An array of nonvolatile memory cells includes a plurality of vertically stacked tiers of nonvolatile memory cells. The frequency offset detector comprises a multiplication circuit which multiplies, by a pseudorandom number bit sequence, a reception signal generated by Fourier transformation of an OFDM modulated signal wherein pilot symbols are dispersed and arranged in accordance with four kinds of patterns and periodically transmitted. The terminals include medium terminals locked by medium locking lances fit into an escaping portion between the operating portion and the locking pieces in the retainer. One or multiple optical amplifiers may be incorporated as well between the optical transmitting module and the optical receiving module.