Where can I find a small laser transceiver ?

[matrix1]

laser transceiver conclusion

Hi All.
i need to make a laser transceiver of range about 500 metres to 5000 metres. i may buy it as off the shelf units or can build it myself if the suitable components are found. But the ready made units must be of small size due to the application location requirements. So please guide me how should i proceed, where could i find desired info and parts. i have read some where about the optics used in such a system, does it help to increase the range of the system?? thanks a lot.

 

[Peter McFly]

Re: Laser Transceiver !!

For the optics, what you want is to maximize irradiance of the transmitter at the reception site, for this you have to reduce the divergence of the transmitted beam as much as possible as long it is kept in mind those limitations affecting the geometry of the beam:

-The angular stability of the xmitter assy & the challenge to align it.

-Atmospheric turbulence cells effects.

-Atmospheric scatterer.

-Quality factor of the laser beam & optics.

-Aperture of optics & wavelenght.

For this one, what limit the minimum beam divergence is diffraction due to xmitter optics aperture & wavelenght (Rayleigh Criterion). So the telescope to be used at xmitter must have an apperture large enough but not overkill in respect to other limitations stated before.

At this point, it is assumed that a wavelenght at NIR (750nm - 1µ band) has been considered due to availability & cost of lasers. Atmospheric propagation at those wavelenght is similar to visible (but not identical as some reader familiar w/t IR photography may point out that NIR has a better penetration thru white haze). So it can be a good advise to get hand on amateur telescope & use it to image reception site from xmitter site at different magnification & get the feel of magnitude of problems ahead! Ultimately this telescope can be a great prototyping optics for your laser xmitter.

A good advise if you gonna experiment w/t NIR: get yourself a CCD quickcam & replace the internal IR cutoff filter by 2 pieces of fully exposed & develloped Kodack Kodacolor film. Those films will block visible but let pass above 800nm. The CCD respond up to slightly over 1µ.

IMO NIR is the best band for horizontal atmospheric propagation in the context of what you want & cost. It is the band of choice for military in daylight for imaging & range finding. MWIR & LWIR band are not "all weather" as some cie tend to pretend. In fact selection of which band to use is a complex matter full of tradeoff to do relative to the climate you live in. I've seen a couple of real life measurement of performance for these 3 bands in different weather conditions/obscurants & believe me, longer wavelenght not always equal miraculous improvement.

Another thing to help you to stay away from the 2 others propagation window: think about the size of the optics, the cost of material making the lens & the possible requirement of a cryogenic detector.

However, it must be remembered that the transmission is done in free air & the system must be made eye safe. There are law for optical transmission of laser beam thru air & we don't want to risk the eyesight of pilot of plane crossing the beam or those poor little bird so dependend of their sight. Either the irradiance everywhere in the beam fall bellow safe limit or we have to go to a safe wavelenght.

If the safe wavelenght is the only solution, operation at 1.5µm would be reasonable. However we begin to slip outside of the NIR propagation windows. Si detector no longer work. See:
**broken link removed**
**broken link removed**

Very good information can be found for many aspect of this project by researching the subjet of Laser Ranger. Also you must absolutly gain acess to "The IR & EOS Handbook"

At this point, it must be emphased that the BW capacity does not only related to laser modulation capacity & detector BW. Atmospheric Scatterer will tend to add temporal dispersion to the signal.

For the detector assy, the optics must limit the FOV (field of view) to maximize ratio of received laser power to background optical noise. This is spatial filtering to increase S/N. So we got the same problems of angular stability & alignment as the xmitter.

As a matter of fact, NIR background is quite high and vary wildly between day & night. A BLIP (background-limited operation) receiver circuit of considerable BW can be built arount PIN photodiode.

With the high backgroung signal, optical spectral filtering must be done w/t a bandpass optical filter as narrow as possible (modulation BW permitting). A dielectric filter is well advised here. Here the choice of the type of xmitter laser come back to haunt us as some type of lasers vary in frequency and/or has multiple frequency modes. There no point having a killer filter if the center wavelenght of the source go outside it. Also pay attention that dielectric filter are sensitive to wavefront geometry/angle & temperature.

As a side note, I have knowledge of sucess at xmitting low bps thru laser pointer, one exemple was between buildings in an university campus but cannot remember link. Anyone tempted to experiment this funny matter must be advised that laser pointer are not made equal when its time to modulate them. Surprisingly, the most cheapest one are reported the most capable at being modulated, but still in low bps range...

However I would be interested to know performance possible using Typical CD writer Laser Diode TE & Voltage stabilized, good optics, blip receiver & adapted filtering.

As a conclusion note, optical transmission thru the atmosphere is a nightmare and the question to ask is: What is the reason not to use microwave?[/u]