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A compact low dispersion fibre Bragg grating is designed and fabricated with the recently developed polarisation control method. Its detuning tolerance is investigated for five different modulation formats at 10 Gbit/s showing a bandwidth utilisation above 89%. Good performance is predicted at 40 Gbit/s for this device.

A photoelectron–photofragment coincidence (PPC) study of the dissociative photodetachment of OHF− at a photon energy of 4.80 eV is presented. The correlated electron kinetic energy (eKE) and translational energy release (ET) into the O + HF + e− products yield information on the potential energy surface close to the transition state of the neutral reaction OH + F → O + HF. The correlation spectrum shows two different features in the energetically allowed O + HF product channel: (a) diagonal ridges, resulting from direct dissociative photodetachment (DPD) and (b) areas with higher ET in the neutral fragments from nonadiabatic dissociation. The total translational energy spectrum (ETOT = eKE + ET) reveals a vibrationally resolved product state distribution. These results are discussed in the context of recent theoretical studies of the dissociative photodetachment of OHF−.

Photoelectron-photofragment coincidence spectroscopy is employed to study the dissociative photodetachment (DPD) dynamics of S 2 O 2 - at 258 nm. Experimental data and theoretical calculations show evidence for photodetachment from a trigonal form of S 2 O 2 -. The vertical detachment energy of this isomer was determined to be 3.73 ′ 0.02 eV. An upper bound of 2.41 ′ 0.14 eV is determined for the enthalpy of the reaction S 2 O 2 - (r) S + SO 2 + e - at 0 K. The observed dynamics are interpreted in terms of dissociative photodetachment of S 2 O 2 - to S( 3 P) + SO 2 ( 1 A l ) + e - , S( 1 D) + SO 2 ( 1 A 1 ) + e - , and S 2 ( 3 ( g -) + O 2 ( 3 ( g -) + e - product channels. The S-atom channels are characterized by a large photofragment kinetic energy release and an anisotropic photofragment angular distribution peaked along the electric vector of the laser. The S 2 channel has a low kinetic energy release consistent with elimination of highly vibrationally excited O 2 from a strained form of the trigonal isomer.

Photoelectron-photofragment coincidence (PPC) spectroscopy has been used to study the dissociative photodetachment of H2O2− and D2O2−. The observed partitioning of photoelectron and photofragment translational energies provides information on the dynamics in the transition state region of the reaction between two hydroxyl radicals: OH+OH→O(3P)+H2O. The data reveal vibrationally resolved product translational energy distributions for both the entrance channel OH+OH and the exit channel O(3P)+H2O upon photodetachment. The total translational energy distribution shows a convoluted vibrational progression consistent with antisymmetric stretch excitation of H2O in the exit channel and OH stretch in the entrance channel. The photoelectron spectra are compared to two-dimensional time-dependent wave packet dynamics simulations based on an anharmonic potential in the anion and a model collinear potential energy surface for the neutral complex. The PPC spectra also yield the dissociation energies D0(H2O2−→H2O+O−)=1...

Photoelectron–photofragment coincidence spectroscopy was used to study dissociative photodetachment of the doubly hydrated clusters of oxide and hydroxide, M−(H2O)2→M+2H2O+e− (M=O, OH). These experiments yield information on the energetics of the parent anion and the dissociation dynamics of the photodetached neutral species. Photoelectron spectra and photoelectron–photofragment coincidence spectra are presented and compared to data for O−(H2O) and OH−(H2O). Unlike the singly hydrated species, no evidence of vibrationally resolved product translational energy distributions is observed. The second hydration energy of O− with both H2O and D2O was also measured to be 0.80±0.08 and 0.81±0.08 eV, respectively. The three-body dissociation dynamics of the neutral clusters produced by photodetachment were studied by measuring the velocities and recoil angles of all the particles in coincidence. The observed partitioning of momentum is consistent with a two-step mechanism or dissociation from a wide range of start...

Photodetachment and dissociative photodetachment processes of cyclopropoxide, c-C3H5O-, and cyclobutoxide have been studied at 532 nm. Photodetachment of c-C3H5O- produces both the ground X(2A‘ ‘) state and the first excited A(2A‘) state of cyclopropoxy radical, c-C3H5O. The X(2A‘ ‘) state is stable at lower levels of excitation, but with increasing internal energy, dissociation into HCO + C2H4 is observed. The A(2A‘) state completely dissociates into HCO + C2H4. Correlated measurements of photoelectron and photofragment kinetic energies provide dissociation energies c-C3H5O- and c-C3H5O into HCO- + C2H4 and HCO + C2H4 of 0.85 ± 0.07 and −0.26 ± 0.07 eV, respectively. Ab initio calculations have been performed to aid the interpretation of the dissociation mechanism. Cyclobutoxide, c-C4H7O-, undergoes only dissociative photodetachment to ground-state vinoxy radical and ethylene. The adiabatic electron affinity (AEA) of c-C4H7O is estimated to be 1.7 ± 0.1 eV. c-C4H7O- and c-C4H7O are both found to be therm...

Dynamics in the transition state region of the bimolecular OH+H2O→H2O+OH hydrogen exchange reaction have been studied by photoelectron–photofragment coincidence spectroscopy of the H3O2− negative ion and its deuterated analog D3O2−. The data reveal vibrationally resolved product translational energy distributions. The total translational energy distribution shows a vibrational progression indicating excitation of the antisymmetric stretch of the water product. Electronic structure calculations at the QCISD level of theory support this analysis. Examination of the translational energy release between the neutral products reveals a dependence on the product vibrational state. These data should provide a critical test of ab initio potential energy surfaces and dynamics calculations.

We report picosecond time-resolved pump–probe photoelectron spectra of the allyl radical, C3H5, and the fully deuterated allyl, C3D5, carried out in order to elucidate the primary photophysical processes upon UV excitation. It is shown that the UV bands of allyl decay in a two-step process: the first step is an internal conversion to the lower-lying A-state within 20 ps or less, while the second step is a very fast decay from the A-state to the electronic ground state through a conical intersection. In addition we report the first zero kinetic energy (ZEKE) photoelectron spectrum of allyl, yielding an ionisation energy of 65762 cm−1.

Enolate anions are intermediates in many organic reactions that involve carbon-carbon or carbon-oxygen food formation. They also play a key role in the development of stereoselective and stereocontrolled syntheses of complex compounds. Enolate radicals are important intermediates in combustion and photochemical smog cycles. In particular, the vinoxy radical, C{sub 2}H{sub 3}O{sup {sm{underscore}bullet}} is a major product of the reaction of odd oxygen and ethylene. The photoelectron spectrum of binoxide, C{sub 2}H{sub 3}O{sup {minus}}, at 355 nm is reported, showing photodetachment to both the X({sup 2}A{double{underscore}prime}) ground and first excited A({sup 2}A{prime}) states of the vinoxy radical. Both direct interpretations and Franck-Condon simulations of the photoelectron spectrum of this simple enolate anion have been used to obtain insights into the energetics and structures of the anion and the ground and first excited state of the neutral radical. Franck-Condon simulations were generated from ab initio geometry and frequency calculations using the CASSCF method and showed good agreement with the vibrational structure visible in the experimental spectrum. The electron affinity (E.A.{sub exp} = 1.795 {+-} 0.015 eV; E.A.{sub calc} = 1.82 eV) and separation energy of the ground and first excited states (T{sub 0,exp} = 1.015 {+-} 0.015 eV; T{sub 0,calc} = 0.92more » eV) obtained from the ab initio calculations are in good accord with the experimental values.« less

The photochemistry and photodissociation dynamics of the propargyl radical, C3H3, upon UV excitation is investigated by time- and frequency-resolved detection of hydrogen atoms. From a statistical analysis of the data, we conclude that formation of cyclopropenylidene, c-C3H2, is the dominant reaction channel. Around 22% of the excess energy is released into the translational degrees of freedom. By varying the excitation energy between 265 and 240 nm, microcanonical rates for the loss of a hydrogen atom can be obtained as a function of excess energy. The experimental rates, on the order of several 106 s−1, are in good agreement with Rice–Ramsperger–Kassel–Marcus (RRKM) calculations, provided a scaling factor is used for the vibrational frequencies, to account for the effects of anharmonicity. The interpretation is confirmed in experiments using monodeuterated propargyl radicals, H2CCCD, indicating a mechanism that proceeds via an initial [1,2] H-shift, followed by cyclization.

The photochemistry and photodissociation dynamics of the allyl radical upon ultraviolet (UV) excitation is investigated in a molecular beam by using time- and frequency-resolved photoionization of hydrogen atoms with Lyman-α-radiation. The UV states of allyl decay by internal conversion to the ground state, forming vibrationally hot radicals that lose hydrogen atoms on a nanosecond time scale. Two channels are identified, formation of allene directly from allyl, and isomerization from allyl to 2-propenyl, with a subsequent hydrogen loss, resulting in both allene and propyne formation. The branching ratio is between 2:1 and 3:1, with direct formation of allene being the dominant reaction channel. This channel is associated with site-selective loss of hydrogen from the central carbon atom, as observed in experiments on isotopically labeled radicals. Ab initio calculations of the reaction pathways and Rice–Ramsperger–Kassel–Marcus (RRKM) calculations of the rates are in agreement with the mechanism and branching ratios. From the measured Doppler profiles a translational energy release of 14±1 kcal/mol is calculated. The calculated value of 66 kcal/mol for the barrier to the 1,2 hydrogen shift from allyl radical to 2-propenyl is confirmed by the experimental data.

The direct observation of the products, kinetics and translational energy release from the photodissociation of the allyl radical, C3H5, upon excitation in the near-uv is reported. A statistical analysis of the data shows that they are in agreement with allene formation being the dominant H-loss reaction channel.

International audience; The photoionization and dissociative photoionization of the allyl radical in the VUV range was investigated using synchrotron radiation. Allyl radicals were generated by flash pyrolysis from allyl iodide and 1,5-hexadiene. Mass spectra show the appearance Of C3H3+ as a fragment in the dissociative photoionization of allyl above 10 eV. Between 10.4 and 10.5 eV the C3H3+ signal reaches 50% of its maximum value. Cyclopropenyl cation, c-C3H3+, is the thermochemically most stable product, but accompanying ab initio computations indicate that formation of linear 1-C3H3+ becomes competitive at small excess energies. The dissociative photoionization of the precursor molecules is discussed as well. An ionization energy of 8.13 +/- 0.01 eV was deduced from threshold photoelectron spectra.

A simple method of displacing a holographic interferogram within a waveguide based on translation along a slightly tilted fringe pattern of the optical interferogram is presented. By way of example, phase-shifted fibre gratings are produced this way. The spatial resolution in fringe shift is separated from the physical spatial displacement of the optical elements and determined primarily by the fringe tilt angle. This relaxes the tolerances in optical components required and can potentially enable spatial displacements as small as 1 nm/μm to be realised.

A precision phase-shifting approach to fabricate various phase-shifted gratings using different combinations of polarized ultraviolet (UV) light is demonstrated. In doing so, the difference between s- and p-polarized light reported by others is confirmed. However, we reveal added complexity for the role of hydrogen and deuterium in the UV-induced process. Previous arguments for the origins are systematically ruled out by reviewing existing literature. We note that the birefringence is made up of at least two components with different thermal stabilities, one consistent simply with molecular hydrogen being present in the system. Overall the birefringence, by deduction, is associated with anisotropy in hydrogen reactions within the fiber. As a result they lead, through known mechanisms of dilation in glass, to anisotropic stress relaxation that can be annealed out, with or without hydrogen remaining, at low temperatures close to 125 degrees C.

An integrated Michelson delay interferometer structure making use of waveguide gratings as reflective elements is proposed and fabricated by direct ultraviolet writing. Successful return-to-zero alternate-mark-inversion signal generation using phase-to-intensity modulation conversion is demonstrated up to 40 Gb/s using the device. Compared to other implementations, the device is compact, inherently stable, and allows for easy customization of the pulsewidth by proper positioning of the gratings on a single coupler structure.

We report on a cross-phase modulation-based Raman-assisted wavelength conversion by a novel notch filtering approach at 160 Gb/s. Error free operation for all channels is shown along with regenerative properties. (2 pages)

We demonstrate a new class of hollow-core Bragg fibers that are composed of concentric cylindrical silica rings separated by nanoscale support bridges. We theoretically predict and experimentally observe hollow-core confinement over an octave frequency range. The bandwidth of bandgap guiding in this new class of Bragg fibers exceeds that of other hollow-core fibers reported in the literature. With only three rings of silica cladding layers, these Bragg fibers achieve propagation loss of the order of 1 dB/m., Comment: 9 pages including 5 figures

In conclusion the polarization control method is superior to any double scan method since it allows much more flexible apodization and inclusion of phase shifts, and since only one exposure is required. In many cases the method performs just as well as the most sophisticated phase mask jitter methods, while still remaining very simple and robust.

We report the application of the polarization control method for the UV writing of advanced fiber Bragg gratings (FBG). We demonstrate the strength of the new method for different apodization profiles, including the Sinc-profile and two designs for dispersion-free square filters. The method has been tested for advanced FBGs with various strengths up to 65 dB.

A systematic study of the thermal decay of Bragg gratings in high and low Ge-content fibers is presented. The resulting annealing curves are discussed in the framework of three different models of UV-induced index changes.

A novel single-step technique for the apodization of planar waveguide Bragg gratings based on the polarization control method is proposed. First results are presented, showing successful side-lobe suppression in the reflection spectrum of the gratings.

We report a study on the spectroscopy and the photochemistry of the allyl radical, C3H5, in the vacuum ultraviolet energy range using synchrotron radiation as the light source and a flash pyrolysis source to generate the radical. Mass spectra recorded over a wide energy range show the appearance of C3H3+ as a fragment in the dissociative photoionization of allyl above 10 eV. The threshold photoelectron spectrum of the allyl radical shows a progression of around 52 meV, corresponding to the CCC bending vibration.

The polarization control method offers a flexible, robust, and low-cost route for the parallel fabrication of gratings with complex apodization profiles including several discrete phase shifts and chirp. The performance of several test gratings is evaluated in terms of their spectral response and compared with theoretical predictions. Short gratings with sidelobe-suppression levels in excess of 32 dB and transmission dips lower than 80 dB have been realized. Finally, most of the devices fabricated by the polarization control method show comparable quality to gratings manufactured by far more complex methods.

We present a flexible and simple method for UV writing of Bragg gratings with advanced apodization profiles including discrete phase shifts. The method is based on a p phase shift between the refractive-index modulation profiles induced by s and p polarization of UV light. By changing the ratio of UV intensity in the two polarizations we are able to control the modulation strength and to induce phase shifts while keeping a constant effective refractive index throughout the Bragg grating. We demonstrate strong UV-written Bragg gratings with Gaussian or sinc apodizations with spectral shapes, in good agreement with theoretical predictions.

Raman-assisted wavelength conversion for ultra-high speed data is characterised in approaches: a novel scheme based on cross-phase modulation using specially designed notch filters and a 160 Gb/s experiment based on self-phase modulation