Near-infrared hollow waveguide gas sensors

The development of a hollow core waveguide (HWG) gas sensor in combination with a fast and compact near-infrared (NIR) spectrometer is presented. The spectrometer operates in the spectral range of 1200-1400 nm and may thus be applied for the detection of gas-phase analytes providing NIR absorptions in that spectral window such as, e.g., methane. Since mid-infrared spectroscopy in combination with HWGs has already been successfully demonstrated for probing hydrocarbons in the gas phase, the present study investigates the achievable sensitivity in the NIR spectral regime. Methane has been selected as an exemplary analyte due to the fact that it shows strong absorption features in the mid-infrared (mid-IR) fingerprint area, but also overtone bands in the NIR. Since the HWG simultaneously serves as a miniaturized absorption gas cell and as an optical waveguide for NIR radiation, a compact yet optical and cost-efficient sensor device was established providing an interesting alternative in target sensing for mid-IR devices. The achieved limit of detection (LOD) was 5.7% (vol./vol.) methane for a 9.5 cm long HWG, 1.6% (vol./vol.) methane for a 39.1 cm long HWG, and 1.3% (vol./vol.) methane for a setup using a 77.4 cm long HWG, which provides the most practical HWG dimensions among the three investigated setups. Limit of quantitation (LOQ) values were calculated at 20.1% (vol./vol.) methane, 8.7% (vol./vol.) methane, and 5.6% (vol./vol.) methane, respectively.     

Infrared hollow waveguide sensors for simultaneous gas phase detection of benzene, toluene, and xylenes in field environments

Simultaneous and molecularly selective parts-per-billion detection of benzene, toluene, and xylenes (BTX) using a thermal desorption (TD)-FTIR hollow waveguide (HWG) trace gas sensor is demonstrated here for the first time combining laboratory calibration with real-world sample analysis in field. A calibration range of 100-1000 ppb analyte/N(2) was developed and applied for predicting the concentration of blinded environmental air samples within the same concentration range, and demonstrate close agreement with the validation method used here, GC-FID. The analyte concentration prediction capability of the TD-FTIR-HWG trace gas sensor also compares well with the industrial standard and other experimental techniques including GC-PID, ultrafast GC-FID, and GC-DMS, which were simultaneously operated in the field. With the advent of a quantum cascade laser with emission frequencies specifically tailored to efficiently overlap benzene absorption as the most relevant analyte, the overall sensor footprint could be considerably reduced to ultimately yield hand-held trace gas sensors facilitating direct and real-time detection of BTX in air down to low ppb levels.     

Self-assembled DNA-based fluorescence waveguide with selectable output

Using the principle of self-assembly, a fluorescence-based photonic network is constructed with one input and two spatially and spectrally distinct outputs. A hexagonal DNA nanoassembly is used as a scaffold to host both the input and output dyes. The use of DNA to host functional groups enables spatial resolution on the level of single base pairs, well below the wavelength of light. Communication between the input and output dyes is achieved through excitation energy transfer. Output selection is achieved by the addition of a mediator dye intercalating between the DNA base pairs transferring the excitation energy from input to output through energy hopping. This creates a tool for selective excitation energy transfer on the nanometer scale with spectral and spatial control. The ability to direct excitation energy in a controlled way on the nanometer scale is important for the incorporation of photochemical processes in nanotechnology.     

Wavelength-selective filter based on a hollow optical waveguide

In this paper, we describe a theoretical and experimental study of a wavelength-selective filter derived from hollow optical waveguides composed of Bragg reflectors with defect layers on a silicon substrate. The defect states of the transmission filter at wavelengths of 1519 and 1571 nm were realized using one-dimensional photonic crystals (1D PCs) formed from a-Si and SiO(2). The transmission spectra of the filter waveguides and the band structure of the defect 1D PCs were calculated using the two-dimensional finite-difference time-domain and transfer matrix methods, respectively. The device exhibited the narrow bandwidths of 0.5 and 1.1 nm for wavelengths of 1571 and 1519 nm, respectively.     

Embossable grating couplers for planar waveguide optical sensors

Planar optical waveguides are an attractive tool for use in analytical chemistry and spectroscopy. Although similar to fiber optics, planar waveguides have been slow to be commercially accepted due to the difficulty of coupling light into the guide. Generally, prism coupling is the method of choice in the laboratory, as efficiencies approaching 80% can be reached. However, prisms are impractical for routine use for several reasons: expensive positioning equipment is required, coupled power is sensitive to environmental fluctuations, and prism coupling prohibits the fabrication of a truly planar device. The use of thin gratings on the surface of the waveguide allows for a two-dimensional structure to be maintained, while providing enough efficiency to be useful as a sensor. Our research efforts focus on developing a technique to make inexpensive, reproducible gratings that are easy to fabricate. By chemically modifying the surface of a commercial grating with a suitable release agent, it is possible to emboss replica gratings onto a variety of waveguide types. The fabrication of embossed gratings will be described, and their performance on glass, ion-diffused, polymer, and semiconductor waveguides will be presented.     

Infrared multiphoton dissociation with a hollow fiber waveguide

A novel scheme for performing infrared multiphoton dissociation (IRMPD) is presented in which a hollow fiber waveguide (HFWG) is used to transmit IR radiation into the ion storage region of a mass spectrometer. Efficient dissociation of oligonucleotide and protein ions is demonstrated on an ESI-FTICR instrument in which IRMPD is performed in the external ion reservoir and on a quadrupole ion trap. Using a simple optical scheme consisting of a single focusing lens and an x, y translator, the 10.6-microm IR laser beam, initially 3.5 mm in diameter, is focused into the vacuum-sealed HFWG. The small internal diameter and the high transfer efficiency of the waveguide allow IR radiation of high power density to be employed for IRMPD. In studies performed on a quadrupole ion trap, a 500-microm-i.d. waveguide was used as a medium to transmit IR radiation directly through a 700-microm orifice in the ring electrode. Efficient IRMPD of both a 12-mer oligonucleotide and the protein melittin were performed at laser powers of 0.5 and 3.2 W, respectively.     

Design of Stark-tuned far-infrared laser with circular hollow waveguide

A novel design of a Stark-tuned far-infrared laser using a circular hollow dielectric waveguide is proposed, and its characteristics are studied. Supplementary electrodes are inserted inside the circular hollow dielectric tube to suppress charge accumulation while keeping field uniformity. In what is believed to be a new design, the mode property is found to be improved, and the angular dependency of the attenuation loss according to the beam polarization is estimated to be much smaller than that of the conventional rectangular hybrid waveguide design. In this new design, DeltaM=0 far-infrared (FIR) transition as well as DeltaM=+/-1 transition can be observed, and the power enhancement for the DeltaM=0 FIR transition is expected.     

A Raman waveguide detector for liquid chromatography

A novel real-time liquid core Raman waveguide detector designed for liquid chromatographic applications is described. The Raman waveguide detector provides enhanced selectivity over typical high-performance liquid chromatography (HPLC) detectors. The waveguide detector also greatly improves the sensitivity of a typical Raman measurement without resorting to surface enhancement or resonance approaches and is compatible with the typical peak width volumes eluted by microbore and minibore HPLC (packed 1-2-mm-i.d. columns). Detection limit enhancements of over 1000-fold have been achieved for the measurement of alcohols in the aqueous phase with the Raman cell utilizing liquid core waveguide technology. The liquid core waveguides demonstrated in this study were constructed using Teflon AF 2400 tubing with a refractive index of 1.29. The low refractive index of the polymer material allowed HPLC separations with Raman detection to be performed with an aqueous mobile phase. A calibration curve for aqueous solutions of 2-propanol was generated and a limit of detection (LOD) of 2 ppm was determined. The Raman waveguide detector is demonstrated for the HPLC analysis of alcohol test mixtures, with LODs in the low-ppm range at the detector. By coupling the temporal separation achieved by HPLC with the vibrational information gleaned from Raman detection, an information-rich multivariate data matrix is obtained that can be deconvoluted to provide chemical speciation even when the HPLC resolution is poor. In this paper, we will discuss the physical and optical design of the Raman waveguide detector and the demonstration of the detector for HPLC detection.     

Design and optimization of waveguide sensitivity in slot microring sensors

The waveguide sensitivity of silicon slot microring sensors and single- and double-slot microrings is analyzed using a combination of the effective index and the Airy-functions-based mode matching methods. The sensing properties of these two cases are investigated under a variety of geometries. The trends of the waveguide sensitivity on each geometrical parameter are obtained. In addition, the influence of asymmetry on the waveguide sensitivity is also investigated. Calculation also illustrates that double-slot microrings offer wider fabrication tolerance than single-slot ones. These results provide a guideline and insights for designing microring geometry to satisfy the desired sensing requirements and performance.     

Orientation effects in waveguide resonance Raman spectroscopy of monolayers

In this paper we study the effect of molecular orientation on the observed Raman spectra in waveguide Raman spectroscopy. Depolarization ratios differ from those in 'normal' Raman spectroscopy due to the polarization properties of the guiding modes that excite the Raman scattering. Waveguide Raman spectra of submonolayers of cytochrome c and NiOEP were recorded using different polarization settings. The observed depolarization ratios strongly deviate from those obtained in bulk Raman spectra. Using the derived equations for the depolarization ratios, the preferred orientation angle of the molecules can be obtained from the observed depolarization ratios. From these results we first conclude that high-quality spectra of sub-monolayers can be obtained using waveguide Raman spectroscopy and secondly, that these spectra can be used to determine the preferred orientation of the molecule with respect to the waveguide surface.     

Rectangular characteristic gratings for waveguide input and output coupling

Normal-incidence planar-optical waveguide-imbedded phase gratings of finite aperture width and length are analyzed with Svidzinskii's (Sov. J. Quantum Electron. 10, 1103 (1980)] two-dimensional Braggdiffraction theory. Svidzinskii's characteristic-grating equations are adapted for the rectangulargrating case, and an overlap integral is used to extend the theory to account for the mode structure of the waveguide. The combined theory is used to optimize the throughput of a system composed of an input grating coupler, a waveguide, and an output grating coupler for both the highly multimode (thickwaveguide) and the few-mode (thin-waveguide) cases.     

Characterization of supported cylinder-planar germanium waveguide sensors with synchrotron infrared radiation

Cylinder-planar Ge waveguides are being developed as evanescent-wave sensors for chemical microanalysis. The only non-planar surface is a cylinder section having a 300-mm radius of curvature. This confers a symmetric taper, allowing for direct coupling into and out of the waveguide's 1-mm(2) end faces while obtaining multiple reflections at the central <30-microm-thick sensing region. Ray-optic calculations indicate that the propagation angle at the central minimum has a strong nonlinear dependence on both angle and vertical position of the input ray. This results in rather inefficient coupling of input light into the off-axis modes that are most useful for evanescent-wave absorption spectroscopy. Mode-specific performance of the cylinder-planar waveguides has also been investigated experimentally. As compared to a blackbody source, the much greater brightness of synchrotron-generated infrared (IR) radiation allows a similar total energy throughput, but restricted to a smaller fraction of the allowed waveguide modes. However, such angle-selective excitation results in a strong oscillatory interference pattern in the transmission spectra. These spectral oscillations are the principal technical limitation on using synchrotron radiation to measure evanescent-wave absorption spectra with the thin waveguides.     

Interrogation of extrinsic Fabry-Perot interferometric sensors using arrayed waveguide grating devices

In this paper, we present details of a solid state interrogation system based on a 16-channel arrayed waveguide grating (AWG) for interrogation of extrinsic Fabry-Perot interferometric sensors. The sensing element is configured in a reflecting mode and is illuminated by a broad-band light source through an optical fiber. The spectrum of light reflected from the sensor is analyzed using an AWG device acting as a coarse spectrometer. Using measurement points from the AWG channels, the original spectrum of the sensing element is reconstructed by a means of curve fitting. This allows sufficient information for the position of the reflection peak (or inverted peak) to be uniquely determined and the value of a measurement quantity obtained. In addition to the theoretical simulations of the proposed measurement system, we provide details of the laboratory evaluation.     

Mechanoluminescent pulse pressure sensors. Processing of output optical signal

The process of mechanoluminescence transformation of a pulse pressure sensor is considered. The process consists in excitation of emission under the action of mechanical loading. An algorithm for use in processing the output optical signal of the sensor that makes it possible to determine an input shock pulse is presented. __________ Translated from Izmeritel’naya Tekhnika, No. 10, pp. 28–31, October, 2007.     

An arrayed waveguide grating based multiplexer and interrogator for Fabry-Perot sensors

In this paper, we propose an interrogation system capable of multiplexing four identical Fabry-Perot (FP) interferometric sensors using two wavelength-division multiplexing devices. One is a 40-channel DWDM channel monitor, the other a four-channel CWDM device. The sensors are connected to the output channels of the CWDM device in order to assign a portion of the spectrum to each sensor. The reflected spectra are then analyzed using the DWDM channel monitor. By monitoring the power incident on each of the DWDM channels, the four sensor reflection spectra can be reconstructed in software and information relating to the measurand obtained. Based on software simulations and previous laboratory experiments with single sensors, it is predicted that this system would be capable of interrogating four EFPI strain sensors simultaneously at frequencies greater than 5 kHz with a resolution of approximately 2 /spl mu//spl epsiv/ and a range of 3000 /spl mu//spl epsiv/.     

Improving near-field throughput in a hollow metallic tapered waveguide via wavefront modulation

Throughput enhancement through a 100?nm-aperture hollow tapered aluminum waveguide via wavefront modulation was investigated numerically. The propagating modes in the waveguide are controlled and efficiently transformed to the fundamental propagating mode with optical wavefront modulations via a centro-symmetrical 9-bit binary optical element. Combined with the adjustment effect of the focal length of the coupling lens, an optimal throughput enhancement of about 11 times is obtained.     

Angular momentum of the fields of a few-mode waveguide: Conversion of the angular momentum

An analysis is made of the transformation of the angular momentum density in the field of an unstable IV vortex of a few-mode optical fiber. It is shown that the effect of mode dispersion of IV vortices is observed as the conversion of the polarization and orbital components of the electrodynamic angular momentum. The angular momentum defect may be recorded experimentally as a mechanical twist of the optical few-mode fiber. Formally the dispersion process resembles the conversion of the signs of the orbital and polarization components of the angular momentum density. A complex pseudopotential, whose real and imaginary parts characterize the field lines and lines of equal pseudopotential, is introduced to describe the energy flux density of the fiber vortex. The conversion of field states with equivalent partial ê ⁺ F ₁(R)exp{−iφ} and ê2⁻ F ₁(R)exp{+iφ} vortices was investigated experimentally. Pis#x2019;ma Zh. Tekh. Fiz. 23, 58–65 (November 26, 1997)     

Extinction and scattering of a guided beam in a hollow dielectric waveguide.

We present a theoretical approach to the problem of mode scattering by a spherical object that is placed inside a circular dielectric waveguide. This approach is based on the separation-of-variables method for each subsystem, namely, the spherical inclusion and the circular dielectric cylinder, and on the concept of the generalized recursive T-matrix algorithm for multilayered structures. We apply the technique to the backward and the forward scattering of a quasi-optical beam in the form of the fundamental HE11 mode by a sphere inside a circular hollow dielectric waveguide. The results calculated for the perfectly conducting spherical objects inside the circular hollow dielectric waveguide are compared with corresponding measured data of the backward-and the forward-scattering characteristics at the 4-mm wave band.     

Raman imaging of carious lesions using a hollow optical fiber probe

Raman spectroscopy using a hollow optical fiber probe with a glass ball lens at the distal end is proposed for detection of early caries lesions. Raman spectroscopy on carious lesions of extracted teeth showed that the probe enables measurement with a high signal-to-noise ratio when combined with a ball lens with a high refractive index. The proposed probe and lens combination detects changes in Raman spectra caused by morphological differences between sound and carious enamel. We also obtained a high-contrast image of an early carious lesion by scanning the tooth surface with the probe.