The effect of laser frequency drift on the response of Phase-sensitive Optical Time-domain Reflectometer

Characterization of Laser Frequency Stability by Using Phase-Sensitive Optical Time-Domain Reflectometry

“…This phase difference varies along the fiber length and allows one to track in time variations in fiber temperature δT, strain δε, and probing laser frequency [22][23][24][25][26][27][28][29]. The overall variation in phase difference is given by the formula…”

“…Here, δΦ Rayleigh are random phase jumps caused by the interference fading [10,[23][24][25][26][27][28][29], α T , α ε , α ν are thermal, strain, and frequency drift coefficients, and δν is a phase shift due to laser frequency change between the probing pulses. In the case of laser white frequency noise, δϕ las is a random quantity with zero mean and standard deviation of √ γ2πτ d ∆ν L determined by laser linewidth and delay time between the pulses in probing pair; the γ factor depends on the DP-ϕOTDR parameters.…”

“…The frequency drift coefficient is proportional to time delay between the pulses α ν = 2πτ d . It was shown in [29] that probe pulses carrier frequency shifts lead to the uniform phase difference changes over the entire fiber length. Taking τ d = 300 ns and λ = 1550 nm, we obtain α T = 3300 rad•K −1 , α ε = 0.25 rad•nanostrain −1 , and α ν = 1.9 rad•MHz −1 .…”