The effects of gamma-radiation on the properties of Brillouin scattering in standard Ge-doped optical fibres

We have experimentally studied the effects of gamma-radiation up to very high total doses on the physical properties of Brillouin scattering in standard commercially available optical fibres. A frequency variation of about 5 MHz for both Brillouin frequency and linewidth has been measured at the total dose of about 10 MGy. The radiation-induced shift has a negligible practical impact and makes Brillouin scattering very immune to radiation, so that distributed sensors based on this interaction exhibit an interesting potential for use in nuclear facilities. © 2006 IOP Publishing Ltd.

High spatial resolution and intrinsically strain-insensitive distributed temperature sensing based on stimulated Brillouin scattering in gas

Flavien Gyger, Luc Thévenaz, Fan Yang

Temperature/strain cross sensitivity is a long-standing issue in Brillouin-based distributed sensors, impairing the reliability of such sensors. So far, all the proposed methods perform the discrimination by measuring two quantities showing distinct responses to temperature and strain. Distributed Raman sensing enables temperature measurement without strain sensitivity. However, due to its weak signal intensity resulting from the principle based on spontaneous scattering, the spatial resolution is typically limited to ∼ 1 m. Here, for the first time, we use stimulated Brillouin scattering in gas-filled hollow-core photonic crystal fibers for distributed temperature sensing and we demonstrate ∼ 1 cm spatial resolution and 0.3°C temperature resolution fully free of strain cross sensitivity. Substantially higher performance is obtained thanks to the higher Brillouin gain, narrower gain linewidth and relaxed optical power restrictions when compared to solid silica single-mode fiber. This opens a new avenue in high performance distributed fiber sensing based on gas nonlinear optics.

SPIE2019

Brillouin Echoes for Advanced Distributed Sensing in Optical Fibres

Stella Foaleng Mafang

Brillouin scattering is particularly efficient and attractive for the implementation of strain and temperature distributed sensing in optical fibres. Recently a trend has been observed that modern advanced applications require a substantial step towards better spatial resolution, while preserving temperature/strain precision over a long range. For this purpose the state of the art does not satisfy all these requirements. In this thesis we present a radically new approach named Brillouin Echoes distributed sensing (BEDS) that allows covering these requirements. In the first part, we propose an updated configuration of the classical existing Brillouin sensor for time domain analysis allowing drastic noise reduction. Then we investigate the limitations (due to non-linear effects) of the classical Brillouin sensor in terms of long distance range measurements. The identified nonlinear effects are pump depletion due to SBS itself, self-phase modulation (SPM), modulation instability (MI), which occurs only in fibres presenting an anomalous dispersion at the pump wavelength and Raman scattering (RS). We propose the modeling of the pump depletion effect to obtain analytical expressions that are useful for the proper design of a BOTDA sensor and for the determination of a very small depletion. The model confirmed by experimental measurements is informative on the conditions maximizing the depletion effect; therefore a standard configuration can be defined to test the value of the depletion in the set-up. Furthermore, we demonstrate that SPM-induced spectral broadening can have a significant effect on the measured effective gain linewidth. Modeling and experiments have undoubtedly demonstrated that the effective gain linewidth can easily experience a two-fold increase in standard conditions when the pulse intensity profile is Gaussian. We showed that the problem can be practically circumvented by using a clean rectangular pulse with very sharp rising and falling edges. The theoretical and experimental analysis of the undesirable effects of MI and forward RS in distributed BOTDA sensors systems gives a simplified expression to predict the critical power for a given distance range. MI turns out to be the dominant nonlinear limitation since it shows the lowest critical power, but it is less critical since it can be avoided to a wide extent by using the fibre in the normal dispersion spectral region such as a DSF in the C-band. On the other hand Raman scattering can be avoided only by limiting the optical pump power and therefore is the ultimate nonlinear limitation in a distributed sensing system. Under similar conditions RS shows a critical power ∼5 times larger than MI. In the second part, we present the new approach Brillouin echo distributed sensing (BEDS) which has proved to be a powerful solution to realize sub-metric spatial resolutions in Brillouin distributed measurements. We have demonstrated both theoretically and experimentally that an optimized configuration is reached when the optical wave is π-phase shifted. The experimental tests have shown a spatial resolution down to 5 cm, with a clear margin for further improvement down to a real centimetric spatial resolution. This optimized configuration produce the best contrast independently of the pulse intensity, with a factor 2 of improvement compared to other techniques based on the same approach (dark pulse, bright pulse). This extends the dynamic range by 3 dB, which corresponds in standard loss conditions to a 5 km extension of the sensing range. An analytical developed model has proved to be an excellent tool not only for optimizing the pumping scheme but also in post-processing the measured data. Finally the potentialities of BEDS technology provide solutions in real contexts. Using the BEDS technology in landslide monitoring at laboratory scale, for the first time it became possible to observe the failure propagation in laboratory scale with an accurate precision. Furthermore, using BEDS we have proposed and demonstrated the possibility of mapping geometrical structure fluctuations along a photonic crystal fibre (PCF). Both long- and short-scale longitudinal fluctuations in the Brillouin frequency shift have been identify and quantify. Observation of Brillouin linewidth broadening in PCF fibre through distributed measurement of the Brillouin gain spectrum using BEDS has allowed fundamental understanding of SBS in PCF fibre and in their design in view of applications to optical-strain/temperature sensing.

EPFL2011

High-Performance CMOS SPAD-Based Sensors for Time-of-Flight PET Applications

Francesco Gramuglia

Detecting light, photon by photon, has been possible since the 1930s, with the invention of the photomultiplier tube (PMT). However, it is only since the 1970s, that solid-state single-photon detectors have emerged and only since 2003 that a new technology, known as single-photon avalanche diode (SPAD) has become CMOS-compatible.The SPAD technology is scalable, low-cost, and it is natively digital, thereby enabling a variety of sensor architectures optimized for applications, such as bioimaging, high energy physics, and medical physics. Thanks to the adaptability of SPADs to many different CMOS technologies, it is possible to achieve systems of different complexity, operating conditions, and, ultimately, cost. The versatility of this device, makes it ideal in very different applications, where it is emerging as the sensor of choice.This thesis focuses on improving CMOS SPAD performance for time-of-flight PET, while other applications, such as FLIM and LiDAR could also benefit from the technology developed here.We have designed SPAD-based sensors in two different silicon standard technology nodes and we have fully analyzed their performance. We have also explored the optimization of light extraction in inorganic scintillator-based detector modules and direct particle detection and we have studied two applications beyond PET.The thesis is organized as follows. After a comprehensive description of SPAD technology, we focus on new SPADs structures in standard deep-submicron technology nodes, 180 nm CMOS and 55 nm BCD. The results achieved with these devices show how SPADs can achieve an unprecedented timing performance of 7.5 ps FWHM at room temperature with a sensitivity peak of 55% thanks to a fully integrated front-end readout system that is shown to play an essential role here.We show that state-of-the-art performance is possible using advanced standard technology nodes, where outstanding timing and sensitivity performance were achieved by systematically engineering the electric field in multiplication and drift regions to achieve optimized carrier generation and transport upon detection.We have demonstrated the first large-scale 3D-stacked frontside-illuminated (FSI) multi-digital silicon photomultiplier (MD-SiPM), whereas we discuss the sensor structure and its electronic components aimed at modularity and scalability, so as to ultimately achieve large-format SPAD sensors.To demonstrate specific aspects of the intended applications, in primis, TOF-PET, we look at coupling between photodetectors and inorganic scintillators, where gamma radiations, originated from electron-positron annihilations, are detected. Using photonic crystals structures, nanoimprinted on the scintillator output surface, we have demonstrated a 40% improvement in light extraction and 25% better energy resolution in BGO crystals.Finally, we explore the use of SPADs for direct time-of-flight coincidence measurements of minimum ionizing particles. We achieved an unprecedented 9.4 ps coincidence uncertainty in this novel and very thought-provoking SPAD application, corresponding to a 6 ps Gaussian sigma timing resolution for a single detector.In summary, this thesis demonstrates new paradigms of SPAD performance, together with a novel 3D integration of FSI SPADs, and novel application solutions. In the future, we thus expect that these achievements will pave the way for further performance improvements and extensive scientific explorations.