Photothermal Phase-Modulating Laser Diode Interferometer with High-Speed Feedback Control

We propose a photothermal phase-modulating laser diode interferometer equipped with a high-speed feedback control system. The CCD camera’s shutter-function raises the modulating frequency, which in turn elicits a higher feedback response, thus enabling us to eliminate nearly all external disturbance. With the recent application of a system purpose-built for discriminating between numerous fringe sequences, we have also reduced processing time.     

Nonlinear Crosstalk Suppression in WDM-SCM Systems

The mechanism of nonlinear crosstalk in WDM-SCM fiber communication systems is investigated. Then the crosstalk suppression method of complementary modulation of twin carriers is introduced and discussed. Aiming at the limitation of this method, a new method named symmetric carrier complementary modulation is presented, and it has shown satisfactory effect in crosstalk reduction.     

相对论速调管放大器中微波放大机制

解析地研究了电子束在相对论速调管放大器（ＲＫＡ）的调制腔和漂移管中的预群聚；第一次用粒子－波相互作用的场方法导出了在辐射腔中预群聚电子束产生辐射的自洽方程；并且计算了线性区的增益．     

Combined Role of Synchronized Amplitude Modulation and Synchronized Phase Modulator for Soliton-Soliton Interaction

In this paper, the interaction between two solitons has been studied by the perturbation approach in the presence of synchronized amplitude modulation and synchronized phase modulation. The results show : the combined role of synchronized amplitude modulation and synchronized phase modulation can suppress effectively the interaction.     

Effect of quasi-periodic gravitational modulation on the convective instability in Hele-Shaw cell

The aim of the present paper is to examine the effect of a quasi-periodic gravitational modulation on the onset of convective instability in Hele-Shaw cell. The quasi-periodic modulation considered here consists in a modulation having two incommensurate frequencies. This study is an extension of a previous work by Aniss et al. [Asymptotic study of the convective parametric instability in Hele-Shaw cell, Phys. Fluids 12 (2) (2000) 262-268] in which only a periodic gravitational modulation was considered. We have shown that for Pr=O(1) or Pr?1, the gravitational modulation has no effect on the convective threshold as expected. However, for Pr=O(ε²), it turns out that a modulation with two incommensurate frequencies has a stabilizing or a destabilizing effect strongly depending on the frequencies ratio.     

High-frequency optical signal generation in a semiconductor laser by incoherent optical feedback

This work examines the feasibility of using incoherent optical feedback to generate high-frequency optical signals in a semiconductor laser. At a roundtrip feedback distance of 30 cm, a signal appears at around 1 GHz while the laser output is kept in single longitudinal mode and single polarization. The proposed method does not use any external modulation or saturable absorber. Some of the signal's properties are discussed as well.     

Angular Multiplexing Gratings Characterization in Dichromated Gelatin and Thoughts on Diffractive Optical Memory

We were mainly concerned here with multiple exposure gratings that allow interactions and coupling between gratings. We started the study of interactions and coupling with five gratings recorded in dichromated gelatin (DCG). Having determined experimentally the characteristic parameters of these five gratings, we used a method developed by Oxford University to synthesize the modulation profile. Entering these parameters of each grating into our computing programs (based on the alternative theory of diffraction in modulated media) and by varying the amplitude modulation, the latter was deduced by adjusting the numerical curve to the experimental one. We then extended the study to nine multiplexed gratings. This study allowed us to know the range of the amplitude modulation such that a certain number of angular multiplexing can be done in DCG. Several undesirable effects which we had experimentally observed appeared then during the reconstruction of any one of the multiplexed images, such as cross-talk effect. We noted the good concordance between experimental and theoretical results. We finally illustrated cross-talk effect in a diffractive memory where, if the amplitude modulation was first calculated, the number of multiplexing for an angular range could then be determined and the angular separation achieved to avoid cross-talk.     

Nonlinear distribution function for a plasma obeying Vlasov-Maxwell system of equations

The nonlinear distribution function of Allis, generalised to include the transverse electromagnetic waves in a plasma, is used to set up the coupled wave equations for the longitudinal and the transverse modes. These are solved, keeping terms up to the cubic order of nonlinearity, by using the method of multiple scales. The equations of wave modulation are derived, which are solved to discuss the nature of the modulational instability and solitary wave propagation. It is found that the solutions so obtained satisfy conditions which are very similar to the well known Lighthill criterion for stability, appropriately modified due to the coupling of the two modes. The role of the average constant current due to any flow of the resonant and trapped electrons in determining the stability, is also discussed.     

Microwave propagation through modulated air plasma

When a microwave propagates through a plasma in which electron density and electron collision frequency periodically vary, the propagating wave is modulated in amplitude and phase. An approximate theory is derived to suit the laboratory experimental conditions. Introducing the amplitude and phase difference, the dependence of electron density and electron collision frequency has been derived for different radio frequency modulation and frequency parameter. A scanning double probe technique is used to measure the exact time variation in the plasma parameters at any fixed position during a single cycle of the applied field. Theoretical values agree with those of experiment.     

Necessary conditions for Schatten class localization operators

We study time-frequency localization operators of the form , where is the symbol of the operator and are the analysis and synthesis windows, respectively. It is shown in an earlier paper by the authors that a sufficient condition for , the Schatten class of order , is that belongs to the modulation space and the window functions to the modulation space . Here we prove a partial converse: if for every pair of window functions with a uniform norm estimate, then the corresponding symbol must belong to the modulation space . In this sense, modulation spaces are optimal for the study of localization operators. The main ingredients in our proofs are frame theory and Gabor frames. For and , we recapture earlier results, which were obtained by different methods.