Hideyo Higuchi

The 1/f Fluctuation of a Traffic Current on an Expressway

It is observed that the fluctuation of a traffic current on an expressway obeys the 1/f law for low spectral frequencies. Under proper assumptions the variation of the car concentration is expressed by the Burgers nonlinear differential equation. The observed power spectrum is accounted for as a characteristic feature of the Burgers turbulence.

Traffic Current Fluctuation and the Burgers Equation

The car concentration on an expressway must obey the Burgers equation if the concentration is linearly related to the drift speed. The power spectral density of the random Burgers flow was numerically evaluated based on this model and compared with the observed spectrum of the car flow. They are in approximate agreement.

Low threshold InGaAsP/InP buried crescent laser with double current confinement structure

An InGaAsP/InP laser diode emitting at 1.3 μm with a crescent shaped active region is described. The active region is completely embedded in InP by a two-step LPE technique, and a double current confinement scheme is incorporated with two reverse biased p-n junctions at both sides of the active layer. A threshold current as low as 20 mA has been achieved in CW operation at room temperature. Fundamental transverse mode operation with linear light output-current characteristics and single longitudinal mode oscillation have been obtained.

InGaAsP/InP buried crescent laser diode emitting at 1.3 µm wavelength

The fabrication procedure, designing of an active region and a p-n-p-n current blocking structure, characteristics and the aging results of an InGaAsP/InP buried crescent (BC) laser diode are described. The BC laser diodes exhibit high laser performances, such as a low-threshold current, a fundamental transverse mode oscillation with linear light output-current characteristics. CW operation at as high as 100°C is achieved with a junction up configuration as a result of the improvement in the current blocking structure. A stable CW operation at 80°C has been realized with a constant optical output power of 5 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. >

Position of the degradation and the improved structure for the buried crescent InGaAsP/InP (1.3 μm) lasers

We have found a degradation of the buried crescent (BC) InGaAsP/InP lasers that occurs when the p‐n junction plane coincides with the surface exposed in the high‐temperature H2 ambient before the melt contact during the liquid phase epitaxial growth. To eliminate the degradation, we have fabricated a new structure of the BC laser and have obtained stable cw operation at 80 °C.

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 uncooled DFB lasers with low distortion for CATV application

Strained-layer multiquantum-well distributed-feedback (MQW-DFB) lasers at a wavelength of 1.3 /spl mu/m operating from -40/spl deg/C to 85/spl deg/C without any coolers are demonstrated. Extremely low threshold current of 17 mA at 85/spl deg/C and operation current of 37 mA at 5 mW and 85/spl deg/C are obtained. To the best of our knowledge, each of them is the lowest value so far reported in InGaAsP-InP based DFB lasers. The lasers also realize low distortion of less than -50 dBc at 65/spl deg/C in 78-channel composite second order distortion (CSO) measurements. An experimental estimation of the leakage current in buried heterostructure presents that the distortion at high temperatures is mostly dominated by the carrier overflow from an active layer, not by the leakage current flowing through current blocking layers. On the basis of the leakage current analysis, a laser structure including the active layer and the current blocking layer is chosen to achieve low distortion over a wide-temperature-range. The uncooled distributed-feedback (DFB) laser we have developed is very useful for several channel transmission in CATV systems.

Internal Loss of InGaAsP/InP Buried Crescent (λ=1.3 µm) Laser

The temperature dependence of the internal loss α of the InGaAsP/InP buired crescent (λ=1.3 µm) laser is presented in the temperature range 20~80°C. α is about 18 cm-1 and no apparent temperature dependence of α is observed in this temperature range. This indicates that the temperature-sensitive behavior of the threshold current in 1.3 µm laser is not due to the internal loss. The decrease in the external quantum efficiency with increasing temperature is attributed to the decrease in the internal quantum efficiency, which is possibly caused by some leakage current.