We demonstrated a high power Yb-Raman combined nonlinear fiber amplifier. The seed is a 30 W dual-wavelength Yb-doped fiber laser formed by 1070 nm and 1120 nm lasers. The gain medium in the amplifier stage is a 45-meters-long Yb-doped fiber, which power scaled the 1070 nm laser and Raman amplified the 1120 nm laser simultaneously. A record 1120 nm power of 732 W is obtained with pump power of 890 W. Numerical study taking into account of both Yb and Raman gain has been conducted to analyze the output characters and power evolution in the amplifier under different conditions. The numerical results are in good agreement with the experiment. This amplifier is also suitable to amplify laser in the wavelength range of 1100-1150 nm.
© 2014 Optical Society of America
Ytterbium doped fiber laser (YDFL) is believed to be the most promising high power laser source in 1 μm reign due to the excellent beam quality and power scaling capability. The wavelength coverage of high power YDFL usually ranges from 1060 nm to 1100 nm corresponding to the highest gain spectrum. Actually, the emitting spectrum of YDFL can stretch to 1.2 μm , and wavelength in the range of 1100-1150 nm can be found lots of applications in remote sensing, spectroscopy, laser star generation, and biology [2–6]. However, due to the small emission cross section, YDFL in the wavelength of 1100-1150 nm would suffer strong gain competition with the peak gain range when increasing the power, preventing the step to power scaling . Recently, Supradeepa et. al demonstrated a 453 W 1117 nm Yb-doped fiber amplifier consisting of a forward-pumped oscillator and a two-stage bidirectionally pumped power amplifier . The power record decreases with the increase in wavelength, for the wavelength of 1147 nm, the highest reported power is 35 W , which is far behind the demand of applications. Yb-doped fiber amplifier configuration may be a way to enhance the output power level, but theoretical study shows the amplified times is limited by amplified spontaneous emission (ASE) and parasitic lasing .
An alternative method for the achievement of emission wavelengths in the 1100-1150 nm range is to use Raman fiber laser or amplifier. Raman effect is one kind of useful nonlinear effect in optical fiber for wavelength transformation by Stokes frequency shifting, which compensates the spectral gaps between the rare-earth (RE) doped fiber lasers . In 2009, Y. Feng et. al reported a 150 W 1120 nm Raman fiber laser core pumped by a 1070 nm YDFL , leading the power of this wavelength band to a hundred-watts level. In this configuration, a high power wavelength division multiplexer (WDM) is required to combine the pump and signal wavelengths into several tens or hundreds meters Raman gain fiber, but the power handling ability of commercial available WDM would become a restriction to higher power.
Most recently, L. Zhang et. al reported a integrated Yb-Raman fiber amplifier . The amplifier was seeded by a pair of Raman wavelengths (1070 and 1120 nm) and the two waves were amplified firstly in the Yb-doped fiber (YDF) simultaneously, then in the following passive fiber, the Stokes wavelength (1120 nm) laser was Raman amplified. This amplifier setup is simple and WDM is not required. In this paper, we want to report a more compact Yb-Raman combined fiber amplifier, in which there is only one length of YDF as the gain medium without passive fiber compared to the experiment in . In the YDF, the Stokes wave is amplified by ion gain as well as the Raman gain resulting in more efficient energy transfer. So, the fiber length in the amplifier stage is shorter than that in  and it is more conformable to amplify single frequency laser in 1100-1150 nm range . Finally, we achieve a record power of 732 W 1120 nm laser at the pump power of 890 W. A numerical model is also proposed to analyze the characters of this amplifier and the results are in good agreement with experiment.
2. Experimental setup
The experimental setup is schematically shown in Fig. 1. The seed is formed by two cascaded YDFLs (cavity A and cavity B), each of which makes up of a pair of fiber Bragg gratings (FBGs) and a length of YDF. The diameters of the core and inner cladding of the double cladding YDF (YDF1 and YDF2) are 10 μm and 125 μm, respectively. An isolator is followed after the seed to ensure the seed would not be affected by the backward light from the amplifier. The fiber of the signal port of the combiner in amplifier stage is 20/400 μm (diameter of core/ inner cladding), so a mode field adaptor (MFA) is employed to connect the seed and the combiner. The pump ports are spliced to six 976 nm laser diodes (LDs) with total available power after the combiner of 890W. The gain fiber used in the power amplifier stage is a 45-meters-long 20/400 YDF. The nominal cladding absorption coefficient is about 1.3 dB/m and the numerical aperture is 0.065.After the YDF, a pump dump is connected to stripe unabsorbed pump light and cladding mode. A home-made endcap is spliced in the end to avoid unwanted end reflection. The fiber used in the endcap is parameters matched 20/400 passive fiber with a length of 1.5 m. The gain fiber and the combiner are set on a water cooled metal plate.
In the seed, the 1120 nm laser power would decrease by a factor of about 25% when pass through cavity B due to the reabsorption of YDF2 in the case of LD B turned off. The 1120 nm laser would be amplified by Yb and Raman gain simultaneously in the YDF2 when LD B is turned on. But the length of YDF2 is short (6 meters long) and the reflectivity of the output coupler (OC) is relatively low (about 10%), so the amplification of the 1120 nm laser would not be significant. Moreover, the 1120 nm laser power can be turned by setting the drive current of LD A. On the other hand, we care more about the seed stability in time-domain. We held the 1070 nm laser in a constant power (20 W or 30 W) and increased the power of 1120 nm laser (from 0 to 15 W), and also held 1120 nm laser power (8W) and increased the 1070 nm laser power (from 0 to 35 W) at the same time. We found there is no obvious power fluctuation or pulse operation in the whole process, showing the seed can work in a steady condition.
3. Numerical simulations
In the power amplifier stage, the dual-wavelength seed is firstly amplified by 976 nm laser pumped Yb-gain and as the power increasing, 1070 nm laser would also become the pump source for the 1120 nm laser through Yb energy transfer as well as stimulated Raman effect. Such power evolution can be numerically studied by a set of rate equations including both Yb and Raman amplifications:
The emission spectrum (from 970 nm to 1180 nm) can be divided into dictate spectral channels with width of Δλ nm. The subscript k represents the kth channel but p, s1 and s2 represent the pump and two signal waves specially, which are 976nm, 1070 nm and 1120 nm, respectively. The superscript ± corresponds to positive and negative directions, respectively. N(z) is the Yb ions concentration distribution along the fiber, for passive fiber N(z) = 0; N2(z) is the excited state population; Psi is the signal power; P(z, λk) is the power of laser λk; σa and σe are Yb absorption and emission cross sections, respectively. Γ is the mode field overlapping factor to the doped area A and we set Γ = 0.7, A = 3.14 × 10−10 m2; h is the Planck constant; ν is frequency; gR is the Raman gain coefficient, which is set to be 0.5 × 10−13 m/W. α is the loss coefficient which is estimated to be 0.0015 m−1 for the signal waves.
In the simulation, the feedback of the output end is neglected. Assuming the seed power is 30 W (the power of 1120 nm laser is 8 W); the pump power is 890 W. The fiber is a 45-meters-long YDF followed by a 1.5-meters-long matched passive fiber and the other parameters are the same with the experiment. Then we can calculate the power distributions of pump and signal waves along the fiber, which is shown in Fig. 2. It can be found that the pump power fast depletes in the first 10 meters, where the 1070 nm laser grows rapidly but the 1120 nm laser increases in a slow step. Obviously power growing of 1120 nm laser happens in the 10-35 meters range. The energy transfers from 1070 nm laser to 1120 nm laser in the YDF through Yb energy transformation and Raman effect. Finally, a high power 1120 nm laser output is obtained by a single amplifier. With other parameters fixed and changing the pump power, the 1120 nm power change can be calculated as well as the total output power (Fig. 3). The total power increases linearly in the calculation, but the 1120 nm power ratio decreases firstly before pump power of 300 W because the 1070 nm laser amplification dominates. With pump power increasing, next order power transformation from 1070 nm to 1120 nm becomes dominating, the output power nearly only contains 1120 nm laser when the pump power is about 900 W. Such power ratio change also can be found in the configuration in .
Figure 2 shows that 976 nm pump power is nearly absorbed in the first 20 meters, so in order to compare this setup to the one that reported in , we calculate the case in which the gain fiber of the amplifier is formed by a 20 m YDF and 26.5 m passive fiber that the same with our experiment in length. The result is displayed in Fig. 4(a). The power evolutions in the first 20 m are the same with Fig. 2, but the energy transformation is slower in the passive fiber compared to our case because of the lack of Yb-energy-transformation. In this configuration, efficient energy transformation requires a passive fiber longer than 40 m (see Fig. 4(b)). As for the same fiber, the more energy transformation mechanism it has, the faster the power transfer happens, which means shorter fiber is required. It is favorable to increase the threshold of nonlinear effects with short fibers, such as Stimulated Brillouin Scattering (SBS) or Stimulated Raman Scattering (SRS) when amplified single frequency lasers  or power scaling broad bandwidth lasers in 1100-1150 nm range.
Assuming there is no Raman effect in the amplifier, i.e. gR = 0 and other parameters are the same as that used in the calculation of Fig. 2. We can find 1070 nm laser transfers slowly to 1120 nm laser in Fig. 5. So the dominated mechanism for power scaling of 1120 nm laser in the YDF is SRS, which is a nonlinear process. It is why we name such setup “nonlinear amplifier”. But different with simple Raman amplifier, the Yb ions can also amplify the 1120 nm (or 1100-1150 nm) laser in the “nonlinear amplifier”. In this calculation there is only about 100 W 1120 nm laser in the output, but higher power could be achieved by using fibers with larger 1120 nm emission cross sections, such as Al-doped YDF .
The power ratio of the 1120 nm laser in the dual-wavelength seed can be optimized for a certain amplifier. Figure 6 is the calculated 1120 nm output power as a function of 1120 nm power ratio in the seed for our experiment. At the pump power of 890 W, the total output power is about 770 W. 720 W 1120 nm laser power can be obtained, when the 1120 nm power ratio in the seed is only 5%. And when the 1120 nm power ratio is over 40% in the seed, the output is nearly 1120 nm laser in our case. But the ratio should be increased if there is more pump power or the gain fiber is shorter. It should be noted that the function of 1070 nm laser in the seed is important for it is an interim product. Locating at the gain peak of Yb ions, 1070 nm laser extracts energy from the pump source firstly and passes to 1120 nm in succession. Lacking of such interim product, the 1120 nm laser can hardly extract energy efficiently for the YDF, which would result in strong ASE or parasitic lasing.
4. Experimental results
In the experiment, the 1120 nm power is 8 W and the total seed power is about 30 W. The measured total output power and 1120 nm power are shown in Fig. 7. The 1120 nm power is achieved by integrated corresponding wavelength band in the output spectrums. At the full pump power, the total output is 773 W and the maximal 1120 nm power is about 732 W. The result is nearly the same with the calculation (Fig. 3). However, when pump power is about 800 W, the 1120 nm power ratio reaches its maximal and then starts to decrease. From the output spectrum (Fig. 8) we can find that a new wavelength peaking at 1175.1 nm appears and partially power begins to transfer to this wavelength. This phenomenon can be attributed to the Raman-assisted-amplified four-wave mixing (FWM). It can be found that the frequency difference of 1175.1 nm and 1120 nm just equals to that of 1120 nm and 1070 nm lasers. The 1175.1 nm seed is initially formed by FWM due to the relatively low threshold (the threshold of spontaneous Raman effect for our amplifier is estimated to be larger than 1500 W) , as the 1120 nm power increasing and the new seed propagating along the fiber Raman amplification dominates. The new wavelength may decrease the efficiency of the nonlinear amplifier, but it contains less than 4% power ratio in this setup. For the applications of pumping Ho-doped fiber laser  or cascaded pumping Raman fiber laser , this 1120 nm laser source is practicable. Actually, this effect can be partially weakened by reduce the FWM. FWM in Yb-doped fiber amplifier has been thorough studied (see [13–15] and references therein). There are also some methods proposed to suppress the FWM in gain fiber, such as using polarization maintained fiber and tuning of the seed polarization [14,15]. But there is no particular report about the character of FWM in the proposed amplifier as we know. How the Raman-assisted-amplified FWM works and how to restrain it in the amplifier deserve to be further studied. We measured the 3 dB bandwidth of the 1120 nm laser in the output and it broadened from 0.2 nm to 1.5 nm at the maximal power, which is attributed to the self phase modulation (SPM)  and FWM  because of the existence of Yb and Raman gain mechanisms.
We also detected the scattering light from the power meter by a photo detector in the output end. The result shows that the signal is nearly in continue mode in despite of the small fluctuation (less than 1%) with a time scale of about 100 ms. There exists pulse operation in the output signal with an average period about several millisecond but the amplitude is as small as 0.5%. Due to the big gain difference of the two signals in Yb-doped gain fiber, we believe the gain competition is too weak to generate catastrophic pulse.
The influence of the 1120 power ratio in the seed on the output 1120 nm power at different pump powers has been experimental studied. In Fig. 9, the seed power is about 30 W for all cases and we measured the output powers and spectrums at different 1120 nm power ratio. It can be found that at low pump power, the amplification of 1070 nm laser dominates and with power increasing, 1120 nm laser amplification dominates, which is consistent to our previous theory analysis. The result of the higher 1120 nm laser power ratio in the seed the faster the 1120 nm power dominating in the output is also in good agreement to our calculation shown in Fig. 6.
In conclusion, we proposed an Yb-Raman gain combined nonlinear fiber amplifier to amplify 1120 nm laser to a record power of 732 W and optical efficiency of 82.2%. In the amplifier, Yb gain and Raman gain both contribute to the amplification of 1120 nm laser. Numerical study has been done to analyze the power evolutions in the amplifier. The influence of the power ratio in the seed has also been analyzed. The numerical results are in good agreement with the experiment. Such amplifier utilizes an interim wavelength locating at the gain peak range of YDF to amplify its Raman Stokes wave in a compact structure. So it can be applied to amplify lasers in wavelength of 1100-1150 nm by using the interim wavelength of 1060-1100 nm. Furthermore, due to the more efficient energy transformation, the gain fiber used in the amplifier is shorter, so it provides a suitable choice to amplify single frequency laser.
This research is sponsored by the National Nature Science Foundation of China under NO. 61322505, the program for New Century Excellent Talents in University, the Hunan Provincial Innovation Foundation for Postgraduate Student and Innovation Foundation for Graduates of the National University of Defense Technology under grant B130702.
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