5 research outputs found
Highspeed multiplexed heterodyne interferometry
Digitally enhanced heterodyne interferometry is a metrology
technique that uses pseudo-random noise codes for modulating the phase
of the laser light. Multiple interferometric signals from the same beam
path can thereby be isolated based on their propagation delay, allowing
one to use advantageous optical layouts in comparison to classic laser
interferometers. We present here a high speed version of this technique for
measuring multiple targets spatially separated by only a few centimetres.
This allows measurements of multiplexed signals using free beams, making
the technique attractive for several applications requiring compact optical
set-ups like for example space-based interferometers. In an experiment using
a modulation and sampling rate of 1.25 GHz we are able to demonstrate
multiplexing between targets only separated by 36 cm and we achieve a
displacement measurement noise floor of < 3 pm/
√
Hz at 10 Hz between
them. We identify a limiting excess noise at low frequencies which is
unique to this technique and is probably caused by the finite bandwidth in
our measurement set-up. Utilising an active clock jitter correction scheme
we are also able to reduce this noise in a null measurement configuration by
one order of magnitude
Towards the LISA backlink: Experiment design for comparing optical phase reference distribution systems
LISA is a proposed space-based laser interferometer detecting gravitational waves by measuring distances between free-floating test masses housed in three satellites in a triangular constellation with laser links in-between. Each satellite contains two optical benches that are articulated by moving optical subassemblies for compensating the breathing angle in the constellation. The phase reference distribution system, also known as backlink, forms an optical bi-directional path between the intra-satellite benches. In this work we discuss phase reference implementations with a target non-reciprocity of at most 2π μrad Hz-1, equivalent to 1 pm √Hz-1 for a wavelength of 1064 nm in the frequency band from 0.1 mHz to 1 Hz. One phase reference uses a steered free beam connection, the other one a fiber together with additional laser frequencies. The noise characteristics of these implementations will be compared in a single interferometric set-up with a previously successfully tested direct fiber connection. We show the design of this interferometer created by optical simulations including ghost beam analysis, component alignment and noise estimation. First experimental results of a free beam laser link between two optical set-ups that are co-rotating by ±1° are presented. This experiment demonstrates sufficient thermal stability during rotation of less than 10-4 K √Hz-1 at 1 mHz and operation of the free beam steering mirror control over more than 1 week. © 2018 IOP Publishing Ltd Printed in the UK