1.Laboratory of Laser Measurement Technology, Academy of Opto-Electronics, Chinese Academy of Sciences, Beijing 100094, China 2.University of Chinese Academy of Sciences, Beijing 100049, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 61475162, 61575105), the Key Research Project of Bureau of Frontier Sciences and Education, Chinese Academy of Sciences (Grant No. QYZDY-SSW-JSC008), and the International Partnership Program of Chinese Academy of Sciences (Grant No. 181811KYSB20160029).
Received Date:05 December 2018
Accepted Date:21 January 2019
Available Online:01 March 2019
Published Online:20 March 2019
Abstract:Over the last decades, passive mode-locked fiber laser has received considerable attention because of ultrashort pulse, compactness, and low cost. As a saturable absorber, nonlinear optical loop mirror (NOLM) has shown the advantages of high damage threshold, possibility of all-PM fiber implementation, short response time and therefore potentially low intrinsic noise. Spectral filtering plays an important role in NOLM mode locked fiber laser, but the influence of filtering parameters on mode locking operation is rarely reported. In this paper, the influence of filtering bandwidth on mode locking operation and on output pulse characteristics are experimentally investigated. A 2 × 2 optical coupler with a splitting ratio of 10 : 90 is introduced at one end of fiber loop to form a loss-imbalanced NOLM, and extracts 90% of intracavity pulse energy as outputs. With this architecture, an all polarization-maintaining figure-8 Er-doped fiber ultrafast laser is achieved. A home-made bandwidth and wavelength tunable bandpass filter is utilized in the cavity, and the filtering bandwidth is defined by 10 dB bandwidth. The clockwise and counter-clockwise mode locked output power are 8.4 mW and 8.6 mW, respectively, with a repetition rate of 2.734 MHz. With a spectral bandwidth of 2.1 nm, the intracavity pulse is shaped by spectral filtering and soliton effect. The 3 dB bandwidth of the clockwise and counter-clockwise mode locked output pulse are 10.1 nm and 1.8 nm, and the values of corresponding full width at half maximum (FWHM) of the direct outputs are 583.7 fs and 2.94 ps, respectively. As the filtering bandwidth increases, the role of filter in spectral shaping weakens, and the parameters of two output pulses become close. When spectral bandwidth is larger than 7.3 nm, the intracavity pulse is shaped by gain spectrum and soliton effect. Both of the clockwise and counter-clockwise output pulses become the transform-limited pulses with almost the same FWHMs of 440 fs. Besides, the wavelength of the figure-8 fiber laser can be adjusted in a range larger than 30 nm by modulating the wavelength of the filter. The tunable mode-locked fiber laser has great potential applications in various application fields. Keywords:figure-eight mode locked laser/ loss-imbalance/ nonlinear optical loop mirror/ spectral filtering