Theoretical and experimental study of passive spatiotemporal shaping of picosecond laser pulses

Abstract

We report the results of theoretical and experimental studies on passive spatiotemporal shaping of cw mode-locked picosecond laser pulses for driving the photocathode of a high-brightness, high-current energy recovery linear accelerator. The temporal pulse shape is modified using birefringent crystals, while a refractive optical system is used to generate a flattop spatial beam profile. An optical transport system is designed and implemented to deliver the flattop pulse onto a photocathode sited 9 m away from the shapers. The alignment tolerances on the beam shaper and the temporal pulse stacker have been studied both theoretically and experimentally. The experimental results agree well with theoretical simulations.

ICFA Beam Dynamics Newsletter

“…The development of the necessary spatial and temporal shaping techniques is an ongoing project: proof-of-principle experiments have been successfully carried out with a laser of similar pulse width, operating at 532 nm and 81.5 MHz. A transport line has been designed and built and the propagation of a shaped pulse through it to the photocathode simulated and tested experimentally [4]. As the performance of the complete photocathode drive system is critical for ERL operation, an extensive set of diagnostics will be in place to monitor and maintain its performance.…”

ERL R&D: Laser and Laser Light Transport

“…There are two approaches to doing this. One method is to use a number N of birefringent crystals to progressively duplicate and timeshift pulses such that the resulting series of 2 N pulses overlap in such a way as to sum to a flat top pulse [2][3][4]. This technique is illustrated for an eight-pulse series in Fig.…”

“…Further experimental details can be found in Ref. [4] Static phase differences between replicas can arise from unequal path lengths traversed by the ordinary and extraordinary pulses, as a result of the tilt of the crystal plane. These phase differences can change over time due to thermal expansion and temperaturedependent birefringence of the crystals, particularly since the optical absorption in this case is significant (32% over all 3 crystals).…”

Adaptive electron beam shaping using a photoemission gun and spatial light modulator

“…This shaper worked very well when supplied with a good quality Gaussian beam with a precise initial size and divergence, but it was not realistic to use this in practice with our nonideal input beam, and more importantly it does not allow for a tunable shape. See also [19]. Similarly, we also tested an engineered diffuser (Thorlabs ED1-C20) which produces a uniform flattop speckle pattern for an arbitrary input beam.…”