Mitigation for turbulence effects in a 40-Gbit/s orbital-angular-momentum-multiplexed free-space optical link between a ground station and a retro-reflecting UAV using MIMO equalization.

We experimentally demonstrate turbulence effect mitigation in a 100-m round-trip orbital-angular-momentum (OAM)-multiplexed free-space optical communication link between a ground transmitter and a ground receiver via a retro-reflecting hovering unmanned aerial vehicle (UAV) using multiple-input-multiple-output (MIMO) equalization. In our demonstration, two OAM beams at 1550 nm are transmitted to the UAV through emulated atmospheric turbulence, each carrying a 20-Gbit/s signal. 2×2 MIMO equalization is used to mitigate turbulence-induced crosstalk between the two OAM channels.

Performance enhancement of an optical high-order QAM channel by adding correlated data to robust neighboring BPSK or QPSK channels.

We numerically simulate and experimentally demonstrate an approach to potentially enhance the performance of a high-order quadrature amplitude modulation (QAM) channel by adding correlated data to other robust binary-phase-shift-keyed (BPSK) or quadrature-phase-shift-keyed (QPSK) channels. The correlated data are introduced by optically multiplying the BPSK or QPSK channels, already modulated with their own data, by the target high-order QAM data of the same baud rate. After joint detection and signal processing, a ∼3  dB optical signal-to-noise (OSNR) improvement is observed in simulations by comparing the performance of the target QAM channel (from 4QAM to 256QAM) with and without the use of channel correlation.

Raman-assisted phase sensitive amplifier using a fiber Bragg grating-based tunable phase shifter.

A low-loss Raman-assisted phase sensitive amplifier (PSA) with a ∼20  dB signal net gain is experimentally demonstrated. The amplitude and phase adjustment for PSA are achieved by using non-uniform Raman gain and a tunable fiber Bragg grating (FBG), respectively. The total component loss of the system is measured to be ∼8  dB.

High-Capacity Free-Space Optical Communications Between a Ground Transmitter and a Ground Receiver via a UAV Using Multiplexing of Multiple Orbital-Angular-Momentum Beams.

We explore the use of orbital-angular-momentum (OAM)-multiplexing to increase the capacity of free-space data transmission to moving platforms, with an added potential benefit of decreasing the probability of data intercept. Specifically, we experimentally demonstrate and characterize the performance of an OAM-multiplexed, free-space optical (FSO) communications link between a ground transmitter and a ground receiver via a moving unmanned-aerial-vehicle (UAV). We achieve a total capacity of 80 Gbit/s up to 100-m-roundtrip link by multiplexing 2 OAM beams, each carrying a 40-Gbit/s quadrature-phase-shift-keying (QPSK) signal.

Using a complex optical orbital-angular-momentum spectrum to measure object parameters.

Light beams can be characterized by their complex spatial profiles in both intensity and phase. Analogous to time signals, which can be decomposed into multiple orthogonal frequency functions, a light beam can also be decomposed into a set of spatial modes that are taken from an orthogonal basis. Such decomposition can potentially provide a tool for spatial spectrum analysis, which may enable stable, accurate, and robust extraction of physical object information that may not be readily achievable using traditional approaches.

Photochemical reaction of hydrogen with germanosilicate glass initiated by 3.4 5.4-eV ultraviolet light.

By measuring the photosensitivity of Ge-doped silica fibers over the 3.4-5.4-eV spectral range and from T=95 K to T=375 K , we demonstrate that different physical mechanisms take place depending on whether H(2) is present.

Increase of photosensitivity in Ge-doped fibers under strain.

Straining a Ge-doped optical fiber increases its UV photosensitivity. The speed of grating fabrication in a standard AT&T telecommunication fiber increases by 18 times, while the final refractive-index change is enhanced by a factor of 5. Measurements reveal opposite changes in the stress of the fiber core induced by UV light in strained and unstrained fibers.

Bragg grating fabrication in germanosilicate fibers by use of near-UV light: a new pathway for refractive-index changes.

Using 334-nm light, we demonstrate side writing of Bragg gratings with an index change of ~10(-4) in germanium-doped fibers. No hydrogen loading of the fibers was required. These gratings have the same temperature stability as gratings fabricated with 240-nm light.

Efficient photobleaching of 390-nm luminescence in germanosilicate preforms by the third harmonic of a Nd:YAG laser.

Permanent photobleaching of blue luminescence with a maximum at ~390 nm in Ge-doped fiber preforms by exposure to the third harmonic (354 nm) of a Nd:YAG laser has been observed. The experimental results show that photodestruction of a germanium oxygen-deficient center can occur without photoionization.

Efficient photoinduced second-harmonic generation in Ce-doped lead germanate glasses.

Efficient photoinduced second-harmonic generation in Ce-doped bulk lead germanate glasses is observed. A linear increase of photoinduced second-order nonlinearity with Ce concentration is obtained. The reported glasses can be encoded for frequency doubling with extremely low (as little as 30 W of peak power at 1.