[en] In order to realize a high repetition rate seeded coherent radiation source, it is necessary to develop a seeding system which works in a continuous mode. Utilizing the longitudinal electric field in a higher transverse mode laser stored in an optical cavity, it is possible to introduce an energy modulation in an electron bunch. Through acceleration and dispersion handling, the modulation at laser wavelength can be converted into a finer density structure. It can be used as the seed of coherent radiation. We are developing a laser system to be used in the laser modulator system. (author)