Multimode Fabry–Perot Interferometer Probe Based on Vernier Effect for Enhanced Temperature Sensing

Abstract

New miniaturized sensors for biological and medical applications must be adapted to the measuring environments and they should provide a high measurement resolution to sense small changes. The Vernier effect is an effective way of magnifying the sensitivity of a device, allowing for higher resolution sensing. We applied this concept to the development of a small-size optical fiber Fabry–Perot interferometer probe that presents more than 60-fold higher sensitivity to temperature than the normal Fabry–Perot interferometer without the Vernier effect. This enables the sensor to reach higher temperature resolutions. The silica Fabry–Perot interferometer is created by focused ion beam milling of the end of a tapered multimode fiber. Multiple Fabry–Perot interferometers with shifted frequencies are generated in the cavity due to the presence of multiple modes. The reflection spectrum shows two main components in the Fast Fourier transform that give rise to the Vernier effect. The superposition of these components presents an enhancement of sensitivity to temperature. The same effect is also obtained by monitoring the reflection spectrum node without any filtering. A temperature sensitivity of -654 pm/°C was obtained between 30 °C and 120 °C, with an experimental resolution of 0.14 °C. Stability measurements are also reported.

A High Sensitivity Temperature Sensing Probe Based on Microfiber Fabry-Perot Interference

“…It has a more compact structure for developing high-performance temperature microprobes. Femtosecond laser [31] or ion beam etching technology [32], as well as high-precision fiber-splicing technology [36,37], can improve its temperature detection limit to as high as 1200 °C, making it suitable for extreme high temperature environments; furthermore, sol coating [35] or temperature-sensitive polymer encapsulation technology [39] can be used for enhance normal temperature microprobes, which will be a promising candidate for implantable microsensors for health or environmental monitoring under 200 °C.…”

“…In addition to the above complex optical fiber structures, single polymer optical fibers have been demonstrated with a temperature sensitivity of ~10 −3 °C [27], where the temperature performance were revealed by the transmission power and the effect of relative and twist have been experimentally obtained [28,29]. Furthermore, their packaging size is hard to reduce further depending on the bending loss of the optical fiber [30], which will seriously limit their application in a narrow space; the latter ones are carried out as reflective structures, where the temperature sensitive cavity was constructed at the end of the optical fiber by laser or ion beam processing, chemical etching or film forming and special fiber splicing technologies [31,32,33,34,35,36,37]. Among them, femtosecond laser processing can machine a refractive index turning point with good repeatability in the optical fiber, which was used as a Fabry-Perot cavity and can work at high temperatures up to 1000 °C [31]; focused ion beams can etch an air cavity at the tip of an optical fiber, based which a Fabry-Perot temperature sensor with a sensitivity of −654 pm/°C has been experimentally demonstrated [32].…”

“…Furthermore, their packaging size is hard to reduce further depending on the bending loss of the optical fiber [30], which will seriously limit their application in a narrow space; the latter ones are carried out as reflective structures, where the temperature sensitive cavity was constructed at the end of the optical fiber by laser or ion beam processing, chemical etching or film forming and special fiber splicing technologies [31,32,33,34,35,36,37]. Among them, femtosecond laser processing can machine a refractive index turning point with good repeatability in the optical fiber, which was used as a Fabry-Perot cavity and can work at high temperatures up to 1000 °C [31]; focused ion beams can etch an air cavity at the tip of an optical fiber, based which a Fabry-Perot temperature sensor with a sensitivity of −654 pm/°C has been experimentally demonstrated [32]. However, the expensive and complex preparation processing, as well as the high technique requirements for engineers have become huge obstacles for commercial production [33].…”

Giant refractometric sensitivity by combining extreme optical Vernier effect and modal interference

“…In a two-mode Fabry–Perot interferometer, the effective refractive index difference between the fundamental mode and a higher order mode that produce an envelope modulation in the reflection spectrum can be calculated through 20 : where and are the wavelength positions of two consecutive maxima (or minima) of the envelope, L is the length of the sensing interferometer, and FSR is the free spectral range of the envelope modulation. For the few-mode sensing interferometer developed, which presents a length of 105 μm, the FSR is estimated to be around 450 nm.…”

Enhancing the Visibility of Vernier Effect in a Tri-Microfiber Coupler Fiber Loop Interferometer for Ultrasensitive Refractive Index and Temperature Sensing

“…The Vernier effect, originally employed in a mechanical Vernier caliper to enhance the accuracy of length measurement, has also been recently used to increase sensitivity in optical fiber sensing by taking advantage of the small-scale difference between two interferometers, which are used as the main ruler and the Vernier, respectively [1]. Several structures have been proposed to realize the Vernier effect, such as two cascaded Mach-Zehnder interferometers [2], optical microfiber coupler modal interferometers [3], two cascaded Fabry-Perot Interferometers [4], reflective θ-shape microfiber resonator [5] and multimode Fabry-Perot Interferometer [6] [7]. It should be noted that in all of the above structures in order to utilize the Vernier effect, the optical lengths of the reference arm and the sensing arm were carefully controlled.…”