Pound-Drever-Hall locking scheme free from Trojan operating points

TL;DR

This paper introduces a refined Pound-Drever-Hall locking scheme that eliminates Trojan operating points, enabling stable laser locking over large perturbations without extra electronics.

Contribution

The authors propose a novel PDH locking method that removes Trojan operating points by phase modulating at half the free spectral range, enhancing robustness and simplicity.

Findings

Successfully implemented on a Yb:YAG laser

Achieved stable locking after large perturbations

Analytical formulation of error signals provided

Abstract

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.