Kevin Birnbaum

100 Tbit/s free-space data link enabled by three-dimensional multiplexing of orbital angular momentum, polarization, and wavelength

We investigate the orthogonality of orbital angular momentum (OAM) with other multiplexing domains and present a free-space data link that uniquely combines OAM-, polarization-, and wavelength-division multiplexing. Specifically, we demonstrate the multiplexing/demultiplexing of 1008 data channels carried on 12 OAM beams, 2 polarizations, and 42 wavelengths. Each channel is encoded with 100 Gbit/s quadrature phase-shift keying data, providing an aggregate capacity of 100.8 Tbit/s (12×2×42×100 Gbit/s).

Fiber coupler for generating orbital angular momentum modes

We propose a fiber coupler consisting of a central ring and four external cores to generate up to ten orbital angular momentum (OAM) modes. Four coherent input lights are launched into the external cores and then coupled into the central ring waveguide to generate OAM modes. By changing the size of the external cores, one can selectively excite a high-order OAM mode. The quality of the generated OAM modes can be enhanced by adjusting the polarization state and the phase of input lights. We show the generation of OAM modes with odd charge numbers of -9 to +9 (i.e., 10 modes totally) with mode purity of >99% using <2 mm long fiber. This fiber coupler design can be extended to enable all-fiber spatial-mode (de)multiplexing.

Phase-shift interference-based wavefront characterization for orbital angular momentum modes

Wavefront characterization for orbital angular momentum (OAM) modes is demonstrated using quadrature phase-shift interference. The phase fronts and intensity profiles of OAM(-2), OAM(-4), OAM(-6), and OAM(-8) are measured. Wavefront correlations between the experimental results and the pure Laguerre-Gaussian modes are calculated to evaluate the measurement. The measured results are in reasonable agreement with the anticipated results based on simulations.

On approaching the ultimate limits of photon-efficient and bandwidth-efficient optical communication

It is well known that ideal free-space optical communication at the quantum limit can have unbounded photon information efficiency (PIE), measured in bits per photon. High PIE comes at a price of low dimensional information efficiency (DIE), measured in bits per spatio-temporal-polarization mode. If only temporal modes are used, then DIE translates directly to bandwidth efficiency. In this paper, the DIE vs. PIE tradeoffs for known modulations and receiver structures are compared to the ultimate quantum limit, and analytic approximations are found in the limit of high PIE. This analysis shows that known structures fall short of the maximum attainable DIE by a factor that increases linearly with PIE for high PIE.

LLCD operations using the Optical Communications Telescope Laboratory (OCTL)

The Optical Communications Telescope Laboratory (OCTL) located on Table Mountain near Wrightwood, CA served as an alternate ground terminal to the Lunar Laser Communications Demonstration (LLCD), the first free-space laser communication demonstration from lunar distances. The Lunar Lasercom OCTL Terminal (LLOT) Project utilized the existing 1m diameter OCTL telescope by retrofitting: (i) a multi-beam 1568 nm laser beacon transmitter; (ii) a tungsten silicide (WSi) superconducting nanowire single photon detector (SNSPD) receiver for 1550 nm downlink; (iii) a telescope control system with the functionality required for laser communication operations; and (iv) a secure network connection to the Lunar Lasercom Operations Center (LLOC) located at the Lincoln Laboratory, Massachusetts Institute of Technology (LL-MIT). The laser beacon transmitted from Table Mountain was acquired by the Lunar Lasercom Space Terminal (LLST) on-board the Lunar Atmospheric Dust Environment Explorer (LADEE) spacecraft and a 1550 nm downlink at 39 and 78 Mb/s was returned to LLOT. Link operations were coordinated by LLOC. During October and November of 2013, twenty successful links were accomplished under diverse conditions. In this paper, a brief system level description of LLOT along with the concept of operations and selected results are presented.

25.6-bit/s/Hz spectral efficiency using 16-QAM signals over pol-muxed multiple orbital-angular-momentum modes

We demonstrate generation/multiplexing/demultiplexing of polarization-multiplexed (pol-muxed) multiple orbital-angular-momentum (OAM) modes. High spectral efficiency of 25.6-bit/s/Hz is implemented using 10.7-Gbaud 16-QAM signals over pol-muxed four OAM modes. An OSNR penalty less than 3.5 dB is observed.

Active laser ranging over planetary distances with millimeter accuracy

Precision laser ranging between planetary bodies will allow advances in the study of fundamental physics and solar system dynamics. Current precision ranging techniques based on retro-reflectors are limited to the Earth-Moon distance. We present a method of active laser ranging over interplanetary distances with asynchronous two-way ranging. The method is validated in real time laboratory experiments and field tests. Sub-millimeter accuracy has been achieved in real-time active laser ranging for interplanetary distances, providing precision improvement well above three orders of magnitude over the current RF techniques. An instrument developed with the approach and deployed in future planetary missions will significantly advance planetary sciences and fundamental physics. For example, implemented in a future Mars lander mission our approach will clarify the Mars interior (liquid or solid), which is still an open question due to limitation of RF ranging precision although extensive efforts have been made o...

Precision optical ranging by paired one-way time of flight

Precision ranging between planetary bodies would provide valuable scientific information, including tests of fundamental physics. Current ranging techniques based on retroreflectors, however, are limited to the Earth- Moon distance due to an inverse fourth power scaling. We present methods for interplanetary distances based on paired one-way ranging, which scales with a more favorable inverse square power. Corrections for clock offset, frequency error, and the Doppler effect are shown. We present the results of tabletop experiments demonstrating sub-millimeter ranging accuracy.

A receiver for the Lunar Laser Communication Demonstration using the optical communications telescope laboratory

We discuss the design and implementation of a receiver for the Lunar Laser Communication Demonstration based on a 12-pixel array of tungsten silicide superconducting nanowire single photon detectors. The receiver was used to close a software communication link from lunar orbit at 39 and 79 Mbps.

Combined laser communications and laser ranging transponder for Moon and Mars

High-resolution active laser ranging systems for Moon, Mars and beyond are analyzed. Both stand-alone laser-ranging transponders, and laser-communications systems configured to provide millimeter-level ranging data are analyzed. It is shown that a combined dual-function laser-communications and laser-ranging system is feasible.