Chip-scale frequency combs for data communications in computing systems

Yoshitomo Okawachi; Bok Young Kim; Michal Lipson; Michal Lipson; Alexander L. Gaeta; Alexander L. Gaeta

doi:10.1364/OPTICA.460175

1. INTRODUCTION

Today’s communication networks require high bandwidths in order to meet the demands of the explosive growth of data processing worldwide. Due to the increase in bandwidth-hungry cloud-based applications including artificial intelligence, machine learning, big data analytics, and neural networks, solutions are needed to optimize interconnects to be able to meet the demands for low cost, low latency, high bandwidth density, and high energy efficiency for the end-user. With the ever increasing demand for high bandwidth density, electronic interconnects remain a critical bandwidth bottleneck. Optical interconnects based on silicon photonic integrated circuits are regarded as a promising approach for meeting data requirements of the future [1–6]. Such systems will rely on wavelength-division multiplexing (WDM) to allow for massive parallelism using a large number of discrete wavelength optical carriers. To meet the demand for increasing bandwidth, optical interconnects have been deployed in data centers for interconnects longer than a few meters. Current WDM communication links largely rely on an array of discrete wavelength, continuous-wave (cw) lasers for use as optical carriers and a single fiber or fiber arrays to realize wavelength-separated or spatially separated channels, respectively, to allow for parallel transmission [Fig. 1(a)] [3,7,8]. However, in order to meet the ever increasing demand for high-bandwidth-density, low-energy interconnects, photonic integrated circuits (PICs) are being considered for board-to-board, board-to-chip, and chip-to-chip interconnects, where the electronics and optics are copackaged in order to reduce the footprint and electrical power consumption [6,9–18]. Copackaged optical interfaces allow optical modulators to be brought close to the electronic switch architecture, allowing for higher bandwidth density while reducing loss in the electrical interconnects. However, integrated lasers are accompanied by thermal management issues due to the high thermal fluctuations that can exist on PICs [10,17]. To overcome this issue, edge coupling of external optical sources, such as an array of narrow-frequency lasers or optical frequency combs (OFCs), is being considered for copackaging while allowing for modularity and thermal isolation [18].

Fig. 1. Multiple wavelength sources for WDM communication. Conceptual schematic of (a) array of discrete wavelength lasers and three types of integrated frequency combs: (b) extended cavity modelocked laser, (c) electro-optic frequency comb, and (d) microresonator-based Kerr frequency comb.

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OFCs have emerged as a technology capable of producing hundreds of optical carriers that can be matched to the WDM frequency grid [Figs. 1(b)–1(d)]. An OFC is described by a set of discrete and equally spaced frequency lines such that the $m$th frequency line is defined by ${f_{ m}} = {f_{0}} + m\!{f_{r}}$, where ${f_{0}}$ is the carrier-envelope offset frequency and ${f_{ r}}$ is the comb-line spacing [19–22]. Over the past decade, OFCs have found diverse applications including fundamental physics, time and frequency metrology, spectroscopy, distance ranging, astronomy, RF and microwave generation, and communications [23]. In particular, the advances in integrated photonics platforms have led to the development of compact on-chip frequency combs that can be used outside of a well-controlled laboratory environment. OFCs can be highly suitable as a source for optical carriers of a WDM communication system due to (i) absolute precision of each of the generated comb-line wavelengths, (ii) high wall-plug efficiency due to a single high-power pump, and (iii) control over comb spacing. Key parameters of frequency combs as carriers in communications include the comb-line power, optical signal-to-noise ratio (OSNR), spectral efficiency, linewidth of the optical carrier, and frequency accuracy of transmitter and receiver combs. Table 1 offers a range of values for some of the key performance metrics, which differ depending on the reach and application. How each of these parameters affects the overall system performance has been extensively investigated in prior work [7,24–27].

Table 1. Performance Metrics of Multiple Wavelength Sources in Communication Systems

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In this review, we provide an overview of the recent developments in chip-based frequency-comb technology towards the potential implementation of a multiple wavelength source for WDM communication systems relevant for short-reach applications (i.e., ${\lt}{100}\;{\rm m}$), including high-performance computing and data centers. The three different approaches highlighted in this review are semiconductor modelocked lasers (MLLs), electro-optic combs, and Kerr frequency combs, where in each case a distinct optical nonlinearity is responsible for generation of the OFC.

2. INTEGRATED SEMICONDUCTOR MODELOCKED LASERS

A. Overview

Modelocked solid-state and fiber lasers are mature technologies that have been used to realize fully stabilized OFCs via supercontinuum generation in nonlinear waveguides [21,28]. The development of fiber lasers has allowed for the realization of a portable compact platform for use beyond the lab environment [28]. However, most of these MLL systems operate with repetition rates up to a few hundred megahertz, which is not suitable for communication applications. Over the past two decades, there has been significant development of modelocked semiconductor lasers, which enables high pulse repetition rates (${\gt}{\rm GHz}$), due to the large gain cross section, which assists in preventing ${Q}$-switching instabilities [29–31]. Furthermore, research efforts on monolithically integrated MLLs have allowed for further miniaturization, which is essential for applications including communications and microwave generation. These lasers are electrically pumped and allow for picosecond and subpicosecond pulses with repetition rates from gigahertz to terahertz.

Modelocking in semiconductor lasers can be implemented through active, passive, or hybrid approaches. To achieve active modelocking, the optical gain is modulated by sending an electrical signal at a frequency near the cavity repetition rate or its subharmonic [32]. Similarly, an electro-absorption modulator can be implemented to spur pulse formation [33]. These approaches are largely limited by the availability of current sources at high RF frequencies and typically result in longer pulses in the picosecond range. Alternatively, a saturable absorber can be inserted within the cavity to allow for passive modelocking. The nonsaturable losses, recovery time, and saturation fluence of a saturable absorber strongly determine the properties of short pulse generation. Furthermore, to reduce the phase noise and timing jitter of a MLL, hybrid approaches have been utilized where the saturable absorber is modulated with an external RF source [34].

B. Integrated Solutions

Since the demonstration of light emission from a gallium arsenide (GaAs) diode laser in the 1960s [35], semiconductor diode lasers have become a widely utilized technology for cw lasers. III-V technology has been integrated with silicon photonics technology via wafer bonding, epitaxial growth, or hybrid approaches [36–38]. The active gain region can be electrically pumped (forward bias) to realize light emission. One design that is used for semiconductor laser diodes is a ridge waveguide, where the ridge section along with the cladding allows for the index contrast required for confinement of the optical mode. Recently, MLLs have been realized by modifying this waveguide geometry and introducing a reverse-biased saturable absorber section [39]. This two-section approach serves as the basic structure for many waveguide-based MLLs and we review recent advances in integrated semiconductor MLLs.

1. Quantum-Well, Quantum-Dot, and Quantum-Dash MLLs

Semiconductor quantum structures can be used to provide both gain and SA action in integrated MLLs. The three different structures that have been used for MLLs are quantum wells, quantum dots, and quantum dashes. A quantum-well system consists of an embedded thin layer of semiconductor material between other layers that have wider bandgaps, creating a potential well with discrete energy levels. There have been many demonstrations of quantum-well-based MLLs where a reduction in cavity losses and an increase in gain saturation energy allows for reduction in the pulse-timing jitter. Research efforts have focused on the laser structure such as the cladding composition and the number of quantum wells to improve these metrics [40], and modelocking is achieved with a two-section approach and 1-ps pulses with a repetition rate of 40.77 GHz are generated with a timing jitter of 1.2 ps. Another approach is to use what is known as colliding-pulse modelocking, in which the saturable absorber is placed at a location where pulses from each direction overlap, allowing for faster saturation and shorter pulses [41]. In Lo et al. [42], two semiconductor optical amplifiers (SOAs) are placed symmetrically with respect to the saturable absorber, multimode interference reflectors (MMRs) are used as end mirrors, and phase modulators are integrated to allow for bandwidth control. Using this approach, they demonstrate a 1.8-THz modelocked spectrum (350-fs pulse duration) with a 21.5-GHz repetition rate and a 450 kHz repetition-rate linewidth. The two-section approach allows for high repetition rates with demonstrations up to 100 GHz [43].

Modelocking has also been demonstrated using a single-section indium arsenide/indium phosphide (InAs/InP) quantum-dot waveguide [44]. Quantum dots are nanometer-scale semiconductor particles where the electrons and holes are constrained, resulting in discrete energy levels. In contrast to the reverse-biased saturable absorber used in the two-stage structures, modelocking here is achieved via self- and cross-phase modulation, four-wave mixing (FWM), and the Kerr-lensing effect, which leads to the generation of 312-fs pulses with a repetition rate of 92 GHz and a repetition-rate linewidth ${\lt}{20}\;{\rm kHz}$.

The quantum dash (QDash) consists of an elongated semiconductor structure that allows for strong carrier confinement. The broad gain bandwidth and low coupling of amplified spontaneous emission (ASE) make this material ideal for on-chip modelocking [45]. A key characteristic of QDash MLLs is that pulse formation can occur without a saturable absorber [46] and modelocking in this system is attributed to FWM in the gain region [47]. Gosset et al. [47] have shown 800-fs pulse duration, 42-GHz QDash MLLs with narrow repetition-rate linewidths of 50 kHz, offering promise for timing jitter as low as 200 fs. Further studies by Merghem et al. have shown reduction of timing jitter down to 130 fs for 1.3 ps pulses at a repetition rate of 47.54 GHz [45]. The single stage structure is ideal for achieving high repetition rates and pulse trains with subpicosecond pulses, with demonstrations of 245 and 346 GHz operation [46,48] and subpicosecond timing jitter [49]. Like the quantum-dot laser, the QDash laser also exhibits higher performance than the two-section quantum-well laser.

2. Extended Cavity MLLs

Key desirable characteristics for MLLs are low phase noise and timing jitter. One approach that has been used to realize low phase noise MLLs is to use an extended cavity that incorporates a passive, low-loss waveguide. While the repetition rate is reduced, the noise properties are also improved due to the increase in the photon lifetime of the cavity [50,51]. One approach utilizes a 4-mm-long ring cavity, where the saturable absorber, SOA, and passive waveguide are all embedded [52], and such a laser has been shown to generate 900-fs pulses (11.5-nm 3-dB bandwidth) with a repetition rate of 20 GHz, a repetition-rate linewidth of 800 kHz (3-dB), and an optical linewidth of 800 MHz near the central optical mode. Another approach implements an intracavity gain-flattening filter based on a Mach–Zehnder interferometer to realize a 900 fs pulse train (15 nm 10-dB bandwidth) with a 300-GHz repetition rate, repetition-rate linewidth of 500 kHz, and an optical linewidth of 29 MHz [53]. A narrower repetition-rate linewidth of 6.1 kHz and an optical bandwidth of 3 nm were demonstrated using a longer (33-mm-long) InP-based ring cavity (Fig. 2) [54]. However, as a consequence of the longer cavity, the repetition rate was reduced to 2.5 GHz.

Fig. 2. (a) Microscope image of the ring-cavity MLL: semiconductor optical amplifier (SOA), saturable absorber (SA), electrical isolation (ISO), electro refractive modulator (ERM), multimode interference coupler (MMI), and passive waveguides (PWG). Measured (b) output spectrum and (c) repetition linewidth of ring-cavity MLL. Figure adapted from Ref. [54].

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A second approach uses a Fabry–Perot (FP) cavity with long passive waveguides typically in a spiral geometry to extend the overall cavity length. Similar to the ring laser, the gain and SA regions are within the extended cavity. The high reflectors at the ends of the cavity are realized using MMRs [55]. Guzmán et al. uses a 41-mm-long monolithic InP extended cavity to realize a 1-GHz repetition-rate MLL with a repetition-rate linewidth of 398 kHz, which is largely limited by the optical loss of the spiral waveguide [56]. To achieve higher repetition rates using an extended cavity configuration, harmonic modelocking has been achieved using three SA sections that divide the cavity into four gain segments with a total length of 1.66 mm. Such a system produces 500-fs pulse trains with 100-GHz repetition rates [57]. Alternatively, a hybrid approach has been implemented, where a III-V reflective SOA section, consisting of gain and SA regions, is coupled to a 30-mm-long silicon nitride (${{\rm Si}_3}{{\rm N}_4}$, SiN) based extended cavity region that consists of a long spiral and a Sagnac-based reflector [51], which produces a bandwidth of 4 nm (6.31 ps pulses) at a repetition rate of 2.18 GHz. The long cavity allows for narrowing of the repetition-rate linewidths down to as low as 31 Hz. One drawback of the InP extended cavity is the relatively high propagation losses (3 dB/cm) and the nonlinear loss due to two-photon absorption [51].

3. III-V on Silicon MLLs

There has been significant development towards heterogeneous integration of lasers with silicon PICs. Silicon PICs utilize mature complementary metal oxide semiconductor technology and readily offer numerous photonic components including modulators, filters, detectors, splitters, and multiplexers with capabilities of wafer-scale fabrication for high-yield, mass production [58]. A key development has been the heterogeneous integration of an electrically pumped optical source, where the III-V gain material is placed on the silicon PIC. Approaches for III-V on silicon integration include bonding [59–62], direct epitaxial growth via intermediate buffer layers [63,64], and micro-transfer printing [38]. Recent developments of a characterization method involving a stepped-heterodyne approach has allowed for phase and amplitude measurements of the laser output indicating that transform limited pulses can be achieved in these systems [65].

For implementation of a MLL on silicon, oxide plasma assisted wafer bonding was used to transfer the III-V epitaxial layer structure, which consists of a gain and SA region, to the silicon-on-insulator wafer, where the silicon waveguides are initially fabricated to interact with the III-V structure via evanescent coupling [66], and the laser cavity is formed by polishing the facets. Using such a system, both passive and hybrid modelocking was demonstrated with repetition rates as high as 40 GHz. The system was further improved to allow for integration with other photonic components by using a racetrack geometry, eliminating the end facets [67], and produced 7-ps pulses at a 30-GHz repetition rate. The main advantage that silicon offers is the low propagation loss in the passive silicon waveguide (0.7 dB/cm), allowing for a reduction in the repetition-rate linewidths [68]. Similar to the extended cavity approaches demonstrated in InP, Keyvaninia et al. demonstrated an extended cavity MLL using the III-V on silicon platform with both a FP and ring-cavity geometry [68]. The III-V region is bonded using divinylsiloxane-bis-benzocyclobutene [60], allowing for evanescent coupling to the silicon waveguide. The FP cavity and ring cavity had a repetition rate of 4.7 GHz and both showed a reduction in repetition-rate linewidth (12 kHz and 16 kHz, respectively) with optical bandwidths up to 9 nm. In addition, Keyvaninia et al. presented an anti-colliding-pulse-type MLL which produced a 3-ps pulse train with a repetition rate of 4.83 GHz and a repetition-rate linewidth of 1.7 kHz [69]. The end mirrors are implemented using high reflectivity distributed Bragg reflectors to allow for further integration with other components in the silicon layer. Wang et al. demonstrated a 1 GHz repetition-rate MLL using an extended silicon waveguide design, producing a 10-nm optical bandwidth, a 450 Hz repetition-rate linewidth, and a 250 kHz optical linewidth [70]. Further repetition-rate linewidth reduction has been achieved by using a III-V on SiN platform [71,72]. SiN has even lower loss than silicon (5 dB/m) [72], enabling low-noise modelocking. Here, the SiN spiral waveguides (two 20-cm-long waveguides) act as the extended cavity and are coupled to a InP/InAlGaAs-based multiple-quantum-well amplifier waveguide through the silicon layer. The generated spectrum spans 3.27 nm with a repetition rate of 775 MHz, a repetition-rate linewidth of 1 Hz, and an optical linewidth ${\lt}{200}\;{\rm kHz}$, which is the lowest for an on-chip MLL to date. The low-loss nature of SiN waveguides offers promise for incorporating rare earth doped laser gain media, with recent work demonstrating a MLL operating at 1.9 µm with a repetition rate of 1.2 GHz and an optical bandwidth of 17 nm [73].

For applications in WDM communications the channel spacing (i.e., the repetition rate) needs to be larger. As seen in both InP and III-V on silicon extended cavities, to achieve the narrow repetition-rate linewidth, long cavities are required, reducing the pulse repetition rate. However, the low propagation losses in silicon allow for additional elements to be incorporated into the cavity, which can permit higher repetition rates while maintaining narrow linewidths. Srinivasan et al. utilized a 2-GHz cavity with an intracavity filter that has a 20-GHz spacing, allowing for low noise harmonic modelocking at a 20-GHz repetition rate with a 52-kHz linewidth and an optical linewidth of 1 MHz, which is ${10} \times$ lower than a 20 GHz cavity without a ring filter [74]. Alternatively, Liu et al. demonstrated a low timing jitter (82.7 fs) and narrow repetition-rate linewidth (1.8 kHz) quantum-dot MLL on silicon (Fig. 3) [75]. The gain region implements a chirped quantum-dot design to allow for broader bandwidths (6.2 nm), and the narrow linewidth performance is attributed to the low ASE noise and low confinement factor in the quantum-dot material. The bandwidth of the generated laser spectrum spans 6.2 nm with a repetition rate of 20 GHz. The average optical linewidth of the modes is measured to be 10.6 MHz. We have highlighted the performance of the various semiconductor MLLs in a table at the end of the section (Table 2).

Fig. 3. (a) Schematic of the quantum-dot MLL. (b) Output spectrum (blue) and optical linewidth of the comb modes (red). (c) BER measurement and (d) eye diagrams for comb lines modulated using a PAM-4 signal. Figure adapted from Ref. [75].

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Table 2. Performance of Select Semiconductor MLLs

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C. Semiconductor MLLs for Communication

MLL sources have applications in various WDM and coherent optical communication systems [76]. One key issue for semiconductor MLLs particularly for applications in communications is the high relative intensity noise (RIN), which is largely attributed to the laser linewidth, which is dependent on the cavity $Q$, and the mode partition noise [77,78]. The linewidth requirement is generally set by the data rate of the system along with the modulation format that is employed [79,80], thus making a system level performance study important. Kurczveil et al. investigated the performance of a quantum-dot laser bonded on silicon, where three comb lines of their 38-GHz output are modulated with a non-return to zero (NRZ) signal [81]. The measured signal-to-noise ratio is ${\gt}{11.5}\;{\rm dB}$, and the bit-error rate (BER) is less than ${10^{- 12}}$ for all channels. In addition, Moscoso-Mártir et al. implemented an eight-channel WDM transceiver based on a semiconductor MLL and studied the performance in the context of data-center interconnects (Fig. 4) [82]. The transmitter consisted of an external quantum-dash semiconductor MLL, a silicon chip consisting of filters to remove the unused lines and resonant ring modulators for on–off-key modulation, and an SOA for optical amplification. The MLL has a 100-GHz repetition rate, and each of the eight channels has power ${\gt}{500}\;\unicode{x00B5}{\rm W}$. Analysis of the transmitter link has shown that the system operates up to 14 Gbps with hard-decision forward error correction (FEC). The QDash MLL was also used to demonstrate a 10.68 Tbps aggregate net data rate but using a 38-channel 16QAM dual-polarization WDM system [83]. Alternatively, Arsenijević et al. demonstrated 160-Gbps data transmission using differential quadrature phase-shift keying (QPSK) and time division multiplexing based on a 40-GHz repetition-rate quantum-dot MLL [84]. Furthermore, using 64 comb lines from a quantum-dot laser on silicon, Liu et al. characterized the data transmission performance using a 32-GBd PAM-4 signal offering promise towards 4.1 Tbps of transmission capacity [75]. Pan et al. used a 100-GHz repetition-rate quantum-dot MLL to demonstrate 64-GBd PAM-4 data transmission using seven wavelength channels [43]. Another issue with semiconductor MLL is the relatively low power per comb line. This can be mitigated with external SOAs. However, the SOA current must be optimized as the ASE from the SOA can be coupled to the MLL cavity affecting the MLL cavity and can affect the modelocking process [85].

Fig. 4. (a) Block diagram of WDM transmitter using QDash MLL as the WDM source. (b) Image of transceiver with I/O. (c) MLL spectrum (blue) and transmitter response (black) showing the eight wavelength channels. Figure adapted from Ref. [82].

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Table 3. Performance of Select EO Modulators and Comb Sources

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3. ELECTRO-OPTIC COMBS

A. Overview

Frequency combs can be generated via the electro-optic (EO) effect where the refractive index of a material is changed in the presence of an applied, near DC electric field. This allows for the generation of sidebands separated by the modulation frequency of the driving field. For the simplest case where a cw laser with frequency ${\omega _0}$ is sent into a phase modulator driven with an electric field $V(t) = {V_0}\sin ({\omega _m}t)$, where ${\omega _m}$ is the modulation frequency, the output in the frequency domain can be expressed as

(1)$$\tilde A(\omega) = {A_0}\sum\limits_{n = - \infty}^\infty {J_n}\left({\pi \frac{{{V_0}}}{{{V_\pi}}}} \right)\delta (\omega - n{\omega _m} - {\omega _0})$$

with ${V_\pi}$ as the half-wave voltage [86]. This expression shows that a discrete spectrum is generated with the frequency of the $n$th comb line given by ${\omega _n} = n{\omega _m} + {\omega _0}$. In order to shape the spectral bandwidth of the generated comb output, various configurations have been considered including cascaded intensity and phase modulators, dual-drive Mach–Zehnder modulators (MZMs), dual-parallel MZMs, and EO cavities [87].

B. Photonic Platforms

Although a key application of EO modulators is their use in imprinting data from the electrical to the optical domain, the past decade has also witnessed tremendous development of integrated modulators for frequency-comb generation. Research efforts have focused on exploring materials with high electro-optic coefficients or low ${V_\pi}$, compact footprint, and low propagation losses. An important efficiency metric for modulators is the voltage-length product ${V_\pi}L$. Here, we review recent developments in integrated EO-comb platforms, including thin-film lithium niobate modulators, silicon modulators, plasmonic modulators, and hybrid modulators. The performance of the various platforms is summarized in a table at the end of the section (Table 3).

1. Thin-Film Lithium Niobate Modulators

Electro-optic modulators based on lithium niobate (${{\rm LiNbO}_3}$, LN) are well established as a key module in fiber-optics technology [105]. These modulators rely on the Pockels effect and often use the relatively large EO tensor component (${r_{33}} = 31\;{\rm pm/V}$) [106]. These widely available commercial devices are fabricated via ion-indiffusion or annealed proton exchange techniques using bulk crystal wafers, resulting in a low index contrast and necessitating a large optical mode [107]. Over the past several years, advancements in thin-film LN have led to the realization of low-loss (${\lt}{0.3}\;{\rm dB/cm}$), high-confinement waveguides with cross sections on the wavelength scale. This allows for higher electro-optic efficiency due to the electrodes being closer to the waveguide to allow for stronger interactions between the optical and electric fields [107]. Using the lithium-niobate-on-insulator (LNOI) platform, modulators with a 3-dB bandwidth of 100 GHz have been demonstrated [88], and high-speed data transmission at 120 Gbps (NRZ) and 220 Gbps (PAM-4) has been shown [108]. Luke et al. have shown that wafer-scale fabrication is possible using deep ultraviolet lithography, offering promise for a scalable and cost-effective LN PIC [109].

One approach for comb generation involves placing the EO modulator inside a FP cavity to allow for resonant enhancement of the optical field [110–112]. More recently, the development of high-${Q}$ resonators has enabled resonant EO combs where both the optical carrier and the microwave frequencies are resonant [89,113–115]. Zhang et al. demonstrated broadband resonant EO comb generation using a phase modulator embedded in a low-loss, dispersion-engineered LN microresonator based on the LNOI platform (Fig. 5) [89]. Here, the comb spans ${\gt}{80}\;{\rm nm}$ such that 900 comb lines were generated with a 10-GHz spacing. The microwave efficiency can be improved by embedding the optical cavity within the microwave cavity [114], and a recent demonstration shows that high pump-to-comb conversion efficiencies of 30% can be achieved by using a coupled-resonator geometry to overcome the change in coupling conditions induced by the microwave drive signal used for comb generation, resulting in a 132-nm bandwidth EO comb with 30.9 GHz spacing [116].

Fig. 5. (a) Microscope image of lithium niobate resonator for EO comb generation. Electrodes (gold) are used for microwave modulation of the resonator. (b) Generated EO comb spectrum. Figure adapted by permission from Macmillan Publishers Ltd.: [89].

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Another approach for EO comb generation involves the use of cascaded intensity and phase modulators. To achieve pulse compression and consequently spectral broadening, such a system uses a time lens [117,118], which takes advantage of the space–time duality of paraxial diffraction in the spatial domain and dispersive propagation in the time domain. This is a well-established technology using fiber-optic components [118–124], with demonstrations of octave-spanning bandwidths [123,124]. The low-loss LNOI platform allows for full integration of the cascaded modulators and a dispersive element, offering potential for a fully integrated comb source with spacings ${\gt}{30}\;{\rm GHz}$ [90]. Since the phase noise of the $n$th comb line due to the microwave signal scales as ${n^2}$, the noise becomes a significant issue as the number of comb lines increases [123]. This issue can be mitigated by using a cavity for phase noise suppression with suppression of up to 40 dB demonstrated experimentally [125].

2. Silicon Modulators

Silicon is a material platform that is used extensively for PICs and has become a mature technology with hybrid photonic and electronic integration done via interposers or through-silicon vias. Frequency-comb generation has been demonstrated using phase modulators based on free-carrier injection. These modulators utilize the index change that results from the free-carrier plasma dispersion effect, which is dependent on the number of carriers and the effective conductivity masses of the electrons and holes [126,127]. Carriers can be injected by sending an electric field across a PIN junction, which consists of heavily doped $p$-type and $n$-type semiconductors that surround an undoped intrinsic semiconductor. PIN modulators have enabled modulation speeds beyond 100 GHz [91,92], and have been implemented for electro-optic conversion and high data rates in integrated platforms [128–132]. Jacques et al. demonstrated 240 Gbps with PAM-8 modulation using a segmented-electrode MZM [132], and Sepehrian et al. showed 232 Gbps with 16-QAM using a silicon IQ modulator [131]. Alternatively, resonator-based modulators have been shown to improve the modulation efficiency in a compact footprint [93,133–135]. Baba et al. achieved 50-Gbps NRZ operation with ${V_\pi}L = 2.8\;{\rm V} \cdot {\rm mm}$ [93].

This technology has been used for frequency-comb generation, and eight comb lines over a 20-dB bandwidth were produced in a 4.5-mm-long traveling-wave phase modulator [94]. The waveguide geometry allows for continuous tuning of the repetition rate, and tuning between 7.5 and 12.5 GHz was demonstrated. In order to further reduce the footprint and RF power, a relatively low $Q$ (3000–10,000) ring resonator modulator was used for comb generation [136]. The low $Q$-factor is accompanied by a photon lifetime ${\lt}{8}\;{\rm ps}$ such that the resonator free-spectral range (FSR) does not dictate the optical bandwidth and is only dependent on the modulation frequency. In order to generate a frequency comb with a 10-GHz spacing, the modulator is driven with two different RF frequencies, a 10-GHz signal with $1.5\;{{\rm V}_{{\rm pp}}}$ and a 20-GHz signal with $2.9\;{{\rm V}_{{\rm pp}}}$. By controlling the bias voltage, seven comb lines are generated within a 20-dB bandwidth. The generated bandwidth can be further increased through such cascading of modulators [137,138].

3. Plasmonic Modulators

Plasmonic structures allow for confinement of the optical mode to subwavelength waveguide geometries allowing for compact photonic devices [139]. In these structures, the refractive index is altered with the presence of an applied field based on the Pockels effect. Assisted by a surface-confined plasmon-polariton mode, the strong localization of the optical and electrical fields in the slot structure allows for large mode overlap between the two fields, resulting in high modulation indices and low power consumption with electro-optic coefficients as high as ${r_{33}} = 390\;{\rm pm} \cdot {{\rm V}^{- 1}}$ demonstrated [140], and ${V_\pi}L = 0.06\;{\rm V} \cdot {\rm mm}$ has been demonstrated [95]. Moreover, plasmonic modulators based on organic EO materials exhibit flat frequency responses up to 360 GHz [96]. Such modulators have been used to demonstrate data transmission up to 120 Gbps (NRZ) [141] and 120 Gbps (PAM-4) (Fig. 6) [142]. While current demonstrations have been limited to the generation of a pair of sidebands [96,142], the platform could be ideal for high efficiency EO comb generation with wider comb spacings.

Fig. 6. (a) Microscope image of silicon Mach–Zehnder interferometer with a plasmonic phase modulator in each arm. (b) Measured eye diagram for three different modulation formats. Figure adapted from Ref. [142].

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4. Indium Phosphide Modulators

III-V materials offer the potential for full integration of both active and passive elements of a transceiver, including the pump laser, optical amplifier, and modulator. There has been much work done on modulators based on InP. The EO effect in III-V materials can arise from several different processes, including the Pockels effect (${r_{41}} = - 1.4\;{\rm pm/V}$) [143], the Franz–Keldysh effect (bulk), quantum confined Stark effect (quantum well), and free-carrier effects [104,144]. Demonstrations of comb generation using InP modulators have implemented the waveguide geometry where a single modulator or several modulators (both intensity and phase) are connected in series. For the single-modulator approach, a traveling-wave-electrode dual-drive MZM [145] and a dual-electrode MZM [146] have been used to generate as many as 29 comb lines [146]. The number of comb lines was further increased by using a cascaded modulator approach on a single chip. Bontempi et al. use a dual-drive MZM and two phase modulators in series to generate 55 comb lines with a 1-GHz spacing [97]. Furthermore, a similar device has been demonstrated that integrates the cascaded modulators with a distributed Bragg reflector laser and an SOA to boost the output [147]. The system offers the potential for supporting higher data rates with Ogiso et al. previously reporting InP modulators with a bandwidth of 50 GHz [98]. The high modulation frequency allows for wider comb spacing, minimizing crosstalk between channels.

5. Hybrid Modulators

Hybrid modulators consisting of a silicon or SiN waveguide combined with an EO material to realize a modulator for comb generation. One approach uses an organic EO material (chromophore DLD164) on a silicon slot waveguide, which has a ${V_\pi}L$ of $2\;{\rm V} \cdot {\rm mm}$ [99]. Here, flat-top spectra with seven comb lines within a 2-dB bandwidth are demonstrated with a spacing of 40 GHz. The organic EO material also offers potential for high-bandwidth operation up to 100 GHz [100]. Other platforms that show a low ${V_\pi}L$ include a SiN ridge waveguide with LN (${V_\pi}L = 30\;{\rm V} \cdot {\rm mm}$, 8 GHz 3-dB bandwidth) [101], ${{\rm BaTiO}_3}$ on silicon (${V_\pi}L = 2\;{\rm V} \cdot {\rm mm}$, 2 GHz 3-dB bandwidth) [102], ${{\rm LiNbO}_3}$ on silicon (${V_\pi}L = 94\;{\rm V} \cdot {\rm mm}$, 40 GHz 3-dB bandwidth) [103], and InP on silicon (${V_\pi}L = 12.5\;{\rm V} \cdot {\rm mm}$, 1.5-GHz 3-dB bandwidth) [104]. More recently, Churaev et al. have shown wafer-scale bonding of thin-film LN to a SiN PIC [148]. While there exists an increase in the ${V_\pi}L$ by a factor of 2 as compared to LN PICs, the platform offers promise as a hybrid low-loss PIC combining ${\chi ^{(2)}}$ and ${\chi ^{(3)}}$ nonlinearities. For data transmission, 112 Gbps data transmission for an NRZ signal has been demonstrated using a LN-on-silicon hybrid modulator [149].

4. KERR FREQUENCY COMBS

A. Overview

A particularly promising approach for producing on-chip frequency combs for data communications utilizes the ${\chi ^{(3)}}$ nonlinearity within microresonators. This process known as Kerr frequency-comb generation is based on a nonlinear optical process of FWM in the presence of group-velocity dispersion (GVD), which can lead to parametric oscillation in a high-$Q$ microresonator pumped with a single-frequency laser source [20,21]. Previously, FWM had been used in a single-pass fiber geometry to generate multiple sidebands using a two-tone pump [150–154], in which the comb spacing is dictated by the frequency spacing of the pump fields, and bandwidths ${\gt}{900}\;{\rm nm}$ have been demonstrated [154]. In a microresonator geometry, the pump power builds up through cavity enhancement and leads to parametric gain for other cavity modes. Once the gain exceeds the propagation and coupling losses, parametric oscillation occurs and under suitable conditions leads to highly cascaded oscillations and phase locking of these modes. Kerr combs are characterized by a rich set of nonlinear dynamics and can be generated in both the anomalous and normal-GVD regimes. Early demonstrations of Kerr comb generation were performed in whispering gallery mode (WGM) resonators composed of silica or fluorides [155,156]. Levy et al. first demonstrated Kerr combs in a planar geometry using a CMOS-compatible material [157]. In ensuing work, it was realized that these combs could be modelocked. In the anomalous GVD regime, modelocking occurs via excitation of dissipative Kerr solitons (DKS) [20,21], while modelocking in the normal-GVD regime arises from interlocking switching waves [158]. To understand the existence range and stability of Kerr combs in the two respective GVD regimes, bifurcation analysis has been performed [159–161]. Kerr comb dynamics have been intensely studied theoretically and can be modeled using a damped driven nonlinear Schrödinger equation (NLSE) known as the Lugiato–Lefever equation (LLE) [162].

With modest changes in the microresonator geometry, Kerr combs can produce mode spacings from 10 GHz to 1 THz with powers per line exceeding 0.5 mW, making them ideal for applications including communications, microwave generation, and optical frequency synthesizers. In particular, Kerr combs are well matched to WDM standards for data centers [6,163], and the first study of the feasibility of using Kerr combs as a WDM source was presented by Levy et al. in 2012, where an open eye and error-free operation (${\rm BER}{ \lt 10^{- 9}}$) was demonstrated under conditions of modelocked operation using a 10 Gbps pseudo-random bit sequence (PRBS) (Fig. 7) [164], and the characterization of noise and coherent properties were performed by Wang et al. [165].

Fig. 7. (a) BER measurements of five filtered Kerr comb lines at 1568, 1573.2, 1576.6, 1585.2, and 1597.5 nm that are modulated with a 10 Gbps NRZ signal. The cw reference measurement acts as a back-to-back baseline. Error-free operation of the comb lines and a minimal power penalty measured at a BER of ${10^{- 9}}$ are denoted with a dashed line. (b) Eye diagrams for the five measured comb lines. Figure adapted from Ref. [164].

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B. Soliton Combs

Microresonators pumped in the anomalous GVD regime lead to the excitation of DKS, which arises as a result of the balance between the ${\chi ^{(3)}}$ nonlinearity and dispersion along with the balance between gain and loss [20]. The generation dynamics have been extensively studied, and a variety of states can be accessed by tuning the pump on the blue-detuned side of the cavity and progressively red-detuning the pump frequency, including Turing rolls, modulation instability (MI) states, and multi- and single-soliton states. The soliton states manifest when the pump reaches the red side of the resonance, and the number of solitons circulating in the cavity can progressively be reduced by controlling the pump-cavity detuning, which manifests as discrete steps in the cavity transmission. In addition, an ordered temporal distribution of pulses forms a multi-soliton ensemble known as a soliton crystal [166]. The generated comb bandwidth is largely dependent on the GVD of the microresonator with higher-order dispersion contributing to dispersive-wave generation that can enable bandwidths spanning an octave [167,168].

1. Soliton Comb Properties

One of the key efforts in DKS studies has been achieving deterministic generation of a single soliton in a steady state. This is attributed to the fact that different numbers of solitons are associated with different average intracavity powers, leading to a thermo-optically induced shift of resonance position that also depends on the soliton number. To overcome this thermal shift, various approaches have been used to rapidly tune the pump-cavity detuning. One approach uses “power kicking,” where a fast pump power modulation combined with a slow laser scan allows for the pump power to be adjusted as the laser is tuned to resonance, enabling access to the single-soliton state [169]. Similar approaches have been demonstrated using a single-sideband modulator [170]. Another approach uses fast thermal tuning of the cavity resonance using integrated heaters [171], which allow for repeatable single-soliton formation by introducing an abrupt red-shift to the overall slow resonance scan analogous to “backward tuning” to stably access the single-soliton state [172]. Other approaches that have been explored include the use of a pulsed pump [173,174], phase modulation [175], capture and stabilization [176,177], mode interactions [178], the photorefractive effect [179,180], and auxiliary lasers [181].

2. Soliton Combs for Communication

In addition to the early work [164,165] mentioned above, studies of BER performance have been done on various Kerr comb states, including Turing rolls, MI, and single soliton, where Turing and single-soliton states show FEC-free error-free operation (${\rm BER}{ \lt 10^{- 9}}$) for a 10 Gbps NRZ signal [182]. Since then, data transmission has been demonstrated with more complex modulation formats, including QPSK and quadrature amplitude modulation. Pfeifle et al. use QPSK with 20 channels and demonstrated capability up to 1.44 Tbps of data transmission [183]. The transmission performance is characterized by selecting individual channels for modulation and detection. In addition, Marin-Palamo et al. have demonstrated 50-Tbps net transmission using 179 comb lines based on interleaving of two DKS combs and 16QAM modulation format (Fig. 8) [184]. Here in addition to the DKS comb in the transmitter, a second DKS comb is used as the local oscillator (LO) in the receiver. Furthermore, using a single 49-GHz soliton crystal comb, Corcoran et al. showed 44.2 Tbps transmission using 64QAM [185]. Alternatively, a hybrid approach combined DKS and EO modulation to increase the comb-line density for high spectral efficiency superchannel transmission [186,187]. Using this approach, Geng et al. have shown 6.885-Tbps transmission [187]. Another approach for creating a superchannel transmission uses lower repetition-rate soliton combs (22.1 GHz) where 50 comb lines are used for 12 Tbps transmission using PM-256QAM [188]. Table 4 summarizes results for several different methods of transmitting data using anomalous-GVD combs.

Fig. 8. (a) Generated single-soliton DKS spectrum. (b) SiN microresonator and cross-section images. (c) Bit-error ratio comparison between DKS comb line and external-cavity laser using 16QAM signal at 40 GBd. Figure adapted by permission from Macmillan Publishers Ltd.: [184].

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Table 4. Performance of Anomalous-GVD Kerr Frequency Combs

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A key figure of merit (FOM) for Kerr combs is the pump-to-comb conversion efficiency, which is critical for applications such as data communications that require sufficient power per line for data transmission across the full link, and high wall-plug efficiency is needed. Extensive theoretical and experimental studies of conversion efficiency have been performed [176,189–191], and efficiencies beyond 20% can be achieved only with high FSR (${\gt}{1}\;{\rm THz}$) and over-coupled resonators [191]. Alternatively, a feedback approach using a hybrid Mach–Zehnder and ring geometry has allowed for the generation of soliton crystal states with pump-to-comb conversion efficiencies of 55% [192]. In addition, conversion efficiencies beyond 50% have been demonstrated by using an auxiliary resonator as a selective frequency shifter [193].

Low comb mode noise is also critical for communication applications. Previous studies have shown that the phase noise of a generated comb line increases quadratically for increasing mode numbers with respect to the pump mode [194], which is largely attributed to the thermorefractive noise in the microresonator [195,196]. The temperature fluctuation lead to fluctuations in the repetition rate of the generated soliton. Researchers have revealed a quiet point for Kerr comb operation [197–199]. This refers to the pump-cavity detuning operating point that balances the nonlinear effects in the cavity including spectral recoil due to dispersive wave formation and minimizes the coupling of the fluctuations in detuning frequency to the soliton repetition rate. Yang et al. demonstrated repetition-rate phase noise reduction of 36 dB [198]. Furthermore, recent demonstrations show that thermorefractive noise can be reduced by laser cooling with an auxiliary laser [200] or via self-cooling by pumping an auxiliary cavity mode [201,202].

There have been investigations for implementing Kerr combs in other aspects of communications. Previously, Marin-Palamo et al. [184] used two separate DKS combs, one for the transmitter and the other as the LO in the receiver. An alternative approach for generating the LO uses a comb line from the master DKS comb for generating the LO comb in the receiver link for coherent detection [203]. The coherence between the master and secondary combs can allow for phase recovery in the receiver. Alternatively, the narrow linewidth comb lines in DKS combs have been used as coherent pumps for wavelength multicasting, enabling superior error-free performance as compared to free-running pump lasers [204,205].

C. Normal-GVD Combs

Kerr combs can also be generated and modelocked in the normal-GVD regime. While such combs have often been described as “dark soliton” combs [206,207], bifurcation analysis has revealed that the dark structures formed in the normal-GVD regime correspond to interlocked wavefronts, or switching waves [158,161]. Switching waves are structures of light that connect the upper and lower homogeneous state solutions of the bistable cavity response. Two switching waves of opposite polarity that lock to one another form the pulsed structure corresponding to a stable, modelocked normal-GVD comb [158,206,208–212]. Key benefits of normal-GVD Kerr combs include the relative ease of accessing high pump-to-comb conversion efficiencies (${\gt}{30}\%$) even for relatively small FSRs (e.g., 100 GHz) and slower power falloffs within the spectral region of interest [207,213], which are more ideal for data communications.

Table 5. Performance of Normal-GVD Kerr Frequency Combs

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1. Background and Comb Properties

The generation of a normal-GVD Kerr comb requires a hard excitation to enable the formation of switching waves. This initial perturbation can be achieved through pump modulation at the resonator FSR [211,214], injection locking [215], or local changes in the dispersion to allow for MI [207,210,212,213,216–219]. The pump modulation demonstration showed not only that harmonic driving can generate normal-GVD combs but also that desynchronization of the pump repetition rate from the cavity FSR can shift the comb spectrum [211]. The injection locking scheme showed that a normal-GVD comb maintains its benefits even under such conditions [215].

The most prevalent method to generate a normal-GVD comb is through the modification of the effective dispersion of a microresonator through mode splittings [207–210,212,213,216–219]. A mode splitting is formed when modes of different mode families spectrally overlap and couple. Common mechanisms of mode splittings include coupling to different polarization modes [220], spatial modes [221], and auxiliary resonator modes [222]. Simulations of such Kerr combs can be achieved via various methods. One approach is to approximate the avoided mode crossing as a simplified two-parameter model [223] that modifies the dispersion operator in the LLE. A more accurate approach simulates the avoided mode crossings, which is important when the splitting strength is strong, through a unitary coupling matrix and a modified NLSE to account for the FSR difference of the mode families [212]. This method can be extended to different types of splitting schemes and accounts for the periodicity of the mode crossings and the power exchange between the modes. Alternatively, the dispersion can be locally modified via a frequency-domain point defect in a photonic crystal resonator. By controlling the strength of this defect along with the pump-cavity detuning, both dark and bright pulses can be induced, allowing for control of the bandwidth and efficiency of the generated comb [224]. Table 5 summarizes results for several different methods of generating normal-GVD combs.

Although the noise properties of normal-GVD combs have not been studied as extensively as those of DKS combs, several demonstrations have shown key properties that show the usability of normal-GVD Kerr combs as WDM sources. Xue et al. demonstrated the low RF noise of a normal-GVD comb [217], and Kim et al. showed the high coherence of individual comb lines (Fig. 9) [213]. Jin et al. showed that the comb lines of a normal-GVD comb can reach a fundamental linewidth on the order of a hertz, and Rizzo et al. showed open eye diagrams at 16 Gbps with some lines achieving natively error-free BERs (${ \lt \!10^{- 12}}$) [225].

Fig. 9. (a) Microscope image of SiN coupled ring resonator with integrated resistive heaters. (b) Simulation of resonances using experimental parameters from [213]. Periodic effects of mode crossings can be observed. (c) Normal-GVD comb spectrum with 201.6 GHz comb spacing and 41% pump-to-comb conversion efficiency. Power levels of 50 µW, 100 µW, and 1 mW are indicated with red, green, and black dashed lines, respectively. Figure adapted from Ref. [213].

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Fig. 10. (a) Microscope image of fabricated 32-channel transmitter chip and zoom-in of all active and passive devices on the chip. (b) Measured BER comparing comb performance to cw laser at 1559.8 nm. (c) Measured BER for multiple comb channels in the C- and L-bands. Figure adapted from Ref. [225].

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2. Normal-GVD Combs for Communication

Several demonstrations of WDM communications have utilized normal-GVD Kerr combs. In one experiment by Fulop et al., 20 lines of a 230-GHz comb were split into even and odd channels where the data in each arm was modulated at 20-GBd using PM-64-QAM. At the receiver side, after 80 km of transmission fiber, a local oscillator was tuned to each data channel to measure the BER of each channel. This coherent communications scheme resulted in a final post-FEC BER below ${10^{- 15}}$ with an aggregate data rate of 4.4 Tbps [219]. A more recent demonstration by Rizzo et al. used a 200-GHz normal-GVD comb for data transmission on a single 32-channel integrated photonic link architecture (Fig. 10) [225]. This method utilized simple intensity modulation and direct detection of each comb line at modest channel speeds to reduce the latency and energy per bit and eliminate the requirement of FEC and a local oscillator used in coherent communications. The photonic link measured a direct BER better than ${10^{- 8}}$ for all data channels at 16 Gbps, yielding an aggregate data rate of 512 Gbps [225].

In addition, the high conversion efficiency and relatively flat spectral profile of a normal-GVD comb make it a strong candidate for direct modulation of the generated lines without an additional amplifier. Even so, the powers generated by a Kerr comb are limited due to the high field confinement and effective nonlinearity. Such a limitation can be overcome by using coherent beam combining, which can be achieved with high efficiency via the mechanism of synchronization [212]. By passively coupling individual combs together, synchronization allows for the repetition rates of individual combs to become equal, and if the pump frequencies are identical, the comb lines will overlap spectrally to allow for coherent combining [212,226]. Recently, Kim et al. showed that not only can normal-GVD Kerr combs be synchronized like their DKS counterparts [226,227], but they also can be coherently combined via on-chip synchronization for a nearly $2 \times$ increase in the comb power to extend the power range of a natively spaced comb with high efficiency [212].

3. Fully Integrated Kerr Comb Source

While the microresonator was fabricated on a compact photonic platform, most previous demonstrations of Kerr comb generation required pumping with a CW laser amplified with an erbium-doped fiber amplifier, resulting in a table-top demonstration. Recently, significant progress has been made towards full integration of Kerr combs. These advancements used a hybrid integration approach with SOA gain chips [228], FP laser diode chips [229,230], and distributed feedback (DFB) laser chips [215,231–233]. Early work by Liang et al. showed that the DFB laser can be injection locked to a cavity mode of a magnesium fluoride WGM resonator and allow for subsequent comb generation [231]. Stern et al. demonstrated hybrid integration of a reflective SOA gain chip and SiN chips and demonstrated DKS generation using two different mechanisms. One approach is to use the microresonator, not only the comb generator but also the end reflection facet of the entire integrated laser structure. The other method used a Sagnac loop as the end mirror before the microresonator, which separates the pump laser portion from the nonlinear element [Figs. 11(a)–11(c)] [228]. Subsequently, Pavlov et al. showed that a Fabry–Perot laser diode chip can be coupled to a WGM microresonator to produce a DKS [229]. Raja et al. demonstrated that the backscattering from a SiN chip to a Fabry–Perot laser diode chip allowed for injection locking and the generation of a DKS comb [230]. Shen et al. fully packaged a DFB laser chip with a SiN chip to show injection locking and DKS generation [Figs. 11(d) and 11(e)] [232]. Furthermore, work by Jin et al. has shown that injection locking can also be used for generating a normal-GVD comb [215]. These approaches show a path towards realizing a fully integrated Kerr comb source that is critical for realizing a compact photonic integrated WDM source that could be not only implemented in data-center transceivers but also copackaged with electronic chips.

Fig. 11. (a) Microscope image of integrated Kerr comb source with the reflective SOA waveguide and the nonlinear microresonator. (b) Photograph of integrated comb source. The RSOA is edge-coupled to the SiN chip. (c) Measured single-soliton Kerr comb spectrum. (d) Image of fully packaged DFB laser and SiN chip in a butterfly package. (e) Optical spectrum of 40 and 15 GHz Kerr comb states. (a)–(c) adapted from Ref. [228]; (d) and (e) adapted by permission from Macmillan Publishers Ltd.: [232].

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To overcome power limitations of the comb source, on-chip amplifiers can be implemented to boost the power of the generated comb lines. One approach is to utilize a III-V gain medium with epitaxial growth on silicon [234,235] or with bonding to silicon [236–238]. Another approach is to use rare earth doping in waveguides with demonstrations in aluminum nitride, silicon nitride, silicon dioxide, and lithium niobate [239–244]. Alternatively, cw parametric gain has been utilized for integrated amplifiers with demonstrations in waveguides [245,246] and resonators [247]. Similar to MLLs, the amplifier properties may require optimization to minimize the ASE being coupled back into the cavity that could cause destabilization of the Kerr comb.

5. CONCLUSIONS AND OUTLOOK

In order to overcome the energy-bandwidth limitations of electronic interconnects, more optical technologies are being deployed in data centers. To meet the demands towards ultra-high-bandwidth links in the Tbps regime, much tighter optoelectronic integration is required for communication at the board and chip level. The recent advances in chip-scale frequency-comb devices offer promise as multiwavelength optical carriers for WDM transceivers Integrated comb sources with 64 and 128 channels in conjunction with higher-order modulation formats such as PAM4 at 20 Gbps can allow for reaching aggregate data rates of 5 Tbps and 10 Tbps, respectively, with a distinct path towards further data rate scaling. In this review, we presented an overview of the chip-scale OFC technology that offers promise for realizing ultra-high-bandwidth integrated photonic transceivers for deployment in future high-bandwidth-density data-center architecture. Key challenges for next-generation chip-scale comb technology include increased power per comb line and comb-line power equalization while achieving a high overall wall-plug efficiency. We envision a multilayer photonic platform that allows for the integration of passive and active photonic elements and for the realization of a photonic transceiver that enables the integration of photonic interconnects in data centers for high-bandwidth density, low-cost, energy-efficient communication networks.

Funding

Defense Advanced Research Projects Agency (HR0011-19-2-0014); Advanced Research Projects Agency - Energy (DE-AR0000843); Air Force Office of Scientific Research (FA9550-15-1-0303).

Acknowledgment

The authors thank Dr. J. K. Jang and Y. Zhao for critically reading the manuscript and helpful discussions. We gratefully acknowledge E. Sahagún (Scixel) for illustrations in Fig. 1.

Disclosures

YO and ALG: Xscape Photonics (I,E,P), ML: Xscape Photonics (I,P).

Data availability

No data were generated or analyzed in the presented research.

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Power per channel	0.5–50 mW
Number of channels	4–160
Channel spacing	$> 50 G H z$
Linewidth	1 kHz–10 MHz
Relative intensity noise (RIN)	$< 140 d B / H z$
Side-mode suppression ratio (SMSR)	$< 30 d B$

Normal-GVD Kerr Frequency Combs
Platform	Method	FSR [GHz]	Bandwidth [nm]	Efficiency [%]
SiN microresonator [207]	Intrinsic mode splitting	231	250	31.8
Fiber ring [211]	Pulsed pumping	0.54	40	–
SiN coupled microresonator [213]	Induced mode splitting	200 and 206	150	40.6
SiN microresonator [215]	Injection locking	5 or 10	10	–
SiN coupled microresonator [217]	Induced mode splitting	378 and 391	240	–
SiN microresonator [219]	Intrinsic mode splitting	230	100	20
${T a}_{2} O_{5}$ photonic crystal resonator [224]	Frequency-domain point defect	200	240	21

Power per channel	0.5–50 mW
Number of channels	4–160
Channel spacing	$> 50 G H z$
Linewidth	1 kHz–10 MHz
Relative intensity noise (RIN)	$< 140 d B / H z$
Side-mode suppression ratio (SMSR)	$< 30 d B$

Normal-GVD Kerr Frequency Combs
Platform	Method	FSR [GHz]	Bandwidth [nm]	Efficiency [%]
SiN microresonator [207]	Intrinsic mode splitting	231	250	31.8
Fiber ring [211]	Pulsed pumping	0.54	40	–
SiN coupled microresonator [213]	Induced mode splitting	200 and 206	150	40.6
SiN microresonator [215]	Injection locking	5 or 10	10	–
SiN coupled microresonator [217]	Induced mode splitting	378 and 391	240	–
SiN microresonator [219]	Intrinsic mode splitting	230	100	20
${T a}_{2} O_{5}$ photonic crystal resonator [224]	Frequency-domain point defect	200	240	21

Chip-scale frequency combs for data communications in computing systems

Abstract

1. INTRODUCTION

2. INTEGRATED SEMICONDUCTOR MODELOCKED LASERS

A. Overview

B. Integrated Solutions

1. Quantum-Well, Quantum-Dot, and Quantum-Dash MLLs

2. Extended Cavity MLLs

3. III-V on Silicon MLLs

C. Semiconductor MLLs for Communication

3. ELECTRO-OPTIC COMBS

A. Overview

B. Photonic Platforms

1. Thin-Film Lithium Niobate Modulators

2. Silicon Modulators

3. Plasmonic Modulators

4. Indium Phosphide Modulators

5. Hybrid Modulators

4. KERR FREQUENCY COMBS

A. Overview

B. Soliton Combs

1. Soliton Comb Properties

2. Soliton Combs for Communication

C. Normal-GVD Combs

1. Background and Comb Properties

2. Normal-GVD Combs for Communication

3. Fully Integrated Kerr Comb Source

5. CONCLUSIONS AND OUTLOOK

Funding

Acknowledgment

Disclosures

Data availability

REFERENCES

Data availability

Cited By

Figures (11)

Tables (5)

Equations (1)

Optica

Semiconductor Modelocked Laser
Platform	Repetition Rate [GHz]	Repetition-Rate Linewidth [kHz]	Bandwidth [nm]	Optical Linewidth [MHz]
Quantum well [42,43]	100	450	14	–
Quantum dot [44]	92	20	11.6	–
Quantum dash [47,48]	346	50	9.3	17
Extended cavity InP [53]	30	500	15	24
Extended cavity InP (harmonic ML) [57]	100	–	5.7	–
III-V on Si [66]	40	–	7	–
Extended cavity III-V on Si [70]	1	0.45	10	0.25
Extended cavity III-V on SiN [72]	0.775	0.001	3.27	0.146
Extended cavity III-V on Si (harmonic ML) [75]	20	1.8	6.2	10.6

EO Modulator/Comb
Platform	$V_{π} L$ [ $V \cdot m m$ ]	Frequency Response [GHz]	Comb Bandwidth [nm]
LN [88–90]	44	100	80
Si [91–94]	2.8	100	1
Plasmonic [95,96]	0.06	360	–
Indium phosphide [97,98]	5	50	2
Organic EO on Si [99,100]	2	100	4
LN on SiN [101]	30	8	–
${B a T i O}_{3}$ on silicon [102]	2	2	–
LN on Si [103]	94	40	–
InP on Si [104]	12.5	1.5	–

Anomalous-GVD Kerr Frequency Combs
Platform	Channel Spacing [GHz]	Channels	Modulation Type	Transmission Speed [Tbps]
SiN [164]	200	5	NRZ	0.05
${M g F}_{2}$ [182]	10	145	NRZ	1.45
SiN [183]	25	20	QPSK	1.44
SiN [184]	50	179	16QAM	50
SiN [185]	25	160	64QAM	44.2
SiN [186]	23	6	QPSK	0.12
SiN [187]	24	180	8QAM	6.885
Silica [188]	22.1	52	PM-256QAM	12