Kilowatt Ytterbium-Raman fiber laser Lei Zhang,1,2 Chi Liu,1 Huawei Jiang,1,2 Yunfeng Qi1, Bing He,1 Jun Zhou,1,4 Xijia Gu,3 and Yan Feng1,* 1
Shanghai Key Laboratory of Solid State Laser and Application, and Shanghai Institute of Optics and fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China 2 University of Chinese Academy of Sciences, Beijing 100049, China 3 Department of Electrical and Computer Engineering, Ryerson University, Toronto, Ontario M5B 2K3, Canada 4
[email protected] *
[email protected]
Abstract: A kilowatt-level Raman fiber laser is demonstrated with an integrated Ytterbium-Raman fiber amplifier architecture. A high power Ytterbium-doped fiber master oscillator power amplifier at 1080 nm is seeded with a 1120 nm fiber laser at the same time. By this way, a kilowatt-level Raman pump laser at 1080 nm and signal laser at 1120 nm is combined in the fiber core. The subsequent power conversion from 1080 nm to 1120 nm is accomplished in a 70 m long passive fiber. A 1.28 kW all-fiber Raman amplifier at 1120 nm with an optical efficiency of 70% is demonstrated, limited only by the available pump power. To the best of our knowledge, this is the first report of Raman fiber laser with over one kilowatt output. ©2014 Optical Society of America OCIS codes: (140.3510) Lasers, fiber; (140.3550) Lasers, Raman; (290.5910) Scattering, stimulated Raman.
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Received 23 May 2014; revised 10 Jul 2014; accepted 10 Jul 2014; published 23 Jul 2014 28 July 2014 | Vol. 22, No. 15 | DOI:10.1364/OE.22.018483 | OPTICS EXPRESS 18483
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1. Introduction Recently, fiber lasers have drawn extensive attention due to their high efficiency, high power scaling capacity and wide emission spectrum. Until now, up to 10 kW continuous wave ytterbium-doped fiber (YDF) laser in close to diffraction-limited beam quality has been reported [1]. Typically, the efficient emission wavelength of a YDF laser is between 1030 nm and 1100 nm. However, some specific wavelengths beyond this wavelength range have important applications. For example, high power 1120 nm laser could be used to pump Raman fiber amplifier at 1178 nm for laser guide star [2] and to pump Tm-doped fiber [3]. Lasers with wavelength ranging from 1150 nm to 1160 nm are attractive, since their second harmonics could find wide applications in medicine, ophthalmology and dermatology [4]. High power narrow linewidth laser at 1262 nm is required for remote sensing of atmospheric oxygen [5]. Extending to even longer spectral range, Ytterbium-and Erbium co-doped fiber lasers operating at 1.5 µm wavelength region also have attractive features, such as eye safety and atmospheric transparency. However, their power scaling is limited by the lower quantum efficiency and the onset of 1 µm lasing [6]. There is an increasing interest in 1645 nm laser, which is used for pumping mid-infrared optical parametric oscillator [7]. Recent research progress shows that all these lasers can be potentially produced with a single laser technology, which is YDF laser pumped Raman fiber laser. With the right pump source, Raman fiber lasers could lase at arbitrary wavelength across the transparency window of optical fibers. Core-pumped Raman fiber lasers with output power of up to 200 W at 1120 nm with high optical efficiency were reported in [8–10]. Codemard et al. demonstrated a 100 W continuous wave (CW) Raman fiber laser operating at 1120 nm, cladding-pumped with an YDF laser [11]. Supradeepa et al reported a 300 W high-efficiency cascaded Raman fiber laser at 1.5 µm with a novel amplifier architecture, core-pumped by a high power YDF laser [12]. In their experiments, the wavelength division multiplexer (WDM) used to combine the Raman pump and seed lasers needs to handle hundreds to thousands watts laser power. For even higher power, this component could be the bottle neck. Most recently, we proposed an integrated ytterbium-Raman fiber amplifier (YRFA) architecture for power scaling of Raman fiber laser [13]. A 300 W all-fiber linearly-polarized single mode amplifier at 1120 nm was demonstrated in a proof of principle experiment. One of the most important improvements for this architecture is the elimination of the WDM that has been used in almost all core-pumped Raman fiber laser. In the new architecture, the Raman seed lasers and pump laser are propagated and amplified in the core of the same fiber. Later, Zhang et al. reported a 730 W 1120 nm laser with a similar configuration, which proves that the new YRFA architecture has a good perspective of power scaling [14]. In this paper, we report a demonstration of a kilowatt level Raman fiber laser with the proposed YRFA architecture. A 1.54 kW Yb-doped fiber master oscillator power amplifier (MOPA) at 1080 nm is seeded again with a 1120 nm fiber laser. The amplified 1080 nm laser is Raman-shifted to 1120 nm in a subsequent piece of Raman gain fiber. Up to 1.28 kW Raman fiber laser at 1120 nm is achieved with an optical efficiency of 70%, limited by the available pump power, which is the first report of Raman fiber laser with over kilowatt power. A
#212613 - $15.00 USD (C) 2014 OSA
Received 23 May 2014; revised 10 Jul 2014; accepted 10 Jul 2014; published 23 Jul 2014 28 July 2014 | Vol. 22, No. 15 | DOI:10.1364/OE.22.018483 | OPTICS EXPRESS 18484
preliminary numerical simulation shows that kilowatt level Raman fiber laser covering 1.1~2 µm is feasible with the YRFA architecture and a state-of-the-art high power Yb fiber MOPA laser. 2. Experiment configuration Figure 1 illustrates the schematic diagram of the kilowatt-level Ytterbium-Raman fiber laser. The Raman Stokes seed laser is a 1120 nm YDF fiber laser [15], which emits 40 W of linearly-polarized laser. Then a 1080 nm laser oscillator consists of a pair of FBGs (R>99% high reflector and R = 10% output coupler) and 20 m of Yb-doped double-clad fiber (a core diameter of 20 µm, a numerical aperture of 0.06, a cladding diameter of 400 µm, and a nominal cladding absorption of 1.2 dB/m at 976 nm), followed by a cladding mode stripper (CMS). The 1120 nm light is coupled into the 1080 nm laser cavity through a pump and signal combiner. The multimode input ends of the combiner are connected to the pigtailed 976 nm laser diodes with total power of 148 W. The 1080 and 1120 nm laser are then coupled into the main YDF booster amplifier for power amplification. The total available pump power for this stage is 1630 W. 12 m 20/400 YDF with the same fiber parameters as those of the 1080 nm oscillator is used as the gain fiber. A second CMS is spliced after the amplifier to remove the residual pump laser in the cladding. At the end of the YDF amplifier, a piece of 70 m-long germanium-doped fiber (GDF) with the matching parameters to the YDF fiber is spliced as a Raman converter (Nufern LMA-GDF-20/400, background loss
