Numerical simulation on spectral compression of frequency-shifting femtosecond pulses in photonic crystal fiber

We present a numerical investigation of nonlinear propagation of femtosecond pulses in photonic crystal fibers (PCFs) by solving the generalized nonlinear Schrodinger equation. The PCFs have a second-order dispersion of -48 ps2/km, nonlinear coefficient of 115 W-1km-1, and third-order dispersion (TOD) ranging from 0.1 ps3/km to 1 ps3/km at 1550-nm wavelength. The simulation results show that efficient spectral compression of ultrashort pulses can be induced in the regime of soliton self-frequency shift (SSFS) in PCFs when the input pulse parameters satisfy the condition 0.9 = N = 1.2 for the soliton order N. It is found that the output spectral width is dependent on the peak power of input pulse and the PCF length. A spectral-compression factor up to 2.2 can be achieved for 50-fs, 1550-nm solitons propagating through 10-m PCF with a TOD of 0.5 ps3/km, where the soliton wavelength shifted from 1550 nm to 1698 nm. The effect of initial pulse chirp on output spectral width can be negligible for large PCF length. Greater spectral-compression factor can be obtained using PCF with larger TOD value. This SSFS-based spectral-compression scheme offers much promise for generation of narrow line-width tunable light sources in photonic applications.