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

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.

Lippmann waveguide spectrometer with enhanced throughput and bandwidth for space and commercial applications

Philippe Giaccari, Benedikt Guldimann, Hans Peter Herzig, Mohammadreza Madi

This article presents an innovative high spectral resolution waveguide spectrometer, from the concept to the prototype demonstration and the test results. The main goal is to build the smallest possible Fourier transform spectrometer (FTS) with state of the art technology. This waveguide FTS takes advantage of a customized pattern of nano-samplers fabricated on the surface of a planar waveguide that allows the increase of the measurement points necessary for increasing the spectral bandwidth of the FTS in a fully static way. The use of a planar waveguide on the other hand allows enhancing the throughput in a waveguide spectrometer compared to the conventional devices made of single-mode waveguides. A prototype is made in silicon oxynitride/silicon dioxide technology and characterized in the visible range. This waveguide spectrometer shows a nominal bandwidth of 256~nm at a central wavelength of 633~nm thanks to a custom pattern of nanodisks providing a μm sampling interval. The implementation of this innovative waveguide FTS for a real-case scenario is explored and further development of such device for the imaging FTS application is discussed.

2018

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

On-chip NIR optical spectrometer based on polymeric waveguide and metallic nano-structures

Hans Peter Herzig, Maurine Malak Karam Awadallah, Toralf Scharf

In this work, we report about optical spectrometry using gold nano-structures printed on a polymer based integrated optical waveguide. The optical waveguide is a single mode buried waveguide, having dimensions of 3x2.2µm 2 . It is made from a combination of photo-polymerizable materials and is fabricated by photolithography on a glass substrate. To sense the electric field inside the waveguide, a gold nano-coupler array of thin lines (50 nm thick and 8 µm length) is embedded on top of the aforementioned waveguide. They are produced by E-beam lithography. The array pitch is 2.872 µm and the number of lines 564, which yields an array of 1.619 mm length. The device is enclosed with a glass superstrate to prevent it from dust and destruction. Both waveguide ports are polished and the output port in particular, is coated with a thin gold layer to assimilate a mirror and hence, it enables the creation of stationary waves inside the structure. The measurement procedure involves light injection using a single mode fiber carrying both visible light (658nm) and infrared light (785nm), used for alignment and measurement purposes respectively. Stationary waves generated inside the guide constitute the spatial interferogram. Locally, light is out-coupled using the nano-couplers and allows measuring the interferogram structure. The resulting pattern is imaged by a vision system involving an opticalmicroscope with a digital camera mounted on-top of it. Signal processing, mainly based on Fast Fourier transform is performed on the captured signal to extract the spectral content of the measured signal.

2014

Tunable and Broadband Nanostructured Photonic Devices

Pierre-Yves Baroni

EPFL2010