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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 27, NO. 6, MARCH 15, 2015
1.5-kW Yb-Raman Combined Nonlinear Fiber Amplifier at 1120 nm Hanwei Zhang, Rumao Tao, Pu Zhou, Xiaolin Wang, and Xiaojun Xu Abstract— A high power Yb-Raman combined nonlinear fiber amplifier employing dual-wavelength seed is demonstrated. The amplifier is seeded by 1070- and 1120-nm signal lasers simultaneously. The mechanism of power evolution of the two signals along the gain fiber is analyzed. By careful design and optimization of the amplifier, we make sure the 1120-nm signal extract majority of the gain in the amplifier. In the end, 1.52-kW 1120-nm laser is obtained with an optical efficiency of 75.6%, which is the highest power ever reported near this wavelength. The compact and efficient amplifier scheme proposed in this letter can be employed for high power amplification of 1100–1200-nm lasers. Index Terms— Yb-doped fiber laser, Raman fiber amplifier, high power.
Fig. 1.
The experimental setup of the amplifier.
I. I NTRODUCTION
H
IGH-BRIGHTNESS laser in the band of 1100-1200 nm can be found plenty of applications in remote sensing, spectroscopy, laser star generation, and biology [1]–[3]. Yb-doped fiber laser (YDFL) seems to be the most direct way to generate lasers in this wavelength range, for the wide emission spectrum of Yb-doped fiber (YDF) [4]. Nevertheless, due to the relatively small emission cross section, power scaling of lasers in the wavelength of 1100-1200 nm directly from YDF is limited by amplified spontaneous emission (ASE) and parasitic lasing in the conventional wavelength range of 1030-1080 nm. By now, the reported highest power of Yb-doped fiber oscillator at 1120 nm is 322 W [5]. Via amplifier configuration, 453 W output at 1117 nm was reported by Supradeepa et al [1]. However, experimental and numerical study shows that the magnification times of lasers in wavelength longer than 1120 nm is small for each amplifier stage due to the ASE [6] and several stages amplifier would make the system more complex and unstable. Raman fiber lasers (RFLs) provide a convenient alternative to generate high power lasers at 1100-1150 nm range. There is no ASE or parasitic lasing in RFL compared to YDFL. In 2009, Feng et al. reported a 150 W 1120 nm Raman fiber laser core pumped by a 1070 nm YDFL [7]. Considering the high power potential of YDFL in 1060-1100 nm, RFL seems to be easier to achieve high power lasers at 1100-1200 nm. Manuscript received September 10, 2014; revised December 12, 2014; accepted December 23, 2014. Date of publication December 25, 2014; date of current version February 19, 2015. This work was supported in part by the National Excellent Doctoral Dissertation of China and in part by the Hunan Provincial Innovation Foundation for Postgraduate Students. The authors are with the College of Optoelectronic Science and Engineering, National University of Defense Technology, Changsha 410073, China (e-mail:
[email protected];
[email protected];
[email protected];
[email protected];
[email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2014.2386973
However, the power record did not update for many years. In order to combine the pump and signal wavelengths into Raman gain fiber, high power wavelength division multiplexer (WDM) is required, but the power handling ability of commercially available WDM becomes a restriction by now. Recently, Zhang et al. proposed an integrated Yb-Raman fiber amplifier, in which a pair of Raman wavelengths (such as 1080 nm and 1120 nm) is firstly amplified in Yb-doped fiber amplifier (YDFA) and then the Stokes wavelength (1120 nm) laser is Raman amplified in the following passive fiber. Finally they obtained 1.28 kW power output at 1120 nm [8]. For such configuration, due to the imperfect spliced point between the YDF and the following passive fiber, the beam quality of the output of YDFA may deteriorate, which would result in the generation of cladding mode that makes the insufficient power transfer to the Stokes wave. Lately, we proposed a more compact Yb-Raman combined nonlinear fiber amplifier, in which Yb and Raman amplification are accomplished in the same YDF [3]. Accordingly, additional passive fiber is not required. In our previous result, we achieved 732 W 1120 nm laser output with less than 20 W 1070 nm laser residual. In this letter, we present an optimized 1120 nm Yb-Raman combined nonlinear amplifier with record output power of 1.5 kW. The YDF used in the amplifier is only 26 m with total nominal cladding absorption coefficient of 15 dB at 915 nm, which is 20 meters shorter than our previous experiment. The scheme of this nonlinear amplifier has the potential for higher power scaling. Furthermore, it can also be applied to amplify other lasers in the range of 1100-1200 nm by improving the seed content. II. E XPERIMENTAL S ETUP OF THE A MPLIFIER Figure 1 is the schematic diagram of the amplifier. The seed of the amplifier consists of two cascaded YDFLs,
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ZHANG et al.: 1.5-kW Yb-RAMAN COMBINED NONLINEAR FIBER AMPLIFIER
Fig. 2. The output power of the 1070 nm, 1120 nm, residual pump and the total power as a function of the pump power.
whose central wavelengths are 1120 nm and 1070 nm (named as cavity A and cavity B), respectively. Both cavities are made up of a pair of fiber Bragg gratings (FBGs) and a length of 10/125 µm double cladding YDF and pumped by 976 nm laser diodes (LDs). The 1120 nm laser formed in cavity A passes through the whole 1070 nm YDFL in the core. The temporal stability of these two lasers has already been demonstrated in [3]. In the amplifier stage, the pump source is formed by two parts. One is four LDs at 915 nm with total power of 1620 W; the other is two LDs at 976 nm with total power of 400 W. The gain fiber used in the amplifier is 20/400 µm (diameters of core/inner cladding) YDF with a length of 26 m. The total nominal cladding absorption coefficient is 15 dB at 915 nm. The fiber used in the seed is 10/125 µm double cladding fiber, so a mode field adaptor (MFA) is employed to connect the seed and the combiner. After the 20/400 µm YDF, a cladding light striper is introduced to get rid of unabsorbed pump light as well as cladding modes. A home-made endcap that coated with anti-reflection films is spliced in the end to avoid unwanted end reflection, the fiber of which is parameters matched 20/400 µm passive fiber with a length of 1 m. The gain fiber and the combiner are set on a water cooled metal plate for efficient heat dissipation. III. R ESULTS AND D ISCUSSION In our previous theoretical study we find that high power ratio of the 1120 nm laser in the dual-wavelength seed could enhance power transfer from 1070 nm to 1120 nm [3]. However, it would also strengthen the Raman-assistedamplified Four Waves Mixing (FWM) effect, which may prevent the 1120 nm power further increasing. It can be understood by the enhancement of the high-order stimulated Raman effect, whose intensity is proportional to the power and the fiber length. So in this experiment the power proportion of 1120 nm laser in the seed is optimized to 30% and the total seed power is 26 W. Such power level is high enough to extract the pump energy for our amplifier. Figure 2 shows the power properties of the amplifier. The total output power increases nearly linearly with the pump power and so does the 1120 nm laser. The maximal output power is 1655 W when 2010 W pump power launched into the amplifier and the power of 1120 nm laser is about 1521 W
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Fig. 3. Spectra of the amplifier. Inset is the spectrum at maximum power in linear scale.
with optical efficiency of 75.6%, which is the highest power reported in 1100-1200 nm range. In the whole process of power scaling, the power of 1070 nm laser is less than 200 W and in the end only 96 W 1070 nm laser is residual in the output, which is lower than the result reported in [8], indicating that this configuration is more efficient. Figure 3 plots the output spectrum of the seed and spectrum at the maximal power (recorded by Optical Spectrum Analyzer, Yokogawa AQ6370C). The power ratio of 1120 nm laser turns from 30% to 96%, when the dual-wavelength seed passes through the amplifier as a result of the Raman amplification. The 3 dB bandwidth of the 1120 nm laser broadens from 0.35 nm to 1.4 nm when the LDs turned on. At the total output power of about 1 kW, a new wavelength peaking at 1175 nm stars to arise. As explained in our previous investigation, this new wavelength results from Raman-assisted-amplified FWM [3]. The power proportion of this signal is only 0.7% at the full power. It is interesting that when the total output power is about 1.5 kW, another two new wavelengths appear which locate at 1091 nm and 1149 nm, respectively. The frequency differences of these two wavelengths to 1120 nm are equal, so we believe it is also caused by FWM. The YDF we used is not polarization maintained fiber so the phase-matching may satisfy in the case of modal birefringence. Moreover, these two wavelengths can also be amplified in the amplifier so FWM processes may be significant [9]. However, such FWM can be suppressed by applying a controlled variation of the phase-mismatch along the fiber [9]. Though there are several new wavelengths in the output, the power ratio of 1120 nm laser still reaches to 91.9%. It is noteworthy that no ASE can be observed from the spectrum, which is the dominant factor that limits the power scaling of YDFL or YDFA that operates in 1100-1200 nm. For comparison, we have also attempted to boost the 1120 nm seed without using auxiliary 1070 nm laser in the amplifier. Strong ASE in 1030 nm-1080 nm arose when output power was only 200 W. So we believe the presented amplifier has more potential to achieve higher power. In the amplifier, 1070 nm laser is amplified first for its high gain in the YDF when pumped by LDs. The power proportion of 1070 nm laser reaches its maximum of over 45% at the pump power of 350 W (Fig. 4). Then another power conversion from 1070 nm to 1120 nm becomes dominant, in which the gain comes from stimulated Raman amplification
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 27, NO. 6, MARCH 15, 2015
Fig. 4. The relationship of power proportions of 1120 nm and 1070 nm lasers with pump power.
Fig. 5.
as well as Yb-amplification [3]. Therefore, 1120 nm laser experiences obvious increase as power growing. However, 1070 nm laser nearly do not decrease when the pump power is in the range of 1350-1600 W (the 1070 nm power proportion is about 10%), though the 1120 nm laser still has little increase. This can be attributed to the inadequate Raman interaction length. In our amplifier, the process of energy transfer from 915 nm to 1070 nm takes place in the segment of gain fiber that close to the pump end, whereas the Raman conversion occurs in the fiber close to the output end. In the experiment the total nominal cladding absorption coefficient at 915 nm is only 15 dB, thus the residual fiber length for Raman amplification is not enough, resulting in the insufficient energy transfer from 1070 nm to 1120 nm. This can also be confirmed by our following experiment. We employed two 976 nm LDs with total power of 400 W into the amplifier. With the power increasing, the proportion of 1070 nm laser in the output restarts to decrease. There is only 5.8% 1070 nm laser residual at the full power and we believe it can be further reduced by increasing the 976 nm pump power. Actually, the absorption of 976 nm pump source in YDF is about three times of 915 nm laser. When 976 nm pump laser is injected into the amplifier, the power of 1070 nm laser would increase faster. Then the position where the Raman amplification happens would move closer to the pump side causing the increase of the efficient Raman interaction length. It demonstrates that choosing different pump scheme is also a method to optimize the output power of the amplifier. The behavior of the amplifier in time-domain is another important character we care about. Usually, the output would be unstable if there were two signals in an YDF amplifier due to the gain competition. But in our amplifier the huge gain difference of the two wavelengths prevents the appearance of the gain competition. Moreover, giant pulse may occur when ASE or parasitic lasing happens due to the interaction of Stimulated Brillouin and Rayleigh backscattering [10]. In the experiment we did not observe any abnormal pulse resulting from ASE or parasitic lasing. Figure 5 is the measured time trace at the output by a detector with bandwidth of 100 MHz and an oscilloscope with bandwidth of 1.5 GHz. The relatively high power fluctuation in the seed may mainly come from the noise of the detector. The stability of the seed can be kept in the amplifier. The output signal can still be in continuous mode despite a slight enhancement of the noise level when
the 976 nm and 915 nm LDs turned on. In microsecond level the temporal behavior is also stable. It shows the amplifier is reliable to achieve kilowatt level power output at the wavelength of 1100-1200 nm range.
The time trace of the amplifier in the output end.
IV. C ONCLUSION In conclusion, we boost the power in 1100-1200 nm band to 1.5 kW by the proposed Yb-Raman combined nonlinear amplifier, which is the highest power result as we know. The optical efficiency is as high as 75.6%. This amplifier uses an interim wavelength locating at the gain peak range of YDF to amplify its Raman Stokes wave in a compact structure. It needs no additional passive fiber for further Raman amplification but fulfilling the Yb and Raman amplification in the same YDF, which makes the system more efficient and compact. By proper design, this configuration can be used to amplify longer wavelength by adding next order Stokes wave (such as 1178 nm) in the seed. R EFERENCES [1] V. R. Supradeepa and J. W. Nicholson, “Power scaling of high-efficiency 1.5 µm cascaded Raman fiber lasers,” Opt. Lett., vol. 38, no. 14, pp. 2538–2541, Jul. 2013. [2] L. R. Taylor, Y. Feng, and D. B. Calia, “50 W CW visible laser source at 589 nm obtained via frequency doubling of three coherently combined narrow-band Raman fibre amplifiers,” Opt. Exp., vol. 18, no. 8, pp. 8540–8555, Apr. 2010. [3] H. Zhang, H. Xiao, P. Zhou, X. Wang, and X. Xu, “High power Yb-Raman combined nonlinear fiber amplifier,” Opt. Exp., vol. 22, no. 9, pp. 10248–10255, May 2014. [4] H. M. Pask et al., “Ytterbium-doped silica fiber lasers: Versatile sources for the 1–1.2 µm region,” IEEE J. Sel. Topics Quantum Electron., vol. 1, no. 1, pp. 2–13, Jan. 1995. [5] H. Zhang, H. Xiao, P. Zhou, K. Zhang, X. Wang, and X. Xu, “322 W single-mode Yb-doped all-fiber laser operated at 1120 nm,” Appl. Phys. Exp., vol. 7, no. 5, p. 052701, Jul. 2014. [6] H. Zhang, H. Xiao, P. Zhou, X. Wang, and X. Xu, “High-power 1120-nm Yb-doped fiber laser and amplifier,” IEEE Photon. Technol. Lett., vol. 25, no. 21, pp. 2093–2096, Nov. 1, 2013. [7] Y. Feng, L. R. Taylor, and D. B. Calia, “150 W highly-efficient Raman fiber laser,” Opt. Exp., vol. 17, no. 26, pp. 23678–23683, Dec. 2009. [8] L. Zhang et al., “Kilowatt Ytterbium–Raman fiber laser,” Opt. Exp., vol. 22, no. 15, pp. 18483–18489, Jul. 2014. [9] J.-P. Fève, “Phase-matching and mitigation of four-wave mixing in fibers with positive gain,” Opt. Exp., vol. 15, no. 2, pp. 577–580, Jan. 2007. [10] A. A. Fotiadi and R. V. Kiyan, “Cooperative stimulated Brillouin and Rayleigh backscattering process in optical fiber,” Opt. Lett., vol. 23, no. 23, pp. 1805–1807, Dec. 1998.
