Stefan Werzinger

Quasi-Distributed Fiber Bragg Grating Sensing Using Stepped Incoherent Optical Frequency Domain Reflectometry

We examine the quasi-distributed interrogation of fiber Bragg grating arrays (FBGA) with the method of stepped incoherent optical frequency domain reflectometry (IOFDR) combined with wavelength scanning of a tunable laser source. The technique of IOFDR provides some advantages over traditional time division multiplexing schemes, because continuous wave modulation of the light intensity instead of short and low energy pulses is used, maximizing the reflected signal received from the sensor fiber. Moreover, an electrical heterodyne demodulation by a microwave vector network analyzer provides a low-noise detection, while achieving high spatial resolutions down to the centimeter range. A quasi-distributed temperature measurement with ten FBGs of 0.5% reflectivity is demonstrated. In the experiment, a root mean square temperature error of 0.3 K and a maximum error of 1 K is observed, using a fiber-coupled power of only -12 dBm. A spatial two-point resolution of 2.14 cm is achieved, which enabled a successful interrogation of FBGAs with 20 and 30 cm spacings. The potential multiplexing capability of the given setup could reach more than 50 gratings in a 30 cm spacing at a measurement rate of 50 s/nm.

An Analytic Connector Loss Model for Step-Index Polymer Optical Fiber Links

The authors report on an analytic connector loss model for step-index polymer optical fibers (SI-POF). The model describes the non-linear dependencies for combinations of the most important intrinsic and extrinsic influence parameters like Fresnel reflections, mismatches in numerical aperture (NA) and core diameters, as well as lateral, longitudinal and angular offsets. As connector losses strongly depend on the mode distribution, expressions for the mode-dependent coupling efficiencies for the various influence parameters are derived. This way, the model can handle any rotationally symmetric mode distribution including the uniform mode distribution (UMD) and the equilibrium mode distribution (EMD) in the steady-state. It is shown that for step-index fibers, lateral and longitudinal offsets can be handled in a similar way, which reduces the overall effort for the mathematical description. For the typical case of identical fibers, the authors also derive easy-to-apply approximations for a combination of lateral and longitudinal offsets under UMD and EMD conditions. The results are in good agreement with ray-tracing simulations and are evaluated for the cases of identical fibers and the worst case parameter constellation for the SI-POF in the IEC A4a.2 fiber class.

Statistical analysis of intrinsic and extrinsic coupling losses for step-index polymer optical fibers

The intrinsic and extrinsic coupling losses of step-index polymer optical fibers are statistically examined by Monte Carlo simulations. In contrast to most existing models that linearly scale individual losses, a comprehensive analytic coupling loss model is used that also considers the interdependencies between mismatches in numerical aperture and core diameter, as well as radial and longitudinal offsets. As a typical example, the coupling losses of A4a.2 step-index multimode fibers are analyzed for an equilibrium mode distribution. The results show considerably less conservative coupling loss estimations than with traditional models, improving link power budgeting.

Model-based compressed sensing of fiber Bragg grating arrays in the frequency domain

We propose a model-based compressed sensing (MBCS) of FBG arrays (FBGA), interrogated with wavelength scanning incoherent optical frequency domain reflectometry. This method measures the frequency response of a FBGA with an electrical vector network analyzer combined with a tunable laser. Instead of the usual inverse discrete Fourier transform (IDFT), we apply a direct estimation of the grating reflectivities with a simple frequency domain model. A reconstruction of 10 gratings spaced by 20 cm is demonstrated. MBCS allows to reduce the number of measurement frequencies from 120 to 8, compared to an IDFT, while using a bandwidth of just 500 MHz.

Spatially resolved fiber Bragg grating sensing using wavelength scanning incoherent OFDR

A spatially resolved interrogation of fiber Bragg grating sensors, using incoherent optical frequency domain reflectometry and wavelength scanning is demonstrated. The method potentially allows a spectrally denser sensor packing and provides centimeter range resolution.

Model-Based Position and Reflectivity Estimation of Fiber Bragg Grating Sensor Arrays

We propose an efficient model-based signal processing approach for optical fiber sensing with fiber Bragg grating (FBG) arrays. A position estimation based on an estimation of distribution algorithm (EDA) and a reflectivity estimation method using a parametric transfer matrix model (TMM) are outlined in detail. The estimation algorithms are evaluated with Monte Carlo simulations and measurement data from an incoherent optical frequency domain reflectometer (iOFDR). The model-based approach outperforms conventional Fourier transform processing, especially near the spatial resolution limit, saving electrical bandwidth and measurement time. The models provide great flexibility and can be easily expanded in complexity to meet different topologies and to include prior knowledge of the sensors. Systematic errors due to crosstalk between gratings caused by multiple reflections and spectral shadowing could be further considered with the TMM to improve the performance of large-scale FBG array sensor systems.

Using coherent optical frequency domain reflectometry to assist the additive manufacturing process of structures for radio frequency applications

Additive manufacturing has already found broad acceptance in rapid prototyping of machinery and is an emerging technology in many other fields such as radio frequency (RF) engineering, where the advantages of the so-called 3D printing technology overcome limitations of established processes and allow entirely new designs. The ability to create almost arbitrary shapes with high precision has proven very useful for antenna design, for example. Using conductive and dielectric ink, RF transmission lines can be 3D printed directly on uneven surfaces. As for RF structures geometrical dimensions are crucial for the resulting RF properties such as impedance, a technique to measure the distance between the printing nozzle and the substrate is necessary. This turns out to be a challenging task since a small spot size is required and transparent (dielectric) as well as reflective (conductor) materials must be detected while maintaining a mechanically flexible and robust system. We propose a distance measurement system based on coherent optical frequency domain reflectometry to accurately measure this distance. The proposed miniaturized coupling optic uses a gradient-index (GRIN) lens with a diameter of less than 3 mm, can be integrated into a printing head easily and is compatible to standard single-mode fibers. In first experiments, we have achieved very promising results that show a good agreement with (destructive) microscopic measurements. Reflective and transparent surfaces can be detected with μm-accuracy.

Effective light coupling in reflective fiber-optic distance sensors using a double-clad fiber

Many fiber optic distance sensors use a reflective configuration, where a light beam is launched from an optical fiber, reflected from a target and coupled back into the fiber. While singlemode fibers (SMF) provide low-loss, high-performance components and a well-defined output beam, the coupling of the reflected light into the SMF is very sensitive to mechanical misalignments and scattering at the reflecting target. In this paper we use a double-clad fiber (DCF) and a DCF coupler to obtain an enhanced multimodal coupling of reflected light into the fiber. Increased power levels and robustness are achieved compared to a pure SMF configuration.

Fiber sensor identification based on incoherent Rayleigh backscatter measurements in the frequency domain

In this work, a fiber identification method based on incoherent optical frequency domain reflectometry (lOFDR) measurements is introduced. The proposed method uses the characteristic interference pattern of IOFDR Rayleigh backscatter measurements with a broadband light source to unambiguously recognize different initially scanned fiber segments. The recognition is achieved by crosscorrelating the spatially resolved Rayleigh backscatter profile of the fiber segment under test with a initially measured and stored backscatter profile. This profile was found to be relatively insensitive to temperature changes. It is shown that identification is possible even if the fiber segment in question is installed subsequent to 300 m of lead fiber.

High resolution position measurement of “flying particles” inside hollow-core photonic crystal fiber

Optically trapped “flying particles” inside hollow core photonic crystal fiber (HC-PCF) can be used as multiparameter sensors of, for example, temperature, radiation levels or external electric fields. They represent a new type of optical fiber sensor, offering a spatial resolution that is only limited by the particle size, while being functionally reconfigurable. Here we demonstrate accurate measurement of the axial position of flying particles using incoherent optical frequency domain reflectometry in combination with model-based estimation processing. The approach allows to measure the particle position inside the HC-PCF with a precision of ∼140 pm.