A bilateral two-port model of a semiconductor laser that includes bidirectional electronic and photonic interfaces is developed by combining an equivalent circuit laser model and a transmission-line laser model (TLLM). The new model includes chip and package parasitics, gain nonlinearities, and a detailed optical cavity model based on optical traveling waves. This model can be applied to a variety of device structures to simulate optical spectra, optical and electrical waveforms, intensity spectra, and noise characteristics. Three examples demonstrate the flexibility of the model: 1) transient response simulation of semiconductor lasers, 2) characteristics of lasers used as detectors, and 3) coherence collapse in lasers with optical feedback. In all cases, the results are in good agreement with experimental measurements.