Comellas, J.; Gene, J. M.; Prat, J.; Junyent, G.; Perdigues, J. M. Optical and quantum electronics Vol. 36, num. 15, p. 1263-1274 DOI: 10.1007/s11082-004-8312-6 Data de publicació: 2004-12 Article en revista
We describe the principle of operation of a new class of optical devices operating in quadratic non-linear media that mix wave front topological charge dislocations nested in focused light beams and produce certain patterns of bright spatial solitons. Central to the device behaviour is the orbital angular momentum of the light beams.
Channel spacing in optical frequency-division multiplexed (OFDM) coherent optical systems with continuous-phase frequency-shift-keying (CPFSK) is obtained both theoretically and experimentally. The bit error ratio is derived taking into account the non-Gaussian interference statistics, expressed in terms of the Marcum Q-function, and the sensitivity penalty is reported as a function of the signal-to-interference ratio and the channel spacing for different IF filters. Laboratory demonstration of a state-of-the-art fully engineered two-channel 2.5 Gb s-1 CPFSK coherent system at 1550 nm has been performed, showing an inter-channel crosstalk measurement in agreement with the theoretical analysis. A 0.1 nm channel spacing is experimentally confirmed.
Second-harmonic generation of light in bulk quadratic non-linear media with intense input beams containing phase dislocations is studied numerically under conditions for TypeI phase-matching. We investigate how, above a threshold light intensity, the input beams self-split along the azimuthal direction into a pattern of separate beams which then form a set of spatial solitons. The mechanism of such a behaviour is the azimuthal modulational instability of the ring-shaped, mutually trapped fundamental and second-harmonic beams containing the phase dislocations.
The propagation of intense light beams in planar optical waveguides made of quadratic nonlinear media under conditions for second-harmonic generation is investigated. It is shown numerically that under appropriate conditions, input light beams break up into several spatial solitons that separate from each other and exit the waveguide at different positions. This paper reports the results of a series of numerical experiments and analytical investigations that were performed to elucidate the dynamics of the self-splitting process and discusses the implications of the results to the implementation of an eigenvalue switching device.
Cascading is the process by which the exchange of energy between optical beams interacting via second order nonlinearities (¿(2)) leads to various effects such as nonlinear phase shifts, the generation of new beams, all-optical transistor action, the formation of soliton-like (solitary) waves, etc. Here we review the fundamentals of the processes and discuss experimental verification of the effects and various related applications.
Hybrid surface plasmon polaritons propagating in a new configuration comprising an ultrathin metal film bounded by a positive birefringent crystal on one side and a glass material on the other side are investigated. The results show that the conditions for existence of such hybrid surface plasmon polaritons have a strong dependence on the structure parameters. This feature opens the way for a new type of optical sensor with directional sensitivity.
An alternative theoretical analysis directed to obtaining a more accurate expression for the critical power in the symmetric nonlinear directional coupler, or other nonlinear, five-layer waveguiding structures, is reported. Our analysis works with the nonlinear supermodes of the waveguide, and it is presented as an improvement of the preceding study by Silberberg and Stegeman . Numerical simulations comparing both techniques, and by means of the beam propagation method, are reported for a few cases. A set of three representative, power dependent parameters is proposed, in order to give a comprehensive view of the power flow between both branches in the device as a function of the excitation conditions.
The cutoff behaviour of graded-index planar waveguides is analysed in terms of the normalized waveguide parameters by using the multilayer technique and the WKB approach. The results show a very high degree of accuracy when checked against exact known values for exponential and hyperbolic index profiles. A set of universal plots showing the general features is given for the cutoff values of both fundamental and first-order modes. The TM polarization is also studied by comparison with the TE one.
It is known that for data transmission above 100 Mbits s-1 multimode fibre systems, working at 1.3 µm wavelength, are more modal-dispersion-limited than loss-limited. Then some form of baseband equalization is highly desirable. Baseband equalizers are usually made in the form of a transverse filter with variable tap gains and tap spacing equal to the symbol spacingT. However, the performance of these equalizers depends critically on the timing phase. To overcome this difficulty, in this paper we analyse the performances of fractional tap-spacing equalizers that have a tap spacing smaller thanT. Results obtained show that this type of equalizer gives satisfactory performance for a broad continuous range of timing phase values. It allows the application of simple clock recovery schemes in the receiver.
For data rates above 100 Mbits s–1 multimode optical fibre systems are generally modal dispersion rather than loss limited and some form of equalization is highly desirable. In this paper a new strategy for the optimum design of an equalized receiver is introduced with the aim of maximizing the repeater spacing. Linear and nonlinear equalizers are briefly reviewed and their relative merits discussed. In the receiver design, Poisson signal statistics and additive Gaussian noise are considered. A new upper bound on the bit error rate which allows an adequate accuracy is proposed. Results obtained for 140 and 560 Mbits s–1 show a noticeable increase of the repeater spacing when the proposed design strategy is introduced.