Miklos Lenner

Vibration Sensitivity Reduction of Photoacoustic Gas Analyzers

Optical gas analyzers are devices for measuring gas concentrations with high precision in industrial environments. Unfortunately mechanical shocks and vibrations can impact the measurement accuracy if the analyzer is installed in noisy environments, e.g. near rotating machines. In this paper, we propose two methods to remove the influence of vibrations by using an auxiliary signal measured via an accelerometer. In the first method, the mechanoelectrical transfer function between the vibration and its effects on the microphone is estimated before installation of the analyzer. In the second method, the mechanoelectrical transfer function is updated continuously under normal sensing operations. The two methods have been implemented within an ABB URAS gas analyzer and corrections are executed in real time. Tests on a vibrating table have confirmed the effectiveness of both methods: a reduction in vibration sensitivity up to a factor twenty is achieved.

Characterization of fiber wave retarders for interferometric fiber-optic current sensors

Fiber-optic wave retarders made from a short section of polarization-maintaining (pm) fiber are crucial components of interferometric fiber-optic current sensors for high-voltage substations [1]. Another potential application of such retarders is in chiral spectroscopy, e. g. for the detection of specific molecules. In a fiber-optic current sensor, as considered here, the magnetic field of the current introduces a differential optical phase shift between left and right circularly polarized light waves during their round trip through a fiber coil that encloses the current conductor (Faraday-effect). Closed-loop fiber gyroscope technology is used to measure the current-induced optical phase shift (Fig. 1). The phase retardation of the retarder which generates the circular waves and its temperature dependence affect the sensor scale factor and are therefore critical to the sensor performance. Using an appropriately designed retarder (Fig. 2) it is possible to compensate for the temperature dependence of the Faraday-effect [1] that is 0.7% per 100°C. The overall retardation can be substantially influenced by splice joints - especially if short beat length fiber is used as fusion splicing modifies the fiber birefringence in the proximity of the joints. Conventional methods to determine the phase and/or group birefringence of pm fibers [2–3] are generally applied to long and homogeneous fiber segments. The alternative method presented here is ideally suited for short fiber sections (of a few millimeters in length) acting as wave retarders and also accounts for the influence of splice joints.

Effects of thermal fiber annealing on the temperature compensation of interferometric fiber-optic current sensors

In this paper, we present an experimental and theoretical study on how thermal fiber annealing influences the temperature dependence of the interferometric fiber-optic current sensor. Such sensors measure the current-induced phase shift (Faraday effect) between left and right circularly polarized light waves in a reflective fiber coil around the current conductor. The fiber-optic phase retarder at the coil entrance that generates the (nearly) circular light waves may also serve to compensate for the temperature dependence of the Faraday effect. Thermal annealing serves to remove disturbing bend-induced fiber birefringence in the sensing coil - but it can also affect the retarder parameters. We investigate, to our knowledge for the first time, how thermal annealing affects the optical phase retardation and its temperature dependence of elliptical-core fiber retarders and show how the retarder parameters must be set prior to annealing in order to achieve a temperature-independent sensor after annealing.

Fiber optic current and voltage sensors for electric power transmission systems

Optical current and voltage sensors have become attractive alternatives to conventional instrument transformers in high voltage electric power transmission systems. The optical sensors offer important benefits such as small size and weight, enhanced performance, and constitute an important part of the transition to digital substations. The sensors must comply with stringent accuracy and reliability requirements. Commonly, substation applications demand accuracy to within ±0.2% over outdoor temperature ranges. Other aspects are insensitivity to shock and vibration and stray fields as well as life times in excess of 30 years. We review the technology of the sensors and present particular measures that were necessary to achieve the required performance. This includes the exploration of different sensing fiber types, inherent temperature compensation, accelerated life tests, and, in case of voltage sensors, adequate high voltage proof insulation and packaging. We discuss the integration of a current sensor into a circuit breaker and show results from a corresponding field test.

Long-term Reliability of Fiber-optic Sensor Components in Harsh Industrial Environments

Applications of fiber-optic technology in harsh industrial environments require high level of reliability of the components. Comprehensive accelerated life tests demonstrated excellent long-term performance of reliability-critical optical components.

Investigation of Lamb waves in solid-liquid layers

Lamb waves are used in non-destructive testing to detect defects or corrosion of walls of pipes or other sensitive constructions. Depending on the used mode of the Lamb wave the propagation of the waves can be influenced by the presence of liquids as it e.g. occurs in liquid filled pipes. The here occurring leaky Lamb waves has therefore already been investigated in the past. In addition also in non-filled pipes liquids can cover the wall either due to condensation out of a saturated gas phase or due to remains of water on a former filled pipe. The liquid would then cover the wall partly as a layer and could also influence the propagation of the Lamb waves. In this paper the propagation of Lamb waves is therefore investigated in partially or fully covered plates with liquid layers showing the damping of the waves at the beginning and along the liquid layers.

Interferometric fiber-optic current sensor with inherent source wavelength shift compensation

Fiber-optic current sensors utilize the Faraday effect in fused silica fiber to measure electric current. Since the effect is wavelength dependent, high sensor accuracy requires a stable source wavelength. Commonly used semiconductor sources require temperature stabilization within ≈0.1°C for adequate wavelength stability that adds extra cost. Here, we theoretically and experimentally demonstrate a novel method for inherent (passive) compensation of source wavelength shifts in interferometric fiber-optic current sensors. The method is based on an appropriately detuned fiber-optic half-wave retarder that generates wavelength dependent cross-coupling between the two orthogonal polarization modes of the sensor which compensates the change of the Faraday effect with wavelength. It is shown that at a wavelength shift over 14 nm (near 1305 nm), which corresponds to a source temperature change of 24°C, the scale factor variation is reduced from 2% to <;0.2%.

A Study on Different Types of Fiber Coils for Fiber Optic Current Sensors

We consider an interferometric fiber optic current sensor with a fiber coil operated in reflection and compare three different techniques to prepare the coil: thermally annealed coils, stress-free packaging of a bare low birefringent fiber in a fused silica capillary, and coils from highly birefringent spun fiber. In particular we theoretically and experimentally investigate how the fiber retarder that generates the near left and right circular light waves in the sensing fiber must be prepared for temperature compensation of the Faraday effect in the three cases. All three methods can achieve accuracy within ±<0.2% over an extended temperature range but they considerably differ in their practical challenges.

Thermal tuning of fiber quarter-wave retarders for temperature compensation of fiber-optic current sensors

The fiber retarder of an interferometric fiber-optic current sensor is fine-tuned by controlled local heat treatment for temperature compensation of the Faraday effect. The retardation is determined from sensor scale factor measurement.