An 8×240 Gbps dense wavelength division multiplexing transmitter with lithium tantalate

Domain: physics
Published: 2025-11-27
Editorial outcome: minor_revision

Abstract
Dense wavelength division multiplexing (DWDM) is regarded as a revolutionary solution that significantly enhances transmission capacity. However, DWDM in electro-optic (EO) material platforms faces bottlenecks including limited channel numbers, large insertion losses (ILs), and limited spectral efficiency. Here, an 8×240 Gbps DWDM transmitter at O band is demonstrated on a lithium-tantalate-on-insulator platform through proposing a robust flat-top optical filter based on a novel coupled Fabry-Perot (FP) cavity. The flat-top optical filter shows overall high performance of a low IL of ~0.1–0.4 dB, a large free spectral range of 12 nm, a high extinction ratio (ER) of ~26 dB, and a large fabrication tolerance. The 8-channel DWDM transmitter is further realized with uniform channel spacings of 200 GHz and a total data capacity of 8×240 Gbps. Therefore, it offers a critical strategy for enhancing the capacity of optical links, holding significant promise for widespread application in fields like data centers and artificial intelligence.

Introduction
With the exponential growth of global data traffic, optical interconnect technologies, characterized by inherent advantages including broad bandwidth and superior power efficiency, have grown indispensable for supporting data centers and artificial intelligence-related applications1–3. DWDM emerged as a revolutionary solution by multiplexing multiple optical signals of distinct wavelengths onto a single fiber, exponentially amplifying transmission capacity, serving as a transformative force from the fourth- to the fifth-generation systems.

Integrated photonics, leveraging its unique benefits of miniaturization, scalability, and high energy efficiency, has emerged as a cornerstone of next-generation optical communication and computing systems4–8. Among these technologies, silicon photonics has garnered significant attention for enabling low-cost and high-speed optical transceivers due to its compatibility with complementary metal oxide semiconductor fabrication processes. Worldwide research efforts have achieved notable advancements, pushing integrated silicon photonic interconnects towards the ~Tbps regime9–15. For instance, a polarization-insensitive silicon intensity modulator based on polarization-insensitive grating couplers and a Mach–Zehnder interferometer (MZI) EO modulator has achieved a maximum speed of 224 Gbps9; ultra-low-energy 800-Gbps inter-chip data links have been demonstrated by three-dimensional photonic/electronic integration15. However, the EO modulation speed of silicon based on the plasma dispersion effect gradually approaches its limitation. Further improving link capacity by increasing the number of multiplexing channels raises issues of complex electronic/photonic circuit designs, a large number of involved lasers/detectors, high power consumptions, strict uniformity controls, etc.

EO materials with intrinsic EO modulation bandwidth towards THz level, such as lithium niobate and lithium tantalate, are considered as alternative solutions to break the limitations of EO modulation speed. Numerous high-performance EO modulators have been demonstrated on the lithium-niobate-on-insulator (LNOI) platform7,16,17. For example, an LNOI MZI EO modulator with a 3-dB bandwidth exceeding 110 GHz achieved a record data rate of 1.96 Tbps/wavelength based on coherent modulation18. An LNOI 4-channel DWDM transmitter based on a Fabry-Perot (FP) cavity realized 400 Gbps data transmission19. Recently, the lithium-tantalate-on-insulator (LTOI) platform with the advantages of low thermo-optic coefficients, low birefringence20, small direct current drift, and high damage threshold21 has also attracted extensive attention for exploring high-performance EO modulators22 and large-capacity optical transmitters20. LTOI is also expected to have cost advantages over LNOI since lithium tantalate has been mass-produced for 5G RF filters.

However, both LNOI and LTOI platforms face challenges in DWDM, including the number of channels, ILs, and spectral efficiency. LNOI and LTOI platforms with a relatively small refractive index contrast need a relatively large bending radius to avoid bending loss20,23. The conventional cascaded microring resonator scheme, which is popular for DWDM in silicon photonics, has a limited multiplexing channel number in the LNOI and LTOI platforms as the free spectral range (FSR) is restricted. Additionally, arrayed waveguide gratings occupying large footprints suffer large ILs in terms of principle because of phase errors as well as the insufficient light collection at the boundaries of Rowland circles and arrayed waveguides24,25. Moreover, DWDM requires channel spacing to be much larger than channel bandwidth to reduce crosstalk between neighboring channels if the channel has a non-flat-top optical transmission, thus, the spectral efficiency is limited. Flat-top optical transmission is a reliable method to break the spectral efficiency limitation. However, current methods for flat-top optical transmission, including cascading high-order rings, integrating arrayed waveguide gratings with multimode interferometers, face challenges of large ILs and small fabrication tolerances26,27. Flat-top optical transmission remains a long-standing difficulty in DWDM for different kinds of integration platforms, even including silicon photonics.

In this work, we demonstrate a large-capacity 8-channel DWDM transmitter composed of an 8-channel DWDM and an EO modulator array on an LTOI platform for the first time. Critical issues of limited FSR, large IL, and limited spectral efficiency are resolved by proposing a flat-top optical filter based on a novel coupled FP cavity. The demonstrated flat-top optical filter exhibits a large FSR of ~12 nm, a low IL of 0.1–0.4 dB, a 3-dB bandwidth of 0.8 nm, an ER of 26 dB, and large fabrication tolerance. Further, a fabrication-tolerant 8-channel DWDM is demonstrated by cascading 8 flat-top optical filters, achieving uniform channel spacings of 200 GHz, 3-dB bandwidths of 0.8 nm, ILs of 0.15–0.98 dB, and crosstalk of <−22 dB without any calibration. EO modulators realize 3-dB bandwidths beyond 67 GHz and data rates of 120 GBaud OOK and PAM4, indicating a total transmission capacity of 1.92 Tbps for the demonstrated 8-channel DWDM transmitter. Therefore, the proposed flat-top optical filter based on a coupled FP cavity provides a novel approach to overcoming technical bottlenecks in DWDM parallel data transmission and processing. The 8-channel DWDM transmitter lays a solid foundation for breaking link capacity limits, finding widespread application in fields like data centers and artificial intelligence.

Results
Figure 1 illustrates a three-dimensional view of the present 8-channel LTOI DWDM transmitter, comprising 8 cascaded flat-top optical filters and 8 MZI EO modulators. Each filter integrates a coupled FP cavity and a TE0/TE1 mode (de)multiplexer based on an adiabatic taper. The asymmetric-toothed coupled FP cavity transforms the incident TE0 mode into TE1 mode when the wavelength is near the Bragg wavelength; the reflected TE1 mode then passes through the mode (de)multiplexer, emerging at the drop port as TE0. This design eliminates the need for an integrated circulator to separate input and reflected light, endowing the flat-top filter with strong scalability for DWDM integration. The transmitter is fabricated on an x-cut LTOI substrate with a 400-nm-thick lithium tantalate layer, 200-nm etching depth, and air cladding. As shown in the inset of Fig. 1b, the waveguide features a 60° sidewall angle, a result of the isotropic etching process.

The detailed illustration of the flat-top optical filter composed of a mode (de)multiplexer and a coupled FP cavity is presented in Fig. 2a. The mode (de)multiplexer, which relies on an adiabatic dual-core taper, was designed following the methodology outlined in our prior study28. Waveguides for adiabatic couplers have input/output widths of (wa1, wb1) = (0.85, 0.60) μm and (wa2, wb2) = (1.70, 0.25) μm, respectively. The taper lengths are set to (L01, L12, L23) = (50, 150, 50) μm, while the gap values are configured as (wg1, wg2, wg3) = (1.5, 0.3, 1.5) μm. The coupled FP cavity consists of three cascaded mirrors derived from asymmetric multimode waveguide gratings (AMWGs). This configuration yields a flat-top optical response, as predicted by coupled FP cavity theory (see Supplementary Information S1). Moreover, the cavity length Lt can be as short as Λ/2 to break the FSR limitation faced by microring cavities (see Supplementary Information S2).

We designed the coupled FP cavity according to the requirement of realizing an 8-channel DWDM with a channel spacing of 200 GHz. The structural parameters are chosen as δ = 850 nm, Λ = 357 nm, η = 0.5, w = 1900 nm, b = 10, N1 = N3 = 80, and N2 = 160. The simulated optical transmission spectrum is shown in Fig. 2b. It can be seen that the designed flat-top optical filter has a large FSR of ~12 nm, a low IL < 0.3 dB, a 3-dB bandwidth of 0.8 nm, and an ER of > 32 dB, verifying that the coupled FP cavity enables a large FSR and box-like optical transmission which helps break the FSR and spectral efficiency limitations.

The optical transmission spectrum can be flexibly adjusted by changing the reflectivity of AMWG1 and AMWG3, determined by teeth number N1 and N3, as well as the coupling strength between the two cavities, determined by teeth number N2. Figure 2c illustrates the simulated transmission spectra of the coupled FP cavity with different N1 and N3 (N1 = N3 = 70, 75, 80, 85, 90, 95) when N2 is set to 180. On the one hand, the 3-dB bandwidth decreases with the increase of the number N1 and N3 because the larger reflectivity of AMWG1 and AMWG3 make the coupled FP cavity with a higher quality factor. On the other hand, the ERs of the optical transmission spectra are enhanced with the increase of N1 and N3, leading to a box-like transmission spectrum. Figure 2d plots the simulated optical transmission spectra with different N2 (N2 = 170, 175, 180, 185, and 190) when N1 and N3 are set to 80. Increasing N2 from 170 to 190 results in the bandwidth expanding from 0.7 nm to 1.5 nm, while the ERs decrease from 25 to 21 dB, respectively. The fabrication tolerance of the coupled FP cavity is also analyzed as shown in Fig. 2e, f. The operation wavelengths and transmission spectra are maintained as the initial values when assuming variations in etch depth Δδ = 0, ±10, and ±20 nm (Fig. 2e) as well as core width Δw = 0 and ±5 nm (Fig. 2f). Figure 2g–i show the simulated light propagation in the coupled FP cavity when operating at the through non-resonance wavelength of 1330 nm (Fig. 2g), resonant wavelength of 1303 nm (Fig. 2h), and the reflected non-resonance wavelength of 1310 nm (Fig. 2i). It can be seen that the launched TE0 mode (forward) is reflected and converted to the TE1 mode (backward) at 1310 nm (which is a non-resonant wavelength). When operating at the resonant wavelength of 1303 nm, resonance appears in the cavities. When operating at the non-resonant wavelength of 1330 nm, all the light passes through the grating because the Bragg reflection condition is not satisfied.

Optical filters with flat-top optical transmission spectra are highly desired to reduce signal distortions, thus enhancing the spectral efficiency. Figure 3a shows the optical transmission spectra of filters based on single and coupled cavities. Figure 3b depicts the frequency spectra of 40, 80, 120, and 150 Gbaud OOK signals, respectively, where the bandwidth increases almost linearly with the increase of data rate. An optical filter will cause signal distortions if the bandwidth of the optical filter is smaller than the bandwidth of the modulated signal. DWDM based on single-cavity filters must increase the 3-dB bandwidth to support high-speed signals, but the channel spacing must be increased at the same time to reduce the crosstalk between neighboring channels. As a result, the spectral efficiency is limited. In terms of DWDM based on coupled-cavity filters with flat-top optical transmission spectra, the coupled-cavity filter has a broader 3-dB bandwidth and lower crosstalk at the same channel spacing as a single-cavity filter, thus supporting higher speed signals. Figure 3c, d show the simulated eye diagrams after transmitting through optical filters based on single (Fig. 3c) and coupled (Fig. 3d) cavities, respectively. Signal distortions can be clearly observed after transmitting through a single-cavity filter when the data rate is 120 GBaud. As for a coupled-cavity filter, no signal distortion is found even when the data rate approaches 150 GBaud.

Figure 4a–c presents the fabricated flat-top optical filter (Fig. 4a), AMWG (Fig. 4b), and the adiabatic dual-core taper (Fig. 4c). For ease of measurement, grating couplers optimized for TE polarization were adopted to enable chip-fiber coupling. The characterization of the fabricated devices was conducted using a broadband superluminescent diode (SLD) as the light source and an optical spectrum analyzer (OSA) for signal detection. As shown in Fig. 4d, the normalized optical transmission spectra of the flat-top optical filter at both the drop and through ports reveal an IL of approximately 0.1–0.4 dB, an ER of around 26 dB, a 3-dB bandwidth of roughly 0.8 nm, and a FSR of about 12 nm. Figure 4e show zoomed-in view of optical transmission spectrum.

Figure 4f shows the measured optical transmission spectra of fabricated flat-top optical filters with different Δδ, indicating that the fabricated flat-top optical filters maintain a similar operation wavelength and box-like transmission spectra even with deviations of Δδ = ±50 nm. Figure 4g shows the measured optical transmission spectra of 5 flat-top optical filters with identical structural parameters fabricated on the same chip. It clearly conveys the information that the flat-top optical filters have large fabrication tolerance because all devices have similar operation wavelengths and transmission spectra. The large fabrication tolerance performance lays a solid foundation for forming a calibration-quasi-free DWDM. Figure 4h presents the measured optical transmission spectra of the fabricated flat-top optical filter across a temperature range of 25–75 °C. The operating wavelength shifts upward as temperature increases. As further quantified in Fig. 4i, the thermo-optic coefficient derived from fitting this wavelength-temperature relationship is approximately 40 pm/°C.

An 8-channel DWDM is designed based on cascading 8 flat-top optical filters in series, as shown in Fig. 5a, the operation wavelengths of 8 flat-top optical filters increase linearly from left to right by carefully optimizing the cavity length Lc. Here, the cavity lengths Lc for the 8 channels with a channel spacing of 200 GHz nm are chosen as Lc = 357, 397, 437, 477, 517, 557, 597, and 637 nm, respectively, when other structural parameters are δ = 850 nm, Λ = 350 nm, duty cycle η = 0.5, w = 1850 nm, b = 10, N1 = N3 = 80, and N2 = 160. The corresponding simulated transmission spectra are plotted in Fig. 5b, indicating the 8-channel DWDM has channel spacings of 200 GHz, ILs of ~0.1–0.4 dB, crosstalk between adjacent channels of −25 dB, and crosstalk between non-adjacent channels of −30 dB. Figure 5c shows an optical microscope image of the fabricated 8-channel DWDM. The measured optical transmission spectra of the 8-channel DWDM without any calibration is shown in Fig. 5d, indicating channel spacings of 200 GHz, ILs ranging from 0.15 to 0.98 dB, and the crosstalk between adjacent and non-adjacent channels of ~−22 dB and ~−30 dB.

Figure 6a shows an optical microscope image of the 8-channel DWDM transmitter. The detailed design and measured results of the EO modulator are described in Supplementary Information S3. The measured results show that the fabricated EO modulator has a 3-dB bandwidth beyond 67 GHz. The experimental setup characterizing the 8-channel DWDM transmitter is depicted in Fig. 6b. Figure 6c illustrates the measured open-eye diagrams corresponding to OOK and PAM4 signals across 8 channels, each operating at a data rate of 120 GBaud. It confirms that the proposed optical transmitter is capable of delivering an overall data capacity of 1.92 Tbps.

Moreover, standard single-mode fibers exhibit near-zero chromatic dispersion around 1310 nm, minimizing distortion of high-speed signals over long fibers. It is highly desired for medium-to-short distance optical communications such as data centers, local area networks (LANs), metro networks, and access networks. Therefore, we performed high-speed transmission over long fibers. Figure 7a plots the measured eye diagrams of 120 GBaud OOK and PAM4 signals for a single channel after traveling through fiber lengths of 1, 3, 5, and 8 km; all eye diagrams keep well-open for different fiber lengths, which indicates excellent stability across different transmission distances. Figure 7b presents the measured eye diagrams of 120 GBaud OOK and PAM4 signals for a single channel under different optical powers of −1, −3, and −8 dBm. Distortion was observed when the optical power dropped to −8 dBm for the 120 GBaud PAM4 signal.

Table 1 delivers a comparative summary of critical performance parameters for photonic transmitters documented in recent research. State-of-the-art designs, which leverage diverse material platforms such as silicon on insulator (SOI), III-V semiconductors, barium titanate (BTO), plasmonic, and LNOI, have shown data rates spanning 10 to 100 Gbps/channel, with their maximum total data rates reaching 1.6 Tbps. Notably, the newly developed transmitter based on LTOI stands out with superior performance as it attains a single-channel data rate of 240 Gbps and a total capacity of 1.92 Tbps.

Methods
Fabrication
The device was fabricated on a 400-nm LTOI platform with a 3-μm-thick silica buffer layer and a 525-μm-thick silicon handle substrate. Electron-beam lithography was first employed to pattern the device structure. This pattern was then transferred into the lithium tantalate layer via Ar+ plasma dry etching, a process that resulted in waveguides with a sidewall angle θ of 60°. Subsequently, ultraviolet lithography was used to define electrode patterns, followed by the deposition of a 10-nm titanium (Ti) adhesion layer and a 500-nm gold (Au) conductive layer via electron-beam evaporation. Finally, a lift-off process was performed to form the complete electrodes.

Characterization
Light emitted from a tunable laser, whose polarization was regulated by a polarization controller, was coupled into the device under test through a grating coupler. Concurrently, electrical signals combined with a static voltage using a bias-tee was then fed into the device to modulate the incoming light. After modulation, the light was coupled out of the device via another grating coupler. The output light signal was first amplified by a PDFA before being collected and analyzed using an electrical sampling oscilloscope.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The authors demonstrate an eight wavelength (channel) DWDM transmitter using lithium tantalate MZI modulators with a\ngrating assisted FP cavity and mode demultiplexer. The device performance is very impressive, but I find the claim that the\ntransmitter is 1.92Tbps to be misleading for several reasons. While the authors did test each channel one by one, this is not\nsufficient to claim that the DWDM transmitter can carry all eight 120 GBaud PAM4 channels simultaneously. In particular, the\nchannel spacing is 200GHz, which should not be enough to support 120GBaud operation without significant crosstalk when\nall channels are on, provided the single side-band spectral content will extend up to 120GHz. Without testing multiple\nchannels at a time, the effects of crosstalk are not captured. If the authors are unable to test multiple channels at a time, then\nI suggest that the simulations in Figure 3 be expanded to capture multiple channel effects.\nI also have the following major comments and questions:\n• The authors stress the importance of a flat-top optical filter shape as to not introduce signal distortion. Given the target\n200GHz channel spacing in this work, what is the optimal filter bandwidth?\n• Each filter seems to produce two usable “resonant” wavelengths (i.e 1303nm and 1315nm), and the authors seem to swap\nbetween the two resonances in their measurements, such as in Fig. 4. Is there any practical difference between the two?\nWas there any reason to not use a single resonant wavelength throughout the paper?\n• The term “FSR” is used to describe the 12nm separation between the resonances. However, the distance between the\npeaks change as we move away from the Bragg condition, like in Fig. 4d where the next peak appears to be 1321nm. Most\nreaders expect FSR to be constant, but that does not appear to be true in this device. Is FSR the correct term to use given\nthe spectral response is not periodic?\n• If possible, please share experimental standalone spectral measurements of a single cavity, as I expect this would be of\ninterest to the readers.\n• The insertion loss of the filter is extremely impressive (0.1dB), but where is the actual measurement behind this claim? Can\ndetails be provided about how such a small loss was measured and normalized (possibly in the Supplementary\nInformation)? Is the drop port loss normalized to the thru port or to another reference structure? How do you define the loss in\nthe case that there is a ripple in the drop port?\n• At a high level, given 0.1dB loss for one filter, wouldn’t the expected loss for Ch8 (8 cascaded filters) be 0.8dB? The\nauthors instead quote a maximum of 0.5dB.\n• In Table 1, the “flat top” column does not seem to be relevant for all the technologies. For example, many of the transmitters\non SOI use cascaded microring modulators, which do not necessarily need a flat-top filter to combine the different channels.\nI suggest using N/A instead of No for the demonstrations where flat-top filter is not relevant.\n• I think the readers would appreciate a comparison with other LTOI modulators, perhaps in the Supplementary material.\nThe following are minor fixes and comments.\n• In Fig. 3, what is the bandwidth (1dB or 3dB) for the simulated single and coupled cavity filters.\n• In Fig. 3c, why does there appear to be more overshoot at higher Baud rates for the single cavity case?\n• Please add a legend to Fig. 4f\n• It would be helpful to match the colors of the channels in Fig 5a,b,c,d.\n• Typo in Fig. 6b (RF amplifiber)\n• Supplementary document has many references in the text that are missing and instead showing “Error! Reference source\nnot found”.", "reviewer_2": "The current work lies in the field of integrated photonics and implementing high capacity WDM transmitters for optical\ncommunications, targeting data rates beyond the current limitations introduced lately by carrier dispersion engineering of\nsilicon photonics. In order to achieve this, the authors utilize lithium tantalate as a fast modulator, combined in an 8-channel\nmutliplexed transmission with a novel flat top designed multiplexer. The work extends a previous high impact publication of\nthe authors, which is appreciated by the reviewer. The work is interesting and of high impact that could possibly merit\npublication. The work is well presented, nicely structured and very well documented, while seem to present a clear advance\ntowards multi-channel lithium tantalate platforms. The only comments are to elaborate more one the trade-offs with existing\nplatforms, including a more detailed comparison in terms of fabrications costs, achievable linerates and power\nconsumptions. For references, some of the references added on high capacity transmitters seem to be rather outdated and\nnot of maximum datarates (e.g. rings on Silicon), not favoring a very fair comparison with existing high-bandwidth\nmodulators. Please provide a more extended literature review on high bandwidth modulators, including indicatively higher\nlinerates achieved on a single channel by lithium tantalate modulators (e.g. Ref A) or InP modulators with even beyond\n200G and 400 data rates per channel, electro-optic and plasmonic modulators.\nOverall the work could merit publications in its current form without any major technical modifications recommended.\n[Ref A] Chengli Wang, Dengyang Fang, Junyin Zhang, Alexander Kotz, Grigory Lihachev, Mikhail Churaev, Zihan Li, Adrian\nSchwarzenberger, Xin Ou, Christian Koos, and Tobias J. Kippenberg, \"Ultrabroadband thin-film lithium tantalate modulator\nfor high-speed communications,\" Optica Vol. 11, Issue 12, pp. 1614-1620 (2024) https://doi.org/10.1364/OPTICA.537730\n[Ref B] Yoshihiro Ogiso, Josuke Ozaki, Kenta Sugiura, Yusuke Saito, and Mitsuteru Ishikawa , \"High Speed InP Modulator\nfor Beyond 200 Gbaud,\""}, "author_response": "We sincerely thank Reviewer #1 for constructive suggestions and the\npositive assessment of our work. We appreciate the opportunity to provide a detailed\nresponse and revisions addressing the raised questions.\nAt first, we change the title of our manuscript from “1.92 Tbps dense wavelength\ndivision multiplexing transmitter with lithium tantalate” into “An 8×240 Gbps dense\nwavelength division multiplexing transmitter with lithium tantalate” to avoid\nmisleading.\nSecond, the experimental setup, as depicted in Figure R1a, was employed to\ncharacterize the modulation crosstalk between different channels. Crosstalk between\ntwo channels was evaluated by measuring the modulation strength when light was\ntransmitted through one channel while an RF signal from an LCA was applied to the\nother. Figure R1b illustrates the modulation strength when light was injected into CH5\nand RF signals were applied to CH1–CH8, demonstrating that the modulation strength\nfrom CH5 was at least 20 dB higher than that from CH1, CH2, CH3, CH4, CH6, CH7,\nand CH8. Additionally, according to the passive characterization results of the\nfabricated DWDM (as shown in Figure 5), the optical crosstalks between adjacent and\nnon-adjacent channels are approximately −22 dB and −30 dB. Further, we numerically\nsimulated the multichannel effects based on the measured RF and optical crosstalk.\nFigures R1c(R1e) and R1d(R1f) show the simulated eye diagrams of 120 GBaud OOK\nand PAM4 signals without(with) crosstalk, respectively. Neglectable influences are\nobserved when comparing eye diagrams with crosstalk introduced or not. We have\nmade the corresponding revision to include multichannel effects in the revised\nmanuscript.\n(a) (b)\n0\n-20\nPC\nRF -40\n-60\n0 20 40 60\nTunable laser LCA\n(c) 120 GBaud OOK (d) 120 GBaud PAM4 (e) 120 GBaud OOK (f) 120 GBaud PAM4\nWithout crosstalk Without crosstalk RF and optical crosstalks RF and optical crosstalks\n2\n)Bd(\nnoissimsnarT\nFrom CH1 to CH5 From CH2 to CH5\nFrom CH3 to CH5 From CH4 to CH5\nFrom CH5 to CH5 From CH6 to CH5\nFrom CH7 to CH5 From CH8 to CH5\nFrequency (GHz)\nFigure R1. Modulation crosstalk characterization. (a) Experimental setup for characterizing modulation\ncrosstalk between different channels; (b) modulation strength when light was sent into CH5 and RF\nsignals were applied to CH1–CH8; (c) simulated eye diagrams of 120 GBaud OOK signal without\ncrosstalk; (d) simulated eye diagrams of 120 GBaud PAM4 signal without crosstalk; (e) simulated eye\ndiagrams of 120 GBaud OOK signal with measured crosstalk; (f) simulated eye diagrams of 120 GBaud\nPAM4 signal with measured crosstalk.\nRevisions:(Title) An 8×240 Gbps dense wavelength division multiplexing\ntransmitter with lithium tantalate\n(Supplemental material, Page 7, lines 93–111)\nS4. Modulation crosstalk between different channels\nThe experimental setup, as shown in Figure S5a, was employed to characterize\nmodulation crosstalk between different channels. Crosstalk between two channels was\nevaluated by measuring the modulation strength when light was transmitted through\none channel while an RF signal from a Lightwave Component Analyzer (LCA) was\napplied to the other. Figure S5b depicts the modulation strength when light is injected\ninto CH5 and RF signals are applied to CH1–CH8, revealing that the modulation\nstrength from the self-channel is at least 20 dB higher than that from adjacent and non-\nadjacent channels. Further, we numerically simulated the multichannel effects based on\nthe measured RF and optical crosstalks (Figure 5). Figures S5c(S5e) and S5d(S5f)\npresent the simulated eye diagrams of 120 GBaud OOK and PAM4 signals\nwithout(with) crosstalks, respectively, indicated negligible differences caused by RF\nand optical crosstalks.\n(a) (b)\n0\n-20\nPC\nRF -40\n-60\n0 20 40 60\nTunable laser LCA\n(c) 120 GBaud OOK (d) 120 GBaud PAM4 (e) 120 GBaud OOK (f) 120 GBaud PAM4\nWithout crosstalk Without crosstalk RF and optical crosstalks RF and optical crosstalks\n3\n)Bd(\nnoissimsnarT\nFrom CH1 to CH5 From CH2 to CH5\nFrom CH3 to CH5 From CH4 to CH5\nFrom CH5 to CH5 From CH6 to CH5\nFrom CH7 to CH5 From CH8 to CH5\nFrequency (GHz)\nFigure S5. Modulation crosstalk characterization. (a) Experimental setup for characterizing modulation\ncrosstalk between different channels; (b) modulation strength when light was sent into CH5 and RF\nsignals were applied to CH1–CH8; (c) simulated eye diagrams of 120 GBaud OOK signal without\ncrosstalk; (d) simulated eye diagrams of 120 GBaud PAM4 signal without crosstalk; (e) simulated eye\ndiagrams of 120 GBaud OOK signal with measured crosstalk; (f) simulated eye diagrams of 120 GBaud\nPAM4 signal with measured crosstalk.\nComment 1: The authors stress the importance of a flat-top optical filter shape as to\nnot introduce signal distortion. Given the target 200GHz channel spacing in this work,\nwhat is the optimal filter bandwidth?\nResponse:Thank you for the comment. As shown in Figure R2a, a perfect square filter\nwith a 200 GHz bandwidth would undoubtedly be ideal when paired with a 200 GHz\nchannel spacing; however, experimentally realizing such an ideal filter remains\nextremely challenging. Regarding the 120 GBaud signal employed in our experiments,\nour simulations on the impact of filter bandwidth (as presented in Figure R2b–R2e)\nindicate that 120 GBaud PAM4 signals necessitate a filter with a 3-dB bandwidth of at\nleast 120 GHz. The flat-top filter we demonstrated, which is based on a coupled Fabry-\nPerot (FP) cavity and features a 3-dB bandwidth of approximately 130 GHz, is fully\ncapable of handling 120 GBaud PAM4 signals.\n(a)\n1\n0\n-200 -100 0 100 200\n(b)\nIdeal filter\n(c)\n3-dB bandwidth\n(d)\n3-dB bandwidth\n(e)\n3-dB bandwidth\n4\nnoissimsnarT\nFrenquence (GHz)\nFigure R2. Simulations on the impact of filter bandwidth. (a) Transmission spectrum of an ideal filter.\n(b)–(e) Simulated eye diagrams of 120 GBaud OOK and PAM4 signals after transmitting through an\nideal filter (b) and filters with different 3-dB bandwidth of 110 GHz (c), 120 GHz (d), 130 GHz (e).\nComment 2: Each filter seems to produce two usable “resonant” wavelengths (i.e\n1303nm and 1315nm), and the authors seem to swap between the two resonances in\ntheir measurements, such as in Fig. 4. Is there any practical difference between the two?\nWas there any reason to not use a single resonant wavelength throughout the paper?\nResponse: Thank you for your comment. The two resonant wavelengths of the cavity\nare induced by certain effective cavity length, the distance between the two resonant\nwavelengths is just the free spectral range (FSR) of the cavity. In addition, either of\nthese resonant wavelengths is suitable for DWDM, and the right resonant wavelength\nwith a longer wavelength was adopted in our experiment. Corresponding revisions have\nbeen made to Figure 4 in the revised manuscript, where all displayed wavelengths are\nadjusted to the right resonant wavelengths.\nRevision:(In result, Page 9, Figure 4 (h) and (i)):\n1316\n1315\n1314\n1313\n1312\n20 30 40 50 60 70 80\n5\n)mn(\nhtgnelevaW\n0\n-25\n-50\n1310 1314 1318\nTemperature (˚Ϲ)\n)Bd(\nnoissimsnarT\n(h) (i)\n40 pm/˚Ϲ\n25 ˚Ϲ 35 ˚Ϲ 45 ˚Ϲ\n55 ˚Ϲ 65 ˚Ϲ 75 ˚Ϲ\nWavelength (nm)\nComment 3: The term “FSR” is used to describe the 12nm separation between the\nresonances. However, the distance between the peaks change as we move away from\nthe Bragg condition, like in Fig. 4d where the next peak appears to be 1321nm. Most\nreaders expect FSR to be constant, but that does not appear to be true in this device. Is\nFSR the correct term to use given the spectral response is not periodic?\nResponse: Thank you for your comment. The FSR of a FP cavity is jointly determined\nby the intermediate cavity length and the effective lengths of AMWGs. For the 8-\nchannel DWDM device, all channels share identical AMWGs with effective lengths on\nthe order of tens of micrometers, while the intermediate cavity lengths range from a\nmaximum of 0.700 μm to a minimum of 0.350 μm. This results in variations in the\nintermediate cavity length that are significantly smaller than the effective lengths of the\nAMWGs. Consequently, the FSR variations across different channels are negligible. As\nillustrated in Figure R3a, the simulated FSRs of all 8 channels remain approximately\n12 nm. Consistently, the experimentally measured FSRs for all 8 channels also maintain\naround 12 nm, as shown in Figure 3b. The slight increase in FSR variations observed\nin the experimental results can be attributed to the difficulty in accurately quantifying\nsuch variations, given that they are much smaller than the 3-dB bandwidth of the filter.\nAs for “like in Fig. 4d where the next peak appears to be 1321nm”, the reason for the\nrises at the wavelengths around 1320 nm and 1295 nm is that the reflection band of\nAMWGs is exactly ~1295–1320 nm, so the rises do not result from FSR.\n13\n12\n11\n10\n1 2 3 4 5 6 7 8\n13\n12\n11\n10\n1 2 3 4 5 6 7 8\nChannel\nRSF\n(a) Simulated (b) experimental\nChannel\nFigure R3. The simulated (a) and measured (b) FSRs of all channels.\nComment 4: If possible, please share experimental standalone spectral measurements\nof a single cavity, as I expect this would be of interest to the readers.\nResponse: Thank you for your comment. We have included the experimental\nstandalone transmission spectrum in the revised manuscript.\nRevisions:(In result, Page 8, Line 191):\nFigure 4(e) zoomed-in view of optical transmission spectrum.\n(In result, Page 9, Line 216-219):\n0\n-25\n-50\n1295 1300 1305 1310 1315 1320\n)Bd(\nnoissimsnarT\n(e)\nWavelength (nm)\nFigure 4. (e) zoomed-in view of optical transmission spectrum;\nComment 5: The insertion loss of the filter is extremely impressive (0.1dB), but where\nis the actual measurement behind this claim? Can details be provided about how such\na small loss was measured and normalized (possibly in the Supplementary Information)?\nIs the drop port loss normalized to the thru port or to another reference structure? How\ndo you define the loss in the case that there is a ripple in the drop port?\nResponse: Thank you for your comment. All measured results in this paper are\nnormalized with respect to the transmission of a straight waveguide on the same chip.\nFor a single cavity, the minimum measured loss is 0.1 dB without considering the ripple\nin our previous manuscript. Considering the measured ripple, we have revised the value\nin the main text to 0.1–0.3 dB in our revised manuscript. Despite losses at this level are\ndifficult to be characterized precisely because different grating couplers with identical\ndesign might have different coupling losses, the low insertion loss feature of a single\nfilter can also be evidenced by the measured results of the 8-channel DWDM as shown\nin Figure R4, where losses range from 0.15 dB to 0.98 dB.\nFigure R4. Measured results of 8-channel DWDM\nRevisions:(In abstract, Page 1, Line 22):\nThe flat-top optical filter shows overall high performance of a low IL of ~0.1-0.4 dB,\n(In result, Page 3, Line 86-88):\nThe demonstrated flat-top optical filter exhibits a large FSR of ~12 nm, a low IL of\n0.1-0.4 dB,\n(In result, Page 8, Line 188-190):\nports of the flat-top optical filters, indicating an IL of ~0.1-0.4 dB,\n(In result, Page 8, Line 209-210):\nindicating the 8-channel DWDM has channel spacings of 200 GHz, ILs of ~0.1-0.4 dB,\n(In conclusion, Page 13, Line 271-272):\nThe proposed filter exhibits a large FSR of ~12 nm, a low IL of ~0.1-0.4 dB, a 3-dB\nbandwidth of 0.8 nm, and high fabrication tolerance.\nComment 6: At a high level, given 0.1dB loss for one filter, wouldn’t the expected loss\nfor Ch8 (8 cascaded filters) be 0.8dB? The authors instead quote a maximum of 0.5dB.\nResponse: Thank you for your comment. The insertion loss of a single filter originates\nfrom a mode (de)multiplexer and a coupled FP cavity. However, for a cascaded filter,\nif light with a wavelength is out of the resonant spectrum of a coupled FP cavity, the\nlight will be reflected by the first AMWG directly instead of passing through the\ncoupled FP cavity. Therefore, the loss of the Nth channel of a cascaded filter comes from\nN mode (de)multiplexers and a single coupled FP cavity. As a result, the expected loss\nof CH8 is not 8 times of CH1. The detail of measured results of the 8-channel DWDM\nis shown in Figure R5, where losses range from 0.15 dB to 0.98 dB. Revisions have\nbeen made in the revised manuscript to accurately express the insertion losses of all\nchannels.\nFigure R5. Measured results of 8-channel DWDM\nRevisions:(In abstract, Page 1, Line 22):\nThe flat-top optical filter shows overall high performance of a low IL of ~0.1–0.4 dB,\n8\n(In result, Page 3, Line 86-88):\nThe demonstrated flat-top optical filter exhibits a large FSR of ~12 nm, a low IL of\n0.1–0.4 dB,\n(In result, Page 8, Line 188-190):\nFigure 4(d) shows the normalized optical transmission spectra at the drop and through\nports of the flat-top optical filters, indicating an IL of ~0.1–0.4 dB,\n(In result, Page 8, Line 209-210):\nindicating the 8-channel DWDM has channel spacings of 200 GHz, ILs of ~0.1–0.4 dB,\n(In conclusion, Page 13, Line 271-272):\nThe proposed filter exhibits a large FSR of ~12 nm, a low IL of ~0.1–0.4 dB, a 3-dB\nbandwidth of 0.8 nm, and high fabrication tolerance.\n(In abstract, Page 1, Line 25-26):\nand ILs of 0.15–0.98 dB, without any calibration\n(In result, Page 3, Line 89-90):\nachieving uniform channel spacings of 200 GHz, 3-dB bandwidths of 0.8 nm, ILs of\n0.15–0.98 dB,\n(In result, Page 9, Line 213-215):\nILs of 0.15–0.98 dB, the crosstalk between adjacent channels of ~−22 dB, and the\ncrosstalk between non-adjacent channels of ~−30 dB.\n(In conclusion, Page 13, Line 274-275):\n3-dB bandwidths of 0.8 nm, IL of 0.15–0.98 dB, and crosstalk levels <−22 dB.\nComment 7: In Table 1, the “flat top” column does not seem to be relevant for all the\ntechnologies. For example, many of the transmitters on SOI use cascaded microring\nmodulators, which do not necessarily need a flat-top filter to combine the different\nchannels. I suggest using N/A instead of No for the demonstrations where flat-top filter\nis not relevant.\nResponse: Thank you for your comment. We have made revisions in the revised\nmanuscript according to the advice.\nRevisions:(In result, Page 13, Line 267-268):\nTable 1 Summary of key parameters and performance metrics for photonic transmitters\nReference Material Single Channel Flat Top Channel Spacing Total Capacity\nThis work LTOI 240 Gbps Yes 200 GHz 1.92 Tbps\nComment 8: In Fig. 3, what is the bandwidth (1dB or 3dB) for the simulated single and\ncoupled cavity filters.\nResponse: Thank you for your comment. We set the 3-dB bandwidth for the simulated\nsingle and coupled cavity filters. The single cavity filter 3-dB bandwidth = 40 GHz and\nthe coupled cavity filter 3-dB bandwidth = 120 GHz.\nFigure 3. Comparison between filters with single and coupled cavities. (a) Optical\ntransmission of filters with single (blue, 3-dB bandwidth = 40 GHz) and coupled (red,\n3-dB bandwidth = 120 GHz)\nComment 9:.In Fig. 3c, why does there appear to be more overshoot at higher Baud\nrates for the single cavity case?\nResponse: Thank you for your comment. An optical filter will cause signal distortions\nif the bandwidth of the optical filter is smaller than the bandwidth of the modulated\nsignal. Thus, in the case of a single cavity, since its 3-dB bandwidth is smaller than the\nbaud rate, greater overshoot distortion will occur as the baud rate increases.\nComment 10:.?It would be helpful to match the colors of the channels in Fig 5a,b,c,d.\nResponse: Thank you for your comment. we have made the corresponding revision in\nthe revised manuscript.\nRevisions:(In result, Page 10, Figure 5):\nResponse: Thank you for your comment. we have made the corresponding revision in\nthe revised manuscript.\nRevisions:(In result, Page 11, Figure 6):\n(b) Bias tee\nV\nClock\nOn/off output\nBias voltage RF amplifier AWG\nESO\nPolarization\ncontroller\n1310 nm\ntunable Laser PDFA\nComment 12:.Supplementary document has many references in the text that are missing\nand instead showing “Error! Reference source not found”.\nResponse: Thank you for your comment. we have made the corresponding revision in\nthe revised manuscript.\nGeneral comment: The current work lies in the field of integrated photonics and\nimplementing high capacity WDM transmitters for optical communications, targeting\ndata rates beyond the current limitations introduced lately by carrier dispersion\nengineering of silicon photonics. In order to achieve this, the authors utilize lithium\ntantalate as a fast modulator, combined in an 8-channel mutliplexed transmission with\na novel flat top designed multiplexer. The work extends a previous high impact\npublication of the authors, which is appreciated by the reviewer. The work is interesting\nand of high impact that could possibly merit publication. The work is well presented,\nnicely structured and very well documented, while seem to present a clear advance\ntowards multi-channel lithium tantalate platforms. The only comments are to elaborate\nmore one the trade-offs with existing platforms, including a more detailed comparison\nin terms of fabrications costs, achievable linerates and power consumptions. For\nreferences, some of the references added on high capacity transmitters seem to be rather\noutdated and not of maximum datarates (e.g. rings on Silicon), not favoring a very fair\ncomparison with existing high-bandwidth modulators. Please provide a more extended\nliterature review on high bandwidth modulators, including indicatively higher\nlinerates achieved on a single channel by lithium tantalate modulators (e.g. Ref A) or InP\nmodulators with even beyond 200G and 400 data rates per channel, electro-optic and\nplasmonic modulators.\nOverall the work could merit publications in its current form without any major\ntechnical modifications recommendeds：\nResponse: We sincerely thank Reviewer #2 for constructive suggestions and the\npositive assessment of our work. We appreciate the opportunity to provide a detailed\nresponse and revisions addressing the raised questions. LTOI shows trade-offs versus\nSOI, LNOI, and InP in terms of cost, data rate, and power. In terms of cost, LTOI is\nlower than InP (with high epitaxy cost) and LNOI, but higher than SOI. For data rate,\nelectro-optic materials like LTOI and LNOI are expected to have higher limit than SOI\n13\nand InP. Regarding the power consumption, the majority power of an optical transceiver\nis consumed by electrical circuits and lasers instead of electro-optical modulators.\nWe have included more comparisons and latest references in the revised manuscript as\nadvised.\nRevisions:(In introduction, Page 2, Line 50-54):\nHowever, the EO modulation speed of silicon based on the plasma dispersion\neffect gradually approaches its limitation. Further improving link capacity by\nincreasing the number of multiplexing channels raises issues of complex\nelectronic/photonic circuit designs, a large number of involved lasers/detectors, high\npower consumptions, strict uniformity controls, etc.\n(In introduction, Page 2, Line 61-66):\nRecently, the lithium-tantalate-on-insulator (LTOI) platform with the advantages\nof low birefringence21, low thermo-optic coefficients, high damage threshold22, and low\ndirect current drift has also attracted extensive attention for exploring high-performance\nEO modulators23 and large-capacity optical transmitters21. LTOI is also expected to\nhave cost advantages over LNOI since lithium tantalate has been mass-produced for 5G\nRF filters.\n(In result, Page 13, Line 267-268):\nTable 1 Summary of key parameters and performance metrics for photonic transmitters\nReference Material Single Channel Flat Top Channel Spacing Total Capacity\n14\nThis work LTOI 240 Gbps Yes 200 GHz 1.92 Tbps\n15"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have done a careful job to address my comments. Thank you for performing the additional experiments. I would\nlike to point out that the additional experiment with the LCA (up to 67GHz) does not match the highest frequency content of\nthe symbol rate (120GBaud) and is not sufficient to dismiss the crosstalk concerns. However, I understand that the authors\nmost likely do not have the capability to experimentally test all channels on at once, and will not pursue this point further.\nThe other comments have been addressed nicely. Thank you for clarifying how the insertion loss was measured, including\nthe ripple amplitude.", "reviewer_2": "The authors have performed an ex ellent work to address reviewers concerns. Acceptance can be recommended."}, "author_response": "We sincerely thank Reviewer #1 for constructive suggestions and the\npositive assessment of our work.\nWe sincerely thank Reviewer #2 for constructive suggestions and the\npositive assessment of our work."}]

Decision letter