Characterisation, Measurement & Analysis
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Optical Reflectometers – How Do They Compare?

Measuring the return loss along a fibre optic network, or within a photonic integrated circuit, is a common and very important technique when characterising a network’s or device’s ability to efficiently propagate optical signals. Reflectometry is a general method of measuring this return loss and consists of launching a probe signal into the device or network, measuring the reflected light and calculating the ratio between the two.

Spatially-resolved reflectometers can map the return loss along the length of the optical path, identifying and locating problems or issues in the optical path. There are three established technologies available for spatially-resolved reflectometry:

  • Optical Time-Domain Reflectometry (OTDR)
  • Optical Low-Coherence Reflectometry (OLCR)
  • Optical Frequency-Domain Reflectometry (OFDR)

The OTDR is currently the most widely used type of reflectometer when working with optical fibre. OTDRs work by launching optical pulses into the optical fibre and measuring the travel time and strength of the reflected and backscattered light.These measurements are used to create a trace or profile of the returned signal versus length. OTDRs are particularly useful for testing long fibre optic networks, with ranges reaching hundreds of kilometres. The spatial resolution (the smallest distance over which it can resolve two distinct reflection events) is typically in the range of 1 or 2 meters. All OTDRs, even specialised ‘high-resolution’ versions, suffer from dead zones – the distance after a reflection in which the OTDR cannot detect or measure a second reflection event. These dead zones are most prevalent at the connector to the OTDR and any other strong reflectors. OLCR is an interferometer-based measurement that uses a wideband low-coherent light source and a tunable optical delay line to characterise optical reflections in a component. While an OLCR measurement can achieve high spatial resolution down to the tens of micrometers, the overall measurement range is limited, often to only tens of centimetres. Therefore, the usefulness of the OLCR is limited to inspecting individual components, such as fibre optic connectors.Finally, OFDR is an interferometer-based measurement that utilises a wavelength-swept laser source. Interference fringes generated as the laser sweeps are detected and processed using the Fourier transform, yielding a map of reflections as a function of the length. OFDR is well suited for applications that require a combination of high speed, sensitivity and resolution over short and intermediate lengths.Luna’s Optical Backscatter Reflectometers (OBRs) are a special implementation of OFDR, adding polarisation diversity and optical optimisation to achieve unmatched spatial resolution. An OBR can quickly scan a 30-meter fibre with a sampling resolution of 10 micrometers or a 2-kilometre network with 1-millimetre resolution.This graphic summarises the landscape of these established technologies for optical reflectometry. By mapping the measurement range and spatial resolution of the most common technologies, the plot illustrates the unique application coverage of OBR.