Quantum-well Electroabsorption Modulator Research Articles

(Invited) Silicon Photonics Based on Ge/Sige Quantum Well (QW) Structures and Ge-Rich Materials for Near-IR and Mid-IR

Silicon photonics is a disruptive technology poised to revolutionize several areas of our society. It has often been considered as the natural step forward to overcome some of the inherent bottlenecks of current microelectronics technology. Importantly, photonic integrated circuits can be implemented using CMOS-like fabrication processes, hence benefiting from a mature technology that provides low-cost mass fabrication using advanced tools. A large pool of Si components has been demonstrated, standing for low-loss waveguides, filters, micro-ring resonators, Mach-Zehnder interferometers, array waveguide gratings or photonic crystals, among others [1]. Similarly, the incorporation of Ge in Si photonic platforms has yield the demonstration of several active optical devices such as high-speed optical modulators and photodetectors with unprecedented performance [2]. In addition, strong electro-absorption and electro-refraction has been demonstrated in Ge/SiGe quantum wells showing strong Quantum Confined Stark Effects (QCSE), allowing for efficient optoelectronic modulation and detection at telecom wavelengths [3]. Interestingly, tuning the QW design allows fine tuning of the absorption band edge of devices over a wideband range. Moreover, light emission from the direct bandgap transition HH1-CΓ1 has also been reported in SiGe/Ge quantum wells, opening the route towards the development of light emitting devices fully integrated in a silicon chip [4]. Noticeably, all the above described activities are meant to develop Si-based photonic platforms in the near-infrared wavelength range, where most applications are foreseen. Lately, however, there is an increasing interest in the exploitation of Ge-based photonic platforms at longer wavelengths up to the mid-infrared [5]. Having Ge a transparency window up to 15 µm, there are good prospects for the development of advanced photonic sensing platforms leveraging from the strong mid-IR molecular absorption bands of several chemical and biological substances. Besides, Ge-based platforms have also been proposed as ideal candidates to exploit the nonlinear properties at mid-IR wavelengths, owing to a higher χ(3) nonlinearity and the absence of two-photon absorption beyond λ ≈ 3.15 µm [6]. Important benefits are also expected in data communications, taking advantage from the two atmospheric transparency windows at λ ≈ 3-5 µm and λ ≈ 8-13 µm and the relaxed wavelength regulations put in place. Thus, in this presentation we will show our recent work on the development of Ge-based photonic devices for telecom and mid-IR applications. For that, a review of the advanced optoelectronic devices demonstrated so far in our group will be presented, including high-speed SiGe/Ge QWs electro-absorption modulators and detectors exploiting the QCSE. Then, further insight on the room temperature luminescence of QWs will be provided, followed by its integration in light emitting diodes in waveguide configuration for planar emission. In addition, a novel mid-IR photonic platform based on Ge-rich graded-index SiGe alloys will be presented. This platform offers an interesting set of advantages over other existing mid-IR approaches such as low-loss propagation and optimal modal area over an unprecedentedly broadband wavelength range [7]. As a consequence, the first demonstration of ultra-wideband mid-IR Mach-Zehnder interferometers operating from λ = 5.5 µm to 8.5 µm was performed using the graded-index SiGe platform [8]. Finally, the third-order nonlinear properties of this platform will be discussed, supported by experimental data on Ge-rich SiGe waveguides using a bidirectional top hat D-scan method [9]. Promising results were obtained, thus providing a promising scenario to develop broadband optical sources based on the supercontinuum generation.

Investigation of InGaAsP Quantum-Well EAM Based Pump-Probe Configuration for Ultrafast Optical Signal Processing

A comprehensive theoretical model for analyzing both static and dynamic properties of arbitrary composition <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm In}_{1-x}{\rm Ga}_{x}{\rm As}_{y}{\rm P}_{1-y}$</tex></formula> quantum-well (QW) electro-absorption modulator (EAM) is proposed. Optical absorption spectra of the QW-EAM considering both excitonic transitions and continuum conduction-band to valence-band transitions are included to evaluate the QW-EAM static performance. The dynamic absorption recovery of EAM is calculated as a function of electric field and pump power. Then, based on the theory of the carrier and absorption dynamics, a 40 Gb/s pump-probe configuration for optical signal processing using <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm In}_{0.53}{\rm Ga}_{0.47}$</tex></formula> As/InP QW-EAM as nonlinear element is thoroughly investigated. Influence of various parameters including pulse width, optical wavelength, optical power, and QW structure on the intensity and phase variation of the output probe signal through nonlinear interactions between pump and probe signals are investigated. The presented model and investigation are suitable for optimizing the device design and describing the properties of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm In}_{1-x}{\rm Ga}_{x}{\rm As}_{y}{\rm P}_{1-y}$</tex></formula> QW-EAM used in ultrafast optical signal processing applications.

High-Performance Quantum Dot Lasers and Integrated Optoelectronics on Si

This paper provides a review of the recent developments of self-organized In(Ga)As/Ga(Al)As quantum dot lasers grown directly on Si, as well as their on-chip integration with Si waveguides and quantum-well electroabsorption modulators. A novel dislocation reduction technique, with the incorporation of self-organized In(Ga,Al)As quantum dots as highly effective three-dimensional dislocation filters, has been developed to overcome issues associated with the material incompatibility between III-V materials and Si. With the use of this technique, quantum dot lasers grown directly on Si exhibit relatively low threshold current ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sub> =900 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) and very high temperature stability ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> =278 K). Integrated quantum dot lasers and quantum-well electroabsorption modulators on Si have been achieved, with a coupling coefficient of more than 20% and a modulation depth of ~100% at a reverse bias of 5 V. The monolithic integration of quantum dot lasers with both amorphous and crystalline Si waveguides, fabricated using plasma-enhanced chemical-vapor deposition and membrane transfer, respectively, has also been demonstrated.

Monolithic Integration of Sampled Grating DBR with Electroabsorption Modulator by Combining Selective-Area-Growth MOCVD and Quantum-Well Intermixing

We present the monolithic integration of a sampled-grating distributed Bragg reflector (SG-DBR) laser with a quantum-well electroabsorption modulator (QW-EAM) by combining ultra-low-pressure (55mbar) selective-area-growth (SAG) metal-organic chemical vapour deposition (MOCVD) and quantum-well intermixing (QWI) for the first time. The QW-EAM and the gain section can be grown simultaneously by using SAG MOCVD technology. Meanwhile, the QWI technology offers an abrupt band-gap change between two functional sections, which reduces internal absorption loss. The experimental results show that the threshold current Ith = 62 mA, and output power reaches 3.6mW. The wavelength tuning range covers 30nm, and all the corresponding side mode suppression ratios are over 30 dB. The extinction ratios at available wavelength channels can reach more than 14 dB with bias of -5 V.

C-band side-entry Ge quantum-well electroabsorption modulator on SOI operating at 1 V swing

An electroabsorption modulator using a side-entry architecture achieved a contrast ratio exceeding 3 dB over a 3.5 nm range in the C-band, using a voltage swing of 1 V and operating at 100°C. Modulation was due to the quantum-confined Stark effect from ten Ge/SiGe quantum wells epitaxially grown on silicon-on-insulator (SOI) wafers. The device exploits an asymmetric Fabry-Perot resonator formed between the totally internally reflecting air-SiGe interface and a frustrated total internal reflection from the buried oxide layer of the SOI substrate.

Analysis of Compressively Strained GaInAsP&amp;#x2013;InP Quantum-Wire Electro-Absorption Modulators

Performance of compressively strained (CS) GaInAsP-InP quantum-wire (QWR) electro-absorption modulators (EAMs) is theoretically studied using an eight-band kldrp model. An empirical relationship is proposed for the quantum-confined Stark shift in QWR EAMs. The accuracy of this relationship is verified by comparing with numerical data. The effects of the variation of different device parameters on the absorption spectra are investigated. The absorption peaks are found to be stronger in narrower QWRs with strain-compensating barriers. Comparison of the extinction ratio with that of similar quantum-well EAMs show that, in spite of the lower in-plane filling factor, QWR EAMs exhibit a higher extinction ratio. Effect of fluctuation of wire width on the absorption spectrum of QWRs has been studied. The proposed QWR EAMs are suitable for photonic integrated circuits (PICs) fabricated by electron-beam lithography, reactive-ion etching, and two-step epitaxial growth. Due to the nature of the integration in such structures, the QWR EAMs are not required to be polarization-insensitive. On the contrary, the QWR EAMs are naturally tuned to the polarization of the output of the CS QWR lasers, fabricated on the same PIC, leading to an enhancement of the absorption strength. Moreover, the QWR EAMs, integrated with QWR lasers, offer low insertion loss.

Ge–SiGe Quantum-Well Waveguide Photodetectors on Silicon for the Near-Infrared

We demonstrate near-infrared waveguide photodetectors using Ge-SiGe quantum wells epitaxially grown on a silicon substrate. The diodes exhibit a low dark current of 17.9 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 5-V reverse bias. The photodetectors are designed to work optimally at 1480 nm, where the external responsivity is 170 mA/W, which is mainly limited by the fiber-to-waveguide coupling loss. The 1480-nm wavelength matches the optimum wavelength for quantum-well electroabsorption modulators built on the same epitaxy, but these photodetectors also exhibit performance comparable to the demonstrated Ge-based detectors at longer wavelengths. At 1530 nm, we see open eye diagrams at 2.5-Gb/s operation and the external responsivity is as high as 66 mA/W. The technology is potentially integrable with the standard complementary metal-oxide-semiconductor process and offers an efficient solution for on-chip optical interconnects.

Monolithic Integration of InGaAs–GaAs Quantum-Dot Laser and Quantum-Well Electroabsorption Modulator on Silicon

Monolithic integration of an In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> As-GaAs quantum-dot laser and an In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.2</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.8</sub> As-GaAs quantum-well electroabsorption modulator on silicon is demonstrated for the first time by using molecular beam epitaxy and focused-ion-beam etching techniques. The laser-modulator coupling coefficient is larger than 20% and the depth of modulation is 45% and 100% at 3- and 5-V reverse bias, respectively

40-Gb/s Widely Tunable Transceivers

We present the first monolithic widely tunable 40-Gb/s transceivers. The devices integrate sampled grating distributed Bragg reflector (SG-DBR) lasers, quantum-well electroabsorption modulators (EAM), low-confinement semiconductor optical amplifiers (SOA), and uni-traveling carrier (UTC) photodiodes for state-of-the-art light generation, modulation, amplification, and detection. A relatively simple high-flexibility fabrication scheme combining quantum-well intermixing (QWI) and blanket metal-organic chemical vapor deposition (MOCVD) regrowth was used to integrate components with performance rivaling optimized discrete devices. The SG-DBR/EAM transmitters demonstrate 30 nm of tuning, 39-GHz bandwidth, low-drive voltage, and low power penalty 40-Gb/s transmission through 2.3 km of fiber. The SOA/UTC photodetector receivers provide 23-28 dB of gain, saturation powers up to 18.6 dBm, and -20.2 dBm of chip-coupled sensitivity at 40 Gb/s. By connecting the transmitters and receivers off-chip, we demonstrate 40-Gb/s wavelength conversion

A single regrowth integration platform for photonic circuits incorporating tunable SGDBR lasers and quantum-well EAMs

A monolithic integration platform is demonstrated for high functionality photonic circuits that include quantum-well electroabsorption modulators, semiconductor optical amplifiers, and widely tunable lasers. The platform is based on the selective removal of a set of active quantum wells located above an optical waveguide layer. The waveguide layer contains a second set of quantum wells to be used in modulator regions. Fabrication requires only a single blanket InP regrowth

New Design of InGaAs–InGaAlAs MQW Electroabsorption Modulator for High-Speed All-Optical Wavelength Conversion

We have optimized the structure of InGaAlAs multiple quantum-well electroabsorption modulators for all-optical signal processing. As wavelength conversion is a key function for all-optical telecommunication network, we have based our optimization upon the main parameters of this function, namely, polarization insensitivity, exciton saturation, and photogenerated carrier sweep-out time. Four samples have been studied with different well tensile strain and barrier height. We find low polarization dependence over a range of applied bias. Less than 10-ps absorption-saturation recovery time at lower biases and smaller saturation power than in the literature are reported.

Peripheral-Coupled-Waveguide MQW Electroabsorption Modulator for Near Transparency and High Spurious Free Dynamic Range RF Fiber-Optic Link

Peripheral coupled waveguide (PCW) design has been deployed in InGaAsP multiple quantum-well (MQW) electroabsorption modulator (EAM) at 1.55-μm wavelength. PCW enhances the optical saturation power and reduces the optical insertion loss and the equivalent V/sub /spl pi// simultaneously. A radio-frequency link using a 1.3-mm-long lumped-element PCW EAM has achieved experimentally a link gain of -3 dB, at 500 MHz and at input optical power of 80 mW. The corresponding two-tone multioctave spurious-free dynamic range (SFDR) at the same bias is measured at 118 dB/spl middot/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> 3/. The single-octave SFDR at the third-order null bias is 132 dB/spl middot/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> 5/.

Absorption recovery in strongly saturated quantum-well electroabsorption modulators

We observe experimentally that a quantum-well electroabsorption modulator, when strongly saturated by a highly energetic optical pulse, may exhibit an absorption recovery time much longer than for excitation with a low-energy pulse. Using a comprehensive drift-diffusion type model, we are able to explain this effect theoretically. The prolongation of the absorption recovery is induced by carrier distribution effects, not by field-induced changes in the dynamical transport parameters such as the time carriers take to escape from the wells.

A cat's eye multiple quantum-well modulating retro-reflector

A new kind of modulating retro-reflector using cat's eye optics and a multiple quantum-well electro-absorption modulator array is described. The device exhibits retro-reflection over a 30/spl deg/ field of view and can support data rates of up to 50 Mb/s using 1-mm pixels. The use of the device in free-space optical communication is discussed.

Low insertion loss and low dispersion penalty InGaAsP quantum-well high-speed electroabsorption modulator for 40-Gb/s very-short-reach, long-reach, and long-haul applications

We present a metal-organic-chemical-vapor-deposition-grown low-optical-insertion-loss InGaAsP/InP multiple-quantum-well electroabsorption modulator (EAM), suitable for both nonreturn-to-zero (NRZ) and return-to-zero (RZ) applications. The EAM exhibits a dynamic (RF) extinction ratio of 11.5 dB at 1550 nm for 3 Vp-p drive under 40-Gb/s modulation. The optical insertion loss of the modulator in the on-state is -5.2 dB at 1550 nm. In addition, the EAM also exhibits a 3-dB small-signal response (S21) of greater than 38 GHz, allowing it to be used in both 40-Gb/s NRZ and 10-Gb/s RZ applications. The dispersion penalty at 40 Gb/s is measured to be 1.2 dB over /spl plusmn/40 ps/nm of chromatic dispersion. Finally, we demonstrate 40-Gb/s transmission performance over 85 km and 700 km.

Modeling of carrier dynamics in quantum-well electroabsorption modulators

We present a comprehensive drift-diffusion-type electroabsorption modulator (EAM) model. The model allows us to investigate both steady-state properties and to follow the sweep-out of carriers after pulsed optical excitation. Furthermore, it allows for the investigation of the influence that various design parameters have on the device properties, in particular how they affect the carrier dynamics and the corresponding field dynamics. A number of different types of results are presented. We calculate absorption spectra and steady-state field screening due to carrier pile-up at the separate-confinement heterobarriers. We then move on to look at carrier sweep-out upon short-pulse optical excitation. For a structure with one well, we analyze how the well position affects the carrier sweep-out and the absorption recovery. We calculate the field dynamics in a multiquantum-well structure and discuss how the changes in the field near each well affect the escape of carriers from that well. Finally, we look at the influence that the separate-confinement heterostructure barriers have on the carrier sweep-out.

Heterodyne technique for measuring the amplitude and phase transfer functions of an optical modulator

We propose a technique based on heterodyne detection for accurately and simultaneously measuring the amplitude and phase transfer functions of an optical modulator. The technique is used to characterize an InGaAsP multiple quantum-well electroabsorption modulator. From the measurements we derive the small-signal /spl alpha/-parameter and the time-dependent chirp for different operation conditions.

Growth optimization for p-n junction placement in the integration of heterojunction bipolar transistors and quantum well modulators on InP

We demonstrate the necessary conditions for successful metalorganic vapor phase epitaxy (MOVPE) growth of InGaAs-InP-based heterojunction bipolar transistor (HBT) layers on p-i-n InGaAsP-InGaAsP quantum-well electroabsorption modulators. Optimization of the doping profile in the uppermost p-cladding layer of the modulator stack was achieved to obtain suitable p-n junction placement after the final HBT growth. Photoluminescence, electron beam induced current traces, scanning electron microscope photographs, and photocurrent spectra of etched diode mesa were utilized to study this process. In addition, the procedure described here will be useful in fine-tuning many other integration designs that include p-n junctions.

Design of InGaAsP multiple quantum-well Fabry-Perot modulators for soliton 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.

Efficient harmonic generation using an electroabsorption modulator

A multiple quantum-well electroabsorption modulator has been assessed as a harmonic generator of millimeter-wave signals for radio-over-fiber systems. It is shown experimentally that the steep nonlinear absorption characteristics of electroabsorption modulators lead to efficient generation of high order harmonics when driven by a sinusoidal voltage. Photocurrent harmonic power levels generated using electroabsorption and Mach-Zehnder modulators are compared with theory.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>