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Optics Letters

Compact high-extinction tunable CROW filters for integrated quantum photonic circuits

Rakesh Ranjan Kumar, Xinru Wu, Hon Ki Tsang

Published Jan 23, 2020• 29 citations
Publisher

Abstract

We describe the use of cascaded second-order coupled-resonator optical waveguide (CROW) tunable filters to achieve one of the highest reported measured extinction ratios of $ {\gt} {110}\;{\rm dB}$>110dB. The CROW filters were used to remove the pump photons in spontaneous four-wave mixing (SFWM) in a silicon waveguide. The SFWM generated quantum-correlated photons that could be measured after the cascaded CROW filters. The CROW filters offer a compact footprint for use in monolithic quantum photonic circuits.

DOI
10.1364/ol.384187

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We describe the use of cascaded second-order coupled-resonator optical waveguide (CROW) tunable filters to achieve one of the highest reported measured extinction ratios of >110  dB {\gt} {110}\;{\rm dB}>110dB>110dB. The CROW filters were used to remove the pump photons in spontaneous four-wave mixing (SFWM) in a silicon waveguide. The SFWM generated quantum-correlated photons that could be measured after the cascaded CROW filters. The CROW filters offer a compact footprint for use in monolithic quantum photonic circuits.
We report an integrated tunable-bandwidth optical filter with a passband to stop-band ratio of over 96 dB using a single silicon chip with an ultra-compact footprint. The integrated filter is used in filtering out the pump photons in non-degenerate spontaneous four-wave mixing (SFWM), which is used for producing correlated photon pairs at different wavelengths. SFWM occurs in a long silicon waveguide, and two cascaded second-order coupled-resonator optical waveguide (CROW) filters were used to spectrally remove the pump photons. The tunable bandwidth of the filter is useful to adjust the coherence time of the quantum correlated photons and may find applications in large-scale integrated quantum photonic circuits.
Integrated photonic circuits have become an attractive platform for the quantum information processing, paving the way for quantum information management with scalable device. In this context, silicon photonics represents the most mature technology to implement the quantum system functionalities, due to its large scalability and compatibility with CMOS technology. Efficient photon-pairs sources based on Spontaneous Four-Wave Mixing (SFWM) and high-performance photodetector have been already demonstrated. The efficient detection of photon-pairs requires a pump filter at the photodetector, preserving the signal-idler pair. Thus, filters with high Extinction Ratio (ER), low Insertion Loss (IL) and narrow rejection Bandwidth (BW) are needed. Here, we propose the design of an ultra-high-performance rejection filter, based on a silicon dual-loaded single input/output Mach-Zehnder Interferometer (MZI), with one branch coupled to a Ring Resonator (RR) and the other to three serially coupled RRs forming a Coupled Optical Resonator Waveguide (CROW). Very high performance (ER = 150.55 dB, IL = 0.104 dB, BW = 0.243 nm), within a footprint of 60 µm × 160 µm, has been calculated, demonstrating its suitability for an efficient suppression of the pump signal. The filter response is also thermo-optically tuneable in a 6 MHz range, with a reconfigurability time of about 8 µs.
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We fabricate silicon waveguide spirals and a ring resonator to generate photon pairs based on a spontaneous four-wave mixing. The coincidence-to-accidental-ratio (CAR) of photon pairs from the silicon waveguides is measured up to 400 after a noise-filtering by using the combination of bandpass filters and pump-rejection filters. The CAR is enhanced up to 700 by adding on-chip pump-rejection MZIs. We observe the CAR of the photon pairs from a silicon spiral is highly depending on the wavelength detuning from the pump wavelength. We discuss the noise sources related to the degradation of the CAR based on our experimental results.