Ultrasensitive detection of amplified spontaneous emission at 710 nm by means of picosecond collinear optical parametric amplification near degeneracy.

We report the experimental results on improving the detection of an ultraweak optical signal using a 355 nm pumped picosecond collinear optical parametric amplification (OPA). The OPA is seeded by the amplified spontaneous emission (ASE) generated in a solution of pyridine-1 dye in ethanol. The gain factor of this amplifier is determined as ~1.5×10(8), and the detection limit is ~1.25 aJ per pulse, corresponding to five photons at 710 nm within the 15 ps pulse width of the pump beam. This is achieved by reducing superfluorescence background noise by means of signal/idler double seeding near degeneracy, amplification under a slightly phase-mismatched condition, and space filtering with increasing observation distance up to 3.2 m. Compared with previous reports, the detection limit is significantly enhanced. The pulse shape of the ASE is also measured with OPA, and it agrees well with that measured by an ultrafast oscilloscope.