Study of coherence tomography based on optical frequency comb

相干扫描显微镜、相干雷达三维传感和扫描白光干涉测量等一系列技术已经被开发出来，用于同时测量不连续物体的三维台阶高度和层析成像。然而，这些技术需要用压电换能器或精密翻译器进行机械扫描。另一方面，光谱干涉测量法可能为高速绝对三维物体形状测量提供了一种强大的手段，而无需移动机械部件。然而，由于具有足够宽范围的可调谐激光器的高成本（例如，外腔激光二极管和染料激光器）以及由于傅里叶变换中大量计算而导致的数据采集速度慢，这些方法的实际适用性仍然受到限制。作为一种基于干涉测量的替代方法，相干函数的合成已被报道为适合于距离测量。该方法通过激光二极管的调频产生伪频率梳。然而，由于频率梳间隔被限制在兆赫数量级，这种方法不适用于表面轮廓测量。在这篇文章中，提出了一种新的基于频率梳的干涉测量技术用于轮廓测量和断层扫描。采用这种方法的光学频率梳可以简单地调整，允许干涉测量在频率间隔高达千兆赫数量级进行。因此，该方法适用于测量几微米到几米的绝对距离，而不需要机械运动。通过扫频实现了精度为9 μm的步进高度测量。还演示了玻璃板的厚度测量。这种轮廓测量方法有望用于表面轮廓测量和断层扫描。

A range of techniques, including coherence scanning microscopy, three-dimensional sensing by coherence radar, and scanning white-light interferometry, have been developed for simultaneous measurement of the three-dimensional step height of discontinuous objects and tomographic imaging. These techniques, however, require mechanical scanning with a piezoelectric transducer or a precision translator. Spectral interferometry, on the other hand, potentially offers a powerful means for high-speed absolute threedimensional shape measurement of objects without moving mechanical parts. The practical applicability of such methods, however, remains limited due to the high cost of tunable lasers with sufficiently wide range (e.g., external cavity laser diodes and dye lasers) and the slow speed of data acquisition because of the large number of calculations in the Fourier transform. As an alternative interferometry-based approach, synthesis of the coherence functions has been reported to be suitable for distance measurements. This method involves pseudo-frequency comb generation by frequency modulation of a laser diode. Yet this method is not suitable for surface profilometry because the frequency-comb interval is limited to the order of megahertz.In this Letter, a novel frequency comb-based interferometry technique for profilometry and tomography is proposed. The optical frequency comb employed by this method can be adjusted simply, allowing interferometry to be performed at frequency intervals of up to gigahertz order. The method is therefore suitable for measurement of absolute distances of several micrometers to several meters without the need for mechanical movement.Step-height measurement with 9 μm accuracy was successfully performed by sweeping the comb interval frequency. Thickness measurement of a glass plate was also demonstrated. This profilometry method is expected to be useful for both surface profilometry and tomography.