A 170 W all-fiber linearly-polarized single-frequency sing-mode ytterbium amplifier at 1064 nm with an optical efficiency of 80% is demonstrated. 3.9 m long ytterbium-doped polarization maintaining fiber with a core diameter of 10 μm is used as the gain fiber, which guarantees a diffraction-limited output with a measured M2 of 1.02. To suppress the stimulated Brillouin scattering, longitudinally varied strains are applied on the gain fiber according to the signal power evolution and the temperature distribution. 7 times increase of the stimulated Brillouin scattering threshold is achieved.
©2013 Optical Society of America
Recently, high-power single-frequency single-transverse-mode linearly-polarized lasers have found widespread applications in science and industry, such as gravitational wave detection, coherent beam combination, active laser remote sensing, and parametric wavelength conversion in resonant cavities. However, due to limitation primarily from the nonlinear effect of Stimulated Brillouin Scattering (SBS), the laser power scaling has been difficult with single transverse mode and narrow linewidth output. In earlier investigations, up to 600 W level high power single frequency fiber amplifier had been demonstrated using bulk free-space components [1–3], facilitating the backward pump configuration. For all-fiber single frequency amplifier, 194 W laser power using large mode area (LMA) Yb-doped SBS-suppressing fiber was achieved . A 300 W level counter-propagation pumped all fiber single frequency amplifier was reported with a demanding pump and signal combiner. The backward pump configuration and the applied temperature gradient increase the threshold of the SBS . With high absorption Yb-doped LMA fiber, Wang et al. reported a 310 W all fiber single frequency laser . However, the polarization state of the laser output is uncertain in these works, since the polarization may rotate and drift in non-polarization maintaining fiber.
For many applications, all fiber linearly polarized laser is required. However, SBS gain is higher for linearly polarized laser propagating in polarization maintaining (PM) fiber . Therefore, power scaling of linearly polarized narrow linewidth fiber amplifier is even more difficult. Up to 174 W polarized output had been achieved through heating of the fiber and optimizing the fiber length . Using the gain competition method, Zeringue et al. reported a 203 W single frequency all fiber linearly polarized amplifier . However, multimode ytterbium-doped fibers have been used in these studies.
In this paper, we report a 170 W all-fiber linearly-polarized single frequency fiber amplifier with a 10 μm-core single-transverse-mode PM ytterbium-doped fiber. We design and apply a strain distribution along the gain fiber according to the signal power evolution and the temperature distribution to suppress the SBS effect. 7 times increase of SBS threshold is achieved. To the best of our knowledge, this is the highest output from a single-frequency true-single-mode PM all-fiber amplifier. The SBS suppression technique can be applied to amplifiers with larger mode area fibers, so as to further scale the single frequency output power.
2. Experiment configuration
The experimental configuration of single frequency amplifier is illustrated in Fig. 1 . A commercial NPRO laser (Innolight Mephisto) is used as seed, which can generate up to 50 mW, ~1 kHz bandwidth, linearly polarized fiber-coupled laser at 1064 nm according to the datasheet. The seed is amplified by two preamplifier stages and a power scaling stage, which are built with all-fiber PM components. The seed light is amplified up to 3 W by the first two stages using Nufern 5/130 μm PM YDF and Nufern 10/125 μm PM YDF as the gain fibers, respectively. High power PM isolators are inserted between the adjacent amplifiers to prevent parasitic oscillation. A 1% diagnostic tap coupler is placed before the 3rd stage to monitor the backscattered optical power and the spectrum. The signal light is combined with the pump light by a PM (6 + 1) × 1 combiner and the 976 nm pump light is provided by ten 25 W laser diode. However, due to the coupling efficiency of the 2 × 1 combiners and (6 + 1) × 1 combiner, the measured available pump power is 210 W after the combiner. The gain fiber for the last stage amplifier is 3.9 m PLMA-YDF-10/125 fiber (Nufern) with a core diameter of 10 μm, a numerical aperture (NA) of 0.075, and nominal cladding absorption of 4.8 dB/m at 976 nm. 10 and 20 steps of longitudinal varied strain (details in section 3) are applied onto the gain fiber to suppress the SBS in two separate experiments. The output fiber is cleaved with an angle of 8° to suppress the parasitic oscillation. A dichroic mirror at the output port is used to separate the signal light from the pump light.
3. SBS suppression scheme
3.1 Longitudinally varied strain for SBS suppression
SBS suppression by applying strain distribution along fiber is well known . However, most of the studies focused on narrow linewidth laser transmission in passive fibers. In high gain fiber amplifiers, the power distribution along the fiber is very uneven. To effectively suppress SBS, a clever design of strain distribution is necessary. In high power single frequency Raman fiber amplifier, 20 times reduction in effective stimulated Brillouin scattering coefficient was achieved by 30 steps of longitudinally varied strain .
We focus on step-wise strain distribution. This means each step contains a piece of fiber with uniform strain. SBS gain spectra of different fiber segments are shifted, but still have some overlap to each other, depending on the difference of applied tensile strain. With a given step number, the best SBS suppression happens when each strain step sees the same SBS light generation at different frequency shift. This can be achieved by adjusting the individual step lengths, since SBS gain is an integration along the fiber length. A standard differential equation model is built for calculating the laser power distribution along the fiber and estimating SBS gain. Considering Yb fiber amplifier with pump going from left, seeded from left, so SBS light propagates from right to left, the equations read :
Where PP and PS are powers of pump light and signal light, respectively. x is the location along the fiber. and are the emission and absorption cross-sections for the 975 nm pump light, which are 2.15 × 10−24 m2 and 2.35 × 10−24 m2. andare the emission and absorption cross-sections for the 1064 nm signal light, which are 2.5 × 10−25 m2 and 2.95 × 10−27 m2. and are the overlap factors between the light-field modes and the Yb distribution, which are 6.4 × 10−3 and 0.69. The SBS light is considered by dividing into spectral channels with a frequency bin size of 2 MHz. is the i-th Brillouin wave power, whileis the SBS gain coefficient at the corresponding frequency, which is a function of fiber location x. SBS gain spectrum at any location is assumed to have a Lorentz shape with a peak gain coefficient of 0.125 m−1W−1 and a bandwidth of 60 MHz. However, the center frequency varies along the fiber due to the strain and temperature distribution. ,andare loss coefficient of pump, signal and SBS light in fiber, which are 0.09 m−1, 0.046 m−1 and 0.046 m−1, respectively. The initiation of SBS light is considered following the method in . We come to a configuration with 3.7 m of fiber and 20 strain steps separated by 80 MHz. Aiming an output power of ~170 W seeded by 3 W and pump power of 210 W, an optimum strain step distribution is calculated as shown in Fig. 2(a) , which depicts the calculated signal power and the strain distribution along the gain fiber.
3.2 Contribution of temperature distribution
The previous investigations indicated that temperature distribution along the fiber played a significant role in influencing the SBS spectrum in high power single frequency amplifier . In our experiment, the high pump power (210 W), the high absorption of the gain fiber (4.8 dB/m) and the small fiber cladding diameter of 125 μm cause a high temperature rise. So the temperature distribution must be taken into consideration to achieve a synergistic effect with the designed strain distribution to suppress the SBS. The temperature distribution along the gain fiber is calculated according to ,Eq. (3), whereand are the wavelengths of the pump light and signal light, and equal to 975 nm and 1064 nm, respectively. The quantum defect of the laser is assumed to be the only source of the thermal load. In our experiment, the gain fibers are spooled on an aluminum metal tube. Two electric fans are used to blow the metal tube to dissipate the heat generated by the laser system. Therefore, the convective heat transfer coefficient h is defined to be 100 W/(m2·K) in the simulation . The core radius a, inner cladding radius b and outer cladding radius c are 5 μm, 62.5 μm and 125 μm, respectively. k1, k2 and k3 are thermal conductivities of core, inner cladding and outer polymer region, which take values of k1 = k2 = 1.38 W/(m·K), k3 = 0.2 W/(m·K). The pump and signal light power distribution along the fiber length is calculated by Eq. (1). As is depicted in Fig. 2(c), the calculated temperature is 336 K at the input end of the gain fiber, and then increases to 422 K at 0.7 m from the input end. This is due to improved pump absorption when the signal power increases. Along the remaining gain fiber, the temperature decreases gradually to 300 K at the output end of the gain fiber because the pump power decreases. Providing the SBS frequency shift with temperature is 1.36 MHz/degree , 166 MHz SBS shift is generated by the temperature differences along the last 3.2 m gain fiber, where the signal power is much higher than in the 0.7 m gain fiber at the input end. So it is important to take the temperature distribution into consideration. Unlike what we did in single frequency Raman amplifier , now we apply the maximum strain to the input end of the gain fiber, and then make a step-decreasing strain distribution, to achieve a synergistic effect with the designed strain distribution.
3.3 SBS suppression
Based on the analysis above, we spool the fiber on a metallic drum with variable strains. Starting from the seed side of the gain fiber, the tension decreases by 100 g successively until the 19th step. The 20th step is unstrained as output fiber. Using the tool provided by Corning , the strain on each segments can be calculated. Then the shift of Brillouin gain spectrum peak can be estimated with a formula ,Table 1 .
Figure 2(b) shows the calculated SBS output spectrum in the amplifier under the strain distribution. Twenty peaks are shown in the SBS spectrum and broaden the spectrum from 60 MHz to 1.6 GHz. The calculated SBS spectrum considering both the temperature and strain effect is shown in Fig. 2(d). Compared to Fig. 2(b), the SBS spectrum is broadened further. And as a result the amplitude of each frequency decreases.
4. Experimental results and analysis
In the first round of experiments, 3.4 m and 3.6 m 10/130 YDF with identical 10-step strain distribution (125 g per stage, the maximum strain is 1.5%, design length of the last step is 0.20 m) are used as the gain fiber, while the lengths of the last unstrained fiber segments are 0.2 m and 0.4 m, respectively. Six 25 W 976 nm diode lasers are spliced directly to the combiner to pump the amplifier. When the signal light power is 2 W, the output power curves are shown in Fig. 3(a) . For the 3.4 m gain fiber, a maximum 127 W laser output is achieved at the full pump power. The power of the backward propagating light is 7.5 mW, corresponding to 0.06 ‰ of the output power. However, for the 3.6 m gain fiber with 0.4 m output fiber segment, when the output power reaches 117 W, the backward light starts to increase quickly to 38 mW and limits the amplifier output. The result shows the importance of the precision of the strain distribution.
In a higher power experiment, a 3.9 m-long 10/125 fiber with 20 strain steps is used as gain fiber. The strain distribution is described in Tab. 1. However, for experimental convenience, the last unstrained output fiber is 20 cm, which is higher than the designed 12.4 cm. And there is another unstrained 10 cm Yb fiber used for fusion with the combiner at the start of the input gain fiber. As depicted in Fig. 3(b), the maximum output power and the backward light power are 171 W and 31.6 mW (0.21‰ of the output power), respectively, limited by the available pump power. The corresponding optical efficiency is 81%. For comparison, we tested a 3.9 m unstrained fiber as the gain fiber. The maximum output power is 24.5 W and the backward light increases quickly to be 38 mW. Therefore, the SBS threshold is improved by 7 times.
The spectra of the 1064 nm signal light are measured with an optical spectrum analyzer (AQ 6370, Yakogawa) and shown in Fig. 3(c). A signal to ASE ratio of > 50 dB is measured at full output. Figure 3(d) shows the backward light spectra at different output powers. It is seen that the backward light consists of Rayleigh scattering of the laser and the SBS light. The SBS light at the highest laser output is 7.8 dB higher than the Rayleigh scattering. A polarization extinction ratio (PER) of > 20 dB and a M2 factor of 1.02 are measured at an output power of 30 W. Measurement at higher power is limited by the setup. A far field spatial profile is show in the inset of the Fig. 3(b).
In summary, we demonstrated a 171 W all-fiber single-frequency single-mode polarization maintaining Yb-doped amplifier, which is the record-high single-frequency output for a gain fiber of 10 μm core. A numerical model including both the strain and temperature is developed to simulate the SBS generation in single frequency Yb doped fiber amplifier. The designed stepwise strain distribution is applied to the gain fiber and 7 times increase of SBS threshold is demonstrated experimentally. The SBS suppression technique can be applied to fibers with larger mode area, which is the natural path for further power scaling.
References and links
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