The " Kama Suthra " guide to 445nm LD positioning

[CDBEAM]

On the learning curve path for optics, I have come to the understanding regarding the Fast and Slow Axis nature of the 445nm LD. I was puzzled because in setting up my test bed-“ Copper is Proper “….the only way I was able to get the prisms to correct the astigmatic beam of the 445 was to align the stripe …horizontally to the prism pair…or so I thought !!!!!!!
Let me explain….when one looks at the colliminated ( FL @ 4mm ) beam of the 445 at 12 Meter….I saw a rectangular output of 14mm tall by 43mm wide. OK….so I logically assume that the Fast Axis, the axis which has more divergence was represented by the 43mm wide portion of the beam profile. And conversely…the Slow Axis was therefore the 14mm wide portion of the beam. When I placed the anamorphic prism pair ( Per Zoof’s template ) in front of the LD output…the beam was corrected. At 12Meter…the resultant beam profile was a square geometry of 14mm. Perfect. Per Pseudonomen’s 137 Divergence calculator….that works out to 1.0 mRad. So…contrary to all I have read…and patient and teaching PM’s from Dave, p!t8ull, and RedlumX ….I said NO…the Fast Axis must strike the prisms…HORIZONTALLY…a logical conclusion based on what works and what I observed.
Well…not so in the “ Alice in Wonderland “ of laser optics !!! It turns out that this beam is a “ Shape Shifter” The beam…just after it leaves the LD IS a ellipse…long axis going up and down….VERTICAL…At this point…the LD IS orientated so the Slow Axis is going up and down….and the Fast Axis is going from side to side. So… with NO prisms…and with a Collimination lens in the beam path…
At 10cm from the LD output....the beam profile looks like " l "
At75cm from the LD output….the beam profile looks like " o "
At 300cm from the LD output...the beam profile looks like " -- "
So what does ALL this mean ?? Simply this….For a Axis module with a 445nm LD pressed in or a bare 445nm diode inserted into a Brass mount ( like the one from LSP/Dave with the LD secured in its holder)…and with the Collimination lens attached….the beam will appear to be a rectangle…..at 12 Meters…this bar will typically be 14mm x 43mm. The correct radial position for the Axis module or bare diode is when the 43mm dimension of the rectangle projection is horizontal. This position is where the prism set corrects the Fast Axis. This position delivers a square geometry…at 12 Meters. What happens at 20 Meters…not as square….but still OK.
A variable for ALL Graphic Laser Show venues is…How far from the projector will the beam strike the termination plane ??? Big Variable !!!!!....but for my guess…..I would speculate that a range from 5 to 30 Meters would be normal. This is just my SWAG !!....Unless you are at Disney or Stone Mountain !!! For my needs…12 Meters is typically the max. The first prism in the prism set can be adjusted to deliver the optimal geometry for your needs. For all you behind me on the learning curve ( If any )….this blah, blah explaination is intended.
Most of this info has been presented before....although NOT in the unique way I have presented my thoughts ( Read mental meanderings ). Thanks for reading and hope this helps anyone else !!!!




Research presented from " Terrawatt Labs "

CDBEAM 2/2011

[megaton]

Hi CDBEAM

Nice info, thanks.
BTW what Zoof template are you using or recommend? (I'm thinking to use 53 degrees).

Cheers for you and woof for Terra,
Ed

[CDBEAM]

The Brewster angle...about 57 degree... (Wow...almost had to feel the wrath of the 15C limit..haha Fat Chance !!)

CDBEAM

[megaton]

Thanks CDBEAM

So 56.775 it is

Trackto says WOOF to Terrawatt
Ed

[logsquared]

An easy way to find the Brewster angle empirically is to rotate the prism while looking at the light reflected away. When the light is at a minimum you should be very close to the Brewster angle. This assumes the incoming ray is un-polarized or p polarized. i usually just set the rig up so the reflected light shines on the wall behind my work table. With the 445nm diodes its real obvious when you've hit the "sweet spot".

[kitatit]

hi CDBEAM,

thanks for sharing your insights

I liked your explanation. Plenty of explicit detail and the language you used was easy for me to understand.

I'm behind you on the learning curve and your explanation is a great heads up on what to look for.

It really helped bridged the gap in knowledge for us optics noobs.

respect.
Kit

[CDBEAM]

Thanx Gents.....We ALL understand a little more when we all help each other. Wish I could contribute more !!!!!!!!!

CDBEAM and Terrawatt

[Doc]

Yes. basically the fast axis is the thin axis near the diode's output. This axis then diverges really quickly; the prisms pre-expand this fast axis to a similar size as the slow axis, thus reducing it's divergence.

[CDBEAM]

Thanx Doc.....I understand.....Still the semantics crack me up !!! One is " Pre Expanding....and this REDUCES divergence "...Yikes...I need the " Brain Boost:\" ...located at one time on the " Forbidden Planet "....but ...for my desire to put optics theory in simplistic terms..by expanding/magnifing the FA...hmmmmm....it is like you expand it on one end....and it piviots around the focal point...and ends up with less over divergence on the other end !!....like a " Titer Toter " !!!!!

Thankfully...I do not have to completely understand the" Second law of Thermodynamics "....to drive my car !! hahaha....

Thanx again....

CDBEAM/Bob

[Elektrotechniker]

Great info but LOL @ the title