Scanner Jitter?

Over the weekend I spent time making fine adjustments to an LOC 826 diode module to allow all the diodes to fit the scanner mirrors without any telescope and I made it. The far field combined beam was slightly less than 1 cm. at 12M. Great! "The projection should now sparkle". It certainly wasn't worse, maybe a little better, but not much. So I set up a test pattern, Slightly misaligned the lasers to produce three single color grids and measured the X and Y line widths. The blue (445) which has three X the divergence in the worst axis ( at 1.75 mrad) was wider at aprox. 3.0 mrad and in its best axis ( at 0.5 mrad) was almost 2.0 mrad. Both the green and the newly tuned red at 0.8 mrad also produced 2.0 mrad lines. I am using an Eye Magic scanner with the 9.5mm aperture mirrors and although shows are typically preformed 18-20 K speeds, even at 10 K there was no improvement in the line widths. An additional fact is when the lasers and scanner are "disabled" the faint residual blue beam is double the diameter of the intercepted static beam.

I also notice that when the static beams are perfectly aligned and then put through the scanner they always need a small adjustment to overlap on the test patterns and when rechecked in static mode they are always slightly misaligned.

Is this due to jitter or position inaccuracy? Is this typical of all of these moderately priced scanners? Can anything be done to reduce or eliminate this? Or are we wasting time trying to reduce divergence below 1-2 mrad because it will be overwhelmed by scanner limitations?

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Originally Posted by planters

Over the weekend I spent time making fine adjustments to an LOC 826 diode module to allow all the diodes to fit the scanner mirrors without any telescope and I made it. The far field combined beam was slightly less than 1 cm. at 12M. Great! "The projection should now sparkle". It certainly wasn't worse, maybe a little better, but not much. So I set up a test pattern, Slightly misaligned the lasers to produce three single color grids and measured the X and Y line widths. The blue (445) which has three X the divergence in the worst axis ( at 1.75 mrad) was wider at aprox. 3.0 mrad and in its best axis ( at 0.5 mrad) was almost 2.0 mrad. Both the green and the newly tuned red at 0.8 mrad also produced 2.0 mrad lines. I am using an Eye Magic scanner with the 9.5mm aperture mirrors and although shows are typically preformed 18-20 K speeds, even at 10 K there was no improvement in the line widths. An additional fact is when the lasers and scanner are "disabled" the faint residual blue beam is double the diameter of the intercepted static beam.

I also notice that when the static beams are perfectly aligned and then put through the scanner they always need a small adjustment to overlap on the test patterns and when rechecked in static mode they are always slightly misaligned.

Is this due to jitter or position inaccuracy? Is this typical of all of these moderately priced scanners? Can anything be done to reduce or eliminate this? Or are we wasting time trying to reduce divergence below 1-2 mrad because it will be overwhelmed by scanner limitations?

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Do you mean that if you put the scanners at zero all the beams are aligned, if you create say a large square they go off alignment?

No. When I interrupt the path before the scanner and using a steering mirror send the combined beam to a target, then the beams are perfectly aligned. In addition the misalignment through the scanner is angular (it increases with distance) and not a step misalignment as if the different wavelengths are reflecting from different surfaces of the scanner mirrors.

i wonder if you're having nearfield alignment issues.

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Originally Posted by planters

No. When I interrupt the path before the scanner and using a steering mirror send the combined beam to a target, then the beams are perfectly aligned. In addition the misalignment through the scanner is angular (it increases with distance) and not a step misalignment as if the different wavelengths are reflecting from different surfaces of the scanner mirrors.

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Quote:

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Originally Posted by planters

No. When I interrupt the path before the scanner and using a steering mirror send the combined beam to a target, then the beams are perfectly aligned. In addition the misalignment through the scanner is angular (it increases with distance) and not a step misalignment as if the different wavelengths are reflecting from different surfaces of the scanner mirrors.

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So if you bypass the scanners you get a nice beam, after the scanners you get a distorted one?

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Originally Posted by swamidog

i wonder if you're having nearfield alignment issues.

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Yeah, sounds like classic symptoms of that.

sounds like your near filed alignment may not be perfect. you need to get your near field 110% perfect before you touch the alignments for far field.

are all of your "dots" PERFECTLY overlapped on your dichros?All of your combined beams need to be PERFECT on your optics BEFORE you try to align your far field.

-Marc

How is it that a beam that is already combined ie post dichroics and is aligned in the far field and travels the same distance will show this phenomenon due to near field misalignment? Nevertheless, the near field is apparently well aligned, they all need to be to fit on the scanner( multi dot diode beams are difficult to align in any case). I guess I could arbitrarily change the near field in a given direction and correct the resulting misalignment in the far field and check if this helps.
My major concern however, is the apparent contribution of the scanner to far field beam width. Whether the far field lines are aligned or not they do not change in width; I've measured this. Of course, no scanners can be perfect and any flexure or position feedback noise/error will add to the width of lines that are being drawn over and over. Might this be more significant for the Eye Magics or is this common and no one has looked closely at this. The Eye Magic scanner is rear mounted which is very convenient, but less robust than the corner cube blocks used by other systems. The feedback loops for these or other motors are a black box to me and might have different performance depending on the model. What do other scanners show? Is the mirror flexing and causing a defocus of the beam? Or is each narrow beam not being precisely overwritten?

try using a little fog/smoke/whatever so you can actually see the beams, not just the spots on the dichros. i'm always surprised at how far off i am by just trying to overlap the spots.

Quote:

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Originally Posted by planters

How is it that a beam that is already combined ie post dichroics and is aligned in the far field and travels the same distance will show this phenomenon due to near field misalignment? Nevertheless, the near field is apparently well aligned, they all need to be to fit on the scanner( multi dot diode beams are difficult to align in any case). I guess I could arbitrarily change the near field in a given direction and correct the resulting misalignment in the far field and check if this helps.
My major concern however, is the apparent contribution of the scanner to far field beam width. Whether the far field lines are aligned or not they do not change in width; I've measured this. Of course, no scanners can be perfect and any flexure or position feedback noise/error will add to the width of lines that are being drawn over and over. Might this be more significant for the Eye Magics or is this common and no one has looked closely at this. The Eye Magic scanner is rear mounted which is very convenient, but less robust than the corner cube blocks used by other systems. The feedback loops for these or other motors are a black box to me and might have different performance depending on the model. What do other scanners show? Is the mirror flexing and causing a defocus of the beam? Or is each narrow beam not being precisely overwritten?

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Scanner jitter DOES NOT do beam position shifts with respect to wavelength on first surface reflective optics. Theta total equaling 2X theta input for a front surface mirror is a law of the universe, period. You put a beam in at -45', it comes out at +45', no matter what, on a first surface mirror. You'd need a dispersive optic for anything else to happen, and first surface coatings are NOT thick enough for a visible dispersion.

Most likely the beams are not aligned in the near field.


If it is not the nearfield alignment it can only be one thing I can think of....


Try on this idea, your laser beam from a DPSS is wobbling with gain shifting from modulation.
I've seen greens move and mode shift like crazy. A loose cavity mirror, or thermal distortion in the intra-cavity crystals, can move the beam

You can pick up cross modulation from surges in the scanner supply getting picked up by the diode drivers, or vice versa.
A modern galvo dumps 3-4 amp surges into the ground plane or back into the rails. This is why Bill Benner released the star grounding paper for projector design.

How much reserve ampacity did you build into your psus? Any possibility a psu has a bad filter cap?

Make a precision bar or bars with a tiny hole in it at your beam height, then slide it along your baseplate to check Alignment. You can also fix pairs of them in place. This is the classic way for doing near field for combining lasers, be they ion or solid state


Steve