A scanning three-dimensional coherent laser radar (ladar) based on the frequency modulated continuous wave (FMCW) is proposed and demonstrated, which can obtain many three-dimensional high-quality images. The system utilizes an electro-optic modulator and an optical filter to output a linear FMCW with a bandwidth of 2 GHz. The flexible and variable rotating double prism is used for beam scanning. The images of flight demonstration are formed by attitude compensation correction. The experiment result validates the performance of our system for airborne three-dimensional scanning imaging.

Using theoretical simulations for optical fiber surface plasmon resonance (SPR) sensors and prism-based SPR sensors coated with negative permittivity material (NPM), we investigated the effect of the permittivity of NPM on the transmitted spectrum of optical fiber SPR sensors and the reflected spectrum of prism-based SPR sensors and then obtained optimum permittivity of the NPM, which can excite the sharpest SPR spectrum in the white light region (400–900 nm).

We experimentally demonstrate the ultra-high range resolution of a photonics-based microwave radar using a high repetition rate actively mode-locked laser (AMLL). The transmitted signal and sampling clock in the radar originate from the same AMLL to achieve a large instantaneous bandwidth. A Ka band linearly frequency modulated signal with a bandwidth up to 8 GHz is successfully generated and processed with the electro-optical upconversion and direct photonic sampling. The minor lobe suppression (MLS) algorithm is adopted to enhance the dynamic range at a cost of the range resolution. Two-target discrimination with the MLS algorithm proves the range resolution reaches 2.8 cm. The AMLL-based microwave-photonics radar shows promising applications in high-resolution imaging radars having the features of high-frequency band and large bandwidth.

A specific system structure of down-looking synthetic aperture imaging ladar (SAIL) is given, and a far-field experiment over 6 km of down-looking SAIL under this system design is carried out. The down-looking SAIL can overcome the influence of atmospheric turbulence to a great extent. By applying this system design, it also has advantages in self-compensating phase modulation. A fine image is obtained after aligning in the orthogonal direction and phase error compensation in the travel direction based on a dominant scatterer. The achieved imaging resolutions in the two dimensions are both better than 5 cm.

In this Letter, we propose an efficient compression algorithm for multi-spectral images having a few bands. First, we propose a low-complexity removing spectral redundancy approach to improve compression performance. Then, a bit plane encoding approach is applied to each band to complete the compression. Finally, the experiments are performed on multi-spectral images. The experiment results show that the proposed compression algorithm has good compressive property. Compared with traditional approaches, the proposed method can decrease the average peak signal noise ratio by 0.36 dB at 0.5 bpp. The processing speed reaches 23.81 MPixels/s at the working frequency of 88 MHz, which is higher than the traditional methods. The proposed method satisfies the project application.

Real-time and high-resolution imaging is demonstrated based on field trial detection of a non-cooperative target using a photonics-based inverse synthetic aperture radar (ISAR). By photonic generation and de-chirping of broadband linear frequency modulation signals, the radar can achieve a high range resolution thanks to the large instantaneous bandwidth (8 GHz at the K band), as well as real-time ISAR imaging using low-speed analog-to-digital conversion (25 MSa/s). A small-size unmanned aerial vehicle is employed as the non-cooperative target, and ISAR imaging is realized with a resolution far better than those achieved by the previously reported photonics-based ISARs. The capability for real-time ISAR imaging is also verified with an imaging frame rate of 25 fps. These results validate that the photonics-based radar is feasible in practical real-time and high-resolution ISAR imaging applications.

To achieve radar and infrared stealth, an infrared stealth layer is usually added to the radar absorbing material (RAM) of stealth aircraft. By analyzing the millimeter-wave (MMW) emissivities of three stealth materials, this Letter investigates the impact of the added infrared stealth layer on the originally “hot” MMW emission of RAM. The theoretical and measured results indicate that, compared with the monolayer RAM, the MMW emission of the bilayer material is still strong and its emissivity is reduced by 0.1–0.2 at almost every incident angle. The results partially demonstrate the feasibility of detecting stealth aircraft coated with this bilayer stealth material.

The output amplitude of the differential circuit is studied for differential discrimination in pulsed laser time-of-flight systems. Based on the studies of the probability of detection and the probability of false alarms, the minimum detectable input signal of differential discrimination can be calculated. The results indicate that the differential discrimination detectability of the small signal will be reduced. A combined discrimination is proposed in this Letter to improve the time resolution of the large signal and ensure the probability of detection of the small signal at the same time. A proper value of the circuit parameter is found to balance the time resolutions of the small and large signals.

This Letter introduces a trigger-controlled Geiger-mode avalanche photodiode (GM-APD). A hierarchical look-back-upon tree recurrence method is given to predict the performance of trigger-controlled GM-APDs under different trigger-count upper limits. In addition, the normalized detection probability is defined to evaluate the detection performance of trigger-controlled GM-APDs in typical weak optical signal detection (impulse noise and continuous noise situations). Theoretical analyses show that the trigger-controlled GM-APD improves the detection performance of GM-APDs in weak optical signal detection via the optimization of the trigger-count upper limit, compared with single-trigger and multi-trigger GM-APDs.

Infrared signatures of aircraft are the basis for detection and monitoring. In past years, most of the studies focused on the aircraft’s infrared signature in the mid-wave spectral region and long-wave spectral region for missile guidance or aircraft survivability studies. For the security of civil aviation, methods and instruments that can detect and monitor aircrafts from space are expected to be developed in the coming years. A short-wave infrared hyperspectral imager aboard the Tiangong-1 spacecraft acquired some civil aircraft’s spectral data. The differences between the aircraft and the background in their spectral signatures are analyzed and discussed. Less absorption in the vapor absorption bands and a reflection spike is discovered at the 1.84 μm spectral band. The result shows that 1.84 μm and other vapor absorption bands can make contributions to aircraft detection in the daytime.

We propose a reflection-type infrared biosensor by exploiting localized surface plasmons in graphene ribbon arrays. By enhancing the coupling between the incident light and the resonant system, an asymmetric Fabry–Perot cavity formed by the ribbons and reflective layer is employed to reshape the reflection spectra. Simulation results demonstrate that the reflection spectra can be modified to improve the figure of merit (FOM) significantly by adjusting the electron relaxation time of graphene, the length of the Fabry–Perot cavity, and the Fermi energy level. The FOM of such a biosensor can achieve a high value of up to 36/refractive index unit (36/RIU), which is ～4 times larger than that of the traditional transmission-type one. Our study offers a feasible approach to develop biosensing devices based on graphene plasmonics with high precision.

We propose a method for simultaneous 3D temperature and velocity measurement of a micro-flow field. The 3D temperature field is characterized with two-color laser-induced fluorescence particles which are tracked with micro-digital holographic particle tracking velocimetry. A diffraction-based model is applied to analyze defocused particles to determine the intensity ratio of two fluorescent dyes on the particle. The model is validated with experimental images. As the result shows that the intensity ratio nearly remains unchanged with respect to depth positions, defocused particles can be used as 3D temperature sensors. Numerical work is carried out to check the method, and 3D temperature and velocity field in a 120 μm×120 μm×80 μm test volume are retrieved.

A static-mode synthetic aperture imaging ladar (SAIL) in which the target and carrying platform are kept still during the collection process is proposed and demonstrated. A target point of 0.5 mm×0.5 mm and a two-dimensional (2D) object are reconstructed in the experiments, in which an optical collimator with a focal length of 10 m is used to simulate the far-field condition. The achieved imaging resolution is in agreement with the theoretical design. The static-mode down-looking SAIL has the capability to eliminate the influence from the atmospheric turbulence and can be conveniently operated outdoors.

Doppler spectrum width, which relates to wind turbulence, is one of the essential parameters of coherent LIDAR. Using the Fourier transform theorem and the definition of correlation function, the power spectrum function in the stationary condition is deduced. The effects of pulse shape, pulse duration and the windowing are included. The spectrum width resulted from the turbulence is given under Kolmogorov turbulence model. Based on the power spectrum theory, the spectrum broadening by different pulse shapes and pulse durations are calculated. To validate the accuracy of the theory and usage in the retrieval of turbulence parameters, the numerical simulations of the echo signal are carried out in turbulence conditions. The statistics characteristics of the spectrum broadening from laser pulse shapes and durations are inverted and compared. The results show that the spectrum broadening varies greatly due to the selection of pulse shapes and pulse durations, and the numerical simulation is in accordance with the theory results.

A laser ranging system using all fiber high speed pseudorandom (PN) coded laser at 1550 nm and photon counting is proposed to realize high spatial resolution. Different lengths of PN code are employed in the optical fiber delay ranging test, the results show the improvement in both ranging accuracy and signal-to-noise ratio (SNR) as PN code trains increase. A ranging accuracy of 3 cm is acquired when transmitting pulses propagate to a target of 1.77 km away and received by an InGaAs/InP avalanche photodiode (APD). Simulation is also carried out under space borne condition based on current system. The system is demonstrated to have a potential for remote ranging and imaging.

An all fiber pulsed coherent Doppler lidar (CDL) system at 1.54 μm wavelength is developed for wind profiles measurements. This lidar affords 43.0-μJ pulse energy at 10-kHz pulse repetition frequency with 500-ns pulse width. The lidar is operated in monostatic mode with 50-mm diameter telescope. The heterodyne mixing signals are acquired with 500 M/s analog to digital converter and 2048 points fast Fourier transform (FFT) is implemented. Line of sight wind speeds are measured with more than 3.0-km range in a horizontal direction and about 1.9 km in the vertical direction with 75-m range resolution. Systematic error of speed measurement of 0.2 m/s is validated.

This paper proposes the post-integration technology based on sub-pixel image registration and image fusion to improve the signal-to-noise ratio (SNR) of remote sensing images without motion degradation caused by satellite vibration. A two-dimensional vibration system is set up to simulate satellite disturbance. Image sequences with different exposure times are captured using a high-speed CMOS camera. The displacement plots are compared with the motion data measured by the grating linear encoder. These plots indicate that the accuracy of the registration algorithm is better than 0.1 pixels. The sub-pixel image fusion shows an improvement in image quality, thus indicating that this technology is powerful for staring imaging systems in geostationary orbit.

We describe a surface plasmon resonance-based fiber sensor based on a side-polished graded-index multimode fiber, in which an Al-doped zinc oxide/gold (AZO/Au) bilayer is deposited on the side-polished surface of the fiber core to improve the detection sensitivity of the device. The AZO/Au layer is used as the active sensing member of the device with a combination of a 75-nm-thick AZO layer and a 40-nm-thick Au layer. Such a device is then applied to the concentration measurement of CH3COONa solutions, as an example showing a good response to concentration variation. The results indicate that the additional AZO layer in the active sensing member may lead to higher detection sensitivity and greater measurement stability in the measurements of solution concentration.

An in-line Fabry-Perot (FP) refractive index (RI) sensor based on an intrinsic FP cavity and fabricated by the etching and fusion splicing method is proposed. The experimental results demonstrate that the sensor possesses a high resolution of 1508 nm/RIU for the measurement of acetylene gas RI. The temperatureresponse measurement shows that the sensor is insensitive to room temperature variations. The FP RI sensor is suitable for applications in biosensing and environmental monitoring because of its high sensitivity and structural simplicity, thereby making it suitable for low-cost mass production.

The capability of the parameters derived from waveform data in discriminating objects is assessed and the effect of the relative calibration of full-waveform data in discriminating land-cover classes is evaluated. Firstly, a non-linear least-squares method with the Levenberg–Marquardt algorithm is used to fit the return waveforms by a Gaussian function. Gaussian amplitude, standard deviation, and energy are extracted. Secondly, a relative calibration method using the range between the sensor and the target based on a radar equation is applied to calibrate amplitude and energy. The change in transmit pulse energy is also considered in this process. A support vector machine classifier is used to distinguish the study area into non-vegetated area (including roads, buildings, and vacant lots), grassland, needle-leaf forests, and broad-leaf forests. The overall classification accuracy ranges from 79.33% to 87.6%, with the combination of the two groups of the three studied parameters. Calibrated data classification accuracy is improved from 1.20% to 6.44%, thus resulting in better forest type discrimination. The result demonstrates that the parameters extracted from the waveforms can be applied effectively in identifying objects and that relative calibrated data can improve overall classification accuracy.

We present a downhole seismic monitoring system and the field test results using a fiber laser seismometer (FLS). The distributed feedback fiber laser is used as the sensing element of the seismometer. Using the interferometric demodulation system, a minimum detectable acceleration of ~ng is achieved. The FLS is installed in a 400-m-depth well in Puer city, Yunnan Province. A micro-earthquake (M=1.2) in Puer area is detected. Compared with the existing underground electrical seismometer, FLS shows a higher signal-to-noise ratio and good reliability, which indicates that the FLS provides a new approach to deep-well seismic monitoring.

A novel microring resonator sensor coated with a single negative metamaterial (SNM) layer is proposed. The dispersion relation of hollow cylindrical dielectric waveguide covered with a SNM layer is derived, and resonant frequencies of the corresponding whispering gallery mode are computed. We find that there is a good agreement between the theoretical results and numerical results of resonant frequencies and electric field distributions. Compared with traditional resonator sensor, the sensitivity and resolution are significantly enhanced by SNM, and the Q factor can be tailored by adjusting the distance between the waveguide and the microring.

This letter describes a method for calculating water-leaving radiance on smooth surfaces at different angles based on Fresnel law and the polarized measurements using an ASD FS3 spectroradiometer. The spectroradiometer is mounted on a goniometer so that it views the water surface from a height of several decimeters at different viewing angles through a linear polarizing filter. The incident angles equal the viewing angles. The water-leaving radiance spectra acquired by other methods are compared with the polarized measurements from the smooth water surface, and the radiance spectra obtained by the proposed method are found to be consistent with the results of the reference methods. The current study provides another effective technique for measuring water-surface reflectance via remote sensing. Moreover, the method does not avoid the sun glint during the detection of water-leaving radiance in cases where the viewing geometric position is clear.

An approach to the simultaneous measurement of refractive-index (RI) and temperature changes using optical ring resonators is proposed and theoretically demonstrated. With a liquid-core silica ring resonator as an example, two different-order whispering gallery modes (WGMs) might differ in not only RI but also temperature sensitivities. Thus, a second-order sensing matrix should be defined based on these WGMs to determine RI and temperature changes simultaneously. The analysis shows that the RI and temperature detection limits can be achieved on the order of 10^{-7} RI unit and 10^{-3} K at a wavelength of approximately 780 nm.

Chaotic laser radar based on correlation detection is a high-resolution measurement tool for remotely monitoring targets or objects. However, its effective range is often limited by the side-lobe noise of correlation trace, which is always increased by the randomness of the chaotic signal itself and other transmission channel noises or interferences. The experimental result indicates that the wavelet denoising method can recover the real chaotic lidar signal in strong period noise disturbance, and a signal-to-noise ratio of about 8 dB is increased. Moreover, the correlation average discrete-component elimination algorithm significantly suppresses the side-lobe noise of the correlation trace when 20 dB of chaotic noise is embedded into the chaotic probe signal. Both methods have advantages and disadvantages.

The continuous monitoring of H2S gas concentration is a common problem in natural gas desulfurization process technology. Tunable diode laser absorption spectroscopy (TDLAS) is a preferred technology for continuous monitoring of gas in industrial sites, because of its high selectivity, high sensitivity, and fast response. We discuss the technical solutions of on-line monitoring of H2S in natural gas desulfurization process technology based on TDLAS, and study the security design of monitoring system in inflammable and explosive areas. We also design a weak photocurrent signal converting circuit and perform experiments on transmission characteristics of different distances. The signal-to-noise ratio (SNR) of laser absorption spectrum does not decrease after the 1 500-m transmission. The detection limit is 300 ppb. The system can be operated stably and reliably, and satisfies the need for continuous monitoring of the H2S in natural gas desulfurization process.

A novel method for online correction of light intensity fluctuation in a practical tunable diode laser absorption spectroscopy (TDLAS) system with wavelength modulation is presented. The proposed method is developed according to the linear relation between peaks at multiple frequencies of sine modulation in the power spectral density of the demodulated second-harmonic (2f) signal and the incident light intensity. Those peaks are demonstrated experimentally and explained as residual power at the first-harmonic and third-harmonic frequencies after 2f demodulation of the residual amplitude modulation signal due to the limited integrating time constant of the lock-in-amplifier. This method can achieve real-time correction of light intensity fluctuations with only little calculation. It can work well in a very large range of light intensity and has great potential applications in the wavelength modulation spectroscopy system.

Displacement sensor based on the polarization mixture and the cavity tuning of the orthogonal polarized He-Ne laser 1.15 \mu m is presented. The power tuning curves of He-Ne laser are irregular, and it is difficult to measure the change in cavity length. The distortion of the curves is caused by the higher relative excitation compared with the He-Ne laser at 633 nm. In view of its potential for the wider displacement measuring range, a new method of displacement sensing is developed. Experiments show that displacement measuring stability based on the method of the polarization mixture is better than that of the power tuning curves. The displacement sensor achieves the measuring range of 100 mm, resolution of 144 nm, and linearity of 7 \times 10^{ 6}.

A 1.55-\mu m all-solid frequency-modulated continuous-wave (FMCW) coherent lidar based on the sinusoidal frequency demodulation technique for range and velocity measurement is presented. Both the nonlinearity of linear modulation waveform and the difficulty in measuring the frequency of sinusoidal modulation system are circumvented by utilizing segmental-processing-average (SPA) techniques. The results demonstrate that the resolutions of range and velocity measurement are better than 2 mm and 0.1 mm/s, respectively. The system has numerous practical and potential applications in space missions.

We present a range-gating delayed detection super-resolution imaging lidar with high accuracy based on the signal intensities of three consecutive delay samples. The system combines the range and signal intensity information from multi-pulse detections to calculate the pulse peak position under the assumption of a Gaussian pulse shape. Experimental results indicate that the proposed algorithm effectively calculates pulse peak position and exhibits excellent accuracy with super-resolution. Accuracy analysis shows that accuracy is best improved by enhancing signal-to-noise ratio, strategically selecting samples, reducing pulse width, and appropriately choosing the delayed periods between samples.

A high temperature sensor based on an ultra-abrupt tapered fiber Michelson interferometer fabricated by the fusion-splicing method is proposed. The sensor consists of a single abrupt taper and the cleaved surface is used as the reflection mirror. The thermal characteristic is investigated at 25 to 1 000 oC. The sensitivity of the sensor is observed to vary with the temperature, that is, 25 and 78 pm/ C at 25–300 and 300–1 000 oC, respectively. The Michelson interferometer sensors have the advantages of simple structure, cost effectiveness, compactness, and simple fabrication process.

A 1 064-nm pulsed coherent Doppler lidar (CDL) prototype is developed to measure short range wind speed in the lower altitude troposphere layer. The CDL system adopts an injection{seeded Nd:YAG laser with the pulse duration of 80 ns, single pulse energy of 0.5 mJ, and pulse repetition rate of 200 Hz. Speed calibration experiments are implemented to obtain a speed accuracy of 0.3 m/s using a hard target. Data analysis results show that the CDL system can obtain a line-of-sight wind velocity at a range of 30 to 500 m with the range resolution of 40 m and 38 pulses accumulation.

We present a novel system for parameter design and optimization of modulated lidar. The system is realized by combining software simulation with hardware circuit. This method is more reliable and flexible for lidar parameter optimization compared with theoretical computation or fiber-simulated system. Experiments confirm that the system is capable of optimizing parameters for modulated lidar. Key parameters are analyzed as well. The optimal filter bandwidth is 200 MHz and the optimal modulation depth is 0.5 under typical application environment.

A low-cost, compact, and lossless temperature sensor based on a twin-core fiber (TCF) is demonstrated and manufactured by splicing two single-mode fibers to the ends of a TCF. The extinction ratio of the comb transmission spectrum is bigger than 15 dB, and the temperature sensitivity of the coupling angle is –0.02 rad/(oC.m) at –30–90 oC and –0.032 rad/(oC.m) at 90–175 oC. Finite element method is used to calculate the supermodes of the TCF, and the result agrees well with the experiment.