Spectral reflectance measurements of InP overgrowth upon RIE prepared gratings provide for in situ measurements of layer thickness to control the coupling constant (K) of the laser mode to the grating. We show that the use of a commercially available reflectometer utilized on a rotating disk reactor provides growth rate measurements of InP overgrowth of InGaAs gratings with 50% duty cycle as well as patterned gratings. Spectral reflectance (SR) determined thickness has been correlated to post-growth measurements such as cross section SEM and XRAY measurements with good results. SR measurements also provide an optical signature of grating overgrowth that has thermally degraded gratings and grating overgrowth that can lead to low photoluminescence of MQW layers. The results of these measurements are improved process control of grating overgrowth in laser diode manufacture, enabling higher yields.