Luna Technologies Webinar Series New Rayleigh-Based Technique for Very High Resolution Distributed Temperature and Strain Sensing Dr. Dawn Gifford Senior Optical Engineer Mr. Ed Valigursky VP of Sales
Overview Introduction to OBR Applications for sensing Introduction to sensing mechanism Sensing examples Live demonstration How it works Summary
Luna s OBR Introduced in the telecom market as industry s only micrometer resolution OTDR designed for testing components, modules, and assemblies Offers unprecedented inspection and diagnostic capabilities with comprehensive testing and inspection in less time with a single instrument Now the same instrument is capable of fully distributed sensing using standard optical fiber
Applications for sensing In-situ temperature monitoring in optical assemblies Touch to locate Industrial process monitoring Strain monitoring in optical cable assemblies Structural health monitoring Mechanical load testing Security Many other possible applications
Sensing Mechanism The OBR monitors changes in the amplitude and phase of the light that is naturally back-reflected from standard optical fiber. These changes can be mapped to applied temperature or strain. Fiber Bragg gratings can be used, but are not required the fiber itself is the sensor.
Rayleigh Backscatter Imperfections in fiber lead to Rayleigh backscatter: Injected Laser Light Backscattered Light Rayleigh backscatter forms a permanent spatial fingerprint along the length of the fiber.
Frequency Shift Mean period between perturbations given by Λ. Λ 0 Strain or temperature change causes Rayleigh fingerprint to stretch. Λ 1 As for a Bragg grating, shift in Λ leads to a shift in optical frequency Measured frequency shift can be calibrated to strain or temperature
Example: Tapered Fiber Bundle Measurement Optics 1550/980 WDM (6) MM Pumps PM TFB Amplifier Optics Local Heating Set-up for measurement of tapered fiber bundle.
Measurement Results Heating in the TFB for several input pump powers. The two bumps represent heating at the splice locations on either side of the TFB and the area in between is heating in the taper region.
Example: Cable Strain Luna OBR 4 m Apply twist to 4-lead graded index multimode cable and observe strain response.
Measurement Results 1500 1000 0.5 Twists 2 Twists 4 Twists 6 Twists 8 Twists 10 Twists Strain x 10 6 500 0 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Position (m) Distributed strain induced in a 62.5 µm core multimode fiber in a four lead cable due to cable twist.
Distributed Sensing Demonstration Splice Connector Luna OBR Switch 8.0 m 1.5 m Touch To Locate Demonstration Network Strain and Temperature Measurement Demonstration Network
Live Demonstration
Measured Rayleigh Scatter Measured Rayleigh Scatter from a fiber spool. With polarization diversity: r( τ ) = E ( τ ) + E ( τ ) s 2 p 2
Data Processing for Sensing Sensing fiber (SMF, PM, etc.) (+) Optical Backscatter Reflectometer Scatter amp. Scatter amp. Distance (meters) Distance (meters) cool reference hot measurement
Processing amp. Sliding window x cool reference amp. Distance (meters) hot measurement Distance (meters) reference x measurement x Slice out 2mm segments of scatter profile along entire fiber length
Processing FFT reference x measurement x FFT reference spectrum, S ref measurement spectrum, S meas ( ν ) ( ν ) S ref S meas crosscorrelation yields spectral shift if there is a T or ε difference between S ref and S meas ν
Processing Summary Record Rayleigh fingerprint as a function of distance along fiber for reference and test case Divide data into segments x and Fourier Transform into the optical frequency domain Measure ν for each segment along the length of the test fiber ν can be calibrated to T or ε
Key Highlights Highly distributed fiber sensing for strain and temperature Very high spatial resolution: down to 2 mm +/- 0.1 ºC, +/- 1 µε resolution at 1 cm 5 s acquisition times at highest resolution Same spectral response as Bragg gratings Does not require specialty fiber or gratings
Overview of Luna Technologies LT a division of Luna Innovations Luna group founded Summer 1992 Luna Technologies: focused on measurement instrumentation for fiber optic components and subsystems World headquarters in Blacksburg, VA World wide sales offices and representation
Need More Information? Contact: Ed Valigursky 770-634-6765 edv@lunatechnologies.com
Swept Wavelength Interferometry Discrete Fourier Transform
Measurement Network (SWI) Measure output power of the interferometer As a function of the input optical frequency. 2 2 [ ω τ ϕ ] P = E r + E + 2 E E r cos ( t) + s lo s lo r
Sensing Linearity 3.5x10-3 3.0 400x10-6 300 λ / λ 2.5 2.0 1.5 λ / λ 200 100 1.0 0 0.5-100 1 2 3 4x10-3 Strain λ/λ = (0.7314±0.0006)(ε + 166±2x10-6 ) SMF-28e λ/λ = (0.7484±0.0005)(ε - 112±2x10-6 ) InfiniCor 300 10 20 30 40 50 60 70 Temperature ( C) λ/λ = (7.09±0.04x10-6 )(T - 23.1±0.2 C) SMF-28e λ/λ = (7.64±0.04x10-6 )(T - 26.4±0.3 C) InfiniCor 300 Spectral shift measured using the OBR as a function of applied strain using a 10 mm integration window. Spectral shift measured using the OBR as a function of applied temperature using a 10 mm integration window.
About Luna Luna Innovations commercializes technology in advanced materials, sensing and instrumentation Founded 1992 with six VA locations Luna Technologies, a division of Luna Innovations: Focused on measurement instrumentation for fiber-optic components and subsystems World headquarters in Blacksburg, VA World-wide sales offices and representation
Current Specifications OBR Specifications Rayleigh Mode Spatial Resolution (typical): 2 mm 20 mm Sensing Range: Strain Resolution: Strain Range: Temperature Resolution: Temperature Range: Sampling Rate: Sensor Capacity: Operating Temperature (Measurement System): 30 m standard (70 m optional) + 1.0 µstrain (@10 mm resolution) + 20,000 µstrain + 0.1 ºC (@10 mm resolution) -50 to 300 ºC (practically limited by packaging) ~ 0.1 Hz = length of sensing fiber / resolution (~3000 sensing locations typical) 10 ºC to 35 ºC