A self-scanned 1024 element photodiode array and minicomputer are used to measure the phase (wavefront) in the interference pattern of an interferometer to lambda/100. The photodiode array samples intensities over a 32 x 32 matrix in the interference pattern as the length of the reference arm is varied piezoelectrically. Using these data the minicomputer synchronously detects the phase at each of the 1024 points by a Fourier series method and displays the wavefront in contour and perspective plot on a storage oscilloscope in less than 1 min (Bruning et al. Paper WE16, OSA Annual Meeting, Oct. 1972). The array of intensities is sampled and averaged many times in a random fashion so that the effects of air turbulence, vibrations, and thermal drifts are minimized. Very significant is the fact that wavefront errors in the interferometer are easily determined and may be automatically subtracted from current or subsequent wavefrots. Various programs supporting the measurement system include software for determining the aperture boundary, sum and difference of wavefronts, removal or insertion of tilt and focus errors, and routines for spatial manipulation of wavefronts. FFT programs transform wavefront data into point spread function and modulus and phase of the optical transfer function of lenses. Display programs plot these functions in contour and perspective. The system has been designed to optimize the collection of data to give higher than usual accuracy in measuring the individual elements and final performance of assembled diffraction limited optical systems, and furthermore, the short loop time of a few minutes makes the system an attractive alternative to constraints imposed by test glasses in the optical shop.

Digital wavefront measuring interferometer for testing optical surfaces and lenses

https://www.cell.com/biophysj/pdf/S0006-3495(95)80396-6.pdf

Evidence for localized cell heating induced by infrared optical tweezers.

Optical-trapping forces exerted on polystyrene microspheres are predicted and measured as a function of sphere size, laser spot size, and laser beam polarization. Axial and transverse forces are in good and excellent agreement, respectively, with a ray-optics model when the sphere diameter is ≥ 10 µm. Results are compared with results from an electromagnetic model when the sphere size is ≤ 1 µm. Axial trapping performance is found to be optimum when the numerical aperture of the objective lens is as large as possible, and when the trapped sphere is located just below the chamber cover slip. Forces in the transverse direction are not as sensitive to parametric variations as are the axial forces. These results are important as a first-order approximation to the forces that can be applied either directly to biological objects or by means of microsphere handles attached to the biological specimen.

Parametric study of the forces on microspheres held by optical tweezers.

Axial trapping forces exerted on microspheres are predicted using a Gaussian beam electromagnetic field model and a ray‐optics model, and compared with experimental measurements. Ray‐optics predicts a maximum trapping efficiency Q= −0.14 for optically trapped polystyrene microspheres in water, compared to a measured value of −0.12 ± 0.014 for 10 μm diam microspheres. When the microspheres are composed of amorphous silica, the predicted ray‐optics Q decreases to −0.11, compared to a Q = −0.034 predicted by the electromagnetic field model, and a measured value of −0.012 ± 0.001 for 1 μm diam microspheres. These results indicate that the two models have applicability in two different size regimes, and thus, are complementary.

/pdf/radiation-trapping-forces-on-microspheres-with-optical-3izwz8iwho.pdf

Radiation trapping forces on microspheres with optical tweezers

Slowing down light on a chip can lead to the development of optical buffers1, filters2,3 and memory elements4 useful for optical interconnects and for resonantly enhanced chip-based nonlinear optics5,6. Several approaches to slow light rely on the phenomenon of light interference in a sequence of coupled resonators1,2,3,4,7,8,9,10,11; however, light interference is also responsible, in disordered structures, for the localization of light, an effect particularly prominent in one-dimensional devices12,13. Until now, the length of the waveguides investigated has been less than the localization length. Here we report the first observation of light localization in compact silicon nanophotonic slow-light waveguides consisting of long sequences of coupled resonators. Our results show that disorder limits how much light can be slowed, and that localization leads to spatially concentrated and locally trapped light in a quasi-one-dimensional waveguide at wavelengths near the band edge.

/pdf/localization-in-silicon-nanophotonic-slow-light-waveguides-eedg93ygjf.pdf

Localization in silicon nanophotonic slow-light waveguides

This note describes a convenient working technique for measuring the minimum size focused spot formed by a lens system. The technique is routinely capable of measurement precision of better than ?lambda. The theory of knife-edge scanning is presented, and the relationship of the measured spot profile to the actual spot profile is shown to be relatively insensitive to the acceptance solid angle of the photodetector. Finally, some typical results of high resolution microscope objectives are presented.

Knife-edge scanning measurements of subwavelength focused light beams.

A new technique, tuned-laser spectroscopy, is described. Absorption data are given for methane and ethane in the region from 2947.8 cm−1 to 2948.0 cm−1 which were obtained using a helium–neon laser. The large resolving powers available have made it possible to carry out the first absolute measurements of the collision diameters for methane–other gas collisions.

M. E. Heller

Papers

Knife-edge scanning measurements of subwavelength focused light beams.

High-Resolution Tuned-Laser Spectroscope