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Here we show that by using an external optical gain instead of electronic gain, a conventional photodiode was able to respond linearly to few-photon pulses. Few-photon pulses could be detected after amplification to ~ 104-photon pulses by stimulated emission in erbium-doped fiber amplifiers. The voltage amplitude of the peak output signal from the photodiode was proportional to the incident photon numbers and no saturation effects were observed. Such a linear external optical gain photodetector was implemented in an ultralow light interference experiment, showing its prospective potential application as a simple but robust solution for the ultralow optical detection.