|Abstract: ||本實驗主要開發鈮酸鋰薄膜波導之不同製備方法，並分析及比較各方法之優缺點。本實驗將x-cut鈮酸鋰薄膜波導設計於單模條件下，並透過軟體R-soft之光束傳播法（Beam propagation method, BPM）模擬直波導、絕熱耦合器(Adiabatic coupler, AC)及晶片上的量子系統(Quantum system-on-chip, QSoC)之模態及光傳播情況。|
模擬結果顯示，於絕熱耦合器(Adiabatic coupler)有效耦合長度為0.4mm，相比於傳統鈮酸鋰20mm-50mm之耦合長度大幅減小了98%; 並且於晶片尺寸5mm1mm0.5mm之QSoC得到25%:25%:25%:25%之分光比。
波導製備方法分為兩部分，共五種方法，第一部份(方法一、二、四)利用黃光微影、濕蝕刻及乾蝕刻技術於鈮酸鋰薄膜上蝕刻出脊型波導;第二部份(方法三)利用雙束聚焦離子系統(Dual beam-focused ion beam system, FIB)，於鈮酸鋰薄膜上蝕刻出線寬0.8m，側壁角度82之脊型波導，並量測TE偏振光總插入損耗~15.77dB，耦合損耗估計為14.19dB，傳播損耗估計為0.35dB/mm；TM偏振光總插入損耗為20.58dB，傳播損耗估計為25.75dB/mm。
另一方面，在與耶拿大學合作下，利用IBEE(Ion-beam enhanced etching)聚焦離子數蝕刻法(方法五)，成功量測到AC及QSoC模態分佈，其AC結構總損耗為22.19dB。
;In order to achieve the waveguiding on the thin-film lithium niobate (TFLN
) substrate, we developed five fabrication methods to compare the advantages and disadvantages in different aspects of these methods.
Our devices are designed under single-mode conditions of 1550-nm band, and we simulated the single mode conditions, the adiabatic couplers (AC) and the quantum system-on-chip (QSoC) structures based on thin-film lithium niobate (TFLN) substrates by using the Beam Propagation Method (BPM) in commercial software of R-soft packages.
We found the effective coupling length of adiabatic couplers (AC) is only 0.4 mm, which is very effective in comparison with standard lithium niobate (LN) ACs with 20-50 mm length, the TFLN-AC coupling length is greatly shortened by 98% in comparison with our previous studies of classical LN ACs. In addition, The QSoC structure size is only 5 mm x 1 mm x 0.5 mm with 25%:25%:25%:25% simulated output splitting ratio, which is also 50 times miniaturization in comparison with the reference of QSoC based on classical LN with 50 mm x 5 mm x 0.5 mm size.
Our devices are designed and fabricated on 0.5-mm-thick x-cut thin-film lithium niobate-on-insulator (LNOI) substrates with thin-film lithium niobate layer of 600 nm above an insulator layer a4700-nm-thick silicon dioxide, and we used five methods to form the ridge waveguides on the surface of TFLN, the etching method is divided into two sections. The first three method (Method 1, Method 2 and Method 4) is wet and dry etching with using photolithography, wet etching and dry etching techniques before the TFLN etching process, these ridge waveguides are etched by inductively coupled plasma-reactive ion etching (ICP-RIE) equipment. For the ICP-RIE etched waveguides, we didn’t observe the stable guiding modes from Method 1 and Method 2, it may due to the high coupling loss and possible the high propagation losses, which may from the uneven channel and rugged surfaces of etched ridge region. The method of the second type (Method 4) is dual beam-focused ion beam system (FIB) etching method. We successfully etched TFLN ridge waveguides with a linewidth of 800 nm and a sidewall angle of 82 degrees by FIB-etching. For chip #FIB9-B, the measured insertion loss is around 15.77 dB of TE-polarized modes, the calculated propagation loss is only 0.35 dB/mm of TE polarization state, and the measured insertion loss ~20.58 dB of TM-polarized modes, the calculated propagation loss is 25.75 dB/mm at TM polarization state.
On the other hand, under the Jena University cooperation project, we used the ion-beam enhanced etching (IBEE) method (Method 5) to etch the TFLN substrates. We successfully measured adiabatic coupler with ~ 22.19 dB insertion loss of TE-polarized modes within the broadband spectral coupling characteristics, and also successfully observed the QSoC mode fields with the power ratio of 10.1%, 32.4%, 50.7% ,and 6.8% for 4 outputs, respectively, which represents and proves the S-bend, Y-branch and directional couplers of QSoC configuration are all have functions.
In the term of future outlook, due to various excellent characteristics of TFLN, the TFLN devices size can be reduced to the hundreds of microns, instead of mm to cm level of classical lithium niobate devices, and according the mode size and excellent electro-optical properties of TFLN, we may imagine in the near future, though the CMOS-compatible electro-optic switching voltages and the standard semiconductor foundries comparable mass-production processes, the TFLN electro-optic modulator (TFLN-EOM) can be further fully integrated with silicon photonics of more variety related technologies, such as integrated photon-detector (PD), integrated laser diode (LD) or even more complex optical circuits, the TFLN EOM can play as key role of important active modulation components in silicon photonics and realize the fully integrated electro-optical circuits (IEOC) in the decade.