Mode-locked fiber laser with multimode fiber as saturable absorber

Hong Yao; Zhang Jing; Ji Haiying; Sun Mengru; Wang Tianshu

doi:10.12086/oee.2021.200362

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Hong Y, Zhang J, Ji H Y, et al. Mode-locked fiber laser with multimode fiber as saturable absorber[J]. Opto-Electron Eng, 2021, 48(5): 200362. doi: 10.12086/oee.2021.200362

Citation:

Hong Y, Zhang J, Ji H Y, et al. Mode-locked fiber laser with multimode fiber as saturable absorber[J]. Opto-Electron Eng, 2021, 48(5): 200362. doi: 10.12086/oee.2021.200362

Mode-locked fiber laser with multimode fiber as saturable absorber

1.
College of Science, Changchun University of Science and Technology, Changchun, Jilin 130022, China
2.
National and Local Joint Engineering Research Center of Space Optoelectronics Technology, Changchun University of Science and Technology, Changchun, Jilin 130022, China
3.
College of Opto-Electronic Engineering, Changchun University of Science and Technology, Changchun, Jilin 130022, China

Fund Project: National Natural Science Foundation of China (61975021)

More Information

Corresponding author: Zhang Jing, E-mail: zhangj@cust.edu.cn

Received Date 09 October 2020

Revised Date 09 March 2021

Published Date 15 May 2021

Abstract

Abstract

We demonstrate a new mode-locking method: multimode interference mode-locking. This method is simple and convenient in construction. It is only necessary to fuse two short pieces of graded-index multimode fiber in a single-mode fiber laser, which uses the mode interference effect of single-mode multimode single-mode (SMS) structure to achieve saturable absorption mechanism. In order to realize the mode-locking of the SMS structure, it is necessary to precisely control the length of multimode fiber. We propose to coil the SMS structure into the polarization controller. By theoretically deriving the polarization controller to adjust the phase of transmission light in a multimode fiber, the saturable absorption effect can be achieved. Under the 263 mV pump power, a stable 24.83 MHz repetition frequency fundamental frequency mode-locked pulse output was realized, where the pulse interval was 40.12 ns, the signal-to-noise ratio was 50.8 dB, and the center wavelength was 1881.7 nm. The conversion between soliton molecules and traditional soliton can be realized by adjusting the polarization controller and pump power. Under the pump threshold of 410 mW, a stable 25 MHz repetition frequency soliton molecular mode-locked pulse output was realized, where the pulse interval was 40.3 ns, the signal-to-noise ratio was 54.4 dB, and the center wavelength was 1887.60 nm.
- fiber laser /
- mode-locked laser /
- single-mode multimode single-mode fiber structure /
- nonlinear multimode interference

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References

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Overview

Overview

Overview: In recent years, the wide application of Thulium-doped fiber ultrafast mode-locked lasers in the wavelength band of about 2 μm due to its compact structure, narrow pulse width and high peak power has attracted great attention. It has important application prospects in material processing, gas detection, biomedicine, laser radar, etc. The ultrashort pulses in fiber lasers can be produced by using passive mode-locking technology produces. The key device that determines the mode-locking performance is the saturable absorber (SA). The SA with SMS structure has the advantages of simple structure, long-term stability, and high damage threshold. These advantages enable the laser to obtain higher pulse energy and peak power. In recent years, researchers have published many related studies on SMS as a SA. In 2013, Nazemosadat and Mafi theoretically proposed to use the SMF-GIMF-SMF (SMS) structure as the SA in the mode-locked fiber laser. In 2015, S. Fu demonstrated a Q-switched all-fiber laser using SMS as a SA. In 2017, Z. K. Wang et al. used stretched GIMF to obtain a stable pulse. In 2018, N. Wang reported the observation of SMS-based soliton, which introduced an internal microcavity in GIMF and used it as a nonlinear optical switch. In 2019, Zhang et al. improved the mode-locking properties of SMS by coiling it on the paddles of polarization controller (PC) in 1.5 μm band. It can be seen from the reports that it is difficult to achieve precise control of the length of SMS-based multimode fiber in practice.

In this paper, we report a method based on nonlinear multimode interference in the 2 μm band, using single-mode fiber-gradient index multimode fiber (GIMF)-single-mode fiber (SMS) which are twined into the PC as mode-locked fiber laser with saturable absorber, and the gain medium is the 2 m Thulium-doped fiber. In the SA structure, two SMSs are fused together, and each SMS is twined into the PC. By properly adjusting the PC's paddles, we can easily achieve mode locking. This structure reduces the control accuracy of GIMF length. Such an all-fiber SA is based on nonlinear multimode interference. Basically, stable mode-locking operation is obtained under the pump threshold of 410 mW. We have obtained a stable soliton molecule with a shortest pulse duration of 40.3 ns, a corresponding repetition frequency of 25 MHz, and a center wavelength of 1887.60 nm. The signal-to-noise ratio of RF spectrum is 54.4 dB. The conversion between soliton molecules and traditional soliton can be realized by adjusting the PC and input power. Using SMS as SA has many potential applications in human eye-safe ultrafast photonics.