Coherent Doppler Lidar
Coherent Doppler lidar provides a means of optically sensing an object’s range as well as its relative velocity along the line-of-sight (LOS). In one particularly powerful application of this technique, coherent Doppler lidar of aerosols in the air can be used to measure down range wind speeds. By measuring the wind speeds along multiple widely spaced LOS, the 3D wind vectors for a volume of air can be calculated. Such information is extremely useful in monitoring wind fields near airports, planning wind farms, and even controlling individual wind turbines.
Because 3D wind sensing lidar requires a small number of widely spaced angles, excellent wavefront quality, and is inherently polarization sensitive, the application is very well suited to using liquid crystal polarization grating (LCPG)-based non-mechanical beam steering. Conventional 3D wind sensing lidar systems rely upon motorized optics or multiple telescopes to provide the required LOS. Using LCPGs, a single transmissive window can generate the LOS while reducing the required size, weight, and power (SWaP).
- Automotive Sensors
- Wind Power Generation
The Challenge
Interest is increasing in integrating 3D wind sensing lidar into SWaP-constrained platforms. For scientific markets, the costs of aerial wind profiling studies could be greatly reduced by using small drones in place of large aircraft. For energy generation markets, wind turbine use can be better optimized with 3D wind sensing integrated into turbine nacelles. One of the critical barriers to this miniaturization is the generation of multiple LOS without resorting to many bulky telescopes or power-hungry motors.
In this effort, we sought to demonstrate that the LCPG beam steering technology provides the efficiency and wavefront accuracy required to meet the needs of coherent Doppler lidar of weakly backscattering aerosol particles.
A block diagram of the coherent lidar system, which relies heavily upon readily available fiber optic components.
Photograph of the coherent Doppler lidar system.
Photograph and schematic illustration of the 4-angle beam steerer used for indoor testing of the non-mechanically steered coherent Doppler lidar system.
The Outcome
We built a frequency modulated continuous wave (FMCW) coherent Doppler lidar system using commercially available fiber optic components and a custom telescope with 4 cm final aperture. This system was characterized indoors using a spinning disk target to assess carrier-to-noise ratio (CNR) with and without LCPG non-mechanical beam steering. The LCPG steering device provide 4 different LOS, which were kept small (±1.4°) in this study to facilitate the indoor tests
- 4
steering angles - 90%
single-pass optical efficiency of LCPG steerer - <λ/60
steered wavefront error (RMS) - <1 dB
total impact of LCPG steerer on lidar CNR
During component characterization, the LCPG steering device was measured to provide 90% of the incident light into the desired steered order, with most of the remainder being absorbed in the liquid crystal cells. Additionally, laser grade end-caps were used in the assembly, resulting in < λ/60 RMS steered wavefront error at 1550 nm across all steering angles. When incorporated into the lidar, the total impact on the lidar CNR was < 1 dB, which supports that there was no degradation to the coherent detection beyond the conventional insertion loss measured on during the component characterization.
Even though roughly 0.5% of the incident light at each LCPG layer is “leakage” that does not get steered, this light is orthogonally polarized to the signal beam and therefore we found no measurable coherent signal generated from the leakage (>30 dB suppression).
Our Approach
The goal of this study was to validate the impact of LCPG beam steering on the sensitive measurement technique of coherent Doppler lidar. To this end, we built and then characterized a FMCW coherent Doppler lidar system as a baseline reference. Similarly, we built and characterized a 4-angle LCPG steering device. Using these data sets, we compared the lidar performance with the LCPG steering device to their independent contributions and assessed the ultimate impact of non-mechanically steering the lidar.
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Build and Characterize FMCW Lidar
We built a monostatic FMCW lidar to characterize the impact of LCPG steering on such systems. Briefly, the system used a 1550 nm single-frequency laser diode, split into a transmit beam and local oscillator. The transmit beam was delivered via commercially available fiber optic components to a telescope with 4-cm clear aperture. This lidar was characterized on an optical bench using a spinning disk target to establish a baseline performance reference. Due to the low-power fiber laser used, the transmitted power was limited to 11 mW and the lidar provided a CNR of 29.7 dB under the stated test conditions.
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Schematic representation of the coherent lidar breadboard used to evaluate PG beam steering contributions to phase noise and insertion loss in a Doppler detection application.
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Fabrication and Characterization of LCPG Steering Device
We built a simple two-stage LCPG steering device with 4-cm clear aperture capable of providing ±1.4° in X and Y, suitable for indoor measurements over modest distances. The individual LCPGs used measured >99.5% diffraction efficiency and < λ/75 RMS diffracted wavefront error. An additional LC cell was used at the exit aperture of the PG steerer and driven to act as a quarterwave retarder to ensure that light propagated to the target in a linearly polarized state and was returned to a circularly polarized state upon backscatter to the PG steerer. The total efficiency of the LCPG steering device was measured to be 90% with steered wavefront error varying between λ/61 - λ/84 RMS at 1550 nm, with the variation arising from addition or subtracting of individual LCPG wavefront errors.
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Steered wavefront error of a LCPG used in this study, showing λ/31 RMS wavefront error at 632.8 nm, or λ/76.5 RMS at the design wavelength of 1550 nm.
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Characterize Impact of LCPG Steering Device on the FMCW Lidar
Using the same test setup as in Step 1, we measured the CNR from the spinning disk target with the LCPG beam steering device deflecting the lidar onto and off of the target. We found the measured CNR to vary slightly between angles to a minimum CNR of 28.9 dB, within 1 dB of the native lidar CNR (29.7 dB). This is in keeping with the measured bi-directional insertion loss of the LCPG beam steering device (0.85 dB). This indicates that there were no additional losses from other aspects of LCPG beam steering, such as polarization, wavefront error, or LC phase ripple effects.
Additionally, we analyzed lidar returns when the lidar was steered off of the target in order to search for “ghost” signals that might be generated from light coupling into the unsteered orders of the LCPG device. Because this light is overwhelming of the opposite polarization to the desired steered beam, no coherent signal contribution was found.
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CNR measurements through the two-axis beam steering stack. (Left) Representative Doppler return with steered beam on target. (Right) Measured Doppler signal with beam steered off of target, showing contribution of any leakage beam to signal. No contribution is detectable.
The Final Results
This study demonstrated the versatility of the LCPG beam steering technology in several ways. First, it showed the capability of steering monostatic sensors through a common non-mechanical beam steering device. Second, it demonstrated that the wavefront quality and polarization control are sufficient for sensitive coherent lidar measurements without unexpected signal degradations. Lastly, it demonstrated that LCPG steering can be an excellent fit for generating multiple LOS in 3D wind sensing applications.
