Zhihui Duan

Dispersion-free optical coherence depth sensing with a spatial frequency comb generated by an angular spectrum modulator

As an alternative to the conventional optical frequency comb technique, a spatial frequency comb technique is proposed for dispersion-free optical coherence depth sensing. Instead of generating an optical frequency comb over a wide range of time spectrum, we generate a spatial frequency comb by modulating the incident angle of a monochromatic plane wave with a spatial light modulator (SLM). The use of monochromatic light combined with the SLM enables dispersion-free depth sensing that is free from mechanical moving components.

Spatial coherence radar applied for tilted surface profilometry

A new method of spatial coherence profilometry is demon- strated. The surface profile is measured by shifting the spatial degree of coherence gradually in its own space of existence, and modulating its phase angle. In each point of the sample we analyze the change of light intensity versus the phase of a Fresnel zone pattern used as the intensity distribution of an incoherent quasimonochromatic source. The tilt of the surface is measured by gradually shifting the Fresnel zone plate on its transverse plane. This shift of the light source rotates the spatial degree of coherence around the coordinate origin until the condition of maximum interference visibility is fulfilled. The method works without any mechani- cal movement and a quasimonochromatic light illuminates the interfero- metric system. Experimental demonstration of the new method is pre- sented.

Dispersion-free absolute interferometry based on angular spectrum scanning

As an alternative to the conventional optical frequency scanning technique, an angular spectrum scanning technique is proposed for absolute interferometry. Instead of sweeping the optical frequency over a wide range of spectrum, we sweep the angular spectrum by changing the incident angle of a monochromatic plane wave with a spatial light modulator (SLM). The use of monochromatic light combined with the SLM enables dispersion-free absolute interferometry that is free from mechanical moving components.

Spatial coherence profilometry on tilted surfaces

The influence of tilted surfaces on the measurement of shape by spatial coherence profilometry is investigated. Based on theoretical analysis and experimental results, the systematic measurement error caused by surface tilt is determined. The systematic measurement error depends not only on the tilt angle but also on the parameters of the experimental setup. The theoretical analysis and the experiments show the similarities and differences between spatial coherence profilometry and white-light interferometry. We also suggest the conditions to obtain correct measurements by use of spatial coherence profilometry.

Synthetic spatial coherence function for optical tomography and profilometry: influence of the observation condition

We address the basic issue of the observation condition in a synthetic coherence function applied to optical tomography and profilometry, which has not been made clear in previous papers. We present a more general theory for interference fringe formation for spatial coherence control with a synthetic source. The generalized theory predicts the existence of the observation condition that can make the measurement insensitive to the tilt of the object, which will open the new possibility of measuring objects with rough surfaces. We present experimental results that quantitatively verify the validity of the principle and the prediction.

LCD-based low spatial coherence interferometer

An interferometer based on the spatial coherence detection has been developed for absolute height measurement. The location of a coherence peak resulting from the illumination of an extended quasi-monochromatic spatially incoherent light source with zone-plate-like spatial structure gives the longitudinal depth information. Besides simplifying the control of the light source structure, LCD-SLM eliminates the mechanical movement. Experiments of step height detection and profilometry of an object with a rough surface are presented that demonstrate the validity of the principle.

Experimental Study of Coherence Vortices: Birth and Evolution of Phase Singularities in the Spatial Coherence Function

In summary, we have presented evidence of coherence vortices for the first time and experimentally investigated the properties of phase singularities in the coherence function. Unlike for conventional optical vortices, the intensity for coherence vortices does not vanish, but their contrasts become zero. Furthermore, the proposed method for synthesizing coherence vortices faciliates direct observation of the detailed local properties of an coherence vortex, and introduces new opportunities to explore other topological phenomena for the coherence function.

Coherence holography: Holographic imaging with coherence function

A technique of imaging with a spatial coherence function, referred to as coherence holography, is proposed. A hologram of a three-dimensional object is recorded with coherent light in the same manner as in conventional holography, but the hologram is illuminated with spatially incoherent light to reconstruct the object as the three-dimensional distribution of a spatial coherence function, rather than as the amplitude or the intensity distribution of the optical field. A simple optical geometry for direct visualization of the reconstructed coherence image is proposed, and the results of preliminary experiments of a computer-generated coherence hologram (CGCH) are presented, which demonstrate the potential of CGCH as a means for optical coherence tomography and profilometry.

Synthesis and spatial evolution of longitudinal coherence vortex

By controlling the irradiance of an extended quasi-monochromatic, spatially incoherent source with a spatial light modulator, we generated a special optical field that exhibits a high degree of coherence with phase singularities for a specific pair of points at specified locations along the axis of beam propagation. Some local properties associated with coherence vortices, such as the Berry anisotropy ellipse describing the anisotropic degree of coherence close to a vortex core and the Dennis angular momentum rule for its associated phase, are also investigated experimentally.

Spatial coherence profilometry

Spatial coherence profilometry is a method for measurement of the geometrical form of objects. In addition to the two lateral coordinates x and y, it measures the longitudinal coordinate z. In this way the complete 3D description of the objects surface is acquired. The main piece of the presented method is a Michelson interferometer illuminated by a monochromatic spatially extended light source. The surface of the object whose geometrical form should be measured is used as one mirror of the Michelson interferometer. By moving of the measured object along the optical axis, the intereference is observable only if the objects surface occurs in the vicinity of the so-called reference plane. The reference plane is given by the position of the object mirror when the Michelson interferometer is balanced. The described effect follows from the form of the spatial coherence function originated by the spatially extended light source. If the intensity at the output of the interferometer is recorded as a function of the position of the measured object, a typical correlogram arises. This correlogram is similar to that known with white-light interferometry. From the maximum of the correlogram, the z coordinate of the objects surface can be determined. Usually a CCD camera is used as the detector at the output of the Michelson interferometer. Then z coordinates of many surface points are parallel measured in the course of one measurement procedure and the 3D description of the objects surface is acquired. The scanning in the lateral direction is not necessary. Thus the described method provides a spatial coherence analogy to white-light interferometry which is based on temporal coherence. Unlike white-light interferometry, the described method does not require a broadband light source, the interferometer is illuminated by a monochromatic light source, usually a laser.