Yue Zuo

The mid-infrared (MIR) spectral regime is central to applications including remote sensing, precision spectroscopy, higher harmonic generation, and free-space optical communication. However, coherent and broadband MIR modulation remains challenging owing to high optical loss, limited bandwidth, and large drive voltages in existing platforms. Here, we overcome the challenges by deploying a suspended thin-film lithium-niobate (TFLN) based electro-optic (EO) platform co-designed with high-performance traveling-wave microwave (MW) electrodes. We demonstrate a record-low V _{π , D C} of 2.3 to 4.3 V over a broadband MIR bandwidth from 2.4 to 3.6 μm, and a 2.3-dB EO bandwidth of 40 GHz (extracted 3-dB bandwidth of 52 GHz), yielding a figure-of-merit of 19.3 GHz/V–more than an order of magnitude higher than the state-of-the-art. We demonstrate, for the first time, high-frequency V _{π , M W} of 4.5-6.5 V in the 27-34 GHz range, and frequency-agile MIR EO frequency comb generation with a 10-dB optical bandwidth over 0.8 THz using a suspended phase modulator of 4-cm active modulation length. We further validate the platform in a free-space optical communication link. Our results establish a monolithic MIR photonic platform capable of powerful EO modulation and spectral synthesis, and present a significant step towards reconfigurable MIR sensing and communication systems on chip.

The full adder (FA) and full subtractor (FS) are key operations used in the arithmetic and logic units of digital processors. Performing the FA and FS optically may enable the direct line-rate processing of high-speed optical data. Previous demonstrations of optical FA and FS have not been shown for phase-encoded input and output data signals. Such phase-encoded data formats could offer improved noise tolerances and longer distances of transmission compared to amplitude-encoded data formats. In this work, we experimentally demonstrate an optical FA and FS for binary-phase-shift-keying (BPSK) data signals at 2.5- and 5-Gbit/s. Our approach utilizes input BPSK data signals and continuous-wave pumps that are encoded at different frequencies, in which one of the input BPSK data signals is negated for the FS case. The data and pumps undergo nonlinear wave mixing in a periodically poled lithium niobate (PPLN) waveguide to generate the FA or FS BPSK outputs at different frequencies. The experimental results show error-free logic operation for both the FA and FS outputs at 2.5- and 5-Gbit/s over 1536 captured bits.

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.

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.

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 μm 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.

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.