Peter Jacquemin

Design of a confocal holography microscope for three-dimensional temperature measurements of fluids in microgravity

The design of a Confocal Scanning Laser Holography (CSLH) microscope applied to microgravity studies of fluids is described. This microscope generates a hologram for each three-dimensional point describing an object and offers a new, non-intrusive means to determine the three-dimensional temperature and composition of objects, which is useful information for heat and mass transfer studies. The holograms are created from the interference of a ‘known’ reference beam to an ‘unknown’ object beam, which contains the phase information from which the object’s index of refraction is determined. The key feature of the microscope for microgravity experimentation is the object remains stationary as the beam is rastered through the object, ensuring a quiescent environment. Additional vibration disturbances due to the motion of optical components are minimized by applying counter balances and by using the Motion-vibration Isolation System (MIM).

Developing a Confocal Acoustic Holography Microscope for non-invasive 3D temperature and composition measurements

A confocal acoustic holography microscope (CAHM) has been designed, simulated and partially verified experimentally to take holograms for non-invasive, three-dimensional measurements of a specimens refractive indices from one view point. The designed and simulated prototype CAHM used a frequency of 2.25 MHz and measured sound speed changes of 16 m/s, temperature changes of 5 degrees C and had a spatial resolution of 660 microm. With future improvements utilizing the latest technologies such as two-dimensional array detectors, Micro-Electro-Mechanical Systems (MEMS), and acoustic lenses, resolutions of 1m/s, 0.5 degrees C, and 150 microm are expected. The CAHM is expected to have many useful applications, including non-invasive mass and heat transfer measurements in fluids and materials and as a medical diagnostic tool to non-intrusively visualize compositions and temperatures within the human body.

A Low Error Reconstruction Method for Confocal Holography using Limited View Tomography to Determine 3-Dimensional Properties

A new method to reconstruct the three-dimensional (3D) refractive index or temperature of a specimen with limited viewing confocal holography is presented here. Scanning is restricted to a single viewpoint window with a scan angle that is limited to the cone angle of the probe beam within the specimen. Tomographic reconstruction typically uses a 160° to 360° scan angle with small angle increments for low reconstruction error. The confocal holography microscope does not allow rotations of the specimen or the microscope optics which restricts the scan angle to 28° for reconstruction. Scanning by translating the focal point of the probe beam within the specimen along the optical propagation axis produces a singular non-invertible reconstruction matrix. Increasing the number of scan positions within the specimen produces redundant data and an ill-conditioned reconstruction matrix with excessive reconstruction error. The reconstruction problem is overcome by combining a particular scanning geometry with boundary conditions. The volume of the specimen is scanned by translating the focal point of the probe beam in xyz-directions within the specimen. The specimen is a BK7 refractive index matching liquid in a 5x5x45mm fluid-cell cuvette. The boundary conditions are defined by the temperatures along the side walls of the fluidcell. The novel reconstruction algorithm is called “wily” because the sparse reconstruction matrix contains not a single non-zero diagonal element and yet it is well conditioned for inversion. Negligible reconstruction error is important to improve the accuracy of the microscope since it is sensitive to minute phase-shifts or fringe translations in holograms. The confocal holography microscope was originally designed to non-intrusively measure the three-dimensional temperature and composition of transparent solids and fluids [1]. An object beam that propagates through the specimen is combined with a reference beam to form a hologram. The fringes in the hologram translate as a phase-shift in response to a change in temperature or composition. Both temperature and composition can be determined from the index-of-refraction [2]. Refractive index information is contained within the scanned holograms. A convergent beam for a single scan position is shown in Fig 1 where a small sphere is positioned at the focal point and another sphere that partially intercepts the central rays is positioned off to the side. The difference in information between the two positions is detected in the holograms. The simulated hologram of Fig 2 shows the phase shifts of the fringes for a high refractive index sphere centered at the focal point of the convergent beam. Fig 3 shows the fringe shift pattern for the sphere that is off to the side from the focal point of the convergent beam. The horizontal lines are due to the wave interference of the overlapping beams outside the sphere region. A cone beam angle of 28o is seen in Fig 1 for a single scan position and in Fig 4 for multiple scan positions. Applying standard methods of limited viewing tomography produces an unacceptable index reconstruction error of 10 -3 . Small increases in cone angle will not sufficiently reduce this reconstruction error when using a single observation window. The “wily” matrix is generated from the propagation of the marginal rays through a 6-row x 8-column grid where the top and bottom rows are boundary conditions as shown in Fig 4. The “wily” algorithm uses the marginal rays of the convergent beam for each scan position through the specimen. Translational scanning of the beam is along the 1 − i and 1 + i positions and step-wise down the optical propagation axis. The scan position for each of the marginal rays within the grid produces a 32x32 path length matrix. Every 288 doi:10.1017/S1431927610063178 Microsc. Microanal. 16 (Suppl 2), 2010

Measurements validating the confocal scanning laser holography microscope.

A confocal scanning laser holography (CSLH) microscope that uniquely combines the concepts of confocal microscopy with holography has been validated for making nonintrusive, full three-dimensional (3D) intensity and phase measurements of objects from a single viewpoint of observation without loss of object information. The phase measurements have been used to determine the 3D refractive indices of a point source heated silicone oil. The refractive indices are converted to 3D temperature measurements, which are useful for heat transfer studies. An important feature of CSLH is its nonintrusive 3D scanning method, which enables its application to the study of Marangoni convection in microgravity with minimal operational vibrations affecting the motion of fluid in the specimen.