STOCHASTIC AMPLITUDE FLUCTUATION IN COHERENT OPTIMAL TIME-DOMAIN REFLECTOMETRY AND ITS REDUCTION TECHNIQUE

When the coherent detection technique is employed in optical time doma in reflectometry (OTDR) (C-OTDR), enhancing the dynamic range is expec ted to be an effective means of improving the minimum detectable signa l level to that of shot noise. In C-OTDR, however, there are fluctuati ons in the OTDR trace, and the problem is the degradation of the resol ution of the loss measurements. The main reasons for such amplitude fl uctuations are: (1) scattering light-powerjags due to the interference between Rayleigh backscattered lights; (2) variations of the heterody ne detection efficiency due to rotations of Rayleigh backscattering po larization; and (3) variations of the detection efficiency due to phas e fluctuations of Rayleigh backscattering light. In this paper, such f luctuations are evaluated stochastically, and the techniques for their reduction are proposed. With regard to cause (1), the amplitude fluct uations can be reduced by integration over the frequency of light, whi le with regard to cause (2), the improvement can be achieved by polari zation diversity detection or integration over the polarization. In ca se (3), phase diversity detection or phase integration is effective in reducing the amplitude fluctuations. Experimental investigations with regard to cause (1) are conducted in detail, and it is shown that the amplitude fluctuations can be reduced by about 3 to 7 times as a resu lt of integration in the frequency domain compared to those without su ch integration.