Detection of ∼1 atto-joule signal pulse at 532 nm using a picosecond collinear BBO optical parametric amplifier

We present the experimental results on pushing the detection of ultra-weak signal using a 355 nm pumped picosecond collinear optical parametric amplifier (OPA). The signal is the second harmonic pulse of a passively/actively mode-locked Nd:YAG laser at 532 nm. The measured gain factor of this amplifier is found to be ∼1.36×108, and the detection limit is ∼1.03 aJ per pulse, corresponding to 3 photons at 532 nm within the 15 ps time window of the pump pulse. This was achieved by increasing the distance between the CCD detector and the OPA. It led to a significant reduction of parametric superfluorescence background so that the amplified spot can be observed under the detection limit condition. Compared with previous reports, the detection limit is improved by one order of magnitude.