A simpler design is far better than having to take lots of steps to ensure multiple perfect mirrors are perfectly aligned and perfectly free from even the slightest spec of dust wich could scatter the light, wich is dificult in a engineering prototype type environment rather than a clean laser bench lab environment and where the prototype is open and handled a lot.
The detector is also on quite a large pcb, although it could be mounted edge on the the beam.
The scalpel blade was actually quite sucesfull in the end becuase I was able to angle it so any scattered light went completly away from the detector. (it was actually painstakingly polished quite well, you could realy see your face in it). The only problem was that there was some slight light from the side of the laser beam wich hit the edge of the mirror and scattered backwards, but was easily eliminated with a hole in some black cardboard, (the detector circuit is exceedingly sensitive as it uses heterodyne and synchronous detection).
The idea of having just one mirror for the detector is by far the best solution I think, as it requires only one surface wich isnt too critical for focus or scattering, unlike the mirrors directly in the laser path would be.
You can see how simple the idea is in ABSE. its a synch to setup too.
I dont recal mentioning using a rear coated mirror.
Of course it would. We are not talking about making a telescope. Sprayon stuff is at least 70% reflective For the same efficiency as a perfect mirror of a given size, just make
30% larger.
--
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Try to find a shop in your area that will cut holes in glass (without messing up the mirror surface). Then you have the easier task of finding a mirror without the hole.
I doubt it will matter. You're not concerned with forming an image, you're just collecting as much power as you can on a detector.
On Wed, 06 Sep 2006 03:48:56 GMT, "colin" Gave us:
"The detector" should be a single transducer package with leads that go to the supporting circuitry on a pcb.
The IR tube I described focussed the incoming IR light onto a 2 mm spot inside a transistor can sized package at the front of the tube. It was mounted on a three rod ring, just like the telescope boys do it, and one does not see those support structures when one looks through a scope.
On Wed, 06 Sep 2006 03:48:56 GMT, "colin" Gave us:
Is the word you are seeking to use "cinch"?
You said your mirror scattered the laser's light badly. A good first surface mirror will NOT do that, so I assumed you were using a shit mirror with glass between the mirror surface and the laser. So, it is either that or you fat fingered the surface or was dumb enough to attempt to "clean" it.
First surface mirrors are bare aluminum, and should NEVER be touched in any way shape or form. Particularly to "clean" one.
On Wed, 06 Sep 2006 12:38:36 +0100, Dirk Bruere at NeoPax Gave us:
the
or
Of course it wouldn't.
So what? A ten percent poor reflection is akin to bit error rate in digital transmission. The detector loses far more than ten percent of what it needed to do its job.
Which is 100% inadequate.
Fucked up logic. Misdirected reflection is misdirected reflection, no matter how big you make it.
Im sorry, I still just dont see where the argument lies for 3 mirrors versus
1 mirror. If the mirror just isnt there it wont be able to scatter the light in the first place. The mirror that reflects the received light doesnt matter too much if it scatters a little bit as any such light subsequently reaching the detector will be very low and its phase will be related to the distance to-target. The wavelength of the light modulation is about 100mm so it will unlikly be significantly out of phase.
More importantly the light directly from the laser that will be a problem if it is scattered becuase it will be about 70db greater than the received light and will have a phase that is totally unrelated to the distance to target.
This is why I chose a straight through path for the laser, its best to just avoid problems where possible rather than have to go to great pains to solve them.
This isnt just a simple intensity measurement, like I said the pcb is quite large, detectors come in all types including SMD they arnt compelled to have long leads. This detector needs to sit close on the pcb and have significant amount of related circuitry next to it with as short as traces as possible due to the fact that it operates at 1-2ghz, its a heterodyne detector wich uses optoelectronic mixing, this is done by applying a 1-2ghz AC signal on top of the 250v bias for the avalanche photodetector, obviously the bias driver and output amplifier need to be right next to it to keep parasitics as low as possible.
Not to mention coaxial in and out leads, it also needs to be in a screened box with just a hole for sensor to peek out of, so it realy cant be in the line of incoming light of a mirror, it has to be to the side, or behind a lense wich anyway I have a good selection of lenses lying here from previous design.
Yes thanks, spot on, knew it didnt look quite right lol.
Ah I see, well the laser light is probably >70db more than the received light so although any scattered light was bad it didnt mean the mirror was quite that awefull, in fact it proved how extremly sensitive the detector was.
The previous design was a bit compact, I often had to remove the screen and optical bits to try and get more and more out of the detector, the design changed many many times. Its inevitable I touched them or they got dusty. Fortunatly the scalpel blade proved to be a robust little thing. It was a 3mm square section of the blade, wich seems to be made of surprisingly good quality stainless steel, soldered to a thin post soldered to the PCB. It was the only reflecting surface used.
The reflector I have now is quite easily removed, it has a slot where the bulb retainer was wich conveniently slides over a metal tab soldered to the PCB, I only had to clean it once very carefully to remove the dust from sawing off the moulded on front lense, sawing it in half and drilling a hole in it, im not sure what the plastic is coated with, probably aluminium as you say, but the cycle lamp must be 20-30 years old and its still realy shiny.
Well from my recent experience it does need to form an image, not necesarily an undistorted one of course, but certainly a relativly small but clearly visible and relativly bright image in the center anyway, wich means it needs to at least be very shiny, and quite close to the right shape. the metal mirror I tried, wich I later found a good way to test by puting a simple led torch some distance away from it, forms a clearly visible quite bright image but a rather large one, so most of the light isnt captured by the 1mm^2 detector. I just did it with a piece of .15 mm sheet steel and a ball pein hammer and a flat metal surface, fortunatly I do have a little bit of skill necesary to do it that way. If I made a wooden dolly of the right shape im sure I could get it close enough.
However I think any DIY method to make one, although possible, would need quite a bit of time to make it good enough. I picked up a CD to play with but actually they dont reflect a great deal of light, they are partly transparent and also much of the light is difracted.
The lamp mirror im using now forms an image wich is quite bright in the center 1mm part of it, although its certainly not a just a well defined dot, I think the much shorter focul length is a big advantage.
It works enough to prove the signal is from the demodulated reflected light rather than crosstalk, as moving the target through 75mm (at about 200mm away) clearly cuases 180' phase change in the signal (at modulation = 1ghz). now I need to try and get it to work at 2ghz.
In other places I have seen telescope guys make their own mirrors using 2 quite large bits of glass rubbed together with grinding paste, then silvered in a home made alu sputtering chamber !
This "image" will be just 1 x 1 pixels, since you have a single detector element. My whole point was that you don't need a really good, spatially resolved image. You just need to focus as much of the power as possible onto a 1 mm^2 area. Aberations become less of a concern than, say, designing a camera or microscope lens where the goal is to produce excellant, undistorted images.
My original suggestion was to
Find a supplier for the mirror you want, without the hole, and then
Find somebody who can cut a hole in the mirror, without ruining the mirror. You're best bet is to find a local glass shop that can do this.
Even though I said the image needn't be as good as, say, in a camera, I can't imagine this method being suitable. How do you get a curved mirror using a flat surface? And even if you had a curved surface, a hammer will not produce the smoothness you need.
Yes, it is possible to grind and polish your own mirror, if you really want to. If it were me, I would just buy it. For example, from Thorlabs in the US: D = f = 25 mm, 42 $US or 44 Euro, part # CM254-025-G01 D = f = 50 mm, 59 $US or 62 Euro, part # CM508-050-G01
Other diameters and focal lengths, plus a reflectance curve, are at
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Again, finding a local glasscutter to make your hole is up to you.
Hi, I was virtualy agreeing with you on that point its not the acuracy of the image, but in trying to get a small spot that is initself effectivly an image of the small laser spot on the target. however with the focul length very much shorter than the distance to target the perfect image would be a pin point, clearly such a perfect image is not needed.
There were lots of interesting sugestions wich I thank you all, I found a few cheap spherical glass mirrors on ebay, and some good off axis aluminium types from scientific places wich were expensive, however the plastic lamp reflector has proved to be an extremly good idea, even a very simple mock up prototype was easy to make/drill etc and has worked considerably better than I was hoping for, for now at least.
most
and
Wel like I said it takes some skill, its like an art, if you bash the metal against the flat surface with a hammer it will squish out and expand where you hit it, if you do it carefully this causes it to deform into a dish shape, even on a flat anvil, if your able to work out how much it needs to deform and where to start etc you can make all sorts of shapes, once you start geting the right shape you concentrate on smoothing out the hammer marks by hitting it more gently. you need some proper metal bashing hammers the right size and shape, (wich havnt been used for banging in nails !) and tons of practice and patience. a polish with brasso made it quite shiny, you cant realy see my hammer marks exept where I was a bit heavy handed, but like I said it would be time consuming to get it good enough. I cant realy show a picture I dont think it would show up the curvature. My dad and brother were both mechanical engineers so I picked up quit a bit.
I just wouldnt have the patience, it takes like 100s of hours lol.
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Thanks, some nice mirrors there at a reasonable price, im not sure what the focul length im using now is but its very short compared to the diameter, I think the only type with such short FL are torch type reflectors wich works great, coated plastic seems quite good enough, especialy as it should be cheap.
They also list lasers, however its a shame they dont spec how fast they are, Im trying to find a faster laser but I cant find such details in the specs for (cheap) visible 5mw types.
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