Electrical diagnostics of the operating conditions in a monitoring amplifier based on a laser diode

Changes in the voltage across a laser diode operating as an amplifier (below the lasing threshold) were investigated as a function of the pump current and of the input optical signal of wavelengths 6–7 nm longer and shorter than the emission wavelength of the laser diode. The inversion threshold, traveling wave and regenerative amplification conditions, and lasing threshold could all be determined from the electrical signal developed by the diode. The sign of the signal changed on increase in the pump current: it was positive in the photodiode regime and negative when the diode operated as an amplifier. The maximum amplitude of the negative signal was 0.6–1 mV.