Optical interference is not only a fundamental phenomenon that has enabled new theories of light to be derived but it has also been used in interferometry for the measurement of small displacements, refractive index changes, and surface irregularities. In a two-beam interferometer, variations in the interference fringes are used as a diagnostic for anything that causes the optical path difference (OPD) to change; therefore, for a specified OPD, greater variation in the fringes indicates better measurement sensitivity. Here, we introduce and experimentally validate an interesting optical interference phenomenon that uses photons with a structured frequency-angular spectrum, which are generated from a spontaneous parametric down-conversion process in a nonlinear crystal. This interference phenomenon is manifested as interference fringes that vary much more rapidly with increasing OPD than the corresponding fringes for equal-inclination interference; the phenomenon is parameterised using an equivalent wavelength, which under our experimental conditions is 29.38 nm or about 1/27 of the real wavelength. This phenomenon not only enriches the knowledge with regard to optical interference but also offers promise for applications in interferometry.

光学干涉不仅是一种基本现象，能够推导出新的光理论，而且还被用于干涉测量中，以测量小位移、折射率变化和表面不规则性。在双光束干涉仪中，干涉条纹的变化可用于诊断导致光程差 (OPD) 变化的任何因素；因此，对于指定的 OPD，条纹变化越大表明测量灵敏度越好。在这里，我们介绍并通过实验验证了一种有趣的光学干涉现象，该现象使用具有结构化频率角谱的光子，这些光子是由非线性晶体中的自发参数下转换过程产生的。这种干涉现象表现为，随着OPD的增加，干涉条纹的变化速度比等倾角干涉的相应条纹快得多；该现象使用等效波长进行参数化，在我们的实验条件下，该波长为 29.38 nm 或约为真实波长的 1/27。这种现象不仅丰富了有关光学干涉的知识，而且为干涉测量的应用提供了希望。