I. INTRODUCTION
Bidirectional transmission on a single fiber has been proposed for cost-sensitive metro and access networks [1]. BOADM structure. A bidirectional optical add–drop multiplexer (BOADM) is the key element in realizing such bidirectional networks. Important requirements for BOADMs in metro networks are low cost, good performance, and high flexibility. Most previously reported BOADMs are for networks with wavelength interleaving [2] or wavelength band allocation schemes [1], [3] and/or contain many components to achieve bidirectional add–drop functionality and bidirectional gain [1]–[5].