Spiral phase contrast (SPC) imaging offers a vital, convenient tool for edge detection in image processing. Despite significant experimental and theoretical progress in this area, SPC imaging with invisible light is still lacking. In contrast to the general SPC scheme, here we construct a nonlinear spatial filter by equivalently imprinting the vortex phase plate onto the potassium titanyl phosphate crystal using second harmonic generation (SHG). The phase or intensity objects are displayed by a spatial light modulator (SLM) and illuminated with 1064 nm infrared light. Then the combination of our nonlinear filter with SHG in the Fourier domain enables concise, yet highly efficient SPC imaging, leading to a visible edge enhancement with invisible illumination. By programming a running dog cartoon with SLM, we also demonstrate the capacity of our scheme to detect edges and contours in real time. Our present scheme could find direct applications in infrared monitoring.

中文翻译：

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非线性光学中的螺旋相位对比成像：使用不可见照明查看相位对象

螺旋相位对比（SPC）成像为图像处理中的边缘检测提供了至关重要的便捷工具。尽管该领域在实验和理论上取得了重大进展，但仍缺乏使用不可见光进行SPC成像的方法。与一般的SPC方案相反，在这里，我们通过使用二次谐波（SHG）将涡旋相位板等效地压印在磷酸氧钛钾晶体上，构造了一个非线性空间滤波器。相位或强度对象由空间光调制器（SLM）显示，并用1064 nm红外光照射。然后我们非线性的组合傅里叶域中的SHG滤波器可实现简洁而高效的SPC成像，从而通过不可见照明增强可见边缘。通过使用SLM对流浪狗卡通进行编程，我们还演示了该方案实时检测边缘和轮廓的能力。我们目前的方案可以在红外监测中找到直接的应用。