利用定向耦合器代替Y形分支结构,在X-cut铌酸锂(LN)基底上设计了一款具有低驱动电压、高调制参数的马赫-曾德尔(M-Z)型电光调制器。采用有限元法对定向耦合器的原理及影响因素进行了详细分析和精确计算,并对电光调制器的关键结构参数进行了优化。结果表明:光波沿着定向耦合器传播时,能量在两波导中来回交替,呈正弦和余弦函数分布,并且耦合长度随着其间距的增大而快速增大。在调制臂长为3 cm,光波长为1550 nm的情况下,单臂调制所获得的半波电压V_π为2.22 V,调制参数S₂₁为-51.13 dB。通过对调制臂进行截面分析,给出了电场E_x分量、电位移矢量D_x分量和光模分布,并求得重叠积分因子为0.486。同时还表明脊形波导结构有利于提高电光作用效率。当脊高为1 μm时,重叠积分因子为0.714,相较于平板波导,其电光重叠积分因子提高了46.91%。

A Mach-Zehnder (M-Z) electro-optical modulator with low driving voltage and high modulation parameters is designed on the X-cut Lithium Niobate (LN) substrate by using a directional coupler instead of traditional Y-shaped branch structure. The working principle and major factors of directional coupler are analyzed and calculated precisely by the finite element method. The major structural parameters of the electro-optic modulator are optimized. The results show that when the light wave propagates along the directional coupler, the energy is alternated back and forth in the two waveguides with a distribution of sine and cosine functions. At the same position, the coupling length increases rapidly with the increase of the gap. When the electro-optical modulation arm length is 3 cm and the optical wavelength is 1550 nm, the half-wave voltage Vπ obtained by the single-arm modulation is 2.22 V and the modulation parameter S21 is -51.13 dB. With the section analysis of modulation arms, the electric field Ex component, the electric displacement vector Dx component and the optical mode distribution are given, and the integral overlap factor is 0.486. It also shows that the ridge waveguide structure can effectively improve the electro-optic efficiency. When the ridge height is 1 μm, the overlap integral factor is 0.714, and the electro-optical overlap integral factor of it is 46.91% higher than the slab waveguide.