Microwave Photonic Subsystems

Technical Overview

Microwave photonics (MWP) involves interactions between the RF/microwave/millimeter-wave and the optical portions of the electromagnetic spectrum. Photonics is utilized for the generation, transmission, detection, processing, and control of microwave signals with direct applicability to antenna systems (e.g., wireless and array), sensing, and instrumentation. This technology also makes it possible to have functions in microwave systems that are complex or even not possible in the radio-frequency domain and also creates new opportunities for telecommunication networks.

Some of the key advantages of microwave photonic links over conventional electrical-transmission media, such as coaxial cables or waveguides, include reduced size, weight, and cost; low and constant attenuation over the entire microwave and millimeter-wave modulation frequency range; immunity to electromagnetic interference; low dispersion; and high data-transfer capacity. The weight and attenuation benefits of microwave photonic links over coaxial cables are particularly compelling: typically 1.7 kg/km and 0.5 dB/km for optical fiber versus 567 kg/km and 360 dB/km at 2 GHz for a coaxial cable.

Program Goals

  • Identify and define critical Department of Defense MWP systems applications in advanced radar systems, sensors, and electronic-warfare (EW) that are enabled by unique Lincoln Laboratory technology related to high-performance hybrid and monolithic integrated photonic and electronic components
  • Investigate and develop advanced photonic components and photonic integrated circuit (PIC) techniques to meet these MWP system requirements and facilitate rapid prototyping for system insertion
  • Provide the government with technical support and guidance on MWP-related technology acquisition for advanced radar, EW, and sensor systems

Research Highlights

Lincoln Laboratory has been developing several MWP subsystems that leverage its technology to realize next-generation wideband EW receivers and simultaneous transmit and receive (STAR) systems.

  • A high-performance analog link (Figure 1) is the building block for MWP subsystems. MWP systems have been driven to the 1.5-μm-wavelength region by the availability of erbium-doped fiber amplifiers (EDFAs) and other photonic components (e.g., lasers, modulators, and photodiodes) developed for the fiber-optic communications industry. However, the required performance of components needed for high spur-free dynamic range (SFDR) analog MWP systems greatly exceeds that of standard digital-telecom components. For instance, to achieve high gain, signal-to-noise ratio (SNR), and SFDR, MWP systems typically require optical sources having high power (>0.4 W) and low noise, and photodiodes having high saturation current and wide bandwidth. Recent achievements include leveraging the Laboratory's low-noise, high-power slab-coupled optical waveguide external-cavity lasers (SCOWECLs) at 1.5-μm to achieve a high-dynamic range X-band delay line with >1 μs of delay for insertion into compact size, weight, and power (SWaP) EW systems.

    Block diagram of a microwave photonic (MWP) antenna remoting system enhanced by Lincoln Laboratory laser and photodetector technologyFigure 1. Block diagram of a microwave photonic (MWP) antenna remoting system enhanced by Lincoln Laboratory laser and photodetector technology.

  • RF photonic techniques extend operating frequencies and bandwidths of RF receivers due to availability of wideband photonic components, e.g. optical modulators and detectors operating up to 100 GHz. We have recently achieved and reported a compact wideband channelized electronic intelligence (ELINT) receiver architecture that leverages high-Q silicon-photonic filter PIC, low-noise SCOW lasers, and high-saturation SCOW photodetectors. The Laboratory demonstrated a prototype system for a two-channel ELINT receiver; the system performed both channelization and photonic frequency down-conversion from X band to baseband with high image rejection using advanced photonic filtering. This level of performance is necessary for ELINT receivers operating in dense signal environments.

  • Simultaneous transmit and receive (STAR) is a critical system need in electronic attack and electronic protection systems and also in a wide array of applications for which high average transmitted power is desirable. The Laboratory's RF system architecture for STAR operation utilizes an adaptive RF canceller. The function of the RF canceller is to adaptively replicate the response of the surrounding environment, adjusting the transfer function, to match an inverted version of the mutual coupling between the transmitter and receiver.

    Currently available RF canceller approaches are inherently narrowband because of the small number of taps and the use of phase shifters for tap adjustment. For wideband cancellation, variable time delay would be ideal; however, this is difficult to implement with required spatial and spectral resolution at RF frequencies. Additionally, the number of taps in the current implementation is limited by the amount of power tapped off at each point on the delay line, and isolation between taps is difficult to obtain in a small form factor.

    At Lincoln, we are developing a photonic tapped-delay-line RF canceller architecture for STAR. In this effort, low-loss photonic taps are achieved through a fiber Bragg grating (FBG) written into the core of a low-loss optical fiber. By leveraging the low propagation loss in the optical fiber (~0.2 dB/km) and wavelength division multiplexing (WDM) technology, we plan to realize a large finite-impulse-response (FIR) tapped-delay-line filter in a compact form factor.


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