A Diodes 'Fast Axis / Slow Axis' demystified / discussion...

[dsli_jon]

Hey Kats -

Ok, so, respectfully carrying this-on, out of the 'Mitsu G71' thread, there... There seems to be a bit of 'subjective-differences' in how people are discussing / defining the 'fast and slow axes' of laser diodes. So this is a thread where we can clarify the 'definition' / understanding, so we can put an entry into the PL Wiki / help n00bs '..use the search, Luke..'

..Not to suggest that people are not always-free to 'express things in their own terms', but simply for the sake of congruity, when discussing this - technically - in threads relating to combining-techniques, ie, knifing, etc, and corrective optics / lenses, etc, etc...

First, I'll clearly state, that I am no 'expert' on the subject, nor do I purport to-be.. In-so-posting this thread, I am also 'challenging' my own-understanding, as you can always learn from others' perspectives / knowledge of a subject.. That-said, I can read pretty well, so... Reference-wise, here's what I am founding this discussion on: http://www.rp-photonics.com/broad_ar...er_diodes.html

The 'relevant points' I distilled-out, were: (blah, blah-edits, mine..

* In the vertical (short) direction, the height (e.g. 1 μm) is ..(blah, blah, blah..) Because of the small aperture size, the beam divergence in this direction is relatively high, with a beam divergence half-angle of.....(blah, blah, blah..) Due to that fast divergence, this is called the fast axis direction.

* In the long direction (slow axis direction), the stripe width may be e.g. 50, 100, 200 μm, or even larger, so that the light is distributed over many spatial modes in this direction. As a result, the beam divergence is much larger than for a.. ..(blah, blah, blah..) although still significantly smaller than for the fast axis direction. ..(blah, blah, blah..) The beam quality in terms of focusability is reduced; ..(blah, blah, blah..) Furthermore, the beam profile may be multi-peaked in the horizontal direction, and the shape of the intensity pattern may depend on the injection current. .....The broader the stripe, the higher is the achievable power, but the worse is the beam quality in the “slow” direction.

..and, when we read 'data' like, '3mm x 1mm', this helps 'clarify the description' in the info, above...

Keep in mind, that as Sir Pit said, this is speaking a) about the *raw*, uncollimated output-profile of a diode - since any correction-lenses or prisms-used - regardless of which make / focal-length / number of elements, etc - will change this 'profile' - So, that's where the dis-congruity seems to creep into discussions, here, since the divergence / shapes, etc change, post-collimation, and especially, post prisms / cylinders / other 'corrective'-optics...

..But, when discussing the FA and SA, as it relates to multi-diode knifing, especially, one should always be thinking of the terms, as they are 'defined', above - from the 'raw-output-perspective'... What 'happens' to the FA and SA, because of our collimator / correction-schemes, is a different-discussion, ya 'folla?

b) Remember, too, that how one 'orients' the diode, as they look at the raw-output, can also introduce some 'confusion'.. If you, for example, read: "...In the vertical (short) direction...[..which is the 'fast axis'] - but you're looking at the long-stripe, ( – ) horizontally on the wall, you might think, now that the 'short-dimension of the rectangle' is this 'fast-axis' - but that's actually the slow... tricky little buggers, eh?

Perhaps the 'easiest-way to remember the difference' is that these two 'terms' (FA and SA) relate to these divergence-planes in a diode beam, not the 'dimensions'... especially, post-collimation / corrective-optics, where everything 'changes'... at least, it looks that way...

Here's a quickie-diagram to help, in-case I'm not 'splainin-it clearly...



Discuss / correct / improve, at-will...
j

[p1t8ull]

First, I'll clearly state, that I am no 'expert' on the subject, nor do I purport to-be

Same here, just fumbling my way through

Jon, your description above sounds 100% good to me and thought my original post in the g71 thread was on the same wavelength? Maybe I didn't translate thought to keyboard too well..

b) Remember, too, that how one 'orients' the diode, as they look at the raw-output, can also introduce some 'confusion'.. If you, for example, read: "...In the vertical (short) direction...[..which is the 'fast axis'] - but you're looking at the long-stripe, ( – ) horizontally on the wall, you might think, now that the 'short-dimension of the rectangle' is this 'fast-axis' - but that's actually the slow... tricky little buggers, eh?

I would clarify.. this is with no collimating lens and the diode with the case pin east or west

[dsli_jon]

^^ ..all 100%, agreed... I think I was the one being a bit 'obtuse' in previous discussions..

Any-other diagrams / explanations / links, welcome..

cheers..
j

[planters]

Thanks Jon,

You were not being the least bit obtuse...maybe a tad verbose ,but exquisitely clear. This terminology is used so much it really needs to be used correctly or not at all. It probably should be used less when talking about the downstream optical chain. Here divergence, aspect ratio, beam dimensions and spot size will be more directly relevant and less likely to introduce misunderstanding. No excuse though, if working with these diodes so regularly it just seams wrong not to understand how they work.

[Galvonaut]

Great description of fast and slow axes. There is so much useful information hidden away in the depths of PL

Keith

[dnar]

Is blah a real technical term?

[Galvonaut]

Is blah a real technical term?

It sure is ....... http://youtu.be/U7L1LxcR0Vs