FREE-SPACE OPTICAL COMMUNICATION: N-slit interferometer extends secure free-space communications to 527 m

Researchers at Interferometric Optics (Rochester, NY), the US Army Aviation and Missile Command (Redstone Arsenal, AL), Science Applications International Corporation (SAIC), and the US Army Space and Missile Defense Command (both in Huntsville, AL) have extended the range of a secure, terrestrial free-space optical-communications system using an N-slit interferometer design from mere centimeters when first demonstrated to a practical 527 m.¹ Furthermore, atmospheric turbulence could be detected over the 527 m propagation distance; the insertion of fine fibers into the optical path was detectable in the laboratory at distances of around 7 m.

The interferometric setup

Empirical interest in N-slit (where N is the number of slits) arrays was first expressed by photonics giants Isaac Newton and Albert Michelson. Because physicist Richard Feynman performed double-slit thought experiments on electrons using Paul Dirac's quantum notation, generalized theoretical equations to describe N-slit interference with a narrow-linewidth laser source were developed in the late 1980s.

Using this interferometric theory, the research team built a simple N-slit interferometer using a 570 μm grating (570-μm-wide slits separated by 570-μm-wide islands) for 7.235 m transmission experiments in the lab, and a 1000 μm grating for 527.33 m transmission experiments through clear, summer night air at 24°C with 66% humidity. Theoretically predicted interferometric configurations denoted by the characters a, b, c, and d (corresponding to illuminating N = 2, 3, 4, and 5 slits of the grating, respectively) were created at the interferometer entrance using an expanded single-transverse-mode 632.8 nm helium-neon laser beam.

In the proof-of-concept experiment at the maximum transmission distance, the patterns produced by the 1000 μm grating were easily detected at the interferometric plane by a calibrated screen and recorded through digital photography. Each of the diffraction patterns was clearly discernable for the characters a through d at an interferometric distance of 527.33 nm as measured by a calibrated laser rangefinder. Predicted and recorded interferograms, in terms of the overall divergence of the interferometric patterns and the position of the spatial features of the interferograms, agree to within 1% to 2%. In addition, digital movie recordings of the interferograms show slight intensity disturbances consistent with atmospheric turbulence and thought to be due to a mild breeze present during experimentation.

A minor intrusion

Experiments using the 570 μm grating and a shorter interferometric distance of 7.235 m were performed using a 1340 pixel CCD array placed at the interferometric plane. The researchers also tested the system security by inserting two fine fibers—a 50-μm-diameter human hair and a 25-to-30-μm-diameter spiderweb fiber—one at a time, close to the detection plane. The minor intrusion in the free-space optical path was clearly visible in the resultant pattern at the detection plane, demonstrating the secure nature of the interferometric terrestrial technique (see figure). More obvious intrusions by conventional thin beamsplitters led to a catastrophic collapse of the signal.

"The N-slit laser interferometer was initially developed to characterize the fidelity of transmission gratings, the optical homogeneity of imaging surfaces, and as a coherent-digital microscopic tool," says researcher Frank Duarte from Interferometric Optics. "The next phase of development for the laboratory version of this instrument is to build an automated infrared version for kilometer-range distances for either secure free-space optical communications or the detection of clear-air turbulence, which is of significant interest for aviation safety."

REFERENCE
1. F.J. Duarte et al., J. Opt., 13, 035710, 1–5 (Feb. 3, 2011).