• DTS - Distributed Temperature Sensing
• DAS - Distributed Acoustic Sensing
• OTDR - Optical Time Domain Reflectometry

Optical fibers are increasingly used in a diverse range of applications. Their ability to transmit information at light speed over long distances and with low loss has made them the primary medium for large volume long range data communication. As such, fiber optic networks can be found in telecommunications systems where they are used for transmitting and receiving purposes. They are also used to deliver a variety of digital services such as internet, HDTV, and video on-demand.

In industrial zones optical fiber can be employed for imaging in hard to reach areas. In addition, being largely immune to electromagnetic or radio frequency interference (EMI and RFI) they offer significant advantages over conventional wiring in environments that may be subject to high levels of radiation.

In recent years techniques have also been developed that allow optical fiber to be used as sensory devices to make temperature, pressure and other measurements. For example, temperature can be determined by activating various sensing materials such as phosphors, semiconductors or liquid crystals with fiber optic links. Their unique capabilities allow fiber optics to be deployed in medicine for imaging and illuminating (such as photoacoustic microscopy and endoscopy), mining (bore hole logging and safe data sharing), automotive (in-vehicle signal and data transmission) and oceanography (hydrophones for seismic waves and SONAR).

The characteristics of light transmission in optical fiber can be affected by physical parameters, such as temperature, strain or pressure. As a result, fibers can be used as linear sensors to locate external physical effects. The process is commonly called distributed temperature sensing (DTS). In DTS light scattering (known as Raman scattering) and wavelength shifting (Stokes Line) are both studied to reveal information about the physical parameters of interest.

A similar process for characterizing optical fiber systems is optical time domain reflectometry (OTDR). The method works by injecting a series of optical pulses into a fiber under test and then collecting light that’s reflected (as either Rayleigh backscatter or Fresnel reflections) from points along the fiber. See figure 1 as an example. In this setup the digitizer acquires the analogue signals coming from the detector that’s receiving the reflected light. The acquired data can then be analysed so that the optical fiber can be characterized. Two key parameters for OTDR systems are their range and resolution. To increase resolution the optical pulses need to be narrow while the sensor and measuring electronics has to be fast enough to resolve each separate event.

With their high sampling rates and resolution Spectrum digitizers are extremely useful devices for acquiring signals from a variety of optical fiber sensing systems. With models offering sampling rates from 5 MS/s to 5 GS/s users can select a unit that best matches their specific application. For example, the fastest sampling rate products can be used to acquire and analyze optical pulses with widths down to the sub-nanosecond range. Spectrum digitizers also offer vertical resolutions up to 16-bit. These models are ideal for applications where increased sensitivity and low-level signal amplitudes may be encountered.

Knowledge

• Learn on wikipdia about Distributed Temperature Sensing by clicking here
• Learn on Wikipedia about Distributed Acousting Sensing by clicking here