Theory of ultrabroadband optical pulse generation by induced phase modulation in a gas-filled hollow waveguide

Abstract

Generation of an ultrabroadband optical pulse with a fluent frequency dependency of the phase is important for creating a monocyclelike optical pulse and for shaping multiwavelength optical pulses. A previously proposed method that uses induced phase modulation between a femtosecond fundamental wave 1 and its second-harmonic wave 2 ϭ 2 1 in a fused-silica fiber is applied to a capillary fiber filled with noble gas. Analytic results of chirps without dispersion but with loss in the fiber are shown, and the optimum conditions relating to a delay time between two pulses and to input peak powers are found for fully covering the spectrum between 1 and 2 . Furthermore, numerical calculations, including dispersion effects of the fundamental and the second-harmonic waves from a Ti:sapphire laser-amplifier system with experimentally realizable parameters, are presented. These calculations show that it is possible to generate an ultrabroadband optical pulse whose spectrum ranges from 300 to 900 THz (330 to 1000 nm) with quasi-linear chirp by this method.

Comparison between theory and experiment of nonlinear propagation for a-few-cycle and ultrabroadband optical pulses in a fused-silica fiber

“…Converting the units of to [watt ] by multiplying and using the relations and , where is the linear index of refraction of the medium, we have (6) where (7) Taking the inverse Fourier transform of (6), we have (8) where…”

“…In the SVEA, we neglect the term in (6). Also, it is assumed that in (6) In addition to the assumptions and is conventionally assumed in the SVEA.…”

“…Thus, when the delay time is fs, both pulses overlap near the fiber exit end. For the capillary fiber IPM using the fundamental and the second-harmonic pulses, we have analytically [6] and experimentally shown [7] that the spectrum overlapping becomes largest when both pulses meet near the fiber exit end. The present results indicate that a similar tendency holds for the fused-silica fiber.…”

“…Recently, we have proposed to use both induced-phase modulation (IPM) and self-phase modulation (SPM) to generate ultrabroadband optical pulses using a single-mode fused-silica fiber [3]- [5] and a capillary fiber filled with noble gas [6], [7]. For the fused-silica fiber, the Raman effect may become important when the pulse duration is close to the Raman response time [8], [9].…”

Generation of intense ultrabroadband optical pulses by induced phase modulation in an argon-filled single-mode hollow waveguide

“…We have analyzed nonlinear chirp from SPM and IPM in the case of two-pulse propagation in a single-mode capillary fiber and found that by choice of a proper delay time between pulses it is possible to generate ultrabroadband pulses whose spectrum covers the near infrared to the near ultraviolet, with a f luent frequency dependence of the phase by means of a practical setup. 9 Since the second-harmonic and the fundamental waves are generated from one common femtosecond pulse, the carrier-phase difference between their waves is constant. 10,11 This enables us to synthesize constructively two spectrally broadened waves at the fiber output.…”