Photonic crystal fiber Bragg grating based sensors:opportunities for applications in healthcare

We review the state-of-the-art in photonic crystal fiber (PCF) and microstructured polymer optical fiber (mPOF) based mechanical sensing. We first introduce how the unique properties of PCF can benefit Bragg grating based temperature insensitive pressure and transverse load sensing. Then we describe how the latest developments in mPOF Bragg grating technology can enhance optical fiber pressure sensing. Finally we explain how the integration of specialty fiber sensor technology with bio-compatible polymer based micro-technology provides great opportunities for fiber sensors in the field of healthcare.

Low-cost fully integrated fiber Bragg grating interrogation system

Fiber Bragg gratings can be used for monitoring different parameters in a wide variety of materials and constructions. The interrogation of fiber Bragg gratings traditionally consists of an expensive and spacious peak tracking or spectrum analyzing unit which needs to be deployed outside the monitored structure. We present a dynamic low-cost interrogation system for fiber Bragg gratings which can be integrated with the fiber itself, limiting the fragile optical in- and outcoupling interfaces and providing a compact, unobtrusive driving and read-out unit. The reported system is based on an embedded Vertical Cavity Surface Emitting Laser (VCSEL) which is tuned dynamically at 1 kHz and an embedded photodiode. Fiber coupling is provided through a dedicated 45° micromirror yielding a 90° in-the-plane coupling and limiting the total thickness of the fiber coupled optoelectronic package to 550 µm. The red-shift of the VCSEL wavelength is providing a full reconstruction of the spectrum with a range of 2.5 nm. A few-mode fiber with fiber Bragg gratings at 850 nm is used to prove the feasibility of this low-cost and ultra-compact interrogation approach.

Phase-shifted fiber Bragg grating for strain measurement at extreme conditions

In this work, a phase-shifted fiber Bragg grating is proposed for strain sensing at extreme temperatures. The grating structure is written in bare standard single mode fiber, using the point-by-point femtosecond laser technique. Strain measurements are performed at temperatures ranging from room temperature up to 900°C. By subjecting the sensor to such extreme conditions, the wavelength of the grating increases. © 2014 OSA.

Cost-effective optical coherence tomography spectrometer based on a tilted fiber Bragg grating

A compact, fiber-based spectrometer for biomedical application utilizing a tilted fiber Bragg grating (TFBG) as integrated dispersive element is demonstrated. Based on a 45° UV-written PS750 TFBG a refractive spectrometer with 2.06 radiant/μm dispersion and a numerical aperture of 0.1 was set up and tested as integrated detector for an optical coherence tomography (OCT) system. Featuring a 23 mm long active region at the fiber the spectrum is projected via a cylindrical lens for vertical beam collimation and focused by an achromatic doublet onto the detector array. Covering 740 nm to 860 nm the spectrometer was optically connected to a broadband white light interferometer and a wide field scan head and electronically to an acquisition and control computer. Tomograms of ophthalmic and dermal samples obtained by the frequency domain OCT-system were obtained achieving 2.84 μm axial and 7.6 μm lateral resolution. © 2014 SPIE.

Enhancing the sensitivity of poly(methyl methacrylate) based optical fiber Bragg grating temperature sensors

In poly(methyl methacrylate) (PMMA)-based optical fiber gratings (POFBGs), the temperature response is determined by thermal expansion and the thermo-optic effect of the fiber. Because thermal expansion introduces a positive change and the thermo-optic effect introduces a negative change in the Bragg wavelength of the POFBG, they cancel out each other to some extent, leading to reduced and varying temperature sensitivity. By pre-straining a POFBG, the contribution of thermal expansion can be removed, and, consequently, the temperature sensitivity of POFBG can be greatly enhanced. Theoretical analysis also indicates a reduced thermo-optic coefficient of POFBG due to restrained linear expansion that matches experimental results.

Improved response time of laser etched polymer optical fiber Bragg grating humidity sensor

The humidity sensor made of polymer optical fiber Bragg grating (POFBG) responds to the water content change in fiber induced by the change of environmental condition. The response time strongly depends on fiber size as the water change is a diffusion process. The ultra short laser pulses have been providing an effective micro fabrication method to achieve spatial localized modification in materials. In this work we used the excimer laser to create different microstructures (slot, D-shape) in POFBG to improve its performance. A significant improvement in the response time has been achieved in a laser etched D-shaped POFBG humidity sensor.

Fiber-optic liquid level monitoring system using microstructured polymer fiber Bragg grating array sensors:performance analysis

A highly sensitive liquid level monitoring system based on microstructured polymer optical fiber Bragg grating (mPOFBG) array sensors is reported for the first time. The configuration is based on five mPOFBGs inscribed in the same fiber in the 850 nm spectral region, showing the potential to interrogate liquid level by measuring the strain induced in each mPOFBG embedded in a silicone rubber (SR) diaphragm, which deforms due to hydrostatic pressure variations. The sensor exhibits a highly linear response over the sensing range, a good repeatability, and a high resolution. The sensitivity of the sensor is found to be 98 pm/cm of water, enhanced by more than a factor of 9 when compared to an equivalent sensor based on a silica fiber around 1550 nm. The temperature sensitivity is studied and a multi-sensor arrangement proposed, which has the potential to provide level readings independent of temperature and the liquid density.

Optical RI sensor based on an in-fiber bragg grating fabry-perot cavity embedded with a micro-channel

We report a linear response optical refractive index (RI) sensor, which is fabricated based on a micro-channel created within a Fabry Perot (F-P) cavity by chemical etching assisted by femtosecond laser inscription. The experimental results show the F-P resonance peak has a linear response with the RI of medium and the measuring sensitivity is proportion to the length of micro-channel. The sensor with 5 μm -long micro-channel exhibited an RI sensitivity of 1.15nm/RIU and this sensitivity increased to 9.08nm/RIU when widening the micro-channel to 35μm. Furthermore, such micro-channel FP sensors show a much broader RI sensing dynamic range (from 1.3 to 1.7) than other reported optical fiber sensors. © 2012 SPIE.

Dynamic strain sensor using a vcsel and a polymer fiber bragg grating in a multimode fiber

We have implemented a dynamic strain sensor using a Polymer Optical Fiber Bragg Grating (POFBG). In this paper, we have investigated an approach for making such systems cheaper through the use of easy to handle multimode fiber. A Vertical-Cavity Surface-Emitting Laser is used to decrease the cost of the interrogation system and a photodetector converts the reflected light into an electrical signal.

Characterization of polymer fiber Bragg grating with ultrafast laser micromachining

We report that the main photosensitive mechanism of poly(methyl methacrylate)-based optical fiber Bragg grating (POFBG) under ultraviolet laser micromachining is a complex process of both photodegradation and negative thermo-optic effect. We found experimentally the unique characteristics of Bragg resonance splitting and reunion during the laser micromachining process providing the evidence of photodegradation, while the mean refractive index change of POFBG was measured to be negative confirming further photodegradation of polymer fiber. The thermal-induced refractive index change of POFBG was also observed by recording the Bragg wavelength shift. Furthermore, the dynamic thermal response of the micromachined-POFBG was demonstrated under constant humidity, showing a linear and negative response of around -47.1 pm/°C.

Fiber Bragg grating inscription in four core fibers and their sensing applications

We inscribe FBGs in all cores of four core fiber simultaneously and investigate their thermal, strain and bending (both direction and magnitude) responses. The influence of fiber core distance on bending sensitivity is also discussed. © 2015 OSA.

High spatial and temporal resolution interrogation of fully distributed chirped fiber Bragg grating sensors

A novel interrogation technique for fully distributed linearly chirped fiber Bragg grating (LCFBG) strain sensors with simultaneous high temporal and spatial resolution based on optical time-stretch frequency-domain reflectometry (OTS-FDR) is proposed and experimentally demonstrated. LCFBGs is a promising candidate for fully distributed sensors thanks to its longer grating length and broader reflection bandwidth compared to normal uniform FBGs. In the proposed system, two identical LCFBGs are employed in a Michelson interferometer setup with one grating serving as the reference grating whereas the other serving as the sensing element. Broadband spectral interferogram is formed and the strain information is encoded into the wavelength-dependent free spectral range (FSR). Ultrafast interrogation is achieved based on dispersion-induced time stretch such that the target spectral interferogram is mapped to a temporal interference waveform that can be captured in real-Time using a single-pixel photodector. The distributed strain along the sensing grating can be reconstructed from the instantaneous RF frequency of the captured waveform. High-spatial resolution is also obtained due to high-speed data acquisition. In a proof-of-concept experiment, ultrafast real-Time interrogation of fully-distributed grating sensors with various strain distributions is experimentally demonstrated. An ultrarapid measurement speed of 50 MHz with a high spatial resolution of 31.5 μm over a gauge length of 25 mm and a strain resolution of 9.1 μϵ have been achieved.

PMMA based optical fiber Bragg grating for measuring moisture in transformer oil

Water contamination can cause serious problems that compromise in transformer's safe operation and reduce its lifetime. Online monitoring of moisture concentration in transformer oil would permit the control of moisture buildup. This letter presents a direct optical measurement of moisture concentration in transformer oil using a poly(methyl methacrylate) (PMMA)-based optical fiber Bragg grating (POFBG). The refractive index and volume of PMMA-based optical fiber vary with the moisture in the surrounding transformer oil, changing the reflecting wavelength of the grating. A sensitivity of POFBG wavelength change to moisture content of 29 pm/ppm is demonstrated in this letter, indicating detectable water content better than 0.05 ppm.

A novel sensor interrogation technique using chirped fiber grating based sagnac loop

We demonstrate a novel and simple sensor interrogation scheme for fiber Bragg grating (FBG) based sensing systems. In this scheme, a chirped FBG based Sagnac loop is used as a wavelength-dependent receiver, and a stable and linear readout response is realised. It is a signijkant advantage of this scheme that the sensitivity and the measurement wavelength range can be easily adjhsted by controlling the chirp of the FBG or using an optical delay line in the Sagnac loop.

In-fiber polymer–glass hybrid waveguide Bragg grating

A 1.2 µm (height) × 125 µm (depth) × 500 µm (length) microslot along a fiber Bragg grating was engraved across the optical fiber by femtosecond laser patterning and chemical etching. By filling epoxy in the slot and subsequent UV curing, a hybrid waveguide grating structure with a polymer core and glass cladding was fabricated. The obtained device is highly thermally responsive with linear coefficient of 211 pm/°C.