Takashi Kajimura

High Relaxation Oscillation Frequency (beyond 10 GHz) of GaAlAs Multiquantum Well Lasers

Direct modulation bandwidth of GaAlAs multiquantum well (MQW) lasers with 5 nm-thick GaAs wells was investigated. It was experimentally found that relaxation oscillation frequency of MQW lasers is beyond 10 GHz, which is twice that of double heterostructure lasers. This result was confirmed by theoretical analysis.

Mode-hopping noise in index-guided semiconductor lasers and its reduction by saturable absorbers

Mode-hopping noise in index-guided semiconductor lasers is investigated. It is found that random switching between lasing modes and output power differences in those modes cause mode-hopping noise. An effective method to suppress such mode-hopping noise is proposed. High Te doping to an n-type GaAlAs cladding layer completely suppresses the noise. Te in GaAlAs forms a DX center that acts as a saturable absorber. This property stabilizes the laser mode and prevents mode competition. The minimum loss difference between lasing and nonlasing modes to suppress mode-hopping noise is also discussed.

Low-noise AlGaAs lasers grown by organometallic vapor phase epitaxy

The noise behavior of self-pulsation lasers is investigated. It is shown that the pulsation frequency is an important factor in achieving low-noise characteristics under optical feedback. To obtain low-noise lasers, an AlGaAs laser which has a ridge-waveguide self-aligned structure grown by OMVPE (organometallic vapor-phase epitaxy) has been developed. Low-noise characteristics (relative intensity noise >

Transverse-mode stabilized Ga 1− xAlxAs visible diode lasers

Transverse-mode stabilized visible diode lasers in the 0.7-microm wavelength region are fabricated by growing a Ga(1-x)Al(x)As double heterostructure on a grooved GaAs substrate (channeled-substrate-planar structure). The diodes operate stably in the fundamental transverse mode and provide nonstigmatic laser beams. They can be collimated with a 0.5-1-mrad beam divergence using a simple graded-index fiber lens. Corresponding to single-longitudinal-mode operation at current levels above 1.1 times the threshold, a coherence length as long as 14 m is obtained in Michelson interference experiments.

High‐power operation of index‐guided visible GaAs/GaAlAs multiquantum well lasers

Stable transverse mode operation has been realized for the first time in visible (780 nm) multiquantum well lasers composed of seven 3‐nm‐thick GaAs wells separated by six 5‐nm‐thick Ga0.8Al0.2As barriers grown by metalorganic chemical vapor deposition. A self‐aligned structure with a built‐in optical waveguide to stabilize the transverse mode is fabricated by a two‐step epitaxial technique. Low threshold current (35 mA), high output power (up to 40 mW) in the fundamental transverse mode, and a very low degradation rate at 70 °C have been confirmed.

Leaky‐mode buried‐heterostructure AlGaAs injection lasers

Leaky‐mode buried‐heterostructure injection lasers are fabricated and operated successfully under pulse bias at room temperature. The lasers consist of a thin active layer confined by parallel n‐ and p‐AlxGa1−xAs layers with a high mole fraction of AlAs (x=0.4) and perpendicular p−‐AlyGa1−yAs layers with a low mole fraction of AlAs (y=0.25). It is shown that most of the laser power can be coupled out into the low‐loss perpendicular layers and emitted from the end facets, resulting in highly collimated beams with a beam divergence of about 1°.

Nonradiative dark regions along surface ripples in GaP LPE layers

A nonradiative dark region along a surface ripple is observed in optically and electrically excited GaP LPE layers. The region originates from the substrate interface and terminates at the ridge of the surface ripple. It is clearly distinguishable from the known one caused by dislocation in its features. The p‐n junction breakdown voltage in this region is lower than that in the radiative region in a LPE layer. This fact suggests that in liquid‐phase epitaxy an impurity concentrated region is formed in the growing process of surface ripples. The existence of the region contributes to the deterioration of the characteristics of devices using thin LPE layers.

Improved Self-Aligned Structure for GaAlAs High-Power Lasers

An improved self-aligned structure using two-step MOCVD technique is developed for GaAlAs high-power lasers. This structure is characterized by the introduction of a GaAlAs layer having low AlAs mole fraction at the regrowth interface to avoid the formation of native oxide film. The lasers operate at levels up to 100 mW in the stabilized fundamental transverse mode under CW operation at room temperature and show high reliability.

High-power 780 nm AlGaAs quantum-well lasers and their reliable operation

A high-power laser in the 780-nm wavelength region was realized by introducing an AlGaAs ternary alloy quantum-well structure. Since there is no axial structure, high reproducibility is expected. By applying the quantum-well structure to the AlGaAs ternary alloy wells, a reduction in the operation current of a laser which oscillates in the 780-nm wavelength region can be achieved. A reduction of the optical power density in the direction perpendicular to the junction plane is examined to lessen facet degradation. Investigations focus on the characteristic temperature and catastrophic optical damage (COD) level. It is found that a triple quantum-well (TQW) active layer structure has superior characteristics for high-power operation. Stable operation for over 1000 h under 50 degrees C and 60 mW conditions was achieved. A relatively high characteristic temperature of around 150 K was also obtained. The results of an aging test are provided. >

Transverse‐mode‐stabilized ridge stripe AlGaInP semiconductor lasers incorporating a thin GaAs etch‐stop layer

Transverse‐mode‐stabilized ridge stripe AlGaInP semiconductor lasers fabricated using a thin GaAs layer as an etch stop are reproducibly realized. The ridge stripe structure is fabricated utilizing the large difference in etching rates between GaAs/AlGaAs and AlGaInP layers. As a result, the thickness control of the cladding layer adjacent to light absorbing layers is significantly improved. Results show that the GaAs layer is effective as an etch‐stop layer even if the thickness is only 1–2 nm. The total internal loss of the lasers is estimated to be about 20 cm−1 and loss due to the insertion of the GaAs layer is almost negligible. A threshold current of about 50 mA and a stable fundamental transverse mode up to an output power of about 10 mW is attained at room temperature under cw operation. Similar results were obtained when an AlGaAs layer was used instead of GaAs.