Although confocal imaging has recently received much attention among researchers exploring high-resolution noncontact three-dimensional (3-D) optical imaging, interference microscopy may offer a simpler and perhaps faster solution, in part because lateral scanning is eliminated. The scheme obtains depth information by measuring the degree of coherence between corresponding pixels in the object and reference plane using the entire available illumination.1 All transverse points are measured in parallel, and both transverse resolution and depth response are comparable with that of a confocal microscope.
The interference microscope with wavelength-to-depth encoding uses a 4-f imaging system on the object arm with a diffractive lens and an objective lens to inversely image the coherent cells in the focal plane of the diffractive lens for the center wavelength of the system.
While the interference-microscopy-based imaging technique can use a variety of architectures, including those based on the Linnik microscope, the Mirau correlation microscope, and the Michelson interferometer, such systems require the use of a piezoelectric translation stage to scan the object along the vertical axis. Researchers at the University of California-San Diego (UCSD; La Jolla, CA) believe there is a way to work around such mechanical depth scanning techniques.
The scientists from the UCSD department of electrical and computer engineering instead propose using an interferometric microscope based on a wavelength-to-depth encoding technique. This method, which combines a diffractive lens with a wavelength-tunable laser, allows the use of separate diffractive and refractive imaging systems in the object and reference arms. High-resolution 3-D profiles of an object are obtained by calibrating the wavelength-to-depth encoding and measuring the coherence function for each image pixel. Vibration does not impact system feedback significantly because the technique measures the occurrence of interference, not the interference phase.
The proof-of-principle experiment includes a wavelength-tunable Ti:sapphire laser that emits linearly polarized quasimonochromatic light. This collimated beam then travels through a rotating ground glass that generates a spatially incoherent optical field, with the coherence area defined in such a way that the light from any two points within the area will interfere. With certain assumptions met, each basic area can be called a coherence cell. Two sets of these will be created in each arm by a beamsplitter, and any coherence cell in one arm will be coherent with only one coherence cell in the other arm. When corresponding cells overlap in image space, the technique will produce high-contrast interference fringes.
The microscope's object arm uses a 4-f imaging system with a diffractive lens and an objective lens to inversely image the coherent cells in a plane designated x1y1, which is the focal plane of the diffractive lens for the center wavelength of the system (see figure). The reference arm includes another 4-f imaging system with a refractive large-aperture achromatic lens and an objective lens, as well as a translation stage mirror. After the coherent cells in the object plane and the reference mirror interact, the result is imaged back to a plane designated x5y5. The overlapping optical fields are then imaged through a lens onto a charge-coupled-device (CCD) detector, where the interference patterns are recorded.
To measure the depth discrimination possible with wavelength encoding, the UCSD scientists fixed the object mirror and fine-tuned the wavelength of emitted light from 830 to 893 nm in increments of 0.25 nm. At each wavelength, they monitored the output power of the laser and recorded the interference fringes. The intensity of the fringes was then normalized according to the laser power and the spectral response of the CCD. Results indicated that an interference microscope based on wavelength-to-depth encoding offers a depth resolution of 0.71 µm with the use of 0.9-NA objective lenses.
Paula Noaker Powell
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