We introduce a light steering technology that operates at megahertz frequencies, has no moving parts, and costs less than a hundred dollars. Our technology can benefit many projector and imaging systems that critically rely on high-speed, reliable, low-cost, and wavelength-independent light steering, including laser scanning projectors, LiDAR sensors, and fluorescence microscopes. Our technology uses ultrasound waves to generate a spatiotemporally-varying refractive index field inside a compressible medium, such as water, turning the medium into a dynamic traveling lens. By controlling the electrical input of the ultrasound transducers that generate the waves, we can change the lens, and thus steer light, at the speed of sound (1.5 km/s in water). We build a physical prototype of this technology, use it to realize different scanning techniques at megahertz rates (three orders of magnitude faster than commercial alternatives such as galvo mirror scanners), and demonstrate proof-of-concept projector and LiDAR applications. To encourage further innovation towards this new technology, we derive theory for its fundamental limits and develop a physically-accurate simulator for virtual design. Our technology offers a promising solution for achieving high-speed and low-cost light steering in a variety of applications.
Our technology uses the acousto-optic effect to turn a transparent medium, such as water, into a programmable optic that steers an incident light beam. Sound is a pressure wave that travels inside a medium by compressing and rarefying it, spatiotemporally changing the medium density. In turn, this changes the refractive index of the medium, which is proportional to the density. We design the pressure profile of the sound wave so that, at any time instant, the spatially-varying refractive index makes the medium behave as a periodic set of virtual gradient-index (GRIN) lenses, each with an aperture equal to the sound wavelength. The GRIN lenses bend light beams incident on the medium, with the GRIN profile determining the beam trajectory. These lenses travel at the speed of sound (1.5 km/s in water) and are reconfigurable at MHz frequencies, allowing us to steer light faster than mechanical devices. To enable flexible steering patterns, we combine this optic with a pulsed laser with a programmable pulse rate. By synchronizing the laser source with the sound waveform, and modulating the phase of the sound waveform, we control both the speed of beam steering and the location of the beam.
We show the (a) schematic and (b) prototype built for showing the proof-of-concept applications and comparing them with a galvo mirror system. We diverge the laser beam, and the expanded beam is focused by the ultrasonically-sculpted refractive index. The beam passes through the galvo mirrors onto the scene. We steer the beam either with the ultrasonically-sculpted lens or the galvo mirrors, but not both, depending on the experiment. The reflected light from the object takes the same path back to the sensor. The SPAD sensor, which is colocated with the laser, does not have any optics in front of it other than the ultrasonically-sculpted lenses. This setup allows us to compare the scanning speed of our system with galvo mirrors, while keeping the aperture the same.
We compare our light steering system with commercially available galvo mirrors, to demonstrate the speed and the new capabilities our system enables. We keep the field-of-view and aperture same for both systems.
We compare our beam steering technique with commercially available galvo mirrors (GVS-212). The "A" shape is made up of 100 points. With a 1 MHz transducer, we are able to project a million points per second (pps), and hence, project ten thousand "A"s per second. Constrained by the laser's low beam power (20 μW at 1 MHz), practically we can only capture "A" at 1 ms exposure. The commercially available galvo mirrors, which are only rated at 1 kpps, only project a streak when driven at 1 kHz, 2 kHz, and 5 kHz. At 50 ms exposure and 1 kHz scan rate, the galvo mirrors only project half the pattern at the rated 1 kHz, and at higher frequencies, the galvo mirrors project a corrupted pattern as we are operating them well beyond their 1 kpps rating.
Each of the above four scenes has two characters, "CV", "PR", "20", "23". "C" and "P" are at approximately 160 cm depth, "2" at 170 cm, and the remaining at 180 cm. The top row shows the peak of the transient measured by the SPAD, and the bottom row shows the depth map in cm. We scan the scene at 100×100 resolution using the raster scanning technique we proposed for an exposure of one second. The Thorlabs galvo mirrors are not capable of scanning these scenes in under a second.
Scanning speed vs. aperture size: For the arbitrary point projector, the scanning speed (points per second, pps) is equal to the frequency (fus) of the ultrasonic transducer voltage. The aperture of the ultrasonically-sculpted lens is equal to the ultrasound wavelength (λus). Therefore, the product of scanning frequency and the lens aperture is always less than or equal to the speed of sound (cus). Increasing the scanning speed decreases the aperture of the ultrasonic lens. This decrease in the aperture size is not a problem for projector applications, but for LiDAR applications, the decrease in aperture decreases light throughput.
Diffraction limit: The numerical aperture of the ultrasonically-sculpted lens is approximately nλus⁄2F, where F is the focal length of the ultrasonically-sculped waveguide. Therefore, the diffraction-limited spot size is Δx≈λfusF⁄ncus, where n is the refractive index of the medium and λ is the wavelength of light.
Scanning speed vs. spatial resolution Spatial resolution (Δx) is inversely proportional to the diffraction-limited spot size, and temporal resolution (ΔT, inversely proportional to scanning speed) is determined by the frequency of the transducer. So, we have the following uncertainty principle between spatial and temporal resolution: ΔxΔT≤λF⁄ncus.
|Parameter||ultrasound speed (cus)||ultrasound frequency (fus)||wavelength of light (λ)||refractive index (n)|
|scan speed||increases||increases||decreases||no effect|
|aperture||increases||decreases||no effect||no effect|
Solid media are preferrable: In this paper, we used water as a medium. We could use polymers to improve the form factor of the system. The speed of sound is typically higher in polymers such as epoxy, which is a desirable quality based on the above table. Unfortunately, epoxy softens at higher pressure and temperature, limiting the range of voltages we could apply and observe the acousto-optic effect. Building transparent, compressible, and temperature-resistant polymers will lead to solid prototypes that have higher speed of sound and hence higher scanspeed, aperture, and spatial resolution.
We thank Hossein Baktash and Lloyd Lobo for help with the hardware prototype, and Andre Nascimento and Amy Lee for help with the physics-based renderer. This work was supported by NSF awards 1730147, 1900821, 1900849, 1935849, gifts from AWS Cloud Credits for Research and the Sybiel Berkman Foundation, and a Sloan Research Fellowship for Ioannis Gkioulekas.