Development of MZ modulators based on silicon photonics, lithium niobate, and III–V compound semiconductors

(2) Silicon-based MZ modulators Silicon itself lacks an electro-optic coefficient—so why adopt silicon as the platform for MZ modulators? The reason lies in the maturity of silicon microelectronics and silicon photonic circuits: silicon-based modulators are compatible with complementary metal–oxide–semiconductor (CMOS) fabrication processes, and integration of modulators with lasers is feasible on silicon platforms. The modulation principle of silicon-based MZ modulators relies on the plasma dispersion effect: an externally applied electrical signal alters the effective refractive index of the MZI phase-shifting arm to achieve phase modulation. As early as 2004, Intel reported a silicon-based MZ modulator with a 1-GHz bandwidth; within the following year, its bandwidth was increased to 10 GHz. Subsequent optimizations by researchers have led to recent designs employing reverse-biased PN-junction waveguides combined with multimode interferometer (MMI) couplers for intensity modulation, and coplanar waveguide (CPW) traveling-wave electrodes to achieve high radio-frequency (RF) bandwidth. For instance, structures such as p-i-n-i-n or laterally doped high–low PN junctions reduce absorption loss and improve modulation efficiency, ultimately achieving electro-optic modulation bandwidths exceeding 30 GHz. Overall, silicon-based MZ modulators benefit from CMOS process compatibility, high process maturity, ease of large-scale integration, and compact device footprints enabling high-bandwidth operation—hence their widespread appeal. However, because the waveguide region of silicon modulators resides within doped areas, significant optical absorption loss arises, preventing elongation of the modulation arms; consequently, insertion loss remains high, Vπ is insufficiently low, and linearity is poor—leaving ample room for further performance improvement. To address these issues, researchers have proposed silicon–organic hybrid (SOH) MZ modulators, which leverage ultra-high-electro-optic-coefficient polymer materials to drastically reduce Vπ. By filling the silicon waveguide trench with electro-optic polymer material, electro-optic modulation is realized, enabling 100-GBd 16QAM modulation. Although SOH modulators exhibit excellent electro-optic properties, several challenges remain: first, coupling between the standard waveguide region and the phase-modulation arm introduces non-negligible coupling loss; mode-field conversion between waveguides is thus essential for SOH modulators; second, SOH modulators still lack sufficient reliability under typical application conditions—especially at elevated temperatures (>80 °C), where they clearly fail to meet various commercial qualification standards. Nevertheless, with ongoing advances in materials science, the prospect of identifying materials that simultaneously possess strong electro-optic response and high thermal stability is becoming increasingly promising. (3) III–V MZ modulators

International Photonics Integration Roadmap (IPSR-I): Development roadmap for polymer photonic materials

IPSR-I is an initiative jointly launched by PhotonDelta (Netherlands) and the MIT Center for Microphotonics, aiming to establish a globally unified roadmap for the integrated photonics industry over the next 5–20 years, thereby helping address major societal challenges. It seeks to drive mass production of photonic integrated circuits (PICs) by optimizing the entire supply chain—from research through to end users. However, its previously published roadmaps for silicon photonics and silicon nitride appear somewhat academically oriented and exhibit notable bias. The polymer photonic materials section is no exception: it was authored primarily by Lightwave Logic, Inc. (LWLG), a leading European company specializing in electro-optic polymers. Given LWLG’s inherent commercial interest, the section emphasizes polymer advantages—such as high bandwidth, low drive voltage, and small footprint—while largely omitting discussion of optical loss and reliability validation. Therefore, this section should mainly be viewed as an informative overview of the current status and analysis. Polymer technology nonetheless retains unique strengths: personally, I am optimistic about applications leveraging additive manufacturing techniques like two-photon polymerization (TPP) to simplify packaging, enable optical–electrical chip interconnects, or support chiplet integration. 1. Technology Platforms and Current Applications The industry currently focuses on business growth for 100-G and 200-G line-rate products while exploring extensions to higher line rates (e.g., 300 G and 400 G). Active electro-optic polymer modulators demonstrate outstanding performance in bandwidth and power efficiency, holding promise for achieving these higher line rates; passive polymers and plastic optical fiber (POF) will likewise serve as short-reach interconnect solutions supporting such rate expansions. 2. Technical Challenges and Development Directions ◆ Enhancing Modulator Performance Modulators face challenges in increasing analog bandwidth (e.g., >100 GHz to meet 300-G/400-G requirements), reducing power consumption (e.g., achieving drive voltages 400 pm/V), reducing waveguide propagation loss of passive polymers (e.g., 350 °C). The figure below presents this roadmap: black markers indicate achievable targets through optimization efforts, whereas red markers denote long-term ideal goals. The roadmap identifies key metrics the entire industry must strive toward—and ultimately achieve at scale—as summarized in Table 1 below. These metrics include targeting 3.2-T optical transceivers, >150-GHz 3-dB bandwidth, electro-optic coefficient >400 pm/V, drive voltage 350 °C, and photonic coupling interfaces (e.g., printed wiring boards [PWB], microlenses) achieving ≤0.5 dB per bond or ≤0.5 dB per interface. The primary goal for modulators is to progressively increase bandwidth from 80 GHz to >150 GHz—while maintaining silicon-photonics-process compatibility and drive voltage <1 V—and to establish comprehensive reliability datasets.

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