[Laser] Prism Tuner

Wed Nov 30 16:18:42 EST 2011

Hi Dave,

That's true there will be some hard reflections from an uncoated prism. And i guess this applies to  
any optical parts that are uncoated. Stray reflections can cause ghosts and in general a lessening  
of contrast / higher noise etc. So you would want to start with a good quality anti-reflection coated  
prism at least. 

As you say, most normal spectrographs use collimated light but this is just a simple minimum parts 

setup that  depends on the main lens to do all the focusing. Another type of spectrograph that uses a single  
element is the "objective prism" type. It has the prism outside in front of the main lens and takes advantage  
of the fact that a very distant source (like a star) will have nearly parallel rays and can be considered  
collimated. One disadvantage with this though is you have to have a prism as large as the main lens.  

Both of these simple spectroscopes are used in astronomy for getting a large number of spectrums  
all at once from a field of stars in the frame. Low dispersion, low resolution, whole field-image. 

Someone on another group pointed out that this simple design (no slit) would "smear" or blurr the  
spectrum produced. In effect, increasing the bandwidth seen by the detector beyond what you  
would expect from a simple calculation based on total spectrum width vs. detector area width.  
At night this might not be much of a problem but in daylight there is much more scattered light and  
nearby objects in the field of view to overlap and interfere.  

All in all i guess if you have to add lenses to collimate the beam or add a slit, then this probably makes  
a regular bandpass filter cheaper. This was mostly about which could do the same thing cheaper - a bandpass  
filter or a prism. And mostly during daytime use. From what i've read, they decided a regular bandpass  
filter would need to use collimated light and so would need two extra lenses - one on each side of the  
filter. But for reasons of the filter itself not having the same bandwidth for rays entering at a large  
angle compared to a collimated ray. This led to another tuning idea of tilting the filter to tune the bandpass  
a small amount.  

As you point out, the photodiode will have a different sensitivity depending on the wavelength. If you were  
using something like this for wavelength and amplitude measurements, you would want to note the  
reduced sensitivity to the blue end of the spectrum. But you would probably want to use a whole-spectrum  
sensitive diode anyway so the difference wouldn't be so great. I can imagine some regular diodes might 

not have  any useful response before you even reach the green/blue parts of the spectrum. 

I was surprised to read, on the other group, that (in daylight) filters seem to help even when using the  
already narrowband LED-as-detectors setups. I guess even they have some noticeable response to out  
of band light.  

-toast

----- Original Message -----

> Message: 1
> Date: Mon, 28 Nov 2011 13:50:55 -0500
> From: Dave <wa4qal at ix.netcom.com>
> Subject: [Laser] Prism Tuner
> To: laser at mailman.qth.net
> Message-ID: <4ED3D80F.9080701 at ix.netcom.com>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
> 
>>  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>
>>  Message-ID:
>>      <1321780279.96364.YahooMailNeo at web37904.mail.mud.yahoo.com>
>>  Content-Type: text/plain; charset=iso-8859-1
>> 
>>  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
> 
> 
> 
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