Long cavity ring fiber mode-locked laser with decreased net value of nonlinear polarization rotation

Abstract

We investigate a new configuration of a mode-locked fiber laser by using a nonlinear polarization rotation-based design to generate soliton pulses with low repetition rate. Unlike with previously reported configurations, we introduce a Faraday mirror after the first half of the cavity length to counteract the nonlinear polarization rotation effects. The total cavity length is 437 m including a 400-m long twisted SMF-28 fiber. The fiber was twisted to cancel the linear birefringence and to ensure that the polarization ellipticity is not altered as the pulse travels along the fiber. The strict control of polarization yields a stable relation between the polarization state of the pulses propagating in the cavity and the regimes of generation. Depending on the polarization state we observed three different emission regimes, the single soliton regime (SR), conventional noise-like pulses (NLP) and noise-like square-waveform pulse (NLSWP). In the SR, a 467.2 kHz train of solitons was obtained with pulse duration of 2.9 ps at 1558.7 nm. IntroductionUltrafast lasers have attracted significant interest due to their potential for applications like surgery [1], high-precision micromachining [2] and multiphoton microscopy (MPM) [3]. In the last case the fiber soliton sources can be especially useful because of their capacity of generating ultrashort pulses in the wavelength range from 1530 nm to 2000 nm using Er, Tm, Ho doped fibers and the effect of soliton self-frequency shift [4]. A high-repetition train of ultrashort pulses causes the effect of thermal accumulation [5,6] which can be beneficial in some cases, however it is undesirable for applications such as MPM. The repetition rate can be reduced by using pulse pickers such as Pockels cells or acousto-optic modulators, however this method is energetically inefficient and increases the complexity of the setup. Because the repetition rate of a mode-locked laser is inversely proportional to its resonator length, the repetition rate can be reduced simply by lengthening the cavity. In contrast to solid-state lasers, which require critical alignment, fiber lasers are more prone to achieve lower repetition rates with the convenience and simplicity of elongating a fiber cavity.The long cavity lasers were studied mainly with the purpose to increase the pulse energy in the configurations with net-normal dispersion. For example, Kobtsev et al. [7]

Sub-200-kHz single soliton generation in a long ring Er-fiber laser with strict polarization control by using twisted fiber

“…In the following the minimal transmission angle is referred to as 0° and the angle of maximal transmission is 100°. In our previously reported works used all-polarization control cavities [25][26][27]33] we experimentally found that the regimes of mode-locking operation depend on the polarization state in CW mode, before mode-locking is started. We will refer to this state as the initial polarization state.…”

“…Recently we proposed a new way to solve the problem of overdriving for long ring cavity lasers using NPR for mode-locking [27]. The handedness of NPR depends on the handedness of the polarization.…”

“…So, if we change the handedness of the polarization after propagation through a section of the cavity, the angle of NPR acquired in this section will be compensated in the remaining part of the cavity. The laser reported in [27] includes two 200-m spans of the twisted fiber. After propagation through the first span we changed the polarization handedness using a Faraday mirror.…”

Picosecond pulse-width ultra-low repetition rate passive mode-locked fibre ring laser

“…High‐resolution in long fibre monitoring is of great interest in this context and a shorter duty cycle is very important [1, 2 ]. A comparison between duty cycle results in previous works [4, 6–9 ] is presented in Table 1. The present work shows the lowest duty cycle while maintaining a low repetition rate and ps‐long pulses.…”

“…A similar experiment has been reported in [4 ], but with a pulse, durations limited to the nanosecond scale, mainly due to dispersion effects within the ultra‐long cavity. While a number of works in the literature demonstrate either short pulse duration [6, 7 ] or low repetition rates [4, 8, 9 ], the present work employed dispersion management over the long‐length ring cavity allowing for stable fundamental mode‐locked pulses to be obtained with picoseconds temporal width and kHz repetition rate.…”