Laguerre–Gaussian (LG) beams with different azimuthal indices are orthogonal to each other regardless of their radial indices. We investigate the potential advantage and impact of multiplexing LG beams with different azimuthal indices and nonzero radial indices, compared to LG beams with different azimuthal indices but zero radial index. Our simulation results show that LG beams with nonzero radial indices could help in receiving higher signal power in a free-space optical link under a certain short transmission distance range, especially when the receiver has a very limited aperture size. Our experimental demonstration shows a more than 6 dB received power improvement when multiplexing four LG beams with nonzero radial indices.

We demonstrate the generation of orbital angular momentum (OAM) beams using high-efficient polarization-insensitive phase masks. The OAM beams generated by the phase masks are characterized in terms of their tolerance to misalignment (lateral displacement or tilt) between the incident beam and phase mask. For certain scenarios, our results show that (a) when the tilt angle is within the range of -20 to +20  deg, the crosstalk among modes is less than -15  dB; and (b) lateral displacement of 0.3 mm could cause a large amount of power leaked to adjacent modes. Finally, OAM beams generated by the phase masks are demonstrated over a two-channel OAM-multiplexing link, each channel carrying a 40 Gbit/s data stream. An optical signal-to-noise-ratio (OSNR) penalty of ∼1  dB is measured without crosstalk at the bit error rate (BER) of 3.8×10-3. With crosstalk, an OSNR penalty of <1.5  dB is observed at the same BER.

An all-optical pilot-tone-based self-homodyne detection scheme using nonlinear wave mixing is experimentally demonstrated. Two scenarios are investigated using (1) multiple wavelength-division-multiplexed channels with sufficient power of the pilot tones and (2) a single channel with a low-power pilot tone. The eye diagram and bit error rate of the system are studied by tuning various parameters such as pump power, relative phase, and pilot-to-signal ratio.

Analogous to time signals that can be composed of multiple frequency functions, we use uniquely structured orthogonal spatial modes to create different beam shapes. We tailor the spatial structure by judiciously choosing a weighted combination of multiple modal states within an orthogonal orbital angular momentum (OAM) basis set, creating desired beam intensity “shapes.” The weights of the OAM beams to be combined forms a Fourier pair with the spatial intensity distribution in the azimuthal direction of the resultant beam. As an example, we simulate and experimentally create various beam shapes by designing the weights of the combined OAM beams. We also find that 6× higher localized power, as compared to traditional beam combining, could be achieved by coherently combining nine orthogonal OAM beams.

All optical signal level swapping and multilevel amplitude noise mitigation are experimentally demonstrated using the three gain regions of optical parametric amplification, i.e., linear, saturation, and inversion. The two-amplitude-shift-keying and eight-quadrature-amplitude-modulation optical communication systems with baud rates of both 10 and 20 Gbaud have been employed to demonstrate the proposed approaches. Less than 1% error-vector-magnitude degradation is observed after signal level swapping. For amplitude noise mitigation, a more than 20% decrease in amplitude error is confirmed.

Orbital angular momentum (OAM) has gained interest due to its potential to increase capacity in optical communication systems as well as an additional domain for reconfigurable networks. This is due to the following: (i) coaxially propagated OAM beams with different charges are mutually orthogonal, (ii) OAM beams can be efficiently multiplexed and demultiplexed, and (iii) OAM charges can be efficiently manipulated. Therefore, multiple data-carrying OAM beams could have the potential capability for reconfigurable optical switching and routing. In this paper, we discuss work involving reconfigurable OAM-based optical add/drop multiplexing, space switching, polarization switching, channel hopping, and multicasting.

We experimentally demonstrate a tunable simultaneous ROADM with crosstalk reduction using optical nonlinear elements in spectrally overlapped 20-25 Gbaud QPSK WDM channels. OSNR gain of 5 dB shows the effectiveness of the approach.

In this paper, recent studies on the potential challenges for an orbital angular momentum (OAM) multiplexing system were reviewed. The design guideline for a practical OAM multiplexing system were investigated in term of (i) the power loss due to the beam divergence and limited-size receiver, and (ii) the channel crosstalk due to the misalignment between the transmitter and receiver.

Free-space optical communications can play a significant role in line-of-sight links. In general, data can be encoded on the amplitude, phase, or temporal position of the optical wave. Importantly, there are environments for which ever-more infounation is desired for a given amount of optical energy. This can be accomplished if there are more degrees-of-freedom that the wave can occupy to provide higher energy efficiency for a given capacity (i.e., bits/photon). Traditionally, free-space optical links have used only a single beam, such that there was little opportunity for a wave to occupy more than one spatial location, thereby not allowing the spatial domain to be used for data encoding. Recently, space- and mode-multiplexing has been demonstrated to simultaneously transmit multiple data-carrying free-space beams. Each spatially overlapping mode was orthogonal to other modes and carried a unique amount of orbital-angular-momentum (OAM). In this paper, we consider that OAM modes could be a data-encoding domain, such that a beam could uniquely occupy one of many modes, i.e., 4 modes would provide 4 possible states and double the bits of information for the same amount of energy. In the past, such OAM-based encoding was shown at kHz data rates. We will present the architecture and experimental results for OAM-based data encoding for a free-space 1.55-mu m data link under different system parameters. Key features of the results include: (a) encoding on several modes is accomplished using a fast switch, and (b) low bit-error-rates are achieved at >Gbit/s, which is orders-of-magnitude faster than previous results.

We experimentally demonstrate and investigate the performance of a multi-input-multiple-output (MIMO) free-space optical link employing orbital-angular-momentum multiplexing through turbulent channel. We show that turbulence effects can be mitigated by exploiting spatial diversity and MIMO equalization.

In this paper, the generation and steering of beams carrying orbital angular momentum utilizing a custom-designed circular antenna array has been demonstrated at 28 GHz. A steering angle as large as 30 degrees for an orbital angular momentum (OAM) beam has been achieved. The effect of number of antennas and the distance from antennas to the array center to the quality of beam generation and beam steering is investigated through both experiments and simulations. Our results indicate that: (1) As the steering angle increases, the mode purity of the generated OAM beams decreases; (2) Increasing the number of antennas improves the OAM mode purity; (3) For a fixed number of antennas, high mode purity is observed for lower order OAM modes; (4) Placing the antennas farther away from the array center allows for reduced divergence of the generate OAM beams.

We experimentally demonstrate and characterize the performance of a 400-Gbit/s orbital angular momentum (OAM) multiplexed free-space optical link over 120 m on the roof of a building. Four OAM beams, each carrying a 100-Gbit/s quadrature-phase-shift-keyed channel are multiplexed and transmitted. We investigate the influence of channel impairments on the received power, intermodal crosstalk among channels, and system power penalties. Without laser tracking and compensation systems, the measured received power and crosstalk among OAM channels fluctuate by 4.5 dB and 5 dB, respectively, over 180 s. For a beam displacement of 2 mm that corresponds to a pointing error less than 16.7 μrad, the link bit error rates are below the forward error correction threshold of 3.8×10(-3) for all channels. Both experimental and simulation results show that power penalties increase rapidly when the displacement increases.

In this paper, we explore the potential benefits and limitations of using transmitter lenses in an orbital-angular-momentum (OAM)-multiplexed free-space optical (FSO) communication link. Both simulation and experimental results indicate that within certain transmission distances, using lenses at the transmitter to focus OAM beams could reduce power loss in OAM-based FSO links and that this improvement might be more significant for higher-order OAM beams. Moreover, the use of transmitter lenses could enhance system tolerance to angular error between transmitter and receiver, but they might degrade tolerance to lateral displacement.

Analogous to time signals that can be composed of multiple frequency functions, we use uniquely structured orthogonal spatial modes to create different beam shapes. We tailor the spatial structure by judiciously choosing a weighted combination of multiple modal states within an orthogonal basis set, and we can tunably create beam phase and intensity "shapes" that are not otherwise readily achievable. As an example shape, we use a series of orbital-angular-momentum (OAM) functions with adjustable complex weights to create a reconfigurable spatial region of higher localized power as compared to traditional beam combining. We simulate a structured beam created by coherently combining several orthogonal OAM beams with different complex weights, and we achieve a >10X localized power density enhancement with 19 beams. Additionally, we can create unique shapes by passing a single beam through a specially designed phase and intensity mask that contains the combination of multiple OAM functions each with complex weights. Using this approach, we experimentally demonstrate a ~2.5X localized power density increase when utilizing 9 functions.

We present a short-range experimental orbital-angular-momentum (OAM) multiplexing millimeter-wave communication system using patch antenna arrays. The dependence of the evolution of OAM beams on the number of array elements and array radius are analyzed. We also find phase delay deviation of 30 degrees leads to mode purity degrading to ∼1% and power deviation of 6 dB reduces the purity by <40%. An obstruction of size <10mm has not much influence on the evolution of OAM beams generated from antenna arrays with r = 8mm, which enables the stacking of multiple antenna arrays. We also use these patch antenna arrays to demonstrate a 60 GHz wireless communication link using two multiplexed OAM modes, each carrying a 500-Mbaud 16-QAM or 2-Gbaud QPSK signal. A channel crosstalk of less than −20 dB and bit-error-rates (BER) of less than 3.8 × 10−3 are achieved.

To increase system capacity of underwater optical communications, we employ the spatial domain to simultaneously transmit multiple orthogonal spatial beams, each carrying an independent data channel. In this paper, we show up to a 40-Gbit/s link by multiplexing and transmitting four green orbital angular momentum (OAM) beams through a single aperture. Moreover, we investigate the degrading effects of scattering/turbidity, water current, and thermal gradient-induced turbulence, and we find that thermal gradients cause the most distortions and turbidity causes the most loss. We show systems results using two different data generation techniques, one at 1064 nm for 10-Gbit/s/beam and one at 520 nm for 1-Gbit/s/beam; we use both techniques since present data-modulation technologies are faster for infrared (IR) than for green. For the 40-Gbit/s link, data is modulated in the IR, and OAM imprinting is performed in the green using a specially-designed metasurface phase mask. For the 4-Gbit/s link, a green laser diode is directly modulated. Finally, we show that inter-channel crosstalk induced by thermal gradients can be mitigated using multi-channel equalisation processing.

We explore the mitigation of atmospheric turbulence effects for orbital angular momentum (OAM)-based free-space optical (FSO) communications with multiple-input multiple-output (MIMO) architecture. Such a system employs multiple spatially separated aperture elements at the transmitter/receiver, and each transmitter aperture contains multiplexed data-carrying OAM beams. We propose to use spatial diversity combined with MIMO equalization to mitigate both weak and strong turbulence distortions. In a 2×2 FSO link with each transmitter aperture containing two multiplexed OAM modes of ℓ=+1 and ℓ=+3, we experimentally show that at least two OAM data channels could be recovered under both weak and strong turbulence distortions using selection diversity assisted with MIMO equalization.

We characterize a low-phase-noise Kerr frequency comb for carrying data channels. Results show pump wavelength detuning region grows slightly at higher pump power and the phase noise stays relatively unchanged over pump wavelength detuning region.

In line-of-sight communication systems, accurate alignment between the transmitter and receiver is important to guarantee sufficient signal power at the receiver. Such alignment is even more important for orbital angular momentum (OAM) multiplexing systems since misalignment between the transmitter and receiver may cause crosstalk among channels. In this paper, we demonstrate the simultaneous generation and tunable steering of two OAM beams utilising a custom-designed circular antenna array at 28 GHz. We achieve a steering angle of up to 35 degrees from the antenna array normal. We find that (i) the steering angle of the generated OAM beams is limited by the emitting angle of the antenna elements, and (ii) a larger steering angle may degrade the mode purity of the generated OAM beams as well as induce inter-symbol-interference to each of the individual channels. Moreover, we demonstrate the transmission of two 1-Gbaud quadratic phase shift keying (QPSK) signal over the two steerable OAM beams with both multiplexed channels achieved bit error rates (BERs) of <3.8 × 10−3.

We experimentally demonstrate the use of orbital angular momentum (OAM) superposition as basis for the generation of a directed energy beam. Different OAM superpositions are tested with a maximum achieved peak power gain of ~2.5.