  The Pound-Drever-Hall (PDH) technique is a popular method for stabilizing the
frequency of a laser to a stable optical resonator or, vice versa, the length
of a resonator to the frequency of a stable laser. We propose a refinement of
the technique yielding an "infinite" dynamic (capture) range so that a
resonator is correctly locked to the seed frequency, even after large
perturbations. The stable but off-resonant lock points (also called Trojan
operating points), present in conventional PDH error signals, are removed by
phase modulating the seed laser at a frequency corresponding to half the free
spectral range of the resonator. We verify the robustness of our scheme
experimentally by realizing an injection-seeded Yb:YAG thin-disk laser. We also
give an analytical formulation of the PDH error signal for arbitrary modulation
frequencies and discuss the parameter range for which our PDH locking scheme
guarantees correct locking. Our scheme is simple as it does not require
additional electronics apart from the standard PDH setup and is particularly
suited to realize injection-seeded lasers and injection-seeded optical
parametric oscillators.
