Assembly and Operating Instructions for Open Cavity HeNe Laser Kits

Version 1.00 (27-Apr-22)

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This document contains notes on the assembly and operation of the various open cavity Helium-Neon (HeNe) laser kits. Rather than having both a High Reflector (HR) mirror and Output Coupler (OC) mirror permanantly attached, these tubes have one or two Brewster-angle windows or perpendicular AR-coated windows in their place. This means that one or both laser cavity mirrors need to be external, and that additional optics can be added inside the cavity.

These are for the true laser geek where just powering a common internal mirror HeNe laser tube or laser head is too mundane and BORING. ;-) And just getting these to lase with external mirrors can be a frustrating experience for the novice - but a truly rewarding one when sucessful. Short of fabricating the laser tube itself (which is orders of magnitude more difficult), this is the closest one can reasonably come to actually building a HeNe laser from scratch.

Melles Griot / Pacific Lasertec High-Q One-Brewster HeNe Laser Tubes

This section applies to all tubes of similar construction including the LHR-563, LHB-564, LHB-568, and LHB-569, as well as the very rare and challenging LGB-563 with a green (544 nm) internal mirror.

IMPORTANT: The one or two Brewster and perpendicular window HeNe laser tubes provided in these kits are more fragile than the already fragile internal mirror tubes. But in particular, those called "High-Q" where one end is all glass need to be treated with special care.

Failure due to screwups in any of these is not covered under the warranty.

Where a power supply with female Alden cable is included, a mating male Alden cable will be provided. It may even already be wired to the required ballast and tube clips. Generally, the red fat HV wiring needs to be well insulated but the black (fat or thin) wiring is near or at ground potential.

Melles Griot / Pacific Lasertec Frit-Sealed One-Brewster HeNe Laser Tube

Serving Suggestion: Melles Griot/Pacific Lasertec 05-LHB-570 Tube in Test Fixture and Photo of Similar Tube. The High-Q tubes can be mounted in a similar way.

For these tubes, the B-window may be at the anode or cathode-end of the tube. In the diagram, it is at the cathode-end while in the photo it is at the anode-end. Power must always be wired with respect to the anode and cathode, NOT where the window is located! However, that will influence how likely it is to have a shocking experience while messing around near the B-window. :( ;-)

Aligning an External Mirror

With no external mirror, it's just a really funky neon sign. ;-) The build a laser, an external HR or OC mirror is needed and it must be aligned so that the cavity is stable. (Other optics like cat's eye reflectors, roof prisms, or cube corners can be used with some lasers in place of a mirror, but with a HeNe, the gain is so low that these would be quite challenging to get to work.) There are two primary requirements for lasing to occur: (1) the cavity geometry must be stable and (2) the round trip gain must be greater than 1.

For the first requirement to be met the cavity formed by the mirror inside the laser tube and the external mirror be stable. It's possible to set up unstable cavities that lase but that is generally not practical or useful with HeNe lasers. However, note that although these kits do not include any convex mirrors, HeNe lasers can be built with a concave and convex mirror providing some specific advantages. The internal mirrors in all these tubes have a 60 cm Radius of Curvature (RoC) unless noted otherwise. All the mirrors included in the kits will form a stable cavity at some range of distances from the internal mirror that is determined by the RoC of the external mirror. The cavity will be stable if:

This diagram below shows the stable area as a function of the dimensionless quantities g1 and g2 which are equal to L/r1 and L/r2, resectively. RoC1 = r1 and RoC2 = r2. While lasing is generally not possible precisely long either axis of the edge of the curves, interesting things occur at the origin. ;-)

So for example with the 60 cm RoC internal mirror and a 30 cm RoC external mirror, the the cavity will be stable up to 30 cm and from 60 cm to 90 cm. In practice, lasing near the limits will be difficult or impossible. With a planar external mirror (RoC = infinity), the range will be from close to the tube window to 60 cm. (Note that to be really pricise, the effective optical distance within the window must also be taken into account, but that can generally be ignored.)

The single pass gain for the LHB-568, LHB-570, and similar tubes is around 3 percent; for the LHB-564 it is 1 to 2 percent. The mirror adds loss based on its reflectivity, cleanliness, and imperfections. There are also losses due to scatter from both the inner and outer surfaces of the Brewster or perpendicular window. And since the bore is not infinitely large, the tail of the intracavity beam brushes the side-wall called resulting in "diffraction losses, but for these tubes with wide bores, they are minimal.

As a first test, the mirror labeled "LO1160 1m OC" should be easiest to use because it has a high reflectance (99.5%) yet will produce a full power beam (>5 mW for the LHB-568 or LHB-570 and >1 mW for the LHB-564), and it will lase from next to the window to up to 60 cm from the internal HR.

Mounting the Tube and Mirror


Pacific Lasertec 05-LHB-568 One-Brewster HeNe Laser Tube on Rail with External Mirror in Kinematic Mount

The tube is installed in a custom Plexiglas enclosure that protects the tube from humans and protects humans from shocking experiences due toh the high voltage. ;-) The "4-screw ring mounts facilitates the adjustment of the alignment of the tube with respect to the rail and mirror.

Optics Cleaning

With regards to optics cleaning, the best approach is to never have to do it, especially the mirror.

Alignment Methods

There are a semi-infinite number of ways of aligning an open cavity laser. Here are three that are effective for the one-Brewster or one-Window tubes: These all assume that the window and mirror are clean enough for lasing to occur and that the distance between the mirrors is well within the range for a stable cavity.