The spontaneous and coherent properties of a laser with strong TE polarization (E parallel to the junction) are measured and discussed in terms of the nonlinear theory of injection lasers. The laser cavity is 330 × 100 × 0.25 μm with a symmetrical double heterojunction, sawed sides, and cleaved facets. Its dispersion is that of the normal modes of a rectangular box. The current dependence of spontaneous emission indicates a modest increase in population inversion across the lasing region, the maximum overdrive papameter reaching only 0.4. The power dependence of the overdrive parameter deduced from the spontaneous emission agrees well with that from the lateral profile of the external beam. Within a rather large uncertainty from several factors, the calculated value of 25% for the extra prorated loss of the TM modes compared to TE modes arising from facet reflection accounts quantitatively for the persistence of complete polarization to maximum safe output. The critical power associated with the optical nonlinearity is around 5 mW per cavity mode, in agreement with other reported values, and is shown to be independent of facet area as predicted.