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Development and Refinement of Long Period Fiber Grating (LPFG) Manufacture and Characterization Techniques Kevin Edmonds Patrick Chan Dr. H.P. Lee Long Period Fiber Gratings Operate as band rejection filters in optic fibers Useful for sensing and signal shaping applications

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Development and Refinement of Long Period Fiber Grating (LPFG) Manufacture and Characterization Techniques

Kevin Edmonds

Patrick Chan

Dr. H.P. Lee

long period fiber gratings
Long Period Fiber Gratings
  • Operate as band rejection filters in optic fibers
  • Useful for sensing and signal shaping applications
  • Two important kinds:
    • CO2 Etched Gratings
    • Acousto-optic Tunable Filters
lpfgs continued
LPFGs Continued
  • Optic fibers with periodically varying indexes of refraction through a section of the fiber
    • We’ll call them interfaces
  • CO2 Etched Gratings have their interfaces written into them with a CO2 laser
  • AOTFs use a standing wave
lpfgs continued continued
LPFGs Continued Continued
  • Important differences:
    • CO2 etched gratings are compact and passive devices, but are non-tunable
    • AOTFs are tunable, but are comparatively bulky and active devices
manufacture of etched gratings
Manufacture of Etched Gratings
  • Point by point method:
    • Place fiber under tension
    • for(int i = 0; i < 20; i++){
      • Heat
      • Stop heat
      • Move laser a little

}

manufacture of etched gratings6
Manufacture of Etched Gratings
  • Modulation Scan
    • Keep laser on entire time, but increase intensity when writing interfaces
    • End result is smoother changes in refractive index, which results in lower insertion loss (signal degradation)
novel approach to manufacture of etched gratings
Novel Approach to Manufacture of Etched Gratings
  • Each time light passes through an interface it gets attenuated at some wavelength
  • If all of the interfaces are the same, we strongly reject one wavelength
novel approach to manufacture of etched gratings8
Novel Approach to Manufacture of Etched Gratings
  • If each wavelength is a little different, we sort-of-strongly reject lots of wavelengths
  • This is a broad-band rejection filter, sometimes called a chirped grating
novel approach to manufacture of etched gratings9
Novel Approach to Manufacture of Etched Gratings
  • One way of writing chirped gratings is to change the writing intensity for each interface
  • Question: What happens if we angle the fiber as we write?
novel approach to manufacture of etched gratings10
Novel Approach to Manufacture of Etched Gratings

Answer: Terrible, horrible things

novel approach to manufacture of etched gratings15
Novel Approach to Manufacture of Etched Gratings
  • Laser falls out of focus at some interfaces and focuses too hard at other interfaces.
  • End result is that some interfaces are written too strongly while others are written too weakly.
  • It’s really just an awful awful nightmare because each test that fails (and there were a lot of tests that failed) means we (by “we” I mean “I”) have to prepare another optic fiber for writing and it turns out that there’s a certain minimum amount of manual dexterity required to work in my lab and it turns out I don’t have that kind of manual dexterity because for each fiber I prepare successfully I break two others and I think I got a splinter once FROM AN OPTIC FIBER HOW MANY PEOPLE IN THE WORLD GET THOSE?? ONLY ONE AND IT’S ME BECAUSE EVERYONE ELSE APPARENTLY HAS THE MANUAL DEXTERITY TO NOT GET SPLINTERS.
novel approach to manufacture of etched gratings16
Novel Approach to Manufacture of Etched Gratings
  • It’s possible to correct this problem in software but that sort of defeats the purpose
  • We conclude that this method is more trouble than it’s worth
interferometric measurement of the vibration amplitude of an aotf to determine mounting efficiency
Interferometric Measurement of the Vibration Amplitude of an AOTF to Determine Mounting Efficiency
  • The actual vibration transferred to the optic fiber is dependent on the connection between the optic fiber and the PZT
  • We’ll call the quality of this connection the mounting efficiency
aotf mounting efficiency
AOTF Mounting Efficiency
  • We would like to quantitatively characterize the mounting efficiency of our AOTFs
  • We use interferometry to measure the amplitude of the vibration of our AOTFs
experimental setup schematic19
Experimental Setup Schematic
  • Laser goes through a beam splitter
    • Beam breaks up into “sample beam” and “reference beam”
    • Sample beam will hit the vibrating sample
    • Reference beam goes through an acousto-optic modulator and gets tuned to some frequency (80MHz in our case)
    • Sample and reference beams recombine and go into photodetector
what the signal looks like
What the signal looks like
  • In the frequency domain we will see three peaks
what to do with this
What to do with this
  • The ratio of the high peak to the lower side peaks is given by l /4pa, where a is the amplitude of the vibrating sample
extraction of a signal from a high noise environment
Extraction of a signal from a high noise environment
  • Our AOTFs may be vibrating very weakly and may therefore be hard to detect
  • We will use a lock-in amplifier in this case
lock in amplifier
Lock-in Amplifier
  • Tool to extract a weak signal from a noisy environment
  • Output given by:
development of a software lock in amplifier
Development of a Software Lock-in Amplifier
  • Successfully reproduced lock-in amplifier functionality in software
  • End result:
measurement results
Measurement Results
  • Successfully able to measure vibration of a dummy sample
future research
Future Research
  • Ensure measurement consistency
  • Signal from photodetector is jittery
  • Automate data detection
  • Do tests with real AOTFs
acknowledgements
Acknowledgements
  • Patrick Chan, Ivan Tomov, Len Szalkowski, H.P. Lee
  • National Science Foundation
  • IM-SURE
  • Said M. Shokair