An Ultra-Coherent Semiconductor Laser Locked to a Short Delay Fiber Mach-Zehnder Interferometer With Active Fiber Noise Cancellation

We propose and demonstrate a technique to vastly reduce the linewidth of a commercially available semiconductor laser operating at 1550 nm by locking it to an all-fiber Mach-Zehnder interferometer (MZI) with a short path length imbalance. The long term linewidth was reduced from 20 kHz to the single and even sub Hz level. Very narrow linewidths can only be measured by beating two separate, nominally identical lasers. To this end, we employ two lasers configured in a unique, counter propagating configuration, both locked to two different fringes of the MZI. The beat between the two locked lasers proves the extremely narrow linewidth of each laser. In order to detect and compensate for the noise caused by mechanical and thermal fluctuations of the fibers, we use the beat signal as an input to a servo loop that feeds a piezo electric fiber stretcher stabilizing the MZI.