50 years since operation "Red Line"

It's hard to believe - especially, I am sure, for those who were involved - that it's been over 50 years since Operation Red Line.

For more details visit the Operation Red Line web site:  
 
http://www.modulatedlight.com/eos/Operation_Red_Line.html


What was Operation Red Line?

Back in early 1963 - just a few months after the invention of the visible Helium-Neon Gas Laser - a group of (mostly) amateur (ham) radio operators that happened to work at EOS (Electro-Optical Systems, later affiliated with Xerox/Parc) in Pasadena, California took on the challenge of doing something that would be both fun and challenging:  Go for the distance record for laser-beam communications.

A 1963 Laser

Figure 1:
On the workbench, the EOS Helium-Neon laser tube.  The Viking II
transmitter - the source of the RF excitation and modulation - may
be seen to the right of the laser.
Click on the image for a larger version.

At this time you couldn't just go out and buy a Helium-Neon laser tube, so they managed to cobble it and the optics together with the blessings of the senior management and a bit of help from their own well-equipped glassblowing and optics shops.

One of the vexing problems with early gas lasers - or practically any gas-discharge tube - was that of contamination/wear of the electrodes used to excite the gas mixture within the laser tube, so rather than mess with trying to engineer a solution to yet another problem, they chose electrode-less RF excitation.

But first, they had to get it to work!

The RF source itself was pretty easy:  Being ham operators, they already had access to a 100-watt class AM transmitters so Bob Legg, W6QYY, offered the use of his Johnson Viking II transmitter.  They chose a 10 meter frequency of 28.620 MHz, because it was a rather high frequency and would make the matching a bit easier, but it would also be more likely to couple into the tube and properly excite the gas - plus there was less chance that the 10 meter band would be open and the unintentional RF radiating from the coupling system would be heard halfway across the world!

In this case the idea was easy, but the implementation was more difficult:  How does one properly excite the gas inside the tube so that it lases?  Electrodes on the glass in the form of self-adhesive copper tape were an obvious choice, but it turned out to be a bit of a challenge to achieve uniform excitation until someone struck on the idea of interleaving elements, with every other element connected together, fed 180 degrees out of phase with the other set via a balanced-wire output from the transmitter coupling.

Finally, lasing!

Figure 2:
The "business" end of the laser, in the lab.  A pair of semi-silvered,
confocal mirrors were used to allow the laser to work.  The laser
assembly was put into a large piece of metal channel so that it
could be transported and maintain a degree of mechanical/optical
alignment.  A "10 power" telescope was used to collimate the
beam to approximately 2 inches (5 cm) diameter to minimize
atmospheric scintillation.
Click on the image for a larger version.

Even though around 100 watts of RF was being input to the matching network, the EOS folks were able to measure only about 125 microwatts of light emerging from the laser itself.  In speaking with those that were there at the time, they believed that with additional work they could have gotten more power out of the laser, but they felt compelled to get it out of the lab as soon as possible since crunching the numbers indicated that 0.125 milliwatts should be more than enough power to cover any distance for which they were likely to find a line-of-sight path!

The RF alone wasn't usually enough to "strike" the tube and cause the gasses within to ionize, but they had on hand a device that they referred to as a "zapper".  About the size of a high-power soldering iron, this gadget was used in the neon light and vacuum tube industry to test for gas within the device being tested by outputting a very high voltage, low-current arc that can ionize the gas within, through the glass via capacitance -  much like the trigger electrode of a xenon strobe tube:  This device is actually visible in Figure 1, sitting on the upper shelf on the far right, to the left of what looks like a water glass.

The mechanical adjustments for the laser were also very finicky.  At each end was a partially-silvered confocal mirror on a micrometer mount and it was very easy for slight temperature variations and flexure of the laser assembly to knock these out of alignment and prevent lasing.  As it turned out, both mirrors were identical so laser light was actually emitted from both the "front" and "back" of the laser, but this "wasted" laser energy from the back was useful in tweaking the laser while it was in operation since this could be done without blocking the light going to the distant end.

Finding a long, line-of-sight path:

Meanwhile, other members of the group were busy poring over maps to find two locations that were line-of-sight with each other and offered some hope of being accessible by vehicles that would be available for use and eventually, a pair of sites emerged as candidates worth checking out on the ground.  The west-ish end of the path, near the Grassy Hollow campground in the San Gabriel mountains and another site in the Panamint range, next to Death Valley and east-ish of the (almost) ghost town of Ballarat.

Getting to the Grassy Hollow site didn't appear to be too much of a problem:  Easy access from good roads near an established campground.  The Panamint site was more of a challenge, but making contact with the sole inhabitant of Ballarat - an old miner who called himself "Seldom Seen Slim" - and a few others that had mining claims in the area - proved fruitful and they were able to get permission to travel across these claims as well as get directions to some of the higher-elevation roads.  After a bit more scouting about they found a location along a road at which there was enough flat-ish area for both vehicles and setting up the gear and it offered a good view back toward Grassy Hollow.

The calculated distance?  A bit over 118 miles according to the maps.

Detecting a distant laser:

Figure 3:
In the lab, detecting the purposely-attenuated laser light using a 12.5-inch
reflector telescope.  This was one of many tests "in the lab" before the
gear was taken into the field.
Click on the image for a larger version.

At this point, one may start to wonder about the "other" end of the laser path.

It wouldn't do just to shine a laser from point "A" to point "B", but it was necessary to be able to communicate - if only one way - over the beam electronically, preferably via voice.  To do this, a means of detection of the (likely) weak beam was required.

Fortunately, one of their number, Parks Squyres, owned a large reflector telescope, a Herring-Cave 12.5 inch (approx. 32 cm) Newtonian.  To it they fitted an external box containing a movable front-surface mirror to direct the gathered light either to an eyepiece or an electronic light detector known as an S-20 "Photomultiplier" tube.  This type of tube, invented in the 1930's, is used today as it is still unsurpassed in its sensitivity to light and it was the natural choice at the time since there were no other types of electronic light detectors that came anywhere close to being sensitive enough for this task!

For testing they took advantage of the fact that the EOS building was laid out such that, using mirrors, they were able to send laser light - severely attenuated to attempt the simulation of atmospheric effects - up one hall and down the other, a distance spanning well over 100 feet.  After a bit of tweaking they were able to get good results transmitting voice from end-to-end of this simulated path, giving them confidence that the entire system would have a reasonable chance of working!

Modulating the laser was actually the "easy" part.  Originally, they had intended to take advantage of the polarized light of the laser itself (its design included a pair of "Brewster's Windows") and use a "Kerr Cell" - a device that can be used to electronically rotate the polarization at extremely high speeds - and using the two together it would theoretically be possible to convey many channels of voice.  In the interest of time and simplicity, however, they used the same Viking II transmitter that was providing the RF excitation to amplitude-modulate the light since it was, after all, an AM transmitter!    Because amplitude modulation (AM) RF was used for modulating the laser, some care had to be taken to avoid excessive excursions to "0%" modulation or else the laser would "go out" and require re-striking with the "zapper"!

Figure 4:
The laser at Grassy Hollow, set up in a tent.  Communications between this, the transmit site and the receive site
about 119 miles away was maintained using the 2 and 6 meter amateur bands.
Click on the image for a larger version.

Time was of the essence:

The EOS folks were on a bit of a "fast track" with this project because they got wind of at least two other groups making preparation to span a fairly long distance.  One of these was associated with Ryan Aircraft and other with the U.S. Military, both of whom were making rather elaborate preparations.  Since this was a purely volunteer effort using donated time, equipment and funds, there was the double pressure of making it work quickly and cheaply!

Soon after getting both the laser and receiver working they scheduled a weekend during which they would go out into the field and make a first attempt:  Friday, May 3, 1963.

Getting to the Grassy Hollow campground was fairly easy:  They just drove their cars and station wagons to the site, set up tents and then fussed with the fickle laser while the receive site group covered the greater distance to Ballarat, dropped off some of their cars in the care of Seldom-Seen Slim and then packed themselves into two four-wheel drive trucks and Bob Legg into his wife's Plymouth Valiant and they bounced their way up the steep roads into the mountains.

Figure 5:
The telescope used to receive the distant laser.  The box to
which the eyepiece was attached also had a front-silvered mirror
that allowed light to be redirected to the photomultiplier tube for
detecting the modulation (audio) on the distant laser light.
Click on the image for a larger version.

After getting to the receive site just after 3 pm on May 3rd - with Bob having to be towed the last little bit up the last, steepest part - they set about putting up tents and setting up the gear.  Soon, they were in contact, via 2 meter and 6 meter radio with Grassy Hollow over the 118+ mile path and they waited for dark.

Pointing the laser:

One of the challenges with a long-distance optical path is that at 118+ miles, even large landmarks at that distance are difficult to discern - not to mention trying to determine the precise location of the other party!  Knowing only generally where the the receive party was, the crew at Grassy Hollow needed more a more precise visual reference on which to base the aiming of the laser and the complications of doing this changed as the daylight faded, old landmarks disappeared and new ones such as city lights came into view.

Aiding this effort the receive site crew set up a very powerful Xenon strobe, but even though this was blindingly bright to those in the Panamint range at the receive site, it was stubbornly invisible from Grassy Hollow despite the use of both binocular and telescope.  Having anticipating the possible failure of the strobe a surplus military aircraft rescue flare was set off by the receive site crew and this was quickly spotted, just before the wind blew its smoke in front of the flare and blocked it from view, but at least those at Grassy Hollow now knew exactly where to look.  On a hunch that the atmosphere was blocking the dominantly green-blue Xenon flash, a humble 100 watt clear incandescent shop light was clamped into place at the focus of the Xenon strobe's reflector and this proved to be visible at the transmit site as well, especially now that they knew where to look!

Figure 6:
At the receive site, the flashlamp/reflector and the telescope.
Click on the image for a lager version.

As it got dark they began the arduous procedure of aiming the laser and something very quickly dawned on everyone:  While considerable attention had been made in the design and alignment of the laser's optics and in achieving good sensitivity of the optical receiver, no-one had really thought too seriously about the practical difficulties of aiming a very narrow beam over a distance of 118+ miles!  Using a number of improvised techniques, the laser crew managed to get the beam "close", setting the elevation with various shims and other pieces onhand, but getting both azimuth (horizontal) and elevation (vertical) dialed in proved to be a hair-pulling task.

Figure 7:
At the transmitter site, making adjustments to the laser.  The laser
itself was very "touchy", often requiring adjustment of the
mirrors at each end of the laser tube to sustain oscillation
(laser action.)
Click on the image for a larger version.

After a bit of fussing, the receive site crew was tantalized by the occasional brief, bright flash from the distant laser but it seemed as though the transmit site crew could never repeat the maneuver - plus the necessary corrections - to get the laser back and on-point!  When the receive site crew queried the Grassy Hollow folks about this on the radio it turned out that they were using two primitive tools to adjust the aiming of the laser:  A large rock tapped at the end of the metal channel in which the laser was mounted for coarse adjustments and a much smaller rock for fine-tuning!

Eventually, after much finessing and hair loss, a reasonably bright and steady beam was obtained at the receive site.

Bob Legg, in a 2008 interview, told me that after successful alignment he had "walked" the beam at the receive site and found it to be about 150 feet across at the 118 mile distance implying an overall beamwidth of approximately 0.014 degrees (about 0.25 milliradians) - a narrow beam, indeed!

Success at last!

At about 11:15 pm on the evening of May 3, 1963 the crew at the receive site was finally able to make out a voice in the weak, fading signal being detected from the distant laser.  A bit more than an hour later and after a bit more tweaking of the laser and its its aiming, signals were somewhat improved and the voice of Jack Pattison, W6POP could be heard coming across the link with reasonable clarity.

* * * * * * * *

Actual audio from a laser transmission

on the evening of May 4, 1963

Other recordings from this event may be found on the "Operation Red Line" web page - (link), near the bottom of the page.

Figure 8:
Success!  The group at the Grassy Hollow transmit site celerates
success in the late evening of May 3, 1963.
Click on the image for a larger version.

In the intervening years lasers have become commonplace, consumer commodity items with several being present in nearly every home - most notably in CD/DVD and Blu-Ray players. Although widely used on fiber-optic communications, they have found only scant use for "through-the-air" free-space optical communications. 

* * * * * * * *

The actual distance of the path: 

At the time the distance was measured using the approximate locations determined from the available maps and it was calculated to be "over 118 miles."

Since that time the precise GPS locations of the two sites have been determined and the calculated distance, using the "Haversine" method, is now known to be approximately 119.145 miles (191.74 km). 

Acknolwledgements:

I'd like to thank Bob Legg for supplying most of the images and many details of Operation Red Line during a 2008 interview.  I would also like to thank Parks Squyres, Ron Sharpless and Dave McGee for providing additional pictures and details to fill in some of the missing pieces.


Of course, there were many others involved in the project, each contributing their own, important part to the success of this project

Please visit the Operation Red Line web page (link) for additional attribution and details!

Links:

If you are interested, be sure to check out the links below for more information about Operation Red Line and optical, through-the-air communications in general.

• Operation Red Line:  http://modulatedlight.org/eos/Operation_Red_Line.html - link - This site has pictures, sound files and a written narrative about this project.

• Operation Red Line photo gallery:  http://modulatedlight.org/eos/operation_red_line_gallery.html - link - A lot of pictures and details from this project!

• The Modulatedlight web page:  http://modulatedlight.org - link - A resource for those who might be interested in optical, through-the-air experimentation.

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This page stolen from ka7oei.blogspot.com