Experimental evidence of a fundamental noise floor at the tens of millihertz level in laser locking onto unbalanced fibre-based Michelson interferometer

摘要

Summary form only given. Large arm length imbalance fibre-based interferometers have shown great potential for laser frequency stabilization and control, with frequency noise power spectral density close to the 10-1 Hz/Hz1/2 level [1]. In order to understand the performance of such frequency stabilization systems, it is important to distinguish the intrinsic noise of the frequency reference, which is ultimately limited by fibre thermal noise, and the locking noise, which is limited by the detection noise but can also be affected by residual amplitude modulation fluctuations in phase modulator or other less well known causes. It is often difficult to measure the out-of-loop locking noise when one laser is locked to a frequency reference. However, the fibre length noise practically cancels when two lasers are locked onto the same interferometer with a small frequency difference, which allows to measure the out-of-loop locking noise to a very low level [2].To prove this hypothesis, we have performed numerical simulations to calculate the effect of the nonlinearity of the optical frequency discriminator on the error signal. The results show indeed a white frequency noise with a level in excellent agreement with the experimental measurement. Thanks to the simulation we have been able to derive relations between the residual laser frequency noise and this white noise level. Those relations can be very useful to optimize the choice of the fibre spool length as a function of the frequency noise of the laser to be locked, in order to minimize this noise.