Takuo Hirono

Study on the dominant mechanisms for the temperature sensitivity of threshold current in 1.3-/spl mu/m InP-based strained-layer quantum-well lasers

We study the basic physical mechanisms determining the temperature dependence of the threshold current (I/sub th/) of InP-based strained-layer (SL) quantum-well (QW) lasers emitting at a wavelength of 1.3 /spl mu/m. We show that I/sub th/ exhibits a different temperature dependence above and below a critical temperature T/sub c/. It is indicated that T/sub c/ is the maximum temperature below which the threshold gain exhibits a linear relationship with temperature. We demonstrate that below T/sub c/ the Auger recombination current dominates the temperature dependence of I/sub th/. On the other hand, above T/sub c/ a significant increase in both the internal loss and radiative recombination current in the separate-confinement-heterostructure region, which is mainly due to electrostatic band-profile deformation, is found to play a major role in determining the temperature sensitivity of I/sub th/. On the basis of the comparison between the theoretical analysis and the experimental results, we conclude that the temperature dependence of the threshold current in 1.3-/spl mu/m InP-based SL-QW lasers is dominated by different mechanisms above and below T/sub c/.

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.

Polarization rotation in semiconductor bending waveguides: a coupled-mode theory formulation

A theoretical model for the bending waveguide polarization rotator has been developed based on the full-vectorial wave equations and the coupled-mode theory. Calculation results from this model are found to agree favorably with measurement data reported in literature. It is found that the polarization conversion efficiency of this device hinges on the degree of asymmetry of the cross-sectional field profile with respect to both the x- and y-axes. Sensitivity analysis, furthermore, revealed that the device characteristics are strongly dependent on the waveguide geometry, in particular, the sidewall tilt angle and the amount of over-etch of the ridge waveguide. Finally, it is also found that the bending waveguide polarization rotator is virtually wavelength independent, making it suitable for wavelength division multiplexing (WDM) applications.

Full-vectorial wave propagation in semiconductor optical bending waveguides and equivalent straight waveguide approximations

Device characteristics of optical polarization rotators are founded upon the vector properties of the Maxwell equations. Recently, a bending waveguide based polarization rotator has been proposed and demonstrated. To provide a rigorous basis for the analysis and design of this polarization rotator, the full-vectorial wave equations for both E/spl I.oarr/ and H/spl I.oarr/-field in bending waveguides are derived. It is found from these wave equations that under a broad range of circumstances, a bending waveguide can be analyzed using the equivalent straight waveguide approximation. Details of the model for optical polarization rotators, which is based on the coupled-mode theory, will be described in a companion paper.

Improvement of kink-free output power by using highly resistive regions in both sides of the ridge stripe for 980-nm laser diodes

Suppressing the lateral expansion of driving current by forming highly resistive regions at both sides of the ridge stripe in ridge-waveguide-type 980-nm laser diodes leads to an enhanced kink-free output power. The highly resistive regions are formed by hydrogen passivation of p-type carrier (Zn) on the plasma exposure in a mixture gas of methane and hydrogen. A simulation predicted a decrease in local gain in the lateral direction at both sides of the ridge stripe. Fabricated laser diodes with the highly resistive regions exhibit kink-free output power of over 500 mW, showing an increase in kink-free power of 85 mW on average with an increase of slope efficiency of about 10% compared to those without highly resistive regions.

Stability and numerical dispersion of symplectic fourth-order time-domain schemes for optical field simulation

The use of a more accurate scheme is effective in reducing the required memory resources in the explicit time-domain simulation of optical field propagation. A promising technique is the application of the symplectic integrator, which can simulate the long-term evolution of a Hamiltonian system accurately. The stability condition and the numerical dispersion of schemes with fourth-order accuracy in time and space using the symplectic integrator are derived for the transverse electric (TE)-mode in two dimensions. Their stable and accurate performance is qualitatively verified, and is also demonstrated by numerical simulations of wave-converging by a perfect electric conductor wall and propagation along a waveguide whose refractive index difference between the core and cladding is more than 9%.

Control of higher order leaky modes in deep-ridge waveguides and application to low-crosstalk arrayed waveguide gratings

In a lightwave circuit using a strongly confined waveguide, higher order leaky modes must be suppressed to obtain ideal characteristics, especially in arrayed waveguide gratings (AWGs). Propagation loss for higher order leaky modes in InP-based deep-ridge waveguides was investigated by simulation and experiment. A highly sensitive loss measurement method based on optical low-coherence reflectometry was used to determine the loss, and the relationship between the loss for the higher order mode and AWG crosstalk was investigated. Optimizing the deep-ridge waveguide parameters, especially the core thickness, the refractive index of the core, and the etch depth under the core, significantly reduced the propagation loss for the higher order leaky mode. The effective elimination of the higher order modes will enable fabrication of low-crosstalk AWG routers.

A narrow beam 1.3-/spl mu/m-super luminescent diode integrated with a spot-size converter and a new type rear absorbing region

The device structure and performance of 1.3-/spl mu/m narrow beam superluminescent diodes (NB-SLDs), which consist of a spot-size converter and a new type rear absorbing region, are reported. A butt-jointed selectively grown spot-size converter (SSC) is employed to realize the narrow beam characteristics. The rear absorbing region is designed as a taper structure with a part of the region is inclined from the active-stripe axes. In order to investigate the effects of both SSC length and active-region length on device performance, two types of NB-SLDs, whose SSC and active-region lengths differ, are fabricated. An electrode to sweep out photoexcited carriers in the absorption region is formed on one device. By comparing the characteristics of these devices, we clarify that a 500-/spl mu/m-active-region device is suitable for high-output power operation, and a 400-/spl mu/m-active-region device is suitable for realizing short coherent length. The light-output power is 13.9 mW at 200-mA-injection current for the former device, and the full-width at half-maximum (FWHM) of the spectrum is 62.6 nm (calculated coherence length is 26.5 /spl mu/m) for the latter device. Very small spectral modulation index (0.015 at 5 mW-output power) is attained by grounding the absorption-region electrode. For the SSC length, a 300-/spl mu/m SSC device shows very narrow far-field patterns (FFPs) and very good fiber-coupling characteristics. The FWHM of horizontal and vertical FFPs are 8.9 and 10.6/spl deg/, respectively. Because of this narrow beam divergence, the coupling efficiency of -1.9 dB to a flat-end 4-/spl mu/m spot-size fiber is obtained without lenses. The alignment tolerance of this device to the fiber for both horizontal and vertical direction is more than 3 /spl mu/m at a loss of when -1 dB from the optimum coupling.

A novel analytical expression of sensitivity to external optical feedback for DFB semiconductor lasers

A novel analytical expression for DFB laser sensitivity to external optical feedback is derived based on coupled-wave theory. The expression reveals that the sensitivity is proportional to the ratio between the output power from the reflector-side facet and the magnitude of Lagrangian of the electromagnetic field in the cavity. The simplicity of the expression helps in estimating the sensitivity of DFB lasers with complex cavity structures. >

Surface-Grating Distributed Bragg Reflector Lasers with Deeply Etched Grooves Formed by Reactive Beam Etching

Two surface-grating distributed Bragg reflector (DBR) lasers, one using a 1.55-?m-wavelength InP-based material system and one using a 0.98-?m-wavelength GaAs/AlGaAs material system, were made using the same reactive beam etching technique following single-step growth of the layer structure for the lasers. A 800-nm-deep grooved grating with a 240 nm period for 1.55-?m-wavelength first-order diffraction and one with a 150 nm period for 0.98-?m-wavelength first-order diffraction were formed using electron-beam lithography and Br2-N2 mixed-gas reactive beam etching. Single-longitudinal-mode operation with a side-mode suppression ratio of over 27 dB was obtained for both lasers; the output power was more than 15 mW for the 0.98-?m one. Between 15 and 55?C, the measured temperature dependence of the lasing wavelength for both lasers was about 0.1 nm/?C, which is one-fifth of the dependence of a Fabry-Perot-type laser. The performance of these lasers indicates the wide applicability of the present etching technique to the fabrication of various wavelength-selective devices.