a dual - output mach - zehnder modulator and configurations of optical transmitters based on the mzm2 which obviate the optical splitter that is typically used to provide keep - alive functionality in redundantly - connected , protected optical networks , sonet add - drop multiplexers , and optical ip routers . the configurations provide equivalent keep - alive utility at a reduced system cost relative to the prior art . the configurations also support enhanced operations , administration , maintenance , and provisioning functionality at little to no additional cost relative to the prior art . instead of being a direct copy of the service signal , the keep - alive of this configuration is an inverted version of the service signal . this inverted version of the service signal is supported at a client by utilization of means for detecting and righting the inverted signal . the inverted state of the signal on the protection path can be used as an inband indicator to notify the client in the event of a failure on the service path . this inband indicator requires little or no additional circuitry to accommodate . another aspect of this invention allows the provisionable substitution of the inverted signal with a preemptive signal to support low - priority traffic when there is no fault on the service path s .