  Optical frequency combs provide the clockwork to relate optical frequencies
to radio frequencies. Hence, combs allow to measure optical frequencies with
respect to a radio frequency where the accuracy is limited only by the
reference signal. In order to provide a stable link between the radio and
optical frequencies, the two parameters of the frequency comb must be fixed:
the carrier envelope offset frequency $f_{\rm ceo}$ and the pulse
repetition-rate $f_{\rm rep}$. We have developed the first optical frequency
comb based on difference frequency generation (DFG) that eliminates $f_{\rm
ceo}$ by design - specifically tailored for applications in cold atom physics.
An $f_{\rm ceo}$-free spectrum at 1550 nm is generated from a super continuum
spanning more than an optical octave. Established amplification and frequency
conversion techniques based on reliable telecom fiber technology allow
generation of multiple wavelength outputs. In this paper we discuss the
frequency comb design, characterization, and optical frequency measurement of
Sr Rydberg states. The DFG technique allows for a compact and robust, passively
$f_{\rm ceo}$ stable frequency comb significantly improving reliability in
practical applications.
