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Abstract
In this paper, we designed, implemented, and characterized compact Mach-Zehnder interferometer-based electro-optic modulators. The modulator utilizes spiral-shaped optical waveguides on Z-cut lithium niobate and the preeminent electro-optic effect which is applied using top and bottom electrodes. Optical waveguides are made of rib etched lithium niobate waveguides with bottom silicon oxide cladding, while SU8 polymer covers the top and sides of the rib waveguides. The proposed implementation resulted in low optical losses < 1.3 dB/cm. Moreover, we achieved compact modulators that fit 0.286 cm and 2 cm long optical waveguides in 110 µm × 110 µm and 300 µm × 300 µm areas, respectively. For single arm modulation, the modulators achieved a VπL of 7.4 V.cm and 6.4 V.cm and 3-dB bandwidths of 9.3 GHz and 2.05 GHz, respectively. Push-pull modulation is expected to cut these VπL in half. The proposed configuration avoids traveling wave modulation complexities and represents a key development towards miniature and highly integrated photonic circuits.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The photonic integrated circuit (PIC) is a revolutionary technology that can realize miniature photonic structures on microchips with ultra-high capacity and data transmission speeds [1,2]. Moreover, PICs exhibit large integration density and compatibility with existing fabrication techniques, promising mass production with high yield and low cost. Applications for PICs are extensive, including optical communications such as in long-haul networks [2] and data centers [3], optical signal processing such as in quantum computing [4], bio-photonics such as in photonic lab on chip [5], and sensing for automotive, agricultural, and astronomical systems [6–8].
Modulators are one of the most important components in PICs. They can control the amplitude, phase, and polarization of optical modes by causing light modulation and side-bands generation with radiofrequency (RF) fields. Both resonant and non-resonant photonic modulators have previously been implemented on different types of substrates such as silicon [9,10], silicon nitride [11], indium phosphide [12], aluminum nitride [13], gallium arsenide [14], and lithium niobate [15,16]. Photonic modulators can be used for microwave to photonic conversion [17,18], frequency comb generation [19], and arbitrary waveform generation [20].
Lithium niobate (LN) has been used for decades for optical communication systems [21] due to its remarkable electro-optic (EO), photo-elastic, piezoelectric, and nonlinear properties [22]. Moreover, LN has a high refractive index (2.13 at 1550 nm) and a wide spectral range of transparency. Conventional LN is used in its bulk form where waveguides are implemented using titanium in-diffusion [23] or proton exchange [24], resulting in weak confinement and large and bulky devices. Thin-film LN (TFLN), a novel technology that became commercially available through ion slicing or wafer bonding/grinding [25,26], can offer exceptional confinement of optical modes and ultra-low loss waveguides compared to their bulk counterparts.
LN EO modulators employ the Pockels effect representing refractive index perturbation as a function of the applied RF electric field. According to the EO tensor of LN, the highest modulation efficiency is achieved when the applied electric field and the polarization of the optical mode are both parallel to the Z-axis of LN [22,27]. Most commonly, LN EO modulators are implemented on X-cut or Y-cut, simplifying the fabrication process using planar electrodes forming a coplanar transmission line that encompasses the optical waveguide [18,28–31]. The main objective of the co-traveling structure is to achieve phase matching between RF and optical fields to extend the bandwidth beyond 100 GHz. Unfortunately, X and Y-cuts have a refractive index that is anisotropic in plane, which requires optical waveguides to run in straight lines to avoid anisotropy and harness the EO effect. It results in long and narrow modulators that are not ideal for highly integrated PICs with a multitude of photonic components. Such a long optical waveguide cannot be modeled as a lumped component and traveling wave methodology must be employed. This is achieved by matching the group velocity of both the RF and optical fields in addition to terminating the transmission line with the system characteristic impedance (usually 50 Ω) to avoid reflections [18]. On the other hand, Z-cut modulators are isotropic in plane, permitting flexible and compact layouts with spiral-shaped optical waveguides that require top and bottom electrodes. Compared to resonant modulators using rings on Z-cut [16,32,33] or race-tracks on X-cut [30], non-resonant Mach-Zehnder interferometer (MZI) modulators are less sensitive to temperature and fabrication variations and have ultra-wide optical BW. Moreover, the proposed approach is applicable for hybrid EO modulators allowing for more compact footprints. Examples of hybrid modulators can be found in [34–39].
In this paper, we utilize the isotropic in-plane property of Z-cut LN to implement highly compact MZI based EO modulators. The modulator utilizes spiral-shaped optical waveguides while the EO effect is employed using top and bottom electrodes. The proposed implementation allows extremely long optical waveguides that are desired to reduce the voltage required to achieve 180° phase shift (Vπ), to fit in a dense and compact area. For example, a 2 cm long optical waveguide can fit in a 300 µm × 300 µm area resulting in miniature and highly integrated photonic circuits. Optical waveguides are made of rib etched LN waveguides with bottom silicon oxide cladding, while SU8 polymer covers the top and sides of the rib waveguides. Gold (Au) and aluminum (Al) electrodes are used on the bottom and top, respectively. The proposed implementation resulted in low optical losses < 1.3 dB/cm. Moreover, we achieved compact 0.28 cm long modulators with VπL < 7.4 V.cm for single-arm modulation (VπL < 3.7 V.cm is expected for push-pull) and bandwidths > 9 GHz that are limited by the R-C time constant and the phase mismatch between the optical and RF fields.
2. Design methodology
Figure 1 shows a mock-up of the proposed design on the isotropic in-plane Z-cut LN. A spiral-shaped optical waveguide is used to scale down the aerial footprint [40–42]. The spiral follows an Archimedean trajectory defined by
where (r,θ) are the polar coordinates of the optical waveguide, a is the minimum radius, W is the photonic waveguide width, and S is the spacing between waveguides, as shown in Fig. 1. To draw the complete optical waveguide, we use two Archimedean spirals connected to each other with an s-curve located in the center. One spiral has n full revolutions, and the other has n+0.5 revolutions. The largest spiral radius corresponds to the electrode radius (rE) and the total length of the spiral (LT), including the s-curve, are formulated in (2) and (3), respectively where the s-curve is assumed to have a circular trajectory for simplicity. The minimum radius, a, needs to be optimized to balance the tradeoff between radiation losses due to curvature and total spiral length for a fixed area footprint. The s-curve connecting the two spirals was designed using Bezier curves [43–45] to minimize its losses below 0.5 dB. Lower total loss can be achieved by increasing the minimum radius at the expense of area and capacitance.
Fig. 1. Mock-up of the proposed compact Archimedean spiral EO modulator. X and Y are LN crystal axes. no is the ordinary refractive index of LN.
The initial wafer was prepared by NanoLN with bottom Au electrodes. The initial stack is made of a silicon handle layer, a total of 140 nm metallization layer made of Au with chromium (Cr) on top and bottom of Au as adhesion layers, 1 µm silicon dioxide (SiO2), and 800 nm optical grade Z-cut LN. Figure 2(a) shows the waveguide cross-section after patterning LN and adding the top cladding and electrodes. SU8 is utilized as the top cladding because it can be spin-coated on top of etched LN to produce a highly uniform surface with a relatively large thickness that can be controlled by the spin coating speed. Moreover, it is optically transparent and has a refractive index nr ≈ 1.57 at 1550 nm and RF dielectric constant around εr ≈ 3 [46–49], both of which are close to SiO2 properties (nr = 1.44 at 1550 nm εr ≈ 3.7). Unlike SU8 that has remarkable insulation properties [50], SiO2 deposited using conventional methods, such as plasma-enhanced chemical vapor deposition (PECVD), has insulation properties that are far from ideal, resulting in diminished performance across frequency. The simulated transverse-magnetic (TM) mode in the proposed waveguide structure is shown in Fig. 2(b), exhibiting a mode index nTM = 1.93 and group index ng-TM = 2.3.
Fig. 2. (a) Cross-section of the implemented modulator on Z-cut LN depicting core dimensions. no and ne are the ordinary and extra-ordinary refractive indices of LN, respectively. (b) Simulated TM mode of the optical waveguide showing the mode shape of the electrical field in Z-direction.
The major disadvantage of the proposed structure in Figs. 1 and 2 is the RF bandwidth (BW), which is limited electrically by the R-C time constant [27] and the phase mismatch between the optical and RF fields. The R-C 3-dB BW can be calculated from f3-dB = 1/(2πRC). The R-C time constant is a result of the resistance of the driving source (R) and the capacitance of the modulator (C) between the top and bottom electrodes. R is usually 50 Ω for RF systems which is ideal for coaxial cables. R can be changed by custom designs of the driving stage, allowing for low output resistance buffers and improved BW. Moreover, C can be reduced to increase the BW by reducing the electrode area shown in Fig. 1, but this will limit the length of the optical waveguide and increase Vπ. Since the electrodes are considered lumped, the optical field acquires different phase shifts as it travels through the spiral waveguide resulting in another BW limitation. The phase mismatch limited 3-dB BW can be calculated from f3-dB = μ/τ, where τ is the traveling time through the spiral waveguide and μ is a constant (μ = 0.3). Other BW limitations arise from the loss tangent of different materials in the stack.
Because we have full control over SU8 top cladding thickness, we can investigate the tradeoffs that result from varying that thickness. Figure 3(a) shows the tradeoff between the modulation efficiency (VπL) and the optical loss. The smaller thickness will result in improved efficiency but higher optical loss. A similar tradeoff is shown in Fig. 3(b) between modulation efficiency and capacitance per unit area, limiting the electrical BW. Due to the compactness of the proposed design, the modulation efficiency figure of merit VπL can be redefined as VπLeff where Leff is the effective length representing the longest length of the modulator’s footprint. Leff can be 15-50 times less than L.
Fig. 3. (a) Simulated modulation efficiency and optical loss vs. the top SU8 cladding thickness. (b) Simulated modulation efficiency and capacitance per unit area vs. the top SU8 cladding thickness. The green points show the selected operation point. Optical loss considered in the simulation is due to top and bottom electrodes.
3. Experimental validation
3.1 Fabrication process
The detailed fabrication procedure is shown in Fig. 4. The process starts with the initial stack that is made of Z-cut single-crystal TFLN (800 nm thick) bonded to a silicon carrier (500 µm) with an intermediate layer of SiO2 (1 µm) and Au/Cr electrodes (140 nm). First, Cr was sputtered, and a photoresist layer (PR) was patterned using electron-beam lithography (EBL) to define PICs components. Both Cr and LN were dry-etched using chlorine-based (Cl2 + O2) and fluorine-based (CHF3 + Ar) plasma, respectively [51], resulting in optical waveguides with ∼70° sidewalls. Next, a photoresist layer is patterned using optical lithography to define openings for the bottom RF ground plane by etching LN and SiO2. After that, SU8 was spin-coated and patterned on top of all photonic components, except the grating coupler (GC) used for fiber-to-chip vertical coupling. SU8 was hard baked after patterning to become a permanent part of the final device. Finally, a PR layer was patterned to lift off 200 nm of sputtered aluminum (Al) which results in lower optical losses near 1550 nm compared to other metals. Note that Al was not available as a bottom electrode.
Figure 5 shows microscope and scanning electron microscope (SEM) images of the fabricated device at different stages of the fabrication procedure. The fabricated waveguide width W = 1 µm and the minimum radius a = 20 µm. Both the number of revolutions n and spacing S were varied in different devices as summarized in Table 1.
Fig. 5. (a) SEM cross-sectional image of the spiral modulator showing two optical waveguides with uniform SU8 top cladding and Al top electrode. (b) Top-view SEM of the spiral optical waveguide before adding the top cladding and electrode. (c) Microscope image of the MZI before adding the top cladding and electrode. (d) Microscope image of the spiral optical waveguide after SU8 patterning and Al deposition.
Table 1. Measurements Summary of seven different MZI Modulators.
3.2 Measurement approach
An unbalanced MZI with a mismatch in the length of ΔL = 200 µm between the two MZI arms was optically measured using the setup shown in Fig. 6(a). The output optical power vs. wavelength is shown in Fig. 6(c), exhibiting > 20 dB extinction ratio and free spectral range (FSR) of 5.06 nm. Knowing that FSR = λ2/(ng-TM ΔL), the group index can be calculated ng-TM = 2.37, confirming the simulation result shown in Fig. 2(b). Waveguide loss was measured to be < 1.3 dB/cm using a ring resonator structure with a 40 µm radius that included both top and bottom electrodes. The measured loss was noticeably higher than the simulated value of 0.21 dB/cm. Scattering from sidewall roughness is not included in the simulation and is believed to be responsible for the extra loss. The simulated loss consists of 0.01 dB/cm of bending loss for the 40 µm radius ring plus 0.20 dB/cm of absorptive loss from the electrodes. Both Al and Au electrodes gave almost this same low optical loss in the simulation. In the experiment, we used Al top electrodes because Au requires a Cr adhesion layer, which simulation predicts will increase the optical loss to 1 dB/cm at 1550 nm. The simulated bending loss increases dramatically at 10 µm radius, which is why we use Bezier curves for the s-curve that results in ∼ 0.5 dB loss. Grating couplers used for fiber-to-chip coupling were designed to have -3.5 dB insertion loss at 1550 nm. Due to fabrication and measurements nonidealities, their measured insertion loss was < -10 dB. An erbium-doped fiber amplifier (EDFA) is used to boost the input laser to compensate for optical losses. A fiber polarization controller (FPC) is used to adjust the input light to the TM-mode.
Fig. 6. (a) Setup used to measure the optical response of the MZI. EDFA, erbium-doped fiber amplifier. FPC, fiber polarization controller. DUT, device under test. DAQ, data acquisition card. (b) Setup used to measure the RF response of the MZI intensity modulator. (c) Optical power of an unbalanced MZI vs. wavelength.
The RF response of the MZI intensity modulator was measured using the setup shown in Fig. 6(b). A two-port vector network analyzer (VNA) with a minimum frequency of 10 MHz was used with Port-1 as a voltage source to apply the RF electric fields, while Port-2 measures optical S21. A photoreceiver with an internal trans-impedance amplifier and a responsivity of 1625 V/W is used.
Usually, Vπ is measured by applying a linearly increasing DC voltage on the electrodes. Unfortunately, the DC response of LN is deficient and unstable mainly due to surface charge accumulation, among other effects [27,52]. Achieving an electro-optic DC response of LN is a challenge, especially for Z-cut modulators where charge redistribution is much more severe compared to state-of-the-art X-cut modulators. While the DC response for X-cut exists but drifts with time, the DC response for our proposed stack was below the measurement floor of our instruments. This problem can be mitigated using the thermo-optic effect, which is stable at DC/low frequencies [39]. To avoid this complication, Vπ was measured at 1 MHz by replacing the VNA in Fig. 6(b) with a function generator connected to Port-1 and a spectrum analyzer connected to Port-2.
3.3 Results
RF measurements were done for seven different MZI modulators with different numbers of revolutions and spacing, consequently, different top electrode diameters. Three of them are shown in Fig. 7. Figures 7(a), (b), and (c) show measurements done by applying a sinusoidal waveform with a swept peak-to-peak voltage (Vpp) at 1 MHz to Port-1 while the RF power is measured at the fundamental (1 MHz) and 3rd harmonic (3 MHz), respectively. Figures 7(d), (e), and (f) show the optical S21 measured directly using the VNA.
Fig. 7. (a), (b), and (c) RF power at 1 MHz and 3 MHz and the fundamental fit for devices #1, #4, and #6, respectively. (d), (e), and (f) Optical S21 for devices #1, #4, and #6, respectively. Parameters for these devices are given in Table 1. The 3-dB point of the optical power is the 6-dB point of the RF power measured by the VNA because it measures the square of the optical power.
The MZI modulator is single-arm modulated where one arm is phase modulated by the applied electric fields while the other arm is not modulated, resulting in an intensity-modulated light signal. Phase shifts due to refractive index perturbation caused by the applied RF electric field (Δφn) and the initial phase mismatch between the MZI arms representing any imbalances (ΔφL) can be expressed as
By fitting the measured RF power at the fundamental using P1RF from (7) and (4), Vπ can be estimated. A summary of measured Vπ, VπL, electrical BW, and different device parameters for all the measured devices is found in Table 1.
4. Conclusions
Efficient, compact, and wideband MZI intensity modulators were demonstrated in this paper. The modulators utilize the strong electro-optic effect on Z-cut lithium niobate using top and bottom electrodes. Rib waveguides were fabricated with 800 nm center thickness and 200 nm rib thickness with 70° sidewalls and improved optical loss of < 1.3 dB/cm. Moreover, the waveguides have SiO2 bottom cladding and SU8 top and sides cladding allowing the top surface to be uniform for the top Al electrode. Ultra-compact intensity modulators with VπL = 7.4 V.cm were achieved for single-arm modulation, corresponding to an expected VπL < 3.7 V.cm for the push-pull configuration. The electrical bandwidth of 9.3 GHz was limited by the R-C time constant and the phase mismatch between the optical and RF fields. The proposed spiral-shaped modulator allows for a 2 cm long optical waveguide to fit in a 300 µm × 300 µm area, albeit at a 2.05 GHz bandwidth. Nevertheless, this spiral modulator architecture can be designed to meet the different application needs of miniature and highly integrated photonic circuits.
Funding
National Aeronautics and Space Administration (80NSSC17K052).
Disclosures
The authors declare no conflicts of interest.
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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