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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, ΔL<inf>2</inf> = − ΔL<inf>1</inf>) 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 (ΔL<inf>2</inf> = ΔL<inf>1</inf>), 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, ΔL<inf>2</inf> and ΔL<inf>1</inf> 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.