[Laser] Optical Repeater in San Diego

[Lee Scheppmann]

Abstract:
On June, 23, 2007 a two way contact was made between members of the San
Diego Microwave Group: Kerry Banke N6IZW and Lee Scheppmann KD0IF, over a 14
km path using a recently installed optical repeater on San Miguel peak near
San Diego. PSK31 was used along with 910 nm laser diodes and PGP style
detectors. The optical repeater was assembled from two LED array beacons,
(the type normally used on communications towers), and driven by a prototype
detector circuit. The PSK31 base-tone was at 750 Hz and the received signal
was greater than 43 dB above the noise floor.  

Background:
Optical communications has been an interest of the San Diego Microwave Group
for several years with a broad range of experiments being conducted by group
members. Equipment has included home-brew transmitters based on laser
pointers, high-powered LEDs and most recently, high-powered laser diodes. On
the receiving end various optical arrangements usually driving a version of
the PGP detector. Experiments have included very high bandwidth, linear
communication, (near 1 GHZ), various digital communication modes, long range
communications, and most recently the linear optical repeater designed by
Kerry N6IZW.

The group maintains a repeater site on San Miguel Mt, (DM12MQ), where San
Diego State University and its PBS affiliate; KPBS-TV has provided the group
with mounting and power. The microwave and optical equipment operates under
flexible, 450 MHz remote control link, provided and maintained by Greg
K6QPV. The first optical equipment on the mountain was a simple
retro-reflector which is useful for equipment alignment. This was followed
by a small, 5 LED, high power array acting as an optical beacon modulated
with 750 Hz. This provides a signal for scintillation measurements and
experiments. This beacon can also be driven from an MP3 player with
repeating message and ID. So far, this small beacon has been received at a
distance of 50 miles.

The Optical Repeater:
Recently, Kerry designed and constructed a first generation repeater with a
simple, single lens input coupled to a PGP detector driving two very high
power Radio Tower LED arrays. The input signal from the PGP detector is
amplified, filtered, run thru an AGC circuit and then coupled to current
limiting output drivers for the arrays. The arrays operate at 16VDC with an
idle current of 3.6 A, rising to 8 amps on modulation peaks. Each
semicircular array is 18" high and 12" in diameter and consists of 324
bright red LEDs with horizontal, cylindrical lens which maintain a narrow
horizontal beam. All of this is packaged in a weather-proof enclosure. This
configuration does a good job of lighting the San Diego area, but
improvements are planned to enable the detector to see a much wider field of
view.

An added feature is the ability to turn on the small 5 LED 750 Hz beacon
which is in sight of the repeater input so that the 750 Hz signal is
retransmitted thru the large array. Recent experiments with this
configuration show a 70 db signal over the noise floor at 8 miles. This is
probably a good indication of the LED array output when driven to maximum
illumination.

The Contact:
Located at a distance about 7 km from the repeater in DM12MS, Kerry N6IZW
and Lee KD0IF set up their equipment and initially focused on the beacon for
alignment. At that distance the beacon was very strong and a webcam view
(fig 1) thru Lee's 135 mm telephoto lens shows the beacon array in the lower
right. The two lights above it are the regular tower beacons on the KPBS
transmitting towers. Both stations received very solid signals from the 750
Hz tone modulation and after Greg K6QPV turned off the tone and opened up
the repeater using the 450 Mhz control link, the first PSK31 contact was
made without a hitch. The received signals were greater than 43 db over the
optical noise floor.


The Equipment:
>From his end Kerry N6IZW runs a home brew 4", single lens into a PGP
detector. On the transmit side he has a 1 watt, 910 nm laser diode into a 4"
single lens collimator.  Lee's equipment consists of a PIN Diode head at the
focal point of an 8 inch SC telescope feeding a PGP circuit. On the transmit
side is a 4", single lens collimator in front of a 1 watt 910 nm laser
diode. Laptops were used by both, running PSK31 software under Spectrum Lab
on Kerry's end and Digipan on Lee's end. Needless to say, tripods and
precise pointing are key ingredients. 

The Next Steps:
Given the high S/N level, and previous experience over a 50 mi path, we
expect that this repeater will be good for at least that distance. More work
needs to be done on the receive optics before we can operate at more
divergent angles. We will also be using the increased output of the beacon
for scintillation studies.


The full article with photos is available. Just email me at lee at kd0if.com

Lee, KD0IF