A photonic QPSK modulation in 2 GHz with an RF signal from a microwave optoelectronic oscillator

This article presents a photonic circuit that achieves direct carrier QPSK modulation of an RF signal from an opto-electronic oscillator (OEO) suitable for satellite data transmitter. The circuit was implemented with three Mach-Zehnder optical modulators. One modulator was used to construct an OEO to generate a carrier with a frequency equal to 2.019 GHz. The others two modulators were used to compose the microwave photonic I/Q modulator. The setup initially achieved a data rate of 50 Mbps in a QPSK scheme with an EVM of 7.5%. Using a root-raised-cosine filter, it was possible to reduce the EVM to 5%. The diagrams of the optoelectronic circuit and the modulation measurements are displayed. After the EVM analysis, the modulated signal was demodulated through an actual satellite receiver. Using the demodulator, it was possible to measure the BER and consequently the degradation analysis. The degradation for a bit rate of 100 Mbps was 0.07 dB using the OEO as RF generator. In this case, the values of Eb/N0 and BER were 11.37 dB and 10^-7, respectively.