Laser-micromachined Fabry-Perot optical fiber tip sensor for high-resolution temperature-independent measurement of refractive index

Abstract

We propose and demonstrate a Fabry-Perot (F-P) optical fiber tip sensor for high-resolution refractive-index measurement fabricated by using 157-nm laser micromachining, for the first time to our knowledge. The sensor head consists of a short air F-P cavity near the tip of a single-mode fiber and the fiber tip. The external refractive index is determined according to the maximum fringe contrast of the interference fringes in the reflective spectrum of the sensor. Such a sensor can provide temperature-independent measurement of practically any refractive index larger than that of air and offers a refractive-index resolution of ~4 x 10(-5) in its linear operating range. The experimental data agree well with the theoretical results.

Note: Optical fiber milled by focused ion beam and its application for Fabry-Pérot refractive index sensor

“…The fiber constitutes the backbone conduit for light guiding, and effective light-fluid interaction is achieved when the microfluidic channel intersects the optical pathway. In recent years, devices such as microchannels, microslots, and FP interferometers have been fabricated in single-mode fibers (SMFs) by the use of femtosecond laser micromachining, [8][9][10][11][12] which can be effectively utilized for RI sensing with low temperature sensitivity and large dynamic range. However, a relative low fringe visibility of the interference pattern is achieved when using this technique for making FP fiber sensors.…”

HCPCF-based in-line fiber Fabry-Perot refractometer and high sensitivity signal processing method

“…2, the fringe contrast of the interference spectrum changes when n EX changes, so λ 2 the RI of the analyte can be determined by measuring the fringe contrast. Usually, the interference fringe that gives the maximum fringe contrast is chosen to obtain a better sensitivity [13]. By locating the absolute minimum R FP (λ 1 ) and the adjacent peak R FP (λ 2 ) as shown in Fig.…”

“…Optical fiber refractometers have attracted widespread attention in the past decades due to many desirable advantages such as the small size, compact structure, high flexibility, immunity to electromagnetic interference, corrosion resistance, and the capacity for in situ and multiplexed operation. Various optical fiber refractometers have been developed including tapered fiber structures [1], optical fiber surface plasmon resonance (SPR) devices [2][3], fiber Bragg gratings (FBGs) [4][5], titled fiber Bragg gratings (TFBGs) [6], long-period gratings (LPGs) [7][8], optical fiber modal interferometers (MIs) [9][10], and fiber Fabry-Perot interferometers (FPIs) [11][12][13][14][15][16]. Among them, the fiber FPIs have advantages of compact probe structure, large measurement range, linear response, low temperature cross-sensitivity, and convenient reflection mode for detection, which make them particularly attractive for RI measurement.…”

“…Normally, the intrinsic fiber FPI is difficult to be fabricated and has a relatively low fringe contrast. The in-line fiber FPI has a hybrid structure combining the characteristics of extrinsic and intrinsic FPIs [13][14][15][16]. It has been a hotspot in recent years due to its good performance and easy fabrication.…”

“…The RI measuring principle of the in-line fiber FPI is the same as the intrinsic FPI, and the RI of the analyte is usually obtained from the maximum fringe contrast of the interference spectrum [13,14]. However, it requires a high signal-to-noise ratio (SNR) to calculate the maximum fringe contrast accurately.…”