Tsukuru Ohtoshi

Linewidth enhancement factor in strained quantum well lasers

The linewidth enhancement factor alpha of strained quantum-well lasers is analyzed by the k-p perturbation method using the effective-mass approximation. It is found that the alpha factor in a strained In/sub 0.80/Ga/sub 0.20/As/InP quantum-well (QW) laser with 1.9% biaxial compression is less than 1.5. For a strained QW laser with p-type modulation doping (MD) of 5*10/sup 18/ cm/sup -3/, the alpha factor is as small as 0.8. It is also demonstrated that the spectral linewidth and wavelength chirping in the strained MD QW laser are significantly less than those in conventional bulk and QW lasers.<<ETX>>

A two-dimensional device simulator of semiconductor lasers

Abstract A two-dimensional simulator for aid in designing semiconductor lasers is developed. Poissons equation and the current continuity equations for electrons and holes as well as the wave equation and rate equation for photons are numerically solved. Heterojunctions and carrier degeneracy are rigorously treated, and analytical results on channeled-substrate-planar lasers are presented to demonstrate the simulator. Reasonable agreement is found between calculated and experimental results, and calculated results clarify precisely the operation mechanism of semiconductor lasers. The present work enables computer simulation for the first time to be a practical design aid in research and development of various kinds of semiconductor lasers.

Dependence of optical gain on crystal orientation in surface‐emitting lasers with strained quantum wells

We analyze theoretically optical gains in vertical‐cavity surface‐emitting lasers (VCSELs) for various crystal orientations. The calculation based on the multiband effective‐mass theory takes into account the effects of anisotropy and nonparabolicity on the valence subband dispersion. It is found that in VCSELs employing InGaAs/InP strained quantum wells (QWs) with non‐(001) orientations except (111), the polarization in the QW plane can be controlled and high gains are obtained. In particular, the gains in VCSELs with (NN1)‐oriented (N≥2) strained QWs are markedly higher than those in the equivalent (001) lasers.

Valence subband structures of (101̄0)-GaN/AlGaN strained quantum wells calculated by the tight-binding method

The effect of biaxial strain on the valence bands in (1010)-GaN/AlGaN quantum wells (QWs) is theoretically investigated, using the sp3 tight-binding method. The effective mass around the valence band edge in unstrained (1010) QWs is reduced to about 1/2 that of (0001) QWs. Under compressive strain, the subband non-parabolicity near the band edge is further reduced due to heavy-hole/light-hole splitting. The optical matrix elements of [1120] polarization in these QWs are twice as large as those in (0001) QWs. The reduced effective mass and large optical matrix elements in the (1010) QWs are an advantage for short-wavelength laser diodes based on wurtzite GaN.

High Relaxation Oscillation Frequency (beyond 10 GHz) of GaAlAs Multiquantum Well Lasers

Direct modulation bandwidth of GaAlAs multiquantum well (MQW) lasers with 5 nm-thick GaAs wells was investigated. It was experimentally found that relaxation oscillation frequency of MQW lasers is beyond 10 GHz, which is twice that of double heterostructure lasers. This result was confirmed by theoretical analysis.

Optical line shape functions in quantum-well and quantum-wire structures

Line shape functions in quantum-well and quantum-wire structures are theoretically analyzed taking non-Markovian relaxation processes into account. For high carrier density (as in laser operation), the line shape functions in low-dimensional systems have a strong convergent characteristic because the carrier-carrier coupling, i.e. the system-reservoir coupling, becomes stronger in the lower-dimensional systems. In particular, the line shape of the quantum wire can be approximated by the Gaussian function. In the case of low carrier density, the lower-dimensional systems have smaller homogeneous broadening widths because of longitudinal optical phonon localization; the transverse relaxation times are 0.1 ps (bulk), 0.3 ps (quantum well), and 0.7 ps (quantum wire). >

Orientation dependence of optical properties in long wavelength strained quantum-well lasers

The dependence of optical properties on crystal orientation is analyzed for long wavelength strained quantum-well (QW) GaAsP-InGaAsP lasers. The calculation is based on the multiband effective mass theory which enables us to consider the anisotropy and the nonparabolicity of the valence-band dispersions. It is found that the optical gain increases as the crystal orientation is inclined from [001] toward [110]. This is due to the reduced valence-band density of states. The differential gain is about 1.6 times larger for the [110]-oriented 1.55-/spl mu/m strained QWs than for equivalent [001]-oriented QWs. It is also shown that the threshold current density in 1.3-/spl mu/m strained QW lasers decreases to two-thirds of that in the [001]-oriented laser as the orientation is inclined away from [001] by 40/spl deg/-90. >

Dependence of optical gain on crystal orientation in wurtzite–GaN strained quantum-well lasers

Optical gains in wurtzite–GaN strained quantum-well (QW) lasers are estimated theoretically for various crystallographic directions. The calculation of the valence subbands is based on the k⋅p theory, where deformation potentials are determined by a semiempirical tight-binding method. It is found that the gains in GaN strained QW lasers with non-(0001) orientations, particularly around the (1015) orientation, are markedly high and anisotropic, unlike those in (0001)-oriented lasers.

Mode-hopping noise in index-guided semiconductor lasers and its reduction by saturable absorbers

Mode-hopping noise in index-guided semiconductor lasers is investigated. It is found that random switching between lasing modes and output power differences in those modes cause mode-hopping noise. An effective method to suppress such mode-hopping noise is proposed. High Te doping to an n-type GaAlAs cladding layer completely suppresses the noise. Te in GaAlAs forms a DX center that acts as a saturable absorber. This property stabilizes the laser mode and prevents mode competition. The minimum loss difference between lasing and nonlasing modes to suppress mode-hopping noise is also discussed.

Analysis of current leakage in InGaAsP/InP buried heterostructure lasers

The mechanism of current leakage at high temperatures in InGaAsP/InP buried heterostructure (BH) lasers with p-n-p-n current-blocking structures is analyzed using two-dimensional computer simulation. It is found that no junction in the blocking layers is reverse-biased and that current confinement is due to electrically floating regions in the blocking structures. To minimize the leakage current in these BH lasers, it is necessary to decrease the device width and the connection length between the blocking and cladding layers and to increase the doping level and thickness of the blocking layers. >