Heterodyne Doppler 1-microm lidar measurement of reduced effective telescope aperture due to atmospheric turbulence

Coherent Two-Photon LIDAR with Incoherent Light

Coherent light detection and ranging (LIDAR) offers exceptional sensitivity and precision in measuring the distance of remote objects by employing first-order interference. However, the ranging capability of coherent LIDAR is principally constrained by the coherence time of the light source determined by the spectral bandwidth. Here, we introduce coherent two-photon LIDAR, which eliminates the range limitation of coherent LIDAR due to the coherence time. Our scheme capitalizes on the counterintuitive phenomenon of two-photon interference of thermal light, in which the second-order interference fringe remains impervious to the short coherence time of the light source determined by the spectral bandwidth. By combining this feature with transverse two-photon interference of thermal light, we demonstrate distance ranging beyond the coherence time without relying on time-domain interference fringes. Moreover, we show that our coherent two-photon LIDAR scheme is robust to turbulence and ambient noise. This work opens up novel applications of two-photon correlation in classical light.

Heterodyne high-spectral-resolution lidar

In this work, a novel lidar technique to perform high-spectral-resolution measurements of the atmospheric backscatter is discussed and the first results are presented. The proposed method, which relies on a heterodyne detection receiver, allows us not only to separate the molecular and the aerosol component of the atmospheric backscatter, but also to investigate the spectral shape of the Rayleigh-Brillouin line. As in the case of the direct-detection high-spectral-resolution lidars, the separation of the different scattering processes would allow an independent system calibration and aerosol extinction measurements. The proposed retrieval technique was successfully tested on the Deutsches Zentrum für Luft- und Raumfahrt airborne Doppler wind lidar system with measurements conducted during different measurement campaigns and under different atmospheric conditions. In light of these results, further ideas for the implementation of a dedicated heterodyne high-spectral-resolution lidar are discussed.

Research on heterodyne detection of a mode-locked pulse laser based on an acousto-optic frequency shift

Heterodyne detection research is carried out on a mode-locked pulse laser using an acousto-optic frequency shifter (AOFS). Theoretical calculation and numerical simulation of the beat frequency of the mode-locked pulse were carried out, and the results show that the frequency offset can be calculated according to the beat period of the pulse waveform, and the offset is the frequency difference between the two adjacent longitudinal modes. Experiments were implemented with a self-built mode-locked laser using an AOFS to simulate the frequency shift of the detection laser induced by the object, the coherent beat frequency of the signal light and local oscillation light that were observed on the surface of the detector. The beat frequency wave was processed by filtering to obtain the frequency shift of the signal light. The experimental results are in good agreement with the numerical simulation.

Bistatic coherent laser radar signal-to-noise ratio

We investigate the signal-to-noise ratio (SNR) for a bistatic coherent laser radar (CLR) system. With a bistatic configuration, the spatial resolution is determined by the overlap of the transmit beam and the virtual backpropagated local oscillator beam. This eliminates the trade-off between range resolution and the bandwidth of the transmitted pulse inherent in monostatic systems. The presented analysis is completely general in that the expressions can be applied to both monostatic and bistatic CLR systems. The heterodyne SNR is computed under the assumption of untruncated Gaussian optics and untruncated Gaussian beam profiles. The analysis also includes the effects of refractive turbulence. The results show that, for maximum SNR, small transmit and local oscillator beam profiles (e-1 intensity radius) are desired.

Laser Doppler optical air-data system: feasibility demonstration and systems specifications

A compact laser Doppler velocimeter with diode-pumped solid-state laser technology at 1.06 µm and a novel solid-block beam-splitting interferometer was demonstrated. This system allows for accurate Doppler velocity measurements even in a vibration environment. A comparison of the backscatter measurements with a backscatter sonde from the University of Wyoming showed excellent agreement. Based on demonstrated performance and technology projection, the system specifications of an optical air-data system at 2.015 µm are determined, and a detailed design concept is presented. In addition, the potential for a multifunctional sensor that can determine air data and detect wind shear and wake vortices is addressed.

Effect of atmospheric turbulence on heterodyne lidar performance

The effect of atmospheric turbulence on heterodyne lidar performance is studied by use of scattering theory. A theoretical analysis is carried out for both bistatic and monostatic lidar systems with independently variable transmitter and receiver parameters in regimes of weak and strong intensity fluctuations. The conditions of validity of a diffuse target model for description of the optical wave scattering by aerosols in a turbulent atmosphere are presented. The equations for signal power degradation and the conditions under which the time-averaged output of a heterodyne lidar does not depend on either turbulent conditions of propagation along the path or the transmitter parameters, including transmitter coherence length, are obtained. A physical interpretation of these results is given, and a comparison with the data of previous theories is made.

Effects of refractive turbulence on coherent laser radar

The leading-order effects of refractive turbulence are calculated for a general coherent laser radar with beam-angle and beam-offset misalignment. The effects of refractive turbulence are important for 10-microm operation in the atmospheric surface layer for typical daytime conditions and paths longer than 3 km. The effects for near-infrared and visible wavelengths are more pronounced. The behavior of different parameter regimes are related to the scintillation scales of the transmitted beam. In certain cases, the small-scale scintillation structure produces a measure of beam misalignment that is more sensitive than the free-space case. The phase approximation of the extended Huygens-Fresnel principle is shown to be correct in the limit of large path-integrated refractive turbulence.

Coherent summation of spatially distorted laser Doppler signals by using a two-dimensional heterodyne detector array

We report what is to our knowledge the first demonstration of phase-sensitive coherent summation of individual heterodyne detector array signals for the enhanced detection of spatially distorted laser Doppler returns. With the use of a 2 x 2 heterodyne detector array, the phase and amplitude of a time-varying speckle pattern was detected, and the signal-to-noise ratio of the Doppler shift estimate was shown to be improved by a factor of 2, depending on the extent of spatial coherence loss. These results are shown to agree with a first-order analysis and indicate the advantage of coherent summation for both short-range laser Doppler velocimetry and long-range atmospheric coherent lidar.

Useful receiver telescope diameter of ground-based and airborne 1-, 2-, and 10-microm coherent lidars in the presence of atmospheric refractive turbulence

The integrated effect of atmospheric refractive turbulence on ground-based and airborne 1-, 2-, and 10-microm coherent lidars with different geometries is calculated as a function of height by using altitude profiles of C(n)(2).

Coherent 1-microm lidar measurements of atmospheric-turbulence-induced spatial decorrelation using a multielement heterodyne detector array

We have employed a coherent 1-microm Nd:YAG lidar system to measure directly, for the first time to our knowledge, the reduced spatial coherence length, p(0), of the lidar returns caused by atmospheric turbulence. Our experiments were conducted by using a 2 x 2 heterodyne detector array, which permitted real-time spatial correlation measurements of the lidar returns at two different detector spacings. The spatial correlation coefficients and spatial coherence length of the lidar returns from a hard target weremeasured during a day-to-night time period when the atmospheric turbulence parameter, C(n)(2), was measured to vary from 2 x 10(-13) to 2 x 10(-14) m(-1/3). These directly measured values of p(0) as a function of C(n)(2) were found to be in good agreement with theoretical predictions.

Enhanced detection of atmospheric-turbulence-distorted 1-microm coherent lidar returns using a two-dimensional heterodyne detector array

We have employed a two-dimensional multielement heterodyne detector array and demonstrated, for the first time to our knowledge, the enhanced detection efficiency of atmospheric-turbulence-distorted 1-microm coherent lidar returns. The heterodyne lidar signal intensity and statistical signal fluctuation were measured for both a 2 x 2 detector array and a single detector as a function of the atmospheric turbulence parameter C(n)(2). The detector array improved the lidar detection efficiency by a factor of approximately 4, and the statistical signal distribution changed from Rayleigh to Gaussian. This improvement is shown to be consistent with a firstorder analysis.