Opt Lett. 2019 Nov 1;44(21):5125-5128. doi: 10.1364/OL.44.005125.
It is typically assumed that the fluctuations associated with a stationary broadband incoherent field propagate in free space at the speed of light in vacuum c. Here we introduce the concept of "coherence group velocity" which, in analogy to the group velocity of coherent pulses, is the speed of the peak of the coherence function. We confirm experimentally that incorporating a judicious spatio-temporal spectral structure into a field allows tuning its coherence group velocity in free space. Utilizing light from a superluminescent diode, we interferometrically measure the group delay encountered by the cross-correlation of a structured field synthesized from this source with the unstructured diode field. By tracking the propagation of this cross-correlation function, we measure coherence group velocities in the range of 12c to -6c.
通常认为，与静止宽带非相干场相关的涨落在真空中以光速 c 在自由空间中传播。在这里，我们引入了“相干群速度”的概念，它类似于相干脉冲的群速度，是相干函数峰值的速度。我们通过实验证实，在一个场中加入适当的时空光谱结构可以调整其在自由空间中的相干群速度。我们利用超辐射发光二极管产生的光，通过干涉测量来测量由该光源合成的结构化场与非结构化二极管场的互相关的群延迟。通过跟踪这个互相关函数的传播，我们测量了相干群速度在 12c 到-6c 的范围内。
