Evanescent wave gas sensing based on modulation spectroscopy

In recent years, optical gas detectors based on a tunable diode laser absorption spectrometer has become more and more important for monitoring trace gases in atmospheric, medical, and industrial processes. In the medical field, blood gases, essentially pO2 and pCO2, are vital parameters for patient respiratory status monitoring. Typically, hypoxaemia or hypercapnia can be very harmful, especially for neonates, and requires continuous monitoring for rapid clinical intervention. Transcutaneous blood gases, permeating through the skin, can be continuously measured in patients by electrochemical sensors applied to the surface of the skin. These sensors suffer specific instability and must be re-calibrated. For this reason, the present thesis explores a new concept of gas sensors, applying light absorption at the surface of an integrated optics waveguide and using the advantages of a modulated spectroscopy technique for sensitivity and selectivity enhancement. In other words, the present thesis sets a mathematical model and demonstrates experimentally a new concept of evanescent wave gas sensing based on modulation spectroscopy. The field of application considered in this book is restricted to gas detection, however the concept can also be applied to the detection of liquids. In particular, we demonstrate the potential applicability of the concept to transcutaneous blood gas monitoring, and especially carbon dioxide monitoring. The system transfer function modelization of the concept includes carbon dioxide absorption spectrum in the near-infrared region (1.6µm), laser diode modulation spectroscopy assuming optical frequency modulation (FM) as well as intensity modulation (IM) induced by current modulation, and integrated optics sensor-head transmission. The modelization evidences that the silicon nitride technology has the advantage of: high evanescent wave sensitivity (8% for the selected TE mode), low propagation loss, possibility of strong curvatures for miniaturization and availability of fiberto- waveguide mode matching technique (taper). In the modelization, we evidence that the optimal waveguide length equals the inverse of the propagation loss which is mainly limited by the waveguide scattering. Also, we demonstrate that the implementation of a fiber-to-waveguide mode matching taper at both waveguide ends, drastically decreases the resonance limitation of the system resolution. Experimentally, the transfer function of the concept is validated and characterized based on a free-space modulation spectroscopy setup whose performance, in terms of second harmonic resolution, is first measured at 3.55 · 10-6 1/√Hz corresponding to 0.275 mmHg/√Hz CO2 partial pressure. In the chosen region of 200-400MHz of frequency modulation, we confirm that the 1/f-noise is negligible and the dominating amplifier noise can be approximated by the thermal noise. In the case of high insertion loss and short interaction length the interferometric noise (or etalon-effect) becomes the system resolution limitation. By the insertion of a prism coupled planar waveguide instead of the free-space cell, we demonstrate the concept of integrated optics evanescent wave sensing based on modulation spectroscopy. The system characterization is completed by the validation of the evanescent wave sensitivity in gas of about 8% for a laterally confined waveguide. The outlook of a transcutaneous blood gas monitoring instrument is also established. We first extrapolate the expected waveguide characteristics to show that a resolution of 1mmHg CO2 partial pressure can be reached in 4.5 to 12sec response-time. Then, the requirement of a maximal sensing area of 10mm diameter is at the origin of two geometries of sensor-head: a crossing spiral and a non-crossing double spiral. We verify experimentally a propagation loss as low as 0.5dB/cm and observe a stringent tolerance for light injection. The latter requires an integrated device for mode matching and pigtailing. After implementation of a taper at both waveguide ends, we demonstrate the drastic reduction of the total coupling loss. The carbon dioxide measurements reveal a relatively deep pattern on the second harmonic signal that tends to prove that polarization and resonances are coupled together most likely at the waveguide level. Applying nitrogen (N2) as reference gas, the pattern compensation results in the carbon dioxide signature measurement in direct detection only. The next generation, now in process, is a patented pigtailed non-crossing double-spiral sensor-head building the first step towards an industrial prototype. The waveguide miniaturization allows an interaction length longer than 21cm on a 3×3mm area. The improvement towards a high resolution system concerns the propagation loss and the coupling loss on the sensor-head, and concerns the polarization control or scrambling on the modulation spectroscopy setup. We finally conclude, following the demonstration on the planar waveguide and the extrapolation of the measured characteristics, that the concept of evanescent wave gas sensing based on modulation spectroscopy, can compensate the lack of sensitivity of an integrated optics sensor-head and can be applied in gas and liquid sensing. Moreover, based on the sensor-head developments and realistic improvements, we also conclude that the perspective of high resolution sensing system, and in particular for carbon dioxide determination, evidences the potential applicability of the concept to transcutaneous blood gas monitoring. This opens a new optical outlook of continuous and non-invasive blood gas monitoring, applied in medicine for patient respiratory status monitoring.

Supercontinuum generation in silicon nitride (Si_3N_4) waveguides for middle infrared spectroscopy

Eirini Tagkoudi

The middle-infrared (mid-IR) spectral range hosts the most intense roto-vibrational absorption lines of many important molecules. Particularly, mid-IR spectroscopy constitutes a unique tool for identifying and quantifying molecular species through their mid-IR spectral fingerprints. Exploiting efficient ways to access the mid-IR is of supreme importance for spectroscopic applications and environmental monitoring. Supercontinuum (SC) sources have been a common way to access this spectral region, using femtosecond fiber lasers and optical fibers as the broadening medium. However, emphasis has recently shifted towards compact SC sources based on integrated platforms. Among the different materials studied for the integration of SC, stoichiometric silicon nitride,

\mathrm{Si_3N_4}

, is one of the most promising. With a transparency window extending from the visible to mid-IR, CMOS compatibility, and a mature fabrication technology combined with flexibility in design,

\mathrm{Si_3N_4}

waveguides represent an attractive solution for on-chip SC generation. The thesis's main aim is to develop a tunable mid-IR SC source based on

\mathrm{Si_3N_4}

waveguides that can efficiently reach the spectral region between 3 - 4

\mathrm{\mu m}

, enabling mid-IR spectroscopy experiments. Towards this goal, nonlinear SC dynamics based on anomalous and all-normal-dispersion (ANDi) pumping regimes are explored in different waveguide designs. In the first part of the thesis, the development of a tunable mid-IR source is presented. Starting from a tunable turn-key femtosecond 2

\mathrm{\mu m}

fiber laser located in the anomalous dispersion regime of cladded waveguides, mid-IR soliton-induced dispersive waves (DWs) are generated. A synergy of proper waveguide geometry and pump positioning enables favorable dispersion profiles towards the mid-IR while suppressing the generation into the visible. Moreover, by switching to a waveguide with different dimensions, the DW can be tuned between the entire 3 - 4

\mathrm{\mu m}

spectral region, reaching record efficiencies up to 35%. The milliwatt output powers of the mid-IR DWs guarantee their utilization in gas spectroscopy. In the second part, the application of the on-chip generated mid-IR source in direct absorption spectroscopy is demonstrated for the first time. The most efficient DW generated at 3.0

\mathrm{\mu m}

is used to detect acetylene in a proof-of-principle spectroscopic experiment. However, due to DWs' limited bandwidth, parallel gas detection is not possible. By leveraging a waveguide that generates a DW centered at 3.5

\mathrm{\mu m}

and fine-tuning the pump wavelength, a larger bandwidth of the DW source is achieved, enabling simultaneous detection of multiple gas-phase species. Using the broad mid-IR DW from a single

\mathrm{Si_3N_4}

waveguide, detection of acetylene, methane, and ethane is successfully demonstrated, with hundreds of ppm detection limit. Finally, a polarization-sensitive SC source based on an un-cladded

\mathrm{Si_3N_4}

waveguide is studied. The waveguide is engineered to yield ANDi at 2

\mathrm{\mu m}

for the TM mode and anomalous dispersion regime for TE mode excitation. A flat and highly coherent ANDi SC or an octave-spanning soliton fission-driven SC source can be selectively generated by simply switching between the pump's polarization states. As a result, the SC has the potential for versatile applications, from broadband on-chip sensing to pulse compression.

EPFL2021

Distributed sensing using polarization sensitive optical low-coherence reflectometry and fiber gratings

Dragan Coric

Optical low coherence reflectometry (OLCR) is non-destructive interferometric technique that allows measuring amplitude and phase of the light reflected from the device under test. OLCR is a powerful tool for the characterization of the various optical devices such as fiber gratings and optical waveguides. This thesis work had explored possibilities to combine the OLCR technique with fiber gratings to perform distributed strain and temperature measurements, and to develop novel sensing techniques as an alternative to classical spectral methods. Special attention is devoted to polarization sensitive measurements and techniques, and OLCR techniques that use only amplitude information, as an alternative to more complicated phase sensitive measurements. Using a polarization sensitive system two methods for the measurement of local birefringence of fiber Bragg gratings (FBG) using OLCR were developed. The first technique uses oscillations in the OLCR amplitude signal to directly obtain the beat length and the birefringence. The second method is based on the measurement of the OLCR phase and inverse scattering algorithm. Both methods were compared with birefringence obtained from spectral measurements, and very good agreement was obtained. The indirect method, based on local Bragg grating determination requires two independent measurements and mathematical reconstruction, but provides a very high spatial resolution of ≈ 25 µm. The grating length limits the minimum measurable birefringence in the direct method, but a good sensitivity of 4×10-6 was obtained, which corresponds to a Bragg wavelength shift of 4 pm. The spatial resolution is in the millimeter range in this case. The novel methods were successfully applied for the distributed measurement of birefringence of FBG under diametric load. New types of tunable devices and sensors – fiber Bragg gratings written in high attenuation fibers (HAF) were developed. Active tuning by heating was achieved by optical pumping of a pure single mode fiber without any mechanical or electrical parts, or deposited light absorption coatings. Tuning can be controlled by the applied pump power, the position of the grating, the HAF attenuation level, and the pumping configuration. The proposed design assures a high repeatability and long lifetime. Using OLCR these devices were successfully applied to measure the liquid level with a spatial resolution of 100 micrometers. Liquid level measurements that use both phase and amplitude, or only amplitude were demonstrated. The same principles could be easily applied to design other fiber grating devices (long-period, tilted etc), and sensors based on hot-wire anemometry, like flow or vacuum sensors.

EPFL2007

Subwavelength Metallic Structures for Sensing

Qing Tan

Following rapid developments in biotechnology and medicine, optical sensing promises to be extremely important in various applications such as drug discovery, environmental monitoring, etc. Refractive index (RI) based optical sensing is straightforward to monitor or analyze the physical and chemical properties of substances. Among diverse configurations developed for RI detection, plasmonics devices using nanoscale patterned metals are particularly promising. Nano-structured metallic devices enable field confinement in a volume smaller than the diffraction limit. Moreover, they present possibilities for device miniaturization and sensor multiplexing on the same substrate. This thesis is dedicated to investigating a sensing platform combining nanostructured metallic cavities and dielectric waveguides. Periodically structured Au films have been explored for their application in local RI sensing. Two structures are considered: the slot waveguide cavity (SWC) and the annular aperture array (AAA). The SWC is composed of a Au film structured by a periodic slot array, which is deposited on a silicon (Si) waveguide. The evanescent field overlap correlates the resonant optical response of the cavity to the transmission spectrum of the Si waveguide. Theoretical studies, fabrication and characterization have been performed to verify the RI sensing ability for extremely subwavelength slot dimensions (30 nm slot width for 20 nm-thick Au film and 700 nm cavity length). The experimental sensitivity, 730 ± 10 nm/RIU, corresponds to the theory and an optimum resolution of 5.8 × 10-5 RIU is anticipated for RI detection. For the AAA device, the patterned Au film is directly embedded in a Si3N4 waveguide. The Fabry-Pérot-like resonance of the Au cavity results in a resonant reflection, enabling a device sensitive to RI variations. For a wide range of analytes, the theoretical sensitivity is 764 nm/RIU in the near-infrared wavelength range. Compared with other resonance based photonic sensors, the two devices exhibit comparable sensitivities to the RI variation. The small metallic cavity implies a local detection on nano scale and the sample volume can be on the order of a femtolitre. Furthermore, the planar configuration permits easy integration with other photonic devices. Parallel sensing using compatible sensors on the same substrate is anticipated for the realization of portable and low-cost sensor systems.

EPFL2012