A comprehensive study of an optical voltage-controlled oscillator (OVCO) is presented. With the help of analyzing the coherent receiving process for the binary phase-shift keying signal in a Costas subcarrier optical phase-locked loops system, the method to evaluate the OVCO performance based on calculating the transfer function of the modulator inside the OVCO is proposed, including the power efficiency and the requirement for received signal bandwidth. In addition, three possible OVCO configurations realized by Mach–Zehnder modulator, IQ modulator, and phase modulator are presented. The means to achieve the optimal performance of the OVCO configurations in practical implementations are theoretically explained, respectively, and their properties and the complexity are summarized. This work provides useful theoretical guides for the OVCO design in a lab experiment or for the production of a commercially integrated OVCO component.
Benefiting from the high spectral efficiency and low peak-to-average power ratio, constant envelope orthogonal frequency division multiplexing (OFDM) is a promising technique in coherent optical communication. Polarization-division multiplexing (PDM) has been employed as an effective way to double the transmission capacity in the commercial 100 Gb/s PDM-QPSK system. We investigated constant envelope OFDM together with PDM. Simulation results show that the acceptable maximum launch power into the fiber improves 10 and 6 dB for 80- and 320-km transmission, respectively (compared with the conventional PDM OFDM system). The maximum reachable distance of the constant envelope OFDM system is able to reach 800 km, and even 1200 km is reachable if an ideal erbium doped fiber amplifier is employed.
As a key component of the subcarrier-based optical phase-locked loop, an optical voltage controlled oscillator (OVCO) suffers a penalty due to various factors, such as the nonoptimal peak drive voltage, the bias voltage deviation, and the inevitable imperfections of the modulator and the driver. We have performed a systematic study to investigate the influence of these factors on the performance of the OVCO. Our theoretical analysis and experimental demonstration show that by setting the peak drive voltage to around 1.172Vπ, employing a proper automatic bias control technology for the Mach–Zehnder modulator, and applying a driver with adequate output saturation voltage, the optimal performance of the OVCO with high power efficiency and stable output can be achieved. Our results may provide useful guides for the design of an OVCO or the production of a commercially integrated OVCO component.
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