Patent ID: 7869600

Claim:
An optical transmitter for transmitting cryptogram by an optical signal, the optical transmitter comprising: a central processing unit configured to operate as: a pseudo-random number generation section for generating a pseudo-random number sequence from an initial key; a basis selection control section for retaining a basis group including a plurality of bases and for selecting a basis from the basis group in accordance with the pseudo-random number sequence; and an optical intensity modulation section for generating an optical signal modulated by an intensity which is based on the selected basis and on a logical value of transmission data, wherein the plurality of bases included in the basis group are positioned such that at least two bases of the plurality of bases each have intensities adjacent to each other across a middle point of a maximum intensity and a minimum intensity, that the intensities adjacent to each other include two logical values to be represented by the transmission data, and that each of the plurality of bases included in the basis group has a high intensity and a low intensity between which a difference is set to be a certain value smaller than half a difference value of the maximum intensity and the minimum intensity.