1. Field of the Invention
The invention concerns an interferometer delay line, in particular a delay line of the type used in an interferometer device such as a FOURIER transform spectrometer in which the aim is not only to stabilize dynamically the optical component during a programmed movement, in order to eliminate the effect of vibrations, but also to compensate for disturbing optical path differences generated independently of the optical component and its movement in the remainder of the interferometer device, and even externally of the latter. The invention also concerns an optical delay line which is part of a coherent recombination stellar interferometer in which the distance (called the base) between the telescopes is in the order of 100 meters and equalizing the optical paths despite the fact that the rotation of the Earth necessitates displacement of the retroreflector device of the optical delay line over a distance of several meters, at a speed of several tens of millimeters per second, with an average positioning accuracy of a few microns and an instantaneous positioning accuracy (during a setting time of a few hundredths of a second) of a few nanometers (vibration amplitude). Thus the constraints with respect to dynamic errors are more severe than those with respect to static errors.
2. Description of the Prior Art
Similar displacement characteristics are also needed when the object is to equalize the optical paths by displacing the telescopes themselves.
European Patent 0 398 772 (KOEHLER) describes a delay line in detail. This reference describes a vibration free displacement control device for static and dynamic control of an optical component which is mobile in one direction within an interferometer device, the control device including a guide track, a carriage carrying a payload including the optical component and mobile along the guide track, a drive unit for driving the carriage along the guide track, and a drive control unit connected to the drive unit to move the carriage in accordance with a predetermined set point law. The payload is connected to the carriage by flexible connecting members allowing relative movement between the carriage and the payload parallel to the mobile direction and at least one actuator acts on at least part of the optical component under the control of at least one stabilization control loop connected to at least a second sensor. In this device the stabilization control loop is a single loop independent of the drive control unit and the second sensor is a single sensor embodying an accelerator carried by the payload which is responsive to acceleration of the payload parallel to the mobile direction. The actuator is a single actuator acting on all of the payload with its other end bearing against the carriage. The stabilization control loop includes a stage for filtering the accelerometer signal and the drive control unit includes a control loop connected to a position sensor adapted to sense the position of the carriage on the guide track.
The prior art includes position sensing devices including a fixed graduated rule and an optical head moving along the rule which is adapted to supply pulses representative of distance increments (or increments of displacement of the head relative to the rule). These heads have electronic processor devices so that the output pulses can correspond to distance increments very much smaller than the gaps between the graduations of the rule. For example, the distance increments are in the order of 15 nanometers for a distance of 4 microns between graduations.
Graduated rules of this kind cooperating with optical heads to provide a resolution as above are available only for limited lengths, typically not more than about one meter. Other rules in the form of a strip are available, in lengths up to around 30 meters, but their precision and their resolution are very much lower than the values stated above and are not compatible with the requirements of an astronomical interferometer delay line.