Introduction
Silicon nitride (SiN) has emerged as a promising platform for photonic integrated circuits (PICs) due to its exceptional properties, including low propagation loss, absence of two-photon absorption in the C-band, and moderate nonlinearity. These advantages have enabled the demonstration of various nonlinear phenomena, such as wavelength conversion and photon pair generation based on the four-wave mixing (FWM) process. However, achieving efficient on-chip rejection of the unused pump immediately after the photon pair source is crucial for enhancing the coincidence-to-accidental ratio (CAR) and improving the overall performance of on-chip quantum photonic circuits.
Proposed Photonic Integrated Circuit
The proposed photonic integrated circuit, as shown in Fig. 1, aims to address this challenge by incorporating a pump rejection filter and a demultiplexer for separating the idler and signal, enabling further on-chip quantum information processing.
The authors utilized a 370 nm SiN layer deposited on a thermally grown SiO2 layer using the Low-Pressure Chemical Vapor Deposition (LPCVD) process. The distributed Bragg reflector (DBR) parameters were chosen such that the forward-propagating pump from the fundamental mode supported input waveguide reflects as the first-order mode (λ01B), which radiates into the slab at the input side. The device was fabricated through single-step e-beam lithography followed by Inductively Coupled Plasma Reactive Ion Etching (ICPRIE), resulting in a slab height of approximately 240 nm. The microscopic image of the fabricated device is shown in Fig. 2.
Characterization and Four-Wave Mixing
The device was characterized using an optical source spectrum analyzer (OSSA: APEX 2043B) with an input optical power of -6.5 dBm. The through response of the microring resonator (MRR) integrated DBR exhibited a stopband extinction close to 50 dB (limited by the noise floor of the OSSA) with a bandwidth of 6 nm, as shown in Fig. 3(a). The tap response after the MRR is displayed in Fig. 3(b), with a loaded quality factor of approximately 70,000 around the λ01B stopband.
For the FWM process, the resonant mode of the pump (λmp) was aligned with the stopband of the DBR, while the resonant mode for the signal (λm-5s) was situated within the device's passband. Initially, the pump was aligned with the resonant wavelength via the tap port, resulting in the generation of an idler (λm+5i), as illustrated in Fig. 4(b). Subsequently, the output fiber was repositioned to the through port, and the FWM spectrum was recorded, corresponding to the through port, as presented in Fig. 4(c). During this phase, the generation of the conjugate idler (λm+5i,c) was observed.
The signal power was enhanced by 7.5 dB, while the pump power was suppressed by 43 dB, resulting in a total pump suppression of 50 dB. The conversion efficiency was found to be -46 dB for 1 mW launched pump power. To measure the maximum extinction ratio of the filter, the wavelength of a high-power tunable laser was shifted from the passband to the Bragg wavelength (λ01B). In this process, a maximum extinction ratio of 65 dB was observed, as depicted in Fig. 5.
Discussions and Conclusions
The authors successfully demonstrated pump-suppressed nonlinear signal generations, achieving an impressive extinction ratio of 65 dB through the utilization of a single-stage Bragg filter. To enhance the conversion efficiency of the resonator, further advancements can be made by reducing the effective area and intrinsic loss of the waveguides. The demonstration highlights the potential of incorporating a single sidewall DBR in SiN-based futuristic large-scale quantum photonic circuits with photon pair sources.
Reference
[2] Goswami, A. Tiwari, R. Goswami, S. S. Bhaka, P. K. Swain, E. Bhattacharya, and B. K. Das, "SiN Photonic Integrated Circuit for Pump Laser Suppressed Nonlinear Signal Generations," in Proc. IEEE, Chennai, India, 2024, pp. 1-6.
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