Machined out of ULE (ultra-low expansion) glass, this special design of Jun Ye and Mark Notcutt at JILA (Colorado) could very well turn our 2051 nm Tm,Ho:YLF laser into a one-Hertz wide Monster of Coherence.
How does it work?
Its length, kept extremely constant, defines a set of Fabry-Perot fringes at fixed frequency which are used in a feedback loop to narrow the linewidth and cancel the drift of a noisy laser.
How is the length kept so constant?
First, the material is chosen to have a coeffecient of thermal expansion of zero a few degrees above room temperature. This temperature is found experimentally and then heavily regulated. Passive control is maximized by surrounding the cavity with an aluminum heat shield, and mounting it on insultating teflon rods in a vacuum chamber kept well below 10-9 Torr (all shown here). The chamber itself will be housed in an insulated aluminum box which is temperature controlled to about 1 mK.
Second, all that passive sheilding goes a long way in reducing the coupling of vibrations on the floor and table into the cavity which would otherwise compress and expand the cavity's length dynamically, impacting any laser locked to it.
Third, the key innovation is the fact that the cavity is vertically supported at the exact midplane: there is an equal amount of mass above the supports as is below (to a percecnt or so). It turns out that vertical shocks will therefore not, to first order, change the bore length.
