The Challenge of Microwave Photonics
Next generation signal processing is required for improved data rates in telecommunication networks. Microwave photonics (MWP) attempts to address this limit by combining radio-frequency engineering with opto-electronics, with integrated photonics being a particularly popular approach for its implementation. The integrated microwave photonics field faces a two-front challenge: on one end, it must implement signal processing functions that typically require sizeable devices on very small chips. This integration is meant to reduce infrastructure size, weight, power usage, and cost. On the other end, it must preform those functions within contemporary communication network standards, such as low noise figures. The more functions are integrated on a single chip, the lower the size, weight, power usage, and cost of the network infrastructure. Simultaneously, the harder it is to keep the signal pure: each function requires its own set of structures and features on the chip, which add reflections, dissipation, and other sources of error, to say nothing of its fabrication complexity and reliability.
The typical approach thus far has been, for example, to implement modulation of wave phase and amplitude on one photonic chip containing a circuit designed for that function, while another chip might act as a notch filter for that modulated output of the first. Single chip implementation of all these functions at a workable noise figure was the always a task for future research. This week, the Nonlinear Nanophotonics group of the University of Twente published a paper in Nature Communications showing how they implemented exactly such a chip .
The chip was integrated on our ultra-low loss silicon nitride platform, TriPleX®. It is composed of two main sections: a universal signal modulator, and a universal filter. The modulator utilises a clever combination of a sophisticated asymmetric Mach-Zehnder interferometer, a tunable attenuator, and a phase shifter, which allows for intensity-to-phase as well as phase-to-intensity modulation at impressive extinction figures. The filter was implemented via a versatile double-injection ring resonator programmable for the execution of notch, bandpass, Fano-like, and all-pass filtering, all at narrow linewidths around 400 MHz and 20 GHz free spectral range. Using the asymmetric double stripe TriPleX® geometry, propagation losses in the circuit were kept to a minimum of 0.1 dB/cm, with 1.1 dB/facet losses at the fiber-to-chip interface. This work constitutes the first time that simultaneous RF notch filtering and linearization were demonstrated on the same chip, allowing for a record-high spurious-free dynamic range of 123 dB.Hz4/5.
This was the brilliant doctoral work of Okky Daulay, under the supervision of David Marpaung. With our advice and experience with TriPleX® manufacturing and design, the research culminated in a single photonic chip capable of programmable optical signal modulation and filtering at an ultra-high dynamic range of more than 120 dB.Hz and a 15 dB noise figure. These are ground-breaking results in the MWP world, a feat that is fast becoming NPLP’s in-house speciality. We look forward to sharing more of the exciting results of our collaboration in 2023!
 O. Daulay et al., ‘Ultrahigh dynamic range and low noise figure programmable integrated microwave photonic filter’, Nat. Commun., vol. 13, no. 1, Art. no. 1, Dec. 2022, doi: 10.1038/s41467-022-35485-x.