How low-coherence interferometry works
Principles of low-coherence interferometry
|Low-coherence interferometry is a non-contact optical sensing technology. An optical probe directs a low-coherence light beam at a sample surface and sends reflected light signals back to the interferometer. When the sample is a stack of semi-transparent films, light is simultaneously reflected back from the top and bottom of each material layer (see diagram).
The reflected optical data from each single scan point is interpreted by the interferometer as an interference pattern and recorded as a depth profile (A-Scan). By scanning the probe in a linear fashion across the sample, a cross-section (B-scan) is obtained. 3D volumetric images can be generated by combining multiple cross-sections.
Note that scanning with low-coherence interferometer probes is collinear: the emitted and reflected light signals travel along the same axis.
How the instrument works
The interferometric detector directs low-coherence light at the sample surface and captures and analyzes the reflected light.
Two main types of low-coherence interferometers – time domain (TD) and frequency domain using swept source – differ by the source of their light and by details of their implementation.
1) Time domain (TD) interferometers use low-coherence light from a super-luminescent diode. The light is fed into a fiber-optic coupler that splits the light beam into two arms (paths), one directed at the sample surface, the other at a scanning reference mirror. The detector then captures the interference of light rays reflected back from these two arms. Constructive interference is observed as an intensity maximum when the optical paths of both arms are exactly equal. By scanning the length of the reference arm to bring forth the appearance of the interference signal, the detector determines the precise position of the reflection point in the sample.
2) Frequency domain interferometers with swept source use light from a fast sweeping laser source instead of a super-luminescent diode. The reference mirror is fixed. The detector captures the spectrum of the interference pattern in time and then converts this spectrum to the time domain using Fourier transformation.
Comparing time-domain and frequency-domain (with swept source) LC interferometers
Both types of interferometers
- are immune to air perturbation and to cutting the beam
- know the absolute distance of the sample surface when they are turned on, with no need to count fringes (unlike interferometers that use coherent laser-light sources)
Time domain low-coherence interferometers
- perform at a scanning speed reaching a few kHz
- are very robust and immune to saturation since the interferometric fringes are encoded in frequency
- maintain sensitivity regardless of scanning depth
Frequency domain low-coherence interferometers with swept source
- perform at high and very high scanning speeds (20 kHz and higher)
- maintain sensitivity with higher speeds
- lose sensitivity with increased scanning depth