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We used a 47 MHz femtosecond fiber oscillator centered at 1558 nm (5 nm bandwidth). Feeding the pulse train into a Mach Zehnder interferometer and adding electro optic modulators (EOMs) in both optical paths, we imprint a π phase shift in one path on every other pulse. We then use a polarization beam splitter to combine the two beams and stabilize the resulting polarization with a unity gain significantly lower than MHz. Therefore after the two beams are combined, the π phase shift stays intact and neighboring pulses have their polarization flipped. In a following PBS horizontal and vertically polarized pulses are separated. The vertical polarized pulses are delayed and combined with the next pulse. This halves the repetition rate basically acting like a pulse picker but without discarding 50% of the output power. The efficiency of the experiment was so far limited by linear losses in the PBS and linear losses due to the pickups for the photodiodes. We will give an update un the latest results and our plans to go forward. |
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