Volume Holographic Imaging (VTI)

Three dimensional volume imaging has attracted much research interest recently for its wide applications, both in the military areas such as 3D LADAR and 3D terra-scope and the civil areas, e.g. molecular biomedical investigation, on-line industrial product inspection, micro-fabrication investigation, etc. A field with fast growing importance is the 3D volume investigation of the objects that show very fast dynamic changes even in the range of microseconds, e.g. molecular biological process, micro-fabrication process, etc.

Confocal microscope is the current most widely used 3D imaging instruments. It utilizes a small pinhole at the confocal plane to obtain depth discrimination. Recent research also proposed new three-dimensional imaging methods such as optical coherent tomography, which utilizes the phase information acquired from the correlation of a broadband ultra-short laser pulse to obtain the 3D spatial information. Another approach is to use the mutual coherence information from pairs of point sources over the object to reconstruct the 3D object.

Basic principle of volume hologram imaging:

Generally speaking, hologram is an optical component in which there exists some refractive index modulation. The most common method to make a hologram is to use some photonic-sensitive material to record the interference of two mutually coherent light beams. For example, you may use a silver halide, and let it exposure to the interference fringes of two laser beams. Then you can develop the exposed silver halide to make an amplitude hologram, or you can bleach it to a phase hologram to future improve the diffraction efficiency.

Conventionally, people use hologram to record the 3D scene of an object, as you may see in a lot of science museums. Most of these holograms are called thin hologram, namely, their have thin thickness, which is comparable to the wavelength of the light. However, we mainly use volume hologram in our research. Volume hologram typically has a thickness of more dozens of wavelengths. Theories are usually used in treating the optical behavior of the volume holograms are three dimensional scalar diffraction theories or coupled wave theory, up to the strength of the volume hologram. However, the easiest way to understand volume hologram is treating it as a 3D diffractive lattice and applying Bragg diffraction theory. Volume hologram is recorded by two coherent beams, in which one is called signal beam and the other is call reference beam. People found if the reconstruction beam used to probe the volume hologram is identical to the original reference beam respected to the wavefront profile and propagating direction, a strong diffraction which propagates along the signal beam propagating direction and preserves identical wavefront profile with the signal beam is generated. In this case, the volume hologram is called Bragg matched. The diffraction vanishes very fast respected to the increasing of the propagating direction deviation and/or wavefront profile distortion between the probing beam and the reference beam. In these cases, the volume hologram is called Bragg mismatched.

We used volume hologram as an agile depth selective lens in imaging system to realize 3D imaging. Advantages of volume holographic imaging (VHI) systems include more design freedom, higher light efficiency, lower building cost, etc. According to different illumination source, our research on volume holographic imaging system mainly can be mainly divided into two categories.