2D Layered Materials: Synthesis, Nonlinear Optical Properties, and Device Applications

As the only method for all-weather, all-time and long-distance target detection and recognition, radar has been intensively studied since it was invented, and is considered as an essential sensor for future intelligent society. In the past few decades, great efforts were devoted to improving radar's functionality, precision, and response time, of which the key is to generate, control and process a wideband signal with high speed. Thanks to the broad bandwidth, flat response, low loss transmission, multidimensional multiplexing, ultrafast analog signal processing and electromagnetic interference immunity provided by modern photonics, implementation of the radar in the optical domain can achieve better performance in terms of resolution, coverage, and speed which would be difficult (if not impossible) to implement using traditional, even state-of-the-art electronics. In this tutorial, we overview the distinct features of microwave photonics and some key microwave photonic technologies that are currently known to be attractive for radars. System architectures and their performance that may interest the radar society are emphasized. Emerging technologies in this area and possible future research directions are discussed.

/pdf/microwave-photonic-radars-1mv4ppmi12.pdf

Microwave Photonic Radars

Gapless linear energy dispersion of graphene endows it with unique nonlinear optical properties, including broadband nonlinear absorption and giant nonlinear refractive index. Herein, we experimentally observed that few-layers graphene has obvious nonlinear absorption and large nonlinear refraction, as investigated by the Z-scan technique in the mid-infrared (mid-IR) regime. Our study may not only, for the first time to our knowledge, verify the giant nonlinear refractive index of graphene (∼10−7  cm2/W) at the mid-IR, which is 7 orders of magnitude larger than other conventional bulk materials, but also provide some new insights for graphene-based mid-IR photonics, potentially leading to the emergence of several new conceptual mid-IR optoelectronics devices.

Broadband ultrafast nonlinear optical response of few-layers graphene: toward the mid-infrared regime

A label-free cardiac biomarker immunosensor based on phase-shifted microfiber Bragg grating.

Photonics-based radar with a photonic de-chirp receiver has the advantages of broadband operation and real-time signal processing, but it suffers from interference from image frequencies and other undesired frequency-mixing components, due to single-channel real-valued photonic frequency mixing. In this paper, we propose a photonics-based radar with a photonic frequency-doubling transmitter and a balanced in-phase and quadrature (I/Q) de-chirp receiver. This radar transmits broadband linearly frequency-modulated signals generated by photonic frequency doubling and performs I/Q de-chirping of the radar echoes based on a balanced photonic I/Q frequency mixer, which is realized by applying a 90° optical hybrid followed by balanced photodetectors. The proposed radar has a high range resolution because of the large operation bandwidth and achieves interference-free detection by suppressing the image frequencies and other undesired frequency-mixing components. In the experiment, a photonics-based K-band radar with a bandwidth of 8 GHz is demonstrated. The balanced I/Q de-chirping receiver achieves an image-rejection ratio of over 30 dB and successfully eliminates the interference due to the baseband envelope and the frequency mixing between radar echoes of different targets. In addition, the desired de-chirped signal power is also enhanced with balanced detection. Based on the established photonics-based radar, inverse synthetic aperture radar imaging is also implemented, through which the advantages of the proposed radar are verified.

Photonics-based radar with balanced I/Q de-chirping for interference-suppressed high-resolution detection and imaging

The human epidermal growth factor receptor 2 (HER2) is a clinically significant molecular marker of breast cancer. Interrogation of a high-resolution and temperature- compensational HER2 antigen biosensor based on a microwave photonics filter (MPF) is proposed and experimentally demonstrated. A microfiber Bragg grating (mFBG) and a regular fiber Bragg grating (FBG) are used as a sensing probe for HER2 antigen-antibody specific binding detection and for temperature compensation, respectively. The key point of this work is to use the frequency domain interrogation scheme based on an MPF for high resolution biosensing detection. Experimental results show that the frequency shift of an MPF notch has a linear relationship with the surrounding refractive index (SRI) change over a range from 1.332 to 1.352, with an SRI resolution up to 2.45 × 10 −6 RIU, which is almost three orders of magnitude better than that based on the conventional wavelength interrogation technology (2.9 × 10 −3 RIU) using the same sensing probe. In situ monitoring is achieved with a limit of detection (LOD) up to 50 ng/mL, with temperature self-compensation ability. The proposed frequency-domain interrogation method based on microwave photonics opens up a new way for fiber-optic biochemical and environmental measurement.

High-resolution and temperature-compensational HER2 antigen detection based on microwave photonic interrogation

A novel microwave photonic phase shifter structure is presented. It is based on the conversion of the optical carrier phase shift into an RF signal phase shift via controlling the carrier wavelength of a single-sideband RF-modulated optical signal into a fiber Bragg grating. The new microwave photonic phase shifter has a simple structure and only requires a single control to shift the RF signal phase. It also has the ability to realize multiple phase shifts. Experimental results demonstrate a continuous 0°–360° phase shift with low amplitude variation of <2  dB and low phase deviation of <5° over a wideband microwave range.

Optical-to-RF phase shift conversion-based microwave photonic phase shifter using a fiber Bragg grating

A new dual-function photonic microwave signal processing structure that has the ability to realize both frequency up/down conversion and RF/IF phase shifting, is presented. In the proposed signal processor, a dual-polarization dual-parallel Mach Zehnder modulator (DP-DPMZM) is used to generate an orthogonally polarized single RF/IF signal and local oscillator (LO) modulation sideband without an optical carrier. The optical phase difference between the two sidebands can be controlled by controlling a DC voltage applied to a LiNbO3 electro-optic phase modulator that is connected after the DP-DPMZM. Beating between the two sidebands at a photodetector generates an RF/IF signal with a phase equal to the optical phase difference between the IF/RF signal and LO sidebands. The dual-function photonic microwave signal processor has a wide bandwidth and does not require a precise control of the laser wavelength. Experimental results demonstrate that a flat >−3 dB down/up conversion efficiency is achieved for an RF signal from 3.5 to 26.5 GHz and for an IF signal from 0.5 to 3 GHz, and a full 360° continuous phase shift of the output IF/RF signal.

Broadband Photonic Microwave Signal Processor With Frequency Up/Down Conversion and Phase Shifting Capability

This paper presents a new technique for realizing continuous 0°-360° RF signal phase shift over a very wide bandwidth. It is based on using single-sideband modulation together with optical filtering to largely suppress one of the RF modulation sidebands over a wide input RF frequency range, and controlling the phase of the optical carrier to shift an RF signal phase. The technique does not require expensive electrical or optical components to realize an RF signal phase shift over 2-40 GHz frequency range with a flat amplitude and phase response performance. This overcomes the current technology limitation in which no reported phase shifter structure has demonstrated the capability of operating in such a wide bandwidth. Experimental results demonstrate only ± 1 dB amplitude variation and ± 5° phase deviation from the desired RF signal phase shift over 2-40 GHz bandwidth and the RF signal amplitude control function. The phase shifter wavelength insensitive performance is also demonstrated experimentally.

Ultra-wide bandwidth photonic microwave phase shifter with amplitude control function.

A new photonic microwave phase shifting structure that can realize a continuous 0° to 360° phase shift with only little frequency dependent amplitude and phase variation over a wide frequency range is presented. It is based on controlling the amplitude and phase of the optical carrier and two antiphase RF modulation sidebands of two phase modulated optical signals by adjusting the wavelength of the optical carriers that fall on the rising and falling edges of an optical bandpass filter with a nonlinear phase response. The new photonic microwave phase shifter has a simple structure. It only requires one normal electro-optic phase modulator and one low-cost commercially available optical filter. It does not involve any electrical component, and has the advantages of small size, lightweight, and robust performance. Experimental results are presented which demonstrate the realization of a continuous 0° to 360° phase shift, with close to flat phase and amplitude response performance over a wideband microwave range.

Xudong Wang

Papers

High-resolution and temperature-compensational HER2 antigen detection based on microwave photonic interrogation

Optical-to-RF phase shift conversion-based microwave photonic phase shifter using a fiber Bragg grating

Broadband Photonic Microwave Signal Processor With Frequency Up/Down Conversion and Phase Shifting Capability

Ultra-wide bandwidth photonic microwave phase shifter with amplitude control function.

All-Optical Photonic Microwave Phase Shifter Based on an Optical Filter With a Nonlinear Phase Response