| 摘要 |
An optical parametric amplifier pumped by ultrashort optical pulses provides time-gated image amplification or time-gated image frequency conversion, resulting in optical sectioning of an object under test and/or background rejection of improperly timed light. An ultrashort laser pulse of one frequency is used to illuminate an object under test. An ultrashort pulse light beam at a signal frequency transmitted through or scattered from the object is optically mixed with an ultrashort laser pulse at a pump frequency in a nonlinear optical medium. This mixing produces an amplified image of a particular optical section of the object at the signal frequency in addition to producing a frequency converted image of the same optical section at an idler frequency. This time-gated amplification can be used in conjunction with a confocal imaging system, or a conventional imaging system. The resolution of optical sectioning is determined by the temporal widths of the signal and pump pulses and by the group velocity walkoff in the nonlinear medium. By illuminating the target with a train of closely spaced ultrashort pulses, an image of multiple sections can be amplified and downconverted within a single laser shot, giving a contour image of the target. The signal light can also be fluorescence from the object, excited by a short laser pulse, either through single-photon or multi-photon absorption. In this case, the signal light is incoherent with respect to the pump light. By using quasi-phase-matched nonlinear optical crystals as the amplifying medium, advantages such as an increased acceptance angle and lower pump thresholds are obtained.
|
