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Abstract
Here we demonstrated an efficient high-power single-frequency thulium-doped fiber ring laser operating at 1720 nm. Three cascaded sub-rings were inserted into the main cavity to significantly enlarge the effective free spectral range. By incorporating a fiber Bragg grating, the single longitudinal mode operation was achieved. The maximum single-frequency output power reached up to 1.11 W under 3.75-W launched pump power, while the slope efficiency with respect to the absorbed pump power was 46.4%. The laser linewidth at maximum single-frequency power was measured of 1.9 kHz. Potential power scaling of the single-frequency output power with different quantity and lengths of the sub-rings was also theoretically investigated.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In recent years, high-power single-frequency fiber lasers which exhibit outstanding performance in narrow linewidth, long coherence length and low noise have attracted intensive attention for their applications in high-resolution molecular spectroscopy, nonlinear frequency conversion, and optical communication [1–5]. In particular, single-frequency fiber lasers based on thulium-doped fiber operating at 1.7 μm regime, where C-H bond exhibits strong absorption while the water molecule is transparent, have been recognize as excellent laser sources and applied in atmosphere sensing, methine gas detection, and medical diagnose [6–9].
Several techniques have been proposed to achieve single longitudinal mode (SLM) operation in fiber lasers. Based on distributed feedback (DFB) and distributed Bragg reflector (DBR) structure, single-frequency fiber lasers with linear cavity configuration have been demonstrated at 1 μm [10], 1.5 μm [11]and 2 μm regimes [12]. However, for 1.7-μm thulium-doped fiber which suffers low gain and strong reabsorption loss, short active fiber of a few centimeters would result in very limited output power and laser efficiency, or even cannot reach lasing threshold. Although the first 1.7-μm single-frequency fiber laser based on thulium-doped fiber has been demonstrated with the DFB structure in 2004 [13], the output power at 1735 nm was merely 1 mW with a quite low efficiency of 0.2%. In 2021, Cen et al demonstrated a 1727 nm DBR single-frequency fiber laser based on 1.8-cm-long heavily thulium-doped germanate fibers [14]. However, the maximum output power was only 12.4 mW with a poor slope efficiency of 4.8%. Although the rare-earth-doped soft glass fiber with higher doping concentration benefits higher optical gain, the improvement of the output power and efficiency at this special wavelength are still limited. On the contrary, the ring cavity fiber lasers incorporated with linewidth-narrowing techniques allow longer active fiber lengths which provides sufficient laser gain and potential output power, which are more suitable for single-frequency power scaling. Moreover, the absence of spatial hole-burning effect also facilitates the realization of high-power single-frequency operation. However, the long cavity length of fiber ring laser generally results in densely-spaced longitudinal modes. To achieve single-frequency lasing in a ring cavity, one needs to insert ultra-narrowband filters, such as an FBG-based Fabry–Perot etalon [15] or a piece of unpumped active fiber based saturable absorber filter [16]. Recently, by using the thulium-doped fiber as the saturable absorber, our team demonstrated a 1720 nm single-frequency fiber ring laser with a maximum single-frequency output power of 400 mW. However, since the thulium-doped fiber exhibits strong absorption at 1.7 μm, the threshold 1570 nm pump power reached up to ∼6 W, which has limited the further power scaling of the single-frequency output [17].
Alternatively, the compound cavity, composed of several passive sub-ring cavities and a main cavity, was recognized as a low-cost and feasible approach to achieve single-frequency operation. The incorporated sub-cavities can substantially enlarge the effective free spectral range (FSR) based on the Vernier effect. The signal needs to meet the resonance conditions of each single sub-ring cavity synchronously, and thus the effective FSR would be the least common multiple of each FSR. Such behavior would significantly enlarge the longitudinal mode spacing. By further combining a narrower-bandwidth fiber filter, such as a fiber Bragg grating (FBG) as the coarse wavelength selector, SLM oscillation at a specific wavelength would be realized. Based on this approach, Yeh et al. demonstrated a 1481-1513 nm tunable single-frequency erbium-doped fiber laser [18]. However, the maximum output power was no more than 3 mW and other details, such as the pump power and laser efficiency were not discussed. In 2011, Yin et al. reported a single-frequency ytterbium-doped fiber laser with a wavelength tuning range of 60-nm based on two cascaded sub-rings [19]. The single-frequency output power was only 4 mW under 100 mW pump power at 976 nm, which was limited by the large cavity loss and a low output coupling. In 2016, Feng et al. proposed a novel passive sub-ring cavity which is composed of a Type-1 ring nested inside a Type-2 ring [20]. Based on this structure, they demonstrated a tunable single-frequency erbium-doped fiber laser with a sub-kHz linewidth. Although the laser exhibited excellent linewidth performance, the extremely low output power (<1 mW) has restricted its practical application.
Overall, single-frequency fiber lasers based on a compound cavity have already been demonstrated with Yb- and Er-doped fibers, the output power and efficiency of which are limited yet. However, due to the low gain and high reabsorption loss at 1.7 μm of thulium-doped fiber, efficient single-frequency fiber lasers working at this wavelength based on such scheme has never been reported. In our previous work, we demonstrated a 1.7-μm thulium-doped fiber ring laser, which reveals that efficient 1.7-μm operation can be achieved with ring-cavity scheme via the optimization of active fiber length and output coupling [21]. In this work, we further optimized the spectral behavior of the ring-cavity fiber laser and demonstrated a watt-level single-frequency thulium-doped fiber laser emitting at 1720 nm. Three sub-ring cavities were introduced into the primary fiber ring to further select SLM. Before the laser became multi longitudinal mode (MLM) under higher pump power, a maximum single-frequency output power of 1.11 W and a slope efficiency of 46.4% with respect to the absorbed pump power were achieved. Furthermore, to improve the single-frequency output power, we have also theoretically investigated the transmission characteristic of the compound cavity with different lengths and quantity of the sub-rings.
2. Experimental setup
The experimental setup of 1720 nm thulium-doped fiber ring laser is depicted in Fig. 1. 3-m commercial thulium-doped fiber (Nufern, SM-TSF-9/125) with a low doping concentration of ∼0.3 wt.% served as the active fiber, which was core-pumped by a 1570-nm fiber laser providing a maximum output power of 6 W. Two wavelength division multiplexers (WDM1 and WDM2) were used to couple the pump into the cavity and remove the residual pump. To achieve a high laser output power, a homemade coupler providing 78% output coupling and 22% feedback, which is very closed to the theoretically optimal output coupling [21], was used as the output coupler. The lower intracavity power resulted from the low feedback has also helped to suppress the MLM oscillation. The circulator was used to ensure the unidirectional operation of the signal and prevents spatial hole-burning. A high-reflectivity (HR) FBG with a 3-dB bandwidth of 0.61 nm was used to determine the laser wavelength and select longitudinal modes preliminarily. Considering the trade-off between simplicity and SLM selection ability, three sub-rings made by three homemade 50/50 fiber couplers were used in experiment. The total insertion loss at 1.7 μm of three couplers was measured of around 1.2 dB. For achieving single-frequency operation with such compound cavity scheme, an effective FSR larger than the FBG bandwidth and a narrow transmission band that allows only one longitudinal mode to oscillate within it are essential. In order to meet the above conditions, the lengths of the three sub-rings are chosen as 0.63 m, 1.33m and 1.53 m, corresponding to the FSRs of nearly 330 MHz, 155 MHz, and 135 MHz, respectively. According to the Vernier effect [22], the combined FSR is around 92 GHz (0.91 nm @ 1720 nm), which is 1.5 times over the 3-dB bandwidth of FBG (∼61 GHz @ 1720 nm). Hence, only one longitudinal mode is able to oscillate with the lowest loss within the FBG band and SLM operation can be theoretically achieved. A polarization controller (PC) was used to adjust the state of signal polarization.
Fig. 1. Schematic of the 1.7-μm single-frequency thulium-doped fiber ring laser. PC: polarization controller.
3. Experimental results and discussion
3.1 Experimental results
We first inserted two sub-rings (sub-ring1 and sub-ring2) into the laser cavity. The longitudinal mode behavior was measured using a scanning Fabry–Perot interferometer (FPI, Thorlabs, SA210-12B) and an oscilloscope (Tektronix, DPO2024B). The FSR and fineness of the FPI are 10 GHz and 150, respectively. Due to the small effective FSR (∼10 GHz, 0.09 nm @ 1720 nm) of the compound sub-cavities, the laser could operate in SLM only near the threshold. The maximum single-frequency output power before it became MLM under higher pump power was around 60 mW under 1.2 W launched pump, while the slope efficiency with respect to the absorbed pump power was 46.8%. Then we inserted the sub-ring3 to further suppress the MLM oscillation. In this case, the output power of the single-frequency fiber laser as functions of both launched pump power and absorbed pump power are shown in Fig. 2. The laser threshold in terms of launched pump power was ∼1.1 W. Due to the large effective FSR, the maximum single-frequency output power was significantly improved to 1.11 W under 3.75-W launched pump power. Under the maximum available launched pump power of 6 W, the laser yielded a maximum MLM output power of around 2 W. The slope efficiency with respect to the launched and absorbed pump power were 41.7% and 46.4%, respectively, which exhibited very little decrease compared with those of the 2-sub-ring case, and were comparable with those our former results which concentrated on the optimization of the 1720-nm laser efficiency [19]. The laser spectrum at the single-frequency output power of ∼1.1 W was measured by an optical spectrum analyzer (OSA, Yokogawa, AQ6375) and presented in the inset. The signal-to-noise ratio (SNR) is around 54 dB and the fraction of amplified spontaneous emission (ASE) noise in the output power is estimated less than 10−3. A higher SNR can be obtained by using a higher feedback, which would decrease the output power and efficiency yet. The lasing wavelength λc, determined by the FBG, is 1719.7 nm and the 3-dB linewidth was measured of around 0.05 nm, which was limited by the resolution of the OSA. At the maximum output power of 2 W, the SNR and 3-dB linewidth recorded by the OSA were 56 dB and 0.05 nm, respectively.
Fig. 2. Output power versus launched and absorbed pump power of the 1.7 μm single-frequency thulium-doped fiber laser. The solid and dashed symbols represent the SLM and MLM operation, respectively. Inset: Laser spectrum at a single-frequency power of 1.11 W.
Figure 3 shows the FPI traces of the single-frequency fiber laser at different pump powers and output powers. As shown in Fig. 3(a), when the pump power was below 3.75 W after reaching threshold, there was only one longitudinal mode within a scanning circle, indicating that a stable SLM operation was achieved. The 3-dB linewidth of a single longitudinal mode is measured of ∼67 MHz, which was limited by the FPI’s resolution. As the pump power further increased, several weak longitudinal modes started oscillating and the SLM operation became slightly unstable, as shown in Fig. 3(b) and Fig. 3(c). When the pump power increased to 6 W that 2 W 1720-nm laser output was obtained, much more longitudinal modes started oscillating and a completely MLM oscillation was recorded (see Fig. 3 (d)). Note that for all the above cases, there is always only one main longitudinal mode and each neighboring longitudinal mode was resolvable. We also investigated the stability of the SLM operation of the thulium-doped fiber laser by monitoring the FPI spectrum. At lower pump power (< 2 W), the laser operated in the mode-hopping free scheme. Mode-hopping occurred gradually as the pump power increased. When the pump power was increased to 3.75 W (output power of 1.1 W), mode-hopping occurred once every three minutes on average. The frequency spacing between mode-hopping hops was less than 500 MHz. Since we did not employ any active thermal management in experiment, the temperature variation induced by the pump power change would inevitably cause the frequency drift. Fortunately, three sub-rings exhibited a negligible temperature variation owing to their low loss (∼0.4 dB). At a fixed pump power, the laser tended to become stable without obvious frequency fluctuation after several minutes of operation. Hence, the temperature change is not a serious issue in current experiment. For further practical applications, effective temperature feedback system could be employed to suppress the frequency drift and mode-hopping.
Fig. 3. Oscilloscope trace of the FP interferometer at output power of (a) 1.11 W, (b) 1.17W, (c)1.27 W, and (d) 2 W. Ppump means the launched pump power, while the Pout means the output power.
We used an electrical spectrum analyzer (Agilent Inc., PXA N9030A) to measure the relative intensity noise (RIN) at a single-frequency power of ∼1.1 W. The resolution bandwidth and view bandwidth were set to 1 kHz and 0.1 kHz, respectively. The laser power launched into the photodetector was attenuated to ∼1 mW, corresponding to a shot noise limit of 156.4 dB/Hz [23]. As shown in Fig. 4 (a), two peaks (-93 dB/Hz @ 90 kHz, -116 dB/Hz @ 180 kHz) were observed within a frequency range of 0-5 MHz. As the pump power increased, the high-frequency peak shifted toward a higher peak, while the frequency of the lower one remained constant, indicating that the high-frequency peak was related to the relaxation oscillation while the lower one was a noise transformed from pump source [24,25]. The RIN of the single-frequency fiber laser was lower than 142 dB/Hz at the frequency above 1 MHz, which benefited from the excellent wavelength and intensity stabilization of 1570-nm pump source. Additionally, the in-band pump scheme has minimized the quantum defect and the thermal noise, which also facilitated the low-noise performance. A weak peak at around 0.6 MHz, which was resulted from the external disturbances or the measurement system, was also observed.
Fig. 4. (a) Measured RIN, (b) laser linewidth and its Lorentzian fitting profile of the single-frequency fiber laser at output power of 1.1 W. Inset: zoomed RIN within 0-0.5 MHz span.
Employing the delayed self-heterodyne method with 50 km delay fiber (SMF-28, Corning), the laser linewidth was measured at an output power of 1.1 W, as shown in Fig. 4 (b). According to the Lorentzian fitted profile, a 20 dB linewidth of 38 kHz was observed, indicating that the full width of half maximum was 1.9 kHz. Furthermore, the power stability of the single-frequency fiber laser has been monitored for 30 minutes. As shown in Fig. 5, under 3.75 W launched pump power, the root mean square (RMS) for output power was less than 0.5%.
Fig. 5. Power stability of the 1720nm single-frequency fiber laser. The launched pump power is 3.75 W.
3.2 Potential further power scaling of the single-frequency output
For further power scaling of the single-frequency output, we theoretical analyzed the reason of the longitudinal mode behavior deterioration with the increasing of the pump power. According to [26,27], for a passive fiber ring cavity, the 3-dB bandwidth can be presented as follows:
Fig. 6. (a), (b), (c) FSR and 3-dB bandwidth of sub-ring1, sub-ring2 and sub-ring3, (d) overall transmission of three sub-cavities, (e) overall transmission after adding one more 0.63-m sub-ring.
It is worth mentioning that the FBG also has some influence on the longitudinal mode behavior. For achieving single-frequency operation with such compound cavity scheme, the bandwidth of FBG needs to be narrower than the effective FSR. This condition would be satisfied easier if a narrower-band FBG with the same reflectivity is used, which means the chosen of sub-rings’ lengths would be more flexible. In addition, as mentioned before, the sub-transmission-peaks limited the further power scaling of single-frequency output in current experiment. If the HR-FBG was replaced by a narrower-band FBG with the same reflectivity, less sub-transmission-peaks would be contained within the FBG reflection band, which would suppress the MLM oscillation. Hence, using a narrower-band FBG is beneficial for a higher single-frequency output power. However, since the replaced FBG has the same reflectivity, the cavity loss for the oscillating mode would remain unchanged, the slope efficiency would not change.
4. Conclusion
In summary, we have experimentally demonstrated a single-frequency ring-cavity fiber laser at 1720 nm with commercially available thulium-doped silica fiber. By utilizing three sub-rings for mode-selecting, 1.11 W stable single-frequency output was obtained. The slope efficiency, SNR and the 3 dB linewidth were 46.4%, 54 dB and 1.9 kHz, respectively. The RIN was lower than -142 dB/Hz when the frequency was above 1 MHz. It is shown that adding more sub-rings into the laser cavity causes only a little decrease on laser efficiency but improves the single-frequency laser power significantly. Further power scaling of the single-frequency output was also discussed.
Funding
National Natural Science Foundation of China (61975146, 62075159); Major Scientific and Technological Innovation Projects of Key R&D Plans in Shandong Province (2019JZZY020206).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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