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.

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Interferometric Fiber Optic Sensors

Optical fibers have been involved in the area of sensing applications for more than four decades. Moreover, interferometric optical fiber sensors have attracted broad interest for their prospective applications in sensing temperature, refractive index, strain measurement, pressure, acoustic wave, vibration, magnetic field, and voltage. During this time, numerous types of interferometers have been developed such as Fabry-Perot, Michelson, Mach-Zehnder, Sagnac Fiber, and Common-path interferometers. Fabry-Perot interferometer (FPI) fiber-optic sensors have been extensively investigated for their exceedingly effective, simple fabrication as well as low cost aspects. In this study, a wide variety of FPI sensors are reviewed in terms of fabrication methods, principle of operation and their sensing applications. The chronology of the development of FPI sensors and their implementation in various applications are discussed.

https://www.mdpi.com/1424-8220/14/4/7451/pdf-vor

Chronology of Fabry-Perot interferometer fiber-optic sensors and their applications: a review.

A fiber-optic Fabry-Perot interferometer was constructed by splicing a short length of photonic crystal fiber to a standard single-mode fiber. The photonic crystal fiber functions as a Fabry-Perot cavity and serves as a direct sensing probe without any additional components. Its pressure and temperature responses in the range of 0-40 MPa and 25°C-700°C were experimentally studied. The proposed sensor is easy to fabricate, potentially low-cost, and compact in size, which makes it very attractive for high-pressure and high-temperature sensing applications.

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High-pressure and high-temperature characteristics of a Fabry―Perot interferometer based on photonic crystal fiber

In-line fiber optic interferometers have attracted intensive attention for their potential sensing applications in refractive index, temperature, pressure and strain measurement, etc. Typical in-line fiber-optic interferometers are of two types: Fabry-Perot interferometers and core-cladding-mode interferometers. It's known that the in-line fiber optic interferometers based on single-mode fibers can exhibit compact structures, easy fabrication and low cost. In this paper, we review two kinds of typical in-line fiber optic interferometers formed in single-mode fibers fabricated with different post-processing techniques. Also, some recently reported specific technologies for fabricating such fiber optic interferometers are presented.

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In-Line Fiber Optic Interferometric Sensors in Single-Mode Fibers

We investigated a novel and ultracompact polymer-capped Fabry-Perot interferometer, which is based on a polymer capped on the endface of a single mode fiber (SMF). The proposed Fabry-Perot interferometer has advantages of easy fabrication, low cost, and high sensitivity. The variation of the Fabry-Perot cavity length can be easily controlled by using the motors of a normal arc fusion splicer. Moreover, the enhanced mechanical strength of the Fabry-Perot interferometer makes it suitable for high sensitivity pressure and temperature sensing in harsh environments. The proposed interferometer exhibits a wavelength shift of the interference fringes that corresponds to a temperature sensitivity of 249 pm/°C and a pressure sensitivity of 1130 pm/MPa, respectively, around the wavelength of 1560 nm.

Simultaneous measurement of pressure and temperature by employing Fabry-Perot interferometer based on pendant polymer droplet

A temperature-insensitive fiber optic sensor for high-resolution measure of refractive index is proposed and demonstrated. The sensor is based on Fresnel reflection modulated by Fabry-Perot interference, which consists of a section of single-mode fiber (SMF) tip coated with a thin film of SU-8 at the end of the fiber tip. The fringe contrast of the interference spectrum reflected from the sensor head is used to determine the extern RI. The experimental results agree well with the theoretical results and show that the fringe contrast is not sensitive to temperature variation. Such a refractive index sensor can offer the refractive index resolution and accuracy of 5 × 10-6 and 5 × 10-5, respectively. By simultaneous tracking the wavelength shift of the interference pattern, the sensor also has the potential capability for the simultaneous measurement of liquid RI and temperature.

High-Resolution and Temperature-Insensitive Fiber Optic Refractive Index Sensor Based on Fresnel Reflection Modulated by Fabry–Perot Interference

Simultaneous strain and temperature measurement using a highly birefringence fiber loop mirror and a long-period grating written in a photonic crystal fiber

A simple fiber-optic sensor based on Fabry-Perot interference for refractive index measurement of optical glass is investigated both theoretically and experimentally. A broadband light source is coupled into an extrinsic fiber Fabry-Perot cavity formed by the surfaces of a sensing fiber end and the measured sample. The interference signals from the cavity are reflected back into the same fiber. The refractive index of the sample can be obtained by measuring the contrast of the interference fringes. The experimental data meet with the theoretical values very well. The proposed technique is a new method for glass refractive index measurement with a simple, solid, and compact structure.

Extrinsic fiber-optic Fabry–Perot interferometer sensor for refractive index measurement of optical glass

Fabry–Perot interference-based fiber-optic sensor for small displacement measurement

A practical sensor with high sensitivity for the temperature measurement by using a highly birefringent (HiBi) fiber loop mirror (FLM) concatenated with a long-period grating (LPG) written in a photonic crystal fiber (PCF) is proposed. Only HiBi-FLM acts as a sensor head, while the PCF-LPG serves as a temperature insensitive filter to provide wavelength dependent optical power transmission. A bandpass filter is used to provide a narrow band light source, thus only one of transmission peaks of the HiBi-FLM is located within a negative linear region of the LPG's transmission spectrum. By utilizing the stable filtering function of the PCF-LPG, the resonant wavelength variation of the FLM with temperature is transferred effectively to the intensity variation of the output light. The temperature applied on the FLM is deduced by a cheap optical power. As a result, the sensitivity of the sensor is about -0.4872 dB/°C. When an optical power meter with the resolution of 0.01 dB, a temperature resolution of 0.02°C is achieved.

Jia-Rong Zhao

Papers

High-Resolution and Temperature-Insensitive Fiber Optic Refractive Index Sensor Based on Fresnel Reflection Modulated by Fabry–Perot Interference

Simultaneous strain and temperature measurement using a highly birefringence fiber loop mirror and a long-period grating written in a photonic crystal fiber

Extrinsic fiber-optic Fabry–Perot interferometer sensor for refractive index measurement of optical glass

Fabry–Perot interference-based fiber-optic sensor for small displacement measurement

A Practical Temperature Sensor with High Sensitivity Utilizing a Fiber Loop Mirror with a Long-Period Grating in a Photonic Crystal Fiber