The invention relates to a telematics method for helicopters. The sending part converts each width of a plurality of digital input signals into a voltage in accordance with a predetermined weight, generates a send signal by adding voltages converted from the plurality of digital input signals, and outputs the send signal. The coarse-tunable equalizing section coarsely flattens the spectrum of the light in the predetermined wavelength range, and includes a plurality of filters each having a large loss and a small reflectance as compared with the fine-tunable equalizing section. Both of the encryption and decryption keys have the same value.