Microwave Photonics combines two world March 6. 2015 Myungjin Shin High-Speed Circuits & Systems Lab Department of Electrical and Electronic Engineering Yonsei University
Contents - Introduction - Devices and material - Radio-over-system - Optical beam forming - Analog-to-digital converters - Arbitrary waveform generation
Introduction - Microwave photonics(MWP): Study of photonic devices operating at microwave frequencies and their application to microwave & optical system - Simple microwave photonic link - Advantage of microwave photonic link - Cost, weight, loss, high data-transfer capacity
Introduction - MWP & optical communication runs in parallel - 3 fundamental historical development in MWP - Low-loss silica multimode & single-mode optical fibers - Fast depletion p-i-n - Avalanche detectors Useful microwave bandwidth response
Device & Material - Modulator - Modulation bandwidth - Semiconductor laser - Lithium niobate interferometric modulator - Semiconductor material modulator - Organic polymer - Electro-absorption modulators(EAMs) - Photodetector - Surface illuminated & Vertically illuminated - Edge illuminated - UTC-PD
Device - Modulation bandwidth - Photon lifetime, differential gain, carrier recombination time, optical output power - Semiconductor laser - Incorporation of quantum well and addition of strain 1.55um InGaAsP quantum-well - Enhance frequency response resonantly - External cavity laser - Monolithic multisection laser - Optical injection locking - 1.55-um InGaAsP distributed-feedback(DFB) laser injection- locked to external cavity diode laser - optically injection locked 1.55-um vertical cavity surface emitting laser(VCSEL)
Material - Lithium niobate interferometric modulator - 3-dB electrical bandwidth 30 ~ 70GHz with drive voltage 3.5V~5.1V - Semiconductor material modulator - Fairly high bandwidth - Low bulk electro-optic coefficient of semiconductor materials & poor overlap of applied electric field & optical mode high driver voltage - Fiber-to fiber coupling loss higher(10dB) than Lithium niobate(<4dB) - Organic polymer - Attractive features for integrated optical application - Bandwidth, drive voltage similar to semiconductor material modulator - Poor power handling capacity & long term bias stability
Material - Electro-absorption modulators(EAMs) - 3-5 compound semiconductors - Bulk semiconductor materials incorporating the Franz-keldysh effect & quantum-well structure - Ability to directly integrated with semiconductor lasers - Large optical loss - Free-carrier absorption & band to band absorption Restrict size - Poor optical power-handling capabilities & very sensitive wavelength and temperature change
Device - Photodetector(in MWP) - High responsivities, bandwidths and optical power-handling capability are required - Surface illuminated & Vertically illuminated - Trade-off bandwidth & device efficiency - Edge-illuminated waveguide - Large bandwidth & efficiency - Space-charge effect electron velocity & electric field ↓ limit input power(saturation) - UTC-PD: Only electrons travelling at a velocity much higher than the saturation velocity contribute to the space charge effect and high output photocurrents can be achieved
Radio-Over-Fiber System - Radio-over-fiber: technology that light is modulated by radio signal and transmitted over an optical fiber - Hybrid fiber-radio(HFR): Integration of fiber-optic and wireless networks form - Benefit of RoF: flexible approach for remotely interfacing to multiple antennas, with the ability to reduce system complexity by using a centralized architecture that incorporates a simplified antenna module located closer to the customer - Challenge of HFR: distribution of the radio signal, while reducing the complexity of the hardware located at the remote antenna site
Example of ROF - Uses of HFR technology: cellular networks, indoor distributed antenna systems and wireless local area networks(WLANs)
Type of RoF - Several possible approaches to transporting radio signals over optical fiber - RF-over-fiber, Intermediate-frequency-over-fiber, baseband-over-fiber
Type of RoF - RF-over-fiber: Radio signal transport directly over the fiber at the wireless transmission frequency, without the need for any subsequent frequency - Intermediate-frequency-over-fiber & baseband-over-fiber - make use of mature and reliable RF and digital hardware for signal processing at the central office and remote antenna site as well as low-cost optoelectronic interference - As wireless network frequency increases the need for frequency conversion complicate the antenna module architecture design
Future RoF - Millimetre-wave opto-electronic mixer for frequency conversion to or from an intermediate frequency - Using wavelength division multiplexing(WDM) in HFR systems as a means of increasing network capacity - Simplifying the antenna-module architecture - Recently single eletro-absorption transceiver in 60-GHz HFR system - Transmission of ultrawideband(UWB) signals in HFR system is being investigated owing to the emergence of new wireless personal area network(WPAN) standards
Optical Beam Forming - Phase array radar system: controlling the relative phase of an RF signal between successive radiating elements of an antenna array, beam can be created that will radiate in a specific direction - Advantage of optical phased array antenna: size, weight, bandwidth, propagation loss, immunity to electromagnetic interference - Beam-forming network required phase shifts at the antenna input - Main issue for phase array radar: distribution of RF signals, phase shifter control, true time-delay(TTD) beam forming &processing of RF signals
Example of optical beam forming - True time-delay(TTD) networks incorporated high-dispersion optical fibers, where the dispersion property of the fiber-optic link used to create variable delays for variable source wavelengths - Above figure show the single wavelength tunable laser used in conjunction with dispersive fiber to create fiber-optic prism
Future beam forming - Switching of the effective optical path length in optically controlled phased-array systems incorporating TTD - By replacing microwave phase shifter into spatial light modulator(SLM) - SLM: object that imposes some form of spatially varying modulation - WDM techniques to simplify the optical beam-forming architecture is promising
Analog-to-digital Converter - In certain analog application, digital signal processing provides superior performance and ADC is a main bottleneck - CMOS digitizer are limited in speed with several factor - Jitter in sampling clock, settling time, sample-and-hold circuit, speed of comparator Can be overcome using parallelism to implement time-interleaved ADC architectures but still limited - Photonic time-stretch(PTS) technique is one of the promising method by slowing down the electrical signal - Have to solve calibration & channel matching, impact of noise from optical source, distortion in optical fiber
Photonic time strech - Process of Photonic time stretch - Broadband transform limited pulse linearly chirped optical pulse(L 1 ) input RF signal envelope of the pulse is stretched in a second fiber(L 2 )
Developed ADC - Dispersion penalty limits the system performance of ADC can be overcome by single-side band(SSB) - Residual phase distortion that must be corrected using an electronic equalizer - SSB relies on the use of microwave hybrid to provide quadrature outputs and because of frequency limitation of hybrid structure SSB has bandwidth limitation - Exploitation of the phase diversity that exists between the two outputs of a dual output Mach-Zehnder modulator can be another method
Arbitrary Waveform Generation - Arbitrary waveform generation is very useful in variety of application - Pulsed radar, UWB, optical & electronic test, measurement - Current electromagnetic AWG is band-limited to below 2GHz MWP signal processing technique can overcome - Generating arbitrary waveform - Fixed and programmable shaping of the spectrum of a supercontinuum followed by a wavelength-to-time mapping in a dispersive fiber link - Combination of chromatic dispersion & nonlinear effects in optical fiber
Block diagram - Supercontinuum spectral shaping source with wavelength to time mapping - Supercontinuum generate broadband optical pulse bandpass-filter spectrally dispersed by a diffraction grating different spectral components impinge different pixel of spatial light modulator(SLM) Spectrally generate waveform one-to-one mapping between time & wavelength which convert spectral modulation into time domain amplitude modulation
Future Task & Challange - Challenge: Improving their RF spectral region of operation, conversion efficiency and dynamic range with low cost - Possible application: Optical packet switched networks & optical probing, terahertz-wave generation & processing for non invasive high resolution sensing & MWP-based quantum key distribution
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