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