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

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.

Evanescent wave gas sensing based on modulation spectroscopy

Daniel Salzmann

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.

EPFL2003

Design, Fabrication and Characterization of Mid-Infrared Strip Waveguide for Laser Spectroscopy in Liquid Environments

Yu-Chi Chang

Optical sensors based on the interaction between molecular vibrations and mid infrared radiations attracted high attentions in the last decades. In this study, we present the development of a waveguide evanescent field sensor based on the absorption spectroscopy in liquid environments. The device consists of a single mode strip waveguide integrated with a microfluidic system. With a quantum cascade laser at the wavelength of 5.8 μm, we demonstrate the detection of cocaine dissolved in tetrachloroethylene (PCE). Mid infrared germanium waveguides on silicon have been proposed but not experimentally demonstrated. Here we realize this new type of waveguide with a monocrystalline germanium layer on a silicon substrate (Ge-on-Si) with standard photolithography and reactive ion etching. It is designed to be singlemode in transverse magnetic polarization at 5.8 μm. We used a method based on the Fabry-Perot resonance to measure the propagation loss, and the achieved lowest loss is 2.4 dB/cm. The bending loss was measured to be 0.12 dB per 90∘bend with the radius of 115 μm. A microfluidic chip made from a UV-curable adhesive is integrated to the Ge-on- Si waveguide. This is the first integration of this waveguide with a microfluidic system. Solutions of cocaine in PCE flow in the microfluidic channel and absorb the light in the evanescent field. A reference waveguide and an integrated Y- junction eliminate the noise from coupling fluctuations, and the limit of detection is calculated to be 5 μg/ml. Mid infrared photonics can be further developed based on the platform of Ge-on- Si waveguides. We demonstrate a detection of refractive index changes with a Mach-Zehnder interferometer integrated with a microfluidic system. Subwavelength structures for antireflection of the waveguide facets are also experimentally tested. We foresee an increasing development and applications based on this waveguide in the mid infrared.

EPFL2012

Mid infrared gas spectroscopy using efficient fiber laser driven photonic chip-based supercontinuum

Camille Sophie Brès, Davide Grassani, Hairun Guo, Clemens Herkommer, Tobias Kippenberg, Eirini Tagkoudi, Fan Yang

Directly accessing the middle infrared, the molecular functional group spectral region, via supercontinuum generation processes based on turn-key fiber lasers offers the undeniable advantage of simplicity and robustness. Recently, the assessment of the coherence of the mid-IR dispersive wave in silicon nitride (Si3N4) waveguides, pumped at telecom wavelength, established an important first step towards mid-IR frequency comb generation based on such compact systems. Yet, the spectral reach and efficiency still fall short for practical implementation. Here, we experimentally demonstrate that large cross-section Si3N4 waveguides pumped with 2 μm fs-fiber laser can reach the important spectroscopic spectral region in the 3–4 μm range, with up to 35% power conversion and milliwatt-level output powers. As a proof of principle, we use this source for detection of C2H2 by absorption spectroscopy. Such result makes these sources suitable candidate for compact, chip-integrated spectroscopic and sensing applications.