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The directional optical transmission characteristics of on-chip integrated optical isolators have wide application value in fields such as optical communication and optical signal processing. At early stage, various schemes of on-chip optical isolators have been developed, such as single-crystal magneto-optical pomegranate scheme, and silicon nitride ($\mathrm{Si_3N_4}$) micro-ring resonators. However, there is still lack of compact on-chip optical isolator solutions. Here, we proposed a compact and integrated silicon optical isolator on a standard silicon on insulator (SOI) substrate, designed by intelligent algorithms and a variety of micro-nano circular vias. A modified genetic algorithm is developed and introduce a segmented design fitness function and establishesa gene library to obtain an ultra-compact optical isolator scheme with a size of only $4.2\text{ μm}\times 3\text{ μm}$. On a standard silicon on insulator substrate, a linear passive isolation scheme was achieved by etching circular holes with five different diameters such as 60 nm, 120 nm, 180 nm, 240 nm and 300 nm.In TE polarization mode, the design achieved an isolation degree of approximately 31 dB and an insertion loss of approximately 2 dB. Furthermore, in TM polarization mode, the design achieved an isolation degree of approximately 38 dB and an insertion loss of 2 dB; Finally,the impact of different size groups on the performance of isolators was analyzed. Results show that the smaller circular hole structure,the better isolation performance. However, at the same time, we also need to consider the real silicon etching process requirements. Too small holes are difficult to etch in practice. We also evaluated the effect of 10 nm, 20 nm and 30 nm etch penetration between circular vias on the isolator performance, and results tentatively show that the etch penetration caused by the current more mature 30 nm etching process is acceptable. Therefore,considering all factors, a 30 nm minimum circular hole size and 30 nm minimum distance adjacent circular hole spacing are recommended. These results have a promoting effect on the development of highly integrated and ultra-small on-chip optical signal directional transmission schemes.
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Keywords:
- Integrated optical isolator /
- ultra-compact /
- genetic algorithms
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图 1 隔离器结构示意图 (a)正向导通示意图, 下方是优化过程中可供选择的五种不同的圆孔孔径参数; (b)反向通光示意图, 下方的结构是1550 nm的TM模式光源经过1500次迭代后最优价值函数对应的隔离器结构分布图
Figure 1. Isolator structure diagram (a) Schematic diagram of the forward guide, below which are the five different round hole aperture parameters that can be selected during the optimization process; (b) Reverse light passing diagram. The structure below is the distribution diagram of the isolator structure corresponding to the optimal value function of the 1550 nm TM mode light source after 1500 iterations.
图 2 遗传算法的流程图
Figure 2. Flow chart of genetic algorithm
图 3 遗传算法的最优结构在正向、反向传播时光场模场分布 和对应的结构分布图(a) (d) (g) 迭代1次; (b) (e) (h) 迭代11次; (c) (f) (i) 迭代201次
Figure 3. The optimal structure of genetic algorithm in the forward and backward propagation of power distribution and the corresponding structure distribution diagram (a) (d) (g) Iteration 1 times; (b) (e) (h) 11 iterations; (c) (f) (i) 201 iterations.
图 4 将光源设置为1550 nmTE偏振的优化结果 (a) 透射率随着迭代次数的变化曲线; (b) 价值函数和隔离度随着算法的迭代的变化曲线; (c) 不同波长通过结构的透射率分布曲线; (d) (g) 正向、反向通光时的电场模场分布图; (e) (h) 正向、反向通光时的磁场分布图; (f) (i) 正向、反向通光时的光场模场分布图
Figure 4. Optimization results of setting the light source to 1550 nmTE polarization (a) Transmittance with the number of iterations; (b) the variation curve of the value function and isolation degree with the iteration of the algorithm; (c) Transmittance distribution curves of different wavelengths through the structure; (d) (g) Mode field distribution of the electric field when the light is transmitted in the forward and reverse directions; (e) (h) Magnetic field distribution in forward and reverse transmission; (f) (i) Power distribution diagram for forward and reverse transmission
图 5 TM偏振的优化结果. (a) (d) 1535 nm处的光场模场分布图; (b) (e) 1555 nm; (c) (f) 1575 nm; (g)隔离度、插入损耗的变化曲线; (h)透射率的变化曲线; (i)不同波长的光源通过结构的透射率和隔离度分布曲线; (j) (k) 1550 nm
Figure 5. Optimization results of TM polarization. (a) (d) Optical power distribution at 1535 nm; (b) (e) 1555 nm; (c) (f) 1575 nm; (g) variation curves of isolation and insertion loss; (h) variation curves of transmittance; (i) transmittance curves and isolation distribution curves of different wavelengths of the light source passing through the structure; (j) (k) 1550 nm.
图 6 四种特征尺寸随迭代次数的变化曲线和刻蚀穿透间距离 (a) 正向透射率; (b) 反向透射率; (c) 隔离度; (d) 价值函数; (e)—(h) 刻蚀10 nm孔间距得到的透射率曲线、示意图和光场模场分布图; (i)—(l) 刻蚀20 nm得到的结果; (m)—(p) 刻蚀30 nm得到的结果
Figure 6. Variation curves of the four feature sizes with the number of iterations and the distance between the etched penetrations (a) forward transmittance; (b) reverse transmittance; (c) isolation; (d) FOM function; (e)–(h) Transmittance curves, schematic and power distribution plots obtained by etching 10 nm pore spacing; (i)–(l) results obtained by etching 20 nm; (m)–(p) results obtained by etching 30 nm.
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