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My method to align a Plano-Concave Resonator For Good Quality TEM00 mode.



The above configuration allows good TEM mode selection and the use of resonant etalon type reflector reduces the number and magnitude of longitudinal modes. Insertion of additional elements like etalons or staturable absorbers as mode selectors will further reduce longitudinal modes. The Plano-Concave resonator is still somewhat sensitive to mis-alignments and a balance of alignment tolerance vs. mode volume should be considered when deciding on HR mirror focal length and separation length. Refer to my spreadsheet for these plano-concave calculations and the contained comment tutorials weights.xls

Setup Resonator:

Both the three mirror Fabry Perot and the two mirror FP can be aligned the same way as given below.

Note: Since the alignment of the two OC mirrors on version 7 are critical for good spatial beam quality then this alignment procedure was determined to give the best results for the ruby laser. Patience is a must as slight adjustments of the OC mirror 1 will generally correct poor tem00 mode due to multiple reflections as long as OC mirror 2 is correctly aligned with HR.

If using a sapphire etalon in the three mirror arrangement, then you must confirm that the sapphire is cut at the zero degree c axis for use in this system (Ruby rods normally have the c axis at 60 degree relative to the rod axis. Sapphire etalons could be either 0 or 90 degree orientations with 1-2 arc second parallel faces. 0 degree will assure minimal double refraction effects. Both the Ruby rod and the sapphire etalon must be aligned before aligning the resonator. Follow the same method for both as indicated in the "ALIGN ROD" procedure.alignrod.htm The sapphire etalon is aligned when it too is the darkest in the cross polarizers. It is aligned with one of the polarizers. In the case of the ruby rod we know its orientation due to its color change but to find out if the sapphire is aligned with the ruby rod may require testing in the laser setup and observing maximum power output. Correctly aligned with the ruby rod will allow both sides of the etalon to cause maximum reflectivity.

1. Setup an alignment laser like a HENE on the HR side. Remove the HR mirror. Center the beam at the HR opening and confirm at the OC Mirror 2 that the beam arrives in the center of that optic as well. Since the beam travels passively through the dye cell, ruby rod, and OC Mirror 1 then all deviations will be taken into account. Ruby rod and dye cell was oriented with an offaxis tilt so that any residual reflections from these components was reflected out of the resonator. Ruby rod when pumped will create some negative lens effects on the system during q switch pulse generation but that effect is usually not critical in the above arrangement.

2. With the above laser beam geometrically aligned to the centers of both end mirrors and the HR mirror removed, Close down the diaphragm aperture and center it on the beam using the XY adjusters for the aperture.

3. Open aperture and remove Resonant OC mirror 2 (outer OC) so that it's reflections are not involved. Adjust kinematic mount for OC mirror 1 while observing the back reflection from OC mirror 1 towards the aperture as it lands on Dye Cell. Observe Dye Cell glass and center reflection onto arriving beam.  

4. Install OC mirror 2 and adjust while observing the back reflection from OC mirror 2 towards the aperture and dye cell The reflection should be centered into the beam observed at Dye Cell glass.

5. Place a white card 45 inches beyond the laser system and secure so it can't move. Observe the HENE laser beam after it leaves the system as it falls on the card. Confirm that the laser beam only has one spot if not adjust either OC and watch as the beam becomes centered. Repeat steps 3 and 4 if you get lost. Steps 3 to 5 confirm that that both OCs are aligned with the laser beam. In Steps 3 and 4 you can use this white card instead of the Dye Cell glass to observe alignment.

6. Tape onto the card a piece of thermal paper like Kentek's zapit paper. With a straight pin poke a small hole centered where the HENE laser beam strikes the paper. Reinstall the HR mirror and observe and just note any lateral displacements of the HENE beam caused by the HR mirror having a slight wedge.

7. Adjust the HR while observing the HENE beam at the paper target and eliminate any extra beam spots if possible by centering these toward the main beam spot. The laser is roughly aligned.

8. Close down Aperture to 1.5mm.   Fire the laser and observe the spot that lands onto the thermal paper. If no spot then either increase power supply or adjust HR. Recommend firing and readjusting just one control on the HR until lasing is observed. Once this happens then adjust the HR until the desired beam is centered on the pinhole made. The system is now aligned. Finally adjust the HENE to be centered on the pinhole as well if the HENE is to be left in place as a coaxial alignment tool for installation of power amplifiers and other optics. If at step 7,  the HENE is adjusted to be centered again on the pinhole after the HR was installed, and the HR adjusted to  eliminate any extra beam spots, then it was found that very mininum laser firing and adjustment would be needed to center the fired beam onto the target pinhole.

9. The output should show good quality spatial beam, circular without any elongations or extra beam lobes. The slightest misalignment of the two OCs can cause these spatial problems and slight adjustment of OC mirror 1 or OC mirror 2 may be necessary. Suggest using a dial indicator on the kinematic mount with a 0.0001 accuracy to verify repositioning of the OC. The use of a dial indicator will help you get back to where you started from if you make adjustments and to fix these issues generally require slight adjustments between the 0.0001 graduation marks. It is possible to adjust to within 20 microns if needed provided a good dial indicator and fixture is used.  Step 9 is a last ditch effort to tweak mirror 1 or mirror 2 if necessary to fix a spatial problem. The best method is by repeating the steps 2 to 8 above. If having trouble you can tyry removing or adjusting with large tilt the inner OC (mirror 1) during that firing procedure. Align the OC mirror 2 for best power and TEM00 mode quality. Then install or adjust mirror 1 to reflect the HENE back to aperture and tweak until good TEM00 mode returns by eliminating mulitple reflections.    


Below are some thermal paper illustrations from previous lasers built.