Fabrication of Integrated Single Frequency DBR Fiber Laser Directly on YDF by Femtosecond Laser

A 1080 nm single-polarization single frequency distributed Bragg reflector (DBR) fiber laser is fabricated by directly inscribing fiber Bragg gratings (FBGs) in ytterbium-doped fiber using a femtosecond laser. The high reflectivity FBG with high birefringence is prepared by femtosecond laser overexposure, serving a polarizing element. The low reflectivity FBG is inscribed by scanning axially along the fiber to narrow the FBG bandwidth. This DBR fiber laser avoided the mode hopping as temperature variation and intracavity fusion loss. A single frequency laser with an output power greater than 11 mW, a linewidth of 32 kHz, and a polarization extinction ratio of 10 dB is achieved at a pump power of 517 mW. The influences of the cavity length and LR-FBG reflectivity on the linewidth and output power are further studied. This provides a reference for optimizing the performance of integrated single-frequency DBR fiber laser prepared by a femtosecond laser.

I. Introduction

Single frequency fiber lasers (SFFLs) have promoted the development of gravitational wave detection, distributed fiber sensing and coherent beam synthesis due to their narrow linewidth, low noise, high stability [1]. Distributed Bragg reflector (DBR) and distributed feedback (DFB) SFFLs are considered promising candidates for these applications due to their compact structure and excellent stability [2]. The DFB-SFFL is formed by writing phase-shifted fiber Bragg grating (PS-FBG) in the active fiber using ultraviolet (UV) laser, which requires stringent preparation techniques. Moreover, its power is limited due to short gain length [3]. In addition, achieved narrow linewidth pulsed laser via extra-cavity modulation, which affects the compactness of the system [4]. DBR-SFFL is formed with a pair of FBGs and an active fiber between them. Compared with DFB-SFFL, it could achieve higher power by increasing the active fiber [5], [6]. However, because the FBG written in the active fiber is week due to its low photosensitivity to using UV laser, long FBG should be prepared to achieve high reflectivity and narrow bandwidth, which results in increased effective cavity length and possibility of mode hopping [7], [8]. Hence, DBR-SFFL is usually achieved by fusing a pair passive FBGs made in photosensitive fiber to a section of active fiber [9], [10], [11]. However, the mismatch in core diameter and loss of fusion point can lead to thermal build-up and ASE light. And the gain length is shorter than the effective cavity length due to the presence of passive FBGs, thereby limiting the output power for a given effective cavity length. More severely, mode-hopping easy occurs as temperature varies, due to the different thermal response coefficient of active fiber and passive fiber [12]. Sophisticated temperature control devices usually are required to suppress mode hopping and wavelength drift [9].