Fibre imaging bundles for full-field optical coherence tomography

An imaging fibre bundle is incorporated into a full-field imaging OCT system, with the aim of eliminating the mechanical scanning currently required at the probe tip in endoscopic systems. Each fibre within the imaging bundle addresses a Fizeau interferometer formed between the bundle end and the sample, a configuration which ensures down lead insensitivity of the probe fibres, preventing variations in sensitivity due to polarization changes in the many thousand constituent fibres. The technique allows acquisition of information across a planar region with single-shot measurement, in the form of a 2D image detected using a digital CCD camera. Depth scanning components are now confined within a processing interferometer external to the completely passive endoscope probe. The technique has been evaluated in our laboratory for test samples, and images acquired using the bundle-based system are presented. Data are displayed either as en-face scans, parallel to the sample surface, or as slices through the depth of the sample, with a spatial resolution of about 30 ï ­m. The minimum detectable reflectivity at present is estimated to be about 10-3, which is satisfactory for many inorganic samples. Methods of improving the signal-to- noise ratio for imaging of lower reflectivity samples are discuss

Objective speckle displacement: an extended theory for the small deformation of shaped objects

This paper describes an extended and improved theory of the displacement of the objective speckle pattern resulting from displacement and/or deformation of a coherently illuminated diffuse object. Using the theory developed by Yamaguchi [Opt. Acta 28, 1359 (1981)], extended expressions are derived that include the influence of surface shape/gradients via the first order approximation of the shape as linear surface gradients. Both the original Yamaguchi expressions and the extended form derived here are shown experimentally to break down as the detector position moves away from the z-axis. As such, improved forms of the expressions are then presented, which remove some of the approximations used by Yamaguchi and can be used to predict the objective speckle displacement over a wide range of detector positions and surface slopes. Finally, these expressions are then verified experimentally for the speckle shifts resulting from object translations

Gas cells for tunable diode laser absorption spectroscopy employing optical diffusers. Part 1: single and dual pass cells

New designs for gas cells are presented that incorporate transmissive or reflective optical diffusers. These components offer simple alignment and can disrupt the formation of optical etalons. We analyse the performance-limiting effects in these cells of random laser speckle (both objective and subjective speckle), interferometric speckle and self-mixing interference, and show how designs can be optimised. A simple, single pass transmissive gas cell has been studied using wavelength modulation spectroscopy to measure methane at 1651 nm. We have demonstrated a short-term noise equivalent absorbance (NEA, 1 sigma) of 2x10(-5), but longer term drift of up to 3x10(-4) over 22 hours

Using integrating spheres as absorption cells: path-length distribution and application of Beer's law

We have modeled the path-length distribution in an integrating sphere used as a multipass optical cell for absorption measurements. The measured radiant flux as a function of analyte concentration is nonlinear as a result, deviating from that expected for a single path length. We have developed a full numerical model and introduce a new analytical relationship that describes this behavior for high reflectivity spheres. We have tested both models by measuring the optical absorption of methane at 1651nm in a 50mm diameter sphere, with good agreement with experimental data in the absorption range 0-0.01cm -1 . Our results compare well with previous work on the temporal response of integrating spheres

Objective speckle displacement resulting from the deformation of shaped objects

This paper describes an extended theory of the displacement of the objective speckle pattern resulting from displacement and/or deformation of a coherently illuminated diffuse object where the influence of the surface shape is included via the linear surface gradients. An experimental system capable of measuring the translational scaling factors, the ratios of speckle shift to object translations, to an accuracy of ± 0.02 and a repeatability of approximately ± 0.008 is described which was used to experimentally measure the speckle shift for a range of detector positions and surface gradients. The original expressions developed by Yamaguchi1 and the new extended expressions2 are then compared with experimental results for measurements on zero surface gradients, i.e. the mean surface lying in the x-y plane. The divergence of Yamaguchi’s expressions from experimental results for off-axis detector positions that was first observed by Světlík3 was confirmed, and the new expressions shown to successfully predict translational scaling factors for off-axis positions. The new expressions are then compared to the experimental results for a range of surface gradient magnitudes and directions, as well as detector positions both on and off-axis, and shown to successfully predict the observed speckle shift.EPSR

Full-field interferometry using infinity corrected optics

In this paper the construction of full-field (imaging) interferometers using infinity corrected optics commonly used in microscopy is discussed, with an emphasis on self-mixing interferometry configurations where the imaged light field is mixed with itself rather than a reference wave. Such configurations are used in speckle shearing interferometry, flow visualisation and quantitative flow measurement. The critical considerations for constructing path-length imbalanced full-field interferometers for these and similar applications are discussed, expressions are derived for key calculations and interferograms from example interferometers are presented. These include the concept of balancing the infinity-spaces of the two arms via the use of a glass block to minimise the optical path difference variation across the interferogram and ensure adequate sampling of the fringes on the detector. Further, the use of tilted glass blocks in single-pass and double-pass arrangements is detailed for the generation and control of spatial carrier fringes without extensive realignment of the interferometer, and for phase shifting

A solution to the slow stabilisation of surface pressure sensors based on the Wilhelmy method

Dynamic measurement of surface pressure is of particular interest in the field of Langmuir monolayers, where the change in surface pressure throughout an experiment can provide information on the properties of the monolayer forming material, or on the reaction kinetics of the monolayer’s interaction with other materials. One of the most common methods for the measurement of dynamic surface pressure is the Wilhelmy plate method. This method measures changes in the forces acting upon a thin plate of material at the air-water interface; this measurement is then converted to surface pressure. One version of this method, which uses filter paper plates at the air-water interface, is particularly popular due to their relatively low cost. However, it has been seen that the use of filter paper plates attached to a Wilhelmy balance requires an initial stabilisation period lasting several hours, during which the readings drift from the original baseline. Here the cause of this drift is explored, considering how changes in the weight of the plate over time influence the assumptions on which the surface pressure is derived from the measurements made by the Wilhelmy balance. A simple method for preventing this drift through pre-soaking of the filter paper plates is presented

A simple method for fabricating phase-shifted fibre Bragg gratings with flexible choice of centre wavelength

A simple technique for fabricating phase-shifted fibre Bragg gratings (PSFBGs) without the use of a phase-shifted phase mask is presented. Two, 3-mm long, standard fibre Bragg gratings (FBGs) were inscribed sequentially in singlemode fibre at the same Bragg wavelength such that the FBGs physically overlapped by one grating period. This induces a spectral-hole in the middle of the reflection spectrum of a standard FBG, equivalent to a π-phase shifted FBG. The flexibility of the technique in writing PSFBGs at any choice of wavelength is demonstrated. The results show that PSFBG devices produced by this method are highly reproducible and the process is fas

Dissolved oxygen sensing using an optical fibre long period grating coated with hemoglobin

A long period grating fiber optic sensor coated with hemoglobin is used to detect dissolved oxygen. The sensitivity of this sensor to the ratio of dissolved carbon dioxide to dissolved oxygen is demonstrated via the conversion of carboxyhemoglobin to oxyhemoglobin on the sensor surface. The sensor shows good repeatability with a %CV of less than 1% for carboxyhemoglobin and oxyhemoglobin states with no measurable drift or hysteresis

Self-mixing interference effects in tunable diode laser absorption spectroscopy

We report the effects of self-mixing interference on gas detection using tunable diode laser spectroscopy. For very weak feedback, the laser diode output intensity gains a sinusoidal modulation analogous to that caused by low finesse etalons in the optical path. Our experiments show that self-mixing interference can arise from both specular reflections (e.g. cell windows) and diffuse reflections (e.g. Spectralon™ and retroreflective tape), potentially in a wider range of circumstances than etalon-induced interference. The form and magnitude of the modulation is shown to agree with theory. We have quantified the effect of these spurious signals on methane detection using wavelength modulation spectroscopy and discuss the implications for real gas detecto