[Laser] Prism Tuner

[Dave]

> Date: Sun, 20 Nov 2011 01:11:19 -0800 (PST)
> 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>
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> 	<1321780279.96364.YahooMailNeo at web37904.mail.mud.yahoo.com>
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>
> 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.jpg
>
> One possible advantage of a prism over a color filter may be that the prism is unaffected by
> the large angles of entrance of the beam from a large lens. Also an advantage of a prism over
> a transmission grating (or reflective grating) is that the prism produces only one "order" or
> spectrum, and all the received power is in one spectrum. A grating would divide up the power
> between several orders.

However, consider that you're going to get a substantial reflection 
(10%?) from the
surface of the prism.  From normal "window" glass, you get about a 4% 
reflection, and
a 4% absorption (VERY rough estimates), but, prisms are usually made 
from high
reflective index glass, which will increase the reflection coefficient 
substantially.

You also usually want the beam entering a prism (or a diffraction 
grating, for that matter)
to be collimated.  That's usually not a problem for a long distance 
propagation path,
although a collector lens may adverse affect this, by focusing the 
light.  However, a
collimating lens behind the collecting lens should be able to collimate 
the light.

> So, we have a converging beam from the lens passing through the prism and on to the focal
> plane of the photodiode. The lens system still produces an image at the focal plane but it is
> spread out in one dimension according to frequency. So, when viewing a white point source, the
> receiver 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 the
> spectrum to spread out and the focal line will be relatively short. There might only be a few
> millimeters between the red and blue focal points. And this would be especially true with short
> focal length systems common in light beam coms.

This really depends upon the size of the prism used.  Normally, though, 
the size of the
prism is minimized due to cost reasons, as well as weight reasons.

> 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.

Do you need to factor in the sensitivity of the photodiode with wavelength?

> 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.

Thin film filters?

> - toast

Dave