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Dual-lens beam compression for optical coupling in superconducting nanowire single-photon detectors

Labao ZhangChao, WanMin Gu, Ruiying XuSen, ZhangLin Kang, Jian Wu

Published Jan 1, 2015• 0 citations
OpenAlex

Abstract

The optical coupling of superconducting nanowire single-photon detectors (SNSPDs) has always been restricted to a single-mode fiber for a limited detec- tion area. In this study, for enhancing photon coupling, a dual-lens system operating at 2.2 K was used to compress the beam size on the basis of the Gaussian beam theory and geometric approximation. A magnification of approxi- mately 0.3 was obtained, and a focused spot with diameter of approximately 10 lm was measured from a multimode fiber. Assisted with the compressed beam, a system effi- ciency of 55 % (1550 nm) was achieved for a SNSPD with a detection area of 10 lm 9 10 lm and 62.5 lm multi- mode fiber coupling. At the same time, a high speed of 106 MHz was measured with the proposed system. The realization of a highly compressed optical beam reduced the optical coupling requirement and helped maintain a high speed for the SNSPD.

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APL Photonics•Oct 1, 2025•6 citations
The optical coupling of superconducting nanowire single-photon detectors (SNSPDs) has always been restricted to a single-mode fiber for a limited detec- tion area. In this study, for enhancing photon coupling, a dual-lens system operating at 2.2 K was used to compress the beam size on the basis of the Gaussian beam theory and geometric approximation. A magnification of approxi- mately 0.3 was obtained, and a focused spot with diameter of approximately 10 lm was measured from a multimode fiber. Assisted with the compressed beam, a system effi- ciency of 55 % (1550 nm) was achieved for a SNSPD with a detection area of 10 lm 9 10 lm and 62.5 lm multi- mode fiber coupling. At the same time, a high speed of 106 MHz was measured with the proposed system. The realization of a highly compressed optical beam reduced the optical coupling requirement and helped maintain a high speed for the SNSPD.
超导的 nanowire 的光联合单个光子的察觉者(SNSPD ) 总是为一个有限察觉区域被限制了为单个模式的纤维。在这研究，为提高光子联合，在 2.2K 操作的一个双透镜的系统被用来根据 Gaussian 横梁理论和几何近似压缩横梁尺寸。约 0.3 的放大被获得，并且有近似 10m 的直径的一个集中的点从 multimode 纤维被测量。帮助了压缩横梁， 55% 的系统效率(1550nm ) 与 10m 的一个察觉区域为 SNSPD 被完成
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.
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).
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.
We report on the development of fiber-coupled 4-element superconducting nanowire single photon detector arrays. The SNSPD arrays consist of four nanowire elements which are placed into a sensitive area with 15 ¥ 15 μm2square. The device was mounted into small chip-mounting block with four tiny connectors which can pass high frequency signals up to 110 GHz, and each element was connected to each connector through CPW line. The single-mode optical fiber with GRIN lens was fixed to fiber-holding block and was joined to chip-mounting block from back side and aligned accurately so that the light spot irradiate to the center of device sensitive area. The fiber coupled package is then installed into closed-cycle cryocooler system and cooled down to 2.3 K. All four elements were able to detect single photon event, and total system DE reached to 8.07%.
Single-photon detectors with high detection rates are the foundation for the high-speed remote laser communication. Although a total detection rate of gigahertz has been achieved with multiple superconducting nanowires, the coupled single-mode fibers are incompatible with most large-aperture optical telescopes, and the uneven response distribution between nanowires also limits the system performance in practice. Here, we report a multimode fiber coupled superconducting nanowire single-photon detector (SNSPD) with a gigahertz detection rate through 16 intertwined niobium nitride nanowires operated at 2.4 K. The SNSPD is coupled with a 62.5 μm multimode fiber through beam compression. Overcoming the interference of speckles in multimode fibers, all nanowires perform highly uniform with narrow distributions of detection efficiencies (5.48% ± 0.136%), achieving a total detection efficiency of 87.6% and a total system detection efficiency of 52.3%, a maximum detection rate of 1.3 Gcps when the efficiency drops by 3 dB, and a highest detection rate of 3.1 Gcps before latching occurs. The significance of the uniform performance of pixels in terms of system speed is verified by modeling the combined detection rate vs the detection efficiency distribution, which is crucial in high-speed laser communication.