Short Rayleigh Length Research Articles

Glass welding with ultra-short laser pulses and long focal lengths A process model

The welding of glasses with ultra-short laser pulses and high-NA optics has matured to a well-established process. The short focal length requires relatively thin work pieces in the order of 1-2 mm, and the short Rayleigh length requires a very precise adjustment of the laser focus relative to the interface to be welded.To make welding of thick samples possible with relaxed alignment requirements, we welded optically contacted fused silica with a focal length of 65 mm. The resulting welding seams can be as deep as 2 mm and more. It turned out, that they grow from the focal point towards the laser source with increasing pulse energy and irradiation time. A thermal model for the growth of the molten zones will be presented and compared to the process with short focal lengths. Furthermore, the results of tensile tests and helium-leakage tests of welded fused silica samples will be presented.

Short Rayleigh length free electron laser: Experiments and simulations

We report experiments at Jefferson National Accelerator Facility (Jlab) and computer simulations performed at the Naval Postgraduate School (NPS) designed to probe the small Rayleigh length regime. We compare the gain, power, and sensitivity to mirror and electron beam misalignments as a function of decreasing Rayleigh length. The agreement is quite good, with experiments and simulations showing comparable trends as the Rayleigh length is decreased. In particular, we find that the gain and power do not decrease substantially at short Rayleigh length, contrary to a common Gaussian-mode filling factor argument. Within currently achievable alignment tolerances, the gain and power are still acceptable for FEL operation.

Stability of a short Rayleigh length laser resonator

Motivated by the prospect of constructing a short Rayleigh length free-electron laser in a high-vibration environment, we demonstrate the use of a collection of rays to study the effect of mirror vibration and distortion on the behavior of the fundamental optical mode of a cold-cavity resonator. We find that the ray collection accurately describes both on-axis and off-axis optical beams. We show that a tilt or transverse shift of a mirror causes the optical mode to rock about the original resonator axis, while a longitudinal mirror shift or a change in the mirror's radius of curvature causes the beam diameter at a mirror to successively dilate and contract on the mirror. Results are in excellent agreement with analytic calculations and wave front propagation simulations as long as the mirrors remain large with respect to the beam diameter.

Single-mode simulations of a short Rayleigh length FEL

Free electron lasers can make use of a short Rayleigh length optical mode in order to reduce the intensity on resonator mirrors. A simulation method is used that includes the dynamics of this rapidly focusing optical mode and the macroscopic and microscopic electron evolution. The amplitude and phase of the optical fields are represented by a single Gaussian mode. The simulation runs in seconds on small laptop computers and can be used for system analysis.

Simulations of high-power free electron lasers with strongly focused electron and optical beams

A high-power free electron laser (FEL) is being designed in collaboration with Jefferson Laboratory, University of Maryland and Advanced Energy Systems, using short Rayleigh-length resonators to increase the spot size at the mirrors and hence avoid mirror damage. A short Rayleigh length implies a very small optical mode waist in the center of the cavity. It may be desirable to strongly focus the electron beam as well, to improve overlap with the intense optical fields in the interaction region. Three-dimensional simulations are used to study the effects of varying the electron beam radius and angular spread to enhance FEL gain and efficiency. The effects of off-axis shifting and tilting of the electron beam are also studied.

A study of the stability of a high-power free electron laser utilizing a short Rayleigh length

In order to avoid mirror damage on a high-power free electron laser (FEL), the design can utilize a short Rayleigh length optical cavity in combination with a short magnetic undulator. The short Rayleigh length increases the mode area and reduces the intensity at the mirrors, and also alters the basic FEL interaction and the stability of the laser itself. In particular, mirror misalignment may significantly affect the behavior of the cavity modes. We present simulations showing the effect of mirror tilt on the performance of 100kW and 1MW FEL designs with short Rayleigh lengths.

The free electron laser interaction with a short-Rayleigh-length optical mode

High-power, short-wavelength free electron lasers (FELs) can make use of a short-Rayleigh-length (SRL) optical mode in order to reduce the intensity on resonator mirrors. The conventional FEL interaction attempts to optimize the coupling between the electron beam and optical mode by minimizing the optical mode volume around the electron beam. In contrast, the SRL FEL focuses optical power in a small region of the undulator, which accelerates the electron bunching process. As a result, the fundamental FEL interaction is significantly altered with a rapidly changing optical field and phase along the undulator. Advantages and disadvantages of FELs designed with an SRL optical mode are discussed.

Simulations of the 100 kW TJNAF FEL using a short Rayleigh length

The TJNAF FEL can be upgraded to operate at 100 kW average power and then explore the use of a short Rayleigh length in order to reduce the power density on the resonator mirrors. The short Rayleigh length can only work with a relatively short undulator. Multimode simulations are used to self-consistently model the optical mode interaction with the electron beam. The steady-state resonator mode is affected by the complex, non-linear electron beam evolution as well as the resonator design.

An undulator with non-adiabatic tapering for the IFEL project

We describe the design of a planar undulator with unusually strong tapering, for the inverse FEL experiment (on the IFEL experiment at the UCLA Neptune Lab. Presented at the 2001 Particle Accelerator Conference, June 18–22, 2001, Chicago, Illinois) to be carried out in Neptune Lab. (Nucl. Instr. and Meth. A 410 (1998) 437) at UCLA. A powerful TW CO 2 laser will be used to accelerate electrons up to 50–60 MeV in 50 cm long undulator. A strong undulator tapering is needed because of the short Rayleigh length of the laser beam. Both the magnetic field and the undulator period are tapered to provide synchronicity of the laser beam interaction with a captured electron bunch along the whole undulator length. The most critical part of the undulator is the region near the laser focus. The main characteristics of the IFEL, such as the percentage of trapped electrons, energy of accelerated electrons and sensitivity to the laser focus transverse position, are given. The general principles of the design of this undulator construction can also be useful for high efficiency FEL amplifiers of intense laser modes.

Free electron lasers with short Rayleith length

To decrease power loading on cavity mirrors, a Gaussian mode with a very short Rayleigh length could be employed. We study the physics of the FEL interaction while shortening the Rayleigh length in the FEL oscillator. The results can be applied to the Industrial Laser Consortium's UV FEL and to high average power FELs in general.