  The noise in optical source needs to be characterized for the security of
continuous-variable quantum key distribution (CVQKD). Two feasible schemes,
based on either active optical switch or passive beamsplitter are proposed to
monitor the variance of source noise, through which, Eve's knowledge can be
properly estimated. We derive the security bounds for both schemes against
collective attacks in the asymptotic case, and find that the passive scheme
performs better.
