Generation of ultrafast laser pulses by electrooptic modulation

Abstract

The passive mode locking traditionally used frequently for generation of ultrafast optical pulses has a problem in generality and controllability although the method can generate short pulses. On the other hand, it is considered that the method based on electrooptic modulation cannot reduce the pulsewidth and the speed is slow even though the controllability is superior. This paper describes the research on the forced mode‐locking method for possible ultrashort and ultrafast properties and the external direct modulation pulse generation method, even though the highly controllable electrooptic modulation is used. First, the forced mode‐locking method is described in which the locking modulator (mode locker) with a pulse‐like transparency in itself and a deflector and a composite resonator are used. Next, a method is described in which short pulses on the order of subpicoseconds can be generated regardless of the laser bandwidth by means of the pulse‐generating modulator outside the laser cavity. (An example is a modified Fabry‐Perot modulator.) Further, an optical pulse synthesizer with a possibility of generating subpico‐second to several tens of femtosecond pulses is described in which the narrowband large signal electrooptic modulator and a deflector are used for generating optical sidebands with a wide optical bandwidth and they are combined for generation of arbitrary‐shaped optical pulse after the phase and amplitude are controlled. The concept is compared with the experiments. Some methods are still under development.