Abstract: Interferometry apparatus which comprises a measurement light beam (2a, 2b) and a reference light beam (2c, 2d) which interact with each other to cause a spatial fringe pattern (24). An optical device (12) is provided which interacts with the spatial fringe pattern (24), such that light is spatially separated into different directions (30, 32, 34, 36). The intensity modulation in two or more directions of the spatially separated light is phase shifted. The optical device may comprise, for example, a diffractive device, a refractive device or a diffractive optical element.

Abstract: Apparatus for measuring deviation of a trajectory from a straight line in the movement of a first body with respect to a second body comprising a transmitter unit mounted on one of the bodies and an optic unit mounted on the other of the bodies. The transmitter unit directs at least one light beam towards the optic unit such that two or more light beams are received within it. One of the units is provided with two or more detectors to detect the light beams transmitted to or reflected from the optic unit. The position of the light beams on the detectors is used to calculate the deviation of a trajectory from a straight line of one of the bodies with respect to the other in at least one degree of freedom. This enables measurement of straightness, pitch, roll, yaw and squareness errors.

Abstract: A laser system is disclosed comprising a laser source for producing a laser beam along a beam path, means to stabilize the frequency of the laser beam, at least one optical component which produces back-scattered light when in use, and a phase modulator for modulating the phase of the back-scattered light. The optical component may be a plane mirror or an optical fibre. The phase modulator may be an oscillating glass block, optionally driven by a motor. Alternatively, the phase modulator is a glass block cyclically compressed by a piezoelectric element.