Nestor O. Farmiga

Optical measurement of depth and duty cycle for binary diffraction gratings with subwavelength features

We describe a new and unique method for simultaneous determination of the groove depth and duty cycle of binary diffraction gratings. For a near-normal angle of incidence, the +1 and -1 diffracted orders will behave nearly the same as the duty cycle is varied for a fixed grating depth. The difference in their behavior, quantified as the ratio of their respective diffraction efficiencies, is compared to a look-up table generated by rigorous coupled-wave theory, and the duty cycle of the grating is thus obtained as a function of grating depth. Performing the same analysis for the orthogonal probe-light polarization results in a different functional dependence of the duty cycle on the grating depth. By use of both TE and TM polarizations, the depth and duty cycle for the grating are obtained by the intersection of the functions generated by the individual polarizations. These measurements can also be used to assess qualitatively both the uniformity of the grating and the symmetry of the grating profile. Comparison with scanning electron microscope images shows excellent agreement. This method is advantageous since it can be carried out rapidly, is accurate and repeatable, does not damage the sample, and uses low-cost, commonly available equipment. Since this method consists of only four fixed simple measurements, it is highly suitable for quality control in a manufacturing environment.

Phase effects of secondary reflections on the performance of reflective liquid-crystal cells

In this paper, the phase effects of secondary reflections in reflective liquid-crystal cells are explored. The existence of such secondary reflections are analytically predicted and experimentally verified. The wavelength dependence of the phenomena is used to explore the magnitude of the effect. Changes of the net retardation due to these secondary reflections are measured to be +/- 3%. Numerical modeling verifies that the root cause of these changes is secondary reflections, and that as much as 10% change can be obtained for different liquid-crystal cell thicknesses and material compositions. The deleterious effects of these secondary reflections are explored from a device performance perspective, and found to be most harmful for incident light with a broad spectral bandwidth.

Collimators and collimator arrays for telecommunication applications

Precision collimators and collimator arrays are increasingly critical components as telecommunication networks increase in bandwidth. In this paper we describe the collimator fabrication process at Corning Rochester Photonics Corporation. We describe replicated and etched collimator arrays and illustrate packaging concepts. Metrology and optical performance data is presented for both individual and arrayed collimators. Microlens surfaces etched into fused silica with surfaces sags of greater than 70micrometers , surface finishes better than 30 Angstroms are presented. Collimator arrays with pointing errors less than 40(mu) Rad, and microlens focal length uniformity error less than +/- 0.5% have been fabricated and the data presented.