Page History

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• The Laserdiode needs proper cooling-contact. Mount a peace of indium foil (50-100um thick) between the laserdiode and the copper heatsink.
• With these handling notes in mind, mount the laser diode on the Micro Chip Laser Block using the screws that came with the diode.
• Then solder 2 cables to the tongues of the diode. We recommend using a solder station with ESD protection. We recommend highly flexible silicone cable in red (plus) and black (minus), See picture for polarity.
• Add zip ties to add pull relief to the cables
• route the cables to the laser diode driver and connect them there

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The height of the lens is given by mounting it on the MC-lensbar parts, but the other constraints must be adjusted by hand. This is a delicate process, be careful!

• Power the laserdiode up without the Grin-lens assembly and led it shine to a IR-detector card at a distance of around 10cm on a metal plate, and mark there with a pen the position of the 808nm beam out of the laserdiode. 
• mount the aluminum MC-lensbar on the cooling block using 2 M3 screws. There is a little bit of play. Try to find the middle position, so that there is a chance to move the parts a bit in every direction later, if needed.
• prepare a small amount of high viscosity two-part epoxy
• add a small drop of epoxy in the middle of the MC-lensbar
• carefully place the lens on the MC-lensbar with pliers, making sure that no epoxy get on the front and back faces of the lens. Also, do not touch the front side of the diode (with the plies or the lens)
• adjust the lens position (see constraints above)
  
• let epoxy harden
• prepare some more epoxy, and use it to secure the lens position

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We now can repeat the diode test from above, this time with the lens attached. We want to see, if the lens is nicely aligned. If not, we can loosen the screws at the MC-lensbar and adjust the position slightly. 

Adjust the lens position (see constraints above) in a way, that the beam of the laserdiode hits the metal plate in the middle of the pen-marking. (in this way, one ensures to align it in the middle).

Mounting the laser resonator

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• right in front of the lens, not touching it, with a distance of ca 0.5mm
• The axis of the lens co-axial to the diode laser output. It must be
  • at the right height
  • at the right position (directly in front of the diode)
  • straight in line with the diode (not tilted)
  • To ensure that, power up the diode at low power (around 1.5A current) and mount one of the copper L-brackets on the microchip laser block (MC-bracket-A) in front of the Grin lens.
  • look carefully with at low radiationi densities with a IR-detector card and at high beam densities ( near the focus of the Grin lens) with the backsie of the IR-detector card) to ensure that the beam out of the grin lens is guided in the middle of the groove of the L-bracket. Therefore the laser will be guided perpendicular and through the middle of the crystal.

This time we can only vary the distance between crystal and lens, the other parameters are predefined by the mounting setup.

For the crystal we must take special precaution that cooling is good. This is archived by putting 0.1mm indium foil around the crystal. This greatly improves the heat conductivity between the crystal and the copper bracket. This works best, if the crystal is covered with indium on all 4  side (obviously not the input and output side), but no foil parts overlapping each other. We recommend the following procedure

• Mount one copper bracket on the microchip laser block (MC-bracket-A).
• cut a indium foil piece the size of 2 sides, that is 6 x 9mm
• Fold the foil piece in an L-shape, covering 2 sides of the crystal
• put the crystal together with the foil onto the bracket, in such a way that the foil is now hidden. In other words: the foil is between the bracket and the crystal
• Now prepare another piece of foil with the same size and make it L-shaped too
• Put the foil on the crystal, covering the 2 side visible sides
• Finally, add the second copper bracket. Make sure the indium foil stays where it is

Adjust the position of the crystal (by moving the 2 brackets using the slot holesith carefully as close to the endface of the bracket to grin lens as possible) and screw them the second bracket in place.

Testing and Fine-Tuning the Microchip Laser

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• Lenses L1+L2 (beam expander) und mirrors M1 and M2 are roughly mounted at the right position and angle (optical amplifier is not included in this build)
• With the micro chip laser running, use the IR viewing plate to fine position all components. Make sure L1+L2 is nicely aligned with the beam and the the beam hits M1 roughly at the center
• Adjust M1 so the beam hits M2 in the center
• Now adjust the beam expander (L1+L2) so that the beam profile is more or less the same in all positions (before/after M1, between M1 and M2 and after M2
• Remove the KTP crystal, L3 and L4 (power off laser in the meantime)
• Adjust M2 so the beam hits M3 at its center
• Adjust M3 so the beam hits M4 at its center
• Now add in L3 and L4, making sure they are nicely aligned with the beam (the laser beam should hit the positions at M3 and M4 from above)
• Add in the KTP again (power off laser in the meantime). The KTP should be behind the focus (around 5mm).

You should now have green light after the KTP crystal. Please note that with the safety glasses you cannot see it. You can use e.g. postit notes as "viewing plates" to make it visible.
If frequency doubling is not working (and you do not green light), go back to the previous steps. Try slightly changing positions of L3, L4 and KTP

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• Beam expander setting (position of L1 and L2)
• Position of L3 and L4
• Alignment of KTP. Try also turning the KTP crystal slightly out of perfect alignment, this can lead to a better efficiency. Also turn the KTP in its angle to get better conversion efficiency.
• Do several iterations of finding the best settings
• Also you can go back to the previous step and check alignment of the components

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