Experimental Analysis of Coherent Velocity Measurement Based on Near-infrared Single-element SPAD Detector
Coherent lidar has advantages of suppressing background noise such as sunlight and detecting sensitivity close to shot noise limit.It is widely used in civil and defense fields such as wind detection,velocity measurement and military target detection.Coherent detection can be divided into heterodyne detection to extract frequency information and homodyne detection to extract phase information.For velocity measurement,heterodyne detection is usually used to extract the Doppler frequency shift of the echo laser from a moving target,and then the velocity of target is retrieved.Conventional heterodyne lidar adopt normal optical detectors,such as PIN detectors,which have limited detection sensitivity for a small number of echo photons.And generally,strong local oscillator laser power is required to suppress thermal and circuit noise,but excessive local oscillator is likely to generate excess shot noise.With the development of Single Photon Avalanche Diode(SPAD)detector with low circuit noise,it not only provides a way for the detection of a small number of echo photons,but also makes it possible to realize heterodyne detection with a weak local oscillator.Researchers have successively adopted InGaAs SPAD array detectors and superconducting nanowire single-photon detectors for near-infrared spectrum,single-element Si SPAD detectors and MPPC detectors for visible spectrum,but there have been few experimental research on heterodyne detection with single-element InGaAs SPAD detector.The heterodyne lidar based on near-infrared SPAD can be integrated in all-fiber structure with an operating wavelength of 1.5 μm,which makes it more suitable for practical working platforms such as airborne.Although the count rate dynamic range of the single-element SPAD is not as good as that of the SPAD array,the current disadvantages of SPAD array,such as low pixel fill-factor,poor uniformity of pixel performance(e.g.,hot pixel),and slow speed of data readout,limit its performance to a certain extent.Besides,compared with superconducting nanowire single-photon detectors,single-element near-infrared SPAD do not require extremely complex and bulky cooling system.Therefore,we established a heterodyne velocimetry experimental system based on a 1.5 μm fiber laser and a single-element InGaAs SPAD detector to analyze the influence of SPAD's dead time,dark count rate and photon count rate for the extracting of beat frequency.The output laser was shifted by 40 MHz using an Acousto-optic Frequency Shifter(AOFS)to simulate the Doppler frequency shift of the echo laser from a moving target.Then,under the experimental set up of 1 μs dead time and 1 ms data acquisition time,we analyzed the influence of different photon count rates on the SNR of the beat frequency spectrum under SPAD's dark count rates of 1.8 kHz,54.4 kHz and 194.4 kHz.The experimental results show that,the SNR increases gradually and then tends to be stable with the increase of the photon count rate.When the photon count rate is close to saturation,harmonic frequency components appear in the low-frequency area of the frequency spectrum as well as the two side regions centered on the beat frequency.The harmonic frequency spacing is basically equal to the photon count rate.The optimal photon count rate which is slightly affected by harmonics is about 90%of saturation count rate of SPAD detector.In addition,as the dark count rate increases,the photon count rate required to extract the beat frequency signal is higher.The experimental results can provide a reference for the development and practical application of all-fiber single-photon Doppler velocity measurement lidar technology.
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