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• AM

o 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• Transmitter

o 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