Distributed Optical Fiber Sensing (DOFS)
Optical fiber is often viewed merely as the modern plumbing powering our connected world, replacing older twisted pair copper connections in data transmission networks. However, due to some unique physical characteristics, fiber optics can and are targeted at a range of distributed sensing applications. Applications that benefit from fiber’s economy combined with an inherent ability to make localized measurements over considerable distances. Metrology modalities divide into four sensing modes:
- Distributed Temperature Sensing (DTS)
- Distributed Acoustic Sensing (DAS)
- Distributed Strain Sensing (DSS)
Fiber optic cables are waveguides that allow concentrated coherent light (provided by either a laser e.g. VCSEL or LED source) to travel with very little attenuation, long distances. Fibers benefit from being physically small and lightweight when compared to equivalent copper cables. Moreover, they are immune to electromagnetic interference.
How DOFS works
Science shows that impurities arising in fibers during their manufacture causes light traversing their length to experience low levels of scattering. Three scattering types are described by their physical manifestation and the methods needed to detect them:
- Rayleigh, Brillouin and Raman scattering
Fortuitously, these scattering processes when combined with suitable signal capture technology create distributed sensors capable of measuring physical characteristics including temperature, mechanical strain, and even acoustic energy throughout the fiber - opening a host of economic metrology applications. The device used to measure these physical characteristics is an interrogator (figure x). It is within the interrogator that digitizer technology is applied enabling a fiber sensor to:
- Detect and localize a thermal hot spot (DTS) along a length of power cable.
- Detect and localize out of bound mechanical strains (DSS) signaling the prelude to a mechanical failure in large built structures (e.g. a road traffic flyover, bridge or damn).
- Detect and localize personnel or vehicular intrusions (DAS) via a hidden fiber boundary that may stretch more than 50 km.
Some interesting DOFS facts
- Light traveling within a fiber travels 1 km in ~ 5 µs – as compared to light in free space which travels 46 % faster (3.33 µs/km).
- Fiber cables offer low signal attenuation over large distances of between 0.19 to 0.33 dB/km depending on wavelength.
- Optical time domain reflectometry (OTDR) enables an operator to evaluate the quality of installed fiber connections, localizing problems including attenuation, and event losses.
- Raman scatter can be used to determine and localize heat spots along a fiber with a precision of a few degrees centigrade.
- Brillouin scatter provides an easy to install perimeter intrusion detection system able to discriminate and localize multiple intrusions of both people and vehicles.
What to consider when selecting a digitizer for DOFS applications
The most important aspect of DOFS applications is to understand that:
- back scatter, irrespective of the specific type to be detected is a low-level signal demanding high system dynamic range (demanding ADC resolution > 10-bit).
- The short light pulses (typically between 5 ns & 20 µs) used for effective metrology mean high speed digitizers are essential to capture laser pulses. Thus, sample rates in the order of a few giga samples are necessary to capture the shortest pulses.
- The selection of a suitable measurement pulse width is determined by considering the target fiber length and assessing both the incident and reflected pulse flight times. Provision of some level of user or autonomous data-based pulse width configurability in a DOFS system may be a critical part of the system design
DOFS system designers need to pay attention to the design of suitable optical front end pulse generator and detector circuits to complement the high-end performance offered by the selected digitizer. However, with local open FPGA resources on hand within the digitizer can provide a master control system for both the data acquisition process as well as laser pulse control and timing.