Generation of squeezed vacuum pulses at 810 nm using a 40-cm-long optical fiber.

We experimentally demonstrate the generation of a squeezed vacuum pulse at 810 nm with a fiber polarization interferometer. During femtosecond laser pulse propagation through an optical fiber in the normal dispersion regime, only self-phase modulation within a short length contributes to pulse squeezing since the laser pulse is immediately broadened. Guided acoustic-wave Brillouin scattering (GAWBS) noise that increases in proportional to the fiber length is also lower with shorter fibers. Consequently, a maximum noise reduction of 2.1 dB (4.8 dB when corrected for losses) is obtained using a 40-cm-long single mode optical fiber.

Generation of squeezed pulses with a Sagnac loop fiber interferometer using a non-soliton femtosecond laser pulse at 800 nm.

We experimentally demonstrate generation of a squeezed vacuum at 800 nm with a Sagnac loop fiber interferometer. When negative dispersion is properly added to an input laser pulse to compensate for the fiber dispersion, the level of squeezing is improved. A squeezed vacuum of 0.45 dB is obtained at a dispersion of -0.0157 ps(2) for the 1.5 m-long fiber loop. Since the squeezed vacuum is degraded by guided acoustic-wave Brillouin scattering (GAWBS), the noise level of the squeezing is improved by -0.3 dB at a liquid nitrogen temperature. We also demonstrate generation of photon number squeezing at -1.3 dB.

Spatiotemporal vector pulse shaping of femtosecond laser pulses with a multi-pass two-dimensional spatial light modulator.

A novel non-interferometric vector pulse-shaping scheme is developed for femtosecond laser pulses using a two-dimensional spatial light modulator (2D-SLM). By utilizing spatiotemporal pulse shaping obtainable by the 2D-SLM, we demonstrate spatiotemporal vector pulse shaping for the first time.

Controlling the dissociative ionization of ethanol with 800 and 400 nm two-color femtosecond laser pulses.

Ethanol molecules were irradiated with a pair of temporally overlapping ultrashort intense laser pulses (10(13)-10(14) Wcm(2)) with different colors of 400 and 800 nm, and the dissociative ionization processes have been investigated. The yield ratio of the C-O bond breaking with respect to the C-C bond breaking was varied in the range of 0.17-0.53 sensitively depending on the delay time between the two laser pulses, and the absolute value of the yield of the C-O bond breaking was found to be increased largely when the Fourier-transform limited 800 nm laser pulse overlaps the stretched 400 nm laser pulse, demonstrating an advantage of the two-color intense laser fields in controlling chemical bond breaking processes.

Dissociative ionization of ethanol by 400 nm femtosecond laser pulses.

The dissociative ionization of ethanol in short-pulsed laser fields at approximately 400 nm is investigated. The yield ratio of the C-O bond breaking with respect to the C-C bond breaking increases sharply as the temporal width increases from 60 to 400 fs, and the yield ratio is two to three times as large as that at 800 nm in the entire pulse-width range of 60-580 fs. The enhancement of the C-O bond breaking of singly charged ethanol at 400 nm and the bond elongation prior to the Coulomb explosion of doubly charged ethanol occurring in the relatively weak light field intensity of 10(12)-10(13) W cm(2) is interpreted by the efficient light-induced coupling among the electronic states at the shorter wavelength of 400 nm. From the double pulse experiment, in which ethanol is irradiated with a pair of short pulses (<80 fs), the most efficient coupling occurs at Deltat=160 fs that is much earlier than Deltat=250 at 800 nm, where Deltat denotes the temporal separation of the two pulses, indicating that the nonadiabatic field-induced potential crossings of singly charged ethanol occurs much earlier at 400 nm than at 800 nm.

Chaotic on off keying for secure communications.

We experimentally demonstrate a chaotic on-off keying method for secure communications by using chaos synchronization in two microchip lasers. The output of the microchip laser in the transmitter is externally modulated with an acousto-optic modulator at ~4 MHz . One encodes a digital message in the chaotic carrier by turning the modulation on and off at 100 kHz. Because the accuracy of synchronization for the slave laser in the receiver tends to be degraded in the presence of external modulation in the injection laser signal, one can distinguish two binary states. The digital message can be recovered as an envelope of the chaotic oscillation when the difference between the two laser outputs of the transmitter and the receiver is calculated.

Optimization of ultrashort-pulse squeezing by spectral filtering with the Fourier pulse-shaping technique.

We propose an adaptive waveform-control approach to optimize photon-number squeezing by the nonlinear fiber spectral filtering method. More than -8-dB squeezing of the sech envelope pulse is numerically predicted when the spectral phase is modulated before the pulse is sent to a fiber. The quantum cross correlations of the photon-number fluctuation become uniformly negative when the pulse is optimally shaped.

Spatial phase information transmission through an optical fiber by coherence function synthesis.

Transmission of one-dimensional spatial phase information by low-coherence light through a single-mode optical fiber is experimentally demonstrated by use of space-time conversion at a 4-f Fourier coherence function shaper and time-space conversion with spectral holography. The dispersion during the fiber propagation can be automatically compensated for with spectral holography. However, space-time coupling caused by the transmitter limits the capacity of information transmittable with one coherence function shaping. A significant advantage in the space-time-space conversion with low-coherence light is that an infinite number of signal channels can be multiplexed with a newly invented delay-time division scheme, which can extend this analog transmission to two-dimensional spatial phase patterns.

Accuracy of chaos synchronization in Nd:YVO(4) microchip lasers

Synchronization of chaotic oscillations generated in two Nd:YVO(4) microchip lasers is experimentally and numerically demonstrated with master-slave coupling schemes. The synchronization performance under some parameter mismatch between the two lasers is quantitatively characterized. Synchronization is always achieved when the lasing frequency of the slave laser is matched to that of the master laser through injection locking. Accurate synchronization of chaos at an average intensity error of less than 2% is attained and maintained for tens of hours. The modulation parameters of the two lasers do not need to be matched for synchronization when the injection power is at a sufficiently high level, because chaos synchronization is based on its injection-locking performance. For accurate synchronization in multimode lasers, the power distribution among longitudinal modes needs to be matched.

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.

Stable mode-locking operation in a Cr:forsterite laser with a five-mirror cavity.

The experimental finding of more-stable mode-locking operation in a five-mirror cavity than in a conventional four-mirror cavity for a Cr:forsterite laser [IEEE J. Quantum Electron. 33, 1975 (1997)] was interpreted by ABCD-matrix formalism. Since the optimum cavity configuration operation for mode-locking operation was attainable in the middle of the stable cavity condition, we conclude that one can easily achieve KLM alignment and stable mode locking with a five-mirror cavity.

Experiments on chaos synchronization in two separate microchip lasers.

Synchronization of chaos generated in two Nd:YVO(4) microchip lasers is experimentally demonstrated with master-slave coupling schemes. For synchronization of chaos, precise locking of the sustained relaxation-oscillation frequency is required, as is optical frequency locking. One needs to match both the pump-modulation parameters for chaos generation and the laser parameters of the two lasers to generate perfectly synchronized chaotic spectra in the master-slave type I coupling scheme.

Low-coherence interferometry with synthesis of coherence function.

Synthesis of a coherence function by manipulation of the spectral phase of low-coherent light with a segmented liquid-crystal phase modulator and its application in a low-coherence interferometry are described. Effects of space-time coupling caused at diffractive gratings and second-order dispersion at the spatial light modulator on the coherence function synthesis are theoretically and experimentally verified. Various coherence functions can be shaped with phase-only masks designed by simulated annealing optimization algorithm. We utilized this technique for a novel optical low-coherence reflectometry without any mechanical movement for scanning optical delay.

Suppression of chaotic oscillations in a microchip laser by injection of a new orbit into the chaotic attractor.

Suppression of chaotic instability arising in a Nd:YVO(4) microchip laser subject to frequency-shifted optical feedback is accomplished by injection of one of the periodic orbits into the bifurcation region of another chaotic system driven by pump modulation. Various periodic patterns, which do not exist in the original chaotic attractor, can be extracted from the chaotic oscillation by use of this nonfeedback chaos-control technique.

Synchronous dual-wavelength operation of a self-injection-seeded narrow-linewidth flash-lamp-pumped Q-switched Ti:Al2O3 laser.

Perfect synchronization of two different wavelength oscillations that are tunable within a range of 770-820 nm is obtained from a self-injection-seeded Q-switched flash-lamp-pumped Ti:Al(2)O(3)laser. In spite of a significant time difference (as much as 2 micros) between the two seed pulses generated in the master oscillator, a common Pockels cell switch synchronizes the two injection-seeded Q-switched pulses growing from spatially separated slave oscillators in a common laser rod within a delay time of <3 ns , depending on the combination of the two wavelengths. A sum output energy of approximately 41 mJ/pulse was obtained for a pair of 780- and 810-nm pulses with a typical pulse width of approximately 30 ns (FWHM).