Materials for Photonic Applications From Sol-Gel*

A review of sol-gel materials developed in our laboratory for photonic applications is presented. These materials include planar and strip waveguides for integrated optics (IO) passive devices, Er doped waveguides for IO amplifiers, films doped with semiconductor quantum dots for optical switching and fullerene doped materials for optical limiting.     

Rare earth doped fluoride waveguides fabricated using molecularbeam epitaxy

We have grown planar waveguides of rare earth doped single crystal fluoride films on insulating and semiconductor substrates using molecular beam epitaxy and have formed channel waveguides by ion milling. Structural and spectral analysis demonstrates that excellent crystallinity is being achieved and that the rare earth ion is incorporated into the film at sites and in charge states similar to bulk laser hosts. Lifetime measurements confirm that the local environment of the dopant ion is essentially that found for bulk materials. Single and higher order optical mode propagation has been demonstrated for the channel waveguides. By exciting individual channels with an 800 nm pump, we have generated strong upconversion fluorescence in Er and Nd doped guides. The ability to fabricate these waveguides On semiconductor substrates substantiates the potential for on chip integration of both IR downconversion lasers and IR pumped upconversion visible and UV lasers with a diode laser pump source. The use of transition metal dopants is possible and would enable tunable operation. Waveguide propagation loss in present devices must be reduced to realize a laser oscillator and we discuss how this is being addressed     

Second-harmonic generation in potassium niobate waveguides

We report on our progress in the formation of waveguides in potassium niobate (KNbO₃) using techniques such as ion implantation and ion sputtering. Different methods for the structuring of channel waveguides are presented, and their advantages and disadvantages are discussed in terms of their optical and nonlinear optical properties. The excellent power-handling capability of KNbO₃ waveguides is compared to other waveguide materials, and we highlight the influence of postimplantation annealing and repoling on the waveguide attenuation and the nonlinear optical coefficient. We also review recent results on second-harmonic generation in KNbO₃ waveguides focusing on blue light generation     

Beginning an Integrated Optics Laboratory for Teaching and Research

This paper describes an inexpensive ($2000) integrated optics laboratory used for teaching and research. It lists and specifies the basic materials needed to fabricate optical waveguides and make guide measurements. Several methods of fabricating optical waveguides are discussed, and the dipping method using glass substrates and a polystyrene solution is favored. Polystyrene waveguides are used to demonstrate some of the fundamental concepts of guided waves: propagation modes, coupling, and attenuation.     

Optical intensity induced shutter in photochromic-doped sol-gelgel-glass waveguides

The dynamic response, modeling and performance of a new optical intensity induced shutter have been studied. Hybrid optical waveguides made of optical fibers and photochromic doped gel-glass have been prepared by sol-gel process. A novel theoretical model explaining their behavior is shown, Experimental validation of the model is also shown. Design parameters of the devices, optical tunability and their application as extrinsic fiber optic intensity induced shutter (EFIS) are theoretically and experimentally demonstrated     

Silicon-on-silicon rib waveguides with a high-confiningion-implanted lower cladding

The realization of single-mode rib waveguides in standard epitaxial silicon layer on lightly doped silicon substrate, using ion implantation to form the lower cladding, is reported. The implanted buffer layer enhances' the vertical confinement and improves the propagation characteristics. Respect to similar standard all-silicon waveguides a propagation loss reduction of about 7 dB/cm, in the single-mode regime, has been measured. A numerical analysis has been performed to evaluate the theoretical attenuation and the transverse optical field profiles. As a result of the presence of the ion implanted buffer layer, an increase of the fundamental mode confinement factor from 0.3 to 0.85 has been calculated. This results in a great enhancement of the coupling efficiency with standard single-mode optical fibers. Moreover, the proposed technique is low cost, fully compatible with standard VLSI processes, and allows a great flexibility in the integration of guided-wave devices and electronic circuits. Finally, the very high thermal conductivity characterizing these waveguides makes them attractive host-structures for electrically and thermally controlled active optical devices     

Novel waveguide structures for enhanced fiber grating devices

An emerging class of fiber waveguide structures is being used to increase the functionality of fiber gratings, enabling new devices critical to the performance of next generation light-wave communications systems. These devices rely on advances in the fabrication of optical fiber waveguides, which go beyond the conventional doped silica design and fall into two general categories: 1) local modifications to the waveguide after fabrication and 2) fibers drawn with modified claddings that include nonsilica regions throughout their length. This paper provides a comprehensive review of emerging fiber waveguide structures that enhance the functionality of optical fiber grating devices. Two examples of technologies that fall into the first category are thin metal films deposited onto the cladding surface, which can be used for thermal tuning and infusion of nonsilica materials into the air regions, which change the waveguide structure and can provide enhanced tunability. The second category is typified by air-silica microstructured optical fibers, which contain air-voids that run along the length of the fiber. These fibers have unique cladding mode properties that can be exploited in fiber grating based devices     

Photosensitive and Rare-Earth Doped Ceramics for Optical Sensing: A Review

This work reviews two inorganic materials classes used for optical sensors: photosensitive glasses and rare-earth doped materials. The underlying physical mechanisms contributing to the photosensitive response of germanosilicate glasses are discussed, along with a summary of optical sensor strategies which have been developed based upon photo-imprinted Bragg gratings. Particular attention is paid to the fabrication and characterization of photosensitive germanosilicate thin-films. Insights into the ability to control the photosensitivity of these materials through manipulation of material compositions and structures are also included. The discussion of rare-earth doped optical sensor materials emphasizes the important contribution of the host material to the observed optical behavior of the rare-earth ions. The use of rare-earth doped materials in applications ranging from temperature sensing to fiber-optic gyroscopes is also described.     

Birefringence properties of magneto-optic rib waveguide as a function of refractive index

We report on the theoretical study of magneto optical rib waveguides with two kinds of silica type matrix doped by magnetic nanoparticles and made by sol–gel process (SiO₂/ZrO₂ film or SiO₂/TiO₂). The mode propagation and the light confinement are simulated using software based on a Film Mode Matching method. The modeling is based on geometrical adjustments of the rib waveguide. We propose from those results magneto optical waveguide geometries for optical integrated applications.     

Er3+-doped channel optical waveguide amplifiers for WDMsystems: a comparison of tellurite, alumina and Al/P silicate materials

Er³⁺-doped tellurite and Er³⁺-doped alumina optical waveguide amplifiers are analyzed both as single amplifiers and as elements of 16-channel wavelength-division multiplexing (WDM) systems; their performances are compared with that from Er³⁺-doped Al/P silica waveguide amplifiers. The amplifier model is based on propagation and population-rate equations and includes both uniform and pair-induced up-conversion mechanisms. It is solved numerically by combining finite elements and the Runge-Kutta algorithm. The analysis predicts that Er³⁺-doped tellurite waveguides exhibit improved gain characteristics compared with alumina and Al/P silica waveguides. Using Er³⁺-doped tellurite waveguide amplifiers, it is suggested that 16 WDM channels may be transmitted to a maximum distance of 4250 km. By using in-line notch gain equalizing filters, the maximum transmission distance can be increased to 5250 km     

Analysis of Hybrid Dielectric Plasmonic Waveguides

Future ultracompact photonic integrated circuits (PICs) will rely on high-index-contrast dielectric materials, which permit a strong confinement of the optical field in the diffraction limit as well as low propagation losses. This is the case of PICs implemented on a silicon-on-insulator (SOI) platform. To achieve confinement beyond the diffraction limit, plasmonic waveguides (based on metal–dielectric interfaces) have been recently proposed. This new kind of waveguide provides a strong enhancement of the field in the metal–dielectric interface, which is of paramount importance for nonlinear functionalities or sensing. Plasmonic waveguides can also be built on SOI wafers. Thus, it can be reasonably thought that high index contrast as well as plasmonic waveguides can coexist in future ultradense PICs. In this paper, a theoretical and numerical study on the performance of several dielectric and plasmonic waveguides is presented. Thanks to their plasmon-coupled supported modes, ultracompact devices as hybrid ring resonators can be devised and integrated with silicon photonic circuits.      

The SHG-response of different phases in proton exchanged lithiumniobate waveguides

Reflection second-harmonic generation from the polished waveguide end face is used to investigate the second-order nonlinear optical properties of as-exchanged and annealed proton-exchanged (PE) waveguides in different H_xLi_1-xNbO₃ phases. A detailed correlation is done between the nonlinear properties, the processing conditions, the refractive index changes, and the optical losses of the waveguides. It is found that for the direct PE samples, where the β₄, β₃, and β₁ phases are generated at the surface, the nonlinearity in the guide is strongly reduced by more than 85% of its bulk value, while for waveguides prepared in the β₂ phase, the nonlinear coefficient is about 55% of the bulk one. A consequence is that the step-like β_i-phase PE LiNbO₃ waveguides with large refractive index increase are advantageous for efficient SHG in Cherenkov configuration. The nonlinearity, strongly reduced after the initial proton exchange, is found to be restored and even increased after annealing. However, this apparent increase of the nonlinearity is accompanied by a strong degradation of the quality of the second-harmonic generation reflected beam in the region of initial waveguides due to beam scattering. The graded proton exchange technique and dilute melt proton exchange have been shown to produce high-quality waveguides with essentially undergraded nonlinear optical properties. It has been also shown that the nonlinear properties of annealed proton exchanged LiNbO₃ waveguides can be effectively recovered by the reverse proton exchange technique. The results obtained are important for the design, fabrication, and optimizing of guided-wave nonlinear optical devices in LiNbO₃     

基于NPS的集成光学型光隔离器的研究

本文介绍了集成光学型光隔离器的两种构型,着重讨论了基于非互易相移(NPS)的集成光学型光隔离器.对于构成集成光学型光隔离器的关键构件磁光波导的理论基础,数值计算及仿真进行了研究,给出了实现波导结构优化的手段.     

Tapered rib adiabatic following fiber couplers in etched GaAsmaterials for monolithic spot-size transformation

Design details and demonstration data are presented for an (Al,Ga)As monolithic tapered rib waveguide achieving modal spot-size transformation. The tapered rib adiabatic following fiber coupler structure (TRAFFIC) achieves two-dimensional (2-D) expansion of the output optical mode of single-transverse-mode semiconductor waveguide modulators and lasers using a one-dimensional (1-D) taper between noncritical initial and final taper widths which are compatible with optical lithographic techniques. Measurements are presented of total mode expansion losses between ~1.5-2.0 dB and semiconductor to single-mode-fiber waveguide coupling losses of ~0.5-1.0 dB for doped pin optical-modulator-type waveguides using the TRAFFIC waveguide. A semiconductor laser with a TRAFFIC tapered-rib mode-expansion section and measured coupling loss between the laser output and single-mode fiber of only 0.9 dB is described. Finally, a TRAFFIC Spot-size transformer for undoped waveguide modulators with total mode expansion losses of 1.84 dB and excellent modal behavior at 1.32-μm wavelength is presented. The TRAFFiC structure is particularly well suited for integration with both active and passive etched rib waveguide devices. Fabrication is relatively simple, requiring only patterning and etching of the tapered waveguide and uniform-width outer mesa waveguide without any epitaxial regrowth     

Waveguide lasers in the C-band fabricated by laser inscription with a compact femtosecond oscillator

Femtosecond laser writing of high-quality optical waveguides by means of a compact diode-pumped cavity-dumped Yb:glass laser oscillator is reported. Waveguides have been written on an erbium-ytterbium co-doped phosphate glass for active device production. A detailed analysis of the optical characteristics of waveguides written with different repetition rates, pulse energies, and translation speeds is presented, and an optimum set of writing parameters is established. Coupling losses as low as 0.1 dB/facet with standard telecom fibers and propagation losses lower than 0.4 dB/cm have been obtained. Characterization of the active properties of the waveguides is also presented, together with the demonstration of waveguide laser action in the whole C-band with peak output power of more than 30 mW.     

Dielectric investigation of photoinduced chromophore degradation in nonlinear optical side-chain polymer electrets

Organic materials with noncentrosymmetric chromophores are known to be susceptible to a number of photochemical processes, including reversible isomerization reactions as well as irreversible photooxidation or photoreduction reactions. Reversible isomerization is the basis for a variety of applications, such as photoinduced poling, optical data storage and optical grating formation. The irreversible processes that involve the destruction of the chromophores have been found useful for the fabrication of optical waveguides, but they also limit the life times of polymeric photonic devices. In this paper, it is demonstrated that dielectric measurements allow for an in-depth investigation of nonreversible chromophore degradation processes in a typical side-chain polymer. The time- and temperature-dependent dielectric function of the polymer at 1 kHz enables us to follow the chromophore-degradation kinetics and to monitor the bleaching depth as a function of time at room and elevated temperatures.     

Theory of design parameters for quasi-phase-matched waveguides andapplication to frequency doubling in polymer waveguides

A theory of dispersion in single-mode symmetric waveguides is presented for phase-matching second harmonic generation (SHG) of the fundamental modes, based on the approximate analytical waveguide theory of Botez. The theory is used to derive new equations for the maximum phase-matching distance allowed when there are random fluctuations in waveguide thicknesses, under critical and noncritical phase-matching conditions. The theory is also used to calculate the overlap integral as a function of the waveguide parameters V^ω and V^2ω. A new expression is derived for the efficiency of SHG in waveguides in terms of waveguide parameters that can be used to optimize SHG. Theoretical results are presented for typical LiNbO₃ and polymer waveguides. Quasi-phase-matched (QPM) waveguides are fabricated from nonlinear optical (NLO) polymers using the techniques of periodic poling and bleaching, and channel waveguides are printed by the bleaching of the NLO polymers. The NLO polymers are characterized for their refractive indexes, optical loss, NLO coefficients, and bleaching characteristics. Phase-matched SHG results are presented for the different fabrication methods over a distance of 0.5 cm, and an assessment is given of the relative strengths and weaknesses of the different fabrication approaches     

Silicon micromachined hollow optical waveguides for sensingapplications

Novel micromachined optical waveguides useful for sensing applications are proposed. The waveguide is designed as hollow-core antiresonant reflecting optical waveguide (ARROW) and can be easily fabricated using standard silicon micromachining techniques. The hollow structure permits to use the core to confine simultaneously the light and the substance to be probed, leading to an increase of the interaction efficiency. Numerical simulations, performed using finite element method technique, show that with a suitable design these waveguides can be used in sensing applications, where the substances under test can be gases or liquids     

Self-Aligned Single-Mode Polymer Waveguide Interconnections for Efficient Chip-to-Chip Optical Coupling

Single-mode (SM) ultrashort optical interconnections between the fibers and waveguides using self-forming polymeric waveguides with low optical losses at 1300 and 1550 nm were demonstrated. The localized refractive index in the SM regime is estimated by measuring the surface topography induced by monomer diffusion during the waveguide formation. A loss less than$-1$dB can be obtained from self-aligning SM-to-multimode (MM) fibers and SM-to-SM fibers interconnections, respectively. A self-formed waveguide-to-fiber interconnection is fabricated and measured with loss less than 0.2 dB at 1550 nm. The polymer waveguide relaxes the positioning requirements for single-mode chip-to-chip optical interconnections, showing great potential to improve the short-term yield and long-term reliability.     

Polymeric optical waveguide films for short-distance opticalinterconnects

We describe three different applications of polymeric waveguide films as short-distance optical interconnects. We fabricated the waveguide films, which were 6.5 cm long and mounted in MT-compatible (MTC) connectors by passive alignment, for MM fiber systems with a 50-μm diameter graded index (GI) core. The average insertion loss of these devices was approximately 0.6 dB at 0.85-μm wavelength. We also fabricated waveguide films with a 350 mirror and an MTC connector for use as 90° out-of-plane optical deflectors, and they exhibited an insertion loss of 1 dB. Two silica planar waveguides for single-mode (SM) fiber systems were also connected by a polymeric waveguide film. Low insertion losses were obtained in both MM and SM films designed to be employed as bending waveguides. This reveals their good potential for use as practical short-distance optical interconnects