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

We propose and demonstrate a thinned fiber based Mach-Zehnder interferometer for multi-purpose sensing applications. The sensor head is formed by all-fiber in-line singlemode-multimode-thinned-singlemode (SMTS) fiber structure, only using the splicing method. The principle of operation relies on the effect that the thinned fiber cladding modes interference with the core mode by employing a multimode fiber as a mode coupler. Experimental results showed that the liquid refractive index information can be simultaneously provided from measuring the sensitivity of the liquid level. A 9.00 mm long thinned fiber sensor at a wavelength of 1538.7228 nm exhibits a water level sensitivity of -175.8 pm/mm, and refractive index sensitivity as high as -1868.42 (pm/mm)/RIU, respectively. The measuring method is novel, for the first time to our knowledge. In addition, it also demonstrates that by monitoring the wavelength shift, the sensor at a wavelength of 1566.4785 nm exhibits a refractive index sensitivity of -25.2935 nm/RIU, temperature sensitivity of 0.0615 nm/°C, and axial strain sensitivity of -2.99 pm/μe, respectively. Moreover, the sensor fabrication process is very simple and cost effective.

All-fiber Mach-Zehnder interferometers for sensing applications

Graphene as a novel material has laid a foundation for its applications in optical fiber sensors, due to its unique properties, especially the optical properties. On the other hand, optical fiber sensors have received world-wide attention due to their high sensitivity, small size, good anti-electromagnetism disturbance ability and other potential advantages. In this paper, the developments of graphene in the applications of optical fiber sensors were reviewed from four aspects. Firstly, the common preparation methods of graphene were introduced. Next, the optical properties of graphene have been concluded. And then, some typical optical fiber chemical and biological sensors based on graphene, such as temperature sensors, biological sensors and gas sensors, were reviewed. It was shown that graphene had a great potential in the optical fiber sensing technology. Furthermore, the deficiencies and challenges of the graphene in the applications of optical fiber sensors were analyzed. In a whole, the unique advantages of graphene have present their versatility and importance in the application fields of optical fiber sensors.

Review on the graphene based optical fiber chemical and biological sensors

A review of the salinity measurement technology based on the optical fiber sensor is presented. The principles of optical fiber measurement, the structures of probes and the characteristics of various sensing structures are concerned. Firstly, this paper discusses the relationship between the salinity and refractive index, and the effect of ion pairs on the refractive index. Secondly, four methods of direct or non-direct measurements of salinity are summarized, including optical refraction method, optical fiber grating, optical interference and surface plasmon effect. Subsequently, the article compares performances of various sensing structures and analyses the advantages and disadvantages of different sensors. Finally, a prospect of salinity measurement requirement and the development direction of fiber-optic sensors in this area are addressed.

Review of salinity measurement technology based on optical fiber sensor

We report a compact fiber in-line Mach-Zehnder interferometer for refractive index sensing with high sensitivity and precise sensing location. One arm of the interferometer contains a microcavity formed by removing part of the fiber core near the core and cladding interface by femtosecond laser micromachining, and the other arm remains in line with the remaining part of the fiber core. Such a fiber in-line Mach-Zehnder interferometer exhibits an extremely high refractive-index-sensitivity of −9370 nm/RIU (refractive index unit) within the refractive index range between 1.31 and 1.335.

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Fiber in-line Mach-Zehnder interferometer fabricated by femtosecond laser micromachining for refractive index measurement with high sensitivity

An approach to achieve simultaneous measurement of refractive index and temperature is proposed by using a Mach–Zehnder interferometer realized on tapered single-mode optical fiber. The attenuation peak wavelength of the interference with specific order in the transmission spectrum shifts with changes in the environmental refractive index and temperature. By utilizing S-band and C/L-band light sources, simultaneous discrimination of refractive index and temperature with the tapered fiber Mach–Zehnder interferometer is demonstrated with the corresponding sensitivities of −23.188 nm/RIU (refractive index unit) and 0.071 nm/ °C, and −26.087 nm/RIU (blueshift) and 0.077 nm/°C (redshift) for the interference orders of 169 and 144, respectively.

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Tapered fiber Mach–Zehnder interferometer for simultaneous measurement of refractive index and temperature

An in-line one-fiber approach to realize simultaneous measurement of salinity and temperature is proposed. The sensor system, which consists of multiplexed polymer-coated fiber Bragg gratings, showed that the polyimide-coated grating responds to variations of both temperature and salinity, while the acrylate-coated grating is only sensitive to the environmental temperature. The experimental results indicated that the temperature sensitivity of the acrylate-coated grating in water was 0.0102nm∕°C for redshifted Bragg wavelength with increasing temperature, and the temperature and the salinity sensitivities of the polyimide-coated grating were 0.0094nm∕°C (redshifted) and 0.0165nm∕M (blueshifted), respectively, which are in excellent agreement with the theoretical analysis.

A multiplexed fiber Bragg grating sensor for simultaneous salinity and temperature measurement

An approach to resolve the cross sensitivity of fiber Bragg gratings (FBGs) is proposed by the adoption of different polymers as the coating materials for gratings. From the different optical responses resulted from the gratings of different polymeric coatings, sensitivity to individual parameter can be exactly revealed. As an application of this approach, simultaneous discrimination of axial strain and temperature with two FBGs of different polymeric coatings is demonstrated with the axial strain and temperature sensitivities of 1.228pm∕μe and 11.433pm∕°C for the acrylate-coated FBG, and 1.170pm∕μe and 11.333pm∕°C for the polyimide-coated FBG, respectively.

Resolving cross sensitivity of fiber Bragg gratings with different polymeric coatings

A new fiber-optic sensor for simultaneous measurement of water-soluble analytes and temperature with polymer-coated fiber Bragg gratings (FBGs) is proposed. As an application of the approach, simultaneous monitoring of the concentration of sugar or potassium chloride (KCl) and temperature has been achieved. Changes in these environmental parameters result in different extents of either red- or blue-shifts of the Bragg resonance wavelengths of the gratings. It has been found that polyimide-coated FBG responds to variations of both temperature and concentrations of soluble analytes, while acrylate-coated FBG is sensitive to environmental temperature only. The experimental results showed that the temperature sensitivity of the acrylate-coated FBG, temperature, sugar, and KCl concentration sensitivities of the polyimide-coated FBG are 0.0102 nm/degC , 0.0094 nm/degC, 0.0012 nm/degBx , and 0.0126 nm/M, respectively. The sensing mechanism of the polyimide-coated FBG lies in the hygroscopic properties of the polyimide coating, which result in the change of the strain of the fiber and, thus, the optical properties of the grating. Since the sensor detects the analytes that swell the polyimide coating and different analytes induce different swelling effects, the sensor can detect different analytes without prior knowledge once a calibration curve is developed.

Polymer-Coated Fiber Bragg Grating Sensors for Simultaneous Monitoring of Soluble Analytes and Temperature

A femtosecond laser trimmed fiber taper as a Mach-Zehnder interferometer in a standard single-mode optical fiber is demonstrated to achieve simultaneous measurement of axial strain and temperature. The experimental results indicate axial strain sensitivities of -1.0 and -1.2 pm/μe (blueshift) and temperature sensitivities of 81.3 and 98.8 pm/°C (redshift) at two interference orders of m1 (95) and m2 (90), respectively.

Liqiu Men

Papers

Tapered fiber Mach–Zehnder interferometer for simultaneous measurement of refractive index and temperature

A multiplexed fiber Bragg grating sensor for simultaneous salinity and temperature measurement

Resolving cross sensitivity of fiber Bragg gratings with different polymeric coatings

Polymer-Coated Fiber Bragg Grating Sensors for Simultaneous Monitoring of Soluble Analytes and Temperature

Femtosecond Laser Trimmed Fiber Taper for Simultaneous Measurement of Axial Strain and Temperature