Electroabsorption properties of InGaAs/InAlAs multiple quantum wellstructures at 1.5 μm

The electroabsorption properties of InGaAs/InAlAs MQW structures are characterised in terms of Δα, Δα/F and Δα/α₀, where Δα is the electroabsorption, α₀ is the residual absorption coefficient under zero bias, and F is the applied electric field. The limitations of these structures for 1.5 μm modulators are primarily due to the relatively small Δα/F values as a result of the small well width. The results are compared with the literature     

Large electroabsorption effect in GaInAs/InP multiple quantum well(MQW) optical modulator grown by OMVPE

To make short, high-speed electroabsorption modulators, it is necessary to use a material and device structure that displays a large change in absorption coefficient, Δα. The authors report a device with Δα=7800 cm^-1, which provides an on/off ratio of 44:1 at λ=1.6 μm with a drive voltage of 10 V     

On the figures of merit for electroabsorption waveguide modulators

ΓΔα/F and Δα/α₀ (where Δα is the absorption change, α₀ is the residual absorption, F is the applied electric field, and Γ is the optical confinement factor in the waveguide) have been separately proposed as the relevant figure of merit for electroabsorption waveguide modulators. Using a quantitative and systematic argument, the authors show that they are both necessary and important to the total performance of the modulator     

Theoretical design optimization of multiple-quantum-wellelectroabsorption waveguide modulators

Optical on-off modulators require low insertion loss, high contrast ratio (CR), small drive power and large bandwidth or bit-rate. A systematic approach to optimize the total performance of these modulators based on the quantum-confined Stark effect is presented here. The approach consists of minimizing the power/bandwidth ratio while satisfying a given CR and insertion loss. Our design consists of a large-core multimode passive waveguide with a thin buried active layer. The passive waveguide is designed to yield a high coupling efficiency to conventional single-mode fibers. The quantum well material structure is designed to maximize Δα/ΔF², while maintaining a sufficiently large Δα/α₀, where Δα is the absorption change, α₀ is the residual absorption at zero bias, and ΔF is the swing of the applied electric field. Our theoretical model shows that i) wider quantum wells give larger Δα/ΔF², and ii) the bandwidth/power ratio as high as 4 GHz/mW can be achieved simultaneously with small insertion loss, For example, with a drive voltage of 3 V, an RC limited bandwidth as high as 60 GHz is predicted, while a contrast ratio of 20 dB and a total insertion loss of 4.5 dB may also be obtained     

Electric-field-induced refractive index changes in InGaAs-InAlAsasymmetric coupled quantum wells

Two kinds of InGaAs-InAlAs asymmetric coupled quantum wells (QW's) were numerically analyzed in an effort to enhance change in the refractive index (Δn) at longer wavelengths where absorption coefficient is small. The analysis reveals the operating voltage, Δn, chirp parameter, Δn/Δk, (Δk: change in the extinction coefficient) and figure of merit, Δn/α, (α: absorption coefficient) can be improved by these two kinds of the QW's. This improvement is caused by an abrupt electric-field-induced increase or decrease in the oscillator strength for the lowest heavy hole exciton     

Design and demonstration of polarization-insensitive Mach-Zehnderswitch using a lattice-matched InGaAlAs/InAlAs MQW and deep-etchedhigh-mesa waveguide structure

The authors have designed and demonstrated a 2×2 Mach-Zehnder switch in view of polarization independence as well as low propagation loss (α) and absorption change (Δα). To obtain polarization-insensitive refractive index change (Δn), a lattice-matched InGaAlAs-InAlAs multiple quantum-well (MQW) with a large detuning wavelength was used. Moreover, to reduce the insertion loss difference between polarizations, we applied a multimode-interferometer 3-dB coupler and a deep-etched high-mesa waveguide structure. This switch, therefore, can provide polarization-independent operation about both driving voltage and insertion loss, which is indispensable to practical optical switching applications. We also paid attention to Δα suppression when we decided the value of wavelength detuning and the length of the phase shift region. We also investigated the wavelength dependence of the switch. Within 1530-1560 nm, which is the erbium-doped fiber amplifier (EDFA) gain band, polarization independence in the driving voltage and the crosstalk was maintained. This result shows that the switch is also applicable in wavelength division multiplexing (WDM) applications     

On the optimization of InGaAs-InAlAs quantum-well structures forelectroabsorption modulators

Several InGaAs-InAlAs multiple quantum-well structures grown by metalorganic vapor phase epitaxy (MOVPE), with various Ga content and quantum-well width, have been investigated for electroabsorption modulators (EAM's). The light-hole heavy-hole splitting, the chirp parameter, the insertion loss and the figure of merit ΓΔα/F of the different InGaAs-InAlAs structures have been evaluated with photocurrent, photoluminescence, absorption and X-ray measurements. It was then possible to experimentally study the influence of different parameters of the multiple quantum-well structures on the device performance. The use of tensile strained barriers are believed to be responsible for the improvement in the figure of merit. Structures with unresolved light-hole and heavy-hole transitions, with negligible chirp, with adequate insertion loss and with extremely high values for ΓΔα/F have been obtained, however, not simultaneously     

Amplitude and phase modulation in a 4 μm-thick GaAs/AlGaAsmultiple quantum well modulator

The experimental results of a 4 μm-thick GaAs/AlGaAs MQW modulator show an ~10:1 on/off ratio with an applied voltage of 20 V (Δα~6×10³/cm at E~50 kV/cm) and ~0.4 πrad of phase shift with an applied voltage of 10 V (Δn~0.04 at E~25 kV/cm). Such high electro-optical modulations have previously been reported only in MQW optical waveguide modulators     

Electroabsorption and electrooptic effect in SiGe-Si quantum wells:realization of low-voltage optical modulators

We have calculated the behavior of the band-to-band absorption coefficient in square, coupled, and graded bandgap Si_0.6Ge _0.4-Si quantum wells as a function of the transverse electric field. It is seen that due to the weak confinement of the electrons (ΔE_c⩽20 meV) the absorption of photons with energy equal to the interband transition energy can be reduced at very small values of the transverse electric field. This phenomenon lends itself to the design of efficient amplitude modulators. In addition, the resulting change in the refractive index is also large and the corresponding linear electrooptic coefficient is calculated to be as large as 1.9×10^-10 m/V in square wells. This effect could prove to be the basis for the realization of efficient Si-based electrooptic modulators. Device designs are discussed     

Hg0.4Cd0.6Te 1.55-μm avalanche photodiodenoise analysis in the vicinity of resonant impact ionization connectedwith the spin-orbit split-off band

The authors describe the electrical and optical characterization of three Hg_1-xCd_xTe avalanche photodiodes manufactured using planar technology with composition parameter x near 0.6. This alloy composition leads to devices that are well suited for 1.55-μm detection. From the noise analysis under multiplication, the authors show the tight dependence of the ratio β/α (of the hole; and electron ionization coefficient, respectively) upon x and the ratio Δ/E_g where Δ is the spin-orbit splitting energy and E _g is the bandgap energy. It turns out that in these alloys around x=0.6, Δ is very close to the bandgap energy so β/α reaches its maximum value. Owing to this property, which is characteristic of II-VI compounds, Hg_1-xCd_xTe is a good candidate for 1.3-μm to 1.6-μm avalanche photodiodes     

Semiempirical relation for curved optical waveguide design in theedge-guided mode regime

A semiempirical relation has been developed for determining the minimum allowable radius of curvature of a circular waveguide with an edge-guided fundamental mode. The specified parameters are the wavelength, the effective refractive index of the waveguide, the lateral index step, and the allowable radiation loss coefficient. This relation was fitted and verified against numerically evaluated solutions of Maxwell's equations in two dimensions for lateral index steps 0.01<ΔN<0.1, for effective indices of refraction 3xGa_1-xAs system), and for radiation losses of 10^-4 <α_r<100 cm^-1. The difference between the results of solving Maxwell's equations and the semiempirical relation over these parameter ranges was determined to be less than 2%     

Architecture, design, and test of continuous-time tunableintermediate-frequency bandpass delta-sigma modulators

This paper examines the architecture, design, and test of continuous-time tunable intermediate-frequency (IF) fourth-order bandpass delta-sigma (BP ΔΣ) modulators. Bandpass modulators sampling at high IFs (~100 MHz) allow direct sampling of the RF signal-reducing analog hardware and make it easier to realize completely software programmable receivers. This paper presents circuit design of and test results from continuous-time fourth-order BP ΔΣ modulators fabricated in AlInAs/GaInAs heterojunction bipolar technology with a peak unity current gain cutoff frequency (f_T) of 80 GHz and a maximum frequency of oscillation (f_MAX) of about 130 GHz. Operating from ±5-V power supplies, a fabricated 180-MHz IF fourth-order ΔΣ modulator sampling at 4 GS/s demonstrates stable behavior and achieves 75.8 dB of signal-to-(noise+distortion)-ratio (SNDR) over a 1-MHz bandwidth. Narrowband performance (~1 MHz) performance of these modulators is limited by thermal/device noise while broadband performance (~60 MHz), is limited by quantization noise. The high sampling frequency (4 GS/s) in this converter is dictated by broadband (60 MHz) performance requirements     

Measurement of chirp parameter in GaInAs/GaInAsP quantum wellelectroabsorption modulators by using intracavity modulation

A useful technique for evaluating the chirp parameter α of electroabsorption modulators is presented. The α parameter of a GaInAs/GaInAsP quantum-well (QW) electroabsorption modulator integrated with an active segment in a Fabry-Perot cavity has been evaluated by measuring the Fabry-Perot fringes. It has been shown that the relationship between absorption changes and refractive index changes is nonlinear and the α parameter is dependent on operating wavelengths     

Effect of intrinsic phase mismatch on linear modulator performanceof the 1×2 directional coupler and Mach-Zehnderinterferometer

The performance of 1×2 directional couplers and Mach-Zehnder interferometers as linear modulators is compared. Models describing the operation of each device, incorporating the intrinsic phase mismatch Δβ₀ which is present in nonideal devices, are developed. Distributions of Δβ₀ are obtained for each device and are shown to be independent of device type. Linear dynamic range and sensitivity measurements showing how Δβ₀ affects performance agree with theory. The additional effect of the coupling coefficient in the 1×2 directional coupler is demonstrated. This information can be used to predict device yield when combined with the measured Δβ₀ distributions     

Use of sigma-delta modulation to control EMI from switch-mode powersupplies

Conducted electromagnetic interference (EMI) is a major cause of concern in switch-mode power supplies (SMPSs) which commonly use standard pulsewidth modulation (PWM). In this paper, sigma-delta (ΣΔ) modulation is proposed as an alternative switching technique to reduce conducted EMI from an SMPS. The result of using ΣΔ modulation is a spread in the spectrum of the conducted emissions so that large concentrations of power at discrete frequencies are avoided. Experimental time-domain waveforms and spectra of the switching function of first-order and second-order ΣΔ modulators are presented to prove the viability of the scheme for EMI mitigation. These modulators are then applied to a DC-DC converter in an off-the-shelf computer power supply and experimental results show a reduction of roughly 5-10 dB·μV in EMI emissions over standard PWM modulators     

Optimal decoding for data acquisition applications of sigma deltamodulators

A class of optimal nonlinear decoding algorithms is proposed for data acquisition applications of sigma-delta (ΣΔ) modulators. The technique is applicable to all current ΣΔ structures, including single and double-loop, cascade, and interpolative modulators. While the performance of the technique is identical to that of other optimal nonlinear decoding schemes such as table lookup, it is considerably simpler to implement. Numerical results are presented to compare its performance to that of linear decoders. Effects of circuit imperfections on performance are examined     

Mismatch shaping for a current-mode multibit delta-sigma DAC

Mismatch shaping allows the use of multibit quantization in delta-sigma analog-to-digital converters and digital-to-analog converters (DAC's) since it noise-shapes the error caused by static element mismatch in a multibit DAC. In this paper, mismatch-shaping techniques for low-pass delta-sigma (ΔΣ) modulators are reviewed, and a mismatch-shaping technique for bandpass ΔΣ modulators is described. The dynamic error caused by frequent element switching is identified as a major source of error in a current-mode DAC with a continuous-time output. Modifying the mismatch-shaping algorithm to account for this effect yields a continuous-time ΔΣ DAC that is insensitive to both element mismatch and element switching dynamics. Experimental results confirm the effectiveness of the proposed techniques     

Comparison of effective α parameters for multiquantum-wellelectroabsorption modulators

A new effective α parameter appropriate for evaluating the transmission performance of multiquantum-well electroabsorption (MQW-EA) modulators is proposed. It is defined by the ratio of the phase change to the loss change between the ON voltage and the voltage at which the exciton peak reaches the wavelength of the light source. Simulation reveals that along various effective α parameters it has the best correlation with the equivalent α parameter obtained from the transmission performance     

A 1.8-mW CMOS ΣΔ modulator with integrated mixer forA/D conversion of IF signals

In this paper, the design of a continuous-time baseband sigma-delta (ΣΔ) modulator with an integrated mixer for intermediate-frequency (IF) analog-to-digital conversion is presented. This highly linear IF ΣΔ modulator digitizes a GSM channel at intermediate frequencies up to 50 MHz. The sampling rate is not related to the input IF and is 13.0 MHz in this design. Power consumption is 1.8 mW from a 2.5-V supply. Measured dynamic range is 82 dB, and third-order intermodulation distortion is -84 dB for two -6-dBV IF input tones. Two modulators in quadrature configuration provide 200-kHz GSM bandwidth. Active area of a single IF ΣΔ modulator is 0.2 mm² in 0.35-μm CMOS     

Responsivity and impact ionization coefficients ofSi1-xGex photodiodes

The spectral response and impact ionization coefficient ratio of Si_1-xGe_x have been determined. Measurements were made on p⁺-i-n⁺ diodes grown by solid/gas source molecular beam epitaxy. The diodes are characterized by reverse breakdown voltages of 4-12 V and dark currents of 20-170 pA/μm² . The long wavelength cut-off of the diodes increases from 1.2 μm to 1.6 μm as x increases from 0.08 to 1.0 with a maximum responsivity of 0.5 A/W in all the diodes tested. The ratio α/β varies from 3.3 to 0.3 in the same composition range, with α/β=1 at x≅0.45. These results have important implications in the use of this material system in various photodetection applications