Naoki Karasawa

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

Wave-propagation equations, including effectively the second derivative in time under the condition of a small difference between the group and phase velocities and the first derivative in position /spl xi/ in the group velocity coordinate, are derived based on the slowly evolving wave approximation. These can describe ultrabroadband optical pulse propagation with not only self-phase modulation (SPM), but also induced-phase modulation (IPM) in the monocycle regime in a fiber. It is shown that linear dispersion effects can be rigorously included in the numerical calculations. Calculations including SPM in a single-mode fused-silica fiber with the Raman effect are performed and compared with experimental results. Also, calculations including IPM in the fused-silica fiber are compared with experimental results. The effects of each term in the calculations on spectra are analyzed and it is shown that inclusion of the Raman effect and the dispersion of the effective core area is important for obtaining better agreement with experiments. It is shown that inclusion of more than third-order dispersion terms is necessary for calculations of monocycle pulse propagation.

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

We experimentally demonstrate the generation of intense ultrabroadband optical pulses whose spectrum ranges from 300 to 1000 nm (700-THz bandwidth) with a well-behaved spectral phase and 23-muJ pulse energy by a novel, simple setup utilizing induced phase modulation (IPM) in an argon-filled single-mode hollow waveguide. Fundamental as well as second-harmonic pulses produced by one common femtosecond pulse from a Ti:sapphire laser-amplifier system are copropagated in the hollow waveguide. The effect of the delay time between the two input pulses on the IPM spectral broadening is clarified and confirmed to agree with the theoretical result. It is found that the compressed pulse duration from this pulse is 1.51 fs if its phase is completely compensated for.

Experimental generation of an ultra-broad spectrum based on induced-phase modulation in a single-mode glass fiber

An ultra-broad spectrum over the range from 480 to 900 nm is experimentally generated by induced-phase modulation (IPM) of two 120 fs intense optical pulses copropagating in a 3-mm single-mode fused-silica fiber for the first time to our knowledge. The center wavelengths of the two pulses are 640 nm and 795 nm, respectively. The Fourier transform of this spectrum yields a transform limited 4 fs optical pulse. This IPM-induced spectrum broadening method opens the way to monocycle optical pulse generation in the near future.

Optical pulse compression to 5.0 fs by use of only a spatial light modulator for phase compensation

We experimentally demonstrate the generation of 5.0-fs optical pulses (2.5 µJ, 1-kHz repetition rate), using only a spatial light modulator for phase compensation. Pulse compression of the broadband pulse (500–1000 nm) from an argon-filled capillary fiber is achieved with a liquid-crystal spatial light modulator without any prechirp compensation. The output pulse width is found to be 4.1 fs by a fringe-resolved autocorrelator fitted with a transform-limited pulse and to be 5.0 fs by second-harmonic generation frequency-resolved optical gating with marginal correction. It is to our knowledge the shortest pulse ever generated by use of only a spatial light modulator for phase compensation.

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

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 ω1 and its second-harmonic wave ω2=2ω1 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 ω1 and ω2. 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.

Sub-5 fs optical pulse characterization

Ultrabroadband optical pulses generated through self-phase and induced-phase modulation effects and ultrashort optical pulses whose phases were compensated for using a 4f pulse shaper with a spatial phase modulator were generated. Interferometric autocorrelation, frequency-resolved optical gating and spectral phase interferometry for direct electric-field reconstruction (SPIDER) measurements were made to characterize these pulses, and the results were compared. The generation of 5.0 fs (2.4 cycle) or shorter optical pulses was confirmed. For much shorter pulses, below-two-cycle or monocycle optical pulses, single-shot characterization excluding the errors due to the pulse-to-pulse fluctuation is essential. The sensitivity of SPIDER, which is the most advantageous characterization technique apart from its low sensitivity, was improved by a factor of about a hundred (~1 nJ/THz-bandwidth). Instead of a chirped reference pulse split from the pulse to be characterized, a powerful external pulse from a Ti:sapphire laser amplifier as a highly intensive chirped pulse was employed. By use of this modified SPIDER, the characterization of an over-one-octave ultrabroadband optical pulse was performed. This modified-SPIDER method is the most promising for characterization of monocycle optical pulses.

High-powered ultrabroadband pulse generation from near-infrared to near-ultraviolet by induced phase modulation in a gas-filled single-mode hollow waveguide

Generation of an ultrabroadband optical pulse with a fluent frequency-dependency of the phase is important, for example, for shaping multi-wavelength optical pulses and monocyclization. When two optical pulses, one fundamental, the other generated by second harmonic generation, are co-propagated in a nonlinear waveguide, induced-phase modulation (IPM) between them, as well as self-phase modulation (SPM), can be used to broaden the spectrum of each pulse. Since the second-harmonic wave as well as the fundamental wave are generated from one common femtosecond pulse, the carrier-phase difference between their waves is constant and these can be synthesized constructively after propagation. For a case where a capillary fiber filled with noble gas is used as an almost-dispersionless, nonlinear waveguide, we analysed the optimum delay between two pulses and powers required for covering spectrum between two pulses and it was predicted that the generation of the optical pulses whose spectrum covers from near-ultraviolet to near-infrared was possible. To verify these predictions, experimental studies have been performed.

Optical pulse compression to 5.0 fs using only the spatial light modulator

Summary form only given. Optical pulses in the 5-fs region have been generated using chirped mirrors for chirp compensation. Chirped mirrors have the advantage of high throughput. However, the difficulty of obtaining a very large bandwidth, the inter-dependence of different phase orders, and the inability to fine-tune the phase in the experimental set-up are disadvantages. On the other hand, the pulse shaping technique (Weiner et al, IEEE J. Quantum Electron. vol. 28, p. 909, 1992) using a liquid crystal spatial light modulator (SLM) for pulse compression has the advantages of large bandwidth (300-1500 nm) and in-situ adaptive phase control. Recently it was used to compress broadband pulses with pre-chirp compensation by the prism pair to obtain sub 6-fs pulses (Xu et al, 2000). Here, we demonstrate experimentally that the pulse shaper with the SLM can be used to compress broadband (500-1000 nm) pulses from the argon-filled capillary fiber without any pre-chirp compensation to generate 5.0-fs pulses. By not using any pre-chirp compensation optics, the optical throughput increases, the alignment becomes easier and the total spectral width is not cut. Also, the important parameter of the phase pattern applied by the SLM for generating pulses close to the transform-limit is identified. To accurately evaluate the pulses thus generated, second-harmonic frequency-resolved optical gating (SH-FROG) is used.

Characteristics of the oscillatory spectrum due to only induced-phase modulation in an argon-filled hollow waveguide accompanied with intense self-phase modulation

We experimentally observed the periodic oscillatory structure in the second-harmonic spectrum when we simultaneously propagated intense fundamental and second-harmonic pulses from a Ti:sapphire laser-amplifier system in an argon-filled capillary fiber. The periodic oscillatory structure in the second-harmonic spectrum is only due to the induced-phase modulation (IPM) which, in general, is hidden because of strong self-phase modulation. It is found that this structure can be well explained by the interference of the electric field components in the second-harmonic pulse. These electric field components are generated based on IPM by the fundamental pulse. The theoretical analysis of the nonlinear pulse propagation agrees well with experimental results. In addition, the measured dependence of the modulated spectral behavior on the delay time, including the spectral feature, the oscillatory period and the range of the delay time between two pulses, indicates the excellent agreement with the numerical analysis which includes the medium and waveguide dispersion and the steepening effect.

Comparison between theory and experiment of nonlinear propagation for a few-cycle and an ultraband optical pulses in a fiber-beyond the slowly-varying envelope approximation

Summary form only given.To describe the propagation of short-duration and broadband optical pulses in a fiber, the conventional slowly-varying envelope approximation (SVEA), has the following two limitations: (1) the approximation of the slowness of the envelope compared with the optical cycle time becomes invalid for an ultrashort pulse, (2) its treatment of linear dispersion of the fiber, i.e., including ordinary up to 3rd-order terms around the center frequency of the pulse, becomes incorrect when the bandwidth of the pulse spectrum becomes extremely large. To solve the first problem, we have derived a nonlinear fiber propagation equation by the method similar to the slowly-evolving-wave approximation. To solve the second problem, we have developed a novel method to include all orders of terms for the linear material and waveguide dispersion in the numerical calculations. In summary, a nonlinear pulse propagation equation that can be used to describe ultrabroadband and small-cycle optical pulses in a fiber is derived and the calculated spectra obtained from it for self phase modulation agrees well with the experimental spectra.