A high-speed optical star network using TDMA and all-optical demultiplexing techniques

The authors demonstrate the use of time-division multiplexing (TDM) to realize a high capacity optical star network. The fundamental element of the demonstration network is a 10 ps, wavelength tunable, low jitter, pulse source. Electrical data is encoded onto three optical pulse trains, and the resultant low duty cycle optical data channels are multiplexed together using 25 ps fiber delay lines. This gives an overall network capacity of 40 Gb/s. A nonlinear optical loop mirror (NOLM) is used to carry out the demultiplexing at the station receiver. The channel to be switched out can be selected by adjusting the phase of the electrical signal used to generate the control pulses for the NOLM. By using external injection into a gain-switched distributed feedback (DFB) laser we are able to obtain very low jitter control pulses of 4-ps duration (RMS jitter <1 ps) after compression of the highly chirped gain switched pulses in a normal dispersive fiber. This enables us to achieve excellent eye openings for the three demultiplexed channels. The difficulty in obtaining complete switching of the signal pulses is presented. This is shown to be due to the deformation of the control pulse in the NOLM (caused by the soliton effect compression). The use of optical time-division multiplexing (OTDM) with all-optical switching devices is shown to be an excellent method to allow us to exploit as efficiently as possible the available fiber bandwidth, and to achieve very high bit-rate optical networks

Closed Analytical Expression for the Electric Field Profile in a Loaded RF Structure with Arbitrarily Varying vgv_{g} and R′/QR'/Q

The design of a detuned and damped accelerating structure implies variations in the geometry which induce in turn a variation of the group velocity v^g and of the impedance per unit length R', divided by the quality factor Q. The resulting differential equation for the longitudinal electric field (fundamental mode) contains coefficients that depend on the distance z along the structure. This report describes a possible method to solve this nonlinear, first order differential equation analytically and how to obtain approximate closed algebraic forms, by using the sequence of Gauss integration methods. Analytical expressions of the longitudinal field profile in a loaded or unloaded accelerating section is deduced for both linear and arbitrary variations of v^g and R'/Q = Q. Simple relations between the average field and the field at the entrance of the structure E(0) make it possible to provide the dependence of the field function E(z) on the design value for ? and on the structure parameters. The results are in good agreement with the direct numerical integration. Applications are presented for particular structure designs

MBTR Simulations for Different Multibunch Structure Models

This paper reports on simulations undertaken with the code MBTR to explore the emittance growth for various multibunch CLIC structures. Two simplified models of long-range dipole fields are considered and compared with that of a damped structure with frequency discriminated wave guide damping. The second model corresponds to a possible design of a multibunch structure, with damping and moderate detuning, recently studied and showing promising characteristics. The reported simulations carried out with the CLIC main beam parameters of the LC97 workshop corroborate this hope. They show that the vertical emittance growth of a 60-bunch train accelerated to 1 TeV is only a fraction larger than the single-bunch emittance growth, in the same conditions

Analytical study of the conjecture rule for the combination of multipole effects in LHC

This paper summarizes the analytical investigation done on the conjecture law found by tracking for the effect on the dynamic aperture of the combination of two multipoles of various order. A one-dimensional model leading to an integrable system has been used to find closed formulae for the dynamic aperture associated with a fully distributed multipole. The combination has then been studied and the resulting expression compared with the assumed conjecture law. For integrated multipoles small with respect to the focusing strength, the conjecture appears to hold, though with an exponent different from the one expected by crude reasoning

Multibunch BNS Damping and Wakefield Attenuation in High Frequency Linacs

In high frequency linacs, where the wakefields are strong, the stability of a train of bunches is critical. It was therefore important for the Compact Linear Collider study (CLIC) to investigate numerically and theoretically this question. Basically, two methods of controlling beam break up have been considered; firstly a multibunch generalization of the BNS damping principle and secondly the attenuation of the long-range fields as it results from damping or staggered tuning of the accelerating sections. Simulation codes have been written for both checking the theoretical predictions and investigating the requirements associated with a possible application to the CLIC main linac

New theory of single bunch stability in a linac with quadrupole displacements

The analytical treatment previously described by Guingard and Hagel (1998) has been extended to include the important effect of magnetic quadrupole transverse displacements, the chromatic variation of the magnetic focusing, the energy spread along the bunch and possible microwave quadrupoles, the last two in relation to BNS damping The analytical treatment previously described has been extended to include the important effect of magnetic quadrupole transverse displacements, the chromatic variation of the magnetic focusing, the energy spread along the bunch and possible microwave quadrupoles, the last two in relation to BNS damping. Both, the longitudinal and transverse equations of motion are solved, the second by using the perturbation method with partial expansions developed for this theory. The localized nature of the quadrupole displacements is preserved by using thin lenses and the super-position principle for the kick effects. The causality principle applied to the downstream beam oscillations due to the kicks is introduced via Heaviside functions. The treatment presented provides formulae for the tuneshift in the bunch and first-order solutions for the transverse beam off-sets within the bunch. It presents a break-through in the recent efforts to solve the problem of the bunch stability theoretically, with realistic beam and linac models

Analytical Treatment of Single Bunch Transverse Dynamics in Linacs with Wakefields

In this paper we present an analytical treatment of the equation of motion of single bunch particles traveling in the linac of a linear collider, in the presence of wakefields. Using a somewhat simplified model, the equation is solved for a Gaussian distribution of charge, a linear variation of the wakefield along the bunch, a smooth focusing and in the absence of acceleration. The specificity of the method consists of preventing the appearance of artificial secular terms and keeping at any stage the intrinsic tune-shift that characterizes the problem and stabilizes the motion. Hence, a first order perturbation becomes sufficient provided that a non-standard perturbation expansion, specially developed for this analysis, be however applied. Solutions for the particle off-sets within the bunch are obtained and their contributions to the effective emittance calculated. The results explain the observed features of beam breakup and BNS damping, and reproduce the bunch dipole-oscillations visible on the animated graphics after simulations with the code MUSTAFA. This treatment provides in addition a closed expression for the tune shift along the bunch and confirms the existence of an optimum BNS damping setting which differs from autophasing in a single bunch mode

Beam Characteristics Versus Cavity Models in CLIC

The luminosity requested for linear colliders with center-of-mass energies exceeding 500 GeV, implies a multibunch operation be considered in order to limit the RF power consumption. In the Compact Linear Collider (CLIC), though the repetition rate is high, a train of at least 20 bunches is necessary to obtain the performance needed for the experiments. Since at high RF frequency the wakefields are large, beam break-up is critical and stability simulations have been carried out for different pre-estimated models of detuned and/or damped cavities. The results obtained give indications about the wakefield level that should not be exceeded along the train, to avoid significant emittance growth. They also show the sensitivity to some specific parameters and the dependence on the scaling of focusing with energy. Eventually, they are used as guide lines for accelerating structure development and as a basis for a possible set of CLIC parameters