Understanding the wavefront is crucial in laser optics, fiber optics, and biomedical applications like refractive surgery, cataract surgery, and ocular surface abnormalities. In these fields, maintaining a smooth, simple wavefront shape is often technically preferable. Wavefront quality is closely related to the degree of spatial coherence, and analyzing it helps predict optical system performance.
In this article, we will demonstrate how to sample and analyze the wavefront in a fiber coupling system.
Let’s consider a fiber coupling system with the following specifications:
- Numerical Aperture (NA): 0.2 (from the fiber)
- Focal Distance: 40 mm
- F/# of the lenses: 2.4
- Waist Radius: 1.56 microns (corresponding to NA 0.2)
In general, wavefront phase profiles change more quickly and are more challenging to sample than the beam intensity profiles.
Note: A lens generally imparts a quadratic wavefront phase across an incident beam. In Zemax, the wavefront phase is plotted from -π to π. If the lens introduces a phase shift greater than π (e.g., 3π/2), the software wraps the phase, which is just a plotting convention and doesn’t indicate actual discontinuities in the wavefront. For example, a phase of 3π/2 will be displayed as π/2.
We have adjusted the sampling before and after the lenses.
Let’s look at the front of the lens, surface 4. The intensity profile looks fine.
Let’s look at the wavefront.
It can be seen that without wavefront phase wrapping, the wavefront introduced by the lens will have a smooth shape and is well sampled. The first few rings in the phase are sampled correctly. Moving toward the edge of the aperture, the slope of the phase is steeper, and the rings are not sampled well. This causes aliasing: the phase is changing quickly, but the function is sampled more slowly, resulting in strange geometric patterns that don’t represent the true wavefront in the lens.
A cross-section of the phase at the front of the lens also shows the undersampled regions clearly. For many fast systems, though, the ray-based Fiber Coupling algorithm is appropriate and POP is not needed. For the majority of fiber coupling systems, the diffraction effects from the edges of the lenses aren’t significant. Every surface in a system must be checked to insure that sampling of the wave front phase profile is adequate. Fast lenses may require very high sampling and very long computation times.
Reference
- https://www.zemax.com/
- The design file used in this article is attached. How to analyze the wavefront in a fiber coupling system