We demonstrate a single-ended colorless coherent receiver using symmetric 3x3 couplers for optical hybrids. We show that the receiver can achieve colorless reception of fifty-five 112-Gb/s polarization-division-multiplexed quadrature-phase-shift-keyed (PDM-QPSK) channels with less than 1-dB penalty in the back-to-back operation. The receiver also works well in a long-haul wavelength-division-multiplexed (WDM) transmission system over 2560-km TrueWave® REACH fiber.

Third-order optical nonlinear properties of four thienyleneethynylenes and two thienylenevinylenes containing from two to five thiophene units were measured. Third-order nonlinear susceptibilities x (~) ( -3w;o,o,o) at the fundamental wavelength of 1064 nm were measured by third harmonic generation (THG) for thin films of the oligomers in poly(methy1 methacrylate) (PMMA) using the Maker fringe technique. The data were reduced to the equivalent molecular second hyperpolarizabilities y ( -3w;w,o,w). Theoretical calculations of the static (zero-frequency) ( y ) values were performed at the semiempirical Austin model one/finite field (AM1/ FF) and intermediate neglect of differential overlap/single and double configuration interaction (INDO/SDCI) sum-over-states (SOS) levels and at the nonempirical valence effective Hamiltonian (VEH)-SOS level. The evolution of y as a function of the torsion angle between the thiophene rings was investigated via the AM1/ FF method. With the INDO-SOS technique, the frequency dependence of y( -3o;o,o,w) was also analyzed. For the ethynylenes, the experimental THG values of y lie between 63 x and 2300 x esu and increase with the number of heterocycle units of the oligomers. These values are affected by three-photon resonance. The resonant THG y-values of the ethynylenes lie within 10% of the corresponding values of the vinylenes. After a simple correction for three-photon resonance, the dependence of y-values on the number of n-electrons is fitted to power laws and compared with the theoretical calculations. The theoretical static y-values indicate a higher response in the vinylene derivatives than in the corresponding ethynylene derivatives. However, the experimental THG results provide the reverse trend: this can be explained by a stronger resonance enhancement of the THG y-values in the ethynylene compounds, as is demonstrated by the results of the frequency-dependent calculations.

Conjugated polymers such as polyacetylenes and polythiophenes have spurred considerable research activity for several years because of their potential applications in, for example, photonic devices''"] and highly conducting organic thin films.14] In order to optimize the electrical and optical properties of such materials, control of both the microscopic and the macroscopic solid-state structures is important, since they collectively define the electronic structure. A clear demonstration of the relationship between structural regularity and improved electrical and optical properties is found in Naarmann's defect-free polya~ety1ene.l'~ Recently, enhanced electrical conductivity in regio-regular poly(3-alkylthiophenes) has also been demon~trated["~I ( Fig. 1).In the present communication the effect of regio-regularity in polythiophenes on the electronic contribution to the third order nonlinear optical susceptibility, ~(~) (-3w;w,w,o), is addressed. The experimental strategy has been to measure the nonlinear susceptibility x (~) of regio-regular poly(3-dodecylthiophene) (RR-PDDT) thin films by third harmonic generation (THG) and compare it with similar THG measurements on regio-irregular (RIR-PDDT i.e.

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Enhanced values of the second molecular hyperpolarizability, γ, are reported for molecules that undergo significant multidirectional charge redistributions between the ground and excited states, e.g., charge is displaced from the periphery to the center, or vice versa, upon excitation, in during large changes in electric moments higher than dipole moments. The γ‐values are compared with those for polyene‐like molecules having a similar number of π‐electrons. It is demonstrated that the “periphery‐to‐center charge‐transfer” design concept is fruitful and should lead to new molecules with high γ‐values.

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Langmuir-Blodgett films consisting of two-dimensional interfaces between corbiene donor and quinone acceptor molecules show a significant nonlinear optical property (second-harmonic generation) that can be attributed to the intermolecular contact across the interface between the donor and acceptor molecules in the film.