Kazuya Takasago

Evaluation of femtosecond pulse shaping with low-loss phase-only masks

We numerically and experimentally evaluate the performance of programmable femtosecond pulse shaping with specified time-dependent amplitude and phase profiles using phase-only masks. With the masks that have discrete phase levels of /spl ges/4 designed by simulated annealing optimization algorithm, various pulses with arbitrarily specified amplitudes, pulsewidths, and pulse intervals are obtained with high accuracy. However, when specifying temporal phase profiles together with the amplitude, the range of shaped output pulses that can be produced with phase-only masks is somewhat limited even with a phase level of 64. The efficiency of transmitted optical power in the experiments is /spl sim/60%, mainly due to the diffraction loss at a grating pair, which is much higher than that using an amplitude-and-phase mask.

Design of Frequency-Domain Filters for Femtosecond Pulse Shaping

As a calibration method for frequency-domain filters in Fourier-transform synthesis of arbitrarily shaped laser pulses, the spatial amplitude and phase profile of the filter were reconstructed from the synthesized temporal pulse shape and its spectrum using a simple iterative method. We also developed a design procedure for phase-only filters to synthesize both temporal phase and amplitude even for asymmetric pulse profiles of femtosecond laser pulses.

Timing jitter in a kilohertz regenerative amplifier of a femtosecond-pulse Ti:Al 2 O 3 laser

We measure the timing error of femtosecond pulses amplified by a Ti:sapphire regenerative amplifier at a 1-kHz repetition rate by use of a modified cross-correlation technique. Linearly frequency-chirped amplified pulses are frequency mixed with transform-limited oscillator pulses. A shift in the sum frequency corresponds to the timing error of each amplified pulse relative to the oscillator pulses. The timing error was measured every 6 ms with approximately 1-fs resolution over a measurable range of 400 fs.

Spatial-phase code-division multiple-access system with multiplexed Fourier holography switching for reconfigurable optical interconnection

A new, to our knowledge, space-variant optical interconnection system based on a spatial-phase code-division multiple-access technique with multiplexed Fourier holography is described. In this technique a signal beam is spread over wide spatial frequencies by an M-sequence pseudorandom phase code. At a receiver side a selected signal beam is properly decoded, and at the same time its spatial pattern is shaped with a Fourier hologram, which is recorded by light that is encoded with the same M-sequence phase mask as the desired signal beam and by light whose spatial beam pattern is shaped to a signal routing pattern. Using the multiplexed holography, we can simultaneously route multisignal flows into individually specified receiver elements. The routing pattern can also be varied by means of switching the encoding phase code or replacing the hologram. We demonstrated a proof-of-principle experiment with a doubly multiplexed hologram that enables simultaneous routing of two signal beams. Using a numerical model, we showed that the proposed scheme can manage more than 250 routing patterns for one signal flow with one multiplexed hologram at a signal-to-noise ratio of ~5.

Reduction of timing jitter with active control in a kHz regenerative amplifier of femtosecond pulse Ti: Al2O3 laser

We measured the timing error of femtosecond pulses amplified by a Ti:sapphire regenerative amplifier operated at a 1 kHz repetition rate using a modified cross-correlation technique. This technique can detect sub-femtosecond timing variation. By actively controlling the amplifier cavity length, we reduced the rms timing jitter of the regenerative amplifier into the sub-femtosecond range.

Accurate Pulse Shaping of Femtosecond Lasers Using Programmable Phase-Only Modulator

Accurate pulse shaping of femtosecond optical pulses is obtained using a 128-pixel liquid crystal spatial light modulator, which manipulates the spectral phase of the optical frequency components in a Fourier-transformed plane. The spectral phase shifts produced by the modulator can be programmed using a simulated annealing optimization code so as to obtain the desired pulse shapes. Significant improvement in the pulse shaping accuracy is achieved by increasing the number of gray levels in the spectral phase mask to 128. Several examples of pulse shaping operation, including high-repetition-rate pulse trains and asymmetric pulse trains, are described.

Timing stabilized regenerative amplifier with spectral-resolved cross-correlation technique

We modified a conventional cross-correlation technique for the timing fluctuation measurement of the pulses generated from a 1-kHz regenerative amplifier. By actively controlling the amplifier cavity length, we suppressed the rms timing jitter of the regenerative amplifier into the sub-femtosecond region.

Laser synchrotron femtosecond x-ray source for novel imaging

A description is given in this paper on the present progress of a compact laser synchrotron femtosecond x-ray source based on the inverse Compton scattering of high energy femtosecond laser pulses by high energy electrons. The present research program is reviewed by the target energy and number of the femtosecond x-ray photons for phase 1 (1996 - 2000) and phase II (2001 - 2004). Possible examples are considered for ultrafast imaging and pump- probe experiments by using this x-ray source.

Timing stabilization of the 1-kHz femtosecond pulses with active control by means of the spectral-resolved upconversion

We modified a cross-correlation technique for the timing error detection of the amplified pulses. By actively controlling the amplifier cavity length, we reduced the RMS timing jitter of the regenerative amplifier into the sub-femtosecond range.

50 kHz, 62μJ femtosecond pulse SHG at 389 nm from a single stage Ti:sapphire regenerative amplifier

We have demonstrated the second harmonic generation with output pulses of a 50 kHz Ti:sapphire regenerative amplifier. 280 fs pulses with an average power of 3.1 W were generated at 389 nm.