Sub-50 fs all-fiber Yb-doped laser with anomalous-dispersion photonic crystal fiber

Summary form only given. An intense research effort has been channelled into improving mode-locked Yb-fiber oscillators in recent years. Despite efforts in all-normal dispersion oscillators, dispersion management is evidently necessary to reach pulse durations below 50 fs. This is implemented most commonly with bulk optical components in Yb-doped fiber lasers. Increased robustness remains a valuable trait, for which all-fiber-integration is highly desirable. Photonic crystal fibers (PCF) with anomalous dispersion have small mode field diameters, enhancing nonlinear effects and usually are birefringent. The first mode-locked laser to incorporate a PCF was reported in 2002 [1]. However, mode-locking was not self-starting owing to the residual birefringence of the PCF Since then, a number of dispersion-managed Yb-doped fiber lasers using PCFs and all-fiber-integrated lasers have been reported. After 10 years, no all-fiber-integrated Yb-fiber laser has been demonstrated to support pulses below 60 fs [2]. Here, we present an all-fiber-integrated, dispersion-managed Yb-doped oscillator incorporating a segment of PCF. The oscillator is self-starting and generates pulses with a spectral bandwidth of 58.2 nm, compressible externally to 42 fs. The pulse evolution had to be carefully designed to achieve this performance: the residual birefringence that prevented self-starting operation of the first fiber laser with PCF [1] is actively employed to construct an intra-cavity Lyot filter to ease initiation of mode-locking [3]. The filtering effect has been characterized using a Sagnac loop seeded by a broadband amplified emissions source (Fig. 1(a)). Numerical simulations were performed to investigate the mode-locking dynamics (Fig. 1(b)). The pulse duration and spectral bandwidth decrease in the first half of the gain fiber, then grow essentially self-similarly. Initial compression in the PCF is followed by stretching, accompanying with spectral broadening and narrowing. The pul- e energy is reduced to ~78 pJ in the PCF to prevent excessive nonlinear effects. This requires an intra-cavity gain of 60 to reach the maximum pulse energy of 4.7 nJ. While the overall evolution is consistent with stretched-pulse operation, influence of self-similar evolution in the gain fiber is observed due to the strong nonlinearity, which counteracts gain narrowing.In conclusion, we have demonstrated an all-fiber-integrated Yb-doped laser with an anomalous-dispersion PCF. The compressed pulse duration is 42 fs, which is the shortest, to the best of our knowledge, from an all-fiber oscillator at 1 μm. These results have been achieved 10 years after the first mode-locked oscillator with PCF was demonstrated, which was plagued with non-self-starting operation and limited long-term stability primarily due to the residual birefringence of the PCF. Here, the birefringence of the PCF is exploited to function as a fiber-integrated Lyot filter for stable and self-starting mode-locking. Nonlinear effects and spectral shaping throughout the cavity is carefully managed to avoid multiple pulsing.