Hirokazu Shimizu

A 0.2 W CW laser with buried twin-ridge substrate structure

An extremely high output power has been obtained with a new structure laser named the buried twin-ridge substrate (BTRS) laser. The very thin active layer formed on a ridged substrate permitted high power output increasing the catastrophic damage level. The buried stripe formed with a blocking layer remarkably improved the current confinement lowering the threshold current. A multilayer coating technique was applied to both facets to increase the front facet output. Fundamental transverse mode is achieved at more than 100 mW in CW with an uncoated laser while the maximum output power attained is as high as 200 mW in CW operation with a multicoated laser.

Low-noise operation of a diode-pumped intracavity-doubled Nd:YAG laser using a Brewster plate

The authors demonstrate noise reduction which has been attained in a diode-pumped intracavity-doubled Nd:YAG laser by using a novel birefringent filter formed by a Brewster plate and a birefringent KTP crystal. It is confirmed that single longitudinal mode operation is obtained by the birefringent filter. As a result, the relative intensity noise obtained is less than -135 dB Hz. This noise reduction technique should play an important role for application of laser-diode pumped intracavity-doubled solid-state lasers. >

Highly-reliable CW operation of 100 mW GaAlAs buried twin ridge substrate lasers with nonabsorbing mirrors

Long-life GaAlAs lasers with 100-mW fundamental-transverse-mode operation are discussed. This excellent degree of operation has been attained by using nonabsorbing mirrors for suppression of mirror degradation and a twin-ridge substrate structure for stabilization of the transverse mode. It has been found that under 100 mW CW operation at 50 degrees C, degradation is insignificant even after 6000 h. >

A novel high-power laser structure with current-blocked regions near cavity facets

A novel high-power GaAlAs laser structure has been developed. In the new structure, current-blocked regions are formed near both facets for suppressing local temperature rise. Furthermore, the active layer is made thin only in the vicinity of the facets in order to enlarge the spot size without significant increase of the operating current. The experimentally fabricated laser with the new structure exhibited a COD power density 1.4 times higher and a degradation rate 1/2 times lower than those of the conventional structure lasers.

Improvement in operation lives of GaAlAs visible lasers by introducing GaAlAs buffer layers

It is shown that drastic reduction of the active-layer stress can be obtained by the introduction of a GaAlAs buffer layer in GaAlAs visible lasers. The life tests of the GaAlAs visible lasers with the optimally designed buffer layers were carried out at 50°C in a dry nitrogen ambient. The results indicate that the introduction of the optimal buffer layer leads to a reduction in the stress induced degradation of GaAlAs visible lasers.

Very low threshold visible TS lasers

Fundamental transverse mode lasers with very low threshold currents have been realized by use of the modified terraced substrate (TS) geometry. The threshold currents of 250 μm cavity length lasers at room temperature are typically 20-30 mA for wavelengths around 750 nm. In the 710 nm laser, which is the shortest lasing wavelength in this work, the threshold is as low as 70 mA in CW operation. The 726 nm laser was operated for only 150 h, while a lifetime longer than 10 000 h is expected for TS lasers with a lasing wavelength longer than 750 nm.

Fundamental transverse and longitudinal mode oscillation in terraced substrate GaAs-(GaAl)As lasers

A new, simple structure for a stripe-geometry laser is proposed. A double heterostructure is fabricated on a terraced substrate, providing a modified rib-waveguide structure in the active layer. A lasing mode is confined and stabilized in the narrow area between two adjacent bends of the active layer. The terraced substrate laser exhibits a stable, single longitudinal mode oscillation as well as a fundamental transverse mode oscillation in CW operation. No kinks have been observed in light-output versus current characteristics. The linearity continues up to ten times the threshold in the pulsed operation at which the power output is 150 mW per facet.

A new chemical etching technique for formation of cavity facets of (GaAl)As lasers

A new chemical etching technique which offers excellent cavity facets of Ga1-xAlxAs lasers is reported. This technique is based on our finding that the crystallographic anisotropy in the conventional etching process of Ga1-xAlxAs multilayers depends strongly on the AlAs mode fractionxin every layer. A suitable combination of the mole fractions in the multilayer is therefore a key factor for obtaining practically vertical walls with sufficient smoothness and flatness as laser cavity facets. In fact, the reflectivity of the etched facet obtained is 28 percent, being compatible to that in the conventional cleaved facets. As a result, a CW operation with threshold current as low as 28 mA and external quantum efficiency as high as 24 percent per facet has been attained with high reproducibility.

A high-power GaAlAs superluminescent diode with an antireflective window structure

A novel high-power GaAlAs superluminescent diode (SLD) structure that introduces an antireflective (AR) window region into the rear side of the SLD is proposed. The light beam which travels backward is emitted from the edge of the active layer and diverges in the window region. Then the beam is reflected at the AR-coated rear facet only by a small percentage, a fraction of which couples into the active layer. Thus, this window structure gives a reduction of the reflectivity at the interface between the active layer and the window region so that lasing oscillation is successfully suppressed. An SLD operation of output power as high as 50 mW is obtained with a stable fundamental spatial mode. The spectral bandwidth at half maximum is about 15 nm over a wide output power range. >

50 mW stable single longitudinal mode operation of 780 nm GaAlAs DFB laser

The achievement of stable single-longitudinal-mode (SLM) operation of a 780 nm GaAlAs distributed-feedback (DFB) laser with output power as high as 50 mW is discussed. The laser employs the buried twin-ridge substrate structure which allows stable fundamental spatial mode operation even at high power levels. The designed coupling strength is 0.5 from the viewpoint of obtaining a low operation current at 50 mW. SLM operation was maintained for powers up to 50 mW at room temperature and in the temperature range from -17 to 37 degrees C at 50 mW. The maximum power attained was 62 mW. >