Absolute measurement of the resonator length of the free electron laser at the S-DALINAC with a modulated semiconductor laser

A simple method for an absolute measurement of the cavity length of a Free Electron Laser (FEL) with a non-contact optical technique is presented. The injection current of a semiconductor laser diode is modulated with a proper frequency and amplitude resulting in a high-frequency (several GHz) intensity modulation of the emitted laser light. This effect is used to determine the absolute distance between the two positions of the FEL cavity mirrors. The method is based on the subsequent measurements of the phase difference (corresponding to the modulation frequency) between two laser beams reflected from different positions of the resonator mirrors. In this way, the length of the optical cavity of the FEL at the superconducting Darmstadt electron accelerator (S-DALINAC) has been determined with an absolute accuracy of less than ±100 μm at a total length of 15 m corresponding to a relative accuracy of better than ±10−5.