Most important thing is frequency, not output power !!

And indeed, quite a few companies have been derived from ELI and scattered around Asia (Korea, Taiwan, China, Japan, Singapore, etc) region. They are supplying HP’s, Zygo’s frequency stabilized He-Ne laser repair service to semiconductor manufacturing equipment, stepper, scanner, CD-SEM, and memory repair machine.

But I’ve never seen any of companies that they can guarantee the absolute frequency and frequency stability.
The center frequency of a He-Ne laser is determined by several factors, mix ratio of Ne isotopes during gas fill process is most significant factor. When they repair using replica tube, they must confirm how far the center
frequency shift from original tube and what is the frequency stability after frequency stabilization.
We can calibrate the laser frequency and guarantee the frequency stability after repair by using frequency standard ¹²⁷I₂ / He-Ne laser.

Collimated beam is also another important factor for laser interferometer system. Most of the commercially available HeNe laser have beam expanding optics to have 3mm, 6mm and 9mm output beam diameter. If bad collimated beam, converged or diverged with elongation distance, severe alignment error will occur in interferometer optics due to poor overlap between reference and measurement beam in heterodyne detection.

We can make beam expander unit to generate a laser beam consisting of parallel wave fronts, constant beam diameter and with negligibly small divergence over a long distance. Collimation test is done by employing shear plate interferometer consists of a piece of window with very flat surfaces having a slight wedge angle between them.