  Decoherence of Josephson qubits can be substantially reduced by tuning their
parameters to optimal operation points, with only quadratic coupling to
fluctuations. We analyze dephasing due to 1/f noise for a two-level system,
detuned from an optimal point, i.e., the crossover to the linear-coupling
regime, both for free induction decay and for spin-echo experiments. Influence
of several noise sources is also discussed.
