Electroabsorptive Fabry-Perot reflection modulators with asymmetricmirrors

A normally-on electroabsorptive surface-normal Fabry-Perot reflection modulator is reported with an on/off ratio of 22, insertion loss of 3.7 dB, and bandwidth of 3.4 nm for an operating voltage swing of 11 V. The asymmetric Fabry-Perot structure is made with asymmetric mirrors using a quarter-wavelength grating of 15 1/2-periods on the bottom and an air-semiconductor interface on the top. The active region is 1.4 μm thick and composed of 100Å GaAs/100 A Al_0.2Ga_0.8As multiple quantum wells. The structure provides both high-efficiency and bandwidth for surface-normal modulators     

Surface-normal electroabsorption reflection modulators usingasymmetric Fabry-Perot structures

The authors discuss a family of surface-normal electrooptic reflection modulators using asymmetric Fabry-Perot (ASFP) structures with GaAs-AlGaAs multiple quantum wells (MQWs) as the active medium. When an optimized top mirror reflectivity (~75%) is used, a voltage swing as low as 2 V is enough to change the device reflectivity by more than 40% with a contrast ratio of 50. A comparison with various Fabry-Perot structures shows that the ASFP is the best structure in terms of its operating voltage requirement and optical bandwidth. A sensitivity analysis shows that tolerances of 2 nm in operating wavelength, 0.5 V in operating voltage, 0.3% in layer thickness control or 10°C in temperature variation can be expected from ASFPs with a finesse ~10     

High-frequency electrooptic Fabry-Perot modulators

Electrooptic modulators built from GaAs/Al/sub x/Ga/sub 1-x/As Fabry-Perot cavities operating up to 6.5 GHz are reported. The measured frequency response agrees well with the one predicted using an equivalent circuit model derived from high-speed electrical measurements. The parasitic capacitances have been reduced to approximately 30 fF by fabricating the devices on semi-insulating GaAs substrates and integrating them with on-wafer bound pads which have dimensions compatible with microwave coplanar probes.<>     

Millimeter-wave asymmetric Fabry-Perot modulators

We have designed, fabricated, and characterized GaAs-AlGaAs (λ=864 nm) asymmetric Fabry-Perot modulators with ≈37 GHz modulation frequency response, comparable to the fastest waveguide modulators. The modulation response saturates at high optical powers due to saturation of the excitonic absorption and heating effects, but the frequency response is independent of the incident optical intensity, since it depends only on the RC time constant, and not on the carrier transit time. The device design takes advantage of the fact that the quantum-confined Stark effect is more pronounced at some distance from the absorption edge to achieve a modulator with ⩾20 dB contrast and ≈3 dB insertion loss for ±2 V operating voltage, but only 21 fF capacitance. The DC bias used to move the operating point off the absorption edge has the additional benefits of improving the linearity and chirp of the device, as well as the saturation intensity. Here we present measurements of the modulation and photocurrent responses of the modulators, calculate the RC and transit times for the device, analyze the saturation mechanisms, and discuss the linearity and chirp of the device from the perspective of a high-speed digital optical communications system     

Temperature-independent low-voltage wide-bandwidth optical modulators

Temperature-independent electro-optic modulators have been constructed using lithium niobate. A frequency response extending to 2 GHz has been achieved with a relatively low drive power. The modulators have been used with a 0.63 ?m laser and are easily aligned on a standard optical bench.     

Fabrication and characterisation of a 4×4 array ofpolymer-based asymmetric Fabry-Perot reflection modulators

The fabrication and optical characterisation of a 4×4 array of asymmetric Fabry-Perot (AFP) electro-optic modulators is reported. The electro-optic material used, a cross-linked polymer based on an azo-benzene derivative, offers the potential for high time-bandwidth modulation. The uniformity of the spatial light modulator is evaluated, and a contrast ratio exceeding 4:1 can be obtained simultaneously at each of the 16 pixel positions     

Low-voltage superlattice asymmetric Fabry-Perot reflection modulator

A normally off transverse superlattice asymmetric Fabry-Perot modulator that has a contrast ratio of more than 26:1 at the Fabry-Perot resonance, and a reflection change of 33% at a wavelength of approximately 20 degrees AA away from Fabry-Perot mode for an operating voltage swing of <3 V is discussed. The structure contains an active region of 52 periods of a 30 AA GaAs-30 AA Al/sub 0.3/Ga/sub 0.7/As superlattice, embedded between top and bottom quarter-wave grating mirrors. The modulation is achieved by reducing the cavity loss at the Fabry-Perot resonance through the field-induced effective absorption edge blue-shift in the superlattice.<>     

Fabry-Perot bistable cavity optimization on reflection

An analysis of bistable Fabry-Perot action in reflection is presented. Optimum values for the cavity reflective coatings and cavity length are determined. Reasons are given why reflection-mode operation rather than transmission-mode operation should be preferred for device applications. Two prototype devices, an all-optical SR (Set-Reset) flip-flop and an all-optical clock, are described.     

Analog characterization of low-voltage MQW traveling-wave electroabsorption modulators

In this paper, high-speed traveling-wave electroabsorption modulators (TW-EAMs) with strain-compensated InGaAsP multiple quantum wells as the absorption region for analog optical links have been developed. A record-high slope efficiency of 4/V, which is equivalent to a Mach-Zehnder modulator with a V/sub /spl pi// of 0.37 V and a high extinction ratio of > 30 dB/V have been measured. A detailed study of the nonlinearity and the spurious-free dynamic range (SFDR) is presented. By optimizing the bias voltage and the input optical power, the SFDR can be improved by 10-30 dB. After minimizing the third-order distortion, an SFDR as high as 128 dB-Hz/sup 4/5/ is achieved at 10 GHz. A simple link measurement was made using this EAM and an erbium-doped fiber amplifier and a 50-/spl Omega/ terminated photodetector. At 10 GHz, a link gain of 1 dB is achieved at a detected photocurrent of 7.6 mA with higher gains at lower frequencies. The dependence of link gains on bias voltage, input optical, and radio frequency powers are investigated in detail.     

Fabry-Perot electroabsorption modulators for high-speed free-space optical communication

Retroreflective modulators are key components in free-space optical communication systems between mobile platforms and users. Wide-aperture surface-normal electroabsorption modulators based on GaAs-AlGaAs quantum wells embedded in an asymmetric Fabry-Perot cavity are designed and fabricated with a high yield process on 10-cm wafers. It is shown that the modulator yield and its speed are improved significantly by a pixellated approach in which monolithic modulators are divided into 4-64 pixels. The fabricated 1.5/spl times/1.5 cm/sup 2/ devices exhibit contrast ratios of 8 dB at a driving voltage of 8 V and a modulation frequency higher than 10 MHz, which provides low noise and fast data transmission for long distance free-space optical communication.     

High power performance of asymmetric Fabry-Perot MQW modulators

We have demonstrated experimentally that a normally-on surface normal asymmetric Fabry-Perot modulator can still operate at high optical powers (equivalent to intensities of >20 kW/cm²). We have identified two distinct operating regimes: one thermally assisted regime where the maximum reflection change increases at high powers and peak values of 75% are achieved, and one nonthermal regime where reflection changes approaching 55% are achieved albeit at higher applied voltages     

Design of InGaAsP multiple quantum-well Fabry-Perot modulators forsoliton control

The use of synchronous optical modulators is effective in reducing the pulse timing jitter in long-distance soliton transmission. The inherently polarization-insensitive characteristics of the Fabry-Perot multiple quantum-well (MQW) electroabsorption modulator make it a potentially suitable device for this application. We investigate the intensity and phase modulation characteristics of symmetric and asymmetric Fabry-Perot modulators, and show that, by positioning the resonant wavelength <30 nm away from the exciton absorption peak to obtain negative chirp operation, both configurations can be used to successfully reduce timing jitter in a 20 Gb/s soliton system     

High tolerances for a low-voltage, high-contrast, low-insertion-loss asymmetric Fabry-Perot modulator

The authors report on the reduction of the operating voltage of a GaAs/AlGaAs multiple-quantum-well asymmetric Fabry-Perot modulator to approximately 4.2 V for a high-contrast ( approximately 15 dB), low-insertion-loss ( approximately 3 dB) device. This improvement on modulator performance makes the device suitable for a practical system and has been accomplished by an increase of the front reflectivity to approximately 43%. The authors discuss general design issues and show that the higher finesse cavity does not have to lead to low fabrication and environmental tolerances. On the contrary, this device's tolerance to fabrication and environmental factors is expected to have been improved.<>     

Very low voltage, normally-off asymmetric Fabry-Perot reflection modulator

Using the electro-absorptive properties of approximately=150 AA quantum wells in an asymmetric Fabry-Perot modulator (AFPM), a normally-off reflection modulator with very low drive voltage was demonstrated. With this device contrasts of more than 6 Db at normal incidence in reflection have been achieved, with a voltage swing of only 3.5 V and >     

The mode control of asymmetric Fabry-Perot optical modulators withmultiple quantum wells

The asymmetric Fabry-Perot (ASFP) mode position with the thickness of different index coating layer is calculated. The reason for the blue shift of the ASFP mode with the increasing thickness of low index coating layer is analyzed and this phenomenon is observed in experiments. With the wet-etching method, the ASFP mode can be tuned to the desired wavelength and thus the deviation of growth can be compensated. This method is used to improve the contrast ratio of modulators. With the ASFP mode located at different positions relative to the unbiased e-hh peak, different modulation characteristics are demonstrated     

Internal biasing by δ-doping for low-voltage, high-bandwidthquantum-well optical modulators

Optical intensity modulators must exhibit low-voltage operation, low insertion loss, high contrast ratio and high electrical bandwidth. Electroabsorption calculations for semiconductor quantum wells predict that internal biasing by strategic δ-doping can produce greatly improved low-voltage operation in waveguide or normal-incidence modulators, for a specified insertion loss and contrast ratio, without compromising electrical bandwidth. Growth of strategically δ-doped electrorefractive intensity modulators is shown computationally to be insensitive to nonreproducibility in layer growth. Internal biasing by strategic doping is of potential value across the whole range of modulator applications     

K-band operation of asymmetric Fabry-Perot modulators

The authors report preliminary high-frequency, small-signal optical measurements of asymmetric multiple-quantum well (MQW) Fabry-Perot electrooptic modulators which indicate that the electrical bandwidth for these devices is about 15 GHz at the onset of saturation, and as high as 21 GHz at low optical intensity-higher than any other measurements published to date. The modulators, 30 μm×30 μm in size, are integrated with on-chip microwave probe pads. The authors detail the fabrication process developed to achieve these high operating frequencies and predict from device models the maximum RC-limited operating frequencies for these devices     

Novel T-rail electrodes for substrate removed low-voltage high-speed GaAs/AlGaAs electrooptic modulators

A novel traveling-wave electrode utilizing capacitively loaded T-rail elements was developed for low-voltage high-speed substrate-removed GaAs/AlGaAs electrooptic modulators. Electrodes with varying dimensions were fabricated and characterized. Electrode phase velocity, characteristic impedance, loss coefficient, and capacitive loading were extracted from the measured s-parameters up to 40 GHz. Electrode was also simulated using a finite-element solver. The measured and calculated electrode capacitance values were found to be in excellent agreement, showing that the electrode can be precisely designed. Approaches were outlined to provide a group velocity-matched very high-speed modulator electrode suitable for a low drive-voltage substrate-removed GaAs/AlGaAs electro-optic modulator     

GaAs-AlGaAs multiquantum well reflection modulators grown on GaAsand silicon substrates

Measurements of GaAs-AlGaAs multiple-quantum-well (MQW) reflection modulators grown simultaneously on GaAs and silicon substrates are presented. Comparable electroabsorption is observed, with contrast ratios of about 4:1 for both modulators at 20 V. The absorption spectrum of the GaAs-on-Si quantum well shows a single exciton peak, which leads to certain improvements in modulator performance. This study is very encouraging for the growth of GaAs MQW modulators on silicon integrated circuit chips for off-chip communication     

Low-power nonlinear Fabry-Perot reflection in CdHgTe at 10 µm

Measurements of the intensity dependence of CO2laser radiation reflected from a polished Cd0.23Hg0.77Te etalon at 77 K results in nonlinear features at power levels on the order of 1 mW, in good agreement with a theory which includes bandgap resonant nonlinearities and Auger carrier recombination. A high-reflectivity rear coating enhances these features, giving a characteristic which demonstrates differential optical gain. The effects of heating are discussed in this unusual case in which the electronic and thermal contributions to the intensity dependence have the same sign.