Instrumental effects on spectrum measurement from a semiconductor diode biased below threshold

Comparison between the F-P mode spectrum from a subthreshold-biased semiconductor diode and its measured counterpart with a grating monochromator has been made. It is shown that the modulation index measured from the recorded spectrum takes its minimum periodically when the slit width is set to allow the light from an integer number of modes to pass through, and the measured wavelengths at the valleys (or peaks) on the recorded mode spectrum are different from their true values by an amount equal to a half of the diode mode spacing if the passband width of the monochromator is larger than the mode spacing