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We introduce a bandwidth enhancement technique for traveling-wave Mach-Zehnder modulators (TW-MZM). This approach reduces the microwave attenuation caused by the loaded PN junction capacitance. We achieved a bandwidth of 61 GHz for a 4-mm long silicon modulator, significantly greater than the 45 GHz bandwidth of a conventional design, while maintaining a comparable V$_\pi$. Conducted simulations and circuit analysis demonstrated the efficacy of the proposed approach. We also compared the performance of our solution with the reference modulator experimentally in transmission tests at various baud rates. The proposed micro-capacitor-enhanced TW-MZM has a lower bit error rate and can reach up to 120 Gbaud while respecting the bit error rate threshold for forward error correction with a 7% overhead.

While single sideband (SSB) modulation with orthogonal frequency-division multiplexing (OFDM) is used to avoid chromatic dispersion (CD)-induced fading, it is only effective in the absence of chirp. We examine the effectiveness of a extremely wideband silicon photonics (SiP) microring modulator (MRM) modulator designed for low chirp operation. We identify and confirm via simulation and experiment two independent sources of signal impairment from MRM chirp. Signal throughput can be as much as doubled when using a low-chirp design. This enables adoption of very small, low power, easily integrated SiP modulators for short-reach applications at very high throughput.

Coherent detection increases system capacity and can be combined with multi-channel structures, such as space division multiplexing (SDM), to push to the 1.6 Tbit/s required by future data center interconnects (DCIs). We present a methodology for finding a power efficient SDM multi-channel structure for a given traveling-wave Mach-Zehnder modulator (TW-MZM) design, as specified by its experimentally measured time and frequency domain properties. We focus on silicon photonics (SiP) solutions in our examples, due to their potential for high-scale integration and low cost. We simulate each channel of an SDM structure, considering the hardware characteristics and digital signal processing (DSP) performance enhancement, for an end-to-end system examination. We find the amount of parallelism that yields the lowest combined laser and RF drive power. The choice of operating point (drive voltage) and the judicious use of DSP and clipping must be optimized independently for each SDM structure. For the two SiP modulators examined, we find an SDM structure with three channels consumes the least amount of power among all multi-channel solutions, and saves up to 59% of consumed power vis-à-vis the single-channel solution.

The use of multiple segments on a silicon photonic modulator can increase bandwidth at the cost of greater complexity in the driving signals. We propose and demonstrate a simple driving scheme for use with dual segment modulators that involves very little additional complexity when equal length segments are used. The increased bandwidth with segmentation scales linearly with the number of segments, but the net bit rate does not; net rate depends on many factors. With an IQ modulator with two 2 mm segments, we demonstrate an improvement of 14% in net bit rate as compared to a single 4 mm segment. We examine the trade-offs in moving to three-segment modulation. We explore implementation penalties and use probabilistic shaping and optical pre-emphasis to achieve a net rate of 1.07 Tb/s with dual polarization transmission over 80 km of fiber at 116 Gbaud using 64QAM modulation.

We demonstrate an all-silicon segmented modulator with traveling-wave electrodes, achieving an electro-optic bandwidth beyond 67 GHz and a $V_\pi$ of 5 V. We show that dividing a long optical modulator into shorter segments can help achieve a higher bandwidth without compromising modulation efficiency or optical loss. We analyze the bandwidth limitation due to the delay mismatch between the driving and optical signals. We show that the impact of misalignment of driving delays between segments on the overall bandwidth of the modulator can be described by a finite impulse response filter. Using this modulator, we achieve optical transmission of 120 Gbaud 8-level amplitude shift keying with coherent detection. Taking into account the forward error correction overhead, this yields 336.4 Gb/s net on a single polarization. Our design has the potential for achieving 1 Tb/s using dual-polarization IQ modulation.

Orbital angular momentum (OAM) modes have low model interactions during fiber propagation at data center distances, and thus are suitable for ultra-high capacity systems at low digital signal processing. Generating OAM modes using free-space setups is useful for proof-of-concept experiments, but is not a scalable solution. We use an optical phased array (OPA) with two-dimensional antennas for on-chip circularly polarized OAM beam generation. Our previous work demonstrated an OAM multiplexer for lower-order modes. In this work, we demonstrate an OAM multiplexer that supports a record of 46 (23 per polarization) simultaneous spatial modes up to OAM order 11. We also improve the crosstalk performance of our multiplexer. We incorporate an intensity tuning capability that substantially improves the OAM quality by enabling a uniform power distribution across the antennas. The worst-case crosstalk for the supported OAM5 to OAM11 are found experimentally to be better than $-12$ dB, with OAM10 achieving $-17.2$ dB.

Modal multiplexing can greatly increase fiber capacity, but often at the cost of high digital processing with multiple-input multiple-output (MIMO) algorithms. The use of orbital angular momentum (OAM) for multiplexing can support MIMO-free reception, but it always operates in an interferencelimited regime. Many previous MIMO-free demonstrations for OAM used a type of optical MIMO that is impractical for deployed systems. We compare the performance of MIMO-free and standard electronic 2 × 2 MIMO in a demonstration with no recourse to optical MIMO. We show that the receiver complexity of these two reception strategies is very similar in hardware and signal processing. We explore commercial coherent receivers with 2 × 2 MIMO, hence this solution for OAM demultiplexing can be deployed effectively with current technology. The 2 × 2 MIMO reception enhances the total bit rate and is more compatible with wavelength division multiplexing (WDM) as it equalizes performance across the C-band. At 1.3 km transmission, the net bit rate spans 426–450 Gb/s across wavelengths with 2 × 2 MIMO, while variability is high for MIMO-free reception, spanning 200–418 Gb/s.

Ring core fibers (RCF) enable high-performance modal multiplexing with low crosstalk and can support orbital angular momentum (OAM) modes. RCFs are challenging to characterize due to the lack of commercial multiplexers, especially for high OAM orders. For fibers supporting large numbers of modes, typical cutback techniques for characterization are extremely wasteful of fiber, especially as one cutback is required for each mode. We show the differential modal loss across modes 3 to 10 was significantly underestimated using an OTDR when exciting modes individually or when exciting all modes indiscriminately. We exploit perfect vortex beams to achieve reliable and nondestructive characterization of mode-dependent loss (MDL) for OAM modes. Perfect vortex beams allow us to maximize the coupling efficiency at each mode launch, increasing the accuracy of MDL estimate. We fabricated fiber with a refractive index difference between the ring core and the cladding of a $5.1\times 10^{-2}$. For this fiber, mode orders 3 to 10 are the most suitable for data transmission and were the focus of our work (the fiber support up to OAM order 13). Such a high index difference can lead to MDL. We demonstrate that the modal loss spans from 2.14 to 4.38 dB/km for orders 3 to 10.

Much attention has been focused on the use of scalar modes for space division multiplexing (SDM). Alternative vector mode bases offer another solution set for SDM, expanding the available trade-offs in system performance and complexity. We present two types of ring core fiber conceived and designed to explore SDM with fibers exhibiting low interactions between supported modes. We review demonstrations of fiber data transmission for two separate vector mode bases: one for orbital angular momentum (OAM) modes and one for linearly polarized vector (LPV) modes. The OAM mode demonstrations include short transmissions using commercially available transceivers, as well as kilometer length transmission at extended data rates. The LPV demonstrations span kilometer length transmissions at high data rate with coherent detection, as well as a radio over fiber experiment with direct detection of narrowband signals.

Advanced optical interconnects require high-speed links, which can be achieved by combining high channel rates with wavelength-division multiplexing (WDM). We report a multi-channel transmitter using cascaded microring modulators (MRMs) in silicon photonics. One MRM works as a flexible-grid optical comb generator, while the others work as channel modulators. With a single-wavelength laser input, we achieve flexible channel spacing (up to 25 GHz) with a tone-to-noise ratio above 54 dB at low power consumption of less than 4.6 mW. We examine experimentally multi-channel transmission modulating data onto adjacent comb lines without significant signal crosstalk. This single-laser, flexible-grid WDM transmitter is a scalable solution: more comb lines can be obtained using uncoupled MRMs in a series. To the best of our knowledge, this is the first demonstration of monolithic integration of a comb generator and multi-channel modulators for ultra-compact, power-efficient WDM photonic interconnects.

Codesign and integration of optical modulators and CMOS drivers is crucial for high-speed silicon photonic (SiP) transmitters to reach their full potential for low-cost, low-power electronic-photonic integrated systems. We present a CMOS-driven SiP multi-level optical transmitter implemented using a commercially available lateral p-n junction process. It uses a Mach-Zehnder modulator (MZM) segmented to increase speed and to lower the required power on a per segment basis to a level achievable with CMOS. A multi-channel driver is designed and implemented in 130 nm RF CMOS, providing a swing of 4 V in a push-pull configuration at 20 Gbaud. Binary data at the CMOS input is manipulated via digital logic to produce the proper per-segment drive signals to generate a four-level pulse-amplitude modulation optical signal. Multi-level modulation is achieved using only binary signals as input (DAC-less). Cosimulation of the optical and electrical circuits shows good agreement with experiment. Reliable transmission is achieved without post-compensation at 28 Gb/s, and at 38 Gb/s when using post-compensation. [ABSTRACT FROM PUBLISHER]

We derive a mathematical description of a perfect vortex beam as the Fourier transformation of a Bessel beam. Building on this development, we experimentally generate Bessel–Gauss beams of different orders and Fourier transform them to form perfect vortex beams. By controlling the radial wave vector of a Bessel–Gauss beam, we can control the ring radius of the generated beam. Our theoretical predictions match with the experimental results and also provide an explanation for previous published works. We find the perfect vortex resembles that of an orbital angular momentum (OAM) mode supported in annular profiled waveguides. Our prefect vortex beam generation method can be used to excite OAM modes in an annular core fiber.

We employ simple analytical models to construct the entire frequency-modulation (FM)-noise spectrum of tunable semiconductor lasers. Many contributions to the laser FM noise can be clearly identified from the FM-noise spectrum, such as standard Weiner FM noise incorporating laser relaxation oscillation, excess FM noise due to thermal fluctuations, and carrier-induced refractive index fluctuations from stochastic carrier generation in the passive tuning sections. The contribution of the latter effect is identified by noting a correlation between part of the FM-noise spectrum with the FM-modulation response of the passive sections. We pay particular attention to the case of widely tunable lasers with three independent tuning sections, mainly the sampled-grating distributed Bragg reflector laser, and compare with that of a distributed feedback laser. The theoretical model is confirmed with experimental measurements, with the calculations of the important phase-error variance demonstrating excellent agreement.