RF Photonics Applications Research Articles

Ultraflat bandpass, high extinction, and tunable silicon photonic filters.

We report on the design and the demonstration of silicon photonic ultraflat bandpass filters with low insertion loss and high out-of-band rejection for an operation near the 1550 nm wavelength band. These filters are based on cascading low (2nd) order Ring-Assisted Mach-Zehnder Interferometer (RAMZI) filter stages. The cascade design enables high out-of-band rejection while keeping the unit cells of each stage low order to be more tolerant to fabrication imperfections. The characterization of filters shows an insertion loss of ∼1 dB, an in-band ripple of <0.1 dB, an out-of-band rejection of >50 dB for a filter 3-dB bandwidth of ∼1.1 nm, and tunable up to ∼6 nm. We also investigate the filter's spur-free dynamic range at high input optical powers, which is important for RF photonics applications, and quantify a dynamic range of >60 dB for a laser power as high as ∼11.6 mW sent to the filter. Such integrated filters are promising for applications in pump wavelength rejection in four-wave mixing photon pair generation, and in RF antenna remoting where multiple RF signals are carried on different coarse wavelengths to be separated.

Ultra-High Bandwidth Radio Frequency and Microwave Photonic Signal Processing Based on Kerr Micro-Combs

Integrated Kerr micro-combs, a powerful source of many wavelengths for photonic RF and microwave signal processing, are particularly useful for transversal filter systems. They have many advantages including a compact footprint, high versatility, large numbers of wavelengths, and wide bandwidths. We review recent progress on photonic RF and microwave high bandwidth temporal signal processing based on Kerr micro-combs with spacings from 49-200GHz. We cover integral and fractional Hilbert transforms, differentiators as well as integrators. The potential of optical micro-combs for RF photonic applications in functionality and ability to realize integrated solutions is also discussed.

Microcomb-Based Photonic RF Signal Processing

Microcombs are powerful tools as sources of multiple wavelength channels for photonic RF signal processing. They offer a compact device footprint, large numbers of wavelengths, and wide Nyquist bands. Here, we review recent progress on microcomb-based photonic RF signal processors, including true time delays, reconfigurable filters, Hilbert transformers, differentiators, and channelizers. The strong potential of optical micro-combs for RF photonics applications in terms of functions and integrability is also discussed.

Integration of Brillouin and passive circuits for enhanced radio-frequency photonic filtering

Signal processing using on-chip nonlinear or linear optical effects has shown tremendous potential for RF photonic applications. Combining nonlinear and linear elements on the same photonic chip can further enable advanced functionality and enhanced system performance in a robust and compact form. However, the integration of nonlinear and linear optical signal processing units remains challenging due to the competing and demanding waveguide requirements, specifically the combination of high optical nonlinearity in single-pass waveguides, which is desirable for broadband signal processing with low linear loss and negligible nonlinear distortions required for linear signal processing. Here, we report the first demonstration of integrating Brillouin-active waveguides and passive ring resonators on the same integrated photonic chip, enabling an integrated microwave photonic notch filter with ultradeep stopband suppressions of &amp;gt;40 dB, a low filter passband loss of &amp;lt;−10 dB, flexible center frequency tuning over 15 GHz, and reconfigurable filter shape. This demonstration paves the way for implementing high-performance integrated photonic processing systems that merge complementary linear and nonlinear properties, for advanced functionality, enhanced performance, and compactness.

RF Photonics: An Optical Microcombs’ Perspective

In the past decade, optical frequency combs generated by high-Q micro-resonators, or micro-combs, which feature compact device footprints, high energy efficiency, and high-repetition-rates in broad optical bandwidths, have led to a revolution in a wide range of fields including metrology, mode-locked lasers, telecommunications, RF photonics, spectroscopy, sensing, and quantum optics. Among these, an application that has attracted great interest is the use of micro-combs for RF photonics, where they offer enhanced functionalities as well as reduced size and power consumption over other approaches. This article reviews the recent advances in this emerging field. We provide an overview of the main achievements that have been obtained to date, and highlight the strong potential of micro-combs for RF photonics applications. We also discuss some of the open challenges and limitations that need to be met for practical applications.

Beat note stabilization in dual-polarization DFB fiber lasers by an optical phase-locked loop.

A fully fibered microwave-optical source at 1.5 µm is studied experimentally. It is shown that the beat note between two orthogonally polarized modes of a distributed-feedback fiber laser can be efficiently stabilized using an optical phase-locked loop. The pump-power-induced birefringence serves as the actuator. Beat notes at 1 GHz and 10 GHz are successfully stabilized to a reference synthesizer, passing from the 3 kHz free-running linewidth to a stabilized sub-Hz linewidth, with a phase noise as low as -75 dBc/Hz at 100 Hz offset from the carrier. Such dual-frequency stabilized lasers could provide compact integrated components for RF and microwave photonics applications.

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RF Performance of Single Sideband Modulation Versus Dual Sideband Modulation in a Photonic Link

Single sideband optical modulation can be used in multiple RF photonic link applications. However, single sideband optical modulation is often thought to provide lower RF output power than traditional dual sideband modulation techniques. While true in one specific case, it is not true in all cases. We theoretically and experimentally analyze the RF performance of a photonic link utilizing a Z-cut dual-electrode Mach-Zehnder intensity modulator with a 90° RF hybrid that produces optical single sideband modulation. The single sideband performance is compared to double sideband modulation using the same Mach-Zehnder modulator in either a push-pull configuration using a 180° RF hybrid or a single-arm drive configuration. The optical sideband powers, RF output power and the output intercept power and spur-free dynamic range of the third-order intermodulation nonlinearity are compared both theoretically and experimentally for each of the three cases. The performance of a double sideband modulated link using an X-cut inherently push-pull Mach-Zehnder modulator is also compared theoretically, assuming the same V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">π</sub> and insertion loss as the dual-electrode Mach-Zehnder modulator.

Microstructure fibre array for RF photonic signal processing applications

A novel microstructure fibre array (MFA), comprising multiple singlemode fibre waveguides reduced to an overall diameter of approximately 30 µm is reported. When used in conjunction with a single photodetector, the array facilitates the efficient and stable electrical summation of a number of RF-modulated optical signals with minimal optical interference effects. The reported MFA has optical insertion loss less than 0.1 dB. An RF filter based on the array exhibits less than 0.4 dB optical insertion loss and excellent temporal stability.

Fully programmable ring-resonator-based integrated photonic circuit for phase coherent applications

A novel ring-resonator-based integrated photonic chip with ultrafine frequency resolution, providing programmable, stable, and accurate optical-phase control is demonstrated. The ability to manipulate the optical phase of the individual frequency components of a signal is a powerful tool for optical communications, signal processing, and RF photonics applications. As a demonstration of the power of these components, we report their use as programmable spectral-phase encoders (SPEs) and decoders for wavelength-division-multiplexing (WDM)-compatible optical code-division multiple access (OCDMA). Most important for the application here, the high resolution of these ring-resonator circuits makes possible the independent control of the optical phase of the individual tightly spaced frequency lines of a mode-locked laser (MLL). This unique approach allows us to limit the coded signal's spectral bandwidth, thereby allowing for high spectral efficiency (compared to other OCDMA systems) and compatibility with existing WDM systems with a rapidly reconfigurable set of codes. A four-user OCDMA system using polarization multiplexing is shown to operate at data rates of 2.5 Gb/s within a 40-GHz transparent optical window with a bit error rate (BER) better than 10/sup -9/ and a spectral efficiency of 25%.

White light sources filtered with fiber Bragg gratings for RF-photonics applications

In this paper we demonstrate that broadband light emitters filtered with fiber Bragg gratings are a suitable light source for RF-photonics applications. A 0.075 nm bandwidth fiber grating illuminated by a light emitter of 40 nm bandwidth has been used as light source to feed a microwave phase shifter based on a chirped fiber Bragg grating. The device operates successfully in the tested frequency range (0.5–6) GHz and has been optically tuned in a range of 4 nm.

Two-Frequency Lasers: from Excess Quantum Noise to RF Photonics Applications

We illustrate the physics of two-frequency lasers by two examples. The first example illustrates the fundamental consequences of the existence of two eigenstates on the laser line width. We indeed show experimentally thatthe non-orthogonality of these two eigenstates results in an increase in the laser quantum noise. We also give a physical explanation of this vectorial excess noise factor. The second example illustrates the capabilities of two-frequency lasers in terms of applications. In the domain of RF frequency generation by optical means, we show how a pulsed two-frequency source can be built for lidar-radar applications and pulsed RF frequency generation.

Offset phase locking of Er,Yb:glass laser eigenstates for RF photonics applications

An optical source of microwaves with very low phase noise for communications and radar systems is realized and tested. It is obtained by dual-frequency operation of a diode pumped Er,Yb:glass laser. An electrooptic crystal inserted inside the resonator permits both to tune the frequency difference between orthogonally polarized eigenstates, and to turn the laser into a voltage controlled oscillator. An optical phase-locked loop is then implemented in the GHz range, resulting in a measured instrument limited 3 dB-linewidth of 10 Hz. The phase noise is shown to be -100 dBc/Hz at 10-kHz offset.

Distributed Balanced Photodetector for RF Photonic Applications

A novel velocity-matched distributed balanced photodetector operating at 1.3 and 1.55 μm wavelengths has been proposed and experimentally demonstrated. Distributed absorption and velocity matching of the optical and microwave signals are employed to achieve high saturation photocurrent. The velocity matching between the incident optical wave and the output microwave signal allows the detector length and effective absorption volume to increase without penalizing the bandwidth. A common-mode rejection ratio greater than 27 dB has been achieved over a wide range of photocurrents. More than 24-dB suppression of the relative intensity noise of the laser source and EDFA added noise has been achieved in a broadband RF link experiment. Shot-noise limited performance has been achieved with significant improvements in the signal-to-noise ratio over a wide range of frequencies and phase mismatch of input RF signals. The experimental results indicate that the distributed balanced photodetector will have a major impact on most RF photonic systems with its superior potential of reaching very high power levels.