Design And Optimization of Integrated Mach_ Zehnder Modulators In X-Cut Lithium Niobate Thin Film


 The Mach-Zehnder electro-optic modulator (MZM) was designed and fabricated using the AFM COMSOL program and the pulse laser deposition (PLD) technique. The PLD technique was used to deposit silicon, silicon oxide and LN thin film. Previously estimated optical properties of the prepared films were employed in the theoretical design of the modulator. Optimum device parameters were obtained, which delivered a considerable figure of merit (Vπ.L) values within the range of 2.12 to 2.25 at a device length of 2 cm.


Introduction
Throughout the past three decades, as the demand for telecommunication facilities increased and the subsequent need for large bandwidths grew accordingly, the need to compensate external modulation in the transmission of ber-optic arrangements has intensi ed [1][2][3]. Within optical transmission systems, an optical modulator is a key component and building block that serves as an information encoding device starting in the electrical to optical domains. Hence, a relatively high-speed optical modulator is considered one of the key features of a successful optical transmission system [3]. Optical modulation depends primarily on free carrier dispersion effects. However, free carrier dispersions in the silicon optical modulation are fundamentally absorptive and nonlinear, thereby resulting in the degradation of the amplitude of optical modulation that might lead to signal distortion. In this work, a number of attempts are proposed to realise a high-performance optical modulator by using a variety of material/nanomaterial platforms to address any issues [4][5][6][7][8][9].
Among the utilised materials, lithium niobate (LiNbO 3 ) is the most preferred material because of its outstanding electro-optic modulation (EOM) features. A lithium niobate (LN) modulator obtains exceptional results for high-data-rate multilevel signal generation. A conventional LN modulator is typically designed using a low-indexed-contrast waveguide (WG) alongside weak optical con nement, whereby the utilised electrodes should be positioned at a relatively far distance from the optical WG. This approach is mainly employed to minimise the absorption loss and thereby results in an augmented drive voltage. Thus, the conventional LN modulator performs low productivity modulation and has a bulky size [10,11].
Silicon photonics are emerging as a competitive platform on silicon-on-insulator (SOI) technology, showing great promise for next-generation photonic integrated circuits that has the advantages of optical interconnects [12]. Silicon photonics are becoming a valuable approach for future optical interconnection development mainly due to its high bandwidth, high-density I/O capability, feasibility of dense integration and correlated compatibility with the processing of complementary metal-oxide-semiconductor CMOS.
Its high bandwidth is mainly attributed to compacted WG dimensions. Si integrated photonics deliver large bandwidth and subsequently low power within intra-chip and on-chip optical interconnections [13].
For nearly a decade, LN has been the most preferred material for optical modulator design because of its outstanding large electro-optic and nonlinear optical coe cients and EOM features [14]. The LN Loading [MathJax]/jax/output/CommonHTML/fonts/TeX/fontdata.js modulator shows good performance in high-bandwidth multilevel signal generation, low loss and low drive voltage [15]. Lately, the use of LN membrane on insulators has shown a potential pathway to produce a WG device that possesses good con nements. Furthermore, the hybrid design of silicon/LN (Si/LN) materials combines Si scalability alongside outstanding LN modulation e ciency. The proposed system relies on a super-mode WG structure that consists of Si and non-patterned LN membrane. This con guration helps optical mode distribution. However, it also decreases the e ciency of the modulation, which results in low electro-optic bandwidth and/or relatively high operating voltage [16,17]. Furthermore, the EOM in this geometry is considered a phase modulator. However, the stated phase modulator is known to be a component in an interferometric geometry of the so-called Mach-Zehnder modulator (MZM), in which the phase modulation in every single arm of the geometry is utilised to provide the required interference, thereby e ciently modulating the optical intensity [18,19].
The main design concept is the utilisation of relatively high-index Si (WG), wavelength (λ) of 1550 nmand refractive index at around 3.48. This condition can deliver an optical mode alongside close horizontal con nement, which results in a constant guided mode during the actual electro-optic modulation performance through the LN thin lm, thereby possibly providing the Si with a con nement modal. In contrast to the effect of carrier dispersion in the Si, the Pockels effect hypothetically tolerates an extreme broadband device alongside a maintained low optical propagation loss [20]. The voltage-length product (Vπ.L) is a well-known gure of merit that can be employed to elaborate the inverse correlation between the voltage needed to modulate the optical intensity as an output of MZM from its maximum to minimum values through the application of π phase shift (de ned as Vπ) and the modulation region physical length L) [21]. Another important gure of merit is 3-dBe, which was also illustrated. The simulated MZM was introduced rstly to validate its higher performance and to investigate new possibilities of the fabricated MZM device. The dimensions that were considered rstly in the design are within the millimetre scale mainly because of the wire-cut dimensions used in the experimental work [22]. This work presents a geometrical simulation of a hybrid Si/LN MZM on the basis of the experimental results. The performance/e ciency, optimum dimension, con nement factor and the effective mode area to satisfy the single mode conditions are demonstrated.

Optical constant estimation
The optical properties of the deposited lms are investigated in the ultraviolet to near-IR wavelength to nd the optical re ection of the deposited lms by using (double-beam UV-vis) spectrophotometer of (Shimadzu UV-vis 1800, Japan) type. The refractive index (n) of the deposited materials was calculated using a developed equation [23,24]

theoretical wave guide design
The MZM modulator design was obtained by employing the estimated optical properties from the experimental part. AFM COMSOL Multiphasic was used to simulate the optical mode and mesh the geometry of the WG. This algorithm can adapt to the arbitrary WG structure, obtain the effective mode indices and the effective mode area distribution in the WG, solve the bending loss and the mode distribution, and enable low Vπ.L and low metal absorption loss [15,25]. Figure 1 shows a schematic of the investigated device.

Results And Discussion
The UV-Vis analysis of the deposited lms on the LN-substrate is presented in Fig. 2(a) within the range of 200 nm to 800 nm; this analysis was accomplished in accordance with the optical re ectance results. The re ectance spectrum of the deposited lm tends to appear at around 320 nm. A pronounced increment in the re ectance spectrum was observed with the wavelength increment up to 400 nm. The re ectance spectrum of the deposited lm was observed to be steady alongside the wavelength increment to 800 nm (within the visible region). A similar behaviour was previously reported in the literature [26,27].
Concurrently, the re ectance spectrum of the deposited lm on the Si-substrate with a wavelength range from 200 nm to 800 nm is outlined in Fig. 2(b). The optical re ectance was noted at around 270 nm in the ultraviolet region, after which a strong increment in the corresponding spectra was perceived after the stated wavelength up to 400 nm. Moreover, within the visible region (400 nm-800 nm), the optical re ectance exhibited a steady behaviour. The results are in agreement with previously reported ndings [27][28][29][30]. However, Fig. 2(c) illustrates a comparison between the Si substrate and LN substrate, showing that the deposited lm on the LN substrate exhibited a higher re ectance tendency than that deposited on Si substrate (blue circled region) with respect to the wavelength increment. This nding clearly indicates the occurrence of a bathochromic shift in the re ectance spectrum of the deposited lm on the LN substrate. This particular observation was noticed within the ultraviolet region. In the visible light region, the deposited lm on the LN substrate also exhibited higher re ectance intensity than the one deposited on the Si substrate; this condition is referred to as a hyperchromic shift. An almost similar behaviour was observed in the mentioned wavelength region (red circled region).
The obtained experimental results were employed in the theoretical simulation were optimum modulator dimensions were decided. Figure 3 (a and b), represents the effective index obtained at SiO2 thickness of 0.5 mm. The increment started to appear at WG thickness of 0.041 and 0.039 (mm) for "without and with LN layer addition", respectively. Constantly, the effective mode area showed an increment pro le with the increment of WG thickness. The con nements of the simulated MZM and MZM-LN are elaborated in the inset of Fig. 3 (a and b). Loading [MathJax]/jax/output/CommonHTML/fonts/TeX/fontdata.js From the experimental results used in the theoretical simulation, the optimum modulator dimensions were determined. Figures 3(a and b) present the effective index obtained at SiO 2 thickness of 0.5 mm.
The increment started to appear at WG thickness of 0.041 and 0.039 (mm) without and with LN layer addition, respectively. The effective mode area showed an increment pro le with the increment of WG thickness. The con nements of the simulated MZM and MZM-LN are elaborated in the inset of Figs. 3(a  and b).
With the limitation of this study concerning the available laboratory equipment, the thickness of the WG was high and therefore cannot be applied in the experiment because of the required high number of laser pulses (10,000 pulses and above), possible crack occurrence and mismatching.
The second design was proposed considering micrometre dimensions and was accomplished by multiplying by a factor of 10 4 . The micrometre MZM based on Si WG design with a variety of thicknesses and widths is demonstrated. The cross section of the SiO 2 layer measured 100 µm, with thicknesses of 0.4, 0.45 and 0.5 µm. The LN thin lm was also added with a thickness of 100 nm. The size of the utilised Si WG was in the range of 0.7 µm to 3 µm, as shown in Figs. 4(a, b). This gure effectively demonstrates the index and effective mode area alongside the increment in the Si WG width and thickness at 0.5 µm SiO 2 layer thickness for the simulated MZM and MZM-LN, respectively. The effective index tended to increase as the thickness and width of the Si WG increased, as observed in both simulated MZM and MZM-LN. However, upon the addition of the LN layer, the effective index values decreased due to the high refractive index of the deposited LN nanostructure, thereby negatively affecting the effective index values, along with the very small particle size of the LND nano lm [31]. Furthermore, the effective mode area increased in both MZM and MZM-LN devices alongside the Si WG width increment. In contrast, the effective mode area exhibited a reduction in the acquired values as the Si WG thickness increased. In the con nement variation, as shown in the inset of Figs. 4(c and d), high con nement was noticed at a low Si WG width, while a high-order mode and low con nement were observed at a larger width.  Figures 5(a, b, c, d) presents the effective index and the effective mode area of the fabricated and simulated MZM-LN device using LN substrate at SiO 2 (n = 1.44) and LN (n = 2.346) at a thickness of 0.5 µm and 100 nm, respectively. The Si WG width and thickness varied in this investigation, and the refractive indices were 3.48 and 3.6111 for the simulated and fabricated MZM-LN, respectively. As demonstrated in Fig. 5(a), the effective index demonstrated higher values than those obtained using the bulk refractive index (Fig. 5[b]) possibly because the refractive index difference (∆n) between the core and clad is higher in the fabricated MZM-LN than in the simulated MZM-LN. Loading [MathJax]/jax/output/CommonHTML/fonts/TeX/fontdata.js Furthermore, a high-order mode was obtained in the effective index of the fabricated MZM-LN. The effective mode areas of both simulated and fabricated devices are elaborated in Figs. 5(a) and 5(b), respectively. At a low Si WG thickness (0.43 µm) of the fabricated MZM-LN, the TE mode is outside the WG geometry due to a high attenuation coe cient [24]. The high-order mode attained in the fabricated MZM-LN could be due to the newly applied refractive index (3.6111), which is higher than that applied in the simulated MZM-LN (3.48). This nding means that the fabricated MZM-LN has a higher effective mode area and in turn lower con nement as compared with that obtained in the simulated MZM-LN (inset of Fig. 5). Figure 6 illustrates the fabricated and simulated MZM-LN device in terms of effective index (a and b) and effective mode area (c and d), respectively, using the Si substrate. The same parameters applied in Fig. 4 are applied in Fig. 6. However, the refractive index introduced to evaluate both the effective index and effective mode area is n = 6.101. Figures 6(a and b) Figs. 7(a and b). A linear relation exists between the n eff and Vπ.L product (Fig. 6 [a]). At high values of n eff , the addressed product seemed to have similar values for the three introduced refractive indices (bulk, Si and LN). The product demonstrated a range from 2.12 to 2.25 at a device length of 2 cm, which is within the acceptable Vπ.L value that was reported previously [32,33]. The variation of the Vπ.L as a function of the Si WG width is shown in Fig. 7(b), in which the obtained values increased along with the increment of the Si WG width. The obtained value of the Vπ.L was similar in the bulk and Si pro les, while a slightly higher value was noted in the LN pro le; this nding indicates a width of 2.5 µm, which was the applied width throughout the deposition process of the MZM-LN device. The utilisation of Si substrate indicated a better Vπ.L value than that of the LN substrate. In summary, optimum MZM performance can be obtained using Si WG with a width of 2.5 µm and thicknesses of 0.43 and 0.44 µm alongside a SiO 2 layer of 0.5 µm thickness. The development of the Si substrate obtained a better Vπ.L value as compared with the LN substrate with respect to the Si WG width (2.5 µm).

Funding
The research reported in this manuscript has been done at the University of Technology, Baghdad, Iraq. Fully funded of our own nancial support.

Disclosures
I am the undersigned on behalf of the authors declare that this manuscript is original, has not been published before, and is not currently being considered for publication elsewhere.
Our manuscript creates a paradigm for future studies to encourage researchers to focus on this area. All Authors were aware of about the paper content and approved its submission.