Transmitter Parameter Optimization (17-Aug-2009)

Transmitter Parameter Optimization

Idea: Erich Gottwald, Harald Rohde, Thomas Treyer

The quality and properties of an optical communication system depend strongly on the quality/properties of the transmitted signal and/or on the adaption of the transmitter signal to the transfer characteristics of the medium (fiber, filters, multiplexers and other optical components).

Current problems are e.g. to reduce penalties by transmission medium characteristics, to mitigate signal parameter degradation at the transmitter site by aging, to minimize influences (temperature, etc.) and imperfect settings of the modulator driving parameters.

In the following, a new idea is proposed, that is applicable to optical broadband access networks and in general to optical networks and point to point links - it is not applicable for pure broadcasting. It relates to an optical communication system and in particular to the control and optimization of the transmitted signal by modulator and/or modulation parameters and characteristics.

In an access network like the coherent UDWDM NGOA system (Ultra Dense Wavelength Division Multiplexing Next-Generation Optical Access) an OLT - ONU (Optical Line Termination - Optical Network Unit) transceiver pair or an ONU-ONU transceiver pair (master and listener) is set to a parameter optimization modus, preferable during a set up procedure. One of them serves as master and sends a stream of special test data to the listener, possibly coded in a special way for better characterization of transmission impairments. The listener receives and demodulates the data and sends it back to the master. The master compares the received and the original data and analyses them in terms of BER (Bit Error Rate) and in a further embodiment analyses them with respect to special bit sequences etc. to get more information to identify sources of impairments.

To find a point where BER=0 and at which the setup is tolerant to parameter variations (gradient = 0, optimum) a contour diagram as a function of TX parameters will be carried out which delivers BER and the gradient. From this n-dimensional data field a stable working point will be estimated. Such data field is exemplarily shown in Figure 1.

Advantages of the proposed idea:

• No special performance measuring units are necessary.
• Solution is based on software measurements.
• No FEC (Forward Error Correction) is necessary.
• Overall optimization of performance including e.g. modulator bias, modulator driving voltage (Electro-Absorption Modulator (EAM), Phase Modulation (PM), Mach-Zehnder Modulator (MZM)); Reduction of influence of receiver imperfections and of signal distortions.

• Reduction of testing costs.

The method of transmitter parameter optimization belongs to a class of methods, which requires a feedback value from the receiver. In the optical domain,...