1. Introduction

During the past years, thulium-doped fiber lasers (TDFLs) have attracted much attention due to their numerous applications in 2 μm waveband, such as gas sensing, clinical surgery, optical communication and coherent LIDAR. For example, there are many gas molecule absorption lines in the waveband of 2 μm, including N₂O, CO₂, etc. Therefore, narrow linewidth single-frequency TDFLs with long coherent length are in great demand for remote atmospheric sensing to improve the resolution, sensitivity and sensing distance [1]. On the other hand, stable continuous wave single-frequency TDFLs are convenient to be modulated into optical carriers for optical communication experiments in 2 μm waveband.

To date, several common methods have been implemented to develop SLM fiber lasers. Lee et al, reported an SLM fiber laser with a passive multiple-ring cavity at 1533 nm [2]. The output power and linewidth of the laser were 23 mW and ~2 kHz, respectively. However, this kind of resonant cavity was complicated and not stable enough since the mode hopping would occur if one of the three different short ring cavities was affected by the surroundings. In 2004, Agger et al, demonstrated the first SLM distributed-feedback (DFB) thulium-doped silica fiber laser emitting at a wavelength of 1735 nm [3], but the maximum output power of laser was only 1 mW due to the low pump absorption. In 2015, Fu et al, presented an SLM and all-fiber distributed Bragg reflection (DBR) laser at a wavelength of 1950 nm [4]. The length of active fiber was only 1.9 cm and pumped by a 793 nm single-mode diode laser. The laser had a maximum output power of 18 mW and a narrow linewidth of ~37 kHz. Subsequently in 2017 [5], they reported a linear cavity SLM fiber laser at 1920 nm using Tm-doped fiber as an ultra-narrow bandwidth filter. More than 60 mW laser power was obtained and the linewidth was measured to be about 40 kHz.

Compared to a long linear cavity, a ring cavity can enable the laser to resonate in a travelling-wave state rather than a standing-wave state. Hence a ring cavity can avoid the spatial hole burning (SHB), which may introduce multi-longitudinal-mode (MLM) competition and oscillation. In this letter, we demonstrate a stable narrow linewidth single-frequency thulium-doped fiber ring laser using a segment of un-pumped PM-TDF as an ultra-narrow bandwidth filter. The fiber laser operates at 1957 nm and its power is amplified to over 400 mW by a homemade TDFA. Furthermore, the linewidth of amplified fiber laser is measured to be about only 20 kHz and in the meantime the OSNR is maintained over 60 dB.

2. Experimental setup

Figure 1 shows the schematic diagrams for the proposed narrow linewidth single-frequency TDFL constructed with a ring cavity and forward-pumped TDFA. The type and length of the gain fiber used in the resonant cavity are the same as these used in the amplifier. The gain fiber is about 4-meter-long single-mode thulium-doped fiber (SM-TDF; OFS, TmDF200), whose nominal peak core absorption at 790 nm is 200 dB/m. We amplify the output power of a commercial 1580 nm DFB diode laser (QDFBLD-1580-20, QPhotonics) up to watt level by the Er:Yb co-doped fiber to pump the SM-TDF via the 1580/1960 nm wavelength-division multiplexing (WDM) coupler. A high-reflection fiber Bragg grating (HR-FBG) is employed as a coarse wavelength selector, determining the resonant wavelength, whose specified central wavelength, reflection bandwidth and reflectivity are 1957.59 nm, 1.32 nm and >99.5%, respectively. To achieve stable SLM lasing operation, a segment of about 2-meter-long PM-TDF (Nufern, PM-TSF-9/125) is spliced with the HR-FBG and serves as an ultra-narrow bandwidth filter. In the un-pumped PM-TDF, two counter propagating waves can induce a standing wave which produces SHB subsequently [6]. The refractive index along the PM-TDF is then modulated due to the SHB effect, creating a dynamic grating. It can be regarded as an ultra-narrow bandwidth filter (or a fine wavelength selector) as the bandwidth for such a dynamic grating is ultra-narrow (compared to that of the HR-FBG) and can be estimated by [7]:

 

Fig. 1 Schematic diagrams for the single-frequency TDFL and TDFA (Pump1/Pump2: 1580 nm pump laser, SM-TDF: single-mode thulium-dope fiber, WDM: wavelength-division multiplexing coupler, HR-FBG: high-reflection fiber Bragg grating, OC: optical coupler, ISO: isolator, PC: polarization controller, PM-TDF: polarization-maintaining Tm-doped fiber, CIR: circulator).

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A compact in-line polarization controller (PC) is used to control the polarization state in the laser cavity, ensuring the stable SLM lasing operation. The circulator (CIR) here not only acts as an isolator to ensure the unidirectional propagation of the light, but also helps form the standing wave in the PM-TDF. The stability of the SLM lasing is also related to the intensity of the standing wave in the PM-TDF [8]. In order to obtain a desired power laser output and guarantee stable lasing operation meanwhile, we carefully select an optical coupler (OC) with 30:70 power ratio, and place it between the WDM and the CIR. It should be noted that an isolator (ISO) is placed after the 30% OC output, avoiding light reflection and consequent instability. As for the TDFA, the forward-pump way to amplify a small signal laser source has the advantage of keeping high OSNR and clean optical spectrum. All the optical fiber devices are spliced together for being compact and minimizing the total loss. The whole single-frequency TDFL is boxed in a case against vibration from the surroundings.

3. Results and discussion

Figure 2(a) shows the output spectral information of the single-frequency TDFL at 226 mW 1580 nm pump, which is measured by an optical spectrum analyzer (OSA) with a resolution of 0.05 nm. The center wavelength of this laser is located at 1957.24 nm, corresponding to the central wavelength of the HR-FBG. In addition, the TDFL achieves an ultra-high OSNR over 60 dB and no obvious amplified spontaneous emission (ASE) is observed within 2 μm waveband. Figure 2(b) shows the laser output power at the wavelength of 1957 nm when the launched pump1 power increases. The laser has a threshold pump power of ~68.7 mW and generates 61.6 mW output power at 689 mW pump1 power. The slope efficiency is estimated to be about 9.5%.

 

Fig. 2 (a) Output spectrum of the single-frequency TDFL when pump1 is at 226 mW, (b) 1957 nm laser output power when the launched pump1 power increases.

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For radio frequency (RF) spectrum measurement, the output laser is injected into a high-speed InGaAs photodetector (PD, ET-5000F). The PD has a frequency bandwidth of 12.5 GHz and is connected to a RF spectrum analyzer (Agilent E4447A) with a resolution of 10 kHz. We increase the pump1 power from 0 mW to 689 mW gradually and meanwhile adjust the PC carefully. If the fiber laser operates in a MLM state, different longitudinal modes will interfere with each other and generate beat frequency signals in RF domain. It is worth to note when the pump1 power is over 431 mW the laser operates in a MLM state. Figure 3(a) shows the spectrum of the beat frequency signals among different longitudinal modes. The interval of beat frequency signals, namely the longitudinal mode interval, is estimated to be about 16.67 MHz, corresponding to approximately 12.3-meter-long ring resonant cavity. This phenomenon may be explained as follows. A stronger pump will result in a stronger dynamic grating due to the increased modulation depth of the index. However, the light will be reflected by the grating, which is caused by the light itself. And a stronger grating will reflect more light so that less light will take part in the formation of dynamic grating, which can weaken the grating. Consequently, a weaker grating will let more light pass the un-pumped PM-TDF, making a strong grating again. It is a dynamic cyclical progress that the performance of grating changes drastically all the time if the pump1 is too large, which decreases the stability of the grating and results in mode hopping and MLM lasing operation. We should control the pump1 power below 431 mW, so that the laser operates in an SLM state. Figure 3(b) shows the measured homodyne frequency spectrum at the wavelength of 1957 nm every 5 minutes. No beat frequency signal is observed in the measured homodyne frequency spectrum within a span of 100 MHz, which confirms that the laser is at stable SLM operation.

 

Fig. 3 (a) Beat frequency signal spectrum when the pump1 power is over 431 mW, (b) Homodyne frequency spectra of 1957 nm laser measured every 5 minutes when the pump1 power is below 431 mW.

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To get a higher laser output power, the laser source is input to a homemade TDFA. The TDFA contains only one stage amplifier with a forward pump. The laser source is maintained in an SLM state by controlling the pump1 power at 357 mW, corresponding to about 15 mW laser output. Figure 4(a) shows the amplified laser output power as a function of pump2 power. The maximal amplified power can exceed 400 mW at 912 mW pump2 power and the slope efficiency is estimated as high as 45.8%. We put a 99:1 optical coupler between the WDM and the SM-TDF in the TDFA to test the backward propagating light through the OSA,and no Brillouin shift is observed when the output power is amplified to over 400 mW. We also observe the noise behavior of the TDFA. Figure 4(b) shows the amplified laser spectra along with the increment of pump2 power. The inset in Fig. 4(b) shows the different laser power. The OSNR of amplified laser is still kept more than 60 dB and no obvious ASE appears within 2 μm waveband, exhibiting an excellent amplification performance.

 

Fig. 4 (a) Amplified laser output power as the launched pump2 power increases, (b) Amplified laser spectra at different pump2 power.

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We cannot measure the linewidth of the single-frequency laser directly through OSA due to its resolution limitation. On the other hand, the self-heterodyne method [9] to measure linewidth is also not suitable for the large wave propagation loss in a tens-of-kilometers long silica fiber which serves as a delay line. Thus we inject the amplified laser as a Brillouin pump (BP) light into a Brillouin fiber ring cavity (which was used in [10] to generate a Brillouin laser (BL)) and detect the beat frequency signal between the BL and BP. Thanks to the linewidth narrowing effect [11], the linewidth of the BL is always much narrower than that of the BP. Furthermore, the full width at half-maximum (FWHM) of the beat frequency signal represents the sum of the BL’s and BP’s linewidths. Hence this FWHM can be deemed as the linewidth of the BP by ignoring the BL’s linewidth contribution. Figure 5 shows the beat frequency spectrum in detail. The 3-dB linewidth is estimated as narrow as 20 kHz.

 

Fig. 5 Beat frequency signal spectrum between the Brillouin laser and Brillouin pump (the linewidth of present amplified fiber laser).

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4. Conclusion

In summary, we have demonstrated a narrow linewidth single-frequency thulium-doped fiber ring laser at 1957 nm. By combining a HR-FBG as the coarse wavelength selector and a piece of 2-meter-long un-pumped PM-TDF as a fine wavelength selector, stable SLM fiber lasing operation has been achieved by controlling 1580 nm pump power under 431 mW. The slope efficiency of the fiber laser is estimated as 9.5% with a threshold pump power of 68.7 mW. Through a homemade high slope-efficiency (~45.8%) TDFA, the laser power can be amplified to over 400 mW without obvious OSNR (>60 dB) degeneration. And the linewidth of amplified fiber laser is measured to be about 20 kHz via beat frequency signal between the BL and BP.