Cascaded Thin-Film Lithium Niobate Modulators

• Cascaded thin-film lithium niobate modulators are integrated electro-optic devices that leverage the strong Pockels effect for high-speed, low-loss modulation in compact footprints.
• They employ architectures such as Mach-Zehnder interferometers and resonant modulators to enhance bandwidth (up to 320 GHz) and reduce the voltage–length product, enabling precise signal control.
• Advancements in LNOI fabrication and electrode engineering underpin scalable, energy-efficient performance, making these modulators central to coherent communications and quantum photonic applications.

Cascaded thin-film lithium niobate (TFLN) modulators refer to architectures in which multiple integrated electro-optic modulation elements—often Mach-Zehnder interferometers (MZI), phase modulators, or resonant modulators—are concatenated within or across photonic circuits, leveraging the unique efficiency, bandwidth, and scalability advantages of thin-film LN. The emergence of TFLN-on-insulator (LNOI) platforms has provided a transformative material and device basis for constructing ultralow-loss, high-speed, and energy-efficient cascaded modulator systems. This article provides a comprehensive analysis of the design principles, performance trade-offs, device architectures, and application domains of cascaded TFLN modulators, as demonstrated in recent literature.

1. Fundamental Device Physics and Modulation Principles

TFLN electro-optic modulators exploit the strong Pockels effect ($r_{33}$) in LN, supporting broadband phase and intensity modulation with low drive voltages and compact footprints. The basic modulation process can be described by the linear phase insertion,

$\Delta \phi = \frac{\pi V_\text{drive}}{V_\pi}$

where $V_\pi$ (half-wave voltage) is set by the overlap of the RF and optical fields and the modulator geometry. The reduction in $V_\pi \cdot L$ (voltage–length product) is a central metric for high-efficiency TFLN devices (Ren et al., 2019, Xu et al., 2020, Chen et al., 2023).

Cascading modulators allows for complex functionalities, such as:

• Generation of wideband electro-optic frequency combs via sequential phase modulation (Ren et al., 2019, Gao et al., 13 Jun 2024),
• In-phase/quadrature (IQ) or dual-polarization modulation through nested MZI configurations (Xu et al., 2020, Wang et al., 2022),
• Multi-stage signal synthesis, pulse carving, or frequency translation via serrodyne or mixer topologies (Assumpcao et al., 7 May 2024),
• Implementation of complex spectral shaping using sequential ring or photonic crystal cavity modulators (Hou, 2023, Larocque et al., 2023).

Cascading can be spatial (series connection along a waveguide), parallel (e.g., IQ branches), or hybrid (serial-parallel mesh arrangements).

2. Material Platform, Fabrication, and Electrode Engineering

Modern cascaded TFLN modulators leverage LNOI (or TFLN-on-sapphire, TFLN-on-Si, or hybrid SiN–LN) substrates. Key fabrication advances affecting cascade integration efficacy include:

• High-index contrast waveguide formation (e.g., 600 nm TFLN, etched 300 nm to form a rib) for tight optical confinement and single-mode operation (Ren et al., 2019),
• Low-loss and smooth sidewall patterning via electron-beam lithography, chemo-mechanical etching (PLACE), or ICP-RIE (Gao et al., 13 Jun 2024, Xue et al., 17 Dec 2024),
• Spot-size converters for low-loss fiber coupling, critical in multi-stage devices (Wang et al., 2022, Assumpcao et al., 7 May 2024),
• Metallization via thick ($>$1 μm) gold, Ti/Au, or composite (Au/ITO) electrodes; thick metals exceed the skin depth at GHz, minimizing RF loss (Ren et al., 2019, Meng et al., 2023).

Segmented or slow-wave electrode designs enable microwave/optical group velocity matching—an essential criterion for high-bandwidth cascades and low RF walkoff ($n_\text{RF} \approx n_g$) (Nelan et al., 2022, Xue et al., 17 Dec 2024). Capacitively loaded, T-rail, or composite (with high-$\epsilon$ or conductive oxide) approaches further optimize the RF field distribution (Chen et al., 2023, Meng et al., 2023).

Summary of performance-defining parameters:

Parameter	Typical Value (State-of-the-Art)	Significance in Cascading
$V_\pi L$	1–4 V·cm	Low value reduces length, voltage, and cascade loss
EO bandwidth ($f_{3dB}$)	40–120 GHz	Sets bit rate and timing dispersion for cascades
Extinction ratio	30–45 dB	Critical for SNR in multi-stage links
Insertion loss	0.5–4 dB per modulator	Limits number of cascaded stages

3. Performance Metrics and Experimental Achievements

Cascaded TFLN modulators exhibit a range of performance enhancements compared to bulk or low-index LN devices:

• Half-wave voltages ($V_\pi$) of 1.3–4.5 V at 5–67 GHz (Ren et al., 2019, Xu et al., 2020, Xue et al., 17 Dec 2024).
• EO modulation bandwidths exceeding 67 GHz and up to 110–320 GHz for optimized designs (Nelan et al., 2022, Li et al., 26 Nov 2024, Rahman et al., 1 Apr 2025).
• On-chip optical loss as low as 0.5–1.8 dB per device, with dual-polarization, IQ, and frequency comb generation in subcentimeter footprints (Wang et al., 2022, Ren et al., 2019, Xue et al., 17 Dec 2024).
• Frequency combs with more than 40 sidebands covering 10 nm, and sideband generation up to 29 lines with dual-arm phase modulator topologies (Ren et al., 2019, Gao et al., 13 Jun 2024).

Ring-assisted (RAMZI) and photonic crystal configurations deliver unprecedented spurious-free dynamic range ($\text{SFDR}=$ 120 dB·Hz$^{4/5}$) and energy-efficient IQ modulation at CMOS voltages (Feng et al., 2022, Larocque et al., 2023).

The voltage–length figure of merit can be analytically related as:

$V_\pi L = \frac{\lambda d}{n^3 r \Gamma}$

where $d$ is the electrode gap, $n$ the refractive index, $r$ the EO coefficient, $\Gamma$ the overlap factor.

4. Device Architectures: From Serial Phase Chains to IQ/PDM Meshes

Cascaded implementations vary according to application:

• Serial phase modulators increase modulation depth for EO comb broadening without incurring high $V_\pi$ or unwieldy insertion loss (Ren et al., 2019, Gao et al., 13 Jun 2024);
• Dual- and multi-arm IQ or IQP (in-phase/quadrature/polarization) encoders leverage parallel cascaded MZMs with thermo-optic shifters and fine RF bias control to encode high-order QAM and polarization-multiplexed signals at 1.6 Tb/s per chip (Wang et al., 2022);
• Ring-pair and photonic crystal cavity cascades overcome the limitations of single-resonator extinction ratio and linewidth, effectively doubling extinction ratio and expanding usable bandwidth without significant loss penalties (Hou, 2023, Larocque et al., 2023);
• Advanced spatiotemporal optical nonreciprocal devices, such as isolators, are implemented as cascaded traveling-wave phase modulators with precisely controlled delay and phase in each stage, enabling 27 dB isolation without magnetic materials (Huang et al., 2022).

Architectures for hybrid systems—such as wafer-bonded LN–SiN or nested lasers/controllers integrating amplitude and phase modulation for combined pulse shaping and frequency shifting—enable dense multiplexing and system-level scaling (Rahman et al., 1 Apr 2025, Assumpcao et al., 7 May 2024).

5. Applications and Enabling Technologies

Cascaded TFLN modulators are central to a range of demanding systems:

• Next-generation coherent optical communication (e.g., 16-QAM/256-QAM at 1.6 Tb/s) exploits cascaded IQ/IQP modulation meshes—requiring high EO bandwidth, low $V_\pi$, and robust extinction ratio (Xu et al., 2020, Wang et al., 2022).
• Frequency-agile quantum photonic networks utilize cascaded phase/amplitude modulators for high-efficiency serrodyne frequency shifting and multiplexed node addressing at >50 GHz (Assumpcao et al., 7 May 2024).
• RF–photonic links, arbitrary waveform generation, and low-noise analog photonics benefit from ultra-linear, cascaded ring-assisted MZIs (RAMZI) and dual-output, ultra-high extinction MZMs (Feng et al., 2022, Nelan et al., 2022).
• Resonant-based coherent modulation for dense WDM exploits cascaded Gires–Tournois etalons enabling multiple wavelength channels within minimal footprint (Kari et al., 15 Feb 2025).

Scalability is fundamentally enabled by:

• Low insertion loss per stage ($<$1 dB in some cases (Chen et al., 2023, Rahman et al., 1 Apr 2025)),
• Engineered electro-optic and microwave overlaps (periodic slow-wave, capacitively loaded, and composite electrodes),
• Hybrid integration with silicon or SiN photonics for wafer-scale manufacturing and low propagation loss (Rahman et al., 1 Apr 2025, Xue et al., 17 Dec 2024),
• On-chip switches, couplers (loss $<$1 dB/facet), and polarization rotator/combiner structures (Assumpcao et al., 7 May 2024, Wang et al., 2022).

6. Limitations, Trade-offs, and Prospects for Large-scale Integration

Despite substantial advances, there exist constraints associated with cascaded architectures:

• Cumulative insertion loss—while minimized via low-loss waveguide and coupling technologies, becomes significant for very deep cascades or high channel count (Wang et al., 2022, Assumpcao et al., 7 May 2024),
• Bias drift and photorefractive or carrier migration effects can degrade long-term stability in LN, necessitating mitigation by tailored poling, annealing, or cladding strategies (Larocque et al., 2023, Zhu et al., 2021),
• Trade-offs in electrode design: achieving group-velocity matching and low drive voltages simultaneously requires advanced slow-wave, composite, or high-permittivity strategies; fabrication complexity may increase accordingly (Xue et al., 17 Dec 2024, Nelan et al., 2022, Chen et al., 2023),
• Resonant topologies (ring, PhC, Gires–Tournois) enable extreme compactness and energy efficiency but impose bandwidth-limiting photon lifetimes unless Q is intentionally reduced (Larocque et al., 2023, Kari et al., 15 Feb 2025).

Future directions highlighted in the literature include:

• Integration of multiple active and passive TFLN building blocks (modulators, switches, couplers, detectors) on single chips (Assumpcao et al., 7 May 2024, Rahman et al., 1 Apr 2025);
• Cavity and apodized-grating engineering for ultralow-loss slow-light enhancement, targeting FOMs of 180+ (Gbps·(dB/V)/(V·cm)) and record-high bandwidths (e.g., 320 GHz predicted) (Li et al., 26 Nov 2024);
• Scalable wafer-scale/hybrid photonic manufacturing leveraging SiN or Si platforms for high-yield, high-throughput device production (Zhu et al., 2021, Rahman et al., 1 Apr 2025).

7. Comparative Summary and Impacts

Cascaded TFLN modulators, due to their material, design, and fabrication innovations, have surpassed legacy bulk LN and many semiconductor-based competitors in modulation efficiency, bandwidth, linearity, footprint, and system-level energy consumption. As shown across contemporary literature, key quantitative milestones include:

• $V_\pi L$ reaching 1.02–1.41 V·cm with composite electrode, high-$\epsilon$ cladding, or slow-light structures (Meng et al., 2023, Chen et al., 2023, Li et al., 26 Nov 2024),
• Cascaded dual-polarization/IQP links supporting 1.6 Tb/s per chip (Wang et al., 2022),
• EO comb generation of $>$40 sidebands in a single device, with robust prospects for further broadening via cascaded phase chains (Ren et al., 2019, Gao et al., 13 Jun 2024),
• 3-dB modulation bandwidths consistently exceeding 67–110 GHz (and up to 320 GHz predicted) (Nelan et al., 2022, Xue et al., 17 Dec 2024, Li et al., 26 Nov 2024, Rahman et al., 1 Apr 2025).

Continued research focuses on further improving group-velocity matching, reducing cumulative loss in massive multi-element cascades, and addressing practical integration issues for quantum, classical communication, and advanced microwave photonic systems. Given the rapid evolution of TFLN photonics and scalable manufacturing routes, cascaded TFLN modulators are positioned as central elements in the future of large-scale, high-performance photonic circuitry and systems.