A photonic-assisted radio frequency (RF) down-converter integrating with the optoelectronic oscillator (OEO)-based high quality local oscillator (LO) has been proposed and experimentally demonstrated. The LO and the RF input signal are mixed at the same phase modulator and photo-detector (PD) of the OEO-loop without any additional modulator or PD. The working bandwidth of the proposed RF down-converter is nearly from 2.5 to 10 GHz. The performance of the proposed down-converter is presented and the spurious frequency dynamic range at frequency of 5.5 GHz with the LO at 6.138 GHz is measured to be 98.4 dB–Hz2∕3. The influences that the frequency range and power of the RF input signal bring to the system are discussed as well.

A novel scheme for the generation of background-free pulsed microwave signals is proposed and experimentally demonstrated based on spectral shaping, frequency-to-time mapping, and balanced photodetection. In the proposed scheme, the optical spectral shaper, which consists of a differential group delay (DGD) element, two polarization controllers, and a polarization beam splitter, has two outputs with complementary power transfer functions. By passing a short optical pulse through the spectral shaper and a dispersive element (DE), a pulsed microwave signal is obtained after balanced photodetection. Thanks to the balanced photodetection, the lowfrequency components (i.e., the background signal) in the electrical spectrum is suppressed, leading to the generation of a background-free pulsed microwave signal. Meanwhile, the spectral power of the obtained microwave signal is enhanced compared to that obtained by single-end detection. Experimental results for the generation of a pulsed microwave signal centered at 12.46 GHz show that the background signal can be suppressed by more than 30 dB, and the spectral power is increased by 5.5 dB. In addition, the central frequency of the obtained background-free pulsed microwave signal can be tuned by changing the DGD introduced by the DGD element, and/or by changing the dispersion of the DE.

An integrated on-chip optical device consisting of two distributed feedback (DFB) lasers and one multimode semiconductor ring laser (SRL) has been numerically investigated. In this optical circuit, the two DFB lasers are injected into the SRL, and with the presence of the four-wave mixing effect and optical feedback, the three semiconductor lasers achieve mutual-locking state. The beating between the output optical spectral lines can generate readily tunable radio frequency signals with high spectral purity.

Stimulated Brillouin scattering (SBS) in optical fibers has long been used in frequency-selective optical signal processing, including in the realization of microwave-photonic (MWP) filters. In this work, we report a significant enhancement in the selectivity of SBS-based MWP filters. Filters having a single passband of 250 MHz–1 GHz bandwidth are demonstrated, with selectivity of up to 44 dB. The selectivity of the filters is better than that of the corresponding previous arrangements by about 15 dB. The shape factor of the filters, defined as the ratio between their -20 dB bandwidth and their -3 dB bandwidth, is between 1.35 and 1.5. The central transmission frequency, bandwidth, and spectral shape of the passband are all independently adjusted. Performance enhancement is based on two advances, compared with previous demonstrations of tunable SBS-basedMWPfilters: (a) the polarization attributes of SBS in standard, weakly birefringent fibers are used to discriminate between in-band and out-of-band components and (b) a sharp and uniform power spectral density of the SBS pump waves is synthesized through external modulation of an optical carrier by broadband, frequency-swept waveforms. The signal-to-noise ratio of filtered radio-frequency waveforms and the linear dynamic range of the filters are estimated analytically and quantified experimentally. Lastly, a figure of merit for the performance of the filters is proposed and discussed. The filters are applicable to radio-over-fiber transmission systems.

A novel method for accurately measuring chromatic dispersion of optical fibers is proposed based on the use of chirped intensity-modulated signals. Unlike the conventional method, the proposed method utilizes the configurable transfer function of optical fibers caused by the residual chirp of intensity modulation, which not only eliminates the chirp error but also improves the measurement range through adjusting the chirp parameter of the intensity modulator. Our method is applicable for measuring both the magnitude and sign of chromatic dispersion of optical fibers or other dispersive devices at different operating wavelengths by using a vector network analyzer.

A serial line scan microscopic imaging system with 1 GHz scan rate is proposed and demonstrated. This method is based on optical time-stretch in dispersive fiber to realize superfast scan imaging. Furthermore, a wavelength division technique is utilized to overcome the trade-off between high frame rate and spatial resolution caused by dispersion-induced pulse overlap. Every single frame is carved into two channels by optical filters and is detected in different wavelength bands separately. Then, both channels are combined to reconstruct the whole frame. By this method, an imaging system with spatial resolution of 28 μm at line scan rate of 1 GHz with chromatic dispersion of 1377 ps∕nm is realized. It has the potential to capture fast, nonrepetitive transient phenomena with a timescale of less than one nanosecond.

An optical length-change measurement technique is proposed based on an incoherent microwave photonic filter (MPF). The optical length under testing is inserted into an optical link of a single-bandpass MPF based on a polarization-processed incoherent light source. The key feature of the proposed technique is to transfer the length measurement in the optical domain to the electrical domain. In the electrical domain, the measurement resolution is extremely high thanks to the high-resolution measurement of microwave frequency response. In addition, since the MPF is a single-bandpass MPF, the optical length is uniquely determined by the central frequency of the MPF. A detailed investigation of the relation between the center frequency of the MPF and the optical length change is implemented. A measurement experiment is also demonstrated, and the experimental results show that the proposed technique has a measurement sensitivity of 1 GHz/mm with a high length-measurement resolution of 1 pm in theory. The proposed approach has the advantages of high sensitivity, high resolution, and immunity to power variation in electronic and optical links.

The designs of magneto-electric (ME) dipole antennas and 4 × 4 planar arrays on low-temperature cofired ceramic (LTCC) substrates are presented for radio-over-fiber (ROF) systems. The ME dipole antenna covers the four 2.16 GHz channels defined in the 60 GHz band from 57 to 66 GHz. It can be used as a 4 × 4 planar antenna array element for high gain performance. The results show that the proposed antenna array achieves a variation of peak gain from 15.0 to 18.1 dBi and a peak gain up to 17.67 dBi at 60 GHz. It is revealed that our design satisfies the 60 GHz standards ruled by IEEE 802.15.3c.

In order to achieve small size, light weight, and immunity to electromagnetic interference, it is desirable to replace bulky coaxial cables with optical fiber in advanced radar front-ends. Such applications require a large dynamic range that is beyond the reach of conventional intensity modulation–direct detection fiber-optic links. A coherent fiber-optic link employing an optical phase-locked loop (OPLL) phase demodulator has been proposed as a solution to this problem. The challenge is the practical realization of the OPLL demodulator that satisfies the stringent loop delay requirement. A novel attenuation counterpropagating (ACP) OPLL concept has been proposed and demonstrated as a solution. In this paper we review the recent progress in realizing chip-scale ACP-OPLL devices. In particular, we focus on the latest measurement results achieved using a hybrid integrated ACP-OPLL, as well as the design and performance potential of a monolithically integrated ACP-OPLL photonic integrated circuit.

Microwave photonics (MWPs) uses the strength of photonic techniques to generate, process, control, and distribute microwave signals, combining the advantages of microwaves and photonics. As one of the main topics of MWP, radio-over-fiber (RoF) links can provide features that are very difficult or even impossible to achieve with traditional technologies. Meanwhile, a considerable number of signal-processing subsystems have been carried out in the field of MWP as they are instrumental for the implementation of many functionalities. However, there are still several challenges in strengthening the performance of the technology to support systems and applications with more complex structures, multiple functionality, larger bandwidth, and larger processing capability. In this paper, we identify some of the notable challenges in MWP and review our recent work. Applications and future direction of research are also discussed.

Three-dimensional (3D) terahertz (THz) imaging or THz tomography has recently proven to be useful for nondestructive testing of industrial materials and structures. In place of previous imaging techniques such as THz pulsed/continuous-wave radar and THz computed tomography, we propose a THz optical coherence tomography using photonics- and electronics-based THz sources, and demonstrate thickness measurement and tomographic imaging in frequency regions from 400 to 800 GHz.

We present several laser sources dedicated to advanced microwave photonic applications. A quantum-dash mode-locked laser delivering a high-power, ultra-stable pulse train is first described. We measure a linewidth below 300 kHz at a 4.3 GHz repetition rate for an output power above 300 mW and a pulse duration of 1.1 ps after compression, making this source ideal for microwave signal sampling applications. A widely tunable (5–110 GHz), monolithic millimeter-wave transceiver based on the integration of two semiconductor distributed feedback lasers, four amplifiers, and two high-speed uni-traveling carrier photodiodes is then presented, together with its application to the wireless transmission of data at 200 Mb∕s. A frequency-agile laser source dedicated to microwave signal processing is then described. It delivers arbitrary frequency sweeps over 20 GHz with high precision and high speed (above 400 GHz∕ms). Finally, we report on a low-noise (below 1 kHz linewidth), solid-state, dual-frequency laser source. It allows independent tuning of the two frequencies in the perspective of the implementation of a tunable optoelectronic oscillator based on a high-Q optical resonator.

An approach to implementing optical single sideband (OSSB) polarization modulation, which is a combination of two orthogonally polarized OSSB modulations with complementary phase differences between the optical carrier and the sideband, is demonstrated based on two cascaded polarization modulators (PolMs). The two PolMs are driven by two RF signals that are 90° out of phase. By properly adjusting the polarization state between the two PolMs, OSSB polarization modulation with large operation bandwidth can be realized. An experiment is performed. OSSB polarization modulation with an operation bandwidth from 2 to 35 GHz is successfully demonstrated. The spectral profile of the OSSB polarization-modulated signal is observed through an optical spectrum analyzer, and its complementary phase properties are analyzed by sending the signal to a photodetector (PD) for square-law detection. Due to the complementary phase differences between the optical carrier and the sideband along the two polarization directions, no microwave frequency component is generated after the PD. The generated OSSB polarization-modulated signal is transmitted through 25 and 50 km single-mode fiber with 50 Mbaud 16 quadrature amplitude modulation baseband data to investigate the transmission performance of the proposed system in radio-over-fiber applications, and very small error vector magnitude degradation is observed. OSSB polarization modulation is also employed to realize a microwave photonic phase shifter. A full-range tunable phase shift is obtained for 2 and 35 GHz microwave signals.

Spurious-free dynamic range (SFDR) limited by intermodulation distortions is a usually accepted measure for dynamic performance of a photonic time-stretched analog-to-digital converter (ADC). In this paper, SFDR improvement in a photonic time-stretched ADC based on third-order predistortion is proposed. The third-order predistortion is achieved optically within an integrated dual-parallel Mach–Zehnder modulator (DPMZM). The mechanism of SFDR improvement with third-order predistortion in the DPMZM is theoretically analyzed. Compared with a conventional scheme without predistortion, the experimental results show that the SFDR improvement of ～26 dB in the proposed scheme is proved.