S. Etemad

Spectrally efficient optical CDMA using coherent phase-frequency coding

We demonstrate feasibility of a spectrally efficient wavelength-division-multiplexing-compatible optical code-division multiple-access system using 16 phase-locked laser lines within an 80-GHz tunable window as frequency bins and an ultrahigh frequency resolution spectral phase encoder-decoder. Coding and decoding using binary [0, /spl pi/] phase chips were demonstrated for four users at 2.5 Gb/s, and a single coded signal was separated from four copropagating signals, with bit-error rate <10/sup -9/.

Fully programmable ring-resonator-based integrated photonic circuit for phase coherent applications

A novel ring-resonator-based integrated photonic chip with ultrafine frequency resolution, providing programmable, stable, and accurate optical-phase control is demonstrated. The ability to manipulate the optical phase of the individual frequency components of a signal is a powerful tool for optical communications, signal processing, and RF photonics applications. As a demonstration of the power of these components, we report their use as programmable spectral-phase encoders (SPEs) and decoders for wavelength-division-multiplexing (WDM)-compatible optical code-division multiple access (OCDMA). Most important for the application here, the high resolution of these ring-resonator circuits makes possible the independent control of the optical phase of the individual tightly spaced frequency lines of a mode-locked laser (MLL). This unique approach allows us to limit the coded signals spectral bandwidth, thereby allowing for high spectral efficiency (compared to other OCDMA systems) and compatibility with existing WDM systems with a rapidly reconfigurable set of codes. A four-user OCDMA system using polarization multiplexing is shown to operate at data rates of 2.5 Gb/s within a 40-GHz transparent optical window with a bit error rate (BER) better than 10/sup -9/ and a spectral efficiency of 25%.

X(3) of trans-(CH)x: Experimental observation of 2Ag excited state

Abstract Quantitative measurements of the third order optical susceptibility ( X (3) ) in the semiconducting gap of trans -(CH) x have been carried out through the technique of third harmonic generation. There is a sharp two-photon resonance in X (3) that has been attributed to the presence of the 2A g excited state at 1.9eV. In contrast to the case of finite polyenes, the position of the 2A g state in trans -(CH) x is very close to that of its optical gap. This result confirms the conjectures that Coulomb correlation in polyacetylene is in the intermediate regime. The magnitude of X (3) parallel to the polymer chain direction is in excess of 10 −9 esu for wavelengths larger than 1.3 μm. This is one of the largest values of electronic X (3) inside the gap of a semiconductor.

Fast bolometric response by high Tc detectors measured with subnanosecond synchrotron radiation

We have measured a fast response by thin‐film YBa2Cu3O7−δ detectors to pulsed, broadband, infrared radiation. Synchrotron light from an electron storage ring was used as the infrared source, providing subnanosecond pulses from far infrared through visible. Pulse responsivities as high as 106 V/J and as fast as 4 ns have been observed. For film thicknesses in the range 400–3200 A, the detector response follows the film absorptivity while the speed varies inversely with thickness, suggesting a bolometric mechanism. Calculations based on such a model are in accord with the data. We find no evidence for any nonbolometric components in the response.

Observation of nonlinear optical transmission and switching phenomena in polydiacetylene‐based directional couplers

Nonlinear optical transmission and switching phenomena have been observed in directional coupler devices fabricated from soluble polydiacetylenes. Effects due to both slow thermal nonlinearities and ultrafast (picosecond) electronic nonlinearities were identified. At the operating wavelength of 1.06 μm used here, the ultrafast electronic nonlinear phenomena originated from intensity‐dependent changes in the imaginary part of the refractive index due to two‐photon absorption effects.

Waveguiding in spun films of soluble polydiacetylenes

In prism coupling experiments, the guiding of 1.06 μm light over 3 cm pathlengths was demonstrated in thin spun films of soluble polydiacetylenes. From measurements of the intensity of scattered light, we estimate the transmission losses in the films were as low as 1 dB/cm. Although spun films appeared isotropic when examined under polarized light, the coupling angle observed for the TM waveguide mode was ∼6° higher than that of the TE mode, implying that the films were istropic in the plane of the film, but the refractive index normal to the plane of the film was lower.

Fluorescence and topological gap of conjugated phenylene polymers

Abstract We propose that strong fluorescence in conjugated polymers requires a dipole-allowed state to be the lowest singlet. Huckel theory for para-conjugated phenyl rings yields an extended, topologically one-dimensional π-system with increased alternation, states localized on each ring, and charge-transfer excitations between them. Exact Pariser—Parr—Pople results and molecular spectra for oligomers support a topological contribution and a lowest dipole-allowed singlet in phenylene polymers.

Measurement of the complex nonlinear refractive index of single crystal p‐toluene sulfonate at 1064 nm

Z‐scan at 1064 nm was used with single, 35 ps pulses to measure the nonlinear refraction and absorption in single crystal PTS (p‐toluene sulfonate). Detailed analysis of the Z‐scan data based on Δn=n2I+n3I2 and Δα=α2I+α3I2 yielded n2=5(±1)×10−12 cm2/W, α2=100(±20) cm/GW, n3=−5(±1)×10−21 cm4/W2 and α3=−5(±1) cm3/GW.2 The resulting two photon figure of merit T for PTS is marginal for high throughput, all‐optical waveguide switching at 1064 nm.

Frequency dependence of the large, electronic χ (3) in polyacetylene

Conjugated polymers possess a 1-D band structure; a property which when coupled to their large intrachain bandwidth allows extensive 1-D delocalization of the electronic wave function. The unique delocalization is predicted to give rise to extremely fast (<1 ps) and large electronic third-order optical susceptibilities [χ(3)]. χ(3) the measure of the change in the index of refraction with the pulse intensity and is the most important material parameter for use in an all-optical device. As the prototype conjugated polymer, polyacetylene provides a unique opportunity for studying the nonlinear optical response of this potentially useful class of optoelectronic materials.

Fabrication of waveguide structures from soluble polydiacetylenes

Abstract High quality films of poly(4BCMU) were spin-cast from cyclopentanone or chlorobenzene; similar poly(3BCMU) films were deposited from warm N,N-dimethylformamide solutions. Two methods are presented for the fabrication of waveguide structures in these films. In the first process, micron-sized features were patterned in thick (>1) polydiacetylene films using deep-UV lithography. In the second method, composite waveguides were fabricated from a patterned glass substrate with a polydiacetylene overlayer. These composite structures were mechanically robust, had minimal losses, and exhibited single-moded behavior at a wavelength of 1.3 μm.