1. Introduction

The global data traffic is witnessing booming growth and has driven modern data centers into the “Zettabyte Era” [1]. Because of this rapid expansion, transmitters across all levels of optical networks are now required to increase the data rates drastically while maintaining a reduced power consumption as well as remaining cost effective. An electro-optic modulator is a critical building block of optical transmitters [2]. Recently, thin-film lithium niobate (TFLN) has emerged as a promising platform for fabricating high-performance electro-optic modulators [3–5]. Compared with the legacy lithium niobate modulators, the TFLN modulators offer noticeable improvements in voltage-bandwidth performance, while retaining the key material advantages, such as a linear response, high extinction ratio, and low optical loss [6].

However, lithium niobate materials cannot generate light efficiently, which is a major drawback for their application in optical transmitters. To date, numerous TFLN lasers have been developed to address this issue. For example, optically pumped TFLN microcavity lasers can be achieved by doping TFLN with erbium ions [7,8]. More importantly, electrically pumped TFLN lasers have also been designed with heterogeneous [9] or hybrid [10], integrating III-V semiconductor gain materials with TFLN photonic integrated circuits (PICs), structures. In particular, hybrid integrated III-V/Si external cavity lasers have been demonstrated with single-mode and tunable lasing wavelengths [11–13], indicating the feasibility of developing tunable optical transmitters on TFLN platforms.

In this study, we demonstrated the first widely tunable optical transmitter realized by the butt-coupling a TFLN intensity modulator with an electrically pumped TFLN/III-V tunable laser. The laser exhibited a side-mode suppression ratio of >60 dB, a linewidth of 475 kHz, and a wavelength-tuning range of over 40 nm. The TFLN modulator could process a voltage-length product of 2.9 V${\cdot}$cm and an electro-optic response of 1.5 dB roll-off at 50 GHz. Further, data transmission of 128 Gb/s OOK and 160 Gb/s PAM-4 signals without digital signal processing was successfully demonstrated. The tunable optical transmitter developed in this study paves the way to high-performance fully integrated TFLN transmitters for next-generation 800 Gb/s intensity modulation-direct detection and > 1.6 Tb/s coherent transmission systems.

2. Design of the TFLN/III-V transmitter and fabrication

Figure 1(a) presents a schematic of the widely tunable TFLN/III-V transmitter and cross sections of different regions. The device consisted of a butt-coupled III-V reflective semiconductor optical amplifier (RSOA) and a TFLN photonic integrated circuit (PIC). The on-chip laser included an RSOA, a spot size converter (SSC), cascaded microring resonators (MRRs), and a broadband distributed Bragg reflector (DBR). The dimension of the TFLN chip and RSOA was 1.2*0.4 cm and 0.2*0.15 cm, respectively. So the dimension of the complete transmitter was approximately 1.4*0.4 cm.The RSOA (Thorlabs, SAF1144C) provided a 3-dB optical gain bandwidth of 37 nm when a current of 200 mA was injected. The front facet of the RSOA was coated with an antireflection layer with a reflectivity of ≤ 0.01%, and its rear facet was covered with a partially reflective layer with a reflectivity of 10%. The III-V waveguide at the coupling facet of the RSOA was tilted by 8° to minimize the end-face reflectance [14–16]. An SSC with a bilayer structure was designed to achieve an efficient light coupling between the RSOA and the TFLN PIC. The simulated mode profiles of the SSC tip and RSOA gain chip are shown in Fig. 1(b) and Fig. 1(c), respectively. A TFLN DBR was used as an on-chip reflector for the laser cavity. The lasing wavelength was selected by leveraging the optical Vernier effect of the two cascading thermos-tunable MRRs with slightly different free spectral ranges (FSRs). The output of the laser was directly coupled to an MZM with a capacitance-loaded traveling-wave electrode featuring a large electro-optic bandwidth [17]. Finally, the modulated optical signal was coupled to a lensed fiber through an SSC.

 

Fig. 1. (a) Schematic of the widely tunable TFLN/III-V transmitter. The simulated mode profile of the SSC tip (b) and RSOA gain chip (c).

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To achieve an efficient optical coupling between the TFLN PIC and the output of the RSOA (an elliptic beam with a diameter of 4.07 µm × 1.91 µm), we designed a trident-waveguide SSC (Fig. 2(a)). The widths of the SSC at the facet were w₁, w₀, and w₁, separated by a distance of gap. To maximize the coupling efficiency, we first calculated the mode overlap with various w₀ and gap values with a fixed w₁ at an initial value of 0.3 µm. Figure 2(b) presents a maximum overlap integral value of 0.88 at w₀ = 0.2 µm and gap = 1.05 µm. Then, we swept w₁ with a fixed w₀ and gap at the optimum values. Finally, the maximum overlap value was found to be 0.91 when w₀ = 0.2 µm, gap = 1.05 µm, and w₁ = 0.22 µm. To match the InP waveguide optical mode and angle at the interface, based on the effective refractive index of the LN SSC, the titled angle is 17.5° according to the Snell’s Law, as shown in Fig. 2(c).

 

Fig. 2. (a) Schematic of the SSC with a tilted angle θ. Simulated power coupling ratio, n_eff, and θ for the SSC with an optimized combination of (b) w₀, gap, and (c) w₁. (d) Simulated transmission spectra of the Vernier filter with a coupling coefficient $\kappa $ = 0.3 (blue curve) and the gain spectrum of the RSOA at 200 mA injection current (red curve). (e) Simulated transmission spectrum of the Vernier filter showing Fabry–Pérot longitudinal modes enveloped by the Vernier modes. SMSR: side-mode suppression ratio. (f) Simulated and measured transmission spectra of a broadband DBR.

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A Vernier filter composed of two cascaded MRRs was employed to achieve a single-mode and tunable lasing wavelength. The circumferences of the two MRRs were designed such that the overall FSR of the Vernier filter exceeded the gain bandwidth of the RSOA. The gain spectrum of the RSOA was centered at 1325 nm with a 3-dB bandwidth of 37 nm at an injection current of 200 mA (Fig. 2(d)). The circumferences of the two MRRs were designed to be 589.5 and 602.05 µm, respectively, resulting in an overall FSR of 61 nm, which ensured that there is only one lasing wavelength in the gain spectrum. The power-coupling coefficients of the MRRs were designed to be 0.3, resulting in an 11-fold length enhancement at the resonance [18]. The side-mode suppression ratios for the Vernier filter and longitudinal mode were determined to be 3.4 and 6.2 dB, as shown in Fig. 2(e), respectively, which were sufficient for the single-mode lasing. Further, the measured quality factor of the MRR was 81900. The DBR was integrated after the Vernier filter to provide an optical feedback. A Gaussian apodization profile is employed to reduce the sidelobe in the reflection spectrum of the DBR. The expression of the sidewall corrugations width was referred our previous work [19]. Here, the measured DBR with a reflectivity of 85% and a 3-dB bandwidth of 38 nm at 1330 nm as shown in Fig. 2(f).

The LN PICs were processed on a X-cut LNOI wafer with a 360-nm-thick LN device layer and 4.7-µm-thick buried oxide layer. Firstly, the LNOI waveguide was defined by electron beam lithography (EBL) and then the patterns were transferred into the LN layer by inductively coupled plasma (ICP) etching. Afterwards, a SiO2 layer was deposited by plasma enhanced chemical vapor deposition (PECVD). Subsequently, Ni-Cr micro-heaters and Au probe pad were deposited. Then a SiO2 layer was deposited again. Finally, the facet of the LNOI chip was end-polished to achieve an efficient edge coupling with the RSOA chip and lensed fiber.

3. Characterization of the TFLN/III-V tunable laser and modulator

The we evaluated the single-mode lasing performance of the widely tunable laser was first evaluated. A directional coupler and a grating coupler were used to monitor the laser output. The RSOA was mounted on a temperature-controlled XYZ precision alignment stage, and butt couped with the TFLN chip. The best alignment between the two chips was achieved by maximizing the light output power from the grating coupler. The coupling loss between the TFLN SSC and RSOA was measured to be 3.5 dB, using TFLN waveguide with the identical SSC but without DBR as the reference. The measured maximum optical power coupled to the grating coupler was ∼2 mW at 200mA, which can be much further improved by increasing the reflectively of the rear facet of the RSOA chip. The maximum optical power measured after the Mach-Zehnder interferometer was of ∼ 1.34 mW. The laser spectrum was analyzed using an optical spectrum analyzer (Anritsu MS9740A) with a resolution of 0.07 nm. Figure 3(a) presents the emission spectrum of the laser at 1328.3 nm, biased at 200 mA. A single-mode lasing with a side-mode suppression ratio of >60 dB was achieved. The delayed self-heterodyne method [20] was used to measure the linewidth of the hybrid laser with a 10.2-km-long SMF for delaying the optical beam, as schematically shown in Fig. 3(c). The normalized linewidth and the corresponding Voigt fitting data are presented in Fig. 3(b) with a linewidth of 475 kHz in the single-mode lasing state.

 

Fig. 3. (a) A typical output spectrum obtained from the laser at 1328.3 nm with an injection current of 200 mA. (b) Measured linewidth and the corresponding Voigt fitting data for the proposed laser. (c) Experimental set-up for measuring the linewidth of lasers. PC, polarization controller; AOM, acousto-optical modulator; OSA, optical spectrum analyzer; PD, photodiode. (d) 31 channels tuning with ≈ 1.3 nm channel spacing. (e) The corresponding lasing wavelength and heating power on MRR1/MRR2.

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Next, the tunability of the lasers wre characterized. The resonances of the two MRRs were thermally tuned using the fabricated NiCr microheaters [21]. The FSRs of MRR₁ (1.30 nm) and MRR₂ (1.2725 nm) were slightly different. The transmittance reached a maximum value, when the resonances of the two MRRs coincided, which determines the lasing wavelength of the hybrid laser. Figure 3(d) shows an example of the superimposed spectra at various lasing wavelengths. The injection current of the RSOA was set at 200 mA. Because the transmission spectra of the two rings at different wavelengths are not perfectly alignment, so the laser output power varies at different wavelengths. Figure.3(e) shows the lasing wavelength is linearly tuned by the heating the MRR1. The responding wavelength tuning range and the laser tuning efficiency is 40.3 nm and 0.48 nm/mW, respectively.

In Table 1, the performance metrics of the state-of-the-art TFLN/ III-V lasers were summarized.

Table 1. Comparison of several performance metrics of TFLN/ III-V lasers

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We then measured ${V_\pi }$ and the modulation bandwidth of the integrated TFLN MZM. The device had 10-mm long capacitance-loaded travelling-wave electrodes terminated by an on-chip RF terminator [23]. We first employed the 100 kHz triangular voltage sweep method to characterize ${V_\pi }$. As shown in Fig. 4(a), ${V_\pi }$ was measured to be 2.9 V (Fig. 4(a)). Furthermore, we measured the modulation bandwidth using a vector network analyzer (Agilent N5227A) and a 50 GHz photodiode (Finisar, XPDV2320R). The electro-optic response (S₂₁) showed only a 1.5 dB roll-off at 50 GHz (Fig. 4(b)).

 

Fig. 4. Measured (a) ${V_\pi }$ and (b) electro-optic response of the 10-mm long TFLN modulator.

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4. Data transmission performance

We investigated the high-speed data-transmission performance of our transmitter using the setup shown in Fig. 5(a). The RSOA was biased at 200 mA and 20 °C. The heating powers on MRR₁ and MRR₂ were 71.9 and 38.9 mW, respectively. The lasing wavelength was 1322.5 nm, and the MZM was driven by an amplified electrical signal generated from an arbitrary waveform generator (AWG, Keysight M8199A, with 256 GSa/s). The modulated optical signal was then passed through a 1:9 coupler with a 10% arm sent to the power meter to measure the output power, and the other arm was passed through another 1:99 coupler. A 1% arm was connected to the optical spectrum analyzer (OSA) to monitor the spectrum. The output from the 99% arm was amplified using a praselectro-opticdymium-doped fiber amplifier (Thorlabs) and passed through a wide-bandwidth sampling oscilloscope (Agilent Technologies, DCA-X 86100D).

 

Fig. 5. (a) Experimental set-up for measuring the eye diagrams. Eye diagram of the OOK signal at (b) 80 Gb/s and (d) 128 Gb/s, and that of the PAM-4 signal at (c) 64 Gbaud (128 Gb/s), and (e) 80 Gbaud (160 Gb/s).

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Initially, OOK modulations were applied to the device, and the optical eye diagrams at 80 and 128 Gb/s are shown in Fig. 5(b) and 5(d), respectively. The measured dynamic extinction ratios were 6.4 and 3.2 dB, respectively. Furthermore, the optical PAM-4 eye diagrams at 128 and 160 Gb/s were successfully obtained, as shown in Figs. 5(c) and 5(e), The measured dynamic extinction ratios were 4 and 3.2 dB, respectively. For the 80 Gbaud (160 Gb/s) PAM-4 modulation experiment as shown in Fig. 3(e), we can calculate an energy consumption of 29.6 fJ/bit. In particular, we did not use additional digital signal-processing algorithms in the experiment to improve the quality of the modulated signal. The high-speed modulation performance was limited by the electronics used in the current setup.

5. Conclusion

In summary, we demonstrated the first hybrid integrated tunable TFLN/III-V transmitter. The hybrid laser exhibited a greater than 60 dB side-mode suppression ratio along with a linewidth of 475 kHz and wavelength-tuning range of over 40 nm. Moreover, the TFLN MZM could process a ${V_\pi }$ of 2.9 V and an electro-optic response of 1.5 dB roll-off at 50 GHz. The transmitter demonstrated a single-lane data rate of 160 Gb/s PAM-4 modulation. These results provide a new route for the development of high-performance optical transmitters for future high-speed and low-power-consumption optical links. The further hybrid integration of the III-V and TFLN chips can be realize by the flip-chip bonding of RSOA and TFLN chips.