Hao Chi

An Approach to the Measurement of Microwave Frequency Based on Optical Power Monitoring

A novel approach to the measurement of microwave frequency based on optical power monitoring is proposed and demonstrated. The microwave signal with its frequency to be measured is modulated on two optical carriers with their wavelengths set at one peak and one valley of the spectral response of a sinusoidal filter. The modulation is performed by a Mach-Zehnder modulator that is biased to suppress the optical carriers. A mathematical expression relating the optical powers from the two wavelength channels and the microwave frequency to be measured is developed. By simply monitoring the optical powers at the outputs of the two wavelength channels, the microwave frequency can be evaluated. A proof-of-concept experiment is implemented. Frequency measurement with good accuracy for microwave signals at different power levels is realized.

Frequency Quadrupling and Upconversion in a Radio Over Fiber Link

In this paper, a novel technique to realize frequency quadrupling and upconversion in a radio over fiber (RoF) link is proposed and experimentally demonstrated. The frequency quadrupling is achieved by using two cascaded Mach-Zehnder modulators (MZMs) that are biased at the minimum transmission point, with a tunable optical delay line placed between the MZMs. By properly adjusting the time delay between the two MZMs, a pair of optical wavelengths with a wavelength spacing corresponding to four times the frequency of the microwave drive signal is generated. The two wavelengths are then sent to a third MZM to which an intermediate-frequency (IF) signal is applied. At the output of the third MZM, a frequency-upconverted signal at the millimeter-wave (mm-wave) band is obtained. The advantages of the technique are that a relatively low-frequency local oscillator (LO) signal is used to generate a high-frequency LO signal and the upconverted signal is more tolerant to the dispersion-induced power fading compared with a conventional RoF link based on double-sideband (DSB) modulation. Experiments are performed to verify the technique.

Analytical Models for Phase-Modulation-Based Microwave Photonic Systems With Phase Modulation to Intensity Modulation Conversion Using a Dispersive Device

Recently, optical phase modulation has been widely used in microwave photonics (MWP) systems, such as radio over fiber systems, photonic microwave filters, optical microwave and millimeter-wave signal generators, and optical subcarrier frequency up-converters. An optical phase-modulated signal can be converted to an intensity-modulated signal in a dispersive optical fiber. Due to the intrinsic nonlinearity of optical phase modulation, for linear applications such as microwave signal distribution and filtering, the modulation index should be kept small to minimize the unwanted modulation nonlinearity. However, for nonlinear applications such as microwave frequency multiplication and subcarrier frequency upconversion, the modulation index should be large to maximize the frequency multiplication and upconversion efficiency. In this paper, for the first time to our knowledge, we develop a thorough theoretical framework for the characterization of phase-modulation-based MWP systems, in which the phase modulation to intensity modulation conversion is realized using a dispersive fiber. Analytical models for the distributions of single-tone and two-tone microwave signals and for microwave frequency multiplication and subcarrier frequency upconversion are developed, which are verified by numerical simulations. The analytical models for single-tone and two-tone transmissions are further confirmed by experiments. The developed analytical models provide an accurate mathematical tool in designing phase-modulation-based MWP systems.

Microwave Frequency Measurement Based on Optical Power Monitoring Using a Complementary Optical Filter Pair

An approach to the measurement of a microwave frequency based on optical power monitoring using a complementary optical filter pair is proposed and investigated. In the proposed system, a microwave signal is applied to a Mach-Zehnder modulator, which is biased at the minimum transmission point to suppress the optical carrier. The carrier-suppressed optical signal is then sent to the complementary optical filter pair, with the powers from the complementary filters measured by two optical power meters. A mathematical expression that relates the microwave frequency and the optical powers is developed. Experiments are performed to verify the effectiveness of the proposed approach. The performance of the proposed system in terms of the frequency measurement range, operation stability, and robustness to noise is also investigated.

Multiplexed Millimeter Wave Communication with Dual Orbital Angular Momentum (OAM) Mode Antennas

Communications using the orbital angular momentum (OAM) of radio waves have attracted much attention in recent years. In this paper, a novel millimeter-wave dual OAM mode antenna is cleverly designed, using which a 60 GHz wireless communication link with two separate OAM channels is experimentally demonstrated. The main body of the dual OAM antenna is a traveling-wave ring resonator using two feeding ports fed by a 90° hybrid coupler. A parabolic reflector is used to focus the beams. All the antenna components are fabricated by 3D printing technique and the electro-less copper plating surface treatment process. The performances of the antenna, such as S-parameters, near-fields, directivity, and isolation between the two OAM modes are measured. Experimental results show that this antenna can radiate two coaxially propagating OAM modes beams simultaneously. The multiplexing and de-multiplexing are easily realized in the antennas themselves. The two OAM mode channels have good isolation of more than 20 dB, thus ensuring the reliable transmission links at the same time.

Instantaneous Microwave Frequency Measurement Using an Optical Phase Modulator

A novel technique for instantaneous microwave frequency measurement using an optical phase modulator is proposed and demonstrated. In the proposed system, a microwave signal with its frequency to be measured is modulated on two optical wavelengths at the phase modulator, with the phase-modulated optical signals sent to a dispersive element, and detected at two photo-detectors. Due to the chromatic dispersion of the dispersive element, the two microwave signals will experience different power fading, leading to different power versus frequency functions. A fixed relationship between the microwave frequency and the microwave powers is established. By measuring the microwave powers, the microwave frequency is estimated. Compared with the techniques using an intensity modulator, the proposed approach is simpler with less loss. Since no bias is needed the system has a better stability, which is highly expected for defense applications. Experimental verification is presented.

Photonic Generation of Phase-Coded Microwave Signal With Large Frequency Tunability

A photonic approach to realizing phase-coded microwave signal generation with large frequency tunability is proposed and demonstrated. Two coherent optical wavelengths are generated based on external modulation by biasing a Mach-Zehnder modulator (MZM) at the minimum transmission point to generate ±1 -order sidebands while suppressing the optical carrier. The two ±1-order sidebands are then sent to a fiber Sagnac interferometer (SI) incorporating an optical phase modulator (PM) and a broadband flat-top fiber Bragg grating (FBG), with one of the sidebands being phase modulated at the PM. A frequency tunable phase-coded microwave signal is generated by beating the two sidebands at a photodetector (PD). The proposed technique is experimentally investigated. The generation of a frequency tunable phase-coded microwave signal at 22 and 27 GHz is demonstrated.

An Approach to Photonic Generation of High-Frequency Phase-Coded RF Pulses

A novel method to generate high-frequency phase-coded RF pulses using all-fiber components is proposed. The system consists of a mode-locked fiber laser (MLFL), a dispersive element, an unbalanced Mach-Zehnder interferometer (UMZI), an optical phase modulator (PM), and a photodetector (PD). The PM is incorporated in one arm of the UMZI. In the system, an ultrashort pulse generated by the MLFL is broadened and chirped after passing through the dispersive element, which is then sent to the UMZI, to get two time-delayed chirped pulses. By beating the time-delayed chirped pulses at the PD, an RF pulse with its frequency dependent on the time delay difference is obtained. The generated RF pulse can be phase coded if an encoding signal is applied to the PM. A theoretical model is presented which is verified by experiments. The generation of RF pulses with binary phase coding is also experimented

Photonic Generation of Microwave Signals Based on Pulse Shaping

A novel approach to generating microwave signals based on optical pulse shaping is proposed and experimentally demonstrated. The proposed system consists of a femtosecond pulse laser source, a Sagnac-loop filter (SLF), a dispersive element, and a photodetector. The spectrum of the femtosecond pulse is shaped by the SLF that has a sinusoidal spectral response. Thanks to the frequency-to-time conversion in the dispersive element, time-domain pulse exhibiting the shape of the optical power spectrum is obtained. Depending on the free-spectral range of the SLF and the total dispersion of the dispersive element, signals with frequencies up to terahertz can be generated. A model to describe the signal generation is developed. Experimental results agree well with the theoretical analysis

All-Fiber Chirped Microwave Pulses Generation Based on Spectral Shaping and Wavelength-to-Time Conversion

An approach to generating chirped microwave pulse based on optical spectral shaping and nonlinear chromatic-dispersion-induced wavelength-to-time mapping using all-fiber components is proposed and demonstrated. In the proposed approach, the spectrum of a femtosecond pulse is shaped by a two-tap Sagnac-loop filter that has a sinusoidal spectrum response. The spectrum shaped pulse is then sent to a dispersive element that has first- and second-order chromatic dispersions. Thanks to the nonlinear wavelength-to-time mapping of the dispersive element, a temporal pulse that has a central frequency in the microwave band with a large chirp is generated, which provides the potential for applications in high-speed communications and radar systems. Numerical and proof-of-concept experimental results are presented.