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)

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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