An optical resonator length of free electron lasers must be exactly matched with the frequency of the repeated electron beam from the accelerator driver so the incident electron beam can overlap with stored light pulses in the resonator. We made the exact resonator length matching by injecting ultrashort laser light pulses in the resonator as external clock signals. The external laser was a mode-locked Ti:Sapphire laser with 80 fs pulse width and synchronized with the accelerator clock signals. With the simultaneous use of a streak camera and a photo diode, we succeeded in making the quick matching within an accuracy of 1 μm or less in a direct and exact manner.
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