1 Attachment(s)
A Diodes 'Fast Axis / Slow Axis' demystified / discussion...
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..' :p
..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... :p 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... :p
Here's a quickie-diagram to help, in-case I'm not 'splainin-it clearly... :p
Attachment 31343
Discuss / correct / improve, at-will... :cool:
j
