D. R. Lyons (Hampton University) Fiber Optic Sensors Group COMPREHENSIVE FINAL REPORT FOR THEPERIOD 4/97-4/99 "Fiber Optic Sensor Components and Systems For Smart Materials and Structures" Final Report for NASA Grant NAG8-1340: The general objective of the funded research effort has been the development of discrete and distributed fiber sensors and fiber optic centered opto-electronic networks for the intelligent monitoring of phenomena in various aerospace structures related to NASA Marshall specific applications. In particular, we have proposed and have been developing technologies that we believe to be readily transferable and which involve new fabrication techniques. The associated sensors developed can be incorporated into the matrix or on the surfaces of structures for the purpose of sensing stress, strain, temperature-both low and high, pressure field variations, phase changes, and the presence of various chemical constituents. These are the most significant milestones accomplished during the two-year funding period: We completed the fabrication and testing of the prototype unit for multiple sensor fabrication shown in Figure 1. This prototype consists of a high resolution, automated writing system composed of a double interferometer setup (one visible and the other UV). A schematic of the actual prototype is shown in Figure 2. This device will be used to register highly accurate and evenly spaced gratings in samples such as our D- fiber pressure sensors, ultra sensitive phase change detectors, shear stress monitors, and high density artificial (human oriented) nervous system. In order to test some of these concepts, we recently verified the ability to measure and write transverse holograms fibers with a priori knowledge of the resonances. This is an unparalleled accomplishment in this field. We have also, partly through Marshall funding, set up 4 high tech fiber optics laboratories with various diagnostic and fabrication capabilities. See laboratory photos in Figures 16-21. Figure 1. Prototype wavelength comparator for high resolution multi-sensor production. "Fiber Optic Sensor Components... 1 D.R.Lyons(HamptoUnniversity) FiberOpticSensoGrsroup Synchronized Rotation Stage 2 MI ,Oscillating UV Diode Detector Synchronized 1 High Output Sabre FReD Laser Operating @244/257nm Figure 2. Double interferometer Bragg grating wavelength standard schematic layout. M2 U.V. Fluorescent Screen/ Photographic Plate 40X Quartz Objective Optical Fiber Rotational Stage /tic FArregqoune-Inocny DLaosuebrled@ 244nm I MI \ Superimposed Image Cylindrical Piano-Convex of Fringes and Fiber Lenses Lens Electronic Shutter Figure 3. Bragg grating system used in phase mask defects and grating-fiber imaging work. Using D-type fibers and the photographic imaging techniques, we derived and filed a patent for an optimization algorithm for phase mask production. See 'Publications and Presentations' in Appendix A [1-3,11,12] and Scientific Patents and Patent Disclosures in Appendix B [1]. The consequences of this direction of research has been the design of a new tunable writing device and to a deeper understanding of the physics surrounding phase mask design and fabrication. See 'Patents and Disclosures' in Appendix B [2]. See Figure 5 for the principle of phase mask grating writing. "Fiber Optic Sensor Components... 2 D. R. Lyons (Hampton University) Fiber Optic Sensors Group theoretical model that completely explains a certain class of typical anomalies in diffraction patterns produced certain commercial phase masks is the subject of a recently accepted paper entitled "Modeling and Observations of Phase Mask Trapezoidal Profiles Using Grating-Fiber Image Reproduction". The details of this model can be found in Appendix D. Figure 4. Image of UV fringes and D-fiber showing holographic grating patterns. •" -1 +1 -2 +2 _fical Fiber -.o Phase Mask Figure 5. Illustration for optimization involving dominant first order beam diffraction. "Fiber Optic Sensor Components... 3 D. R. Lyons (Hampton University) Fiber Optic Sensors Group Figure 6. Phase Mask Phase Mask Transverse Holographic Period = 1.059p.m Period = 0.566t.tm 0 = 25 degrees The AFM images showing the two phase masks used in our paper entitled "Modeling and Observations of Phase Mask.Trapezoidal Profiles.Using..." are included for completeness. See Appendix A [1 1] and Figure 7. These images were then fitted to our model to mathematically generate the fringes of Figure 8. Note the striking similarities. '_'Figure 7. AFM Images of .566_m and 1.059_tm Phase Masks JL/L/- / Calculated Relative Inter_ity 0.35 0.3 0.9-5 0.9- 0.15 0.1 0.05 order 0 1 2 Figure 8. Trapezoidal phase mask pattern for .5661.tm mask and corresponding Mathematica fringe pattern. "Fiber Optic Sensor Components... 4 D. R. Lyons (Hampton University) Fiber Optic Sensors Group The fiber optic shear stress monitor mentioned at the end of last year's report was also built and tested. It produced very promising results in that we were able to directly correlate wavelength shifts in our sensor due to stress. These shifts are indicators of both wind speed and the stress in a structure resulting from these shearing forces. We produced several designs for optical fiber wind shear sensor one of that uses D-fibers with Bragg gratings embedded into structural surfaces. It incorporates compressed air flowing across a surface containing a matched pair sensor unit, whose modulated twin unit is outside of the region being measured, shear forces then generate a modulated error signal when incorporated into a phased locked system. Although the technical complexity of this experiment extended beyond the scope of this research grant, the work is still currently being partially funded under another NASA grant. In addition, a recent experimental setup along with a conceptual design for an evanescent-field pressure and phase change sensor are shown in Figures 9 and 10. Flow AI Bar w/V-_,roove Meter 1l Bragg Signal 0!Pi, No Flow Pressure f/ ..... High Speed Row Gauge ,, II • jJl n_.. _,' P Wavelength Shift w/Flow Figure 9. Shear Stress Experimental Setup Fluid density Optical Chopper J I Ntuanrarbolwe-linewlaistehr Reference To Modulated signal toLock-ln Differential Input Lock-ln Amplifier _- Error signal proportional to dynamic fluid density in perturbed region Figure 10. Evanescent field pressure sensor that could detect the invisible presence of ice on aircraft wings "Fiber Optic Sensor Components... 5 D. R. Lyons (Hampton University) Fiber Optic Sensors Group We have experimentally demonstrated that these sensors have reasonable responses and sensitivities. We also have a number of new designs for optimizing the response of these sensors for various configurations. Sensitivity enhancements also include the choice of retrofit materials in which the sensors are embedded, amplification of the strain transfer to the D-fiber sensors using gradation of material, and Bragg wavemeteer produced evanescent field coupling for ultra-sensitive operation. In addition, we will continue low priority work, begun during this grant, on the development of a miniature FO spectrum analyzer for readout verification and characterization of distributed Bragg filters within a given fiber unit. This verification method involves the device shown in Figure 11 along with its response to a single frequency, actively stabilized He-Ne laser. Figure 12 shows the our conceptual design of the resulting SMFOSA unit that is to be used for Bragg filter characterization. The motivation for this endeavor is to deliver compact readouts along with grating units to NASA centers that have interest in them. Epoxy [91--- Gauge Length .... I_ _s_ _'_Piezo Ceramic Modulator s=10-30 SMFOSA Coherent Spectrum Analyzer Figure 11. Single mode fiber optic spectrum analyzer for use as a compact Bragg grating readout. Photodetector Epoxy Brag9filler / _ PlezoCeramicModulator s=10-30Jam, RT=R.,=(50-90% Reflectivity @633nm) Figure 12. Bragg grating detection using in-house constructed SMFOSA. Figures 13 and 14 show the experimental diagram of our Ti:Sapphire laser system used for grating characterization and a typical Bragg resonance which it was used to detect. This system has been used as a narrow linewidth source and we expect it to remain an invaluable resource in the future with respect to the continuance of certain of the project "Fiber Optic Sensor Components... 6 D. R. Lyons (Hampton University) Fiber Optic Sensors Group • objectives. An additional tunable diode laser for Bragg filter characterization was purchased, in part through the funding supplied under NAG8-1340. n(clad)--, 5n _ n(cor_,)\ Tunable ] , Ti:Sapphire Laser] ',-,) ; ' ,,,) _,Bragg t Photodetector_ WaveScan System l it I !,R 1 __j/ '__ ....... I I I I -- - _--2> t \N Figure 13. Tunable laser scanning system for D-fiber gratings using a Ti:Sapphire laser. 18h H loaded SM Fiber 2 3 I I r [ I :3 2 O "O G N °I ''_l'i!_*'", (Eu A L= O z i 820 822 824 826 828 830 Wavelength(nm) Figure 14. Output from Ti:Sapphire scanning laser system. • Finally, our laboratory has continued the development fiber optic sensing devices based upon the simple Fabry-Perot design depicted in figure 15. "Fiber Optic Sensor Components... 7 D. R. Lyons (Hampton University) Fiber Optic Sensors Group 2mm long hollow-core fiber Single-mode optical fiber Epoxy ,7 I_ a [,11 t _1_ Gauge Length a= 10 - 30 m (gap spacing) Figure 15. Single mode fiber optic Fabry-Perot sensor routinely fabricated inour laboratories. Laboratory Resources Made Available in Part or Whole Due to NAG8-1340 Funding: The following photos show portions of the laboratories and some of the experimental setups including the computer controlled Ti:Sapphire laser system used for high resolution characterization of Bragg grating sensors (Figure 16), the Coherent commercial spectrum analyzer system used as a standard for comparison to the first experimental trace shown in Figure 11 (Figure 17), our new multi-displinary fiber optic sensors laboratory (Figure 18), the Northrop-Grumman Fiber Optic Sensors Lab originally donated to Hampton University in 1993 (Figure 19) with an AFM setup (Figure 20), members of the Fiber Optic Sensors and Smart Structures Group (Figures 21 and 22), and the wavemeter Development Group (Figure 23). Figure 16. A Tunable Ti:Sapphire laser system with WAVESCAN used for Bragg grating diagnostics. "Fiber Optic Sensor Components... 8 D. R. Lyons (Hampton University) Fiber Optic Sensors Group Figure 17. Coherent, Inc. commercial spectrum analyzer used to evaluate the SMFOSA in Figure 9a. Figure 18. New muitidisplinary fiber optic sensors laboratory where distributed sensor writing device is located along with other grating diagnostics. "Fiber Optic Sensor Components... 9 D. R. Lyons (Hampton University) Fiber Optic Sensors Group Figure 19. Northrop-Grumman Fiber Optic Sensors Laboratory originally donated in 1993. Figure 20. The atomic force microscope used to generate images in phase mask defects work. "Fiber Optic Sensor Components... 10