  A novel microwave photonic scheme for secure data transmission in optical
networks is proposed. The security of the scheme is guaranteed by physical
encryption and decryption via the temporal Talbot effect in dispersive mediums.
First, the original data is randomized in the digital domain by performing an
exclusive OR operation using a random matrix. Subsequently, a time-varying
multi-tone electrical signal, which represents the randomized data matrix, is
modulated onto an optical carrier. The optical signal after modulation is then
phase-modulated by a temporal Talbot array illuminator (TAI) signal, and the
optical signal after discrete quadratic phase modulation will lose its original
appearance in the frequency domain and be further dispersed in the first
dispersive medium. Due to the dispersion that does not match the TAI signal
exactly, the waveform after the first dispersive medium is a noise-like signal.
Hence, the physical encryption of the original data is successfully achieved.
As the optical signal passes a second dispersive medium that makes the total
dispersion match the TAI signal, the temporal waveform of the noise-like signal
after photodetection is transformed into pulses. "1" and "0" in the randomized
data matrix are represented through the presence and absence of pulses, and the
physical decryption is achieved. By further processing the recovered data
matrix using the random matrix, the original data can be recovered. The
physical layer security of the proposed scheme and its fiber transmission
capability are demonstrated. 8-Gbit/s data is transmitted, encrypted, and
decrypted using two dispersive mediums and an optical fiber of 10 to 200 km,
and error-free transmission is achieved. Many factors that affect the
encryption, decryption, and transmission performance of the system have been
analyzed.
