An optical delay line of a configuration in which a number of components
such as a circulator and an optical coupler is reduced is to be provided.
To this end, the optical delay line has a configuration in which, after an
optical fiber diffraction grating having a core subjected to refractive
index modulation and a cladding partly cleared of its outer circumference
and an optical fiber having a cladding partly cleared of its outer
circumference are brought close to each other so that the cores have
mutually parallel directions of an optical axis to fabricate a directional
coupler, both ends of the optical fiber are connected into an optical
fiber loop. In this way, an optical delay line to generate a specific time
delay for controlling optical signals can be provided in small dimensions
and with a small number of components.
