  We present a cascaded system consisting of three non-degenerate optical
parametric amplifiers (NOPAs) for the generation and the enhancement of quantum
entanglement of continuous variables. The entanglement of optical fields
produced by the first NOPA is successively enhanced by the second and the third
NOPAs from -5.3 $dB$ to -8.1 $dB$ below the quantum noise limit. The dependence
of the enhanced entanglement on the physical parameters of the NOPAs and the
reachable entanglement limitation for a given cascaded NOPA system are
calculated. The calculation results are in good agreement with the experimental
measurements.
