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Silicon Microring Resonator Design with a Dispersive Directional Coupler for Spectrally Pure Photon Sources

Arnab Goswami, Bijoy Krishna Das

Published May 6, 2025• 0 citations
Publisher

Abstract

Heralded single photon sources generated through spontaneous four wave mixing in silicon microring resonators are highly demanding for large-scale integrated quantum photonics. However, the spectral purity of conventional microring resonators is limited to 93%, which impacts the quality of quantum interference within photonic circuits. The wavelength dependent coupling between ring and bus waveguides plays a significant role in the generation/heralding of single photons from the resonator. In this letter, we have presented a detailed investigation of the dispersive nature of relatively long directional couplers in order to design a microring resonator in all-pass configuration for chip-scale generation of single photons with maximum purity. In our studies, we have shown that a photon purity level close to 99% (λ ∼ 1550 nm) can be achieved by the use of a dispersive directional coupler in a race-track microring resonator in all-pass configuration, without affecting the photon heralding rate. The design parameters of the device are optimized such that it can be fabricated in commercial CMOS foundries.

DOI
10.1364/opticaopen.28935896.v1

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Heralded single photon sources generated through spontaneous four wave mixing in silicon microring resonators are highly demanding for large-scale integrated quantum photonics. However, the spectral purity of conventional microring resonators is limited to 93%, which impacts the quality of quantum interference within photonic circuits. The wavelength dependent coupling between ring and bus waveguides plays a significant role in the generation/heralding of single photons from the resonator. In this letter, we have presented a detailed investigation of the dispersive nature of relatively long directional couplers in order to design a microring resonator in all-pass configuration for chip-scale generation of single photons with maximum purity. In our studies, we have shown that a photon purity level close to 99% (λ ∼ 1550 nm) can be achieved by the use of a dispersive directional coupler in a race-track microring resonator in all-pass configuration, without affecting the photon heralding rate. The design parameters of the device are optimized such that it can be fabricated in commercial CMOS foundries.
Heralded single photon sources generated through spontaneous four wave mixing in silicon microring resonators are highly demanding for large-scale integrated quantum photonics. However, the spectral purity of conventional microring resonators is limited to 93%, which impacts the quality of quantum interference within photonic circuits. The wavelength dependent coupling between ring and bus waveguides plays a significant role in the generation/heralding of single photons from the resonator. In this letter, we have presented a detailed investigation of the dispersive nature of relatively long directional couplers in order to design a microring resonator in all-pass configuration for chip-scale generation of single photons with maximum purity. In our studies, we have shown that a photon purity level close to 99% (λ ∼ 1550 nm) can be achieved by the use of a dispersive directional coupler in a race-track microring resonator in all-pass configuration, without affecting the photon heralding rate. The design parameters of the device are optimized such that it can be fabricated in commercial CMOS foundries.
One of the key building blocks of quantum photonic systems is a source of indistinguishable photons. Silicon ring resonators can be used as photon pair sources by taking advantage of silicon's large third order nonlinearity with a process known as spontaneous four wave mixing. These sources are capable of producing pairs of indistinguishable photons but typically suffer from an effective 50% loss. By slightly decoupling the input waveguide from the ring, the desired photons generated in the ring can be directed to the drop port. Thus, the ratio between the coincidences from the drop port and the total number of coincidences from all ports (coincidence efficiency) can be significantly increased, with the trade-off being that the pump is less efficiently coupled into the ring. This asymmetric coupling scheme was used to demonstrate a dual-bus microring source with a coincidence efficiency of ~96%. Unfortunately, the penalty to pump power was quite large, with a required ~10x increase in the pump power to maintain the same pair generation rate.\nTo further improve upon this result, a microring source was designed with multipoint evanescent couplers to induce a strong spectral dependence in the coupling coefficients. This coupler design can be used on both sides of the ring resonator so that resonances supported by one of the couplers are suppressed by the other. This is the ideal configuration for a ring-based photon-pair source as it can only support the pump photons at the input side while only allowing the generated photons to leave through the output side. Asymmetric coupling was also used in this configuration but in a different way than in the previous source. In this case, the input side of the ring was configured to be critically coupled for maximal resonance enhancement while the output side was configured to be over coupled to minimize photon-pair loss within the ring. This device design was used to demonstrate a photon-pair source with coincidence efficiencies of 0.9981+-6.425x10^-4 and 0.9979+-1.665x10^-3 for degenerate and non-degenerate pump configuration respectively.\nIn addition to the source development, work was done to move some of the pump rejection filtering onto the photonic chip. While the fabricated device only exhibited an extinction of 50 dB, the root cause of the limited extinction was explored and a new filter design was proposed to overcome these limitations. With the combination of the microring source with its greatly improved efficiency and the movement toward reducing the required off-chip filtering, this work represents a significant advancement in the scalability of integrated quantum photonic systems which can enable high performance quantum computing and communication systems.
The intrinsic third-order nonlinearity in silicon has proven it to be quite useful in the field of quantum optics. Silicon is suitable for producing time-correlated photon pairs that are sources of heralded single-photon states for quantum integrated circuits. A quantum signal source in the form of single photons is an inherent requirement for the principles of quantum key distribution technology for secure communications. Here, we present numerical simulations of a silicon ring with a 6 μ m radius side-coupled with a bus waveguide as the source for the generation of single photons. The photon pairs are generated by exploring the process of degenerate spontaneous four-wave mixing (SFWM). The free spectral range (FSR) of the ring is quite large, simplifying the extraction of the signal/idler pairs. The phase-matching condition is considered by studying relevant parameters like the dispersion and nonlinearity. We optimize the ring for a high quality factor by varying the gap between the bus and the ring waveguide. This is the smallest ring studied for photon pair generation with a quality factor in the order of 10 5 . The width of the waveguides is chosen such that the phase-matching condition is satisfied, allowing for the propagation of fundamental modes only. The bus waveguide is pumped at one of the ring resonances with the minimum dispersion (1543.5 nm in our case) to satisfy the principle of energy conservation. The photon pair generation rate achieved is comparable to the state of the art. The photon pair sources exploiting nonlinear frequency conversion/generation processes is a promising alternative to atom-like single-photon emitters in the field of integrated photonics. Such miniaturized structures will benefit future on-chip architectures where multiple single-photon source devices are required on the same chip.
Photonic chips with silicon nitride (Si3N4\mathrm{Si_3N_4}Si3​N4​) microring resonators are well established as heralded single-photon sources, but their operation as frequency-degenerate twin-photon sources has not previously been demonstrated. Here, we realise a twin-photon source at telecommunication wavelengths in a Si3N4\mathrm{Si_3N_4}Si3​N4​ ring microresonator via an inverse four-wave mixing (FWM) process, in which two photons from spectrally distinct pumps are converted into a pair of identical twin photons. The measurements show a maximum coincidence-to-accidental ratio (CAR) of 5.4±0.65.4\pm0.65.4±0.6. In addition, the microresonator functions as a heralded single-photon source through pump-degenerate spontaneous four-wave mixing (SFWM), exhibiting a spectral purity of P=0.67±0.05P=0.67\pm0.05P=0.67±0.05 and a heralded anti-bunching of gh(2)(0)=0.0042±0.0015g^{(2)}_h(0)=0.0042\pm0.0015gh(2)​(0)=0.0042±0.0015. Together, these results demonstrate both photon-generation schemes on a single integrated Si3N4\mathrm{Si_3N_4}Si3​N4​ platform, highlighting its potential for scalable, tailored quantum light generation.
The generation of pure single-photon states and their characterization are fundamental in quantum information processing applications like quantum computing. In this work, we design integrated optical directional couplers on a silicon nitride platform for heralded single-photon characterization. We assume the generation of such photons in silicon nitride waveguides through the spontaneous four-wave mixing process. We show that manipulating the waveguide geometry and dielectric contrast makes it possible to obtain nearly wavelength-independent directional coupler designs that perform as 50:50 beamsplitters over a broad bandwidth around the central design wavelength. Assuming two independent but identical heralded single-photon sources, we compare the visibility obtained by simulating the Hong-Ou-Mandel interference for three different 50:50 beamsplitters: an ideal one (perfectly wavelength-independent), an engineered one (nearly wavelength-independent), and an unengineered one (significantly wavelength-dependent). The purity obtained with the engineered beamsplitter closely matches the purity obtained with the ideal one, for which the interference visibility is a direct measure of the quantum state purity. This finding implies that the proposed engineered beamsplitter is suitable for characterizing heralded single-photon states, provided its bandwidth is 23 nm.
Recently, many researchers focus on the integration of single photon sources on an optical chip toward the realization of large-scale quantum information processing.Identical single photons are required to obtain good interference between photons for quantum information processing, for which high "spectral purity" of single photons is required.An appealing scheme is employing the spontaneous four-wave mixing (SFWM) in a silicon micro-ring resonator (MRR) because it allows us to generate high-purity photons without external spectral filters.However, the purity value is limited to 0.92 due to the dispersion property of an MRR We have developed a new scheme to further increase the spectral purity from an MRR by manipulating the spectral phase distribution of the pump pulse.Via exploring several strategies on the spectral phase modulation, we found that the purity of single photon can be increased up to approximately 0.96 breaking the conventional limitation of 0.92 in a standard MRR.We further explore the experimental feasibility of the scheme toward future implementation.