Haydn Martin

Near common-path optical fiber interferometer for potentially fast on-line microscale-nanoscale surface measurement.

We introduce a new surface measurement method for potential online application. Compared with our previous research, the new design is a significant improvement. It also features high stability because it uses a near common-path configuration. The method should be of great benefit to advanced manufacturing, especially for quality and process control in ultraprecision manufacturing and on the production line. Proof-of-concept experiments have been successfully conducted by measuring the system repeatability and the displacements of a mirror surface.

Vibration compensating beam scanning interferometer for surface measurement

Light beam scanning using a dispersive element and wavelength tuning is coupled with fiber-optic interferometry to realize a new surface measurement instrument. The instrument is capable of measuring nanoscale surface structures and form deviations. It features active vibration compensation and a small optical probe size that may be placed remotely from the main apparatus. Active vibration compensation is provided by the multiplexing of two interferometers with near common paths. Closed loop control of a mirror mounted on a piezoelectric transducer is used to keep the path length stable. Experiments were carried out to deduce the effectiveness of the vibration compensation and the ability to carry out a real measurement in the face of large environmental disturbance.

Rapid phase-shifting fiber interferometer with optical stylus.

Optical fiber interferometry holds many advantages for the online measurement of high-precision surfaces. Here, a fiber interferometer with a wavelength-scanning probe is reported. Such an interferometer requires active stabilization against the effects of temperature drift and vibration. A method of multiplexing dual wavelengths into the same fiber, combined with rapid phase shifting and real-time phase calculation, is investigated. Experimental data show the successful stabilization of the interferometer regardless of environmental perturbation.

High resolution position measurement from dispersed reference interferometry using template matching.

This paper describes a method to extract high resolution position data from a dispersed reference interferometry (DRI) by applying a template matching technique to the acquired spectral interferograms. Calculation of the correlation coefficient between windowed spectral interferograms acquired from the DRI apparatus and a set of numerically calculated template interferograms allows the absolute determination of position with nanometer resolution. Both the operating principle of the DRI apparatus and implementation of the template matching method is presented. Experimental validation of the method is provided through the demonstration of position tracking and an assessment of linearity, repeatability and noise performance.

Actively stabilized optical fiber interferometry technique for online/in-process surface measurement

In this paper, we report the recent progress in optical-beam scanning fiber interferometry for potential online nanoscale surface measurement based on the previous research. It attempts to generate a robust and miniature measurement device for future development into a multiprobe array measurement system. In this research, both fiber-optic-interferometry and the wavelength-division-multiplexing techniques have been used, so that the optical probe and the optical interferometer are well spaced and fast surface scanning can be carried out, allowing flexibility for online measurement. In addition, this system provides a self-reference signal to stabilize the optical detection with high common-mode noise suppression by adopting an active phase tracking and stabilization technique. Low-frequency noise was significantly reduced compared with unstabilized result. The measurement of a sample surface shows an attained repeatability of 3.3 nm.

Surface profile measurement using spatially dispersed short coherence interferometry

Improved online techniques for surface profile measurement can be beneficial in high/ultra-precision manufacturing, in terms of enabling manufacture and reducing costs. This paper introduces a spatially dispersed short-coherence interferometer sourced by a super luminescent diode. This technique uses a broadband light source, which is spatially dispersed across a surface using a reflective grating and a scan lens. In this way, the phase data pertaining to surface at height is spectrally encoded. The light reflected from the surface is interfered with a reference beam in a Michelson interferometer, after which the resulting fringes are interrogated by a spectrometer. Phase shifting interferometry is used to extract the spectrally encoded phase information by analysing four captured frames using a Carre algorithm procedure; in this way, surface height can be determined across a profile on a sample. The short coherent light utilized in this interferometric technique means it has the potential for an application as a remote probe through an optical fibre link. This paper describes the concept of a spatially dispersed short coherence interferometer and provides some of the initial experimental results.

Development of an optical system for the non‐invasive tracking of stem cell growth on microcarriers

The emergence of medicinal indications for stem cell therapies has seen a need to develop the manufacturing capacity for adherent cells such as mesenchymal stem cells (MSCs). One such development is in the use of microcarriers, which facilitate enhanced cell densities for adherent stem cell cultures when compared with 2D culture platforms. Given the variety of stem cell expansion systems commercially available, novel methods of non‐invasive and automated monitoring of cell number, confluence, and aggregation, within disparate environments, will become imperative to process control, ensuring reliable and consistent performance. The in situ epi‐illumination of mouse embryonic fibroblasts and human mesenchymal stem cells attached to Cytodex 1 and 3 microcarriers was achieved using a bespoke microscope. Robust image processing techniques were developed to provide quantitative measurements of confluence, aggregate recognition, and cell number, without the need for fluorescent labeling or cell detachment. Large datasets of cells counted on individual microcarriers were statistically analyzed and compared with NucleoCounter measurements, with an average difference of less than 7% observed from days 0 to 6 of a 12‐day culture noted, prior to the onset of aggregation. The developed image acquisition system and post‐processing methodologies were successfully applied to dynamically moving colonized microcarriers. The proposed system offers a novel method of cell identification at the individual level, to consistently and accurately assess viable cell number, confluence, and cell distribution, while also minimizing the variability inherent in the current invasive means by which cells adhered to microcarriers are analyzed. Biotechnol. Bioeng. 2017;114: 2032–2042.

Burg algorithm for enhancing measurement performance in wavelength scanning interferometry

Wavelength scanning interferometry (WSI) is a technique for measuring surface topography that is capable of resolving step discontinuities and does not require any mechanical movement of the apparatus or measurand, allowing measurement times to be reduced substantially in comparison to related techniques. The axial (height) resolution and measurement range in WSI depends in part on the algorithm used to evaluate the spectral interferograms. Previously reported Fourier transform based methods have a number of limitations which is in part due to the short data lengths obtained. This paper compares the performance auto-regressive model based techniques for frequency estimation in WSI. Specifically, the Burg method is compared with established Fourier transform based approaches using both simulation and experimental data taken from a WSI measurement of a step-height sample.

Development of a spatially dispersed short-coherence interferometry sensor using diffraction grating orders: publisher’s note

This publishers note amends the author listing and the funding and acknowledgment sections in Appl. Opt.57, 6391 (2017)APOPAI0003-693510.1364/AO.56.006391.

Towards the development of a hybrid-integrated chip interferometer for online surface profile measurements

Non-destructive testing and online measurement of surface features are pressing demands in manufacturing. Thus optical techniques are gaining importance for characterization of complex engineering surfaces. Harnessing integrated optics for miniaturization of interferometry systems onto a silicon wafer and incorporating a compact optical probe would enable the development of a handheld sensor for embedded metrology applications. In this work, we present the progress in the development of a hybrid photonics based metrology sensor device for online surface profile measurements. The measurement principle along with test and measurement results of individual components has been presented. For non-contact measurement, a spectrally encoded lateral scanning probe based on the laser scanning microscopy has been developed to provide fast measurement with lateral resolution limited to the diffraction limit. The probe demonstrates a lateral resolution of ∼3.6 μm while high axial resolution (sub-nanometre) is inherently achieved by interferometry. Further the performance of the hybrid tuneable laser and the scanning probe was evaluated by measuring a standard step height sample of 100 nm.