1. Introduction

High-power single-frequency fiber lasers at 1.5 μm are attractive for some potential applications, such as Doppler LIDAR, coherent and spectrum beam combining, atom clock, atom cooling and trapping [1–4]. Especially, the fiber laser at 1560 nm can generate the second harmonic generation around 780 nm, which is the transition wavelength of the D₂ line of rubidium (Rb) atom used in the atom clock. In order to obtain high frequency-conversion efficiencies, low-noise, kHz-linewidth, Watt-level, and linearly-polarized (LP) laser output is particularly desirable [5, 6].

Most of the reported high-power single-frequency LP lasers use either free-space linear cavity with some bulk-optic polarizing components [7, 8] or complex multi-stages master-oscillator power amplifier (MOPA) schemes [9–11]. However, the free-space cavity configurations break the all-fiber structure and need careful alignment, which could cause high amplified spontaneous emission (ASE) noise, widen laser linewidth up to MHz, and deteriorate polarization-extinction ratio (PER) [11–13]. Also, the typical MOPA setups comprise a low-power (several milliwatts) laser diode with tens of kHz linewidth and several (from two to four stages) fiber amplifiers, which complicate the laser setup and then result in a low PER, high cost, and the unwanted ASE produced may even lead to parasitic lasing or pump diodes damage [9–11, 14, 15].

Therefore, in a real laser system, the MOPA configuration with only one-stage and simple all-fiber design is better. However, in order to solve the limitation on the power scaling of MOPA laser, it is essential to suppress the stimulated Brillouin scattering (SBS). Furthermore, to increase the robustness and reliability of laser system, it is important to properly manage unwanted coupling light, generating all along the chain of optical components, which can damage the components or burn down the optical link instantaneously.

In our previous work, a laser oscillator structure with an efficient 300 mW low-noise single-frequency laser output at 1535 nm was achieved, which was based on a 2-cm-long heavily Er³⁺/Yb³⁺-codoped phosphate fiber (EYPF) [16]. In this paper, we developed a one-stage all-fiber 10.9 W single-frequency LP-MOPA laser at 1560 nm.

2. Experimental setup

The LP-MOPA laser configuration is presented in Fig. 1 . It consists of a homemade 1560 nm single-frequency LP seed laser and an all-fiber one-stage power amplifier. The seed laser was established by a polarization-maintaining (PM) narrow-band fiber Bragg grating (FBG) and a wideband FBG that are fusion spliced to the end faces of a 1.8-cm-long EYPF. The EYPF was fabricated using a fiber-drawing tower based on the rod-in-tube technique, more details of the fiber properties and the seed laser can be found in our previous works [16, 17]. The linewidth of the seed laser was narrower than 2.0 kHz, the PER was 26 dB, the signal-to-noise ratio (SNR) was more than 75 dB, and its output power can be adjusted from 0 to 20 dBm.

 

Fig. 1 Experimental setup of the LP-MOPA laser. PM HISO – polarization-maintaining high-power isolator.

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The LP-MOPA laser was designed to operate on the fiber slow axis. Two 0.1/99.9 tap PM fused couplers (30 dB PMCs) with 2 × 2 ports were used to monitor continuously optical spectrum and average power of backward propagating light. A cladding-mode stripper (CMS) was installed to strip the residual pump light in the cladding after the double cladding fiber, which is a splice covered by high-refractive index adhesive.

A 7-m-long PM Er³⁺/Yb³⁺-codoped double cladding fiber (PM-EYDF, DCF-EY-10/128-PM, CorActive) was employed for the power amplifier. The fiber had a 10 μm diameter core, 0.20 core numerical aperture (NA), a 128 μm diameter inner cladding, and the cladding absorption was 2.1 dB/m at 915 nm. The fiber laser was backward-pumped by five 915 nm pump diodes pigtailed with a 0.22 NA 105/125 μm multimode fiber, providing a maximum pump power of 37 W. The signal and pump light were launched to the fiber by a (6 + 1) × 1 PM combiner, and one of the pump ports was used to monitor on-line optical spectrum or unwanted coupling light. The insertion loss and pump efficiency of the combiner were 0.4 dB, and 92%, respectively.

A 0.5-m-long 10/125 μm single-mode PM fiber was used as an output delivery fiber and the output fiber end face was polished to an angle of 8 degrees. In order to eliminate the heat accumulation, all the PM components were placed on an optical table and the PM-EYDF was resting on an aluminum plate, with no active cooling.

3. Results and analysis

As we know, the primary limitation on the output power from the LP-MOPA laser is the onset of SBS, which scatters signal light in the reverse direction and reduces the output power. With textbook values for the Brillouin gain coefficient, the SBS threshold estimated is only ~5 W, thus it was necessary to raise the threshold in the power amplifier. The PM-EYDF with no active cooling technique has been used purposely, which can induce the temperature gradient along the fiber [13], leading to an effective thermal Brillouin gain broadening and reduction of the total Brillouin gain. The backward propagating power is plotted in Fig. 2(a) as a function of the laser output power. It was found that the power level in the backward direction grew up linearly and did not exhibit any sudden increase, indicated there was still below the SBS threshold. The spectrum of the backward propagating light was also recorded using an optical spectrum analyzer (OSA) with a resolution bandwidth of 0.02 nm, as shown in the inset of Fig. 2(a). With the output power at 10.9 W, the spectrum is dominated by Rayleigh scattering signal, more than 25 dB above the Stokes and anti-Stokes Brillouin scattering peaks. Meanwhile, the temperature along the PM-EYDF surface was measured by an IR thermal camera. The hottest temperature point is about 95 °C, which cannot induce thermal degradation of the polymer based outer cladding. With the values for Brillouin gain coefficient (0.9 × 10⁻¹¹ m/W) in [18], the SBS threshold estimated was pushed back to ~26 W in such single-mode core fiber.

 

Fig. 2 (a). Backward propagating power measured as a function of the laser output power. Inset: Spectrum of the backward propagating light. (b). Coupling power measured as a function of the launched pump power with different input powers. Inset: Spectrum of the unwanted coupling light.

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In order to improve the robustness and reliability of such all-fiber one-stage MOPA system, some methods have been present to deal with the unwanted coupling light, which involves ASE, residual pump light and core light leaking or being reflected into the cladding. First, the PM combiner was manufactured using side-pump technique and provided a high isolation of 25 dB at 1560 nm. With the above isolation value and the neglect of ASE powers, the coupling power of the unwanted coupling light estimated to ~43 mW at an output power of 10.9 W. Furthermore, reducing this light guided in the multimode fiber of the pump ports, an alternative approach to suppress the ASE is to increase the input powers of the seed laser. The coupling power from the monitoring pump port of the combiner, as a function of the launched pump power with different input powers of the seed laser, is plotted in Fig. 2(b). When the input power was 10.0 dBm and the pump power was 37 W, the coupling power reached the maximum value of 150 mW, which could be a risk for the pump diodes to perform at high power (the typical resist power level of 915 nm pump diode is approximately 100 mW). However, when the input power was increased to 20.0 dBm, it can be seen that the coupling power dropped to around 60 mW, which was safe power-handling level of the pump diodes and was in good agreement with the estimated value.

At the laser output power of 10.9 W, the spectrum of the unwanted coupling light was also recorded and is shown in the inset of Fig. 2(b). The signal light at 1560 nm is clearly observed in the spectral regime, where the two ASE peaks and 915 nm pump peak are more than 30 dB below the leaking or being reflected signal, and there were no sign of parasitic lasing. In addition, an all-fiber 1064/1550 nm PM fused WDM rather than a filter type WDM was installed to filter out the 1µm band ASE from the excited Yb³⁺, which can protect the seed laser and the components from being destroyed in a real system.

Figure 3(a) shows the LP-MOPA laser output power and the gain as a function of the launched pump power with different input powers of the seed laser. It can be found that the laser output power increased linearly with the pump power. When the input power was 10.0 dBm and the pump power was 37 W, the gain could reach the maximum value of 30.0 dB. However, the corresponding laser output power and optical-to-optical conversion efficiency (CE) were only 10.0 W and 27.3%, respectively. When the input power was increased to 20.0 dBm, the gain dropped off dramatically to ~20.5 dB. On the other hand, the highest output power of 10.9 W and the maximum CE of 29.5% were achieved. It should be noted that the maximum output power was limited by the available pump powers without any roll-over.

 

Fig. 3 (a). Laser output power and gain as a function of the launched pump power with different input powers. (b). Output spectrum of the fiber laser. Inset: Power stabilities of the fiber laser for 2 h.

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The output spectrum of the fiber laser was recorded with a resolution bandwidth of 0.1 nm by an OSA and is plotted in Fig. 3(b). Compared with that of the seed laser, the SNR of the fiber laser with an output power of 10.9 W was slightly deteriorated due to the ASE noise, still to be 70 dB. The power stabilities of the laser output power at 10.1 W was investigated, as shown in the inset of Fig. 3(b). It’s obvious that the output power instability was less than 3% over 2 hours, which is caused by the small fluctuations in the pump laser power and ambient temperature. When the fiber laser worked for half an hour, the laser output power obviously became very stable and the power instability was reduced to about ± 1%, which is satisfactory.

To perform further characterization of the LP-MOPA laser, the single-frequency nature of the laser was verified by a scanning Fabry–Perot interferometer with a resolution of 7.5 MHz and a free spectral range of 1.5 GHz. Figure 4(a) and the inset of Fig. 4(a) show the linewidths of the two lasers, which were measured by a self-heterodyne method using a 50 km fiber delay. The linewidth of the seed laser is approximately 2.0 kHz FWHM (full width at half maximum). However, the linewidth of the fiber laser with an output power of 10.9 W is 3.5 kHz, which revealed the laser linewidth was broaden slightly, owing to the self-phase modulation caused by intensity fluctuations of relaxation oscillations [19].

 

Fig. 4 (a). Linewidth of the fiber laser. Inset: Linewidth of the seed laser. (b). DOP measured of the fiber laser over 30 seconds period.

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The polarization state of the LP-MOPA laser was analyzed using an optical polarization analyzer. The measured degree of polarization (DOP) is shown in Fig. 4(b). The results indicated that the polarization state of the laser output is stable. The DOP was at the level of 99.2% with 0.6% variation over 30 seconds period, owing to thermal stress in the fiber and ambient vibration, corresponding to a PER of > 24 dB.

4. Conclusions

In conclusion, a one-stage all-fiber 1560 nm single-frequency LP-MOPA laser with the output power of 10.9 W has been obtained. The laser provides a linewidth of 3.5 kHz and a PER of 24 dB. The measured SNR is more than 70 dB and the optical-to-optical conversion efficiency is 29.5%. Without detrimental SBS and the damaging effects of unwanted coupling light were observed at the highest output power. The results indicate that the fiber laser might be a promising candidate as an efficient narrow-linewidth laser source for nonlinear frequency conversion.