Measurement systems such as systems based on interferometric combination of beams of light, require highly accurate alignment of optical components such as mirrors to achieve fringes. Moreover, the stability of such alignment mechanisms must be even higher. Alignment devices based on flexures provide accurate alignment with excellent resolution, without play or hysteresis.
TNO TPD developed mechanisms for adjustment of mirrors in two degrees of freedom, meaning two rotations, and used these mechanisms in setups to achieve picometer stability. The paper describes the design process and the development of a set of alignment mechanisms. Theoretical and practical aspects are mentioned. First the design aspects for designing stable mounts are given, and then two mechanisms are described. The mechanisms consist of a monolithic adjustment mount for a mirror that is made by wire erosion in such a way that the mirror can rotate about two axes. Adjustment screws in combination with a lever and a gear provide easy and accurate adjustment of the rotation of the mirror. The combination of flexures result in a virtual point of rotation that is positioned on the centre of the mirror surface. In this case, the optical path length of the deflected light path will not change. Two degree of freedom rotation mechanisms have a generic design, so the design can be used in multiple instruments. The measurement systems show high stability of the components.
For high accuracy alignment of optical components in optical instruments TNO TPD has developed dedicated, monolithic, flexure-based alignment mechanisms, which provide accuracies below 0.1 μm or 0.1 μrad as well as stabilities down to tens of picometers per few minutes after locking.
High resolution, high stability alignment mechanisms consist of an adjustment mechanism and a locking device.
Complex monolithic flexure-based mechanisms are designed to align specific degrees of freedom. They are realized by means of spark erosion. The benefits of these mechanisms are no play, no hysteresis, high stiffness, a simplified thermal design, easy to assemble. Using alignment mechanisms a passive system can be maintained.
Locking after alignment is mandatory to guarantee sub-nanometer stability. However, a high accuracy alignment will be disturbed again due to drift during locking if the locking device is not properly designed. Several low-drift locking devices have been designed and developed.
The dedicated alignment mechanisms presented here are based on: (a) the results of an internal ongoing research program on alignment and locking and (b) experience with mechanisms developed at TNO TPD for high precision optical instruments, which are used in e.g. a white light interferometer breadboard (Nulling) and an interferometer with picometer resolution for ESA’s future cornerstone missions “DARWIN” and “GAIA”.