  We have implemented a simple and cost-effective digital system for long-term
frequency stabilisation and locking to an arbitrary wavelength of the
single-frequency ring CW Ti:Sapphire laser. This system is built around two
confocal Fabry-Perot cavities, one of which is used to narrow short-term line
width of the laser and the other to improve long-term stability of the laser
frequency. The second interferometer is also in the path of the radiation from
an external-cavity diode laser stabilized to an atomic transition. Our system
is an extension of a commercial Tekhnoscan laser lock. This system has been
successfully used in experiments on high-resolution laser spectroscopy of
ultracold Rydberg atoms.
