Get started with a Krypton Ion OC, or gulp, a Alexandrite / or low nm range Ti Saph OC, but Alex has far more gain than ruby, so the transmission may be too high.
676 is not far from 694, usually Ion laser OCs have a design bandwidth and moderate transmission slope flatness at least 30 nm outside their design range.
Otherwise, you are looking for un-obtainium or a custom coated optic.
Like from these folks:
About us | Markets, Portfolio, Production & References (pgo-online.com)
They are one of the few who will run a single optic. At least they used to claim one offs.
One of the problems of asking for custom coatings, is if you don't know the single pass gain, desired radius, and stored energy details of your rod, no one will want to coat for you as a hobbyist. Been there, done that, and have the Tee Shirt. Many optics producers have been sued when cavity optics design goes wrong, thus ending with damage on a high power rod, so... usually the answer is NYET!
A long radius, hard coated, SP 124 or 125 sized , HENE OC mirror might work, but certainly would not have much transmission and may not release enough energy to protect the rod. Unless you have a Q-Switch, you may find that long pulses and Ruby rods don't get along well, due to heating of the faces / thermal lensing. Keep the rep rate down to 10s of seconds, if not minutes, or more when testing, else thermal lensing/breaking . Al2O3 does not conduct heat well.
Random mirrors won't help you. This takes some calculations, and, in your case, assumptions on your rod's original use, was it an oscillator or amplifier when designed? Doping will be less for the amplifier rod.
You can search Ebay for Ruby parts / optics from JK Lasers in Rugby UK till the Cows go home, and not ever find one. I worked on a one oscillator two amplifier JK for a few months maybe 10 (or was it 110?) years ago.
Good luck, that is a TOUGH problem. How long is this rod?
There is this guy, Anderson Lasers, Inc - Main Page , he liked oddball things like Ruby...
Steve
Last edited by mixedgas; 06-22-2022 at 18:00.
Qui habet Christos, habet Vitam!
I should have rented the space under my name for advertising.
When I still could have...