Dual-slope linear optical frequency discriminator for flexible, high performance frequency modulated direct detection (FM-DD) microwave photonics links

We present a linear optical frequency discriminator (FD) filter with (i) selectable dual-slope functionality and (ii) switchable slope (positive or negative) signs for flexible frequency-modulated / phase-modulated direct-detection (FM/PM-DD) microwave photonics links (MPLs). The FD uses only two Mach-Zehnder interferometers (MZIs) and one Y-branch combiner. When the optical path-length differences (OPLDs, ΔL2 = − ΔL1) of the two MZIs have equal magnitude but opposite sign, the FD filter (refer to as FD-1) has a linear slope value of −0.63 and a corresponding full bandwidth utilization (BWU). On the other hand, when the two OPLDs (ΔL2 = ΔL1), have equal magnitude and same sign, the FD filter (refer to as FD-2) has a linear slope value of −1.23 but its BWU is reduced to ∼ 0.4 BWU. The second dynamic feature, switchable slope (positive or negative) sign is achieved if the both OPLDs, ΔL2 and ΔL1 are made tunable. Both slopes have excellent linearity performance when the power split ratio r of the Y-branch combiners is set to 0.113. The link gain is enhanced by the increase in the slope value. The dual-slope FD filter opens up the possibility of a flexible and dynamic MPLs and systems.