10$^{-15}$-level laser stabilization down to fiber thermal noise limit using balanced photodetection

We demonstrate a self-homodyne detection method to stabilize a continuous-wave 1550-nm laser to a 1-km optical fiber delay line, achieving a frequency instability of 6.3x10^-15 at a 16-ms averaging time. This result, limited by fiber thermal noise, is achieved without the need for a vacuum system, highlighting the potential of our approach for ultra-stable laser systems in non-laboratory environments. The system utilizes only a few passive fiber optic components and a single balanced photodetector, significantly simplifying the laser stabilization process while maintaining high performance. The entire optical setup is compactly packaged in a portable metal air-tight enclosure.