Environment-Dependent Eu2+-Activated Ba3CaK(PO4)3 toward White-Light Emission by Chemical Cosubstitution of the (BO3)3- Anion Group.

Investigating the phosphors doped with single activators in a single component to realize white-light emission is urgently desired for phosphor-converted white-light-emitting diodes. In this work, on the basis of the chemical unit cosubstitution strategy, the new borophosphate phosphors Ba3CaK(PO4)3-x(BO3)x:0.02Eu2+ with a mixed anion group were prepared. Coupling structure refinement and photoluminescence analyses, Ba3CaK(PO4)3-x(BO3)x consists of five different cationic sites with different coordination environments, with Eu2+ occupying the three sites for Ba2+. In the process of partial substituting (BO3)3- for (PO4)3-, because of the greatly distorted coordination field generated from the difference in the geometric configurations between the two anion groups, a red shift and broadening of the emission bands occurs, resulting in a color-adjustable emission from blue to white. A phosphor-converted light-emitting diode has been successfully fabricated with the incorporation of an as-prepared Ba3CaK(PO4)2.6(BO3)0.4:0.02Eu2+ phosphor and a 405 nm near-ultraviolet chip, which exhibits Commission International de I'Eclairage chromaticity coordinates of (0.31, 0.37) and a correlated color temperature of 6295 K. As demonstrated in the present work, an approach adopted from phosphate to borophosphate is conducted to develop high-quality phosphors.

Orthogonally polarized frequency comb generation from a Kerr comb via cross-phase modulation.

We experimentally demonstrate that a single microresonator can emit two orthogonally polarized individually coherent combs: (i) a strong polarized soliton comb and (ii) an orthogonally polarized continuous wave seeded weaker comb, generated from the first one via cross-phase modulation, sharing the repetition rate of the soliton comb. Experimental results show that the power of the transverse electric-polarized seed can be well below the threshold of comb generation (e.g., 0.1 mW). In addition, simulations show that a dark pulse could be generated in the anomalous dispersion regime by a bright soliton when the two orthogonally polarized modes have the same group velocity in the microresonator.

Coherent optical wireless communication link employing orbital angular momentum multiplexing in a ballistic and diffusive scattering medium.

We experimentally investigate the scattering effect on an 80 Gbit/s orbital angular momentum (OAM) multiplexed optical wireless communication link. The power loss, mode purity, cross talk, and bit error rate performance are measured and analyzed for different OAM modes under scattering levels from ballistic to diffusive regions. Results show that (i) power loss is the main impairment in the ballistic scattering, while the mode purities of different OAM modes are not significantly affected; (ii) in the diffusive scattering, however, the performance of an OAM-multiplexed link further suffers from the increased cross talk between the different OAM modes.

Switchable detector array scheme to reduce the effect of single-photon detector's deadtime in a multi-bit/photon quantum link.

We explore the use of a switchable single-photon detector (SPD) array scheme to reduce the effect of a detector's deadtime for a multi-bit/photon quantum link. The case of data encoding using M possible orbital-angular-momentum (OAM) states is specifically studied in this paper. Our method uses N SPDs with a controllable M × N optical switch and we use a Monte Carlo-based method to simulate the quantum detection process. The simulation results show that with the use of the switchable SPD array, the detection system can allow a higher incident photon rate than what might otherwise be limited by detectors' deadtime. For the case of M = 4, N = 20, a 50-ns deadtime for the individual SPDs, an average photon number per pulse of 0.1, and under the limit that at most 10 % of the photon-containing pulses are missed, the switchable SPD array will allow an incident photon rate of 2250 million counts/s (Mcts/s). This is 25 times the 90 Mcts/s incident photon rate that a non-switchable, 4-SPD array will allow. The increase in incident photon rate is more than the 5 times increase, which is the simple increase in the number of SPDs and the number of OAM encoding states (e.g., N/M = 20/4).

Experimental demonstration of beaconless beam displacement tracking for an orbital angular momentum multiplexed free-space optical link.

In this Letter, we experimentally demonstrate beaconless beam displacement tracking for free-space optical communication link multiplexing multiple orbital angular momentum (OAM) beams, where the data-carrying OAM beams are used for position detection. 400 Gbit/s data transmission is demonstrated under emulated lateral displacement of up to ±10 mm with power penalties of less than 3 dB for all channels. Channel crosstalk is reduced by the beam tracking system to below -18 dB. Moreover, we investigate using a Gaussian beacon for beam displacement tracking, and achieve similar channel crosstalk and power penalties, compared with using the beaconless beam tracking.

400-Gbit/s QPSK free-space optical communicationlink based on four-fold multiplexing of Hermite-Gaussian or Laguerre-Gaussian modes by varying both modal indices.

We experimentally demonstrate the four-fold multiplexing of Hermite-Gaussian (HGmn) or Laguerre-Gaussian (LGlp) modes to achieve a 400 Gbit/s quadrature-phase-shift-keyed (QPSK) free-space optical communication link. In this experiment, both modal indices for the HG and LG modes are simultaneously utilized to achieve a larger potential orthogonal modal space. Moreover, we also investigate and compare the effects of aperture size, lateral displacement, and rotation on the system. We measure the degree of performance degradation due to the following: (1) a receiver aperture with a limited size causes power loss and crosstalk for both HG and LG modes; (2) a lateral misalignment between the transmitter and receiver which leads to crosstalk for LG modes, but might not affect HG0n or HGm0 modes due to axial symmetry; and (3) a rotational misalignment between the transmitter and receiver which causes crosstalk for the HG modes, but does not tend to affect the LG modes due to azimuthal symmetry.

Demonstration of a 10 Mbit/s quantum communication link by encoding data on two Laguerre-Gaussian modes with different radial indices.

We experimentally demonstrate a 10 Mbit/s free-space quantum communication link using data encoding on orthogonal Laguerre-Gaussian (LG) modes with the same azimuthal index but different radial indices. Data encoding on two LGℓp modes (i.e., for ℓ=0, we encode ["0", "1"] as [p=0, p=1], and for ℓ=1, we encode ["0", "1"] as [p=0, p=1]) is demonstrated by employing directly modulated laser diodes and helical phase holograms. The quantum symbol error rate (QSER) of <5% is achieved at an encoding rate of 10 Mbit/s. Moreover, the influence of the circle radius (R) of the receiver phase pattern on registered photon rates and QSERs is investigated. Our results show that a receiver phase pattern whose R does not match the beam size of the LG modes would induce higher cross talk between the two encoded quantum branches.

Experimental utilization of repeated spatial-mode shifting for achieving discrete delays in a free-space recirculating loop.

We demonstrate an optical recirculating delay loop by shifting the spatial mode order of orbital-angular-momentum (OAM) beams in the free-space. The desired delay can be selected at the loop output by exploiting the orthogonality of the OAM modes. When sending a 20-Gbaud quadrature-phase-shift-keyed (QPSK) signal through the delay system, three recirculations are demonstrated, each with an additional delay of 2.2 ns. Around 0.5 and 2 dB system penalties are measured for the second and third recirculations, respectively. We also simulate the performance of our approach under different scenarios.

Spatially multiplexed orbital-angular-momentum-encoded single photon and classical channels in a free-space optical communication link.

We experimentally demonstrate spatial multiplexing of an orbital angular momentum (OAM)-encoded quantum channel and a classical Gaussian beam with a different wavelength and orthogonal polarization. Data rates as large as 100 MHz are achieved by encoding on two different OAM states by employing a combination of independently modulated laser diodes and helical phase holograms. The influence of OAM mode spacing, encoding bandwidth, and interference from the co-propagating Gaussian beam on registered photon count rates and quantum bit error rates is investigated. Our results show that the deleterious effects of intermodal crosstalk effects on system performance become less important for OAM mode spacing Δ≥2 (corresponding to a crosstalk value of less than -18.5 dB). The use of OAM domain can additionally offer at least 10.4 dB isolation besides that provided by wavelength and polarization, leading to a further suppression of interference from the classical channel.

Localization from the unique intensity gradient of an orbital-angular-momentum beam.

We propose and simulate the use of the unique intensity gradient of beams carrying orbital angular momentum (OAM) for tracking and localization of an object. We design a three-pixel detector structure to efficiently determine the intensity gradient of an OAM beam. The resultant intensity gradient is then used to calculate the offset direction and distance of the target object from the center of the OAM beam. Our simulation results indicate the following: (i) an OAM-based localization system can have a stronger control signal than the one generated from a Gaussian beam; (ii) an OAM+2 beam may generate a ∼5× stronger localization feedback signal but operates over half the target capture area as an OAM+1 beam; and (iii) our scheme will generally have two orders of magnitude lower accuracy but ∼2× larger coverage area as the distance from the beam emitter to the target increases from 200 to 1000 m.

Dual-pump generation of high-coherence primary Kerr combs with multiple sub-lines.

We experimentally generate high-coherence primary Kerr combs with multiple sub-lines by using dual pumps and demonstrate the application of a primary comb state in multichannel communications. We find that more than 10 primary comb lines can be generated within the spectrum of modulation instability gain in our microring resonator. The generation is also verified by numerical simulations and the measured linewidth confirms the high coherence of the generated primary comb lines. We also demonstrate the high-coherence characteristics in a coherent communication experiment, in which each comb line is encoded with 20 Gbaud quadrature phase-shift-keyed signals.

Tunable insertion of multiple lines into a Kerr frequency comb using electro-optical modulators.

We experimentally insert a flexible number of electro-optical (EO) comb lines into a Kerr frequency comb via EO modulation and demonstrate the use of combined Kerr and EO combs as light sources in coherent communications. The number of EO lines inserted into the Kerr comb can be varied by changing the modulation frequency. Additionally, the inserted EO comb is found to have similar coherence to that of the Kerr comb, as indicated by their linewidths. The high coherence of both the Kerr and EO combs has further been demonstrated in a communication experiment in which the comb lines are encoded with 10 Gbaud quadrature phase-shift-keyed signals. The increased number of comb lines after EO modulation indicates the possibility of obtaining more data channels in optical communications.

Orbital angular momentum beams generated by passive dielectric phase masks and their performance in a communication link.

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.

Spatial light structuring using a combination of multiple orthogonal orbital angular momentum beams with complex coefficients.

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.

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. As a proof-of-concept example, we measure an object's opening angle using orbital-angular-momentum (OAM) -based complex spectrum, achieving a >15 dB signal-to-noise ratio. Moreover, the dip (i.e., notch) positions of the OAM intensity spectrum are dependent on an object's opening angle but independent of the opening's angular orientation, whereas the slope of the OAM phase spectrum is dependent on the opening's orientation but independent of the opening angle.

Demonstration of a 280 Gbit/s free-space space-division-multiplexing communications link utilizing plane-wave spatial multiplexing.

We demonstrate a 280 Gbit/s free-space space-division-multiplexing communications link incorporating a set of independent tilted truncated plane-waves, each generated by a single mode fiber placed at the back-focal plane of a spherical lens. Each of the seven tilted plane-wave channels are encoded with a 40 Gbit/s 16-QAM signal. Our approach comprises two identical linear fiber-arrays placed approximately 5 m apart. As each fiber array is placed at the back-focal-plane of a spherical lens, each fiber array is effectively placed in a conjugate image plane of the other. A channel crosstalk of less than 26 dB is shown, with a bit-error-rate below the FEC threshold of 3.8×10(-3).

Experimental demonstration of a 200-Gbit/s free-space optical link by multiplexing Laguerre-Gaussian beams with different radial indices.

We demonstrate a 200-Gbit/s space-division multiplexing system using two Laguerre-Gaussian (LG) beams with different radial indices (LGℓ=0,p=0 and LGℓ=0,p=1). With a proper design of the radial change of the demultiplexing pattern, the channel crosstalk could be minimized and both channels could achieve a bit error rate of 3.8×10-3. Moreover, the multiplexing of four LG beams with different azimuthal indices and different radial indices (e.g., LGℓ=0,p=0, LGℓ=0,p=1, LGℓ=1,p=0, and LGℓ=1,p=1 beams) is also demonstrated with a <-12 dB channel crosstalk, potentially enabling a 400-Gbit/s data transmission.

Effect of a breather soliton in Kerr frequency combs on optical communication systems.

In this study, we numerically investigate the effect of Kerr-comb-generated breather soliton pulses on optical communication systems. The breather soliton pulse amplitude and spectrum envelope oscillate periodically in time. Simulations show that the spectrum of each comb line in the breather soliton state has multiple sub-teeth due to the periodic oscillation of the comb spectrum. In the simulation, the comb output is modulated with different formats. We find that the sub-teeth distort quadrature phase-shift-keyed signals but have less of an effect on on-off-keyed signals.

Experimental characterization of a 400 Gbit/s orbital angular momentum multiplexed free-space optical link over 120 m.

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.

Atmospheric turbulence mitigation in an OAM-based MIMO free-space optical link using spatial diversity combined with MIMO equalization.

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.