[Laser] Some beginner questions

[Art]

At 05:01 AM 10/15/2006, you wrote:

>Hi from a newcomer!
>I am Paolo, IK1ZYW, from Torino, Italy. Any other Italians here?
>
>My interest in laser comms started when I showed a colleague (with 
>whom I share interest in QRP and SOTA - www.sota.org.uk - ) an offer 
>of 3 laser pointers for 3 USD (at www.ledshoppe.com). Those are 
>declared to be 5mW/2mile red lasers, so we started rambling about 
>attempting a SOTA Mile-Per-Watt laser record with a minimalist RTX.


Welcome Paolo,



>I've read through the 2006 archives and quickly glanced at the FAQ 
>and other webpages. I have a few questions, that have certainly 
>already been answered elsewhere. Please point us to the right direction.
>
>I understand photodetectors from CDROM drives are not good for long 
>distance because of their small capture surface. How about those 
>built into VCRs? All my samples are in black containers, so perhaps 
>they're not for visible light? IR? Are they any good if combined 
>with a red laser?

CDROM units are ok, but have small active area and thus are more 
suitable for wideband data transmission (less shunt capacitance 
associated with the physical diode junction). Also, they are 4 
photodiodes combined in a quadrant array, designed to allow the 
electronics to track the center of the relatively narrow data tracks 
on the CD (positional information can be obtained which is relative 
to how close the light source is aimed at the particular track the CD 
is receiving at that moment). They are not good choices for most of 
us although they can be used for those on budgets.

The remote control receivers used with VCR's are meant for receiving 
subcarrier  modulated pulses, with the modulation occurring at ~44 
Khz and the pulses in the 20 to 100 msec ranges. They are crude and 
aren't terribly sensitive. Further, due to the high carrier frequency 
needed, they are less sensitive than baseband receivers-but enjoy 
immunity from ambient light that happens at 50/60 and/or 100/120 Hz.

>I have seen OPT202 for a reasonable price, but had not the chance to 
>see a datasheet. How about these detectors? Should I wait for those 
>mentioned on previous posts (xx1 or xx3)?

The OPT units are the best deal by far, IMHO. If you get a 5 pin 
inline package unit, the bandwidth controlling resistor is 
unavailable to the user, so the better deal is the DIP packages which 
allow the user to install an appropriate value to optimize 
sensitivity for the bandwidth they are using.

They are low cost (ebay) and use little power. To get best results, 
you need to shield them completely, just as though it was an rf 
circuit and mechanically (small air currents induce noise).

The best of these units is the opt-301, which comes in a metal can 
with a clear window to admit light. These are easier to use becasue 
less shielding is required and they have slightly better sensitivity. 
However, they cost more::>


>About the receiver, is it better with FET/BJT or some op-amp 
>arrangment? Any "beginner" circuit to start with?

K3PGP has a FET front end using totally discrete components. Whether 
it's better or not is a good question. It certainly takes much time 
to fabricate and is larger and heavier than any of the OPT devices. 
It is less user programmable however, the ultimate bandwidth (and 
thus the usable sensitivity) is more dependent on the photodiode and 
FET's electrical characteristics than the OPT-xxx type receivers.

For very serious work, a photomultipler tube is needed. They use high 
voltage and need special handling. If you over expose them to high 
light levels (even microseconds of ambient room light levels), they 
are useless for days or weeks until they recover. However, they are 
30 db more sensitive than the best photodiodes. They use much more 
power, typically 100 to 150 ma from a 12 volt power supply. So, they 
are much harder to use in portable operations. Unless you get very 
serious about DX, you should avoid them.

>Thank you in advance for your answers,


Before leaving the topic, I'd like to say a few words about the 
unique opportunity you have to set distance records in association 
with your mountain climbing interest. The ultimate restriction 
limiting DX for most of us Earthlings is the clarity of the air-or 
more precisely, the amount of water vapor in the air. In other words, 
you can do much more distance with one milliwatt of laser from summit 
to summit than I could do with 100 milliwatts here on the East Coast 
(near sea level) USA. Colder air is drier air, and at higher 
elevations, you have less molecules of air to trap water vapor to 
begin with!!!!!!  By going to higher altitudes where the air is 
colder and clearer, you could potentially have very long line of 
sight paths and be able to communicate over longer distances than 
someone who can't hike to mountain tops.

I think the current world record is 170 miles, made in Arizona from 
hilltop to hilltop. The air is much cleaner there, but getting up to 
seriously high elevations such as the EU Alps could potentially 
enable much longer distance contacts.

If you know of line of sight paths that are 300+ miles apart and have 
climbers that can climb both peaks at the same time (carrying laser 
gear), then breaking the current world DX record for lasers would be 
very doable.

Enjoy.

Art

PS:Forget about the claims of laser sellers regarding 'range'. The 
range they quote is subjective and is relative to the eyes response 
to different colors. For instance, a 1 milliwatt 780 NM laser pointer 
might have a 'range' of 500 feet, a 1 milliwatt helium neon laser 
(632.8 NM) would have a 'range' of 1500 feet and a 1 milliwatt green 
laser might have a 'range' of 3 miles. The point I am trying to make 
is that the 'range' rating has nothing at all to do with the actual 
communications distance you can achieve with that particular laser::>



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