Laser Communications

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Laser Communications
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Laser Classes

• Class I Sealed systems
• Class II Output lt1mW
• Class IIIa Output 1mW - 5mW
• Class IIIb Output 5mW 500mW
• Harmful to eyes, diffuse viewing OK
• Class IV Output gt500mW
• Harmful to skin and eyes, diffuse viewing
  hazardous

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Diode Lasers

• Laser diodes emit an elliptical beam with
  astigmatism
• Better units will include corrective lenses for
  astigmatism and to make the dot appear round
• Neither of these problems are inherently bad for
  DX purposes but correcting them also improves
  divergence, a big win (more gain).

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Human Spectral Response
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Perceived Intensities
Wavelength Multiplier Color
532 nm 28 Green
635 nm 7 Red-Orange
640 nm 5 Red-Orange
650 nm 3 Red
660 nm 2 Red
670 nm 1 Red
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Laser Diode
Laser Diodes include Photodiodes for feedback to
insure consistent output
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Pointer Design
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Pointer Innards
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Modulation

• AM
• Easy with gas lasers, hard with diodes
• PWM (Pulse Width Modulation)
• Used by Ramsey in their kit
• PFM (Pulsed FM)
• Potentially the highest bandwidth (gt100kHz)

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Gain Systems

• Transmitter
• Maximum output power
• Minimum divergence
• Receiver
• Maximum lens area
• Clarity
• Tight focus on detector

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Filters

• Sun shade over detector
• Shade in front of lens
• Detector spectral response
• Colored filters
• Absorb 50 of available light
• Difficult to find exact frequency

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Mounting Systems

• Mounts and stands need only be as accurate as
  beam divergence
• Good laser diodes will be 1-2mR (milliRadian)
• A 32 pitch screw at the end of a 2' mount will
  yield 1mR per revolution. Since quarter turns
  (even eighth turns) are possible, this is more
  than accurate enough
• Higher thread pitches allow shorter mounts which
  may be more stable (against wind, vibration,
  wires)
• 1mR is 1.5' of divergence every 1000', 3' at 2000
  ', etc.

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Pointing

• GPS and Compass
• Scopes and Binoculars
• Strobe lights, large handheld floods, headlights
• HTs to yell when laser light is seen at remote
  location

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Weak Signal Modes
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Laser DX

• 1991 - June 08 - WA7LYI and KY7B - 153.97 miles !
  
• Equipment used 18 inch fresnel lens into
  Photomultiplier tube
• Transmitter 15 mw helium cadmium laser (442 nm)

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Applications

• Transmit voice for miles line-of-sight
• Use weak signal modes for cloud scatter
• Transmit video with cheap pens
• Transmit high speed data without WEP
• Blind flies for easy extermination