Reflective Modulator Research Articles

Laser welding for fiber pigtailing with long-term stability and submicron accuracy

Laser welding as a means for optical packaging, providing long-term stability and submicron precision, has been implemented for fixing single-mode fibers or fiber arrays to integrated optical waveguides of GaAs and InP optoelectronic chips. Fixing accuracies better than 0.5 μm are achieved by applying a new method of corrective welding. Devices such as high-power LEDs featuring low spectral modulation, 980-nm laser diodes, integrated reflective Mach-Zehnder modulators (GaAs), and 1 x 2 InP switches have been packaged and used in system test beds.

Performance predictions for GaAs franz‐keldysh effect optical reflection modulators based on measured absorption data

AbstractPerformance predictions for a simple reflection modulator based on measured absorption data are presented, and then resonant enhancement is described. Calculated and measured performance for a resonant transmission device is presented with excellent agreement. An optimized resonant reflection modulator is designed giving 17‐dB contrast for 2.1‐dB insertion loss at — 30‐V bias. © 1995 John Wiley & Sons, Inc.

Analogue optical links for detector front ends at the LHC

Analogue lightwave links are being developed for the readout of tracking detectors at the CERN Large Hadron Collider (LHC). The links are based on arrays of reflective electro-optic intensity modulators, mounted on detectors and connected by optical fibres to remotely located transceivers (CW lasers and photoreceivers). The modulators are essentially passive devices with negligible power requirements. The transceivers are integrated optics devices of a novel design. Modulator prototypes have been fabricated and tested. The results achieved so far and the key system issues are reviewed.

Diffraction from a short-cavity Fabry-Pérot: application to photorefractive quantum wells

Diffraction efficiencies from absorption and index gratings in short-cavity Fabry-Pérot multiple quantum well structures are strong functions of film thickness. Diffraction efficiencies of transmitted and reflected waves vary by an order of magnitude by adjusting the device thickness to bring the Fabry-Pérot resonance condition close to the excitonic absorption. Asymmetric Fabry-Pérot quantum-well reflection modulators produce the largest diffraction efficiencies, with input diffraction efficiencies approaching 6%. Analytic expressions for the diffraction efficiencies are derived in the Raman-Nath regime for a single thin film and for multilayer structures. These results apply in general for dynamic holography and transient dielectric gratings in thin films, but is illustrated specifically for photorefractive semiconductor quantum wells.

Normally-on GaAs/AlAs multiple-quantum-well Fabry–Perot reflection modulators for large two-dimensional arrays

We report normal-incident normally-on 90 Å GaAs/21 Å AlAs multiple-quantum-well Fabry–Perot reflection modulators operating at the same wavelength with the same reverse bias over much of a 2 in. wafer. The fabricated modulators achieve ON/OFF reflectance changes larger than 48% and ON/OFF contrast ratios larger than 14.7 over a diameter of 31 mm at the wavelength of 845 nm with a fixed bias of 7 V. Over a diameter of 40 mm, these modulators achieve reflectance changes larger than 43.2% and contrast ratios larger than 5.

Predicted performance of Franz-Keldysh effect optical reflection modulators and comparisons with similar multiple quantum well-based devices

In the paper the electroabsorption and electrorefraction spectra are calculated for GaAs, InGaAsP and InGaAs. These are compared both with experimental results and data from the literature giving very good agreement. From these calculations the behaviour of simple reflection modulators operating at 885 nm, 1.32 mu m and 1.76 mu m is quantified for a 3 dB insertion loss. This produces contrasts (on:off) of approximately 3 dB at -30 V bias for all three semiconductors above. By using a Fabry-Perot cavity formed by the end faces of the modulator devices these contrasts are shown to increase to 20 dB for GaAs and InGaAsP, and 6.9 dB for InGaAs at the same bias. This potential performance is compared to multiple quantum well devices which also make use of cavity effects.

Dynamic holographic phase gratings in multiple-quantum-well asymmetric Fabry–Perot reflection modulators

Dynamic holographic pi-phase gratings can be written in a multiple-quantum-well asymmetric Fabry-Perot reflection modulator by spatial modulation of the quantum-confined exciton absorption. Operating near the Fabry-Perot resonance, the phase grating quenches the zero-order reflection and enhances the first-order diffraction efficiency. The diffraction efficiency of these structures as diffractive optics elements is not sensitive to the contrast ratio, which eases the constraints for device fabrication. Numerical simulations for multiple-quantum-well structures are presented that fully include refractive-index changes in response to a spatial modulation of the exciton absorption, which can be caused by any of several techniques, including free-carrier gratings, absorption bleaching, electro-optic effects, and photorefractive effects.

A low-voltage, high-reflectance-change normally off refractive GaAs/Al/sub 0.2/Ga/sub 0.8/As MQW reflection modulator

We present our optimization of a normally off refractive GaAs/Al/sub x/Ga/sub 1-x/As multiple-quantum-well (MQW) reflection modulator with respect to the on/off reflectance change, on/off contrast ratio, and operating voltage. We use optical transfer matrices, theoretically calculated refractive indices, and absorption coefficients to simulate the operation of a normal-incident Fabry-Perot MQW modulator. Our calculations suggest that a normally off refractive GaAs/Al/sub 0.2/Ga/sub 0.8/As MQW reflection modulator with a reflectance change of 42.9% and an on/off contrast ratio of 1539 for an operating voltage of only 2.44 V can be fabricated by molecular-beam epitaxy (MBE).< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Experimental and theoretical studies of a novel hetero-nipi reflection modulator

A novel GaAs/Al/sub 0.35/Ga/sub 0.65/As hetero-nipi all-optical reflection modulator has been investigated, both theoretically and experimentally. The modulation is found to be dominated by the refractive index changes that occur in the quantum wells as a result of optically induced electric field changes via the quantum-confined Stark effect. Good agreement is found between the theoretical and experimental behavior, with a maximum absolute experimental modulation of 11% and a maximum contrast ratio of 4.4:1 being observed at room temperature at optical pump densities of only 1.9 mW/cm/sup 2/. A numerical model of the device is developed, which predicts large reflectivity modulations of greater than 53% with a contrast ratio of 25:1 in optimized device structures.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

LOCNET: a fiber in the loop system with no light source at the subscriber end

An optical subscriber loop with simultaneous video and bidirectional data transmission over the same fiber is presented. A reflective modulator replaces the light source at the customer end. The system has been tested and experimental results are described.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Optical properties of GaAs/AlxGa1−xAs multiple quantum wells versus electric field including exciton transition broadening effects in optical modulators

A systematic study of the refractive index and absorption coefficient of GaAs/AlxGa1−xAs quantum wells as a function of electric field is presented. In this model, the effective-mass mismatches between GaAs and AlxGa1−xAs have been considered. For the interband optical absorption, we have considered the effect of the final-state interactions, and found an analytical function well describing this effect. The line-shape functions and their appropriate linewidths due to the inhomogeneous broadening mechanisms have been considered as a function of layer thicknesses, Al fraction, electric field, and residual doping density in the intrinsic multiple-quantum-well (MQW) region of a p-i-n optical modulator. The calculated absorption coefficient as a function of wavelength and electric field agrees very well with the experimental data. The calculated refractive index is indirectly verified by comparing the calculated reflectance spectra with the measured data of a MQW reflection modulator.

High‐speed modulation of GaAs/AlGaAs multiple‐quantum‐well (MQW) asymmetric Fabry‐Perot (ASFP) reflection modulator

AbstractWe report the high‐frequency performance of GaAs/AlGaAs multiple‐quantum‐well‐ (MQW‐) based asymmetric Fabry‐Perot (ASFP) reflection modulator. The measured frequency response bandwidth of the modulator was ∼600 MHz at an applied voltage of only 5 V at a wavelength of ∼854 nm. The differential contrast of the modulator was 4.0 at an applied voltage of 12.5 at a wavelength of 863 nm. © 1994 John Wiley &amp; Sons, Inc.

Comparison of electroabsorption in tensile-strained and lattice-matched GaAs(P)/AlGaAs quantum wells

The performance of tensile-strained GaAsP/AlGaAs multiple-quantum well (MQW) reflection modulators is compared to that of similar lattice-matched GaAs/AlGaAs devices operating in the same wavelength range. The tensile-strained modulators utilize ∼95 Å GaAs0.92P0.08 quantum wells, which are designed to make use of the field-induced merging of electron-to-light-hole (e-lh) and electron-to-heavy-hole (e-hh) excitonic absorption edges achievable in tensile strained wells. Unstrained ∼46 Å GaAs quantum wells yielding similar excitonic gaps are utilized in the lattice-matched devices, but the strained and unstrained modulators are otherwise virtually identical. Room-temperature differential reflection spectra reveal increased modulation depths at low drive voltages in the tensile-strained devices, consistent with electroabsorption enhancements expected in these structures from merging of the elh and ehh transitions.

Zero chirp quantum well asymmetric Fabry–Perot reflection modulators operating beyond the matching condition

We demonstrate asymmetric Fabry–Perot reflection modulators using both InGaAs and GaAs quantum wells which operate beyond the matching condition and exhibit zero phase change (zero chirp) when switched. These devices are structurally similar to devices which can operate before the matching condition, and with incorporation of only a small etch step enable planar realization of zero phase change modulators operating before and beyond the matching condition which reflect 180° out of phase. We also show how observation of the reflectivity spectrum can provide quick information about the voltage necessary to obtain this zero chirp behavior. These structures provide a basis for low chirp optical switching, beam steering, and spatial light modulation.

Electroabsorption modulators operating at 1.3 μm on GaAs substrates

This paper describes the growth and device performance of electroabsorption modulators on GaAs substrates operating near 1.3 μm, the dispersion minimum for silica fibres. The key to the successful molecular beam epitaxial (MBE) growth of these devices was the incorporation of a linearly-graded buffer layer beneath the InGaAs/AlGaAs multi-quantum-well active layer. Both transmission and reflection modulators are produced. For transmission devices, larger modulation is achieved when the buffer is graded more slowly: The maximum modulation reported was 22% for ΔT/TOcorresponding to a 0.86 dB contrast ratio with an insertion loss of roughly 5 dB at 1.34 μm. Antireflection coating a transmission modulator yields a reasonable reflection modulator. However, improved performance is reported for a reflection modulator using a novel technique of integrating the bottom quarter-wave mirror into a buffer with linearly-graded In composition. At 1.33 μm, a normally-off reflection modulator with an integrated mirror exhibited a ΔR/ROof 73%, a constrast ratio of 2.38 dB, and an insertion loss of 4 dB.

Active antireflection coating electrooptic modulator

A new type of multiple quantum well (MQW) reflection modulator, based on the multilayer active antireflection coating (A-ARC) is proposed and compared theoretically with the conventional MQW modulator based on the asymmetric Fabry-Perot etalon (AFP). It is shown that both a large on-off ratio and a low operating voltage can be achieved. It will be demonstrated that the A-ARC device matches the performance of the recently reported asymmetric Fabry-Perot modulator while requiring smaller device thickness. It will also be shown that A-ARC MQW modulators exhibit superior tolerance to the unavoidable thickness variations, thus allowing fabrication of large-area uniform arrays of modulators.

Mirror quality and efficiency improvements of reflective spatial light modulators by the use of dielectric coatings and chemical-mechanical polishing

To date, silicon backplane spatial light modulators have been characterized by poor-quality mirrors. Hillock formation during metal sintering has been identified as the source of this problem. Here hillock elimination is achieved by constraining the metal with a low-temperature plasma-enhanced chemicalvapor deposition silicon dioxide coating. A double-layer metallization procedure increases the silicon area available for circuitry and improves the mirror fill factor. Second-layer metal mirrors require a flat, intermediate dielectric substrate. Chemical-mechanical polishing is demonstrated to provide the flatness necessary to achieve high optical quality.

Wavelength dependence of saturation and thermal effects in multiple quantum well modulators

Experiments are described in biased AlGaAs/GaAs quantum well reflection modulators demonstrating that the degradation in continuous wave modulator performance due to absorption saturation at wavelengths λ≳λ0, is significantly reduced compared to that at λ0, the zero field exciton wavelength, which in this case is 850 nm. This is of importance in the selection of operating wavelengths for the self-electro-optic effect device (SEED) used in photonic switching or optical interconnect applications.

All-optical measurement of the giant ambipolar diffusion constant in a hetero-nipi reflection modulator

We have measured the in-plane ambipolar diffusion constant in a hetero-nipi reflection modulator using a novel all-optical technique. This technique allows us to monitor the time evolution of an excess carrier population at any point on the sample without the need for electrical contacts. The diffusion is found to be enhanced by almost an order of magnitude over that in bulk material (to values of 45 cm2 s-1) due to the 'giant ambipolar diffusion' mechanism arising from perturbations to the periodic nipi potential. This has important implications in the use of nipi structures as fast optical switching elements as it ensures uniform switching across an individual pixel in a device where the in-plane diffusion dynamics are typically the limiting factor in the switching speed.

Measurement of modulation saturation intensity in strain-balanced, undefected InGaAs/GaAsP modulators operating at 1.064 μm

We measure high modulation saturation intensities in both strain-relaxed InGaAs/GaAs multiple-quantum-well (MQW) modulators (107 kW/cm2) and strain-balanced InGaAs/GaAsP MQW modulators (31 kW/cm2) operating at 1.064 μm, measured with a Nd:YAG laser. This compares with 16 kW/cm2 for GaAs/Al0.3Ga0.7As thin barrier MQW modulators and 65 kW/cm2 for GaAs/Al0.02Ga0.98As shallow MQW modulators operating near 850 nm (reflection modulators). The advantage of the strain-balanced system is that it results in an undefected material with sharper excitonic features and better morphology compared to the strain-relaxed system. This study shows that the InGaAs/GaAsP system may be used to obtain long wavelength operation with these advantages without incurring an unreasonable penalty for high power operation due to the higher barriers.