  Up-to-date network telemetry is the key enabler for resource optimization by
capacity scaling, fault recovery, and network reconfiguration among other
means. Reliable optical performance monitoring in general and, specifically,
the monitoring of the spectral profile of WDM signals in fixed- and flex- grid
architectures across the entire C-band, remains challenging. This article
describes a two-stage spectrometer architecture amenable to integration on a
single chip that can measure quantitatively the spectrum across the entire
C-band with a resolution of 1 GHz approximately. The first stage consists of a
ring resonator with intra-ring phase shifter to provide a tuneable fine filter.
The second stage makes use of an AWG subsystem and novel processing algorithm
to synthesize a tuneable coarse filter with a flat passband which isolates
individual resonances of a multiplicity of ring resonances. Due to its maturity
and low loss, CMOS compatible Si$_3$N$_4$ is chosen for integration. A
fabricated ring resonator functioning over the entire C-band with 1.3 GHz FWHM
bandwidth resonances tunable over a complete free spectral range of 50 GHz is
experimentally demonstrated. The complete system operation is demonstrated
using an industry standard simulation tool and AWG constructor data. The
operation of the circuit is invariant to the optical path length between
individual components substantially improving robustness to fabrication process
variations.
