Intra-datacenter Applications Research Articles

First Demonstration of an O-Band Coherent Link for Intra-Data Center Applications

First Demonstration of an O-Band Coherent Link for Intra-Data Center Applications

800G/$\lambda $ Self-Homodyne Coherent Links With Simplified DSP for Next-Generation Intra-Data Centers

Assuming a low-latency forward error correction (FEC)-based bit-error-rate (BER) threshold, we demonstrate and provide a detailed analysis of a 120 Gbaud/DP-16QAM single-carrier self-homodyne coherent link in the C-band. Through theoretical and simulation analysis as well as experimental investigation, we find that a minimum receiver DSP based on a simple I-Q correction scheme is needed to compensate for the combined effect of fiber dispersion, I-Q phase rotation, transmitter I-Q impairment, and receiver skew in combination with a finite analog to digital converter (ADC) resolution. Based on the simplified DSP, we demonstrate link budgets of 4.5 dB (for 16 amplified dense wavelength division multiplexing (DWDM) channels) and 5.2 dB (unamplified single-channel case). The amplified DWDM self-homodyne coherent system provides a 12.8 Tb/s capacity per fiber pair and is useful for bandwidth-demanding intra-data center applications.

Simultaneously precise frequency response and IQ skew calibration in a self-homodyne coherent optical transmission system.

The self-homodyne coherent detection (SHCD) system is becoming more popular in intra-data center applications nowadays. However, for a high-speed SHCD system, the device imperfection such as transmitter (Tx) and receiver (Rx) side in-phase (I)/quadrature-phase (Q) time skew and bandwidth limitation will greatly restrict the transmission performance. The current mainstream calibration methods for traditional optical transceivers rely on the effect of frequency offset and phase noise to separate the Tx and Rx imperfection, which is not compatible with the SHCD system. In this paper, we have proposed and demonstrated a highly precise calibration method that can be applied in dual-polarization (DP) SHCD system. Based on the specially designed multi-tone signals, the amplitude/phase frequency response (AFR/PFR) of the transceiver and the Tx/Rx IQ skew can be obtained by just one measurement even after long-distance fiber transmission. By using a 4 MHz linewidth distributed feedback (DFB) laser, a DP SHCD transmission system combined with a 20 GHz optical transceiver and two 10 km standard single-mode fibers is experimentally constructed. The test results indicate that the measurement error of the AFR/PFR and Tx/Rx skew are within ±1dB/±0.15rad and ±0.3ps respectively, and the dynamic range for IQ skew calibration can reach dozens of picoseconds. The measured bit error rate value of 46GBaud DP-16QAM signals/35GBaud DP-64QAM signals are improved from 2.30e-2 to 2.18e-3/9.59e-2 to 2.20e-2 with the help of the proposed calibration method.

Experimental demonstration of advanced modulation formats for data center networks on 200 Gb/s lane rate IMDD links.

This work contributes experimental demonstrations and comprehensive comparisons of various modulation and coding techniques for 200 Gb/s intensity modulation and direct detection links including four-level pulse amplitude modulation (PAM-4), PAM-6, trellis-coded modulation (TCM) over PAM and discrete multi-tone (DMT) transmission. Both C-band Mach-Zehnder modulator and O-band electro-absorption modulated laser transmitters were examined for intra-data center applications based on state-of-the-art commercial components.

Hardware Efficient Adaptive Equalizer for Coherent Short-Reach Optical Interconnects

We propose a novel adaptive equalization (AEQ) algorithm for short-reach coherent optical interconnects targeting intra-datacenter applications. The algorithm consists of 1-tap $2\times 2$ complex-valued multiple-input multiple-output (MIMO) finite impulse response (FIR) filter for polarization division de-multiplexing, four N-tap real-valued FIR filters for digital equalization, and a post three-tap T-spaced $4\times 4$ real-valued MIMO FIR filter for the timing skew mitigation. Up to 8-km standard single-mode fiber (SSMF) transmission experiments with 28-Gbaud PDM-16QAM signals verifies that less than the 1-dB receiver sensitivity penalty can be tolerated in comparison with conventional $4\times 4$ MIMO AEQ, while the number of multipliers can be reduced by ~59% if both algorithms use 25 taps. The tolerance of timing skew and chromatic dispersion for the AEQ algorithm is numerically investigated.

400-Gb/s DMT-SDM Transmission Based on Membrane DML-Array-on-Silicon

We report on space-division multiplexing (SDM) transmissions of up to 400 Gb/s over a homogeneous four-core fiber using discrete multitone (DMT) modulation for intra-datacenter applications and 200/400 GbE links. The transmission system is enabled by a compact, SDM-channel scalable, multi-core fiber (MCF) pluggable, and energy-efficient SDM transmitter composed of a 4-channel 1.3-μm membrane directly modulated laser (DML) array-on-silicon integrated with a fiber-bundle type fan-in with loss of less than 1 dB and negligible optical and electrical crosstalk (XT). By simultaneously modulating all four lasers, 200-Gb/s (50-Gb/s/channel) and 400-Gb/s (100-Gb/s/channel) are achieved over a 425-m MCF link for KP4 and soft-decision forward error correction, respectively, with a power reduction of ~6× compared with conventional DML transmitters. Additionally, numerical simulations based on a detailed rate-equations model predict the feasibility of the system for MCF transmissions up to 10 km even with moderate levels of XT.

Half-Terabit Single-Carrier Direct-Detect Transceiver, Formats, and DSP: Analysis and Demonstration

In this paper, we extend our recent work and further demonstrate and analyze a single-carrier short-reach direct-detect transceiver and two modulation formats, dual-polarization pulse amplitude modulation 4 plus 4-phase modulation (DP-PAM4+4PM) and DP-PAM4+3PM, for intradata center applications delivering 504 and 462 Gb/s, respectively. To the best of the authors’ knowledge, this is the first demonstration of a transceiver delivering half a terabit per second on a single carrier with direct detection at a BER of 1.8 $\times\, 10^{\mathbf {-3}}$ , below the threshold of hard-decision FEC of 6.69% overhead. We thoroughly detail the receiver digital signal processing that leverages the performance of this transceiver and formats. We derive the adaptation of all filters and coefficients that are updated using the stochastic gradient descent algorithm. Even if the transceiver and formats are meant to be operated in the O-band, we present several performance results in the C-band, where chromatic dispersion is large. We present the performance of the 504 Gb/s single carrier over varying received signal powers and for three different MIMO/MISO filters lengths, and show that the required received power decreases by as much as 2.4 dB when 10 neighboring symbols are included in the DSP compared to 4. We also demonstrate that, when operated in the C-band, the DP-PAM4+4PM format delivers 448 Gb/s of error-free data payload over 500 m and DP-PAM4+3PM delivers 400 Gb/s of error-free payload over 1 km, independently of the state of polarization of the received signal. Finally, we experimentally compare at 84 GBaud the BER degradation of the PAM4- $\hat{x}$ , PAM4- $\hat{y}$ , and PM tributaries of the DP-PAM4+4PM and DP-PAM4+3PM formats from 0 to 1 km, demonstrating much worst degradation of the PAM4 bearing phase modulation caused by phase to intensity conversion via chromatic dispersion.

Comparison of single-/few-/multi-mode 850 nm VCSELs for optical OFDM transmission.

For high-speed optical OFDM transmission applications, a comprehensive comparison of the homemade multi-/few-/single-transverse mode (MM/FM/SM) vertical cavity surface emitting laser (VCSEL) chips is performed. With microwave probe, the direct encoding of pre-leveled 16-QAM OFDM data and transmission over 100-m-long OM4 multi-mode-fiber (MMF) are demonstrated for intra-datacenter applications. The MM VCSEL chip with the largest emission aperture of 11 μm reveals the highest differential quantum efficiency which provides the highest optical power of 8.67 mW but exhibits the lowest encodable bandwidth of 21 GHz. In contrast, the SM VCSEL chip fabricated with the smallest emission aperture of only 3 μm provides the highest 3-dB encoding bandwidth up to 23 GHz at a cost of slight heat accumulation. After optimization, with the trade-off set between the receiving signal-to-noise ratio (SNR) and bandwidth, the FM VCSEL chip guarantees the highest optical OFDM transmission bit rate of 96 Gbit/s under back-to-back case with its strongest throughput. Among three VCSEL chips, the SM VCSEL chip with nearly modal-dispersion free feature is treated as the best candidate for carrying the pre-leveled 16-QAM OFDM data over 100-m OM4-MMF with same material structure but exhibits different oxide-layer confined gain cross-sections with one another at 80-Gbit/s with the smallest receiving power penalty of 1.77 dB.

336 Gb/s in Direct Detection Below KP4 FEC Threshold for Intra Data Center Applications

We demonstrate direct detection of single polarization and dual polarization PAM4 signals delivering 168 and 336 Gb/s below the KP4 FEC threshold of $2.2\times 10^{{ {-4}}}$ . The 84 Gbaud 2 bits and 4 bits per symbol transceivers operate at a bit error rate (BER) that is compliant to the RS(544, 514) FEC encoding for error-free operation, as considered for next generation PAM4 400GBase Ethernet standard targeting data center applications. For the DP-PAM4 format, we show that the BER is independent of the state of polarization of the optical signal. To the best of our knowledge, this is the first experimental demonstration of a single carrier delivering 336 Gb/s in direct detection at a BER compliant to the physical coding sublayer of 400GBASE interfaces. Finally, using an intensity modulator in series with a phase modulator, we demonstrate chromatic dispersion pre-compensation of 84 Gbaud PAM4 signals, producing a 40-fold BER improvement after 10 km of single-mode fiber.

Silicon and hybrid silicon photonic devices for intra-datacenter applications: state of the art and perspectives [Invited

We review the state of the art and our perspectives on silicon and hybrid silicon photonic devices for optical interconnects in datacenters. After a brief discussion of the key requirements for intra-datacenter optical interconnects, we propose a wavelength-division-multiplexing (WDM)-based optical interconnect for intra-datacenter applications. Following our proposed interconnects configuration, the bulk of the review emphasizes recent developments concerning on-chip hybrid silicon microlasers and WDM transmitters, and silicon photonic switch fabrics for intra-datacenters. For hybrid silicon microlasers and WDM transmitters, we outline the remaining challenges and key issues toward realizing low power consumption, direct modulation, and integration of multiwavelength microlaser arrays. For silicon photonic switch fabrics, we review various topologies and configurations of high-port-count N-by-N switch fabrics using Mach–Zehnder interferometers and microring resonators as switch elements, and discuss their prospects toward practical implementations with active reconfiguration. For the microring-based switch fabrics, we review recent developments of active stabilization schemes at the subsystem level. Last, we outline several large challenges and problems for silicon and hybrid silicon photonics to meet for intra-datacenter applications and propose potential solutions.