Optical Reflectometers – How Do They Compare?
Measuring the return loss along a fiber optic network, or within a photonic integrated circuit, is a common and very important technique when characterizing 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 fiber. OTDRs work by launching optical pulses into the optical fiber 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 fiber optic networks, with ranges reaching hundreds of kilometers. 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 specialized ‘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 characterize 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 centimeters. Therefore, the usefulness of the OLCR is limited to inspecting individual components, such as fiber optic connectors.
Finally, OFDR is an interferometer-based measurement that utilizes 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 polarization diversity and optical optimization to achieve unmatched spatial resolution. An OBR can quickly scan a 30-meter fiber with a sampling resolution of 10 micrometers or a 2-kilometer network with 1-millimeter resolution.
This graphic summarizes 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.
Interested in Learning More?
See what you’ve been missing with traditional optical instrumentation, such as OTDRs, in this white paper.