Carrier envelope phase (CEP) stabilization has proven an indispensable tool in attosecond science [1, 2] and precision frequency metrology . Although fully CEP stabilized mode-locked fiber lasers and Ti:sapphire lasers are commercially available, CEP stabilization and all-optical frequency synthesis are still far away from general applicability to any given mode-locked laser. Stabilizability ultimately relies on the availability of at least 10 photons per pulse in the detection scheme . Astro-combs, e.g., are rather demanding in this regard as they require ultra-high repetition rates. Moreover, despite their high pulse energies, thin-disk lasers have proven notoriously difficult to stabilize. Here we demonstrate that introduction of active optical gain into the infrared arm of an f-to-2f interferometer  can boost the signal by up to 20 dB, making stabilization of notoriously unstabilizable mode-locked lasers possible, e.g., high repetition-rate oscillators or thin-disk lasers. Our experimental setup is illustrated in Fig. 1(a).
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