Abstract

Outline  Benefits of Microwave Photonics ○ Requirements for RF Sensing applications  Fiber Optic Remoting Links ○ Performance metrics ○ Improving performance ▪ Low Biasing ▪ Linearization  RF Photonic Frontends ○ Efficient integration of antennas  Summary Microwave Photonics For RF Sensing W4B.1.pdf  Multidisciplinary field bringing together worlds of microwave engineering and opto-electronics ○ Over four decades old  Initially spurred by interest in exploiting benefits of optical fiber for transporting ○ Communications ○ Ultrafast measurement systems ○ THz photonics ○ Signal processing ○ Sensing, surveillance, and radar Microwave Photonic Systems For RF Sensing W4B.1.pdf  Sensing and surveillance networks are becoming increasingly more complex  Photonics can significantly enhance the overall performance of these networks W4B.1.pdf  Fiber remoting of receive signals ○ High fidelity, linear transport of RF signals ○ Wide operating frequency range and instantaneous bandwidth ○ Optical distribution/switching  Fiber remoting of transmit signals ○ RF signal generation techniques with low phase noise ○ Multiband RF signal generation ○ Control of wideband phased array signals  Photonic RF front-ends ○ Low conversion loss over wide frequency range  Digital receiver ○ High sampling rate, low jitter, analog-to-digital conversion over wide bandwidths W4B.1.pdf  Benefits of optical fiber for transporting RF signals ○ Low loss and low distortion propagation ▪ Weak frequency dependence from MHz to multi-GHz ○ Reduced cabling size and weight ▪ Fiber: < 0.5 dB and 1.7 kg per km; Coax: 360 dB at 2 GHz and 567 kg per km ○ Signal isolation/EMI resistance ○ Design flexibility ▪ Increased bandwidth and flexible routing with WDM  Photonic devices can support a wide range of RF frequencies  N ISR apertures interfaced with multiple receivers High Performance Fiber Remoting Links for RF Sensing W4B.1.pdf  RF signal can be encoded onto the amplitude or phase of optical carrier ○ Intensity Modulation/Direct Detection (IMDD) ○ Phase Modulation/Coherent I/Q Demodulation  Coherent links more complex to implement but capable of achieving better performance ○ O/E process dominant nonlinearity W4B.1.pdf ○ Gain ○ Noise Figure ○ Linearity ○ Spurious Free Dynamic Range W4B.1.pdf  Two main types of IMDD links Spurious-Free Dynamic Range (SFDR) is a key performance figure of merit Improving Linearity for Wideband Sensing Networks  Challenge: Achieving high SFDR over a large operational (multi-band) and/or instantaneous bandwidth ○ Inherent non-linearity of E/O conversion process  Variety of techniques demonstrated to linearize link ○ Pre-distortion, Post distortion compensation ○ Feedforward, Feedback linearization ○ Downconversion with digital linearization ○ Novel optical modulator designs W4B.1.pdf  Demonstrated technique for improving Noise Figure and SFDR  Moves EOM bias point away from quadrature towards null ○ Operating point defined by bias angle, φ ○ Reduces amplitude of optical carrier ○ For same EOM output optical power, link gain will be greater than quadrature bias ▪ Relaxes performance constraints on PD ○ Corresponding decrease in link NF and increase in SFDR ○ No impact on third-order intercept point  2nd-order distortion products increase in magnitude ○ Link operational bandwidth limited to single octave W4B.1.pdf  Laser RIN noise and shot noise decrease with EOM bias angle ○ At some bias angle, receiver thermal noise dominates  At high optical input powers, NF of quadrature-biased link is dominated by laser RIN ○ Cannot be further reduced with increasing optical power W4B.1.pdf  SFDR improves moving away from quadrature bias point  At some bias angle, receiver thermal noise dominates Low Biased Millimeter- Wave Fiber Link for RF Sensing  High power DFB laser followed by fiber power amplifier ○ Link is shot noise limited  Low biased, wideband electro-optic modulator ○ Improved thermal stability with GaAs semiconductor structure  Input 3^rd order intercept point = 21 dBm at 35 GHz W4B.1.pdf  > 118 dB-Hz^2/3 SFDR over 28 – 38 GHz  150 MHz instantaneous bandwidth  No apriori knowledge of distortion required  Can correct for noise as well as nonlinear distortions  Provides transient and static nonlinearity suppression  Can be applied to directly and externally modulated fiber links W4B.1.pdf  Linearization performed with correction signal derived from distorted signal ○ Nonlinear E/O encoding process creates distorted signals (error) ○ Signal Cancellation Circuit isolates distortion/error ○ Error Cancellation Circuit cancels distortion  Distortion corrected in real-time  Can adapt to any instantaneous bandwidth for sub-octave as well as multi-octave applications  Input signals ○ Pulsed X-band carrier at 10 kHz repetition rate and 50 % duty cycle ○ CW signal at 1 MHz offset from pulsed carrier W4B.1.pdf  Directly modulated transmitter and error correction lasers  Opportunity for reduction in form factor and power dissipation Linearity of FF Linearized Directly Modulated Link W4B.1.pdf  > 20 dB suppression of distortion at 3.2 GHz  > 7 dB improvement in SFDR to 113 dB-Hz^2/3 W4B.1.pdf  Downconversion of RF signal at end of fiber remoting link  Digitization followed by digital signal processing (DSP) to improve linearity  Opportunity to enhance link performance and relax component requirements ○ Dynamic range of ADCs reduces with bandwidth Downconverting Link Approaches W4B.1.pdf - Electronic ○ Also require wideband PD  Link linearity increases with optical power into modulator  Link linearity independent of modulator switching voltage ○ Dominated by RF mixer linearity W4B.1.pdf ○ Enables lower bandwidth PD ○ Perfect isolation between RF and LO ports  Link linearity increases with optical power into modulator  Link linearity improves with low biasing W4B.1.pdf  Distortion in link primarily due to the encoding optical modulator (EOM1): 𝐼𝑜𝑢𝑡 𝑡 = ^{𝜂𝑃𝑙𝑎𝑠𝑒𝑟 𝑇} sin ^𝜋 𝑉_𝑖𝑛 + 𝜙 2 𝑉_𝜋  Downconverting optical modulator (EOM2) provides mixing without introducing any in-band distortion  Original undistorted input signal can be recovered by applying inverse sine to output current: 𝑉 = 𝜋 −₁ 𝐼𝑜𝑢𝑡 (𝑡) 𝑉_𝑖𝑛 sin − 𝜙 𝜋 𝐼𝑎𝑣𝑔  Arcsine linearization suitable for arbitrary input signals W4B.1.pdf  Input RF signal of 9 GHz downconverted to 100 MHz  39 dB suppression of third-order intermodulation distortion W4B.1.pdf W4B.1.pdf DSP Linearization Operating on Modulated Signals  Input RF signal of 9 GHz with 1 MHz QPSK W4B.1.pdf W4B.1.pdf  Phase modulation (PM) provides linear E/O encoding  Coherent detection using Inphase (I) and Quadrature (Q) demodulation ○ Linear recovery of phase and amplitude information  Detection and digitization of I and Q signals separately followed by DSP W4B.1.pdf  126.8 dB-Hz^2/3 at 1 GHz Efficient RF Photonic Frontends for RF Sensing W4B.1.pdf Challenge for Photonic RF Frontends W4B.1.pdf  Efficient integration to reduce insertion loss and maximize link budget ○ Optical → Antenna ○ Antenna → Optical  High isolation between Transmit and Receive path in RF photonic frontend ○ Requires antennas with ultra-low return loss over a wide bandwidth  Printed antenna technologies capable of efficient integration with LiNbO₃ W4B.1.pdf  Single layer radiator  Based on ‘traveling wave’ concepts ○ Quasi Yagi printed antennas ○ Circular slot antennas ○ Tapered slot antennas  Highly efficient, broad bandwidth responses ○ ‘Independent’ of material  Difficult to integrate into modulator package ○ Require modifications to wafer and EOM package W4B.1.pdf ○ Can use slots and cavities  Highly efficient, broadband ○ Multi-layering helps achieve this  Modifications to wafer ○ Can be minor (or none)  Modifications to packaging ○ Can be minor  Two styles ○ Direct contact ▪ Require modifications to the wafer ○ Indirect contact W4B.1.pdf  Based on proximity coupled patches  Multiple dielectric layers and two vertically coupled patches W4B.1.pdf W4B.1.pdf  Broadband X-band antenna integrated with LiNbO₃ modulator W4B.1.pdf

© 2018 The Author(s)