[Laser] Thoughts on receiver design

Mon, 22 Mar 2004 15:15:24 -0800

The frequency response of the cheap lasers with series resistor is 
100MHz+.  I did a demo of a simple laser pointer TV link where I biased the 
laser through a 100 ohm resistor to a power supply and then capacitively 
coupled about 0 dBm of channel 3 audio/video modulated RF from a VCR to the 
laser pointer. The receiver was just a PIN diode connected directly across 
the TV   RF input connector with the TV set for channel 3. It worked well 
up to  channel 6 as well but fell off when I jumped to channel 7.  Using a 
sweep generator showed useable response to 125 MHz but I don't know if it 
was limited by the Laser or detector. I recently purchased some 1 ns 
response PIN detectors so will try checking the response again.
  - Kerry N6IZW -

At 06:01 PM 3/22/2004 -0500, you wrote:
>[email protected] wrote:
>
>>I haven't frequency tested my laser pointers yet, but they
>>don't seem to have any trouble responding at 800 Hz, and
>>based on the shape of the waveform at that frequency, I
>>expect that they'll go a lot higher.  Laser diodes themselves
>>can work well up into the GHz range (or even higher!), but
>>the circuitry in a laser pointer to regulate the current to the diodes 
>>usually has some capacitors which tend to limit the maximum 
>>frequency.  Of course, you could always design your own current regulator 
>>to get around the frequency limitations (and, you'll have to if you're 
>>going to use raw
>>diodes, rather than laser pointers).
>>
>>
>If you go with the really cheap pointers you will find that the "current 
>regulator" consists of a 47-ohm chip resistor.  The little laser keychain 
>units can be had for around $1/ea off of ebay if you buy in quantities of 
>10 or more.  I they shouldn't have any trouble with being modulated into 
>the RF range unless there is some weird capacitance in the dye that I 
>don't know about.
>
>Andy K0SM/2
>
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