Add–drop filter (ADF) is a key component in optical integrated circuits that can be used in all-optical communication networks and wavelength division multiplexing (WDM) systems. The quality factor, coupling efficiency, transmission efficiency and coupling length are important parameters in add–drop filters. Photonic crystal (PC) optical devices have become popular among researchers because their structure is suitable to embed into optical circuits. This paper covers a comprehensive review of the principle structure of ADF, coupled mode theory (CMT), types and recent applications in WDMs, accelerometer and bio/chemical sensors. Although there are some different categories of photonic crystal ring resonator-based ADF in general, all of them can be divided into photonic to two class of non-circular and circular. This article is reported a comprehensive study about ADF and improvement of these ADF.

分插滤波器（ADF）是光集成电路中的关键组件，可以在全光通信网络和波分复用（WDM）系统中使用。品质因数，耦合效率，传输效率和耦合长度是分插滤波器的重要参数。由于光子晶体（PC）的结构适合嵌入光路中，因此已在研究人员中流行。本文全面介绍了ADF的原理结构，耦合模式理论（CMT），类型以及WDM，加速度计和生化传感器中的最新应用。尽管一般来说，基于光子晶体环形谐振器的ADF有一些不同的类别，但它们都可以分为光子非圆形和圆形两类。