We report advances in tunable thin film technology and demonstration of multi-cavity tunable filters. Thin film interference coatings are the most widely used optical technology for telecom filtering, but until recently no tunable versions have been known except for mechanically rotated filters. We describe a new approach to broadly tunable components based on the properties of semiconductor thin films with large thermo-optic coefficients. The technology is based on amorphous silicon deposited by plasma-enhanced chemical vapor deposition (PECVD), a process adapted for telecom applications from its origins in the flat-panel display and solar cell industries. Unlike MEMS devices, tunable thin films can be constructed in sophisticated multi-cavity, multi-layer optical designs.
The ability of photonic crystals to mold the flow of light in new ways can lead to a variety of novel and improved designs of optical nano-components and nano-devices in photonics. Two examples will be presented: a) Using linear materials, a polarization independent waveguide is designed in a 3D photonic crystal. It is demonstrated that this system provides lossless guiding of light at length-scales approaching the wavelength of the light itself, offering a promising platform for the design of integrated high performance polarization-insensitive waveguide networks. b) Using nonlinear materials, a cylindrical photonic crystal fiber is designed that can exhibit all-optical switching without the need for an axial periodicity. It is shown that this property stems from the unique structure of the cylindrical photonic crystal guided-mode dispersion relation, and can lead to significant improvements in manufacturing ease, operating power usage, and device size requirements, making such a system ideal for integrated all-optical signal processing.
The chief properties and possible applications of periodic waveguides and their leaky modes are presented in this paper. After summarizing the basic physics of the guided-mode resonance, computed leaky-mode field patterns are provided to illustrate their structure and the high local focal field enhancement obtainable. An example fabricated bandstop filter is found to exhibit 90% efficiency, 1 nm linewidth, and low sidebands. Computed spectra for a single-layer bandpass filter operating at 1.55 μm wavelength yield low sidebands, extending 100 nm, and an angular aperture of ~1.7°. Resonant vertical-cavity surface-emitting lasers (VCSEL) are presented in which multilayer Bragg-stack mirrors are replaced with leaky-mode resonance layers. The use of guided-mode resonance mirrors provides optical power flow across and laterally along the laser active region. The round-trip gain is thereby increased resulting in high laser efficiency and relaxed mirror reflectivity constraints. As the GMR mirror achieves high reflectivity at resonance, the laser wavelength is locked at the resonance wavelength principally defined by the period. Example resonant VCSEL embodiments are shown along with their computed characteristics. Resonant biosensors are addressed last. The high parametric sensitivity of the guided-mode resonance effect, a potential limitation in filter applications, can be exploited for sensors as illustrated by several examples.
Dielectric microspheres, with their morphology dependent resonances, are used to resonantly couple light from half optical fiber couplers. The dropped channels are observed in the elastic scattering and the transmission spectra. The excitation of the microsphere with the nearly Gaussian beam in the half optical fiber coupler provides spatially and spectrally selective, and enhanced light coupling. The filter drops approximately 10% (0.5 dB) of the power at the resonance wavelength. A tunable single mode distributed feedback diode laser is used as the infrared excitation source. The generalized Lorenz-Mie
theory, describing the illumination of the microsphere with a Gaussian beam, is used to interpret the experimental results.
Two-photon photopolymerization, as an important laser nanofabrication scheme, is applicable to production of micromechanical and optoelectronic devices of various dimensions. Here we report our latest progress in using this technology for fabrication of functional photonic crystals, including the achievement of high reproducibility writing by using finely quantified pixels, high-fidelity shaping through shrinkage pre-compensation, and attainment of complex three-dimensional diamond-lattice structures. These works would be essential for advanced functions of photonic crystal-based polymer optoelectronic devices.
Micro-stereolithography (μSL) is capable of fabrication of highly complex three-dimensional (3D) microstructures by selectively photo-induced polymerization from the monomer resin. However, during the evaporative drying of structures from liquid resin, the 3D microstructures often collapse due to the capillary force. In this work, a theoretical model is developed to analyze the deflection and adhesion between thin polymer beams under capillary force. The
detachment length of the test structures and adhesion energy of a typical μSL polymer (HDDA) are obtained experimentally which are important for MEMS structure design. Finally, we successfully developed a sublimation process to release the 3D microstructures without the adhesion.
We report on a polymer-based planar lightwave circuit platform that enables high levels of integration. The materials used represent the state of the art in optical polymers, and include properties such as ultra-low loss (0.1 dB/cm in single-mode waveguides at 1550 nm), widely tunable refractive index contrast (0-35%), and large thermo-optic coefficient (-3.2×10-4/°C). The large index contrast values enable compact photonic microcircuits. The circuits are produced photolithographically, and can have a variety of inorganic materials integrated in them (e.g., by insertion in slots or by flip-chip mounting), resulting in a platform that can support functions that span the range of the building blocks needed in optical circuitry, while using the highest-performance material for each function. In this manuscript, we focus on the polymeric microcircuits, which provide interconnects, static routing elements such as couplers, taps, and multi/demultiplexers, as well as thermo-optically dynamic elements such as tunable couplers, switches, variable optical attenuators, and tunable notch filters. We demonstrate complex-functionality polymeric photonic microcircuits based on this technology, including fully reconfigurable optical add/drop multiplexing subsystems on a chip that perform channel switching, power monitoring, load balancing, and wavelength shuffling.
Recent progress toward wavelength-scale photonic crystal lasers is summarized. Lasing characteristics of two possible configurations of the unit-cell photonic crystal laser that has a central node through which current could be supplied. The very high quality factor in excess of 100,000 is theoretically expected from a square lattice unit-cell photonic crystal resonator. Applications of photonic crystals to other forms of active devices are also briefly discussed.
We discuss various practical points in the design, fabrication and characterization of form birefringent retardation plates in GaAs. The role of the substrate in the device performance is presented, together with the importance of using anti-reflection coatings. Also, we discuss the specific case of metallic reflection gratings in GaAs substrates and the resulting enhanced retardation. Finally we present the results of thermal tuning of a nominally half-wave subwavelength retardation plate.
Unfettered and near-instantaneous access to massive amounts of information has become so commonplace that we are all starting to take it for granted. But the decreasing cost of storing data and the increasing storage capacities of ever smaller devices have been key enablers of this revolution. Current storage needs are being met because improvements in conventional technologies---such as magnetic hard disk drives, optical disks, and semiconductor memories---have been able to keep pace with the demand for greater and
However, there is strong evidence that these surface-storage technologies are approaching fundamental limits that may be difficult to overcome, as ever-smaller bits become less thermally stable and harder to access. Exactly when this limit will be reached remains an open question: some experts predict these barriers will be encountered in a few years, while others believe that conventional technologies can
continue to improve for at least five or perhaps even ten more years. In either case, however, one or more successors to current data storage technologies will be
needed in the near future. An intriguing approach for next generation data-storage is to use light to store information throughout the three-dimensional volume of a material. By distributing data within the volume of the recording medium, it should be possible to achieve far greater storage densities than current technologies can offer.
In this talk, I review progress to date in these various volumetric storage technologies, describe the open engineering challenges, and speculate wildly on their prospects for the near future.
A very thin image capturing system called TOMBO (Thin Observation Module by Bound Optics)was developed with compound-eye imaging and post digital processing. With the prototype system, some excellent results have been obtained. In this paper, we focus on a multispectral imaging system as an application of the TOMBO.
In the system, it is possible to observe specific points on the target by multiple photodetectors with a special arrangement of the system. A filter array is inserted in front of the image sensor to observe the spectral distribution of the target. A captured compound image is reconstructed by an extended version of the pixel rearrange method. The pixels of the captured image are geometrically rearranged onto a multi-channel virtual image plane. Experimental results of the image reconstruction show effectiveness of the proposed algorithm.
Nanophotonic ICs promise to play a major role in the future of opto-electronic signal processing and telecommunications. But for these devices, which consist of large numbers of wavelength-scale photonic components, to be successful, reliable and cost-effective mass-fabrication technology is needed. Photonic components, and among them photonic crystals, require a high degree of accuracy, which translates to low fabrication tolerances. Today, similar demands are made for high-end CMOS components, made of Silicon, for which a large manufacturing base is installed.
We demonstrate the fabrication of nanophotonic components, like photonic crystal waveguides and photonic wires, using state-of-the-art CMOS processing tools. The foremost of these is deep UV lithography at 248nm and 193nm, combined with dry-etch processes. To maintain compatibility with standard CMOS processes, we use Silicon-on-Insulator (SOI) as our material system. SOI is transparent at telecom wavelengths and provides a good substrate for high-index contrast optical waveguides. Moreover, recent studies have shown that nanophotonic components in SOI are less sensitive to surface roughness than similar components made in III-V semiconductor.
Although deep UV lithography cannot attain the resolution of e-beam lithography, this can be compensated with thorough process characterization, and the technique offers more speed because of its parallel nature. We will illustrate this with experimental results, and will also discuss some of the issues that have arisen in the course of this project.
In this paper we report results obtained in the design and fabrication of diffractive optical elements (DOEs) with minimum feature size down to tens of nanometers by the use of e-beam and x-ray lithography. The DOEs are patterned using e-beam lithography and replicated by x-ray lithography. Since in our days there is an increased interest for extreme ultraviolet and x-ray microscopy our work has been focused toward the fabrication of DOEs mainly for these applications. Different types of zone plates (ZPs) were fabricated for x-ray beam focusing: high resolution ZPs for high resolution beam focusing, multilevel phase ZPs to increase the diffraction efficiency in the desired order and high aspect ratio ZPs for hard x-rays. Recently we have extended the concept of the ZPs to a more general category of DOEs which beside simple focusing can perform new optical functions in the range of x-rays. In particular, the intensity of the beam after the DOE can be distributed with almost complete freedom. We have designed and fabricated DOEs that focus the beam in an array of spots disposed either in plane or along the optical axis. This type of DOEs has been tested successfully in x-ray differential interference contrast microscopy. The possibility to introduce a specified phase shift between the generated spots is demonstrated in this paper by preliminary results obtained from computer simulations and experiments performed in visible light.
The development of replication processes of arrays of diffractive optical elements (DOEs) on planar substrates serving as optical light-guides (OLGs) by soft nano-lithography are described.The master DOE arrays were recorded by holographic interferometry in photoresist on the OLG substrate. These master OLGs carrying nanometer surface-relief
grating structures were then transferred into a thermally curable Si-elastomer serving as the molding tool for the production of the replica. The replica were formed by casting UV-curable photopolymers onto a planar substrate inserted into the mold. The replication process variables were then designed and evaluated by the statistical Taguchi technique. The replica OLGs were compared to the master OLGs both in terms of surface flatness and grating fidelity. The results show that optimal processes yielded both good OLG flatness (<λ/2)and high grating fidelity (~1).
The high-density grating is routinely used in spectral expansion of optical information processing system. But usually it is hard to examine such gratings directly. The well-used Moire interferential pattern method can only obtain the overall coarse result. While in practice,the local quality of a grating is highly interesting. In this paper we use a nano-probe fiber to scan the near field of a grating. With the Talbot effect of a grating,we can take a picture of 5 x 5 square-micron area for analyzing the local quality. In our experimental setup,the Talbot image of a grating is coupled into a fiber detector with the fiber tip of 50 nanometers. With the Talbot effect, the surface profiles of the gratings are detected. Three gratings are examined in our experiment with the line widths of 600 lines per millimeter for all gratings. From the experimental results we can see that a well made grating can yield a sharp image at the Talbot distance. Experimental results demonstrate that this nanotechnology-based Talbot detection method can be widely applied in grating examination.
In this paper,analysis of a microlens arrays between fiber arrays an vertical cavity surface emitting laser arrays is reported with a variety of optical devices in transverse and vertical core pitch matching from 25 μm to 127 μm. Using 850nm wavelength 8 x 8 VCSEL arrays, we scale down the physical size in area from 4mm2 to 43 μm2 ,and minimize the longitudinal coupling length to 1.335mm. The optical power coupling efficiency can be increased above 90% with a flexible optical alignment range 30 μm of our proposed microlens arrays.It is necessary for the requirements of micro-optics package and minimized volume between fiber array and array chips.
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 cm2 . 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.
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.