M. Kadulová

Measurement of chromatic dispersion of polarization modes in optical fibres using white-light spectral interferometry

We report on a white-light interferometric technique for a broad spectral range measurement (e.g. 500–1600 nm) of chromatic dispersion of polarization modes in short-length optical fibres. The technique utilizes an unbalanced Mach–Zehnder interferometer with a fibre under test of known length inserted in one of the interferometer arms and the other arm with adjustable path length. We record a series of spectral interferograms by VIS–NIR and NIR fibre-optic spectrometers to measure the equalization wavelength as a function of the path length difference, or equivalently the differential group index dispersion of one polarization mode. The differential group dispersion of the other polarization mode is obtained from measurement of the group modal birefringence dispersion. We verify the applicability of the method by measuring the chromatic dispersion of polarization modes in a birefringent holey fibre. We apply a five-term power series fit to the measured data and confirm by its differentiation that the chromatic dispersion agrees well with that specified by the manufacturer. We also measure by this technique the chromatic dispersion of polarization modes in an elliptical-core fibre.

Wide spectral range measurement of modal birefringence in polarization-maintaining fibres

We report on a substantially improved white-light spectral interferometric technique for measurement of the group and phase modal birefringence in polarization-maintaining fibres (PMFs) over a wide wavelength range (e.g. 480–1600 nm). The technique utilizes a tandem configuration of a Michelson interferometer and a PMF placed between Glan–Taylor polarizer and analyzer. Spectral signals are recorded by VIS–NIR and NIR fibre-optic spectrometers to measure the equalization wavelength as a function of the path length difference adjusted in the interferometer, or equivalently, the wavelength dependence of the group modal birefringence in the PMF. Moreover, a new procedure is used to specify the sign of the group modal birefringence. A polynomial fit is applied to the measured data to determine also the wavelength dependence of the phase modal birefringence in the PMF over a wide spectral range.

Highly birefringent fiber-based temperature sensor utilizing the wavelength interrogation

Spectral interferomeric methods utilizing the interference of polarization modes in a highly birefringent fiber to measure temperature are analyzed experimentally and theoretically. First, we consider an experimental setup comprising a white-light source, a polarizer, a sensing birefringent fiber, an analyzer and a spectrometer. Temperature sensing by this method is based on the wavelength interrogation, that is the position of a chosen spectral interference fringe in a channeled spectrum is measured as a function of temperature. Employing the setup, we carried out temperature sensing in the range from 300 to 370 K when a part of the sensing fiber is exposed to temperature changes. A wavelength shift of a selected spectral interference fringe is measured and the temperature sensitivity reaches −0.11 nm/K. Second, we consider a setup with another interferometer (represented by a polarizer, a birefringent quartz crystal and an analyzer) to increase the sensitivity of the temperature sensing. In this setup, the resultant channeled spectrum is with envelope which shifts with temperature. We analyze the new sensor theoretically and show that temperature sensing is once again possible by using the wavelength interrogation and the temperature sensitivity to be reached is 0.68 nm/K.

Spectral interferometry-based dispersion characterization of microstructured and specialty optical fibers using a supercontinuum source

Two spectral interferometric techniques employing a supercontinuum source are used for dispersion characterization of birefringent microstructured and specialty optical fibers over a broad spectral range (e.g. 500-1600 nm). First, a technique employing an unbalanced Mach-Zehnder interferometer is used for measuring the chromatic dispersion and zero-dispersion wavelength of one polarization mode supported by a microstructured optical fiber. Second, a technique employing a tandem configuration of a Michelson interferometer and a fiber under test is used for measuring the group modal birefringence dispersion of the fiber and the chromatic-dispersion difference as a function of wavelength. From these measurements, the chromatic dispersion and the zero-dispersion wavelength of the other polarization mode supported by the microstructured optical fiber are retrieved. We revealed from four measurements the dependence of the zero-dispersion wavelength on the geometry of air-silica microstructured optical fiber. We also measured by the second technique the zero-chromatic-dispersion difference wavelength for elliptical-core optical fibers. We revealed from four measurements that the dispersion parameter can be tuned by the fiber geometry.

Broadband measurement of dispersion in a two-mode birefringent holey fiber by spectral interferometric techniques

We present the results of broadband dispersion measurement of a two-mode birefringent holey fiber (BHF). First, a spectral interferometric technique employing an unbalanced Mach-Zehnder interferometer with the fiber in the test arm is used for measuring the wavelength dependence of the group effective index of the fundamental mode supported by the fiber. Second, a spectral interferometric technique employing a tandem configuration of a Michelson interferometer and the BHF under test is used for measuring the group modal birefringence dispersion for two lowest-order linearly polarized (LP) modes supported by the BHF. The data measured over a broad spectral range are fitted to polynomials to obtain the dispersion of the phase modal birefringence for both LP modes. We reveal that the results are in agreement with a general model of birefringence in air-silica BHFs.

Infrared thermographic measurement of the surface temperature and emissivity of glossy materials

Determination of the surface temperature and emissivity of glossy materials is a complicated task due to the relatively wide range of emissivity values and the reflection of infrared radiation from surrounding objects. As a consequence, standard methods used in infrared thermography are not applicable. In this article, an alternative method is proposed for the measurement of the surface temperature and emissivity of glossy materials used in the external structures of buildings that is based on an external source of thermal radiation. It is shown that the method gives quite accurate values of emissivity of both low- and high-emissivity glossy materials, whereas the surface temperature of low-emissivity glossy materials is less accurate and strongly depends on the accuracy of the used thermal camera.

Spectral interferometric fiber optic temperature sensor with enhanced sensitivity

Spectral interferometric techniques utilizing the interference of polarization modes in a highly birefringent (HB) elliptical-core fiber to measure temperature are analyzed experimentally. First, an experimental setup comprising a white-light source, a polarizer, a sensing birefringent fiber, an analyzer and a spectrometer is considered. Temperature sensing by this method is based on the wavelength interrogation. Second, the above setup is extended by a birefringent quartz crystal to increase the sensitivity of the temperature sensing. Third, the above setup is extended by an analyzer, and the combination of a polarizer, a birefringent quartz crystal and an analyzer represents another interferometer, which is used to increase the sensitivity of the temperature sensing. In this case the Vernier effect is present and the resultant spectrum is with an envelope, which is utilized in temperature sensing. We reached a sensitivity of 0.56 nm/K in the third setup, compared to -0.12 nm/K and -0.19 nm/K in the first and the second setup, respectively.

Temperature sensing using the spectral interference of polarization modes of a highly birefringent fiber

A spectral-domain interferometric technique using the interference of polarization modes of a highly birefringent (HB) elliptical-core fiber to measure the temperature is presented. The method is based on the wavelength interrogation, i.e., the position of a chosen spectral interference maximum as a function of temperature is measured. Temperature sensing is carried out in a range from 300 to 370 K in an experimental setup comprising a white-light source, a polarizer, a delay line, a sensing HB fiber, an analyzer and a spectrometer. As the delay line, a birefringent quartz crystal of a suitable thickness is utilized to resolve a channeled spectrum in a range as wide as possible. A part of the sensing HB fiber, which is placed in a chamber, is exposed to temperature changes, and first, the polarimetric sensitivity to temperature is measured. It is revealed that the HB fiber is suitable for temperature sensing at a wavelength of 600 nm. Second, the shift of the wavelength position of the chosen spectral interference maximum with temperature is measured. It is revealed that the temperature sensitivity is higher at shorter wavelengths.

Fiber-optic refractive index sensor based on surface plasmon resonance

A fiber-optic refractive index sensor based on surface plasmon resonance (SPR) in a thin metal film deposited on an unclad core of a multimode fiber is presented. The sensing element of the SPR fiber-optic sensor is a bare core of a step-index optical fiber made of fused silica with a deposited gold film. First, a model of the SPR fiber-optic sensor based on the theory of attenuated total internal reflection is presented. The analysis is carried out in the frame of optics of multilayered media. The sensing scheme uses a wavelength interrogation method and the calculations are performed over a broad spectral range. Second, in a practical realization of the sensor with a double-sided sputtered gold film, a reflection-based sensing scheme to measure the refractive indices of liquids is considered. The refractive index of a liquid is sensed by measuring the position of the dip in the reflected spectral intensity distribution. As an example, the aqueous solutions of ethanol with refractive indices in a range from 1.333 to 1.364 are measured.

Highly sensitive displacement measurement utilizing the wavelength interrogation

Spectral interferometric methods utilizing the interference of two beams in a Michelson interferometer to measure the displacement are analyzed theoretically and experimentally. First we consider an experimental setup comprising a white-light source, a dispersion balanced Michelson interferometer and a spectrometer. The position of one of the interferometer mirrors is controlled via a piezo positioning system and the displacement measurement is based on the wavelength interrogation, i.e., the position of a selected interference fringe in the resultant channeled spectrum is measured as a function of the mirror displacement. Second we consider a setup with another interferometer, included in the Michelson interferometer, to increase the sensitivity of the displacement measurement. In this setup, the resultant channeled spectrum is with envelope which shifts with the displacement of the interferometer mirror. We analyze the new measurement method theoretically and show that the displacement measurement is once again possible by using the wavelength interrogation and the sensitivity is substantially increased. We also realized the new measurement setup in which the position of the interferometer mirror is controlled via a closed-loop piezo positioning system and confirmed the theoretical results.