Introduction

Single-frequency laser (SFL) sources near 2 μm have received intense research interest in recent years [1–8], as the laser radiation around 2 μm is eye safe and coincides with some absorption lines of several chemical compounds (H₂O, CO₂, N₂O, etc), it is also the ideal pump of mid-infrared lasers [9]. Especially, the SFL with wavelength tunable operation is very attractive for many practical applications, such as high-resolution spectroscopy, passive component test, fiber sensor, and dense wavelength-division-multiplexing (WDM) [10]. At present, a lot of researches have been working on the SFL near 2 μm wavelength field. Schemes to achieve single-frequency operation include bulk Fabry–Perot cavities [3–5], distributed-feedback (DFB) [1, 2] and distributed Bragg reflector (DBR) cavities [7, 8]. But the laser linewidth has not been tested adequately. The laser linewidth is an important parameter in the single-frequency application areas, such as the high-resolution sensing, coherent telecommunication and laser LIDAR [11].

In 2007, Geng et al. [7] reported an efficient single-frequency Tm³⁺-doped fiber laser (TDFL) with a maximum output power of 50 mW and linewidth of more than 3 kHz by using the heavily Tm³⁺-doped germanate glass fiber as the active fiber in a DBR cavity. Subsequently in 2009 [8], they reported a single-frequency narrow-linewidth TDFL with a maximum output power of about 40 mW and linewidth of less than 3 kHz by using the Tm³⁺-doped silicate glass fiber as the active fiber. Certainly, the low output power can be overcome by manufacturing a heavily Tm³⁺-doped concentration glass fiber and pumped by a higher power, which has been confirmed at both the 1.0 and 1.5 μm wavelengths [12, 13]. Nevertheless, the Tm³⁺-doped concentration is limited by the effect of concentration quenching and the maximum gain coefficient is less than 2 dB per centimeter. However a much higher power narrow-linewidth laser at the 2.0 μm band is still preferable.

Furthermore, the short cavity length of the fiber laser makes it impossible to install extra tunable filters or short length Fabry-Perot etalon to achieve a wide wavelength-tuning range. Alternatively single-frequency operation can be obtained by means of traveling-wave ring [14, 15] and loop cavities [16, 17]. This type of cavity structures can realize high slope efficiencies, high output powers with a relatively long active fiber. Moreover, narrower linewidth and wider wavelength-tuning range could be achieved owing to the long cavity architecture. While comparing with the traditional short cavity DFB and DBR fiber lasers, the mode-hopping is still a serious problem.

In this letter, we demonstrate a single-frequency narrow-linewidth fiber ring laser by using a self-developed heavily Tm³⁺-doped single-mode germanate glass fiber as the laser active medium and the saturable absorber (SA). Over 200 mW stable single longitudinal mode continuous wave laser at 1.95 μm was achieved. The laser linewidth is less than 7 kHz. And a wavelength-tuning range from 1949.55 to 1951.23 nm was also demonstrated through changing the tension on the fiber Bragg grating (FBG) outside the cavity. The mode-hopping was controlled by assembling the SA fiber into a copper tube, whose temperature was strictly controlled with a resolution of 0.05°C.

Germanate glass fiber and the experimental setup

The heavily Tm³⁺-doped germanate glass single-mode fiber was drawn in house at 1000°C using the rod-in-tube technique [18]. The Tm³⁺ ion concentration is ~4.5 × 10²⁰ cm⁻³. The fiber has a core diameter of 8.6 μm with a numerical aperture (NA) of 0.145 at 2.0 μm, and the cladding diameter is 125 μm. The cross section shown in the inset of Fig. 1(a) is featured via an amplified CCD viewer. Figure 1(a) shows the absorption cross section of the SFL. It is straightforward to recognize that a 795 nm single-mode laser would be the best pump source due to the cross-relaxation process that a single exciting Tm³⁺ ion in the ³H₄ level will generate two Tm³⁺ ions in the ³F₄ upper laser level, potentially yielding 200% of quantum efficiency for 2 μm laser operations [19]. But to our best knowledge, the maximum output power of the commercial 795 nm single-mode lasers are below 200 mW. And high power single-mode fiber laser at 1.5 μm are very mature. Then we used the 1568 nm single-mode fiber laser as the pump source and the measured absorption coefficient is 0.734 dB/cm. The propagation loss coefficient is less than 0.05 dB/cm at 1310 nm and the net gain coefficient is estimated to be 2.3 dB/cm. Figure 1(b) shows the measured emission cross sections of the germanate glass fiber. The maximum 3-dB bandwidth is about 250 nm.

 

Fig. 1 (a) Absorption cross section of single-mode Tm³⁺-doped germanate fiber; inset: the cross section of the germanate glass fiber. (b) Emission cross section of the germanate glass fiber.

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Based on the heavily Tm³⁺-doped germanate glass fiber, we demonstrated an all-fiber wavelength-tunable single-frequency narrow-linewidth TDFL, as shown in Fig. 2. Two pieces of heavily Tm³⁺-doped germanate glass fibers were used in the cavity, one of which was 14.5-cm long and was used as the laser active medium, while another 6.8-cm one as the SA. A single-mode 1568 nm laser was pumped into the fiber ring through a 1550/1950 nm WDM. The FBG with a 3-dB bandwidth of 0.06 nm was used as the laser output mirror and the reflectivity at the center wavelength (1.95 μm) is 83.4%. The 1.95 μm laser passes through the Tm³⁺-doped fiber SA and interferes with the reflected portion, a spatial hole burning (SHB) occurs which leads to the refractive index changes [20]. The periodic variations of the refractive index create a superstructured Bragg grating acting as a narrow filter in the laser cavity. The SA was assembled into a copper tube, whose temperature was strictly controlled with a resolution of 0.05°C. An optical spectrum analyzer (OSA, Yokogawa AQ6375) with a wavelength resolution of 0.05 nm and an optical power meter (OPM, Field Mate Area Meter) with a power resolution of 0.1 mW were used to measure the spectra and powers of the laser, respectively.

 

Fig. 2 Schematic of the Tm³⁺-doped narrow-linewidth single-frequency fiber laser.

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Experimental results and discussions

The center wavelength of the laser is 1950.06 nm and the SNR is higher than 68 dB, as shown in Fig. 3(a). Figure 3(b) shows the laser output power versus the absorbed pump power. The lasing threshold is 80 mW and with the pump power increasing the output power rises linearly versus the absorbed pump with a slope efficiency of 34.8%. When the absorbed pump power is 680 mW, the output power is up to 206 mW. Much higher output power could be expected with a further increase in pump power. However it’s beyond the limitation of the handling power of the components used in the cavity. From the Fig. 1(a), taking into consideration the high absorption coefficient of Tm³⁺-doped germanate glass at wavelength larger than 1600 nm, much higher output power will be obtained if the pumped source is at the wavelength of more than 1600 nm.

 

Fig. 3 (a) Spectrum of the Tm³⁺- doped germanate glass fiber laser. (b) 1.95 μm laser output power versus the absorbed pump power at 1568 nm.

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Single-frequency characteristics of the output laser was confirmed by using the scanning Fabry–Perot interferometer (FFPI, SA200-18B) with a free spectral range (FSR) of 1.5 GHz and resolution of 7.5 MHz. When the temperature of the SA was properly settled, a stable single-frequency laser emission can be achieved with an optimized length of the SA fiber. Figure 4(a) shows, within a scanning cycle, the laser operates in multimode when only the FBG was used in the cavity. By varying the length of the SA fiber, we observed a stable single-longitudinal-mode operation with a 6.8 cm long Tm³⁺-doped germanate fiber, as shown in Fig. 4(b).

 

Fig. 4 (a) Longitudinal mode characteristics when only the FBG was used in the cavity. (b) Longitudinal mode characteristics when a 6.8 cm Tm³⁺-doped germanate fiber was used as the SA.

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The linewidth of the output laser was measured by the self-heterodyne method using a 3 km fiber delay. Figure 5(a) shows the heterodyne signal spectrum recorded by a radio frequency (RF) spectrum analyzer (Agilent N9320A). After a Lorentzian fitting, it is about 32 kHz with −20 dB from the peak, which indicates the laser linewidth is 1.6 kHz full width at half maximum (FWHM). However, the linewidth resolution of the self-heterodyne measurement with a 3 km fiber delay is about 66 kHz [21]. In 2010, Domenico et al. reported a simple formula to estimate the laser linewidth through the frequency noise [22]. In order to obtain a more accurate laser linewidth, the frequency noise has been measured at the absorbed pump power of 600 mW because the laser linewidth is influenced by the pump-noise [23]. An imbalance Michelson fiber interferometer with a 100 m optical path difference was used to measure laser frequency noise. Figure 5(b) shows the measured frequency noise spectral density and the modulation index line β, from which we can calculate the laser linewidth to be 6.76 kHz using the method that had been reported by Domenico et al. Therefore the laser linewidth in our experiments is less than 7 kHz.

 

Fig. 5 (a) Linewidth of the Tm³⁺-doped germanate glass fiber laser measured by the self-heterodyne method. (b) Frequency noise spectral density and the modulation index line β.

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In order to tune the output laser wavelength, the FBG was gradually tensioned and a wavelength tuning range of about 1.7 nm was achieved from 1949.55 to 1951.23 nm, as shown in Fig. 6. The wavelength shift increases linearly with the tension at a slope coefficient of 1.25 nm/N. Once the tension was removed, the output laser wavelength immediately shifted back to the original wavelength. In order to control the mode-hopping during changing the tension on the FBG, the SA fiber was assembled into a copper tube, whose temperature was strictly controlled with a resolution of 0.05°C. Much larger tunable wavelength range is expected by inserting a tunable filter or a short length Fabry–Perot etalon into the fiber ring cavity.

 

Fig. 6 Optical spectra of the single-frequency all-fiber ring laser when tension is applied on the NB-FBG.

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Conclusion

In conclusion, a narrow-linewidth single-frequency fiber laser at 1.95 μm has been demonstrated in an all-fiber ring cavity. The maximum output power is more than 200 mW and the slope efficiency with respect to the absorbed pump power at 1568 nm is 34.8%. The laser linewidth is less than 7 kHz. The tunability of wavelength was demonstrated through applying a tension on the FBG outside the cavity and the achieved wavelength-tuning range is from 1949.55 to 1951.23 nm. It will be a potential laser in the high-resolution sensing, coherent telecommunication and laser LIDAR application areas.