[Laser] CONCLUSION TO >>> focal length question

[F1AVYopto at aol.com]

SV8GXC wrote :

IN MY  OPINION THE BPW34 WITH THE 120 DEG. 60% WIDER FROM THE INPUT ANGLE OF 
46.8DEG  IS GOOD CHOISE. SO THE PHOTODIODE WILL RECEIVE ALL THE INCOMING LIGHT 
WITH NO  LOSSES
PLEASE WRITE ME YOUR OPINION AND IF MY THOUGHTS ARE CORRECT & IF  I 
UNDERSTOOD CORRECTLY YOUR POINTS....
 
All  is correct and well understood. 
The  BPW34 is a good choice for medium and low speed  modulation.  
Its  7,5 mm² sensitive area is well matched to the Fresnel lens focusing area 
 always bigger than the one provided by a good glass lens. 
The  focal point quality and size depends on the engrave lines number per  
inch. 
You  will notice the signal level does not depend critically of the focal 
point  adjustment. 
Until  the lighted area does not overlap over the sensitive area limits, the 
signal  intensity will stay the same. 
With  a laser this is not true for a very short path but, above one km, the 
light  coherence is lost in the received beam and the effect in the detector is 
the  same the one from a LED light. 
The  enormous gain with the laser use is only from the TX lens that can give 
a very  narrow beam. 
73  Yves