JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 30, NO. 16, AUGUST 15, 2012 2589 Transform-Limited, Injection Seeded, Q-Switched, Ring Cavity Fiber Laser Renjie Zhou, Wei Shi, Member, IEEE, Fellow, OSA, Eliot Petersen, Arturo Chavez-Pirson, Member, IEEE, Mark Stephen, and Nasser Peyghambarian, Member, IEEE Abstract—We report an Er-doped, actively Q-switched, fiber laser, generating transform-limited pulses based on single-fre- quency fiber laser seeded ring cavity. The output pulsewidth can be tuned from hundreds of nanoseconds to several microseconds by changing the repetition rate or the open time of the electrical pulse trigger. This injection-seeded, Q-switched ring cavity fiber laser can be operated over the whole C-band. In addition, a theoretical model is developed to numerically study the pulse characteristics by changing the acousto-optic modulator trans- mission as well as several cavity parameters, such as the cavity length and loss. The numerical results are in good agreement with the experimental results. Index Terms—Erbium lasers, fiber lasers, tunable lasers, Q-switched laser. I. INTRODUCTION F IBER lasers have compact design and stable operation compared with traditional solid-state lasers. In particular, a large variety of monolithic pulsed fiber lasers have been de- veloped in recent years. These fiber laser pulses are suitable for applications in remote sensing [1], LIDAR systems [2], spec- troscopic sensing [3], [4], laser frequency conversion [5], [6], etc. Especially for applications involving spectroscopic sensing and LIDAR, the coherence length and resolution depends on the linewidth and the pulse duration [2], so narrow-linewidth pulses with tunable wavelength and duration are desired. There are many ways to achieve Q-switching with narrow linewidth and wavelength tuning. The use of a diffraction grating de- vice to achieve wavelength tuning with single frequency and narrow linewidth has been demonstrated previously [7], [8]. Unfortunately, this method needs precise free space alignment and a bulky diffraction grating impacts the cavity size and packaging. Alternatively, a tunable fiber Bragg grating (FBG) can be integrated into the fiber laser cavity to achieve active Q-switching with narrow linewidth. Cuadrado-Laborde et al. Manuscript received January 20, 2012; revised May 12, 2012; accepted May 15, 2012. Date of publication June 01, 2012; date of current version July 18, 2012. This work was supported by the U.S. NASA through Contract NNX10CA53C. R. Zhou was with NP Photonics, Inc., Tucson, AZ 85747 USA. He is now with the Department of Electrical and Computer Engineering, University of Illi- nois at Urbana-Champaign, Champaign, IL 61820 USA. W. Shi, E. Petersen, A. Chavez-Pirson, and N. Peyghambarian are with NP Photonics, Inc., Tucson, AZ 85747 USA, and also with Optical Sciences Center, University of Arizona, Tucson, AZ 85721 USA. M. Stephen is with the Goddard Space Flight Center, NASA, Greenbelt, MD 20771 USA. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JLT.2012.2201446 [9] have demonstrated transform-limited pulses by acoustically perturbing the FBG. A narrow-linewidth pulse is obtained from this laser cavity. However, this method relies on mechanical means to change the FBG transmission spectrum, which needs additional bulky elements, and the tuning speed is limited. Most recently, Williams et al. [10] have demonstrated optical tuning of an FBG spectrum to achieve high-speed Q-switching. However, this method relies on a sophisticated grating in- scription technique using femtosecond lasers and the loss due to the grating could be high, decreasing the efficiency. The FBG approach can achieve narrow-linewidth pulses, but the output wavelength cannot be easily tuned within the cavity. Most of the aforementioned approaches employed linear cavity configurations. However, a ring cavity configuration can avoid spatial hole burning by forcing the wave propagation in either a clockwise or counter-clockwise direction. In this situation, standing waves will not be formed, so single-frequency op- eration is not limited by the linear cavity modes. By limiting the amplified spontaneous emission (ASE) signal in the cavity, a single-frequency (narrow-linewidth) pulse can be achieved. Most recently, Popa et al. [11] have demonstrated the use of a graphene saturable absorber with a tunable filter in a ring cavity to limit the ASE spectrum and achieve wavelength tuning, but the linewidth depends heavily on the bandwidth of the filter, which makes it difficult to achieve transform limited pulses. In recently years, Dragic [12], [13] has demonstrated that by introducing a single-frequency seed into the cavity, the ASE signal can be suppressed, forcing the laser to operate on the frequency of the narrow-linewidth seed. Although Q-switched pulses were demonstrated with this approach, the pulse’s tem- poral profile exhibits multipeak characteristics, which is not desired for many applications. In this paper, we demonstrate the use of single-frequency seed injection into an erbium-doped single-mode fiber laser ring cavity to achieve transform-limited pulsed output. By tuning the seed wavelength, the output signal wavelength can also be tuned, without changing the cavity configuration. An output pulse with a smooth single peaked temporal profile is achieved. The Q-switching is achieved by using an acousto-optic modu- lator (AOM) in the cavity, which allows fast and controllable switching performance. By controlling the AOM drive signal repetition rate, output pulsewidths ranging from hundreds of nanoseconds to several microseconds are achieved. The trans- form-limited linewidth of the fiber laser pulses was verified by using a fiber-based Fabry–Perot. In order to understand the origin of pulse characteristics, we have developed a numer- ical model and simulated the pulse formation from the cavity. The AOM transmission and several cavity parameters that can 0733-8724/$31.00 © 2012 IEEE