Shizhuo Yin

Fiber Optic Sensors

Fiber optic sensors have become indispensable tools for biomedical study because of their unique features such as (1) high sensitivity, (2) small footprint and endoscopic compatibility, (3) multiple agent distributive sensing capability, and (4) immunity from electromagnetic interference. In this chapter, we will provide a brief overview on the basic principle of fiber optic sensors, types of fiber optic sensors, and their applications to biomedical sensing. As a result of the page limitation, important work in this area may not be included in this chapter. Readers may find those contents in the listed reference material. Keywords: fiber optic sensors; light; sensing mechanism; biomedical sensing

Calibration-based phase-shifting projected fringe profilometry for accurate absolute 3D surface profile measurement

In this paper, an accurate calibration-based phase-shifting measurement technique for measuring the absolute 3D surface profiles is presented, in which the system distortions for each detection location are calibrated individually. Thus, this approach offers higher accuracy than that of conventional global system model-based calibration technique. By comparing the experimental results from this technique with the data from Zeiss Universal Precision Measuring Center (Model UPMC 550), it is found that the absolute measurement accuracy for a bowl size object (about 160 mm in diameter and 40 mm in depth) is about 5 μm. This experimental result proves that, indeed, this calibration-based phase-shifting measurement technique has a good enough accuracy for precise engineering surface (such as gear gauge surface) measurement.

Chromatic confocal microscopy using supercontinuum light.

We report on a novel chromatic confocal microscope system using supercontinuum white light generated from a photonic crystal fiber. The chromatic aberration of a pair of singlet lenses is employed to focus the different spectral components of the supercontinuum at different depth levels. An effective depth scanning range of 7 microm is demonstrated. The corresponding depth resolution is measured to be less than 1 microm (FWHM).

Fiber bragg grating strain sensor demodulation with quadrature sampling of a mach-zehnder interferometer.

A simple and highly sensitive phase-demodulation technique is proposed, and its use for a fiber Bragg grating strain sensor is experimentally demonstrated. Sampling a phase-modulated Mach-Zehnder output with controlled time delay produced two quadrature data streams that have relative quadrature phase difference (90 degrees ). The Bragg wavelength-dependent phase information is extracted by application of digital arctangent function and phase unwrapping to the quadrature signals. By use of this technique with a reference grating, strain sensing at as much as a 30-kHz sampling rate was obtained with strain resolution of 3.5 microstrains and 6 nanostrains/[square root]Hz in quasi-static and dynamic strain measurements, respectively.

Analysis of a widely tunable long-period grating by use of an ultrathin cladding layer and higher-order cladding mode coupling

A widely tunable long-period grating in single-mode fiber is analyzed by use of an ultrathin cladding layer and higher-order cladding mode coupling. The numerical simulation shows that a 225-nm tuning range in the newly designed ultrathin long-period grating (cladding thickness, 35 microm) with third-order cladding mode coupling can be obtained. The analyzed tuning range is seven times wider than those of the other known long-period gratings. We believe that the proposed highly sensitive long-period grating will be widely used as a gain-flattening filter for ultrawideband optical amplifiers and fast tunable filters in dynamic optical communication systems.

High-temperature sensing using whispering gallery mode resonance in bent optical fibers

A new mechanism for high-temperature sensing with single-mode optical fibers is proposed. This sensing mechanism is based on whispering gallery mode (WGM) resonance in bent optical fibers. The interference fringes between the core mode and WGMs were induced by bending a cladding-thinned fiber and forming a loop in the fiber. The wavelength shift of the resonance peaks as a function of temperature is investigated. It is demonstrated that the single loop of bend ring in the fibers can be used as a transducer for high-temperature sensing without any internal structure.

Submicrometer displacement sensing using inner-product multimode fiber speckle fields.

A multimode fiber sensor using the intensity inner product of speckle fields is presented. The sensitivity and the dynamic range of the displacement sensing are quantitatively analyzed. We show that the sensitivity of displacement can be in the submicrometer range. Experimental performances show that the results are consistent with the calculated results.

A highly sensitive long period grating based tunable filter using a unique double-cladding layer structure

In this paper, we present a highly sensitive long period grating (LPG) with a unique double-cladding layer structure including an ultra-thin inner silica cladding layer and a low refractive index liquid crystal outer cladding layer. By using an ultra-thin cladding layer (28 μm in diameter) reduced by chemical etching, the LPG has a single resonant band over a wide range of wavelength and higher sensitivity to the environmental refractive index change. In addition, the high thermal-optic effect of liquid crystal makes it very easy to tune the resonant wavelength by controlling the temperature of the liquid crystal. Thus, a highly sensitive tunable filter can be made. The experimental results show that a tuning efficiency of 2.1 nm/°C is achieved. To the best of our knowledge, this is the highest tuning efficiency ever reported.

Giant electro-optic effect in nanodisordered KTN crystals

The electro-optic (EO) effect in nanodisordered potassium tantalate niobate (KTN) crystal is quantitatively investigated. It is found out that the EO coefficient of nanodisordered KTN crystal depends not only on the cooling temperature but also on the cooling rate. A larger EO coefficient can be obtained by employing a faster cooling rate. A Kerr EO efficient (s(11) - s(12) = 6.94 × 10(-14) m(2)/V(2)) is obtained at a cooling rate of 0.45 °C/s. The enhanced EO efficient by employing a faster cooling rate will be greatly beneficial for a variety of applications such as laser Q switches, laser pulse shaping, high-speed optical shutters, and modulating retroreflectors.

Multiple parameter vector bending and high-temperature sensors based on asymmetric multimode fiber Bragg gratings inscribed by an infrared femtosecond laser.

We present a multiple parameter integrated fiber sensor that can detect vector bending and ambient temperature simultaneously with a single asymmetric multimode fiber Bragg grating. Multimode Bragg gratings were fabricated in an all-silica core fiber by an infrared femtosecond laser, which showed multiple transmission dips in the transmission spectrum. Bending and ambient temperature fluctuations affect the shapes of multiple transmission dips in different ways. In bending, different dips have different sensitivities. On the other hand, temperature fluctuations tended to influence the dips uniformly across different dips. By analyzing the changing spectrum of dips, one can distinguish the changes induced by bending or temperature fluctuations. Furthermore, the high thermal stability of Bragg gratings inscribed by an infrared femtosecond laser can make this double parameter fiber sensor work in very harsh, high-temperature environments.