dc.description.abstract |
The need for precision dimensional metrological techniques is always increasing, including
high precision displacement measuring metrology used to precisely and accurately
position stages, components and machines. Laser interferometry is widely considered
the most precise technique available for single dimensional displacement measurements,
however extending this to multiple dimensions typically involves complex
and costly interrogation systems. In this thesis, a series of novel multi-dimensional stage
encoder designs and experimental results are presented as an application of a state-of-the-
art metrological technique, that uses sinusoidal wavelength modulation and range-dependant
signal processing to multiplex the signals from multiple interferometers into
a single interferometric signal. Using range-resolved interferometry (RRI), a series of interferometers
comprising measurements in multiple dimensions are multiplexed onto a
single photodetector, which are then are independently and concurrently demodulated
and evaluated. This technique is applied to novel designs for a 2-dimensional displacement
encoder, a 3-dimensional displacement encoder and a dual-beam angle encoder,
where unlike prior work which has typically required complex optical setups involving
polarisation-sensitive optical components and detectors, uses only minimal, simple bulk
optic components to evaluate multiple dimensions simultaneously. In this work, nonlinearities
below 1 nm along with typical displacement noise levels below 0.4 nm/√Hz
are presented for experimental results of ±50 µm controlled stage motions, showing results
which are highly comparable to existing techniques.
Further to this, in order to make high precision results with confidence, high-stability
lasers are required. In non-modulated and weakly modulated wavelength regimes,
there are a significant number of techniques available for laser stabilisation, for both
short-term and long-term requirements and linked to highly stable references. However,
for widely-wavelength modulated techniques such as RRI, where the modulation
depth is several orders of magnitude greater than standard reference widths, the vast
majority of existing techniques are either unsuitable or significantly less-effective. In
this thesis, a novel technique for long-term stabilisation of a widely wavelength modulated
laser to a high stability reference standard is presented. Introduced as "swept
absorption line locking", the principle behind this technique, in addition to experiments
to test the efficacy of this technique are presented. Included in this, is comparison to
a highly-stabilised helium-neon laser, which is often considered a "gold-standard" for
stability, where co-linear displacement measurements of a Michelson interferometer are
performed and compared to determine the RRI evaluation wavelength. The results presented
here show fractional stabilities as low as 6×10−⁸ over timescales of ∼1000 seconds
using this novel stabilisation technique, and the comparison to the HeNe laser interferometer
shows good agreement between the calculated RRI evaluation wavelength and
the expected stabilised value within calculated uncertainties. |
en_UK |