Akira Suguro

Optical pulse compression to 3.4 fs in the monocycle region by feedback phase compensation

We compensated for chirp of optical pulses with an over-one-octave bandwidth (495-1090 nm; center wavelength of 655.4 nm) produced by self-phase modulation in a single argon-filled hollow fiber and generated 3.4-fs, 1.56 optical-cycle pulses (500 nJ, 1-kHz repetition rate). This was achieved with a feedback system combined with only one 4-f phase compensator with a spatial light modulator and a significantly improved phase characterizer based on modified spectral phase interferometry for direct electric-field reconstruction. To the best of our knowledge, this is the shortest pulse in the visible-to-infrared region.

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.

Pulse compression of white-light continuum generated by induced phase modulation in a conventional glass fiber

The 530–880-nm continuum pulse with a greatly asymmetric temporal profile over 500 fs and a spectral phase variation over 150 rad, which was generated by induced phase modulation (IPM) as well as self-phase modulation in a conventional fused-silica fiber, was compressed to 7.8 fs by a feedback technique. Fundamental (a center wavelength of 800 nm, a duration of 80 fs, a pulse energy of 64 nJ) and signal pulses (a center wavelength of 670 nm, a duration of 80 fs, a pulse energy of 65 nJ) produced by one common femtosecond source with an optical parametric amplifier were copropagated in the fiber under an optimum delay time between the two pulses. The computer-controlled feedback system that combines a 4-f phase compensator with a spatial light modulator and a modified spectral phase interferometry for a direct electric-field reconstruction, automatically compensated for not only the conventional nonlinear chirp (group-delay dispersion and its higher-order dispersion) but also the frequency-independent group-delay (first-order phase dispersion), both of which are essential for pulse compression by use of the IPM effect.

Spectral-Phase Characterization and Adapted Compensation of Strongly Chirped Pulses from a Tapered Fiber

A computer-controlled feedback system that combined a modified spectral-phase interferometer as a direct electric-field reconstruction apparatus and a 4-f pulse shaper with a spatial phase modulator as a programmable chirp compensator was developed. The system enabled us to successfully characterize the complex spectral phase and temporal intensity profile of a weak peak-intensity pulse from a tapered fiber with a high sensitivity comparable to that of the conventional interferometric autocorrelator. In addition, we were able to compress for the first time 185 fs fiber input pulses to 16 fs with subpulses originating from the beat between two splitting-spectral components of the fiber output.

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.

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.

Femtosecond-time-resolved highly selective molecular-vibration excitation using a novel vibrationally synchronized pumping technique with frequency difference resonance

Selective stimulated Raman scattering using a single-color femtosecond N-pulse train of finite pulse duration leads to a low excitation efficiency in the high frequency region. This low efficiency of Raman mode excitation is due to spectral modulation which stems from the finite duration of the pulses in each train. To overcome the low efficiency of excitation in the high frequency region, a vibrationally synchronized pumping technique with frequency difference resonance is proposed. In this technique, both the pulse-repetition rate and the center frequency difference of carrier-phase-locked two-color beams consisting of a femtosecond N-pulse train are chosen to be resonant with the frequency of a specific vibrational or rotational mode of interest. In addition, a formula describing the optimum pulse number N for efficient selective excitation is derived.

Generation of carrier-envelop-phase stabilized 3.3-fs optical pulses

The carrier envelope phase (CEP) stabilizer of the 12-fs Ti:sapphire oscillator with chirp-mirror compensation consists of the so-called f-to-2f interferometry, the phase-detection and phase-locking electronics with a frequency divider of f/sub rep//4 and the controller of the CW pump power through a feedback based on the acousto-optic modulator (AOM). Another frequency divider of f/sub rep//72000 was used to derive the multi-pass amplified pulse repetition rate through a Q-switched trigger of pump pulses. By this system combined with the CEP measurement technology, the new field of the electric-field wavepacket photonics will be opened.

Generation and Characterization of 3.4-fs Optical Pulses with Over-One-Octave Bandwidth

We feedback-compensated for chirp of optical pulses with over one-octave bandwidth (495–1090 nm) generated by self-phase modulation in a single argon-filled hollow fiber, and generated 3.4-fs, 1.56 optical-cycle pulses.

Delay-time dependent adatom desorption from the Si(111)-7/spl times/7 surface using femtosecond optical pulse pair excitation

Using femtosecond pulse-pair excitation at 800-nm, delay-time dependent adatom desorption from the Si(111)-7/spl times/7 surface was observed by scanning tunneling microscopy. Trends in the desorption yield and site selectivity were detected as a function of delay time.