Wojtek J. Bock

Dispersion effects in elliptical-core highly birefringent fibers

Modal birefringence and its sensitivity to temperature and hydrostatic pressure were measured versus wavelength in three elliptical-core fibers and one fiber with stress-induced birefringence. We carried out the measurements in the spectral range from 633 to 843 nm by using interferometric methods. In fibers with elliptical cores all the measured parameters showed high chromatic dependence, whereas in fibers with stress-induced birefringence this dependence was weak. We modeled the dispersion characteristics of two elliptical-core fibers by using the modified perturbation approach first proposed by Kumar. The modification consists of introducing into the expression for the normalized propagation constants an additional perturbation term that contains information about stress-induced birefringence. The results of modeling show that the temperature and pressure sensitivity of elliptical-core fiber are associated primarily with variations in stress induced by these parameters. The agreement between measured and calculated values of sensitivity in the worst case was equal to 20% for modal birefringence and temperature sensitivity and 50% for pressure sensitivity. Lower agreement between measured and calculated values of pressure sensitivity is most probably associated with uncertainties in the material constants used in modeling.

Digital demodulation system for low-coherence interferometric sensors based on highly birefringent fibers

A new type of demodulation system for low-coherence interferometric sensors based on highly birefringent fibers is presented. The optical path delay introduced by the sensor is compensated in four detection channels by quartz crystalline plates of appropriate thickness. The system can be used to decode a single-point sensor with a resolution of 2.5 x 10(-3) or two serially multiplexed sensors with decreased resolution. In a multiplexed configuration each sensor is served by two detection channels. By tilting the quartz plates, we can tune the initial phase shift between interference signals in successive channels to differ by pi/8 or pi/4, respectively, for a single-point or a multiplexed configuration. We transferred the sinusoidal intensity changes into digital pulses by appropriate electronic processing, which eventually allows for an unambiguous phase-shift measurement with a resolution of 1/8 or 1/4 of an interference fringe. The system performance for the measurement of hydrostatic pressure changes and simultaneous changes of hydrostatic pressure and temperature is demonstrated. The pressure sensors are based on side-hole fiber to ensure high sensitivity and an operation range of 2.4 MPa. A new configuration for temperature compensation of hydrostatic pressure sensors is proposed, which is better suited for dynamic pressure measurements. In this configuration the sensing and compensating fibers are located in the same compartment of the sensor housing.

Dynamic pressure sensing with a fiber-optic polarimetric pressure transducer with two-wavelength passive quadrature readout.

We describe the combination of a polarimetric pressure sensor with a two-wavelength passive quadrature demodulation system allowing for dynamic pressure sensing in the 10-MPa range with unambiguous fringe counting. Furthermore, continuous phase measurement with the arctan method applied to the quadrature interference signals after automatic offset subtraction is demonstrated for the first time, to our knowledge. A single low-coherent superluminescent diode is used as a light source, and a polarizing beam splitter in combination with two adjustable interference filters of slightly different central wavelengths serves for the creation of the quadrature signals. Results of initial experiments with 60-ms pressure relaxation-time constants with the fringe-counting technique demonstrate the performance that was predicted theoretically. The measured pressure sensitivity exhibits excellent agreement with the previous research of Bock and Urbanczyk [IEEE Trans. Instrum. Meas. 44, 694-697 (1995)] using a polarimetric readout. The fringe-contrast variation and the measurement range obtained experimentally show the fiber dispersion to influence dephasing (deviation from quadrature) and visibility decrease significantly with increasing pressure.

Temperature desensitization of a fiber-optic pressure sensor by simultaneous measurement of pressure and temperature

A fiber-optic hydrostatic pressure sensor initially temperature compensated by optical means is further desensitized below the limits associated with second-order effects by the method proposed in this paper. We achieved this goal by using an integrated system of two coherence-multiplexed separate sensor components for simultaneous measurement of hydrostatic pressure and temperature and by on-line numerical processing of measurement data delivered simultaneously from both sensor parts. The system is based on highly birefringent fibers, employs electronic scanning, and can be used for quasi-static measurements.

Prototype of the side-hole HB optical fiber

A standard polarization maintaining optical fiber (SMPM- single mode polarization maintain) has either an elliptical core or/and an elliptical inner cladding (bow-tie, panda). These kind of fibers usually display low or modest sensitivity to pressure and high sensitivity to temperature. The side-hole optical fibers having two hollow spaces symmetrically placed on both sides of the core are characterized by much higher sensitivity to pressure. This paper presents results of investigations on sensitivity to pressure and on sensitivity to temperature, which can attain the values of 140 rad/MPa*m and 1 rad/K*m, respectively.

Influence of dispersion on sensitivity of highly birefringent fibers to temperature and hydrostatic pressure

The influence of dispersion on the sensitivity of highly birefringent fibers to temperature and hydrostatic pressure was experimentally investigated. In fibers with geometric birefringence that shows high dispersion, great differences were observed between group and phase sensitivities to temperature and hydrostatic pressure. This difference may reach 400% in the case of temperature response. In contrast, in weakly dispersive fibers with stress-induced birefringence these differences were of the order of 8% and 14%, respectively, for temperature and pressure. The influence of the dispersion effect on the temperature compensation of white-light interferometric sensors based on highly birefringent fibers was also discussed.

Contrast analysis for a fiber-optic white-light interferometric system

The behavior of a system of fiber-optic white-light interferometric sensors is analyzed. Analytical expressions for contrasts of all the interference patterns that may occur at the setups output are derived as a function of coupling coefficients between successive sensors. Two cases of exemplary systems consisting of highly birefringent optical fibers, one with nondichroic the other with some polarizing fibers, are analyzed.

Multiplexing of electronically scanned white-light interferometric strain sensors based on HB fibers

Application of electronic scanning to a sequence of coherence-multiplexed strain sensors is proposed in this paper. The sensors consist of a cascade of HB fibers spliced with their polarization axes rotated by certain angles. We theoretically analyzed the behavior of such chains as a function of their alignment and found the optimal alignment ensuring readout of every sensor with the maximum S/N ratio. The performance of parallel and serial multiplexing configurations was experimentally studied. As a receiving interferometer we used a Wollaston prism followed by a CCD camera and an image processing system while a stepped delay line made of calcite and crystalline quartz was used to match the group delays between sensing and receiving interferometers.

Characteristics of Bragg grating imprinted in highly birefringent side-hole fiber

We present characteristics of Bragg grating imprinted in highly birefringent side-hole fiber. Using specially developed methods we measured spectral sensitivity of the grating (dλB/dX) and the phase sensitivity (dΦ/dX) of the host side-hole fiber to temperature and hydrostatic pressure for each polarization mode. The measurement results for both, the grating and the host fiber, show high difference in responses to pressure and almost identical responses to temperature versus mode polarization. This feature of the Bragg grating imprinted in the side-hole fiber can be potentially used for simultaneous sensing of these two parameters by interrogating wavelength shifts for orthogonally polarized modes. We also present the results of modeling of the phase sensitivities to pressure and temprature for the host side-hole fiber and compare them with experimental values.

Impact of hydrostatic pressure on modal birefringence in photonic crystal holey fibers

In this paper we analyzed the influence of hydrostatic pressure on modal birefringence in photonic crystal holey fibers. We calculated the spectral dependence of modal birefringence B(λ) and its sensitivity to hydrostatic pressure dB(λ)/dp in the holy fiber with birefringence induced by the lack of hexagonal symmetry in the cladding. The contribution of geometrical effect related to deformation of the holey structure as well as the stress-related contribution to the overall pressure sensitivity were analyzed separately. Our results show that both factors decrease modal birefringence, which results in negative sign of pressure sensitivity. Furthermore, we show that the pressure sensitivity of the analyzed structure is of the same order as sensitivities of the commercially available highly birefringent fibers with stress applying elements like Panda or Bow-Tie.