Towards inline spatially resolved temperature sensing in thermal ablation with chirped fiber Bragg grating

Abstract

We investigate the theory and feasibility of an inline spatially resolved temperature sensor, suitable for thermal ablation monitoring. The sensor is based o a chirped fiber Bragg grating (CFBG). The CFBG is modelled as a chain of Bragg gratings, each sensitive to local temperature variations. By using a combination of iterative and statistical optimization techniques, it is possible to use demodulate the CFBG, in case of a Gaussianlike spatial temperature profile. A feasibility test based on CFBG simulation shows that the CFBG returns error <1 mm on cells damage threshold spatial estimation and good noise resilience.

A Fiber Optic Probe for Tumor Laser Ablation With Integrated Temperature Measurement Capability

“…Some studies are aimed at the use of Chirped FBGs (CFBGs), while others exploit the Optical Frequency Domain Reflectometry (OFDR). CFBGs are characterized by a variable period of the refractive index perturbation, so that different wavelengths are reflected at different points along the grating length; this way, it is in principle possible to relate the shape of the grating spectral response with the temperature profile distribution: the technique is promising, although it cannot be considered reliable for practical clinical applications yet [24], [25]. OFDR exploits the changes in the amplitude, frequency and phase of continuously back-scattered Raman, Brillouin or Rayleigh light along the fiber to recover temperature and deformation profiles over lengths that can extend up to several kilometers [26].…”

Review of Chirped Fiber Bragg Grating (CFBG) Fiber-Optic Sensors and Their Applications

“…The method shown in [ 35 , 45 , 46 , 47 , 48 , 49 ] and outlined in Figure 1 b provides a model of the FBG that can be defined as incoherent, as it assumes that the FBG can be modeled as a chain of M uniform FBGs each having different Bragg wavelength and with no intrinsic standing waves existing between each grating element. Calling L the grating length, and M the number of gratings, the length of each short grating element is L g = L / M .…”

“…This is expressed, using CMT, as [ 7 ]: where δn eff is the amplitude of the refractive index modulation. As in [ 35 , 49 ], the parameters of the refractive index modulation ( n eff , δn eff , k ) are assumed to be constant over the whole grating length and therefore they do not have a spatial dependency. The Bragg wavelength of each i -th layer, instead, is spatially varying.…”

“…The Bragg wavelength of each i -th layer, instead, is spatially varying. In a linearly chirped FBG [ 35 , 36 , 37 , 38 , 39 , 46 , 47 , 48 , 49 ] the reference value for the Bragg wavelength has a linear dependence upon the grating length: which can be converted into the discretized grating model as: …”

“…From the sensing perspective, the CFBG behaves as a cascade of FBGs, whereas each Bragg wavelength shifts as the grating is exposed to a temperature or strain variation [ 4 ]. As shown in [ 35 , 49 ], each i -th Bragg wavelength is sensitive to the strain variation Δ ε i and/or to the temperature variation Δ T i observed on the i -th element of the CFBG. For small values of strain and temperature [ 6 ], a linear sensitivity is observed: …”