1 1 A ug 2 02 0 Cross-Phase Modulation Instability in PM ANDi Fiber-Based Supercontinuum Generation

We demonstrate broadband supercontinuum generation in an all-normal dispersion polarization-maintaining photonic crystal fiber and we report the observation of a cross-phase modulation instability sideband that is generated outside of the supercontinuum bandwidth. We demonstrate this sideband is polarized on the slow axis and can be suppressed by pumping on the fiber’s fast axis. We theoretically confirm and model this nonlinear process using phasematching conditions and numerical simulations, obtaining good agreement with the measured data. © 2020 Optical Society of America All-normal dispersion (ANDi) optical fibers have recently emerged as attractive platforms to improve the noise and coherence of supercontinuum (SC) generation beyond the limits of anomalous SC generation (SCG) [1-4]. ANDi SCG is based on two fully coherent nonlinear effects: self-phase modulation (SPM) and optical wave breaking (OWB) [2,5] while anomalous SCG is typically susceptible to or even generated by incoherent nonlinear effects [6]. Despite this, ANDi SCG has its own limitations, and being both very sensitive to Raman noise [2-4,7] and requiring low and flat fiber dispersion engineering that is technically challenging to achieve [1]. When pumping with femtosecond pulses, it has been shown that other factors should be considered including polarization modulation instability (PMI) or the amplitude noise of the laser, which both can drastically degrade the relative intensity noise (RIN) and coherence [3,4,8]. These factors limit the available parameter space for coherent SCG, however, fs-pumped ANDi SCG still has significant potential to generate temporally coherent SC with realistic laser parameters, a feature that is hard in the anomalous dispersion regime. This gives such systems potential in a range of fields including optical coherence tomography (OCT), optical metrology, photoacoustic imaging, and spectroscopy [9-12]. In this work, we investigate SCG in a PM-ANDi silica photonic crystal fiber (PCF) with a femtosecond stable optical parametric oscillator (OPO) with intention to suppress PMI. However, in doing this, we discovered the generation of a sideband outside the SC bandwidth which was not observed in previous PM-ANDi SCG [13,14]. We identify this sideband as the result of cross-phase modulation instability (XPMI) process that builds up from coherent SCG and OWB. As it is described in [15,16], XPMI is usually observed when a beam is launched at a 45 ◦ angle from the principal axis of a highly birefringent fiber. This beam is then split into two linearly polarized modes on each axis that will nonlinearly interact with each other to generate two frequency-detuned and cross-polarized four-wave mixing (FWM) sidebands [15]. However, this XPMI process has never been observed before through the stimulation of a fs-SCG but only via spontaneous generation of the interaction of picosecond or nanosecond pulses. Our results show that we can generate a stimulated XPMI sideband in a PM-ANDi PCF using femtosecond pulses. As expected, this sideband is most powerful while pumping the fiber at 45 off the axes. We also demonstrate this sideband can be completely suppressed when pumping the fiber on the fast axis. Significantly, we note that while this sideband is observable outside the SC bandwidth with a low power pump, at higher powers the bandwidth of the supercontinuum will cover the sideband. The experimental setup used to observe and analyze SC and XPMI generation in the PMANDi PCF is shown in Fig. 1. As a pump laser, we used a Ti:Sa femtosecond pulsed laser (Coherent Chameleon) tunable from 680 nm 1080 nm, delivering 200 fs pulse duration at a 80 MHz repetition rate with a maximum average power of 450 mW at 1040 nm. The output power is controlled using a variable neutral density filter (ND). A half-wave plate is used to turn the input polarization state at the fiber input while the polarizer at the output of the fiber is used to observe the spectral content of the light of each axis. A 40x microscope objective is used to couple the light into the 40 cm of PM ANDi PCF – the NL-1050-NE-PM from NKT Photonics. This fiber has a relative hole size of d/L = 0.45, a small hole-to-hole pitch of 1.44 μm, and a nonlinear coefficient of γ=26.8 W−1km−1 at 1040 nm. A set of 2 aspheric lenses is used to collimate the output beam and then focus it to the multimode pick-up fiber.