There is a trend for the development of mid-infrared laser spectroscopy with remote interrogation, multiplexed multi-point detection, and integrated photonics systems. The merging of semiconductor lasers with hollow-core fibers is one of the promising strategies for these purposes. In this talk I will discuss recent progress on mid-infrared laser spectroscopy performed in a novel type of hollow-core negative curvature fiber (HC-NCF). Sensitive CO, N2O, and H2CO detection between 2.3 and 3.6 µm using absorption spectroscopy will be presented. Photothermal sensing in the HC-NCF will also be discussed using the mid-infrared-pump and near-infrared-probe scheme recently proposed by our group.
Freeform optics are known for their advantages regarding optical performance and system integration. The use of additive manufacturing methods for the rapid production of freeform optics opens up new possibilities for optical metrology. By easily varying the shape and size of optical elements, optical systems specifically adapted to various applications can be fabricated cost-effectively. We present cost-effective freeform polymer optics for the application in Raman spectroscopy which combines laser focusing, Raman scattering collection and a mounting thread within one component. The aspheric surfaces of the optics were designed in a customized simulation tool and optimized regarding to Fresnel losses. The prototypes were fabricated by using a polymer-based Multi-Jet Modeling process. These prototypes were evaluated regarding their geometrical and optical properties and were successfully implemented in a compact and custom-designed Raman spectroscopy system. The system was built based on a continuous wave excitation laser emitting at 785 nm with a maximum output power of 0.5 W and a spectrometer providing a Stokes Raman shift resolution of 6.7 cm-1.
We report a quartz-enhanced photoacoustic sensor (QEPAS) for nitric oxide (NO) detection using a mid-infrared fibercoupled quantum cascade laser (QCL) near 5.2 μm. The QCL radiation was coupled into an InF3 fiber (100 μm core diameter) for light delivery to the quartz tuning fork, a tiny piezoelectric element converting the acoustic wave induced mechanical vibration to the gas-absorption associated electrical signal. This mid-infrared fiber can achieve nearly single-mode light delivery for the target wavelength. The off-beam configuration was adopted for the fiber-coupled detection considering its simpler installation, optical alignment and comparative sensitivity to the traditional on-beam setup.
This article presents the experimental and modeling study of quartz-enhanced photoacoustic detection of nitrogen monoxide (NO) using the off-beam configuration and a distributed-feedback (DFB) quantum cascade laser (QCL) at 5.26 μm as the excitation source. Trace gas monitoring of NO is one of the important subjects for both environmental protection and human health monitoring. Quartz-enhanced photoacoustic spectroscopy (QEPAS) with on-beam configuration is mostly adopted for gas detection. In comparison, the off-beam approach has not only comparative detection sensitivity but also significant advantage of simpler installation and optical alignment. We optimized the sensor performance by adjusting the horizontal and vertical distances between the micro-resonator (mR) and the QTF prongs. Pressure and humidity are two important factors affecting the photoacoustic signal. The effects of both parameters on the NO concentration determination were investigated.
Hydrogen peroxide (H2O2) is a relevant molecular trace gas species, that is related to the oxidative capacity of the
atmosphere, the production of radical species such as OH, the generation of sulfate aerosol via oxidation of S(IV) to
S(VI), and the formation of acid rain. The detection of atmospheric H2O2 involves specific challenges due to its high
reactivity and low concentration (ppbv to sub-ppbv level). Traditional methods for measuring atmospheric H2O2
concentration are often based on wet-chemistry methods that require a transfer from the gas- to liquid-phase for a
subsequent determination by techniques such as fluorescence spectroscopy, which can lead to problems such as sampling
artifacts and interference by other atmospheric constituents. A quartz-enhanced photoacoustic spectroscopy-based
system for the measurement of atmospheric H2O2 with a detection limit of 75 ppb for 1-s integration time was previously
reported. In this paper, an updated H2O2 detection system based on long-optical-path-length absorption spectroscopy by
using a distributed feedback quantum cascade laser (DFB-QCL) will be described. A 7.73-μm CW-DFB-QCL and a
thermoelectrically cooled infrared detector, optimized for a wavelength of 8 μm, are employed for theH2O2 sensor
system. A commercial astigmatic Herriott multi-pass cell with an effective optical path-length of 76 m is utilized for the
reported QCL multipass absorption system. Wavelength modulation spectroscopy (WMS) with second harmonic
detection is used for enhancing the signal-to-noise-ratio. A minimum detection limit of 13.4 ppb is achieved with a 2 s
sampling time. Based on an Allan-Werle deviation analysis the minimum detection limit can be improved to 1.5 ppb
when using an averaging time of 300 s.
Sensitive detection of nitric oxide (NO) at ppbv concentration levels has an important impact in diverse fields of
applications including environmental monitoring, industrial process control and medical diagnostics. For example, NO
can be used as a biomarker of asthma and inflammatory lung diseases such as chronic obstructive pulmonary disease.
Trace gas sensor systems capable of high sensitivity require the targeting of strong rotational-vibrational bands in the
mid-IR spectral range. These bands are accessible using state-of-the-art high heat load (HHL) packaged, continuous
wave (CW), distributed feedback (DFB) quantum cascade lasers (QCLs). Quartz-enhanced photoacoustic spectroscopy
(QEPAS) permits the design of fast, sensitive, selective, and compact sensor systems. A QEPAS sensor was developed
employing a room-temperature CW DFB-QCL emitting at 5.26 μm with an optical excitation power of 60 mW. High
sensitivity is achieved by targeting a NO absorption line at 1900.08 cm-1 free of interference by H2O and CO2. The
minimum detection limit of the sensor is 7.5 and 1 ppbv of NO with 1and 100 second averaging time respectively . The
sensitivity of the sensor system is sufficient for detecting NO in exhaled human breath, with typical concentration levels
ranging from 24.0 ppbv to 54.0 ppbv.
A trace gas absorption sensor for formaldehyde (H2CO) detection was developed using a continuous wave, room
temperature, low-power consumption interband cascade laser (ICL) at 3.6 μm. The recent availability of ICLs with
wavelength ranged between 3−4 μm enables the sensitive detection of trace gases such as formaldehyde that possesses a
strong absorption band in this particular wavelength region. This absorption sensor detected a strong formaldehyde line at
2778.5 cm-1 in its v1 fundamental band. Wavelength modulation spectroscopy with second harmonic detection (WMS-2f)
combined with a compact and novel multipass gas cell (7.6 cm physical length, 32 ml sampling volume, and 3.7 m optical
path length) was utilized to achieve a sensitivity of ~6 ppbv for H2CO measurements at 1 Hz sampling rate. The Allan-
Werle deviation plot reveals that a minimum detection limit of ~1.5 ppbv can be achieved for an averaging time of 140
A quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor system was developed for the sensitive detection
of hydrogen peroxide (H2O2) using its absorption transitions in the v6 fundamental band near 7.73 μm. The recent
availability of distributed-feedback quantum cascade lasers (DFB-QCLs) provides convenient access to a strong H2O2 absorption line located at 1295.55 cm-1. Sensor calibration was performed by means of a water bubbler that generated titrated average vapor concentrations. A minimum detection limit of 75 parts per billion (ppb) was achieved at a pressure
of 80 torr for a 1 sec data acquisition time. The long-term repeatability and stability of the sensor was investigated by
measuring time-varying H2O2 mixtures for ~2 hrs. An Allan deviation analysis was performed to investigate the long-term performance of the QEPAS sensor system, indicating the feasibility of a minimum detection limit of 12 ppb using the optimum data averaging time of 100 sec.