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

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.

Injection-seeded high-power Yb:YAG thin-disk laser stabilized by the Pound-Drever-Hall method

“…During pulse extraction, PD PDH saturates so that the lock is not functional for ca. 40 µs after pulse extraction [26]. Once the photodiode has recovered, the resonator always re-locks correctly on resonance to the seed frequency thanks to our modified PDH scheme and is immediately ready for the next trigger.…”

“…injectionseeded) by stabilizing the resonator length to the seed laser (Toptica DLPro at 1030 nm) frequency via PDH locking. We use a modified PDH-scheme that guarantees automatic locking on resonance even after large perturbations [26]. This is achieved by modulating the phase of the seed laser light at half the free spectral range (FSR) of the TDL resonator to obtain the error signal shown in Fig.…”

“…3. This error signal has only one zero crossing between two neighboring resonances and is thus free from "Trojan" lock points [26]. This allows the servo-controller to always re-lock the TDL resonator correctly on resonance.…”

“…The TDL technology was chosen mainly since it allows short pulse build-up times, large pulse energies and good beam quality, as shown in earlier work of our group [22][23][24][25]. The design of our new TDL presented here is based on three key innovations: 1) A PDH-based injection-seeding process, modified so that the TDL resonator is locking correctly on resonance with the seed even after large perturbations [26]. This makes the injection-seeding process very robust.…”

A compact 20-pass thin-disk multipass amplifier stable against thermal lensing effects and delivering 330 mJ pulses with M² < 1.17

“…In this system a single-frequency Yb:YAG thin-disk laser (TDL), composed of an oscillator followed by the here presented amplifier, is used for pumping mid-IR optical parametric oscillators (OPOs) and optical parametric amplifiers (OPAs). The input laser pulses to the amplifier are provided by an injection seeded TDL oscillator which is frequency stabilized via the Pound-Drever-Hall method [23,24]. The TDL oscillator delivers single-frequency pulses of up to 50 mJ energy and a pulse length between 50-110 ns with nearly diffraction limited beam quality.…”