We present a novel locking scheme for active length-stabilization and frequency detuning of a cavity optomechanical device based on the optical spring effect. The error signal is generated by utilizing the position measurement of a thermally driven intra-cavity nanomechanical device and employing its detuning-dependent frequency shift caused by the dispersive coupling to the cavity field. The scheme neither requires external modulation of the laser or the cavity nor does it demand for additional error signal readout, rendering its technical implementation rather simple for a large variety of existing optomechanical devices. Specifically, for large-linewidth microcavities or in situations where other locking schemes appear unfavorable conceptually or are hard to realize technically, the optical spring lock represents a potential alternative for stabilizing the cavity length. We explain the functional principle of the lock and characterize its performance in terms of bandwidth and gain profile.
REFERENCES
Our SiN trampoline oscillator was manufactured by Norcada.
Note that this VCR is thermally driven opposed to a VCO that is operated in saturation and thus exhibits limit cycle oscillations to reject amplitude noise.
HF2LI was from Zurich Instruments.
Gdet (Gbp) denotes the output phase (frequency) modulation index vs input frequency modulation index as a function of the modulation frequency.
The measurement also contains the minus-sign (+180°) of the negative feedback. It was subtracted out as it is explicitly not contained in Gdet.
In our case, the linewidth Γm = 10.8 mHz can cause cavity length noise on the order of 4.5 fm and thus does not limit the precision of our lock.
