Development And Application Of Optical Communication Band Narrow-linewidth Lasers

Optical frequency transfer via optical fibers is the highest accuracy frequency transfer means. 2014, Germany MPQ-PTB joint team to achieve a transmission stability of the order of 1E-20 via the 1000 kilometers field fiber, for the future use of optical fibers for long-distance optical clock than the technical basis. In the research of optical fibers Frequency transfer, utilizing the narrowlinewidth laser as the laser source of optical frequency transfer, is one of the keys to achieve ultra-high precision frequency transfer. Compatible with existing optical communication network is an inevitable choice of extending the scope of fiber optic frequency transfer application. Therefo re, the research of ultra-narrow linewidth lasers at optical communication wavelengths have important applications in optical frequency transfer via optical fibers. So, surrounding the Optical frequency transfer demand for the light source, We report the recent development of a 1550 nm stable laser system and its preliminary application in optical frequency transfer via laboratory fibers.In the experiment, the conventional Pound-Drever- Hall(PDH) frequency stabilizat io n 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, with a finesse of 344000 and 296000 respectively. The frequency of the laser source is actively stabilized to the first reference cavity by piezo and external frequency shifter simultaneously and the total control bandwidth is measured to be 50 k Hz. 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 applied to bring the first laser beam to the second reference cavity which unfortunately induced unexpected phase noise by environmental distortions. The laser linew id t h broadened is determined to be 0.27 Hz by the beat note measurement between the input and output beam 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 2.7Hz and a frequency stability of 2.5×10-14/s, which corresponding to a single laser linewidth of 1.9Hz and 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 kmlong fiber spool, while a frequency transfer stability of 2.4×10-16/s is achieved by similar setup except that the laser source is a k Hz-level linewidth lasers. 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, include the miniaturization of the optical setup, optimization of the control bandwidth and shorten of the response time of control loop.