Yuzo Yoshikuni

Quasicontinuous wavelength tuning in super-structure-grating (SSG) DBR lasers

The paper describes the design of a super-structure-grating distributed Bragg reflector (SSG-DBR) laser for broad quasicontinuous wavelength tuning with stable single-mode operation. The phase distributions and the effective coupling coefficients of SSGs are optimized to obtain both broad tuning range and high mode selectivity. A computer-aided simulation of wavelength tuning, where the effects of the waveguide loss increase and inhomogeneous gain spectrum are included, provides an optimum cavity structure and indicates the possibility of more than 70 mm quasicontinuous tuning in a 1.55 /spl mu/m InGaAsP-InP SSG-DBR laser. Experimental results for 34 nm quasicontinuous tuning with a properly designed device are also presented.

Broad-range wavelength-tunable superstructure grating (SSG) DBR lasers

A broad tuning range is demonstrated in tunable DBR lasers with a novel superstructure grating (SSG). These SSG reflectors were fabricated by electron-beam lithography, and grating performance was evaluated by measuring the transmittance characteristics of a ridge-type SSG reflector. Incorporating this SSG reflector into distributed-Bragg-reflector (DBR) laser results in lasers that can be tuned over a wide wavelength range by using coarse and fine tuning mechanisms. Wavelength-tunable SSG DBR lasers were fabricated and operated under low-threshold CW conditions. Wavelength tuning ranges of 50 and 100 nm were designed and experimentally measured to be 50 and 83 nm, respectively, with single-mode operation. >

A three-dimensional fourth-order finite-difference time-domain scheme using a symplectic integrator propagator

A new explicit fourth-order finite-difference time-domain (FDTD) scheme for three-dimensional electromagnetic field simulation is proposed in this paper. A symplectic integrator propagator, which is also known as a decomposition of the exponential operator or a general propagation technique, is directly applied to Maxwells equations in the scheme. The scheme is nondissipative and saves memory. The Courant stability limit of the scheme is 30% larger than that of the standard FDTD method. The perfectly matched layer absorbing boundary condition is applicable to the scheme. A specific eigenmode of a waveguide is successfully excited in the scheme. Stable and accurate performance is demonstrated by numerical examples.

Theoretical analysis of pure effects of strain and quantum confinement on differential gain in InGaAsP/lnP strained-layer quantum-well lasers

The pure effects of both strain and quantum confinement on differential gain of InGaAsP/InP strained-layer quantum-well lasers (SL-QWLs) are studied on the basis of valence band structures calculated by k/spl middot/p theory. Using an InGaAsP quaternary compound as an active layer makes it possible to distinguish the effect of strain (both tensile and compressive) from the quantum-confinement effect when keeping the emission wavelength constant. The essential features of strain-induced changes in the valence band structures are extracted from the k/spl middot/p results by four characterization parameters: the averaged density of states (DOS), the subband energy spacings, the joint density of electron and hole states, and the squared optical matrix elements. Each of them is then directly correlated to differential gain in SL-QWLs. In tensile-strained quantum wells, all of these factors are significantly improved compared with unstrained wells, while only the averaged DOS is improved in compressive-strained wells. Due to these characteristic features, it is concluded that the intrinsic potential of tensile-strained QWLs for improving differential gain is twice as high as that of compressive-strained ones. On the basis of the essential features of the strain-induced changes in valence band structures, we also discuss basic design principles for SL QWLs with larger differential gain. >

Super-structure-grating (SSG) for broadly tunable DBR lasers

The reflection characteristics of super-structure-grating (SSG) reflectors are demonstrated theoretically and experimentally. The periodic multireflection peaks with high reflectivities of 68 approximately 98% are theoretically shown in the reflection spectrum. The experimental characteristics of a ridge-type SSG reflector are in good agreement with the theoretical design.<<ETX>>

Narrow spectral linewidth under wavelength tuning in thermally tunable super-structure-grating (SSG) DBR lasers

Narrow spectral linewidth characteristics are demonstrated in thermally wavelength tunable super-structure-grating distributed Bragg reflector (SSG-DBR) InGaAsP-InP lasers. SSG reflectors were fabricated with a multiple-phase-shift insertion method using electron-beam lithography. The phase distributions of the SSGs were optimized to obtain uniform reflection peaks. Incorporating thin-film Pt heaters into the lasers enables us to tune the lasing wavelength by the thermal effect. The device has a wide quasi-continuous tuning range of 40 nm with CW operation at room temperature. The spectral linewidths, frequency fluctuations, and tuning speed in thermal wavelength tuning were studied compared with those in electrical tuning. Experimental results showed that thermal tuning does not influence frequency fluctuations and spectral linewidths, but the tuning speed was slow. Narrow spectral linewidths below 400 kHz were achieved in a wide quasi-continuous tuning range of 40 mm. >

Linewidth enhancement factor in InGaAsP/lnP modulation-doped strained multiple-quantum-well lasers

Reduction of the linewidth enhancement factor /spl alpha/ is studied in InP-based strained multiple-quantum-well (MQW) lasers. Theoretical analysis shows that the /spl alpha/-parameter is greatly reduced in modulation-doped strained MQW lasers and may be zero while keeping positive gain. The experimental evaluation exhibits a very small /spl alpha/-parameter of around 1 in InGaAsP/InP modulation-doped strained MQW lasers. As a result of the small /spl alpha/-parameter, the linewidth-power product is effectively reduced in 1.5-/spl mu/m DFB lasers with the modulation-doped strained MQW structure. A narrow spectral linewidth around 100 kHz was also obtained reproducibly in 1.5-/spl mu/m modulation-doped strained MQW DFB lasers. >

Measurements of a semiconductor waveguide using a low-coherence interferometric reflectometer

A low-coherence interferometric reflectometer is studied as a tool to measure both semiconductor waveguide loss and reflectivity. Accurate estimation is given by using the integral values of the interference envelopes observed as reflected power from the sample. This estimation is free of influence from coupling between the sample and reflectometer or dispersion in the semiconductor materials. The experimental results are in good agreement with those obtained by the Fabry-Perot method. The facet reflectivity measured in 0.5-mm-long very short samples shows waveguide thickness dependence agrees with theoretical results. This approach shows good repeatability and accuracy for short samples.

Influence of buried structure on polarization sensitivity in strained bulk semiconductor optical amplifiers

In order to achieve an accurate design of polarization-insensitive semiconductor optical amplifiers based on tensile strained bulk InGaAsP, the reduction of strain in the active layer of the buried heterostructure and its influence on polarization sensitivity are analyzed numerically for the first time. The gain calculation, including the strain distribution in the active layer, is examined based on the k /spl middot/ p method for the different active layers. It is found that the strain introduced during the epitaxial growth is strongly reduced after regrowth of the burying layer. In an active layer having the aspect ratio of 1 : 4, the strain reduction causes more than a 0.5-dB deviation in the polarization sensitivity of the gain. From a comparison with the experimental results, it is shown that including the effect of the burying layer in the calculation gives an accurate determination of the amount of strain for the polarization independence.

Lasing wavelength changes due to degradation in buried heterostructure distributed Bragg reflector lasers

Changes in the wavelength and lamp characteristics due to the degradation of distributed Bragg reflector (DBR) lasers are investigated and the lasers are confirmed to be reliable enough for application as the light sources in WDM systems. The change in wavelength characteristics is due to degradation of the DBR or phase control region and the change in lamp characteristics is due to the degradation of active region. These changes in the characteristics are caused by diminished recombination carrier lifetime. The wavelength stability is strongly correlated with the injected current density. The lamp characteristics is confirmed to be almost as stable as in conventional Fabry-Perot lasers. The applicability of DBR lasers as the light source for wavelength division multiplexing (WDM) systems is demonstrated for actual-use conditions.