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Laser Communications
Laser Classes
• Class I – Sealed systems• Class II – Output <1mW• Class IIIa – Output 1mW - 5mW• Class IIIb – Output 5mW – 500mW
o Harmful to eyes, diffuse viewing OK• Class IV – Output >500mW
o Harmful to skin and eyes, diffuse viewing hazardous
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).
Human Spectral Response
Perceived Intensities
Red1670 nm
Red2660 nm
Red3650 nm
Red-Orange5640 nm
Red-Orange7635 nm
Green28532 nm
ColorMultiplierWavelength
Laser Diode
Laser Diodes include Photodiodes for feedback to insure consistent output
Pointer Design
Pointer Innards
Modulation
• AMo Easy with gas lasers, hard with diodes
• PWM (Pulse Width Modulation)o Used by Ramsey in their kit
• PFM (Pulsed FM)o Potentially the highest bandwidth (>100kHz)
Gain Systems
• Transmittero Maximum output powero Minimum divergence
• Receivero Maximum lens areao Clarityo Tight focus on
detector
Filters
• Sun shade over detector• Shade in front of lens• Detector spectral response• Colored filters
o Absorb ~50% of available lighto Difficult to find exact frequency
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.
Pointing
• GPS and Compass• Scopes and Binoculars• Strobe lights, large handheld floods,
headlights• HTs to yell when laser light is seen at
remote location
Weak Signal Modes
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)
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