We demonstrate a high power all-fiber single frequency Tm-doped fiber amplifier. The maximum output power reached 102 W and the central wavelength was 1.97 μm. The single frequency laser signal from a seed laser was amplified based on a monolithic master oscillator power amplifier (MOPA) configuration. The slope efficiency was about 50% against the absorbed pump power. Neither parasitic lasing nor nonlinear effect was observed in the monolithic fiber amplifier. The SBS threshold of the single frequency Tm-doped fiber amplifier was analyzed and estimated. The output power is not limited by the SBS threshold and could be further improved by increasing the pump power. To the best of our knowledge, this is the first demonstration of average power exceeding 100 W from monolithic all-fiber laser near 2 μm wavelength.
© 2013 Optical Society of America
Eye-safe single frequency (SF) fiber lasers has attracted much intense attention due to its extensive applications including LIDAR, communication, medicine, environment monitoring, nonlinear frequency generation, and so on. To date the correlative research mostly focuses on 1.5 μm and 2 μm band [1–4]. Tm-doped fiber (TDF) possesses a broad emission band from 1700 nm to 2100 nm and is suitable for producing laser near 2 μm. Moreover, the advantages of fiber laser, such as high robustness, outstanding compactness and good beam quality, make it a favorable choice to achieve practical SF laser near 2 μm [5–11]. There have been several demonstrations on the SF laser around 2 μm based on TDF, employing distributed-feedback (DFB) technology [5, 6], distributed Bragg reflector (DBR) [7, 8], and saturable absorber (SA) . Due to the limits such as absorbed pump power and toleration power of the fiber components, the direct output power of SF fiber laser near 2 μm is low. The output power should be amplified further for practical applications, and power amplification of SF Tm-doped fiber laser (TDFL) is required [11–16].
In fact, the specific energy-level structure of Tm3+ ions enables the cross relaxation process and makes the TDF’s quantum efficiency close to two. The double cladding pump technology further improves the performance of TDFL and makes high power Tm-doped fiber amplifier (TDFA) available. In 1998, Jackson et al. reported the high power TDFA with output power of 5.4 W . In the succedent years, the output power of TDFA mounted up significantly and even exceeded 1 kW in 2010 [18, 19]. In 2012, Tang et al.  reported a narrow-bandwidth 2 μm fiber laser with maximum output power of 137 W, Shah et al.  demonstrated a 100 W polarized and narrow linewidth Tm-doped fiber MOPA, and the linewidths of the lasers were both around sub-nanometer level. As for SF TDFA, the output power also increased enormously and the maximum output power reached 608 W with bulk configuration in 2009 . The great advantages of all-fiber configuration mentioned above make monolithic all-fiber MOPA an attractive approach to achieve favorable SF fiber laser output near 2 μm range. For monolithic all-fiber SF TDFA, Meleshkevich et al.  reported a 10 W output employing 1.5 μm laser as the pump source in 2005, and the amplifier was optimized for 1800 ~2010 nm band. In 2007, Gapontsev et al.  demonstrated a monolithic 20 W SF TDFA at 1.93 μm. Up to now, the output power of monolithic SF TDFA near 2 μm is far below 100 W as far as we know.
In this letter, we experimentally demonstrate a monolithic SF TDFA with output power of 102 W. A SF laser at 1.97 μm was employed as the seed laser in the all-fiber master oscillator power amplifier (MOPA) system. The backward power and the backward light’s spectrum indicated that the amplifier operated well under the stimulated Brillouin scattering (SBS) threshold. Neither nonlinear effect nor parasitic lasing was observed in the fiber amplifier. The SF output power could be further scaled up by increasing the pump power launched into the amplifier.
2. Experimental setup
Figure 1 depicts the pre-amplifier of the SF TDFA. The SF seed laser based on ultra-short cavity  had a linewidth of less than 100 kHz and output power of 40 mW with central wavelength of 1.97 μm. The first stage of the pre-amplifier consisted of a 1550 nm fiber laser, a wavelength division multiplexer (WDM, 1550/2000 nm), a piece of single mode TDF, and a polarization-independent isolator (ISO). The 2.5 m long single mode TDF had core diameter of 9 μm and cladding diameter of 125 μm, the core NA was 0.15. The output power of the seed laser was amplified to 140 mW in the first stage of the pre-amplifier. In the second stage of the pre-amplifier, two 793 nm laser diodes (LDs), a (2 + 1) × 1 signal-pump combiner, a piece of 8 m long double cladding TDF (DC TDF) and an ISO were employed. The DC TDF’s diameters of core and inner cladding were 10 μm and 125 μm, respectively. The NA of the fiber core was 0.15 and that of the inner cladding was 0.46. The absorption at 793 nm was about 3 dB/m and the output signal power was finally amplified to 3 W.
The pre-amplified signal laser was launched into the main-amplifier, as Fig. 2 shows. Five 793 nm LDs were employed to act as the pump source. The signal laser and the 793 pump light were launched into the DC TDF (core diameter 25 μm and inner cladding diameter 250 μm) via a (6 + 1) × 1 signal-pump combiner. The absorption of the DC TDF at 793 nm was about 8 dB/m and the length of the active fiber was 3 m. The core NA was 0.08 and cladding NA was about 0.46. The output end of the signal-pump combiner and one end of the DC TDF were fusion spliced together and coated with low refractive index polymer to confine the pump power in the inner cladding. The output end of the DC TDF was fusion spliced to one end of a piece of 0.5 m matched passive fiber. The fusion splice joint between the active and passive fiber was covered with high refractive index gel to dump the unabsorbed pump power out. The other end of the passive fiber launched the amplified SF signal laser out with an angle cleaved facet to avoid the unwanted feedback (4% Fresnel reflection). The fusion spliced joints at both ends of the DC TDF, the whole DC TDF and the output end of the passive fiber were fixed on the water-cooled heat sink to prevent thermally-induced damage. The DC TDF was coiled to about 12 cm diameter to guarantee the good beam quality of the MOPA system. Since all the components of the MOPA system were fiberized and fusion spliced together, the SF TDFA is a strictly monolithic fiber system with great stability and compactness. The output power was measured by a power meter and the spectra were detected by an optical spectrum analyzer (OSA) with resolution of 0.05 nm. The spare port of the six port of the signal-pump combiner was used to monitor the backward power and spectrum.
3. Experimental results and analysis
Figure 3 depicts the increment of the output power versus the absorbed pump power. The final output power of the SF amplifier reached 102 W. The linear fit of the data shows that the optical-optical conversion efficiency is 50%. The slope efficiency can be improved by optimizing the length of active fiber of the amplifier. The trend of the output power is far from saturated and indicates more output power if the launched pump power is increased. For the time being, the output power is only limited by available pump LDs.
The output spectrum of the SF MOPA is shown in Fig. 4. There is neither ASE nor parasitic lasing observed in the wide range of the spectrum. The inset of Fig. 4 depicts the fine structure of the output spectrum. The optical signal-noise ratio exceeds 40 dB and indicates a favorable amplifying efficiency of the monolithic fiber amplifier, since there were no narrow-band filtering components employed in the system to remove the potential ASE.
The backward power and spectrum of the SF MOPA are depicted in Fig. 5. The backward power mainly contains the Rayleigh scattering of the signal laser as well as the SBS light (if SBS is generated). For a MOPA with 100 W output power, a few mW backward power from amplified Rayleigh scattering is reasonable. The power data show that the backward power increases as the output power scales up, and the trend of the backward power is approximately linear and no sharp increment is monitored. From the inset of Fig. 5 one can estimate approximately that there is no obvious SBS spectrum existing in the backward spectrum, as the resolution of OSA is only 0.05 nm and the wavelength shift at 2 μm is about 0.11 nm. Hence the output power of this SF MOPA system is well below the SBS threshold.25, 28], Eq. (1) can be simplified as:
The effective mode area Aeff is defined in the Eq :
The peak Brillouin gain coefficient gB can be calculated by the following Eq :Table 1 .
The polarization factor K = 2 indicates the complete polarization scrambling of the pump laser. Using the parameters in Table 1, the estimated threshold of SBS in the SF amplifier system reaches 460 W, which is far above the output power that obtained in out experiment. The equations above are deduced upon the assumption that the fiber is single-mode fiber. However, the active and passive fibers used in the experiment are actually few-mode fibers, so the threshold may be higher than the estimated value using Eqs. (1)-(4) [24, 29]. Moreover, the thermally broadened SBS gain spectrum due to end pumping could also suppress the SBS gain and thus elevates the threshold of SBS . Hence the practical SBS threshold in the experiment may be well above 460 W, providing that other limits, such as thermal damage and facet damage, are effectively managed. In addition, the 608 W SF output was achieved by bulk configuration and the fiber’s parameters (core diameter and fiber length) are similar with those of our MOPA , which means the estimated SBS threshold should be even higher. Thus more output power can be achieved in our MOPA without the limit of SBS effect.
Since the fiber used in this experiment was few-mode fiber, which has a V number of about 3.19 for 2 μm lasers. In addition, the fiber was coiled to:12 cm diameter to favor single-mode operation, the output beam quality is guaranteed and M2 should be less than 1.5 [30–32].
A monolithic SF TDFA based on MOPA configuration is experimentally demonstrated. The central wavelength located at 1.97 μm and the maximum output power reached 102 W, which is believed to be the highest output power of SF TDFA employing strictly all-fiber configuration. The backward power and spectrum were monitored and there were no ASE, SBS or parasitic lasing observed in the amplifier. The SBS threshold of the SF TDFA was estimated roughly. The amplifier’s output power was well below SBS threshold and further improvement of the output power is available if more pump power can be launched into the amplifier.
This work is supported by the Graduate Student Innovation Foundation of National University of Defense Technology, and National Natural Science Foundation of China (Grant No. 11274386).
References and links
1. N. Park, J. W. Dawson, K. J. Vahala, and C. Miller, “All fiber, low threshold, widely tunable single‐frequency, erbium‐doped fiber ring laser with a tandem fiber Fabry-Perot filter,” Appl. Phys. Lett. 59(19), 2369–2371 (1991). [CrossRef]
2. C. Yang, S. Xu, S. Mo, C. Li, Z. Feng, D. Chen, Z. Yang, and Z. Jiang, “10.9 W kHz-linewidth one-stage all-fiber linearly-polarized MOPA laser at 1560 nm,” Opt. Express 21(10), 12546–12551 (2013). [CrossRef] [PubMed]
3. C. Gao, M. Gao, Y. Zhang, Z. Lin, and L. Zhu, “Stable single-frequency output at 2.01 microm from a diode-pumped monolithic double diffusion-bonded Tm:YAG nonplanar ring oscillator at room temperature,” Opt. Lett. 34(19), 3029–3031 (2009). [CrossRef] [PubMed]
4. P. F. Moulton, G. A. Rines, E. V. Slobodtchikov, K. F. Wall, G. Frith, B. Samson, and A. L. G. Carter, “Tm-doped fiber lasers: fundamentals and power scaling,” IEEE J. Sel. Top. Quantum Electron. 15(1), 85–92 (2009). [CrossRef]
6. N. Y. Voo, J. K. Sahu, and M. Ibsen, “345-mW 1836-nm single-frequency DFB fiber laser MOPA,” IEEE Photon. Technol. Lett. 17(12), 2550–2552 (2005). [CrossRef]
7. J. Geng, Q. Wang, T. Luo, S. Jiang, and F. Amzajerdian, “Single-frequency narrow-linewidth Tm-doped fiber laser using silicate glass fiber,” Opt. Lett. 34(22), 3493–3495 (2009). [CrossRef] [PubMed]
8. Z. Zhang, A. J. Boyland, J. K. Sahu, W. A. Clarkson, and M. Ibsen, “High-Power Single-Frequency Thulium-Doped Fiber DBR Laser at 1943 nm,” IEEE Photon. Technol. Lett. 23(7), 417–419 (2011). [CrossRef]
9. X. He, S. Xu, C. Li, C. Yang, Q. Yang, S. Mo, D. Chen, and Z. Yang, “1.95 μm kHz-linewidth single-frequency fiber laser using self-developed heavily Tm3+-doped germanate glass fiber,” Opt. Express 21(18), 20800–20805 (2013). [CrossRef] [PubMed]
11. W. Shi, E. B. Petersen, D. T. Nguyen, Z. Yao, A. Chavez-Pirson, N. Peyghambarian, and J. Yu, “220 μJ monolithic single-frequency Q-switched fiber laser at 2 μm by using highly Tm-doped germanate fibers,” Opt. Lett. 36(18), 3575–3577 (2011). [CrossRef] [PubMed]
12. M. Meleshkevich, A. Drozhzhin, N. Platonov, D. Gapontsev, and D. Starodubov, “10 W single-mode single frequency Tm-doped fiber amplifiers optimized for 1800-2020-nm band,” Proc. SPIE 5709, 117–124 (2005). [CrossRef]
13. D. Gapontsev, N. Platonov, M. Meleshkevich, Mishechkin, Shkurikhin S. Agger, P. Varming, and J. H. Poylsen, “20W single-frequency fiber laser operating at 1.93 μm,” Lasers and Electro-Optics, CLEO 2007. Conference on. IEEE1–2 (2007).
15. Q. Fang, W. Shi, K. Kieu, E. Petersen, A. Chavez-Pirson, and N. Peyghambarian, “High power and high energy monolithic single frequency 2 μm nanosecond pulsed fiber laser by using large core Tm-doped germanate fibers: experiment and modeling,” Opt. Express 20(15), 16410–16420 (2012). [CrossRef]
16. L. Pearson, J. W. Kim, Z. Zhang, M. Ibsen, J. K. Sahu, and W. A. Clarkson, “High-power linearly-polarized single-frequency thulium-doped fiber Master-Oscillator Power-Amplifier,” Opt. Express 18(2), 1607–1612 (2010). [CrossRef] [PubMed]
18. P. F. Moulton, G. A. Rines, E. V. Slobodtchikov, K. F. Wall, G. Frith, B. Samson, and A. L. G. Carter, “Tm-doped fiber lasers: fundamentals and power scaling,” IEEE J. Sel. Top. Quantum Electron. 15(1), 85–92 (2009). [CrossRef]
19. T. Ehrenreich, R. Leveille, I. Majid, K. Tankala, G. Rines, and P. Moulton, “1kW all-glass Tm:fiber laser,” Proc. SPIE 7580, 758016 (2010).
21. L. Shah, R. A. Sims, P. Kadwani, C. C. C. Willis, J. B. Bradford, A. Pung, M. K. Poutous, E. G. Johnson, and M. Richardson, “Integrated Tm:fiber MOPA with polarized output and narrow linewidth with 100 W average power,” Opt. Express 20(18), 20558–20563 (2012). [CrossRef] [PubMed]
23. J. W. Dawson, M. J. Messerly, R. J. Beach, M. Y. Shverdin, E. A. Stappaerts, A. K. Sridharan, P. H. Pax, J. E. Heebner, C. W. Siders, and C. P. J. Barty, “Analysis of the scalability of diffraction-limited fiber lasers and amplifiers to high average power,” Opt. Express 16(17), 13240–13266 (2008). [CrossRef] [PubMed]
24. G. P. Agrawal, Nonlinear Fiber Optics (Fourth Edition) (Academic Press, Rochester, New York, 2007).
25. Y. Aoki, K. Tajima, and I. Mito, “Input power limits of single-mode optical fibers due to stimulated Brillouin scattering in optical communication systems,” J. Lightwave Technol. 6(5), 710–719 (1988). [CrossRef]
27. W. Shi, E. B. Petersen, M. Leigh, J. Zong, Z. Yao, A. Chavez-Pirson, and N. Peyghambarian, “High SBS-threshold single-mode single-frequency monolithic pulsed fiber laser in the C-band,” Opt. Express 17(10), 8237–8245 (2009). [CrossRef] [PubMed]
28. G. D. Goodno, L. D. Book, J. E. Rothenberg, M. E. Weber, and S. B. Weiss, “Narrow linewidth power scaling and phase stabilization of 2-μm thulium fiber lasers,” Opt. Eng. 50(11), 111608 (2011). [CrossRef]
29. A. Mocofanescu, L. Wang, R. Jain, K. Shaw, A. Gavrielides, P. Peterson, and M. Sharma, “SBS threshold for single mode and multimode GRIN fibers in an all fiber configuration,” Opt. Express 13(6), 2019–2024 (2005). [CrossRef] [PubMed]
31. J. Liu, J. Xu, K. Liu, F. Tan, and P. Wang, “High average power picosecond pulse and supercontinuum generation from a thulium-doped all-fiber amplifier,” Opt. Lett. (to be published).
32. P. Ma, P. Zhou, Y. Ma, R. Su, X. Xu, and Z. Liu, “Single-frequency 332 W, linearly polarized Yb-doped all-fiber amplifier with near diffraction-limited beam quality,” Appl. Opt. 52(20), 4854–4857 (2013). [CrossRef] [PubMed]