Using binary etch and sub-wavelength DUV photolithography, we have designed and fabricated a variety of flat optical devices (lenses, vortex phase plates, vortex plus lens, and diffusers) useful in telecommunications and other areas. The devices provide the precision and low cost associated with modern semi-conductor manufacturing and offer unique functional performance. Since the design method involves selective binary removal of substrate material we designate derivative devices partial etch phase (PEP) devices. Design principles and fabricated device performance are described.
We describe monolithic advanced-function diffraction grating arrays for instantaneous ultrawide spectral coverage and
other uses that have inherent spectral and spatial self-calibration features. This new technology is made possible by
recent advances in deep ultraviolet (DUV) reduction-lithographic fabrication.
We demonstrate the use of deep ultraviolet (DUV) reduction photolithography, today's foremost commercial
nanofabrication technology, in the patterning of integrated nanophotonic filters based on etched channel waveguide
gratings. DUV photolithographic fabrication is seen to enable control over individual grating lines at the level of
nanometers enabling spectral engineering of the filter function in unprecedented fashion. Novel filter apodization
approaches are introduced and demonstrated that uniquely leverage DUV nanofabrication power. The demonstrated
filter functions are highly relevant for coarse wavelength division multiplexing and fiber to the premise applications.
With the ever increasing bandwidth of optical communications systems, it is critical to allow multiple users to access the available bandwidth. Code division multiplexed access (CDMA) is especially attractive in local area networks due to the large number of subscribers possible, the security and the simple architecture. Here we presents novel encoding/decoding structures using anti-symmetric gratings with application in optical CDMA and optical encryption.
One integrated pair of OCDMA encoder and decoder based on holographic Bragg reflector technology was designed and fabricated to simultaneously provide two multiple wavelength-hopping time-spreading optical codes. We have successfully demonstrated an encoding/decoding operation of two codes in 1.25-Gbps OCDMA testbed. A double-pass scheme was employed, enabling the implementation of much longer code length.
We report on the demonstration of several integrated slab-waveguide-based concentric Fabry-
Perot resonators employing holographic Bragg reflectors as curved 2D cavity mirrors. The cavities,
fabricated in a low-loss silica-on-silicon slab waveguide using high-fidelity deep ultra violet
photolithographic fabrication, exhibit Q-factors approaching up to 10<sup>6</sup> which are competitive with silicabased
ring resonators. The Q-values achieved indicate very high slab waveguide homogeneity providing
wavefront stability and extremely low loss from the volume-holographic lithographically scribed mirrors.
Compared to silica-based ring resonators, the folded Fabry-Perot resonator design allows access to a substantially larger free spectral range by cavity shortening. Pathways to the implementation of highreflectivity
and low loss distributed reflectors promise new directions in photonic integration with
applications in sensing, filtering, and signal transport.
Planar lithographic holography is a new direction in holographic development that draws on the unique merging of three previously unconnected disciplines, volume holography, waveguide optics and DUV projection-photolithographic fabrication. The approach has only recently become practical with the advent of high-resolution photolithography (<250 nm resolution) and waveguide materials of sufficient quality. Using planar lithographic holography, it is for the first time possible to fully harness the spectral and spatial signal processing power of volume holography with the added benefit of doing so in a robust fully-integrated environment. This makes possible the creation of integrated holographic devices for a broad range of optical signal processing applications.
Recent advances in semiconductor fabrication tools, which now support 100-nm pixilation and centimeter-scale spatial coherence, create intriguing new opportunities in integrated photonics. Application of the latest generation of fabrication tools allows for the implementation of broad new photonic device function based on 2D distributed diffractive structures such as holographic Bragg reflectors (HBRs) - devices that provide generalized spatial routing of signals within a planar waveguide circuit (e. g. silica-on-silicon) while at the same time providing powerful spectral filtering function. HBRs and other 2D distributed diffractive devices promise to open disruptive pathways to integrated photonic solutions characterized by high performance, small footprint, and extremely low cost especially when fabricated via stamping/nanoimprinting
Recent advances in the development of two-dimensional holographic Bragg reflectors in planar lightwave circuits have demonstrated the feasibility of highly customizable multi-wavelength filters based on photonics nanostructures programmable to recognize spectra with up to 2000 spectral lines. Such filters rival or exceed performance of free space gratings, thin film filters and Bragg gratings and may be monolithically integrated with detectors in III-V active materials or as passive devices in silica. This new technological platform holds a great promise of being next-generation optical engine for spectral signature recognition in the field of remote sensing, biological, chemical and defense applications.
Integrated holographics is a novel photonics technology made possible by recent advances in semiconductor manufacturing technology and planar waveguide fabrication. The technology's corner stone, the holographic Bragg reflector (HBR), is a slab-waveguide based, nanoscale, refractive-index structure that merges, for the first time, powerful features of holography, such as single-element spectral and spatial signal processing and overlay of multiple structures, with a highly integrated environment. As a building block for photonic circuits, the HBR's holographic signal mapping comprises a unique and novel way of on-chip signal routing and transport that is free-space-like but fully integrated. Signals propagate and overlap freely as they are imaged from active element to active element - an architecture that eliminates the need for constraining electronics-style channel-waveguides and associated space requirements and opens the door to unique integrated photonic circuits of very compact footprint. Photolithographic HBR fabrication was recently demonstrated to provide complete amplitude and phase control over individual HBR diffractive elements thus offering the powerful ability to implement almost arbitrary phase-coherent spectral filtering functions. This is enabling to a broad range of optics-on-a-chip devices including compact multiplexers, tailored passband optical filters, optical switch fabrics, spectral comparators, and correlator-based optical look-up tables.
We report on wavelength-division-multiplexing (WDM) based on lithographically-fabricated slab-waveguide-contained planar holographic Bragg reflectors (HBRs). Partial HBR diffractive contour writing and contour displacement are successfully demonstrated to enable precise bandpass engineering of multiplexer transfer functions and make possible compact-footprint devices based on hologram overlay. Four and eight channel multiplexers with channel spacings of ~50 and ~100 GHz, improved sidelobe suppression and flat-top passbands are demonstrated. When a second-order apodization effect, comprising effective waveguide refractive index variation with written contour fraction, and the impact of hologram overlap on the hologram reflective amplitude are included in the simulation, excellent agreement between predicted and observed spectral passband profiles is obtained. With demonstrated simulation capability, the ability to fabricate general desired passband profiles becomes tractable.
We propose and demonstrate a powerful new approach to spectral bandpass engineering (apodization) of one-dimensional channel-waveguide Bragg reflectors. Bandpass engineering is accomplished via precise photolithographic control over the transverse width and longitudinal placement of individual grating lines which, respectively, provide unique line-by-line diffractive amplitude and phase control. Several channel waveguide gratings exhibiting complex filtering functions based on the present apodization method have been fabricated and modeled. They include an essentially polarization -insensitive 4-nm wide flat-top filter with steep roll-off and a multi-passband spectral decoder, useful, e.g., for optical spectral code-division multiplexing or spectral signature recognition. When a second-order apodization effect, comprising effective waveguide refractive index variation with grating-line transverse width, is included in the simulation, extraordinary agreement between predicted and observed spectral passband profiles is obtained.
A dual-channel, integrated, multiplexer, based on holographic Bragg reflector (HBR) devices and exhibiting flat-top, 4-nm-wide channels is demonstrated. Theory calibrated by the achieved performance indicates that HBR waveguide grating devices can be implemented to provide fully integrated and high performance multiplexer solutions for CWDM and FTTH applications. The enabling HBR devices can be regarded as mode-specific photonic crystals, i.e. photonic crystals whose spatial structure is tailored to interact with a specific signal mode or a very narrow range of such modes. Unlike standard bandgap-based photonic crystals, mode-specific photonic crystals may be effectively implemented with low-refractive-index-contrast and hence low-loss materials.
We demonstrate that holographic Bragg reflector grating structures, photolithographically scribed in planar waveguides, support a unique approach to apodization and overlay that uses fixed-depth etching and
partial contour writing to achieve continuous reflective amplitude control.
Planar holographic Bragg reflectors (HBR’s) are slab-waveguide-based computer-generated two-dimensional in-plane refractive-index holograms. The slab waveguide allows signals to propagate freely in
two dimensions, a geometry that enables HBR’s to offer powerful holographic function in the form of simultaneous spectral and spatial signal processing in a single element. Owing to their planar structure
HBR’s are fully consistent with photolithographic fabrication which provides complete amplitude and phase control over individual diffractive elements thus providing a flexible means to precisely engineer device spectral transfer functions. Here, we report on a photolithographically-fabricated silica-on-silicon slab-waveguide-based HBR that provides 17 GHz, essentially Fourier-transform limited, spectral resolution in a device footprint of only 0.3 cm<sup>2</sup> . The device maps the input beam to a spatially distinct output with diffraction-limited performance. Our results conclusively establish that the silica-on-silicon format and
submicron photolithography can provide fully coherent planar holographic structures of centimeter scale.