writing a long period grating

Hallo friends,

I bother you for help.

I am trying to write a LPG into a photosensitive fiber type Nufern GF1 by point to point technique (i.e. I am using a shutter to block the beam and making a pattern with period 400um). I have used a power of

50mW and the beam is focused into a spot of 200um, the exposure time is 3min per cell, 60 cells . The result is: nothing!

Does anybody have an idea about what I do wrong?

I have some dubts already, but I am not sure whether they are correct or not:

1) the photosensitivity is still too low and I will never get anything with this laser (I cannot send in more power) 2) I do not make beam filtering to avoid the waste of power. Does this limit the focusing due to abberations?

Thanks in advance to anybody who can give me some hints!


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Some questions: What kind of UV laser, in particular wavelength? Which wavelength range to you cover with your measurement setup? The exact resonance wavelength of LPGs depends on several issues. Fiber hydrogen-loaded or not? Smaller focus of course will help

This of course depends on the mode field of the UV laser. But my first guess would be that such non ideal conditions would degrade the spectrum of the resulting LPG and not prevent their existence.

Just to be sure: the 200 micrometer are the 1/e^2 diameter and not the radius?

Reply to
Frank Knappe

Hi Frank!

We already met a number of times, so I know you may be the right person to help me to get out of the mud!

1) I am using an Ar ions laser at 244nm, which I believe is very similar to that you were using 2) to detect the presence of the LPG I make the measurement in tx with a broadband souce (two LED at 1.3 and 1.55um, thus covering the range 1.25-1.60 with a gap around 1.4um) and an OSA 3) unfortunately no H2 loading (but the fiber is sold as "photosensitive" and I have recorded bragg gratings with reflectivity ~99%) 4) yes, I am talking about the beam diameter measured at 1/e^2. Is it too big? Do you have an idea of to choose the writing parameters (power, exposure time)?



Reply to

Hello Massimo!

Perhaps I can give you some hinds, but LPG never were my speciality.

Do you monitore online during the writing? Is there really nothing happening or are there small chnages in the transmission (less then 1dB dips perhaps)?

The fiber is definitiv photosensitive. And also more photosensitive than a SMF28. But hydrogen loading gives you a factor of 100. That's a lot.

Well, easy answer: power as much as possible, exposure time as ong as possible. This will yield the highest index changes.

LPGs are different to FBGs:

- FBGs are getting stronger if you increase the length, for LPGs this is different. After a certain length the power from the cladding modes is coupled back to the core.

- The transmission spectrum of an LPG is very sensitive to nearly everything (which makes them great for sensors, if you can cope with the cross correlations)

- Snce you have to know the dispersion characteristics of all the cladding modes, the design of LPGs is more complicated. So perhaps your period is yielding an LPG at a totally different wavelength.

- For an FBG the period is given by some kind of interference pattern. How well defined is the period of the dots in your case?

I've only made some prood of principle experiments with LPGs. For writing them I used a narrw focussed beam (about 5 micron) and moved in in a sneaky way over the fiber which was mounted on high precision translation stages.

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Reply to
Frank Knappe

Hi Frank,

sorry for my late reply but had to stop LPG for a while.

I bother you some more about the way you managed to focus to such a small spot. Did you spatially filtered the beam? In that case don't you loose too much power? Do you make a beam circularization with cylindr. lens prior to beam expansion and focusing?

I hope people do not become upset since the discussion is becoming personal...anyway every idea is welcome!



Reply to

I was either using a 10mm best form lens or a x20 UV objective lens.

Nope. I was quite lucky that our Coherent UV laser UV laser had a good beam profile. And for the writing of LPGs the real beam focus profile isn't that important. It was more important for my other purposes, but even in that cases it was good enough.

For the writing of LPGs with such a small spot you will have to take into account the culindirc lens effect of the fiber itself. Or if you just want to show the principle effect you don't care about. That's how I have done it. What I have done was just a proof of principle experiment during a free afternoon.

Reply to
Frank Knappe

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