Method for Probing the Orbital Angular Momentum of Optical Vortices in Electromagnetic Waves from Astronomical Objects

We present an efficient method for probing the orbital angular momentum of optical vortices of arbitrary sizes. This method, based on a multipoint interferometer, has its most important application in measuring the orbital angular momentum of light from astronomical sources, opening the way to interesting new astrophysics. We demonstrate its viability by measuring the orbital angular momentum of Laguerre-Gaussian laser beams.

Figure 1

Geometry and notation of a generic multipoint interferometer consisting of $N$ points, uniformly distributed over a circle of radius $a$ in the $xy$ plane. The points are indicated by open dots and the angular coordinate of the $n$th point is ${\alpha }_n=2\pi n/N$.

Figure 2

Far-field intensity patterns behind an MPI of $N$ points illuminated by an LG beam with azimuthal mode index $l$ calculated from Eq. (2) (linear gray scale, white corresponds to high intensity). $N=2$, $l=0$ reproduces the results for Young’s experiment with two pinholes. $N=16$ hints at the fact that the observed patterns converge to a Bessel function of order $|l|$. At intermediate values the pattern is very dependent on the azimuthal mode index $l$, and therefore the OAM of the field at the location of the MPI.

Figure 3

Simulated far-field intensity patterns behind an MPI of $N$ points illuminated by an LG beam with azimuthal mode index $l$ calculated from Eq. (2). The patterns for $l$ values of opposite sign are mirrored in the $x$ axis. For an odd number of points one can determine the sign of $l$.

Figure 4

Setup to measure interference patterns behind a multi-pinhole interferometer. A helium-neon laser (HeNe), a filter wheel (FW), two mirrors (M1 and M2), and a fork hologram (FH) are used to create LG beams and a set of mirrors (M3 and M4) and a diaphragm (D) are used to select an LG beam to illuminate the multi-pinhole interferometer (MPI). A CCD camera and a lens (L) are used to record the far-field image.

Figure 5

Measured far-field intensity patterns behind an MPI of $N$ points illuminated by an LG beam with azimuthal mode index $l$. The measured patterns show excellent agreement with the calculated ones (compare Fig. 2).