When the river becomes narrower or the two rivers collide and merge, the speed of the water flow will increase and become stronger. The flow of water and the flow of light are similar in many ways, and this has motivated a group of researchers from Singapore South-South Polytechnic University and University of Nebraska-Lincoln to investigate whether optical flow shows the same characteristics in waveguides. That is, a linear structure that can transmit electromagnetic waves between the endpoints.
According to a report in the journal Applied Physics Letters, they tested the infrared sensing characteristics of this fiber-optic coupler and established a sensor that has a sensitivity that is higher than that of a conventional fiber-based biochemical sensor. 20 Times.
By using a tricky approach to increase the sensitivity of the optical fiber to small changes in refractive index in the surrounding environment, changes in the concentration of molecules on the fiber or near the surface of the fiber can be detected. Although the general idea is well known, this particular method solves an existing sensitivity problem.
The basic concept of the research team's fiber optic coupler sensor is “interference of light and transmission along the fiber coupler,†explains Lei Wei, an assistant professor at the School of Electrical and Electronic Engineering and Engineering at Nanyang Technological University.
Interference is a common natural phenomenon. When light, sound, and even water waves are in the same space, it can happen.
"In our work, when light travels along a fiber, two different fiber-optic conduction modes can be excited in the coupler and interference can occur," Wei added.
Sensors based on optical interference are currently used for many applications including in the field of biometry, and the use of optical fibers makes these devices very compact and economical.
“But it has been reported that one of the drawbacks of most current fiber biochemical sensors is that they exhibit high sensitivity only when the surrounding refractive index (RI) and the fiber's refractive index (1.44) are close to each other,†Wei explained. Say. "In practice, the detected biomolecules are actually in the form of prepared aqueous solutions, with an RI that is quite close to the refractive index of water (1.33). Therefore, the analysis of the target for achieving high sensitivity and detection of small molecules is fundamental. This is challenging for fiber-based sensors.
During the group's research, they created an optical fiber coupler sensor capable of ultra-high sensitivity "low-refractive-index sensing in the 1.33 refractive index range, by precisely designing parameters to minimize 'even super-mode' The difference in refractive index with the 'Ultra Supermodel' group," he added.
What is called super-mode? A typical optical fiber coupler is composed of two parallel, closely spaced microfiber two-input (P1 and P2) and two output (P3 and P4) ports, two transition cones, and a central unified Waist shaped area. The injection mode field generated by the input mode is called "super mode" and includes odd and even distributions. When the odd-even supermodule travels along the fiber coupler, the exchange of power and resulting interference spectrum can be obtained at the output.
The team's experimental results show that its sensors are more sensitive ~20X than conventional fiber optic biosensors.
"The resulting ultra-sensitive mechanism not only creates a wide range of applications in the chemical and bio-sensing fields, but also broadens the scope of other interferometric fiber-optic sensors, even acoustic sensors," Wei said.
Due to its ultra-high sensitivity, this group of researchers is the ideal choice for target analysis and small molecule detection.
"It can also achieve a variety of applications in the fields of medical diagnosis and environmental monitoring, such as the early detection of cancer and other diseases, and the detection of heavy metals in water," Wei pointed out. "In the near future, we hope to develop into a new sensing platform with ultra-high sensitivity and fast response time, and it is feasible and low-cost to realize the application in the doctor's office."
The team's next step is to "array the sensors and further explore their sensory properties for cancer biomarkers," Wei said.
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