Collimating a 1535 nm Source: Aperture, Divergence and What You Cannot Fix
Divergence numbers on our datasheets look large next to what a good telescope can produce. That's not a defect. A 0.3 mm emitting aperture has a diffraction limit of roughly 6 mrad at 1535 nm, and our 7–8 mrad parts are close to it.
So the raw beam is about as tight as its aperture allows. Getting below that is an optical design job, and the design has a price.
The exchange rate
A beam expander trades diameter for divergence. Magnify the beam ten times and the divergence drops ten times, but the output aperture has to grow ten times too, and every millimetre of that has to be filled with clean wavefront.
Going from 8 mrad to 1 mrad needs about 8× magnification and something like a 5 mm output. To reach 0.3 mrad you're looking at 10 mm of aperture. There's no way around the arithmetic.
Clipping is the usual mistake
Put a 12 mm beam through a 10 mm window and you don't get 10 mm of performance. You get a truncated aperture, which widens the far-field pattern and throws energy into rings, plus a hot edge that's bad for coatings.
Size the last element for the beam you actually have, not the one on the drawing.
Alignment gets harder as magnification rises
A 10× expander also magnifies angular error by 10. Half a milliradian of tilt at the input becomes five at the output. On a gimbal, that's the difference between a spot and a smear.
Check boresight after thermal cycling, not just at assembly. The ER300 and larger units are heavy enough to move a light mount when the temperature changes.
What we can and can't change
We can supply a source with a given raw divergence and beam diameter, and tell you the M² range. We don't design your telescope. If you send the required far-field divergence and the aperture you have, we can tell you whether the source supports it — or whether you need a smaller emitter and more magnification.
Talk to us directly: WhatsApp +86 133 0819 5797 (message opens with the application line prefilled) or email Taylor@erdimail.com.