This tutorial provides an overview of free-space optical (FSO) communications operating in the mid-infrared (mid-IR) wavelength bands. In the mid-IR region, there are two atmosphere transmission windows: (mid-wave infrared (MWIR) at 3–5 µm and long-wave infrared (LWIR) at 8–12 µm) with relatively low atmospheric absorption. Compared to near-infrared (near-IR) wavelengths (e.g., telecom C-band at ∼1.55 µm), mid-IR tends to be more resilient to atmospheric degradation effects (e.g., fog-induced scattering and atmospheric turbulence). Thus, using mid-IR wavelengths can enable more robust FSO links through challenging atmospheric conditions. This tutorial aims to review recent developments and advances in mid-IR FSO communications. Various devices and approaches for mid-IR data transmitters and receivers are discussed. Advanced demonstrations for single-channel and multi-channel-multiplexed high-capacity mid-IR FSO links are reviewed. This tutorial also discusses the challenges and limitations of current techniques, as well as the outlook for future research.

With the iterative upgrade and popular application of new information technologies such as 5G, cloud computing, big data, and artificial intelligence (AI), the global data traffic and the demand for computing power has ushered in explosive growth [...]

Space-time wave packets (STWPs) with correlated spatial and frequency degrees of freedom exhibit time-dependent spatial interference, thereby giving rise to interesting dynamic evolution behaviors. While versatile spatiotemporal phenomena have been demonstrated in freely propagating fields, coupling spatiotemporal light into multimode fibers remains a fundamental experimental challenge. Whereas synthesizing freely propagating STWPs typically relies on a continuum of plane-wave modes, their multimode-fiber counterparts must be constructed from the discrete set of fiber modes whose propagation constants depend on fiber structures. Here, we demonstrate STWPs with axially controllable motion of the transverse profile and reconfigurable group velocity in graded-index multimode fibers. This is accomplished by introducing a linear association between frequency comb lines and corresponding fiber modes. The synthesized STWPs present dynamic rotation and translation with a 4.8-ps period. Simultaneously, the group velocity can be tuned from positive subluminal and superluminal to negative values (e.g., 0.870, 1.35, 10, and –3.3 × 108 m/s, respectively). Su et al. demonstrate ultra-fast light pulses in multimode fiber with dynamically evolving spatial profiles and tunable group velocities ranging from subluminal to superluminal and negative values, achieved through correlated space-time wave packets.

We experimentally demonstrate optical ranging through underwater scattering using a range(z)-dependent angular rotation of a spatially structured beam. This beam is generated by combining two Bessel modes, and each Bessel mode carries a unique pair of orbital angular momentum (OAM) order (l(i), i = 1 or 2) and longitudinal wavenumber (k(z)(li) , i = 1 or 2). With two OAM orders l, a transverse petal-like intensity pattern is formed. With two different k(z) values, the relative phase between the two Bessel modes linearly changes along the longitudinal position, and thus, the angular rotation of the petal-like intensity pattern changes with the propagation distance. By measuring the angular rotation of the reflected beam profile, the distance information is retrieved. In our experiment, the scattering extinction coefficient (gamma) is varied from 0 to 9.4 m(-1) with ranging distance up to 0.4 m. The experimental results show that (a) the measurement error (delta(z)) for our scenario is < 10 mm in clean water and (b) the petal-like intensity profile maintains its shape and delta(z) <20 mm is achieved under scattering conditions.

In this Letter, we demonstrate turbulence mitigation of four mode-division-multiplexing (MDM) quadrature-phase-shift-keying (QPSK) channels in a pilot-assisted self-coherent free-space optical (FSO) link using a photodetector (PD) array and digital signal processing (DSP)-based channel demultiplexing. A Gaussian pilot beam is co-transmitted with four 1-Gbaud QPSK channels carried by four orbital angular momentum (OAM) modes. The pilot beam experiences similar turbulence-induced wavefront distortion to the data beams. At the receiver, the turbulence distortion is mitigated by its conjugate during the pilot–data mixing in a PD array. Subsequently, we demultiplex the four channels by applying in DSP a fixed matrix on the signals. Results show that our approach has <3-dB turbulence-induced power penalty at a 7% forward error correction (FEC) limit under a turbulence strength of 2 w 0 / r 0 = ∼4.4. The same turbulence can cause >18-dB penalties for a local oscillator (LO)-based coherent MDM system.

Free-space optical (FSO) communications in the mid-infrared (mid-IR) wavelength region has gained increasing interest due, in part, to the lower atmospheric loss than at lower wavelengths. In this paper, we review multi-channel mid-IR FSO communications using channel multiplexing to increase the system data capacity. We discuss different channel multiplexing schemes and channel degradation effects for mid-IR beam propagating through the atmosphere. Additionally, we review different techniques for mid-IR data transceivers in mid-IR FSO links. Finally, we describe several recent multi-channel mid-IR FSO link demonstrations resulting in increased total data rates.

In general, space-time wave packets with correlations between transverse spatial fields and temporal frequency spectra can lead to unique spatiotemporal dynamics, thus enabling control of the instantaneous light properties. However, spatiotemporal dynamics generated in previous approaches manifest themselves at a given propagation distance yet not arbitrarily tailored longitudinally. Here, we propose and demonstrate a new versatile class of judiciously synthesized wave packets whose spatiotemporal evolution can be arbitrarily engineered to take place at various predesigned distances along the longitudinal propagation path. Spatiotemporal synthesis is achieved by introducing a 2-dimensional spectrum comprising both temporal and longitudinal wavenumbers associated with specific transverse Bessel-Gaussian fields. The resulting spectra are then employed to produce wave packets evolving in both time and axial distance - in full accord with the theoretical analysis. In this respect, various light degrees of freedom can be independently manipulated, such as intensity, polarization, and transverse spatial distribution (e.g., orbital angular momentum). Through a temporal-longitudinal frequency comb spectrum, we simulate the synthesis of the aforementioned wave packet properties, indicating a decrease in relative error compared to the desired phenomena as more spectral components are incorporated. Additionally, we experimentally demonstrate tailorable spatiotemporal fields carrying time- and longitudinal-varying orbital angular momentum, such that the local topological charge evolves every ~1 ps in the time domain and 10 cm axially. We believe that our space-time wave packets can significantly expand the exploration of spatiotemporal dynamics in the longitudinal dimension, and potentially enable novel applications in ultrafast microscopy, light-matter interactions, and nonlinear optics.

We experimentally demonstrate a 10-Gbit/s free-space communication link using a single Laguerre-Gaussian (LG) beam with tunable radial and azimuthal modal indices generated by a photonic integrated circuit comprising two concentric uniform circular antenna arrays (UCAs). To tune the azimuthal modal indices ℓ of the generated beam, the azimuthal phase gradient inside each UCA is tuned. To tune the radial mode p of the generated beam, the amplitude ratio and phase difference between the two concentric UCA are tuned. To implement the above functions, the integrated device is composed of (a) two concentric UCAs where the inner (outer) UCA has 4 (8) optical antennas, (b) one Mach-Zehnder interferometer to control the amplitude ratio between the two UCAs, (c) one phase shifter to control the phase distribution between the two UCAs, and (d) phase shifters to control the azimuthal phase gradient of the inner and outer UCA. In our experiment, (a) the two modal indices of the generated beam are independently tuned (ℓ ={0,+1},p={0,1}), (b) the measured mode purity of the generated beam ranges from 23% to 38% among different target LG modes, and (c) a 10-Gbit/s chip-to-free-space optical link carried by the generated tunable LG beam is demonstrated.

We demonstrate a 10-Gbit/s mid-IR coherent link through fog using an OPO to generate a ~100-mW QPSK channel at ~3400 nm. Our link achieves BERs below the 20% FEC limit under fog that causes ~18-dB mid-IR power loss, corresponding to ~40-dB loss for ~1550 nm.

This publisher's note contains a correction to Opt. Lett.48, 6452 (2023)10.1364/OL.506270.

We experimentally demonstrate an optics-based half-adder of two 4-phase-shift-keying (4-PSK) data channels using nonlinear wave mixing. The optics-based half-adder has two 4-ary phase-encoded inputs (i.e., SA and SB) and two phase-encoded outputs (i.e., Sum and Carry). The input quaternary base numbers {0,1,2,3} are represented by 4-PSK signals A and B with four phase levels. Along with the original signals A and B, the phase-conjugate signal copies A* and B*and phase-doubled signal copies A2 and B2 are also generated to form two signal groups SA(A, A*, A2) and SB(B, B*, B2). All of the above signals in the same signal group are (a) prepared in the electrical domain with a frequency spacing of Δf and (b) generated optically in the same IQ modulator. When combined with a pump laser, group SA mixes with group SB in a periodically poled lithium niobate nonlinear (PPLN) device. At the output of the PPLN device, both the Sum (A2B2) and the Carry (AB + A*B*) are simultaneously generated with four phase levels and two phase levels, respectively. In our experiment, the symbol rates can be varied between 5 Gbaud and 10 Gbaud. The experimental results show that (i) the measured conversion efficiency of two 5-Gbaud outputs is approximately -24 dB for Sum and approximately -20 dB for Carry, and (ii) the measured optical signal-to-noise ratio (OSNR) penalty of the 10-Gbaud Sum and Carry channels is <10 dB and <5 dB, compared with that of the 5-Gbaud channels at the BER of 3.8 × 10-3.

We experimentally demonstrate “automatic” turbulence mitigation of a self-coherent FSO MDM link with four 1-Gbaud QPSK channels using a transmitted pilot beam and PD array without power-splitting losses or detector-bandwidth sharing. Results show <3-dB turbulence-induced penalty, as compared to a >16-dB penalty for a conventional LO-based MDM system.

mospheric turbulence can cause critical problems in many applications. To effectively avoid or mitigate turbulence, knowledge of turbulence strength at various distances could be of immense value. Due to light-matter interaction, optical beams can probe longitudinal turbulence changes. Unfortunately, previous approaches tended to be limited to relatively short distances or large transceivers. Here, we explore turbulence probing utilizing multiple sequentially transmitted longitudinally structured beams. Each beam is composed of Bessel-Gaussian ( BG_𝓁=0,k_z ) modes with different k_z values such that a distance-varying beam width is produced, which results in a distance- and turbulence-dependent modal coupling to 𝓁0 orders. Our simulation shows that this approach has relatively uniform and low errors (<0.3 dB) over a 10-km path with up to 30-dB turbulence-structure-constant variation. We experimentally demonstrate this approach for two emulated turbulence regions ( 15-dB variation) with <0.8-dB errors. Compared to previous techniques, our approach can potentially probe longer distances or require smaller transceivers.

Networks can play a key role in high-speed and reconfigurable arithmetic computing. However, two performance bottlenecks may arise when: (i) relying solely on electronics to handle computation for multiple data channels at high data rates, and (ii) the data streams input to a processing node (PN) are transmitted as phase-encoded signals over an optical network. We experimentally demonstrate the operation of optically-assisted reconfigurable average of two 4-phase-encoded data channels at 10- and 20-Gbaud rates. Our input signals are two streams of 2-bit numbers representing a binary floating-point format, and the operation results in 7-phase-encoded output signals represented by 3-bit numbers. The average operation is achieved in three stages: (1) phase encoding and division-using an optical modulator to encode the data streams; (2) summation-using a highly nonlinear fiber (HNLF); and (3) multicast-using a periodically poled lithium niobate (PPLN) waveguide to multicast back the result into the original signal wavelengths. The experimental results validate the concept, and the measured penalties indicate that: (i) the error vector magnitudes (EVMs) of optical signals increase at each stage and reach ∼18-21% for the final multicast results, and (ii) compared to the inputs, the optical signal-to-noise ratio (OSNR) penalty of output is ∼6.7 dB for the 10-Gbaud rate and ∼6.9 dB for the 20-Gbaud rate at a bit error rate (BER) of 3.8e-3.

We experimentally demonstrate STWPs in fiber by synthesizing various spatial modes onto frequency comb lines. Fiber STWPs are tailored to: (a) dynamically rotate/translate with a 4.8- ps period, and (b) have a tunable group velocity of 10.05, 0.88, and -3.24×10 8 m/s.

There are various performance advantages when using temporal phase-based data encoding and coherent detection with a local oscillator (LO) in free-space optical (FSO) links. However, atmospheric turbulence can cause power coupling from the Gaussian mode of the data beam to higher-order modes, resulting in significantly degraded mixing efficiency between the data beam and a Gaussian LO. Photorefractive crystal-based self-pumped phase conjugation has been previously demonstrated to "automatically" mitigate turbulence with limited-rate free-space-coupled data modulation (e.g., <1 Mbit/s). Here, we demonstrate automatic turbulence mitigation in a 2-Gbit/s quadrature-phase-shift-keying (QPSK) coherent FSO link using degenerate four-wave-mixing (DFWM)-based phase conjugation and fiber-coupled data modulation. Specifically, we counter-propagate a Gaussian probe from the receiver (Rx) to the transmitter (Tx) through turbulence. At the Tx, we generate a Gaussian beam carrying QPSK data by a fiber-coupled phase modulator. Subsequently, we create a phase conjugate data beam through a photorefractive crystal-based DFWM involving the Gaussian data beam, the turbulence-distorted probe, and a spatially filtered Gaussian copy of the probe beam. Finally, the phase conjugate beam is transmitted back to the Rx for turbulence mitigation. Compared to a coherent FSO link without mitigation, our approach shows up to ∼14-dB higher LO-data mixing efficiency and achieves error vector magnitude (EVM) performance of <16% under various turbulence realizations.

We demonstrate a 1.12-Tbps error-free data transmission throughout inverse- designed multimode photonic circuits using spectrally flattened microcombs as a multiwavelength laser source.

Spatiotemporal sculpturing of light pulse with ultimately sophisticated structures represents a major goal of the everlasting pursue of ultra-fast information transmission and processing as well as ultra-intense energy concentration and extraction. It also holds the key to unlock new extraordinary fundamental physical effects. Traditionally, spatiotemporal light pulses are always treated as spatiotemporally separable wave packet as solution of the Maxwell's equations. In the past decade, however, more generalized forms of spatiotemporally nonseparable solution started to emerge with growing importance for their striking physical effects. This roadmap intends to highlight the recent advances in the creation and control of increasingly complex spatiotemporally sculptured pulses, from spatiotemporally separable to complex nonseparable states, with diverse geometric and topological structures, presenting a bird's eye viewpoint on the zoology of spatiotemporal light fields and the outlook of future trends and open challenges.

When a Laguerre Gaussian (LG) beam propagates through the dynamic air-water interface, the aerosol above the water and the water surface curvature could induce various degradations, resulting in modal power coupling. Such degradations include (i) beam distortion, which affects the spatial amplitude and phase of the beam, and (ii) beam wandering, which induces misalignment between the receiver and the beam. Our results for a transmitted LG11 beam show that: (i) with the increase of the sine-shape water curvature frequency from 4 to 8 Hz, the modal coupling to adjacent modes increases from & SIM;-11 to & SIM;-7 dB; (ii) with the increase of aerosol attenuation from 1-2 to 3-5 dB, the modal coupling increases from & SIM;-12 to & SIM;-8 dB; (iii) the combination of curvature and aerosol effect could induce stronger modal coupling compared to the single-effect cases. Furthermore, we study the contributions of beam distortion and beam wandering to modal coupling. We find that: (i) there is & SIM;-8 dB modal coupling from LG11 to adjacent modes under the simulated beam-wandering-only case (the beam wandering corresponds to the one under 8 Hz curvature); (ii) the higher modal coupling for the experimental result under the 8 Hz curvature (& SIM;-7 dB) might be due to beam distortion. Additionally, we demonstrate a 1-Gbit/s on-off-keying (OOK) link carried by a single LG11 beam under aerosol and curvature effects.