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Fiber Optics | Spectrum

Fiber Optics

  • 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.

Schematics of Optical Time Domain ReflectometryA 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.
 

Spectrum Product Features

  • Sampling rates from 5 MS/s to 5 GS/s
  • Very high SNR and SFDR
  • Resolution up to 16 bits
  • Fast data acquisition including segmented and FIFO streaming modes
  • Signal processing (hardware and software averaging) with SCAPP GPU support
  • On-Board Block Average

Matching Card Families

  • M4i.22xx: 8 bit 5 GS/s to 1.25 GS/s digitizer
  • M4i.44xx: 14/16 bit 500 MS/s to 130 MS/s digitizer
  • M2i.59xx: 16 Bit 125 MS/s to 5 MS/s Digitizer

Useful Links

  • At the Changsha University of Science & Technology in China they have used a Spectrum M4i.2210-x8, 1.25 GS/s, Digitizer to acquire high speed pulses in a Brillouin Optical Time-domain Reflectometer (BOTDR) using Discrete Fourier Transforms (DFT). A paper discussing experimental results that show improved spatial resolution and accurate location of distributed sensing is available here
  • The State Key Laboratory of Laser Interaction with Matter, Northwest Institute of Nuclear Technology, China, is using a 1.25 GS/s M4i.2212-x8 Digitizer together with a customized long fiber ring etalon to make measurements and analysis of diode laser modulation wavelengths at high accuracy and response rates. An article discussing  their process can be found here
  • The School of Biomedical Engineering, Tohoku University,  Japan is using a 5 GS/s M4i.2230-x8 Digitizer to achieve optical resolution photoacoustic microscopy with sub-micron lateral resolution for visualization of cells and their structures. Details and results of their experimental setup can be found here
  • At the Technical University of Dortmund, Germany, physicists have developed a technique to determine photon correlations of optical light fields in real time. The system uses a high speed Spectrum M4i.2234-x8 digitizer to acquire signals from a balanced homodyne detector. The full details including the experimental setup and results can be found here
  • The University of Bern’s Institute of Applied Physics in Switzerland is testing and developing algorithms used for image reconstruction in optoacoustic imaging applications. Test signals are acquired using an M4i.4420-x8, 250 MS/s, 16 bit, digitizer and a research paper discussing their findings can be downloaded from here
  • At Switzerland’s Institute of Pharmacology and Toxicology and Faculty of Medicine, at the University Zurich, they are using a burst-mode laser triggering scheme and an M4i.4420-x8, 250 MS/s, 16 bit, digitizer to perform rapid acquisition functional optoacoustic micro-angiography. A paper discussing the developed system, and how it greatly enhances the performance and usability of optoacoustic microscopy for dermatologic and micro-angiographic studies, can be found here
  • The MOE Key Laboratory of Laser Life Science and Institute of Laser Life Science, at the South China Normal University, in China has developed a Photoacoustic Imaging (PAI) pen that can be handheld (performing forward detection and lateral detection) to extend the application of photoacoustic (PA) microscopy to areas such as the oral cavity, throat, cervix, and abdominal viscera. The experimental setup uses an M4i.4450-x8 500 MS/s, 14 bit, digitizer to acquire the sensor signals. A paper discussing the PAI pen and the test results can be found here
  • The University College London has developed a method for large area all-optical ultrasound imaging using robotic control that involves the use of an M4i.4420-x8 250 MS/s, 16-bit digitizer. A white paper on the development can be found here
  • Find out how Nanyang Technological University, Singapore, uses a high speed 16 bit Spectrum digitizer M4i.4420-x8 for Photoacoustic Imaging by clicking here
  • See how the Department of Medical Physics and Biomedical Engineering, at University College London, UK, use an M4i.4420-x8 high-resolution digitizer in a miniature all optical ultrasonic 3D endoscopic imaging system by clicking here

Knowledge

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