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IEEE Transactions on Applied Superconductivity

Characterization of Coupling Efficiency and Absorption Coefficient for Fiber-Coupled SNSPD With an Optical Cavity

Shigehito Miki, Taro Yamashita, Mikio Fujiwara, Masahide Sasaki, Zhen Wang

Published Dec 9, 2010• 15 citations
Publisher

Abstract

We report the characterization of optical coupling efficiency and the absorption coefficient of fiber-coupled superconducting nanowire single photon detector with an optical cavity, which limit the system detection efficiency. The small lenses were applied to compact fiber-coupled package and the efficient optical coupling higher than 95% were confirmed experimentally. The absorption coefficient was also verified and reached to$\sim$60% at 1550 nm wavelength and optimum polarized condition. The system detection efficiencies at 1550, 1310 and 810 nm were 28, 40 and 3%, respectively, which are related to the absorptance dependencies on wavelengths.

DOI
10.1109/tasc.2010.2091486

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We report the characterization of optical coupling efficiency and the absorption coefficient of fiber-coupled superconducting nanowire single photon detector with an optical cavity, which limit the system detection efficiency. The small lenses were applied to compact fiber-coupled package and the efficient optical coupling higher than 95% were confirmed experimentally. The absorption coefficient was also verified and reached to∼\sim∼60% at 1550 nm wavelength and optimum polarized condition. The system detection efficiencies at 1550, 1310 and 810 nm were 28, 40 and 3%, respectively, which are related to the absorptance dependencies on wavelengths.
We report on the enhancement of system detection efficiency in a superconducting nanowire single-photon detector (SNSPD) by applying the optical cavity structure. The nanowire was made using 4-nm-thick NbN thin films and covered with an SiO cavity and Au mirror designed for 1300-1600nm wavelengths. The device is mounted into fiber-coupled packages, and installed in a practical multichannel system based on GM cryocoolers. System detection efficiency depends on the absorptance of cavity structure, and reached 28% and 40% at 1550nm and 1310nm wavelengths, respectively. These values were considerably higher than an SNSPD without optical cavity.
We developed superconducting nanowire single-photon detectors with an optical cavity (OC-SNSPDs) for multichannel systems. For efficient coupling, the devices were installed in compact fiber-coupled packages after their substrate thickness was reduced from 400 to 45 microm. The measured detection efficiency (DE) measurement at different substrate thicknesses and the estimation of optical coupling efficiency indicated that approximately 98% of the input light beam could be radiated on a 15 x 15 microm2 nanowire area from behind the substrate. The DEs of a NbN OC-SNSPD system were observed to be 9.5% and 25% at 1550 nm and 1310 nm, respectively (dark-count rate: 100 c/s).
Ever since superconducting nanowire single-photon detectors (SNSPDs or SSPDs) were demonstrated, they have attracted a considerable attention due to the detectors’ promising performance that includes high detection efficiency (DE), low dark count rate (DCR), broadband sensitivity, and low timing jitter. The system detection efficiency (SDE) has been improved to be over 90% with a wavelength of 1550 nm in a fiber-coupled system by embedding the SNSPD in optical cavities. However, the implementation of optical cavity usually reduces the bandwidth of SNSPDs. As a result, there are some recent works on increasing the bandwidth of SNSPD. Furthermore, the size of the active area of meandered nanowires has to be compatible with the core size of an optical fiber such that good optical coupling can be achieved; this setup requires a long nanowire with a large kinetic inductance, thus resulting in a small counting rate and a large timing jitter, particularly for multi-mode fiber-coupled devices. Nevertheless, a particularly impressive waveguide-coupled SNSPD has been developed in recent years. By placing an SNSPD atop an optical waveguide that has been fabricated on various substrate materials, photons propagating in the waveguide have been found to be evanescently absorbed by nanowires that are only a few tens of microns long. The on-chip DE of this device has been estimated to be over 90%, thus indicating that it has both high absorptance and high intrinsic DE. If the waveguide and the nanowire are correctly dimensioned, the absorptance of this device can reach over 90% for a wavelength range of more than 700 nm. Furthermore, the length of the nanowire is shorter than that of the fiber-coupled SNSPD, thus leading to a higher count rate and lower timing jitter (~18 ps). However, unsatisfactory coupling losses between the optical fiber and the waveguide in such devices can decrease its SDE (~10%), thus limiting its applicability to quantum photonic integrated circuits (QPIC). Recently, we proposed a type of SNSPD that was coupled with microfibers (MFs), and simulations for this SNSPD indicated that its absorption would be as high as 90%. Microfibers and nanofibers, tapered adiabatically from standard fibers, with diameters close to or smaller than the wavelength of the guided light were demonstrated to have low transmittance losses by Tong et al. in 2003. MFs have been shown to have many interesting properties, such as strong evanescent fields, tight optical confinement, wideband abilities, and small masses; these properties are appealing for applications such as optical sensing and coupling. MFs also provide seamless connections with high transmittance from standard fibers; therefore, MF-coupled SNSPDs can achieve both high SDEs and wideband compatibility. We proposed and demonstrated the MF-coupled SNSPDs successfully. The simulation results indicate that with optimal device structure, the optical absorption with efficiency < 90% can be realized over a wavelength range of 350 nm to 2150 nm. The fabricated MF-coupled SNSPD, shows unparalleled broadband system detection efficiencies (SDEs) of more than 50% from 630 nm to 1500 nm. The SDEs reach 66% at 785 nm and 45% at 1550 nm. These results pave the way for ultra-broadband weak light detection with quantum-limit sensitivity.
Increasing the detection efficiency (DE) is a hot issue in the development of the superconducting nanowire single photon detector (SNSPD). In this work, a cavity-integrated structure coupled to the SNSPD is used to enhance the light absorption of nanowire. Ultra-thin Nb films are successfully prepared by magnetron sputtering, which are used to fabricate Nb/Al SNSPD with the curve of 100 nm and the square area of 4 × 4 μm 2 by sputtering and the lift-off method. To characterize the optical and electrical performance of the cavity-integrated SNSPD, a reliable cryogenic research system is built up based on a He 3 system. To satisfy the need of light coupling, a packaging structure with collimator is conducted. Both DE and the dark count rates increase with l b . It is also found that the DE of SNSPD with cavities can be up to 0.17% at the temperature of 0.7K under the infrared light of 1550 nm, which is obviously higher than that of the SNSPD directly fabricated upon silicon without any cavity structure.
We designed superconducting nanowire single-photon detectors (SNSPDs) integrated with silver optical antennae for free-space coupling and a dielectric waveguide for fiber coupling. According to our finite-element simulation, (1) for the free-space coupling, the absorptance of the NbN nanowire for TM-polarized photons at the wavelength of 1550 nm can be as high as 96% by adding silver optical antennae; (2) for the fiber coupling, the absorptance of the NbN nanowire for TE-like-polarized photons can reach 76% including coupling efficiency at the wavelength of 1550 nm by adding a silicon nitride waveguide and an inverse-taper coupler.
Satellite-ground quantum communication requires single-photon detectors of 850-nm wavelength with both high detection efficiency and large sensitive area. We developed superconducting nanowire single-photon detectors (SNSPDs) on one-dimensional photonic crystals, which acted as optical cavities to enhance the optical absorption, with a sensitive-area diameter of 50 μm. The fabricated multimode fiber coupled NbN SNSPDs exhibited a maximum system detection efficiency (DE) of up to 82% and a DE of 78% at a dark count rate of 100 Hz at 850-nm wavelength as well as a system jitter of 105 ps.