In Situ Temperature Measurement of an Optical Fiber Submitted to Electric Arc Discharges

Abstract

Type S thermocouples were assembled in situ by applying high intensity electric arc discharges to the contact junction of two platinum (Pt) and Pt-10% rhodium (Pt-10% Rh) wires, inserted on a silica capillary. The electrically insulated thermocouples built in this way were afterwards employed to estimate the temperature of an optical fiber subjected to arc discharges. For typical values of the arc discharge parameters used to arc-induce long-period fiber gratings (electric current = 9 mA and arc duration = 1 s), a capillary peak temperature value of 1420 C 40 C was obtained by extrapolation of the experimental data for the limit situation of having a thermocouple with negligible diameter. The temperature profiles in the capillary and in an optical fiber were calculated based on a heat transfer model implemented by a finite element algorithm and fitted to the experimental temperature distribution in the Pt and Pt-10% Rh wires. The correspondent peak temperatures computed for the capillary and for the fiber were 1450 C and 1320 C, respectively. A good agreement between the capillary temperature values determined graphically and numerically was obtained.Index Terms-Electric arc discharge, long-period fiber gratings (LPFGs), optical fiber components, temperature measurement.

Estimation of the fiber temperature during an arc‐discharge

“…Recently, based on blackbody radiation, we estimated a value of (1400 Ϯ 50)°C for the fiber temperature [5]. In a previous work carried out using electrically insulated thermocouples [6], a value of ϳ1320°C was obtained for the temperature attained by an optical fiber during an arc discharge. In that approach, the basic requirements were as to guarantee that the arc discharge remained unperturbed and that the temperature sensor had dimensions comparable to those of an optical fiber.…”

“…Afterwards, based on the new equations for the temperature dependence of the thermal conductivity of the platinum-rhodium alloys, we re-evaluated the results obtained in Ref. 6. Also, we present for the first time details on the computer simulations and the temperature dependence of all physical parameters used in the calculations.…”

Arc-Induced Long-Period Gratings

“…Recently, Fernandez et al [23] have demonstrated that LPGs written in pure-silica-core fibers using the electric-arc technique possess high resistance to γ -radiation. We have recently estimated the temperature distribution in an optical fiber submitted to an electric arc discharge [24]. Finally, we have also shown that the increase of the electric current leads to an increase of the strain sensitivity of arc-induced LPGs [25].…”

“…The formation of LPGs in germanium-doped fibers with no applied tension and for high currents (10-16 mÅ) and short duration times has also been attributed to this same mechanism [7,10]. However, assuming that the diffusion coefficient of Ge is D < 5.9 × 10 −16 m 2 ·s −1 [37] and for a fiber temperature of 1350 • C [24] a diffusion length of L < 0.03 µm is obtained for an exposure time of 1 s. The estimated refractive index change is on the order of 10 −5 , not considering the change due to waveguide modulation.…”

“…More recently, two other approaches have also been presented. One, based on the blackbody Downloaded by [York University Libraries] at 23:05 17 November 2014 radiation emitted by a multimode fiber while being heated by an arc discharge [44] and the other based on electrically insulated thermocouples [24]. Figure 3 shows the experimental set-up employed to measure the temperature inside a silica capillary with built-in thermocouples.…”