Fabrication and evaluation of a diamond diffractive fan-out element for high power lasers.

Fabrication and evaluation of diamond binary diffractive fan-out elements is demonstrated. The diffractive optical elements (DOEs) are designed for two different wavelengths, 633 nm and 10.6 microm. The DOE splits an incident beam into 16 spots that form a ring pattern. The surface reliefs were fabricated by photolithographic methods followed by plasma etching, which produced well-defined patterns with smooth surfaces. One DOE was optically evaluated with a HeNe laser, operating at a wavelength of 633 nm, and showed good performance. The DOE designed for a wavelength at 10.6 microm was tested together with a carbon dioxide laser. The light pattern was used to microstructure a 10 mm thick PMMA piece with very good results.

Diamond micro-optics: microlenses and antireflection structured surfaces for the infrared spectral region.

Fabrication and evaluation of a subwavelength grating in diamond, designed to reduce the Fresnel reflection, is demonstrated. The antireflection (AR) structures are designed to reduce the surface reflection at an illuminating wavelength of 10.6 microm. With this AR-treatment, where no other material is introduced (i.e., no thin film coating), the unique properties of diamond can be fully used. The fabricated AR structures were optically evaluated with a spectrophotometer. The transmission through a diamond substrate with AR structures on both sides was increased from 71% to 97%, with a theoretical value of 99%. Microlenses in diamond are also demonstrated. The lenses are evaluated with interferometers and show good performance. The micro-optical structures were fabricated by electron-beam lithography or photolithographic methods followed by plasma etching.

Study of benzocyclobutene as an optical material at elevated temperatures.

Temperature changes of micro-optical elements can be expected in applications such as telecommunication and laser machining. We present a comparative study of diffractive gratings fabricated in benzocyclobutene (BCB) and in a conventional photoresist under the influence of elevated temperature. We measured variations in both optical diffraction efficiency and topography by heating the samples to 300 degrees C in air in a specially built oven. Our main findings show that gratings fabricated in BCB exhibit small changes, both optically and topographically, during the first 45 min, whereas the conventional photoresist gratings change drastically within just a few minutes.

Transfer of continuous-relief diffractive structures into diamond by use of inductively coupled plasma dry etching.

The transfer of continuous-relief diffractive structures from resist into diamond by use of direct-write electron-beam lithography followed by dry etching in an inductively coupled plasma is demonstrated. The gases used for the diamond etching are O(2) and Ar. The chemical-vapor-deposited diamond substrate is of optical quality. Our results show that the transfer process generates fairly smooth etched structures. Blazed gratings with periods of 45mum and Fresnel lenses have been manufactured. The blazed gratings have been optically evaluated with a femtosecond laser operating at 400 nm. The diffraction efficiency was 68% in the first order, with a theoretical value of 100%. We intend to investigate the transfer process further and then to fabricate diffractive and refractive elements for use with Nd:YAG high-power lasers.

Fabrication and simulation of diffractive optical elements with superimposed antireflection subwavelength gratings.

With the aim of reducing surface reflections and increasing the diffraction efficiency we investigated the superposition of subwavelength phase gratings onto blazed phase gratings. With direct-write electron-beam lithography bare blazed gratings and blazed gratings carrying subwavelength gratings were fabricated and their optical performances compared. For TE polarization the subwavelength-carrying gratings showed a maximum diffraction efficiency of 90.6%, whereas the corresponding maximum value for the bare grating was 86.3%. The experiment was simulated with rigorous diffraction theory.

Spatial-mode control of vertical-cavity lasers with micromirrors fabricated and replicated in semiconductor materials.

Micromirrors were fabricated in gallium phosphide by mass transport to provide spatial-mode control of vertical-cavity surface-emitting lasers (VCSEL's). The concave mirrors were used in an external-cavity configuration to provide spatial filtering in the far field. Single-mode cw lasing was demonstrated in 15-microm-diameter VCSEL's with currents as high as 6 times threshold. The fabrication process was extended to micromirrors in gallium arsenide by use of a replication and dry-etch transfer process.

Replication of continuous-relief diffractive optical elements by conventional compact disc injection-molding techniques.

Continuous-relief diffractive optical elements have been replicated by use of conventional compact disc injection-molding techniques. Two continuous-relief microstructures, a blazed grating and a fan-out element, were chosen to evaluate the replication process. Original elements were fabricated by direct-write electron-beam lithography. Optical measurements and atomic force microscopy were used for investigating the replication fidelity.

Diffractive microlenses replicated in fused silica for excimer laser-beam homogenizing.

A diffractive beam homogenizer, based on an array of square, off-axis, continuous-relief diffractive microlenses, for use with an excimer laser has been studied. We originally fabricated the homogenizer by direct-write electron-beam lithography, from which we made replicas in UV-grade fused silica by hot embossing and reactive ion etching. Atomic force microscopy measurements of original and replicated elements showed the accuracy of the replication fidelity. One of the replicated homogenizers was evaluated together with a KrF excimer laser. The homogenized beam had a flat-top profile with 31% of the beam energy contained within an area where the beam intensity was above a threshold level of 90% of the maximum intensity.

Measuring and modeling the proximity effect in direct-write electron-beam lithography kinoforms.

The proximity effect in successively developed direct-write electron-beam lithography gratings is measured. The grating relief shapes are obtained from the measured power in several of the gratings' diffraction orders. Describing the proximity effect by a convolution with a double Gaussian point-spread function, we determine the parameters of the point-spread function. The writing part of the point-spread function is found to increase significantly with increasing development time, the background part much less.

Proximity-compensated blazed transmission grating manufacture with direct-writing, electron-beam lithography.

Proximity-compensated, as well as uncompensated, blazed transmission gratings with periods of 4, 8, and 16 µm were manufactured with direct-writing, electron-beam lithography in positive resist. The compensated gratings performed better than the uncompensated ones. For the 4-µm compensated grating the measured diffraction efficiency was 67%. It was 35% for the uncompensated grating. The compensation was made by repeated convolutions in the spatial domain with the electron-beam point spread function. We determined this function by retrieving the phase from the measured diffraction pattern of the uncompensated gratings.

Successive development optimization of resist kinoforms manufactured with direct-writing, electron-beam lithography.

It is shown that multilevel SAL 110 resist kinoforms can be developed stepwise. Measurements of the kinoform diffraction pattern, performed between the development steps, permitted correct final developments to be made. No significant relief shape degradation was observed for development times as high as 25 min. The results imply that the electron-beam exposure doses, and hence the exposure time, can be reduced by a factor of 3 compared with doses used currently.

Properties of high-pass resolution perimetry targets.

Relationships between high-pass resolution perimetry ring targets and conventional perimetry and acuity targets were explored by optical analytical techniques and by comparative measurements in normal humans. High-pass resolution and acuity targets produced closely proportional resolution measurements, showing that the critical ring detail is the width of the bright core. High-pass resolution and conventional perimetry thresholds were not equally well correlated. From a purely optical point of view, high-pass resolution targets appeared to lose somewhat more contrast on defocusing. On the other hand, high-pass resolution appears to show less variability in practical, clinical use.