DEVELOPMENT OF A PHOTONIC CRYSTAL FIBRE LASER AMPLIFIER FOR PARTICLE BEAM DIAGNOSTICS

We present the latest results on the development of a high power fibre laser system for an ILC-specification laser-wire project. The laser consists of a crystal oscillator with wavelength ~ 1 μm that can be synchronised to an external frequency reference followed by chirped pulse amplification in ytterbium doped double clad fibre and photonic crystal fibre. This system produces 1 μJ pulses in an adjustable burst envelope at a chosen frequency. These pulses are further amplified in a large mode area rod-type photonic crystal fibre, allowing amplification to high pulse energies whilst maintaining a single spatial mode. The fibre is pumped in pulsed mode by a specially commissioned 400 W diode laser fixed at the absorption peak of ytterbium at 976 nm, independent of pumping regime. Pumping in a pulsed mode allows the high energies required for laser-wire at MHz repetition rates to be created without the need for active cooling of the laser. The light will be frequency doubled to ~ 500 nm to achieve improved laser-wire resolution. INTRODUCTION A laser-wire is a non-invasive particle beam profile measurement system that uses a high power laser focussed to a small spot to scan across a particle beam. Inverse-Compton scattered photons proportional to the overlap of the laser and particle beam are measured further along the accelerator giving the particle beam profile [1-3]. This paper focuses on the development of a high power fibre based laser system for this purpose. A summary of the requirements for a system suitable for a laser-wire are shown in Tab. 1. Table 1: Required Laser Parameters Parameter Value Repetition Rate 6.49 MHz Pulse Energy 50 100 μJ Pulse Duration ~ 1ps Beam Quality M < 1.1 Wavelength ~ 500 nm Whilst pulse energies of 100 μJ at 6.49 MHz are beyond current fibre laser technology, the system will be operated in a 1ms burst at 5 Hz. This reduces the average power required to within current technology, although this has not yet been demonstrated. EXPERIMENT A test system was built to investigate fibre amplification as shown in Fig. 1. Experiments were conducted using standard step index double clad single and multimode fibres as well as a large mode area rodtype photonic crystal fibre. Figure 1 : Experimental setup. The laser output from a commercial CPA system (Amplitude Systèmes, Bordeaux) [4] was coupled into the core of each fibre from one end and the pump into the inner cladding, in both cases using aspheric coupling lenses. The seed and pump beams were separated using dichroic mirrors with high reflectivity at 1037 nm and high transmission at the pump wavelength of 976 nm. As shown in Tab. 1, a requirement is that the M parameter (beam quality) be < 1.1 and ideally as close to 1 (theoretically perfect) as possible. The M parameter is the ratio of the measured laser beam to that of a Gaussian. For this measurement a 2 m single mode fibre (6.5 μm diameter core) was used. This only supports the lowest order mode which, in a cylindrical waveguide, is a Bessel function and can be well approximated by a Gaussian [5]. To measure the M of the system, the laser output of the fibre was focussed using a 300 mm lens with an iris placed directly before the lens. This ensures a minimal amount of background from any seed light not coupled into the core but carried in the surrounding inner cladding. A CCD camera was used to measure transverse intensity profiles at various z positions through the focal region of the laser beam as shown in Fig. 2. The diameter at 4σ was measured 64 times and the average and corresponding z position plotted. This graph was then fitted to give the M value of the laser beam. The Power meter or M stage Dichroic Mirror Pump Aspheric Lens Single Mode Fibre