Adjusting the operating point for optical modulation

With transmission on optical glass fibers, modern communication is able to transport large data volumes. In such fast transmission paths, externally modulated lasers are usually used instead of directly modulated lasers to control the amplitude, phase or frequency of the light waves.

State of the art

The carrier wave is often modulated using Mach-Zehnder modulators (MZM) based on lithium niobate. Here, light is guided through an optical waveguide, split and the light of one or both arms is modulated by an electrical high-frequency signal. Both arms are merged again at the exit. In order to achieve the highest possible linearity and distortion-free data transmission, the optimum operating point has to be set and corrected if necessary.

Technology

Scientists of the Institute for Data Processing and Electronics (IPE) at KIT have developed a novel concept for the efficient and simple adjustment of the operating point of MZM. They extend common MZM by a special control device with signal comparison unit, which measures and pre-modulates the input signal after splitting it into two arms, in contrast to the state of the art. Basically, this is a targeted phase shift to obtain an optimum operating point as a basic setting for continuous light. In this case, affecting environmental conditions for the partial signals are already compensated by this controlled thermo-optical modulation. As a result, a light signal that almost ensures the correct operating point is transmitted to the subsequent standard MZM.

Advantages

By using this modulation pre-stage with quasi-static signals, no high-frequency electronics are needed to control and compare the light waves, as is the case with conventional operating point controls. Negative environmental influences can be modulated by relatively simple means.

Options for companies

A Mach-Zehnder modulator according to the described concept can be realized in particular by monolithic integration on photonic chips based on silicon. KIT is looking for companies to further develop or use the operating point control.