Yoshio Cho

High‐repetition‐rate optical pulse generator using a Fabry‐Perot electro‐optic modulator

A high‐repetition‐rate optical pulse generator which employs a Fabry‐Perot electro‐optic modulator as the output coupler of a laser resonator was constructed. Using this generator with a He–Ne 6328‐A laser tube, 21‐psec optical pulses at a repetition rate of 2.7 × 109 pps were experimentally obtained with an average power level of 0.5 mW. In addition, it is demonstrated that the width of the pulses obtained from this generator can be narrowed without regard to the gain‐linewidth limitation. This kind of pulse generator may be particularly useful for obtaining short optical pulses from low‐gain gas lasers, and it is possible to obtain 1010‐pps picosecond pulses from a He–Ne 6328‐A laser.

Extraction of quasi-straightforward-propagating photons from diffused light transmitting through a scattering medium by polarization modulation

The utilization of light polarization is proposed to extract quasi-straightforward-propagating photons from diffused light transmitting through a scattering medium under continuously operating conditions. Removal of a floor level normally appearing on the dynamic range over which the extraction capability is maintained is demonstrated. By use of pulse-based observations this cw scheme of extraction of quasi-straightforwardpropagating photons is directly shown to be equivalent to the use of a temporal gate in the pulse-based operation.

Optical waveguides using GaAs-AlxGa1−xAs multiple quantum well

Abstract For the purpose of using the multiple quantum well structure as an optical waveguide of nonlinear-optical devices, GaAs-Al x Ga 1- x As MQW structure is investigated theoretically. The composition ratios of the composing compound semiconductors can provide flexibility for the design of those devices. The numerically calculated result of the field distributions of the MQW waveguide structures showed that they can be approximated by replacing the MQW core section with a uniform material whose refractive index equals the root-mean-square value of the refractive indices of the MQW core section when the MQW structural (quantum wells and barriers) dimensions are less than ∼ 200 A for the near-infrared region.

Observation of chaos in a semiconductor laser with delayed feedback

Abstract By feeding the output from a semiconductor laser back onto its own output facet with an appropriate delay, the oscillation was able to be brought into a chaotic regime. Observed waveforms and accompanying spectra did not show distinct behavior to reach chaos, but the dimensionality test of the waveforms gave a strong evidence that the observed waveforms were chaos.

TM-mode propagation and form birefringence in a GaAs-AlGaAs multiple quantum well optical waveguide

Abstract On optical waveguides having GaAs-AlGaAs MQW structures, the linear optical guiding property is investigated theoretically, particularly on TM-modes. Dispersion properties and field distributions are calculated either exactly or in a three-layer slab waveguide approximation with a virtual uniform core section. The form birefringence property of the waveguide is discussed. The three-layer slab waveguide treatment gives sufficient approximations for GaAs-AlGaAs MQW optical waveguide having small MQW period.

An intensity/phase autocorrelator for the use of ultrashort optical pulse measurements

Abstract An autocorrelator capable of providing both the phase and the intensity correlations within a single run is described. In this autocorrelator, a mechanical scan scheme was employed, wherein an alternative repetition of slow and fast scans was used for separating the intensity and the phase correlations. The constructed autocorrelator was sufficiently stable to retain the phase correlation component. The simultaneous measurement of the phase and the intensity correlation can provide the possibility to determine quickly the existence of a transform-limited relation in an observed pulse.

Improved expression for the time-bandwidth product of picosecond optical pulses from gain-switched semiconductor lasers

An improved expression for the time-bandwidth product (DeltatDeltaf) of picosecond optical pulses from gain-switched semiconductor lasers was obtained by a series of numerical simulations based on rate equations without making any assumption for a particular pulse shape and chirp condition. The relation can be expressed by [Equation] as a function of the linewidth-enhancement factor alpha. The influence of the spontaneous emission factor beta on the relation is also examined.

Picosecond HEMT Pholodetector

A short and narrow gate GaAs/AlGaAs MESFET with a HEMT structure was tested as a picosecond photodetector. Its impulse response was measured by the autocorrelation technique and found as fast as 22 ps in FWHM.

Electrooptic-distributed Bragg-reflection modulators for integrated optics

The electrooptic-distributed Bragg-reflection modulators having periodically corrugated surfaces are proposed. Principles of distributed-feedback (DFB) and distributed Bragg-reflector (DBR) modulators are outlined and discussed. Calculations for the power per unit bandwidth indicate that the DBR modulator is much more effective than the DFB modulator.

Spin Relaxation of Electrons in Strained-GaAs-Layer Photocathode of Polarized Electron Source

The luminescence polarization method using a mode-locked Ti:sapphire laser and a streak camera is applied to the measurement of the spin relaxation time and the lifetime of electrons in the strained-GaAs-layer photocathode of a polarized electron source. The spin relaxation time and the electron lifetime are 105 ps and 45 ps at room temperature, respectively. Electron-hole scattering is thought to be the main mechanism of the spin relaxation of our strained-GaAs photocathode.