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A high-power single-frequency, single-polarization, thulium-doped all-fiber master-oscillator power-amplifier (MOPA) is demonstrated by using all-polarization-maintaining (all-PM) thulium-doped fiber and all-PM-fiber components. The MOPA yielded 210 W of single-frequency, linear-polarized laser output at central wavelength of 2000.9 nm with a polarization extinction ratio (PER) of >17 dB. No indication of stimulated Brillouin scattering (SBS) could be observed at the highest output power level, and the output power was only currently limited by available pump power. To the best of our knowledge, this is the first demonstration of average output power exceeding 200 W from a single-frequency, single-polarization, thulium-doped all-fiber laser at 2 µm wavelength region.
© 2014 Optical Society of America
1. Introduction
The interest for the development of stable highly-integrated high-power single-frequency laser sources at 2 µm wavelength for applications of Doppler-Lidar, coherent beam combining and mid-infrared (mid-IR) frequency conversion has been increased greatly over the past decade [1]. Single-frequency thulium-doped fiber lasers, which extend the wavelength range of fiber lasers to 1.8~2.1 µm [2–4], could be considered as one of the most important sources of single-frequency laser radiation that has been developed and were intensively investigated for the last several years [5–9]. Most notably, Goodno et al. demonstrated a master-oscillator power-amplifier (MOPA) based on a single-frequency distributed feedback (DFB) diode laser and a four-stage thulium-doped fiber amplifier, which produced 608 W of average output power at 2040 nm [10]. Wang et al. demonstrated a single-frequency thulium-doped all-fiber MOPA system with average output power of 102 W as well [11]. However, these reports used non-polarization-maintaining (non-PM) fiber and non-PM components in the final thulium-doped fiber power amplifier, which means these single-frequency thulium-doped fiber MOPAs are not practical for some applications, such as gravitational wave detection, coherent polarization beam combination, and frequency conversion in nonlinear crystals. So far, the maximum average output power of single-frequency, single-polarization, thulium-doped fiber MOPA is still limited within 100 W levels because of the stimulated Brillouin scattering (SBS) gain is higher in PM fiber than non-PM fiber, the power scaling of single-frequency, single-polarization fiber laser is even more difficult [12–15]. In 2009, Pearson et al. reported a single-frequency, single-polarization, thulium-doped fiber MOPA with maximum output average power of 100 W based on a single-frequency thulium-doped fiber DFB laser and three-stage thulium-doped fiber amplifiers [16]. In recent work, Shah et al. reported a narrow-linewidth, single-polarization, thulium-doped fiber MOPA with maximum output power of 109 W [17], and the spectrum linewidth is sub-nanometer level. However, the report used a free-space pump [16], which means the configuration of the single-frequency, single-polarization, thulium-doped fiber MOPA has less reliability.
In this contribution, we report on a 210 W single-frequency, single-polarization, thulium-doped all-fiber MOPA by using all-PM thulium-doped fiber and all-PM-fiber components. The PER at the highest output power was measured to be >17 dB. Neither SBS nonlinear effect nor parasitic lasing was observed during amplification process. The slope efficiency of the final PM thulium-doped fiber power amplifier was ~53%, and the output power was only currently limited by available pump power. This kind of high-power single-frequency, single-polarization, thulium-doped all-fiber MOPA represents an attractive technology for the generation of high-power mid-IR laser via nonlinear frequency conversion.
2. Experimental setup and results
The high-power single-frequency, single-polarization, thulium-doped all-fiber MOPA consists of a stable low-power single-frequency distributed feedback (DFB) diode laser and four-stage cladding-pumped PM thulium-doped all-fiber amplifiers. The schematic setup of the MOPA is shown in Fig. 1. The single-frequency oscillator was a PM-fiber-pigtailed DFB diode laser with output linewidth of <2 MHz and maximum average output power of 3.5 mW at central wavelength of 2000.92 nm. A high-power broadband PM isolator was inserted to protect the single-frequency butterfly package DFB diode laser and the first PM thulium-doped fiber preamplifier against backward traveling light. For improved long term reliability, all-fiber set-up was chosen for the all fiber preamplifiers and the fiber power amplifier. The active fiber of the first fiber preamplifier was 3 m PM thulium-doped double-clad single-mode fiber (Nufern, Inc. cladding-absorption of 4.7 dB/m at 793 nm) pumped with a fiber-pigtailed multimode diode at 793 nm. The core of the active fiber has a diameter of 10 µm and a numerical aperture of 0.15, and its inner cladding has a diameter of 130 µm and a numerical aperture (NA) of 0.46. The pump source was a fiber-pigtailed high-power multimode diode laser (BWT Beijing Ltd., China.) at 793 nm with fiber core of 105 µm (NA = 0.22), and the maximum output power of 12 W. A PM (2 + 1) x1 pump combiner was used to deliver pump light to the PM thulium-doped double-clad single-mode fiber. It was pumped in a co-propagating scheme to protect the 793 nm pump diodes.
Fig. 1 Schematic setup of the high-power single-frequency, single-polarization, thulium-doped all-fiber MOPA.
In the experiment, the first PM thulium-doped fiber preamplifier produced 100 mW average output power at pump power of 2 W. The output spectrum of the first fiber preamplifier was measured by an optical spectral analyzer (YOKOGAWA AQ 6375) with resolution of 0.05 nm, the center wavelength and the full width at half maximum (FWHM) were 2000.92 nm and <0.05 nm respectively. The polarization extinction ratio (PER) was measured to be >20 dB by using a mid-IR linear polarizer (Thorlabs Inc. LPMIR100). The output narrow linewidth laser from the first fiber preamplifier were amplified by the second PM thulium-doped fiber preamplifier in order to provide high-enough power for the third large-mode-area (LMA) PM thulium-doped fiber preamplifier, as shown in Fig. 1. The amplification gain medium is a 3 m PM thulium-doped double-clad fiber, characterized by the same parameters as first fiber preamplifier mentioned above. Two fiber-pigtailed multimode diodes at 793 nm are employed as the pump source, and the total output power of 24 W. The second PM fiber preamplifier produced 5 W average output power at pump power of 17 W. The reason for the low slope efficiency in the second amplifier is the thulium-doped double-clad fiber wasn’t cooled in a water-cooled heatsink, so the slope efficiency of the second fiber preamplifier was about 30%.
In the third PM thulium-doped fiber preamplifier and the final PM thulium-doped fiber power amplifier, a segment of 4.5 m LMA PM thulium-doped double-clad fiber were used as the gain medium. The fibers have a core diameter of 25 µm, a core NA of 0.09, inner cladding diameter of 400 µm and a NA of 0.46. The cladding absorption at a wavelength of 793 nm is specified with 2.4 dB/m. To improve the beam quality in the third fiber preamplifier and the final fiber power amplifier, the LMA PM thulium-doped double-clad fiber was cooled to 10°C in a water-cooled heatsink and the coil bending radius of the LMA PM active fiber in the power amplification process is maintained less than 10 cm. In the third thulium-doped fiber preamplifier, a high-power (6 + 1) x1 pump combiner was used to deliver pump light to the LMA PM thulium-doped fiber from six fiber-pigtailed high-power temperature stabilized multimode diode laser modules, which give the total output power of 135 W at 793 nm in a 0.45 NA 25/400 µm PM double-clad passive fiber.
Figure 2 shows the third LMA PM thulium-doped fiber preamplifier average output power versus incident pump power. The average output power increased almost linearly with the rise of incident pump power. The maximum average output power was 75 W for 135 W incident pump power, which corresponds to slope efficiency of 55%. The pump source of the final PM thulium-doped fiber power amplifier from six temperature stabilized multimode diode laser modules emitting at 793 nm was delivered by a multimode fiber with a fiber diameter of 105 µm and a NA 0.22, which match to the pump fiber of the pump combiner. The total output power of these laser diode modules was 330 W. A high-power PM (6 + 1) × 1 pump combiner was used to deliver pump light to the LMA PM thulium-doped double-clad fiber with a coupling efficiency of 91%. The output end of the LMA PM thulium-doped fiber was spliced to 0.5 m PM passive fiber with matched core, and the output facet was cleaved at 8° to frustrate parasitic lasing. A dichroic mirror was used to separate residual pump light from the signal light.
Fig. 2 Average output power of the third LMA PM thulium-doped fiber preamplifier with the increase of incident pump power.
The final LMA PM thulium-doped fiber power amplifier average output power versus the incident pump power is plotted in Fig. 3. The maximum average output power was 210 W, which corresponds to slope efficiency of ~53% and the output power increased almost linearly with the increase of incident pump power. The residual pump power at 793 nm wavelength in the final PM thulium-doped fiber power amplifier was measured to be about 20 W at the highest average output power. Due to the quasi-three level nature of the laser, cooling the LMA thulium-doped fiber to the lower temperature was very critical for achieving high slope efficiency in the final PM fiber power amplifier. The slope efficiency is slightly lower than that achieved in the references [18,19]. In addition, the slope efficiency of the fiber power amplifier can be improved further by optimizing the length of the LMA PM thulium-doped fiber [20,21]. The output power fluctuation is less than ± 1%. Because of the stable single-frequency DFB diode laser and all-fiber highly-integrated PM components for the four-stage thulium-doped fiber amplifiers, the high-power MOPA was stable and reliable. Figure 4 shows the optical spectrum of the final LMA PM thulium-doped fiber power amplifier at highest average output power. The center wavelength is 2000.92 nm, which is almost same as those of the DFB diode laser. The spectrum shows no signs of parasitic lasing or significant levels of amplified spontaneous emission. Single-frequency operation of the PM thulium-doped all-fiber MOPA was confirmed by a scanning Fabry Perot interferometer with a Free-Spectral-Range (FSR) of 10 GHz and a finesse of 150, as shown in the Fig. 5. Therefore, the laser linewidth of single-frequency thulium-doped fiber MOPA should be <0.8 pm.
Fig. 3 Average output power of the final LMA PM thulium-doped fiber power amplifier with the increase of incident pump power.
Fig. 4 Optical spectrum of the final LMA PM thulium-doped fiber power amplifier at highest average output power.
Fig. 5 Scanning FP interferometric spectrum of the LMA PM thulium-doped all-fiber MOPA verifying single-frequency operation.
To verify that the LMA PM thulium-doped all-fiber MOPA was operating below the SBS threshold, the backward output power and optical spectrum were continuously monitoring during amplification process. The backward propagating average power is plotted in Fig. 6 as a function of the forward output power from thulium-doped all-fiber MOPA. The power level in the backward direction does not exhibit any sudden increase which indicates that the fiber power amplifier is still below the SBS threshold. The PER at the highest average output power was measured to be >17 dB. In the experiment, the first and second PM thulium-doped fiber preamplifiers are strictly single-mode operation. The output beam-quality was measured using a scanning slit beam profilers system (BeamScope-P8). A near-field intensity profile of the laser beam is shown in Fig. 7. The beam quality factor was measured to be M2 of 1.6 at average output power of 200 W.
Fig. 6 Backward average output power of the LMA PM thulium-doped all-fiber MOPA with the increase of forward output power from MOPA.
Fig. 7 M2 measurement performed using a scanning slit beam profilers system. Inset, near-field beam intensity profile at average output power of 200 W.
3. Conclusion
In summary, we have demonstrated a 210 W single-frequency, single-polarization, thulium-doped all-fiber MOPA by using all-PM fiber and all-PM-fiber components, which represent a significant output power increase and a record power level compared to reported single-frequency, single-polarization thulium-doped fiber MOPA [16]. The slope efficiency for the final LMA PM thulium-doped fiber power amplifier was ~53% with respect to incident pump power. The PER of the MOPA at the highest average output power was measured to be >17 dB. Neither SBS nonlinear effect nor parasitic lasing was observed during amplification process. The output power was currently limited by available pump power. To the best of our knowledge, this is the highest average output power ever reported for single-frequency, single-polarization, thulium-doped fiber laser at 2 µm wavelength region.
Acknowledgment
The authors acknowledge the financial support from the National Natural Science Foundation of China (NSFC, Nos. 61235010 and 61177048).
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