[Laser] reflex optical switch

[Tim Toast]

Hi all,
 
 In case a single laserdiode transceiver doesn't work out...
 
 The advantage of having one main optic with a transceiver got me thinking about some way to 
use that with two separate devices. The old 35mm film cameras used to have what is called a 
"reflex mirror". It is used to sight through the main lens for focusing and framing the shot instead 
of having a separate viewfinder lens system. This lets you see exactly what the frame will look 
like regardless of what lens you are using. The reflex mirror intercepts the light from the lens 
before it reaches the film and diverts it to the viewfinder for focusing etc..Then when the shutter 
is clicked, the mirror rapidly moves up and out of the way so the main lens can focus onto the 
film directly and take the exposure. Afterwards, the mirror pops back down and the camera is 
ready to frame the next shot. The length of the path through the lens to the film is the same as 
the length through the lens, reflected by the mirror to the focusing screen. In other words 
the two paths are equal - this is so focusing can be accurately judged through the viewfinder. 
At any rate, this is why they are called "Single Lens Reflex" cameras or SLR's.
 
The idea of the reflex mirror as a light switch is apparent when you see it operate. It has two 
modes, either up or down in the case of the camera. The mirror movement has an adjustable 
stop that precisely defines the down position so that film and viewfinder are aligned the same. 
The up position of the mirror is not as important as it just needs to be fully out of the way 
when taking the picture.
 
Adapting this to an optical transceiver you just need a small mechanism to snap the mirror 
back and forth between two positions. You can do it two ways really, one way is like in the 
camera where you have the mirror pivot from one edge so that it is either down or up -out of 
the way. So the beam will either be diverted to one position or the mirror will swing out of the 
way and the beam will go straight through unaffected. This may be the best way have the 
photodiode used with no mirror and have the laserdiode in the mirror "on" position. Then you'll 
have no path loss due to the mirror in receive mode. 
 
Another way is to mount the mirror is on its axis in the middle along one edge. So it will pivot to 
two positions, A and B - directing the beam out to the sides of the device. So both paths will 
have reflected off the mirror to their focal points. If you are using an optical switch to compare 
two photodiodes, or LED's etc.you might want to have the switch operate like that so the path 
loss will be the same either way.
 
 
more later
 
-toast