Lidar is an important device for detecting atmospheric parameters. In this paper, a single channel F-P etalon is used as a narrow-band filter to lock the transmittance of 355 nm wavelength emitted laser by control the transmittance of seed laser with 1064 nm wavelength. Not only the background noise can be suppressed and the signal-to-noise ratio can be improved, but also the transmittance of the locked target in the etalon can be tracked to achieve higher detection accuracy. During the experiment, transmittance is controlled at target transmittance with 0.01 precision.
The Fabry-Parot etalon can be widely used in the lidar for Doppler wind measurement and aerosol detection. Due to the very wide application of the FP etalon, the standard transmission curve of the F-P is a very important parameter. The general FP etalon measures the transmittance curve by tuning the wavelength of the laser source or tuning the angle of incident light. The FP etalon generally measures the transmittance curve of the etalon by tuning the wavelength of the laser source or use the frequency comb source. Moreover, the tuning of the wavelength and the angle of incident light are nonlinear, and the measurement accuracy is insufficient, and the frequency comb source is very expensive. This paper proposes a new method for testing the transmittance curve of a FP etalon using a similar frequency comb source. The whispering gallery mode is a typical similar frequency comb laser source which has multiple frequency components, but not equal intervals. The spacing of the frequency of the whispering gallery mode is not equal, but the spacing of the frequency can be determined. So, the transmittance curve of the FP etalon can be measured at one time. And the frequency interval and spectral range are tunable easily. This new method greatly reduces the cost of measuring transmission curve, improve measurement accuracy and effectiveness and has great theoretical and practical value.
As the increasing demands of the environmental protection, meteorological monitoring, ecological detection and other fields, lidars are needed to simultaneously measure a variety of atmospheric parameters (especially simultaneous measurement of various pollutants, such as volatile organic solvent VOCs). Multi-wavelength Raman lidar, differential absorption lidar (DIAL) and high spectrum resolution lidar (HSRL) can be implemented these fields. But either technique needs multiple laser sources to meet their requirements. This paper proposes a new lidar based on Frequency Comb Light Source, which can provide multiple frequency components through a single light source for simultaneously measuring various multiple atmospheric parameters. The frequency comb laser light source is used to emit seed light of frequency ωc from the seed laser, and then after passing through the frequency comb laser, a series of equally spaced spectral frequency components are emitted centered on ωc; then emitted into the atmosphere, and the laser reacts with the atmosphere. The echo signal enters the receiving optical path through the telescope, and is sent through the beam splitter or the discriminator to detect by the detector. Based on different pollutants have different absorption cross sections, similar differential absorption lidar(DIAL), the λon and λoff can be determined. The lidar using the frequency comb light source can be easy designed into a compact structure, which is convenient to carry and maintain. It’s also a very advantageous for the lidar miniaturization and industrialization with a wide application prospect.
KEYWORDS: Fabry–Perot interferometers, Temperature metrology, Optical filters, LIDAR, Refractive index, Transmittance, Optical components, Receivers, Signal to noise ratio
Cloud-Aerosol plays a very important role in the Earth's atmospheric system,so accurate data of global Cloud-Aerosol detection are of great importance to the climate.The 532nm detection channel of the Space-borne Lidar for Cloud- Aerosol has a low signal-to-noise ratio during the daytime,so it cannot be effectively detected during the daytime.In order to improve the signal-to-noise ratio of the 532nm detection channel,a high-stability"sandwich solid-etalon" structure is used in series with a narrow-band interference filter.A filter is designed with a high-precision thermal control structure,it's optical performance is detected.The results show that the performance parameters of the filter meet the design requirements: the mechanical structure is compact and reliable; the temperature control accuracy can reach 0.1 °C;the temperature tuning coefficient is 3.49pm/°C,that is, the temperature tuning range of +/-2.3 °C, which satisfies the central wavelength tuning range of 16pm; the incident angle should be controlled within +/-4.2mrad to meet the peak transmittance of more than 75%
As the core component of lidar, APD detector is used to realize photoelectric conversion of laser echo signal. The detector needs to work at a stable temperature to ensure its application performance. A high gain amplifier circuit is designed. The temperature signal is collected by the MCU and the semiconductor refrigerator is controlled. The precision control of the temperature is realized through the PID algorithm. The adjustable high voltage module is integrated, and the APD bias voltage and temperature can be controlled through the serial port of the MCU. Reducing the volume of the APD detector, ensuring the wide temperature range of the detector and improving the integration degree of the lidar.
Aerosol and water vapor are two important atmospheric parameters. The accurate quantification of diurnal variation of these parameters are very useful for environment assessment and climate change studies. A moveable, compact and unattended lidar system based on modular design is developed for aerosol extinction coefficients and water vapor mixing ratios measurements. In the southern suburbs of Beijing, the continuous observation was carried out by this lidar since the middle of the year of 2017. The lidar equipment is presented and the case study is also described in this paper. The observational results show that the lidar kept a very good status from the long-time continuous measurements which is suitable for networking especially in meteorological research field.
A system for collecting data of Side-Scatter lidar based on Charge Coupled Device (CCD),is designed and implemented. The system of data acquisition is based on Microsoft. Net structure and the language of C# is used to call dynamic link library (DLL) of CCD for realization of the real-time data acquisition and processing. The software stores data as txt file for post data acquisition and analysis. The system has ability to operate CCD device in all-day, automatic, continuous and high frequency data acquisition and processing conditions, which will catch 24-hour information of the atmospheric scatter’s light intensity and retrieve the spatial and temporal properties of aerosol particles. The experimental result shows that the system is convenient to observe the aerosol optical characteristics near surface.
Because of the existence of blind zone and transition zone, the application of backscattering lidar in near-ground is limited. The side-scatter lidar equipped with the Charge Coupled Devices (CCD) can separate the transmitting and receiving devices to avoid the impact of the geometric factors which is exited in the backscattering lidar and, detect the more precise near-ground aerosol signals continuously. Theories of CCD side-scatter lidar and the design of control system are introduced. The visible control of laser and CCD and automatic data processing method of the side-scatter lidar are developed by using the software of Visual C #. The results which are compared with the calibration of the atmospheric aerosol lidar data show that signals from the CCD side- scatter lidar are convincible.
A high sensitivity and global covered observation of carbon dioxide (CO2) is expected by space-borne integrated path differential absorption (IPDA) lidar which has been designed as the next generation measurement. The stringent precision of space-borne CO2 data, for example 1ppm or better, is required to address the largest number of carbon cycle science questions. Spectral purity, which is defined as the ratio of effective absorbed energy to the total energy transmitted, is one of the most important system parameters of IPDA lidar which directly influences the precision of CO2. Due to the column averaged dry air mixing ratio of CO2 is inferred from comparison of the two echo pulse signals, the laser output usually accompanied by an unexpected spectrally broadband background radiation would posing significant systematic error. In this study, the spectral energy density line shape and spectral impurity line shape are modeled as Lorentz line shape for the simulation, and the latter is assumed as an unabsorbed component by CO2. An error equation is deduced according to IPDA detecting theory for calculating the system error caused by spectral impurity. For a spectral purity of 99%, the induced error could reach up to 8.97 ppm.
A high-spectral-resolution lidar (HSRL) can measure vertical profiles of the aerosol backscatter ratio and the aerosol extinction coefficient simultaneously. Fabry-Perot (F-P) interferometer used as a spectrum filter can provide fine frequency resolution with an important impact on HSRL retrieval accuracy. However, in practical applications, the spectral characteristics of the interferometer are affected by many factors. In this article, the impact of several factors on F-P interferometer spectral properties, such as the temperature change, the beam divergence angle, the absorption loss, surface defects and non-parallelism of the interferometer, were analyzed in depth. The performances of F-P interference spectrum filters are modeled and simulated by analyzing the characteristics of lidar echo, respectively using Gaussian model and Bruneau model. Finally, designed the sandwich structure of F-P etalon encapsulation, and the finite element method is used for thermal analysis.
KEYWORDS: Control systems, Lithography, Digital signal processing, Control systems design, Signal processing, Feedback control, Device simulation, Mathematical modeling, Electromagnetism, Servomechanisms
Aiming at the high precision and efficiency requirements of Z-direction locating in Talbot lithography, a control system based on Voice Coil Motor (VCM) was designed. In this paper, we built a math model of VCM and its moving characteristic was analyzed. A double-closed loop control strategy including position loop and current loop were accomplished. The current loop was implemented by driver, in order to achieve the rapid follow of the system current. The position loop was completed by the digital signal processor (DSP) and the position feedback was achieved by high precision linear scales. Feed forward control and position feedback Proportion Integration Differentiation (PID) control were applied in order to compensate for dynamic lag and improve the response speed of the system. And the high precision and efficiency of the system were verified by simulation and experiments. The results demonstrated that the performance of Z-direction gantry was obviously improved, having high precision, quick responses, strong real-time and easily to expend for higher precision.
The spectrum-integral Talbot lithography (STIL) was introduced into the fabrication of one-dimensional micro gratings using the broad-band UV illumination in this paper. In the process of spectrum-integral Talbot lithography, the self-images and π-phase-shifted images generated by different wave lengths overlap and integrate collectively to enormously extend the continuous depth-of-focus area since a certain distance away from the mask. As a result, the route of STIL proves to be of great potential for periodic frequency-doubling in good contrast without any complex improvement and operation to the traditional proximity lithographic system of UV mask aligner.
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