Wavelength-tunable Q-switched fiber laser based on a 45° tilted fiber grating

Wavelength tunable Q-switched fiber lasers have important applications in the fields of communication, medicine, ranging finding and laser processing. Generally speaking, Q-switched fiber lasers can be divided into two types using either active or passive system. Compared to actively Q-switched fiber lasers which need an acoustic-optic or an electro-optic modulator in the laser cavity, the fiber lasers with passively Q-switched technology have the advantages of compact structure, simple design and low fabrication cost. There are several kinds of saturable absorbers which can be used to realize passively Q-switched in fiber lasers, including semiconductor saturable absorber mirrors (SESAMs), transition metal-doped crystals and single-wall carbon nanotubes (SWNTs). However, the method of using nonlinear polarization rotation (NPR) technology which is an artificial saturable absorber to realize passively Q-switched can not only maintain all-fiber structure of fiber lasers, but also change the width of Q-switched pulses by adjusting the states of polarization controllers. By regulating commercial bandpass filter, the central wavelength of the Q-switched pulses can be adjusted continuously in a larger range.

    A continuously tunable Q-switched all-fiber Er-doped laser based on a 45°-tilted fiber grating and tunable bandpass filter is demonstrated. The 45°-tilted fiber grating is used to achieve the nonlinear polarization rotation (NPR) along with two polarization controllers (PCs). In this experiment, the fiber grating is equivalent to an ideal in-fiber polarizer because it has strong polarization-dependent loss (PDL), and then the Q-switched pulses can be easily observed by properly adjusting the polarization controllers. The threshold of pump power to generate Q-switched pulses in the fiber laser is 116 mW. When the pump power increases from 116 mW to 661 mW gradually, the average output power of Q-switched pulses increases from 0.282 mW to 4.884 mW and the repetition rate enhances from 23.7 kHz to 119.0 kHz gradually. Under the pump power of 655 mW, stable Q-switched pulses with central wavelength of 1548 nm, average output power of 4.45 mW, repetition rate of 105 kHz, and signal to noise ratio (SNR) of 39.89 dB are obtained. Furthermore, the Q-switched optical spectrum can be continuously tuned from 1512 nm to 1552 nm by simply rotating the tunable bandpass filter with 655 mW pump power. As far as we know, this is the widest tunable range of tunable Q-switched fiber Lasers based on nonlinear polarization rotation effect and tunable bandpass filter.
 

 
Fig.1  Schematic diagram of fiber grating coupling principle with different tilt angles


Fig. 2  Loss characteristics of 45 degree tilted optical fiber grating at a spectral range of 1525 nm~1605 nm

 
Fig.3  Tunable characteristics of Q - switched optical fiber lasers in the 1512 nm~1552 nm Range

About Team
Professor Mou Chengbo's team at Shanghai University belongs to the Ministry of Education's Key Laboratory of Special Optical Fiber and Optical Access Network. The team was set up in 2016, led by professor Mou Chengbo, the 12th batch of national "Youth Thousand Program Project" with one doctoral student, eight master students and one graduate student. Many of them have won national scholarships, first-class academic scholarships and other honors, and the scientific research atmosphere is active and lively. Based on optical fiber lasers, the team has been devoted to the realization of all - optical fiber ultrafast lasers and pulse dynamics research for a long time, focusing on optical fiber devices and laser polarization characteristics, new fiber devices sensing and fiber lasers application. The team has published many academic papers in IEEE J. Sel. Top, Quantum Electronics, Opt. Express and other journals and has participated in international conferences and presented more than 10 invited reports in recent years. He and his team were responsible for the National Natural Science Foundation Youth Project, Shanghai University Young Oriental Scholar Project, National Youth Thousand Program Project and so on.

Article
Hu Xiaolin, Yan Zhijun, Huang Qianqian, et al. Wavelength-tunable Q-switched fiber laser based on a 45° tilted fiber grating[J]. Opto-Electronic Engineering, 2018, 45(10): 170741.

DOI: 10.12086/oee.2018.170741