# Fiber optic coupling question

1. May 16, 2008

### jeffmoss1

I hope this post in the appropriate forum.

I have a spectrograph and a CCD imaging chip that I am using for an optics experiment. The light coming into the spectrograph is through a 10 micron fiber optic. In order put the light on the CCD imaging chip the way I want it, I believe I need to use the small diameter optic.

The light exiting the fiber optic is not intense enough.

Currently, the light source is a deuterium lamp with a fiber optic connector port (black box with a port and an on-button,) but eventually the light will need to be collimated through a medium and focused back into the 10 micron optic.

Is the only way to get a more intense light to have a more intense source, or is there a way to focus light into a smaller beam and make it more intense? Can I do it with lenses or do I need a stronger lamp?

As far as I know, this deuterium lamp was not designed for a 10 micron fiber, but I also don't know if there would be a difference.

2. May 16, 2008

### mgb_phys

So you just have a 10um fibre pointing at a lamp?

Yes you can probably do better focussing the light onto the fibre with a lens.
Edmunds scientific or someone will have pre-built assemblies if your fibre has a standard connector on the end.

Last edited: May 16, 2008
3. May 16, 2008

### jeffmoss1

yes, as far as I know, the optic is just pointed at the lamp, though I don't really know. It is a:

"High Power UV-VIS, Fiber Light Source" made by Hamamatsu

To me it's just a box with a connector port and an on-button. I don't really know the specifics (maybe I should start finding those out :-))

4. May 16, 2008

### mgb_phys

Just think about the solid angle.
If the end of the fibre is 2inches from the bulb the area of a sphere of R=50mm is 630mm^2
The area of the end of your 10um fibre is only 0.00008 mm^2 so the amount of light going into the fibre is approximately none !

You need something like a 25mm diameter lens focussed onto the end of the fibre, the correct focal length will depend on the NA of the fibre but anything is better than nothing.

Last edited: May 16, 2008
5. May 16, 2008

### jeffmoss1

there might be a collimator or some lens inside the bulb. I do get some output from the fiber that I can read on the computer after the spectrograph, but it is dim. I guess if that's the case then I've answered my own question.

I'll keep fiddling.

thanks for the reply

6. May 16, 2008

### Redbelly98

Staff Emeritus
Is it one of these?
http://jp.hamamatsu.com/resources/products/etd/pdf/L10671_TLSO1067E01.pdf
http://www.sales.hamamatsu.com/assets/pdf/parts_L/L10290_TLSO1059E02.pdf

Hamamatus almost certainly has put some focusing optics, at least on the input end, in these units. It is just hard to imagine that a good outfit like them would not.

Does the manual say anything about a collimation or alignment procedure? You might need to make adjustments somewhere, hopefully only one time, in order to align the optics. If there's nothing in the manual, then call their tech support.

Good luck!

Mark

7. May 18, 2008

### jeffmoss1

yes its the second one!

I looked inside and there are some lenses of some kind. I think they are design for a much bigger fiber though. My lab-mate thinks we need a high quality aspheric lens to focus the light into the tiny 10 micron fiber.

It looks like we're going to be using this lamp, collimating the light through our experiment, and then catching it with the a focusing lens of some kind and focusing it into the 10 micron fiber, then into the spectrograph and onto the CCD.

I think this will be difficult.

8. May 18, 2008

### Redbelly98

Staff Emeritus
To focus most of the light onto a 10 um diameter fiber:

$$10 \mu m = 2.44 \ \lambda \ \mbox{f/\#}$$

So for green light at 0.55 um wavelength, a lens with f/D of 7.5 or less is sufficient. This shouldn't require special aspheric optics, just good alignment.

9. May 18, 2008

### Redbelly98

Staff Emeritus
I'm remembering now that optical alignment is a tricky business if you haven't done it before. You may need somebody experienced with that, in person in your lab, to get this working properly. In a forum like this, we can advise you on what kind of lens to get and stuff like that, but to align things hands-on -- it's really best to have somebody right there to show you.

Are you near a large university where some professor does experimental research with optics or lasers or something like that? You might call the professor and ask if you could hire a senior grad student or post-doc to come over and help you out.

Last edited: May 18, 2008