查看更多>>摘要：The characteristics of plasmas play an important role in femtosecond laser filament-based applications.Spectroscopic analysis is used to experimentally investigate the plasma density and its temperature of the air filament under different pulse repetition rates.In our experiments,the measured average plasma density of the filament is 1.54 × 1017 cm-3 and the temperature of the plasma is about 5100 K under 100 Hz pulse repetition rate.The plasma density decreases to 1.43 × 1017 cm-3 and the temperature increases to 6230 K as the pulse repetition rate increases to 1000 Hz.The experimental observation agrees with the numerical simulation by solving the nonlinear Schrödinger equations with repetition rate related"low density hole"correction.
查看更多>>摘要：Two-dimensional[2D]van der Waals materials have attracted tremendous attention due to their versatile physical proper-ties and flexible manipulation approaches.Among the various types of van der Waals materials,α-In2Se3 is remarkable for its intrinsic 2D ferroelectricity and high-performance opto-electronic properties.However,the study of the α-In2Se3 system in terahertz[THz]radiation is scarce,although it is promising for electrically controlled THz field manipulation.We inves-tigate the α-In2Se3 in different thicknesses and report that the THz generation efficiency induced by femtosecond laser pulses can be largely improved by reducing the thickness from the bulk.Furthermore,we reveal the surge current in thin film coupled with THz emission exhibits a different Auger recombination mode,which is helpful in understanding the mecha-nism and provides insights into the design of 2D highly efficient THz devices.
查看更多>>摘要：Metalenses are essential components in terahertz imaging systems.However,without careful design,they show limited field of view and their practical applications are hindered.Here,a wide-angle metalens is proposed whose structure is optimized for focusing within the incident angles of±25°.Simulation and experiment results show that the focusing effi-ciency,spot size,and modulation transfer function of this lens are not sensitive to the incident angle.More importantly,this wide-angle metalens follows the ideal Gaussian formula for the object-image relation,which ensures a wider field of view and better contrast in the imaging experiment.
查看更多>>摘要：High accuracy and time resolution optical transfer delay[OTD]measurement is highly desired in many multi-path appli-cations,such as optical true-time-delay-based array systems and distributed optical sensors.However,the time resolution is usually limited by the frequency range of the probe signal in frequency-multiplexed OTD measurement techniques.Here,we proposed a time-resolution enhanced OTD measurement method based on incoherent optical frequency domain reflec-tometry[I-OFDR],where an adaptive filter is designed to suppress the spectral leakage from other paths to break the resolution limitation.A weighted least square[WLS]cost function is first established,and then an iteration approach is used to minimize the cost function.Finally,the appropriate filter parameter is obtained according to the convergence results.In a proof-of-concept experiment,the time-domain response of two optical links with a length difference of 900 ps is successfully estimated by applying a probe signal with a bandwidth of 400 MHz.The time resolution is improved by 2.78 times compared to the theoretical resolution limit of the inverse discrete Fourier transform[iDFT]algorithm.In addition,the OTD measurement error is below±0.8 ps.The proposed algorithm provides a novel way to improve the measurement res-olution without applying a probe signal with a large bandwidth,avoiding measurement errors induced by the dispersion effect.
查看更多>>摘要：A sub-Nyquist radar receiver based on photonics-assisted compressed sensing is proposed.Cascaded dictionaries are applied to extract the delay and the Doppler frequency of the echo signals,which do not need to accumulate multiple echo periods and can achieve better Doppler accuracy.An experiment is performed.Radar echoes with different delays and Doppler frequencies are undersampled and successfully reconstructed to obtain the delay and Doppler information of the targets.Experimental results show that the average reconstruction error of the Doppler frequency is 5.33 kHz using an 8-μs radar signal under the compression ratio of 5.The proposed method provides a promising solution for the sub-Nyquist radar receiver.
查看更多>>摘要：The stable long-distance transmission of radio-frequency[RF]signals holds significant importance from various aspects,including the comparison of optical frequency standards,remote monitoring and control,scientific research and experi-ments,and RF spectrum management.We demonstrate a scheme where an ultrastable frequency signal was transmitted over a 50 km coiled fiber.The optical RF signal is generated using a two-section distributed feedback[DFB]laser for direct modulation based on the reconstruction equivalent chirp[REC]technique.The 3-dB modulation bandwidth of the two-sec-tion DFB laser is 18 GHz and the residual phase noise of-122.87 dBc/Hz is achieved at 10-Hz offset frequency.We report a short-term stability of 1.62 × 10-14 at an average time of 1 s and a long-term stability of 6.55 × 10-18 at the measurement time of 62,000 s when applying current to the front section of the DFB laser.By applying power to both sections,the stability of the system improves to 4.42 × 10-18 within a testing period of 56,737 s.Despite applying temperature variations to the transmission link,long-term stability of 8.63 × 10-18 at 23.9 h can still be achieved.
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