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A Cheap and Simple Experimental Wide-Band Laser Link
K.Banke N6IZW and C. Houghton WB6IGP
San Diego Microwave Group
For a number of years Chuck and I have been demonstrating our 35 KHz Laser
Pointer optical communicators at Field Day, to Ham clubs , and making points
during the 10 GHz & Up contest. I have been interested in moving to a wide band
capable system for some time and did a very crude demo a few years back of an
experimental optical link capable of being useable up to 125 MHz. At that time I
wasn’t sure if the frequency limiting component was the laser pointer or the pin
photo diode as I had no known hi-speed photo detectors. Recently I found a
substantial quantity of BPX-63 photodiodes in a local surplus shop and bought a
couple to evaluate hoping they would at least give the 100+ MHz response of the
diodes I had been using. To my surprise the diodes showed useable signals up to at
least 1 GHz when reverse biased to around 50V. The reverse bias is a key point.
The diodes I had been using showed no improvement in frequency response with
reverse bias. With these new diodes, the high frequency response improved by 10s
of dBs as the bias was increased in the range of 40-50V. This is all feeding into a
50 ohm load of course. The down side of applying the bias is that the broadband
noise generated by the diode increases dramatically. The spec sheet shows
maximum reverse bias as 7 volts so we are running way out of spec to get the
speed. The capacitance of the BPX63 diodes at 0 volts bias is 100 pF. I then was
fortunate enough to purchase a good quantity of SFH203 photodiodes from an
auction off the Internet. These diodes are rated for 50V max reverse bias, 11 pF
capacitance at 0V reverse bias and 5 ns rise time with 20V reverse bias. In our
units we have added an ERA-3 MMIC amp directly following the photodiode for
driving cables. One crude but impressive initial demo I made using the link was
connecting my cable TV source to the Laser and connecting my TV to the
photodiode. I was able to tune nicely through the cable TV channels. Other demos
were made using the link to convey the RF between a pair of 2 meter, 70cm and
even 1296 MHz radios. Another interesting experiment demonstrating the high
frequency response of the detector setup is to point an HeNe laser at the detector. A
strong carrier will be seen at a frequency which is equal to twice the transit time of
light through the length of the laser due to longitudinal mode beating. I have two
lasers of different lengths with one producing 437 Mhz and the other 635 MHz.
The signals slowly ( over a few seconds) sweep back and forth about 100 KHz
probably due to warm up of the tubes and related mode changes.
Figure 1 below is a schematic of the link.
Figure 1
Laser pointer circuit The laser pointer is one of the cheap, 3 cell variety selling for less than $2 these
days on the Internet. They are suited for this application as they contain only a
switch, series current limiting resistor and Laser diode. For this application I used a
sharp wire cutter to remove the soft Aluminum tube. I also used the cutter to
remove the spring contact and switch to reduce the amount of inductance in series
with the Laser diode. A coax was then soldered to the circuit board of the Laser
pointer leaving the internal resistor in the circuit. The assembly is then pushed in to
a piece of ½” Brass hobby tubing 1.1 inches long. The shield connection is
carefully bridged to the \Brass tube to supply the ground connection. This needs to
be done quickly to prevent overheating the Laser diode assembly. Heat sinking the
front of the tube is probably a good thing to do. Figures 2 through 6 show the Laser
Pointer modifications.
Figure 2 - Unmodified laser pointer
Figure 3 – Peeling back the housing
Figure 4 – Laser pointer without housing
Figure 5 - Removed spring and added coax
Figure 6 – pressed inside ½” hobby tubing
The Laser diode needs to be biased to have about -4V applied to the coax (which is
connected to the laser diode series resistor. This allows an RF level of 0dBm to be
applied without damaging the Laser diode. A voltage of about -2V can be applied if
the RF level is around +10 dBm. The 330 ohm resistor is used to drop the voltage
from the -12V bias supply to the required -4V as well as provide some RF
isolation. Other combinations of supply voltage and resistor can be used to provide
the -4V.
Figure 7 – Laser diode bias resistor & cap
Photodiode circuit The majority of the photodiode circuit is the positive 45V bias supply. The LT1074
is actually designed as a buck converter but I used it in a crude, regulated boost
circuit since we have a good number of these devices in our group.
Figure 7 shows the frequency response of the link at the output of the ERA-3
amplifier.
The ripples are an artifact of my crude tracking generator assembled for this photo.
Figure 8 – Frequency response of laser link showing 1-1000 MHz
Figure 9 – Photodiode
with ERA-3 amp and reverse bias circuit
The article showing the mechanical construction for our original 35 KHz optical
communicator can be found on the web at:
http://www.qsl.net/wb9ajz/laser/laser.htm
under “A Safe LED Optical Transceiver System” .
This article with color pictures is available at: http://www.hamradio.com/sbms/sd/projindx.htm
Contact Chuck Houghton, WB6IGP at: [email protected] for access to the
photodiodes and some of the other components available for this project.
Data sheets for the SFH203 photodiodes can be found at: http://www2.fhswf.de/fbe/daten_fbei_download_barfuss/SFH203.pdf
Data sheets for the BPX63 photodiodes can be found at:
http://www.soem.ecu.edu.au/units/scp3313/bpx63.pdf