Microwave photonics, which brings together the worlds of radiofrequency engineering and optoelectronics, has attracted great interest from both the research community and the commercial sector over the past 30 years and is set to have a bright future. The technology makes it possible to have functions in microwave systems that are complex or even not directly possible in the radiofrequency domain and also creates new opportunities for telecommunication networks. Here we introduce the technology to the photonics community and summarize recent research and important applications.

Microwave photonics combines two worlds

A software radio is defined as a set of digital signal processing (DSP) primitives, a metalevel system for combining the primitives into communication system functions (transmitter, channel model, receiver, etc.), and a set of target processors on which the software radio is hosted for real-time communications. The performance of enabling hardware technologies is related to software radio requirements, portending a decade of shift from hardware radios toward software intensive approaches. Computational models and architecture are discussed, stressing the need for topological consistency of radio functions and host architectures. A layered topology-oriented design approach encapsulated in a canonical open architecture software radio model is presented. The model provides a unified mathematical framework for quantitative analysis of algorithm structures, host architectures, and system performance for CAD. >

Software radios: Survey, critical evaluation and future directions

Wide-band switches and true-time delay (TTD) phase shifters have been developed using distributed microelectromechanical system (MEMS) transmission lines for applications in phased-array and communication systems. The design consists of a coplanar waveguide (CPW) transmission line (W=G=100 /spl mu/m) fabricated on a 500 /spl mu/m quartz substrate with fixed-fixed beam MEMS bridge capacitors placed periodically over the transmission line, thus creating a slow-wave structure. A single analog control voltage applied to the center conductor of the CPW line can vary the phase velocity of the loaded line by pulling down on the MEMS bridges to increase the distributed capacitive loading. The resulting change in the phase velocity yields a TTD phase shift. Alternatively, the control voltage can be increased beyond the pull-down voltage of the MEMS bridges such that the capacitive loading greatly increases and shorts the line to ground. The measured results demonstrate 0-60 GHz TTD phase shifters with 2 dB loss/118/spl deg/ phase shift at 60 GHz (/spl sim/4.5-ps TTD) and 1.8 dB loss/84/spl deg/ phase shift at 40 GHz. Also, switches have been demonstrated with an isolation of better than 40 dB from 21 to 40 and 40 to 60 GHz. In addition, a transmission-line model has been developed, which results in very close agreement with the measured data for both the phase shifters and switches. The pull-down voltage is 10-23 V, depending on the residual stress in the MEMS bridge. To our knowledge, this paper presents the first wide-band TTD MEMS phase shifters and wide-band switches to date.

Distributed MEMS true-time delay phase shifters and wide-band switches

The two main trends in the progress of ultrawide-band/high-frequency photodetectors (PD's), improving the bandwidth-efficiency product and obtaining a high saturation current, are reviewed. With respect to achieving large bandwidth-efficiency, the limiting factors and potentials of edge-coupled (waveguide, waveguide-fed, traveling-wave, periodic-traveling-wave), resonant-cavity, and refracting-facet photodiodes, as well as the avalanche photodiode are discussed. Regarding high-saturation current, the author estimated how much the space-charge effect limits the saturation current and two ways to reduce the space-charge effect are outlined. One way is to distribute the photocarriers along the edge-coupled PD's and the other is to increase the carrier velocity using a uni-traveling carrier structure. The waveguide-photodiode-based technologies that we have developed are also presented; namely the design and fabrication of a 100-GHz waveguide photodiode (WGPD), uni-traveling carrier WGPD, 60-GHz packaging, and a 20-GHz large-core WGPD for planar lightwave circuit integration. A 50-Gb/s receiver opto-electronic integrated circuit technology based on the WGPD is also presented.

Ultrawide-band/high-frequency photodetectors

Analog Optical Links presents the basis for the design of analog links. Following an introductory chapter, there is a chapter devoted to the development of the small signal models for common electro-optical components used in both direct and external modulation. However this is not a device book, so the theory of their operation is discussed only insofar as it is helpful in understanding the small signal models that result. These device models are then combined to form a complete link. With these analytical tools in place, a chapter is devoted to examining in detail each of the four primary link parameters; gain, bandwidth, noise figure and dynamic range. Of particular interest is the inter-relation between device and link parameters. A final chapter explores some of the trade offs among the primary link parameters.

Analog Optical Links: Theory and Practice

We describe active and nonlinear wave propagation devices for generation and detection of (sub)millimeter wave and (sub)picosecond signals. Shock-wave nonlinear transmission lines (NLTL's) generate /spl sim/4-V step functions with less than 0.7-ps fall times. NLTL-gated sampling circuits for signal measurement have attained over 700-GHz bandwidth. Soliton propagation on NLTL's is used for picosecond impulse generation and broadband millimeter-wave frequency multiplication. Picosecond pulses can also be generated on traveling-wave structures loaded by resonant tunneling diodes. Applications include integration of photodetectors with sampling circuits for picosecond optical waveform measurements and instrumentation for millimeter-wave waveform and network (circuit) measurements both on-wafer and in free space. General properties of linear and nonlinear distributed devices and circuits are reviewed, including gain-bandwidth limits, dispersive and nondispersive propagation, shock-wave formation, and soliton propagation. >

Active and nonlinear wave propagation devices in ultrafast electronics and optoelectronics

Results of the first fabrication and measurement of travelling-wave photodetectors are reported. The devices have bandwidths as high as 172 GHz, the highest reported for a p-i-n photodetector, and bandwidth-efficiency products as large as 76 GHz, the largest reported for any photodetector without gain. Comparisons with vertically illuminated and waveguide photodetectors fabricated on the same wafer establish the superior performance of travelling-wave photodetectors. Microwave loss on the travelling-wave photodetector structure is identified as a bandwidth limitation. >

Travelling-wave photodetectors with 172-GHz bandwidth and 76-GHz bandwidth-efficiency product

Nonlinear transmission lines (NLTL's) fabricated with Schottky diodes on GaAs were used to electrically generate 3.7-V step functions that had a measured 10%-90% fall time of 0.68 ps. These NLTL's were integrated on wafer with sampling circuits that had a measured 3-dB bandwidth of 725 GHz. Key to circuit performance are the use of low-loss, high-wave-velocity elevated coplanar waveguide transmission lines and the elimination of active device pad parasitics by contacting devices above the plane of the wafer. >

DC - 725 GHz sampling circuits and subpicosecond nonlinear transmission lines using elevated coplanar waveguide

Traveling-wave resonant funnel diode (TWRTD) pulse generators comprising transmission lines periodically loaded by GaAs/AlAs resonant tunnel diodes (RTD's) are fabricated. The TWRTD pulse generators have convolved transition times of 3.5 ps when measured with an active probe. Using identical RTD's, TWRTD pulse generators can attain smaller transition times than those obtained with switching circuits employing a single RTD. >

A traveling-wave resonant tunnel diode pulse generator

We report both broadband monolithic transmitter and receiver IC's for MM-wave electromagnetic measurements. The IC's use a nonlinear transmission line (NLTL) and a sampling circuit as a picosecond pulse generator and detector. The pulses are radiated and received by planar monolithic bow-tie antennas, collimated with silicon substrate lenses and off-axis parabolic reflectors. Through Fourier transformation of the received pulse, accurate 30-250 GHz free space gain-frequency and phase-frequency measurements are demonstrated. Systems design considerations are discussed, and a variety of MM-wave broadband transmission measurements are demonstrated. >

S.T. Allen

Papers

Active and nonlinear wave propagation devices in ultrafast electronics and optoelectronics

Travelling-wave photodetectors with 172-GHz bandwidth and 76-GHz bandwidth-efficiency product

DC - 725 GHz sampling circuits and subpicosecond nonlinear transmission lines using elevated coplanar waveguide

A traveling-wave resonant tunnel diode pulse generator

A broadband free-space millimeter-wave vector transmission measurement system