Recently, the Department of Space and Astronautical Laser Technology and Systems of Shanghai Institute of Optics and Precision Machinery (SIPM), Chinese Academy of Sciences (CAS), has made important progress in the research of fiber interferometer laser frequency stabilization. For the first time, the research group adopts different polarization axes of a polarization preserving fiber to construct a dual interferometer frequency stabilization system, which is used for locking the laser frequency and compensating the frequency fluctuation caused by the fiber temperature by taking advantage of the different responses of the phase shifts of the two polarization components to the temperature, respectively. The results are published in Optics Letters under the title "Temperature insensitive FDL-stabilized laser using a PMF-based dual interferometer". The results were published in Optics Letters.
The application of ultra-stabilized lasers in the field of precision measurements places increasing demands on the performance of lasers. All-fiber frequency-stabilized lasers based on fiber delay lines have attracted attention due to their high compactness and reliability, and their ability to achieve fast broadband frequency tuning. Today the short-term frequency stability of such ultra-stabilized lasers is mainly limited by the intrinsic thermal noise of the fiber, while the long-term stability deteriorates rapidly due to temperature perturbations. Vacuum multilayer heat shielding and multistage temperature control measures are more often used to suppress temperature disturbances, which increase the complexity of the system and thus limit the wide application of frequency-stabilized lasers, and new approaches are urgently needed to solve this problem.
Fig. 1 Schematic diagram of dual interferometer frequency stabilized laser
Bias-preserving fibers can simultaneously transmit beams with two polarization states orthogonal to each other and keep the polarization state of the transmitted light stable. Since the fast and slow axes of a bias-preserving fiber have different thermo-optic coefficients, they respond differently to temperature. The team exploited this property by using the fast and slow axes of the bias-preserving fiber to transmit laser light simultaneously, forming a two-way fiber interferometer with different parameters. The laser frequency is locked to one of the interferometers, and fluctuations in the fiber temperature cause changes in the interferometer's optical range, which in turn causes fluctuations in the frequency of the stabilized laser. The phase difference signals extracted from the two interferometers can be characterized as fluctuations in the optical range difference of the laser transmission in the two polarization directions of the fiber, which are highly correlated with the temperature changes in the fiber path. Using the extracted phase difference signal to compensate for the frequency variation of the frequency-stabilized laser can suppress the frequency fluctuation caused by the same temperature fluctuation by a factor of more than 25. In this way, the temperature sensitivity of the frequency-stabilized laser can be significantly improved, the long-term frequency stability can be enhanced, and the fiber interferometer frequency-stabilized laser can be promoted to be used in the detection of gravitational waves in space and other fields.
Figure 2 Frequency fluctuation (a) and frequency stability (b) before and after compensation of frequency-stabilized laser
