A test apparatus has at least one optical source, a high-speed photodetector, a microcontroller or processor, and electrical circuitry to power and drive the at least one optical source, photodetector, and microcontroller or processor and for measuring the frequency response of a multimode optical fiber under test. The test apparatus can utilize an optical pulse waveform with a light adapter to measure of the channel under test. It can also uses a correction method to de-embeed a chromatic bandwidth of the source from the encircled flux modal chromatic bandwidth. The correction method can use correction functions obtained for different type of VCSELs to estimate the optical channel bandwidth when used with VCSEL transceivers.