A photonic implementation of a wavelength meter typically applies an interferometer to measure the frequency-dependent phase shift provided by an optical delay line. This work shows that the information to be retrieved is encoded by a vector restricted to a circular cone within a 3D Cartesian object space. The measured data belong to the image of the object space under a linear orthogonal map. Component impairments result in broken orthogonal symmetry, but the mapping remains linear. The circular cone is retained as the object space, which suggests that the conventional conic section fitting for the wavelength meter application is a premature reduction of the object space from R³ to R². The inverse map, constructed by a learning algorithm, compensates impairments such as source intensity fluctuation and errors in delay time, coupler transmission, and photoreceiver sensitivity while being robust to noise. The simple algorithm does not require initial estimates for all parameters except for a broad bracket of the delay; further, weak nonlinearity introduced by uncertain delay can be corrected by a robust golden search algorithm. The phase-retrieval process is invariant to source power and its fluctuation. Simulations demonstrate that, to the extent that the ten parameters of the interferometer model capture all significant impairments, a precision limited only by the level of random noise is attainable. Applied to measured data collected from a fabricated Si₃N₄ wavelength meter, greater than an order of magnitude improvement in precision compared with the conventional method is achieved.

A panoramic ultra-high resolution photonic integrated circuit spectrometer is under development by the authors. The architecture comprises a tunable ring resonator (RR) stage and an AWG stage. The resolution defines the bandwidth of the RR, determined by the cross-coupled power and hence the gap between the access and ring waveguides. The AWG channel frequency spacing determines the required free-spectral range (FSR) and hence the perimeter of the ring resonator. The specified <1 GHz resolution combined with an FSR of 50 GHz renders accurate simulation difficult, obstructing the design process. In this report, a simplified design rule to determine the minimum gap between straight access waveguides and a circular ring waveguide is proposed. Realistic assumptions such as the existence of local bisymmetry and adiabatic mode evolution throughout the coupling region permit a simple mode solver to determine the relationship between the cross-coupled power and the minimum gap size. A parameter extraction method is also formulated for add-drop rings equipped with two nominally identical couplers that disentangles the loss and coupling ring parameters from intensity-only transmission measurements. The proposed rule is applied to the design of ring resonators fabricated on a Si₃N₄ platform. The parameter extraction method is used to analyze the measured characterization data of the ring resonators. The results show good agreement within ~43 nm between the design rule and the gaps size determined by the parameters extracted from the measured data and provide experimental confirmation of the technological viability of the ring resonators required by the spectrometer.

This paper demonstrates the photonic generation of millimetre-wave carriers by frequency octupling of a radio frequency (RF) carrier using a circuit with two parallel arms each containing two polarization modulators in series. The low RF drive power required and the absence of optical filters make this architecture suitable for generating highly stable and tunable optical millimetre-waves. A transfer function representation is used to analyse the operation and a simulation using Virtual Photonic Inc. software package is presented as a proof of concept. The simulation results show very high unwanted optical and electrical side harmonic suppression ratio for a wide range of modulation index compared to the prior art. Further, considering the practical scenario, operating tolerance against the imbalances in the key parameters is established. The unwanted RF sidebands are more than 19 dB and 30 dB below the desired signal power even when a deviation of ±3° is considered in individual RF phase and polarizer angle respectively.

A photonic frequency shifter based on generalized Mach-Zehnder interferometer (GMZI) architecture is presented and experimentally validated. The circuit consists of four Mach-Zehnder modulators (MZM) in a 4 × 4 network bounded by two 4 × 4 multimode interference couplers and functionally equivalent to two parallel dual-parallel MZM (DP-MZM). The circuit can offer static bias free operation, virtual connectivity control of the components, and spatial separation of up- and down-converted carriers, which can be collected from separate ports without using any optical demultiplexing filters. Thus, the design permits remote heterodyning (advantages which cannot be obtained using a commercial DP-MZM or filter based optical frequency shifter). Experimental investigation shows deviation from ideal performance due to possible fabrication error and poor fiber-chip coupling. A carrier suppression of >20 dB and spurious sideband suppression >12 dB relative to the principal harmonics is achieved without any tuning for bias adjustment. In addition to the frequency conversion, the integration feasible circuit can also perform as a sub-carrier generator, IQ modulator, and frequency multiplier.

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 similar to 1.4 GHz. 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 a novel processing algorithm to synthesize a tuneable coarse filter with a flat passband which isolates individual resonances of a multiplicity of ring resonances. The spectrometer is capable of scanning the entire C -band with high resolution using only one dynamic control. Due to its maturity and low loss, CMOS compatible Si3N4 is chosen for fabrication of the ring resonator and two cyclic AWGs. Complete spectrometer operation is demonstrated experimentally over a selected portion of the C -band. A novel virtual channel synthesis algorithm based on the weighted summation of the AWG output port powers relaxes the conventional AWG design requirement of a flat passband and sharp transition to stopband. The operation of the circuit is invariant to the optical path length between individual components and the algorithm corrects to some extent fabrication process variation impairments of the AWG channel spectra substantially improving robustness.