Development and application of communication band narrow linewidth lasers

Ultra-stable lasers at optical communication wavelengths have important applications in developing optical frequency transfer via optical fibers. We report the recent development of a 1550 nm stable laser system built at National Time Service Center and its preliminary application in optical frequency transfer via laboratory fibers. In the experiment, the conventional Pound-Drever-Hall(PDH) frequency stabilization technology is implemented to achieve the ultra-stable laser at the wavelength of 1550 nm. The output of a single laser source is split and locked onto the resonant frequency of two independent reference cavities, of 344000 and 296000 respectively. The frequency of the laser source is actively stabilized to the first reference cavity by piezo and external frequency shifters simultaneously and the total control bandwidth is measured to be 50 kHz. Then the laser frequency is shifted and stabilized to the second reference cavity by an acousto-optical modulator. A 5 m long single-mode fiber is used to bring the first laser beam to the second reference cavity which unfortunately induces unexpected phase noise by environmental distortions. The laser linewidth broadened is determined to be 0.27 Hz by the beat note measurement between the input and output beams of the fiber. To evaluate the frequency stability of the laser, the frequency control signal within the control bandwidth of the second stable laser system is analyzed by a spectrum analyzer and a frequency counter. The control signal shows a Lorentz linewidth of 2.7 Hz and a frequency stability of 2.5 +/- 10(-14) /s, corresponding to a single laser linewidth of 1.9 Hz with a frequency stability of 1.7 +/- 10(-14) /s if the two stable lasers have similar frequency stability. Applying this ultra-stable laser system as the laser source for the fiber-based optical frequency transfer, a short-term frequency transfer stability of 7.5 +/- 10(-17) /s is demonstrated through a 50 km-long fiber spool, while a frequency transfer stability of 2.4 +/- 10(-16) /s is achieved by a similar setup except that the laser source is a kHz-level linewidth laser. In the experiment an Agilent 53232 A frequency counter is applied to record the beat note signal in the auto mode. In the end, we discuss the possible improvements of the stable laser system, including the miniaturization of the optical setup, optimization of the control bandwidth and shortening of the response time of control loop.