A design method of a dual-wavelength field-widened Michelson interferometer (FWMI) used as a spectral discriminator of a high-spectral-resolution lidar (HSRL) is proposed. The theoretical principle of the dual-wavelength FWMI is elaborated in detail. The influence of dispersion on the frequency discrimination performance is considered. The dual-wavelength field-widened design is adopted to compensate for refractive index, so that FWMI has a larger receiving field of view at 355 nm and 532 nm. The specific parameters of the dual-wavelength FWMI suitable for 355 nm and 532 nm are given, and performance evaluation and tolerance analysis are carried out. The evaluation results show that the dual-wavelength FWMI has a receiving field of view of more than 6° and excellent and stable performance at both wavelengths, and the requirements of processing precision and assembly precision are not high. Finally, Monte Carlo simulation is used to analyze the inversion error of optical parameters of the HSRL system using the dual-wavelength FWMI. The results show that the retrieval errors of aerosol backscattering coefficient and extinction coefficient are 1.82% and 11.39% at 532 nm, and 1.62% and 2.95% at 355 nm, respectively.

To solve the problem that there exist low precision and spatial resolution in the retrieval algorithm of land aerosol optical depth (AOD), a deep learning-based deep belief network (DBN) is proposed to realize the retrieval of land AOD with a spatial resolution of 30 m. The training samples for the algorithm include the AERONET site data with global long time series as well as the observation geometric data and apparent reflectivity data from Landsat 8 OLI which are corresponding to the former in space and time. To ensure the estimation accuracy and stability of retrieval, the process method for the AERONET site data, the spatial-temporal matching method for satellite and site data, and the setting of the DBN structure are investigated. The AERONET site data, independent of the training samples, are used to test the AOD estimation results at 550 nm for different surface types as a whole. In addition, the small-scale accuracy verification is conducted in the study area. The results demonstrate that the root mean square error and the mean absolute error of the proposed method are 0.11 and 0.072, respectively. The proposed method can break the situation in which the retrieval of AOD based on the existing methods relies excessively on other remote sensing products or time-phase data, and it effectively improves the efficiency and spatial resolution in the retrieval of AOD.

In order to simultaneously obtain the total atmospheric transmittance and precipitable water vapor at night under clear sky and cloudless view conditions, a research platform for stellar radiation measurement is built by combining telescope, multi-band filter, and near infrared enhanced CCD radiometer. First, the stellar radiation is collected by telescope. The radiation is filtered by sub-band filter, and the star image is collected by CCD. The gray value of the acquired image is further extracted, and the total atmospheric transmittance is calculated by Langley calibration method. At the same time, the precipitable water vapor is calculated by using the improved Langley method. The measurement results are compared with those of lidar and microwave radiometer to verify the reliability of the measurement method. The work in this paper enriches the means of simultaneous measurement of total atmospheric transmittance and precipitable water vapor at night, and has certain application value for space remote sensing and meteorological research.

We simulate the detection signal of a modulated pulsed underwater light detection and ranging (LiDAR) system and use a fast independent component analysis iterative algorithm to separate the target in the detected signal from the backscatter signal. Our approach succeeds in recovering the echo signals of the weak target in muddy water submerged by strong backscattering and in improving the signal-to-noise rate. To obtain the time delay of the target reflection with respect to the transmitting signal in one step, we used single and triple modulation frequencies as references to demodulate the LiDAR echo signals. The peak position corresponds to the time delay between the target and the detector. After the iterative separation by independent component analysis, the underwater laser detection range can be increased by four to five attenuation lengths.

Limb radiation detection is a new type of satellite detection technology, which mainly uses ultraviolet-visible-infrared solar scattering spectrum to obtain the vertical resolution profile information of trace gases such as ozone. In this paper, based on wavelength pairing spectral analysis method, multiplicative algebraic reconstruction technology, and SCIATRAN forward model, the ozone number density with vertical height of 10-40 km and resolution of 1 km is retrieved by using the limb radiation of SCIAMACHY instrument. First, according to the absorption characteristics of ozone in Chappuis-Wulf band, the mechanism of wavelength pairing is analyzed, and the wavelength and inversion range of ozone inversion are determined by combining the limb radiation weighting function. Then, the influence of several typical wavelength pairing combinations on the inversion results are analyzed, and a group of better wavelength pairing combination is determined. Finally, the error sources in the inversion process of iterative algorithm, such as iteration times, prior profile, tangent height and NO2 concentration are inversed in different degrees. The error results show that these parameters will affect the inversion results of ozone profile with vertical height less than 20 km.

In order to improve the target's detection accuracy of the acquisition, tracking, and pointing system used in satellite-to-ground optical communications, the analysis and evaluation with high precision are needed for the random errors influencing the position determination of the beacon spots on the flat-panel detector. Considering a variety of random factors influencing spot detection, a noise equivalent angle (NEA) model for the centroid localization which is not dependent on the point spread function of target signals is established and the influences of different factors on the NEA are simulated. The results show that the change trend of NEA value under each point spread function of target signals is similar. It increases with the increase of noise, decreases with the increase of signal intensity, and increases with the increase of spot radius. The NEA values calculated under different point spread functions of target signals are different, but the maximum difference is less than 30%. The research result provides a new measurement method and a theoretical analysis basis of random errors existing in target position detection, and is of great significance to the establishment and maintenance of long-distance satellite-to-ground optical communication links.

In this paper, we proposed a new scheme to generate D-band mm-wave vector signals based on an intensity modulator. With this scheme, the eight-fold-frequency D-band mm-wave vector signals were generated based on an intensity modulator and the precoding technology. After wireless transmission for a certain distance, the signals were input to the receiving end, and then were demodulated and analyzed for error. Due to the center-carrier suppression of the scheme, the signals were successfully transmitted without the assistance of any optical filter, greatly reducing the link cost of the radio-over-fiber system. Furthermore, we experimentally demonstrated the generation and transmission of 152 GHz 4 GBaud quadrature phase shift keying (QPSK) signals, and the bit error rate of the QPSK signals received after 1 m wireless transmission was lower than the threshold of soft-decision forward error correction (1.0×10 -2).

In order to stably and accurately detect the hydrogen concentration in a high-temperature and high-humidity environment, we proposed a new fabrication method of fiber Bragg grating (FBG) hydrogen sensors in this paper. Firstly, a polydopamine coating, assembled on the surface of FBG by auto-polymerization, was used to adsorb the palladium ions in the palladium chloride solution and form palladium seeds, thus enhancing the adhesion of the palladium seeds on the fiber surface. Secondly, a reducing agent was used to provide a reduction site for the palladium ions so that the palladium seeds were grown into a dense palladium film. Thirdly, a layer of SiO2 superhydrophobic film was coated on the surface of the palladium film to enhance its stability in a high-humidity environment. Finally, a temperature compensation unit was introduced to eliminate the influence of temperature on the measurement of hydrogen concentration. Furthermore, the effects of polydopamine thickness, the type of the reducing agent, the thickness of the palladium film, temperature, and humidity on the hydrogen-sensitive response characteristics of the sensor were experimentally studied. It is found that the sensor can respond stably and accurately to the changes in hydrogen concentration at the temperature of 30-70 ℃ and the relative humidity of 20%-90%, with a sensitivity of 10.80 pm/% and a relative error of less than 7.2%.

In order to realize the whispering gallery microcavity with compact structure and stable performance, this paper proposes an in-fiber parallel dual microdisk structure, in which the whispering gallery microcavity and the optical waveguide that excites the whispering gallery mode are both in the fiber core. Finite time domain difference method is used to study the variations of the symmetric mode and antisymmetric mode in the mode splitting with the coupling distance between two microdisks. The research results show that there is a competitive relationship between the symmetric mode and the antisymmetric mode. As the coupling distance increases, the transmittance of the antisymmetric mode first increases and then decreases, and the resonance wavelength is red shifted; the symmetric mode is exactly the opposite. The influence of manufacturing process accuracy on mode splitting is analyzed by calculation. The research results can be used to design high-sensitivity fiber-optic temperature sensors in fiber micro-disks and other fields.

Aiming at the problem of low accuracy of blood vessel segmentation caused by the small and blurred outline of fundus retinal blood vessels, a retinal blood vessel segmentation method using wavelet transform to fuse the contour feature and detailed feature of the blood vessel under a multi-scale framework is proposed. The contrast between the blood vessel and the background is enhanced by preprocessing, the contour feature and detail feature of the blood vessel are extracted in a multi-scale framework, and image post-processing is performed. The wavelet transform is used to fuse the two feature images, the maximum value of the corresponding pixel of each scale is calculated to obtain the blood vessel detection image, and finally the Otsu method is used for segmentation. Through the test experiment of the DRIVE data set, the average accuracy, sensitivity, and specificity are 0.9582, 0.7086, and 0.9806, respectively. The method in this paper can accurately segment the contour of the blood vessel while retaining more branches of small blood vessels, and the accuracy is high.

Fourier ptychographic microscopy (FPM), a new microscopic imaging technology, skillfully combines the concepts of phase recovery algorithm and synthetic aperture, solving the problem of difficult combination of a large field of view with high resolution. In traditional calculations, the FPM process is often approximated as coherent imaging. Specifically, LED is regarded as a point light source and the coherent transfer function as the constraint of spectrum support domain for the optimal solution. However, strictly, LED is an extended incoherent light source, so this improper approximation will degrade the reconstructed image quality. For this reason, a new transfer function, apodized coherent transfer function weighted by the Bessel function (B-AC), is proposed as a support domain constraint in this paper. Experimental results show that the B-AC constraint is more suitable for the FPM imaging system and obviously reduces the ringing effect caused by the constraint of the coherent transfer function. Furthermore, the reconstructed image quality and robustness are better than those under the constraints of coherent transfer function and apodized coherent transfer function.

Fourier ptychographic microscopy (FPM) uses illumination of LED array with angle change to overcome the resolution limitations of low numerical aperture objectives. In an original FPM system, position errors of an LED array severely impact an image reconstruction process. Therefore, accurately correcting the position of an LED array is very important to improve the quality of reconstructed images. To solve this problem, the study proposes a position correction method based on a genetic annealing optimization algorithm. First, the influence of the following three factors’ relative positions on an incident wave vector is analyzed: the LED array, sample, and numerical aperture of objective lens. Next, the genetic annealing optimization algorithm is utilized in estimating global error parameters for LED array error locations. Finally, global error parameters are used to quickly and accurately correct the position of the LED array during the reconstruction process. Both simulation and experimental results indicate that the proposed method can significantly improve the quality of FPM reconstructed images.

Photonic integrated interferometric imaging system has the imaging characteristics of small-scale, low weight, low power consumption, and high resolution, which is expected to replace the traditional large aperture telescopes to realize remote detection. The principle of optical interferometry imaging is studied, and the restoration model of space object interferometric imaging is established. The influence of microlens array arrangement on imaging quality is studied, and the design method of microlens array is proposed. The effect of optical coherent baseline matching on spatial target spectrum coverage is studied, the coherent baseline matching method that can cover high, medium, and low spectrum coverage efficiently is presented. Finally, effects of target image simulation restoration under different microlens array arrangements and interference baseline matching approaches are compared. The results show that the proposed arrangement of microlens array and interference baseline matching method can improve the spectrum coverage of space target and improve the restoration quality of target images.

In this paper, a general method for calculating the axial resolution (δz) of optical coherence tomography (OCT) system based on the Wiener-Khinchin theorem was proposed. The power spectral density (PSD) distribution of a light source was subject to inverse Fourier transform to obtain the self-coherence function of the source, and the full width at half maximum of the function was employed to calculate the δz. The δz values of the OCT systems illuminated by the Gaussian and non-Gaussian spectrum sources were calculated by this method, respectively, and its validity for the above sources was verified by comparing the calculated values with the nominal values of products given by the manufacturers. Furthermore, an ultra-broadband white light source was taken as an example to form a non-Gaussian spectrum by edge filtering with two filters. An experimental setup was built to measure the δz, and the result was very close to the calculated one with the proposed method. In conclusion, the method proposed in this paper can be applied to the δz calculation of OCT systems illuminated by non-Gaussian spectrum sources, providing a basis for the parameter consideration and component selection during system design.

Air gap Fabry-Perot (F-P) etalon is a common passive optical device, which, as a filter, has been widely used in optical communications, photoelectric measurement, and optical sensing. Based on the classical multi-beam interference theory, the spectrum of an F-P etalon has periodical Lorentz peaks. However, during the calibration and fitting of the transmission spectrum of a fiber air gap F-P etalon, left-right asymmetry and broadening of the transmission peaks are found. These unexpected phenomena lead to a large fitting error if the classical Lorentz formula is employed. In this study, we firstly analyze the relationship between peak asymmetry and broadening, and then investigate the influence of the divergence angle of a fiber collimator and the assembly error of the etalon. Finally, taking the divergence angle and the incident deflection angle into consideration, we propose a multi-peak superposition fitting formula based on the traditional F-P interference theory. With the new formula, a fitting variance of 0.008 and a coincident accuracy of 0.9998 are achieved for the F-P transmission spectra. Compared with the fitting results of Lorentz lineshape, the fitting accuracy is improved by about 15 times. In conclusion, an accurate fitting method for the transmission spectra of fiber air gap F-P etalons is provided, which has important guiding significance for the design and assembly of the F-P etalons.

A hybrid demultiplexer for mode-wavelength division based on nanowire waveguides and one-dimensional photonic crystal nanobeam cavity is proposed. The device consists of a wavelength division multiplexer (WDM) and a mode division multiplexer (MDM), in which the WDM is composed of two one-dimensional photonic crystal nanobeam cavities, however the MDM adopts a silicon-based nanowire waveguide structure. The parameters of the hybrid demultiplexer are calculated using the three-dimensional finite-difference time-domain (3D-FDTD) method. The results show that the four channels of the fundamental mode (TE0) and the first-order mode (TE1) at wavelengths of 1570.0 nm and 1573.2 nm can be demultiplexed by this device. The insertion loss and the channel crosstalk are smaller than 0.37 dB and -18.4 dB, respectively. The free spectral range can reach 400 nm. The proposed hybrid demultiplexer can be applied to a mode-division and coarse-wavelength-division multiplexing system.

Based on a semiconductor optical amplifier (SOA) and a high-speed fiber Fabry-Perot filter, we built a short ring cavity for producing high-speed swept laser. During the filter sweeping from the long wavelength to the short wavelength, if the bias current of the SOA was switched off, we could acquire a swept laser with a duty cycle of 50%. With the help of an interleaver, the swept laser with a duty cycle of 100% could be obtained. Furthermore, after a second SOA was applied, the output power of the swept laser was further improved. Finally, it was experimentally demonstrated that the swept laser featured a swept frequency of 500 kHz, a center wavelength of 1550 nm, a sweep range of 67 nm, an effective coherence length of 6.5 mm, and an average output power of greater than 20 mW.

Aiming at the problem of poor localization accuracy or failure of traditional visual odometry in low-texture scenes, we propose a method of RGB-D visual odometry that combines point and line features. This method unites the unique characteristics of points and lines, and combines the photometric residual of point features and the local gradient fitness error of line features to construct a joint error function that is robust to low-texture scenes. The Gauss-Newton iteration method is used to perform nonlinear iterative optimization of the joint error function to obtain the accurate pose of each frame. The proposed method is evaluated on the public real-world RGB-D dataset and synthetic benchmark dataset. Experimental results show that, compared with other state-of-the-art algorithms, the proposed algorithm has better accuracy and robustness.

The Sn-doped CsPbBr3 quantum dots were synthesized by the hot injection method in this paper. The transmission electron microscope (TEM) and X-ray diffractometer (XRD) characterization results show that doping with a small amount of Sn can partially replace Pb and passivate the CsPbBr3 quantum dots, reducing the surface defects of quantum dots and improving the photoluminescence quantum yield (PLQY) of the quantum dots. Specifically, in the case of nPb∶nSn=9∶1, the PLQY of the quantum dots is increased from 21.0% of the undoped CsPbBr3 quantum dots to 40.4% of the Sn-doped ones. However, with the rise in Sn content, some heterogeneous phases appear in the XRD patterns, the photoluminescene weakens, and the PLQY decreases from 40.4% of small amount of Sn-doped (nPb∶nSn=9∶1) quantum dots to 10.4% of CsPb0.6Sn0.4Br3 quantum dots. In conclusion, CsPb0.9Sn0.1Br3 doped with a small amount Sn has the strongest photoluminescence and electroluminescence, with a photoluminescence peak at 511 nm, a PLQY of 40.4%, an electroluminescence peak at 512 nm, and an electroluminescence brightness of 343.0 cd/m 2 which is 2.5 times that of the undoped CsPbBr3 quantum dots. The experiments in this paper demonstrate that doping CsPbBr3 with a small amount of Sn (CsPb0.9Sn0.1Br3) can decrease the surface defects of quantum dots and improve the photoluminescence and electroluminescence properties of the quantum dots.

At present, coronary stent implantation has become an important treatment for coronary atherosclerotic heart disease. It is difficult to identify and detect drug-eluting coronary stent fracture (CSF). Intravascular optical coherence tomography (IVOCT) has unique advantages in CSF identification and detection due to its extremely high imaging resolution. Given the above situation, we propose an accurate reconstruction method of coronary stents in IVOCT. On the basis of the fact that the metal stents have imaging shadows, after the vessel boundaries are segmented, the intensity values of a stent and its shadows at a certain depth are accumulated to generate a one-dimensional array. Then, the array is arranged according to the pullback order to produce a stent reconstruction image. Compared with other stent reconstruction methods (such as three-dimensional imaging, longitudinal image cutting), the proposed method can not only maintain the overall structure of the stents but also avoid the requirement of spatial imagination on the operators. In conclusion, the method proposed in this paper can quantitatively identify and analyze CSF, providing different IVOCT imaging parameters for different stent structures.

In order to achieve the high quality alignment of an imperfect imaging off-axis three mirror system, a design method of aberration compensation system placed in front of the focus of the three mirror system is proposed. Based on Gaussian optics, the relationship between the parameters of the compensation system and the spatial position of the image points is derived. In addition, the initial structure of the compensation system is designed according to the third-order aberration theory. An off-axis three mirror system with an effective aperture of 400 mm and an F number of 3.9 is adopted to test the proposed method and the feasibility of this method is verified. The proposed method can effectively improve the optimization efficiency and is suitable for various types of reflection systems. The final design results can achieve the high-quality alignment of an imperfect imaging off-axis three mirror system with a relatively small aperture, effectively reduce the assembly-alignment cost, and improve the assembly-alignment efficiency.

With the development of aerospace field, there exists an increase in the need of space environment simulation and solar radiation simulation has also attracted much attention. At present, most of the mature solar simulators designed and used are horizontal solar simulators, whose collimator mirrors usually adopt the form of splicing. However, the vertical solar simulators possess the advantage of space-saving and become gradually mature. In this paper, according to the general requirements of solar simulators, a large diameter overlooking multi-dimensional adjustable structure of collimating mirrors used for a high irradiation vertical solar simulator is designed. In addition, according to the characteristics of high radiation and low background working environment, a multi-dimensional adjustable flexible hoisting structure is designed. In order to improve the overall debugging performance of the system, a single-block whole mirror structure is adopted to reduce the structural complexity. According to the above designs, the calculation and simulation analysis are conducted and the results show that the flexible hoisting structure can be used to effectively balance the stress deformation caused by the collimating mirror's own gravity and the environmental temperature, and finally to meet the use requirements of irradiance and irradiation uniformity for the solar simulators.

In this study, a multi-freeform collimating lens is designed based on a total internal reflection structure to effectively obtain a wide range of emitted light from the semiconductor light-emitting diodes to ensure efficient use of light energy. The initial structure of this type of lens is based on Snell's law and is designed using algorithms based on optical principles such as the law of conservation of energy. The initial structure of the lens is imported into the Creo software. Subsequently, a 360° rotation is performed to obtain a three-dimensional solid model. The obtained structure is transferred to the TracePro optical software for Monte Carlo ray tracing simulation. A circle spot is obtained at a distance of 20 m from the receiving board. Further, the normal vector of the lens model is corrected. The optimized lens exhibited light transmittance up to 182 cd/lm and a beam angle of ±1.621° after the simulation. Compared with the traditional total internal reflection structure lens, the optimized lens exhibits an improved ability to control narrow beams.

As the basic component of photonic integrated circuits, optical reflectors are widely used in plenty of fields, such as quantum communications, smart grids, aerospace and so on. On-chip optical reflectors with high reflectance and low temperature sensitivity shall greatly simplify the photonic integrated circuit systems and improve the reliability and stability of the photonic integrated circuits. Therefore, we propose an on-chip optical reflector based on silicon on insulator with high reflectance and low temperature sensitivity. Our scheme is based on a Sagnac loop and can obtain ultra-high reflectance with a wavelength range of 3.41 nm (reflectance more than 90%) and high reflectance with a wavelength range of 32.85 nm (reflectance more than 80%). The reflectors are heated by the on-chip micro-electrodes, and the results show that when the power of the micro hot electrode is gradually increased from 0 mW to 6 mW, the wavelength shift of the reflector in the wavelength region of 1566.5-1568.58 nm is less than 0.045 nm. The reflectance change is less than 0.19 dB. The reflector has the advantages of small size, light weight, simple fabrication, high reflectance, low loss, and temperature insensitivity. It can be widely used in communication and signal processing fields such as lasers, microwave photonic filters, and optical transmission networks.

Based on the Huygens-Fresnel principle, spatial frequency spectrum theory, and the law of energy conservation, we derive an inclination factor of the integral formula for far-field diffraction from a small diffractive source with a planar wavefront and provide an expression to calculate the light field distribution on the far-field hemispherical surface. Then, according to the characteristics of the wavefront transformation of a collimating objective satisfying the sine condition, a calculation formula is recommended for a collimating light field from a small diffractive source with a planar wavefront. Furthermore, applications of this formula are shown through the transformation characteristics of Gaussian beams of simple rectangular coordinate polarization and simple cylindrical coordinate polarization.

The accuracy of on-orbit absolute radiometric calibration of optical remote sensors directly affects the breadth and depth of quantitative applications, and thus the on-orbit vicarious calibration of the reflectance-based, irradiance-based, and radiance-based methods based on large and uniform sites plays an important role. However, due to a limited number of sites, low calibration frequency, low site reflectance, and single-point calibration cannot achieve full dynamic range calibration, the calibration accuracy is limited to 5%-8%. The improvement of the spatial resolution of optical remote sensors makes it possible for the targets-based absolute radiometric calibration with good spectral flatness and Lambert property. In this paper, the calibration principle, process and influencing factors of the gray-scale targets method are studied, and a simplified method for radiative transfer calculation is proposed. Furthermore, considering the response nonlinearity and dark current of high-resolution multispectral cameras, we apply the first-order function response model with bias to test a multispectral camera three time and obtain the calibration gain and bias, with the calibration uncertainty being better than 5%. In addition, the reflectance inversion validation is carried out on the laid color targets, and the results show that the absolute difference within 5%-70% reflectance is less than 0.01. In conclusion, the method proposed in this paper can realize the absolute radiometric calibration of optical satellite remote sensors in a large dynamic range and solve the common problem that the accuracy of radiometric calibration is generally not high in low-end quantitative applications.