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We present a C-band Mach-Zehnder modulator with meandered phase shifters and a compact footprint of 432 × 260 μ m 2 on the silicon-on-insulator platform. Electrode, p-n junction, and optical transit time are considered when performing the electro-optic bandwidth (EO BW) simulations. The simulation results prove that the dominant bandwidth limiting factor for this type of modulator is optical transit time. The insertion loss of the modulator without bias is 2.1 dB. The measured half-wave voltage (V π ) and 3-dB EO BW at −0.5 V bias are 6.4 V and 7.7 GHz, respectively. 53 Gbaud PAM-4 transmission over 2 km of standard single-mode fiber is achieved at a bit error rate (BER) below the 6.7% overhead hard-decision forward error correction BER threshold of 3.8×10 −3.

We present the design and characterization of O-band and C-band silicon photonic (SiP) traveling wave Mach-Zehnder modulators (TW-MZM) allowing 220 Gbps/λ net rate operation. The designed modulators show over 45 GHz 3-dB E-O bandwidth with a single-segment design. In the O-band, with simple linear feed forward equalization, we transmit net 203 (200) Gbps signal over 2 km (10 km) of single-mode fiber (SMF) below the hard-decision forward error correction (HD-FEC) BER threshold of 3.8 × 10−3. With the aid of nonlinear Volterra equalizer and one 2.3Vpp driving signal, we transmit net 225 (216) Gbps PAM8 signals assuming 20% overhead soft-decision FEC with a normalized general mutual information (NGMI) threshold of 0.8798 over 2 km (10 km) of SMF. The C-band design enables net 220 Gbps in B2B and net 215 Gbps over 500 m of SMF above the specified NGMI threshold. These results are the highest reported net rate for SiP MZM in an intensity modulation direct-detection (IM/DD) system, fabricated entirely in a commercial foundry.

We report 108 Gb/s Duo-Binary PAM-8 (DB-PAM-8) transmission using direct-detection aided by Volterra equalizer. The bit-error-rate (BER) performance of the DB-PAM-8 signal has been evaluated through error counting by varying the bit rate, received optical power, and reach. The experimental results suggest that up to 108, 102, and 84 Gb/s signals can be propagated in back-to-back (B2B), and over 10, and 20 km of standard single-mode fiber (SMF), respectively, below the low-density parity-check forward error correction (LDPC-FEC) threshold. Furthermore, we derive a probabilistic model of the BER for multilevel DB-PAM-M signaling that is verified using our measurements. The model of this important performance metric turned out to be accurate enough, especially at low BER values, where its use is more convenient.

In a practical 4- $\lambda $ coarse wavelength division multiplexing (CWDM) intensity modulation direct detection (IM/DD) system, different wavelength channels achieve different transmission performances due to 1) inter-channel component differences and 2) chromatic dispersion (CD) – chirp interaction. Large channel performance discrepancies could significantly degrade the average pre- forward error correction (pre-FEC) bit error rate (BER) performance of the system. To improve the system’s robustness against channel performance discrepancies, we propose to employ a symbol-level wavelength-interleaver in conjunction with four-dimensional - 4-level pulse amplitude modulation - trellis coded modulation (4D-PAM4-TCM). We experimentally compare the performances of the proposed wavelength-interleaved 4D-PAM4-TCM system and a conventional 4D-PAM4-TCM system in a 4- $\lambda \,\,\times100$ Gb/s CWDM IM/DD link over 0–20 km. We observe that the wavelength-interleaved 4D-PAM4-TCM system outperforms the conventional 4D-PAM4-TCM system in terms of receiver sensitivity required at the KP4 FEC threshold when sizable channel performance discrepancies exist in such systems.

Photonic integrated circuits (PICs) suffer from birefringence due to high-index contrast. Polarisation handling devices improve the performance of the PICs by reducing the polarisation-dependent dispersion and loss. Furthermore, there is a growing interest in building polarisation division multiplexed transceivers using PICs, which require polarisation management. The authors provide an overview of the recent work on developing polarisation handling devices such as polarisation beam splitters and polarisers in indium phosphide and silicon-on-insulator platforms for optical communications and sensing applications. These devices expand the PICs library of polarisation handling devices and can be used to design more complex circuits with advanced or new functionalities.

Rapid advances in high‐resolution chip lithography have accelerated nanophotonic device development on the silicon‐on‐insulator (SOI) platform. The ability to create sub‐wavelength features in silicon has attracted research in photonic band and dispersion engineering and consequently made available a wide array of device functionalities. By drawing on recent demonstrations, the authors review how periodic, sub‐wavelength structures are used for passive wave manipulation in SOI device design. The optical response is evaluated for both orthogonal polarisations at the telecom wavelengths of 1310 and 1550 nm. The results offer a versatile toolkit for the integration of these features in conventional nanophotonic device geometries. Notable benefits include a fine control of dispersion, wavelength and polarisation selectivity, and broadband performance.

We demonstrate 100 Gb/s/ $\lambda $ duo-binary PAM-4 (DB-PAM-4) transmission over 50 km standard single-mode fiber (SMF) in the O-band using a 20G silicon-based transmitter and a 25G optically-amplified PIN+TIA-based receiver for 100G PON downstream applications. Receiver sensitivities of −15, −14.5 and −12 dBm are achieved at a bit error rate (BER) below the low-density parity-check forward error correction (LDPC-FEC) threshold (i.e., $1\times 10^{-2}$ ) in back-to-back (B2B), and after 20 and 50 km, respectively using a 131-tap linear feed-forward equalizer (FFE) at the receiver. According to the results, a power budget of 20 dB can be achieved for 20 km transmission. Furthermore, we report the performance of 99 Gb/s (33 Gbaud) PAM-8 transmission and show that DB-PAM-4 provides 5.3 dB better sensitivity compared to PAM-8 at the LDPC-FEC threshold.

We report a C-band compact (100 × 420 µm2) and power efficient (0.9 pJ/bit) SiP meandered Mach-Zehnder modulator with low optical loss (2.09 dB) and demonstrate DAC-less and DSP-free 32 Gbps OOK transmission with Q=6.64 dB.

We demonstrate adiabatic couplers (ADCs) with design-intended splitting ratios (SRs) for the silicon-on-insulator (SOI) platform. The operational principle, numerical simulations, and experimental results on the designed ADC are presented. In the operational principle, we derive the analytical relationship between the SR and the effective index-difference, based on which we calculate the corresponding SRs under various output waveguide width-differences at a central wavelength of 1310 nm. For the simulation process, a segmented optimization method is employed to analyze the adiabatic transition process through the ADC. From the experiment results, broadband ADCs with design-intended power SRs, from 7%/93% to 50%/50%, are fabricated and then tested over a bandwidth of 100 nm, i.e., from 1260 to 1360 nm for the fundamental transverse electric (TE) mode. Furthermore, the thermal stability of the ADC is investigated. When increasing the stage temperature from 20 $^\circ$ C to 50 $^\circ$ C, the variances of the SRs for different wavelengths across O band are within 1.3%, which makes the design a potential candidate for performance robustness against temperature variations.

We design and experimentally demonstrate an ultra-broadband and compact two-mode multiplexer based on subwavelength-grating-slot-assisted adiabatic coupler for the silicon-on-insulator platform. The introduction of subwavelength-grating slot effectively increases the refractive index of the gap region between two strip waveguides and enhances the coupling between them, leading to high modal transmission over a short mode-evolution region, particularly at the short-wavelength regime. As a result, our two-mode multiplexer has a compact design footprint, with a mode-evolution region length of only 25 $\mu$ m and an entire device length of only 55 $\mu$ m. Simulation shows that the mode-division multiplexing (MDM) link formed by the designed two-mode multiplexers has a record bandwidth of 740 nm spanning from 1260 nm to 2000 nm, where the crosstalk is lower than −18.5 dB and the insertion loss (IL) is lower than 0.32 dB. The MDM link has been fabricated using electron beam lithography and achieves measured crosstalk lower than −18.8 dB and IL lower than 2.6 dB, over a 100 nm bandwidth from 1260 nm to 1360 nm that covers the O-band and a 130 nm bandwidth from 1500 nm to 1630 nm that covers the C-band and the L-band.

Providing a low-cost, reliable and end-to-end quality of service (QoS) guaranteed connectivity is going to be a major challenge in optical transport due to the immense growth in number of connected devices and traffic volume triggered by fifth-generation mobile networks (5G) and cloud networks. First, we discuss the architectures of the 5G transport network and the data center network, to capture the full value of connectivity. The paper offers an overview of the network requirements and how the integrated photonic platforms, along with the advanced modulation formats, play a key role to support the unprecedented requirements of 5G and massive cloud deployment. Second, we provide an overview of the recent work on the development of high-speed silicon photonic (SiPh) modulators to generate and transport PAM-4 signals. Third, recent advancements of the photodetectors, optical hybrids, and integrated coherent receivers on the silicon-on-insulator (SOI) platform are also described. The results suggest that, in the first access and aggregation segments of the network, integrated silicon-based modulators aided by PAM-4 modulation formats can support the stringent requirements of new generation transport networks in terms of speed, footprint and power consumption.

Four level pulse amplitude modulation (PAM-4) has become the modulation format of choice to replace on–off keying (OOK) for the 400 Gb/s short reach optical communications systems. In this paper, we investigate the possible modifications to conventional Mach–Zehnder modulator structures to improve the system performance. We present three different silicon photonic Mach–Zehnder modulator architectures for generating PAM-4 in the optical domain using OOK electrical driving signals. We investigate the transfer function and linearity of each modulator and experimentally compare their PAM-4 generation and transmission performance with and without use of digital signal processing (DSP). We achieve the highest reported PAM-4 generation and transmission without the use of DSP. The power consumption of each modulator is presented, and we experimentally show that multielectrode Mach–Zehnder modulators provide a clear advantage at higher symbol rates compared to conventional Mach–Zehnder modulators.

We demonstrate 25 and 50 Gb/s four-level pulse amplitude modulation (PAM-4) transmission over 43- and 21-km standard single-mode fiber (SMF), respectively, using a silicon photonic 120° hybrid-based integrated simplified coherent receiver in the $C$ -band. The integrated receiver is composed of an edge coupler, a vertical grating coupler, a polarization splitter–rotator, a $3\times 3$ multimode interference coupler-based 120° hybrid, and three germanium p-i-n photodetectors. For 25 Gb/s PAM-4 transmission, we achieved the receiver sensitivities of −18, −17.5, −16.6, −14.5, and −9.8 dBm in back-to-back (B2B) and after 10.5, 21, 31, and 43 km, respectively, at a bit error rate (BER) below the hard-decision forward error correction (HD-FEC) threshold (i.e., $3.8\times 10^{-3}$ ) without using any receiver equalizer, while for the 50 Gb/s PAM-4 transmission, the receiver sensitivities of −13.6 dBm in B2B, and −13.5 and −11.6 dBm after 10.5 and 21 km are obtained using a 31-tap linear feed-forward equalizer (FFE), respectively. Furthermore, we achieved up to 44 Gb/s receiver equalizer-free PAM-4 transmission over 10.5 km below the HD-FEC. The characterization of the receiver at different wavelengths within the $C$ -band is provided as well.

We experimentally demonstrate a compact broadband polarization beam splitter (PBS) based on a multimode interference (MMI) coupler with internal photonic crystal (PC) for the silicon-on-insulator platform. The internal PC structure is optimized to be reflective to the transverse electric polarization and transparent to the transverse magnetic polarization over a broad wavelength range. A detailed study of the device operation, including the photonic band gap and the influence of the internal PC structure on each mode of the MMI coupler, is presented. The designed PBS has been fabricated using electron beam lithography and the feature size used in our design is CMOS compatible. The fabricated device achieves measured extinction ratios higher than 20 dB and insertion losses lower than 2 dB for both polarizations over a 77 nm wavelength range from 1522 to 1599 nm that covers the entire C -band, with a device length of only 71.5 $\mu$ m.

We present a silicon photonic dual parallel multielectrode Mach-Zehnder modulator (MEMZM) based transmitter targeting 200 Gb/s four-level pulse amplitude modulation (PAM4) short reach transceivers. The MEMZMs have an average V π and electro-optic (EO) bandwidth of 5 V and 38 GHz, respectively. The transmitter is characterized versus receiver equalizer taps, received signal power, driving voltage swing, crosstalk voltage swing, bitrate, and reach. Results reveal that using only a three-tap equalizer at the receiver, 100 Gb/s PAM4 net rate per lane can be achieved at a bit error rate (BER) below the KP4 forward error correction (KP4-FEC) threshold of 2.4 × 10 -4 . Moreover, up to 128 Gb/s can be received at a BER below the KP4-FEC threshold using only 2 Vpp and 1 Vpp driving the MEMZM segments. Then, both MEMZMs are driven simultaneously to assess the crosstalk impact on the BER performance at parallel operation. Driven by four binary signals, we demonstrate 200 Gb/s PAM4 transmission over up to 10 km of single mode fiber at a BER below the KP4-FEC threshold.

We present a systematic comparison of PAM-2 (NRZ), Duobinary-PAM-2, PAM-4, and Duobinary-PAM-4 (duo-quaternary) signaling in the context of short-reach photonic communications systems using a Mach–Zehnder modulator as transmitter. The effect on system performance with a relaxed and constrained system’s opto-electronic bandwidth is analyzed for bit rates ranging from 20 to 116 Gb/s. In contrast to previous analyses, our approach employs the same experimental and simulation conditions for all modulation formats. Consequently, we were able to confidently determine the performance limits of each format for particular values of bit rate, system bandwidth, transmitter chirp, and fiber dispersion. We demonstrate that Duobinary-PAM-4 is a good signaling choice only for bandwidth-limited systems operating at relatively high speed. Otherwise, PAM-4 represents a more sensible choice. Moreover, our analysis put forward the existence of transition points: specific bit rate values where the BER versus bit rate curves for two different formats cross each other. They indicate the bit rate values where, for specific system conditions, switching from one modulation to another guarantees optimum performance. Their existence naturally led to the proposal of a format-selective transceiver, a component that, according to network conditions, operates with the most adequate modulation format. Since all analyzed modulations share similar implementation details, signaling switching is achieved by simply changing the sampling point and threshold count at the receiver, bringing flexibility to IM/DD-based optical networks.

Nonreciprocity is a fundamental requirement of signal isolation in optical communication systems. However, on chip isolator designs require either post-processing steps or external magnetic biasing, which are impractical for commercial applications. This raises the need for standalone devices which support nonreciprocal functionality using standardized fabrication techniques. Here, we report the first design of an electromagnetic coil surrounding a waveguide which exclusively employed the complementary metal-oxide-semiconductor (CMOS) process flow. The coil supported an electric current up to 14 mA. In simulations, it generated an alternating magnetic flux density up to 1.16 mT inside a strip waveguide and thereby induced a rotation of 50.71 picodegrees for the fundamental transverse-magnetic mode at a wavelength of 1352 nm. Our analysis further revealed methods to increase the rotation by orders of magnitude. It demonstrated the scope of manufacturing processes and serves as a building block for the development of a commercially viable, on-chip optical isolator.

We review design considerations for silicon photonic single-segment and multi-segment Mach-Zehnder modulators for net 200 Gbit/s/lane intensity modulation direct detection applications. We consider constraints from both the technology and future standards.

A comb-like delayed electrode design is proposed for a slow-light based Mach- Zehnder Modulator. The electro-optic bandwidth of the modulator is estimated considering optical and RF group indices, PN junction, and electrode design.

The performance of adiabatic couplers depends on the width and separation of the constituent waveguide pair. We investigate the influence of these parameters using linear, exponential, and quadratic tapering in the mode-evolution region. A quadratic separation and exponentially varying width provide superior performance for a constrained length.

We propose and experimentally demonstrate low-power and broadband variable optical attenuators (VOAs) on the silicon photonics platform. The VOAs are based on a Mach–Zehnder interferometer structure. Different variations were fabricated using electron beam lithography using different input and output couplers, heater length and width, operating wavelength, and substrate undercut. Experimental results for the C-band VOA show that, including the substrate undercut, a 3X improvement can be achieved in the power consumption, at the expense of a reduction in electrical bandwidth, where 5- and 20-dB attenuation can be achieved using only 5.5 and 8 mW. Moreover, the VOA can operate over more than 80-nm optical bandwidth with ripples below 1 dB. Furthermore, we study the impact of the heater dimensions on the optical attenuation and VOA bandwidth. Finally, we present similar results for an O-band VOA design.

We demonstrate 25-Gbit/s four-level pulse amplitude modulation transmission over 28-km standard single-mode fiber using an off-the-shelf 10-Gbit/s electro-absorption modulated laser and a 3 × 3 fiber splitter based simplified coherent receiver in the C-band. No digital signal processing and dispersion compensation have been used either at the transmitter or at the receiver. Furthermore, the use of an intensity modulated signal allows us to avoid the phase and frequency locking of the local oscillator. We achieved receiver sensitivities of -18.25, -17, and -13.4 dBm at a bit error rate below the hard-decision forward error correction threshold (i.e., 3.8×10 -3 ) and power budgets of 20.80, 19.55, and 15.95 dB in back-to-back, after 20.7 and 28 km, respectively. We also demonstrate polarization independent operation by slightly modifying the architecture of the simplified coherent receiver.

The performance of faster-than-Nyquist intensity modulation and direct detection (IM/DD) systems can be severely degraded by the inter-symbol interference (ISI) due to the component bandwidth limitation and the faster-than-Nyquist filtering. In addition, nonlinearities caused mainly by optical modulation and square-law detection can further deteriorate signal quality. Here, we propose to incorporate transmitter-side Tomlinson-Harashima precoding to mitigate the ISI and a Volterra nonlinear equalizer (VNLE) to mitigate the nonlinear distortions for PAM4 faster-than-Nyquist systems. We experimentally demonstrate a single wavelength 145 Gb/s PAM4 IM/DD transmission with bandwidth limited commercial components and sub-Nyquist sampling at 70 GSa/s. The optimal VNLE filter length is also investigated to balance the implementation complexity and system performance. In addition, compared with the scheme using transmitter-side duobinary precoding, receiver-side VNLE and maximum likelihood sequence estimation, both performance improvement and implementation complexity reduction are achieved.