Characterization of Autofluorescence in Optical Fibers for NV-based Sensing Applications

TL;DR

This paper addresses fiber-induced autofluorescence that overlaps NV center emission and degrades SNR in NV-based sensing. It employs a systematic spectral characterization of common VIS optical fibers, examining material, assembly, and excitation-wavelength factors, with Raman scattering below $600\,\mathrm{nm}$ and NBOHC defect emission peaking near $645\,\mathrm{nm}$ as key contributors. The study identifies low-background fibers (e.g., AFM200L, UM22-100, FP200ERT) and demonstrates how assembly choices (notably epoxy type) and polishing quality affect background levels, offering practical guidelines for sensor design. The findings enable improved SNR in fiber-coupled NV sensors by guiding fiber selection, assembly practices, and operating conditions, with data openly available for reuse.

Abstract

Optical fibers are crucial for guiding light in various sensing applications. Especially for quantum sensors such as the nitrogen-vacancy (NV) center in diamond, they enable light control and device miniaturization. However, fluorescence and scattering within the fiber, often referred to as fiber background, autofluorescence, or autoluminescence, can overlap spectrally with the NV centers' fluorescence, degrading the signal-to-noise ratio and thus limiting sensor sensitivity. Here, we investigate the optical spectra of standard optical fibers, considering material dependencies, physical influences, and their fluorescence scaling with excitation power and wavelength. Our results identify spectral components and fiber types with minimal unwanted background signals, guiding the selection of optimal fibers for NV-based quantum sensing.

Figures (7)

• Figure 1: Typical fluorescence spectrum (orange) of a diamond containing multiple NV centers longpass-filtered at $600nm$ (FELH0600) and measured spectrum (blue) of an FG050LGA optical fiber during transmission of $520nm$ laser light longpass-filtered at $550nm$ (FELH0550). Both graphs are individually scaled to emphasize the overlap between the two spectra. The exact scaling of the fiber and NV-diamond spectra also varies with fiber length, number of NV centers, and diamond–fiber coupling efficiency.
• Figure 2: Common elements of a bare optical fiber and assembly in a fiber optic connector. (a) Elements of a bare optical fiber, consisting of a core (light blue), cladding (gray), and protective coating (red). (b) Assembly of an optical fiber inside a ceramic fiber connector ferrule. (c) Optical setup and measurement positions. The optical setup consists of essentially two beam paths: Firstly, a fiber-coupled laser source (Laser $\lambda_\mathrm{exc}$), a fiber coupler (FC), a $\lambda/2$-plate, a polarizing beam cube (PBC), a bandpass (BP) filter, an optional neutral density (ND) filter, a dichroic mirror shortpass (DMSP), and another FC for coupling light in and out of the fiber under investigation (Test fiber). The power of the excitation laser is adjusted using the optional ND filter and laser current, and it is fine-tuned with the $\lambda/2$-plate and PBC. Secondly, it consists of a beam path for measuring losses and the intrinsic light of the fiber between the test fiber and the spectrometer. For the analysis of losses, this path consists of a calibration laser (Cal. laser, CPS650F, Thorlabs), an FC with an attached calibration single-mode (SM) fiber (Cal. fiber, P1-460B-FC-2, Thorlabs), which then connects via a fiber-to-fiber connector (F/F) to the test fiber. Afterwards, the calibration light and intrinsic fiber light are transmitted through the test fiber and the connected FC, reflected at the DMSP, and, depending on the setup configuration, filtered from excitation light by a notch filter (NF) or a long-pass filter (LP). Lastly, the remaining light is transmitted and focused onto the camera of the spectrometer via two fiber couplers, a blazed grating, and a lens. The marked positions p1-p4 indicate the measured positions used to determine losses, primarily due to coupling efficiencies.
• Figure 3: Impact on the optical spectra of the used materials for a fiber assembly. (a) Measured optical spectra of the same optical fiber using different epoxy glues (Table \ref{['tab:Fiber_combinations']}, Nos. 5-7) and a connector fully filled with Epo-tek 353ND without any fiber inserted. (b) Optical spectra of a commercially fully assembled fiber (Table \ref{['tab:Fiber_combinations']}, No. 4) and a self-assembled fiber (No. 5). (c) Optical spectrum of an empty connector in comparison to the spectrum of the fiber in Table \ref{['tab:Fiber_combinations']}, No. 5, with and without a furcation tubing. (d) Spectra before and after scratching a fully assembled fiber tip (Table \ref{['tab:Fiber_combinations']}, No. 7).
• Figure 4: Influence of external factors on the fiber background spectrum. (a) Detected spectra after cutting the length of the fiber stepwise. The fiber was equipped with a connector only on one side, and the protective tubing was shortened together with the optical fiber. (b) Detected spectra when bending the fiber around a cylinder with varying diameter. (c) Influence of rapid cooling on the received light spectrum. Here, a large portion of the fiber was placed in a container filled with liquid nitrogen. The orange cooling spectrum was taken a few seconds after inserting it in the liquid nitrogen, while the green fully cooled spectrum was measured after thermalization. (d) Influence of misalignment of the fiber coupling. The unit $°$ represents the turns in $°$ on a mirror before the fiber, while $0°$ represents pre-aligned position. Above a rotation of $\approx200°$, the measured spectra remain constant and no more measurable light is guided.
• Figure 5: Measured optical spectra of a fiber (Table \ref{['tab:Fiber_combinations']}, No. 5) for varying laser power (a). For each spectrum, local maxima are marked. In (b), the retrieved intensities of each local maximum are plotted against excitation power and fit with a linear regression. The colors of the markers in (b) correspond to the markers in (a).