1. Introduction

High power Er doped fiber lasers at the wavelength of ~1.55 μm are very attractive for many applications, such as light detection and ranging and free space communications due to its eye-safe property and high transparency in atmosphere. As high power systems, multi transvers mode 297 W Er:Yb fiber laser at 1567 nm directly pumped by 975 nm laser diodes (LD) [1] and 264 W Er:Yb fiber laser at 1585 nm in-band pumped by 1535 nm Er:Yb fiber lasers were reported [2]. As high power single frequency MOPA systems, 100 W Er:Yb fiber MOPA with 1546-1566 nm tunable range [3] and a 56.4 W linearly polarized Er:Yb all fiber MOPA [4] were reported. As a broad band tunable laser systems, Er fiber ring laser with 1520-1600 nm tunable range [5], a linearly polarized narrow linewidth ~4 W Er:Yb fiber laser with a 1533-1600 nm tunable range [6] and broad linewidth (~0.6 nm with multi peaks) multi transverse mode ~30 W Er:Yb fiber laser with a 1562-1627 nm tunable range [7] were reported. Among them, high power narrow-linewidth linearly polarized Er:Yb fiber MOPA systems are favourable for several applications requiring a narrow linewidth and/or polarized stable laser property. However, high power narrow-linewidth linearly-polarized light source above the wavelength of 1600 nm has not been reported so far. There are several interesting potential applications, e.g. as a pump source for a high power stable mode-locked mid-infrared Cr²⁺ ultrashort pulse laser [8,9] and Tm doped solid state laser using strong narrow in-band absorption line [10], and 1627 nm system for a frequency doubled 813.5 nm magic wavelength laser system for optical lattice clock application [11]. The main issues of a high power narrow-linewidth linearly-polarized Er:Yb fiber amplifier above 1600 nm are a drop of the emission cross section at signal wavelength [12] and increase of excited state absorption [13]. They lead to an increase of the saturation intensity, difficulty of the suppression of amplified spontaneous emission (ASE)/parasitic lasing of Er and Yb ions and stimulated Brillouin scattering resulting low conversion efficiency, limited output power and critical damage of the fiber components.

In this paper, we report a 1610 nm narrow-linewidth linearly-polarized Er:Yb all-PM-fiber MOPA system. The MOPA consists of a DFB laser diode seed source and three amplifier stages with three different gain fibers. A maximum output power of 8 W with a linewidth of 25 MHz and a polarization extinction ratio (PER) of 28 dB were obtained. At the maximum output power, no ASE component around 1.55 μm or 1μm was observed. To our knowledge, this is the highest output power of the narrow-linewidth linearly-polarized Er:Yb MOPA system at the wavelength of 1610 nm (above ~1600 nm) ever reported.

2. Experiment

A schematic picture of the all-PM-fiber MOPA is depicted in Fig. 1. The MOPA consists of a DFB LD seed source and three amplifier stages with different gain fibers. The gain fibers used in the experiment are listed in Table 1. All the fiber used in the set-up is polarization maintaining (PM) fiber. The DFB laser seeds a linearly polarized 40 mW signal at the wavelength of 1610 nm. It has ≥10 MHz linewidth with ~45 dB side mode suppression ratio (FITEL inc.) so that it would have nearly single frequency laser property. The seed signal was coupled into the first amplifier stage through a 1610 nm fiber isolator. The first amplifier stage employed the Er doped single clad alumino-silicate glass fiber (core diameter 6.7 μm, NA 0.22, CorActive inc.). Compared with the phospho-silicate glass Er:Yb fiber used in the following second and third amplifier stages (see Fig. 1 and Table 1), the Er fiber used in the first amplifier stage has a 2.4 times larger emission cross section at the wavelength of 1610 nm and does not suffer from ASE or parasitic lasing of Yb ions. The gain fiber enables saturation amplification at 1610 nm with the 40 mW signal power directly from the DFB laser with negligible ASE. The Er fiber was forward-pumped by a 1480 nm 500 mW LD through a WDM1 (filter type, 1610 nm/1480 nm). The amplified signal from the first amplifier was led to the following second amplifier stage through similar fiber isolator (1610 nm) and WDM2 (1610 nm/1064 nm). The second amplifier stage employed Er:Yb co-doped phospho-silicateglass double-clad fiber (core diameter 6 μm, cladding diameter 128 μm, fiber length 15 m, NA 0.2). The Er:Yb fiber allows us to use a high Er doping level and high power 915-975 nm LD direct pumping. However, the emission cross section of the Er:Yb fiber at the wavelength of 1610 nm is small (Table 1) and the co-doping of the Yb could cause the unfavorable ASE/parasitic lasing at 1μm. Consequently, the gain fiber requires the high power seed signal by aforementioned first amplifier stage with the Er doped fiber. The isolator (1610 nm) and WDM2 (1610 nm/1064 nm) were also used to suppress ASE/parasitic lasing from Er and Yb ions, respectively. As the isolator was optimized for the wavelength of 1.6 μm range, it could have residual feedback for the wavelength of 1 μm, we used the WDM2 to filter the 1 μm fluorescence. The WDM2 was fused type instead of a filter type WDM to avoid unexpected feedback from the fiber facet of the filter type WDM. The Er:Yb fiber was backward-pumped by a 915 nm 20W LD through a pump combiner. The splice region between the gain fiber and the WDM was covered in high-index gel to remove the residual pump light. The amplified signal from the second amplifier was led to the third amplifier through a similar WDM3, ASE filter, and Isolator. The ASE filter has above 20 dB loss for the stop band of 1450-1598 nm and ~0.7 dB insertion loss for the signal wavelength range of 1599-1620 nm. The third amplifier stage employed another Er:Yb co-doped phospho-silicate glass double-clad fiber (core diameter 10 μm, cladding diameter 128 μm, fiber length 5 m, NA 0.2). The gain fiber has a relatively short fiber length and large mode field area (about three times larger than the gain fiber used in the second amplifier stage) leading to high SBS threshold with a penalty of slightly multi-mode propagation property. Therefore, a bending loss with the radius of 4~8 cm (changed along the fiber) was applied to suppress higher order modes. In addition, a single mode fiber (PM1550) isolator was employed after the gain fiber that ensure the single mode output property. The Er:Yb fiber was forward-pumped by 915 nm 40 W LD through a pump combiner. The splice region between the gain fiber and the isolator was covered in high-index gel to remove the residual pump light.

 

Fig. 1 Schematic picture of the narrow-linewidth linearly-polarized 1610 nm Er:Yb all-fiber MOPA. WDM: fused type 1064 nm/1610 nm, Isolator: center wavelength of 1610 nm with ~30 dB isolation level, APC: Angle polished fiber facet. Cross marks indicate splice sections where covered in high index gel to remove the residual pump light.

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Table 1. Gain fibers used in the experiment.

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3. Result and discussion

At the first amplifier stage, the output power and the slope efficiency of 300 mW and 47% were obtained, respectively. Due to the insertion loss of the isolator and WDM2 following the first amplifier, the launched signal power for the second amplifier was 227 mW.

The output power property and the spectra of the second amplifier stage are shown in Fig. 2(a) and 2(b), respectively. The maximum output power of 3.8 W and the slope efficiency of 20% were obtained. The ASE was strongly suppressed during the amplification and was estimated to below 100 mW (estimated by integrating all the spectral components), which is less than 3% against the 1610 nm signal power of 3.7 W. The maximum output power was limited by the available pump power of the LD. In addition the estimated effective fiber length in the second amplifier was 6 m (the evolution of the signal power along the fiber was calculated by rate equation) leading the estimated SBS power threshold of ~3.0 W (We assume peak Brillouin gain value of 5 × 10⁻¹¹W/m with 30 MHz bandwidth) [14, 15]. The maximum output power of 3.7 W was already above the estimated SBS threshold so that we did not try to increase the pump power by adding extra pump LD. The reason why we did not observe SBS with the output power of 3.7 W which is higher than the estimated SBS threshold would be that in the calculation we did not take into account the stress and temperature distributions along the fibers for, which could easily increase the SBS threshold to be above twice higher value [16].

 

Fig. 2 (a) Power property of the second amplifier stage. (b) Spectra at the various output power levels of the second amplifier stage.

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The output power property and the PER as functions of pump power obtained from the third amplifier stage are shown in Fig. 3(a).The maximum output power of 8 W at the maximum pump power of 28 W and the slope efficiency of 21% were obtained. The PER was 28 dB, which was almost constant at the different pump power levels. The spectrum is shown in Fig. 3(b). The ASE was suppressed more than 40 dB against 1610 nm signal which is below the sensitivity of our optical spectrum analyser (AQ6370 YOKOGAWA Inc.).

 

Fig. 3 (a) Power property and PER of the third amplifier stage. (b) Spectra obtained of the third amplifier stage.

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The measurement results of the self-heterodyne detection are shown in Fig. 4. The measured linewidths after the second-amplifier and the third amplifier were 15 MHz and 25 MHz, respectively.

 

Fig. 4 Result of the linewidth measurement by self-heterodyne detection at each output power.

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As a comparison work, we also demonstrated the experiment without the second amplifier stage and the ASE filter (in other words, amplification of the second amplifier stage with the gain fiber used in the third amplifier stage.). The Er:Yb fiber was backward-pumped by the 915 nm 20 W LD through the pump combiner. Then, the amplified signal was led to the WDM3 (1610 nm/1064 nm) and isolator (1610 nm). The output power property as a function of pump power and the measured spectrum at each output power were shown in Fig. 5(a) and 5(b), respectively. The maximum output power of 4 W and the slope efficiency of 20% were obtained. Above the pump power level of 10 W, however, the rapid growth of ASE around 1.55 μm and the significant rollover of 1610 nm signal power were observed. It should be caused by the difference of the core diameters and length of the fiber used in the second and third amplifier stages. Compared with the Er:Yb fiber used in the third amplifier stage, the Er:Yb fiber used in the second amplifier stage has smaller core diameter of 6 μm corresponding to about three times smaller mode filed area, and three times longer fiber length of 15 m that enhances the stimulated emission at 1610 nm (lower saturation power) and increases the reabsorption loss and reduces the gain around ~1.55 μm for ASE.

 

Fig. 5 (a) Power property (b) Spectra at the various output powers of the MOPA without the second amplifier stage and the ASE filter.

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With the second amplifier stage and the ASE filter, the third amplifier stage showed the maximum output power of 8 W at the incident pump power of 28 W. The ASE was suppressed to below the sensitivity of our OSA and no growth of SBS was observed. For the measured linewidths and estimated effective fiber length of 4 m in the third amplifier stage, the SBS power thresholds was estimated to be 16 W. During the amplification, no evidence of the ASE/parasitic lasing of Yb ions around 1 μm was observed, either. Above the pump power of 28 W, we observed fiber fusing starting at the splicing point between the gain fiber and isolator, the escape point of the residual pump power of third amplifier. It was probably thermally induced by the residual pump power which limited the maximum pump power of the third amplifier stage. We believe a higher output power could be available with a better thermal management at the splicing point. The PER of ~28 dB is limited by the PER of the PM fiber components used in the experiment (>22 dB in spec sheet). The measured linewidths from the third amplifier stage were broader than that from the second amplifier stage. The reason of the linewidth broadening is not clear. As the linewidth were constant at the different power levels of the third amplifier stage (see Fig. 4), it might not be caused by nonlinear process such as self-phase modulation. It could be could be caused by the noise from the current driver of pump LD for the third amplifier, but further experiment is required to prove it.

4. Conclusions

In conclusion, we have succeeded in the development of the 1610 nm high power narrow-linewidth linearly-polarized Er:Yb all-PM-fiber MOPA system. The MOPA consists of a DFB laser diode seed source and three amplifier stages with three different gain fibers. The first amplifier stage employed the Er doped single clad alumino-silicate glass fiber (core diameter 6.7 μm, NA 0.22) which has low saturation power (high emission cross section) at 1610 nm and no Yb ion co-doping. The second amplifier stage employed the Er:Yb co-doped phospho-silicate glass double-clad fiber (core diameter 6 μm, cladding diameter 128 μm, fiber length 15 m, NA 0.2) which has the medium saturation power at 1610 nm and high reabsorption loss around 1.55 μm gain band enabling efficient ASE suppression at the wavelength range. The third amplifier stage employed the Er:Yb co-doped phospho-silicate glass double-clad fiber (core diameter 10 μm, cladding diameter 128 μm, fiber length 5 m, NA 0.2) which has the high SBS threshold at the penalty of the high saturation power. An ASE filter and WDMs were also used to suppress ASE and parasitic lasing of Er and Yb ions. The maximum output power of 8 W, the linewidth of 25 MHz, and the polarization extinction ratio of 28 dB were obtained with the MOPA system. At the maximum output power, neither ASE nor parasitic lasing from Er and Yb were observed. To our knowledge, this is the highest output power of the narrow-linewidth linearly-polarized Er:Yb all-PM fiber MOPA system at the wavelength of 1610 nm (above ~1600 nm) ever reported. We believe the output power could be increased by further optimization. In addition, the developed system could work at the wavelength range of 1530-1620 nm where the Er fiber shows large gain and our current fiber component such as ASE filter has low insertion loss, by replacing the seed DFB LD. Above the wavelength of 1620 nm, our current ASE filter does not work and the excited state absorption of the Er fiber would be problem [13]. However, above 10 W output power operation at the wavelength of 1627 nm has already been reported with the phospho-silicate Er:Yb fiber laser [7] so that we think a high power narrow-linewidth Er:Yb all-PM-fiber MOPA system could work till ~1627 nm by further optimization. Very recently we have tuned the signal wavelength to 1611.5 nm by increasing the temperature of the DFB laser and it showed similar output power property, and it is used as a pump source for our in-band pump Tm doped solid state laser system [10].