Nanyang Technological University Develops A New Method To Produce Ultra-intense, Ultra-fast Lasers

Currently, lasers emitted in the mid-infrared range can identify substances in the air within minutes - whether they are greenhouse gas pollutants, toxins, explosives or gases associated with diseases found in human breath.
High-power mid-infrared lasers generated in ultrafast pulses are in high demand because they underpin highly sensitive devices that can safely detect from a distance even trace amounts of substances that would otherwise go unnoticed or be difficult to identify.
Recently, scientists led by NTU Singapore have developed a new method for producing intense and ultrafast lasers. This method, they say, "holds the promise of creating precise devices that can speed up the sniffing out of trace contaminants and harmful gases."
However, the current conventional methods for generating such lasers have their own shortcomings: one method requires lab conditions free of disturbances that can misalign precision-calibrated equipment (e.g., vibrations, temperature/humidity variations) - meaning that the lasers can't be used outside the lab.
The other method can generate lasers while coping with environmental disturbances such as vibration, but they are not strong enough to accurately detect trace amounts of substances. New research from Nanyang Technological University has addressed these challenges.
The results are published in Lasers ＆ Photonics Reviews.
The researchers used specially fabricated hollow optical fibers to generate very bright laser light in the mid-infrared range by adjusting the thickness of the fiber's neutron structure.
Chang Wonkeun, an assistant professor at Nanyang Technological University's School of Electrical and Electronic Engineering, who led this latest study, said, "Our approach paves the way for the development of portable, powerful and fast mid-infrared laser generators that do not rely on well-controlled and vibration-free environments to maintain operation. "
"This means we can pair them with detectors and use them in the field to help test and identify a wide variety of unknown substances. At the same time, there is no need to spend extra time sending samples to the lab for testing, even in trace amounts."
Advantages of detection
Mid-infrared lasers, with wavelengths ranging from 2um-20um, offer advantages over other lasers in detecting substances. Many different types of molecules absorb lasers in the mid-infrared range in a unique way, more than other wavelengths, a characteristic that can be used to identify unknown substances. In addition, even if water is present in these substances, unlike other lasers, the accuracy of identifying substances using mid-infrared lasers is not affected by water molecules.
One way to generate high-power mid-infrared lasers in rapid bursts is to - emit bright and ultra-fast near-infrared radiation through an optical fiber, which has a short wavelength. Mid-infrared lasers produced by optical fibers with solid glass centers are usually not very strong, which makes accurate detection of small amounts of matter difficult.
In order to produce high-intensity mid-infrared lasers, an interference-free environment is usually required, which limits the use of lasers to the laboratory and makes it difficult to realize specific applications. Professor Zhang of Nanyang Technological University solved these problems by using hollow glass fibers. He discovered this when he used computer simulations to determine the type of near-infrared radiation that might be produced when it passes through the hollow fibers.
Wavelength conversion
Unlike conventional optical fibers, this tubular hollow fiber has a ring of smaller glass tubes around the hollow center of the fiber. Simulations showed that by varying the wall thickness of the fiber's miniature tubes, it would be promising to convert near-infrared lasers into powerful, ultrafast mid-infrared lasers.
His team then conducted experiments in which the centers of the hollow core fibers were filled with argon gas, and the scientists were able to confirm the predictions of the simulations. They created a mid-infrared laser with peak power in the megawatt range and a wavelength of 3um-4um, a million times more powerful than a standard light bulb.
This laser conversion occurs because the near-infrared laser interacts with the shape of the optical fiber, which transforms the laser into mid-infrared light by exciting argon gas molecules. The thickness of the microtubes is related to a little more than twice the wavelength of the mid-infrared laser light produced - so a minitube with a wall thickness of 1.6um produces laser light at a peak wavelength of about 3.7um.
Prof. ssambastien fsamvrier (from the University of Limoges), a long-time researcher on mid-infrared lasers, said the Nanyang Technological University team's method of generating lasers "contrasts sharply with devices that usually involve complex nonlinear arrangements".
Prof. ssambastien fsamvrier said, "In addition, since optical fibers can be spliced with each other, these results pave the way for the generation of mid-infrared lasers that are not affected by any moving mechanical parts."