Ruby Rod

For many years I used a helium neon laser as an alignment laser for my ruby laser. The beam from the helium neon laser always produced a visible path through the ruby rod.

In May of 2021, I was examining a ruby laser rod and I became concerned over apparent flaws I was detecting in the rod. After the beam from a helium neon laser was passed through the length of this rod, it appeared to undergo significant distortion (a separate issue, unrelated to the subject of this page). I shared photographs of my activity with Sam Goldwasser. Sam commented on what appeared to be scattering within the rod - the beam path of the helium neon laser was clearly visible in the rod. Sam added that this effect could not be scattering if these were actual laser rods, as such an effect would be intolerable. This became a source of concern for me, so I reflected upon my experience with other lasers. I seemed to remember the helium neon laser making a visible path in the ruby from my existing (old) ruby laser. The rod from this old laser was removed and tested, and indeed the helium neon laser beam produced a visible path in it. I tested a non-ruby laser rod, and a visible path from the helium neon laser did not appear in it (I believe the rod was Nd:Glass), nor did a visible path appear, for the most part, within undoped sections at the ends of the ruby rods I tested. In addition, I noticed a somewhat distinct difference in color from the path in the ruby rod and nearby scattered helium neon laser light - I thought the helium neon laser light appeared much oranger while the beam path in the ruby was noticeably redder. In my opinion therefore, light from the path was not scattered light from the helium neon laser beam.

Had Sam not brought this to my attention, I never would have noticed the actual difference in color. In fact, I never really gave this any consideration or second thought during the countless times I performed resonator alignments on my ruby laser - I probably just dismissed any apparent differences in color as differences in intensity. Sam offered to run some spectroscopic tests on one of the rods, and I took him up on his offer. The test results did indeed confirm that the beam path through ruby was not scattering. Below are the results of that test.

Tested with 5 mW HeNe:

nmRel Intensity

Looking at the following diagram ...

It's amazing I've gone for over a decade without noticing this. The light from the beam path is obviously not the result of scattered 632.8nm laser light from the He-Ne laser (Sam says the 632nm measurement might be due to a reflection). I think it's safe to say that the measurements of 694.1nm and 692.5nm are in fact the same wavelengths as 694.3nm and 692.7nm respectively (interestingly, 692.7 is apparently a lasing wavelength based upon the linked diagram). The margin of separation is the same - 0.2nm. Either the chart is off by 0.2nm or the spectrometer is off by 0.2nm. Either way, the results are quite conclusive.

As a side note - 694.3nm is the common wavelength associated with ruby. I wonder if both 694.3 and 692.7 are simultaneously present in a basic working laser. Although I know of only one additional laser wavelength in ruby, the difference is significant enough to rule out entirely (628nm). I do think it would be obvious if 628nm were present in a basic or hobby-level working ruby laser, so I think it's safe to assume that the 628nm line lases under a significantly different set of requirements.

One question remains: is the 632.8nm He-Ne beam pumping chromium ions directly to the metastable energy level E2, or is it pumping them to some intermediate energy level between E3 and E2, before decaying to E2 from this intermediate energy level?

If there is an intermediate level, it leads me to a possible conclusion - perhaps E3 and E4 just happen to be energy levels capable of storing enough energy to populate E2 to the lasing threshold, rather than being the only energy levels capable of transferring energy to E2.

I was thinking about florescence on common surfaces - fluorescence under UV or blacklight is obvious because invisible wavelengths are resulting in fluorescence at visible wavelengths. A green laser pointer can cause yellow, orange or red fluorescence to occur in dyes, household products and upon surfaces. But fluorescence resulting from red light will be at red or infrared wavelengths (with red being hard to distinguish and infrared being invisible). If anything has ever fluoresced under red light, in the last 35 years that I have had access to a He-Ne laser, I wasn't aware of it. I think I might have possibly made the mistake of overlooking the reality or not realizing the possibility that fluorescence can occur between two closely spaced wavelengths - something that is perhaps easy to overlook and not readily discernible.