Abstract

Second-harmonic generation (SHG) in the limit of large phase mismatch, given by Δβ=β₂−2β₁, effectively induces a Kerr-like nonlinear phase shift on the fundamental wave (FW). The phase mismatch determines the sign and magnitude of the effective Kerr nonlinearity, making large negative phase shifts accessible. This se\f-defocusing nonlinearity can be used to compress a pulse when combined with normal dispersion, and problems normally encountered due to self- focusing in cubic media are avoided. Thus, having no power limit, in bulk media a self-defocusing soliton compressor can create high-energy near single-cycle fs pulses [1], However, the group-velocity mismatch (GVM) between the FW and second harmonic (SH), given by the inverse group velocity difference d₁₂=l/v_g,1−l/v_g,₂, limits the pulse quality and compression ratio. Especially very short input pulses (< 100 fs) experience a Raman-like effect with a characteristic time T_R,SHG=2|d₁₂|/Δβ [1c]. Here we address the limits imposed by GVM by using thermally-poled silica photonic crystal fibers (PCFs) for cascaded quadratic (χ⁽²⁾:χ⁽²⁾) soliton compression. In standard silica fibers strong effective quadratic nonlinearities around 1 pm/V have been created with poling. PCFs are instead interesting because they have very strong wave-guide dispersion that can be tailored: for SHG index-guiding silica PCFs with a triangular hole-arrangement can have zero GVM for any FW wavelength λ₁>780 nm by adjusting the PCF hole pitch Λ and hole diameter d [2]. Our simulations predict that high-quality compression to few-cycle pulses in poled PCFs is possible. Such a waveguided geometry can extend the compression technique to lower-energy pulses and produce uniformly compressed beams.

© 2007 IEEE