We experimentally demonstrate a >Tbit/s mid-infrared (mid-IR) free-space optical (FSO) communication system by combining wavelength-division multiplexing (WDM), polarization-division multiplexing (PDM), and mode-division multiplexing (MDM). We transmit 16 multiplexed data channels simultaneously by employing 4 wavelengths at ∼3.4 μm, 2 polarizations, and 2 orbital angular momentum (OAM) modes with orders of ℓ = +1 and ℓ = +3. The channels are generated in the C-band at the transmitter and converted to mid-IR through difference frequency generation (DFG) nonlinear process. At the receiver, they are converted back to C-band through DFG and detected in the C-band. With each channel carrying a 32-Gbaud QPSK signal, a total data rate of 1.024 Tbit/s is achieved. Our experimental results show that (i) the channel crosstalk between different wavelengths, polarizations, and modes are less than -19 dB, -16 dB, and -15 dB, respectively, (ii) bit-error rates of all multiplexed channels can reach below the 20% forward error correction threshold, and (iii) there is <1-dB optical signal-to-noise ratio (OSNR) penalty induced by channel multiplexing in a single domain (WDM, PDM, or MDM alone), while the OSNR penalty increases to ∼2 dB when combining the three multiplexing schemes.

In general, as compared to direct detection, coherent detection with a local oscillator (LO) can enable advanced complex modulation formats and provide a higher receiver sensitivity. Previous demonstrations of coherent detection in mid-infrared (MIR) free-space optical (FSO) links were achieved by first converting the MIR data to the C-band and then using a C-band detector with a C-band LO for data reception. However, the nonlinear wavelength conversion typically requires a high-power pump (e.g., > watts), which increases the system power consumption. Here, we demonstrate coherent detection of MIR data channels at ∼3.4 μm using a detector in the native MIR domain without wavelength conversion of the channels. Specifically, at the receiver, we mix the data channel with a frequency-offset MIR LO in a MIR mercury cadmium telluride (MCT) detector for coherent heterodyne detection. We demonstrate the coherent detection of (i) a single 1-Gbaud data channel with various modulation formats, including OOK, QPSK, 16 QAM, and 64 QAM, and (ii) five multi-subcarrier-multiplexed 1-Gbaud 16-QAM channels. Our results show ∼14-dB improvement in receiver sensitivity compared to direct detection using the same detector.

Underwater optical ranging is of increasing interest for high-resolution measurements over several meters, with timeof- flight (ToF) being one key approach. Besides the temporal information used in this approach, utilizing the spatial degree of freedom might further enhance ToF ranging performance. In this work, we experimentally demonstrate enhanced underwater ToF ranging performance by spatially structuring a 520-nm light beam carrying a temporal ToF waveform. The transmitted beam, composed of two Bessel-Gaussian (BG) modes, features a distance-dependent rotation of the transverse intensity profile with multiple rotation periods over the measurement range. Upon reflection from the object, the beam is captured by a ToF camera for simultaneously acquiring the temporal phase delay and spatial intensity profile for ToF-based ranging and structured-beam-based ranging, respectively. The ToF result provides coarse ranging to identify the correct rotation period, while the rotation-angle measurement of the structured beam enables fine distance retrieval within that period. Experimental results show that the combined approach reduces the mean absolute error (MAE) from 65.4 mm to 3.5 mm over 0-0.8 m under scattering water with attenuation coefficient γ = 2.3 m-1, compared to the ToF-only approach using a Gaussian beam. Multiple independent distance measurements further show a reduction in standard deviation (SD) from >30 mm to <4 mm, indicating improved stability of the combined approach under scattering conditions.

We experimentally demonstrate 2 Gbit/s probabilistically shaped (PS) 16-QAM signals at 4.5-μm in a mid-infrared FSO link under fog, achieving ~0.4 dB shaping gain over uniform distributions. 3 Gbit/s PS-64-QAM transmission is also demonstrated.

Free-space optical (FSO) communication in the mid-infrared (mid-IR) region has attracted increasing interest due to its lower atmospheric attenuation compared to near-infrared (near-IR) wavelengths. To further enhance link capacity, mode-division multiplexing (MDM) can be employed to transmit multiple orthogonal spatial channels simultaneously. Previous schemes for mid-IR MDM have converted Gaussian data beams from the near IR into the mid-IR before changing the beams into orthogonal modes. However, such a scheme employs a separate nonlinear wavelength converter for each mode (i.e., N converters for N channels), potentially causing high system complexity. In this paper, we demonstrate using a single wavelength converter from near-IR to achieve a mid-IR mode-division multiplexing (MDM) link. Two orbital angular momentum (OAM) channels with tunable mode orders are generated using spatial light modulators (SLMs) at 1550 nm. The channels are multiplexed and simultaneously converted to 3.4 mu m via difference frequency generation (DFG) in a single PPLN crystal. We demonstrate mid-IR intensity modulation/direct detection (IM/DD) and coherent heterodyne communication. A total of 2-Gbit/s on-off keying (OOK) and 4-Gbit/s 16-quadrature amplitude modulation (QAM) MDM links are achieved, all with bit error ratios (BERs) below the 7% forward error correction (FEC) threshold.

An optical-tapped-delay-line (OTDL) can implement various processing functions on data that is transmitted over an optical network, including format conversion, complex filtering, and pattern recognition. Previous demonstrations of OTDL functions have either processed: (i) one data channel with one or more OTDL functions, or (ii) multiple data channels with one OTDL function per channel. In this work, we experimentally demonstrate three OTDL functions simultaneously on two optical data channels to obtain six optical data outputs. The inputs include: (i) two quadrature-phase-shift-keying (QPSK) channels, and (ii) two on-off-keying (OOK) channels. In each case, delayed data copies and pumps encoded with the OTDL function weights undergo nonlinear wave mixing in a periodically poled lithium niobate (PPLN) waveguide. Six optical data outputs are simultaneously generated at six different frequencies. In the QPSK case, the average symbol correctness over 3072 symbols is: (i) ~99.5% at 7.5-Gbaud and ~98.8% at 10-Gbaud for format conversion, (ii) ~98.1% at 7.5-Gbaud and ~94.3% at 10-Gbaud for complex filtering, and (iii) 100% at both baud rates for pattern recognition. In the OOK case, the average symbol correctness at 7.5-Gbaud is ~99.8% for format conversion, ~99.9% for complex filtering, and 100% for pattern recognition.

Conventional ranging techniques in the radio frequency including millimeter-waves typically involve generating pulsed or frequency-modulated continuous-wave (CW) signals for obtaining range information. These signals typically need temporal modulation of the waveform and therefore require high-bandwidth components. We present a single-frequency CW ranging approach, based on Bessel beams, in the millimeter-wave frequency band (28 GHz), which utilizes longitudinally varying spatial information of the electric field intensity to determine the propagation distance of the beam. The beam exhibits petal-like intensity lobes that rotate with propagation whose rotation angle is proportional to the propagation distance. The effect of various transmitter parameters, such as aperture size and longitudinal wavenumber of Bessel beams, on the rotational property of the petal-like beam is analyzed through simulations. The longitudinally varying petal-like intensity patterns are experimentally demonstrated over a 15 cm propagation distance for two different rotation periods of 50 cm and 10 cm.

Space-time wave packets (STWPs) with correlated spatial and frequency degrees of freedom give rise to time dependent spatial interference effects, enabling interesting and potentially useful dynamic evolution behaviors. In free space, a range of novel spatiotemporal phenomena has been demonstrated. However, all such effects have been observed mainly in freely propagating fields, and coupling spatiotemporally structured light into a multimode fiber remains challenging. In 2025, our recent work [Su et al. (2025)] proposes, simulates, and experimentally demonstrates dynamical STWPs with axially controllable motion of the transverse intensity profile and reconfigurable group velocity in graded-index multimode fibers. We introduce a linear association between optical frequency comb lines and the corresponding fiber modes. The key concept relies on the fact that the propagation constants are linearly dependent on both frequency and mode order in parabolic multimode fibers. We review the results of this work that synthesized wave packets present dynamic rotational and translational motion with a period of 4.8 ps. Simultaneously, the group velocity of these STWPs 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) in multimode fibers supporting up to 55 modes.

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.

Mode-division multiplexing (MDM) may enhance the total capacity of free-space optical (FSO) communication links by transmitting independent data channels on orthogonal spatial modes. A photodetector (PD) array can be a promising MDM receiver architecture due to its compactness and reconfigurability. Previous PD array-based MDM receivers have primarily focused on demultiplexing one-dimensional (1-D) modal sets. In contrast, employing a two-dimensional (2-D) modal set-i.e., modes characterized by two spatial indices-may enable a larger number of orthogonal modes, thereby supporting more parallel data channels and higher link capacity. In this work, we experimentally demonstrate a reconfigurable PD array-based MDM FSO receiver capable of supporting 2-D spatial mode sets. The multiplexed data channels are demultiplexed and recovered using multiple-input multiple-output (MIMO) digital signal processing (DSP). We demonstrate MDM links employing different types and numbers of 2-D spatial modes, including (i) a 60-Gbit/s link multiplexing six 2-D Laguerre-Gaussian (LG) modes detected by ten PD elements, and (ii) a 40-Gbit/s link multiplexing four 2-D Hermite-Gaussian (HG) modes detected by seven PD elements. In both cases, all channels achieve bit-error rates (BERs) below the forward error correction (FEC) threshold of 3.8e-3. We further demonstrate the capability of the PD array-based receiver to mitigate atmospheric turbulence effects in a 40-Gbit/s link multiplexing four 1-D orbital-angular-momentum (OAM) modes. With turbulence-compensating MIMO DSP, the error vector magnitudes (EVMs) of all four channels are reduced to below 35%, whereas they exceed 50% without DSP-based compensation.

We simulate and experimentally demonstrate a longitudinally rotating-petal optical beam generated by integrated circular antenna arrays on silicon. Two concentric eight-antenna rings are phase-tuned to form a two-petal core that rotates by ~1.8π over a 5.6-mm propagation distance.

Abstract Underwater optical ranging has attracted growing attention due to its potential for higher accuracy than acoustic-based approaches. Alternative to typical ranging systems using the temporal/frequency information of lightwaves, a ranging scheme that utilizes spatially structured beams for distance retrieval has been demonstrated recently. In this approach, the beams are composed of Bessel-Gaussian modes carrying different orbital angular momentum orders and longitudinal wavenumbers, exhibiting a two-petal-like transverse intensity profile that rotates along the longitudinal propagation direction. This technique has shown relatively accurate ranging performance in scattering media by retrieving the rotation angle of the structured beam, and it operates using only a continuous-wave laser, without the need for high-bandwidth modulation or detection. This paper reviews the background and recent progress in underwater ranging using longitudinally structured beams and provides a perspective on several research topics for the future implementation of this approach.

We experimentally demonstrate an optical full adder and full subtractor for 5-Gbaud phase-encoded signals using nonlinear wave mixing. The phase-encoded outputs are generated at different wavelengths and exhibit error-free logic results over 1536 symbols. © 2026 The Author(s)

Typical radar transmits conventional Gaussian waves to detect targets. However, the reflected power may be low at certain target orientation angles due to destructive interference of the reflected waves, resulting in a failed detection of the target. Previous works have shown spatially structured Orbital Angular Momentum (OAM)-carrying waves can compensate for the power decrease of Gaussian waves, therefore increasing the reflected power from the target. But previous works assumed that the direction of the target is known and is perfectly aligned with incident radar waves. In this paper, we investigate the effect of misalignment on power enhancement using OAM combined with Gaussian through simulation and experiment. In our case, a maximum enhancement of 8.1 dB is achieved without misalignment. It remains 3.8 dB at maximum misalignment in one direction but drops to approximately 0 dB in the other direction.

The surge in deployment of Low Earth Orbit (LEO) satellites over the last two decades has resulted in a number of incompatible communication protocols, limiting efficient inter-satellite communication. In response, we present the first reconfigurable silicon photonic link supporting both coherent and intensity modulation with direct detection (IM-DD). Our link design supports transmission and reception of on-off keying (OOK), binary phase-shift keying (BPSK), and quadrature phase-shift keying (QPSK) modulated signals across two polarization states, enabling a single transceiver to accommodate multiple common modulation formats. We demonstrate successful operation of the custom transmitter, polarization multiplexer, and receiver portions of our link, including: 1) transmission of OOK, BPSK, and QPSK at symbol rates up to 10 GBaud with an error vector magnitude (EVM) <22% in all cases, 2) polarization cross-talk suppression of over 21 dB, and 3) reception of OOK, BPSK, and QPSK at rates up to 5 GBaud with resolved EVM <17%. This innovation can address the pressing need for standardized communication within LEO satellite constellations, improving interoperability and efficiency in satellite communications.

Singular optics is a branch of modern electromagnetics and optics that investigates solutions to Maxwell’s equations that exhibit nontrivial topological features under various boundary conditions. These solutions give rise to light fields containing singularities, points or regions at which certain optical properties, such as phase or polarization, become undefined. Over time, singular optics has evolved into a unifying framework for understanding and engineering optical fields that possess phase, polarization, coherence, and spatiotemporal singularities, each characterized by quantized topological properties. Such structured light fields enable high-dimensional information encoding, robust light–matter interactions, and sensitive probing of complex media, thereby impacting optical communication, imaging, sensing, and materials processing. Parallel advances in theory, fabrication techniques, detection hardware, and computational methods have created a diverse and rapidly expanding landscape, underscoring the need for an integrated and forward-looking perspective. This roadmap synthesizes emerging applications of singular optics across multiple platforms, offering a concise overview of current developments and highlighting key physical concepts, new architectures, and transformative technologies that bridge subfields. In addition to reflecting the insights of leading contributors to these research directions, it also surveys selected recent advances, providing a concise overview of current trends and a foundation for shaping the future of singular optics and its applications.

In free-space optical (FSO) communication links, receivers located at different distances may have access to the same transmitted data, thus raising concerns about unintended signal detection by eavesdroppers. In this paper, we demonstrate distance-dependent channel recovery in a 40-Gbit/s QPSK mode-multiplexed FSO link, in which the data-channel signal-to-noise ratio (SNR) is sufficiently high only within prescribed spatial regions. This is achieved by tailoring the longitudinal wavenumbers (kz) of optical beams to produce distance-dependent intensity distributions within a finite receiver aperture. The structured beams are synthesized by superimposing multiple Bessel modes with equally spaced longitudinal wavenumbers, thereby enabling direct control over the beam intensity profile within a receiver aperture as a function of distance. Mode-division multiplexing (MDM) is employed to transmit two independent data channels on two orthogonal spatial modes in the link. We demonstrate two scenarios using the two modes with distinct longitudinal intensity distributions: (i) detection of both data channels within the same intended longitudinal region, and (ii) detection of both data channels, with each channel recovered only within a separate intended longitudinal region. Measurements of channel matrices and bit-error-rate (BER) performance confirm a power contrast exceeding 25 dB between the intended and unintended locations. We also investigate the impact of receiver aperture size (0–2 mm) on link performance, showing that at unintended distances the crosstalk suppression between the two channels remains at approximately 10 dB even for large apertures, compared with approximately 30 dB at the intended locations. Finally, we simulate structured beams synthesized with different numbers of longitudinal wavenumbers, indicating that increasing the number of longitudinal wavenumbers improves longitudinal intensity shaping.

We experimentally demonstrate a beam shaping technique for simultaneously mitigating crosstalk effects due to turbulence and misalignment in a 50 Gbit/s/channel quadrature phase shift keying (QPSK) modulated two-channel mode-division-multiplexing (MDM) free-space optical (FSO) communication link. Co-propagating probe beams on a different wavelength close to the data channels are used to measure the channel crosstalk caused by turbulence and misalignment. We modify the transmitted beams to be a superposition of different orbital angular momentum (OAM) modes using the inverse modal coupling of the channel, which reduces the crosstalk between the two data channels at the receiver. Our experimental results show that when both turbulence and misalignment are present: (a) crosstalk reduces by > 9 dB between OAM +1 and +2 to <-14 dB when considering both turbulence and misalignment; (b) the proposed approach induces an optical signal-to-noise ratio (OSNR) penalty of similar to 2 dB for both data channels compared to the back-to-back (B2B) scenario near the 7% forward error correction (FEC) threshold.