Zhuo Meng

Measurement of the refractive index of human teeth by optical coherence tomography

We describe a novel method based on optical coherence tomography (OCT) for the accurate measurement of the refractive index of in vitro human teeth. We obtain the refractive indices of enamel, dentin, and cementum to be 1.631+/-0.007, 1.540+/-0.013, and 1.582+/-0.010, respectively. The profile of the refractive index is readily obtained via an OCT B scan across a tooth. This method can be used to study the refractive index changes caused by dental decay and therefore has great potential for the clinical diagnosis of early dental caries.

Long-range vibration sensor based on correlation analysis of optical frequency-domain reflectometry signals

We present a novel method to achieve a space-resolved long- range vibration detection system based on the correlation analysis of the optical frequency-domain reflectometry (OFDR) signals. By performing two separate measurements of the vibrated and non-vibrated states on a test fiber, the vibration frequency and position of a vibration event can be obtained by analyzing the cross-correlation between beat signals of the vibrated and non-vibrated states in a spatial domain, where the beat signals are generated from interferences between local Rayleigh backscattering signals of the test fiber and local light oscillator. Using the proposed technique, we constructed a standard single-mode fiber based vibration sensor that can have a dynamic range of 12 km and a measurable vibration frequency up to 2 kHz with a spatial resolution of 5 m. Moreover, preliminarily investigation results of two vibration events located at different positions along the test fiber are also reported.

Compensation of laser frequency tuning nonlinearity of a long range OFDR using deskew filter

We present a simple and effective method to compensate the optical frequency tuning nonlinearity of a tunable laser source (TLS) in a long range optical frequency-domain reflectometry (OFDR) by using the deskew filter, where a frequency tuning nonlinear phase obtained from an auxiliary interferometer is used to compensate the nonlinearity effect on the beating signals generated from a main OFDR interferometer. The method can be applied to the entire spatial domain of the OFDR signals at once with a high computational efficiency. With our proposed method we experimentally demonstrated a factor of 93 times improvement in spatial resolution by comparing the results of an OFDR system with and without nonlinearity compensation. In particular we achieved a measurement range of 80 km and a spatial resolution of 20 cm and 1.6 m at distances of 10 km and 80 km, respectively with a short signal processing time of less than 1 s for 5 × 10(6) data points. The improved performance of the OFDR with a high spatial resolution, a long measurement range and a short process time will lead to practical applications in the real-time monitoring, test and measurement of fiber optical communication networks and sensing systems.

A novel method for determining and improving the quality of a quadrupolar fiber gyro coil under temperature variations

We introduce a parameter called pointing error thermal sensitivity (PETS) for quantitatively determining the quality of a quadrupolar (QAD) fiber coil under radial temperature variations. We show both analytically and experimentally that the pointing error of a fiber gyro incorporating the fiber coil is linearly proportional to the final radial thermal gradient on the coil, with PETS as the proportional constant. We further show that PETS is linearly proportional to another parameter called effective asymmetric length of the coil. By thermally inducing different radial thermal gradients on the fiber coil and measuring the corresponding pointing errors in a gyroscopic measurement setup, we can confidently determine the PETS of the fiber coil and its associated effective asymmetric length caused by imperfections in coil winding. Consequently, we are able to precisely trim the coil to achieve best thermal performance.

Complete Characterization of Polarization-Maintaining Fibers Using Distributed Polarization Analysis

We present methods and processes of using a ghost-peak-free distributed polarization crosstalk analyzer (DPXA) to accurately obtain all polarization related parameters of polarization-maintaining (PM) fibers. We show that by first inducing a series equidistant periodic polarization crosstalk peaks along a PM fiber and then measuring the positions and the widths of these peaks using the analyzer, all birefringence related parameters of the PM fiber, including group birefringence, group birefringence variation along the fiber, group birefringence dispersion, and group birefringence temperature coefficient, can be accurately obtained. We further show that the DPXA has the ability to identify and eliminate polarization crosstalk contributions of connectors or splices in the measurement system and therefore can be used to obtain high accuracy measurement of the polarization extinction ratio (PER) of PM fibers. Finally, we propose a set of parameters based on the distributed polarization analysis to quantitatively evaluate the quality of PM fibers. We believe that the methods and processes described in this paper can be widely applied in the industry for the complete characterization of PM optical fibers.

Long Measurement Range OFDR Beyond Laser Coherence Length

We present a novel method to significantly extend the measurement range of an optical frequency domain reflectometry (OFDR) beyond the laser coherence length for both discrete reflection (such as Fresnel reflection) and Rayleigh backscattering measurements. A key to the method is to increase the laser frequency tuning speed while measuring the phase noise of the reflected optical signal that carries the location and reflectivity information. Using this technique in a conventional OFDR with a noise floor of -120 dB and a dynamic range of 53 dB, we achieved a measurable distance of 170 km for Fresnel reflection and 120 km for Rayleigh backscattering using a laser with a coherence length of only 13.6 km.

Method for improving the resolution and accuracy against birefringence dispersion in distributed polarization cross-talk measurements

We present a novel method for improving the spatial resolution and amplitude accuracy of distributed polarization cross-talk measurements in a polarization maintaining (PM) fiber against its birefringence dispersion. We show that the broadening of measured polarization cross-talk peaks caused by birefringence dispersion can be restored by simply multiplying the measurement data with a compensation function. The birefringence dispersion variable in the function can be obtained by finding the widths of measured cross-talk envelopes at known distances along the fiber. We demonstrate that this method can effectively improve spatial resolution and amplitude accuracy of the space-resolved polarization cross-talk measurements of long PM fibers.We present a novel method for improving the spatial resolution and amplitude accuracy of distributed polarization cross-talk measurements in a polarization maintaining (PM) fiber against its birefringence dispersion. We show that the broadening of measured polarization cross-talk peaks caused by birefringence dispersion can be restored by simply multiplying the measurement data with a compensation function. The birefringence dispersion variable in the function can be obtained by finding the widths of measured cross-talk envelopes at known distances along the fiber. We demonstrate that this method can effectively improve spatial resolution and amplitude accuracy of the space-resolved polarization cross-talk measurements of long PM fibers.

Accurate method for measuring the thermal coefficient of group birefringence of polarization-maintaining fibers.

We present a method to accurately measure the group birefringence variation with temperature in high-birefringence polarization-maintaining (PM) fibers using a distributed polarization analyzer. By analyzing polarization cross-talk peaks purposely induced at both ends of a PM fiber, the temperature coefficient of group birefringence can be accurately obtained. We confirm the theoretical prediction that the group birefringence of PANDA and TIGER PM fibers decrease linearly with temperature from -40 °C to 80 °C, and find that the temperature coefficients are -5.93 × 10(-7) °C(-1) and -5.29 × 10(-7) °C(-1) for two types of PANDA fibers, and -5.36 × 10(-7) °C(-1) for a TIGER fiber.

Tomographic Inspection of Fiber Coils Using Optical Coherence Tomography

We report the first study of using optical coherence tomography (OCT) to inspect the winding quality of fiber optical gyro coils. We use a swept-source OCT system to scan a quadrupole-wind fiber coil for obtaining its two-dimensional and three-dimensional (3D) tomographic images. The defects beneath the surface of the fiber coil, which otherwise are invisible by video inspection, can be clearly identified with such tomographic images. The winding quality of each layer can also be independently visualized with the reconstructed 3D images from different angles or in different sections. We believe that the proposed method will be useful to ensuring the quality production of each and every fiber gyro coil.

Measurements of the thermal coefficient of optical attenuation at different depth regions of in vivo human skins using optical coherence tomography: a pilot study

We present detailed measurement results of optical attenuations thermal coefficients (referenced to the temperature of the skin surface) in different depth regions of in vivo human forearm skins using optical coherence tomography (OCT). We first design a temperature control module with an integrated optical probe to precisely control the surface temperature of a section of human skin. We propose a method of using the correlation map to identify regions in the skin having strong correlations with the surface temperature of the skin and find that the attenuation coefficient in these regions closely follows the variation of the surface temperature without any hysteresis. We observe a negative thermal coefficient of attenuation in the epidermis. While in dermis, the slope signs of the thermal coefficient of attenuation are different at different depth regions for a particular subject, however, the depth regions with a positive (or negative) slope are different in different subjects. We further find that the magnitude of the thermal coefficient of attenuation coefficient is greater in epidermis than in dermis. We believe the knowledge of such thermal properties of skins is important for several noninvasive diagnostic applications, such as OCT glucose monitoring, and the method demonstrated in this paper is effective in studying the optical and biological properties in different regions of skin.