burner [1,2]. Solid oxide (soot) particles are formed in the reaction zone and are either deposited on the inner surface of the substrate tube, forming a layered ...
Internal reaction temperatures of a modified chemical vapour deposition (MCVD) optical fibre preform lathe dynamically measured with regenerated fibre Bragg gratings. Mattias L. Åslund1, Albert Canagasabey2, Yang Liu2, Kevin Cook1, John Canning1, Amer Ghias2, Gang-Ding Peng2 1 2
Interdisciplinary Photonics Laboratories (iPL), School of Chemistry, University of Sydney, 2006, NSW, Australia School of Electrical Engineering and Telecommunications, University of New South Wales, 2052, NSW, Australia
The temperature profile of the reaction zone inside the substrate tube of a modified chemical vapour deposition (MCVD) optical fibre perform lathe has been characterised using regenerated fibre Bragg gratings (FBGs) to allow for the development of new dynamic deposition methods.
number of different temperature dependent reactions are allowed to occur consecutively – building layered particles as the chemicals propagate down the tube.
Modified chemical vapour deposition (MCVD); Regenerated fibre Bragg gratings; High temperature sensors, Bragg gratings, Photosensitivity, Optical thermocouple
I. INTRODUCTION The modified chemical vapour deposition (MCVD) process is the most common method to produce optical fibre performs. In this process, halide chemicals (e.g. SiCl4, GeCl4) undergo oxidation inside rotating ultra-pure silica substrate tubes as a result of heat provided by an external traversing oxy-hydrogen burner [1,2]. Solid oxide (soot) particles are formed in the reaction zone and are either deposited on the inner surface of the substrate tube, forming a layered structure, or exit the tube as waste. The composition, and in extension refractive index, of each layer can be changed by adjusting the feed composition. The solid rod preform is formed when the substrate tube is collapsed by increasing the burner temperature, which simultaneously sinters the layers. The preform is then softened in a furnace and pulled to an optical fibre in a draw tower. The reaction kinetics of the particle formation itself is controlled by fluid drag and thermophoretic effects. A better understanding of the dynamic temperature gradients experienced by the chemicals inside soot tubes as the laminar gas flow approaches the traversing hot-zone will allow for improved control of the ensuing chemical reactions. Improvements that can be achieved this way are plentiful and include parameters such as improved dopant distribution uniformity and control of dopant concentrations. This would translate into a reduction of refractive index undulations and refractive index step error margins. It could also allow for the deposition of completely novel types of dopants e.g. by playing with complex multi-stage transverse isotherm zones, where a
Figure 1. Internal longitudinal tempearature distribution of MCVD substrate tube as a function of burner position as measured with K-type thermocouple (black line) and ultra high temperature stable regenerated FBG (red line)
In this contribution we investigate the internal MCVDreactor temperatures in detail using ultra-high temperature stable regenerated fibre Bragg gratings (FBGs) as probes, both as single units and also in arrays. With this method the actual temperatures can be measured in-situ at extreme temperatures with great resolution instead of relying on speculative theoretical modeling. These FBGs are destined to replace electrical versions in a range of sensing applications where electronic solutions are not an option, or are preferably avoided altogether; including for instance applications where there is a risk of explosion, where the distance to the point of measure is very long, in areas of high radiation, or where the signal is corrupted by high electrical field interference. We do this for a range of realistic conditions, including transverse/longitudinal position, gas-flow, traversing speed and multiple passes with thermal hysterises effects.
II.
EXPERIMENTS AND RESULTS
The substrate tube used for the experiments was a silica MCVD tube (OD/ID 25/19 mm, synthetic fused silica), fixed in both ends onto an MCVD lathe fitted with a rotary-seal gas supply. The experiments were carried out using thermally stabilised regenerated FBGs. The seed FBG (Rmax > 50 dB) was direct written using an ArF laser (Ȝ = 193 nm, fpulse = 7 mJ/cm2, fcum = 4.8 J/cm2) and a phase mask (ȁ = 1052 nm, L = 1 cm) into a H2 -loaded (P-H2 = 200 Bar, 24 hrs, T = 80 ºC) high-NA germanosilicate optical fibre. Regeneration followed the method reported previously [2]. After regeneration, the regenerated grating was stabilised at 1100 °C for 45 minutes.
speed of 100 mm/min and rotation speed of 40rev/min. The measured temperatures are shown in Fig. 1, as a function of burner position, where the black line denotes the temperature measured with the thermocouple and the red line the temperature estimated from the Bragg wavelength of the FBG. The FBG and the thermocouple were positioned at the zero point and the burner traversing was carried out repeatedly between -290 – 200 mm in the downstream direction only (from negative to positive). In the graph it can be seen that the temperature is decreasing from ~500-600 °C at the starting point to ~400 °C at a position around 150 mm, which is due to thermal lag from the previous scan. We can also see that there is a drop in temperature from the end of the scan to the beginning of the next; this is due to the time it takes the burner to traverse back to the starting point. The peak internal temperature is ~1200 °C, which is a ~700 °C drop in comparison to the outside temperature measured with the pyrometer. The extreme points of the two curves match closely, whereas the temperatures during ramping are slightly offset, which is attributed to slight hysterises due to thermal mass of the metal tube surrounding the FBG.
Figure 2. Internal transverse tempearature distribution of rotating MCVD substrate tube.
To reduce chirping when subjected to the major temperature gradients near the reaction zone of the MCVD lathe, the FBG was inserted into a short length of stainless steel tubing (OD/ID 3/1 mm, L = 12 mm). To maintain minimum noise from reflected light by circumventing angle-cleave fusing at high temperatures, a short length of dummy-fibre was spliced onto the FBG-containing fibres with the cores offset to each other. The packaged FBG sensor was interrogated in reflection mode with a C band swept wavelength system based on an amplified spontaneous emission (ASE) source and a wavelength tunable filter [4]. The tunable filter was scanned at a frequency of 20Hz while the reflection spectrum was monitored continuously with a detector. This system provides a continuous measurement of the temperature experienced by the FBGs as a function of time. The whole system was calibrated using a standard K-type thermocouple positioned next to the FBG in the centre of the tube to provide two separate references between the outside tube-temperature, measured with the lathe pyrometer, and the temperature inside the soot-tube. The lathe was set at an outside temperature of 1900 °C with a standard traversing
Figure 3. Longitudinal tempearature distribution of MCVD substrate tube from cold start (red line) and repeated traversing (blck line).
The radial temperature distribution was then measured by positioning the burner flame stationary at the zero point and allowing the temperatures to stabilise (other settings the same as above). The FBG was then moved around radially whilst the Bragg wavelength was monitored; care was taken to allow the temperature to stabilise. The corresponding temperature profile derived from the Bragg wavelengths is shown in Fig. 2(a). Note that the semi-circular burner is heating the bottom half of
the tube and that the rotation is clock-wise in the graph. In Fig. 2(b) the horizontal and vertical temperature distribution is shown in black squares and red triangles respectively as a function of radial position. In the graphs it can be seen that the radial temperature distribution inside the tube does not vary significantly, and that the time-lag for the heat to propagate through the tube corresponds to roughly half the revolution time. Due to thermal lag, the temperature distribution varies significantly for different length preforms and scanning speeds, so the difference between a cold start and a repeat scan was measured to illustrate the extreme conditions (shown in Fig. 3(a)). A measurement of the thermal distribution as a function of scan speed was also carried out (shown in Fig. 3(b)) using similar conditions. In Fig. 3(a) the cold start distribution is shown in red and the repeat scan is shown in black. The curves follow each other closely near the hot zone, but the repeat scan reaches a ~50 degrees higher peak temperature. In Fig. 3(b) the temperature distribution for a repeat scan at 100, 75 and 50 mm/min is shown in black, red and blue respectively, where it appears as is the temperature distributions tend to bulge out at slower scan speeds and also reach higher peak temperatures. To verify predictions that the internal gas flow rate plays a minor role [1], the peak temperature as a function of gas flow was detected for repeat scans; results are shown in the graph inset in Fig. 3(b). In the graph the peak temperatures varies within 8 degrees and do not correlate with the gas flow rate; the values of the measurement are within the error margin of the external pyrometer.
the side shielded from the stationary flame by the N2 curtain. This was to determine the thermal spread across this region. The FBGs were interrogated in reflection mode using the same C-band swept wavelength and the results are shown in Fig. 4. The positioning of the FBG array gratings with respect to the burner mid-point and one of the nitrogen curtains are also shown in Fig. 4. A maximum temperature of ~900°C was observed near the flame, somewhat less than that measured by the pyrometer ~1445 °C on the outside of the tube, the sharp drop was attributed to the stationary flame. It was determined that the Nitrogen curtain had little impact on the temperature. DISCUSSION AND CONCLUSIONS In conclusion, the dynamic temperature distribution inside the reaction zone of a substrate tube in an MCVD lathe has been characterized in detail for a range of realistic conditions. The results provide the foundations for research into novel preform types and improved refractive index profile control in current production. At the same time, the prospect of an all-optical photonic ultra-high temperature sensor to replace thermocouples has been explored. Early experiments without metal tubing to reduce thermal chirping saw major spectral broading paired with reduction of reflectivity, which prevented peak internal temperatures above ~800 °C to be measured. Nevertheless, these results clearly show that very hot and spatially small regions with large temperature gradients can be accurately probed using an all-photonic thermocouple with metallic temperature equalisation to millimeter resolution. ACKNOWLEDGMENTS Funding from the Australian Research Council (ARC) and an International Science Linkage Grant from the Department of Industry, Innovation, Science and Research (DIISR), Australia is acknowledged. REFERENCES [1]
[2]
[3] Figure 4. Longitudinal tempearature distribution of MCVD substrate tube measured across burner Nitrogen curtain using a distributed array of ultra high temperature stable regenerated FBGs.
To allow for distributed sensing, a sensor array consisting of four regenerated FBGs was tested; it was fabricated using the same methods described earlier but set at different Bragg wavelengths. The feature that was chose for a study was the N2 cooling curtains on either side of the flame of the lathe burner, positioned ~8cm apart. The four gratings were positioned such that two were on the flame side of one N2 and the other two on
[4]
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