An inversely designed integrated spectrometer with reconfigurable performance and ultra-low power consumption

Despite the pressing demand for integrated spectrometers,a solution that deliver high-performance while being practically operated is still missing.Furthermore,current integrated spectrometers lack reconfigurability in their performance,which is highly desirable for dynamic working scenarios.This study presents a viable solution by demonstrating a userfriendly,reconfigurable spectrometer on silicon.At the core of this innovative spectrometer is a programmable photonic circuit capable of exhibiting diverse spectral responses,which can be significantly adjusted using on-chip phase shifters.The distinguishing feature of our spectrometer lies in its inverse design approach,facilitating effortless control and efficient manipulation of the programmable circuit.By eliminating the need for intricate configuration,our design reduces power consumption and mitigates control complexity.Additionally,our reconfigurable spectrometer offers two distinct operating conditions.In the Ultra-High-Performance mode,it is activated by multiple phase-shifters and achieves exceptional spectral resolution in the picometer scale while maintaining broad bandwidth.On the other hand,the Ease-of-Use mode further simplifies the control logic and reduces power consumption by actuating a single-phase shifter.Although this mode provides a slightly degraded spectral resolution of approximately 0.3 nm,it prioritizes ease of use and is wellsuited for applications where ultra-fine spectral reconstruction is not a primary requirement.

Self-mode-locking optoelectronic oscillator with ultrashort time delay

The optoelectronic oscillator(OEO)is a typical time-delay system with rich nonlinear dynamical characteristics.Most of the previous research on OEOs has been focused on analyzing the properties of OEOs with a long time delay,which makes it difficult to realize mode locking without additional phaselocking mechanisms.We have achieved,for the first time to our knowledge,a self-mode-locking OEO and generated stable microwave frequency combs by analyzing the characteristics of OEOs with an ultrashort time scale.In the experiment,the self-mode-locking OEOs with fundamental mode,second-order harmonic,and sixth-order harmonic were realized by adjusting the system parameters,all of which produced uniform square wave signals with tunable duty cycles,steep rising and falling edges,and periods of less than 20 ns.The self-fundamental-mode-locking OEOs with different time delays were also implemented and experimentally realized.Furthermore,the experiment revealed the self-hybrid mode-locking OEO,which is the coexistence and synchronization of the three measured self-locking modes in one OEO cavity,demonstrating the complex nonlinear dynamical behaviors of the OEO system and enabling the generation of periodic nonuniform hybrid square wave signals.The realization of the self-mode-locking OEO and the generation of flexible and stable square wave signals at ultrashort time scales enrich the study of OEO nonlinear dynamics in the realm of complex microwave waveform generation,offering promising applications in areas such as atomic clocks,radars,communications,and optoelectronic neural networks.

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

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 photonicsbased 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 dechirped 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.

Advances in cost-effective integrated spectrometers

The proliferation of Internet-of-Things has promoted a wide variety of emerging applications that require compact,lightweight,and low-cost optical spectrometers.While substantial progresses have been made in the miniaturization of spectrometers,most of them are with a major focus on the technical side but tend to feature a lower technology readiness level for manufacturability.More importantly,in spite of the advancement in miniaturized spectrometers,their performance and the metrics of real-life applications have seldomly been connected but are highly important.This review paper shows the market trend for chip-scale spectrometers and analyzes the key metrics that are required to adopt miniaturized spectrometers in real-life applications.Recent progress addressing the challenges of miniaturization of spectrometers is summarized,paying a special attention to the CMOS-compatible fabrication platform that shows a clear pathway to massive production.Insights for ways forward are also presented.

High-accuracy optical time delay measurement in fiber link [Invited]

Optoelectronic components and subsystems such as optically controlled phased array antennas,distributed radar networks,interferometric optical fiber hydrophones,and high-speed optoelectronic chips demand highaccuracy optical time delay measurement with large measurement range and the capability for single-end and wavelength-dependent measurement.In this paper,the recent advances in the optical time delay measurement of a fiber link with high accuracy are reviewed.The general models of the typical time delay measurement technologies are established with the operational principle analyzed.The performance of these techniques is also discussed.

Instantaneous frequency analysis of broadband LFM signals by photonics-assisted equivalent frequency sampling

We propose a photonics-assisted equivalent frequency sampling(EFS)method to analyze the instantaneous frequency of broadband linearly frequency modulated(LFM)microwave signals.The proposed EFS method is implemented by a photonic scanning receiver,which is operated with a frequency scanning rate slightly different from the repetition rate of the LFM signals.Compared with the broadband LFM signal analysis based on temporal sampling,the proposed method avoids the use of high-speed analog to digital converters,and the instantaneous frequency acquisition realized by frequency-to-time mapping is also simplified since real-time Fourier transformation is not required.Feasibility of the proposed method is verified through an experiment,in which frequency analysis of Kα-band LFM signals with a bandwidth up to 3 GHz is demonstrated with a moderate sampling rate of 100 MSa/s.The proposed method is highly demanded for analyzing the instantaneous frequency of broadband LFM signals used in radar and electronic warfare systems.

Background-free pulsed microwave signal generation based on spectral shaping and frequency-to-time mapping

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.

Optical single sideband polarization modulation for radio-over-fiber system and microwave photonic signal processing

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.

Switchable microwave photonic filter using a phase modulator and a silicon-on-insulator micro-ring resonator

A switchable microwave photonic filter(MPF) using a phase modulator(PM) and a silicon-on-insulator microring resonator(MRR) is proposed and demonstrated. By adjusting the polarization controller between the PM and the MRR, the filtering function of the MPF can be switched between a band-stop filter and a band-pass filter. In a proof-of-concept experiment, an MPF with a rejection ratio of 30 dB(or 15 dB) for the band-stop(or band-pass) response and a frequency tuning range from 9.6 to 20.5 GHz is achieved.

Ultra-high resolution microwave photonic radar with post-bandwidth synthesis

We demonstrate microwave photonic radar with post-bandwidth synthesis, which can realize target detection with ultra-high range resolution using relatively small-bandwidth radio frequency(RF) frontends. In the proposed radar, two temporal-overlapped linear frequency-modulated(LFM) signals with the same chirp rate and different center frequencies are transmitted. By post-processing the de-chirped echoes in the receiver, a signal equivalent to that de-chirped from an LFM signal with the combined bandwidth is achieved. In a proof-ofconcept experiment, two LFM signals with bandwidths of 8.4 GHz are exploited to achieve radar detection with an equivalent bandwidth of 16 GHz, and a range resolution of 1 cm is obtained.

Photonic-assisted single system for microwave frequency and phase noise measurement

We propose a photonic-assisted single system for measuring the frequency and phase noise of microwave signals in a large spectral range. Both the frequency and phase noise to be measured are extracted from the phase difference between the signal under testing and its replica delayed by a span of fiber and a variable optical delay line(VODL). The system calibration, frequency measurement, and phase noise measurement are performed by adjusting the VODL at different working modes. Accurate frequency and phase noise measurement for microwave signals in a large frequency range from 5 to 50 GHz is experimentally demonstrated.

All-fiber-photonics-based ultralow-noise agile frequency synthesizer for X-band radars

We propose and demonstrate an agile X-band signal synthesizer with ultralow phase noise based on all-fiberphotonic techniques for radar applications. It shows phase noise of-145 dBc∕Hz(-152 dBc∕Hz) at 10 kHz(100 kHz) offset frequency for 10 GHz carrier frequency with integrated RMS timing jitter between 7.6 and 9.1 fs(integration bandwidth: 10 Hz–10 MHz) for frequencies from 9 to 11 GHz. Its frequency switching time is evaluated to be 135 ns with a 135 pHz frequency tuning resolution. In addition, the X-band linearfrequency-modulated signal generated by the proposed synthesizer shows a good pulse compression ratio approximating the theoretical value. In addition to the ultrastable X-band signals, the proposed synthesizer can also provide 0–1 GHz ultralow-jitter clocks for analog-to-digital converters(ADC) and digital-to-analog converters(DAC) in radar systems and ultralow-jitter optical pulse trains for photonic ADC in photonic radar systems.The proposed X-band synthesizer shows great performance in phase stability, switching speed, and modulation capability with robustness and potential low cost, which is enabled by an all-fiber-photonics platform and can be a compelling technology suitable for future X-band radars.

GaAs-based polarization modulators for microwave photonic applications

GaAs-based polarization modulators （PolMs） exhibit the unique characteristic of simultaneous intensity and complementary phase modulation owing to the linear electro-optic （LEO） effect determined by crystallographic orientations of the device. In this paper, we reviewed the principle of operation, the design and fabrication flows of a GaAs-based PolM. Analytical models are established, from which the features of a PolM are derived and discussed in detail. The recent advances in PolM-based multifunctional systems, in particular the PolM-based optoelectronic oscillator （OEO） are demonstrated with an emphasis on the remarkable development of applications for frequency conversion, tunable microwave photonic filter （MPF）, optical frequency comb （OFC）, arbitrary waveform generation （AWG） and beamforming. Challenges in practical implementation of the PolM-based systems and their promising future are discussed as well.

Broadband instantaneous multi-frequency measurement based on chirped pulse compression

A broadband instantaneous multi-frequency measurement system based on chirped pulse compression,which potentially has a sub-megahertz(MHz)accuracy and a hundred-gigahertz(GHz)measurement range,is demonstrated.A signalunder-test(SUT)is converted into a carrier-suppressed double-sideband(CS-DSB)signal,which is then combined with an optical linearly frequency-modulated signal having the sweeping range covering the+1 st-order sideband of the CSDSB signal.With photodetection,low-pass filtering,and pulse compression,accurate frequencies of the SUT are obtained via locating the correlation peaks.In the experiment,single-and multi-frequency measurements with a measurement range from 3 to 18 GHz and a measurement accuracy of<±100 MHz are achieved.