Tunable Fiber Laser System in Visible Wavelength Range for Photocathode Drive

The photoinjector is the key component of electron accelerators such as Linac Coherent Light Source (LCLS) at the SLAC National Accelerator Laboratory. The driving laser system that emits laser pulses on the photocathode inside the photoinjector determines the basic properties of the electron beam like sequence in time, their duration, and the amount of bunch charge. Traditional driving laser system consists of femtosecond ultrafast laser amplifier at 1064nm that is quadrupled to 258nm. The laser pulses also need to stretch to picosecond before delivering to photocathode. The whole laser system is complicated, expensive, and not easy for the maintenance. The pulse energy suffers substantial loss to be less than 1 micro-joule after frequency quadrupling and pulse stretching. Next generation photoinjectors for high brightness light sources demands a pulsed laser system that is tunable in visible range and have MHz repetition rate as well as 10s of micro-joule pulse energy. Raytum Photonics proposes to develop a state-of-art fiber laser system that is tunable in visible range with high pulse energy. The laser system consists of a tunable seed module that can tune the wavelength from 1250nm to 1390 nm, a 40 GHz intensity modulator that can generate a 25-30ps pulse, a Pr doped or Pr-Yb co-doped fluoride fiber amplifier to boost the average power to more than 20W and a second- harmonic generation module to covert the final output to visible range and tunable from 625 nm to 695 nm. Our laser system will generate the pulse train with pulse energy of more than 10 micro-joules and repetition rate in MHz. The advantages of fiber laser system is featured in the several aspects: 1) unlike traditional OPA or OPO laser system, the direct wavelength tuning of semiconductor laser is more reliable and easier to operate; 2) the direct intensity modulation to generate pulse provides flexibility of timing properties like pulse width, pulse shape and repetition rate. The direct ps pulse generation does not require the pulse stretcher used in fs lase system. 3) The fiber amplifier is well-known for its ability of power scaling. 100 W or even kW fiber laser amplifier with fs pulse width has been demonstrated in 1um wavelength domain. During Phase I, Raytum Photonics will concept design and breadboard demonstrate the fiber laser system with the following features: 1) tunable diode laser working around 1300nm and shows the pulse generation using high speed intensity modulator; 2) Pr doped or Pr-Yb doped fiber amplifier with average output power at W level and shows the power scalability; 3) demonstration of second harmonicgeneration with tuning fundamental laser wavelength using either traditional BBO crystal or PPLN crystal. The temperature of crystal needs to adjust for the requirement of phase matching when the fundamental wavelength is tuned. All development work will conduct in Raytum Photonics facility. We are also going to collaborate with SLAC to have a preliminary test with the laser system we develop.