Akira Takemoto

1.3- mu m distributed feedback laser diode with a grating accurately controlled by a new fabrication technique

The coupling constant that determines the characteristics of distributed feedback laser diodes (DFB LDs) is controlled by employing the metalorganic chemical vapor deposition (MOCVD) technique and inserting a barrier layer between the active layer and the guiding layer. It is shown that the measured coupling constant is in good agreement with the designed coupling constant. Lasers with a small coupling constant have a large slope efficiency. Lasers with the above structure are expected to have a long life, comparable to that of conventional DFB LDs. >

Distributed feedback laser diode and module for CATV systems

Harmonic distortion of distributed feedback laser diodes (DFB-LDs) for analog transmission systems is investigated. It is shown that, under a modulation frequency of less than 1 GHz, the harmonic distortion depends on the nonlinearity of the light output power-current (P-I) curve under the continuous wave (CW) condition, which is determined by the coupling constant kappa L, and that the distortion can be minimized at kappa L approximately 1. A 1.3 mu m wavelength InGaAsP DFB-PPIBH (p-substrate partially inverted buried heterostructure) LD and its module, with low distortion by the control of a coupling constant, have been developed. >

0.78- and 0.98-/spl mu/m ridge-waveguide lasers buried with AlGaAs confinement layer selectively grown by chloride-assisted MOCVD

The 0.78- and 0.98-/spl mu/m buried-ridge AlGaAs laser diodes (LDs) with a high Al-content AlGaAs confinement layer selectively grown by using a Cl-assisted MOCVD are demonstrated. By employing the AlGaAs confinement layer, the threshold current and the slope efficiency of the 0.78-/spl mu/m LD are improved by /spl sim/40%, compared to those of the conventional loss-guided LD with the GaAs confinement layer. In addition, the stable fundamental mode up to 150 mW and the small astigmatic distance less than 1 /spl mu/m are obtained. The 0.78-/spl mu/m LD also shows the excellent high-power and high-temperature characteristic such as 100 mW CW operation at 100/spl deg/C and the reliable 2,000-hour operation under the condition of 60/spl deg/C and 55 mW. In the 0.98-/spl mu/m LD, the narrow beam with the low aspect ratio of 1.86 and the stable fundamental transverse mode over 200 mW are exhibited. As a result, the 0.98-/spl mu/m LD realizes the high fiber-coupled-power of 148 mW. Moreover, the high-power and high-temperature operation of 150 mW at 90/spl deg/C is obtained. In the preliminary aging test, the LDs have been stably operating for over 900 hours under the condition of 50/spl deg/C and 100 mW. >

Analysis of characteristic temperature for InGaAsP BH lasers with p-n-p-n blocking layers using two-dimensional device simulator

The threshold current and the characteristic temperature of 1.3-/spl mu/m InGaAsP-InP buried heterostructure (BH) lasers with the p-n-p-n blocking layers have been numerically analyzed using a two-dimensional (2-D) device simulator. The simulation model includes optical gain, intervalence absorption, radiative spontaneous-emission current. Auger recombination current, Shockley-Read-Hall recombination current, and heterobarrier leakage current. In addition to these components, the leakage current flowing through the p-n-p-n blocking layer which was ignored so far is also included. The analysis of the current components reveals that the increase in the threshold current with temperature is due to Auger recombination and the leakage current through the p-n-p-n blocking layer. The calculated T/sub 0/ value containing all the components is 54 K at room temperature and 29 K above 80/spl deg/C, which is consistent with observed T/sub 0/ values. When the leakage current through the p-n-p-n blocking layer is ignored in the calculation, the T/sub 0/ value is improved to 90 K and a decrease in the T/sub 0/ value is not observed. This result is consistent with conventional calculations. When Auger recombination is ignored. the T/sub 0/ value increases to 110 K at room temperature. However, the threshold current increases beyond the exponential relationship I/sub th/=I/sub 0/ exp(T/T/sub 0/) and the T/sub 0/ value decreases to 34 K at high temperature. This is due to a large increase rate of the leakage current through the p-n-p-n blocking layer. The reduction of Auger recombination is effective in decreasing the threshold current while the reduction of the leakage current through the p-n-p-n blocking layer is effective in improving T/sub 0/ values at high temperature, since T/sub 0/ values correspond to the increase rate of the threshold current.

Wavelength dependence of characteristics of 1.2-1.55 mu m InGaAsP/InP p-substrate buried crescent laser diodes

Laser diodes with the p-substrate buried-crescent structure have been fabricated for the 1.2-1.55- mu m wavelength region. The dependence of laser characteristics on wavelength has been measured. Up to 70 degrees C, the increasing rates of the threshold current with temperature are similar, while, above 70 degrees C, a shorter-wavelength laser shows a larger increasing rate. At the same full width at half maximum of the far-field pattern perpendicular to the junction plane, the external differential quantum efficiency of the 1.55- mu m laser diode is only 10% smaller than that of the 1.3- mu m laser. The absorption loss coefficients in the active layer of the 1.2-, 1.3-, and 1.55- mu m laser are estimated to be 26, 34, and 73 cm/sup -1/, respectively. >

Theoretical and experimental analysis of leakage current in InGaAsP BH lasers with p-n-p-n current blocking layers

The dependence of the leakage current in 1.3-/spl mu/m InGaAsP buried heterostructure (BH) lasers with p-n-p-n current blocking layers on well number, mesa width, and carrier density has been analyzed using a two-dimensional device simulator and compared with the electroluminescence (EL) emitted from InP layers. The analysis of the minority carrier flow reveals that the electron current flowing through the p-n-p-n current blocking layers is the dominant component of the leakage current. The measured EL intensity has two peaks at both sides of the n-blocking layer apart from the active layer. The EL intensity decreases with increasing well number and carrier density of the p-blocking layer, and increases with increasing mesa width. These results are consistent with the simulations.

1.3-/spl mu/m strained MQW-DFB lasers with extremely low intermodulation distortion for high-speed analog transmission

The intermodulation distortion and the noise characteristics of 1.3-/spl mu/m strained multiquantum-well distributed feedback (MQW-DFB) lasers have been investigated under the modulation frequency of 1.9 GHz in connection with the device structure. In this study, a strained MQW with strain-compensated layers has been introduced in order to increase in the quantum-well number and well width. This causes increase in the differential gain, resulting in increase of the resonance frequency (FR). The FR as high as 5.1 GHz/mW/sup 1/2/ has been obtained which is in good agreement with the theoretical calculation. In addition to the strained MQW structure, a new buried heterostructure entirely grown by MOCVD, named as FSBH (facet selective growth buried heterostructure), has been developed to minimize the leakage current which degrades L-I characteristics at high bias current causing the high distortion. The third-order-intermodulation distortion (IMD3) of -88 dBc and relative intensity noise (RIN) of -152 dB/Hz have been obtained under a two-tone test at 1.9 GHz. This suggests that this newly developed laser is quite suitable for high-speed-subcarrier multiplexing transmission.

Structural and electrical characterization of oxidated, nitridated and oxi-nitridated (100) GaAs surfaces

Oxidation by the UV & ozone process, nitridation by the nitrogen helicon-wave-excited plasma process, and the combination of these processes are applied to (100) GaAs wafers. An atomic force microscope, X-ray photoelectron spectroscopy, a transmission electron microscope, photoluminescence and electrical characteristics (current–voltage and capacitance–voltage) were used to analyze the influences of these processes on the structure and composition of the surfaces and the interfaces. Metal–insulator–semiconductor (MIS) diodes and Schottky diodes were fabricated in order to investigate the electrical influences of these processes. The oxidation slightly disorders GaAs surfaces. Nitridation of a bare surface creates about a 2-nm-thick strongly disordered layer, which strongly deteriorates the electrical and photoluminescence characteristics. Nitridation of oxidated wafers (oxi-nitridation) forms firm amorphous GaON layers, which contain GaN, with very flat and sharp GaON/GaAs interfaces, where crystal disorder is hardly observed. It improves the current–voltage (I–V) and capacitance–voltage (C–V) characteristics and the photoluminescence intensity. Results of the structural and the electrical characterizations qualitatively coincide well with each other.

Narrow-beam and power-penalty-free 1.3-/spl mu/m laser diodes with monolithically integrated waveguide lens formed by selective-area epitaxial growth

A narrow-beam has been realized in a 1.3 /spl mu/m Fabry-Perot laser diode monolithically integrated with a tapered waveguide lens. The beam divergences in the perpendicular and horizontal directions are reduced down to 12/spl deg/ and 11/spl deg/ by a selective area epitaxial growth technique. The threshold current has been kept as low as 14 mA comparable to the conventional ones. Neither kinks in the L-I curves nor changes of far-field patterns are observed in the wide temperature range from -40 to 80/spl deg/C. Furthermore, high cut-off frequency over 4 GHz and power penalty-free characteristic under 622 Mb/s-50 km transmission have been confirmed.

Extremely narrow spectral linewidth and low chirping of the MQW-DFB-PPIBH laser diode

The low threshold current of 9 mA, the high side-mode suppression ratio of more than 45 dB, the extremely narrow spectral linewidth of 1.1 MHz, and the low chirping of 2.8 AA at -20 dB at 2 Gb/s nonreturn to zero (NRZ) modulation have been achieved in the multiple quantum well (MQW) distributed feedback (DFB) p-substrate partially inverted buried heterostructure (PPIBH) laser diode. The spectral linewidth of 1.1 MHz is the narrowest value among 300- mu m-length solitary laser diodes. These results suggest that the MQW-DFB laser diodes are a promising light source for longer distance and higher bit-rate optical communication systems and coherent optical communication systems. >