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Ultrasensitive Quantum Cascade Laser spectroscopy for the heterodyne detection of exhaled biomarkers

Final Report Summary - QCLASER NOSE (Ultrasensitive Quantum Cascade Laser spectroscopy for the heterodyne detection of exhaled biomarkers)

Introduction

Continuous wave quantum cascade lasers (cw-QCLs) are infrared sources which became popular for numerous applications concerning i.e. military and security purposes, medical research, environmental studies, industrial processes, analytical chemistry, and metrology. For spectroscopic applications, single mode operation of the source is required. Generally, this is achieved by implementing a distributed feedback (DFB) structure on the laser chip. However, DFB QCLs are typically designed for operation at a single target frequency and can be only tuned over a narrow spectral range (0.1 - 2 cm-1). If the tuning range is sufficient to resolve a targeted ro-vibrational absorption line of a single molecule, for each specific wavelength (and for each molecule) another QCL has to be designed and manufactured.

In this project, we focused on the development of a single widely continuous tuneable, mode-hop-free external cavity QCL (EC-QCL) for the detection of several medically relevant molecules. We implemented several key issues to improve the EC architecture in order to achieve wide tuning range (> 200 cm-1 within the 6 - 9 µm spectral region), fast tuning speed (>1 kHz full scanning range) and high resolution (< 0.001 cm-1).

EC-QCL setup

External cavity build around infrared semiconductor lasers are used to combine the broadband emission gain of a laser with the spectral selectivity of a grating for single mode operation. The external cavity, developed in this project consists of the following parts: L - a QC laser chip, l - a collimating lens (AR coated for 7 - 14 µm with focusing distance of 4 mm), G - a aluminium diffraction grating (150 groves/mm, blazed at 10.6 µm), and M - a gold mirror. The QCL chip shows a gain profile centred at 8 µm and operates at -30 °C. The first order diffraction of the collimated QCL beam is coupled back from the grating providing the optical feedback for lasing, while the 0th order diffraction creates an optical output beam. The mirror is mounted on the rotational platform together with the diffraction grating at 90 degree angle. This arrangement allows keeping the optical beam in fixed alignment while the wavelength tuning is performed. The rotating platform (grating and mirror) is controlled by a linear translation for slow and wide wavelength tuning and a piezo actuator for fast scanning. In the same way, the EC laser length can be adjusted. All the components are placed into a compact A4-size housing box, flushed with nitrogen gas to avoid water condensation. The flexibility of the mechanical designed of our set-up arrangement allows the instrument to be used with other QC lasers at other wavelengths without changing the EC configuration.

Main results

The main goal of this project concerned sensing of relevant molecules for biomedical applications. The use of the EC-QCL has two main advantages: to work in the mid-infrared region where most of the molecules exhibit strong absorption features (allows high sensitivity), to cover a wide spectral range in order to detect larger molecules (allows to measured broad absorption features). To demonstrate these advantages, the laser was coupled with an advanced spectroscopic sensing method: integrated cavity output spectroscopy (ICOS). The developed system is able to cover a huge tuning range of 250 cm -1 in the mid-infrared range from1150 to 1400 cm-1 (8.6 - 7.1 µm) with a fast tuning speed (> 100 Hz). For molecules having broadband absorption features, fast scanning is performed. As an example, the spectrum of acetone was recorded in 2 sec and compared with a simulated spectrum (PNNL data base). By increasing the recording time up to a few minutes, the full spectral region can be scanned at high resolution. These two modes of operation demonstrate the potential of EC-configuration to detect any kind of molecules absorption in the spectral region of 8 µm.

Conclusion and impact

Within this project, a compact EC-QCL sensor has been developed. The laser was used to detect a variety of different gases within a single, broad wavelength scan, in seconds. The detection has been done with ICOS making the setup relevant for the detection of low concentrations of exhaled biomarkers in real-time. In the near future, such an online monitoring system for biomarkers in breath could have a strong impact on disease management may. The potential of the easy-to-use system can have also its future in the field of preventive medicine, by allowing monitoring the relationships between lifestyle (diet, physical activity) and risk factors for diseases.

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