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Abstract
An ultra-compact and efficient acousto-optic modulator based on a thin-film lithium niobate-chalcogenide (ChG) hybrid platform was designed and realized. In this approach, π phase-shift Bragg grating has an ultra-short effective interaction length of only ∼ 300 µm and a compact footprint of 200 × 300 µm2. The strong microwave-acoustic coupling and superior photo-elastic property of the ChG allow us to achieve a half-wave voltage of Vπ = 1.08 V (4.07 V) for the π phase-shift Bragg grating (waveguide Bragg grating), corresponding to VπL = 0.03 V·cm (0.09 V·cm). This acousto-optic modulator exhibits a compact size, and low power consumption, and can be used for on-chip optical interconnects and microwave photonics.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Acousto-optic (AO) is a versatile technology that can be used to precisely and quickly control the wavelength, amplitude, and propagation direction of light waves [1–4]. In fact, traditional AO devices based on bulk piezoelectric materials face challenges in terms of size and energy consumption, making them incompatible with rapidly evolving photonic integrated circuits. To meet the requirements of low power consumption and high integration, there has been a growing interest in the research of high-performance on-chip AO devices [5–9]. In recent years, researchers have developed AO modulators based on various material systems such as aluminum nitride [10,11], zinc oxide [12], and gallium arsenide [13,14]. Among these materials, lithium niobate (LN) shows great potential as one of the most attractive materials for AO devices due to its piezoelectric advantages [15,16]. In addition, it possesses exceptional electro-optic, nonlinear, and AO properties, a wide transparency window, and a relatively high refractive index [17,18].
To date, several impressive on-chip AO modulators based on thin-film lithium niobate (TFLN) with different waveguide structures have been reported [19–21], such as TFLN-based Mach-Zehnder interferometers (MZIs), microring resonators, and photonic crystal AO modulators, all of which have shown strong modulation effects [22,23]. However, as an AO medium, the moderate elasto-optic (EO) coefficient of LN (p31 = 0.17) limits the modulation efficiency of the device, while chalcogenide glass (ChG) exhibits a higher refractive index and EO coefficient (As2S3: p11≈p12≈0.3) than LN [24]. Therefore, the TFLN-ChG heterogeneously integrated waveguide platform is expected to demonstrate a more efficient AO modulation scheme. In our previous work, we investigated push-pull MZI and microring AO modulators based on TFLN-ChG hybrid waveguide platforms, and an effective half-wave voltage-length product Vπ·L of 0.02V·cm is experimentally demonstrated [25,26].
In addition to modulation efficiency, the compact size of the on-chip AO modulator is also an important evaluation factor. For representative integrated AO modulators based on MZI or microring resonators, large-sized unit components such as splitters, couplers, and curved waveguides are often used to form planar interference waveguides, resulting in a relatively large device footprint [27,28]. Therefore, the development of AO modulators without curved waveguide structures can reduce the device footprint. For example, photonic crystal nanobeams as collinear and compact waveguide configurations have been tried to improve AO interactions due to the advantages of photon-phonon resonance [29,30]. Waveguide Bragg grating (WBG) and Fabry-Perot (F-P) cavities are also expected to show efficient AO modulation performance [31,32].
In this letter, we propose an AO modulator based on a TFLN-ChG integrated waveguide platform, in which the photon transmission structure is based on two kinds of waveguide grating: waveguide Bragg grating (WBG) and π phase-shifted Bragg grating (π-PSBG) to optimize and obtain high AO modulation efficiency. We employ a carefully designed interdigital transducer (IDT) for efficient microwave-to-acoustic conversion, achieving a Vπ·L as low as 0.03V·cm. Thanks to the utilization of a π-PSBG with a length of just 300 µm, our AO modulator exhibits a compact device structure with a footprint of only 200 × 300 µm2. This indicates that our modulator achieves comparable modulation efficiency to current state-of-the-art on-chip AO modulators while occupying a significantly smaller footprint. By leveraging the TFLN-ChG integrated waveguide platform and the compact device footprint, we open up a new possibility for achieving high-performance AO modulation in a miniaturized footprint.
2. Structures and analysis
Figure 1 depicts the schematic diagrams of the two proposed AO devices, which utilize waveguide gratings implemented on X-cut TFLN wafers (manufactured by NANOLN Ltd.). The waveguide direction corresponds to the Y direction of the crystal, while the aperture direction of the IDT aligns with the Z direction. The hybrid waveguide is composed of an 850 nm Ge25Sb10S65 (one of the ChGs) rectangular waveguide and a 400 nm TFLN slab. The refractive index of amorphous ChG (n = 2.19) is slightly larger than that of the LN crystal (ne = 2.13) at 1550 nm. The waveguide width ranges from 1.4 µm to 3.0 µm, with the latter representing the width of the waveguide tapers at the input and output ends of the waveguide. The structure diagrams of WBG and π-PSBG are shown in Figs. 1(a) and 1(b), respectively. The WBG consists of a photonic waveguide grating with a constant period of Λ and cycle number of N, while the π-PSBG device is made of a central segment for phase shift plus two uniform half-cycle WBG where the length of the phase-shifted region is exactly Λ. An IDT is placed beside the waveguide to excite the surface acoustic wave (SAW) that causes the waveguide to deform. A cross-section of the designed device is shown in Fig. 1(c), where the ChG strip and TFLN are placed on top of a 2 µm SiO2 layer.
Fig. 1. (a) Schematic of the proposed integrated WBG AO modulator; (b) Schematic of the proposed integrated π-PSBG AO modulator; (c) Cross-sectional view of the proposed devices.
To analyze the device performance and optimize its structural parameters, we conduct numerical simulations for the acoustic and optical modes. Figure 2(a) displays the numerical simulation result of the dominant Sxx strain component of the SAW mode at 0.844 GHz on the TFLN-ChG heterogeneous-integration waveguide platform. The acoustic wave propagates along the surface of the TFLN and reaches the ChG waveguide, causing the deformation of the waveguide. Since ChG is amorphous, it exhibits strong absorption and scattering properties during SAW transmission, resulting in almost complete energy dissipation of SAW through the ChG waveguide.
Fig. 2. (a) Numerical simulation results of the dominant Sxx strain components of the SAW modes in the heterogeneous-integration waveguide platform; (b) Electric field of the fundamental TE mode in waveguide of 1.4 µm width; (c) Electric field of the fundamental TE mode in waveguide of 2.0 µm width; (d) Simulated transmission spectrum of WBG with 600 grating cycles; (e) Simulated transmission spectrum of π-PSBG with 600 grating cycles.
The optical modes in the waveguide are depicted in Figs. 2(b) and 2(c). The TE modes embody effective refractive indices of 2.06 and 2.08, corresponding to waveguide widths of 1.4 and 2.0 µm, respectively. According to the calculation formula of WBG,
where λ, neff, and Λ represent the wavelength of the photon in vacuum, the effective refractive index, and the period of the grating, respectively.The grating period is set to 370 nm, so that the resonant wavelength of the simulated transmission spectra of WBG and π-PSBG is between 1.53 and 1.54 µm. In the case of the π-PSBG device, the length of the phase shifter is δL = Λ, yielding only a single transmission peak. The parameters of the WBG are as follows: Λ = 370 nm, the corrugation depth δ = 600 nm, and the core width WWBG = Wπ-PSBG = 1.4 µm. Two devices were proposed with different parameters to achieve an optimized extinction ratio and modulation effect. One has 600 grating cycles with IDT acoustic aperture of 220 µm, while the other device has 800 grating cycles with IDT acoustic aperture of 300 µm. Considering our previous research works [25,26], the number of electrodes for IDT is designed to be 60 pairs, and the difference in the number of IDT electrodes is due to impedance matching resulting in an increase of the interaction length from 120 µm to 220 µm or 300 µm. The distance between the IDT and the photon waveguide is 11.4 µm which is the result of simulation and experimental optimization [25]. Figures 2(d) and (e) show the simulated transmission spectra for WBG and π-PSBG respectively, and their cycle number is 600. It can be seen that in WBG, the transmission spectrum has a transmission valley, while the π-PSBG device, yields only a single sharp transmission peak and the simulated Q-factor is 5200.
3. Fabrication
The fabrication process primarily consists of three key steps [Fig. 3(a)], commencing with a 400 nm X-cut TFLN wafer procured from NANOLN. An 850 nm thick Ge25Sb10S65 membrane is then thermally evaporated onto the TFLN wafer. Subsequently, electron-beam lithography (EBL) is employed to create WBG/π-PSBG structures as a template, utilizing an electron-beam resist (ARP 6200.13). After that, the photonic waveguide pattern is transferred onto the Ge25Sb10S65 film via reactive ion etching. Lastly, IDT is fabricated through a lift-off process, which involves a second EBL and gold deposition. The gold electrodes possess a thickness of 100 nm, on a 10 nm Ti adhesive layer previously deposited. During the spin coating process of ARP 6200.13, the thickness is precisely controlled at 400 nm, with a spinning speed of 4000 rpm. Figure 3(b)-3(d) visually presents scanning electron microscopy (SEM) images showcasing the details of the IDT and waveguide.
Fig. 3. (a) Fabrication process of TFLN-ChG grating AO modulator; (b) SEM images of WBG AO modulator with an IDT and an optical waveguide; (c) SEM image of the π-PSBG; (d) SEM image of the WBG.
4. Device characterization
The fabricated hybrid WBG and π-PSBG are characterized by analyzing the transmission spectrum using a Keysight tunable laser, which is swept at wavelengths around 1550 nm. The experimental measurement setup is depicted in Fig. 4(a). The tunable laser is connected to the input and output of the device under test using two lensed fibers, each having a mode field diameter of 3 µm. A fiber polarization controller is employed to adjust the fundamental TE mode fed into the waveguide. The output light is divided into two beams by a 3 dB beam splitter. One is used for power monitoring and the other is for detection by a photodiode or spectrometer. The measured transmission spectra are presented in Fig. 4(b)-(e). The tested transmission spectrum exhibits a 5 nm blue shift compared to the simulated one. This discrepancy is thought to result from the grating structure, which is not strictly rectangular due to the device's fabrication tolerance. The extinction ratio of the device with 800 grating cycles increases from 20 dB to 25 dB compared to the waveguide with 600 grating cycles. The tunable laser power is set to 0 dBm, while the maximum transmitted power is -12 dBm. This indicates that the fiber-to-fiber insertion loss of the AO modulator is 8 dB after subtracting the 3 dB beam splitter and 1 dB polarization controller loss in the system. The illustrations beside Fig. 4(d) and 4(e) correspond to magnifications of the resonant wavelength region of their transmission spectrum, respectively. As the grating cycles increase, the resonance is enhanced. Therefore, the full width at half maximum of the resonant peak narrows from 0.28 nm to 0.12 nm, and the Q value rises from 5457 to 10914. This trend indicates that the Q value can continue to increase. However, there is a trade-off between the Q factor and extinction ratio for π-PSBG, which requires a reasonable coupling design to obtain the best transmission performance.
Fig. 4. (a) Schematic diagram of the device measurement system; TL, tunable laser; VNA, vector network analyzer; DUT, device under test; PC, polarization controller; PD, photodiode; BS: 3 dB beam splitter; OPM: optical power meter; (b) WBG transmission spectrum with 600 grating cycles; (c) WBG transmission spectrum with 800 grating cycles; (d) π-PSBG transmission spectrum with 600 grating cycles; (e) π-PSBG transmission spectrum with 800 grating cycles; The left and right illustrations are enlarged images of the resonant wavelengths in Figs. (c) and (e), respectively.
Microwave acoustic conversion was evaluated by measuring the microwave reflection coefficient (S11) spectrum. The blue lines in Figs. 5(a) and (b) represent the S11 spectra of IDTs corresponding to 220 µm and 300 µm acoustic apertures (corresponding to 600 and 800 cycles, respectively). It can be seen that the impedance of 60 pairs of IDTs is well-matched at the frequency of 0.844 GHz, and the corresponding acoustic resonance S11 is -38 dB. The input efficiency of microwave energy through IDT reaches 99%. When the acoustic aperture width is increased to 300 µm, the S11 reflection is slightly enhanced, but the input efficiency remains above 98%.
Fig. 5. (a) Acoustic S11 and opto-acoustic S21 spectra of WBG (gray line) and π-PSBG (red line) with 600 grating cycles; (b) Acoustic S11 and opto-acoustic S21 spectra of WBG (gray line) and π-PSBG (red line) with 800 granting cycles.
To characterize the AO modulation effect of the device, the opto-acoustic S21 spectrum is further measured using the experimental setup shown in Fig. 4(a), where driving Port 1 of the VNA is connected to the IDT and detecting Port 2 is connected to the photodiode. The laser wavelength is set at half of the maximum transmission value in the spectra, as shown in Fig. 4. For WBG with 600 (800) grating cycles, the laser wavelength is chosen to be 1526 (1525) nm, and the corresponding optical power is -18 dBm (-17.6 dBm). For the π-PSBG with 600 (800) grating cycles, the laser wavelength is chosen to be 1529 nm (1527.5 nm), and the corresponding optical power is -17.7 dBm (-21.6 dBm). The red and gray curves in Fig. 5 represent the S21 spectra of π-PSBG and WBG AO modulators with 0 dBm microwave and optical input power respectively. It can be seen that for both 600 and 800 grating cycles, the S21 of π-PSBG device is about 3 dB larger than that of the WBG, which is primarily due to the strong resonant Q-factor of π-PSBG. An effective half-wave voltage Vπ can be calculated by using the formula of half-wave voltage in MZI waveguide structure, which can be compared with other studies [21],
where RPD is the sensitivity of the photoreceiver and Irec is half of the direct current maximum optical power. It should be noted that formula (2) can also be applied to evaluation methods for structures other than MZI, and it has been applied in related research [22]. The performances of the AO devices are depicted in Table 1. It can be seen that the 800-cycle π-PSBG AO device exhibits the best modulation efficiency, and its Vπ (Vπ·L) can be as low as 1.08 V (0.03 V·cm) even if its modulation length is slightly larger than that of the others.
Table 1. Summary of four AO devices in this work.
To benchmark the performance of our devices, Table 2 summarizes the latest work on integrated AO modulators based on TFLN. It can be seen that MZI has the potential to achieve ultra-high modulation efficiency, especially for push-pull structures. However, they typically have phase-shifted waveguides longer than 1 mm in length and therefore take up a relatively large footprint. For racetrack resonator modulators, the bending radius is still greater than 100 µm due to the anisotropy of LN, and the suspended phonon cavity also introduces the complexity of the manufacturing process. In contrast, our AO modulators offer the advantages with an ultra-compact footprint (only 300 µm long), no suspension structure and low energy consumption.
Table 2. Summary of AO modulators on TFLN.
5. Conclusions
In the present study, we propose and demonstrate two kinds of integrated AO modulators on TFLN-ChG hybrid waveguide platform, in which the photon transmission structure is based on WBG and π-PSBG. The 800-cycle π-PSBG exhibits efficient microwave-to-acoustic conversion, with Vπ·L as low as 0.03 V·cm at an RF frequency of 0.844 GHz and an ultra-compact footprint of 200 × 300 µm2. This is primarily due to the outstanding EO property of ChG and the high Q value of π-PSBG. Importantly, the AO modulator demonstrated here is simple to fabricate and can be easily integrated with other TFLN components without etching LN. Our device provides a more miniaturized AO modulation candidate for photonic integrated circuits, which can be applied in optical interconnect and microwave photonics.
Funding
National Natural Science Foundation of China (62105377, 62175095, 62205383, 62375291); Science Foundation of Guangzhou City under Grant (2024A04J9898).
Disclosures
The authors declare no conflicts of interests
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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