OT-LUNA Blog-Luna to Present Papers at the SPIE Defense + Commercial Sensing Conference

https://lunainc.com/luna-innovations-papers-presented-spie-defense-commercial-sensing-conference/

Luna Innovations will take part in two paper presentations at the SPIE Defense + Commercial Sensing Conference this week at the Baltimore Convention Center. “Distributed polarization state sensing with optical frequency domain reflectometry” will be presented 11:20-11:40 am Tuesday, April 16. Optical Frequency Domain Reflectometry (OFDR) is notable for its ability to characterize a fiber distributed reflection profile with high spatial resolution and high sensitivity in a single laser sweep, allowing for the measurement of strain and temperature with millimeter spatial resolution over short ranges (tens of meters up to a few kilometers). The paper investigates how spatially resolved characterization of the polarization properties of light reflected or transmitted by a fiber can enable additional sensing capabilities, including distributed polarization mode dispersion and polarization extinction measurements, with applications in distributed pressure, electric field and transverse stress sensing.

“Single-mode sapphire fiber optic distributed sensing for extreme environments” will be presented 9:50-10:10 a.m. Thursday, April 18. The authors have developed a single-mode sapphire sensor for distributed temperature and flow measurement to address the extreme environments encountered in energy applications. The sensor is designed to also detect and localize fouling and deposits that accumulate on its surface over time. Optical Frequency Domain Reflectometry (OFDR) was employed to yield these distributed measurements.

Temperature accuracy was on the order of 5°C for measurements ranging from room temperature to over 1000°C. Spatial resolution of 11 mm was attained and enabled visualization of the temperature gradients along a fiber passing through a furnace. The effects of cooling flow were characterized for steady operation. Dynamic cooling flow tests showed that the presence of simulated deposits on the outside of the sensor resulted in slower time response in the vicinity of the deposit. This technique could be used to determine the presence and location of deposits along the length of the sensor assembly.

OT-LUNA Blog-Optical Reflectometers – How Do They Compare?

http://lunainc.com/optical-reflectometers-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.