Fourier-transform spectral interferometry for in situ group delay dispersion monitoring of thin film coating processes

Abstract

A fast Fourier-based measurement system to determine phase, group delay, and group delay dispersion during optical coating processes is proposed. The in situ method is based on a Michelson interferometer with a broad band light source and a very fast spectrometer. To our knowledge, group delay dispersion measurements directly on the moving substrates during a deposition process for complex interference coatings have been demonstrated for the first time. Especially for the very precise production of chirped mirrors it is advantageous to get information about the phase properties of the grown layer stack to react on errors and retrieve more information about the coating process.

Improved LIDT values for dielectric dispersive compensating mirrors applying ternary composites

“…Deposition errors are corrected online by a re-optimization tool (ReOpt) applying the transmission data recorded after termination of each layer. The last layers of the dispersion compensating mirrors are terminated by the in situ group delay dispersion (GDD) measurement system, which mainly consists of a fiber based white light interferometer (WLI) and a fast spectrometer [13].…”

“…Especially taking into account the high sensitivity of the designed GDD to the film thickness accuracy in the final layers, a novel monitoring strategy is applied. In this algorithm, the conventional BBM is used for layer termination until the fifth last layer of the design is finished, and afterwards the aligned interference of the novel in situ white light interferometer (WLI) is used to control and switch the last layers [13]. Starting the coating process again the last layers are terminated by the described in situ GDD measurement system.…”

“…In summary the fiber based WLI in combination with a fast high resolution spectrometer displays the current in situ group delay dispersion curve every time the moving sample is passing the collimator head. The detailed setup and principle is presented in [13]. For a precise layer termination the current in situ GDD measurement curve is continuously compared to the theoretic design GDD curve of the particular layer.…”

“…The current system is based on a Michelson interferometer in the NIR-wavelength range [13]. In previous studies it was shown that the monitored phase in the NIR range can be used to improve significantly the fabrication of CM`s in the NIR as well as in other spectral wavelength ranges as for instance in the UV [4; 13].…”

Approaches toward optimized laser-induced damage thresholds of dispersive compensating mirrors applying nanolaminates

“…High end deposition processes like Ion-Beam-Sputtering (IBS) in combination with precise monitoring techniques are required to maintain a stable deposition process [10]. Recently, the monitoring of IBS processes is significantly improved by controlling the group delay dispersion insitu on the moving substrates, applying a fiber based white light interferometer [11]. In addition, the monitoring concept is well suited to control chirped mirrors target specifications in a wavelength range beyond the bandwidth of the in-situ phase monitor [12,13].…”