Menelaos K. Poutous

Anti-reflective surface structures for spinel ceramics and fused silica windows, lenses and optical fibers

Anti-reflective surfaces structures (ARSS) have been successfully fabricated on fused silica windows, lenses and fibers, and spinel ceramics. The reflection loss for spinel was reduced from 7% per surface to 0.9%. For fused silica with ARSS, the reflection loss was reduced to 0.02% near 1 µm. Pulsed laser damage thresholds at 1.06 µm were measured and thresholds as high as 100 J/cm2 were obtained for fused silica windows of up to 10 cm in diameter with ARSS and 850 J/cm2 for silica fibers with ARSS on the end faces. Spinel samples with ARSS showed damage thresholds more than two times higher than that of spinel with traditional AR coatings.

Integrated Tm:fiber MOPA with polarized output and narrow linewidth with 100 W average power

We report on a Tm:fiber master oscillator power amplifier (MOPA) system producing 109 W CW output power, with >15 dB polarization extinction ratio, sub-nm spectral linewidth, and M2 <1.25. The system consists of polarization maintaining (PM) fiber and PM-fiber components including tapered fiber bundle pump combiners, a single-mode to large mode area mode field adapter, and a fiber-coupled isolator. The laser components ultimately determine the system architecture and the limits of laser performance, particularly considering the immature and rapidly developing state of fiber components in the 2 μm wavelength regime.

Spatial and spectral beam shaping with space-variant guided mode resonance filters

Novel all-dielectric beam shaping elements were developed based on guided mode resonance (GMR) filters. This was achieved by spatially varying the duty cycle of a hexagonal-cell GMR filter, to locally detune from the resonant condition, which resulted in modified wavelength dependent reflection and transmission profiles, across the device aperture. This paper presents the design, fabrication, and characterization of the device and compares simulations to experimental results.

Performance of conformal guided mode resonance filters

Guided mode resonance (GMR) filters are highly functional micro-optics capable of narrowband spectral filtering. GMR devices have previously been demonstrated on flat substrates using a wide range of materials and configurations. In this Letter, we apply a soft lithographic technique followed by the deposition of dielectric layers to generate GMR filters on a concave lens surface. Resonances of the resulting conformal GMR filters are experimentally measured and characterized, and the results are compared to the performance of similar GMR filters fabricated on flat surfaces.

Novel method for the fabrication of spatially variant structures

Spatially varying grating structures formed at the subwavelength scale behave as a layer with an artificial effective refractive index that is dependent on the local fill fraction. We describe a novel technique to pattern gratings with a spatially varying fill fraction using a simple two-step exposure process. The first exposure forms a partial latent image of a grating in the photoresist. The resist is then saturated by overlaying an exposure with an analog spatially varying intensity, generated by using a phase-only masking technique. The cumulative exposure dose from the two steps was designed so that the point of minimum intensity will still develop the photoresist through, in all the spaces in the grating. By varying the exposure window around the saturation dose, the fill fraction of the patterned gratings was modulated; thus, the size of the space cleared at any location in the grating is a scalable function of the local cumulative dose delivered. Constant feature height is achieved across the patterned area by keeping the second exposure dose below the resist threshold exposure value. The exposure process was modeled numerically to predict the relationship between the local dose and patterned fill fraction. This technique enables rapid, low-cost fabrication of apodized grating structures for applications in diffractive optics technology.

Correlation of Fabrication Tolerances with the performance of Guided-Mode-Resonance Micro-optical components

Large-scale fabrication of micro-optical Guided-Mode-Resonance (GMR) components using VLSI techniques is desirable, due to the planar system integration capabilities it enables, especially with laser resonator technology. However, GMR performance is dependent on within-wafer as well as wafer-to-wafer lithographic process variability, and pattern transfer fidelity of the final component in the substrate. The fabrication of lithographs below the g-line stepper resolution limit is addressed using multiple patterning. We report results from computational simulations, fabrication and optical reflectance measurements of GMR mirrors and filters (designed to perform around the wavelength of 1550nm), with correlations to lithographic parameter variability, such as photoresist exposure range and etch depth. The dependence of the GMR resonance peak wavelength, peak bandwidth are analyzed as a function of photolithographic fabrication tolerances and process window.

Comparative discrimination spectral detection method for the identification of vapors using overlapping broad spectral filters.

We present a comparative discrimination spectral detection approach for the identification of chemical vapors using broad spectral filters. We applied the method to flowing vapors of as-received and non-interacting mixtures for the detection of the volatile components of a target chemical in the presence of interferents. The method is based on measurements of the overall spectral signature of the vapors, where the interferent spectrum largely overlaps the target spectrum. In this work we outline the construction of a set of abstract configuration-space vectors, generated by the broadband spectral components from sampled chemical vapors, and the subsequent vector-space operations between them, which enable the detection of a target chemical by comparative discrimination from interferents. The method was applied to the C-H vibrational band from 2500 to 3500 cm−1, where there is large spectral signal overlap between the chosen target chemical and two interferents. Our results show clear detection and distinction of the target vapors without ambiguity.

Two-dimensional guided mode resonance filters fabricated in a uniform low-index material system

We demonstrate the fabrication, simulation, and experimental results of a buried, homogeneous narrowband spectral filter with a periodic, hexagonal unit cell of air pockets, encapsulated in a fused silica substrate. The leaky waveguide is formed by depositing SiO(x) on an etched fused silica grating via plasma-enhanced chemical vapor deposition. Design principles of guided mode resonance filters were utilized to achieve a resonance with 60% reflectivity at a wavelength of 1.741 μm. The device demonstrates resonance with FWHM of 6 nm.

Polarization selective, graded-reflectivity resonance filter, using a space-varying guided-mode resonance structure.

We designed, fabricated, and tested, polarization selective, graded-reflectivity resonant filters; based on a radial-gradient spatially-distributed, guided-mode resonance device architecture. The demonstrated filters have polarized spectral-resonance responses, distributed across their aperture extent, in the range between 1535 nm and 1540 nm wavelengths. Spectral sensitivity was observed on device tests, for wavelength changes as low as 0.2 nm. Using multiple lithographic exposures and biasing exposure methods, the devices were engineered to have a sub-aperture region, with no hard boundaries or diffraction anomalies.

Guided-Mode Resonance Filters for Wavelength Selection in Mid-Infrared Fiber Lasers

A narrowband mid-infrared (mid-IR) guided-mode resonance filter (GMRF) is designed and fabricated using a Hafnium Dioxide film/quartz wafer material system. The fabricated GMRF is then integrated into an erbium (Er)-doped Zr-Ba-La-Al-Na (ZBLAN) fluoride glass fiber laser as a wavelength selective feedback element. The laser operated at 2782 nm with a line-width less than 2 nm demonstrates the viability of GMRFs for wavelength selection in the mid-IR.