We have developed an integrated Tm:fiber master oscillator power amplifier (MOPA) system
producing 100 W output power, with sub-nm spectral linewidth at -10 dB level, >10 dB
polarization extinction ratio, and diffraction-limited beam quality. This system consists of
polarization maintaining fiber, spliced together with fiberized pump combiners, isolators and
mode field adaptors. Recent advances in PM fibers and components in the 2 μm wavelength
regime have enabled the performance of this integrated high power system; however further
development is still required to provide polarized output approaching kilowatt average power.
We report on a Tm:fiber master oscillator power amplifier system producing 100 W output power, with
>10 dB polarization extinction ratio and diffraction-limited beam quality. To our knowledge, this is the
highest polarized output power from an integrated Tm:fiber laser. The oscillator uses polarization
maintaining (PM) single mode fiber with 10/130 μm core/cladding diameters, and the amplifier uses large
mode area PM fiber with 25/400 μm core/cladding diameters. The oscillator and amplifier are pumped
using 793 nm diodes spliced with pump combiners, and the oscillator is spliced to the amplifier via a
mode field adaptor.
This paper presents a narrow spectral filter based on a monolithic material system. Guided-mode resonance is
achieved by embedding a periodic array of air holes within a similar-index material. Microvoids created in the lowindex
substrate during deposition of the waveguide give a relatively high index contrast for guided-mode resonance.
One and two-dimensional gratings are used to examine polarization dependence of the device. Theoretical and
experimental results are provided, demonstrating a roughly six nanometer resonance at the full width half-maximum for
In this paper, we will present the concept, fabrication methods, and simulation results of a novel type of Graded
Transmissivity Optic based on a space variant Guided Mode Resonance Filter (GMRF). This GMRF comprised of a
single dielectric layer deposited on a transparent substrate. The layer is PECVD grown Silicon Nitrirde with a
subwavelength grating (SWG) partially etched through it. The unetched portion of the layer is termed the waveguiding
region. When light is incident upon the GMRF at the resonant wavelength, the SWG couples light into a waveguide
mode. However, due to the SWG on the waveguide, this mode is leaky and re-couples the light back towards the source.
The resonance of the GMRF is a function of the optical properties of the materials used; the thickness of the dielectric
layers; and the period and fill-fraction of the SWG. The resonance will change across the device by slowly varying the
thickness of the waveguiding layer. Previous work has varied the resonance across the structure by varying the fill
fraction of the grating. The methods involved in the previous work made that process usable for only a very narrow
range of wavelengths, however this new method will be scalable to a larger wavelength range. The waveguiding layer
will be sculpted using Additive Lithography and ICP etching. Afterwards the SWG will be patterned into the Silicon
Micro-Optics has expanded to include a wide variety of applications for spectral filtering, polarization filtering and beam
shaping. Recently, a new class of optical elements have been introduced that can combine the spectral, polarization, and
beam conditioning into the same optical element. This engineered optical functionality results in a 3D Meta-Optic
structure that relies on sub-wavelength features to essentially engineer the electromagnetic fields within the structure;
thereby, resulting in highly dispersive structures that spatially vary across the optical element. This talk will summarize
recent results in the design, fabrication and applications of 3D Meta-Optics.
Beams from three frequency stabilized master oscillator power amplifier (MOPA) thulium fiber laser systems were
spectrally beam combined using a metal diffraction grating. Two of the laser oscillators were stabilized with guided
mode resonances filters while the third was stabilized using a gold-coated diffraction grating. Each system was
capable of producing a minimum of 40 W output powers with slope efficiencies between 50-60 %. The three lasers
undergoing combination were operating at wavelengths of 1984.3, 2002.1, and 2011.9 nm with spectral linewidths
between 250-400 pm. Beam combining was accomplished by spatially overlapping the spectrally separated beams
on a water-cooled gold-coated diffraction grating with 600 lines/mm. Beam quality measurements were completed
using M2 measurements at multiple power levels of the combined beam. Power levels of 49 W were achieved before
thermal heating of the metal diffraction grating cause degradation in beam quality. The combining grating was
~66% efficient for the unpolarized light corresponding to a total optical-to-optical efficiency of 33% with respect to
launched pump power.
Guided mode resonance filters (GMRF) were used to spectrally-stabilize and line-narrow the output
spectrum from Tm fiber lasers operating in the 2 μm wavelength regime. The GMRFs were placed in the output path
of an amplified spontaneous emission (ASE) light source and the transmitted light was measured as a notch in the
spectrum on resonance. The GMRFs were characterized to determine their peak reflectivity, resonance wavelength,
and spectral linewidth of each element. These measurements showed various resonance wavelengths and linewidths
varying from 0.50-1.5 nm depending on the individual GMRF parameters. Using GMRFs as feedback elements in
Tm fiber laser oscillators resulted in output powers up to 10 W and slope efficiencies of 30-45% with respect to
launched 790 nm pump power. In order to scale to higher powers and maintain narrow linewidths, a master
oscillator power amplifier (MOPA) setup was employed with a GMRF stabilized master oscillator. In addition to the
laser and amplifier characteristics, thermal and damage testing of the GMRFs is reported.
Eye-safe, high power tunable narrow linewidth lasers are important for various applications such as atmospheric
propagation measurements. We have investigated two techniques of generating narrow linewidth thulium 2-μm fiber
lasers, utilizing a reflective volume Bragg grating (VBG), and a guided mode resonance filter (GMRF) as a cavity end
mirror. A stable narrow linewidth (50 pm), tunable (from 2004 nm to 2054 nm) thulium doped fiber laser using a
reflective VBGg was demonstrated. A CW power of 17 W was achieved. Using a GMRF as an end mirror we showed a
narrow linewidth (~30 pm) laser with an output power of 5.8W, and at a slope efficiency of 44%.
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.
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.
Evolution in nature has produced through adaptation a wide variety of distinctive optical structures in many life forms.
For example, pigment differs greatly from the observed color of most beetles because their exoskeletons contain
multilayer coatings. The green beetle is disguised in a surrounding leaf by having a comparable reflection spectrum as
the leaves. The Manuka and June beetle have a concave structure where light incident at any angle on the concave
structures produce matching reflection spectra.
In this work, semiconductor processing methods were used to duplicate the structure of the beetle exoskeleton. This was
achieved by combining analog lithography with a multilayer deposition process. The artificial exoskeleton, 3D concave
multilayer structure, demonstrates a wide field of view with a unique spectral response. Studying and replicating these
biologically inspired nanostructures may lead to new knowledge for fabrication and design of new and novel nano-photonic
devices, as well as provide valuable insight to how such phenomenon is exploited.
Optical properties of periodic structures formed at the sub-wavelength scale differ significantly from those of the
bulk materials in which these structures are formed. Prior research has shown that periodic structures at the subwavelength
scale possess a polarization sensitive artificial effective refractive index. This effective index is
dependent upon both the duty cycle for a constant period and the period to wavelength ratio. Artificial diffractive
structures have been formed in structured media by spatially varying the duty cycle dependent refractive index
variation. In this paper we describe a novel technique for the patterning and fabrication of sub-wavelength structures
with the effective refractive index spatially varying across the optic using a combination of additive lithography and
analog optics technology that our group has previously developed. A two dimensional grating was formed in the
resist by delivering a partial exposure dose and superimposed with an analog intensity profile generated from a
phase mask to saturate the resist exposure. The exposure was tailored such that the point of least intensity will still
completely expose the photoresist in any of the holes in the array. The local size of the opening created upon
development is dependent upon the amount of controlled over-exposure. The optic was then transferred into the
desired substrate by dry etching. The exposure process is studied by modeling and diffractive and refractive
structures with analog phase functions are demonstrated. The optical response of the fabricated structures as a
function of duty cycle variation is analyzed by numerical modeling.
Using a simplified fabrication process, we present the experimental verification of the performance of a 3-D photonic crystal optical transmission filter. Inherent to this unique fabrication approach to the realization of narrow line width, highly efficient optical transmission filters, is the ability to spatially vary the transmission characteristics across the filter aperture. This differentiates this type of filter from conventional dielectric based space variant optical transmission filters which require additional processing at intermediate steps within the dielectric film deposition process. The multilayer stack consists of alternating high and low refractive index dielectric material grown on either side of a high index dielectric spacer layer, which produces a narrow transmission notch in the center of a large stop band. The nano-structuring of a square lattice array of holes and subsequent etching of the pattern through a dielectric stack provides the ability to spatially vary the location of the narrow transmission peak within the wide stop band based off of variation of the hole diameter or lattice constant of the array.