Optical and Quantum Sensing and Precision Metrology II
Long wavelength, small quantum interferometer module: shrinking MIR OCT systems via quantum entanglement
Gunnar Blume, David Feise, Alexander Sahm, Philipp Hildenstein, Nils Werner, Johannes Zender, Fabian Wendt, Felipe Lucas Gewers, Emma J. Pearce, Sven Ramelow, Katrin Paschke
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 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.
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.
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.
Quantum-enhanced optical systems operating within the 2-2.5 µm spectral region have the potential to revolutionize emerging applications in communications, sensing and metrology. However, until now, sources of entangled photons have been realized mainly in the near-infrared 700-1550 nm spectral window. Above 2 µm lies an atmospheric transparency window with nearly one-third of the solar blackbody radiation as what is typical at telecom wavelengths. There is, therefore, a growing interest to deliver sources and detectors operating in this wavelength range for free-space optical communications and in emerging fields of high-sensitivity metrology, specifically, gravitational wave detection [1,2]. Here, using a 130 fs pulsed laser at wavelength 1.05 μm to pump custom-designed non-linear crystals for spontaneous parametric down-conversion (SPDC) process and tailored superconducting nanowire single-photon detectors (SNSPD), we demonstrate for the first-time two-photon quantum interference and polarization entanglement in the mid-infrared, at ≃ 2.1 μm.
Optical coherence tomography (OCT) is an appealing technique for bio-imaging, medicine, and material analysis. For many applications, OCT in mid- and far-infrared (IR) leads to significantly more accurate results. Reported mid-IR OCT systems require light sources and photodetectors which operate in mid-IR range. These devices are expensive and need cryogenic cooling. Here, we report a proof-of-concept demonstration of a wavelength tunable IR OCT technique with detection of only visible range photons. Our method is based on the nonlinear interference of frequency correlated photon pairs. The nonlinear crystal, introduced in the Michelson-type interferometer, generates photon pairs with one photon in the visible and another in the IR range. The intensity of detected visible photons depends on the phase and loss of IR photons, which interact with the sample under study. This enables us to characterize sample properties and perform imaging in the IR range by detecting visible photons. The technique possesses broad wavelength tunability and yields a fair axial and lateral resolution, which can be tailored to the specific application. The work contributes to the development of versatile 3D imaging and material characterization systems working in a broad range of IR wavelengths, which do not require the use of IR-range light sources and photodetectors.
Mid-infrared (mid-IR) light scatters much less than shorter wavelengths, allowing greatly enhanced penetration depths for optical imaging techniques such as optical coherence tomography (OCT). However, both detection and broadband sources in the mid-IR are technologically challenging. Interfering entangled photons in a nonlinear interferometer enables sensing with undetected photons, making mid-IR sources and detectors obsolete. Here we implement mid-IR frequency-domain OCT based on ultra-broadband entangled photon pairs spanning from 3.3 to 4.3 µm. We demonstrate 10 µm axial and 20 µm lateral resolution 2D and 3D imaging of strongly scattering ceramic and paint samples. By intrinsically being limited only by shot noise, we observe times more sensitivity per integration time and power of the probe light. Together with the vastly reduced footprint and technical complexity, our technique can outperform conventional approaches with classical mid-IR light sources.