Versatile short-wave and mid-infrared sources based on wideband parametric conversion

The mid-infrared part of the optical spectrum is of high interest in a wide range of applications such as environmental gas monitoring, contaminant detection in the chemical, food or pharmaceutical industry, medical diagnosis, or defense and security. Relevant molecules can readily be identified through their mid-infrared absorption spectra, as the latter contains the fundamental resonances of a number of pollutant and toxic gases. Consequently, spectroscopic apparatus, light detection and ranging systems or free-space communication links all benefit from the progress accomplished by mid-infrared technologies over the last years. However some shortcomings in the light emitters capabilities are still to be addressed. In this research work, we aim at designing a mid-infrared laser as versatile as possible and based on nonlinear wavelength conversion. The conversion relies on third-order parametric effects in waveguides such as optical fibers made of various types of glass, or integrated semiconductor chips. The objective is to leverage mature communication-band components to generate and shape the seed optical signals, then mixed in the abovementioned waveguides to down-convert them towards midinfrared. The wavelength conversion is performed in two stages, and the first stage consists of a parametric source emitting in the short-wave infrared range. This thesis mostly focuses on the design and realization of this stage. As such, it is closely linked to the field of nonlinear fiber optics, where the use of silica is preponderant. We build on the research performed over the last years on parametric amplifiers, initially used for the re-amplification of communication signals, and we combine it with technologies dedicated to short-wave infrared fiber lasers. As such, we are able to build wavelength tunable and modulation-capable short-wave infrared sources, significantly more powerful and versatile than previous broadband parametric converter designs. The end of the dissertation is then dedicated to the solutions that are then envisioned to realize the second conversion stage, towards mid-infrared. Very promising numerical and experimental results indicate a successful outcome to the project, confirming the validity of the laser concept.

High spectral resolution waveguide spectrometer with enhanced throughput for space and commercial applications

Mohammadreza Madi

This dissertation presents the results concerning the development of an innovative high spectral resolution waveguide spectrometer, from the concept to the prototype demonstration and the test results. The main innovative aspects of this instrument are the enhanced throughput and the large spectral bandwidth in a static design without mechanism. These innovations are based on the customized pattern of nano-samplers fabricated on the surface of a planar waveguide, which allow enhancing the throughput and to increase the measurement points of the interferogram necessary for increasing the spectral bandwidth in a static way. On the other hand, the propagation in the unconfined direction of the planar waveguide is controlled using an adapted front-end optics to maintain the superposition of forwarding and reflected beams to form the interferogram under the nano-samplers. Polymeric- and silicon oxynitride-based planar waveguides were manufactured, evaluated and tested for studying mode profile in planar waveguides. The nano-samplers are gold nanodisks of few tens of nanometers in diameter and thickness, optimized for sampling the visible light coupled in the waveguide module of the spectrometer. A waveguide spectrometer prototype is made in silicon oxynitride/silicon dioxide technology and characterized in visible range at 633 and 685 nm using stable monochromatic laser sources. This waveguide spectrometer shows a nominal bandwidth of 256 nm thanks to a custom pattern of nanodisks providing 0.25 ÎŒm sampling interval at central wavelength of 633 nm. The targeted prototype in the near future is a highly integrated spectrometer, with an approximate footprint of a few cubic millimeters, excluding the volume required by the front-end optics and the detector electronics. This new type of instrument can be adapted for various spectral ranges where the material for the waveguide, the nano-samplers and the detectors are available and compatible (from ultraviolet to mid-infrared). This instrument has the potential to be used for numbers of space applications, for instance for the applications where the sample is in a close proximity to the spectrometer (e.g. mineralogy from a rover or active gas detection in a capsule or space station), or in remote sensing applications (e.g. atmospheric observations from a satellite). In addition, the principle idea is to group several of such spectrometers to allow hyperspectral imaging: a 1D array of spectrometers to form an acquisition line to image with lateral scanning, heading towards a full cube of spectrometers in a 2D image acquisition scheme to probe directly a complete scene (the concept of "FPAS" - focal plane array spectrometer). This technology is also adaptable for vast commercial applications in tele-surveillance, optical metrology, genomics and medical domain. At the system level, the full instrument optimization is taken into account in this dissertation, including front-end optics and the detector. Different subsystems are evaluated, developed and tested in order to integrate and demonstrate a complete prototype.

EPFL2016

Photosensitivity and luminescence of chromium and bismuth doped fibers

Christian Ban

Optical fiber materials with broad-band gain in the visible or in the telecommunication window are of great interest for optical communication or the biomedical domain in order to built integrated tunable lasers, amplifiers, ultra-short pulse lasers, or broadband light sources. Possible activator ions in compact solid-state host materials for the generation of broadband luminescence, gain, and laser emission are transition-metal (TM), bismuth and rare-earth (RE) ions. Emission lines in TM and bismuth exhibit luminescence with typically several hundred of nanometers of spectral bandwidth. This is a much broader range than can be obtained from the emission of RE ions, which is restricted to less than hundred nanometers. Chromium and bismuth-doped fibers are of great interest as they are potential candidates for broadband light sources and tunable lasers. The idea of this work was to investigate the photosensitivity and luminescence properties of Cr and Bi-doped silica optical fibers in collaboration with the Fiber Optic Research Center at the Russian Academy of Sciences (FORC-RAS) who fabricated the fibers and glass samples. The photosensitivity of Cr-and Bi-doped fibers with different dopant concentrations was investigated using an ArF excimer laser emitting at 193 nm. No refractive index change was observed in pristine Cr-doped fibers. After H2-loading, refractive index change up to 2.82×10-3 was obtained in Cr-Ga-doped fiber. The hydrogen loaded annealed fiber showed an index change of 1.4×10-3. The Bi-Al-doped fibers showed permanent photo-induced index changes up to 2.0×10-4 (highest Bi concentration). Using H2-loading index changes up to 2.2×10-3 were measured without reaching saturation. Such index changes are almost comparable to index changes in H2-loaded Ge-silicate fibers and allow fabricating very strong Bragg grating fiber laser reflectors directly in the doped fibers. Stress measurements were performed to distinguish between color center and compaction that contribute to the photo-induced refractive index change. Stress changes in SMF-28 standard telecom optical fibers irradiated by a femto-second laser emitting at 264 nm showed a contribution of compaction of 70% for non hydrogen loaded fibers whereas no stress change was observed in hydrogen loaded fiber for the same photo-induced refractive index. No stress change indicates the absence of compaction leaving color centers solely responsible for the refractive index change. Irradiated annealed H2-loaded Ga-Cr-doped fiber showed no stress change as well. Although generally assumed for H2-loaded fibers, these are the only two examples that prove a color center based photosensitivity responsible for fiber Bragg gratings reported so far. In contrary, in H2-loaded Bi-Al-doped fibers the refractive index change is related to color centers and compaction. A contribution of compaction of about 41 % and 11 %, was evaluated for the pristine and the H2-loaded Bi-Al-doped fiber. Strategies to increase the luminescence, i.e. the number of active centers include e.g. temperature annealing (Cr3+ to Cr4+), changing concentrations of dopants (Bi) and/or the co-dopants (Al, P, Ge), and fiber fabrication conditions. In this work new strategies were followed: UV irradiation (193 nm), hydrogen loading, and H2-loading followed by UV irradiation. Measurements of absorption and luminescence of Bi-doped fibers with different treatments were performed. The absorption was measured from 360 to 1700 nm. The spectrum was decomposed into 8 Bi-related and 1 OH band. The irradiation increased the absorption by ∼100 dB/m and induced two supplementary absorption bands. UV irradiation combined with hydrogen loading induced three additional bands as compared to the pristine fiber. The luminescence was investigated using visible and infra-red pump lasers. The emission takes place mainly from 600 to 900 nm (3 bands) and from 900 to 1300 nm (3 bands). The luminescence shape and intensities of the pristine and irradiated fibers were similar. H2-loading combined with irradiation increased the ratio of NIR to visible fluorescence intensity depending on the pump wavelength. A side luminescence measurement showed 18 and 13 dB luminescence increase of the 1130 nm and 1390 nm bands. This luminescence enhancement might be due to an increase of the number of active bismuth centers. Up to 1.8 dB/m on-off gain was observed in the visible band around 700 nm under 676 nm excitation in the pristine fiber. However, the absorption increases simultaneously. As a result the net-gain was always negative with a trend towards a positive value below 700 nm. High reflective fiber gratings and a narrow band transmission filter were fabricated in a single mode fiber with cut-off around 920 nm. All gratings were used to realize a Bi-all-fiber laser operating at 1179 nm with a reduced linewidth of < 100 pm. Narrow linewidth lasers have a great potential for frequency doubling to the yellow.

EPFL2009

Tunable and Broadband Nanostructured Photonic Devices

Pierre-Yves Baroni

The main topic of this thesis is the fabrication and characterization of structures smaller than the wavelength of light, for operation in the visible or near infrared spectral range. In the first part of this thesis, the fabrication of periodic structures in one and two dimensions with an interference photolithography technique is described in detail. The structure periodicities that have been fabricated with this process ranges between 270 nm and a few microns on areas that can be as large as 100 cm2. Typical structure thicknesses are approximately equal to the period. In particular, a square lattice of pillars with a period of 270 nm has been created on the (non-flat) surface of a quartz microlens array and exhibits anti-reflective properties. Experimental results show a 15 % attenuation of the reflectivity and a 3 % enhancement of the transmissivity over the visible spectral range. In the second part of the thesis the structures are fabricated by e-beam lithography to ensure very precise devices shapes. The experimental results are obtained in the infrared range with two different structures, called photonic crystals. The first structure, a superprism, is a triangular lattice of pillars infiltrated by liquid crystals. A displacement of the output light spot is measured to be 20.5 µm for a wavelength variation of 27 nm. The structure length is 70 µm. The device, based on standard silicon technology, should allow integration of the device as a multiplexer/demultiplexer system into optical micro-circuits. The second structure, a tunable resonant cavity, is a wavelength filter working in the near infrared spectrum. The active area is composed of a photonic waveguide with a triangular lattice made of sub-micrometer holes. Additional nanostructuring in the light waveguide acts as a resonant cavity. The tunability of the device is obtained due to the liquid crystals which are infiltrated into the nanostructure. A 32 nm shift of the transmitted light peak inside the photonic band gap is measured by changing the temperature from room temperature to 45 °C.

EPFL2010