Shigeyuki Akiba

λ/4-shifted InGaAsP/InP DFB lasers

\lambda/4 -shifted InGaAsP/InP DFB lasers were studied theoretically and experimentally. The effect of reflectivities at the end of a DFB region and that of a \lambda/4 shift position were analyzed in terms of stability of single-longitudinal-mode operation and asymmetric power distribution. The shift of the \lambda/4 -shift position from the center to a certain place in the DFB region, with the end reflectivities less than several tenths of a percent, seemed most effective for efficient power extraction and reproducible DSM operation. The devices emitting at 1.5 μm range were fabricated by using negative and positive photoresists and employing one-step holographic exposure. They exhibited single-longitudinal-mode operations just at or closely around the center of the stopband, i.e., the Bragg wavelength. The slight wavelength deviations from the center were found to be attributed to the accidental phase-shift variations from the optimal value. Concerning such deviations in the fabricated devices, a simple and useful criterion, for example, P_{0}/P_{1} \geq 2-3 at I/I_{th} = 0.9 , for stable DSM operation was presented. Statistically, single-longitudinal-mode operations were observed in 95 devices out of 100, and the theoretical prediction was verified. The side-mode-suppression ratios under high-speed direct modulation were 35 dB or more.

In 1-x Ga x As y P 1-y /InP DH lasers fabricated on InP

In 1-x Ga x As y P 1-y /InP double heterostructure (DH) laser diodes with emission wavelengths of 1.25-1.35\mu m at room temperature were fabricated on

Effect of mirror facets on lasing characteristics of distributed feedback InGaAsP/InP laser diodes at 1.5 µm range

The effect of mirror facets on lasing properties of distributed feedback (DFB) InGaAsP/InP laser diodes was studied theoretically and experimentally. A DFB laser with a window region was found to be suitable to examine the effect of mirror facets. The effective reflectivity of a window structure was calculated to be very small, typically as small as 0.03 percent for a few tens of micrometers of the window region. These small effective reflectivities were experimentally confirmed. Three kinds of DFB lasers, i.e., a double-window (DW), a single-window (SW), and a Fabry-Perot (FP), were discussed. Two modes with a separation corresponding to a so-called stopband appeared in a DW-DFB laser, in which almost no reflection at both ends was estimated. On the other hand, the threshold and the resonant wavelength of an SW- and an FP-DFB laser were found to be sensitive to the phases of corrugation at the facets. It turned out, however, that the mirror facet contributed to the single-mode operation due to an asymmetric resonant spectrum and to the reduction in the threshold. Although a low-threshold-current FP-DFB laser was experimentally obtained, the coincidence between the gain peak and the Bragg wavelengths was essential in this type. The SW-DFB laser seemed the most promising among the three types in terms of the stability of the single-mode operation.

High-speed electroabsorption modulator with strip-loaded GaInAsP planar waveguide

We describe fabrication and characterization of an electroabsorption (EA) light modulator (LM) with a strip-loaded GaInAsP planar waveguide. The EA LMs were fabricated from hydride vapor-phase-epitaxy (VPE) grown wafers. The electroabsorption, the insertion loss, the electrical properties, and the modulation characteristics were investigated for the EA LMs. The drive voltage at a 99-percent modulation depth has been noticeably reduced to 4.5 V by optimizing the thicknesses of the epitaxial layers. The total capacitance of 1.5 pF was obtained by inserting an insulating film under the wire-bonding pad and by improving the mount design. Consequently, a 3-dB bandwidth of 3.8 GHz has been achieved and a pulse modulation operation under 2 Gbit/s nonreturn-to-zero (NRZ) pseudorandom pattern has also been confirmed. Moreover, the dynamic spectra of the EA modulators were measured for the first time. A spectral broadening factor α has been determined to be 1-4 from a relative strength of the sideband to the carrier and it has been experimentally found to decrease with increasing the electric field inside the absorptive waveguide. As the other measures of merit for the EA LMs, the extinction ratio over 23 dB and the insertion loss of 10-14 dB including a coupling loss due to an end-fire method were obtained. As a whole, these results have exhibited that the EA LM is a promising external modulator which will be monolithically integrated into a gigabit per second optical source with a dynamic single-mode laser.

Monolithic integration of InGaAsP/InP distributed feedback laser and electroabsorption modulator by vapor phase epitaxy

Monolithic integration of a 1.55-μm InGaAsP/InP distributed feedback (DFB) laser and an electroabsorption (EA) modulator was studied. The difference between the lasing photon energy and the bandgap energy of the modulator waveguide was designed to be 30-40 meV, taking into account the linewidth-enhancement factor and the zero-bias absorption loss. The integrated devices were grown by three-step vapor phase epitaxy (VPE). The CW threshold current at 20°C of the DFB laser part with a buried heterostructure was 30-60 mA and the breakdown voltage of the modulator part with a strip-loaded stripe geometry was 20-40 V, and these values indicated satisfactory crystal quality in the VPE epitaxial layers. The operating voltage of the modulator to give on:off ratios of 10:1 and 100:1 was 1.5- 4 V and 2.5-6.5 V, respectively, depending on the length in the range 200-500 \mu m. A 3-dB bandwidth of about 2.5 GHz and a linewidth-enhancement factor of about 1.6 were obtained for the integrated modulator.

Measurement of spontaneous-emission factor of AlGaAs double-heterostructure semiconductor lasers

In this paper, experimental determination of the spontaneous-emission factor of an injection laser is presented. For a lot of conventional AlGaAs DH stripe lasers, spontaneous-emission factors of about 10-5were obtained from the measurement of the light intensity of a lasing mode versus the injected current. These values are in good agreement with the theoretical prediction based upon the classical wave theory. Brief discussion of the magnitude of the spontaneous-emission factor is given relating to the direct modulation characteristics of a semiconductor laser, which shows that the damping oscillation can be decreased in a laser consisting of very small active region.

1.5 µm range InGaAsP/InP distributed feedback lasers

Lasing characteristics of InGaAsP/InP distributed feedback (DFB) lasers in the 1.5 μm range were studied theoretically and experimentally. Wave propagation in five-layer DFB waveguides were analyzed to estimate the effect of the structural parameters on threshold conditions. A brief consideration on designing a low threshold laser and its lasing wavelength was made. DFB buried heterostructure lasers with fundamental grating emitting at 1.53 μm were prepared by liquid phase epitaxial techniques. CW operation was confirmed in the temperature rangeof -20° to 58°C, and a CW threshold current was as low as 50 mA at room temperature. A stable single longitudinal mode operation was observed both in dc condition and in modulated condition by a pseudorandom pulse current at 500 Mbits/s. No significant increase in the threshold current was observed after 1400 h continuous CW operation at 20°C.

A multiwavelength fiber ring-laser employing a pair of silica-based arrayed-waveguide-gratings

A multiwavelength fiber ring-laser employing a pair of silica-based arrayed-waveguide-gratings (AWGs) is proposed to easily implement a stable multiwavelength operation for a variety of research needs. A fixed multiwavelength operation is available when the free-spectral-range (FSR) of the AWGs is wide enough to cover the gain bandwidth of deployed erbium-doped fiber amplifiers (EDFAs). A tunable multiwavelength operation is also available when the FSR is narrower than the gain bandwidth. 15 channels with 1.6-nm spacing in the fix type, and five channels with about 40-nm discretely tunable range at 1.4-nm spacing in the tunable type and almost 50-dB signal to amplified spontaneous emission (ASE) ratio in both types are demonstrated.

Electrical and optical interactions between integrated InGaAsP/InP DFB lasers and electroabsorption modulators

Electrical and optical interactions occurring in InGaAsP/InP integrated light sources composed of DFB lasers and electroabsorption modulators have been studied. Static measurements indicated that an isolation resistance between the laser and the modulator should be as large as 100 k Omega or more in order to reduce a lasing wavelength shift associated with biasing the modulator. The useful guided light out of the modulator was about 1-2 percent of the laser output. At high frequency modulation, asymmetric sidebands were observed in dynamic spectra measured at the modulator facet, which were caused by additional modulation of the laser part. Such additional modulation was relatively large at the relaxation oscillation frequency of the laser, but it was reduced by connecting an RF bypass condenser in parallel to the laser diode and by depositing an antireflection coating at the modulator facet. After eliminating the additional modulation, an integrated device showed a high frequency response of 5.7-GHz 3-dB bandwidth and a small linewidth enhancement factor of alpha =0.8. >

Novel wavelength converter using an electroabsorption modulator: conversion experiments at up to 40 Gbit/s

We have proposed a novel wavelength converter using an electroabsorption modulator. High-quality conversion has been demonstrated at 20 Gbit/s over 30 nm of wavelength range. The potentialities for high speed (>40 Gbit/s) and broadcasting applications were also demonstrated.