Hello there Steve,
I'm wondering if you can solve a question that I have about the FACed diodes from LaserTree. Back when these came out some members purchased them and had a difficult time obtaining the expected square beam profile without using a very large 100mm FL lens. What I am confused about is why that is, they seemed to be behaving as if a FACed diode needs to be given distance allowed before collimating to allow the fast axis and slow axis to reach the desired profile shape but this doesn't make sense as the fast and slow axis should already be corrected right out of the diode.
Not steve but I think I can help here.
Only the fast axis is corrected on these diodes. So a lens is still needed to collimate. You need a long FL lens becuase the fac has narrowed the fast axis in an attemp to more closely match the slow axis. Think about a standard correction scheme for a m.m. diode. Say a 8mm fl collimator then prisms or a cylinder pair to expand the slow axis. Lets say the ration of the cylinder pair is 8X. So, 8mm(collimator) X 8 = 64mm. To get the same slow axis divergence with the fac as with the standard cylinder correction a 64mm fl lens in needed. The fast axis with a 64mm lens with no fac would be too tall at the 64mm lens. The fac narrorws the fast axis beam to fit on the lens. The power of the fac lens (fiber diameter) dictates the angle of the beam, and therefore the size of the beam when it hits the 64mm lens.
One last question, there was some confusion about whether to use the "line" or "square" versions of their FACed diodes. It seemed to me that the square version would result in what is accomplished otherwise by cylindrical correction lenses/prisms, leaving me wondering what the "line" versions were for? This would seem the opposite of "fast axis correction", in fact exaggerating the problem of unequal diverging fast and slow axis.
The "linear" fac has a more powerful fac lens so the divergence is very small after the fac (like 1deg or so). The "squared" has a weaker fac lens and more closely matches the natural slow axis (about 8-10deg).
Viewing the raw output of the FACed diode on the wall with no other lenses, the linear looks like a line and the square is more square.
The final beam divergence comes down to ratios. The emitter area of a P73 is about 1um X 45um. But, the 1um diverges at ~40 deg and the 45um diverges at about 10deg. If you use the "square" FACed diode the output at the lens is squared but the far field spot is still rectangular. This is because the FAC reduced the fast axis by 4X (40deg/10deg) So, the far field line that was 1mm x 45mm will now be 1mm x 11mm. This assuses you adjust the collimator lens FL to compensate for the new fac expansion.
Similarly, The the "linear" diode will adjust the fast axis beam by about 40X (40deg/1deg) This in theory makes the spot ~1mmX 1mm realative to the uncorrected diode. However the output beam will be a rectangle just opposite of what the uncorected diode will produce.
For a regular pointer the "inear" is what you want. I found that for knife edging the linear has a bit too much "correction" for long FL collimators (100mm plus) I had laser tree make a custom FACed diode that is about half way bettween the 'square"and "linear"
I made some test modules with the special FACed diodes and have them in the for sale section. Ify anybody want me to make a seperate thread about how they work let me know.
