Single-Photon Detectors Could Improve Low-Power OCT Sensitivity

A detection technology used in quantum optics also could be used to perform OCT (optical coherence tomography) with lower light power than previously possible, potentially improving the imaging quality available from OCT.

“For clinical applications, being able to perform OCT with low light power is crucial because safety standards limit the light intensity levels that can be used,” research team leader Sylwia Kolenderska, from the University of Auckland, said. “In some cases, these power levels are not high enough to achieve good image quality.”

While developing an OCT method based on quantum light, researchers at the University of Auckland discovered that superconducting single-photon detectors (SSPDs) could be used in a standard OCT arrangement to enhance sensitivity. When they replaced standard OCT detectors with SSPDs, they found that this setup allowed good image quality with power levels up to 1 million times lower than those currently used in OCT instruments.

Researchers used a technology borrowed from quantum optics to perform optical coherence tomography (OCT) with much lower light powers than previously possible. Two views of their optical setup are shown. Courtesy of Andrzej Romanski.

Incorporating SSPDs into a standard OCT system required some changes to the typical optical setup. Modern OCT instruments work by discerning the wavelengths reflected from an object. This wavelength discrimination can be performed by using a single pixel detector while the light source produces one wavelength at a time, or it can be done with a diffraction grating that splits the light into different wavelengths and a camera that detects these wavelengths.

The researchers used a fiber instead of a grating to separate different wavelengths, which traveled at different speeds down the fiber. At the fiber’s output end, they used the SSPD to capture the different wavelengths as they arrived. This allowed the researchers to acquire the light spectrum they needed to reconstruct OCT images.

To demonstrate the new detection scheme, the researchers acquired OCT images of a stack of three types of glass and a piece of onion, which represented a biological sample. They obtained good-quality images of both samples at light intensity levels at least five orders of magnitude lower than those set by safety standards.

“Our results show that the new detection approach could allow quality OCT imaging of different parts of the body, especially sensitive organs such as the eyes, without worrying about going above the safety levels in terms of light power,” Kolenderska said. “In fact, the SSPD would be damaged beyond repair long before even 1% of the safety level is reached.”

The researchers did, however, observe artifacts in the OCT images they acquired. These appeared because the system detected all kinds of interactions between photons, not just the ones needed to reconstruct an actual image. The researchers are experimenting to find the best way to prevent these artifacts without compromising imaging speed, which would be important to maintain for clinical applications.

The research was published by Optics Letters, a publication of OSA, The Optical Society (www.doi.org/10.1364/OL.393162).