Femtosecond pulses from coherently combined parallel chirped pulse fiber amplifiers Leo A. Siiman, Tong Zhou, Wei-Zung Chang, and Almantas Galvanauskas Center for Ultrafast Optical Science, University of Michigan, Ann Arbor, MI 48109-2099 lsiiman@eecs.umich.edu Abstract: Active coherent combining of femtosecond pulses from parallel chirped-pulse fiber amplifiers is demonstrated. This opens a new path for simultaneously increasing both energy and average power of ultrashort pulses from fiber based systems. OCIS codes: (140.3510) Lasers, fiber; (140.3298) Laser beam combining 1. Introduction Although fiber lasers offer a remarkable potential for average power scaling, but pulse energies are limited due to the relatively small transverse aperture of a fiber core. Consequently, femtosecond pulses from fiber chirped-pulse amplifier (FCPA) systems are limited in energy to approximately a millijoule level at most; with insignificant prospect of achieving any substantially larger pulse energies by simply increasing fiber amplifier core size. However, orders-of-magnitude increase in pulse energy is necessary, in order to access emerging applications associated with high-intensity laser-plasma interactions, such as laser-plasma accelerators, laser wakefield X-ray generation, etc. Use of fiber lasers for these applications would be highly desirable, since this would enable to overcome critical average-power limitations of existing solid-state laser drivers. Coherent combining of multiple fiber laser outputs offers a way to exceed the performance of a single fiber. However, up to now only cw long-pulse coherent combining schemes have been demonstrated [1, 2], all based on narrow-spectrum signal combining. Therefore, it remains unclear whether active coherent combining schemes can be extended to broad-band ultrashort-pulse amplifiers, where both the phase and delay matching between parallel channels has to be achieved. Here we achieve coherent combining of two parallel FCPA channels, thus demonstrating the extension of coherent-combining schemes into femtosecond-pulse domain. 2. Experiment The complete experimental setup is shown in Fig. 1. An All Normal Dispersion (ANDi) fiber laser oscillator generates seed pulses at 1057 nm with 10 nm spectral bandwidth and 46 MHz repetition rate, which are stretched in a standard folded-configuration Martinez-type pulse stretcher. A 50:50 beam splitter then divides the stretched pulses into two separate PM single-mode fiber amplifier paths. Each path begins with a fiber coupled electro-optic lithium niobate phase modulator and consists of standard single mode PM fiber amplifiers pumped by telecom- standard 600mW pump diodes. The total length of fiber per path is about 10 m. Each fiber amplifier path is set to emit equal amplified signal power at a level below the onset of nonlinear effects in single mode fiber. The output of one path includes an additional delay line for matching the pulse delay of the two paths. A second 50:50 beam splitter is used for recombining the amplified pulses. Consequently, this scheme acts as a Mach-Zhender interferometer, producing 100% power combining after the re-combining splitter when both beams are in phase, and 50% of total power when both beams are not phased coherently. The combined output is sent to a standard Treacy- type diffraction grating compressor. The resulting combined and compressed pulses are monitored through 3.5% Fresnel reflection from a glass window, which is directed into a photodetector for phase-locked-loop feedback control. The feedback loop uses a reference modulation signal, set to 5 kHz, to generate an error signal for controlling the phase modulator that compensates for drifts in the relative phase difference between the two paths. Although only one phase modulator is needed to correct for the relative phase difference between the paths, the other (unused) phase modulator is used here to balance the dispersion introduced by lithium niobate. This modulator will be used for the further extension of the work to larger channel numbers. Compressed pulse duration is characterized using standard second-harmonic autocorrelator. 3. Results and Discussion Figure 2 shows the measured output power from the compressor versus time for phase locked and free running operation. We see that the phase locked loop is able to compensate phase drifts. As expected, the locked power is approximately 4 times larger than the power from each individual path, consistent with the complete coherent combining of the two beams. OSA/ CLEO 2011 CMD2.pdf