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Optics Express

Compact cryogenic self-aligning fiber-to-detector coupling with losses below one percent

Aaron Miller, Adriana E. Lita, Brice Calkins, Igor Vayshenker, Steven M. Gruber, Sae Woo Nam

Published Apr 25, 2011• 166 citations
Publisher

Abstract

We present a compact packaging technique for coupling light from a single-mode telecommunication fiber to cryogenic single-photon sensitive devices. Our single-photon detectors are superconducting transition-edge sensors (TESs) with a collection area only a factor of a few larger than the area of the fiber core which presents significant challenges to low-loss fiber-to-detector coupling. The coupling method presented here has low loss, cryogenic compatibility, easy and reproducible assembly and low component cost. The system efficiency of the packaged single-photon counting detectors is verified by the "triplet method" of power-source calibration along with the "multiple attenuator" method that produces a calibrated single-photon flux. These calibration techniques, when used in combination with through-wafer imaging and fiber back-reflection measurements, give us confidence that we have achieved coupling losses below 1% for all devices packaged according to the self-alignment method presented in this paper.

DOI
10.1364/oe.19.009102

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We present a compact packaging technique for coupling light from a single-mode telecommunication fiber to cryogenic single-photon sensitive devices. Our single-photon detectors are superconducting transition-edge sensors (TESs) with a collection area only a factor of a few larger than the area of the fiber core which presents significant challenges to low-loss fiber-to-detector coupling. The coupling method presented here has low loss, cryogenic compatibility, easy and reproducible assembly and low component cost. The system efficiency of the packaged single-photon counting detectors is verified by the "triplet method" of power-source calibration along with the "multiple attenuator" method that produces a calibrated single-photon flux. These calibration techniques, when used in combination with through-wafer imaging and fiber back-reflection measurements, give us confidence that we have achieved coupling losses below 1% for all devices packaged according to the self-alignment method presented in this paper.
We applied a recently developed fiber coupling technique to superconducting single photon detectors (SSPDs). As the detector area of SSPDs has to be kept as small as possible, coupling to an optical fiber has been either inefficient or unreliable. Etching through the silicon substrate allows fabrication of a circularly shaped chip which self aligns to the core of a ferrule terminated fiber in a fiber sleeve. In situ alignment at cryogenic temperatures is unnecessary and no thermal stress during cooldown, causing misalignment, is induced. We measured the quantum efficiency of these devices with an attenuated tunable broadband source. The combination of a lithographically defined chip and high precision standard telecommunication components yields near unity coupling efficiency and a system detection efficiency of 34% at a wavelength of 1200 nm. This quantum efficiency measurement is confirmed by an absolute efficiency measurement using correlated photon pairs (with λλλ = 1064 nm) produced by spontaneous parametric down-conversion. The efficiency obtained via this method agrees well with the efficiency measured with the attenuated tunable broadband source.
Superconducting photon detectors have emerged as a powerful new option for detecting single photons. System detection efficiency that incorporates the quantum efficiency of the device and system losses is one of the most important single-photon detector performance metrics for quantum information applications. Superconducting transition-edge sensors (TESs) are microcalorimeters that have the ability of distinguishing single photons with negligible dark counts. In addition, TESs are capable of directly measuring the photon number in a pulse of light. We have achieved near-unity system detection efficiency with TESs at particular wavelengths in the near-infrared by using multilayer structures that enhance the absorption of light into the active device material. We describe the design of the multilayer structure enabling high detection efficiency TESs including issues and requirements for obtaining detection efficiency values higher than 99 %. We describe the device fabrication and finally, show recent results of devices optimized for high detection efficiency using the multilayer structure.
Superconducting single photon detectors are promising candidates for cutting-edge applications like space-to-ground communication and quantum-key-distribution. The challenge is to transfer the assembly technology from university research to industrial processes. Especially the positioning of the optical fiber with respect to the active area of the superconducting detector are open questions. We demonstrate the operation of a superconducting nanowire single photon detector (SNSPD) in a closed cycle cryostat. The thermal coupling between superconducting detector chip and closed-cycle cryostat is investigated. The possibility to mount the optical fiber in an RIE-ICP-etched hole in a silicon carrier wafer is demonstrated. Different illumination wavelength and intensities are used to validate the SNSPD assembly in the closed cycle cryostat. Further, an advanced package of the silicon carrier wafer and the SNSPD-chip is introduced. In this package, the SNSPD-chip is mounted via flip-chip technology on the silicon carrier wafer. The striven flip-chip position accuracy of ± 1 μm ensures the accurate coupling between optical fiber and active area of the SNSPD.
Transition-Edge Sensors (TESs) are superconducting photon detectors offering high quantum efficiency, energy resolution, and photon-number discrimination, making them attractive for quantum sensing applications. We report on the design, fabrication, and characterization of a Ti/Au/Ti TES optimized for multimode fiber coupling in NV-center experiments. The device integrates a bi-layer anti-reflective coating optimized for 700 nm, the emission wavelength range of NV centers in which we are interested. Optical characterization in a cryogenic setup demonstrated an energy resolution below 2 eV and photon-number resolution capability. A dedicated calibration setup was used to evaluate the system detection efficiency (SDE), yielding0.85±0.020.85 \pm 0.020.85±0.02. These results confirm the suitability of large-area TESs for efficient multimode fiber coupling and highlight their potential for improving the performance of NV-center-based quantum sensing experiments.
A method for calibration of single-photon detectors without the need of input photon flux calibration is presented. The method relies on the use of waveguide-coupled single photon detectors and a series of photon-counting measurements using a single-photon source. It is shown that the method can yield relative uncertainties of less than 1% including counting statistics and fiber splice loss uncertainties with a total measurement time of about 1 h under the assumptions that the fractional losses of the waveguide-coupled detectors are known or zero and the loss along the waveguide is constant. It is also shown that the fractional losses of the waveguide-coupled detectors can be determined if they are equal. However, in this case an input photon flux calibration is required.
Efficient packaging and fiber coupling of a superconducting niobium nitride nanowire single photon detector is in itself a real challenge and is often a limiting factor to reach a high system quantum efficiency. Our approach makes use of a controlled expansion alloy, Carpenter Invar 36, that has been adequately heat treated to enhance its characteristics for cryogenic operation. This insures the integrity of the optical coupling done at room temperature while providing a good attenuation for electromagnetic interference due to the high relative permeability of this nickel-iron alloy. The small form factor of this optical fiber pigtailed package makes it versatile and could be easily integrated with a commercial cryogen-free system or simply dipped into a standard helium transport Dewar. We report on our experimental methodology to evaluate the optical coupling quality and present the optoelectronic characterization of two devices packaged in this way.