Natural Ruby Lasers
Natural rubies haven't been used in lasers,
it is not a suitable laser material due to many factors:
Iron Impurities
When you shine a bright green or violet or ultraviolet light on a laser
grade sample of ruby, it will fluoresce bright red at 693.4 nanometers.
According to Nassau (1980), most natural rubies contain too many iron
impurities which quench the red fluorescence from the chromium ions.
I can just imagine myself sneaking in a portable UV blacklamp to shine on
multi-million dollar rubies on public display in various museums vaults
;-)
Defects
According to Schawlow (1961) defects (imperfections) in synthetic ruby
limits the effective lasing volume so that less than 10 % of the area at
the surface of cylinder produces laser action.
Although ruby manufacturing techniques have improved since then,
natural rubies have many more defects and would probably have an even
smaller effective lasing volume (if at all). Ruby crystal is non-isotropic
i.e. the index of refraction and other properties depend on
orientation.
Any defects or grain boundaries could potentially split the beam into
multiple components or have other deleterious effects which could
preclude lasing.
Cylinders too Short
Ruby is the rarest form of corundum and the probability of finding defect
free specimens longer than a certain length is low. The length of the
amplifying medium is critical to laser design;
mirrors
are added at the ends to increase the 'effective' path through the gain medium.
Below a certain threshold length, laser action is unlikely due to
a combinations of factors including various loss mechanisms versus
density of chromium ions and realistic optical pump power densities.
The first ruby laser was 0.05 percent Cr (by weight) and during a single pulse
of the flash lamp all the ions were raised to the excited metastable
upper laser state. Since the laser medium was 'saturated' or maximally
charged, in this case increased pump power would not have compensated
for reductions in length. Shawlow (1961) was able to successfully increase
the Cr ion dopant concentration by factor of 10 and still obtain
laser action. However the pump power photon density must correspondingly
be raised to produce a population inversion. Which raises another
concern; thermal dissipation capabilities.
Power Dissipation
Natural ruby is often polycrystalline, and according to theory the thermal
conductivity can be up to 10 times lower than for single crystals.
Although ruby has one of the highest melting points, laser action begins
to suffer even at moderately warm temperatures due to the thermal
deactivation of the upper laser level by excessive photon collisions
(sound quanta which carry heat) This thermal competition significantly
reduces population inversions. Therefore the crystal must remain below a
certain temperature for efficient laser action. The lower cooling rate
due to the low thermal conductivity translates to a much lower pulse
repetition rate. This allow the coolant to remove the heat generated by
the non-radiative transitions from the pump absorption level (4T1,4T2)
to the upper laser level (E).
Conclusion
Even if by luck you could obtain a sample of natural ruby which
was relative free from defect and impurities and you managed
to get it to lase, the output power would be much lower than
theoretically possible by at least an order of magnitude.
(due to the low pulse repetition rate, smaller dimensions etc...)
You certainly could not get it to operate in continuous mode (CW).
Alternatives
The chromium ion within the sapphire crystal matrix can be used in a
three level laser scheme. Contemporary lasers have drastically reduced
pump power requirements because they use the more efficient four level
laser scheme. With higher efficiency comes higher output power due to
better cooling. Also continuous mode (CW) operations becomes practical and
inexpensive. The neodymium ion operates as a four level laser based on
perturbed 4f electrons, because this electron shell is well shielded the
host material can be anything from crystalline YAG (Yttrium Aluminum Garnet)
to glass. In chromium the 3d levels are less shielded which is the main
reason why ruby lasers are prey to lattice defects.
The best solid state laser in term of efficiency, durability and
tuneability is the titanium sapphire laser (Ti:Al2O3) pumped by
diode laser(s). It seems that we have come full
circle back to corundum (sapphire) as the ideal solid host material !
References
- Ruby: Variety of sapphire doped with chromium.
- Other names: corundum, aluminum oxide.
- Chemical Structure: Al2O3 + Cr
- Properties: The most common form is polycrystalline and because it has a of
hardness 9 (diamond is 10) it is used in abrasives (emery paper etc...)
- Nassau,K.: 1980, Scientific American, Oct, p.124
- Shawlow,A.L.: 1961, Scientific American, Jun, p.52
Back to Lasers in the Movies