Atmospheric turbulence causes laser scintillation, beam wanders, beam spreading, and angle-of-arrival fluctuation. The atmospheric coherence length (r0) proposed by Fried is a parameter related to the wavefront-phase structure function, an important parameter for characterizing the intensity of atmospheric optical turbulence. Hence, ground-based in-situ measurement of r0 is of great significance for studying the optical wave propagation effect in the atmosphere. The differential image motion monitor (DIMM), a traditional measurement method of r0, has been widely studied in that it is capable of avoiding measurement errors caused by the vibration of the observation and the tracking equipment and unstable tracking. However, in the scene of long-distance optical turbulence measurement, DIMM needs a power supply and working staff to maintain light beacon images in the center of CCD image sensors. In this study, the original passive beacon is converted into an active illumination beacon to realize single-ended optical turbulence measurement inspired by DIMM. This lidar-style r0 measurement still faces the uncertainty of reflecting media in atmospheric turbulence. Therefore, this paper proposes a fold-path r0 measurement method based on laser-active illumination of a 3M reflective film. With the help of a large-area 3M film, the observed area and distance can be enlarged, and the laser source and CCD imaging system can be integrated into one single-ended unit. Upon the construction of this new layout experiment system, the results of the fold-path r0 measurement can help validate the fold-path optical wave propagation model and explore long-range single-ended optical turbulence telemetry.

First, the theory of atmospheric coherence length on the fold path link is summarized. Then, the results of the all-day laser-active illumination imaging data are analyzed through the switching of the active illumination beacon and conventional 650 nm laser beacon for the measurement of r0. In addition, the comparative experiments of the traditional DIMM and fold-path DIMM are conducted on a 1.1 km optical propagation link. The system is mainly composed of the YSL-SC-PRO-M fiber laser, 3M microcrystal prism array, beam expander system, Meade ACF14F8 telescope (the receiver's aperture is Φ=355 mm, and its sub-aperture is Φ=120 mm), and Allied Vison GT 1920 CCD. One DIMM device is adopted to switch between different beacons to measure r0, so as to eliminate the measurement error caused by different DIMM devices. Echo beam wave wander experiments are carried out on the same 1.1 km optical link with an atmospheric coherence meter recording atmospheric optical turbulence conditions to validate the jitter turbulence inversion methods of the active illumination beacon. A CMOS camera (with a resolution of 1936 pixel×1096 pixel) coupled with a telescope (Φ=200 mm, f=1200 mm) is utilized to record the laser speckle images. A 10 nm optical filter is set up in front of the camera to reduce the sky background radiance. All the above units are mounted on a sturdy platform. A square 1 m 3M film pasted on a flat carbon fiber board is placed at a certain distance perpendicular to the laser beam. After that, turbulence-degraded laser speckle images are obtained by the fold path imaging method, and the multi-day laser-active illumination imaging data is analyzed.

Comparative statistical analysis of fold-path DIMM and traditional DIMM measurement results is shown in Fig. 3. The data of the two different methods exhibit good consistency, and fold-path DIMM can also reveal the turbulence strength variation at the transition moment. The comparison with another traditional DIMM measurement result shows that the deviation of results obtained by the two methods is about 2.5%. The measurement result of fold-path DIMM is slightly smaller than that of traditional DIMM (Fig. 4), which means the laser speckle reflected by the prisms embedded in the film has been severely degraded. Thus, the backward transmitted laser speckle should be non-coherent differing from the coherent beacon employed in traditional DIMM. The comparison results of Cn2 inversion by centroid drift and DIMM under the circumstances of unfocused and focused beams indicate that the results of the focused beam are consistent with those of DIMM measurement (Figs. 5 and 6). The coefficient of determination R2 under the unfocused condition is 0.88 while that under the focused condition is 0.94. Therefore, this paper presumes that a large laser divergence angle results in laser energy missing at the observing termination COMS, which is not reflected by 3M films.

In this paper, the 3M reflective film is regarded as a reflective surface with a certain correlation length and root-mean-square height characteristics. With high reflectivity, fine particles, uniform reflection, and easiness of being spliced into a large-area array, the 3M reflective film is employed as a cooperative target of laser-active illumination imaging. Compared with the results of the traditional DIMM, those of fold-path DIMM have an average deviation of 2.5% throughout the day because backward transmitted laser speckles shall be considered non-coherent. The relative position of the laser emission system and the telescope receiving system is fixed, which simplifies the complexity of the experimental optical path aiming. The results show that the coefficient of determination of the image motion method is ca. 0.94, which allows the system to be further simplified for the telemetry of optical turbulence. Additionally, this transceiver method can eliminate the influence of platform vibration. Combined with a pulsed laser and a high-sensitivity camera, it can be applied to measure the atmospheric coherence length at different distances. Relevant experimental results are conducive to validating the fold-path optical wave propagation model and exploring long-range single-ended optical turbulence telemetry.