Search

In this paper, we present an overview of the current efforts toward integration on chip of fiber Bragg grating (FBG) sensor interrogators. Different photonic-integration platforms are discussed, including monolithic planar lightwave circuit technology, silicon on insulator (SOI), indium phosphide, and gallium arsenide material platforms. Furthermore, various possible techniques for wavelength metering and methods for FBG multiplexing are discussed and compared in terms of resolution, dynamic performance, multiplexing capabilities, and reliability. The use of linear filters, array waveguide gratings (AWG) as multiple linear filters, and AWG-based centroid signal processing techniques are presented as well as interrogation techniques based on tunable microring resonators and Mach–Zehnder interferometers for phase sensitive detection. FBG sensor interrogation based on SOI platform using active and passive phase sensitive detection is also described, demonstrating specifically the potential of passive phase demodulation for high-speed dynamic strain measurements. This paper finally presents the challenges and perspectives of photonic integration to address the increasing requirements of several industrial applications.

U-PHOS (Upgraded PHP Only for Space) is a project developed within the REXUS/BEXUS programme framework, by a team of students from the University of Pisa with the goal to analyse and characterize the behaviour of a Pulsating Heat Pipe (PHP), one of the most attractive two phases passive systems for thermal management in space applications. The PHP consists of a sealed serpentine capillary tube filled with a working fluid. The heat is efficiently transported by means of the combined action of phase change and capillary forces, so no extra equipment is required. The project aims at investigating the thermal response of such a device under a milli-gravity condition, in order to assess its effectiveness in space conditions. To do so, the power delivered to the system, the internal pressure of the PHP and in particular its temperature in several points must be measured. The temperature measurements are performed by an innovative fibre sensing solution based on arrays of fibre Bragg Gratings (FBGs), which act as temperature sensors at specific locations along only one optical fibre. This paper intends to describe the system built by the students, focusing on the FBG temperature measurements systems. A comparison between the adopted solution and other possible measurement systems is performed, highlighting advantages and drawbacks.

The authors experimentally demonstrate the feasibility of interrogating Fiber Bragg Grating (FBG) sensors using an integrated unbalanced Mach–Zehnder Interferometer and active phase demodulation on silicon-on-insulator platform. The use of an external arrayed waveguide grating at the output of the circuit allows the interrogation of multiple FBGs through wavelength division multiplexing. Signal processing employing the phase-generated carrier demodulation technique is used to extract the wavelength shift from the signal patterns, allowing accurate dynamic FBG interrogation. The performance of the proposed integrated FBG interrogator is validated by comparing it with a commercial FBG readout unit based on a spectrometer and used as a reference. Experimental results demonstrate a dynamic strain resolution of 72.3 nϵ/√Hz.

In this paper we present an overview of the current efforts towards integration of Fiber Bragg Grating (FBG) sensor interrogators. Different photonic integration platforms will be discussed, including monolithic planar lightwave circuit technology, silicon on insulator (SOI), indium phosphide (InP) and gallium arsenide (GaAs) material platforms. Also various possible techniques for wavelength metering and methods for FBG multiplexing will be discussed and compared in terms of resolution, dynamic performance, multiplexing capabilities and reliability. The use of linear filters, array waveguide gratings (AWG) as multiple linear filters and AWG based centroid signal processing techniques will be addressed as well as interrogation techniques based on tunable micro-ring resonators and Mach-Zehnder interferometers (MZI) for phase sensitive detection. The paper will also discuss the challenges and perspectives of photonic integration to address the increasing requirements of several industrial applications.

We present a Fiber Bragg Grating sensor interrogator in silicon-on-insulator (SOI) platform based on interferometric wavelength-shift detection. A commercial FBG readout unit is used as a reference for validation of dynamic strain measurement.

We experimentally demonstrate the feasibility of Fiber Bragg Grating sensors interrogation using integrated unbalanced Mach-Zehnder Interferometers (MZI) and phase sensitive detection in silicon-on-insulator (SOI) platform. The Phase- Generated Carrier (PGC) demodulation technique is used to detect phase changes, avoiding signal fading. Signal processing allows us to extract the wavelength shift from the signal patterns, allowing accurate dynamic FBG interrogation. High resolution and low cost chips with multiple interrogators and photodetectors on board can be realized by exploiting the advantages of large scale fabrication capabilities of well-established silicon based industrial infrastructures. Simultaneous dynamic reading of a large number of FBG sensors can lead to large volume market applications of the technology in several strategic industrial fields. The performance of the proposed integrated FBG interrogator is validated by comparing with a commercial FBG readout based on a spectrometer and used as a reference.

In this paper we present a validation of distributed Raman temperature sensing (RDTS) at the CERN high energy accelerator mixed field radiation test facility (CHARM), newly developed in order to qualify electronics for the challenging radiation environment of accelerators and connected high energy physics experiments. By investigating the effect of wavelength dependent radiation induced absorption (RIA) on the Raman Stokes and anti-Stokes light components in radiation tolerant Ge-doped multi-mode (MM) graded-index optical fibers, we demonstrate that Raman DTS used in loop configuration is robust to harsh environments in which the fiber is exposed to a mixed radiation field. The temperature profiles measured on commercial Ge-doped optical fibers is fully reliable and therefore, can be used to correct the RIA temperature dependence in distributed radiation sensing systems based on P-doped optical fibers.

In this paper, we experimentally demonstrate a hybrid distributed acoustic and temperature sensor (DATS) based on Raman and coherent Rayleigh scattering processes in a standard singlemode fiber. A single commercial off-the-shelf DFB laser and a common receiver block are used to implement a highly integrated hybrid sensor system with key industrial applications. Distributed acoustic sensing and Raman temperature measurement are simultaneously performed by exploiting cyclic Simplex pulse coding in a phase-sensitive OTDR and in Raman DTS using direct detection. Suitable control and modulation of the DFB laser ensures inter-pulse incoherence and intra-pulse coherence, enabling accurate long-distance measurement of vibrations and temperature with minimal post-processing.

We propose and experimentally demonstrate the feasibility of a highly efficient FBG-based quasi-distributed sensing system employing dual-wavelength cyclic pulse coding. Significant improvement in the measurement range, resolution and TDM multiplexing capabilities can be achieved, as well as crosstalk reduction with respect to a single wavelength TDM-based FBG interrogation scheme. The mechanism of noise reduction by quasi-periodic cyclic coding is experimentally demonstrated, pointing out significant improvement in accuracy with respect to dual-wavelength single pulse TDM-based FBG interrogation. The proposed technique can also enhance the sensing range of hybrid fiber optic sensor systems in which continuous monitoring of distributed and discrete points are simultaneously measured over the same sensing fiber.

We describe hybrid distributed optical fiber sensor schemes for temperature, strain and vibration measurements. In particular, we first introduce the use of cyclic Simplex coding in hybrid Raman/BOTDA distributed sensors for fast long-distance strain and simultaneous temperature measurements over the same SMF, using a single narrowband source. The same advanced coding technique is also proposed to demonstrate a hybrid distributed acoustic and temperature sensor (DATS) based on Raman and coherent Rayleigh scattering processes in a standard SMF, using a single commercial off-the-shelf DFB laser and a common receiver block.

A field trial has been performed at the CERN high-energy accelerator-mixed (CHARM) field facility, newly developed for testing devices within accelerator representative radiation environments, to validate the use of Raman-based optical fiber sensors for distributed temperature measurements in highly radiative environments. Experimental results demonstrate that Raman distributed temperature sensors, operating in loop configuration on radiation-tolerant optical fibers, are able to compensate for wavelength-dependent losses and are, therefore, robust to harsh environments, in which a mixed-field radiation, including protons, neutrons, photons, and other particles, is potentially altering the fiber material properties. The temperature profile measured on commercial radiation-tolerant optical fibers shows that a temperature resolution

A distributed fiber-optic temperature sensor technique inherently allowing for system calibration, compensating time-dependent variations of the fiber losses as well as local external perturbations, is proposed using a loop-scheme together with Raman anti-Stokes-only measurement. A temperature resolution enhancement with respect to a standard loop configuration is shown by experiments, providing a robust and reliable high-performance sensing technique for long sensing ranges.

We propose and experimentally demonstrate a fully hybrid distributed sensing scheme that uses a single narrow-band laser to perform fast measurement of the BFS using BOTDA and simultaneous temperature measurement based on spontaneous Raman scattering over 10 km of single mode fiber. The use of cyclic pulse coding effectively reduces the pump peak power levels required for accurate Raman-based distributed temperature measurement, enhancing at the same time the speed of the BFS measurement in the BOTDA technique.

Cyclic pulse coding is proposed for fast implementation of long-range Brillouin optical time domain analysis and simultaneous static Raman based temperature measurement over 10 km of standard single mode fiber using a narrowband laser source only.

In this paper we perform field validation of distributed Raman temperature sensing (RDTS) for oil leakage detection in soil. The capability of the distributed Raman sensor in detecting and locating, with high accuracy and spatial resolution, drop leakages in soil is demonstrated through a water leakage simulation in a field trial. The future trends and the high potential of the Raman DTS technology for oil and gas leakage detection in long pipelines is then outlined in this paper by reporting lab experiments demonstrating accurate meter scale temperature measurement over more than 50 km of standard single mode fiber. The proposed solution, based on distributed Simplex coding techniques, can be competitive in terms of cost and performance with respect to other distributed sensing technologies.

We propose and experimentally demonstrate the use of cyclic pulse coding to improve the performance of hybrid Raman/fiber Bragg grating (FBG) fiber-optic sensors, for simultaneous measurement of distributed static temperature and discrete dynamic strain over the same sensing fiber. Effective noise reduction is achieved in both Raman optical time-domain reflectometry and dynamic interrogation of time-division-multiplexed fiber FBG sensors, enhancing the sensing range resolution and providing real-time point dynamic strain measurement capabilities. The highly integrated sensor scheme employs broadband apodized low-reflectivity FBGs, a single narrowband optical source, and a shared receiver block.

A cyclic pulse coding technique is proposed and experimentally demonstrated for fast implementation of long-range Brillouin optical time-domain analysis (BOTDA). The proposed technique allows for accurate temperature and strain measurements with meter-scale spatial resolution over kilometers of standard single-mode fiber, with subsecond measurement times.

Hybrid sensing is becoming a key technology in application fields requiring simultaneous distributed temperature and discrete strain measurements. Two hybrid fiber optic sensing techniques effectively combining Raman and FBG-based sensing are presented in this paper.

A novel method to improve the measurement range, resolution, and multiplexing capability of time division multiplexed (TDM) fiber Bragg grating (FBM) sensors, based on advanced cyclic pulse coding, is proposed and experimentally demonstrated. The mechanism of noise reduction by quasi-periodic cyclic coding is quantitatively demonstrated, pointing out significant improvement in dynamic strain resolution with respect to single pulse TDM FBG interrogation. The use of cycling pulse coding allows for enhanced dynamic performance of real time strain measurement with respect to other coding techniques. The proposed technique can also enhance the sensing range of hybrid optical fiber sensor systems in which continuous static temperature/strain distributions and discrete dynamic strain in specific critical points are simultaneously measured over the same sensing fiber.

We propose and experimentally demonstrate a hybrid fiber optic sensing technique that effectively combines Raman optical time domain reflectometry and in-line time-division-multiplexing for fiber Bragg grating (FBG) dynamic interrogation. The highly integrated proposed scheme employs broadband apodized low reflectivity FBGs with a single narrowband optical source and a shared receiver block, allowing for simultaneous measurements of distributed static temperature and discrete dynamic strain, over the same sensing fiber.

We propose and experimentally demonstrate a hybrid fiber optic sensing technique that effectively combines Brillouin optical time-domain analysis and a time -domain multiplexing interrogation technique for Fiber Bragg Gratings (FBGs). The highly-integrated proposed scheme employs broadband apodized low-reflectivity FBGs with a single optical source and a shared receiver block, allowing for simultaneous measurements of distributed static and discrete dynamic temperature and strain, over the same sensing fiber. Keywords: Brillouin scattering, Fiber Bragg Gratings, fiber optic sensors, temperature sensing, strain sensing. 1. INTRODUCTION During the last years, distributed optical fiber sensing based on stimulated Brillouin scattering (SBS) has become an interesting approach for temperature and strain sensing in many industrial applications. In particular, distributed sensing with meter and sub-meter spatial resolution along several tens of km has been demonstrated using Brillouin optical time-domain analysis (BOTDA) technique [1]. Even though the sensing capabilities provided by BOTDA systems are attractive in many applications, the relatively long measurement time (of the order of a few minutes) makes this technology mainly suitable for static measurements, limiting its potential range of applications. Although the distributed information provided by BOTDA sensing is essential for static strain and temperature monitoring, there are several applications, such as for example in industrial Oil&Gas production plants monitoring, which also require dynamic strain measurements at specific critical points, ad ding in this way crucial information on the structure integrity (e.g. vibrations). Distributed sensing based on either spontaneous Brillouin scattering (SpBS) or SBS has been recently combined with point based Fiber Bragg Grating (FBG) sensors [3,4]. In particular, BOTDA technique has been combined with wavelength-division-multiplexed FBG point sensors, using different demodulation techni ques for distribute d and discrete sensing, as well as distributed Raman amplification, increasing cost and complexity of the whole sensor system [4]. In this paper we propose a highly-integrated hybrid sensing system that effectively combines the advantages of both standard BOTDA and in-line time-division-multiplexed (TDM) FBG-based sensing, allowing for simultaneous dynamic discrete strain measurements and distributed (static) strain/temperature sensing using a single optical fiber, a common pulsed narrowband optical source and a shared receiver unit.

We present advanced techniques for high performance Raman distributed temperature sensing systems, based on graded index multi-mode and standard single-mode fibers. Advanced coding allows for ultra-long sensing capabilities with meter or sub-meter scale spatial resolution

In this paper we demonstrate distributed Raman temperature sensing (RDTS) in a loop scheme employing anti-Stokes light intensity only. Using a single-channel receiver and anti-Stokes traces measured in loop configuration, we implement RDTS with inherent compensation of fiber wavelength-dependent losses, as well as local external perturbations. Experimental results show a signal-to-noise ratio enhancement with respect to a standard RDTS in loop configuration, providing a robust and reliable high-performance sensor for long sensing ranges. Keywords: Raman scattering, fiber optic sensors, temperature sensing.

We experimentally demonstrate the use of cyclic pulse coding for distributed strain and temperature measurements in hybrid Raman/Brillouin optical time-domain analysis (BOTDA) optical fiber sensors. The highly integrated proposed solution effectively addresses the strain/temperature cross-sensitivity issue affecting standard BOTDA sensors, allowing for simultaneous meter-scale strain and temperature measurements over 10 km of standard single mode fiber using a single narrowband laser source only.

We experimentally investigate the benefits of a new optical pulse coding technique for long-range, meter and submeter scale Raman-based distributed temperature sensing on standard single-mode optical fibers. The proposed scheme combines a low-repetition-rate quasi-periodic pulse coding technique with the use of standard high-power fiber lasers operating at 1550 nm , allowing for what we believe is the first long-range distributed temperature measurement over single-mode fibers (SMFs). We have achieved 1 m spatial resolution over 26 km of SMF, attaining 3°C temperature resolution within 30 s measurement time.

A long-range fast-measurement Raman-based distributed temperature sensor (RDTS) with a meter spatial resolution exploiting a quasi-periodic pulse-coding technique is proposed in this paper. We experimentally demonstrate the feasibility of temperature sensing over 26 km standard single-mode fiber with 1 m spatial resolution obtaining 3 °C of temperature resolution within 30 s of measurement time. Experimental results employing 1023-bits codewords are also reported, showing a notable range enhancement and enabling distributed sensing over 58 km distance with 2 m spatial resolution and 4 °C of temperature resolution within 5 min measurement time.

We propose and experimentally demonstrate the feasibility of an integrated hybrid optical fiber sensing interrogation technique that efficiently combines distributed Raman-based temperature sensing with fiber Bragg grating (FBG)-based dynamic strain measurements. The proposed sensing system is highly integrated, making use of a common optical source/receiver block and exploiting the advantages of both (distributed and point) sensing technologies simultaneously. A multimode fiber is used for distributed temperature sensing, and a pair of FBGs in each discrete sensing point, partially overlapped in the spectral domain, allows for temperature-independent discrete strain measurements. Experimental results report a dynamic strain resolution of 7.8 nε/√Hz within a full range of 1700 με and a distributed temperature resolution of 1°C at 20 km distance with 2.7 m spatial resolution.

We propose and experimentally demonstrate an integrated hybrid optical fiber sensing technique that combines distributed Raman-based temperature measurements with a time-domain approach for FBG dynamic interrogation using a shared optical source and receiver block.

We describe the use of pulse coding for long-range distributed Raman and Brillouin optical time domain analysis (BOTDA) sensors and for implementing hybrid Raman/BOTDA distributed sensors for simultaneous strain and temperature measurement on the same fiber.