A range of dot-in-a-well structures designed for long-wavelength (>1.3 μm) emission is studied in polarization- and time-resolved differential transmission measurements. Quantum beats observed in differential transmission are employed to measure the fine structure splitting (EFS) of the bright exciton states. A strong dependence of EFS on In content in the InGaAs well surrounding the dots is observed. Large magnitudes of EFS up to 87 μeV are found. © 2006 American Institute of Physics.