Review of Photonics Presentation to IEEE Foothill Held on September 6, 2014
| November 15, 2014 | Posted by Frank Gomez under CN, COMSOC, CS, EDCAS, MTT/APS |
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The IEEE Foothill Section held a successful technical meeting in the Photonics area on Saturday September 6, 2014 at Cal Poly Pomona. The presentation, titled “Fiber Optic Sensing Technology and Applications” was given by Dr Hao Zhao from Bandweaver Corp, based in Zhejiang (south of Shanghai), PROC.
Dr Zhao described how he helped start this company a little over 10 years ago, right after the bubble in Telecom had deflated, and there was much less need for on-going production of optical fiber communications systems. Fortunately, there were many ongoing research efforts to investigate alternate uses for fiber optical cables, and the technology of fiber Bragg gratings was advancing in many research laboratories. What began as a curiosity, the near total reflection of light from a fiber due to photosensitivity, would be investigated thoroughly to pin down many physical parameters.
Starting from the measurements made with optical fibers for telecommunication purposes, expertise in attenuation measurements was acquired. When investigations were made to demonstrate that periodic gratings (index of refraction variations) could be reliably made in doped fibers by use of ultraviolet band lasers, then new fields of measurements opened up for use of gratings and Bragg reflection effects. These were related to the various photon / phonon interactions: Rayleigh (elastic) Scattering; Brillouin Scattering (acoustic phonon branch of the crystal); and Raman Scattering (optical phonon branch of the crystal).
The basic idea behind these measurements, and the basis for the instrumentation systems that Bandweaver produces, is straightforward. A laser signal passes through a beam splitter mirror into the long optical fiber, which is typically many Km in length. The return signal, a composite of the backscatter signals from Bragg gratings along sections of the optical fiber, is passed through the same beam splitter to a series of filters and then into a detector. Then a FFT program is run to separate out the (small) laser wavelength shifts. As Dr Zhao noted for his optical fibers, Rayleigh scattering yields no frequency shift; Brillouin scattering may yield a typical +/- 11 Ghz shift; while Raman scattering may yield a typical +/- 13 THz frequency shift. (Recall that the initial laser signal (the fo in this system) will be about 10,000 Ghz. This gives some perspective on the wavelength shifts that are to be measured.) Therefore, based on the observed physical effects, Rayleigh scattering is used for acoustic and vibration sensing; Brillouin scattering for temperature and strain sensing; while Raman scattering is used for temperature sensing only.
These ideas form the basis of the Beamweaver instrumentation products. Their Distributed Temperature Sensor (DTS) product uses Raman scattering, with a length of fiber cable <30 Km, and a measurement time minimum requirement of 3 seconds, with a measurement position accuracy of 1 m location, and a temperature accuracy < +/- 1.0 C. This temperature resolution is set by the noise standard variation.
This DTS has been used with their Dynamic Cable Rating algorithm (DCR ) product for High Voltage cable assessment. Here the optical Bragg fiber cable is mounted on the outside of the electrical power cable. From the optical fiber, outer cable temperature is measured. Then the DCR calculates the temperature in the electrical cable.( As noted, such a functioning system could have prevented the catastrophic cable insulation blowout as occurred in New Zealand a few years ago.) This system was verified in 2007 with tests at the Shanghai Power Cable Research Institute, and in 2009 at EPRI China Electric Power Research Institute. As a check, these tests used an external temperature sensor reading for the cable at the defined position. The system error was verified to be <2 C.
These Bandweaver equipment products have been installed in multiple locations in China. One is the Li Shui Bridge, where the fiber Bragg cables are buried in concrete. Multiple measurements of vibration can be continuously made and monitored. Other structural monitoring applications are on the drawing board, such as a new highway in China which has 75% of its length either on bridges or in tunnels, and which would require continuous monitoring.
We also heard an interesting discussion from Dr Zhoa on the formation of this company (Bandweaver) in China. This covered their looking for an engineering niche for productivity, their growth into a multi-skills required organization, their ongoing marketing efforts, their interactions with financial regulators, and their planned collaborations with university investigators in Canada and here in the USA. We received a good snapshot of an engineering production effort and its associated required business effort.
We thank Dr Hao Zhoa for an informative presentation on a current problem-solving area in Photonics, and our own IEEE Foothill Associate Chair for ED/CAS /Photonics, Dr Bo Chen, for making all the meeting arrangements.