Coherent homodyne crosstalk in optical systems is examined. A general approach is introduced to theoretically model the crosstalk of single and multiple interferers in DFB and FP laser systems with a finite modulation extinction ratio, with PIN and APD receivers, in external modulation systems. The theoretical results of this model are verified experimentally and are compared with other theoretical models. In addition, the trade off between modulation extinction ratio and crosstalk tolerance is investigated and optimized. The general mathematical model is implemented in a computer simulator program, creating an inclusive and pragmatic engineering tool. This computer simulator dramatically improves the design of next generation crossconnects, switched and routers.