  We propose a method to continuously frequency shift a target laser that is
frequency stabilized by a reference laser, which is several hundreds of
nanometers detuned. We demonstrate the technique using the 5S 1/2 to 5P 3/2 to
29D 5/2 Rydberg transition in 87Rb vapor and lock the 482 nm target laser to
the 780 nm reference laser using the cascaded electromagnetically induced
transparency signal. The stabilized frequency of the target laser can be
shifted by about 1.6 GHz by phase modulating the reference laser using a
waveguide-type electro-optical modulator. This simple method for stable
frequency shifting can be used in atomic or molecular physics experiments that
require a laser frequency scanning range on the order of several GHz.
