Photonic generation of terahertz dual-chirp waveforms ranging from 364 to 392†GHz.

In this paper, we propose and experimentally demonstrate a photonic scheme based on frequency doubling and photo-mixing to generate dual-chirp signals in the terahertz (THz) band. A broadband dual-chirp THz signal with 28 GHz bandwidth, ranging from 364 GHz to 392 GHz, is successfully generated in the proof-of-concept experiment, resulting in a chirp rate of 0.028 GHz/ns for both up chirp and down chirp signals. THz dual-chirp signals featuring a large bandwidth are beneficial to enable high resolution and high accuracy by mitigating the range measurement error induced by the range-Doppler coupling effect. Therefore, the proposed system is expected to have a great potential for future THz radar applications.

Branching TiO2nanowire arrays for enhanced ethanol sensing.

Nanostructure modulation is effective to achieve high performance TiO2-based gas sensors. We herein report a wet-chemistry route to precipitate directly branched TiO2nanowire arrays on alumina tubes for gas sensing applications. The optimized branched TiO2nanowire array exhibits a response of 9.2 towards 100 ppm ethanol; whilst those of the pristine TiO2nanowire array and the branched TiO2nanowire powders randomly distributed are 5.1 and 3.1, respectively. The enhanced response is mainly contributed to the unique porous architecture and quasi-aligned nanostructure, which provide more active sites and also favor gas migration. Phase junctions between the backbone and the branch of the branched TiO2nanowire arrays help the resistance modulation as a result of potential barriers. The facile precipitation of quasi-aligned arrays of branched TiO2nanowires, which arein situgrown on ceramic tubes, thus provides a new economical synthetic route to TiO2-based sensors with excellent properties.

Method of fiber transfer delay measurement based on phase quantization and delay synthesis.

A novel method, to the best of our knowledge, of fiber transfer delay (FTD) measurement based on phase quantization and delay synthesis is proposed and demonstrated. By detecting the differential phase shifts of a set of frequency-multiplied RF signals transmission through the fiber link with and without the FTD under the test, the ${2}\pi $2π phase ambiguity problem can be solved. To avoid the phase quantization error near the digital quantization boundary, a self-check and error-correction method is proposed so as to greatly improve the reliability of measurement. In the experiment, the measurement repeatability around 0.018 ps within a period of 80 s is achieved for a back-to-back fiber link, and a test resolution of 0.03 ps is proved with a motorized tunable delay line. The system is available for measurement of a large FTD range up to 100 µs with no dead zone.

Enhanced isopropanol sensing of coral-like ZnO-ZrO2 composites.

Both p-type ZrO2 and n-type ZnO are widely adopted oxides towards trace gas detections; however, their combinations to achieve an enhanced gas sensing performance are rarely reported. Herein, we adopted a simple solution combustion technique to synthesize ZnO-ZrO2 composites for isopropanol sensing. The one-step combustion achieved coral-like macro/mesoporous hierarchical architectures. It is found that, when the Zr/Zn molar ratio is less than 0.02, all Zr atoms were doped into ZnO crystallites; whilst ZrO2 appeared when the ratio is beyond 0.03. When utilized to detect trace isopropanol in air, the response increases linearly with the increasing concentration of the target gas in the range of 10-1000 ppm. At the optimal operation temperature of 350 °C, the largest slope (0.18 ppm-1) is recorded for the ZnO-ZrO2 composite with a Zr/Zn molar ratio of 0.04 and the slope is 23 times that of pure ZnO (0.0078 ppm-1). It exhibits also a fast response time and recovery time of 19 s and 8 s, respectively, under 100 ppm isopropanol. The impressive gas sensing property can be contributed to both the macro-/mesoporous structure, which facilitates an intimate contact between the target gas and the sensing site, and the p-n junction induced built-in electric field, which favors the charge separation.

Photonics-enabled compressive sensing with spectral encoding using an incoherent broadband source.

In this Letter, we propose an approach to achieving photonics-enabled compressive sensing of sparse wideband radio frequency signals in which an incoherent broadband source is applied, and the mixing and integration functions are realized in the optical domain. A spectrum shaper is employed to slice and encode the spectrum of the broadband light according to a predetermined random sequence. Because of the dispersion-induced group delay, the mixing between the incoming signal and the random bit sequence is achieved. At the output of the spectrum shaper, an array of photodetectors is employed to realize down-sampling, and the input sparse signal can be captured in a single-shot mode. Since no pulsed laser is employed, our scheme obviates the need for time-domain synchronization between the repetitive ultra-fast pulses and the random sequence. Experimental demonstrations and numerical results are presented to verify the feasibility and potential of the approach.

Experimental generation of linearly chirped 350 GHz band pulses with a bandwidth beyond 60 GHz.

We present in this Letter the experimental generation of linear frequency modulated (LFM) terahertz pulses with large bandwidths by using an optical interferometer-based photonic scheme and cutting-edge terahertz transceiver technology. The LFM pulses exhibiting a bandwidth in excess of 60 GHz centered at 350 GHz are successfully generated in the experiment, which represents the first demonstration of large time-bandwidth products (TBWPs) in the terahertz region above 300 GHz, to the best of our knowledge. The achieved TBWP of up to 527 has great potential in many prospective applications such as high-resolution radar sensing and imaging.

Highly sensitive demodulation of a vibration-induced phase shift based on a low-noise OEO.

A highly sensitive demodulation approach of a vibration-induced phase shift based on a low-noise optoelectronic oscillator (OEO) is proposed and experimentally demonstrated. The vibration-induced optical phase variation is directly converted to the electrical oscillating signal of the OEO with carrier phase-shifted double-sideband (CPS-DSB) modulation, which is realized by cascading a dual-output Mach-Zehnder modulator (DOMZM) and a fiber interferometer. Theoretically, within a CPS-DSB modulated OEO, the minimum detectable optical phase shift is determined by the phase noise achievable, and the sensitivity of the optical phase shift demodulation no longer depends on its frequency. A proof-of-concept OEO oscillating at 100 MHz with ultralow phase noise is built for demonstration. The achieved minimum detectable optical phase shift is 0.58 µrad/√Hz at 1 kHz and 0.21 µrad/√Hz at 10 kHz, which are the best results ever reported, to the best of our knowledge.

Frequency-dependent noise figure analysis of continuous photonic time-stretch system.

In this paper, the frequency-dependent noise figure of a continuous photonic time-stretch system is theoretically analyzed and experimentally demonstrated. Mathematic analysis discloses that the frequency-dependent noise figure in the continuous photonic time-stretch system is mainly owing to the dispersion-induced phase shift in the link. In the experiment, a completely continuous signal is reconstructed with time-interleaved joining of four sections of time-stretched signals from optical wavelength-division de-multiplexing channels. The result of the frequency-dependent noise figure property agrees well with that of theoretical analysis.

All-positive-coefficient microwave photonic filter with rectangular response.

In conventional approaches to realizing microwave photonic filters (MPFs) with rectangular response, both positive and negative coefficients should be included in the systems, which is a difficulty in incoherent MPFs. Electrical or optical ways to realize the MPFs with bipolar taps, such as those using balanced detectors or based on two opposite modulation slopes of dual-input Mach-Zehnder modulators, usually involve more components compared with the MPFs with unipolar taps. In this Letter, a design of MPFs with rectangular response using all positive coefficients is proposed. There is only one optical link as well as one photodetector utilized in the approach, which largely simplifies the filter structure. Experimental demonstrations of 55-positive-tap MPFs with different passbands are presented to verify the feasibility and potential of the approach.

Harmonics analysis of the photonic time stretch system.

Photonic time stretch (PTS) has been intensively investigated in recent decades due to its potential application to ultra-wideband analog-to-digital conversion. A high-speed analog signal can be captured by an electronic analog-to-digital converter (ADC) with the help of the PTS technique, which slows down the speed of signal in the photonic domain. Unfortunately, the process of the time stretch is not linear due to the nonlinear modulation of the electro-optic intensity modulator in the PTS system, which means the undesired harmonics distortion. In this paper, we present an exact analytical model to fully characterize the harmonics generation in the PTS systems for the first time, to the best of our knowledge. We obtain concise and closed-form expressions for all harmonics of the PTS system with either a single-arm Mach-Zehnder modulator (MZM) or a push-pull MZM. The presented model can largely simplify the PTS system design and the system parameters estimation, such as system bandwidth, harmonics power, time-bandwidth product, and dynamic range. The correctness of the mathematic model is verified by the numerical and experimental results.

The Capacity Gain of Orbital Angular Momentum Based Multiple-Input-Multiple-Output System.

Wireless communication using electromagnetic wave carrying orbital angular momentum (OAM) has attracted increasing interest in recent years, and its potential to increase channel capacity has been explored widely. In this paper, we compare the technique of using uniform linear array consist of circular traveling-wave OAM antennas for multiplexing with the conventional multiple-in-multiple-out (MIMO) communication method, and numerical results show that the OAM based MIMO system can increase channel capacity while communication distance is long enough. An equivalent model is proposed to illustrate that the OAM multiplexing system is equivalent to a conventional MIMO system with a larger element spacing, which means OAM waves could decrease the spatial correlation of MIMO channel. In addition, the effects of some system parameters, such as OAM state interval and element spacing, on the capacity advantage of OAM based MIMO are also investigated. Our results reveal that OAM waves are complementary with MIMO method. OAM waves multiplexing is suitable for long-distance line-of-sight (LoS) communications or communications in open area where the multi-path effect is weak and can be used in massive MIMO systems as well.

Local topological charge analysis of electromagnetic vortex beam based on empirical mode decomposition.

The topological charge of an electromagnetic vortex beam depends on its wavefront helicity. For mixed vortex beams composed of several different coaxial vortices, it is significant to investigate the local topological charges. Fourier transform based methods are restrained by the uncertainty principle and cannot achieve high angular resolution and mode resolution simultaneously. In this paper, an analysis method for local topological charges of vortex beams is presented based on the empirical mode decomposition (EMD). From EMD, the intrinsic mode functions (IMFs) can be obtained to construct the bases of the electromagnetic wave, and each local topological charge can be respectively defined. With this method the local value achieves high resolution of both azimuth angle and topological charge, meanwhile the amplitudes of each orbital angular momentum (OAM) modes are presented as well. The simulation and experimental results confirm the validity of the EMD based method.

Analysis of compressive sensing with optical mixing using a spatial light modulator.

Compressive sensing (CS) in a photonic link has a high potential for acquisition of wideband sparse signals. In CS it is necessary to mix the input sparse signal with a pseudorandom sequence prior to subsampling. A pulse shaper with a spatial light modulator (SLM) can be used in photonic CS as an optical mixer to improve the speed of mixing. In this approach, the sparse signal is modulated on a chirped optical pulse and the pseudorandom sequence is recorded on the SLM within the pulse shaper. The optical mixing in the frequency domain is realized based on the principle of frequency-to-time mapping. In this paper, we investigate the performance and limitations of photonic CS with an SLM in detail. A theoretical model to describe optical mixing based on frequency-to-time mapping is presented. We point out that there is an upper limit on the length of the pseudorandom sequence recorded on the SLM that can be mixed with the sparse signal due to the condition of the far-field approximation of the frequency-to-time mapping. Since the length of the pseudorandom sequence is one of the major factors that affect the signal recovery performance in CS, this limitation should be fully considered in the system design of the CS with optical mixing in the frequency domain. We present numerical and experimental results to verify the theoretical findings. Discussion on the performance improvement is also presented.

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.

Orbital angular momentum mode-demultiplexing scheme with partial angular receiving aperture.

For long distance orbital angular momentum (OAM) based transmission, the conventional whole beam receiving scheme encounters the difficulty of large aperture due to the divergence of OAM beams. We propose a novel partial receiving scheme, using a restricted angular aperture to receive and demultiplex multi-OAM-mode beams. The scheme is theoretically analyzed to show that a regularly spaced OAM mode set remain orthogonal and therefore can be de-multiplexed. Experiments have been carried out to verify the feasibility. This partial receiving scheme can serve as an effective method with both space and cost savings for the OAM communications. It is applicable to both free space OAM optical communications and radio frequency (RF) OAM communications.

Space-frequency analysis with parallel computing in a phase-sensitive optical time-domain reflectometer distributed sensor.

For a phase-sensitive optical time-domain reflectometer (ϕ-OTDR) distributed sensor system, space-frequency analysis can reduce the false alarm by analyzing the frequency distribution compared with the traditional difference value method. We propose a graphics processing unit (GPU)-based parallel computing method to perform multichannel fast Fourier transform (FFT) and realize the real-time space-frequency analysis. The experiment results show that the time taken by the multichannel FFT decreased considerably based on this GPU parallel computing. The method can be completed with a sensing fiber up to 16 km long and an entry-level GPU. Meanwhile, the GPU can reduce the computing load of the central processing unit from 70% down to less than 20%. We carried out an experiment on a two-point space-frequency analysis, and the results clearly and simultaneously show the vibration point locations and frequency components. The sensor system outputs the real-time space-frequency spectra continuously with a spatial resolution of 16.3 m and frequency resolution of 2.25 Hz.

High-sensitivity temperature sensor based on an optoelectronic oscillator.

A high-sensitivity temperature sensor based on an optoelectronic oscillator (OEO) is proposed and experimentally demonstrated. The shift of the oscillation frequency in the OEO is inversely proportional to the variation of temperature. An injection-locking method is employed to set up the initial oscillation frequency of the sensor to overcome the uncertainty of the initial oscillation frequency. The experiment results show that high sensitivity of 43.91 kHz/°C with an accuracy of ±0.12°C is achieved between 20°C and 240°C. The system features high temperature sensitivity, wide dynamic range, and high reliability.

Compressive sensing in a photonic link with optical integration.

In this Letter, we present a novel structure to realize photonics-assisted compressive sensing (CS) with optical integration. In the system, a spectrally sparse signal modulates a multiwavelength continuous-wave light and then is mixed with a random sequence in optical domain. The optical signal passes through a length of dispersive fiber, the dispersion amount of which is set to ensure that the group delay between the adjacent wavelength channels is equal to the bit duration of the applied random sequence. As a result, the detected signal is a delay-and-sum version of the randomly mixed signal, which is equivalent to the function of integration required in CS. A proof-of-concept experiment with four wavelengths, corresponding to a compression factor of 4, is demonstrated. More simulation results are also given to show the potential of the technique.

Microwave spectrum sensing based on photonic time stretch and compressive sampling.

An approach to realizing microwave spectrum sensing based on photonic time stretch and compressive sampling is proposed. The time stretch system is used to slow down the input high-speed signal and the compressive sampling based on random demodulation can further decrease the sampling rate. A spectrally sparse signal in a wide bandwidth can be captured with a sampling rate far lower than the Nyquist rate thanks to both time stretch and compressive sampling. It is demonstrated that a system with a time stretch factor 5 and a compression factor 8 can be used to capture a signal with multiple tones in a 50 GHz bandwidth, which means a sampling rate 40 times lower than the Nyquist rate. In addition, the time stretch of the microwave signal largely decreases the data rate of random data sequence and therefore the speed of the mixer in the random demodulator.

Electro-optic modulator feedback control in phase-sensitive optical time-domain reflectometer distributed sensor.

We propose a novel method to control the electro-optic modulator (EOM) applied in the phase-sensitive optical time-domain reflectometer (ϕ-OTDR) distributed sensor system, which uses the data of the OTDR curves rather than applying an independent control module. We explain the relationship between the accumulation value of the OTDR curve and the EOM's extinction ratio, and utilize this relationship to feedback control the EOM. The experimental results show that it can compensate the drift of the EOM, and make the modulator run with a high extinction ratio for a long time. And this method can also ratify a small jump of the EOM's bias point.