Shoji Hirata

Effective speckle reduction in laser projection displays

Two alternative methods for effectively reducing the speckle noise in laser based rear and front projection displays are studied and demonstrated. Firstly, widening of the laser emission spectrum to nearly 4nm by tailoring the structures of 640nm band AlGaInP/GaInP based edge emitting broad-area red laser diode arrays, realized a reduction of speckle contrast from 20% to an acceptable level of 5%. Designing the power/wavelength distribution of the multiple emitters to achieve a flat top profile for the total spectrum plays an important role in maximizing the speckle reduction effect. This approach could easily be adapted to laser diodes arrays of other materials or wavelengths. Secondly, relative oscillation movement within the screen layers can also reduce the speckle contrast to the acceptable level of 5%. The moving layer material and the oscillation period has been studied and optimized to maximize the reduction level for both rear and front projection configurations. The two speckle reduction techniques were evaluated on a front projector utilizing a red laser assisted UHP lamp hybrid light source, and showed significant enhancement in view ability by the suppression of speckle. Combining the two techniques lead to further reduction in speckle and scintillation noise to a supreme level of 2%, enabling a practical solution for wide color gamut and high efficiency laser projection displays.

AlGaAs/GaAs distributed feedback laser diodes grown by MOCVD

Abstract AlGaAs/GaAs distributed feedback (DFB) laser diodes emitting at 880 and 760 nm were fabricated by a two-step MOCVD growth technique. Epitaxial growth was carried out in a conventional atmospheric-pressure MOCVD system with a vertical reactor. A mesa-stripe geometry and separate-confinement heterostructure (SCH) was employed for the laser structure. The cladding layer ( x = 0.35, 0.48) was successfully regrown on the guiding layer ( x = 0.15, 0.25) corrugated with a 0.25 μm period (2nd order grating). Continuous wave (CW) operations were achieved up to 53°C for devices emitting at 880 nm and up to 10°C for devices emitting at 760 nm. A threshold current as low as 55 mA was obtained for a device emitting at 880 nm under CW operation at room temperature.

A submilliampere-threshold multiquantum-well AlGaAs laser without facet coating using single-step MOCVD

An extremely low threshold current, of 0.88 mA under continuous wave (CW) operation was obtained for a three-quantum-well AlGaAs-GaAs laser without facet coating at room temperature. This laser was fabricated using only single-step metalorganic chemical vapor deposition (MOCVD) on a nonplanar GaAs substrate. The energy conversion efficiency from input electric power to light output power was 42% at 1 mW/facet, which is the highest value for all types of lasers. The laser beam shape was nearly round with an aspect ratio of 0.86. >

Highly reliable 7 W operation of 644 nm wavelength laser diode arrays with top-hat near field pattern

AlInP/GaInP 7 W laser arrays with a uniform intensity distribution and a low smile of 1 /spl mu/m have been developed for a display system. The estimated lifetime exceeds 10000 hours under CW operation.

Monolithic-integrated two-wavelength laser diodes for digital-versatile-disk/compact-disk playback

We have developed a monolithic two-wavelength laser diode, which emits 650 and 780 nm wavelengths. This device, which has a separated-confinement-heterostructure multi-quantum-well active region and a gain-guiding tapered-stripe structure, is fabricated using only two steps of metal organic chemical vapor deposition. The operating currents at 5 mW are 57.0 and 61.5 mA for the 650 and 780 nm elements, respectively. The relative intensity noise of the 650 and 780 nm elements was below −130 dB/Hz up to 70 °C without high-frequency superposition circuits.

Growth model of coupled-fractal networks

We extended the dielectric breakdown model in order to generate networks of interacting objects, which are considered as coupled-fractal networks. In the proposed model, each object grows to be a random fractal if isolated, but connects with others if glued. The model is constructed based on a growth hypothesis in which the growth rate of each object is a product of the probability of receiving source materials from faraway and the probability of receiving adhesives from other grown objects.

AlGaAs/GaAs distributed feedback lasers with first‐order grating fabricated by metalorganic chemical vapor deposition

Distributed feedback AlGaAs/GaAs lasers with first‐order grating (1295 A) were fabricated and successfully operated under continuous wave conditions at room temperature. A coupling coefficient of 115 cm−1 and a threshold current of 47 mA were obtained. A stable single‐mode oscillation was maintained up to 100 °C.

Effect of the threshold reduction on a catastrophic optical mirror damage in broad-area semiconductor lasers with optical feedback

We experimentally and theoretically studied degradation phenomena and their mechanism in broad-area semiconductor lasers (BA-LDs) with optical feedback (OFB). We made two types of BA-LDs (one is consisted of AlGaAs emitting at 808nm in TE mode, and another one is consisted of AlGaInP emitting at 642nm in TM mode), and investigated conditions of the degradations caused by an optical feedback. The both types of BA-LDs showed degradations depending on feedback rate and output power. For example, both BA-LDs were damaged with about 20% of intensity feedback rate at half of an output power of a catastrophic optical mirror damage (COMD) levels. To describe a theoretical model for the degradation, the optical power at a front facet of the BA-LDs was calculated and compared with the COMD level of the solitary BA-LDs. In the theoretical model, we included a threshold reduction caused by the OFB. We found that the degradation was explained by a constructive interference between internal and the feedback optical fields. The BA-LDs are damaged when a coherent sum of those fields exceeds the solitary COMD level. We found that the threshold reduction decreases a critical value of the feedback rate corresponding to the damage at low output power regime, and also found that there is an optimum reflectivity of the front facet. The theoretical results show a good agreement with experimental results. According to this model, we can avoid the damages induced by the OFB in the various applications.

Low threshold current AlGaAs/GaAs rib-waveguide separate-confinement-heterostructure distributed-feedback lasers grown by metalorganic chemical vapor deposition

AlGaAs/GaAs rib-waveguide separate-confinement-heterostructure (SCH) distributed-feedback (DFB) lasers emitting at 880 nm were fabricated by a two-step atmospheric pressure metalorganic chemical vapor deposition (MOCVD) growth technique. A CW threshold current as low as 18 mA and an output power of more than 10 mW per facet at room temperature were obtained. Also, single longitudinal and fundamental transverse modes were maintained up to more than twice the threshold current level.

Characteristics of red-emitting broad area stripe laser diodes with zinc diffused window structures

We have applied zinc diffused window structures to 640 nm broad area stripe laser diodes (BALDs) for the first time. A solid-phase zinc diffusion technique was used for a thick single quantum well (SQW) in GaInP employing the short wavelength and disordered active layer possessed a blue shift of 58 nm in photoluminescence spectrum. We fabricated 10 mm arrays including twenty-five BALDs and each BALD consists of a 60 μm ridge stripe and a 1000 μm cavity. An initial catastrophic optical damage (COD) level of the window laser was increased by four times of a conventional none-window laser. A long-term reliability under automatic current control was investigated for initial output powers of 13W and 15W which overcome a previous demonstration of 7.2 W. Measured degradations within a period of 1000-hours were 5 % or less, in contrast a half-life period of our conventional none-window laser with an initial output power of 10 W was only 120-hours. Therefore the window structure improved the BALD in terms of the COD level and the long-term reliability.