Development and Refinement of Long Period Fiber Grating (LPFG) Manufacture and Characterization TechniquesKevin Edmonds
Patrick ChanDr. H.P. Lee
Long Period Fiber Gratings
Operate as band rejection filters in optic fibers
Useful for sensing and signal shaping applications
Two important kinds:CO2 Etched GratingsAcousto-optic Tunable Filters
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
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
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 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
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 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 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 Gratings
Answer: Terrible, horrible things
Experimental Setup Schematic
Actual Setup
Actual Setup Continued
Actual Setup Supercontinued
Novel Approach to Manufacture of Etched Gratings
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.
•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.
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
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
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 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
Actual Setup
What the signal looks like
In the frequency domain we will see three peaks
What to do with this
The ratio of the high peak to the lower side peaks is given by /4a, where a is the amplitude of the vibrating sample
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
Tool to extract a weak signal from a noisy environment
Output given by:
Development of a Software Lock-in Amplifier Successfully reproduced lock-in amplifier
functionality in software
End result:
Measurement Results
Successfully able to measure vibration of a dummy sample
02468
101214161820
1 2 3 4 5 6 7 8 9 10
PZT Amplitude (*10 V)
Vib
rati
on
Am
pli
tud
e
Series1
Future Research
Ensure measurement consistency
Signal from photodetector is jittery
Automate data detection
Do tests with real AOTFs
Acknowledgements
Patrick Chan, Ivan Tomov, Len Szalkowski, H.P. Lee
National Science Foundation IM-SURE Said M. Shokair