How to align plane-plane resonators for pulsed lasers

Mirror alignment is one of the most important steps in getting a laser system to work correctly. However, different resonator types require different alignment strategies - and present unique difficulties. In this vein, the simplest resonator type, the plane-plane resonator, can actually be very hard to align correctly, since it is metastable: It can only be considered truly stable when aligned perfectly. Since this is not possible in the real world, true stability can never be reached with a plane-plane resonator, one can only approach stability.

Short note: An optical resonator is considered stable, when it is possible for photons to bounce back and forth between the mirrors indefinitely without leaving the cavity. One can very easily imagine that two plane mirrors would have to be perfectly parallel for this to be true.

Alignment becomes even more difficult with increasing resonator lengths because the same angular imperfection in the mirror adjustment creates bigger deviations from the optical axis at the other mirror.

However, this resonator type comes with its own advantages: Generally, resonators with less stability tend to have higher mode volumes (=volume of the laser beam inside the resonator) which leads to more atoms in the laseractive material being excited and therefore higher output powers. This can be seen in the picture below. The usage of the laseractive medium is very high with the plane-plane resonator but is reduced when other resonator types are used.

Note that the latter two resonator types can (depending on the resonator length and mirror curvatures) be un-/metastable as well. However, this property is inherent only for the plane-plane resonator.

Another redeeming quality is that plane mirrors are easier and therefore cheaper to manufacture. This combination explains why the plane-plane resonator is still widely used, especially in pulsed lasers where the light beam does not have a lot of time to leave the resonator whilst bouncing back and forth between the plane mirrors. A typical example of this laser type are Nd:Yag lasers. But how can one accurately align a pulsed infrared laser with such a difficult resonator type? This is what I will show you in the following paragraphs.

The alignment process

You’ll need:

• A collimated secondary laser, for example a HeNe laser. A diode laser of sufficient power should also work.

• An adjustable mirror for the secondary laser. A generally reflective mirror (aluminium, silver, molybdenum, maybe gold, dielectric) works well, no need for specialized equipment.

• A pinhole, diameter 1-2mm (diameter not critical). A piece of plastic with a drilled, clean hole works fine.

• The laser cavity itself, of course. The rod doesn’t need to be removed, as long as it doesn’t absorb the secondary laser beam too much. Its alignment is not critical.

The first step will be to fix the adjustable mirror (M1) in front of the laser cavity so that its center coincides roughly with the optical axis of the resonator. Then you can fix the secondary laser to a rigid surface across the room. The higher the distance between the laser and M1, the more precise the alignment will be, but divergence will make the spots on the pinhole (see second step) more blurred. For me, 4 meters worked fine. Mount the pinhole in front of the laser so that it shines right through. Now you can adjust the secondary laser + pinholes so that the beam hits M1 centrally.

In the second step you adjust M1, so that the guide beam is parallel to the table top and goes through the middle of the cavity. Since M1 is adjustable, the secondary laser and the cavity can be at different heights from the floor, no optical table needed.

The third step is the actual alignment. To understand how this works, let’s look at the following picture:

Since the outcoupler (OC) and the high reflector (HR) of the cavity have some reflectivity for the guide laser wavelength even if the coating is not necessarily made for it, some part of the guide beam will inevitably be reflected back into the direction of the source. Some Nd:Yag laser mirrors even have coatings which are reflective at 633nm just for this purpose, but you don’t strictly need this.

Because the mirror will not be perfectly aligned to the guide beam, the reflections will not go right through the pinhole again, but will show up next to the hole. Depending on the initial alignment and the pinhole plate diameter, the dots might not even show up on the plate because the reflected beams deviate too far from the guide beam. In this case you can use a piece of paper to “search” the reflected beams in the room. Moving the cavity mirrors will of course move the corresponding reflected beams. Since the optical path from the pinhole to the HR and back to the pinhole is longer than the path to the OC and back, its corresponding dot will be slightly more blurred. When you align the cavity with the crystal in place, it will also be less bright. Dimming the room lights helps if you have trouble seeing the dots.

Now you can align the mirrors so that the reflected beams both go right through the pinhole. Using a HeNe laser, I could clearly see interference patterns, but this is not strictly required.

Once the dots are aligned, the cavity is ready to lase. This method has worked quite reliably for me. Because the optical path from the pinhole to the cavity is very long, the alignment is very precise.

I hope these instructions are helpful! Thank you so much for reading.

~N