Researchers from the Changchun Institute of Optics, Fine Mechanics and Physics of the Chinese Academy of Sciences, in collaboration with the National University of Singapore, have engineered a dynamic "laser needle" to solve a century-old problem in optical imaging.

By utilizing a liquid crystal spatial light modulator (LC-SLM), the team successfully extended the depth of focus for Optical Coherence Tomography (OCT) systems, allowing for high-resolution imaging deep within complex materials without the need for mechanical refocusing. The research results were published in Optics & Laser Technology.

Optical Coherence Tomography is often described as the "optical ultrasound" of the microscopic world. It is a cornerstone technology in eye surgery, skin cancer detection, and the inspection of multi-layered semiconductor chips. However, OCT has long been haunted by a frustrating trade-off: to see finer details (high lateral resolution), the "clear" zone of the image becomes extremely thin. If a sample is thicker than a few hundred micrometers, the deeper structures inevitably appear as a blurry mess. Traditionally, scientists had to physically move the lens or the sample to stay in focus, a process that is slow, imprecise, and unsuitable for rapid industrial pipelines.

To shatter this barrier, the research team designed a system that reshapes a standard laser beam into an ultra-thin, elongated "needle beam." This needle maintains a consistent, tiny diameter over a much longer distance than a conventional laser. The "brain" behind this transformation was a liquid crystal spatial light modulator—a device that acts like a high-tech magic mirror, manipulating the phase of light waves with microscopic precision. By "twisting" the light, the researchers were able to stretch the focal point into a long, continuous line of clarity.

The research process focused on maximizing both the length and the brightness of this laser needle. The team implemented a genetic optimization algorithm to strategically place 64 individual focal points along the beam's path, fusing them into a single, uniform needle. They also addressed a common flaw in liquid crystal devices known as "zero-order diffraction," which usually creates ghost images and uneven lighting. By applying a specialized intensity calibration and tilt-term correction, the scientists ensured that the beam delivered perfectly uniform energy across its entire extended range.

During the experimental phase, the researchers tested their invention on a variety of challenging targets, including resolution charts and multilayer transparent tapes. The results were striking. While a standard OCT beam became blurred after only 788 micrometers, the optimized needle beam maintained a sharp, 14-micrometer resolution across a depth of 5,000 micrometers. When imaging microspheres hidden inside a gel, the new system captured clear details of the beads' bases that were completely lost to traditional imaging methods.

The reality of this breakthrough is far-reaching. In the semiconductor industry, it provides a non-contact method to measure the thickness of multi-layered films with sub-micrometer accuracy. In medicine, it could lead to faster, more accurate biopsies by allowing doctors to see deeper into biological tissue in real-time. By removing the need for mechanical scanning, this "laser needle" paves the way for a new generation of high-speed, high-precision scanners that can see through the layers of our world with unprecedented clarity.