Optimization of Phase-Sensitive Transparent Detector for Length Measurements

Abstract

A phase selective partly transparent detector (PSTD) enables length measurement with nm-accuracy by sampling an optical standing wave. The PSTD consists of two transparent n-i-p photodiodes of amorphous silicon (a-Si:H) which are embedded between three transparent conductive oxide (TCO) layers. The two photodiodes measure the intensity of an optical standing wave by means of absorption layers with thicknesses below 50 nm and thus, provide two photocurrents which are proportional to the intensity at their individual positions. For an optimization of the device performance, simulations based on a standard electromagnetic formalism were performed. The considered thin-film structure is a glass/TCO/n-i-p/TCO/n-i-p/TCO layer sequence. The aim was to design a layer stack which avoids significant distortions of the standing wave while the phase shift between the photocurrents approximately amounts to 90 , since this will minimize the measurement error. The comparison of experimentally determined and simulated data shows that a further adjustment of the fabricated PSTD into an ideal thickness scheme is necessary to enhance the device performance.

Interferometry in a passive Fabry-Perot cavity with the detection of a standing wave

“…This approach needs a component able to track the interference maxima and minima along the beam axis inside of the cavity. In [8,9,10] a transparent photodetector has been reported even in a design with two active domains separated by a distinct spacing suitable for generation of quadrature signals usual in displacement interferometry. Suitable balance between the losses caused by the detector to the beam passing through and its sensitivity has to be found when it should be placed into a passive resonant cavity (Figure 1).…”

Interferometry with referencing of wavelength

“…This approach needs a component able to track the interference maxima and minima along the beam axis. In [23,24,25] a transparent photodetector has been reported even in a design with two active domains separated by a distinct spacing suitable for generation of quadrature signals usual in displacement interferometry. Suitable balance between the losses caused by the detector to the beam passing through and its sensitivity has to be found when it should be placed into a passive resonant cavity (Figure 3).…”

Fundamental optical simulations of light trapping in microcrystalline silicon thin-film solar cells

“…The back side ZnO layer has a thickness of 80 nm. The optical properties of µc-Si:H and the transparent conductive oxide (ZnO) are described by their optical constants n(λ) and k(λ) and can be found in numerous publications of our institute that have led to a successful description of layers [18], sensors [19] and solar cells [20] based on amorphous and microcrystalline silicon thin-films. For simplicity the silver back contact is calculated as an ideal metal because (i) the optical properties of the evaporated silver at the ZnO/metal interface are unknown and differ significantly from the silver-"bulk" material [7] and (ii) speed-up of the very time-consuming calculations.…”