Affection of optical phase-locked loop on the coherence properties improving of laser

In order to achieve the coherent combination of independent lasers by using optical phase-locked loop (OPLL), the working mechanism of OPLL is analyzed. By calculating the autocorrelation function and the cross-correlation function of lasers, the performance of phase-locking system has been analyzed while a non-ideal laser is employed as the reference laser. The results prove that the coherence of an ordinary laser can be improved obviously by using an OPLL. Furthermore, some main parameters of the phase-locking system have been discussed in detail to provide a theoretical basis for the design of the OPLL system. The results show that if the linewidth of an ordinary laser is 2 MHz, to meet the requirement of efficient coherent beam combining, the time constant of OPLL should be less than 10 ns and the linewidth of reference laser ought to be less than 20 kHz.