Fiber Sensing: Angled optical fiber end performs large-temperature-range sensing

Temperature sensing is ubiquitous in science, and both electronic and optical sensors are available. However, fiber-optic sensors offer immunity to electromagnetic interference and easy, flexible routing in remote-sensing applications.

State-of-the art fiber Bragg grating (FBG)-based temperature sensors offer 10 pm/°C sensitivity, but only operate over a range of temperatures from -200 to +150°C. Likewise, microstructured photonic-crystal fiber or hollow-core fiber sensors offer 18.7 pm/°C sensitivity, but only operate on the high end up to +460°C. Other alternatives that offer better sensitivity include whispering-gallery-mode and surface-plasmon-resonance sensors. Unfortunately, these options have limited temperature sensing range (usually from 0 to +100°C).

But a new fiber-optic sensing design from researchers at Tianjin University (Tianjin, China) uses a simple angled fiber end that relies on interferometric principles to achieve high sensitivity over a very broad range of temperatures from -40 to +140°C and 250 to 900°C, with sensitivity values that increase as temperature rises—specifically, 9.5 pm/°C at -40°C and 17.86 pm/°C at +900°C (see figure).¹

Angled fiber interferometry

To fabricate the sensor, a standard Corning SMF-28 single-mode optical fiber with approximate 8.2-µm-diameter core and 125-µm-diameter cladding is inserted in a ceramic ferrule and polished to an angle of 60° from the original fiber surface. Since total internal reflection occurs at 43° for a silica/air interface according to Snell’s law, light that traverses through the fiber to its angled end is totally internally reflected, reaches the core/clad interface, and splits into two beams that then travel along different paths before recombining along the fiber core with an optical path difference that can be translated into a phase difference.

This angled end acts like a Michelson interferometer, producing fringes with a spacing of 7.5847 nm for a 1550 nm wavelength. And as the angled fiber end is exposed to temperature variations, thermal expansion of the fiber produces refractive-index changes and subsequent wavelength changes that correlate back to temperature.

The broad range of the sensor is because of the fact that the thermo-optic coefficient and thermal expansion coefficient of the fused silica optical fiber are constant and nonzero over a large temperature range. And the sensor is easily fabricated using standard optical-fiber polishing equipment and 1 µm diamond lapping film followed by a finer 0.3 µm diamond lapping film.

In the experimental setup, light from an amplified-spontaneous-emission (ASE) source with a spectral bandwidth of 1525 to 1605 nm was input to the fiber through a circulator, which also routes the reflected fiber signal to an optical spectrum analyzer (OSA) with a 0.05 nm resolution to measure the interference spectrum.

Temperature was controlled in the experiment in two different ranges by two different types of equipment: first, a Fluke 9171 metrology well temperature calibrator with a -45 to +155°C range and 0.01°C precision, and second, a Lindberg BF51866C Blue Moldatherm Box Furnace with range from 200 to 1100°C and 1°C precision. Experiments confirmed the 9.5 pm/°C and 17.86 pm/°C temperature-sensitivity values at the two extreme temperature ends of the range of -40 to +900°C.

REFERENCE

1. T. Wang et al., Opt. Fiber Technol., 45, 19–23 (2018).