定义：
入射光为宽带光的干涉仪。

白光干涉仪用在低相干性的干涉中，通常为迈克尔逊干涉仪，采用白光光源，具有很宽的带宽。光源不一定需要在可见光谱区域。光源的时间相干性很低，空间相干性很高。将灯泡发出的光射入单模光纤中就可以同时得到很高的空间相干性和很宽的带宽，但是这样得到的功率较低。采用超辐射光源，例如超发光二极管就可以使光的亮度提高好几个数量级。有时还可以采用可调谐激光器。 
白光干涉仪的探测器可以是光电探测器，在时间域上采集信号，也可以采用光谱仪。 

白光干涉仪的应用 
白光干涉仪可以用于很多方面。主要应用包括： 

• 测量色散。其中色散的光学元件放置在干涉仪的一个臂上，然后通过扫描一定范围内臂长差的变化来控制得到探测信号。臂长差在0附近时会产生干涉摆动，这时臂长差很大时信号仍然是恒定的。色散很强的情况下，得到的干涉谱很宽。对得到的干涉谱进行傅里叶变换算法，可以得到测试期间的复反射系数或复透射系数，对其进行数值微分就可以得到与波长相关的群时延和色散。 
• 测距。与采用窄线宽激光器的干涉仪相比，白光干涉仪不存在模糊的问题。一个典型的例子是测量表面形状。例如，可以采用迈克尔逊干涉仪，再加上一个CCD相机作为探测器。同样的，记录不同臂长差时的影响。每一个像素点都表征了在给定的位置处，零臂长差附近的干涉摆动。与窄带干涉仪不同之处在于，这里不需要解相过程，因此可以处理非常粗糙的表面。 
• 类似的，也可以测量在光子集成回路内的反射。

Definition: interferometers using broadband light inputs

A white light interferometer, used e.g. in the context of low-coherence interferometry, is an interferometer, typically a Michelson interferometer, which works with a white light source, i.e. with a light source with broad optical bandwidth. The light source does not necessarily operate in the visible spectral range, really generating white light: broadband infrared sources are also sometimes called white light sources. The temporal coherence of the used light has to be fairly small, whereas a high spatial coherence is normally needed. The high spatial coherence combined with a broad bandwidth can most easily be obtained by launching light from a bulb into a single-mode fiber, but this leads to a very small launched power. The radiance (brightness) can be increased by many orders of magnitude by using a superluminescent source such as a superluminescent diode. In some cases, a wavelength-swept laser is used.

The detector in a white light interferometer can either be a photodetector which integrates contributions of different wavelengths and records the signal in the time domain, or a spectrometer (→ spectral phase interferometry).

Figure 1: Simulated signal as obtained in a z scan with a white light interferometer. Zero optical path length difference occurs at z = 2.6 μm.

Application of White Light Interferometry

White light interferometry is used for different purposes. The main applications are:

Medical Imaging

Medical imaging is possible with the technique of optical coherence tomography, which is essentially based on light white interferometry, at least in its original form.

Dispersion Measurements

For the measurement of chromatic dispersion of a dispersive optical element, that element is placed in one interferometer arm, and the detector signal is monitored while scanning the relative arm length through some range. Around zero arm length difference, interferometric wiggles occur, whereas the signal is about constant for large arm length differences. With strong dispersion, the recorded interferogram becomes broader. By applying a Fourier transform algorithm to the recorded interferogram, it is possible to retrieve the complex reflection or transmission coefficient of the device under test, and numerical differentiation reveals the wavelength-dependent group delay and chromatic dispersion.

Distance Measurements

Compared with interferometers based on narrow-linewidth laser sources, the typical ambiguity issues are avoided. A special case is the measurement of surface profiles. For example, a Michelson interferometer with a CCD camera as detector may be used. Again, images are recorded for different arm length differences. Each pixel displays the interferometric wiggles around the point of zero arm length difference at the given transverse location. Unlike the situation in a narrow-band interferometer, no phase-unwrapping procedure is required, so that even rough surfaces can be easily handled.

Reflection Measurements

Reflections within a photonic integrated circuit can also be detected with white light interferometry.

Bibliography