Flexible method for shaping the laser beam extends the depth of focus for OCT imaging

WASHINGTON — Researchers have developed a new method to flexibly create a variety of needle-shaped laser beams. This long, narrow beam can be used to enhance optical coherence tomography (OCT), a non-invasive and versatile imaging tool used for scientific research and various types of clinical diagnosis.

“Needle-shaped laser beams can effectively expand the depth of focus of OCT systems, improving lateral resolution, signal-to-noise ratio, contrast, and image quality over a long depth range,” said study team leader Adam de la Zerda of the Stanford University School of Medicine. . “However, before now, the application of certain needle-shaped beams was difficult due to the lack of a common and flexible generation method.”

In Optica, the Optica Publishing Group’s journal for high-impact research, researchers describe their new platform for creating needle-shaped beams of different lengths and diameters. It can be used to create different types of beams such as one with a very long depth of field or one that is smaller than the light diffraction limit, for example.

The needle-shaped beams produced by this method can be useful for a variety of OCT applications. For example, utilizing long and narrow beams can enable high-resolution OCT retinal imaging without any dynamic focusing, making the process faster and thus more comfortable for the patient. It can also expand the depth of focus for OCT endoscopy, which will increase the accuracy of diagnosis.

“The fast high-resolution imaging capability of the needle-shaped beam can also eliminate the adverse effects that occur due to human movement during image acquisition,” said the paper’s first author, Jingjing Zhao. “It can help determine melanoma and other skin problems using OCT.”

Flexible solution

As a non-invasive imaging tool, OCT has constant axial resolution throughout its imaging depth. However, its axial resolution, which is determined by the light source, has a very small depth of focus. To overcome this problem, OCT instruments are often constructed so that the focus can be moved along the depth to capture a clear image of the entire area of ​​interest. However, this dynamic focusing can make imaging slower and not work well for applications where the samples are not static.

OCT typically uses an objective lens that produces a single focal point with a short single focal depth. To increase the depth of focus, the researchers used a diffraction optical element known as a phase mask that uses microstructures to create various light patterns that produce multiple focal points along the axial direction. They designed a phase mask with randomly distributed and specially patterned groups of pixels to create a new focus that is different from the original. The entire mask phase can then be used to produce densely spaced foci in the axial direction, forming needle-shaped beams with long focal depths.

“Flexibility is the main advantage of this new approach,” Zhao said. “Both the beam length and diameter can be changed flexibly and accurately by modifying the focus location and the phase difference between any two adjacent foci.” This flexibility was made possible thanks to the computational models that the researchers developed to reveal the relationship between beam properties and design parameters of multiple foci in a precise quantitative manner. They also developed high-performance fabrication procedures to fabricate diffraction optical elements based on model calculations.

Choose the right block

To test their model, the researchers created beam shapes suitable for imaging different types of samples. For example, to image individual cells in all layers of the human epidermis, they created a needle-shaped beam with a diameter of less than 2 microns (cellular resolution) and a length of at least 80 microns (epidermal thickness). They were also able to capture dynamic, high-resolution images of heartbeats in live drosophila larvae, which are important model organisms for studying heart disease. This requires a beam that is 700 microns long and 8 microns in diameter to visualize organ structures over a long range of depths.

The researchers are now working to improve the approach by replacing the objective and diffraction optical elements currently used to fabricate needle-shaped beams with single flat metalens based on their model. These metalens can be placed on the skulls of mice to observe the dynamics of neurons in the brains of mice in real time, for example.

New jobs may also find applications beyond improving OCT. “The needle-shaped beam can be used to increase the resolution of all microscopy systems, including particle manipulation with optical tweezers, materials processing, confocal microscopy, multiphoton microscopy, photolithography, and photoacoustic tomography,” Zhao said. “Our model can also be applied to electromagnetic waves for terahertz imaging and even mechanical waves used in ultrasound imaging.”

Papers: J. Zhao, Y. Winetraub, L. Du, A. Van Vleck, K. Ichimura, C. Huang, SZ Aasi, KY Sarin, A. de la Zerda, “A flexible method for producing needle-shaped beams and their application in optical coherence tomography,” 9.7.

DOI: 10.1364/OPTICA.456894.

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