1,721,148 research outputs found
Theoretical study of parametric frequency and wavefront conversion in nonlinear holograms
No full textThird-order mixing of arbitrary beams of pump and second-harmonic light gives rise to a spatially periodic dc polarization whose noninversion symmetry and periodicity are correct for quasi-phase matching of the pump and second harmonic. Under appropriate conditions in certain materials, e.g. optical fibers, a chi(2) forms in proportion to this dc polarization, resulting in a nonlinear hologram that can be used to recreate a second-harmonic image from a reference pump beam. Distributed feedforward and feedback reconstruction of plane waves by slab-shaped nonlinear holograms is analyzed, and expressions for the field profiles and conversion efficiencies are derived, in the DFB case at high fixed pump intensities, multistable states of conversion efficiency exist. The higher order states display catastrophes in their transverse amplitude distributions
Photonic Bloch waves and field microstructure in nonlinear gratings: An intuitive approach
No full textCoupled wave theory is commonly used in numerical simulations of reflection from nonlinear grating structures. By contrast, the complementary photonic Bloch wave (PBW) approach has rarely been adopted, perhaps the only example being recent work on gap solitons. Bloch wave theory offers an alternative physical intuitive picture that encourages one to think in terms of field microstructure, leading to a range of simple explanations for the behaviour of light in linear gratings. In this paper the dispersion relation and field microstructure of Bloch waves are found, and used to clarify the physical mechanisms that lead to regions of bistability, instability and oscillation for incidence of a monochromatic plane wave on a nonlinear grating half-space
Temporal and spatial instabilities and nonlinear beam-steering of light in periodic media
No full textThe presentation will treat the propagation of light in nonlinear periodic structures using an approach based on the normal modes of linear periodic structures - the Photonic Bloch waves (PBW's) [1,2]. These waves exhibit rich and complex behaviour in the presence of optical nonlinearities, in part because they consist of a group of backward and forward waves superimposed and sharing the same group velocity or evanescent decay rate [3,4]. The nonlinear dispersion relation of these waves is straightforward to obtain and can be solved analytically [4]. Its simplicity makes it easy to assess the susceptibility of nonlinear PBW's to modulational instability (MI). In the presence of MI, signals injected at sidebands of the pump wave experience gain or loss via degenerate four-wave mixing. In the temporal domain this means that spurious signals can grow from noise at frequencies ranging from MHz to THz, depending on the strength of the linear grating [5]. The result is oscillation and instability; a spin-off of the MI analysis is that regions of stability and bistability are easily identified. Perhaps even more intriguing is the behaviour in the spatial domain, where temporal frequency translates to spatial frequency or angle; the MI sidebands in this case grow and propagate in directions different from that of the pump wave. The pump wave itself exhibits bistability in certain regimes, leading to nonlinear beam steering where the propagation direction depends on both the pump power level and the route by which it is reached. The general behaviour of nonlinear PBW's in two-dimensional periodic media involves both temporal and spatial effects, giving rise to spatial and temporal beam fan-out and instabilities. Once their physics has been fully understood, exotic applications of these effects - in addition to established work on gap solitons [3] - are certain to emerge. At present much still remains to be done on understanding the complex behaviour of nonlinear PBW's; the talk will feature recent results from work-in-progress
All-optical high gain transistor action using second-order nonlinearities
No full textIt is shown that the output stage of a phase-matched frequency doubler is highly sensitive to the phase and amplitude of a weak injected second harmonic signal. Such devices may be regarded as all-optical transistors with very high small signal gain, resulting in strong modulation of the pump light. Gains as high as 60dB appear to be experimentally feasible in the recently reported highly efficient quasi-phase-matched frequency doublers formed by periodic domain inversion in LiNbO3-optic host crystal
Nonlinear optics in fibres : a history of overturned assumptions and new physics?
No full textFibre optics has repeatedly challenged our assumptions and surprised us with novel nonlinear phenomena and unexpected physical effects. Some striking recent examples, including optically-induced changes in material properties, are discussed
Bloch wave analysis of dispersion and pulse propagation in pure distributed feedback structures
No full textThe excitation and behaviour of monochromatic and pulsed optical Bloch waves in pure distributed feed-back structures are discussed and analysed. The Bloch wave approach, based on a detailed knowledge of the natural optical modes of the periodic structure, is complementary to the more commonly used coupled-wave approach. The inter-relationship between dispersion, field micro-structure and group velocity is discussed, and the effects of group-velocity and higher-order dispersion on pulse propagation treated. Questions about the usefulness of DFB structures for dispersion correction and soliton formation are addressed
Optical switching using second order nonlinearities
Full text linkIt is shown that single-step phase-matched parametric conversion, seeded with small amounts of second harmonic (SH), can yield nonlinear pump phase-shifts, and that the output state is extremely sensitive to the phase of the injected SH
Glass and waveguide poling
No full textGlass, e.g. silica glass, is one of the dominant materials in information technology because of its low fabrication cost compared to crystalline materials, and its superior optical properties such as high transparency and high optical damage threshold. However inversion symmetry of the glass matrix ensures the absence of optical effects based on second-order nonlinearity such as linear electrooptic effect and parametric frequency conversion. The ability to modulate a material's refractive index with an applied field, as in the electrooptic or piezoelectric effect, is necessary for making optical switches and electric field sensors. Frequency conversion of coherent radiation through parametric processes, such as second harmonic and sum or difference frequency generation, is also desirable to produce a large range of wavelength from fibre lasers and for construction of tunable laser sources. The development of a practical second-order nonlinearity in silica and related materials would add both modulators and frequency converters to the list of active fibre components. The advantages of integrability, e.g. monolithic integration of the above devices into optical fibres, and manufacturabilty would ensure widespread use of these capabilities. Until fairly recently, second harmonic generation in specially treated glasses and glass fibres has been of more scientific than practical interest, owing to small levels of nonlinearity (several orders of magnitude less than in lithium niobate) that could be induced. A recent breakthrough is the observation of high second-order nonlinearities of the order of 1 pm/V in glasses and 0.2 pm/V in optical fibres using a variety of different techniques similar to those which are used for making polymer electrets: thermal poling, corona poling and electron implantation. These values of nonlinearity are large enough to be useful for parametric frequency conversion. The nonlinear coefficients, especially for the electrooptic effect, are still small, and so require long interaction lengths. This is not a significant problem in fibre applications where the issues are cost, integrability and packaging, not length. It is worth also noting one more important advantage of poled glass in comparison with nonlinear crystals, namely that the bandwidth of phase-matched second harmonic conversion in poled silica (0.78 nm.cm) is an one order of magnitude larger than in an equal length of periodically poled lithium niobate (0.06 nm.cm). Moreover the group velocity mismatches are about 130 fs/mm and 1.8 ps/mm respectively. This may be of great importance in short pulse work where large acceptance bandwidths and long interaction lengths are required. A better understanding of the physical mechanisms of glass poling may lead to even higher values of nonlinearity, perhaps competitive with the best nonlinear crystals. In the talk we review recent progress in glass and waveguide poling
Narrow-band in-line fiber filter using surface-guided Bloch modes supported by dielectric multilayer stacks
No full textSurface-guided Bloch modes (SGBMs) confined to the interface between a dielectric multilayer stack and an external medium are highly dispersive with wavelength. This makes attractive their use in a filter configuration based on evanescent coupling between two dissimilar guides. The coupling between a fiber mode and a SGBM is modeled theoretically, predicting a narrow bandwidth for small coupling lengths
Properties and applications of poled glass
No full textWhen glass fibres were first proposed for optical communications in the late 1960's, one attractive selling point was their extreme insensitivity (if not complete immunity) to electromagnetic interference, which - in addition to a much larger bandwidth - gave them considerable advantages over co-axial cable. Early research concentrated on reducing the optical absorption to the lowest possible levels, eventually achieving values <0.1 dB/km in the main communications windows at 1.3 and 1.5µm. At this point there seemed no reason to suppose that germanium doped silica glass of extremely high purity should be anything other than an almost perfect optical transmission medium. Ambitious, however, to extend the capabilities of optical fibres, researchers went on to study whether more complex functions such as amplification, modulation, wavelength conversion and lasing could be incorporated, the idea being to avoid the need for optical-electronic-optical repeaters and modulators. This effort has resulted in the successful demonstration of all-optical diode-laser-pumped in-fibre lasing and amplification by incorporation of rare-earth dopants in the core glass. These developments are already revolutionising communications system design
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