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.

/pdf/interferometric-fiber-optic-sensors-2akpm29ydq.pdf

Interferometric Fiber Optic Sensors

DOI: 10.1002/adom.201801433 Scientific American selects plasmonic sensing as the top 10 emerging technologies of 2018.[15] Almost every single new plasmonic or photonic structure would be explored to test its sensing ability.[16–29] These works tend to report the sensing performance of their own structure. Some declare that their sensitivity breaks the world record. However, there is still a missing literature on what the world record really is, the gap between the experiments and the theoretical limit, as well as the differences between metal-based plasmonic sensors and dielectric-based photonic sensors. To push plasmonic and photonic sensors into industrial applications, an optical sensing technology map is absolutely necessary. This review aims to cover a wide range of most representative plasmonic and photonic sensors, and place them into a single map. The sensor performances of different structures will be distinctly illustrated. Future researchers could plot the sensing ability of their new sensors into this technology map and gauge their performances in this field. In this review, we focus on optical refractive index (RI) sensors with no fluorescent labeling required. We will utilize two parameters to characterize and compare the performance of optical RI sensors: sensitivity to RI change (denoted by symbol SRI) and figure of merit (in short, FoM). For simplicity, we restrict our discussions to bulk RI change, where the change in RI occurs within the whole sample. There is another case where the RI variation occurs only within a very small volume close to the sensor surface. This surface RI sensitivity is proportional to the bulk RI sensitivity, the ratio of the thickness of the layer within which the surface RI variation occurs, and the penetration depth of the optical mode.[6] The bulk RI sensitivity defines the ratio of the change in sensor output (e.g., resonance angle, intensity, or resonant wavelength) to the bulk RI variations. Here, we limit our discussions to the spectral interrogations and the bulk RI sensitivity SRI is given by[3,5–7,30]

Optical Refractive Index Sensors with Plasmonic and Photonic Structures: Promising and Inconvenient Truth

Full characterization of the sensitivity of the LPFG is a precursor to practical device design, and knowledge of the sensitivity of the Long-period fiber gratings to the parameters of its physical environment processed is clearly important. We present a theoretical and experimental investigation into the sensitivity of long-period fiber gratings as a function of processed parameters by high-frequency CO 2 laser pulses.

Sensitivity characteristics of long-period fiber gratings

A surface plasmon resonance-based fiber-optic sensor for simultaneous measurement of refractive index and temperature of liquid samples is proposed and experimentally demonstrated. The sensor consists of a gold-coated MM-SM-MM optical fiber structure, whose sensitive section was partially covered with polydimethylsiloxane (PDMS) to generate two independent SPR resonance dips in the fiber transmission spectrum. One of the dips is generated by the bare gold-coated fiber section whose wavelength resonance is tuned by the refractive index and temperature of the surrounding medium. The other dip that is exclusively used to monitor the temperature variations of the liquid sample, whose central wavelength at 900 nm corresponds to PDMS refractive index at 20 °C, is produced by the polymerized gold-coated fiber section. The high refractive index and temperature sensitivity achieved, 2323.4 nm/RIU and −2.850 nm/°C respectively, the small size, the ease fabrication process, and the bio-compatibility of the proposed device are appealing characteristics that makes it ideal for practical bio-sensing applications.

Simultaneous measurement of refractive index and temperature using a SPR-based fiber optic sensor

Fiber Bragg grating has embraced the area of fiber optics since the early days of its discovery, and most fiber optic sensor systems today make use of fiber Bragg grating technology Researchers have gained enormous attention in the field of fiber Bragg grating (FBG)-based sensing due to its inherent advantages, such as small size, fast response, distributed sensing, and immunity to the electromagnetic field Fiber Bragg grating technology is popularly used in measurements of various physical parameters, such as pressure, temperature, and strain for civil engineering, industrial engineering, military, maritime, and aerospace applications Nowadays, strong emphasis is given to structure health monitoring of various engineering and civil structures, which can be easily achieved with FBG-based sensors Depending on the type of grating, FBG can be uniform, long, chirped, tilted or phase shifted having periodic perturbation of refractive index inside core of the optical fiber Basic fundamentals of FBG and recent progress of fiber Bragg grating-based sensors used in various applications for temperature, pressure, liquid level, strain, and refractive index sensing have been reviewed A major problem of temperature cross sensitivity that occurs in FBG-based sensing requires temperature compensation technique that has also been discussed in this paper

https://www.spiedigitallibrary.org/journals/optical-engineering/volume-59/issue-6/060901/Fiber-Bragg-grating-sensors-for-monitoring-of-physical-parameters/10.1117/1.OE.59.6.060901.pdf

Fiber Bragg grating sensors for monitoring of physical parameters: a comprehensive review

We demonstrate the fabrication of long-period fiber gratings (LPFGs) written in the two-mode fiber (TMF) by CO2 laser. Both uniform and tilted LPFGs were fabricated to provide the light coupling between LP01 mode and LP11 mode with a coupling efficiency of more than 99%. The writing efficiency and the bandwidth of the LPFG mode converter can be adjusted by changing the tilt angle of the tilted TMF-LPFGs. The torsion sensitivity of conventional and tilted LPFG mode converters were measured to be 0.37 nm/(rad/m) and 0.50 nm/(rad/m), respectively. Two orthogonal vector modes (the HEeven 21and HEodd 21 modes) and corresponding orbital angular momentum state were successfully obtained at the resonance wavelength. The proposed LPFG mode converter could be used as not only a high efficiency wavelength tunable mode converter in the mode division multiplexing system but also a high sensitive torsion sensor in the field of optical sensing.

Mode converter based on the long-period fiber gratings written in the two-mode fiber

We demonstrate the fabrication of the helical long-period grating (HLPG) in a two-mode fiber (TMF) with CO2 laser. The torsion characteristics of the helical gratings are experimentally investigated. It was found that the resonance wavelength of the TMF-HLPG linearly shifts with a twist sensitivity of 0.47 nm/(rad/m), which is an order magnitude higher than that of the conventional long-period fiber gratings (LPFGs). The temperature sensitivity of the TMF-HPFG was measured to be 23.9 pm/°C, which is one third of that of the conventional LPFGs. The proposed TMF-HLPG-based twist sensor would have a promising application for the twist monitor.

High Sensitivity Twist Sensor Based on Helical Long-Period Grating Written in Two-Mode Fiber

We investigated an all-fiber mode converter based on long-period fiber gratings (LPFGs) written in the few-mode fiber. Mode conversion between the fundamental core mode and different higher-order core modes (LP11, LP21, and LP02 modes) can be realized via a single LPFG with an efficiency of 99% at the resonance wavelength. Moreover, optimized mode conversion between the LP01 and LP21 modes can be realized by cascading two LPFGs with different grating pitches. The maximum conversion efficiency is estimated to be ∼99.5% at 1553 nm. The orbital angular momentum states with different topological charges (±1,±2) are demonstrated experimentally. The all-fiber LPFG mode converters could have promising applications in the mode-division multiplexing optical communications.

All-fiber mode converter based on long-period fiber gratings written in few-mode fiber

An in-fiber Mach-Zehnder interferometer (MZI) was fabricated and characterized for solution refractive index (RI) sensors. The MZI consists of two cascade double cladding fibers (DCFs) in a standard single mode fiber (SMF). The DCFs serve as the in-fiber couplers which split and combine light propagating in the core and the outer cladding region. Since the cladding mode is excited, the interference spectrum is sensitive to the ambient RI variation. Within the RI range from 1.3333 to 1.4535, the sensor characteristics were characterized. The sensitivity of 31 nm/RIU and 823 nm/RIU were obtained for the lower RI (1.34) and the higher RI (1.44), respectively. With the mass-producing of DCF and the easy fabrication process of the sensor head, the proposed in-fiber MZI is a potential alternative for the RI sensor application.

In-Fiber Mach–Zehnder Interferometer Based on Double Cladding Fibers for Refractive Index Sensor

A chiral long-period grating (CLPG) was fabricated successfully by scanning CO2 laser in a twisted conventional single-mode fiber. The strong mode coupling can be achieved in a short grating length. The sensing characteristics of the CLPGs were investigated experimentally. Compared with the conventional long-period fiber gratings, the CLPGs were found to have the similar temperature and refractive index sensitivities and much higher torsion, bending, and strain sensitivities. The high sensitivity of the CLPG can be attributed to the screw-type waveguide structure induced by the scanning laser in the twisted fiber. The proposed CLPGs could have great potential applications as filters and optical sensors.

Yunqi Liu

Papers

Mode converter based on the long-period fiber gratings written in the two-mode fiber

High Sensitivity Twist Sensor Based on Helical Long-Period Grating Written in Two-Mode Fiber

All-fiber mode converter based on long-period fiber gratings written in few-mode fiber

In-Fiber Mach–Zehnder Interferometer Based on Double Cladding Fibers for Refractive Index Sensor

High Sensitivity Chiral Long-Period Grating Sensors Written in the Twisted Fiber