[Laser] Observing the longitudinal mode beating in HeNe lasers

[Kerry Banke]

I recently have been putting together an experimental wide-band optical 
communicator using modified laser pointers and surplus pin photodiodes.  We 
purchased  a good number of very  cheap pin photodiodes locally and found 
that by reverse biasing them to about 50 VDC they will respond beyond 1 
GHz  & plan to make them available to laser  experimenters . I'm in the 
process of putting together an article for the Microwave Update 2004 and 
will eventually  post the info on a web site. I happen to see a brief 
comment last week  about using the longitudinal mode beating of an HeNe 
laser as a easy source for a high frequency modulated optical signal to 
check photodiodes.  I hadn't  used my HeNe units basically since laser 
pointers became available. I fired two of them up and found they produce 
strong, clean   CW  carriers at 437 MHz for my larger unit and about 645 
MHz for a shorter one.  On my spectrum analyzer the signals slowly  (  a 
few seconds) sweep back and forth about 100 KHz I guess do to mode changes 
with temperature as the lasers heat up.  I can readily  hear the one  sweep 
through my 70 cm  radio.  According to the information I saw the frequency 
is F=C/2L  where F is frequency, C is the speed of light and L is the 
length of the laser. I have my pin photodiode mounted on a salvage pcb and 
connected to an ERA-3 (Minicircuits) MMIC amp (3 Ghz) using microwave type 
SMT construction to gain all the bandwidth I can.  The signals are maybe 60 
dB out of the noise on my analyzer.

Does anyone have a good source on the web for more info on longitudinal 
mode beating in HeNe lasers?

  - Kerry N6IZW -