THE LASER SYSTEM FOR THE ERL ELECTRON SOURCE AT CORNELL UNIVERSITY * Dimitre G. Ouzounov # , Ivan V. Bazarov, Bruce Dunham, and Charles Sinclair, LEPP, Cornell University, Ithaca, NY 14853 Shian Zhou and Frank W. Wise, Department of Applied Physics, Cornell University, Ithaca, NY 14853. Abstract A master oscillator-power amplifier (MOPA) laser system has been developed to meet the requirements of the Cornell ERL high brightness electron photoinjector. The system is comprised of a Yb-fiber laser oscillator, a single-mode fiber pre-amplifier and a double-clad, large mode area fiber amplifier. The system provides 5 watts average infrared power in a train of 3-ps pulses at 50 MHz. These pulses are efficiently frequency-doubled to produce 2-ps pulses at 520 nm with more than 40 nJ energy per pulse. We demonstrate an efficient longitudinal pulse shaping technique that allows us to achieve 20- to 40-ps-long, nearly flat-top pulses with sharp rise and fall times. We characterize the noise properties of the fiber oscillator and discuss the route to upgrade the system to achieve 130-watt average infrared power and a repetition rate of 1.3 GHz. INTRODUCTION Cornell University is developing a high brightness, high average current electron source for the injector of an Energy Recovery Linac (ERL) based synchrotron radiation source. The source is a very high voltage DC electron gun with a negative electron affinity photoemission cathode [1]. The production of high charge, low emittance electron beam imposes challenging requirements on the laser illuminating the photoinjector. The necessary lasers are state-of-the-art, operating at GHz repetition rates, with high pulse energies, temporally and spatially shaped pulses, and well synchronized to an external master clock [1]. The Cornell photoinjector is designed to deliver a 100 mA average current at repetition rate of 1.3 GHz. The drive laser has to match at this repetition rate and to provide 15-nJ-pulses at a central wavelength of ~500 nm with a duration of 20 to 40 ps. To handle nonlinear space charge effects, the pulses must to be shaped to flat-tops in both space and time [2]. As an intermediate step, to meet the initial needs for injector development, we built a laser system that operates at the 26 th sub-harmonic of 1.3 GHz with pulses meeting all the requirements of the final system. The system is a MOPA type with a fiber oscillator and two fiber based amplification stages. The fundamental pulse width is ~ 2.5 ps FWHM and nearly Gaussian. This pulse is split and differentially delayed in a series of birefringent crystals of differing thicknesses to produce a nearly flat top temporal profile with fast rise and fall times. The final pulse width is measured by cross- correlation. The pulses are transversely shaped by a commercial aspheric lens system. All laser parameters essential to the performance of the electron source the have been demonstrated, and a full power system operating at the 50 MHz repetition rate is installed for electron beam measurements. LASER SYSTEM DESCRIPTION The system schematic is shown in Fig. 1. The seed is a Yb-doped fiber laser built in a ring cavity. The net dispersion is large and anomalous and the laser operates in the soliton regime. The output of the oscillator is first amplified in a single-mode Yb-fiber pre-amplifier. The second amplification stage is a double-clad, large mode area Yb-doped fiber. This fiber (Liekki Yb1200- 30/250DC-PM) is polarization maintaining with a core diameter of 30 micron and a fiber length of 3.5 meters. The amplifier is pumped by a fiber-coupled semiconductor laser diode stack (LIMO) operating at 976 nm that can deliver 25 watts maximum power. We pump the amplifier with up to 18 watts which should allow us to run the laser system stably for a long time. Figure 1. Schematic of the laser system. A BBO Pockels cell with essentially no acousto-optical ringing is used to reduce the electron beam duty factor for beam measurements without changing the parameters of the individual pulses reaching the photocathode. The ___________________________________________ *Work supported by NSF PHY-0131508 # dgo4@cornell.edu MOPAS044 Proceedings of PAC07, Albuquerque, New Mexico, USA 07 Accelerator Technology Main Systems 530 T24 Lasers 1-4244-0917-9/07/$25.00 c 2007 IEEE