Matej Njegovec

This work presents a compact fiber-integrated sensor designed for high-resolution measurement of electric currents and magnetic fields using a magneto-optic liquid microcell. The primary goal is to develop a miniature, customizable sensor capable of operating in challenging environments where electrical isolation, spatial constraints, and resistance to ambient electromagnetic noise are critical. The sensing element is based on barium hexaferrite nanoplatelets suspended in n-butanol, whose birefringent properties vary in response to external magnetic fields. Microchannels inscribed into standard optical fibers enable precise filling of the microcell with minimal liquid volume, facilitating scalable sensor fabrication. The device measures both direct and alternating currents by detecting the surrounding magnetic field generated by a current-carrying wire. A ferrite toroid core and tunable bias magnetic field improve sensitivity and define three distinct sensor configurations. Theoretical models for each configuration were experimentally validated, demonstrating resolution down to 40 μA in the highest sensitivity setup and an AC measurement bandwidth of 2100 Hz. Temperature stability tests from 25 °C to 50 °C revealed a drift of about –0.004% of the total unambiguous measuring range per °C. These results highlight the sensor’s potential for integration into systems requiring miniature, interference-immune, and adaptable current sensing solutions.

This paper presents an all-silica microwire optical sensor designed for both fast response time and high-resolution temperature detection. The sensor consists of a thin optical microwire created at the tip of an optical fiber, configured as a temperature sensitive Fabry-Perot interferometer. For the purposes of achieving rapid response times, the sensing section of the sensor has a diameter of 11 μm. Experimental bench tests were set up to evaluate the sensor’s static and dynamic performance. The results indicate a sensor system resolution of about 5 mK and demonstrate a bandwidth of 38 Hz that corresponds to a response time of 4ms in still air. The sensor’s functionality is shown through a basic periodic gas compression method detecting small temperature fluctuations. In addition, the sensor was demonstrated for operation up to 600 degrees Celsius.

This paper presents an all-fiber, miniature Fabry-Perot gas sensor based on photothermal absorption spectroscopy with a custom-made and cost-efficient interrogation system. The sensing gas microcell has a diameter of 125 μm, a length of 1 mm, and allows for free gas exchange within the optical resonator through micromachined slits. High light intensity and confinement is ensured by delivering the excitation light directly into the gas microcell through a lead-in single-mode fiber. This enhances the photothermal effect and provides a short system response time. The interrogation system utilizes the modulation of an excitation laser diode with a fixed frequency while locking the probe laser onto the gas microcell’s quadrature point and observing the variations of the reflected optical power. To show the potential of the proposed system, nitrogen dioxide was measured in dry air. Thereby, a limit of detection of 20 ppm could be achieved for 10 seconds’ integration time. Furthermore, the small dimensions of the sensor allow for improved dynamic performance with photothermal modulation frequencies as high as 7 kHz.

This paper presents a fiber optic sensor for the detection of corrosion processes that occur on metal surfaces. In the proposed sensor design, a sensing fiber with an inscribed high-order Bragg grating is attached to the observed metal surface. A broadening of high-order Bragg grating spectral characteristics is observed and used to detect small and highly localized longitudinal strain variations that occur along the fiber as a result of corrosive rust flakes’ formations beneath the sensing fiber. The proposed approach provides a straightforward fabrication method, the possibility for unobtrusive mounting, including mounting of the sensor beneath the corrosion-protective layers, and consistent corrosion detection yields.

This paper describes a Fabry–Pérot sensor-based measuring system for measuring fluid composition in demanding industrial applications. The design of the sensor is based on a two-parametric sensor, which enables the simultaneous measurement of temperature and refractive index (RI). The system was tested under real industrial conditions, and enables temperature-compensated online measurement of emulsion concentration with a high resolution of 0.03 Brix. The measuring system was equipped with filtering of the emulsion and automatic cleaning of the sensor, which proved to be essential for successful implementation of a fiber optic RI sensor in machining emulsion monitoring applications.

Corrosion-induced optical fiber microbending is demonstrated within this article as an efficient method for the design of sensors for the detection and localization of corrosion events on monitored metal surfaces. The proposed sensors were demonstrated in a single point and fully distributed configurations while applied successfully on bare metal surfaces and underneath corrosion-protective coatings. The intrinsic simplicity in the design and manufacturing of the proposed sensors, reliable corrosion detection, and corrosion localization are the main attributes of the proposed microbend corrosion sensing concept.

This paper presents a short response time, all-silica, gas-flow-velocity sensor. The active section of the sensor consists of a 16 µm diameter, highly optically absorbing micro-wire, which is heated remotely by a 980 nm light source. The heated microwire forms a Fabry–Perot interferometer whose temperature is observed at standard telecom wavelengths (1550 nm). The short response time of the sensor allows for different interrogation approaches. Direct measurement of the sensor’s thermal time constant allowed for flow-velocity measurements independent of the absolute heating power delivered to the sensor. This measurement approach also resulted in a simple and cost-efficient interrogation system, which utilized only a few telecom components. The sensor’s short response time, furthermore, allowed for dynamic flow sensing (including turbulence detection). The sensor’s bandwidth was measured experimentally and proved to be in the range of around 22 Hz at low flow velocities. Using time constant measurement, we achieved a flow-velocity resolution up to 0.006 m/s at lower flow velocities, while the resolution in the constant power configuration was better than 0.003 m/s at low flow velocities. The sensing system is constructed around standard telecommunication optoelectronic components, and thus suitable for a wide range of applications.

This paper presents a frequency-modulated optical signal generator in the THz band. The proposed method is based on a fast optical frequency sweep of a single narrowband laser diode used together with an optical fiber interferometer. The optical frequency sweep using a single laser diode is achieved by generating short current pulses with a high amplitude, which are driving the laser diode. Theoretical analysis showed that the modulation frequency could be changed by the optical path difference of the interferometer or optical frequency sweep rate of a laser diode. The efficiency of the optical signal generator with Michelson and Fabry–Perot interferometers is theoretically analyzed and experimentally evaluated for three different scenarios. Interferometers with different optical path differences and a fixed optical frequency sweep rate were used in the first scenario. Different optical frequency sweep rates and fixed optical path differences of the interferometers were used in the second scenario. This paper presents a method for optical chirp generation using a programmable current pulse waveform, which drives a laser diode to achieve nonlinear optical sweep with a fixed optical path difference of the interferometer. The experimental results showed that the proposed signals could be generated within a microwave (1–30 GHz) and THz band (0.1–0.3 THz).

This paper presents a method for gas concentration determination based on the measurement of the refractive index dispersion of a gas near the gas resonance in the near-infrared region (NIR). The gas refractive index dispersion line shape is reconstructed from the variation in the spectral interference fringes’ periods, which are generated by a low-finesse Fabry-Perot interferometer during the DFB diode’s linear-over-time optical frequency sweep around the gas resonance frequency. The entire sensing system was modeled and then verified experimentally, for an example of a low concentration methane-air mixture. We demonstrate experimentally a refractive index dispersion measurement resolution of 2 × 10⁻⁹ refractive index units (RIU), which corresponds to a change in methane concentration in air of 0.04 vol% at the resonant frequency of 181.285 THz (1653.7 nm). The experimental and modeling results show an excellent agreement. The presented system utilizes a very simple optical design and has good potential for the realization of cost-efficient gas sensors that can be operated remotely through standard telecom optical fibers.