After the invention of lasers, in the past 50 years progress made in laser-based display technology has been very promising, with commercial products awaiting release to the mass market. Compact laser systems, such as edge-emitting diodes, vertical-cavity surface-emitting lasers, and optically pumped semiconductor lasers, are suitable candidates for laser-based displays. Laser speckle is an important concern, as it degrades image quality. Typically, one or multiple speckle reduction techniques are employed in laser displays to reduce speckle contrast. Likewise, laser safety issues need to be carefully evaluated in designing laser displays under different usage scenarios. Laser beam shaping using refractive and diffractive components is an integral part of laser displays, and the requirements depend on the source specifications, modulation technique, and the scanning method being employed in the display. A variety of laser-based displays have been reported, and many products such as pico projectors and laser televisions are commercially available already.

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Laser-based displays: a review

We have demonstrated a new type of dynamic measurement system that is comprised of a micropolarizer array and can work with any type polarization interferometer to measure a variety of physical properties. The unique configuration overcomes many of the limitations of previous single frame, phase-shift interferometer techniques. In particular it has a true common path arrangement, is extremely compact, and is achromatic over a very wide range. We demonstrated high quality measurement with both a Twyman-Green and Fizeau type interferometer. The technique is useful for many applications where vibration or motion is intrinsic to the process.

Pixelated Phase-Mask Dynamic Interferometer

In accordance with the invention, the oxygenation of blood-profused tissue is measured by shining light into the profused tissue and analyzing the light reflected within the tissue. The light is reflected by cell walls in the tissue and is partially absorbed by hemoglobin in the blood. Since the extent of absorption is sensitive to the extent of hemoglobin oxygenation, measurement and processing of the reflected light provides a measure of the oxygenation of the blood. In one embodiment, the method is applied to measure the oxygenation of blood within the tympanic membrane (ear drum).

Method and apparatus for tissue oximetry

This invention relates to a method of diagnosing or treating a biological subject, such as a person or animal, comprising the steps of subjecting at least a microsample of the subject's tissue to a physiological perturbation and measuring the response of the microsample to the perturbation using optical coherence tomography (OCT). In an exemplary embodiment, the concentration of glucose in the microsample is perturbed, as by providing the subject with a high glucose drink, and subsequently monitoring at a high sample rate in a microsample by OCT. Pathology, such as diabetes, can be diagnosed by deviation of the concentration vs. time response over several cells (micro-oscillation) from the micro-oscillation in the cells of a healthy subject. Other applications include diagnosing or treating de-hydration and diseases that cause changes in the osmolyte concentrations and thus the osmotic pressure in the cells in tissue.

OCT based method for diagnosis and therapy

Shearography is a full-field speckle interferometric technique used to determine surface displacement derivatives. For an interferometric technique, shearography is particularly resilient to environmental disturbances and has hence become an invaluable measurement tool outside of the optics laboratory. Furthermore, the inclusion of additional measurement channels has turned shearography from a qualitative inspection tool into a system suitable for quantitative surface strain measurement. In this review article we present a comprehensive overview of the technique, describing the principle of operation, optical configurations, image processing algorithms and applications, with a focus on more recent technological advances.

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Shearography technology and applications: a review

In laser projection systems the observer in the far field of the image points on the screen will recognize serious speckle noise There are many methods to reduce or eliminate speckles in the near field by reducing or eliminating temporal or spatial coherence of the laser But for the far field it is hardly possible to change the coherence properties of laser sources so that speckles will disappear We propose a new method for eliminating speckles in the far field by using a diffractive optical element The intensity modulation depth in the far-field speckle pattern can be reduced to a few percent while good beam quality is preserved

Speckle Reduction in Laser Projection Systems by Diffractive Optical Elements

We describe two new techniques for spatial phase stepping using a computer-generated holographic optical element (HOE). The techniques are implemented with electronic speckle pattern interferom- etry, although they could be applied to other optical metrology methods. The first technique uses an HOE that introduces a known phase step between the 61 diffracted orders, without being translated. The interfer- ence phase corresponding to object deformation can be calculated from a single TV frame, and the technique is therefore suitable for the mea- surement of transient deformations. We apply the technique to measure the phase changes caused by refractive index variations in an evolving thermal plume. The second technique requires the HOE to be used in conjunction with a phase grating. In this technique, the HOE and the phase grating are used sequentially for the two exposures required in speckle interferometry (made before and after object deformation) and is therefore suitable for making dynamic deformation measurements with a double-pulsed interferometer. A root mean square (rms) precision of 2p/20 rad for the phase measurements is obtained. © 1999 Society of Photo-Optical Instrumentation Engineers. (S0091-3286(99)00712-6)

Spatial phase-stepped interferometry using a holographic optical element

Solutions and analyses of fractional-Talbot array illuminations

Transient deformation measurement with ESPI using a diffractive optical element for spatial phase-stepping

We propose an optical pickup that acquires data from both layers of
a dual-layer digital versatile disk simultaneously. An adaptive
optical element that uses liquid crystals creates two axial foci
separated by a spacing of 55 µm, which is the distance between
the two layers. The spacing between the foci can be varied by the
adaptive element. The separation of the reflected light into TE and
TM polarized light, corresponding to each of the layers, is made by
dielectric gratings that are characterized by high aspect
ratios. Electron-beam lithography and reactive ion etching
techniques were used to produce the submicrometer structures. All
fabricated elements were assembled in a pickup system, whose properties
were measured.

Lingli Wang

Papers

Speckle Reduction in Laser Projection Systems by Diffractive Optical Elements

Spatial phase-stepped interferometry using a holographic optical element

Solutions and analyses of fractional-Talbot array illuminations

Transient deformation measurement with ESPI using a diffractive optical element for spatial phase-stepping

Integrated optical pickup system for axial dual focus