A Question about the EOM used for PDH locking

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When locking a ~30 kHz linewidth laser to a ~100 kHz linewidth ultrastable cavity using PDH locking, the modulation frequency of the EOM typically ranges from 10-20 MHz. Both fiber and free space EOMs can achieve this modulation, but the choice may impact the locking performance and resulting linewidth. The discussion centers on whether a fiber EOM or a resonant free space EOM provides advantages in achieving a better lock and lower linewidth. Power losses during fiber coupling are not a concern, emphasizing the importance of modulation characteristics and stability. Ultimately, the decision should consider the specific requirements of the laser system and desired locking precision.
Malamala
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Hello! I am locking a ~30 kHz linewidth laser to a ~100 kHz linewidth ultrastable (ULE) cavity, aiming to reduce the laser linewidth to ~ 1 kHz using PHD locking. The feedback signal is sent to the laser current (it is an external cavity diode laser). For PDH the modulation of the EOM used to create the sidebands is usually 10-20 MHz, which can be achieved either with a fiber EOM or with a resonant free space EOM (my power is very small - few mW around 700 nm). Is there any advantage to using a fiber vs a free space EOM? Power losses during fiber coupling are not an issue, my question is mainly focused on whether one of these options allows a better lock and hence a lower linewidth.
 

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