IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 4, APRIL 2004 1029 Femtosecond Neodymium-Doped Fiber Laser Operating in the 894–909-nm Spectral Range M. Rusu, S. Karirinne, M. Guina, A. B. Grudinin, and O. G. Okhotnikov Abstract—We demonstrate a practical ultrafast Nd-doped fiber laser operating in the 894–909-nm spectral range. Using purposely designed semiconductor saturable absorbers, a truly self-started mode-locking regime of operation with clean transform-limited 360-fs pulses was achieved. Index Terms—Mode-locked lasers, neodymium, optical fiber, quantum-well devices. U LTRAFAST fiber lasers are currently becoming very useful and reliable tools in many practically important applications such as micromachining, biomedicine, terahertz generation, and others. The main features of fiber-based de- vices—high efficiency, reliability, and small footprint—make them very attractive for applications traditionally occupied by conventional ultrafast lasers. Important activities in devel- opment of practical ultrafast fiber lasers have been recently focused on the use of semiconductor saturable absorber mirrors (SESAMs) as intracavity intensity discriminating element, and Yb-doped fiber as a gain medium [1], [2]. The SESAMs have been extensively used to mode-lock a large variety of laser systems [3], [4]. The passive mode-locking mechanism provided by an SESAM is very efficient and allows for imple- mentation of very compact linear cavity lasers. Yb-doped fibers offer high efficiency, low threshold, and virtually unlimited ability to scale power up [5], [6]. Yb-doped fiber lasers can be easily tuned in a 1030–1120-nm spectral range. However, some applications require a wavelength close to 900 nm. This can be achieved with Nd-doped fibers [7], [8]. The quasi-three-level F I transition of a Nd- ion-doped laser around 900 nm has attracted much attention in recent years as a master source for generation of high power radiation at 460 nm (blue laser) and as a high power single-mode pump for 980-nm fiber lasers [9], [10]. However, the stimulated emission cross section is an order of magnitude smaller than that of commonly used four-level transition at 1064 nm. In addition, the lower laser level has a significant thermal population at room temperature resulting in losses due to reabsorption of laser light. Conse- quently, quasi-three-level laser systems require considerable pumping before exhibiting gain [6]. Optical fiber-based sys- tems provide promising alternative to the bulk configuration because of their excellent mode confinement and long inter- Manuscript received September 22, 2003; revised December 2, 2003. This work was supported by the Finnish Academy under the TULE-QUEST Project. M. Rusu, S. Karirinne, M. Guina, and O. G. Okhotnikov are with the Op- toelectronics Research Centre, Tampere University of Technology, FIN-33101 Tampere, Finland (e-mail: Oleg.Okhotnikov@orc.tut.fi). A. B. Grudinin is with the Fianium-NewOptics Ltd., Southampton, SO31 4RA, U.K. Digital Object Identifier 10.1109/LPT.2004.824951 Fig. 1. Cavity configuration for a tunable mode-locked Nd fiber laser. action length that allow us to extract sufficient gain even for quasi-three-level transitions [7], [8]. In this letter, we report on a passively mode-locked Nd -doped fiber laser operating at the three-level F I transition near 900 nm. A schematic description of laser setup is shown in Fig. 1. The cavity contains a grating pair for intracavity dispersion com- pensation, a 1-m-long Nd -doped fiber with angled-cleaved end to suppress intracavity reflections, a wavelength-division multiplexer, and a 92% reflectivity fiber loop mirror. The cavity was terminated by the loop mirror from one end and by an SESAM from the other. The neodymium-doped silica fiber (NA , cutoff wavelength 800 nm) is pumped by a pigtailed single-mode laser diode operating at 808 nm. The unsaturated fiber absorption at 808 nm was 37 dB/m. The 808/900-nm pump wavelength-selective coupler and the loop mirror were made of fiber with a cutoff wavelength of 735 nm. The maximum launched pump power was 100 mW. The large normal group-velocity dispersion introduced into the cavity by the fiber is offset by the anomalous dispersion of the grating pair, so that total intracavity dispersion was anomalous which resulted in easy self starting of mode-locked operation. Mode locking was initiated and stabilized by the SESAM. The SESAM was grown by solid-source molecular beam epitaxy on n-type GaAs (100) substrate satisfying an antiresonant design [3]. The sample includes a bottom mirror comprising 20 pairs of AlAs–Al Ga As quarter-wave layers forming a distributed Bragg reflector (DBR). The DBRs stopband had a center wavelength of 910 nm and approximately 100-nm bandwidth (860–960 nm). The absorber region consists of six 6-nm-thick In Ga As quantum-wells separated by 16-nm GaAs barriers. The quantum-wells are placed between a 0.1- m GaAs buffer layer and a 50-nm GaAs cap layer. The device was grown at 650 C with the exception of the active region that was grown at 520 C. The photoluminescence emission peak from quantum-wells was measured to be at 930 nm. The saturation fluence and nonlinear reflectivity have been estimated to be 3 J/cm and 2%–3%, respectively. Recently, similar absorber mirrors were successfully used in broadly tunable mode-locked Yb-fiber lasers [2]. 1041-1135/04$20.00 © 2004 IEEE