TIPRx: A Transmit-Isolating Photonic Receiver

In this paper, we present a new type of photonic interface that is capable of conveying a receive signal from an antenna while simultaneously conveying a transmit signal to the same antenna. We call this interface a TIPRx, which is short for transmit-isolating photonic receiver. We first present the TIPRx theory of operation, and then focus on the key figures of merit: the degree of suppression of the transmit signal at the receive port, which is known as the transmit-to-receive (Tx/Rx) isolation, and the bandwidth over which this isolation is achieved. For a TIPRx using a conventionally designed Mach-Zehnder modulator that has a constant interaction between the modulator electrodes and the optical waveguides, we find that the Tx/Rx isolation has a sin(x)x response. We then show that the Tx/Rx isolation can be significantly enhanced, at only a modest decrease in receive noise figure, by tapering the interaction between the modulator electrodes and optical waveguides. Finally, we report on experimental results of some TIPRx prototypes, which have achieved better than -30 dB of Tx/Rx isolation from 2.5-20 GHz and better than -40 dB from roughly 10-20 GHz.