Immunity to Harsh Environments

Optical Holography is inherently sensitive to small changes in optical wave phase that can be caused by airflow, acoustic wave, apparatus vibrations, etc. This sensitivity may be very useful, when, for example, studying micro-displacements or micro-deformations of objects under stress. But it may impose serious limitations for using holography for 3D measurements in technological environments.

We may consider 2 types of vibrations: “fast” and “slow”.  We call vibrations “fast” if their period is much shorter than hologram exposure time. The “fast” vibrations will “smear” hologram interference patterns, which would reduce hologram efficiency. In extreme cases the hologram won’t be recorded at all. We would therefore reduce hologram exposure time so that “fast” vibrations will become “slow” ones. We call vibrations “slow” if their period is much slower than exposure time. In this case, the vibration effect can be approximated by continuous shift of wave’s phase during the exposure.

However, “slow” vibrations can cause another problem that “conventional” holography would struggle with: they may create unknown phase shifts. This becomes a real problem for on-axis multiple phase-step holography, where multiple holograms are taken with predefined phase-shifts between reference and signal waves.

When unknown phase-shifts are added to predefined ones, this results in “coupling” between amplitude and phase signals, and huge errors in both reconstructed phase and reconstructed depth result. This “unknown phase-shift problem” can be solved by an off-axis holography approach, but the downside is an inevitable loss in lateral image resolution when such a method is used.

 We developed comprehensive methods to address unknown phase-shifts and holds the key to these problems.