Sapphire Photonic Crystal Fiber Sensor

TL;DR

This paper presents the design, fabrication, and testing of a sapphire photonic crystal fiber Bragg grating sensor capable of high-temperature measurement up to 1200°C, emphasizing improved manufacturability and reliability.

Contribution

It introduces a femtosecond laser direct writing method with a spatial light modulator for efficient fabrication of sapphire fiber sensors with enhanced reliability and reduced cost.

Findings

Achieved a temperature sensitivity of 19.0-32.3 pm/°C from 25-1200°C.

Fabricated devices up to 7 cm long with a propagation loss of 0.7 dB/cm.

Reduced fabrication time sixfold compared to similar waveguides.

Abstract

Sapphire optical fiber shows great promise for remote sensing in extreme environments approaching 2000 degC, by using laser-processing to form a single-mode waveguide within it. However, for practical application, longer devices with high manufacturability and reliability are required. We report the design, modeling, fabrication, and optimization of an index-guiding sapphire photonic crystal fiber Bragg grating temperature sensor. The device is fabricated using femtosecond laser direct writing to inscribe both the photonic crystal waveguide and the Bragg grating. A spatial light modulator was used to compensate for the mismatch between the immersion objective and the high-index oil used. This improved the aspect ratio and suppressed cracking during fabrication, for higher reliability. The design results in a 6-fold reduction in fabrication time over an equivalent depressed cladding waveguide, significantly reducing the cost of manufacture. Devices up to 7 cm long were fabricated and spliced to standard single-mode fiber. The propagation loss was estimated to be 0.7 dB/cm and the Bragg gratings had a bandwidth of approximately 0.12 nm. Devices were tested in a furnace showing a temperature sensitivity of between 19.0-32.3 pm/degC over a range 25-1200 degC. These longer devices have the potential to enable practical high precision extreme temperature monitoring in many applications, with lower manufacturing cost and higher reliability.