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A novel heterodyne interferometer for scanning optical microscopy

Offside, Mario Jeff; (1991) A novel heterodyne interferometer for scanning optical microscopy. Doctoral thesis (Ph.D), UCL (University College London). Green open access

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A phase sensitive scanning optical microscope is described which can measure surface height changes of about 1 Å. The system is based on a heterodyne version of the Michelson interferometer, and has been designed to reject phase noise caused by vibration in the optics and the sample. A specially constructed objective lens is used to direct two laser beams onto the object surface. The first beam forms a tightly focused spot to probe the sample structure and the second remains collimated, acting as a large area on sample reference beam. In the simplest implementation, the objective may be fabricated by drilling a hole in a lens singlet. The configuration allows the relative areas illuminated by the two beams to be varied both arbitrarily and independently, thus guaranteeing an accurate absolute phase measurement. This is an important advantage over existing techniques, in which the range of suitable samples is restricted by the limited size of the on sample reference beam. The two beams reflected from the sample are interfered with a third frequency shifted beam, so forming two heterodyne Michelson interferometers in parallel. The light from each interferometer is detected separately, resulting in two AC signals. The phase of these signals are then compared to provide the object surface phase structure. Path length fluctuations due to microphonics are common to both interferometers and are cancelled by this comparison. Results from a bench top version of the system are presented which demonstrate the principle of the technique and a detailed study is made of the factors limiting the sensitivity of the phase measurement. The conclusions of this study have been applied to the design of a prototype microscope and this has been used to record micrographs of a number of representative samples. In addition the particular imaging characteristics of the system are discussed using a combination of geometrical optics and a transfer function approach.

Type: Thesis (Doctoral)
Qualification: Ph.D
Title: A novel heterodyne interferometer for scanning optical microscopy
Open access status: An open access version is available from UCL Discovery
Language: English
Additional information: Thesis digitised by ProQuest.
Keywords: Pure sciences; Michelson interferometers
URI: https://discovery.ucl.ac.uk/id/eprint/10123866
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