Ultrafast all-optical modulation of infrared radiation viametal-semiconductor waveguide structures

We present a novel optical-optical semiconductor switching technique for application to infrared laser beam modulation and ultrashort infrared laser pulse switching. This method relies on the ultrafast optical excitation, with femtosecond above-bandgap laser radiation, of an air-filled metal-clad semiconductor waveguide. Guided electromagnetic wave analysis combined with time-varying dielectric properties of the semiconductor layer are used to investigate the ultrafast switching speed of the structure. The device is capable of modulation at various infrared wavelengths. In particular, we investigate intensity modulation of the quasi-TE₁₀ mode for 10.6-μm laser radiation. At an electron-hole photoinjection density of ~1.8×10¹⁸ cm^-3, an extinction ratio of 83 dB is demonstrated. This ratio is significantly higher than that exhibited by current optical-optical semiconductor switches. Potential applications to all-optical Mach-Zehnder metal-clad semiconductor modulators and self-limiting switches are also discussed     

Phase and amplitude modulation of light by light in silicon-on-insulator waveguides

Waveguided light modulation induced by visible light irradiation in oxidised silicon-on-insulator waveguides is reported. Both induced absorption and phase modulation were characterised. Phase modulation was measured in a waveguided Mach-Zehnder configuration. Less than 200 mW of power was required for 180/spl deg/phase change.     

Vertically integrated silicon-on-insulator waveguides

A SiO₂-Si-SiO₂-Si-SiO₂-Si structure produced by the separation by implantation of oxygen (SIMOX) process used for dual vertically integrated waveguiding in silicon at λ=1.3 μm is discussed. Independent waveguiding is observed when 2-μm-thick Si cores are separated by 0.36-μm-thick SiO₂ . Coupled waveguiding is found for an 0.12-μm intercore oxide thickness     

Low-loss silicon-on-insulator photonic crystal waveguides

Silicon-on-insulator-based 2D photonic crystal waveguides have been optically characterised. The measured total insertion loss is below 19 dB in the waveguides with unpolarised light. With the length mask technique, it is found that the photonic crystal waveguides show propagation losses below 4 dB/mm     

Ultrafast all-optical switching using highly nonlinear chalcogenideglass fiber

Ultrafast all-optical switching with a switching power of 14 W was demonstrated in a Kerr shutter configuration using a single-mode As₂S₃-based glass fiber only 48 cm long. The nonlinear refractive index of the fiber was evaluated from the switching characteristics to be n₂=4.2×10^-14 (cm²/W), which is higher by two orders of magnitude than silica glass fiber     

Fabrication and characterization of narrow-band Bragg-reflectionfilters in silicon-on-insulator ridge waveguides

We describe the design, fabrication and measurement of an integrated-optical Bragg grating filter, operating at a freespace wavelength of 1543 nm, based upon a silicon-on-insulator (SOI) ridge waveguide. The measured spectral response for a 4-mm long grating has a bandwidth of 15 GHz (0.12 nm), and shows good agreement with theoretical predictions     

Ultrafast all-optical multiple quantum well integrated optic switch

An all-optical integrated Mach-Zehnder interferometer switch is implemented using selective area disordering of a multiple quantum well structure. Ultrafast all-optical switching with an adjustable switching window ranging from 2 to 100 ps is demonstrated. The switch contrast ratio in this preliminary device is measured to be 9 dB     

Single-mode and polarization-independent silicon-on-insulator waveguides with small cross section

The fabrication restrictions that must be imposed on the geometry of optical waveguides to make them behave as single-mode devices are well known for relatively large waveguides, with shallow etch depth. However, the restrictions for small waveguides (/spl sim/1 /spl mu/m or less in cross section) are not well understood. Furthermore, it is usually a requirement that these waveguides are polarization independent, which further complicates the issues. This paper reports on the simulations of the conditions for both single-mode behavior and polarization independence, for small and deeply etched silicon-on-insulator (SOI) waveguides. The aim is to satisfy both conditions simultaneously. The results show that at larger waveguide widths, waveguide etch depth has little effect on the mode birefringence because the transverse-electric (TE) mode (horizontal-polarized mode) is well confined under the rib region. However, at smaller rib widths, the etch depth has a large influence on birefringence. An approximate equation relating the rib-waveguide width and etch depth to obtain polarization-independent operation is derived. It is possible to achieve single-mode operation at both polarizations while maintaining polarization independence for each of the waveguide heights used in this paper but may be difficult for other dimensions. For example, a 1-/spl mu/m SOI rib waveguide with an etch depth of 0.64 /spl mu/m and rib width of 0.52 /spl mu/m is predicted to exhibit such characteristics.     

Low threshold and tunable all-optical switch using two-photon absorption in array of nonlinear ring resonators coupled to MZI

In this paper a novel structure based on Mach-Zehnder Interferometer (MZI) and an array of nonlinear ring resonators for all-optical switching including very low threshold, fast and all-optically tunable is presented. In this proposal, an array including N ring resonators is coupled to one arm of MZI to introduce the required nonlinear phase shift to switching. Also, two-photon absorption phenomenon is used and will be proposed as another alternative for manipulation of optical switching properties. The proposed idea needs small chip area (∝1/Ntraditional cases) for implementation. We show that the switching threshold intensity can be decreased with increase of N, decrease of the coupling coefficient and increase of the two-photon absorption coefficient. In this structure with traditional optical fibers with the low nonlinear index of refractions and using an array including 15 ring resonators with 6 mm diameter for each rings only 3 mW for the switching threshold power is required. Also, using erbium-doped fiber for implementation of the rings with the high nonlinear index of refractions and five ring resonators including 0.3 mm diameter, the switching threshold power can be reduced to μW level. If the nonzero two-photon absorption coefficient is considered, the switching threshold will be very smaller than the above-mentioned values. We show that with changing the optical amplifier gain the switching threshold can be tuned. So, the proposal can be used as a functional block for the integrated optical switch with very small and tunable threshold.     

Singlemode and polarisation free conditions for small silicon-on-insulator waveguides

An analysis based on simulation of polarisation dependence and the singlemode condition for small and deeply etched silicon-on-insulator waveguide dimensions is presented. Examples of designs that satisfy both requirements simultaneously have been evaluated using the three-dimensional full vectorial beam propagation method.     

Optimization of silicon-on-insulator based optical switch using tapered waveguides

Optimized 2×2 switches based on silicon-on-insulator (SOI) were demonstrated. In the design, single mode rib waveguidesand multimode interferences (MMIs) are connected by tapered waveguides to reduce the mode coupling loss between thetwo types of waveguides. The average insertion loss of the switches is about -16.9 dB and the excess loss of one is measuredof -1.3 dB. The worst crosstalk is larger than 25 dB. Experimental results indicate that some of the main characteristics ofoptical switches are improved in the modified design, which is according with theoretic analysis. The novel design can beused to improve the characteristics of optical switch matrixes based on 2×2 switch units.     

Ultrafast all-optical switching in an asymmetric Fabry-Perot deviceusing low-temperature-grown GaAs

We report the first ultrafast all-optical switching using low-temperature-grown bulk semiconductor material (LT-GaAs) in a compact asymmetric Fabry-Perot device. We obtain 3-ps response times, 2.8-dB insertion loss, 40-nm bandwidth, and 15-dB contrast ratios using 200-fJ/μm² average switching energy flux     

Ultrafast time-domain technology and its application in all-optical signal processing

In this paper, we review recent advances in ultrafast optical time-domain technology with emphasis on the use in optical packet switching. In this respect, several key building blocks, including high-rate laser sources applicable to any time-division-multiplexing (TDM) application, optical logic circuits for bitwise processing, and clock-recovery circuits for timing synchronization with both synchronous and asynchronous data traffic, are described in detail. The circuits take advantage of the ultrafast nonlinear transfer function of semiconductor-based devices to operate successfully at rates beyond 10 Gb/s. We also demonstrate two more complex circuits-a header extraction unit and an exchange-bypass switch-operating at 10 Gb/s. These two units are key blocks for any general-purpose packet routing/switching application. Finally, we discuss the system perspective of all these modules and propose their possible incorporation in a packet switch architecture to provide low-level but high-speed functionalities. The goal is to perform as many operations as possible in the optical domain to increase node throughput and to alleviate the network from unwanted and expensive optical-electrical-optical conversions.     

Time-resolved nonlinear absorption modulation in GaAs/AlGaAsmultiple-quantum-well waveguides at 1.06 μm

Time-resolved nonlinear absorption modulation due to two-photon absorption (TPA) and free carrier absorption (FCA) was demonstrated in GaAs/AlGaAs waveguides at 1.06 μm. For a peak input intensity of 4.5×10¹¹ Wm^-2, there was a maximum absorption modulation of 35%. This was caused by TPA, so that the recovery time was limited by the laser pulsewidth alone. At a peak input intensity of 1.5×10¹² Wm^-2, a 65% absorption modulation was observed. This consisted of two components: an instantaneous effect due to TPA and also a slower component, attributed to FCA, whose recovery time was limited by the carrier lifetime. A theoretical model was also derived which provided a good fit to the experimental data     

1 × 2 MOEMS switch based on silicon-on-insulator andpolymeric waveguides

A new planar optical switch concept based on a silicon-on-insulator microelectromechanical system (MEMS) structure with post-processed polymeric waveguides is reported. The actuation voltage is 20 to 50 V, and the switching time as low as 30 μs. A crosstalk of -32 dB has been measured     

Anisotropic two-photon transitions in GaAs/AlGaAs multiple quantumwell waveguides

The two-photon absorption coefficient and induced nonlinear refractive index in both TE and TM modes of GaAs/AlGaAs multiple-quantum-well (MQW) strip-loaded channel waveguides are measured in the wavelength range from 1490 nm to 1660 nm. The two-photon absorption coefficient ranges from almost zero to 12 cm/GW, and the nonlinear refractive index is always on the order of 10^-13 cm ²/W in the wavelength window investigated. Strong nonlinear anisotropy is observed. The values of the two-photon absorption coefficient are obtained by calibrating the inverse transmission measurements. Those of the nonlinear refractive index are obtained by monitoring the signal spectrum broadening due to self-phase modulation. The wavelength dependences of the measured two-photon absorption coefficient and nonlinear refractive index are reasonably consistent with the theoretical predictions     

Novel piezoelectric-barrier heterostructure for all-optical light modulation

We propose a novel quantum heterostructure especially designed to be used as an optically-controlled light modulator device with moderate optical power densities and without the need of electrical contacts. The device exploits the piezo-electric effect in strained semiconductor layers and its principle relies on the fact that upon optical excitation, a substantial space charge field perpendicular to the quantum well plane is established in every period of the heterostructure, owing to efficient photocarrier separation in piezoelectric barrier layers. We present preliminary results which demonstrate a photo-induced electric field of more than 30 kV/cm with about 100 W/cm² inducing large spectral shifts in the photo-luminescence spectra of a CdHgTe quantum well.     

基于PPLN波导的全光译码器的设计与研究

随着互联网的发展,网络数据流量极速增长,全光网(AON)概念的提出有利于解决可用带宽受限、信号传输速率较低等问题。其中,全光逻辑信号处理为AON的重要组成部分。根据周期性极化铌酸锂(PPLN)波导的二阶非线性效应原理,将产生的输出信号连续送入不同的PPLN波导进行叠加处理,设计出全光2线-4线译码器的波导级联结构,通过数值计算仿真得到了波形图和眼图,分析了全光2线-4线译码器输出信号的半高全宽(FWHM)、峰值功率、延迟时间和消光比。仿真结果表明:采用PPLN波导级联的方式实现了2线-4线译码器在光域中的逻辑功能,同时保证了光信号的传输质量,为PPLN波导的全光信号处理提供了新的器件类型。     

Low-power all-optical 8-to-3 encoder using photonic crystal-based waveguides

Photonic Network Communications - In this paper, we propose an all-optical 8-to-3 encoder based on photonic crystal. The structure includes 17 waveguides to make the appropriate connections between...