With the rapid growth of data traffic in local area networks, wireless mobile communications, and data centers^[1], high speed, low-cost, and highly compact transceivers have attracted considerable interest in recent years. In order to exploit the data transmission capacity of an optical connection, wavelength-division multiplexing (WDM) and parallel single-mode fiber 4-lane (PSM4)^[2] are mainly adopted. In these applications, a multi-lane transmitter optical sub-assembly (TOSA), with integration of lasers, modulators, and WDM filters, can help miniaturize an optical transceiver. Recent integration technologies for multi-lane TOSA, such as thin-film filter assembling^[3], indium phosphide based integration^[4,5], silicon photonic integrated circuits^[6], and planar lightwave circuit based hybrid integrated^[7], have been extensively studied and adopted in practical applications. Among then, silicon photonics integration is a highly promising platform for next-generation optical interconnect devices, due to its small footprint, low power, and complementary metal–oxide–semiconductor compatibility^[8]. Meanwhile, in order to realize a higher bit rate (e.g., 200 Gb/s per wavelength) the simple on-off keying (OOK) signal format may no longer be a viable approach because the expected electro-optic (EO) bandwidths for the modulator and the photo-detector in this case need to be more than 100 GHz^[9], which could be a huge challenge for the device design. Therefore, advanced modulation formats, such as multi-level pulse amplitude modulation (PAM), quadrature amplitude modulation, discrete multi-tone modulation, are considered and have been widely investigated^[10,11]. Considering these aspects, a four-channel silicon photonic transmitter operating at an aggregated data rate of 800 Gb/s is demonstrated^[8], but with a complex co-design and co-optimization together with the driver due to the limited bandwidth of a silicon modulator.

Thin-film lithium niobate (TFLN) platform^[12] has emerged recently as a new photonic integration platform for optical communications^[13], quantum photonics^[14], and microwave photonics^[15], due to its excellent material properties, including linear Pockels effect, wide transparent window, and excellent temperature stability. High-speed EO modulator, a key functional component of an optical transmitter, has been proposed using different kinds of configurations on the TFLN platform, such as Mach-Zehnder interferometer (MZI) modulators^[16], Michelson interferometer modulators^[17], Brag grating modulators^[18], Fabry-Perot modulators^[19], with a possible bandwidth of >100 GHz and insertion loss of <1 dB. Recently, a high-power PSM4 transmitter on the TFLN platform employing an external flip-chip bonded distributed feedback laser is demonstrated^[20], showing an EO bandwidth of 50 GHz with a half-wave voltageV_π of 4.3 V for each channel.

In this paper, an monolithic four-channel coarse WDM (CWDM) transmitter chip on the TFLN platform is demonstrated, which includes an CWDM device using angled multimode interferometer (AMMI) structure^[21] and four MZI modulators^[22]. Such a TFLN based CWDM device exhibits a compact size and a large fabrication tolerance. The MZI modulators here employ a periodic capacitively-loaded travelling (CLTW) electrode design, and exhibit an excellent modulation performance with a half-wave voltage of ~1.7 V and an EO bandwidth of >40 GHz for each channel. 256 Gb/s (4 × 64 Gb/s) OOK signal transmission and 400 Gb/s (4 × 100 Gb/s) PAM-4 signal transmission are experimentally demonstrated.

Fig. 1(a) shows a three-dimensional (3D) view of the proposed four-channel CWDM transmitter on the TFLN platform, which integrates a four-channel CWDM device and four EO modulators (Ch. 1-Ch. 4). External laser lights at a standard CWDM grid (i.e., 1271, 1291, 1311, and 1331 nm) are coupled in the transmitter using four grating couplers (GCs), respectively, whose peak coupling efficiencies are aligned to the wavelengths of the corresponding channels in order to obtain the highest signal light. The four input GCs and a common output GC are designed with periods of 737, 747, 756, 764, and 752 nm and gaps of 516, 500, 476, 458, and 500 nm, respectively. With the help of the integrated CWDM device of the AMMI structure, the laser lights after modulation from the 4 channels are then combined and coupled out through a common GC. Compared to traditional CWDM structures, such as array-waveguide grating, cascaded MZI, or planar concave gratings, the AMMI structure here maintains a higher fabrication tolerance^[21], and avoids bending waveguides, which are difficult to design in the anisotropic TFLN platform^[23]. The TFLN waveguide has a ridge height of 200 nm, (i.e., half of the total LN thickness). As shown inFig. 1(b), the width of the AMMI structureW_AMMI is 17.54μm and the lengthL_AMMI is optimized as 1943μm. The tilted angleθ and channel distance ΔL are set as 0.17 rad and 37μm, respectively, to ensure a filtered wavelength spacing of 20 nm, as well as a low insertion loss (IL) and a small crosstalk. More details on the design of this AMMI based CWDM device can be found in Ref. [21].

For the modulators here, a balanced MZI structure with a push-pull electrode configuration is adopted. 3dB multimode interferometer couplers are used as the light splitter and combiner. In the modulation sections, the TFLN waveguide has a width of 1.5μm, and the CLTW electrode with a periodic T-segment structure is adopted, which has been shown to present a good voltage-bandwidth product for modulation^[22,24]. The width of the unloaded signal electrode is set large enough as 75μm to ensure low radio-frequency (RF) conductor loss, and the periodic T-segment structure can effectively block the electrical current flow perpendicular to the electrode, and keep the distributed capacitance constant. In order to achieve velocity matching between RF and optical waves, the substrate undercut etching technique is introduced, as shown inFig. 1(c). The detailed parameters of the whole modulator structure are given as (r,c,s, t,h,g,hw,hl, tl, tu, hr) = (47, 3, 2, 5, 15, 1.6, 9, 39, 0.9, 1.1, 35)μm, as also shown inFigs. 1(c) and1(d). A high modulation efficiency (i.e., a half-wave-voltage-length product ofV_πL = 1.54 V·cm) can be obtained without introducing any additional optical absorption losses from the electrodes.

The four-channel CWDM transmitter is fabricated on an x-cut lithium niobate-on-insulator wafer from NANOLN, where a 400 nm thick LN film is bonded on a 3μm buried oxide (BOX). The waveguide and GC patterns were defined using an electro-beam lithography system (Raith Voyager), and 200 nm thick lithium niobate was etched using an inductively coupled plasma reactive ion etching (ICP-RIE) process. A SiO₂ over-cladding layer was deposited using plasma enhanced chemical vapor deposition. Then, holes between the T-segment structures were patterned using ultra-violet contact lithography, and the SiO₂ cladding layer, the lithium niobate slab layer, and the BOX layer were etched in turn to expose the silicon substrate. Next, the CLTW electrode made of gold was fabricated using lift-off processes. Finally, the holes were patterned again, and silicon etching using the ICP-RIE technology was performed to remove the silicon substrate beneath the modulator waveguides. An SF₆ based chemistry with no bias power was used for the isotropic silicon dry-etching. The etching selectivity of such a recipe to photoresist and SiO₂ are therefore high.Fig. 2 shows some optical images and scanning electron microscope images of the fabricated transmitter chip.

The CWDM device and GCs was first characterized using a separated testing structure. The measured and simulated spectral responses of the device are shown inFig. 3(a). One can see that the present CWDM device exhibits a 3 dB bandwidth of 12 nm, an IL of <0.9 dB, and an averaged crosstalk of 18.15 dB at the peak wavelengths, which are also well matched to the simulated results. The losses of the four input GCs and the output GC at their central wavelengths are 4.2, 3.4, 3, 2.6, and 3 dB, respectively, as shown inFig. 3(b). The fabricated four-channel CWDM transmitter including four modulators of a modulation section length of 10 mm as shown inFig. 2(a) was then measured. Four-channel laser lights at the corresponding CWDM wavelengths were coupled in the chip through four GCs. The on-chip losses are 3.15, 2.75, 3.44, and 1.52 dB for Ch. 1 to Ch. 4, respectively, which includes the losses of the modulator and the CWDM device.Fig. 4 shows the half-wave voltageV_π measurements for the four modulators using a 100 kHz triangular voltage sweep. The values ofV_π are 1.68, 1.70, 1.66, and 1.70 V for Ch. 1 to Ch. 4, respectively, which indicates a good uniformity for the four modulators. However, the imbalanced insertion losses measured here for the four channels mainly come from slight misalignments of CWDM wavelengths to the laser lights.

For high-speed performances of the modulators, the employed CLTW electrode is expected to give a low microwave loss. The electric-electric (EE)S₁₂ andS₁₁ responses of the electrode were measured using a vector network analyzer (VNA). Before measurements, the VNA, together with the microwave probes, was first calibrated using a ground-signal-ground calibration kit. As shown inFigs. 5(a) and5(b), the measured EE responses for the four modulators show a similar trend with transmissions of <–6.4 dB (i.e., a level corresponding to the 3 dB EO bandwidth for an index and impedance matched travelling wave modulator^[22]), and reflections of <–15 dB within 67 GHz frequency.

In the present chip, electrodes in adjacent channels are placed side by side with a pitch of 350μm, which might induce unwanted microwave crosstalks. The same VNA was used to test the EE crosstalk characteristic of the fabricated four-channel CWDM transmitter. As for Ch.X, the crosstalk response can be measured by connecting the input and output RF ports of the VNA to Ch.Y and Ch.X (X,Y = 1, 2, 3, 4), respectively. The normalized crosstalk from Ch.Y to Ch.X can be calculated by subtracting the EE transmission response of Ch.X shown inFig. 5(a). As shown inFigs. 6(a)–6(d), the measured crosstalks become worse at higher frequencies, and are higher from adjacent channels than those from non-adjacent ones. Generally, the crosstalks are maintained less than –10 dB for frequencies of <40 GHz for all the four channels.

Next, the small-signal EO modulation responsesS₁₂ of the fabricated CWDM transmitter were measured as shown inFig. 7. The driving signal was fed into the modulator using one microwave probe on one side of the CLTW electrode, and the 50 Ω termination is applied on the other side with another probe. The modulated light was detected using a high-speed photodiode (PD, Finisar XPDV3120) with a bandwidth of 70 GHz and a responsivity of 0.25 A/W at O-band. It can be observed that the 3 dB EO bandwidths for all channels are better than 40 GHz. The fluctuations beyond 40 GHz are due to the limited responsivity of the PD and the lack of an optical pre-amplifier before detection, which limits the signal-to-noise in measurements here. A simulated EO response is also included inFig. 7 using the measured electrode parameters inFig. 5, which indicates that the expected 3-dB EO bandwidth should be larger than 67 GHz.

The data transmission performances using the fabricated transmitter were also tested with a setup shown inFig. 8(a). A pseudo-random bit sequence (PRBS) with a length of 2¹⁵–1 were generated using a 256 GS/s arbitrary waveform generator (AWG, Keysight M8199A) with an analog bandwidth of 70 GHz. The PRBS signal was amplified by a linear RF amplifier (SHF 807) to a peak-to-peak voltage of ~1.41 V, and fed to the fabricated device together with a direct-current bias through a bias-T. The modulated light was directly detected by the high-speed PD without any optical pre-amplifier. Finally, the eye diagrams were recorded using a sampling oscilloscope (Agilent 86100D) with a bandwidth of 80 GHz. The eye diagrams for 64 Gb/s OOK signal in the four channels of the CWDM transmitter are shown inFigs. 8(b)–8(e) with good eye openings. By adjusting the bias voltages, the best dynamic extinction ratios (ERs) of 6.1, 7.3, 8.2 and 5.8 dB can be achieved for Ch. 1 to Ch. 4, respectively. Moreover, the eye diagrams for 100 Gb/s (50 GBaud) PAM-4 signal for the four channels are also shown inFigs. 9(a)–9(d). Good eye openings are observed for Ch. 2 and Ch. 3, while for Ch. 1 and Ch. 4 the eye diagrams are worse. This might be due to the limited coupling wavelength bandwidth of the output GC, which exhibits relatively large coupling losses for Ch. 1 and Ch. 4 wavelengths. Using a broadband edge coupler with higher coupling efficiencies^[25] to reduce the optical coupling loss can improve the quality of eye diagrams at 100 Gb/s.

In summary, we have introduced a compact monolithically integrated on-chip four-channel CWDM transmitter on the TFLN platform. The CWDM device is based on the AMMI structure, which enables an ultra-low insertion loss of <0.9 dB, 3 dB bandwidth of 12 nm, and averaged crosstalk of 18.15 dB for the four standard CWDM channels. The EO modulators are included in the chip using a CLTW electrode, which helps achieve an EO modulation bandwidth of >40 GHz, and enables 100 Gb/s PAM-4 signal transmission. The demonstrated on-chip four-channel CWDM transmitter on the TFLN platform will be a viable solution for 400 G and future 800 G-1.6 T optical transceiver applications to achieve high-speed optical interconnects.