The current status of the praseodymium doped fluoride fiber amplifier (PDFFA), the preferred fiber amplifier at 1.3 micrometers , is reviewed. Small-signal gains of 30dB are achievable, together with output powers of 200mW, and a noise figure of 4-7dB. 1.3- micrometers transmission offers the advantage of operating near the zero dispersion wavelength of standard telecommunications fiber, allowing high speed links with simple transmitters.
The threshold of polarization instability for solitons in birefringent fibers has been calculated analytically. The problem was studied when the polarization state of the soliton is oriented along one principal axis and the polarization state of the perturbation is oriented along the other. The instability threshold corresponds to the soliton field intensity which causes the exponential growing of the perturbation. We have shown for the first time that this problem can be solved exactly under the assumption that the influence of group velocity difference between the soliton and the perturbation on the instability threshold is negligible. Analytical expression has been obtained for the threshold of the soliton instability and its duration. The polarization instability increment near threshold has also been calculated. It was shown that obtained analytical expression are valid under a wide range of birefringent values. On the basis of these expressions, the general analysis of soliton propagation in birefringent fibers has been made and estimations of the instability threshold have been made. The results of this work can be used for the study of ultrafast all optical switching as well as for the analyses of soliton propagation in optical fibers.
Wavelength routing crossconnects are considered the core of WDM optical networks. They consist of optical switches independently rearrangeable for each wavelength channel and for any input- output configuration so that any path can be chosen almost arbitrarily by the network users. In general, the implementation of the wavelength routing function requires complex switch arrays. Very simple wavelength-selective crossconnects can be realized by using acousto-optic switches (AOS), because of their unique ability of processing several optical signals simultaneously and their low driving power consumption, less than 10 mW/channel. AOS's can be considered a particular evolution of acousto-optical tunable filters, whose integrated optic version on lithium niobate has been developed in several research institutions around the world in the past decade. This paper reviews the last accomplishments of AOS's, whose specifications are directly tied with optical network requirements, the foremost challenge being a strong suppression of crosstalk. Dilated AOS's can reduce interport crosstalk to below -30 dB and apodization of acousto-optic interaction can reduce interchannel crosstalk to below -15 dB during multiwavelength operation.
Since the 1990 discovery that porous silicon emits bright photoluminescence in the red part of the spectrum, light-emitting devices (LEDs) made of light-emitting porous silicon (LEPSi) have been demonstrated, which could be used for optical displays, sensors or optical interconnects. In this paper, we discuss our work on the optical properties of LEPSi and progress towards commercial devices. LEPSi photoluminesces not only in the red- orange, but also throughout the entire visible spectrum, from the blue to the deep red, and in the infrared, well past 1.5 micrometers . The intense blue and infrared emissions are possible only after treatments such as high temperature oxidation or low temperature vacuum annealing. These new bands have quite different properties form the usual red-orange band and their possible origins are discussed. Different LED structures are then presented and compared and the prospects for commercial devices are examined.
This paper demonstrates that the characteristics of light- emitting diodes (LEDs) based upon MEH-PPV, (more fully known as poly(2-methoxy,5-(2'-ethyl-heroxy)-1,4-phenylene-vinylene)), are determined by tunneling of both the holes and the electrons through interface barriers caused by the band offset between the polymer and the electrodes. It is shown that manipulating these offsets can control the useful operating-voltage of the device as well as its efficiency. A Model is developed which clearly explains the device characteristics of a wide range of diodes based upon MEH-PPV. The turn-on voltage for an ideal device is shown to be equal to the band-gap, i.e. 2.1eV for MEH-PPV, and is slightly lower at 1.8eV for an indium-tin oxide/MEH-PPV/Ca device. If there is a significant difference in the barrier height, the smaller of the two barriers controls the I-V characteristics while the larger barrier determines the device efficiency. In ITO/MEH-PPV/Ca devices, the barrier to hole injection is 0.2eV and the barrier to electron injection is only 0.1eV. We show that reducing the hold barrier to 0.1eV using a polyaniline anode halves the operating voltage and increases device efficiency.
Optical spectroscopy of single molecules as guests in a solid matrix represents a new and powerful method to study truly local effects of single guest molecules and their interaction with the host. The work of W.E. Moerner et al.  and M. Orrit et al.  showed that single molecule spectroscopy (SMS) is possible by using a low concen— tration of guest molecules and probing a small volume of the sample with a highly sensitive fluorescence excitation spectroscopy. At low temperatures the inhomogenous broadening normally exceeds strongly the homogenously broadened zero—phonon lines of the single guest molecules. This gives rise to a spectral separation of the guest molecules on the frequency scale. In other words the slightly different environment of each guest molecule shifts the transition frequency in a specific way so that by tuning a narrow band laser over the corresponding frequency range isolated fluorescence excitation spectra of single molecules can be observed. At higher concentrations and larger probed volumes single molecules can still be detected, however, in this case they are found only at the wings of the inhomogenously broadened line.
The studies of the photorefractive properties of the two polymer composites based on poly(N-vinylcarbazole) (PVK) are reported. The polymers were obtained by doping PVK with 2,4,7 trinitro-9- fluorenone as a sensitizer agent and two different azo dyes as nonlinear chromophores. Best results were obtained for the polymer doped with 2,5-dimethyl-4-p-nitrophenylazoanisole, which showed a maximum diffraction efficiency of 34% at 40 V/micrometers external electric field strength in 105 micrometers thick samples. The influence of different geometries of the hologram writing- retrieval on the diffraction efficiency is described. Net amplification of one of the writing beams in two-beam coupling within the material has been observed. Recording of the holograms of 2-dimensional objects in real time has been demonstrated with good resolution and image brightness. All the results were obtained with a low power visible laser diode emitting at 675 nm.
The programmable interconnection between N input and N output channels based on a matrix of microholograms is considered. Such a system can be used for optical switching having high speed, about gigabits-per-second. An example of such a system using bacteriorhodopsin film is investigated both theoretically and experimentally. The thickness of bacteriorhodopsin was 50 micrometers and the cell size 3cmx2cm. To maintain interconnects each microhologram was regenerated by means of a routing system composed of a He-Ne laser, deflectors and optical elements. Experimentally, 20 channels were used. The diameter of the microhologram was 1mm, and the diffraction efficiency was about 2%. The tests and calculations show the possibility of arranging 104 switching channels with speed about 1 gigabit per second.
The thermal crystallization process dynamics in a reversible recording medium on the base of chalcogenide semiconductors was studied by means of numerical simulation. The Kolmogorov-Avraami equation was used for nonisothermal crystallization process simulation. The spatial-temporal distribution of crystalline phase in the writing area was obtained. The writing modes under which a uniform distribution of crystalline phase in the irradiated area is achieved were investigated.
Simple light emitting diodes can be constructed using fluorescent organic materials. Conjugated polymers can be used both for charge transport and for light emission. It is considered necessary for maximum device efficiency to balance the rates of electron and hole injection. We report the synthesis of a poly(cyanoterephthalylidenene) that was designed to exhibit an increased electron affinity. Electrochemical measurements showed a significant shift in the oxidation and reduction potentials due to the cyano functionality. The use of this polymer in a range of electroluminescent devices is described. Internal quantum efficiencies of up to 4% can be achieved in a bilayer device using stable electrode materials. The route used to synthesize this polymer is amenable to considerable variation in the subunits employed. This allows tuning of both the band-gap and the electron affinity of the resulting polymer.
Polymer fluorescence indicates a lowest singlet excitation S1 of odd parity, while an even-parity S1 favors radiationless decay. The optical gap Eg defines the lowest odd-parity singlet, 11Bu, and the two-photon gap Ea fixes the 21Ag state. The 1B/2A ordering of conjugated polymers reflects different single-particle gaps, 4t(delta) for alternating chains, at nearly constant correlations. Topological and heteroatom contributions to the effective alternation of poly-p- phenylenevinylenes and polythiophenes are discussed, polymer spectra is analyzed in terms of molecular (pi) -electron theory. Even-parity states at Ea and biexcitons derived from two- electron excitation across Eg indicate intermediate correlations and alternations, thereby limiting the accuracy of band, strong-correlation, and large-(delta) descriptions. Correlated states in the dimer limit illustrate the 2A/1B crossover of Hubbard or excitonic systems.
Time-resolved difference absorption spectra of J-aggregates in ethyleneglycol/water glass were measured by femtosecond pump- probe spectroscopy. the induced absorption near the J-band at 20 K was assigned to the transitions from n-exciton states to (n + 1)-exciton states (n >= 1). The decay time of the n(>= 2)-exciton states is determined to be about 200 fs. For applications of the J-aggregates as nonlinear optical deices, we present a new fabrication method, called vertical spin-coating, to prepare highly oriented 1D J-aggregates dispersed in polymer films. The films are stable even at room temperature. Linear dichroic spectra of the oriented J-aggregate of 1,1'-diethyl- 2,2'-quinocyanine bromide were measured. The dichroic ratio at the peak of J-band was 5 to 10, depending on the preparation conditions. Precise measurement of the dichroism at the J-band revealed that the J-band is composed of two bands with transition dipole moment perpendicular to each other. Large change in a static dipole moment upon electronic excitation was unexpectedly observed in the oriented J-aggregates by electromodulation spectroscopy. The difference absorption spectra due to the Kerr effect were induced by changes in a static dipole moment and a polarizability. The change in the static dipole moment associated with the transition from the ground state to the exciton state were measured with the applied AC field of Hz and the polarization parallel and perpendicular to the 1D axis of the oriented J-aggregates.
The Heterostructure Field Effect Optical Modulator (HFEOM) is a waveguide modulator that operates via band filling of quantum wells using charge transfer from an adjacent n+ charge sheet. The control of this charge transfer is with a gate electrode as in a field effect transistor. The band filling of the quantum wells produces a blue-shift of the absorption edge that is used to modulate the incident light. This device is compatible in both growth and processing with the associated in- plane laser and field effect transistor. The initial high speed results of HFEOMs in the InGaAs/GaAs material system are presented using a double quantum well active region. This structure has demonstrated a 35:1 extinction ratio for a 2 volt swing (-1 V to +1 V) on a 300 micrometers long device along with excellent wavelength compatibility with a 400 micrometers in-plane laser fabricated from the same wafer. Capacitance limited modulation bandwidths of 1.2 GHz and 1.6 GHz are measured for 5 micrometers and 2 micrometers rib widths respectively.