Katsuhito Shima

Transmission characteristics of DFB laser modules for analog applications

In this paper analog transmission characteristics of highly linear 1.3- and 1.55-μm distributed feedback (DFB) laser modules are described. To obtain high-power and high-performance DFB lasers, the bias dependence of distortion characteristics is discussed. The carrier-to-noise ratio, the composite second-order distortion, and the composite triple beat of the DFB laser module which is developed for analog CATV applications are presented. The analog transmission characteristics, such as CNR degradation, after fiber transmission, dispersion-induced distortion, and the influence of reflection, have been made clear. The stable high linearity of the DFB laser also has been confirmed by the composite distortion measurement under an accelerated aging test.

Catastrophic degradation level of visible and infrared GaAlAs lasers

Burn‐off output powers of catastrophic failures have been studied for GaAlAs double heterostructure (DH) lasers with facet coatings in the wavelength range of 750–870 nm. It was found that the burn‐off power density does not depend on the lasing wavelength when the laser is operated under pulsed condition, whereas it decreases slightly as the wavelength becomes shorter under cw condition.

Buried convex waveguide structure (GaAl) As injection lasers

A new channeled substrate (GaAl)As double‐heterostructure laser with mode control as well as internal current confinement is described. The narrow active region (3–3.5 mm) surrounded by GaAlAs is buried in the etched channel, around which a reverse‐biased heterojunction is formed. The threshold current is as low as 20 mA cw, and highly stable fundamental‐mode lasing is observed.

Improvement of photoluminescence property of (GaAl)As double-heterostructure laser wafer with buffer layer

The effect of a buffer layer on a (GaAl)As double‐heterostructure wafer is examined by a photoluminescence (PL) technique. It is found that a buffer layer improves the radiative efficiency and lasing characteristics. There exists a good correlation between the buffer‐layer thickness, and the PL intensity of an active layer and the threshold current density.

Buried convex waveguide structure (GaAl)As injection lasers

The fabrication technique, analysis of the waveguide property, and lasing characteristics of buried convex waveguide structure (BCS) lasers are described. An improved BCS laser structure having a truncated convex active region was produced by using a novel selective etching technique. It was found that the truncated convex waveguide is very effective in suppressing higher order mode lasing when compared with a convex waveguide. The improved BCS lasers showed stable fundamental transverse mode lasing up to 20 mW/facet, typical threshold current of 20 mA, and an external differential quantum efficiency of 28 percent/facet.

Effect of growth terraces on threshold current density of (GaAl)As double-heterostructure laser

The variation of threshold current density Jth caused by the optical scattering loss due to growth terraces is experimentally and theoretically examined. It is found that Jth of the lasers in which growth terraces are parallel to the cavity direction is about 20% smaller than that of the lasers in which growth terraces are perpendicular to the cavity direction when the active‐layer thickness d is 0.1 μm. The optical scattering loss due to growth terraces was estimated to be 50 cm−1 at d=0.1 μm when the cavity contains 10–15 terraces whose height and length are about 100 A and 1μm, respectively.

Visible GaAlAs Laser with a Buried-Convex Waveguide Structure

A new buried-heterostructure visible GaAlAs laser, called a buried-convex waveguide structure (BCS) laser, has been developed. BCS lasers exhibit a highly stable transverse and single longitudinal mode operation in the 750 to 870 nm wavelength range. Very low threshold currents of 10–20 mA are obtained for the wavelength at about 750 nm. The output beam of BCS lasers has an aspect ratio of 2.0–2.5 and has no astigmatism. The relative intensity noise of BCS laser is very low at a level below -150 dB/Hz and is constant in the temperature range between 10 and 50°C. The maximum output power ofBCS lasers with facet coating is almost independent of the lasing wavelength between 750 and 870 nm.

High speed optical components for lightwave systems

As lightwave transmission systems for comprehensive networks have deployed, optical components have been required to increase their performance functionality, to be more compact in size, to be lower cost, and to be highly reliable. Many activities for developing key optical components have been continued and we are now beginning to see cost effective solutions on 10Gb/s components and practical 40Gb/s components which are all key enablers for building such broadband lightwave systems. In this paper, we will address the state-of-the art of technologies for 10Gb/s and 40Gb/s optical components.

AGING CHARACTERISTICS OF InGaAsP DFB LASER

Distributed feedback (DFB) lasers have been applied to wideband and long-haul optical transmission systems. For the applications, the long term performance on transmission characteristics is important. Some life-test results have been reported [1,2], however, the reliability of DFB lasers have not been clear yet. This paper presents the high temperature aging results including 1.8 Gbit/s transmission experiments. This is the first report which describes transmission characterisites as well as lasing characteristics of long aged DFB lasers.

Improvement of GaAs‐GaAlAs double‐heterostructure laser wafer by Ga1−xAlxAs buffer layer

The optically induced degradation in GaAs‐GaAlAs double‐heterostructure (DH) laser wafers is examined at elevated temperatures. The degradation of the photoluminescence properties by optical pumping is considered to originate at the interface between an n‐GaAlAs clad layer and a p‐GaAs active layer, as it is greatly reduced by the insertion of a thin Ga1−xAlxAs buffer layer (x∼0.05, ∼0.05 μm thick) between these two layers. The degradation rate during optical pumping is correlated to laser life. The lasers fabricated from the DH wafers with the Ga1−xAlxAs buffer layer are still operating after 2000 h at 70 °C, whereas lasers without the buffer layer are inoperative after a few hundred hours.