Dissipative solitons in a compact Er-doped all-fiber figure-of-9 laser

Highlights

• High performance DS pulse generation is observed in a compact Er-doped all-fiber figure-of-9 laser for the first time.

• The NALM-based fiber laser can generate long-term stable, self-started, linearly polarized pulses.

• The proposed DS fiber laser is compact, robust, cost-effective and long-term stable.

• We proved both numerically and experimentally that the essential dissipative mechanism for the DS generation is provided by spectral filtering effect of the NALM loop, which simplifies the laser structure and gives extra freedom for the tuning of DS’s spectral bandwidth.

Abstract

We designed and demonstrated a compact all-fiber dissipative soliton (DS) fiber laser. The proposed Erbium doped fiber laser is mode-locked by nonlinear amplified loop mirror (NALM), which has a typical figure-of-9 structure and operates in normal net-cavity dispersion regime. The NALM-based fiber laser is constructed by only three polarization maintaining fiber components, which can generate long-term stable, self-started pulses with 0.29 nJ pulse energy and 354 fs pulse width under 55.6 MHz repetition rate. It is proved both numerically and experimentally that the essential dissipative mechanism for the DS generation is provided by spectral filtering effect of the NALM loop. Our results clearly show that the DS's spectral bandwidth can be modulated by changing length of the fiber in the NALM loop.

Introduction

During the past decades, ultrafast fiber lasers have benefit diverse fields including optical frequency metrology, material processing, optical frequency combs, optical sensing and supercontinuum generation, due to their inherent excellences such as compactness, robustness, cost effectiveness and high beam quality [1], [2], [3]. All these advantages have distinguished fiber laser from other existing laser technologies and given users superior experience. However, due to the small core size of optical fiber, the accumulation of excessive nonlinearity (e.g., self-phase modulation (SPM)) becomes a fundamental challenge to the pulse energy scale-up of ultrafast fiber lasers [4].

Resulting from the balance between group-velocity dispersion (GVD) and SPM, solitary waves can be stabilized in mode-locked fiber lasers with anomalous cavity dispersion. With the increase of output pulse energy, excessive SPM would break the balance and lead to pulse-splitting [5]. Therefore, the single pulse energy of soliton generated by a fiber laser is usually limited to less than 0.1 nJ. Dissipative soliton (DS) fiber lasers are proved to have great potential for producing optical pulses with extremely high energy and peak power. In contrast to conventional soliton, the mechanism of DS depends on a composite balance between several effects including gain, loss, GVD and nonlinearity [2]. Wise et al. at Cornell University experimentally designed the first DS fiber laser by employing all-normal-dispersion structure in 2006 [6]. Since then, the output energy of ultrafast fiber oscillator has been scaled-up by more than three order of magnitude, making itself become comparable to traditional ultrafast solid-state lasers, e.g., Ti:Sapphire lasers [7], [8], [9].

Nonlinear amplified loop mirror (NALM), as an artificial saturable absorber (SA), has been proved to be an effective approach to generate long-term stable high energy ultrafast pulses from a fiber oscillator [10], [11]. In contrast to real SAs like SESAM and graphene, who have slow time response and suffer from low damage threshold [12], and nonlinear polarization rotation (NPR), whose performance is sensitive to environmental disturbance [13], NALM has many advantages including fast time response and high environmental stability. However, for a long time being, NALM-based fiber lasers struggled with the problem of self-starting. In 2013, Aguergaray et al. demonstrated one solution to promote self-starting of a figure-of-8 NALM fiber laser with a specific dual-gain structure [14], [15]. The reported NALM laser can produce a 10 MHz train of linearly polarized, 4.2 nJ self-started pulses that can be de-chirped down to 120 fs. Yet, the dual-gain design increases the complexity of the NALM laser cavity. Since 2016, a new-type NALM fiber laser was reported, which is named as figure-of-9 to distinguish it from the traditional figure-of-8 [16], [17], [18], [19]. A nonreciprocal phase shifter is inserted into the NALM loop to promote the self-starting ability. Thanks to the all-polarization maintaining (PM) configuration and close to zero group velocity dispersion (GVD) design, such laser can produce long-term stable dispersion-managed soliton with ultra-low noise, which has been proved to be a promising seed oscillator for the space application oriented optical frequency comb [20]. Recently, Suga et al. confirmed that different pulses including soliton, dispersion-managed soliton and DS could be generated from an Er-doped figure-of-9 fiber laser with proper design of the net-cavity dispersion [21]. Yet, a comprehensive study of Er-doped figure-of-9 DS fiber laser has not been reported yet. Since DS is quite different from dispersion-managed soliton in the sense of pulse properties and mechanism, the study of Er-doped figure-of-9 DS fiber laser is of important research significance.

In this contribution, we propose an all-PM fiber figure-of-9 Er-doped DS fiber laser working in normal net-cavity dispersion regime. The NALM-based fiber laser constructed by three PM fiber components can generate long-term stable, self-started pulses with 0.29 nJ pulse energy and 354 fs pulse width under 55.6 MHz repetition rate. It is proved both numerically and experimentally that the essential dissipative mechanism for the DS generation is provided by spectral filtering effect of the NALM loop.