Abstract:In this paper, an acousto-optic tunable flat top filter is proposed by using one-dimensional coupled-cavity photonic crystal, and two kinds of materials, i.e. magnesium fluoride and tellurium dioxide, are selected. Based on the theory of acousto-optic effect, the thickness and refractive index of one-dimensional coupled cavity photonic crystal acousto-optic medium are changed by varying the ultrasonic frequency. After the parameters of acousto-optic medium are changed, the central wavelength of flat top filter of transmission spectrum shifts toward the short wave direction, thus realizing tunable filtering function. Based on the transfer matrix method and the theory of acousto-optic effect, the theoretical model of the flat top filter is established. The rectangularity, passband bandwidth, insertion loss and tunability of flat top filter are simulated by COMSOL software. The results show that the tuning effect can be achieved by applying a certain frequency of ultrasound in the case of different ultrasonic amplitudes. The decreasing trends of transmittance are basically identical in the process of changing ultrasonic frequency in the case of different amplitudes. When the central wavelength of the flat top filter increases from 1510 nm to 1514 nm, the transmissivity corresponding to the central wavelength increases sharply from 37% to 90%; when the central wavelength of the flat top filter continuously increases to 1562 nm, the transmittance corresponding to the central wavelength increasing gently from 90% to 97% in the case of different amplitudes. Considering the transmissivity required to exceed 90% in the passband of flat top filter and the cost of ultrasonic generator, the ultrasonic wave with an amplitude of 0.4 nm is selected as the research object. The flat top filter with 5–6 nm central wavelength and 1514–1562 nm tunable flat top filter can be realized by applying ultrasonic wave with the frequency in a range of 6–11 MHz. In the tunable range, the highest insertion loss is only 2.23 dB, the lowest is only 0.78 dB, and the lowest rectangularity is 1.4. In a practical flat top filter with machining error within 5 cm, the deviation of center wavelength, rectangularity, insertion loss and passband bandwidth of flat top filter are all very small. The flat top filter has the characteristics of easy design and integration, flat passband, wide tunable range, stable passband bandwidth, low insertion loss and high quality factor. It has important applications in optical communication fields such as optical switch, tunable fiber laser and fiber sensing. Keywords:flat top filter/ acousto-optic effect/ one-dimensional coupled-cavity photonic crystals/ tunable
其中, ${P_{{\rm{out}}}}$是平顶滤波器在通带波段内某一波长下的输出功率, ${P_{{\rm{in}}}}$是平顶滤波器在通带带宽内某一波长下的输入功率. 考虑到材料本身损耗的影响, 平顶滤波器插入损耗与中心波长的关系如图7所示, 其中实线和短线分别表示入射光在这一中心波长下对应的通带带宽内的最大插入损耗和最小插入损耗. 随着中心波长的漂移, 施加不同超声波频率时通带带宽内对应的最大插入损耗和最小插入损耗变化不大. 最大插入损耗在1.72—2.23 dB范围内, 最小插入损耗在0.78—1.07 dB范围内. 图 7 平顶滤波器中心波长与插入损耗的关系图 Figure7. Relationship between center wavelength of flat top filter and insert loss.
25.4.加工精度对平顶滤波器的影响 -->
5.4.加工精度对平顶滤波器的影响
在工程上, 制备一维光子晶体的过程中, 由于工艺水平的限制难免会出现几何偏差, 加工精度会影响到平顶滤波器的中心波长、通带带宽、矩形度、插入损耗等参数. 一维光子晶体几何尺寸偏差与相关参数的关系如图8所示, 正数代表一维光子晶体厚度较标准增加, 负数代表一维光子晶体厚度较标准减少. 图8(a)表示一维光子晶体几何尺寸偏差与平顶滤波器中心波长和通带带宽的关系图, 实线表示通带带宽, 短线表示中心波长. 可以看出随着一维光子晶体几何尺寸偏差从减少–50 nm到$50\;{\rm{nm}}$, 平顶滤波器中心波长向长波方向漂移, 通带带宽呈增加趋势. 在$ \pm 10\;{\rm{nm}}$的加工精度范围内平顶滤波器中心波长漂移不超过$0.6\;{\rm{nm}}$, 通带带宽在5—6 nm范围内. 图8(b)表示一维光子晶体几何尺寸偏差与平顶滤波器矩形度的关系图, 可以看出在$ \pm 10\;{\rm{nm}}$的加工精度范围内平顶滤波器矩形度出现了一定波动, 在这一范围内不低于1.55, 满足平顶滤波器的要求. 图8(c)表示一维光子晶体几何尺寸偏差与平顶滤波器插入损耗的关系图, 实线表示通带带宽内最大插入损耗, 虚线表示通带带宽内最小插入损耗. 可以看出在$ \pm 10\;{\rm{nm}}$的加工精度范围内最大插入损耗不超过$2.28\;{\rm{dB}}$, 相较原平顶滤波器的最大插入损耗仅增加$0.05\;{\rm{dB}}$. 图 8 一维光子晶体几何尺寸偏差与相关参数的关系 (a) 一维光子晶体几何尺寸偏差与平顶滤波器中心波长和通带带宽的关系图; (b) 一维光子晶体几何尺寸偏差与平顶滤波器矩形度的关系图; (c) 一维光子晶体几何尺寸偏差与平顶滤波器插入损耗的关系图 Figure8. Relationship between geometric size deviation and related parameters of one-dimensional photonic crystal: (a) Relationship between geometric dimension deviation of one dimensional photonic crystal and central wavelength or passband bandwidth of flat top filter; (b) relationship between the geometric size deviation of one-dimensional photonic crystal and rectangularity of flat top filter; (c) relationship between the geometric size deviation of one-dimensional photonic crystal and the insert loss of flat top filter