1. Introduction

In the past few decades, Tm-doped fiber laser attracted significant attentions because of its applications in the area of eye-safe LIDAR, spectroscopy, communication, mid-IR source generation, and optical coherence tomography. Up to date, different types of Tm-doped fiber laser have been reported including wavelength tunable [1–3], high power [4–6], pulsed [7–9], dual-wavelength [10, 11], et al. Beyond the above-mentioned Tm-doped fiber lasers, multiwavelength laser source possessing the advantages of cost-effectiveness and muilti-channel is very attractive in various applications such as spectroscopy. However, so far, there have been few works reported in this area [12, 13]. The main challenge to realize stable multiwavelength laser lies in the homogeneous gain-broadening of rare earth ions at room temperature. In order to overcome the drawback, different methods have been proposed including nonlinear polarization rotation [14–16], hybrid gain [17], nonlinear amplifier loop mirror [18, 19] and four-wave mixing (FWM) [20, 21]. However, most of these works were demonstrated in 1 μm or 1.5 μm band because of the readily available components. In contrast, Tm-doped silica fiber offers a broadband gain ranging from 1.8 μm-2.1 μm [22], but most of the components at this band are expensive or not commercially available, e.g. dispersion compensation fiber [23] and therefore limiting the development of multiwavelength laser source in this band. Recently, Wang et al demonstrated a multiwavelength Tm-doped fiber laser source by using polarization rotation and FWM [12], while Peng et al reported a mutilwavelength source by employing nonlinear amplifier loop mirror [13]. In these two schemes, nonlinear effects (FWM and self-phase modulation) are the main mechanisms to overcome the homogeneous gain-broadening at room temperature. However, because the nonlinear coefficient of standard single-mode fiber (SMF) is rather small at longer wavelength, hundreds of meters length of SMF length was utilized in their setups as a result making the multiwavelength source not that flexible.

Highly Germania-doped fiber (HGDF) consisting of a germania-silica core within a silica cladding has attracted extensive interest in the past years [24–29]. Particularly, HGDF is a promising candidate for the applications at longer wavelength e.g. ~2 μm because of its extraordinary properties. On one hand, compared with SiO₂, the absorption of GeO₂ is much smaller near 2 μm band and therefore HGDF can have lower transmission loss than standard SMF [26]. On the other hand, the larger nonlinearity of GeO₂ makes HGDF a good choice for nonlinear applications [30]. To date, different HGDFs have been employed for pulse generation toward longer wavelength [27–29]. However, the role of HGDF for multiwavelength continuous wave generation has yet to be addressed.

In this paper, we reported an all-fiber multiwavelength Tm-doped laser source by introducing a section highly germania-doped highly nonlinear fiber (HG-HNLF) in the laser cavity. Because of the relatively high nonlinear coefficient in this special fiber, gain competition is mitigated significantly by FWM within only 50 m HG-HNLF. Pumped by an amplified laser diode at 1570 nm, we successfully obtain the multiwavelength lasing with 0.86-nm wavelength spacing at room temperature. The power fluctuations and wavelength drifts of the lasing line are less than 1.6 dB and 0.05 nm, respectively. The performances with or without the HG-HNLF are also compared, which demonstrate that HG-HNLF plays an important role for the broadband multiwavelength generation.

2. Experimental setup and operation principle

The experimental setup of the multiwavelength laser is depicted in Fig. 1. A 4-m long Tm-doped fiber (Nufern, SM-TSF-9/125) with a core diameter of 9 μm, cladding diameter of 125 μm and NA of 0.15 is pumped by an amplified 1570 nm seed light via 80% port of a 2 × 2 20/80 coupler as gain medium. A section of 50-m HG-HNLF is positioned right after the Tm fiber to provide intensity dependent gain. The HG-HNLF with core diameter about 4 μm and cladding diameter 125 μm is directly spliced with the Tm fiber and SMF. According to our measurement, the splicing loss is about 9 dB due to the core size and NA mismatching. An isolator (ISO) is used for forcing unidirectional operation. A comb-like filter for multi-channel lasing is constructed by an 7.2-m polarization maintaining fiber loop mirror (PMF-LM) with birefringence of 5.7 × 10⁻⁴, and polarization controller (PC1) is used to adjust the transmission properties of the loop mirror. Finally, PC2 is employed to control the state of polarization (SOP) of the laser cavity, and the output is obtained from the 80% port of the 20/80 coupler.

 

Fig. 1 Schematic diagram of the proposed Tm-doped multiwavelength laser source. PC: polarization controller; PMF: Polarization maintaining fiber; ISO: Isolator.

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The HG-HNLF (provided by Fiber Opitcs Research Center, Russian Academy of Sciences) is the key component for ensuring stable multiwavelength lasing. When the oscillating light travels along the HG-HNLF, cascaded FWM process generates in the fiber. During the process, lasing lines with high intensity can transfer energy to adjacent low-energy lasing lines which are well-defined by the comb-liked filter and vice versa. Finally, the lasing lines achieve self-stability and therefore the mode competition caused by homogeneous broadening can be alleviated at room temperature. The HG-HNLF used here possesses 4-μm core diameter with 75 mol. % GeO₂ doping concentration. The cladding diameter is 125 μm as well. Based on the Sellmeier functions for GeO₂-SiO₂ glasses in [31], the NA is estimated to be 0.57 at 1900 nm. Also, we perform simulation by using COMSOL Multiphysics, and find that only the 0th-order and 1st-order core-guided mode groups are supported. Therefore, when the HG-HNLF is spliced with SMF-28 without any core offset, fundamental mode can dominate the transmission performance [32]. To compare the nonlinear performance of SMF-28 and the HG-HNLF, we estimate the nonlinear coefficients of these two fibers. For SMF-28, the core diameter is 8.5 μm with 3 mol. % GeO₂ doping concentration in the core region. For the nonlinear refractive index of GeO₂-doped silica, a linear relation with silica of n⁽²⁾ = (0.0552Conc + 2.44) × 10⁻²⁰ m²/W is used to roughly estimate nonlinear refractive index of core and cladding, respectively, where Conc is the mole concentration [33]. Please note that though this relation is obtained by taking the consideration of GeO₂ distribution, it is roughly reasonable to be used here because the nonlinear refractive index of germania-glass should be about 3 times higher than that in SiO₂, as mentioned in [30]. Following to the full vector model [34], both the nonlinear coefficient and effective area can be estimated. According to our calculation, the effective area is estimated to be 10 μm² and the nonlinear coefficient of our HG-HNLF is ~21.8 w⁻¹·km⁻¹ at 1900 nm which is nearly 30 times that of the SMF-28 with ~0.66 w⁻¹·km⁻¹. On the other hand, though the HG-HNLF with such high doping level suffers from larger scattering loss, its transmission loss 20 dB/km is still comparable to the value 12 dB/km in SMF-28. This is because the material loss of GeO₂ is much lower than SiO₂ in the gain bandwidth of Tm-doped fiber [30]. Therefore, it can be expected a short length of HG-HNLF could provide sufficient nonlinear effect so that the cavity length of a multiwavelength laser can be significantly reduced.

3. Results and discussion

During the experiment, we gradually increased the pump power together with adjusting PC1 and PC2. It is found that the lasing threshold is at 408 mW pump level. However, both the power and wavelength of lasing lines are unstable in this condition. Further increasing the pump power to be higher than 865 mW, stable multiwavelength lasing spectra were observed indicating FWM contributes to the gain balance. Figure 2 shows the lasing spectrum obtained by 1.28-W pump power. For OSNR measurement, we define the noise floor to be at −42 dBm which is closed to the “modulation dips” of the lasing peak. Since there are several distinctive lasing lines positioned near the overall lasing spectrum peak, we fit the spectrum and choose the peak of the envelope as the signal power level. In this case, the obtained optical signal-to-noise ratio (OSNR) is as high as 32 dB which is higher than the values reported in [12, 13]. The total number of lasing lines is 9, 22, and 36 within 10-dB, 20-dB, and 30-dB bandwidth, respectively. The wavelength spacing is ~0.86 nm which is determined by the length of PMF. It also can be found that the multiwavelength lasing spectrum has structure and not smooth. In order to find out the reason, we first disconnect the laser cavity and measure the transmission properties of the comb filter by pumping the Tm fiber directly to produce ASE noise. The measured corresponding spectrum is illustrated in Fig. 3(a). The extinction ratio of the filter is about 14 dB near 1880 nm. However, there are several unwanted low transmission wavelength region from the measured spectrum. Next, we measure the ASE spectrum right after the Tm fiber, as shown in Fig. 3(b). It can be find that there are some gain dips in the ASE spectrum and these dips match well with the low transmission wavelength region of the filter spectrum. Therefore, we can confirm that the structure of the multiwavelength lasing spectrum is caused by the unsmooth gain profile near 1880 nm of the Tm fiber. This kind of unsmooth gain profile may come from the absorption of the host material (silica) of Tm fiber or be generated during fiber fabrication process.

 

Fig. 2 The output lasing spectrum under the pump power of 1.28 W.

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Fig. 3 (a) Measured spectrum of comb filter by launching ASE noise, and (b) measured spectrum of ASE noise produced by pumped Tm fiber.

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To investigate the stability of the proposed multiwavelength Tm-doped fiber laser, we monitored the laser output over 30 min without any external facilities for stabilization. The output spectra were demonstrated in Fig. 4(a). It is found that the power fluctuation is within 1.6 dB, while the wavelength drift is less than 0.05 nm, indicating a stable operation status. In order to further illustrate the stability, the power fluctuations of five selected lasing lines are shown in Fig. 4(b).

 

Fig. 4 (a) Output spectra measured within 30 minutes, and (b) power fluctuations of five selected lasing lines.

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The laser output power is measured at different pump power, shown in Fig. 5(a). Good linear relationship is obtained. The output spectra with different pump power, 865 mW, 1033 mW, and 1200 mW are also demonstrated in Fig. 5(c)-5(d), respectively. It can be found that the lasing lines near 1880 nm possess much higher power than the adjacent ones at low pump power, shown in Fig. 5(b). However, with increased pump power, the power difference among the lasing lines becomes smaller, as shown in Fig. 5(c)-5(d). This is because higher power enhances the conversion efficiency of FWM and therefore the total cavity gain is shared with different lasing lines more efficiently.

 

Fig. 5 (a) Laser output power against input pump power, (b-c) laser output spectra with pump power of 865 mW, 1033 mW, and 1200 mW, respectively.

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In order to further highlight the contribution of the HG-HNLF, we remove the HG-HNLF from the cavity for comparison. It is found that without the HG-HNLF the lasing threshold is reduced to 285 mW because of the reduction of total cavity loss. Further increasing the pump power, multiwavelength lasing is achievable. However, only several distinctive lasing peaks can be obtained. Furthermore, both power and wavelength of these lasing lines are unstable. This is caused by the homogeneous gain-broadening of the rare-earth ions at room temperature. Figure 6 (a) demonstrates the output spectrum with 780 mW pump and 158 mW output. Obviously, the power cannot transfer to the adjacent fringes without the help of the HG-HNLF. The output power versus different pump power is illustrated in Fig. 6 (b). A higher slope efficiency of 34.6% can be obtained because of the significant splicing loss reduction. Furthermore, we also replace the HG-HNLF with 50-m length SMF-28. Slightly different from the case in Fig. 6(a), multiwavelength lasing performance can be obtained easily with the same pumping level, since FWM contributes the power transfer between adjacent lasing lines to some extent as shown in Fig. 6(c). However, the nonlinear effect in such a short fiber length is still too weak to permit stable operation. In this condition, a slope efficiency of about 32.3% is achieved, shown in Fig. 6(d).

 

Fig. 6 Laser output spectra with 780 mW pump under the condition of (a) with neither 50-m HG-HNLF nor 50-m SMF and (c) with 50-m SMF. Laser output power versus pump power under the condition of (b) with neither 50-m HG-HNLF nor 50-m SMF and (d) with 50-m SMF.

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To highlight the advantages of utilizing HG-HNLF, we summarize the key points as follows. On one hand, as mentioned before, the cavity length is reduced to the order of several tens meters by using HG-HNLF. Though longer SMF can be coiled, shorter cavity length ensures an improved immunity to external environment changes for example, temperature gradient. Secondly, the nonlinear effect in fiber is mainly determined by three key points, namely, power, length, and nonlinear coefficient. For SMF, the intrinsic low nonlinearity limits the pump power and fiber length reduction. Therefore, sufficient pump power and fiber length are required to permit several tens of lasing lines with good performance, simultaneously. While for HG-HNLF, benefiting from the much higher nonlinearity, the choice of pump power and fiber length can be more flexible for different applications. For instance, the required nonlinear fiber length can be further reduced by increasing pump power. Under the condition of low interactive power, the performance can be optimized by choosing properly fiber length as well. Currently, the challenging by employing HG-HNLF comes from the large loss when splicing with Tm fiber and SMF because of the core size and NA mismatching. Nevertheless, even such additional attenuation existing in the laser cavity, the multiwavelength lasing performance is obvious in our experiment indicating great potential of this fiber. By using advanced splicing techniques such as taper splicing to further reduce the cavity loss [35–37], we believe the performance of our laser can be significantly improved.

4. Conclusion

In summary, we have proposed and experimentally demonstrated an all-fiber multiwavelength Tm-doped laser assisted by a section of HG-HNLF inserted in the laser cavity. Because of the high nonlinearity of the fiber, FWM effect provides intensity-dependent gain to mitigate the impact of gain competition within only 50-m length of HG-HNLF resulting significant cavity length reduction comparing with the configurations employing standard SMF. By using a PMF-LM based comb filter, 9, 22, and 36 lasing lines with considering 10-dB, 20-dB, and 30-dB bandwidth, respectively is obtained at room temperature with a wavelength spacing of 0.86 nm.