Nonlinear polarization dynamics in a weakly birefringent all-normal dispersion photonic crystal fiber: toward a practical coherent fiber supercontinuum laser

Abstract

Dispersion-flattened dispersion-decreased all-normal dispersion (DFDD-ANDi) photonic crystal fibers have been identified as promising candidates for high-spectral-power coherent supercontinuum (SC) generation. However, the effects of the unintentional birefringence of the fibers on the SC generation have been ignored. This birefringence is widely present in nonlinear non-polarization maintaining fibers with a typical core size of 2 µm, presumably due to the structural symmetry breaks introduced in the fiber drawing process. We find that an intrinsic form-birefringence on the order of 10−5 profoundly affects the SC generation in a DFDD-ANDi photonic crystal fiber. Conventional simulations based on the scalar generalized nonlinear Schrödinger equation (GNLSE) fail to reproduce the prominent observed features of the SC generation in a short piece (9-cm) of this fiber. However, these features can be qualitatively or semi-quantitatively understood by the coupled GNLSE that takes into account the form-birefringence. The nonlinear polarization effects induced by the birefringence significantly distort the otherwise simple spectrotemporal field of the SC pulses. We therefore propose the fabrication of polarization-maintaining DFDD-ANDi fibers to avoid these adverse effects in pursuing a practical coherent fiber SC laser.

Overcoming temporal polarization instabilities from the latent birefringence in all-normal dispersion, wave-breaking-extended nonlinear fiber supercontinuum generation

“…It is known that in weakly birefringent fiber, polarization modulation instability can lead to coupling between the effective orthogonal polarization modes of the fiber [6,[9][10][11]. The stability of SC from weakly birefringent fibers is maximized by launching with a pulse polarization orientation that coincides with the highest preserved polarized output power.…”

“…While attempts have been made to induce birefringence through high stress levels [6,18], the optimal solution is PM, all normal dispersion, nonlinear fiber. In the absence of such a PM nonlinear fiber, one can potentially induce enough asymmetrical strain in the fiber to create strong birefringence.…”

“…In the absence of such a PM nonlinear fiber, one can potentially induce enough asymmetrical strain in the fiber to create strong birefringence. Tu et al used a pinch plate along a 27 cm length of PCF fiber and largely eliminated the polarization modulation instability defects from the measured power spectrum and reduced the depolarization from 60% to 20% [6]. For fibers where longer lengths and WB are desired, a small expandable spool could be employed.…”

“…Fibers exhibiting normal dispersion relax the constraints on the launched pulse duration, and permit stable SC generation with a much broader range of input pulse parameters. Photonic crystal fibers (PCF) with all normal dispersion have been used to create few-cycle pulses [2], octave-spanning SC [3], white-light SC [3,4], and high compression ratio pulses [2,[5][6][7]. Solid, ultrahigh-numerical-apertures (UHNA) fibers with all normal dispersion profiles are less frequently used, e.g.…”

“…A combination of low level laser noise and quantum noise from the orthogonal polarization state results in the continuous random fluctuation of the SC output polarization state. Polarization dynamics in birefringent fiber with pulses launched in the all normal dispersion regime have been widely reported [6,[9][10][11] however, the reported dynamics primarily deal with the stable coupling of power into spectral sidebands.…”