Search

This work is devoted to a feasibility analysis for the development of fiber optic humidity sensors to be applied in high-energy physics applications and in particular in the experiments actually running at the European Organization for Nuclear Research (CERN). On this line of argument, due to the wide investigations carried out in the last years aimed to assess the radiation hardness capability of fiber optic technology in high energy physics environments, our multidisciplinary research group has been recently engaged in the development and assessment of fiber optic sensors based on polyimide-coated Fiber Bragg Gratings (FBGs) to perform relative humidity (RH) monitoring at temperatures below 0°C as well as in presence of strong ionizing radiations. Data here reported, obtained during a deep experimental campaign carried out in the laboratories of CERN, demonstrate the amazing RH sensing properties of such sensors in the temperature range −15–20°C as well as their radiation hardness capability up to (at least) 10kGy dose of ionizing radiations.

The approximation that the current is constant in any point of the emitter is not true for solar cells operating under concentrated sunlight. The transverse paths followed by photogenerated carriers to reach metal grid electrodes causes voltage drops leading to non-uniform distributed surface potential and a consequent reduction of the conversion efficiency. Moreover, concentrating lens often provide non-uniform light intensity thus worsening non-uniform current distribution. The correct design of surface geometry of the cell (metal grid, emitter depth, sheet resistance) should always take into account the above effects. Unfortunately a lack of reliable models still exists. In this paper we analytically solve the three-dimensional semiconductor equations with proper boundary conditions and taking into account, as source term, a distributed generation contribute. The reliability of the model has been tested by numerical simulations and experimentally verified with a lock-in thermography technique (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this paper, we present experimental demonstration of using chirped-pulsed frequency modulation technique (C-PFM) in order to increase the multiplexing capability of a multipoint fiber Bragg grating (FBG) sensor network without increasing the hardware complexity of the acquisition system. This technique, extensively used in radar applications, is based on the amplitude modulation of the emitted signal (pulsed) by a chirped frequency signal and on the use of an adaptive filter for the demodulation of the reflected signals. Here, the same principle has been used to develop a FBG multisensor interrogation system by employing a proper frequency chirped modulation. The optical signals reflected from the FBG sensors are first interrogated by using an in-fiber edge filter and discriminated by a novel adaptive numerical filtering. The potentiality and limitation of such technique are discussed. Preliminary experimental results are presented.

This contribution deals with a feasibility analysis for the development of radiation tolerant fiber optic humidity sensors based on long period grating (LPG) technology to be applied in high-energy physics (HEP) experiments currently running at the European Organization for Nuclear Research (CERN). In particular, here we propose a high-sensitivity LPG sensor coated with a finely tuned titanium dioxide (TiO2) thin layer (similar to 100 nm thick) through the sol gel deposition method. The sensor characterization in the relative humidity (RH) range [0-75] % at four different temperatures (in the range -10 degrees C - 25 degrees C) was carried out to assess sensor performances in real operative conditions required in typical experiments running at CERN. Experimental results demonstrate the very high RH sensitivities of the proposed device (up to 1.4 nm/%RH in correspondence of very low humidity levels), which turned out to be from one to three orders of magnitudes higher than those exhibited by fiber Bragg grating (FBG) sensors coated with micrometer thin polyimide overlays. The radiation tolerance capability of the TiO2-coated LPG sensor is also investigated by comparing the sensing performances before and after its exposure to 1Mrad dose of gamma-ionizing radiation. Collected results demonstrate the strong potentialities of the proposed technology in light of its future exploitation in HEP applications as robust and valid alternative to currently used commercial hygrometers.

This work investigates the performance and the radiation hardness capability of optical thermo-hygrometers based on Fibre Bragg Gratings (FBG) for humidity monitoring in the Compact Muon Solenoid (CMS), one of the four experiments running at CERN in Geneva. A thorough campaign of characterization was performed on 80 specially produced Polyimide-coated RH FBG sensors and 80 commercial temperature FBG sensors. Sensitivity, repeatability and accuracy were studied on the whole batch, putting in evidence the limits of the sensors, but also showing that they can be used in very dry conditions. In order to extract the humidity measurements from the sensor readings, commercial temperature FBG sensors were characterized in the range of interest. Irradiation campaigns with ionizing radiation (gamma-rays from a Co-60 source) at incremental absorbed doses (up to 210 kGy for the T sensors and up to 90 kGy for the RH sensors) were performed on sample of T and RH-Sensors. The results show that the sensitivity of the sensors is unchanged up to the level attained of the absorbed dose, while the natural wavelength peak of each sensor exhibits a radiation-induced shift (signal offset). The saturation properties of this shift are discussed.

The single mode condition in large cross section rib waveguides is of great interest because almost every kind of active and passive integrated optoelectronic device or sensor is designed to sustain only the fundamental mode of propagation for better matching with optical fibers. In this paper we present a criterion to determine the single mode condition for a large cross section rib waveguides, by comparison between the numerical solutions found with Neumann boundary conditions and Dirichlet boundaries conditions applied when solving the eigenvalues problem.

The Drever-Pound-Hall technique is a powerful tool to stabilize the laser frequency or to lock a cavity to a laser by controlling its length in the order of fraction of wavelength1-3 . It had been widely applied as method to interrogate fiber optical cavity based sensors, as strain sensors or refractive index sensors, since it allows to reach very high sensitivity, especially in dynamic range, only theoretically limited by the laser shot noise4-7 . In this paper we present a detailed analysis on the possibility to use the DPH technique for the simultaneous detection of detuning of two or more cavities each lying on a different output branch of a splitter, by interrogating them using only the single input channel of the splitter. More precisely, starting from a reflection configuration of the present technique, where the error signal to control the cavity length is extracted by the signal reflected by the cavity, we analyze all the possible configurations to simultaneously interrogate and discriminate the different cavities, using the same input channel to have not overlap and interference between the signals reflected by each of them. The interest of this kind of analysis resides in the possibility to design very compact and less invading sensors that requires bidirectional detection of the involved physical quantity or, the simultaneous and independent control of several parameters (like, for example, bidirectional strain sensors, that simultaneously detect the strain along two orthogonal directions, or magnetic field sensor able to determine the intensity of the field along perpendicular directions, or, refractive index sensor temperature calibrated8 ). Using a single interrogation/detection channel the sensor can be placed far away from the interrogation/detection apparatus and connected to the latest only by means of a single optical fiber, instead to have more signal detection channels.

One of the fundamental advantages in the employment of the Fiber Bragg Grating (FBG) sensors lies in their capability to allow measurements in extreme environmental conditions with also very high immunity toward external electromagnetic interference factors. The behavior of a polymer-coated FBG sensor, tested in cryogenic conditions at the laboratories of the European Organization for the Nuclear Research (CERN) in Geneva, has been analyzed and will be discussed in this paper. Magnets used in the Large Hadron Collider (LHC) for the High Energy Physics Researches at CERN need in fact extreme cooling conditions to preserve the internal superconductivity highly crucial for their performances. The magnets, built with NbTi based superconductors, are cooled with liquid helium and they operate at 1.9 K. The aim of the present work is to estimate the thermal expansion coefficient of two polymers based on epoxy and methacrylate (PMMA) used as coating of FBGs, in the temperature range from 4 K to 300 K and to check their suitability for the use in temperature monitoring of the superconducting magnets. A standard numerical derivative method has been employed to estimate thermal expansion coefficients; moreover the correlated fluctuation analysis (CFA) based procedure is proposed, as a reliable alternative, to overcome numerical derivative drawbacks at very low temperatures within the range of 4–20 K. The calculated values of thermal expansion coefficient for both systems are in agreement with literature data on similar material.

A calibration method for fiber Bragg grating (FBG) cryogenic temperature sensors based on look-up-table was proposed and demonstrated experimentally. The operating principles of different kind of FBG cryogenic temperature sensors are introduced. A statistical characterization of the data was carried out to verify the quasi-static condition of the measurement system by checking the stability of the measurement. Once verified this condition, the sensitivity curves of the sensors were determined by correlating the wavelength shift of the FBGs with the reference sensor measurements. On the basis of the sensitivity curves, the look-up-table (LUT) have been determined. The experimental data shows that the LUT fitting approach provides good and reliable performance in terms of accuracy and processing time.

In this paper we present a preliminary study to realize an integrated photonic crystal double cavities refractive index sensor calibrated in temperature. The studied conguration allows to realize a very compact device with only one interrogation channel, since the monitored signals are the cavities re ected signals. The sensitive elements used are the modulation of the cavities linewidth due to temperature and refractive index change, measured by means of the cavities detuning. The appeal of such type of devices, respect to the corresponding ones in optical bers, is the possibility to expand the conguration to create on the same chip the detector and the requested signal processing devices. The reliability of the proposed conguration is related to the interrogation technique, based on the radio-frequency phase modulation of the impinging laser light. This techniques was widely demonstrated in the last years 1-4 and initially borrowed by the cavity frequency stabilization and locking Pound-Drever-Hall methods. 5 Here we demonstrate as it is possible to use it for simultaneously detection of the detuning of two cavities with only one interrogation channel.

In this work, we study the effect of anisotropic magnetostriction on Terfenol-D based fiber Bragg grating magnetic sensors under different types of magneto-mechanical loading. We demonstrate that a Terfenol-D rod is subjected to axial strain also when the magnetic field is perpendicular to the rod axis and as consequence we question the hypothesis of uniaxial behavior that is normally found in the literature for these sensors. In order to address this issue, we propose a calibration model that introduces transverse sensitivity coefficients that take into account the effect of the transverse magnetic field.

Electrically Induced Bragg Reflectors can be very attractive to realize programable waveguides networks. Their practical realization is nevertheless intrinsically connected to the capability to make a peculiar comb-structure electrode on the top of the waveguides with typical period of 200 nm (corresponding to the Bragg length) and a tolerance of few nanometers. In this work, the experimental fabrication of these comb-structure electrodes by means of electron beam lithography is described. We fabricated large areas 1D periodic gold structures with nanometer resolution by using a high resolution electron beam lithography (EBL) process and a post-processing technique based on lift-off. These electrodes can be employed as Induced Bragg Reflectors in a multilayer structure for a not permanent periodically modulation of the effective refractive index of the guiding structure. The desired structures are obtained with nanometric resolution by means of EBL, digging furrows of rectangular section in both a polymetilmethacrylate (PMMA) and in α-chloromethacrylate and α-methylstyrene (ZEP) layer spin-coated on silicon, then evaporating a metal layer (Au) on the top and then by lift-off of metal. The EBL technique allows a very accurate control of the dielectric distribution of the exposed area able to produce feasible, high efficiency periodic and photonic band-gap structures. The resulting 1D gratings are made of metal lines 100 nm wide with a period of 200 nm and, 120 nm wide with a period of 250 nm, respectively. Large area structures (up to 1 mm x 6 μm) have been realized with nanometre resolution and they have been characterized by scanning electron microscopy (SEM). These structures will be used in a future work of ours to realize 40 GHz switching speed modulator by inducing a Bragg Reflector with a reverse biased vertical InP/InGaAsP p-i-n diode according to the predictions of the grating reflectivity spectra and of the transient response.

In this paper we present the activity that our research group is making on Fiber Optic Sensors (FOS) applications to monitor high-energy physics (HEP) experiments. Starting from the consideration that Fiber Optic radiation hardness has been widely proven, we have applied the technology of Fiber Optic Sensors to this very relevant field of interest. Here, we give the experimental evidences of the possibility to use such a class of sensors also in these very complex environmental side conditions. In particular, regarding the Compact Muon Solenoid (CMS) experiment at the CERN, we have monitored temperatures and strains in different locations by using Bragg Grating sensors, and we are now starting the development of a new class of Relative Humidity sensor based on Fiber Optic technology. Preliminary results are very encouraging, letting us consider the use of FOS technique as a robust and effective solution for monitoring requirements in HEP detectors for other physical and environmental parameters.

In this paper we present a novel approach to temperature sensing with optoelectronic devices which relies on the usage of bare silicon as the transducing material. The device is composed by a single mode input waveguide, a MMI region where a number of higher order modes is also allowed to propagate and two output waveguides. The refractive index variation in the MMI section due to temperature shifts induces different phase velocities of the various propagating modes. The position of the input and output waveguides together with the length and width of the MMI section are chosen in order to maximize the sensitivity of the device. Analytical calculations are presented together with BPM simulations aimed to the maximization of the sensitivity of the sensor as a function of its geometries.

In this paper, with the help of numerical simulations, we show how it is possible to create a Distributed Bragg Reflector by electrically inducing periodical refractive index variations in unperturbed III-V rib-waveguides to design different kind of devices. This approach allows to induce the optoelectronic devices by a suitable reconfiguration of the potential applied to the electrode matrix, i.e. to have a defined routing/switching matrix, or, for dynamic applications, realize intensity modulators, capable of reaching a theoretical 40 GHz switching speed.

Here we present a fiber optic seismic waves sensor based on in-fiber Bragg gratings. Fiber Bragg Grating sensors have been demonstrated to have very high sensitivity to dynamical strain in the sub-micro-strain range and very extended dynamical response from static to very high frequency. The seismic sensing system is based on the integration of three FBGs dynamical strain sensors in a mechanical structure acting as an inverse pendulum. Polar symmetry of the mechanical system and 120° placement of the FBG sensors guarantee a directional capability of the seismic sensor. Design, manufacturing and preliminary dynamical testing of the seismic sensor are discussed.

In this paper a significant improvement of the C-PFM multiplexing and reading technique for fiber Bragg grating sensors (FBGs) is introduced and experimentally validated. Compared to ordinary time division multiplexing (TDM) the C-PFM technique is able to give a far better space resolution for an equal pulse duration, but can be improved further in terms of crosstalk between successive FBGs responses by means of a clever pulse windowing as it will be explained in the following. Experimental data confirming this theory will be showed and discussed.

An electrically induced Bragg Reflector Modulator 2.5 mm long has been designed in InP/InGaAsP rib waveguide. By means of an in house code based on a suitable simulation strategy which takes advantage of state-of-the-art electronic simulator such as Silvaco/ATLAS and a general purpose FEM solver such as Comsol Multiphysics, predictions of DC response the transient analysis had been make. The code allows us to use the same grid to evaluate all the quantity of interest, the effective refractive index included. The simulations results show that such a modulator can theoretically reach ultra 40 GHz switching speed.

In this work we aim to realize a step-by-step electro-optical probe which exploits the linear (Pockels) electro-optic effect to survey the electromagnetic field on the surface of RF integrated circuits. This probe measures the variation of light polarization induced by a lithium niobate crystal immersed in the electric field provided by the DUT. The measurements will demonstrate the main features of this system which can be summarized in non-invasiveness, wide bandwidth, linearity and small spatial resolution. The LiNbO 3 crystals have been developed by SELEX Sistemi Integrati S.p.A. Roma, and the whole research activity has been carried out under the sponsorship of the CRdC Nuove Tecnologie per le Attivita Produttive, the Campania Region Centre of Competence on New Technologies.

A low-cost all-fiber Bragg grating sensing system for temperature and strain measurements is presented. Broadband interrogation and grating-based optical filtering are used to detect a strain- or temperature-induced Bragg wavelength shift with a resolution of 1 us and 0.1 degrees C. The passive nature of the grating demodulation enables dynamic measurements, limited only by the electronic circuitry involved in the system read out. The same system is also used for dynamic strain detection. A fiber grating is bonded to a piezoceramic actuator in different configurations. A laser-triangulation-based system is used as a reference measurements up to 10 kHz, while dynamic strain measurements by a Fiber Bragg grating (FBG) are carried out up to 50 kHz, demonstrating a dynamic resolution of approximate to 40 n epsilon/(Hz)(1/2).

In this paper, we present a novel approach to increase the multiplexing capability of a multipoint fiber Bragg grating (FBG) sensor network. Our approach is based on the possibility of exploiting the chirped-pulsed frequency modulation technique (C-PFM) to the optical pulses used to read a FBG sensor network. This technique, extensively used in radar applications, in the case of optical sensors is based on the amplitude modulation of the emitted signal (pulsed) by a chirped frequency signal and on the demodulation of the reflected signal by means of adaptive filter. The main parameters for a multiplexing systems are the minimum distance between two adjacent sensors and the cross-talk achievable. We show that the use of a well designed time shaping windowing of the transmitted pulse can improve both such parameters.

In this paper, we show some preliminary numerical results in order to increase the multiplexing capability of a multipoint fiber Bragg grating (FBG) sensor network without increase the system complexity. We base the new approach on the possibility of exploiting the chirped-pulsed frequency modulation technique (C-PFM). This technique, extensively used in radar applications, is based on the amplitude modulation of the emitted signal (pulsed) by a chirped frequency signal and on the demodulation of the reflected signal by means of adaptive filter. FBG sensors are instrumental to obtain cheap and easy handling multipoint sensing arrays.

In this paper, for the first time, the performance of an electro-optical router, working at a wavelength of 1.55 mum and based on multimode interference effects together with the electro-optic effect of smectic A* liquid crystal, is theoretically and numerically discussed. The device comprises: a single-mode input optical channel waveguide, a bimodal active region with a liquid crystal over-layer, and an output Y-branch to separate the two output channels. The active region is designed to allow a pi shift between the two modes that it supports, by means of the electro-optic effect of smectic A*, in order to steer light from one output channel to the other. The principal advantages of such a device include a fast tuning speed, low power consumption, low cost and small device size.

An electrically controlled filter for the dynamic selection of channels in DWDM optical network is proposed. The device is realized by combining the electrical injection of free carriers with a Bragg grating realized over the top of a Silicon on Insulator RIB waveguide. Apodization of the duty-cycle of the grating has been investigated in order to reduce the crosstalk between adjacent channels; in this way the performances of the filter in terms of spectral bandwidth for 100 and 50 GHz DWDM spacing become attractive.

In this paper we propose an electrically controlled filter for the selection of channels in monitoring DWDM optical network. The highly selective filtering operation needed in this kind of communication systems is obtained with an all silicon device by combining the eletrical injection of free carriers in a forward biased P-i-N diode with a Bragg grating realized over the top of a Silicon on Insulator RIB waveguide. The combination of this fast electron device and the interference principle present under the periodic corrugation of the waveguide allows very fast reconfiguration of the filter. In order to increase filter performaces in terms of selectivity and crosstalk between adjacent channels, apodization of the duty-cycle of the grating is investigated; in this way the performances of the filter in terms of spectral bandwidth for 100 GHz and 50 GHz DWDM spacing become attractive and comparable to ITU specifications.

We show numerical simulations in order to increase the multiplexing capability of a multipoint fiber Bragg grating (FBG) sensor network. We base the new approach on the possibility of exploiting the chirped-pulsed frequency modulation technique (C-PFM). This technique, extensively used in radar applications, is based on the amplitude modulation of the emitted signal (pulsed) by a chirped frequency signal and on the demodulation of the reflected signal by means of adaptive filter. Last year, we have worked to improve the performances of such approach.

The self-imaging principle has been applied in the design of a novel configuration for an electro-optical router, working at the wavelength of 1.55 micron. The device is composed by: a single mode input optical channel waveguide; a bimodal active region, having a liquid crystal as over-layer; an output Y branch, to separate the two output channels. The active region is designed to allow a π shift between the two modes that it supports, by means of electro-optic effect of smectic A*, in order to steer light from one output channel to the other one. In this paper, for the first time, the performances of an electro-optical router, based on the multimode interference effects together with the electro-optic effect of smectic A* liquid crystal, are theoretically and numerically discussed.

In this paper, the optoelectronic characterization of the polymerization process of thermoset-based composites is described. As is well known, in the last decade, these kinds of material in the light of their low weight/mechanical strength ratio have been widely used in many industrial areas such as automotive, aeronautic and aerospace. Because of the dependence of their properties on the manufacturing stage, real-time monitoring of the curing process has been indicated as the key point for improving the quality and reducing manufacturing process costs. In fact, in situ identification of the status of the processed material would allow the implementation of an on-line control of the manufacturing stage, leading to the transformation of the classical process in a real scientific operation. Based on this line of argument, a less-contact optical technique has been used to monitor the refractive index variation of an epoxy-based resin due to the polymerization process. Starting from preliminary experimental results, a fibre optic sensor has been designed and developed in order to perform in situ cure monitoring by refractive index measurement. A theoretical model has been developed and validated by comparison with calorimetric characterization.

This paper reports on the theoretical and numerical description of an optoelectronic router integrated in a silicon-on-silicon waveguide structure. The device is based on the mode-mixing principle together with the injection-induced optical phase shift. The structure consists of a single-mode all-silicon input waveguide, followed by a two-mode section, which acts as the active region, and a single-mode output Y-branch to separate the two output channels. The fundamental mode from the input waveguide excites both the fundamental mode and the first higher-order mode in the active region. The spatial interference between these propagating modes produces a periodically repeated optical intensity distribution along the propagation axis. Moreover, the active region is designed to allow a π shift between the two modes when a bipolar mode field effect transistor, which injects and controls the free carrier plasma inside the active region, is driven from the OFF state to the ON state. By doing so, it is possible to steer light from one output channel to the other. Electrical and optical simulations have been carried out for the evaluation of the performance of the device. These simulations show optical propagation losses around 3 dB cm-1, an overall crosstalk of -10 dB, a transient rise time of 8.2 ns and a fall time of 7.2 ns.

In this paper we propose an optoelectronic Y-switch based on a three terminal active device and on the mode-mixing principle. It consists of a 1400 micron long asymmetrical rib waveguide designed to sustain only the fundamental and the first propagating mode; along this waveguide the difference in effective indices between these modes cumulates to 2(pi) and the light emerges on the same side where it has been injected. In the ON state, the presence of an electron-hole plasma injected and spatially controlled with a Bipolar Mode Field Effect Transistor device (BMFET) forces the fundamental mode and the first propagating mode to be shifted by an additional -(pi) thus allowing the light to emerge from the other side of the waveguide. Numerical simulation of losses, cross-talk and switching speed are presented.

Optical properties of plasma lenses strongly depend on the magnetic field distribution. Its measurement with conventional magnetic probes is quite critical for such application. In this paper the measurement of the magnetic field in a Z pinch discharge, based on the Faraday effect in optical fibers, is described. It is shown that it is possible, using a quite simple and relatively low cost experimental setup, to detect the field dynamics inside the discharge tube assuring low disturbance, a large bandwidth, and a high signal to noise ratio. Applications to several kinds of plasma lenses and further development are discussed. >

The authors describe an interferometric technique for the measurement of the recombination lifetime of electron-hole pairs as a function of their concentration, which can be measured with an error smaller than 10%. In addition, the approach is much more sensitive than the other optical methods described in the literature. >