High-gain Regime Research Articles

Pulse propagation solution for a quantum free electron laser

We calculate the exact solution of the propagation equations describing a quantum free electron laser. The solution is obtained by evaluating the residue in the essential singularities of the Fourier-Laplace transformed optical field, and is expressed as a complete series of special functions. The classical and quantum limits of the solution are discussed and an asymptotic form, valid in the high-gain regime, is obtained.

Interplay of the chirps and chirped pulse compression in a high-gain seeded free-electron laser

In a seeded high-gain free-electron laser (FEL), where a coherent laser pulse interacts with an ultrarelativistic electron beam, the seed laser pulse can be frequency chirped, and the electron beam can be energy chirped. Besides these two chirps, the FEL interaction introduces an intrinsic frequency chirp in the FEL even if the above-mentioned two chirps are absent. We examine the interplay of these three chirps. The problem is formulated as an initial value problem and solved via a Green function approach. Besides the chirp evolution, we also give analytical expressions for the pulse duration and bandwidth of the FEL, which remains fully longitudinally coherent in the high-gain exponential growth regime. Because the chirps are normally introduced for a final compression of the FEL pulse, some conceptual issues are discussed. We show that to get a short pulse duration, an energy chirp in the electron beam is important.

Long and short light pulses propagation in a long free electron laser: Effects and possible applications in high-field physics

We have studied propagation of Gaussian electromagnetic pulses through a long undulator free electron laser (FEL). Our analysis takes into account both the active and dispersive properties of the laser active medium. We have introduced the definitions of the FEL operating bandwidth and characteristic time. With the last definition a natural time scale has been established, and this allowed us to differentiate the input signals as long and short pulses. We have established that the results of the conventional theories, based on the model of a monochromatic wave, are applicable for pulses of finite but long duration. The propagation of pulses of finite lengths has been examined analytically in the (a) small spectral gain and (b) high spectral gain regimes. We have discovered a stabilization effect in the case (a). Namely, short pulses, which have random initial lengths at the laser entrance, will have the same duration at laser exit. On the other hand, the propagation of short pulses results in a nearly harmonic modulation of their envelopes. In the case (b) we have calculated the light phase and group velocities and ascertained that the laser active medium is like a plasma medium in the amplification regime; whereas it is simultaneously similar to plasma and resonant atom medium in the absorption regime. In the amplification regime, the pulse propagation is accompanied by a smooth increasing of its duration. On the contrary, the FEL can operate as a slight compressor or explosive stretcher in the absorption regime. Besides, the amplification and absorption regimes provide a linear chirping of light pulses and controlling the pulse peak amplitude. Thus the long FELs might be in principle employed to control the amplitude, phase, duration and profile of superintense short pulses, which damage ordinary atom media.

Dispersion relation and growth rate for a high gain ion-channel FEL with a helical wiggler pump

A theoretical description of a wiggler pumped ion-channel free electron laser (WPIC-FEL) in high-gain regime is presented. The analysis involves a perturbation of the Vlasov–Maxwell equations about the constant-velocity helical trajectories, and the general deriving currents are derived for this configuration. The complete dispersion equation is then obtained for a mono-energetic beam. An extensive numerical analysis is presented for a wide range of operating parameters.

A Reexamination of Deep Diffused Silicon Avalanche Photodiode Gain and Quantum Efficiency

Traditionally the measured gain of an avalanche photodiode (APD) has been considered the product of two parameters. The electron multiplication process being one and the quantum efficiency (QE) the other. The multiplication process is often considered wavelength dependent and the QE being bias independent. We propose a new examination of these related parameters where the APD gain is considered intrinsic, defined as being the amount of electron multiplication each photoelectron undergoes based only on the applied bias and is thus independent of the incident photon wavelength, and where the QE, which can be defined as the number of generated photoelectrons per number of incident photons, is considered intrinsic being a function of not only wavelength, but also bias. This is a more logical, and physically real, perspective of APD behavior. We introduce a technique to measure the intrinsic gain and the intrinsic QE of deep diffused silicon APDs. Once the intrinsic gain vs. bias of the APD is measured, it becomes possible to use this measurement as an absolute parameter. With the intrinsic gain known, we show how the intrinsic QE of an APD, for a given wavelength, changes as a function of bias. We show that when the APD is operated from the low to high gain regime, light at 400 nm to 800 nm experiences an increase in QE. These fundamental, dynamic and operational properties of APDs are critical when considering the wavelength(s) that are of interest for a given application

Analytical description of free-electron-laser oscillations in a perfectly synchronized optical cavity

We present an analytical description of free-electron-laser (FEL) oscillations in a perfectly synchronized optical cavity by solving the one-dimensional FEL equations. It is shown that the radiation stored in the cavity eventually evolves into an intense few-cycle optical pulse in the high-gain and low-loss regime despite the lethargy effect. The evolution of the leading slope of the optical pulse, which is defined from the front edge toward the primary peak, is found to play an important role in generating the intense few-cycle optical pulse. The phase space evolution of electrons which interact with the leading slope is solved analytically in a perturbation method, leading to an analytical solution for the optical pulse evolution. The peak amplitude and the pulse length at saturation are found to scale with the electron beam density and optical cavity loss. Those scalings agree well with the intense few-cycle pulses recently observed in a high-power FEL.

Self-amplified spontaneous emission FEL with energy-chirped electron beam and its application for generation of attosecond x-ray pulses

Influence of a linear energy chirp in the electron beam on a SASE FEL operation is studied analytically and numerically using 1-D model. Explicit expressions for Green's functions and for output power of a SASE FEL are obtained for high-gain linear regime in the limits of small and large energy chirp parameter. Saturation length and power versus energy chirp parameter are calculated numerically. It is shown that the effect of linear energy chirp on FEL gain is equivalent to the linear undulator tapering (or linear energy variation along the undulator). A consequence of this fact is a possibility to perfectly compensate FEL gain degradation, caused by the energy chirp, by means of the undulator tapering independently of the value of the energy chirp parameter. An application of this effect for generation of attosecond pulses from a hard X-ray FEL is proposed. Strong energy modulation within a short slice of an electron bunch is produced by few-cycle optical laser pulse in a short undulator, placed in front of the main undulator. Gain degradation within this slice is compensated by an appropriate undulator taper while the rest of the bunch suffers from this taper and does not lase. Three-dimensional simulations predict that short (200 attoseconds) high-power (up to 100 GW) pulses can be produced in Angstroem wavelength range with a high degree of contrast. A possibility to reduce pulse duration to sub-100 attosecond scale is discussed.

Analytical solution for phase space evolution of electrons operating in a self-amplified spontaneous emission free electron laser

I present an analytical solution for the phase space evolution of electrons in a self-amplified spontaneous emission (SASE) free-electron laser (FEL) operating in the linear regime before saturation in the resonant case by solving the one dimensional FEL equation together with the solution of the cubic equation, which represents the evolution of the SASE FEL field. The electrons are shown to be bunched around $\ensuremath{\pi}/6$ ahead of a resonant electron every resonant FEL wavelength in the high gain regime. The phase relation is similar to that in a low gain FEL where an electron beam above resonance is injected, explaining the positive FEL gain. The analytical solutions agree well with numerical simulations and are applied to obtain the coherent optical transition radiation (OTR) intensity produced from electron microbunching at FEL wavelength. The coherent OTR intensity is shown to be proportional to FEL intensity.

Recoil-induced resonances in the high-gain regime

An optically dense, anisotropic cloud of ultracold atoms is used to observe recoil-induced resonances in the high-gain regime. Optical gains as large as 30 are realized, limited primarily by pump depletion. We demonstrate a light-amplified all-optical switch based on this effect. The switch exhibits contrasts exceeding 35 dB.

Model for high-gain fiber laser arrays

Recent experiments have shown that a small number of fiber lasers can spontaneously form coherent states when suitably coupled. The observed synchrony persisted for a long time without any active control. In this paper, we develop a dynamical model for fiber laser arrays that is valid in the high gain regime. In the limiting case of a single laser analysis and simulations are presented that agree with physical expectations. Using simulations to examine array behavior we report results that are in qualitative agreement with laboratory observations.

Quantum spatial correlations in high-gain parametric down-conversion measured by means of a CCD camera

We consider travelling-wave parametric down-conversion in the high-gain regime and present the experimental demonstration of the quantum character of the spatial fluctuations in the system. In addition to showing the presence of sub-shot noise fluctuations in the intensity difference, we demonstrate that the peak value of the normalized spatial correlations between signal and idler lies well above the line marking the boundary between the classical and the quantum domain. This effect is equivalent to the apparent violation of the Cauchy–Schwartz inequality, predicted by some of us years ago, which represents a spatial analogue of photon antibunching in time. Finally, we analyse numerically the transition from the quantum to the classical regime when the gain is increased and we emphasize the role of the inaccuracy in the determination of the symmetry centre of the signal–idler pattern in the far-field plane.

Detection of Sub-Shot-Noise Spatial Correlation in High-Gain Parametric Down Conversion

Using a 1 GW, 1 ps pump laser pulse in high-gain parametric down conversion allows us to detect sub-shot-noise spatial quantum correlation with up to 100 photoelectrons per mode by means of a high efficiency charge coupled device. The statistics is performed in single shot over independent spatial replica of the system. Evident quantum correlations were observed between symmetrical signal and idler spatial areas in the far field. In accordance with the predictions of numerical calculations, the observed transition from the quantum to the classical regime is interpreted as a consequence of the narrowing of the down-converted beams in the very high-gain regime.

X-ray laser on the relativistic ion and pump laser beams

In this paper, the possibility of lasing in the X-ray domain by means of a relativistic high-density ion beam and a counter-propagating pump laser field in a high-gain regime is investigated. This deliberation is based on the self-consistent set of Maxwell and relativistic quantum kinetic equations describing the X-ray generation process at which a pump wave resonantly couples two internal ionic levels and the necessity of initial inverse population for lasing vanishes.

Limits to the power scalability of high-gain optical parametric amplifiers

We investigate the power scaling of optical parametric amplifiers in the high-gain regime. With Gaussian or similarly shaped pump beams, the generated beams tend to become narrower than the pump beam, and backconversion distorts the intense central part of the beams before the peripheral parts of the pump beam are efficiently converted. Good beam quality and high conversion efficiency can nevertheless be combined when diffraction is strong enough to keep the diameter of the generated beams comparable to that of the pump beam. In practice, this requires narrow pump beams and consequently limits the peak power. Recent experiments have already exceeded this limit. As a solution, we find that a two-stage optical parametric amplifier can combine good beam quality and efficiency for much wider beams than a single-stage optical parametric amplifier.

Simultaneous near-field and far-field spatial quantum correlations in the high-gain regime of parametric down-conversion

We study the spatial correlations of quantum fluctuations that can be observed in multi-mode spontaneous parametric down-conversion in the regime of high gain. A stochastic model has been solved numerically to obtain quantitative results beyond the stationary plane-wave pump approximation. The pulsed shape of the pump beam and other features of the system, such as spatial walk-off and diffraction are taken into account. Their effect on the spatial quantum correlations predicted by the plane-wave pump theory is investigated, both for near field and far field measurements, in a type I and in a type II phase-matching configuration.

Multiphoton multimode polarization entanglement in parametric down-conversion

We study the quantum properties of the polarization of the light produced in type II spontaneous parametric down-conversion in the framework of a multi-mode model valid in any gain regime. We show that the the microscopic polarization entanglement of photon pairs survives in the high gain regime (multi-photon regime), in the form of nonclassical correlation of all the Stokes operators describing polarization degrees of freedom.

Free-electron laser without inversion in the high gain regime

A ‘phased’ optical klystron drift region developed to obtain free-electron lasing (FEL) without inversion in the small-gain small-signal regime is here applied to the high-gain regime. Numerical simulations show improvement in the gain and the tolerance to electron beam energy spread compared to conventional FEL. Longitudinal phase space dynamics has been studied to show the effect of the drift region on the electron beam.

Comparison of the coherent radiation-induced microbunching instability in a free-electron laser and a magnetic chicane

A self-amplified spontaneous emission free-electron laser (SASE FEL) is a device which is based on the creation of a very intense, relativistic electron beam which has very little temperature in all three phase planes. The beam in this system is described as having ``high brightness,'' and when it is bent repetitively in a magnetic undulator, undergoes a radiation-mediated microbunching instability. This instability can amplify the original radiation amplitude at a particular, resonant wavelength by many orders of magnitude. In order to obtain high brightness beams, it is necessary to compress them to obtain higher currents than available from the electron source. Compression is accomplished by the use of magnetic chicanes, which are quite similar to, if much longer than, a single period of the undulator. It should not be surprising that such chicanes also support a radiation-mediated microbunching interaction, which has recently been investigated, and has been termed coherent synchrotron radiation (CSR) instability. The purpose of this paper is to compare and contrast the characteristics of the closely related FEL and CSR microbunching instabilities. We show that a high-gain regime of the CSR instability exists which is formally similar to the FEL instability.

High-gain FEL on the coherent Bremsstrahlung of a relativistic electron beam in a crystal

In this paper a possible way to achieve lasing in the X-ray domain due to coherent Bremsstrahlung of an electron beam in the crystal is considered. The high-gain regime is investigated when charged particles move close to the crystal lattice plane or axis. The consideration is based on the self-consistent set of the Maxwell–Vlasov equations. We illustrate the resulting formulae with some speculative examples.

Scheme for attophysics experiments at a X-ray SASE FEL

We propose a concept for production of high power coherent attosecond pulses in X-ray range. An approach is based on generation of eighth harmonic of radiation in a multistage HGHG FEL (high-gain high-harmonic free electron laser) configuration starting from shot noise. Single-spike phenomena occurs when electron bunch is passed through the sequence of four relatively short undulators. The first stage is a conventional “long” wavelength (0.8nm) SASE FEL which operates in the high-gain linear regime. The 0.1nm wavelength range is reached by successive multiplication (0.8nm→0.4nm→0.2nm→0.1nm) in a stage sequence. Our study shows that the statistical properties of the high-harmonic radiation from the SASE FEL, operating in linear regime, can be used for selection of radiation pulses with a single spike in time domain. The duration of the spikes is in the range of 400–600attoseconds. Selection of single-spike high-harmonic pulses is achieved by using a special trigger in data acquisition system. The potential of X-ray SASE FEL at TESLA at DESY for generating attosecond pulses is demonstrated. The use of a 10GW-level attosecond X-ray pulses at X-ray SASE FEL facility will enable us to track process inside atoms for the first time.