A chopped quantum-well polarization-independent interferometricswitch at 1.53 μm

We have theoretically designed and realized a phase shifter for a low-loss Mach-Zehnder interferometric switch. The phase shifter is based on 0.85% tensile strained InGaAs-InP chopped quantum-well material. We realized a Mach-Zehnder interferometric switch with polarization-independent switching voltages as low as 3.3±0.05 V at 1525 nm for a switch with a 4-mm-long phase shifting section. The wavelength sensitivity of the switch is 0.036 V/nm for TE and 0.053 V/nm for TM polarization. Calculations of the electro-refraction in the -0.85% strained chopped quantum-well (QW) material based on the 4×4 Luttinger-Kohn Hamiltonian show that the electro-refraction due to the quantum-confined Stark effect (QCSE) for TM polarization is equal to the sum of the mutually comparable QCSE electro-refraction and the Pockels effect for TE polarization in waveguides along the [11¯0] axis. Our first-principle model for calculating the electro refraction is an accurate design tool for predicting device performance in complicated layer structures. The shortest possible phase shifter with a <-25 dB crosstalk penalty due to electro-absorption unbalance can be as short as 2.2 mm. This compact switch is predicted to have a 6-V switching voltage and a 15-nm window for polarization-independent switching with a <-25-dB crosstalk penalty. With a slight increase of the strain, this chopped QW material can be used for polarization independent switching around 1550 nm     

Electro-optic modulation in germanosilicate fibre with UV-excitedpoling

The authors report what is, to their knowledge, the highest electro-optic effect measured in a germanosilicate fibre ever reported. The electro-optic effect based on a permanent second-order nonlinearity has been induced by UV-excited poling at 193 nm with an electric field >8×10⁵ V/cm. Polarisation dependence of the V_πL product in a birefringent fibre has been observed, and the lowest V_πL product obtained for the TE mode of 32 Vcm, comparable to that based on r₂₂ of LiNbO₃, has been measured at 633 nm with a phase modulation at frequencies up to 5 MHz     

Electro-optic diffraction grating for light beam modulation and diffraction

The interaction between a strong traveling microwave signal and an optical beam in an electro-optic material is described in the limit of very high microwave dielectric constant. The interaction produces effects analogous to those produced by a moving diffraction grating. When the optical beam is wider than the wavelength of the microwave signal, the first grating order is resolved from the zero-order or main beam. Under this condition two types of devices become possible: 1) a beam deflector which can position an optical maser beam on, for example, 10⁵distinct points with negligible crosstalk and with address times of order 10^-7s, 2) a baseband light intensity modulator which is founded on the fact that light deflected into the first-order beam by the microwave signal is removed from the main beam. The amount deflected into the first-order beam is proportional to the microwave power; the intensity modulation follows the microwave envelope. The power required for a given modulation depth is inversely proportional to the seven halfs power of the dielectric Constant. As an example, for a not unrealistic choice of dielectric constant of 10⁴, complete transfer from the zero-order to the first-order beam requires 5 watts of microwave power. The interaction length is of order one centimeter and the interaction bandwidth is essentially unlimited. As a baseband modulator the maximum instantaneous bandwidth is of order 10 percent of the subcarrier frequency. Experimental verification is provided in an earlier paper [1].     

Microwave modulation of a quantum-well laser with and withoutexternal optical injection

We compare three microwave modulation methods experimentally and theoretically using a semiconductor quantum-well (QW) Fabry-Perot test laser: (1) direct microwave current modulation of the test laser (electrical modulation); (2) optical modulation by an external single-wavelength pump laser with a modulated optical injection power; and (3) electrical modulation of the test laser that is injection locked by an external single-wavelength pump laser with a constant injection power. This is the first direct comparison of the three modulation methods on the same QW laser, to the best of our knowledge. The bandwidth of optical absorption modulation is 7.7 GHz, which is 1.45 times the direct electrical modulation bandwidth (5.3 GHz) at a bias current of 30 mA in the test laser. On the other hand, the electrical modulation of the test laser under injection-locking condition has a significantly higher modulation bandwidth (10.5 GHz) than both the electrical and optical modulation methods     

Electric-field-induced refractive index variation in quantum-well structure

The refractive index variation in a quantum-well structure by an electric field is given theoretically. The calculated variation is ?1% for an applied field of 3.1×105 V/cm in a 300 ?-thick GaInAsP/InP single quantum well, which is about 39 times larger than the bulk value. A semiconductor quantum-well structure is found theoretically to be a new material with a larger electro-optic coefficient. Application to a new optical switching device is also suggested.     

A novel SAL-PINSCH quantum-well laser structure for a pinched beamdivergence

Summary form only given. An edge-emitting strained AlGaAs/InGaAs/GaAs quantum-well laser structure is reported. It has a periodic index separate confinement heterostructure (PINSCH) optical confinement layers for a small beam divergence and high output power. Preliminary measurements of AR/HR-coated self-aligned ridge waveguide lasers show a CW output power of up to 350 mW and a 20° transverse beam divergence at a 980-nm lasing wavelength. This low beam divergence results in a high coupling efficiency of 51% into single-mode fibers. The expanded optical field in PINSCH confinement layers significantly pinches the transverse beam divergence and increases the maximum output power     

Quantum capture and escape in quantum-well lasers-implications ondirect modulation bandwidth limitations

Analytic expressions for carrier capture and escape currents into quantum wells are derived. The authors find that the escape rate can be as large as the capture rate under typical operating conditions in quantum-well lasers so that the damping and inertia of the relaxation oscillation are significantly increased in these lasers. Implications for the limitation of the modulation bandwidth of quantum-well lasers and its dependence on the quantum-well structure are discussed     

The influence of Coulomb enhancement on the modulation propertiesof quantum-well lasers

The phenomenon of Coulomb enhancement, resulting from the consideration of many-body effects, is included in a detailed calculation of the gain of a quantum-well (QW) laser, which is then used to predict the laser's modulation response. Carrier transport in the separate-confinement heterostructure is taken into account. The modulation response is compared to experimental data and to predictions from calculations using only the free-carrier gain. The comparison shows that the inclusion of Coulomb enhancement in the theoretical calculations leads to better agreement between simulated and experimental data     

A novel GaInNAs-GaAs quantum-well structure for long-wavelengthsemiconductor lasers

We propose a novel quantum-well (QW) structure for GaInNAs-GaAs lasers that can emit 1.3 μm or longer wavelength light. The idea is insertion of lattice-matched GaInNAs intermediate layers between well and barrier, which is effective for elongating the emission wavelength and reducing well thickness. It is shown that 1.3-μm emission is achievable by using the proposed GaInNAs-GaAs QW with a well thickness thinner than that of conventional rectangular GaInNAs QWs. This structure will relax the design limitation of strained GaInNAs layers     

应变层InGaAsP量子阱激光器结构的调制光谱研究

利用光调制反射谱(PR)对1.55μm应变层InCaAsP三量子阱激光器结构进行了研究,在样品的波导层观察到了Franz-Keldysh振荡。利用Bastard包络函数方法和Kane模型从理论上计算了该应变层InGaAsP四元合金三量子阱内电子和空穴的能级和跃迁能量,计算结果与实验数据符合得很好,得到了In_(0.758)Ga_(0.242)As_(0.83)P_(0.17)与In_(0.758)Ga_(0.242)As_(0.525)P_(0.475)四元合金应变界面的导带不连续性。     

Self-consistent calculation of mode spectra and modulation responsein wurtzite GaN quantum-well lasers

The photon rate-equation formalism is used to evaluate the multimode photon density, the output lasing power, and the modulation frequency response in pseudomorphically strained wurtzite GaN quantum-well lasers. The formalism is based on a self-consistent methodology that couples an envelope function (or k·p) Hamiltonian with Poisson's equation. From this approach, we consider (1) the band structure under the influence of large piezoelectric fields and with many-body effect; and (2) the stimulated and spontaneous emissions for each Fabry-Perot mode. Our calculations predict a threshold current density of approximately 1 kA/cm² and an intrinsic 3-dB modulation bandwidth of 11.7 GHz at 40-mW output power for a 50-Å pseudomorphically strained GaN-Al_0.2Ga_0.8N single-quantum-well (SQW) laser. Our estimation of the threshold current density represents the theoretical limit and is compatible with recent experimental results in InGaN multiple-quantum-well (MQW) lasers     

The effect of interdiffusion on the intersubband optical propertiesin a modulation-doped quantum-well structure

The linear and nonlinear (based on optical field intensity) intersubband absorptions in conduction band, and its change in refractive index in AlGaAs-GaAs interdiffused quantum wells (QWs) are presented. The calculation of the electron energy levels and the envelope wave functions in a modulation doped interdiffused QWs with screening effects are considered. QW interdiffusion shows a wavelength tunability of the intersubband absorption peaks and refractive index dispersions. This shifting of the transition energies is also demonstrated here to be a useful technique for broad-band and multicolor photodetector application. In addition, it can serve to remove noise, such as minor peaks and dispersions, in the optical spectra     

High-power single-mode operation in DFB and FP lasers usingdiffused quantum-well structure

Distributed feedback (DFB) and Fabry-Perot (FP) semiconductor lasers with step and periodic interdiffusion quantum-well structures are proposed for high-power single-longitudinal-mode operation. It is shown that the phase-adjustment region formed by the diffusion step (i.e., step change in optical gain and refractive index) counteracts the influence of spatial hole burning, especially for DFB lasers with large coupling-length products biased at high injection current. Furthermore, it is found that with careful design of the diffusion grating (i.e., grating period and amount of diffusion extent) of FP lasers, side-mode suppression ratio can be enhanced and threshold current density can be minimized to a satisfied level     

Mobility modulation in a quasi-one-dimensional Si-MOSFET with adual-gate structure

The electrical transport characteristics of a quasi-one-dimensional Si-MOSFET with a dual-gate structure are studied. In this device, the width of the one-dimensional channel can be changed continuously using the field effect and the intervals between one-dimensional subbands as well. By making part of the channel narrower, strong oscillations in differential conductance, even negative differential conductance, have been observed at 4.2 K, indicating the enhanced modulation of the electron mobility by inter-subband scattering suppression     

Effects of strain on the high speed modulation of GaAs- andInP-based quantum-well lasers

A small-signal numerical analysis of pseudomorphic GaAs- and InP-based Fabry-Perot quantum-well lasers using calculated optical gain spectra with strain effects included is reported. Examination of the effect of lifetime broadening shows that the resonance frequency increases at a rate of ~250-MHz/meV reduction in the lifetime broadening for a GaAs-based strained layer laser. The modulation speed is limited by either device heating or facet damage. If the limitation is imposed by the optical power then the modulation speed increases as the laser cavity becomes shorter and the number of quantum wells increases. If the limitation is imposed by the injection current density, however, then the modulation speed decreases for the laser with shorter cavity length. The highest modulation speed is given by an optimum well number. A resonance frequency of ~16 GHz is predicted for a pseudomorphic GaAs-based laser with 30% excess In and average output power of ~5 mW     

Theory of resonant modulation at millimeter wave frequencies ofinhomogeneously biased monolithic quantum-well lasers

We show theoretically that resonant modulation at millimeter wave frequencies of inhomogeneously biased monolithic quantum-well laser diodes provides an enhancement in the modulation efficiency of >14 dB over that of the homogeneously biased laser, accompanied by a corresponding 7 dB increase in the noise and a comparable decrease in the signal-to-intermodulation distortion. We derive simple expressions for the enhancement in the modulation efficiency, the noise and intermodulation distortion, and discuss the limits of this approach and the trade off between these quantities. The theoretical results presented here are in good agreement with recently published experimental work     

Optical gain in a strained-layer quantum-well laser

The optical gain and the refractive index change of a uniaxially stressed GaAs-Al₂Ga_1-xAs quantum-well laser is studied theoretically using the multiband effective mass theory (k -p method) and density matrix formalism with intraband relaxations. It is found that uniaxial strain of the quantum well substantially alters the subband structures and the optical gain of the quantum-well laser. In particular, the gain of the TM mode increases while the gain of the TE mode decreases with increasing stress. Thus, the threshold current either decreases or increases with the stress, depending on whether the laser is operating in a TM or TE mode     

Monolithic integration of a quantum-well laser and an opticalamplifier using an asymmetric twin-waveguide structure

We demonstrate the monolithic integration of a 1.55 μm wavelength InGaAsP-InP multiple-quantum-well (MQW) laser and a traveling-wave optical amplifier using an asymmetric, vertical twin-waveguide structure. The laser and amplifier share the same strained InGaAsP MQW active layer grown by gas-source molecular beam epitaxy, while the underlying passive waveguide layer is used for on-chip optical interconnections between the active devices. The asymmetric twin-waveguide structure uses the difference in modal gains to discriminate between the even and odd modes