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Integrated microwave photonics

David Marpaung, Chris Roeloffzen, Rene Heideman, Arne Leinse, Salvador Sales, Jose Capmany

arXiv:1211.4114v1physics.optics

TL;DR

Microwave photonics seeks flexible, high-performance analog signal processing while reducing the bulk, cost, and power demands of conventional implementations. This review surveys integrated microwave photonics technologies and recent advances, including integrated filters, low-loss waveguides, beamforming, and coherent detection. The reviewed work demonstrates integrated functionality alongside advances such as 0–50 GHz filter tuning and ultra-low waveguide propagation loss, while fiber-loop implementations can produce super modes.

  • Problem

    Conventional MWP systems rely on discrete optoelectronic devices and fiber components that are bulky, expensive, power-consuming, and inflexible, while links also incur loss, noise, and distortion.

  • Method

    The article reviews recent advances in integrated microwave photonics, covering key integrated technologies and their prospective applications.

  • Results

    Integrated MWP demonstrations include 0–50 GHz filter tuning, 0.045 dB/m waveguide loss, 0.12 dB/cm ring-resonator loss, and coherent detection achieved at 300 MHz.

  • Takeaways & Limitations

    Photonic integration supports low-cost, advanced analog optical front ends with small footprints, low loss, tunability, and microwave signal-processing functionality.

  • Takeaways & Limitations

    Fiber loops used in many optoelectronic oscillators produce super modes in the phase-noise spectrum.

Abstract

from arXiv · show

Microwave photonics (MWP) is an emerging field in which radio frequency (RF) signals are generated, distributed, processed and analyzed using the strength of photonic techniques. It is a technology that enables various functionalities which are not feasible to achieve only in the microwave domain. A particular aspect that recently gains significant interests is the use of photonic integrated circuit (PIC) technology in the MWP field for enhanced functionalities and robustness as well as the reduction of size, weight, cost and power consumption. This article reviews the recent advances in this emerging field which is dubbed as integrated microwave photonics. Key integrated MWP technologies are reviewed and the prospective of the field is discussed.

1. Introduction

Microwave photonics combines radio-frequency engineering with optoelectronics to provide flexible analog signal processing and functionalities difficult or impossible in the microwave domain. Integrated microwave photonics incorporates these capabilities into photonic circuits to improve flexibility and reduce system size, cost, and power consumption.

  • Microwave photonics combines radio-frequency engineering and optoelectronics to realize functionalities complex or impossible in the radio-frequency domain.
  • Civil applications demand high speed, bandwidth, dynamic range, tunability, electromagnetic-interference immunity, and compact, lightweight, low-power devices.
  • Wideband, highly flexible analog front ends are needed before digital signal processors, and microwave photonics addresses this need through photonic signal processing.
  • Traditional MWP systems based on discrete optoelectronic devices and fiber components are bulky, expensive, power-consuming, and inflexible.
  • Integrated microwave photonics incorporates MWP components and subsystems into photonic circuits to support low-cost, advanced analog optical front ends.

2. Fundamentals of microwave photonics

Microwave photonics uses optical links to convert, transport, process, and analyze RF signals. Its performance is governed by conversion losses, noise, and nonlinear distortion, while two-tone measurements provide key link metrics.

  • MWP links and systems: An MWP link connects an electrical-to-optical modulation device to an optical-to-electrical photodetector, while added optical-domain processing forms an MWP system.Common links use intensity modulation-direct detection, though phase or frequency modulation with direct or coherent detection is also used.
  • Link performance: E/O and O/E conversions add loss, noise, and distortion, and RF loss scales quadratically with optical loss, making low optical loss essential.Thermal noise, shot noise, relative intensity noise, and, with optical amplifiers, amplified spontaneous-emission noise contribute to link noise.
  • Link performance: The main link figures of merit are gain, noise figure, intercept points, and spurious-free dynamic range, reflecting losses, noise, and nonlinearities.Link gain measures RF-to-RF power transfer and is often negative because modulation and detection efficiencies are limited.
  • Noise figure: Low-biasing a low-Vπ Mach-Zehnder modulator with high optical power and a high-power photodetector has enabled sub-10 dB noise figure.Balanced detection can further reduce common-mode relative intensity noise through matched fiber paths.
  • Nonlinearity and SFDR: A two-tone test reveals intermodulation products generated by nonlinear link components, with IMD2 at f1 ± f2 and IMD3 at 2f1 ± f2.IMD2 can interfere with signals in multioctave systems but can be filtered more readily in narrowband sub-octave systems.
  • Nonlinearity and SFDR: SFDR combines noise and nonlinearity by measuring the range between signal-noise equality and nth-order intermodulation-noise equality.For example, an SFDR3 of 110 dB.Hz2/3 corresponds to 70 dB in 1 MHz bandwidth; low bias can improve SFDR3 while sharply reducing IIP2.
  • Applications: MWP links can replace coaxial connections and separate sensitive receiver processing from antennas deployed in harsh environments.This exploits optical-link advantages including size, weight, flexibility, and flat attenuation across the relevant frequency range.

3. Integrated microwave photonics

Integrated microwave photonics addresses the bulk, cost, power, reliability, and flexibility limitations of discrete MWP systems by incorporating components and subsystems into photonic circuits. Its development must balance demanding analog performance with lower-volume markets and accessible fabrication.

  • Motivation: MWP systems offer broad bandwidth and reconfigurability, but insufficient dynamic range, reliability, cost, and power consumption have limited deployment beyond laboratory setups.Improving these factors could allow MWP to replace microwave solutions for processing applications, not only coaxial cables.
  • Motivation: Discrete MWP systems are bulky, expensive, power-consuming, and inflexible, motivating integrated microwave photonics based on photonic circuits.Discrete lasers, modulators, and detectors connected by fiber pigtails also reduce sturdiness and reliability.
  • Distinct requirements: Integrated MWP shares the direction of photonic integration but differs from digital PIC applications because it handles analog signals and targets lower-volume markets.Analog PICs therefore require high performance, often exceeding expectations for digital applications, alongside different production strategies.
  • Technology and access: The reviewed PIC technologies are assessed by performance and availability, with low PIC insertion loss needed to optimize link gain while maintaining a healthy noise figure.Fabrication-access initiatives such as ePIXfab, Jeppix, and OpSIS help users access silicon photonics and indium phosphide technologies.

4. Photonic integration technology

The review compares PIC platforms for integrated microwave photonics, emphasizing their distinct capabilities, losses, integration trade-offs, and demonstrated MWP functions. InP, silica, silicon, and TriPleX technologies each offer different combinations of active functionality, low loss, compactness, and MWP suitability.

  • Platform landscape: PIC platforms include InP, LiNbO3, silica, silicon nitride, and SOI, each offering different strengths such as active functionality, low-loss routing, electronic integration, or packaging.The review frames platform selection around the requirements of integrated MWP systems.
  • 4.1. Indium Phosphide (InP): InP integrates light generation, amplification, modulation, detection, attenuation, switching, and passive functions, but its passive propagation loss can exceed silica- or silicon-based waveguides by an order of magnitude.A reported InP waveguide loss is 1.4 dB/cm, and active gain may be needed in cascaded resonator filters; SOAs can add noise that limits SFDR.
  • 4.2. Silica PLCs: Silica PLCs provide low propagation loss and have demonstrated beamforming networks, integrated frequency discriminators, and arbitrary waveform generators, but low index contrast requires large bends and footprints.A phosphorus-doped silica-on-silicon waveguide reached 0.85 dB/m at λ = 1550 nm with 0.7% index contrast.
  • 4.3. Silicon photonics: Silicon photonics offers CMOS-manufacturing compatibility, strong SOI optical confinement, miniaturization, and large-scale integration, while silicon modulators, detectors, and lasers remain insufficient for stringent MWP requirements at 1550 nm.SOI waveguide loss varies substantially with dimensions and processing; rib and strip geometries present different loss and bending-radius trade-offs.
  • 4.4. TriPleX™ technology (Si3N4/SiO2): TriPleX single-stripe waveguides achieve 0.045 dB/m propagation loss, while double-stripe waveguides support programmable, frequency-selective MWP filters.The double-stripe filters use asymmetric MZIs with one or two ORRs and thermo-optic tuning, with footprints of 0.3×1.5 cm and 0.4×1.5 cm.

5. High dynamic range microwave photonic link

High-dynamic-range MWP links use phase or frequency modulation with either direct frequency-discriminator detection or coherent optical phase-locked-loop detection. Integrated implementations target linear demodulation while improving link performance and compactness.

  • Modulation and demodulation: Phase and frequency modulation are pursued to improve MWP link performance and potentially realize an ideal Class-B link.Phase modulation is intrinsically highly linear and does not require biasing, while frequency modulation has frequency-dependent modulation depth.
  • Direct detection with frequency discriminator: Direct detection converts phase modulation to intensity modulation with an optical discriminator whose filter can be tailored for linearity or noise suppression.The discriminator provides an additional design degree of freedom for enhancing link performance.
  • Direct detection with frequency discriminator: A cascaded-MZI phase discriminator achieved -19.5 dBm OIP3 at 0.11 mA photocurrent, improving OIP3 by 6.7 dB over an MZI with the same received photocurrent.The silica-on-silicon FIR lattice filter used six stages and had a 120 GHz FSR.
  • Direct detection with frequency discriminator: At 10 mA photocurrent, the cascaded-MZI discriminator reached 19.7 dBm OIP3 and a shot-noise-limited SFDR of 125 dB·Hz2/3.At 0.11 mA, the reported shot-noise-limited SFDR was 112 dB·Hz2/3.
  • Coherent detection with OPLL: Coherent OPLL links detect received optical phase against a local oscillator and feed back through a receiver phase modulator to reproduce the transmitted signal.The approach is potentially high performance but difficult at multi-GHz rates because loop gain, stability, and delay must be managed.
  • Coherent detection with OPLL: Integrated coherent receivers demonstrated SFDR values of 122 dB·Hz2/3 at 300 MHz and 126.8 dB·Hz2/3 at 1 GHz using coherent detection architectures.The 122 dB·Hz2/3 result used compact PIC/EIC integration, whereas the 126.8 dB·Hz2/3 architecture used coherent I/Q detection followed by digitization and DSP.

6. Microwave photonic filters

Microwave photonic filters reproduce microwave filtering functions while adding photonic advantages such as low loss, wide bandwidth, fast tunability, and reconfigurability. Integrated implementations use coherent or incoherent architectures and increasingly compact, tunable photonic components.

  • Microwave photonic filters perform functions equivalent to ordinary microwave filters while offering low loss, high bandwidth, EMI immunity, fast tunability, and reconfigurability.
  • In FIR filters, the tap weights and inter-sample delay determine the periodic electronic transfer function, whose free spectral range is f_FSR = 1/T.A dispersive delay line with tunable optical-carrier spacing changes T and enables tunability.
  • Requirements of microwave photonic filters: N = 2 produces a notch filter, whereas N > 2 produces a bandpass filter; changing T shifts the spectral period and filter positions.
  • Requirements of microwave photonic filters: Filter phase and amplitude coefficients support spectral-response tuning, while integrated ring-resonator and semiconductor-based phase shifters can switch below one microsecond.
  • Requirements of microwave photonic filters: 40 to over 60 samples are available in multiwavelength FIR schemes, with a current record of 61 dB main-to-secondary sidelobe rejection.
  • Integrated microwave photonic filters: Integrated demonstrations include InP-InGaAsP programmable pole-zero filters, silicon multi-cavity coherent filters, SOI-ring-resonator phase shifters, and photonic-crystal-waveguide filters.The photonic-crystal-waveguide approach demonstrated 0–50 GHz tuning using delays up to 170 ps in a 1.5 mm waveguide.

7. Optical delay line and beamforming

Integrated optical delay lines and phase shifters provide compact building blocks for microwave photonic beamforming. These approaches address fiber bulk and wideband beam squinting, with integrated designs supporting tunable true-time delays and complex phased-array systems.

  • Reconfigurable optical delay lines and wideband tunable phase shifters are fundamental to microwave photonic filters and optical beamforming.
  • Optical beamforming: Wideband phased arrays suffer beam squinting when constant phase shifts produce frequency-dependent beam pointing; variable delays can compensate it.
  • Optical beamforming: Integrated beamformers reduce fiber-based bulk and include discretely tunable, continuously tunable, and narrowband phase-shifter-based architectures.
  • Optical beamforming: Cascaded tunable optical ring resonators enable continuously tunable beamformers for applications such as satellite communications and phased-array antennas.
  • Optical beamforming: A 16×1 Ku-band beamformer was designed for more than 4 GHz instantaneous bandwidth and a maximum time delay of 290 ps.
  • Optical beamforming: A 22 mm×7 mm beamformer using 40 optical ring resonators supports a 2048-element phased-array system, with propagation loss as low as 0.1 dB/cm.

8. Microwave signal generation

Integrated microwave photonics extends microwave signal generation beyond electronic bandwidth limits through programmable on-chip spectral shaping, pulse shaping, and optoelectronic oscillation. Demonstrations include arbitrary waveforms, UWB pulses, and ultra-pure microwave generation, while bulk optics, fiber devices, and fiber loops remain important limitations.

  • Integrated microwave photonics reviews arbitrary waveform generation, ultrawideband pulse shaping, and stable carrier generation using optoelectronic oscillators.
  • 8.1. Arbitrary waveform generation: Electronic arbitrary waveform generators are limited to 5.6 GHz bandwidth and frequencies up to 9.6 GHz, motivating photonic alternatives.
  • 8.1. Arbitrary waveform generation: A programmable silicon photonic spectral shaper uses eight thermally tunable add-drop microring resonators to generate diverse microwave waveforms.The rings use 500 nm×250 nm silicon nanowires, with a typical 5 µm bending radius and approximately 3.5 dB/cm propagation loss.
  • 8.1. Arbitrary waveform generation: The waveform generator shapes a mode-locked laser spectrum, applies wavelength-to-time mapping through 5.5 km of fiber, and detects the resulting microwave waveform.Reported outputs include apodized profiles, multiple π phase shifts, two-tone waveforms, and frequency-modulated waveforms.
  • 8.1. Arbitrary waveform generation: Integrated pulse-shaping demonstrations include 40 GHz and 80 GHz pulse trains from a 12-tap silica-on-silicon FIR filter built from cascaded Mach–Zehnder interferometers.
  • 8.2. Impulse radio UWB pulse shaping: Cascaded optical ring resonators generate UWB monocycles and doublets by implementing temporal differentiation or microwave-photonic bandpass filtering.
  • 8.2. Impulse radio UWB pulse shaping: 52% power efficiency was achieved for an IR-UWB pulse formed by combining a Gaussian pulse with a delayed copy and filtering it using a silicon optical ring resonator.
  • 8.3. Optoelectronic oscillator and optical comb generation: Optoelectronic oscillators provide ultra-pure microwave generation with low phase noise and frequency versatility, but fiber loops are bulky, temperature-sensitive, and can produce super modes.

9. Other emerging applications

Integrated microwave photonics is also being applied to computing, frequency conversion, photonic analog-to-digital conversion, and frequency measurement. These applications exploit PIC-enabled bandwidth, reconfigurability, lightweight implementation, and compact form factors.

  • PIC technologies support photonic differentiation, all-optical temporal integration, and other fundamental computing functions.
  • An on-chip silicon electro-optical mixer demonstrated RF upconversion from 1 GHz to 10.25 GHz using slow-light-enhanced Mach–Zehnder modulation.
  • A silicon photonic ADC chip incorporating a modulator, wavelength demultiplexers, and photodetectors produced 3.5 ENOB for a 10 GHz input.
  • Photonic instantaneous microwave frequency measurement attracts interest because of its wide bandwidth, fast reconfigurability, lightweight implementation, microwave transparency, and small form factor.The paper identifies military and security applications as potential use cases.

10. Prospective: what’s next for integrated MWP?

The paper anticipates continued growth of integrated microwave photonics, especially for PIC-based filtering and nonlinear signal processing. Future progress is expected to build on demonstrations involving resonators, photonic crystals, four-wave mixing, and on-chip stimulated Brillouin scattering.

  • Integrated microwave photonics is described as an emerging field with a bright future, with microwave-photonic filters expected to remain leading signal-processing applications.
  • Future integrated MWP work is expected to expand PIC-based filters using resonators and photonic crystals, alongside nonlinear-optics approaches.
  • Four-wave mixing has been reported for enhancing MWP-link gain and filtering, while on-chip chalcogenide SBS has been demonstrated for delay lines and MWP filters.
  • The review presents integrated MWP as a developing area spanning signal processing, silicon photonics, phase modulation, and photonic integrated circuits.
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