[Laser] Prism Tuner

Mon Nov 21 11:37:49 EST 2011

Hi Tim,

Yeah, that's it. I see you're on the UKN.... list also. Looks like they have a pretty active group over there. But, then again, when you live in a country the size of Rode Island, go figure. 

Anyway, if you have a "Big Lots" in your area check out the candy section. They have cans of 'Creme de Pirouline' (chocolate sticks) for a $1.00 per can. If you don't like the sticks, throw them away and keep the can. The can is 65mm x 160mm and you don't even have to clean them. (sticks come in a sealed package, inside the can). Just a thought I'm headed out to buy out the rest of them, about 20 left I think.

Just as a side note. I had purchased 2 sets of Ramsey's laser communicators and have not completed them at this moment but I am thinking I might convert them to driving LEDs instead. Your thoughts.

Happy lightning. 

Mike

--- On Mon, 11/21/11, Tim Toast <toasty256 at yahoo.com> wrote:

From: Tim Toast <toasty256 at yahoo.com>
Subject: Re: [Laser] Prism Tuner
To: "Free Space LASER Communications" <laser at mailman.qth.net>
Date: Monday, November 21, 2011, 8:24 AM

Hi Mike,

If i get your meaning - Have the receiver lens focused on the fiber tip and use the fiber to route the light to the 
spectroscope, so there is more room to work with it?? That might bethe only way to use this with very short 
focal length lenses and  small prisms. Fiber could come in handy for other reasons too, no telling. I know they
use fiber to avoid having a heavy instrument attached directly to a telescope - which are often delicately balenced 
and have limits on how much weight you can add.

-toast

>________________________________
> From: MICHAEL COUTURE <mikecouture at bellsouth.net>
>To: Tim Toast <toasty256 at yahoo.com>; Free Space LASER Communications <laser at mailman.qth.net> 
>Sent: Sunday, November 20, 2011 9:51 PM
>Subject: Re: [Laser] Prism Tuner
> 
>Hi Tim,
>
>Hopefully someone with a good Physics background will take a shot at this. Regarding the concern of blocking the the beam or the width, how about a short length of glass fiber optic material? At the opposite end place the detector. Well?
>
>Mike C.
>(sandbar sw FL)
>
>--- On Sun, 11/20/11, Tim Toast <toasty256 at yahoo.com> wrote:
>
>
>>From: Tim Toast <toasty256 at yahoo.com>
>>Subject: [Laser] Prism Tuner
>>To: "laser mailinglist" <laser at mailman.qth.net>, "DX Optical" <Optical_DX at yahoogroups.com>, "Nanowaves UK" <UKNanowaves at yahoogroups.com>
>>Date: Sunday, November 20, 2011, 4:11 AM
>>
>>
>>Hi All,Seeing some talk about narrow optical filters recently, i wanted to bring up an old idea about using a glass prism to tune or pre-select the optical passband.here's a diagram: http://www.aladal.net/toast/prismtuner1.jpgOne possible advantage of a prism over a color filter may be that the prism is unaffected bythe large angles of entrance of the beam from a large lens. Also an advantage of a prism overa transmission grating (or reflective grating) is that the prism produces only one "order" orspectrum, and all the received power is in one spectrum. A grating would divide up the power between several orders.So, we have a converging beam from the lens passing through the prism and on to the focalplane of the photodiode. The lens system still produces an image at the focal plane but it isspread out in one dimension according to frequency. So, when viewing a white point source, thereceiver would produce a narrow line in the focal plane with blue at
 one end and red at the other. -R----G-----B-Placing the prism just in front of the focal point, in effect, turns the receiver into a full aperture or slitless spectrograph. Instead of a "normal" spectrograph where you have a very narrow entrance slit that the main lens is focused on, you take away the slit and focus the lens directly on the detector via the prism.With just a short distance between the prism and the focal points, there isn't much room for thespectrum to spread out and the focal line will be relatively short. There might only be a fewmillimeters between the red and blue focal points. And this would be especially true with shortfocal length systems common in light beam coms.One interesting thing about this is that the bandwidth is mostly determined by the physical diameter of the sensitive area of the photodiode used. Ignoring the nonlinear nature of the prism's spectrum for a moment - the distance between the red and blue focus divided by
 the diameter of the photodiode equals the bandwidth. So if you have a diode with a diameter of one millimeter and the focal line from the prism is 10mm in length, then the bandwidth is about 1/10th of the full 300nm wide spectrum or 30nm.  In practice, the spectrum produced from a prism is compressed toward the red end and expanded toward the blue end. So with a fixed size photodiode, the bandwidth would be narrower at the blue end and wider at the red end. But you could ignore that for the most part and just calibrate the tuning scale by using known wavelength light sources. So, i hope this comes in handy somehow. It may be that there are some factors that make regular color filters better than a prism in the long run. The one that worries me the most is the short focal lengths used commonly because there's a limit for how steep the angles can be and still not block any light from the edges of the beam by the prism, while having the focal points
 available outside of it. All this assumes there's a need for cheap, quick or continuous tunability. If nothing else, it could be useful as a wavelength measuring device that can also be used in the IR or UV with a broadbanded photodiode.- toast______________________________________________________________Laser mailing listHome: http://mailman.qth.net/mailman/listinfo/laserHelp: http://mailman.qth.net/mmfaq.htmPost: mailto:Laser at mailman.qth.netThis list hosted by: http://www.qsl.netPlease help support this email list: http://www.qsl.net/donate.html     
______________________________________________________________
Laser mailing list
Home: http://mailman.qth.net/mailman/listinfo/laser
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:Laser at mailman.qth.net

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html