Differential Absorption Lidar Detection Research Articles

Wavelet modulus maxima method for on-line wavelength location of pulsed lidar in CO_2 differential absorption lidar detection

Differential absorption lidar (DIAL) is an excellent technology for atmospheric CO2 detection. However, the accuracy and stability of a transmitted on-line wavelength are strictly required in a DIAL system. The fluctuation of a tunable pulsed laser system is relatively more serious than that of other laser sources, and this condition leads to a large measurement error for the lidar signal. These concerns pose a significant challenge in on-line wavelength calibration. This study proposes an alternative method based on wavelet modulus maxima for the accurate on-line wavelength calibration of a pulsed laser. Because of the different propagation characteristics of the wavelet transform modulus maxima between signal and noise, the singularities of a signal can be obtained by detection of the local modulus maxima in the wavelet transform maximum at fine scales. Simulated analysis shows that the method is more accurate than the general method such as quintic polynomial fitting and can steadily maintain high calibration precision at different signal-to-noise ratios (SNRs). Last, 16 groups of real experiments were conducted to verify the simulated analysis, which shows that the proposed method is an alternative for accurately calibrating an on-line wavelength. In addition, the proposed method is able to suppress noises in the process of wavelength calibration, which gives it an advantage in accurate on-line wavelength calibration with a low SNR.

Precise wavelength control of a single-frequency pulsed Ho:Tm:YLF laser.

We demonstrate wavelength control of a single-frequency diode-pumped Ho:Tm:YLF laser by referencing its wavelength to an absorption line of carbon dioxide. We accomplish this wavelength control by injection seeding with a cw Ho:Tm:YLF laser that can be tuned over or stabilized to carbon dioxide or water vapor lines. We show that the pulsed laser can be scanned precisely over an absorption line of carbon dioxide by scanning the injection seed laser wavelength. We locked the pulsed laser to within 18.5 MHz of the absorption line center by stabilizing the injection seed on the line center. The single-frequency pulsed output, intended for use as a transmitter for differential absorption lidar detection of atmospheric carbon dioxide and water vapor and for coherent detection of wind, is 100 mJ per pulse at a 5-Hz repetition rate.

Comparison between super low ionization ratio and reach through avalanche photodiode structures

Avalanche photodiodes (APD) are solid state quantum detectors suitable for low level light detection in the visible and near infrared regions. These devices are commercially available from many manufacturers and are fabricated using several different solid state structures. In this paper a comparison between the well developed reach through structure (RTS) aPd and the relatively newer super low ionization ratio, k, (SLIK) APD structure will be discussed. The comparison is based on the experimental characterization of two APDs with these structures. The characterization setup and experiments will be described which include the calibration of the APDs, temperature response, bias voltage response, and noise measurements including noise-equivalent-power and detectivity D*. For this comparison the selected RTS APD is the EG&G C30955E, currently used by the lidar atmospheric sensing experiment (LASE) instrument. The selected SLIK ApD is the EG&G C30649E, which is proposed for LASE development. The characterization experiments where done in order to develop an advanced atmospheric water vapor differential absorption lidar detection system.

Optimal bandwidth for topographical differential absorption lidar detection

A detected laser signal backscattered from a tilted target is modeled with a laser-pulse shape as a response of a high-pass filter to an exponential input that describes the gain buildup within the laser cavity before a laser pulse is emitted and a single-pole low-pass RC filter for the electronic amplifier. The model is used to maximize the signal-to-noise ratio of the detected peak signal with a proper choice of the integration time constant τ as a function of the laser-pulse shape and the tilt angle of the backscattering target.

High pulse repetition frequency, multiple wavelength, pulsed CO_2 lidar system for atmospheric transmission and target reflectance measurements

A multiple wavelength, pulsed CO(2) lidar system operating at a pulse repetition frequency of 200 Hz and permitting the random selection of CO(2) laser wavelengths for each laser pulse is presented. This system was employed to measure target reflectance and atmospheric transmission by using laser pulse bursts consisting of groups with as many as 16 different wavelengths at a repetition rate of 12 Hz. The wavelength tuning mechanism of the transversely excited atmospheric laser consists of a stationary grating and a flat mirror controlled by a galvanometer. Multiple wavelength, differential absorption lidar (DIAL) measurements reduce the effects of differential target reflectance and molecular absorption interference. Examples of multiwavelength DIAL detection for ammonia and water vapor show the dynamic interaction between these two trace gases. Target reflectance measurements for maple trees in winter and autumn are presented.