Stephane Lessard

Experimental study of 112 Gb/s short reach transmission employing PAM formats and SiP intensity modulator at 1.3 μm

We present a Silicon Photonic (SiP) intensity modulator operating at 1.3 μm with pulse amplitude modulation formats for short reach transmission employing a digital to analog converter for the RF signal generator, enabling pulse shaping and precompensation of the transmitters frequency response. Details of the SiP Mach-Zehnder interfometer are presented. We study the system performance at various bit rates, PAM orders and propagation distances. To the best of our knowledge, we report the first demonstration of a 112 Gb/s transmission over 10 km of SMF fiber operating below pre-FEC BER threshold of 3.8 × 10(-3) employing PAM-8 at 37.4 Gbaud using a fully packaged SiP modulator. An analytical model for the Q-factor metric applicable for multilevel PAM-N signaling is derived and accurately experimentally verified in the case of Gaussian noise limited detection. System performance is experimentally investigated and it is demonstrated that PAM order selection can be optimally chosen as a function of the desired throughput. We demonstrate the ability of the proposed transmitter to exhibit software-defined transmission for short reach applications by selecting PAM order, symbol rate and pulse shape.

Experimental Parametric Study of a Silicon Photonic Modulator Enabled 112-Gb/s PAM Transmission System With a DAC and ADC

A packaged silicon photonic traveling wave Mach-Zehnder modulator operating at 1310 nm is presented and studied. The modulator is a series push-pull device and requires a bias applied to the common n-doped region. Effects of varying the bias voltage on the modulators Vπ bandwidth, and on chip-insertion loss are studied, and its impact on transmission performance are experimentally investigated for PAM-4 and PAM-8 formats at a throughput of 112 Gb/s, over varying distances of 0, 2, 10, and 20 km. Residual chromatic dispersion is shown to have no impact on performance up to 20 km. The PAM RF driving waveform is generated by an 8-bit digital-to-analog converter, while direct detection is done with a PIN+TIA followed by an 8-bit analog-to-digital converter. This IM/DD system utilizes digital signal processing with transmitter spectral compensation and receiver residual equalization. The performance impact for a varying number of transmitter precompensation taps and receiver equalization taps is studied, which has a direct impact on the transceivers power consumption. Using PAM-4, a BER below the FEC limit is obtained for either combinations of (transmitter, receiver) tap lengths: (19,19), (7,27), or (35,7), allowing flexibility in power consumption distribution.

224-Gb/s 10-km Transmission of PDM PAM-4 at 1.3 μm Using a Single Intensity-Modulated Laser and a Direct-Detection MIMO DSP-Based Receiver

A polarization-division-multiplexed (PDM) intensity-modulation/direct-detection (IM/DD) system enabled by a novel multiple-input and multiple-output DSP operating in the Stokes space following a DD receiver is demonstrated. Modulating the intensity of the two orthogonal polarization states of a single laser enables doubling the maximum achievable bit rate per wavelength channel, which halves the number of required laser sources in a transceiver using PDM and WDM to achieve an aggregate bit rate compared to using only WDM. Quantitatively, 224 Gb/s is experimentally transmitted over 10 km using a single 1310-nm laser and a silicon photonic intensity modulator using 56-Gbaud PDM PAM-4 with a BER of 4.1 × 10-3. Also, PDM enables halving the baud rate needed to achieve 112 Gb/s resulting in 20-km transmission at low BERs (10-5 -10-6), using either 56-Gbaud PAM-2 or 28-Gbaud PAM-4. These low pre-FEC BERs achieved at 112 Gb/s allow reducing the FEC overhead required compared to a single polarization system that employs twice the baud rate to achieve the same bit rate. Though the transceiver was implemented using discrete components, it can be fully integrated on a SiP chip, enabling its practical realization for short-reach optical interconnects inside datacenters. Finally, in addition to the experimental results, we perform simulations to further investigate the performance of the receiver. In particular, we studied the impact of varying the splitting ratios of the two couplers in the proposed front-end and concluded that using 67/33 couplers instead of 50/50 couplers renders the performance completely independent of the state of polarization of the received signal.

107 Gb/s PAM-4 transmission over 10 km using a SiP series push-pull modulator at 1310 nm

Using a series push-pull SiP modulator at 1310 nm, transmission using PAM-4 at 53.5 GBaud over 10 km of SMF-28 fiber is realized at a BER of 3.6×1 0^-3.

1λ × 224 Gb/s 10 km transmission of polarization division multiplexed PAM-4 signals using 1.3 μm SiP intensity modulator and a direct-detection MIMO-based receiver

Enabled by MIMO DSP and direct-detection receiver, two polarization states of a single laser are intensity modulated with 56 Gbaud PAM-4 signal using a packaged 1310 nm SiP modulator to transmit 224 Gb/s over 10 km with BER of 3×10-3.

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 2.2×10-4. 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.

A Silicon Photonic Broken Racetrack Resonator for Large-Scale Tuning of FSR

We propose and demonstrate experimentally a novel racetrack resonator, in which the perimeter is broken into two sections that are coupled together by forming directional couplers. Simulation and experimental results confirm the performance of the broken racetrack geometry. We demonstrate that the free spectral range of the resonator shrinks from 5.7 to 0.3 nm by testing seven devices with discrete variations in the displacement of the inner segment.

Glass interposer for short reach optical connectivity

We propose a glass interposer containing femtosecond laser-scribed waveguides to interconnect silicon photonic chips. The glass interposer has an insertion loss of about 1.5 dB/cm, and simplifies alignment of silicon photonic chips. Our experiment shows that the insertion loss for the grating coupler/inscribed glass interface was only 0.5 dB higher than the estimated coupling loss of grating coupler to SMF. The 3 dB coupling degradation occurs after 5 µm of in-plane displacement between the laser-inscribed waveguide and the grating coupler.

Trends in High-Speed Interconnects for Datacenter Networking: Multidimensional Formats and Their Enabling DSP

The unprecedented growth in datacenter (DC) traffic, which constitutes the major part of today’s IP traffic, is driving the quest for faster inexpensive short-reach optics to be deployed inside these large warehouse-sized DCs in the so-called intra-DC interconnects over reaches up to 10 km.