Although the optimization of a static Fabry–Perot interferometer (FPI)—used as a Doppler shift discriminator in wind lidar—has been proposed, it cannot be applied to the scanning FPI used in the high-spectral resolution lidar for temperature detection. After a comparison, the optimal scanning implementation is chosen and a new optimization scheme is proposed. The free spectral range (FSR) of the FPI is determined by the width of the Rayleigh spectrum. Then, for analytical purposes, the transmission of Rayleigh backscattering through an FPI is simplified to be a superposition of a Gaussian function and a constant background. The maximum likelihood estimation and the Cramer–Rao bound theory are used to derive an analytic expression of the temperature error. Thus, the effective reflectance of the FPI can be optimized. Finally, assuming known atmospheric temperature–pressure–density profiles, backscattering raw signals are simulated using the optimized parameters of the FPI and some other key system parameters of our existing lidar system. Comparisons between the assumed and retrieved temperature profiles revealed that error <2 K can be achieved in the altitude range of 15 to 40 km, even with the disturbance of aerosol contamination.
Direct detection Doppler wind lidar (DWL) has been demonstrated for its capability of atmospheric wind detection ranging from the troposphere to stratosphere with high temporal and spatial resolution. We design and describe a fiber-based optical receiver for direct detection DWL. Then the locking error of the relative laser frequency is analyzed and the dependent variables turn out to be the relative error of the calibrated constant and the slope of the transmission function. For high accuracy measurement of the calibrated constant for a fiber-based system, an integrating sphere is employed for its uniform scattering. What is more, the feature of temporally widening the pulse laser allows more samples be acquired for the analog-to-digital card of the same sampling rate. The result shows a relative error of 0.7% for a calibrated constant. For the latter, a new improved locking filter for a Fabry–Perot Interferometer was considered and designed with a larger slope. With these two strategies, the locking error for the relative laser frequency is calculated to be about 3 MHz, which is equivalent to a radial velocity of about 0.53 m/s and demonstrates the effective improvements of frequency locking for a robust DWL.
A modification method is described for Rayleigh Doppler lidar wind retrieval. Compared to the double-edge theory of Korb et al. [Appl. Opt.38, 432 (1999)] and the retrieval algorithm of Chanin et al. [Geophys. Res. Lett.16, 1273 (1989)], it has a greater sensitivity. The signal-to-noise ratio of the energy monitor channel is involved in error estimation. When the splitting ratio of the two signal channels is 1.2, which usually happened during wind detection, it will improve the measurement accuracy by about 1% at 30 km altitude for a Doppler shift of 250 MHz (44 m/s ). Stabilities of retrieval methods, i.e., errors caused by the spectrum width deviation including laser pulse, Rayleigh backscatter, and filter transmission curve are first discussed. The proposed method increases the resultant precision by about 15% at 30-km altitude assuming an 8-MHz deviation in full width at half maximum of the Fabry–Perot interferometer.
A novel direct detection method for simultaneous distributed measurement of the temperature and strain along a single
mode fiber based on the optical time-domain reflectometer technique is proposed. The Raman backscattering is sensitive
to temperature but not to strain, thus the temperature distribution along the fiber can be determined. A twin-channel
Fabry-Perot interferometer is designed; of which, the Brillouin channel is used to analyze the strain information within
measurement range of 2000 με strain and 400 °C temperature. The frequency center, spectrum bandwidth and intensity
changes of Brillouin backscattering are transformed into transmission changes after passing through the Fabry-Perot
interferometer. With the look-up table of response functions of detected temperature, the strain profile along the fiber can
be known. The Rayleigh channel is design to preset and measure the outgoing laser frequency relative to the Brillouin
channel. By modulating the voltage added to the piezoelectric tube of the Fabry-Perot interferometer, the frequency
offset of outgoing laser to the Brillouin channel can be preset; the temperature dynamic range can be extended.
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