Why do we use low-coherence light in Optical Coherence Tomography?

[red65]

Hello , I study the principles of optical coherence tomography, where we emit light and by the refraction that we detect we reconstruct and image, but I don't understand why we use low coherence light , if i want to measure the refracted light i would prefer to have coherent light so that the difference in the scattered light will depend only on the form and distance of the object that refracted the light .
can anyone tell me where is the flaw in my reasoning, thanks.

 

[tech99]

As far as I can see, it is a radar, looking at the reflections from the various layers of the retina. If we use a pure coherent source, we have in effect a CW radar. This does not allow distance determination; the source must be modulated to obtain distance information. An ordinary radar uses a pulse modulated source to do this. With OCT, by using a non coherent source, the source is modulated by noise. By comparing the noise envelope of the signal reflected from the retina and that from a reference mirror, a peak will be obtained when the two paths are equal. It looks as if the mirror is moved back and forth slightly to explore the reflected signal intensity from various depths.