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We demonstrate a novel and flexible InP monolithically integrated optical circuit with multiple applications. The circuit is an interferometric delay loop and its particular configuration allows to perform a number of different functions, namely, optical buffering, differential phase-shift keying (DPSK) demodulation, intensity modulation, and differential XOR logic operation. By properly controlling the current supplied to the active elements in the circuit loop (i.e., a semiconductor optical amplifier and a variable optical attenuator), the relative phase of the propagating signals can be adjusted, changing the interference condition between the input signal and its delayed copies. In this way the four different functions are enabled. Experimental results are reported for all the different functions of the device. When the circuit is operated in buffer configuration, up to 13 circulations (corresponding to a 1.62 ns delay) of an input bit at 12.5-Gb/s data rate are shown before the output signal degrades due to excessive ring losses. Its behavior as a current-controlled DPSK demodulator is demonstrated for 8-Gb/s signals, and the resulting bit error rate measurements are compared with those of a thermally tuned commercial demodulator, showing no significant power penalty. A proof-of-concept demonstration as an intensity modulator is reported for relatively low-frequency signals at 10 and 20 MHz, being limited by electrical components of the setup. Finally, error-free operation for a 1-, 2-, and 4-b differential XOR logic gate has been demonstrated at 8, 16, and 32 Gb/s, respectively.