Optical parametric amplification (OPA) is a nonlinear optical process that amplifies weak input signal light. In particular, the OPA process selectively amplifies the coherent light signals (i.e., coming at a defined time, space, frequency, and polarization state), but not the incoherent ambient light. 

Similar to the OPA-based detection, multiple optical imaging detection methods have been developed to distinguish weak imaging signal from a strong background, such as lock-in amplification and heterodyne (interferometric) detection. These methods were widely applied in coherent Raman scattering (CRS) microscopy and optical coherent tomography (OCT). Nevertheless, they have a reduced sensitivity compared to conventional PMT-based image acquisition, resulting in deterioration of image qualities.  Compared to these optical detection methods, the OPA-based detection offers a high gain ratio and a low noise figure (NF) that overcomes the abovementioned issues and provides similar or even higher sensitivity compared to PMT-based imaging.

In label-free optical imaging, near-infrared (NIR) excitation wavelength is preferred due to the deep penetration depth and low phototoxicity. However, PMTs have a low response in the NIR range, where OPA has demonstrated its distinct advantage. In our recent experiments, by using a periodically poled lithium niobate (PPLN) crystal as the OPA medium, the optical signal at 1465 nm was amplified with a gain of 92 dB (Fig. 2 A), much higher than the typical values of a commercial PMT (~ 60 dB). In addition, OPA has an excellent noise figure (NF) down to the quantum limit. Because of the high gain and low NF, OPA allowed us to detect a 23 pW optical signal at 1465 nm under a 5 MHz bandwidth (Fig. 2 B, C). This detection limit is much lower than a commercial NIR PMT (R5509-73, Hamamatsu PMT K.K.).