1. Introduction

Single-frequency fiber lasers have attracted intense attention for their narrow linewidth, low noise and robustness, demonstrating various promising applications, such as high-sensitivity sensing, high-precision metrology, high-resolution spectroscopy, coherent lidar and so on [1–3]. To date, the research on single-frequency fiber lasers has mainly concentrated on 1 μm, 1.5 μm and 1.9 μm region by approaches using ytterbium (Yb³⁺), erbium (Er³⁺) and thulium (Tm³⁺) doped fiber [4,5]; also, single-frequency fiber lasers operating at 9XX nm [6,7], 1120 nm [8] and over 2000 nm [9] have been achieved to meet more application demands. Recently, the superior spectral characteristics of 1.7-μm laser were discovered, including its high absorption by a C–H vibrational overtone and low absorption by water. Hence, 1.7-μm lasers, especially those with emitting wavelengths under 1750 nm, have become excellent laser sources for atmospheric sensing, medical surgery and transparent plastics processing [10–14]. Among common rare-earth dopants, Tm³⁺ can exhibit pronounced fluorescence in the 1.7 μm region. Benefiting from the mature technology of manufacturing Tm³⁺-doped fiber (TDF), Tm³⁺ fiber lasers become the most practical scheme for the 1.7-μm laser generation. Until now, the research on 1.7-μm lasers mainly focused on wavelength tuning [15–17] and power scaling [18–20], and the output power has reached the tens-of-watts level in 2019. Despite this, only a few reports realized 1.7-μm single-frequency fiber lasers, and the output power was still limited to milliwatt level with low efficiencies. These approaches were all based on TDF in a short linear cavity. The first 1.7-μm single-frequency fiber laser was proved in 2004 in a distributed-feedback configuration, in which the maximum output power was only 1 mW with 590-mW launched pump power at 790 nm [21]. The laser slope efficiency was then improved to 1.8% at 1740 nm with an in-band pump source at 1600 nm in 2006, but the output power was only 0.5 mW [22]. The inefficient power transfer mainly resulted from the low pump absorption. Even when using a heavily-doped TDF for higher pump absorption and laser gain, the 1.7-μm laser operation is still inefficient in the short-cavity fiber oscillator. Recently, based on a 1.8-cm-long germanate fiber with 5 wt.% Tm₂O₃ doping concentration, a single-frequency fiber laser at 1726nm with a maximum output power of 12.4 mW and a slope efficiency of 4.81% was demonstrate [23].

Realizing an efficient 1.7-μm TDF laser is not straightforward, and enabling powerful single-frequency laser output is much more difficult. The stimulated emission cross sections of Tm³⁺ at wavelengths around 1.7 μm are nearly half of those at 1.8–1.9 μm [4]; therefore, parasitic lasing in the longer wavelength region must be avoided. Meanwhile, a long active fiber is required for sufficient 1.7-μm laser gain, and short linear cavity configuration can barely enable an efficient 1.7-μm single-frequency output with high output power. On the other hand, the 1.7-μm emission requires a high population inversion of Tm³⁺ because of the strong TDF reabsorption loss at 1.7 μm. Hence, sufficient pump intensity is required. To address these challenges, a ring-cavity fiber laser with core-pump configuration is preferred for the realization of the efficient single-frequency output due to the possibility of using long active fiber and avoiding spatial hole-burning. In our early work, we demonstrated hundred-milliwatt-level 1.7-μm fiber ring lasers by using multimode interference filters for wavelength selection and tuning [24], but the laser efficiencies was no more than 10.3%. Then, by using a fiber Bragg grating (FBG) and optimizing the active fiber length and the output coupling, 1.7-μm multi-watt ring-cavity fiber laser was realized [20]. However, the spectral linewidths of those lasers were of GHz-level due to the lack of an effective mode selective mechanism.

In this paper, considering the small free spectral range (FSR) of the ring-cavity fiber laser, a piece of unpumped TDF was used as a saturable absorber (SA) for frequency selection. Due to the strong absorption at 1.7 μm, narrow-band gratings were induced in the SA fiber core, and enabled the 407-mW single-frequency 1720-nm output with a laser linewidth of 4.4 kHz. The strong saturable absorption also resulted in variable cavity losses at 1720nm, resulting in optical bistability of both power and longitudinal mode behavior. To the best of our knowledge, our scheme achieves the highest output power and the narrowest linewidth among single-frequency fiber oscillators at 1.7 μm. Furthermore, this is the first report of optical bistability in TDF lasers as far as we know, in which the 4.8-W bistability operating range is also a breakthrough.

2. Experimental setup

A schematic of the laser is presented in Fig. 1. The pump source was a 1570-nm single-mode fiber laser. Two pieces of commercial TDF (both are Nufern SM-TSF-9/125) were used as the active fiber (TDF1) and the SA (TDF2), respectively. The TDF has a low doping concentration of 1.37×10²⁵ m^-3 and its absorption coefficient at 1570 nm is ∼6 dB/m [25]. A 1570/1720nm wavelength division multiplexer (WDM1) coupled the pump into TDF1, and another one (WDM2) exported the residual pump. An optical circulator was spliced to the WDMs and TDF2, forcing a clockwise laser operation in the resonator. An FBG with a reflectivity of 70% at 1720nm and 3-dB bandwidth of 0.13 nm served as an output coupler. The forward- and backward-propagating beams in TDF2 formed a standing wave field, which can narrow the laser linewidth [26,27]. The FBG together with TDF2 worked as a narrow-band mode selective filter. The end face of FBG was angle cleaved to avoid 1.8 μm parasitic lasing. The laser power meter and the optical spectrum analyzer (OSA) used in the experiment were Ophir 12A and Yokogawa AQ6375 (resolution 0.05 nm).

 

Fig. 1. The schematic of the single-frequency 1720-nm fiber ring laser.

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3. Experimental results

3.1 Without TDF2

Without TDF2 in the cavity, the maximum laser output power at 1.7 μm was 1.8 W under the maximum available pump power of 7.5 W, with an experimentally optimized TDF1 length of 2.3 m. A shorter active fiber could decrease pump absorption and a much longer active fiber caused significant laser reabsorption, both of which reduced the output power. The slope efficiency versus launched pump power was 26%, which was mainly limited by the small output coupling. The center wavelength of the laser was 1719.68 nm. The signal-to-noise ratio (SNR) was 60 dB between the 1720-nm laser and the 1840-nm amplified spontaneous emission (ASE) peak, as shown in Fig. 2. The laser operated in multi-longitudinal mode (MLM), but the laser linewidth at maximum output power was smaller than 0.05 nm, which was further recorded to be no more than 4.5 GHz by a Fabry-Pérot interferometer (FPI, Thorlabs SA210-12B, FSR of 10 GHz and resolution of 67 MHz).

 

Fig. 2. The optical spectrum of the multi-longitudinal mode 1720-nm laser at 1.8-W output power without using TDF2. Inset: the linear laser spectrum measured by the OSA with a resolution of 0.05 nm.

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3.2 With TDF2

3.2.1 Output power bistability

To narrow the laser linewidth, 0.75-m-long TDF2 was spliced in the resonator, in which the forward- and backward-propagating 1720nm signals formed an auto-tracking narrow-band filter. The laser output power and the residual pump power as functions of launched pump power are plotted in Fig. 3, where those of the free-running case (without TDF2) are also presented for comparation. When 0.75-m TDF2 was used as the SA, much pump remained before reaching the oscillation threshold of 6.1 W (State a). Once the pump power reached 6.1 W, the 1.7-μm laser started oscillating and the output power rose to 1.2 W immediately, while the residual pump power decreased to 0.6 W (State b). Further increasing the pump power to 7.5 W resulted in a maximum laser power of 1.6 W and the slope efficiency was 24% (State c). When the input pump decreased from its maximum value, the output power decreased at the slope efficiency of 24% until the output power dropped to 87 mW at the pump power of 1.3 W (State c and d). With a pump power below 1.3 W, the 1.7-μm laser switched off; after that, the laser would not oscillate unless the pump power was raised to 6.1 W again. As TDF2 brought additional cavity losses, the maximum laser output power of 1.6 W was a bit lower than that of the free-running scheme (1.8 W) under the same pump power of 7.5 W, while the residual pump power of 0.8 W was slightly higher than that of 0.6 W without SA.

 

Fig. 3. Output power and residual pump power versus launched pump power with 0.75-m TDF2 and without TDF2. The solid symbols represent single-frequency operation; the letters of a, b, c, d and the arrows represent the process of pump turning up (arrows pointing to the right) or turning down (arrows pointing to the left) with output power bistability. The dashed lines are visual guides.

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This power bistability originated from the strong saturable absorption of the unpumped TDF2, which caused large-variable 1720-nm signal losses. The unpumped TDF2 in this experiment resulted in a significant loss at 1.7 μm when the laser was not oscillating, which however became much weaker due to the saturable absorption in the SA after the laser started oscillating. The oscillation changed the cavity losses so that optical bistability was induced. Although various unpumped rare-earth-doped fibers have been used as SAs in 1-μm, 1.5-μm and 2-μm lasers [9,28,29], optical bistability was only observed in several 1.5-μm lasers with very long (more than 8 m) Er³⁺-doped fibers, whose maximum output power were only tens of milliwatts with bistable ranges smaller than 1 W (in terms of pump power) [30–33]. In our experimental arrangement, due to the large absorption cross sections of Tm³⁺ in the 1.7-μm wavelength region, the optical bistability was observed in TDF for the first time and a much more pronounced output power bistability (4.8-W-wide bistability range in terms of pump power) was achieved.

3.2.2 Single-frequency output and longitudinal mode bistability

When using 0.75-m TDF2, the center wavelength of the output laser was still 1719.68 nm. The longitudinal mode behavior and the evolution of laser linewidth was characterized using the FPI, as shown in Fig. 4. SLM operation could be observed when the pump power was no more than 2.7 W after the laser was switched on. At this time, the laser linewidth of ∼67 MHz was recorded, limited by the FPI resolution. When the pump power was over 2.8 W, the laser transformed to MLM operation, as demonstrated in Fig. 4(a). Since the cavity FSR of ∼20 MHz (∼10-m cavity length) was small than the FPI resolution, the FPI could not distinguish the neighboring longitudinal modes. Hence, the FPI interference waveform was a huge envelope, which reflected the gigahertz-level linewidth. Figure 4(b) depicts the evolution of laser linewidth as a function of launched pump power, where longitudinal mode behavior also exhibited bistability, which means there is a pump power range that both SLM operation and MLM operation can be obtained separately with a same pump power. After being switched on, the laser operated in MLM. It would turn into SLM as the pump power was reduced to below 2 W. After that, the laser would keep operating in SLM with increasing pump power till it reached the SLM switch-off threshold of 2.8 W, which is denoted as P_SLM-off. In other words, once the laser operated in MLM, we need to re-establish the SLM operation by decreasing the pump power to below the P_SLM-on of 2 W, rather than the P_SLM-off of 2.8 W.

 

Fig. 4. (a) FPI traces and (b) linewidth evolution with longitudinal mode bistability of the 1720-nm laser. P_SLM-on and P_SLM-off represent SLM switch-on and switch-off thresholds. The black arrows represent the process of increasing the pump power (pointing to the right) or reducing the pump power (pointing to the left). The TDF2 length was 0.75 m. The dashed lines are visual guides.

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The unpumped rare-earth-doped fibers can be used as SAs and work as narrow-band filters [26,27]. We infer the longitudinal mode bistability in our experiment is related to different frequency-selecting mechanisms of the SA in SLM operation and MLM operation. When the laser operates in SLM, counter-propagating single-frequency beams induce auto-tracking reflection gratings in the SA, which have higher absorption for frequencies other than the laser frequency that produces the gratings [26]. The gratings and the saturable absorption together result in a high P_SLM-off. When the laser operates in MLM, the frequency selection is less effective because the gratings are washed out; single-frequency operation is forced only by the saturable absorption so the value of P_SLM-on is lower than the P_SLM-off. As mentioned above, this SA has high absorption at 1720nm, hence enabling an outstanding frequency selection capability and resultant high-power operation of the single-frequency laser.

Repeated experiments were conducted to study the properties of the longitudinal mode bistability. We found that this 1720-nm oscillator had fixed thresholds of P_SLM-on and P_SLM-off, which can divide the laser output into three regions: SLM-only region (P_pump<P_SLM-on, with P_pump the incident pump power), MLM-only region (P_pump>P_SLM-off) and longitudinal mode bistable region (P_SLM-on<P_pump<P_SLM-off), depicted in Fig. 4(b). In the SLM-only region, stable single-frequency output was realized, which means the SLM laser had strong resistance to environmental disturbance, and mode hopping was barely observed. In the longitudinal mode bistable region, the output could be single-frequency, but it was less resistant to environmental disturbance. In this region, mode hopping may occur every one or two minutes, and a severe disturbance like moving the TDF2 could irreversibly change the single-frequency laser to MLM operation. In the MLM-only region, the laser linewidth was close to the free-running linewidth without using SA at a same output power, implying a weak frequency-selecting result of the SA.

The single-frequency output power as a function of pump power is plotted in Fig. 5. The maximum single-frequency output power obtained was 407 mW, under a launched pump power of 2.7 W. As the laser would be switched off when the launched pump power was lower than 1.3 W, the minimum single-frequency power was 87 mW. The slope efficiencies with respect to launched and absorbed pump power were 21.7% and 22.7%, respectively. The residual pump power was only 308 mW with 2.7 W incident pump, revealing a pump absorption of 9.4 dB. The power fluctuation at the maximum power of 407 mW was 0.6% (RMS) over 20 minutes. The laser spectrum at 407-mW output power is shown in the inset of Fig. 5. The SNR between 1720nm laser and ASE peak was 52 dB, which is lower than the SNR of 60 dB of the 1.6-W MLM output. The reason is the stronger reabsorption on the 1.7 μm signal under a lower power. Higher cavity feedback can be used to further improve the SNR.

 

Fig. 5. Single-frequency output power of the 1720-nm laser as functions of absorbed pump power and launched pump power. Inset: optical spectrum of the single-frequency laser at 407 mW output power (OSA resolution of 0.05 nm). The TDF2 length was 0.75 m.

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To measure the laser linewidth, a delayed self-heterodyne system was set up with 50 km delay fiber (Corning, SMF-28). The heterodyne signal recorded by a radio frequency spectrum analyzer (Agilent N9030A) was shown in Fig. 6. The 20-dB linewidth of the Lorentzian fitting curve was 88.9 kHz, indicating a 3-dB spectral linewidth of 4.4 kHz. The narrow linewidth was attributed to the small intrinsic linewidth of a long-cavity fiber laser.

 

Fig. 6. The recorded heterodyne signal and the Lorentzian fitting curve at the output power of 407 mW. The TDF2 length was 0.75 m.

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3.2.3 Influence of the TDF2 length on optical bistability

As discussed above, the variable absorption of the SA determined the output power bistable range, while the longitudinal mode bistability also had specific thresholds. Therefore, we experimentally investigated the influence of the TDF2 length on the thresholds of optical bistability on both laser power (output power bistability) and single-frequency operation (longitudinal mode bistability), as presented in Fig. 7. Figure 7(a) shows that when the TDF2 length was over 0.5 m, optical bistability occurred, and longer TDF2 resulted in wider bistable range. For the case with 0.75-m-long TDF2, the power bistable region was 1.2-W wide (87–1245 mW) in terms of output power and 4.8-W wide (1299–6110 mW) in terms of pump power. Increasing the TDF2 length raised the laser switch-on threshold significantly but raised the laser switch-off threshold slightly. Figure 7(b) illustrates the longitudinal mode bistability within the range of output power bistable region. In the figure, the red pattern represents SLM-only region; the cyan pattern indicates MLM-only region; the yellow part stands for the longitudinal mode bistability region, where both SLM and MLM can appear. Higher single-frequency laser power and wider longitudinal mode bistable range were expected with longer TDF2. However, further increasing the TDF2 length to over 0.75 m resulted in a high laser switch-on threshold beyond the maximum pump power available and the laser could not start oscillating. Figure 7(c) shows the thresholds of optical bistability. The single-frequency operating ranges (the red and yellow patterns) were always inside the power bistable operating range in our experiments. When the TDF2 length was shorter than 0.5 m, the laser could barely operate in SLM, but the power bistability still existed. This was unusual compared with other single-frequency fiber lasers. For instance, in Ref. [31], output power bistability was achieved in hybrid Brillouin-erbium fiber laser, but the bistable operating range was within the single-frequency operation range. In our former work [9], optical bistability was not observed in the 2004-nm fiber laser though 6.5-m-long unpumped TDF with the same fiber parameters was used to realize the single-frequency operation. The difference is due to the wavelength-dependent absorption of the SA fiber. The SA fiber has very different absorption coefficients at different wavelengths. When the laser was running at the wavelengths with high absorption coefficient, the SA is likely to result in optical bistability as well as high-power SLM operation. However, for wavelengths at the edge of the SA absorption spectrum, a much longer SA fiber can also force the laser to operate in SLM, but the requirement for power bistability is hard to be met. Generally, employing highly absorptive SA fiber is a promising approach for single-frequency fiber lasers with numerous kinds of dopants and corresponding wavelengths, as well as for fiber lasers based on nonlinear gain.

 

Fig. 7. The regions and thresholds of optical bistability with different lengths of TDF2: (a) regions of output power bistability (within 7.5-W pump power), (b) regions of longitudinal mode bistability (within the output power bistable region), and (c) thresholds of output power bistability and longitudinal mode bistability. The patterns with different colors in (c) represent different longitudinal mode behaviors, which have been explained in detail in the legends of (b). The dashed lines are visual guides.

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4. Conclusions

In summary, a single-frequency Tm³⁺-doped fiber laser at 1720 nm was demonstrated. We proposed to use unpumped active fiber to obtain SLM laser output at a highly absorbing wavelength of TDF. With 0.75-m TDF2, the maximum single-frequency output power reached 407 mW with a SNR of 52 dB and a pump absorption of 9.4 dB. The slope efficiency of the single-frequency laser reached 22.7% (versus absorbed pump power), and the laser linewidth was as narrow as 4.4 kHz. Due to the strong saturable absorption effect, optical bistability both on power and longitudinal mode behavior was observed. The origins of optical bistability were discussed and the optical bistability characteristics with different lengths of unpumped TDF were experimentally studied. The results show that, when using the unpumped TDF as the 1720-nm SA, the ring-cavity TDF fiber laser is capable of powerful single-frequency output as well as pronounced optical bistability.