[Q] back reflection from fiber collimator

laser wrote in news: snipped-for-privacy@x35g2000prf.googlegroups.com:

I considered Brewster's angle, and went hunting, and found this:

Googling for [fibre coupled laser brewster angle] might get you some good stuff. That linked page is a patent describing a combination of AR coating and less-than-Brewster to get good power and minimal reflection. There might be other schemes, and searching in the context of Brewster's angle seems likely to find some more.

innews: snipped-for-privacy@x35g2000prf.googlegroups.com:

this:

Confused, Why back reflection concerns, of course you want to minimize reflections. You will also get reflections from the cleaved or polished output end of fiber as well as both surfaces of the collimating lens. The best you can do is AR coat the fiber end, and both sides of the lens. Are you worried about a beat frequency or something like that? These reflections are minimal. Is the cost incurred to create a brewster scheme or something more exotic justify end result? Why is it (back reflection) such a big concern to you?

innews: snipped-for-privacy@x35g2000prf.googlegroups.com:

this:

Back reflections are a concern because if they re enter the cavity they will effect the gain in an unpredictable manner.

innews: snipped-for-privacy@x35g2000prf.googlegroups.com:

this:

Your statement is true, however, you need more information before making a broad statement like that. I know that is possible but that is why I asked the question. VERY few cavity designs will be adversely affected by low power reflections. As usual...more information is needed to resolve question. Scope too broad.

innews: snipped-for-privacy@x35g2000prf.googlegroups.com:

this:

>

And there are always optical isolators. :)

Also look into APC fibers and orient all other optics at a slight angle if possible.

Note that for the newbies here, it's not so much that the laser will do totally funky things with a bit of backreflection. However, if the frequency and stability are critical, then an extremely small amount of backreflection can be disasterous. For example, with a frequency stabilized HeNe laser, even 1 part in 10E-7 or even less of the output power getting back into the cavity can have a detectable effect.

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innews: snipped-for-privacy@x35g2000prf.googlegroups.com:

this:

agreed, single frequency applications and stabilized single frequency applications are probably the only cavity designs that will be adversely affected by low power reflections and generally speaking...again...gain will be less effected than stability would be. Need more information from original poster on type of laser, cavity, medium, design, and so forth or else the "experts" will just be speculating and spewing forth gray matter....IMHO

Sorry you didn't like the reply. I do not need more information for a generally true statement. From your question, you seemed to be asking why back reflections were a problem.

Sam Goldwasser wrote in news: snipped-for-privacy@plus.seas.upenn.edu:

This is by far the easiest way. It's what I plan to do when mounting two diodes for combining with a polarised beamsplitter. Lining things up looks neat but is a bad idea in that case.

When driving diodes hard it's very critical to avoid retroreflections, if you strongly overdrive a DVD diode for a show device, for example, you can kill it with the reflection off a Lasercheck ND filter.

Of course we all know you shouldn't be overdriving a DVD diode for a show device. :)

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Sam Goldwasser wrote in news: snipped-for-privacy@minus.seas.upenn.edu:

But this world is so full of misguided individuals. >:) We will do these things. Not that I've ever done a public show this way. It's not a bad idea though, power is power, so safety aside, the main thing is replacing the things fast and easily like projector lamps. At current costs and difficulty in getting high power narrow low divergence red beams, that's a viable idea. At least it might be, if selling raw diodes didn't make better sense financially than actually trying to sell lasers built with them.

Stand corrected, rewording my statement. What type of laser architecature do you have that you need to be concerned about low power back reflections? Generally speaking, single frequency or stabilized single frequency lasers are usually mostly adversely affected by low power reflections. However, these "problems" may be corrected by opto isolators, AR coating all reflecting surfaces or mounting reflecting surfaces off angle from lasing system. Will that work?