Interferometric fiber optic sensors

Abstract

Fiber optic interferometers to sense various physical parameters including temperature, strain, pressure, and refractive index have been widely investigated. They can be categorized into four types: Fabry-Perot, Mach-Zehnder, Michelson, and Sagnac. In this paper, each type of interferometric sensor is reviewed in terms of operating principles, fabrication methods, and application fields. Some specific examples of recently reported interferometeric sensor technologies are presented in detail to show their large potential in practical applications. Some of the simple to fabricate but exceedingly effective Fabry-Perot interferometers, implemented in both extrinsic and intrinsic structures, are discussed. Also, a wide variety of Mach-Zehnder and Michelson interferometric sensors based on photonic crystal fibers are introduced along with their remarkable sensing performances. Finally, the simultaneous multi-parameter sensing capability of a pair of long period fiber grating (LPG) is presented in two types of structures; one is the Mach-Zehnder interferometer formed in a double cladding fiber and the other is the highly sensitive Sagnac interferometer cascaded with an LPG pair.

Keywords: Fabry-Perot interferometers; Mach-Zehnder interferometers; Michelson interferometers; Sagnac interferometers; fiber interferometers; fiber-optic sensors.

Figure 1.

(a) Extrinsic FPI sensor made by forming an external air cavity, and (b) intrinsic FPI sensor formed by two reflecting components, R₁ and R₂, along a fiber.

Figure 2.

(a) Schematic of an extrinsic FPI liquid RI sensor system based on a PCF lens, and (b) its reflection spectrum measured with an air cavity [35].

Figure 3.

(a) Fourier spectra of the fabricated FPI fiber sensor measured with liquid (upper inset) and without liquid (lower inset) at the cavity, and (b) the RI of the liquid calculated from the Fourier spectrum and plotted with respect to the labeled RI. A series of liquid solutions of RIs from 1.400 to 1.438 with a step of 0.002 were applied into the cavity [35].

Figure 4.

(a) The microscope image of an implemented double cavity FPI fiber sensor, and (b) its reflection spectrum [38]. The length of HOF is ∼70 μm, and the length of MMF is ∼360 μm. SMF; single mode fiber, HOF; hollow optical fiber, MMF; multi mode fiber.

Figure 5.

(a) Fourier spectra measured with air (black curve) and several RI solutions (color curve). Inset is the magnified image of the red dotted region; (b) Intensity variation of the third Fourier peak plotted with respect to the labeled RI of the solutions [38].

Figure 6.

The schematic of an MZI. A beam is split into two arms, the reference and the sensing arms, and then recombined by using two fiber couplers.

Figure 7.

Configuration of various types of MZIs; the methods of using (a) a pair of LPGs, (b) core mismatch, (c) air-hole collapsing of PCF, (d) MMF segment, (e) small core SMF, and (f) fiber tapering.

Figure 8.

(a) Transmission spectra of an LPG pair under temperature variations; the spectrum was shifted with the same phase; (b) The amount of the spectrum shift with temperature; it was shifted toward longer wavelength with the sensitivity of ∼39 pm/°C; (c) The spectra under strain variations; only the phase was changed without affecting the envelop curve; (d) The amount of the phase shift measured with strain [48].

Figure 8.

(a) Transmission spectra of an LPG pair under temperature variations; the spectrum was shifted with the same phase; (b) The amount of the spectrum shift with temperature; it was shifted toward longer wavelength with the sensitivity of ∼39 pm/°C; (c) The spectra under strain variations; only the phase was changed without affecting the envelop curve; (d) The amount of the phase shift measured with strain [48].

Figure 9.

(a) Basic configuration of a Michelson interferometer and (b) schematic of a compact in-line Michelson interferometer.

Figure 10.

(a) Schematic of the PCF-based Michelson interferometer designed for temperature-insensitive liquid RI measurement, and (b) its spectral responses to RI and temperature [63]. A part of the core mode beam can be coupled to the cladding mode(s) by collapsing the PCF in a short length. Both beams reflected by the common mirror make interference. The silica rod fusion-spliced at the end of the PCF blocks the liquid.

Figure 11.

Schematic of the sensor based on a Sagnac interferometer.

Figure 12.

(a) Experimental setup of the Sagnac interferometric sensor combined with an LPG pair MZI sensor, and (b) the measured transmission spectrum [85]. It can measure temperature and strain simultaneously.

Figure 13.

(a) Temperature sensitivity of the hybrid Sagnac-MZI sensor, and (b) its strain sensitivity. From their different sensitivities, the temperature and the strain applied on the sensor could be separated [85].