[Laser] Optical feedback for transimpedance amplifier

[Kerry Banke]

Yves - How are you doing and what are you experimenting with these days? We have recently been experimenting with some IR LED panels we built which have 100 Leds at 940 nm operating at .1A each.  The experiments are NLOS at 3 miles aimed at the optical horizon at both ends with clear skies.  The signal last night was 30 dB above the noise in 1 Hz BW.  The Leds have a 3dB BW of 20 deg.  This is in the middle of an absorbtion line but seems to work ok for experiments in low humidity.  The Rx is a 12" cast Aluminum dish ( 39 GHz) lined with shiny Aluminum tape just to obtain more apperature . The detector is an SFH206 7mm squared active area diode feeding a K3PGP type preamp.  The frequency response is not optimum for the 450 Hz tone we are using right now. Several more dB can be gained by lowering the beacon frequency but we want to stay away from the 120 Hz of course.  We have an 830 nm long pass filter ahead of the detector.
   
  This brings me to my question for you. Have you seen any designs for a transimpedance amplifier using optical feedback to the detector diode?  I have been thinking about trying something like that with a modified K3PGP amplifier. My thought is to only supply enough light feedback to deal with the desired modulation at low light levels to improve frequency response.
  Any thoughts on this? Thanks
  73 - Kerry N6IZW -