Pulsed Heterodyne Research Articles

Methodology for deriving the telescope focus function and its uncertainty for a heterodyne pulsed Doppler lidar

Abstract. Doppler lidars provide two measured parameters, radial velocity and signal-to-noise ratio, from which winds and turbulent properties are routinely derived. Attenuated backscatter, which gives quantitative information on aerosols, clouds, and precipitation in the atmosphere, can be used in conjunction with the winds and turbulent properties to create a sophisticated classification of the state of the atmospheric boundary layer. Calculating attenuated backscatter from the signal-to-noise ratio requires accurate knowledge of the telescope focus function, which is usually unavailable. Inaccurate assumptions of the telescope focus function can significantly deform attenuated backscatter profiles, even if the instrument is focused at infinity. Here, we present a methodology for deriving the telescope focus function using a co-located ceilometer for pulsed heterodyne Doppler lidars. The method was tested with Halo Photonics StreamLine and StreamLine XR Doppler lidars but should also be applicable to other pulsed heterodyne Doppler lidar systems. The method derives two parameters of the telescope focus function, the effective beam diameter and the effective focal length of the telescope. Additionally, the method provides uncertainty estimates for the retrieved attenuated backscatter profile arising from uncertainties in deriving the telescope function, together with standard measurement uncertainties from the signal-to-noise ratio. The method is best suited for locations where the absolute difference in aerosol extinction at the ceilometer and Doppler lidar wavelengths is small.

基于远程脉冲激光外差探测的匹配滤波过程研究

基于远程脉冲激光外差探测的匹配滤波过程研究

A method to enhance the SNR of pulsed heterodyne LADAR based on radio frequency modulation

The performance of conventional incoherent pulsed LADAR is limited by the emitting power of laser as well as the sensitivity of detector, which makes it hard to increase the operating range. In this paper, a set of pulsed heterodyne LADAR design is proposed by combining efficient radio frequency modulation in laser pulses and pulsed heterodyne detection. Finally, the SNR of this design is analyzed, which shows a superiority of 7dB to 12dB over conventional incoherent pulsed LADAR and a better appeal to long-range detection.

Weibull Wind-Speed Distribution Parameters Derived from a Combination of Wind-Lidar and Tall-Mast Measurements Over Land, Coastal and Marine Sites

Wind-speed observations from tall towers are used in combination with observations up to 600 m in altitude from a Doppler wind lidar to study the long-term conditions over suburban (Hamburg), rural coastal (Hovsore) and marine (FINO3) sites. The variability in the wind field among the sites is expressed in terms of mean wind speed and Weibull distribution shape-parameter profiles. The consequences of the carrier-to-noise-ratio (CNR) threshold-value choice on the wind-lidar observations are revealed as follows. When the wind-lidar CNR is lower than a prescribed threshold value, the observations are often filtered out as the uncertainty in the wind-speed measurements increases. For a pulsed heterodyne Doppler lidar, use of the traditional –22 dB CNR threshold value at all measuring levels up to 600 m results in a $$\approx $$ 7 % overestimation in the long-term mean wind speed over land, and a $$\approx $$ 12 % overestimation in coastal and marine environments. In addition, the height of the profile maximum of the shape parameter of the Weibull distribution (so-called reversal height) is found to depend on the applied CNR threshold; it is found to be lower at small CNR threshold values. The reversal height is greater in the suburban (high roughness) than in the rural (low roughness) area. In coastal areas the reversal height is lower than that over land and relates to the internal boundary layer that develops downwind from the coastline. Over the sea the shape parameter increases towards the sea surface. A parametrization of the vertical profile of the shape parameter fits well with observations over land, coastal regions and over the sea. An applied model for the dependence of the reversal height on the surface roughness is in good agreement with the observations over land.

Chirped pulse heterodyne for optimal beat note detection between a frequency comb and a continuous wave laser.

Chirped pulse heterodyne is proposed to maximize the signal-to-noise ratio (SNR) when measuring the beat note between an optical frequency comb and a continuous wave (CW) laser. The noise model reveals that all the comb power within the largest possible detection bandwidth can be used to increase the SNR. The chirped comb/CW interference experiment is shown to be equivalent to CW/CW interference, using the comb's spectrally available power. The approach can also greatly alleviate dynamic range issues when detected pulsed heterodyne signals. A beat note SNR of 68.3 dB in a 100 kHz bandwidth is achieved.

光子脉冲外差探测系统的测距精度

光子脉冲外差探测系统的测距精度

Study on electro-optic Q switch for <inline-formula><math display="inline" overflow="scroll"><msub><mi>CO</mi><mrow><mn>2</mn></mrow></msub></math></inline-formula> waveguide lasers

We describe some of the characteristics of intracavity CdTe Q-switch modulation in a CO2 laser, which could be used in a pulsed heterodyne imaging lidar. We analyze the thermal influence of intracavity CdTe crystal on output power stabilization and frequency shift, and con- clude that sufficient cooling and temperature control on CdTe modulation is the vital requirement for laser stabilization both in power and fre- quency. We also calculate the frequency chirps due to CdTe thermal index varieties during individual Q-switch pulse duration. The results show that the chirps may not obviously affect the passive frequency stabilization of the laser. For this laser, Q-switch pulses with peak power 360 W and full width at half maximum (FWHM) of 150 ns at 41.67 KHz are achieved. The experimental results also indicate that the passive frequency stabilization of the laser is better than 1.76310 28 and power stabilization in the EH11 mode is better than 5% for more tha n1ho f operation. © 2003 Society of Photo-Optical Instrumentation Engineers.

Investigation of the pulsed heterodyne of an electro-optically Q-switched radio-frequency-excited CO_2 waveguide laser with two channels

We present the results of our theoretical and experimental investigation of the pulsed heterodyne of an electro-optically Q-switched radio-frequency-excited CO(2) waveguide laser with two channels. We used a six-temperature model for CO(2) lasers to describe the process of dynamic emission in an electro-optically Q-switched laser. The calculated pulse waveforms are in good agreement with the experiment, which we used as the basis for our theoretical study of a pulsed laser heterodyne and its Fourier transform frequency spectrum. The experimental results are consistent with the theoretical analyses.

Collective Thomson Scattering Diagnostic for Fast Ion and Deuterium to Tritium Ratio Measurement in Open Magnetic Systems

An advanced diagnostic technique to measure the ion temperature and fast ions in open magnetic field systems is described. The method for the determination of deuterium to tritium ratio and the helium ash density in DT burning plasmas is also proposed. The measurement is made by small angle collective Thomson scattering (CTS) using a high power pulsed CO2 laser and heterodyne receiver system. The axial profiles can be measured by an axially injected beamline. Attenuation and refraction in the plasma are unimportant even for large devices. Scattered spectrum of the expected DT fusion plasma with currently developed laser and receiver system is presented. The component of scattered laser power nearly perpendicular to the magnetic field giving rise to ion cyclotron modulation of the scattered spectrum. Spectrum from pure deuterium and from D-T mixed plasma show the possibility of fuel ratio measurement. The scattered spectrum of D-T plasmas with thermalized helium ash is also calculated. Experimental arrangement to improve the S/N ratio in low density open magnetic systems are also discussed. The possibility of ion temperature and modulation measurement using a homodyne receiver system in GAMMA 10 are also evaluated.

Four-element receiver for pulsed 10-µm heterodyne Doppler lidar

A four-element photomixer receiver has been tested in a 10-mum heterodyne Doppler lidar. It addresses a reduction of the variance of the power scattered off distributed aerosols targets at ranges as long as 8 km. An improvement in performance is expected when the four independent signals recorded on every single shot are combined. Two summation techniques of the four signals have been implemented: a coherent summation of signal amplitude and an incoherent summation of intensities. A phasing technique for the four signals is proposed. It is based on a more suitable correlation time with discernible self-consistent packets (SCP's). The SCP technique has been successfully tested, and the results obtained with a coherent summation of the four signals, i.e., variance reduction, carrier-to-noise ratio improvement, and velocity accuracy improvement, are in agreement with theory.

Application of upconversion detection to pulsed CO_2 lidar

In this paper the application of an upconversion detector to pulsed CO(2) lidar is investigated. In this device a nonlinear IR crystal would be used to convert 10-microm lidar radiation into the visible region for conventional detection with a photomultiplier tube. A pulsed CO(2) lidar can be substantially improved with an upconversion detector configured for rejection of thermal background radiation using a narrowband filter for the up-converted signal or a cold filter front end. The sensitivity of the upconversion detector with the narrowband visible wavelength filter is estimated to be 2 orders of magnitude better than that of the usual direct detection diode. The cold filter can improve upconversion detection to nearly the signal-shot limit. These upconversion detectors are not limited by speckle noise as is a pulsed heterodyne detector.

Blood flow in arteries determined transcutaneously by an ultrasonic doppler velocitymeter as compared to electromagnetic measurements on the exposed vessels

We have compared the instantaneous mean blood velocity measured transcutaneously from total cross-sections of arteries in dogs by the use of a new pulsed heterodyne ultrasound doppler equipment, with instantaneous blood flow as measured simultaneously by an electromagnetic flowmeter applied on the exposed vessels. The experiments were performed on the common carotid and the femoral arteries in a variety of flow situations, always with a close fit between the two curves. The results show that the output from the ultrasound equipment is proportional to flow.

Infrared laser radar technique using heterodyne detection for range-resolved sensing of air pollutants

An infrared, pulsed heterodyne laser radar using differential absorption technique with tunable lasers is proposed as a practical means of measuring dispersed air pollutants with range resolution. A system analysis shows that this scheme offers an adequate sensitivity for ranges up to several km.