[Laser] How much power? Ideed!

[James Whitfield]

I have been reading with interest ( and perhaps a little concern ) the discussion on the power ratings of laser diodes pulled from scrapped CD/DVD recorders.  I have some thoughts and questions about how much power do you really need, but first I want to emphasize safety and the public perception of the safety of amateur experimentation with optical communication.

I am glad that we have progressed beyond the expense and high voltage hazards of the early days of laser experimentation.  I am also very thankful that there were no public reports of a nutjob with a HeNe laser pointed at the flight path of commercial airline traffic.  We have passed into the days of easy access to (comparitively) inexpensive laser devices that operate at low power and safe voltages.  The growing hazard is the intensity of the light being emitted.  This should be particularly evident from the discussion of the fast writing speed and temperatures reached in the discs being recorded.  Take the time to compare the size of a bit on CDs and DVD to the light sensitive cells in the back of your eye ( or more importantly the eye of some neighbor kid several blocks down the street that you don't know you need to warn not to look at your experimental setup ).  Calculate the write time of a single bit on a DVD compared to the reflex time for you to blink from a bright light.  

In short, THINK SAFETY.


Now for some comments about laser diodes extracted from disc recorders:  I did not notice any discussion of the duty cycle at the power levels discussed.  For a simple write once system, there may only be one power setting and the write duty cycle may be limited.  On the other hand, an eraseable disc may require a continuous duty cycle at a different power setting.

The thermal environment may also need to be considered.  Was the laser diode mounted on a heat sink or in an air flow path to remove heat?  Even if the device is rated at 150mw, can you really run it at 50 percent duty cycle for an hour to test an incremental frequency shift keying daylight beacon without first packing it in three pounds of dry ice?

On the flip side of the discussion, how much power do you really need for the type of communications experimentation you are doing?  Part of the problem of answering that question is that we don't have any easy to understand guidelines.

I still consider the "gold" standard to be the heliograph.  That was a device that was intended to be used in daylight and received by the human eye.  The rule of thumb was that the effective range of a heliograph mirror was 10 miles for each inch of mirror diameter.  Perhaps some of you can translate that into something equivalent to the devices we are using today.  How sensitive is the human eye.  How fast can you send Morse code to be received by the human eye?  Translate that into bit rate.  How "bright" is a laser?  Or perhaps the question to ask is how many milliwatts of laser light translate to inches of heliograph mirror diameter?  How do you translate minimum visibility of a laser by the eye to signal strength of an electronic light sensor?  And how do you adjust that depending on the different colors of light?  How to translate the light power needed for heliograph Morse to that needed for PSK31, voice, video, or even today's weak signal methods?

To use a line of sight example, the longest earthbound two way light communication that I am aware of, was 183 miles back in the mid 1890s by US Army Signal Sargeants.  I believe they used 8 inch diameter mirrors.  I assume they also used telescopes or field glasses, but I have no historical reference.  If they did, it would have had no more light gathering power than the common 60mm refractors today's toy stores.  How many milliwatts of laser would it take to match the light output of an eight inch heliograph mirror?


Interestingly, a light source on the Moon would in some ways be easier to see.  There is less air if you look up than if you look at hilltop less than five and a quarter miles away.  The best viewing of the Moon takes place after the Sun goes down, so there is less "noise".  If there were a Moon Base or just a rover up there with the equivalent of a laser pointer, 5 mW and 1.5 mR beamwidth, it could send a spot down that would be 360 miles across.  How much of a telescope would you need to see that?

James
 n5gui