Basics of conoscopic holographyIn classical holography, a hologram is created by recording an interference pattern formed between an object beam and a reference beam using a coherent light source. The two beams propagate at the same velocity (same refractive index), but follow different geometric paths. This means that when overlapped, the phase difference between the two beams depends only on the geometric path difference. This phase difference is responsible for the creation of a measurable interference pattern that can later be used to reconstruct the original light field.
In conoscopic holography, however, a light beam that traverses an optically anisotropic crystal is split into two beams that share the same geometric path but have orthogonal polarization modes. The refractive indices of these two beams generally differ from each other. Therefore, after the two beams exit the crystal an interference pattern is generated. The features of this pattern depend on the distance from the light's source.
Since both beams propagate through the same geometric path, conoscopic holography is highly stable in comparison to interferometry-based measurement techniques. Moreover, it is also possible to perform measurements using incoherent light.
How does the sensor work?The sensor emits an eye-safe laser beam, which is focused by an objective lens, and hits the specimen being measured. Part of the scattered light travels back from the specimen into the sensor, and enters the conoscopic unit that contains the optically anisotropic crystal. The resulting interference pattern is detected, and signal processing algorithms are then used to retrieve the distance information from the measured data.
Figure 1 - Path of light inside Optimet sensor