Multiple-pass cell for very-long-path infrared spectrometry

Compact optical multipass matrix system design based on slicer mirrors

High path-to-volume ratio (PVR) and low-aberration-output beams are the two main criteria to assess the performance of multipass absorption cells. However, no substantial progress has been reported for large-numerical-aperture-coupled multipass cells, which is due to the accumulated aberrations caused by a large number of off-axis reflections. Based on Chernin's design, in this study, we modified Chernin's four-objective multipass matrix cell by using slicer mirrors to eliminate alignment difficulty and decrease the system volume. A generalized design routine based on user requirements is also proposed. Based on the automatic modeling tool package (Pyzdde) connected with Zemax and boundary conditions of the parameters selection proposed, a low-aberration-output beam and a high PVR are easily obtained compared with other multipass cells schemes. In one demo design, 108 passes (5×7 matrix spots) in a base length of 300 mm are presented. The PVR and peak-to-valley value wavefront errors are 67.5 m/L and 0.92 μm, respectively. Finally, a tolerance analysis of this optical multipass system is also presented. This work may provide better broadband optical absorption cells in terms of response time and a better detection sensitivity in versatile applications.

Biconic White multipass cell design based on a skew ray-tracing model

A biconic White multipass cell (bi-WMPC), designed as a compact, zero-geometrical-loss, anastigmatic optical system, capably used with an extended divergent source, is proposed. First, a skew ray-tracing model is developed for a conventional White-type multipass cell (WMPC), based on which the astigmatism is calculated, together with its sensitivity coefficients in relation to all-important optical structural parameters in various configurations. Next, a generalized bi-WMPC is designed to suppress the astigmatism, which leads to much smaller reflection spots on the field mirror compared to the conventional design. The demonstration of an optimized bi-WMPC initialized from a commercial WMPC (52 pass with a 0.8 m base length) suggests a 53 fold reduction of the wavefront error value from 79.187λ to 1.493λ, as well as the path-to-volume ratio (PVR) increase from 20.8 to 35.2 m/L. These improvements are critical for the design of a compact WMPC with a path length of tens to hundreds of meters. Presently developed skew ray-tracing models for a WMPC can also be applied to other freeform surfaces to further compensate the inherent aberrations induced by multiple off-axis reflections.

Mid-Infrared Trace Gas Sensor Technology Based on Intracavity Quartz-Enhanced Photoacoustic Spectroscopy

The application of compact inexpensive trace gas sensor technology to a mid-infrared nitric oxide (NO) detectoion using intracavity quartz-enhanced photoacoustic spectroscopy (I-QEPAS) is reported. A minimum detection limit of 4.8 ppbv within a 30 ms integration time was demonstrated by using a room-temperature, continuous-wave, distributed-feedback quantum cascade laser (QCL) emitting at 5.263 µm (1900.08 cm^-1) and a new compact design of a high-finesse bow-tie optical cavity with an integrated resonant quartz tuning fork (QTF). The optimum configuration of the bow-tie cavity was simulated using custom software. Measurements were performed with a wavelength modulation scheme (WM) using a 2f detection procedure.

Novel focal point multipass cell for absorption spectroscopy on small sized atmospheric pressure plasmas

A novel focal point multipass cell (FPMPC) was developed, in which all laser beams propagate through a common focal point. It is exclusively constructed from standard optical elements. Main functional elements are two 90(∘) off-axis parabolic mirrors and two retroreflectors. Up to 17 laser passes are demonstrated with a near-infrared laser beam. The number of laser passes is precisely adjustable by changing the retroreflector distance. At the focal point beams are constricted to fit through an aperture of 0.8 mm. This is shown for 11 beam passes. Moreover, the fast temporal response of the cell permits investigation of transient processes with frequencies up to 10 MHz. In order to demonstrate the applicability of the FPMPC for atmospheric pressure plasma jets, laser absorption spectroscopy on the lowest excited argon state (1s5) was performed on a 1 MHz argon atmospheric pressure plasma jet. From the obtained optical depth profiles, the signal-to-noise ratio was deduced. It is shown that an elevation of the laser pass number results in an proportional increase of the signal-to-noise ratio making the FPMPC an appropriate tool for absorption spectroscopy on plasmas of small dimensions.

A new device for formaldehyde and total aldehydes real-time monitoring

A new sensitive technique for the quantification of formaldehyde (HCHO) and total aldehydes has been developed in order to monitor these compounds, which are known to be involved in air quality issues and to have health impacts. Our approach is based on a colorimetric method where aldehydes are initially stripped from the air into a scrubbing solution by means of a turning coil sampler tube and then derivatised with 3-methylbenzothiazolinone-2-hydrazone in acid media (pH = -0.5). Hence, colourless aldehydes are transformed into blue dyes that are detected by UV-visible spectroscopy at 630 nm. Liquid core waveguide LCW Teflon® AF-2400 tube was used as innovative optical cells providing a HCHO detection limit of 4 pptv for 100 cm optical path with a time resolution of 15 min. This instrument showed good correlation with commonly used techniques for aldehydes analysis such as DNPH derivatisation chromatographic techniques with off-line and on-line samplers, and DOAS techniques (with deviation below 6%) for both indoor and outdoor conditions. This instrument is associated with simplicity and low cost, which is a prerequisite for indoor monitoring.

Multipass cell based on confocal mirrors for sensitive broadband laser spectroscopy in the near infrared

We report on broadband absorption spectroscopy in the near IR using a multipass cell design based on highly reflecting mirrors in a confocal arrangement having the particular aim of achieving long optical paths. We demonstrate a path length of 314 m in a cell consisting of two sets of highly reflecting mirrors with identical focal length, spaced 0.5 m apart. The multipass cell covers this path length in a relatively small volume of 1.25 l with the light beam sampling the whole volume. In a first application, the absorption spectra of the greenhouse gases CO(2), CH(4), and CO were measured. In these measurements we used a femtosecond fiber laser with a broadband spectral range spanning the near IR from 1.5 to 1.7 μm. The absorption spectra show a high signal-to-noise ratio, from which we derive a sensitivity limit of 6 ppmv for methane observed in a mixture with air.

A new, low temperature long-pass cell for mid-infrared to terahertz spectroscopy and synchrotron radiation use

A new cell has been designed for accurate spectroscopic measurements in the 80-400 K temperature range with variable path lengths from 3 to more than 141 m. The spectral coverage at these temperatures ranges from the visible to less than 10 cm(-1), thanks to the use of diamond windows. The design of the cryostat and vacuum setups allows vibration-free operation. The equipment provides temperature homogeneity and pressure control to better than 2% over the 100-400 K and the 0.1-1000 mbar ranges. Remote-controlled opto-mechanical systems enable in situ adjustments as well as changes of the optical path length within half an hour, in order to optimize measurement time in an open user facility. It allows then to meet the specific requirements of high resolution measurements on the Far-Infrared AILES beamline at SOLEIL as well at the LISA facility, in Créteil, in the mid-IR. This new instrument opens up the way for many experiments in the field of high-resolution gas-phase IR spectroscopy, in particular, in quantitative spectroscopy for atmospheric applications: measurements of absorption line parameters (absolute intensities, cross sections, and pressure-induced widths) using Fourier transform spectroscopy. The design and performance of the equipment are briefly presented and illustrated on spectroscopic examples.

Approximate analytic astigmatism of unit-magnification multipass system

We develop a way to estimate the approximate analytic astigmatism with a high accuracy for any unit-magnification multipass system (UMS). The coaxial optical transmission model for UMS is simplified based on the system's features. Furthermore, astigmatism is derived as a distinct form of vector addition and, thus, feasible analytic astigmatism can be obtained. The effectiveness of our method is verified by simulations for a Bernstein-Herzberg White cell. In our cases, the relative error of optimization for astigmatism correction by our method is smaller than 5 per thousand, which is only one-tenth of that by Kohn's method. Our method significantly improves the efficiency for astigmatism correction, and further benefits the optical design of a UMS.

Generalized method for calculating astigmatism of the unit-magnification multipass system

A generalized method to accurately calculate astigmatism of the unit-magnification multipass system (UMS) is proposed. A practical coaxial optical transmission model is developed for the UMS. Astigmatism analysis is then made convenient by a 4 by 4 general transfer matrix. Astigmatism correction is significantly promoted, and hence further improvement in imaging quality can be expected. Good agreement between numerical simulations and Zemax ray tracing results verifies the effectiveness of this method. The resulted RMS spot size of this method is only 25% to 64% of other previous methods based on the golden section search for minimum astigmatism in real design cases. This method is helpful for the optical design of the UMS.

Infrared spectroscopic methods for the study of aerosol particles using White cell optics: Development and characterization of a new aerosol flow tube

A description of a new aerosol flow tube apparatus for measurements in situ under atmospherically relevant conditions is presented here. The system consists of a laboratory-made nebulizer generation system and a flow tube with a White cell-based Fourier transform IR for the detection system. An assessment of the White cell coupled to the flow tube was carried out by an extensive set of experiments to ensure the alignment of the infrared beam and optimize the performance of this system. The detection limit for CO was established as (1.0+/-0.3) ppm and 16 passes was chosen as the optimum number of passes to be used in flow tube experiments. Infrared spectroscopy was used to characterize dry aerosol particles in the flow tube. Pure particles composed of ammonium sulfate or sodium chloride ranging between 0.8 and 2.1 mum for size diameter and (0.8-4.9)x10(6) particles/cm(3) for density number were generated by nebulization of aqueous solutions. Direct measurements of the aerosol particle size agree with size spectra retrieved from inversion of the extinction measurements using Mie calculations, where the difference residual value is in the order of 0.2%. The infrared detection limit for ammonium sulfate aerosol particles was determined as d(p)=0.9 mum and N=5x10(3) particles/cm(3) with sigma=1.1 by Mie calculation. Alternatively, Mie calculations were performed to determine the flexibility in varying the optical length when aerosol particles are sent by the injector. The very good agreement between the values retrieved for aerosol particles injected through the flow tube or through the injector clearly validates the estimation of the effective optical path length for the injector. To determine the flexibility in varying the reaction zone length, analysis of the extinction spectra as function of the position of the injector was carried out by monitoring the integrated area of different absorption modes of the ammonium sulfate. We conclude that the aerosol loss in the flow tube reactor is negligible and that the aerosol particles remain on-axis for the length of the flow tube.

Simple, stable, and compact multiple-reflection optical cell for very long optical paths

A new type of multiple-reflection optical cell is presented. One of the main advantages of this type of cell is that it can be made of standard mirrors without particular tolerance while allowing a great number of reflections and thus a large optical path, only limited by the reflection coefficient of the mirrors. The configuration is simple, compact, stable, and cheap. This cell consists of three mirrors as in a White cell but its principle is different. It behaves as a multiplier of a Herriott cell from which it inherits the opto-mechanical stability qualities. The Herriott cell and the White cell are two particular cases of this type of cell. As examples, a demonstrator and an absorption cell contained in a volume of 5 l are presented. The first device is usable with a laser in visible light. The second device is usable with an infrared laser diode for the detection of atmospheric trace species.

High-luminosity multipass cell for infrared imaging spectroscopy

An original imaging multipass cell for infrared spectroscopy has been designed and built. The cell design is aimed at overcoming intrinsic sensitivity limitations associated with the low specific spectral radiance power of blackbody sources. Owing to the implemented low f number, the detector collects a large amount of the energy emitted over a wide angle by a blackbody source. In addition, the adopted optical configuration allows maintenance of the same spatial distribution of the radiance pattern at the cell entrance and exit (imaging capability) within an aperture area of several square millimeters. This feature allows the use of uncollimated blackbody-type emitter arrays and infrared sensor arrays coupled with linear, spectrally variable filters, and performance of spectroscopic measurements of infrared absorption for low concentrated gases detection. In the present design the cell has an f number of about 2, and the optical path is ten times larger than the cell length.

Theoretical and practical consideration of the construction of a zero-geometric-loss multiple-pass cell based on the use of monolithic multiple-face retroreflectors

The theory of the multiple-pass cell based on the use of retroreflectors is presented. As a result of this study, it is shown that it is possible to construct an enhanced White cell with zero geometric loss. Starting from theoretical considerations of the design of a new monolithic multiple-face retroreflector, a multiple-pass cell is proposed. Ray-tracing simulations indicate that this cell is easy to align and has zero geometric loss over a very long optical path.