PURPOSE: To compress light pulse width by putting two light guides closely to each other in a nonlinear medium whose refractive index varies with the square of the electric field intensity of a light wave.

CONSTITUTION: A coupling area 23 wherein the nonlinear medium is interposed between the adjacent light guides 21 and 22 is formed. The low input power part of a light pulse 24 inputted to the light guide 21 is coupled with the light guide 22, and the high power part is not coupled with the light guide 22 and appears as the output of the light guide 21. For the purpose, the length of the light guides 21 and 22 in the coupling area 23 is specified to normalized porpagation length Z=π/2, and when a light pulse shown by a waveform 24 is inputted, the high power part if not coupled with the light guide 21, and only the low power part is coupled with the light guide 22. Consequently, a light pulse shown by a waveform 25 is outputted to the light guide 21 and a light pulse shown by a waveform 26 is outputted to the light guide 22. Then the light pulse width of the output of the light guide 21 is compressed.