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Resonant Photoelastic Modulation for Time-of-Flight Imaging

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Time-of-flight (ToF) imaging is an imaging modality that has found wide-spread use in areas such as machine vision, robotics, target tracking, and autonomous vehicles. Many of the areas that use ToF imaging systems need these imaging systems to have high performance. To employ high performance ToF imaging systems on a large scale, they need to be low cost.

One imaging modality that can achieve cost-effectiveness and high performance is phase-shift based flash lidar. These flash lidar systems consist of an intensity modulated light source and an image sensor. The region corresponding to the field of view of the image sensor (determined by imaging optics), is illuminated with intensity modulated light. Any targets present within this region reflect light, and this light is collected by the image sensor. As light travels from the light source to the target, reflects from the target, and returns to the image sensor, the phase of the intensity modulated light shifts. Analyzing the phase change of received light per image sensor pixel enables a distance map to be reconstructed for the scene.

ToF imaging applications usually require meters level imaging range and centimeter scale axial resolution with high refresh rates. To meet these requirements, the intensity modulation frequency needs to be on the order of megahertz frequencies or greater. Detecting the phase of megahertz modulated light, however, requires specialized image sensors. Specialized image sensors with high spatial resolution are expensive. This makes it difficult to employ specialized image sensors on a large scale.

To significantly lower the cost of the imaging system while retaining the performance of expensive phase-shift based flash lidar systems, we have developed a new optical device that we refer to as a longitudinal piezoelectric resonant photoelastic modulator. This device is capable of converting megahertz level intensity modulation frequencies to hertz level tones while preserving the phase of the megahertz level tones. This allows us to use standard image sensors (such as CMOS and CCD cameras), which are low cost and have millions of pixels, in the flash lidar system.

The modulator that we have designed consists of a thin wafer of lithium niobate coated on both sides with transparent surface electrodes. Lithium niobate is a transparent crystal that is piezoelectric, and we use this effect to modulate light passing through the modulator. Applying an electrical signal that corresponds to the fundamental shear acoustic mode of the wafer confines an acoustic standing wave in the wafer. The polarization of light passing through the wafer is modulated at the applied electrical signal frequency through the photoelastic effect. Sandwiching this polarization modulator between polarizers and placing in front of an image sensor allows light to be intensity modulated at the applied electrical signal frequency.

The modulator that we have designed consists of a simple design and integrates easily with any image sensor. This allows us to add the missing depth dimension to any image sensor with minimal hardware additions.

Modulator design and fabrication: a Lithium niobate wafer coated with transparent surface electrodes and connected to an RF power supply. b Side-view of the modulator. c Fabricated modulator mounted and wirebonded to a PCB. d Simulated shear strain amplitude distribution when the fundamental mode of the wafer is excited through the surface electrodes
Time-of-flight demonstration. a Schematic of the time-of-flight imaging setup. b Dimensions of the targets used for the imaging experiment. c Reconstructed depth map per pixel by the camera.

[1] Atalar, O., Van Laer, R., Sarabalis, C. J., Safavi-Naeini, A. H., & Arbabian, A. (2019). Time-of-flight imaging based on resonant photoelastic modulation. Applied optics, 58(9), 2235-2247.

[2] Atalar, O., Van Laer, R., Sarabalis, C. J., Safavi-Naeini, A. H., & Arbabian, A. (2020, May). Lithium Niobate Resonant Photoelastic Modulator for Time-of-Flight Imaging. In CLEO: Applications and Technology (pp. AM3K-7). Optical Society of America.
[3] Atalar, O., Van Laer, R., Safavi-Naeini, A. H., & Arbabian, A. (2022). Longitudinal piezoelectric resonant photoelastic modulator for efficient intensity modulation at megahertz frequencies. Nature Communications, 13(1), 1-8.
[4] Atalar, O., Yee, S., Safavi-Naeini, A. H., & Arbabian, A. (2022). YZ cut lithium niobate longitudinal piezoelectric resonant photoelastic modulator. arXiv preprint arXiv:2204.04745.