Coupled-resonator optical waveguides: Q-factor and disorder influence

Coupled resonator optical waveguides (CROW) can significantly reduce light propagation pulse velocity due to pronounced dispersion properties. A number of interesting applications have been proposed to benefit from such slow-light propagation. Unfortunately, the inevitable presence of disorder, imperfections, and a finite Q value may heavily affect the otherwise attractive properties of CROWs. We show how finite a Q factor limits the maximum attainable group delay time; the group index is limited by Q, but equally important the feasible device length is itself also limited by damping resulting from a finite Q. Adding the additional effects of disorder to this picture, limitations become even more severe due to destructive interference phenomena, eventually in the form of Anderson localization. Simple analytical considerations demonstrate that the maximum attainable delay time in CROWs is limited by the intrinsic photon lifetime of a single resonator.     

掺镁铌酸锂质子交换光波导光学性能的研究

本文用质子交换反应在5mol％MgO:LiNbO_3晶体中引起了e光折射率的增加,晶体中MgO的存在并不影响LiNbO_3质子交换光波导阶跃型折射率分布的特点。用常规的质子交换法制备的MgO:LiNb·O_3波导存在着损耗大和有效折射率不稳定的缺点,经过退火处理后,这些缺点可明显地得到改善。     

Structural and optical properties of cobalt doped ZnO nanocrystals

Zn_1−xCo_xO nanocrystals with nominal Co doping concentrations of x = 0–0.1 were synthesized through a simple solution route followed by a calcining process. The doping effects on the structural, morphological and optical properties were investigated by means of X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Raman, absorption and luminescence spectroscopy. The results indicated that a small amount of Co ions were incorporated into ZnO lattice structure, whereas the secondary phase of Co₃O₄ was segregated and precipitated at high Co doping concentrations, the solid solubility of Co ions in ZnO nanocrystals could be lower than 0.05. The spectra related to transitions within the tetrahedral Co²⁺ ions in the ZnO host crystal were observed in absorption and luminescence spectra.     

Structural and optical properties of a NaCl single crystal doped with CuO nanocrystals

A cupric oxide (CuO) nanocrystal-doped NaCl single crystal and a pure NaCl single crystal are grown by using the Czochralski (Cz) method. A number of techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray (EDX) analysis, Fourier transform infrared (FT-IR) spectroscopy, Raman spectroscopy, optical absorption in the UV-visible range, and photoluminescence (PL) spectroscopy are used to characterize the obtained NaCl and NaCl:CuO crystals. It is observed that the average radius of CuO crystallites in NaCl:CuO crystal is about 29.87 nm, as derived from the XRD data analysis. Moreover, FT-IR and Raman spectroscopy results confirm the existence of the monoclinic CuO phase in NaCl crystal. UV-visible absorption measurements indicate that the band gap of the NaCl:CuO crystal is 434 nm (2.85 eV), and it shows a significant amount of blue-shift (△Eg = 1 eV ) in the band gap energy of CuO, which is due to the quantum confinement effect exerted by the CuO nanocrystals. The PL spectrum of the NaCl:CuO shows a broad emission band centred at around 438 nm, which is consistent with the absorption measurement.     

Coupled-resonator optical waveguide: a proposal and analysis

We propose a new type of optical waveguide that consists of a sequence of coupled high-Q resonators. Unlike other types of optical waveguide, waveguiding in the coupled-resonator optical waveguide (CROW) is achieved through weak coupling between otherwise localized high-Q optical cavities. Employing a formalism similar to the tight-binding method in solid-state physics, we obtain the relations for the dispersion and the group velocity of the photonic band of the CROW's and find that they are solely characterized by coupling factor k(1) . We also demonstrate the possibility of highly efficient nonlinear optical frequency conversion and perfect transmission through bends in CROW's.     

Morphology and optical properties of Mg and Sr doped CaF2 nanocrystals

Magnesium (Mg) and Strontium (Sr) doped Calcium fluoride nanocrystals were synthesized by co-precipitation method. The cubic structure of the samples was confirmed by Powder X-ray diffraction. The average crystallite size of Mg doped samples was found to be ~ 25 nm whereas in Sr doped one it was ~ 35 nm. The morphological features revealed that the nanocrystals were agglomerated, crispy and porous. The as-prepared samples showed the presence of hydroxyl groups. The optical absorption spectrum of as-prepared Mg doped samples showed a strong absorption band peaked at ~ 233 nm whereas the Sr doped one showed a prominent absorption peak at 248 nm. A strong PL emission was observed at ~ 300 nm in Mg doped samples. However, the Sr doped samples showed two prominent emissions at ~ 345 and 615 nm.     

Luminescent properties of doped dielectric nanocrystals

Taking into account electron-phonon interaction effects, we have studied the radiative characteristics of doped nanoellipsoids in a dielectric medium with an arbitrary fraction of nanoparticles in a nanocomposite. The obtained analytical expressions for the radiative characteristics allow it to be concluded to what extent they are susceptible to changes in the phonon subsystem and electron-phonon interaction caused by the spatial confinement effect.     

Theory of optical waveguides

This report summarizes the activities at the 3rd International Workshop on Optical Waveguide Theory, held at the Scuola Superiore Guglielmo Reiss Romoli, L'Aquila, Italy, on 9–11 September, 1978.     

Mode-locking of neodymium lasers by glasses doped with PbS nanocrystals

Nonlinear optical absorption properties of silicate glasses doped by the PbS nanocrystals were investigated by means of the picosecond pump-probe absorption spectroscopy and absorption bleaching technique. The glass samples were used as saturable absorbers for passive mode-locking of Nd:YAG and Nd:glass lasers. Application of the PbS doped glass together with the active mode-locking and laser cavity quality control enabled the stable of generation of 28 ps and 5 ps duration pulses in Nd:YAG and Nd:glass lasers respectively. PACS 42.60.Fc; 42.70.Hj; 78.47.+p     

Rod type photonic crystal optical line defect waveguides with optical modulations

This paper reports the design and principles of two dimensional rod-type photonic crystal (PhC) line defect waveguides for bandgap based optical waveguiding, static modulation and high speed dynamic optical switchings. Experiments were carried out for both high aspect ratio and slab type configurations. The differences in waveguiding mechanisms for the two configurations resulting from the presence of bottom cladding systems, without out-of-plane symmetries are compared for their advantages and disadvantages. In particular, the designs of non-top-clad optical waveguides of layout sizes within micrometers and operational frequencies centered at the optical communication wavelength of 1550 nm, were investigated for the feasibility of large scale integration by batch fabrication process techniques – such as sub-micrometer optical lithography etc. Based on such techniques, specifically designed dispersions of line defect PhC waveguides within a missing row of PhC rods were accompanied by optical testing structures of suitable coupling modes. Optical measurements of waveguiding coefficients were therefore enabled for the different configurations, together with further static and dynamic modulations. PACS 42.70.Qs     

Active semiconductor optical switches with blue-shifted channel waveguides

This paper presents a theoretical design study of active optical switches constructed from III-V semiconductor multiple-quantum-well waveguides. New designs are given for 4 × 4 matrix switches, 1 × 4 tree switches, and 4-delay optical transversal filters. The designs feature ～ 0 dB insertion loss and ～ 30 dB crosstalk isolation. Low injection currents are predicted because pumping is eliminated on ～ 70 percent of the channel waveguide area by an induced-disorder treatment that blue-shifts the material's absorption edge. The active networks also feature etched facet reflectors that facilitate the dense packing of guides.     

Active semiconductor optical switches with blue-shifted channel waveguides

This paper presents a theoretical design study of active optical switches constructed from III-V semiconductor multiple-quantum-well waveguides. New designs are given for 4 × 4 matrix switches, 1 × 4 tree switches, and 4-delay optical transversal filters. The designs feature ∼ 0 dB insertion loss and ∼ 30 dB crosstalk isolation. Low injection currents are predicted because pumping is eliminated on ∼ 70 percent of the channel waveguide area by an induced-disorder treatment that blue-shifts the material's absorption edge. The active networks also feature etched facet reflectors that facilitate the dense packing of guides.     

Splicing of optical waveguides with lossless graded-index tapers

Marcatili has proposed a class of graded-index (GRIN) tapers which do not radiate along their length. We investigated the splicing of such tapers to dissimilar waveguides, and derive a simple formula for the coupling efficiency, which clearly reveals the loss due to mismatch of spot-size and the curvature of the wavefronts.     

Lead sulfide colloidal nanocrystals with strong optical nonlinearity

Colloidal PbS nanocrystals have been synthesized by a developed procedure. UV–Vis absorption and Z-scan technique was also applied to study the nonlinear optical properties of prepared lead sulfide nanocrystals at 532 nm wavelength. The nonlinear refractive (n₂) and absorption (β) were determined which are confirming the strong nonlinearity at 532 nm of colloidal PbS nanocrystals. The obtained results have not been reported before. In this study, only a weak thermal optical nonlinearity was observed and the dominating nonlinear response is resulted by the electronic origin. The nonlinear optical properties of prepared sample supported wide applications in nanophotonics.     

Measurements on high-bandwidth optical waveguides

Frequency response measurements on six high-bandwidth optical waveguides made by the doped deposited silica process are reported. One of these had an extrapolated bandwidth in excess of 3 GHz-km. From plane wave measurements the actual or optimum waveguide profile may be determined given the other. Data taken on a germanium borosilicate waveguide is in agreement with interference measurements.     

Dichroic optical waveguides and lasers

The transverse structure of modes in optical waveguides and resonators filled with dichroic media is investigated. Two families of Hermite-Gaussian modes, corresponding to two orthogonal polarizations of radiation, are found for transversely one-dimensional field distributions and for approximating the characteristics of the medium by quadratic functions of the transverse coordinate. Angular selectivity is displayed by extraordinary waves and is manifested in a decrease in the increment of the mode upon an increase in the mode number. With account of the absence of deformation in the lowest mode due to selection, this implies that resonators containing dichroic media (including photon crystals) are promising for reducing the angular divergence of laser beams.     

Losses in diffused optical waveguides

It has been shown that the attenuation characteristics of TE modes in diffused waveguides are different from those of a conventional slab. A simple semiempirical analysis is performed on the base of a ray-optics picture assuming a parabolic index distribution. Analytical expressions for surface scattering and volume loss are derived. The analysis precisely describes the modal loss behaviour of Ag⁺ ion-exchanged waveguides in glass (new results) and Nb diffused waveguides in LiTaO₃ (data reported by other authors).     

Thermal stresses in optical waveguides

Birefringence of optical waveguides is closely related to the anisotropy of thermal stresses in the core. Closed-form solutions for estimating the thermal stresses in the cladding layers and in the core of an embedded channel waveguide are presented. The solutions are verified by finite-element simulation. It is found that the thermal stress in the core in the direction perpendicular to the wafer cannot be ignored, and one can tune the core stress anisotropy in the plane normal to the light-propagation direction by varying the thermal-expansion mismatch between the upper cladding and the substrate.     

Leaky modes in optical waveguides

Solutions of the scalar wave equation are presented giving, explicitly, the excitation and subsequent attenuation of leaky modes in optical waveguides.