High performance multifunction integrated optic circuits base on thin- film lithium niobate

doi:10.21203/rs.3.rs-4274796/v1

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High performance multifunction integrated optic circuits base on thin- film lithium niobate

https://doi.org/10.21203/rs.3.rs-4274796/v1

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This paper introduces a revolutionary Multifunction Integrated Optic Circuit (MIOC) design utilizing thin-film lithium niobate, surpassing traditional bulk waveguide-based MIOCs in terms of size, half-wave voltage requirements, and integration capabilities. By implementing a sub-wavelength grating structure, we achieve a Polarization Extinction Ratio (PER) exceeding 29 dB. Furthermore, our electrode design facilitates a voltage-length product (VπL) below 2 V·cm, while a double-tapered coupling structure significantly reduces insertion loss. This advancement provides a pivotal direction for the miniaturization and integration of optical gyroscopes, marking a substantial contribution to the field

MIOC

thin-film lithium niobate

sub-wavelength grating

The fiber-optic gyroscope (FOG) is one of the most successful fiber optic angular velocity sensor and has been used across various platforms, such as aerospace, navigation, geological exploration, oil exploration, and earthquake monitoring[1]. The multifunction integrated optic circuit (MIOC) is a crucial optical component in the fiber optic gyroscope (FOG) as it is responsible for performing essential functions such as polarization filtering, light splitting, and phase modulation[2]. A highly efficient MIOC is paramount in achieving a FOG that is highly sensitive, compact, and easily integrated.

The MIOC is generally fabricated with the lithium niobate (LiNbO3) crystal because of advantages of large electro-optic coefficient, reliable physical and chemical characteristics[3]. Typically, the conventional MIOCs are obtained through the process of Annealing Proton Exchange (APE), which results in a naturally high extinction ratio[4]. However, its poor optical confinement ability leads to bulky modulators with low modulation efficiency (Vπ L > 10V cm)[5]. In recent years, the thin film lithium niobate (TFLN) has emerged as a promising platform for excellent and compact electro-optic modulator due to its advantages of low optical loss, ultrafast modulation and high modulation efficiency(Vπ L < 1.5V ∙cm)[6–8].

Hower, the fabrication of TFLN-based optical waveguides technically have the challenge of high polarization extinction ratio[9]. Researchers have explored various solutions, such as sub-wavelength gratings[10, 11], evanescent coupling[12, 13], and multimode interference (MMI)[14, 15], to address this issue. However, the results of these approaches have not met the requirements for applications such as fiber optic gyroscopes. Up to now, it is still a challenge to design MIOC with low optical loss, high modulation efficiency, and high extinction ratio in a small footprint, simultaneously.

Here, we introduce a novel design approach for Multifunction Integrated Optic Circuits (MIOC) utilizing thin-film lithium niobate, which, to our knowledge, is unprecedented. This design achieves a high polarization extinction ratio (PER) through the implementation of a sub-wavelength grating scheme, in conjunction with a carefully designed ridge waveguide that ensures single-mode operation. To minimize optical loss, a double-layer anti-taper coupling structure has been developed. The resulting MIOC demonstrates superior performance characteristics, including a high PER (exceeding 29 dB), a low half-wave voltage (Vπ less than 2V), and a compact form factor (less than 2 cm), distinguishing it from traditional MIOC designs. The insights gained from this research contribute significantly to advancing the development of high-performance MIOC devices.

The proposed MIOC is based on 400-nm X-cut LNOI with the main propagation along the optical axis Y. A schematic of the proposed device is shown in Fig. 1. The proposed device included three double-layer anti-taper couplers, a sub-grating and two phase modulators. Double-layer couplers was use to reduce optical loss. The sub-grating was served to enchance polarization extinction ratio. Two phase modulators are employed to modulate the two arms of light, and both of them share the same ground-signal-ground (GSG) electrodes. The principles and performance of the proposed device are discussed below.

2.1 Single-Mode

The modulation structure is formed by ridge waveguides and cladded by air (Fig. 2(a)). The sidewall angle is set to 65°according to our LNOI etching process. The main structural parameters are: the rib width and the rib height, we evaluate the mode cut-off condition of the ridge waveguide using the Finite Difference Eigenmode (FDE) solver. The result can be observed in Fig. 2(b), When both rib width and parameter rib height are increased, the mode cutoff condition of the ridge waveguide changes from only TE mode (region i) to supporting both the TE and mode (region ii). Figure 2(b) shows that the olnly TE mode condition is more easily met under shallow etching, but entails greater losses. Under comprehensive consideration, Rib width = 0.8µm and Rib height = 0.2µm were selected where the ridge waveguide can only support the TE mode (see region Ⅰ in Fig. 2(b)), while the TM mode is cut off and radiated primarily to the Si substrate.

2.2 Sub-grating

Subwavelength gratings (SWGs), which are composed of periodically arranged dielectric segments with a pitch much smaller than the input light wavelength, have been widely investigated for developing high-performance integrated photonic devices with compact size, low loss, and large bandwidth.

Figure 3a shows the schematic diagram of a proposed SWG based TE-mode-pass filter. From the above analysis, the thickness of SWG structure is chosen to be 200 nm as well, for a strong mode confinement in the lithium niobate slab. For a TE-mode-pass filter, the hybrid SWG waveguide is expected to work in the Bragg regime for TE mode and the subwavelength regime for TM mode. Thus, the input TM mode is reflected whereas the TE mode is converted to the localized Bloch mode and propagates with low loss[16].

$$\Lambda =\frac{\lambda }{{2 \cdot {n_{ef{f_{TM}}}}}}>\frac{\lambda }{{2 \cdot {n_{ef{f_{TE}}}}}}$$

where Λ is the grating pitch, 𝜆 is the central wavelength of photonic stop band for TE mode, n_TE and n_TM are the effective indices of TE and TM modes in the SWG waveguide.

The SWG structure is optically equivalent to a homogeneous medium with a refractive index given by[17]:

$$n_{{SWG}}^{2}=n_{{LN}}^{2} \cdot f+n_{{air}}^{2} \cdot (1 - f)$$

where n_LN and n_air are the refractive indices of lithium niobate and air clad, respectively. The f is the filling factor of the SWG, which is expressed as f = Wt/Λ where Wt is the width of the SWG segment. Here, the f is designed to be 0.8, for an easy-to-fabricate minimum feature size.

Then, the effective refractive indices of the Bloch modes in the SWG waveguide are calculated by using a Finite Difference Eigenmode solver. The parameters in the SWG structure are designed to be w1 = 0.8µm, w2 = 0.3µm, w3 = 2µm, L1 = L2 = 5µm, Λ = 456nm, wt = 364.8nm. The transmission of TE and TM modes as a function of grating period number is simulated at a wavelength of 1550 nm, by using the 3D finite-difference timedomain (3D-FDTD) method, as shown in Fig. 3b. To achieve both low loss and high polarization extinction ratio (PER), the grating period number is chosen to be 20. The simulated insertion loss for TE mode is 0.8 dB and the PER is about 42 dB.

2.3 Double layer inverse cone coupler

The coupler is designed to couple with an PANDA polarization maintaining (PM) fiber with mode field diameter (MFD) of 6.5 µm. A cladding SiO2 layer with 3 µm covers the whole taper area deposited by PECVD (Cladding SiO2 is transparent in Fig. 4). The coupling loss includes the power coupling loss at region Ⅰ(in Fig. 4.a) as well as the mode conversion loss from region Ⅰ to region Ⅳ(in Fig. 4.a). The cross-section view and corresponding mode field distribution of at different region of the coupler are shown in Fig. 4(b).

We use the Finite Difference Eigenmode (FDE) solver to simulate the coupling loss between the PM fiber and the coupler by optimizing the w1 parameter. As shown in Fig. 5a, the simulation result indicates that the coupling loss at region Ⅰis about 0.4dB. The, we use the Eigenmode Expansion (EME) solver to simulate the mode conversion loss between the region Ⅰ and region Ⅳ by optimizing the main parameter L1 as Fig. 5(b) shows. To minimize the coupling loss and the footprint of the coupler, we set the L1 to be 200 µm. Finally the coupling loss of 0.4 dB per facet is obtained for TE light in simulations after optimization. The optimized values are listed in the Table 1.

Table 1

Double layer inverse cone coupler parameter list
Parameter	value	Parameter	value	Parameter	value
L₁	200µm	L₂	50µm	L₃	50µm
w₁	0.19µm	w₂	1.3µm	w₃	1.5µm
w₄	5µm	w₅	0.15µm	w₆	0.8µm

For the fabrication of devices, we utilized 400 nm-thick lithium niobate thin films acquired from NANOLN, with a silica layer thickness of 4.7 µm. The fabrication process for the biconical coupling structures involved intricate steps, including two rounds of electron beam lithography (EBL) and two cycles of inductively coupled plasma reactive-ion etching (ICP-RIE). The ICP-RIE was conducted at powers of 600W for the source and 100W for the bias, using pure argon as the etching gas, and achieving a ridge angle of 65°. For the electrode structures, a magnetron sputtering technique was employed to deposit 50 nm of Ti and 300 nm of Au. The Ti layer served to mitigate absorption losses that could arise from direct contact between the Au metal and the waveguides. The desired metal patterns were then formed using ultraviolet lithography. Finally, the waveguides were cleansed with a solution of H2O2 to H2SO4 in a 3:1 ratio for 10 minutes to remove surface impurities. The device structure is illustrated in the figures provided. Figure (b) depicts the sub-wavelength grating structure, where the deep green image represents the sub-wavelength grating structure as observed under an electron microscope. Figure (a) and figure (c) presents a top view of the double layer inverse cone coupling structure as observed under an electron microscope.

Measurement was conducted using a tunable laser as the light source, operating in the 1550 nm wavelength range. When the waveguide meets the TE single-mode condition, a polarization extinction ratio (PER) of 15 dB was achieved. Incorporating sub-wavelength grating structures further enhances the PER. The test results for the TE PASS sub-wavelength gratings are illustrated in the figures, indicating a PER greater than 29 dB at 1550 nm. This performance surpasses that of polarization controllers based on thin-film lithium niobate devices reported in current literature. This significantly broadens the application of thin-film lithium niobate in Y-junction waveguide chips.

The electrode spacing plays a significant role in relation to the half-wave voltage. With an electrode distance of 14 µm, the half-wave voltage is 6 V, whereas reducing the electrode gap to 7 µm lowers the half-wave voltage to 2 V. The facet structures introduce considerable coupling losses during testing, resulting in an overall device insertion loss greater than 10 dB. The high losses are attributed to two main factors: the complexity of the fabrication process, with a minimum etch linewidth of 190 nm, where the precision of the process significantly affects waveguide losses, and the use of large mode field diameter fibers (6 µm mode field diameter) for testing. Previous reports of lower coupling losses were achieved by coupling thin-film lithium niobate to fibers with smaller mode fields (3 µm mode field diameter)[18]. Addressing the challenge of coupling to large mode fields remains a key issue in research. Future improvements may involve utilizing silicon nitride auxiliary schemes to enhance coupling efficiency.

This paper presents a novel design approach for Multifunction Integrated Optic Circuits (MIOC) based on thin-film lithium niobate, focusing on the key performance indicators of the MIOC: loss, half-wave voltage, and polarization extinction ratio (PER). Key device designs were proposed and experimentally validated. To control losses, a dual-layer inverted taper structure was designed to reduce coupling losses. For half-wave voltage control, adjustments in electrode spacing and structure enabled achieving a half-wave voltage of less than 2V, making the MIOC optically compatible with CMOS device processes, which is significant for future photonic integration. In terms of PER control, an innovative approach utilizing sub-wavelength gratings was proposed to manage the PER, achieving a PER greater than 29 dB at 1550 nm. This strategy is not only applicable to MIOCs but also to other applications requiring polarization control.

The proposed solutions mark a significant advancement in the application of thin-film lithium niobate in fiber optic gyroscopes, validated both theoretically and experimentally. This research provides valuable directions for future studies, with optimization potential in addressing the coupling compatibility between thin-film lithium niobate and large mode field fibers. The findings of this study are poised to significantly propel the development of compact, integrated optical gyroscopes.

Author Contribution

Guo hongjie wrote the main manuscript text and prepared figures ,All authors reviewed the manuscript

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No competing interests reported.

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High performance multifunction integrated optic circuits base on thin- film lithium niobate

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Version 1

Abstract

Figures

1. Introduction

2. Design and simulation

2.1 Single-Mode

2.2 Sub-grating

2.3 Double layer inverse cone coupler

3. Fabrication and measurement

4. Conclusion

Declarations

Author Contribution

References

Additional Declarations

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