Figure 1: SEM image of a 2x1 array of photonic crystal VCSELs
Arrays of photonic crystal VCSELs exhibit many novel and desirable behaviors when their optical modes are coherently coupled. However, tuning these arrays into coherence can be a challenge. The PDRG has been studying various properties of these systems and how they relate to coherent coupling.
Figure 2: Total optical power measurements
One way we can discern coherence is in the optical power measurement. When the elements of an array couple, they tend to exhibit a power enhancement that appears as higher-power “ridges” along the diagonal of power measurements, as illustrated in Figure 2.
Figure 3: An incoherent far-field image
Figure 4: A partially coherent far-field image
Figure 5: A coherent far-field image
Another way of analyzing coherence is to observe the far-field beam shape. When a pair of VCSELs optically couples, the fields in the individual cavities will cause interference in the far-field that shows itself as a series of far-field interference fringes. This is illustrated in Figures 3-5.
Figure 6: Calculated far-field visibility parameter
Figure 7: Calculated far-field Fourier analysis side-peak ratio parameter
The presence of interference fringes can be quantified in a variety of methods, including the far-field visibility parameter (a measure of the interference fringe maxima and minima) that is illustrated in Figure 6, as well as a far-field Fourier transform side-peak ratio (another measure of how strong the interference fringes are) that is illustrated in Figure 7. The dots on Figures 6 and 7 relate where the Figures 3-5 are located.
There is a variety of measurements (including optical spectra and differential resistance) and ways of analyzing these measurements. The manuscript for “Computational methods for VCSEL array characterization and control” (linked below in Resources) explains some of the data analysis methods for analyzing coherence and cites relevant literature.
Resources
- “Computational methods for VCSEL array characterization and control” (DOI) (Abstract) (Presentation) (Manuscript)