Spectrometer | EPFL Graph Search

A spectrometer (spɛkˈtrɒmɪtər) is a scientific instrument used to separate and measure spectral components of a physical phenomenon. Spectrometer is a broad term often used to describe instruments that measure a continuous variable of a phenomenon where the spectral components are somehow mixed. In visible light a spectrometer can separate white light and measure individual narrow bands of color, called a spectrum. A mass spectrometer measures the spectrum of the masses of the atoms or molecules present in a gas. The first spectrometers were used to split light into an array of separate colors. Spectrometers were developed in early studies of physics, astronomy, and chemistry. The capability of spectroscopy to determine chemical composition drove its advancement and continues to be one of its primary uses. Spectrometers are used in astronomy to analyze the chemical composition of stars and planets, and spectrometers gather data on the origin of the universe. Examples of spectromete

Analysis of Femtosecond Electron Bunches at the SwissFEL Injector Test Facility

Bennie Smit

The SwissFEL (Swiss Free Electron Laser) is a free electron laser which is being built at the Paul Scherrer Institute. The SwissFEL will be a user facility to study processes on unexplored scales of space and time. In order to achieve these aims, X-ray pulses on the order of femtoseconds must be generated. For such short X-ray pulses, the electron bunch length must be comparable to the final X-ray pulse length. In order to achieve such a short electron bunch length, it must be monitored at every compression stage. This work proposes two novel methods to measure such electron bunches, namely a novel spectral method that analyses the frequency content of electron bunches that becomes easier with shorter bunch length and a transverse deflecting cavity method which can conceptually measure a zero bunch length. Because the SwissFEL was not built at the time of the PhD, all experiments were performed at the SwissFEL Injector Test Facility (SITF). The SITF is a 200 MeV test bed linear electron accelerator. Theoretically the bunch length is limited due to the uncorrelated energy spread and the higher order components of the bunch compressor. However, due to the limited electron energy in combination with the short electron bunches, space charge effects dominate the beam in the transverse phase space as well as in the longitudinal phase space. A transverse deflecting cavity (TDC) measurement was used to measure a reference bunch length to which the spectral fluctuations could be correlated. In a TDC the electric fields are oriented perpendicular to the beam direction and streaks the head and tail in opposite directions onto a screen. In a conventional TDC measurement, the projected beam size onto the temporal axis is the current profile, provided that the intrinsic beam size is small. However, in the SITF the intrinsic beam size was large due to the aforementioned space charge effects. This led to the development of a novel bunch length analysis method in a space charge dominated beam. In the proposed analysis method, a dispersive section in combination with a TDC was used to visualise the longitudinal phase space. The longitudinal phase space was sliced in the momentum domain to reveal the difference between the intrinsic and the streaked beam in a momentum slice. The bunch length could be extracted by recording the times for all slices. Both the simulations and experiments suggest the method enables the measurement of bunch length in a space charge dominated beam in which the transverse intrinsic beam size is large. The measured minimal bunch length from simulations and experiments are of the same order, 19.5 fs to 24.5 fs, respectively. Since the TDC at PSI has a specified resolution of 20 fs at 200 pC, this method may be used to reduce the resolution. The spectral content of femtosecond electron bunches contains electron bunch length information. As an electron beam collides with an optical transition radiation (OTR) foil, radiation is emitted. The radiation is collected by means of a mirror setup into an optical fibre and into an optical spectrometer. The spectral range of the spectrometer is 316 THz (950 nm) to 1364 THz (220 nm). From OTR theory a flat OTR spectrum was expected and measured. As the electron bunch length was reduced below 70 fs, periodic fluctuations started to appear on the spectra. These fluctuations broaden with decreasing bunch length and show a good agreement with the bunch length measured with the TDC.

EPFL2015

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

Diffusion de neutrons de 14 MeV par le carbone 12

Claude Joseph

The angular distributions of the differential cross-sections for scattering of 14,1 MeV neutrons from 12C, Q = 0(0+); – 4,43(2+); – 7,65(0+) and – 9,63(8–) MeV, have been measured using a time-of-flight spectrometer. An indication can be found of the excitation of a wide level at about 10 MeV. A Monte-Carlo programme has been worked up and used to correct the results for multiple scattering; it calculates time-of-flight spectra which can be compared with the measured ones in order to deduce the best values of the cross-sections. The results are finally confronted with other experimental determinations and existing theoretical predictions.

s.n.1967