High Average Power, High Energy, Femto-second Fiber Chirped Pulse Amplification System

1. High Average Power, High Energy, Femto-second Fiber Chirped Pulse Amplification System F. He, J. H. V. Price, A. Malinowski, A. Piper, M. Ibsen, D. J. Richardson Optoelectronics Research Centre University of Southampton, Southampton, UK J. W. Dawson, C. W. Siders, J. A. Britten, C. P. J. Barty Lawrence Livermore National Laboratory, 700 East Avenue, Livermore, USA

2. Outline • Background and motivation • Experimental setup • Fiber oscillator • Chirped Fiber Bragg Grating (CFBG) • Large Mode Area (LMA) fiber • Modelling tools for bandwidth optimization • Dielectric grating compressor • Experimental results • Conclusion and future work

3. Background and Motivation • Fiber Chirped Pulse Amplification (CPA) system with high-quality, high-energy femto-second pulses, at high average power. • Applications: industrial materials processing, etc. • Fiber systems: high gain, compact and robust, less thermo-optical problems at high power. • Limitations: nonlinear effects • Compact • Fiber oscillator and stretcher • The key for achieving high Pave + E • Dielectric grating

4. Experimental setup Novel technologies: • Mode-Locked Fiber Oscillator • CFBG (designed with both 2nd and 3rd order dispersion compensation matched to the compressor) • Modelling tools for bandwidth optimization • Dielectric Grating

5. Mode locked fiber oscillator pump SESAM HR Yb+3 Fiber • Diode pumped • Robust self start (SESAM) • Polarization rotation mode-locking • Pulse energy ~ 60pJ at ~50 MHz • Low amplitude noise See: L. LEFORT, et Al., Optics Letters, v27 pp291 (2002)

6. Chirped Fiber Bragg Grating • Fiber stretcher • Designed with both 2nd and 3rd order dispersion compensation matched to the compressor • Bandwidth ~12nm • ~2.5dB reflectivity variation • Minimal spectral ripple (measured at 0.01nm resolution) See also: G.IMESHEV, Optics Letters, v29 pp679 (2004)

7. Large Mode Area Fiber • Core: diameter=20µm, N.A.=0.06 • Inner cladding: diameter=200µm, N.A.=0.5 • V=3.77 • Output M2~1.06 at bending diameter~11cm • Slope efficiency ~77%

8. Bandwidth Modelling • A modelling tool was used for predicting the performance of complex Yb-fiber amplifier systems (prediction accuracy shown for single amplifiers below). • Simulations were applied to the design of our broad bandwidth multi-stage amplifier system. See: F.HE, et Al., Optics Express v14 pp12846 (2006)

9. Dielectric Grating • Advantages: • High efficiency • High compression factor (larger size compared with silica transmission gratings) • High damage threshold (~10 times of gold gratings) • High average power handling • Specifications of the grating in use • Groove density ~1780 l/mm • Size ~10cm*20cm • Efficiency ~95% See: J.BRITTEN, et Al., QELS 2005, JFB5

10. (10 MHz) Experiment results • Average output power ~135 W (at final amplifier output) • Pulse energy ~13.5 µJ (at repetition rate of 10 MHz) • Pulse FWHM ~360 fs (assuming Gaussian shape) • Time bandwidth product ~0.6 • Compressor overall transmission efficiency ~52% • Spectral clipping due to the size of the grating (performance has been improved now)

11. Progress and Future work • Compressor optimization • Spectral clipping is removed by applying larger gratings and mirrors • Nonlinear effects • SPM • B-integral ~ π @13.5µJ • Pulse distortion observed @20µJ • SRS • We observed at pulse energy ~ 200µJ • Future work • System with high average power (>100W) and high pulse energy (>100 µJ).

12. Conclusion • We demonstrated an Yb-fiber CPA system incorporating a CFBG stretcher, a cascaded chain of bandwidth-optimised amplifiers and a dielectric grating compressor. • The system produced 135 W average power with pulse energy of 13.5 µJ. The recompressed pulse duration was 360 fs. • Robust source of high pulse energies at high average powers should further broaden the applications of femto-second fiber-based CPA systems.