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 applications1,2,3. 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 systems4,5,6,7,8. 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 regime9,10,11,12,13,14,15. 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 solved 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.

Fig. 1: An 8-channel LTOI DWDM transmitter.
Fig. 1: An 8-channel LTOI DWDM transmitter.The alternative text for this image may have been generated using AI.
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a Schematic illustration of an 8-channel DWDM transmitter, b the cross-section of the LTOI photonic waveguide.

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).

Fig. 2: Design of the flat-top optical filter.
Fig. 2: Design of the flat-top optical filter.The alternative text for this image may have been generated using AI.
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a The present schematic diagram of the flat-top optical filter; b simulated optical transmission spectrum of the coupled FP cavity with structural parameters of δ = 850 nm, Λ = 350 nm, duty cycle η = 0.5, w = 1850 nm, b = 10, N1 = N3 = 80, and N2 = 160; c simulated optical transmission spectra of the coupled FP cavity with N2 = 180 and N1 = N3 = 70, 75, 80, 85, and 90; d simulated optical transmission spectra of the coupled FP cavity with a fixed period number of N1 = N3 = 80 and N2 = 170, 175, 180, 185, and 190; e simulated optical transmission spectra of the coupled FP cavity by assuming variations of etch depth Δδ = 0, ±10, and ±20 nm; f simulated optical transmission spectra of the coupled FP cavity by assuming variations of core width Δw = 0 and ±5 nm; g–i simulated light propagation profiles in the coupled FP cavity at the non-resonant wavelength λ = 1330 nm (g), the resonant wavelength λ = 1303 nm (h), and the reflected non-resonant wavelength λ = 1310 nm (i).

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 makes 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.

Fig. 3: Comparison between filters with single and coupled cavities.
Fig. 3: Comparison between filters with single and coupled cavities.The alternative text for this image may have been generated using AI.
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a Optical transmission of filters with single (blue, 3-dB bandwidth = 40 GHz) and coupled (red, 3-dB bandwidth = 120 GHz) cavities. b Frequency spectra of 40, 80, 120, and 150 GBaud OOK signals. c Simulated eye diagrams of 40, 80, 120, and 150 GBaud OOK signals after transmitting through a single-cavity filter. d Simulated eye diagrams of 40, 80, 120, and 150 GBaud OOK signals after transmitting through a flat-top optical filter.

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 super luminescent 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.

Fig. 4: Measured results of the fabricated flat-top optical filter.
Fig. 4: Measured results of the fabricated flat-top optical filter.The alternative text for this image may have been generated using AI.
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a–c images of the flat-top optical filter (a), AMWG (b), and the mode (de)multiplexer (c); d measured results of the fabricated flat-top optical filter; e zoomed-in view of optical transmission spectrum; f measured results of the flat-top optical filter with corrugation depths of 0.90, 0.85, and 0.80 μm; g measured optical transmission spectra of five flat-top optical filters with identical design; h measured optical transmission spectra for the fabricated flat-top optical filter at different temperatures; i measured (dots) and fitted (line) wavelength shifts as a function of temperature.

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.

Fig. 5: Simulated and measured results of the 8-channel DWDM.
Fig. 5: Simulated and measured results of the 8-channel DWDM.The alternative text for this image may have been generated using AI.
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a Schematic illustration of the 8-channel DWDM by cascading 8 flat-top optical filters; b simulated optical transmission spectra of coupled FP cavities with cavity lengths of Lt = 357, 397, 437, 477, 517, 557, 597, and 637 nm, respectively; c optical microscopic image of the fabricated DWDM; d measured optical transmission spectra of the fabricated DWDM.

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.

Fig. 6: Characterization of the 8-channel DWDM transmitter.
Fig. 6: Characterization of the 8-channel DWDM transmitter.The alternative text for this image may have been generated using AI.
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a Optical microscopic image of the 8-channel DWDM transmitter. b Experimental setup for obtaining eye diagrams. c Measured 120 GBaud OOK and PAM4 eye diagrams. Pr-doped fiber amplifier (PDFA), electrical sampling oscilloscope (ESO), arbitrary waveform generator (AWG), Radio Frequency (RF), Multimode Interference (MMI).

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.

Fig. 7: Measured eye diagrams under different transmission distances and output powers.
Fig. 7: Measured eye diagrams under different transmission distances and output powers.The alternative text for this image may have been generated using AI.
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a Measured eye diagrams of 120 GBaud OOK and PAM4 signals for a single channel after transmission through fiber links of varying lengths; b measured eye diagrams of 120 GBaud OOK and PAM4 signals for a single channel under different optical power conditions.

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.

Table 1 Summary of key parameters and performance metrics for photonic transmitters
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Discussion

In summary, this work demonstrated an 8-channel DWDM transmitter implemented on an LTOI platform with a total capacity of 1.92 Tbps. A key innovation is the introduction of a flat-top optical filter based on a novel coupled FP cavity, overcoming existing challenges of DWDM transmitters, including FSR, ILs, and spectral efficiency. The proposed 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, and high fabrication tolerance. By cascading 8 flat-top optical filters, we successfully demonstrated a fabrication-tolerant 8-channel DWDM with uniform channel spacings of 200 GHz, 3-dB bandwidths of 0.8 nm, IL of 0.15–0.98 dB, and crosstalk levels <−22 dB. Moreover, the channel spacing of DWDM can be further reduced simply by changing the period number N of each AMWG to change the quality factors of the FP cavities. The demonstrated DWDM transmitter achieves 1.92 Tbps capacity in total by carrying 120 GBaud PAM4 signals for all 8 channels. Therefore, the proposed coupled FP cavity-based flat-top optical filter offers a breakthrough solution to critical challenges in DWDM parallel data transmission and processing. It not only paves the way for overcoming link capacity constraints but also holds significant potential for applications in artificial intelligence and data center technologies.

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.