A thermo optical type variable optical attenuator suitable for application in an array type variable optical attenuator that realizes low crosstalk between neighboring waveguides in the array is provided. The device contains corresponding interface means for this. The detection sensor device includes a light emitting member and a light receiving member which receives the light emitted from the light emitting member and output a detection signal in accordance with the intensity of received light. Each MUD log includes information for one or more epochs, where the information for each of the epochs indicates a set of one or more regions that have been modified during the corresponding epoch. The data item is then read from the located storage device. The housing is cylinder-shaped and hollow, having a first housing end and a second housing end with a shaft-receiving opening.