Study On Optical DPSK Modulation And Stability Of MZ Interferometer

Modulation plays an important part in the communication system. The differential phase shift keying modulation (DPSK), one kind of phase modulation, transmits digital signals by the difference between fore-and-aft signal elements' carrier phases. It will be the most potential modulation format in the future communication networks with high speed and large capacity, thanks to its advantage of high sensitiveness and instinctively resisting damages to signals. Besides, it overcomes anti-phase state appearing in phase shift keying modulation (PSK).Nowadays, optical phase modulations are mostly based on LiNbO₃ modulators in the form of MZ interferometer which modulates optical signals by electrical signals. Electron devices restrict high bit rates in present telecommunication. The electric bottleneck inhibits rates not more than 40Gbit/s.The paper proposes a scheme of DPSK modulation system based on SOA-MZI. Its novelty lives in the whole modulation process carried out in optical area and getting rid of the restriction of electric bottleneck. It uses semiconductor optical amplifiers (SOAs) as modulators. The outputs of two beams of the fiber MZ interferometer are controlled by the input signals injected into SOAs and we get the modulation signals in the end. SOAs can be used as modulators due to their cross gain modulation (XGM) effect, one of their nonlinear effects. The paper has proved the scheme is right by theoretical analysis of transmission process.However, the most important factor restricting the scheme's realization is the MZ interferometer's instability. So we do some researches about the MZ interferometer's stability and propose a phase compensation circuit. The basic principle of the compensation circuit is that the phases of the improved MZ interferometer's three outputs are different of 120°in turn. We deal with two outputs of the three so as to control the third output. The circuit has been proved right and usable by software simulation and building the circuit practically. Then the experiment system has been set up after the compensation circuit in the form of printed circuit board (PCB) was designed, jointed and debugged. The input was optical signal whose wavelength was 1553.3nm and power was 1.001mW. When the environment changed, the output of optical fiber MZ interferometer was very instable without the compensation circuit. The maximum undulation range could exceed 40mV. With the compensation circuit, the output was much smoother and the maximum undulation range was not more than 5 mV. According to the experiment, the circuit is good at making the third output stable. This will promote the optical fiber MZ interferometer's practical use greatly.Meanwhile, the paper tested the responsibility of PINs used in the experiment. The calculation process has proved that the value of the responsibility could be greater than one, as long as quanta efficiency is less than one.