelength operation is attributed to the FWM process. We obtain narrow linewidths (typically < 40 pm), high SNR (> 55 dB), high output power (< 1.7 W), and a record narrow wavelength separation of 0.12 nm.

2. Experiment and results

Figure 1(a) illustrates a schematic of the multi-wavelength YDFL. The gain medium comprises 20 m of twin-core fiber. The twin-core fiber has two optically contacted large cores, coated with a low refractive index polymer. A scanning electron microscope (SEM) image of the fiber is shown in Fig. 1(b). The single-mode Yb-doped signal core has a diameter of 5.3 μm and an NA of 0.13. The multimode absorption coefficient is 0.77 dB/m at the pump wavelength. The diameter of the pump core is 118 μm. The advantage of this coupling method is the ability to increase the output power from the fiber laser by coupling multiple pump sources at the input end [10, 11], or by exploiting the physical separation between the pump and signal paths in which the independent access to the pump path allows for power scaling by distributing the pump power along the fiber length. The efficient light coupling between the pump and active cores had been described recently in Ref. [12], and the application of this form of light coupling for pumping an L-band EDFA was demonstrated in Ref. [13].

In our experiments, we use bidirectional pumping and each end of the pump core is spliced to a multi-mode laser diode capable of delivering up to 7 W at 915 nm (we use equal pump powers from each laser diode). A pair of FBGs is spliced at the two ends of the single-mode Yb-doped signal core to form readily a standing-wave cavity. One of the gratings, denoted FBG_A, has high reflectivity (> 99.9%) with a 3 dB bandwidth of 1.02 nm and is centered at 1090.28 nm. The output grating structure is based on either two superimposed FBGs or physically separate FBGs with offset wavelengths to obtain a dual-wavelength output. We use gratings with different wavelength spacings, denoted FBG_B,1, FBG_B,2, and FBG_B,3, whose characteristics are summarized in Table 1. In the case of the superimposed gratings, the wavelength spacing is fixed; for the physically separate gratings, the wavelength spacing can be adjusted by thermally tuning one or both of the FBGs. All of the FBGs were written in hydrogen-loaded Corning HI-1060 fiber using phase masks and UV exposure from a KrF excimer laser. The laser output spectrum is measured using an optical spectrum analyzer with 15 pm resolution (Ando AQ-6310B).

 

Fig. 1. (a). Schematic of dual-wavelength YDFL. (b). SEM image of the cross-section of the twin-core fiber coated with lower index polymer. The left core is for pumping and the right has a Yb-doped core.

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Table I. Characteristics of the superimposed gratings used in the output grating FBG_B.

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Figure 2 shows the output spectra of the YDFL using FBG_B,1 for various pump powers. At low pump powers (e.g. 2 W), only single-wavelength operation is obtained. Occasionally, a second wavelength appears, but operation is unstable due primarily to gain competition in the homogeneously broadened gain medium. Above 4 W of pump power, multiple peaks appear at the output. For example, at 4 W, the two main lasing peaks at 1090.06 nm and 1090.51 nm are associated with the two superimposed gratings while the additional peaks appearing at 1089.61 nm and 1090.97 nm are due to FWM. The 3 dB linewidths of the main peaks are both 40 pm and the SNRs are > 55 dB (the measurement is limited by the OSA resolution of 15 pm); the power difference is < 0.2 dB. As the pump power increases, the stability improves. Figure 2(b) shows repeated scans of the laser output spectrum taken over several minutes for a pump power of 8 W. The worst-case peak-to-peak power fluctuation observed in either of the two main peaks is 0.74 dB. For a pump power of 10 W, there are 6 output peaks and the total output power is 1.7 W (the slope efficiency is about 18% and can be improved with better splicing). The following confirm the presence of FWM: (1) the number of additional peaks depends on the pump or output power, (2) the peaks are all separated by ≈ 0.45 nm, and (3) the peaks span a spectral range of over 2.25 nm exceeding the bandwidth of FBG_A. As in the case of multi-wavelength EDF lasers, the FWM process helps stabilize multi-wavelength operation [4, 5]. In particular, degenerate FWM causes energy (power) to be transferred among the lasing wavelengths thereby suppressing gain competition and promoting stability. However, in contrast to [4, 5], in our case, FWM occurs directly in the gain fiber and not in an additional length of HNLF. Since there are no polarization controllers or polarization selective components in our laser cavity, we cannot exploit an effect such as PHB to stabilize multi-wavelength operation as in [8,9]. Indeed, the laser does not have stable operation until a minimum pump power is provided (or output power is achieved), at which time we not only obtain the two main lasing peaks associated with the superimposed gratings, but the additional peaks from FWM. Polarization controllers can be used to optimize the polarization states to improve the FWM efficiency. Since no polarization controllers are used in the laser cavity, we expect the lasing wavelengths to have elliptical polarization of varying angles.

 

Fig. 2. (a). Laser output spectrum using FBG_B,1 for different total pump powers. (b). Repeated scan of laser output spectrum for 8 W of total pump power.

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Figure 3 illustrates the laser output spectrum when using FBG_B,2 for a pump power of 10 W (FBG_A was tuned slightly so that it overlapped the spectrum spanned by FBG_B,2). Again, the two main lasing peaks at 1090.06 nm and 1090.75 nm are due to the two superimposed gratings. The corresponding linewidths are 40 pm and 60 pm, and the SNRs are > 58 dB and > 59 dB, respectively. The additional peaks at 1088.68 nm, 1089.37 nm, 1091.45 nm, and 1092.14 nm are due to FWM; all peaks are spaced by ≈ 0.69 nm. The total output power is 1.8 W. In this case, a slightly higher pump power is required to obtain the multiple peaks arising from FWM (the stability is similar to before). This may be attributed to the decreased efficiency of FWM owing in part to the larger phase mismatch caused by the larger wavelength separation.

 

Fig. 3. Laser output spectrum using FBG_B,2 for 10 W total pump power.

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In order to obtain the closest wavelength spacing for dual-wavelength lasing, we tune the shorter wavelength grating in FBG_B,3. Figure 4 shows the laser output spectra at 10 W of pump power for the nominal spacing of 0.25 nm and when the spacing is reduced to 0.12 nm. In both cases, the two main lasing peaks exhibit very narrow linewidths (here < 30 pm) and high SNR (limited by the OSA resolution). The corresponding output power is 1.85 W and the stability is comparable to that described above. When the wavelength spacing is reduced further, however, the stability degrades and there are large variations in the output power and one of the main peaks occasionally stops lasing. While a narrower wavelength spacing in principle improves FWM due to the reduced phase mismatch, there is increased gain competition due to the homogeneous broadening which becomes more dominant and degrades laser stability (note that for larger wavelength spacings, the FWM process is less efficient due to the greater phase mismatch, however, gain competition due to homogeneous broadening reduces).

 

Fig. 4. Laser output spectrum using FBGB,3 for 10 W total pump power: (a) nominal spacing of 0.25 nm and (b) 0.12 nm.

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3. Conclusion

In summary, we have reported the observation of FWM in a high-power, dual-wavelength twin-core YDFL. The FWM process, which occurs directly in the gain fiber, serves to stabilize dual-wavelength operation. In particular, we achieve stable dual-wavelength operation with narrow linewidths (typically < 40 pm), high SNR (> 55 dB), and a record narrow wavelength spacing of 0.12 nm. In addition, we obtained the highest output power (1.85 W) so far reported for dual-wavelength operation. The use of twin-core fiber allows the physical separation of the pump and active signal cores, and provides the advantages of distributing the pump power as well as simplicity in defining the laser cavity. We believe that operation can be scaled to additional wavelengths using sampled FBGs or other suitable multi-wavelength filters.