Abstract
Optical probing is an indispensable tool in research and development. In fact, it has always been the most natural way for humankind to explore nature. However, objects consisting of transparent materials with a refractive index close to unity, such as low-density gas jets, are a typical example of samples that often reach the sensitivity limits of optical probing techniques. We introduce an advanced optical probing method employing multiple passes of the probe through the object to increase phase sensitivity, and relay-imaging of the object between individual passes to preserve spatial resolution. An interferometer with four-passes was set up and the concept was validated by tomographic characterization of low-density supersonic gas jets. The results show an evident increase of sensitivity, which allows for the accurate quantitation of fine features such as a shock formed by an obstacle or a barrel shock on the jet boundary in low ambient gas pressures. Despite its limitations in temporal resolution, this novel method has demonstrated an increase in phase sensitivity in transmission, however, it can also be employed to boost the absorption or polarization contrast of weakly interacting objects in both transmission and reflection setups, thus, upgrading the sensitivity of various optical characterization methods.
Highlights
Optical probing is an indispensable tool in research and development
Optical interferometry is a family of probing techniques that utilize the interference between optical waves imprinted in the phase of the probing beam for precise measurement of small distances, observation of refractive index variations, visualization of surface irregularities, determination of wavelengths, and many other characterization methods[1]
The gas jets employed for laser wakefield electron acceleration (LWFA) in particular, range from axisymmetric types generated by cylindrically symmetric nozzles to complex non-rotationally symmetric jets produced by nozzles of various shapes or by gas streams perturbed by obstructions, which generate gas distributions tailored to specific purposes, e.g. pressure ramps and/or s hocks[8,9,10,11,12]
Summary
Optical probing is an indispensable tool in research and development. it has always been the most natural way for humankind to explore nature. The gas jets employed for LWFA in particular, range from axisymmetric types generated by cylindrically symmetric nozzles to complex non-rotationally symmetric jets produced by nozzles of various shapes or by gas streams perturbed by obstructions (razor blades, wires), which generate gas distributions tailored to specific purposes, e.g. pressure ramps and/or s hocks[8,9,10,11,12] The use of such complex gas jet distributions allows for better control over the laser-plasma parameters, which results in electron acceleration with improved shot-to-shot stability, increased bunch charge, smaller energy spread, etc. Relay imaging in the object arm of a double-pass Michelson interferometer introduced in16 allowed increase of sensitivity without affecting the high spatial resolution of the method Another trend is the use of tomography for three-dimensional reconstruction of tailored gas density distributions[17,18,19,20]. Both trends decrease the phase shift of the probe beam, often reaching the sensitivity limits of the characterization method, which is even more critical if the working gas refractive index approaches unity, such as in the case of helium
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