I have a ~10-100pW 850nm signal(free space or fiber coupled) that I need to amplify. I do not need a large gain, in fact for my purpose anything >0dB will work. I have looked into few off-the-shelf optical amplifiers but they all have a minimum input power threshold, mostly around -30dBm. My input signal is many orders weaker than that. Could you please suggest an implementation, if at all it is possible?
Vincent@Vincent Lecoeuche has provided a good answer, I just make a supplement.
According to the power level of 100 pW, I suppose it to be a pulsed source. In this case, you may use a modulated SOA as preamplifier to selectively amplifier the signal and suppress the ASE geneartion.
The minimum input power is limited by the noise of the amplifier. So if it is given by about -30dbm , then in absolute level pmin= 1 microwat. So your signal is still much less than the power related to the signal to noise ratio. Your signal is smaller than the power. Them you may search for lower noise amplifiers. This may be expensive. You can then use such amplifiers to amplify your signal and use signal processing methods to select your signal from noise. One of the main methods are the correlation. The other method may be the very narrow band path filters.
Best wishes
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