Traveling-wave Photodetectors Research Articles

Superconductive Traveling-Wave Photodetectors

This paper introduces a novel family of devices for ultra-fast, ultra-sensitive and ultra-low-noise optoelectronics, namely superconductive traveling-wave photodetectors (STWPDs). STWPDs can be optimized for ultra-wideband/ultra-sensitive photodetection, photomixing and microwave/THz waveform generation. An STWPD is a distributed device, both electrically and optically, which consists of a superconductive thin film kinetic-inductive photodetector integrated with a microwave/THz transmission line and an optical waveguide. We will present the principles of operation of STWPDs, a framework to calculate the input-output relation and the effects of the modal characteristics of the optical propagation on the device performance. Moreover, we will elaborate on how one can design an STWPD for each of the aforementioned applications. Different strategies for the design of the optical waveguide as well as some important design rules will also be discussed. We will report the current status of STWPDs and point out some key issues that should be met in the future for further development.

Embedded Grating ${\hbox{n}}^{+}{\hbox{-i-}}{\hbox{n}}^{+}$ Traveling-Wave Photodetectors

This paper describes the design of embedded-grating n+-i-n+ traveling-wave photodetectors (TWPDs). In the ideal case of perfect electrically matched photodetectors, the TWPD bandwidth is determined by the velocity mismatch between the group velocity of the optical beam and the phase velocity of the electrical photogenerated signal, as well as by the attenuation of the RF signal propagating along the electrodes. Since the group velocity is determined once the layer's thickness is selected, which, in turn, is optimized for coupling the light into the photodetector, only an accurate electrode design allows for the velocity matching. In this paper, we propose a new structure that utilizes a grating integrated within a single-mode waveguide in order to tailor the optical group velocity and match it to the phase velocity of the photogenerated signal.

InP-Based High-Speed Photodetectors

This paper provides an overview on recent advances of InP-based photodetectors for high-speed fiber-optic applications, with emphasis on evanescently coupled waveguide photodiodes, balanced receivers, and distributed traveling wave photodetectors.

A Novel Analytical Model as a Design Tool for Uni-Traveling-Carrier Traveling Wave Photo Detectors Approaching 1 THz

The generation of sub-millimeter waves and terahertz signals is a difficult challenge, and a few approaches are being studied by several groups. We are investigating the approach of heterodyning two optical signals in a uni-traveling-carrier photo detector, which has been identified as the device of choice for very high frequencies. In this paper we present a novel analytical model, which was developed as a design tool for the above device. The model was validated by designing, fabricating and testing a new 100 GHz photo detector. In addition, performance limits for the UTC traveling wave photo detector are calculated by the new model, and it is shown that this device has the potential of delivering more than 1 mW of power at frequencies approaching 1 THz.

Theory of Superconductive Traveling-Wave Photodetectors

This paper studies the theory of traveling-wave photodetection in superconductive multilayer optical waveguides, as a general platform for ultrafast, ultrasensitive, and ultralow-noise optoelectronics. The kinetic inductance theory of superconductive thin films and the modified transmission line theory are employed to investigate the distributed photodetection and signal generation mechanisms in superconductive traveling-wave devices. A general model for calculation of the optical responsivity of superconductive traveling-wave photodetectors is developed and operation regimes of interest are highlighted. Moreover, the inclusion of a photoconductive layer in the device structure and some important loading schemes are discussed and their effects on the device performance, such as quantum efficiency and electrical bandwidth, are addressed.

Parallel-Fed Traveling Wave Photodetector for ${>}100$-GHz Applications

A fully packaged traveling wave photodetector (TWPD) with monolithically integrated optical power splitter for high-speed high-power applications in excess of 100 GHz is demonstrated. The detector module provides a responsivity of 0.24 A/W with a polarization dependence of only 0.2 dB at 1.55 mum wavelength and is suitable for the detection of RZ data rates in excess of 80 Gbit/s with unsaturated peak voltages of >0.5 V. Chip-based measurements show a broadband impedance match, a 3 dB bandwidth of 80 GHz and a maximum electrical output power of -2.5 dBm at 150 GHz.

Hybrid drift‐diffusion–TLM analysis of high‐speed and high‐output UTC traveling‐wave photodetectors

AbstractThe scope of the hybrid drift‐diffusion (DD)–TLM method is extended to include the analysis of unitraveling‐carrier (UTC) photodetectors. The extended method takes into account the overshoot velocity at which electrons in the collector of an UTC photodiode drift. The method is used for large and small signal analysis of the UTC traveling‐wave photodetector (TWPD) and near‐ballistic (NB) UTC TWPD. It is shown that there is a trade‐off between the UTC TWPD's bandwidth and saturation current that has to be considered in the design. A higher bias voltage reduces the bandwidth, but also increases the saturation current of the photodetector. The presented UTC TWPD has bandwidths of 110 and 83 GHz and saturation currents of 15 and 30 mA under 1 and 5 V biases, respectively. To improve the performance under high bias voltage, the NB‐UTC TWPD is investigated. A maximum RF power of 6.5 dBm was obtained at 110 GHz under 5 V bias voltage. Copyright © 2007 John Wiley & Sons, Ltd.

Effect of carrier transit time on the bandwidth of traveling-wave photodetector

We present an impulse response of traveling-wave photodetector considering the photo-excited carrier transit time as well as the velocity mismatch between lightwave and microwave. The transit time effect on the bandwidth is found to be more significant than the velocity mismatch effect, if the thickness of the intrinsic absorption region is over 0.2μm. The effect of the intrinsic absorption region thickness is also examined. The optimal thickness of the intrinsic absorption region is determined considering the transit time, the optical confinement factor, and the velocity mismatch.

High-Speed Miniaturized Photodiode and Parallel-Fed Traveling-Wave Photodetectors Based on InP

A periodic parallel-fed traveling-wave photodetector (TWPD) with monolithically integrated power splitter is presented. The device exhibits an impedance match to the 50-Omega environment as well as a velocity match between the optical and electrical signals and shows a 3-dB bandwidth up to 85 GHz. The electrical output power reaches +10 dBm at 10 GHz. The device is based on evanescently coupled low-capacitance waveguide-integrated p-i-n photodiodes (PDs) with optimized optical matching layer which reveal a bandwidth of 145 GHz when employed as a stand-alone photodetector

Design and Fabrication of Monolithic Distributed Traveling-Wave Photodetectors Integrated With Polymer Optical Waveguides

We have designed and fabricated monolithic distributed balanced traveling-wave metal-semiconductor-metal photodetectors (MSM PDs) distributed along single-mode polymer optical waveguides. We demonstrate that monolithically integrated polymer optical waveguides on a semiconductor can provide efficient optical interconnects in an optoelectronic circuit. A responsivity of 0.44 A/W and a 3-dB bandwidth of 10 GHz were obtained in a PD array with five MSM PDs. A high photocurrent of 7.3 mA was measured in an array having eight MSM PDs. Impulse response measurements were performed and the results were compared with frequency domain results.

An analytical small-signal model for submicrometer n/sup +/-i-n/sup +/ traveling-wave photodetectors

A fully analytical small-signal model for investigating the frequency response of submicrometer n/sup +/-i-n/sup +/ traveling-wave photodetectors is derived. In order to properly account for nonstationary nonlocal phenomena characterizing submicrometer devices, the simple diffusive description of the carrier drift flux in a local driving field discussed by Torrese et al. in 2002 has been replaced by the hydrodynamic approach. The influence of the bias voltage on the transit time, as well as the bandwidth limitation due to the electrical wave reflections at both input and output ends of the detector are discussed.

A hybrid drift-diffusion-TLM analysis of traveling-wave photodetectors

An efficient hybrid method for numerical analysis of traveling-wave photodetectors (TWPDs) is presented. The method is a combination of two-dimensional semiconductor and three-dimensional full-wave electromagnetic (EM) simulators. The time-domain drift-diffusion method is used to determine the photogenerated currents at the cross section of the device. Those currents are then defined as sources in the EM analysis to calculate the microwave bandwidth and output current of the device. In the analysis, the effects of the carrier velocity and lifetime, optical power, bias voltage, velocity mismatch, and microwave loss are taken into account. The method is tested in case of GaAs and low-temperature-grown GaAs-based TWPDs. A very good agreement between the theoretical and experimental data is observed. It was also possible to predict the saturation behavior of the TWPD under high optical illumination due to the field screening effect.

Traveling-Wave Photodetector with Asymmetrically Heterostructured Intrinsic Region

Bandwidth limitation by carrier transit time was analyzed mathematically when traveling-wave photodetector's intrinsic region consists of heterostructure. Because of the smaller hole's velocity than electron's, the hole transit length is crucial for determining the large bandwidth. In order to compensate the small transit velocity of hole, we propose that the intrinsic region should be designed asymmetrically. In this study we found that appropriate asymmetric structure enhanced bandwidth by about 100% of symmetric structure and proved experimentally the effect of asymmetric intrinsic region.

Traveling-Wave Photodetectors With High Power–Bandwidth and Gain–Bandwidth Product Performance

Traveling-wave photodetectors (TWPDs) are an attractive way to simultaneously maximize external quantum efficiency, electrical bandwidth, and maximum unsaturated output power. We review recent advances in TWPDs. Record high-peak output voltage together with ultrahigh-speed performance has been observed in low-temperature-grown GaAs (LTG-GaAs)-based metal-semiconductor-metal TWPDs at the wavelengths of 800 and 1300 nm. An approach to simultaneously obtain high bandwidth and high external efficiency is a traveling-wave amplifier-photodetector (TAP detector) that combines gain and absorption in either a sequential or simultaneous traveling-wave structure.

Nonlinear Behaviors of Low-Temperature-Grown GaAs-Based Photodetectors Around 1.3->tex<$mu$>/tex<m Telecommunication Wavelength

We observed distinct bandwidth degradation behaviors in low-temperature-grown GaAs (LTG-GaAs)-based traveling-wave photodetectors (PDs) under /spl sim/1300-nm telecommunication wavelength operation. Compared with the bandwidth degradation behaviors of different excitation wavelengths (/spl sim/800 and /spl sim/1550 nm) in LTG-GaAs-based PDs, the saturation behaviors at the studied wavelength are more serious and can be attributed to "hot electron" effect of photogenerated carriers. The disclosed unique material properties of LTG-GaAs are important for its applications in ultrafast optoelectronics and understanding its carrier dynamics with the defect states.

Backward-wave cancellation in distributed traveling-wave photodetectors

This paper describes the design, fabrication, and measurement of backward-wave-cancelled distributed traveling-wave photodetectors. One of the fundamental issues in traveling-wave photodetectors is the generation of backward-waves, which reduces bandwidth or, in the case of matched input termination, reduces their radio-frequency (RF) efficiencies by up to 6 dB. We report a traveling-wave photodetector with multisection coplanar strip transmission lines. The reflections at the discontinuities of the transmission line cancel the backward propagating waves exactly. The bandwidth reduction due to backward-waves is eliminated without sacrificing the RF efficiency. We have demonstrated a broadband backward-wave-cancelled traveling-wave photodetector with three discrete photodiodes. The photodetector is realized in InGaAs/InGaAsP/InP material systems and operates at 1.55 /spl mu/m. A 3-dB bandwidth of 38 GHz and a linear RF output of -1 dBm at 40 GHz have been achieved. The experimental results agree very well with the theoretical calculations.

Numerical analysis and design of a new traveling-wave photodetector with an asymmetric I-layer cross section

We propose a new type of the traveling-wave photodetector (TWPD) that has an asymmetric intrinsic layer cross section to enhance bandwidth. In this paper, analysis and design of TWPDs are presented using the finite-difference time-domain method. A full-wave analysis is performed by a numerical technique and impulse responses are obtained in a linear regime. The simulation and experiment results show that 3-dB bandwidth is increased by the difference between electron and hole saturation velocities.

Bandwidth Enhancement of Traveling-Wave Photodetector by Impedance Mismatch

In this paper, we report bandwidth enhancement phenomena of traveling-wave photodetectors (TWPDs) by output impedance mismatch with an open input facet. The impulse response obtained by the extended finite difference time domain (FDTD) method shows that internal reflection of microwaves with a negative reflection coefficient at the load enlarges the bandwidth of TWPDs.

Edge-coupled membrane terahertz photonic transmitters based on metal–semiconductor–metal traveling-wave photodetectors

Ultra-high-speed photodetectors and printed-circuit antennas construct photonic transmitters. In this letter, we demonstrate a terahertz (THz) photonic transmitter: edge-coupled membrane photonic transmitters based on metal–semiconductor–metal traveling-wave photodetectors, which are fabricated with low-temperature-grown GaAs photoabsorption layers. With a membrane-based and edge-coupled structure, the demonstrated photonic transmitters can eliminate the requirement of Si lenses and attain an over 20 times higher optical-to-THz power conversion efficiency 2×10−4 than vertical illuminated photonic transmitters with Si lenses at the same operation frequency.

Ultrahigh-power-bandwidth product and nonlinear photoconductance performances of low-temperature-grown GaAs-based metal-semiconductor-metal traveling-wave photodetectors

Maximum-output-power and bandwidth performances are usually two tradeoff parameters in the design of high-speed photodetectors (PDs). In this paper, we report record high-peak output voltage (/spl sim/ 30 V) together with ultrahigh-speed performance (1.8 ps, 190 GHz) observed in low-temperature-grown GaAs (LTG-GaAs)-based metal-semiconductor-metal (MSM) traveling-wave photodetectors (TWPDs) at a wavelength of 800 nm. Ultrahigh-peak output power and ultrahigh-electrical bandwidth performances were achieved due to superior MSM microwave guiding structure, short carrier trapping time, and the capability to take high bias voltage (/spl sim/ 30 V) with a LTG-GaAs layer. Under such a high bias voltage, a significant nonlinear photocurrent increase with the bias voltage was observed. The nonlinear photoconductance and ultrahigh-output power-bandwidth performances opens a new way in the application of high-performance optoelectronic mixers and photomixer devices.