Abstract: The transfer modulation measuring system is made up of an adjustable multifrequency generator, a laser emission circuit, an optical detector and a spectrum analyzer. The generator includes oscillators at predetermined frequencies belonging to a discrete frequency distribution. Component signals at selected predetermined frequencies may be time-division multiplexed or frequency-division multiplexed into an electrical composite signal which modulates the laser emission. The system comprises also two optical fiber connectors and a so-called priming optical fiber which excits uniformly the propagation modes at the entrance end of the optical fiber to be measured. During a calibration phase, the connectors are joined end-to-end and aligned optically. The spectral lines at predetermined frequencies are successively adjusted to a same predetermined level on the screen of the spectrum analyzer by means of potentiometers adjusting the amplitudes of the component signals.