Chromatic delay in light emitting diodes

When the current through the junction of a light emitting diode (LED) is modulated, there is a delay between the modulation and the response of the optical signal to this modulation. We have recently observed that this delay is not the same at different parts of the LED's optical spectrum. This effect, the relative delay between spectral components, we call chromatic delay. This paper reports the results of measurements of chromatic delay for several LED's of various kinds. This effect amounts to delays of the order of 200-300 psec between half power points of the LED spectrum. Thus it establishes a lower limit on the rise time achievable from these devices.