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Optical and Quantum Sensing and Precision Metrology II

Efficient MIR sensing with undetected photons: high brightness, miniaturized SPDC module for real world applications

Philipp Hildenstein, Gunnar Blume, Alexander Sahm, Nils Werner, David Feise, Atta Ur Rehman Sherwani, Emma J. Pearce, Björn Kemper, Álvaro Barroso, Jürgen Schnekenburger, Sven Ramelow, Katrin Paschke

Published Mar 19, 2025• 0 citations
Publisher

Abstract

Sensing applications in the mid-infrared (MIR) spectral range are challenging due to the high cost and low performance of the available light sources and detectors. This issue can be bypassed by utilizing the process of spontaneous parametric down-conversion (SPDC) to generate tailored MIR-NIR photon-pairs. The nature of SPDC enforces the generation of frequency-entanglement between these paired photons which can be used for spectroscopy: Within a nonlinear interferometric configuration, the MIR radiation is employed for the illumination of a specific material, whereas the NIR radiation is analyzed using a conventional spectrometer. This technique, which is referred to as sensing with ”undetected photons”, so far relied on a breadboard optical setup that is both space-consuming, heavy and complex. Here we present the design, manufacturing and application of a miniaturized quantum light module, with a footprint of less than 80 x 60 mm², which is powered by a direct emitting diode laser at 720 nm. The module contains the nonlinear crystal as the quantum light source, the optical path for the NIR beam, and mechanical connectors leading to the MIR sensing arm. It also provides an interconnection to a spectrometer for the information-carrying NIR photons. This study presents insights into the optical design of the module, its manufacturing and the nonlinear optics quantum light source. The module emits a MIR-NIR photon-pair rate of 1.7 × 109 s−1 W−1, which allows for the implementation of time-efficient sensing applications. The design is optimized for a first application - the detection of synthetic polymers in aqueous solutions, which will be illustrated by concepts for the detection of micro-plastics in wastewater in the field with a mobile device. The potential for integrating a sophisticated, non-linear optical interference configuration into a user-friendly micro-module paves the way for the development of industry-ready setups. This enables a variety of other applications, which are covered by sensing with undetected photons.

DOI
10.1117/12.3041088

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Sensing applications in the mid-infrared (MIR) spectral range are challenging due to the high cost and low performance of the available light sources and detectors. This issue can be bypassed by utilizing the process of spontaneous parametric down-conversion (SPDC) to generate tailored MIR-NIR photon-pairs. The nature of SPDC enforces the generation of frequency-entanglement between these paired photons which can be used for spectroscopy: Within a nonlinear interferometric configuration, the MIR radiation is employed for the illumination of a specific material, whereas the NIR radiation is analyzed using a conventional spectrometer. This technique, which is referred to as sensing with ”undetected photons”, so far relied on a breadboard optical setup that is both space-consuming, heavy and complex. Here we present the design, manufacturing and application of a miniaturized quantum light module, with a footprint of less than 80 x 60 mm², which is powered by a direct emitting diode laser at 720 nm. The module contains the nonlinear crystal as the quantum light source, the optical path for the NIR beam, and mechanical connectors leading to the MIR sensing arm. It also provides an interconnection to a spectrometer for the information-carrying NIR photons. This study presents insights into the optical design of the module, its manufacturing and the nonlinear optics quantum light source. The module emits a MIR-NIR photon-pair rate of 1.7 × 109 s−1 W−1, which allows for the implementation of time-efficient sensing applications. The design is optimized for a first application - the detection of synthetic polymers in aqueous solutions, which will be illustrated by concepts for the detection of micro-plastics in wastewater in the field with a mobile device. The potential for integrating a sophisticated, non-linear optical interference configuration into a user-friendly micro-module paves the way for the development of industry-ready setups. This enables a variety of other applications, which are covered by sensing with undetected photons.
Optical coherence tomography (OCT) in the mid infrared (MIR) spectral range offers an elegant way of non-destructive optical testing of coatings that are opaque at visible and near infrared (NIR) wavelengths. However, light sources and detection equipment in the MIR are somewhat cumbersome due to low emission intensity and detectivity. Here we use spontaneous parametric down-conversion (SPDC) in a periodically poled KTP crystal pumped with 700 mW at 660 nm. The SPDC produces MIR photons near 3.8 µm and entangled NIR photons near 800 nm. All three wavelengths are used in a Michelson-type quantum interferometer. Only MIR photons interact with the device under test (DUT) and share their interference information with the NIR via the entanglement. The NIR photons are then analyzed with a conventional silicon-based spectrometer, thus, allowing MIR "sensing with undetected photons". We were able to shrink the quantum interferometer module to a footprint of 80 x 60 mm² including the 660 nm pump laser. The external MIR beam path offers a length of 235 mm ± 1.5 mm to the DUT and a single mode fiber port to the spectrometer. Using a mirror as a DUT we achieved a visibility of the quantum interference of up to 39%.
Optical coherence tomography (OCT) in the mid infrared (MIR) spectral range by using spontaneous parametric down conversion (SPDC) in a nonlinear interferometer offers an elegant way for miniaturizing said OCT systems in the MIR by detecting correlated near infrared (NIR) photons only. This “sensing with undetected photons” was recently miniaturized and allowed non-destructive optical testing of ceramics in an industrial environment. The demonstrated quantum interferometer module featured a footprint of 80 × 60 mm2. The module included a 700 mW continues wave 660 nm pump laser, a 2.5 mm long ppKTP crystal for type-0 colinear SPDC, a 3.8 µm MIR free space emission port to the device under test and a NIR fiber port to connect to a NIR grating spectrometer with a 50 nm wavelength span near 800 nm. While the SPDC at maximum pump power resulted in about 108 photons per second detected on the NIR spectrometer, we explored whether optimized focusing conditions could potentially further improve this. To this end, we devised an experiment to investigate the impact of a range of different pump focusing conditions on the SPDC-brightness, while always matching the effective focusing parameter for the signal-photon fiber coupling. Experimentally, this was achieved by employing a three-lens beam expander telescope in the collimated part of the pump (and signal) beam, carefully monitoring the collimation condition and optimizing the NIR fiber coupling. Thus, we were able to obtain a maximally optimized SPDC brightness and quantum interference visibility for a large range of different focusing conditions. The maximum brightness was observed for a pump beam diameter of approximately 30 µm in a 2.5 mm long ppKTP crystal, corresponding to a focusing parameter of about 0.7. This result supports the optimized designing and miniaturizing of SPDC experiments and thereby allows a wider application in real-world measurement systems.
Recent years have seen the development of quantum sensing concepts utilizing nonlinear interferometers based on correlated photon pairs generated by spontaneous parametric down-conversion (SPDC). Using SPDC far from frequency degeneracy allows a “division of labor” between the mid-infrared photon for the strongest sample interaction and the correlated near-infrared photon for low-noise detection. The small number of photons provided by SPDC and the resulting inferior signal-to-noise ratio are, however, a limiting factor preventing the wide applicability of the novel sensing concept. Here, we demonstrate a nonlinear interferometer based on pump-enhanced SPDC with strongly improved emission rates while maintaining broadband spontaneous emission. For validation of the concept, we demonstrate high-resolution mid-infrared spectroscopy with near-infrared detection, showcasing improved accuracy. Although the number of mid-infrared photons is about five orders of magnitude smaller than in classical spectrometers, the sensitivity of the quantum spectrometer becomes comparable, marking an essential step toward real-world applications.
Metrology with undetected photons is an emerging technique that leverages quantum effects and photon correlations (entanglement) to retrieve valuable information in a target spectral range (e.g., mid-infrared, mid-IR) using measurements in an easily accessible domain (e.g., visible, near-IR). The underlying quantum process of spontaneous parametric down-conversion (SPDC) is utilized to generate non-degenerate correlated signal and idler photons to serve as detection and probing photons, respectively. Sensing with undetected photons enables important advantages, such as ultra-low probe powers, room-temperature operation, and shot-noise-limited detection. In this contribution, we apply a quantum nonlinear interferometer based on an SPDC source to perform applied mid-IR spectroscopy, mid-IR microscopy, and mid-IR optical coherence tomography (OCT) as among the most promising techniques for quantum-based routine non-destructive testing. Moreover, we characterize the system, benchmark it against classical systems, and provide a prospective outlook for this new technology.
The emissions of two identical spontaneous parametric down-conversion (SPDC) sources pumped coherently, e.g. with the same beam (c.f. Fig. 1(a)), show interference in both, the signal and idler light. This phenomenon results from the indistinguishability of the photon pairs emitted by the identical sources [1]. If a medium between the two SPDC sources absorbs the idler light, the photon pairs from the first and second SPDC source become distinguishable and the interference contrast decreases for both, signal and idler light. By measuring the signal light spectrum in the visible or near-infrared (NIR) range and the corresponding interference contrast, we obtain the information about the idler light absorption of the analyte in the mid-infrared (MIR). Hereby, we enable spectroscopy in the MIR using a standard silicon detector, making detection easier and more efficient than using typical MIR detectors [2].
We report an experimental demonstration of wavelength variable generation and detection of photon pairs in the visible and mid-infrared (MIR) regions over a wide spectral range of 2–5 µm via spontaneous parametric downconversion. Despite the recent increase in interest in such a photon-pair source, there have been few detailed evaluations of emitted photons generated via the downconversion process in the low gain regime, due to the lack of suitable single-photon detectors. By changing the angle of the nonlinear crystal, we continuously tune the phase-matching condition and generate photon pairs as signal photons in the wavelength range of 600–965 nm and idler photons in the wavelength range of 1186–4694 nm. We evaluate the generated photon pairs using single and coincidence counts by superconducting nanowire single-photon detectors up to a wavelength of 2 µm and detect the intensity using an InSb detector with a lock-in detection system up to 5 µm. From this analysis, a pair generation rate of 10 5 s − 1 per mW of pump power is experimentally obtained for this wavelength range. This work provides a basis for the realization of applications such as heralded MIR single-photon sources, infrared imaging, and infrared spectroscopy based on quantum technologies in the MIR region.