Blue Road Research Fiber Optic Grating Sensors and Applications Blue Road Research Session 1 Session 5 Session 4 Session 3 Session 2 Session 6 Session 8 Session 7

Blue Road Research Criteria for a Successful Fiber Optic Sensor Application Blue Road ResearchSession 6, Page 1 Meets and important application need Is unique, superior solution Is economically compelling

Blue Road Research Fiber Grating Sensors Blue Road ResearchSession 6, Page 3 Key parameters - strain and temperature Competitive technology - electrical resistive strain gauges and thermocouples/low cost ($20) - well known, difficult to embed and successfully operate Today - fiber gratings are high cost items ($200 each) being used to measure strain and temperature in embedded materials The future - cost competitive with electrical strain gauges with options for 3 axis strain measurement, environmentally superior performance

Blue Road Research Fiber Grating Sensor Prospects Blue Road ResearchSession 6, Page 4 Fiber gratings are likely to drop into the $25 to $40 range for small quantity buys in two to three years enabling direct competitiveness with electrical strain gauges.

Blue Road Research Fiber Grating - Holographic Method Blue Road ResearchSession 6, Page 6 Induced grating pattern Fiber Laser beams

Blue Road Research Fiber Grating - Phase Mask Method Blue Road ResearchSession 6, Page 7 Induced grating pattern Fiber Laser beam Phase mask

Blue Road Research Means to Write Fiber Gratings Blue Road ResearchSession 6, Page 8 Long exposure side imaged interference pattern »United technology / operates to approximately 500 deg C »Good spectral characteristics /reflectance Short pulse side imaged interference pattern »Naval Research Lab / operates to 800 deg C »Demonstrated manufacture during draw »First gratings low of quality

Blue Road Research Means to Write Fiber Gratings(continued) Blue Road ResearchSession 6, Page 9 Phase masks »Moderate temperature »Good performance »Canadian Communication Research Center Line by line »Higher temperature to 800 deg C »Canadian Communication Research Center Phase masks / bending fiber »Brown University / Bragg Technology »Wider bandwidth, good performance

Blue Road Research Major Historical Milestones Blue Road ResearchSession 6, Page 10 K.O. Hill, CRC 1978, discovery of photosensitivity. Lam and Garside, McMaster U. 1981, observed photosensitivity as a two photon effect. Meltz, Morey, and Glenn, UTRC 1989, side writing technique with UV laser. Hill and Snitzer, CRC and Rutgers 1993, mask technique for writing fiber gratings. LeMaire, AT&T 1993, Hydrogen loading. Archambault, Reekie, Russell, Southampton U. 1993, single pulse, type 2 grating, and draw tower exposure. (Source - 3M Bragg Grating Technologies)

Blue Road Research Bragg Grating Exposure Blue Road ResearchSession 6, Page 11 (Source - 3M Bragg Grating Technologies)

Blue Road Research Photoinduced Index Change Blue Road ResearchSession 6, Page 12

Blue Road Research 0.1% Chirp, Peaks at 0.25 and 0.75L Blue Road ResearchSession 6, Page 13

Blue Road Research Grating Reflection with 1nm Bandwidth and Reduced Sidelobes Blue Road ResearchSession 6, Page 14

Blue Road Research Coupling to Cladding Modes with 4 o Blaze Blue Road ResearchSession 6, Page 15

Blue Road Research Wavelength-Selective Light Coupling from Fibers with Bragg Gratings Blue Road ResearchSession 6, Page 16

Blue Road Research Bandpass Filter with Fiber Gratings in Michelson Arrangement Blue Road ResearchSession 6, Page 17

Blue Road Research Transmission of Fiber Bragg Grating Pair as Fabry-Perot Interferometer Blue Road ResearchSession 6, Page 18

Blue Road Research External Cavity Laser Diode Blue Road ResearchSession 6, Page 19 (Source - 3M Bragg Grating Technologies)

Blue Road Research External Cavity Laser Diode Blue Road ResearchSession 6, Page 20 (Source - 3M Bragg Grating Technologies)

Blue Road Research Temperature Relation for Grating Sensors Blue Road ResearchSession 6, Page 21  / = (  +  )  T  = expansion coefficient = 0.55x10 -6 o C -1 for silica  = thermooptic coefficient for fiber core material = 8.31x10 -6 o C -1 estimated for GeO 2 doping*  / = 8.86x10 -6  T  = nm/ o C at 820 nm * From S. Tahahashi and S. Shibata, Journal of Non-Crystalline Solids 30(1979)

Blue Road Research Strain Relation for Grating Sensors Blue Road ResearchSession 6, Page 22  / = (1 - p e )  p e = photoelastic constant = (n 2 / 2)[p 12 - (p 11 + p 12 )] = 0.22 for silica  / = 0.78   = 6.4 nm / 1% at 820 nm

Blue Road Research Fiber Grating Wavelength Shift Blue Road ResearchSession 6, Page 23 (Source - 3M Bragg Grating Technologies)

Blue Road Research Reflectivity Over Test Period at 650 o C Blue Road ResearchSession 6, Page 24

Blue Road Research Bandwidth Over Test Period at 650 o C Blue Road ResearchSession 6, Page 25

Blue Road Research Transmission Plots Before and After High Temperature Exposure Blue Road ResearchSession 6, Page 26 (Source - 3M Bragg Grating Technologies)

Blue Road Research Temperature and Strain Cycling Blue Road ResearchSession 6, Page 27 Use preannealed FBG 4 hour cycles, 21 o C to 427 o C 512 cycles over 2048 hr.. No change measured in FBG spectrum Apply dynamic strain in tension load Maximum strain 2500 microstrain 1.4 million cycles No change measured in FBG spectrum Temperature Cycle TestStrain Cycle Test

Blue Road Research Fiber Grating Demodulators Blue Road ResearchSession 6, Page 28 Open loop versus closed loop Open loop single grating approach requires broadband grating Brown University is working on broadband chirped gratings PZT stacks and designs for adequate modulation exist at modest voltages

Blue Road Research Low Cost Approaches Blue Road ResearchSession 6, Page 29 Overcoupled coupler Miniature Mach-Zehnder Fiber grating spectral filter

Blue Road Research Overcoupled Beamsplitter Layout Blue Road ResearchSession 6, Page 30 Light source Fiber grating Ratioed output Detectors Overcoupled beamsplitter

Blue Road Research Overcoupled Coupler Issues Blue Road ResearchSession 6, Page 31 Thermal drift more severe with higher sensitivity Polarization mixing issues Packaging of sensitive devices »very long overcoupled couplers are fragile

Blue Road Research Miniature Mach-Zehnder Blue Road ResearchSession 6, Page 32 More rugged than overcoupled coupler approach with comparable sensitivity Superior thermal and polarization properties to overcoupled coupler Smooth spectral profile Needs further thermal and polarization improvements - close to ready

Blue Road Research Grating Sensor with Fiber-Interferometric Wavelength Discriminator Blue Road ResearchSession 6, Page 33

Blue Road Research Fiber Grating Spectral Filter Blue Road ResearchSession 6, Page 34 Can be tailored to match desired dynamic range and sensitivity Athermal package for operation over -40 to 80 o C (less than 0.1 nm drift) Relatively polarization independent Suitable for a demodulator with approximately 100 microstrain sensitivity and +/ microstrain range

Blue Road Research Fiber Grating Spectral Filter Demodulator Blue Road ResearchSession 6, Page 35 Fiber grating spectral filter Light source Receivers Beamsplitters

Blue Road Research Chirped Fiber Grating Spectral Filter Blue Road ResearchSession 6, Page 36

Blue Road Research 1550 nm Grating Demodulation Kit Blue Road ResearchSession 6, Page nm ELED light source (2) 3 dB beamsplitters Chirped fiber grating filter (2) receivers Patch cords FC connector cleaner 1 single axis grating sensor Data CD with manual Optional DAQ card & software Optional Carrying case

Blue Road Research High Speed Grating Demodulators Blue Road ResearchSession 6, Page 38 Stand-alone configuration Three bandwidth options »1kHz, 10 kHz, 2 MHz Flexible design for varied applications Integrated light source and spectral filters

Blue Road Research Fiber Grating System Blue Road ResearchSession 6, Page 39 Modulated reference fiber grating Detector Light source Fiber gratings 1 2

Blue Road Research Fiber Fabry-Perot Tunable Filters Blue Road ResearchSession 6, Page 40 Air gap 1-2 microns PZT actuator Mirrors Cavity length 20 microns for 50 nm FSR

Blue Road Research Fabry-Perot Detector/Fiber Blue Road ResearchSession 6, Page 41 Bimorph actuator Capillary tube Fiber Silicon detector Silicon dioxide Silicon layer

Blue Road Research Fiber Bragg Grating Sensor Array with Fiber Fabry-Perot Demodulator Blue Road ResearchSession 6, Page 42

Blue Road Research Shift in FFP Control Voltage and Bragg Wavelength with Applied Strain to FBG Sensor Element Blue Road ResearchSession 6, Page 43

Blue Road Research Bragg Grating Axial Strain and Temperature Sensor Blue Road ResearchSession 6, Page 44 Measures  T and  1 for surface mounted applications Overlaid Bragg gratings at two wavelengths »850 and 1300 nm Output spectrum contains two peaks » b1 = f 1 (  1,  T) and b2 = f 2 (  1,  T)

Blue Road Research Multi-Parameter Bragg Grating Blue Road ResearchSession 6, Page 45 Two overlaid Bragg gratings created in birefringent fiber » 1, 2 Birefringent fiber can transmit two orthogonal polarization modes »p,q Reflected spectrum will contain four peaks » p1, q2, p2, q2 Four peaks can be used to determine three axis of strain and temperature »  1,  2,  3,  T

Blue Road Research Bragg Grating in Birefringent Fiber Blue Road ResearchSession 6, Page 46 Two polarization modes with different values of n (n p, n q ) Two distinct Bragg peaks p q p q p0 = 2d 0 n p0 q0 = 2d 0 n q0

Blue Road Research Response of Birefringent Fiber to Applied Strain and Temperature Blue Road ResearchSession 6, Page 47 When the fiber is subjected to  or  T, will shift due to change in d (elongation) and n (stress-optic effect)

Blue Road Research Response of Birefringent Fiber to Applied Strain and Temperature Blue Road ResearchSession 6, Page 48 If we assume  23 =0, the equations are linear in  and  T

Blue Road Research 3 Axis Strain and Temperature Blue Road ResearchSession 6, Page 49 Polarization preserving fiber axes Dual overwritten fiber gratings

Blue Road Research Two Overlaid Gratings in Birefringent Fiber Blue Road ResearchSession 6, Page 50 If we add a second grating to the fiber at a different wavelength, we will obtain two additional peaks in the reflected spectrum The response of these new peaks will be different due to the wavelength dependence of fiber properties (p ij, etc.) The response of the four peaks to strain and temperature can be expressed as:

Blue Road Research Determining Three Axes of Strain and Temperature Blue Road ResearchSession 6, Page 51 Provided K is well conditioned, we can determine the strains (  1,  2,  3 ) and temperature (  T) from the change in wavelength of the four peaks using:

Blue Road Research Experimental Setup to Test the Three Axis Strain and Temperature Sensor Blue Road ResearchSession 6, Page 52 3 Axis strain and temperature sensor couplerWDM 1300 nm light source 1550 nm light source Variable FP etalon Etalon controller Receiver Data acquisition unit

Blue Road Research Experimental Setup to Test the Three Axis Strain and Temperature Sensor Blue Road ResearchSession 6, Page 53 WDM 1550 nm 1300 nm Z 50/50 splitter Fiber grating sensor FC/PC connector sleeve Optical Spectrum Analyzer PC running Blue Road Research software GPIB interface

Blue Road Research Axial Loading of 3 Axis Fiber Grating Sensor Blue Road ResearchSession 6, Page 54

Blue Road Research Transverse Loading of 3 Axis Fiber Grating Sensor Blue Road ResearchSession 6, Page 55

Blue Road Research Transverse Loading of 3 Axis Fiber Grating Sensor Blue Road ResearchSession 6, Page 56

Blue Road Research Transverse Loading of 3 Axis Fiber Grating Sensor Blue Road ResearchSession 6, Page 57

Blue Road Research 3 Axis Demodulation Kit Blue Road ResearchSession 6, Page 58

Blue Road Research Possible Applications of 3 Axis Sensor Blue Road ResearchSession 6, Page 59 Aerospace Biomedical Geotechnical Civil structures

Blue Road Research Civil Structure Applications for 3 Axis Sensor Blue Road ResearchSession 6, Page 60 Fiber Optic Strain Sensors Suspension Bridge Fiber Grating Load Cells Drawbridge

Blue Road Research Civil Structure Applications for 3 Axis Sensor (continued) Blue Road ResearchSession 6, Page 61 Scouring Sensors Load cellsScouring Fiber Optic Strain Sensor

Blue Road Research Civil Structure Applications for 3 Axis Sensor (continued ) Blue Road ResearchSession 6, Page 62 Embedded Fiber Optic Strain Sensors Roadside Demodulation box Speed and Weight Embedded Fiber Optic Strain Sensors Traffic Control

Blue Road Research Long Period Fiber Gratings Blue Road ResearchSession 6, Page 63 Temperature dependence 0.04 to 0.05 nm/ o C (short period grating is about 0.01nm/ o C) Strain dependence very fiber specific, examples: Grating A nm/m , Grating B nm/m  (short period gratings are 1.0 to 1.8 nm/m  for the 1.3 to 1.5 micron range) Extremely sensitive to bending which can override the grating Reference: Vengsarker et al, JLT, p.53, Jan 96