Tunable infrared modulator and switch using Stark shift in stepquantum wells

A tunable infrared modulator and/or switch using intersubband Stark shift in a step quantum well is discussed. The device utilizes the intersubband absorption and the large change of separation between energy levels in the step quantum well under an applied electric field. The incident infrared beam on the device is either absorbed or transmitted depending on the energy separation of the levels, and thus the modulation can be achieved by adjusting the energy levels with an applied electric field. The extremely short lifetime of the intersubband transition makes this modulator suitable for application in high-speed long-wavelength optical communications     

Quantum confined light modulators

A general framework for the operation of a large class of quantum confined light modulators is offered, supported by a discussion of the theoretical background. The design of a quantum confined modulator for a specific host system allows a wide choice of modulation mechanism, geometrical design configuration and field-controlled optical parameter. In this short review, we discuss specific combinations of optical property, semiconductor material system and geometry suitable for selected digital and analogue applications. Excitonic optical effects are emphasised as providing electric-field control of both optical absorption and refractive index in multiple quantum well structures. Geometrical aspects are also discussed, including phase modulation in a horizontal waveguide configuration and absorption modulation in transmission and reflection-mode vertical modulators. Self electrooptic effect devices (SEEDs) figure prominently across the range of optoelectronic system components and are used to illustrate various modulator functions in lightwave logic operations and neural networks.     

Field effects on the refractive index and absorption coefficient in AlGaAs quantum well structures and their feasibility for electrooptic device applications

Electroreflectance and electroabsorption measurements have been carried out to clarify field effects on the refractive index and absorption coefficient in AlGaAs quantum well structures. The observed electroreflectance spectra show very clear exciton-induced features at room temperature. A maximum variation of the refractive index in each quantum well at a photon energy near the lowest excitonic transition gap is obtained to be 4 percent induced by a 10⁵V/cm field modulation. Electroreflectance and electroabsorption spectra are shown to demonstrate a relation between dispersion curves of the field-induced variations in the refractive index and absorption coefficient in the quantum well structure. Based on the obtained results, operation of an electroabsorption modulator with the capability of small frequency chirping and an efficient optical switch are discussed from the practical point of view.     

Field-induced optical effect in a five-step asymmetric coupledquantum well with modified potential

A five-step asymmetric coupled quantum well (FACQW) is demonstrated and analyzed for its field-induced optical effect. A strong combined exciton absorption peak is caused by e1hh2 and e2hh1 transitions for the TE mode and e1lh2 and e2lh1 for the TM mode at a small negative applied electric field, while two separated exciton absorption peaks appear at positive applied electric field. The large change of the exciton absorption strength produces large positive refractive index change whose value is larger by over one order of magnitude compared to that in a rectangular quantum well (RQW) when the operation wavelength is away from the absorption edge. A tensile-strained FACQW is employed to improve the polarization dependence. The difference of the refractive index change for TE and TM modes is under 2% while that in the unstrained FACQW is larger than 50%. A modified FACQW structure is also studied for a negative refractive index change. The strong combined exciton absorption peak without a red shift is observed in the measurement of photocurrent spectra of the FACQW at room temperature. The absorption properties of the FACQW under high applied electric field are also observed and analyzed. The measured results are in good agreement with bur numerical analysis. Finally, ridge-waveguide Mach-Zehnder modulators have been fabricated by using the FACQW and RQW structures. The half-wave voltage of the FACQW modulator is as low as 3 V while that of the RQW modulator is 8 V. The result proves that the FACQW structure indeed has larger refractive index change than that of the RQW structure. These properties obtained with the FACQW family have a great potential for application to ultrafast and low-voltage optical modulators and switches     

Refractive index modulation based on excitonic effects inGaInAs-InP coupled asymmetric quantum wells

The effect of excitons in GaInAs-InP coupled asymmetric quantum wells on the refractive index modulation, is analyzed numerically using a model based on the effective mass approximation. It is shown that two coupled quantum wells brought in resonance by an applied electric field will, due to the reduction in the exciton oscillator strengths, have a modulation of the refractive index which is more than one order of magnitude larger than in a similar quantum well structure based on the quantum confined Stark effect, but with no coupling between the quantum wells. Calculations show that combining this strong electrorefractive effect with self-photo-induced modulation in a biased-pin-diode modulator configuration, results in an optical nonlinearity with a figure of merit of 20 cm³/J at a wavelength of 1.55 μm. This value is large compared to optical nonlinearities originating from band edge resonance effects in III-V semiconductor materials     

Proposal for quantum-dot electroabsorption modulator

Quantum-dot, electroabsorption materials are proposed to obtain low internal loss, large absorption modulation, and negative alpha parameters (blue chirped pulses). These characteristics come from the discrete state absorption associated with three dimensional confinement in quantum dots compared to the band absorption of quantum well, quantum wire, and bulk materials. In addition, type II quantum-dot structures are also proposed to obtain the same optical modulation characteristics with the potential for greater immunity to saturation effects.     

Monolithic integration of an amplifier and a phase modulatorfabricated in a GRINSCH-SQW structure by placing the junction below thequantum well

We present results of a novel semiconductor optical amplifier device in which an amplifier and a phase modulator are integrated into a fundamental transverse-mode ridge waveguide. By placing the p-n junction below the quantum well (QW) during epitaxial growth and utilizing the effect of the electric field on the depletion width, the phase of the light from an integrated optical amplifier-modulator is varied. For a modulation reverse bias voltage range of 0.4 V, we have demonstrated a 360°/mm phase shift with less than 1.2 dB of amplitude modulation     

电光双向强度调制器的隔离度研究

设计和制作了应用于微波全双工收发系统的马赫-曾德尔型电光双向强度调制器。根据电光调制器中电信号对光信号的调制叠加原理,通过计算和仿真,分析了因调制电极设计电场与实际电场分布差异导致的器件隔离度劣化。通过对比不同调制电极结构的分析设计和仿真优化结果,得到3.5GHz以上频段隔离度优于-30dB的电光双向强度调制器设计结构。制备出的电光双向强度调制器在5~17GHz范围内隔离度优于-30dB。     

Saturable absorber intensity modulator

We propose and demonstrate a reflection-type optical modulator, with surface-normal architecture that exploits saturation of absorption in semiconductor quantum wells pumped optically. The modulation section, composed of quantum wells placed within a Fabry-Perot cavity, is exposed to an intensity-modulated beam generated by an in-plane laser integrated monolithically on the same wafer. Since the modulation section and the in-plane laser share the same medium, an efficient coupling between the control beam and the signal beam is achieved. Design guidelines and device performance are presented. We demonstrate that the modulator provides an efficient light modulation mechanism that is sufficient to actively mode-lock an erbium-doped fiber laser.     

Quantum interference, Stark, and carrier density infraredelectrooptical modulation based on intersubband transitions inasymmetrical quantum wells

A new approach toward studying electrooptic modulation utilizing intersubband transitions in quantum wells is presented. Using first-order perturbation theory for analyzing the effect of a dc electric field on the linear susceptibility, an understanding of the mechanisms which give rise to intersubband electrooptic susceptibility is presented. This includes modulation due to the dc Stark effect, modulation due to coherent interference of the envelope states, and modulation of the carrier densities in populated subbands. We study several structures that maximize the electrooptic susceptibility of a particular origin and discuss the suitability of the various schemes for practical realizations. Finally, we derive a figure of merit for each type of modulator, taking into account the linear intersubband absorption, and show that highly efficient near-infrared modulators that operate at a wavelength of 1.5 μm can be realized     

光通信高速率调制系统设计及性能仿真测试

为了奠定未来光通信高效实行的基础,需要研究高速高质量的激光调制系统。本文在一般调制系统设计原则基础之上,从核心器件入手,选定了以DFB激光器、EAM为主的调制整体框架;之后进行了激光器周边辅助电路、调制器驱动电路以及调制器自身结构的精细化设计,形成了高速调制系统设计框图;利用光通信仿真平台Optisystem在临界调制速率1.12 Gbits、高速5 Gbits、超高速10 Gbits三种梯度下进行了该系统的性能测试。结果显示:1.12 Gbits到5 Gbits测试性能优异且稳步提升,表明该系统能够满足现阶段一般条件下的光通信高速调制需求;但10 Gbits下系统表现不够稳定,说明适应超高速条件下的调制系统还需进一步深入研究。该调制系统的设计和仿真对后续实测工作的开展具有重要指导意义。     

All-binary InAs/GaAs optical waveguide phase modulator at 1.06μm

We demonstrate the use of multiple [(InAs) (GaAs)]/GaAs short-period strained-layer superlattice quantum wells for optical waveguide modulation at 1.06 μm. We achieve π phase modulation with 2.3 V applied (V_π×L=4.6 V mm, or 39°/V mm) in the presence of negligible absorption change using this all-binary modulator     

Combining self-phase modulation and optimum modulation conditionsto improve the performance of 10-Gb/s transmission systems using MQWMach-Zehnder modulators

For 10-Gb/s transmission over nondispersion shifted fiber, the combined use of self-phase modulation (SPM) and joint optimization of the bias and modulation voltages to increase the dispersion limited transmission distance is considered for multiple quantum well Mach-Zehnder modulators. For the dual drive (push-pull) modulation format, the dependence of the receiver sensitivity on fiber length and average transmitted optical power is determined for both conventional and π phase-shift modulators with either symmetric or asymmetric Y-branch waveguides. When SPM is negligible and the optical extinction ratio is maximized, the modulator design must he considered carefully in order to increase the transmission distance. By combining SPM and optimum modulation conditions, the dependence of the system performance on the modulator design is reduced substantially. For an average transmitted optical power of 12.5 dBm, the receiver sensitivity for transmission over 140 km of fiber varies by only 0.3 dB for the different modulator designs. This compares with a variation of 3.1 dB for maximum extinction ratio modulation     

Widely tunable 10 Gbit/s separate absorption and modulation Mach-Zehnder wavelength converter

A fully monolithic separate absorption and modulation region wavelength converter requiring no bias tees has been fabricated. The device consists of a transmitter comprising a sampled-grating DBR laser and series-push-pull Mach-Zehnder modulator, and a receiver composed of a linear semiconductor optical amplifier and a quantum well pin photodetector. The wavelength converter has a 13 GHz bandwidth and demonstrates error-free operation at 10 Gbit/s with unity gain     

Anisotropic electroabsorption and optical modulation in InGaAs/InAlAs multiple quantum well structures

The first measurements to be made of large anistropic electroabsorption and modulation of long-wavelength light propagating along the plane of InGaAs/InAlAs multiple quantum well (MQW) structures grown by molecular beam epitaxy (MBE) are reported. Photocurrent response of waveguide p-i-n diodes is studied for incident light polarization parallel and perpendicular to the MQW layers. Photocurrent increase with reverse bias throughout the entire photoresponse spectrum is observed for both polarizations. The MQW p-in optical modulator shows a capacitance-limited pulse response of 250 ps and the modulation depth is 14 percent.     

Electric field switching in a resonant tunneling diodeelectroabsorption modulator

The basic mechanism underlying electric field switching produced by a resonant tunneling diode (RTD) is analyzed and the theory compared with experimental results; agreement to within 12% is achieved. The electroabsorption modulator (EAM) device potential of this effect is explored in an optical waveguide configuration. It is shown that a RTD-EAM can provide significant absorption coefficient change, via the Franz-Keldysh effect, at appropriate optical communication wavelengths around 1550 nm and can achieve up to 28-dB optical modulation in a 200-μm active length device. The advantage of the RTD-EAM over the conventional reverse-biased p-n junction EAM, is that the RTD-EAM has, in essence, an integrated electronic amplifier and, therefore, requires considerably less switching power     

Carrier screening of electric field and electroabsorption saturation in InGaAs-GaAs quantum well structure

Saturation of the electroabsorption is obtained in the quantum confined Stark effect (QCSE) at low optical intensities and low electric fields, in a strained InGaAs-GaAs quantum well modulator structure operating at the exciton energy. This effect is attributed to carrier-induced screening of the internal electric field, and its implications for devices using the QCSE are discussed.<>     

Acoustooptic modulator using the Stark effect and stress-inducedbandgap changes

An analysis of the semiconductor multiple-quantum-well (MQW) acoustooptic modulator is performed. The strain and electric-field components generated by surface acoustic waves (SAW's) in a multilayered semiconductor piezoelectric structure are presented. The effects of a SAW propagation on the modulator optical parameters are studied. Two kinds of SAW effects in the modulator structure are taken into account: the variation of the excitonic energies of the MQW due to strain and the companion electric field generated by the SAW is determined. The hole's effective mass change induced by the SAW is analyzed and included in the Schrodinger equation. The optical absorption coefficient spectra of the modulator as a function of the SAW power is presented. Experimental results concerning optical absorption variations of quantum-well structures produced by various SAW powers are shown     

Modulation of intersubband absorption in tunnel-coupled quantum wells in electric fields

Modulation of absorption of middle-infrared radiation in double tunneling-coupled quantum wells in longitudinal electric fields is studied. A specific feature of the quantum wells is the small separation in energy between the two lower levels. As a consequence, the levels may exhibit “anticrossing” even in low transverse electric fields. An interpretation of the change in intersubband absorption is suggested. The interpretation is based on the assumption that a transverse electric-field component may appear in the structure. The change in the absorption coefficient is calculated taking into account the redistribution of electrons between size-quantization subbands and the changes in the temperature of electrons in the subbands in the longitudinal electric field, as well as the changes in the optical matrix elements, the energies of transitions, and the concentrations of electrons in the subbands in the transverse electric field. The possibility of using the structure for the efficient modulation of middle-infrared light with the photon energy 136 meV is shown.     

Optical charge modulation as an internal voltage probe for CMOS ICs

The use of optical charge modulation of refractive index as a probe for internal voltages in CMOS integrated circuits is examined. The Kramers-Kronig relations are used to calculate the index of refraction from absorption coefficient measurements. Both free-carrier depletion and the internal electric field of the p-n junction modulator are shown to affect the measurement sensitivity of this technique. The sensitivity is calculated from experimental results and is dependent on which device, NMOS or PMOS, is under test. These results contradict the predictions of a previous model which was developed using the Drude theory of free-carrier modulation of the refractive index