Optica Publishing Group recently published a study on a quantum interference-inspired approach to light detection and ranging (LiDAR) that achieves optical coherence tomography (OCT) depth resolutions without requiring high levels of stability. This increased resolution allows the use of LiDAR for 3D facial recognition, small feature detection, and tracking.
OCT is a 3D imaging method that gives micron-scale depth resolution in bio-imaging. The optical coherence tomography's resolution exceeds the LiDAR's imaging resolution, which is often limited to millimeter scale resolution because of the impulse response of the detection electronics. However, optical coherence tomography is not feasible in incoherent systems.
Significance of Optical Coherence Tomography Technology in Bio-Imaging
OCT is a 3D imaging technology that obtains depth information from the interference of light sources with short coherence lengths. Due to its capacity to produce an image within a scattering medium, OCT is utilized in many bio-imaging applications, such as cardiology, ophthalmology, dermatology, and oncology.
Every interface within a medium, often biological tissue, produces a reflection that interferes with a reference field. The position of these interference maxima helps calculate the distance light has traveled, the Time-of-Flight (ToF), and the medium depth. In this manner, high-resolution images of tissues or organs are produced.
Optical coherence tomography obtains high-resolution retinal cross-sectional pictures. Layers inside the retina can be distinguished, and the thickness of the retina can be assessed to aid the diagnosis or early detection of retinal disorders and diseases.
Optical coherence tomography testing has become standard for the diagnosis and treatment of the majority of retinal diseases. OCT uses light beams to determine the thickness of the retina. This test uses no radiation or X-rays, and the scan does not harm or cause discomfort to the patient.
Coherence Nature of Optical Coherence Tomography
OCT is an inherently coherent process since it relies on detecting the interference fringes resulting from the optical field's cyclic phase regardless of whether they are detected in the spectral or temporal domain.
As a result, it cannot be used in incoherent systems, such as circumstances in which there is mechanical movement on the wavelength scale throughout the acquisition time. In LiDAR, these conditions frequently prevent interferometric stability from being attained, and even slight variations in air currents might result in a significant phase distortion.
Resolving depth profiles at the millimeter scale and lower is difficult with conventional single photon LiDAR because the detector's timing jitter is in the range of tens of picoseconds or larger for SPADs, limiting the technology.
Using Two-Photon LiDAR Technology to Improve Depth Resolution
The research explores the quantum interference-inspired approach to LiDAR that obtains OCT depth resolutions without requiring high degrees of stability.
The possibility of employing two-photon interference between weak coherent states of light for LiDAR applications was investigated. This could provide OCT's micron-scale depth resolution while simultaneously being immune to phase noise and capable of operating in high-loss settings.
The study used the ability of two photons to become entangled at a beam splitter in an interferometer to quantify two-photon interference by sending one pair into the three-dimensional environment while holding the other in a controlled delay line.
A galvanometer mirror system scans the beam in the transverse plane, and full 3D imaging is produced.
A further measurement was carried out to record each detected photon's individual time tags and investigate the correlations between surrounding pulses up to a decorrelation time.
Important Findings of the Study
The depth imaging system had an effective impulse response of 70 microns. This would make it possible to distinguish between ranging and multiple reflections with significantly higher resolution than with traditional LiDAR systems.
Detecting photon pair correlation-states provides a more accurate measurement of the photon's return time than conventional interference, resulting in a higher depth resolution.
The theoretical depth resolution was as small as approximately 7 microns. The axial resolution can be boosted further by using shorter pulses.
The depth scan was completed in 75 steps with a 50 ms integration time per point, resulting in an image acquisition time of roughly ten hours. However, the proposed two-photon approach decreases the acquisition time to 36s while increasing the amount of photon correlation resources.
Due to second-order dispersion, the 100 fs pulse broadens by about 14 fs during 100 m propagation in air. This can be corrected by either the reference or signal arm and optimized without knowing the propagation distance.
Removing the transverse scanning components in favor of a full-field imaging system can lead to further advancements.
This research paves the way for high-precision LiDAR imaging and ToF sensing devices such as non-line-of-sight imaging and imaging through scattering media.
Murray, R., & Lyons, A. (2022). Two-photon interference LiDAR imaging. Optics Express, 30(15), 27164-27170. https://opg.optica.org/oe/fulltext.cfm?uri=oe-30-15-27164&id=478802
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