Imaging using volume hologramsArnab SinhaGeorge BarbastathisMassachusetts Institute of TechnologyDepartment of Mechanical EngineeringRoom 3-46677 Massachusetts AvenueCambridge, Massachusetts 02139E-mail: [email protected] LiuOndax Incorporated850 East Duarte RoadMonrovia, California 91016Demetri Psaltis,FELLOW SPIECalifornia Institute of TechnologyDepartment of Electrical Engineering1200 East California BoulevardMS 136-93Pasadena, California 91125Abstract. We present an overview of imaging systems that incorporatea volume hologram as one of the optical field processing elements in thesystem. We refer to these systems as volume holographic imaging (VHI)systems. The volume hologram is recorded just once, and the recordingparameters depend on the functional requirements of the imaging sys-tem. The recording step offers great flexibility in designing application-specific imaging systems. We discuss how a VHI system can be config-ured for diverse imaging applications ranging from surface profilometryto real-time hyperspectral microscopy, and summarize recent develop-ments in this field.©2004 Society of Photo-Optical Instrumentation Engineers.[DOI: 10.1117/1.1775230]Subject terms: imaging systems; volume holography.Paper VHOE-B02 received Dec. 10, 2003; revised manuscript received Jan. 29,2004; accepted for publication Feb. 27, 2004.1 IntroductionTraditional optical imaging systems, such as photographiccameras, microscopes, telescopes, and projection lenses arecomposed of an optical train, i.e., several lenses in succes-sion. The role of the lenses is to transform the optical fieldsuch that the resulting field distribution at the image planemeets the functional requirements of the system. For ex-ample, in traditional photographic imaging the goal is tocreate a projection of a 3-D field onto a 2-D receptor plane共photosensitive film or digital sampling plane兲. Within theconstraints of projective geometry, the 2-D image is in-tended to be geometrically similar to the original 3-D ob-ject. The dependence of the selection of the optical train onthe goal of the instrument can be seen by comparing amicroscope and a telescope. In the microscope, one aimsfor lateral magnification from an object plane at a finitedistance, whereas in the telescope the object is at infinityand the goal is angular magnification.1So the two systemsare very different in the way they transform ray bundles 共orequivalently, spatial frequencies兲.Nevertheless, traditional optical systems are very similarwith respect to certain other features. Most prominentamong these features is defocus, which is directly related todepth information 共that is, the third spatial dimension兲.Inclassical optics, a defocused object creates a blurred image,independent of the type of optics used 共even though theblur transfer function is of course highly dependent on thespecific choice of optics兲. Information about the third di-mension is lost in the process, but it can be partially recov-ered with digital postprocessing even from a single cameraimage, for example depth from defocus,2depth fromshading,3,4etc., or from multiple cameras.5The confocalmicroscope6is exceptional because the confocal pinholealmost eliminates out-of-focus light at the expense of fieldof view. Other optical instruments, such as coherence im-agers in the space domain7–10and time domain,11laserradar,12and Radon transform tomographers13acquire depthinformation via different mechanisms and tradeoffs.We describe a new type of optical element, a volumeholographic lens.14The volume holographic lens is a pre-recorded volume hologram15that is incorporated into theoptical train in addition to the other traditional lenses thatare already present in the train. The traditional refractivelenses perform simple 2-D processing operations on theoptical field16as it passes through the optical train and isincident on the volume holographic lens. The volume ho-lographic lens processes the optical field in 3-D on accountof its thickness,17i.e., it has Bragg selectivity.18The fielddiffracted by the volume holographic lens is measured toobtain the specific information that is required about theoptical field.The volume holographic lens is manufactured by record-ing a 3-D interference pattern of two 共or more兲 mutuallycoherent beams, as shown in Fig. 1共a兲. The recording isindependent of the object to be imaged, although the selec-tion of the type of hologram to be recorded 共e.g., the typeof reference beam兲 can be based on prior information aboutthe type of objects to be imaged 共e.g., the average workingdistance, reflective versus fluorescent, etc.兲. Simple record-ing schemes include interfering a spherical reference 共SR兲or planar reference 共PR兲 beam with a planar signal beam torecord holograms 共see Fig. 2兲 in the transmission, reflec-tion, or 90-deg geometry.14After recording is complete, thehologram is fixed;19,20no further processing is done on thehologram 共just like the fixed lenses in an imaging instru-ment after they are ground and polished兲. Despite the ap-parent simplicity of recording, these holograms offerunique imaging capabilities that are not available in tradi-tional lenses.1959Opt. Eng. 43(9) 1959–1972 (September 2004) 0091-3286/2004/$15.00 © 2004 Society of Photo-Optical Instrumentation EngineersDuring imaging, the recorded holograms are probed bythe incident illumination, as shown in Fig. 1共b兲.IfanSRhologram is used, the imaging system is referred to as SR-VHI. Similarly, a PR-VHI system refers to a system thatcontains a planar reference volume hologram. The holo-gram diffracts the Bragg-matched17,18components of theincident illumination. The diffracted field is monitored by adetector or a detector array. The diffracted field intensitycaptured by the detector is the ‘‘image’’ formed by the VHIsystem, and can be used to determine the required objectinformation like the 3-D spatial and/or spectral characteris-tics of the object of interest.This work is arranged as follows. In Sec. 2, we describevarious classes of imaging systems with particular empha-sis on their VHI implementations. In Sec. 3, we presentvarious VHI systems that we have demonstrated and dis-cuss each in detail. Finally, we conclude in Sec. 4 withsome directions for future work in VHI.2 Classification of VHI Systems2.1Type of Object/IlluminationThe material properties of the object and the type of illu-mination determine the nature of the image as