Koichi Hamanaka

Continuous wave operation of a surface‐emitting AlGaAs/GaAs multiquantum well distributed Bragg reflector laser

The first cw operation of an AlGaAs/GaAs distributed Bragg reflector laser was achieved at room temperature with a threshold current as low as 38 mA. Surface emission exceeding 4 mW was obtained with an external differential quantum efficiency of 9% and a beam divergence of 0.17°×10°. A two‐dimensional laser array with 3×4 gratings was also fabricated and an output power exceeding 500 mW was obtained under pulsed condition.

Laser Doppler velocimeter with a novel optical fiber probe

A practical laser Doppler velocimeter with optical fibers in the whole system was developed. The novel optical probe designed for this LDV is constructed of a graded-index rod lens attached to the end of an optical fiber. Since the laser beam from the probe is well collimated, the velocity accuracy and sensitivity are significantly improved. Mechanical vibration measurements were also carried out with this LDV; vibration amplitude down to 1.0 microm p-p can be measured at a frequency of 120 Hz with high accuracy.

Edge‐ and surface‐emitting distributed Bragg reflector laser with multiquantum well active/passive waveguides

A novel AlGaAs/GaAs distributed Bragg reflector laser utilizing a multiquantum well structure both as an active waveguide and as a low loss passive waveguide was fabricated. The threshold current was 102 mA at room temperature, and the differential quantum efficiency for the edge‐ and surface‐emitted light was 16% and 3.7%, respectively. No mode hopping was observed in a temperature range of 35 K. A 0.17°×17° far‐field pattern was obtained for the surface‐emitted light.

Widely separate wavelength switching of single quantum well laser diode by injection-current control

It is demonstrated for the first time that by injection‐current control, the lasing wavelength was able to be switched from the lowest (n=1) quantized state transition to the second (n=2) transition for a single quantum well laser diode. The separation in energy was as wide as >50 meV. This novel function of a laser was realized as a result of appropriately increased threshold gain. This diode, moreover, acted as a multiple wavelength‐emitting laser.

cw surface‐emitting grating‐coupled GaAs/AlGaAs distributed feedback laser with very narrow beam divergence

Room‐temperature cw operation of a GaAs/AlGaAs surface‐emitting grating‐coupled distributed feedback laser is reported. By using a transverse junction stripe scheme which provides a window structure for the surface‐emitted light, cw surface emission having a very narrow beam divergence angle of 0.13° and an output power of more than 3 mW was observed.

(11n)-oriented BiSrCaCuO thin film formed on SrTiO3(110) substrate by RF magnetron sputtering

A superconducting BiSrCaCuO thin film was grown on a SrTiO3(110) substrate at 660°C by sputtering a single target. The RHEED observation showed that the c-axis of the film meets at an angle of about 42° with the substrate surface. The XRD measurement showed that the film has a main peak at 2 θ=31.2°. From these results, the film is considered to chiefly consist of a 2212 phase with (117) orientation and/or a 2212/2223 superlattice phase with (118) orientation. The film surface has a stripelike texture which probably corresponds to the film orientation. The zero-resistivity temperature of the film was lower by 20 K than that of the film formed on the SrTiO3(100) substrate.

Ultrafast switching characteristics of a bistable surface‐emitting multiple quantum well distributed Bragg reflector laser

We describe an ultrafast switching operation of a bistable surface‐emitting distributed Bragg reflector laser. The rise time was as small as 12 ps and the fall time was 90 ps. Both are much smaller than those of conventional bistable laser diodes. Ths was realized by the effect of the multiple quantum well structure and a strong detuning.

Reduction of spectral linewidth in AlGaAs/GaAs distributed feedback lasers by a multiple quantum well structure

We demonstrate the reduction of the spectral linewidth in AlGaAs/GaAs distributed feedback lasers by a multiple quantum well structure for the first time. The spectral linewidth‐output power product was 54 MHz mW and was eight times smaller than that of conventional double heterostructure distributed feedback lasers. The narrowest spectral linewidth was 10.5 MHz at 5 mW.

Ridge waveguide AlGaAs/GaAs distributed feedback lasers with multiple quantum well structure

Ridge waveguide AlGaAs/GaAs distributed feedback lasers with multiple quantum well structure were fabricated for the first time. The threshold current of 28 mA, which is the lowest ever reported among AlGaAs/GaAs distributed feedback lasers, was obtained at room temperature. Stable single longitudinal and transverse mode oscillation was observed over the wide temperature range. The dynamic linewidth was also measured and it was five to six times smaller than that of a double heterostructure distributed feedback laser.

Surface morphology and crystal structures of as-grown BiSrCaCuO thin films prepared by RF magnetron sputtering from three targets

As-grown superconducting BiSrCaCuO thin films were grown at 650°C on MgO(100) substrates by RF magnetron sputtering from three targets. The substrate was rotated over the three targets: Bi3Sr2Ca2Cu3Ox, Bi2Sr3Ca3Cu3Ox and Bi2Sr2Ca2Cu4.5Ox. By adjusting the staying time of the substrate over each target, it was possible to precisely control the chemical composition of the thin film. This paper describes the superconducting properties, surface morphology and crystal structures of sputtered films formed by varying the staying time. X-ray diffraction measurement showed that the crystal structure could be changed by varying the staying time. The film with the optimum composition showed zero resistivity at 75 K and had an exceedingly smooth surface. This film had the mixed phase which is considered to be a stacked structure consisting of a high-Tc phase and a low-Tc phase.