Photonic allpass filter: A versatile building block for all-optical signal processing

The allpass filter, since inception in the 1960s, has found its way into a wide range of digital signal processing applications. The structure of the filter also lends itself to robust analog lattice implementations using traditional passive electronic components. In the late 2000s, the versatility of the allpass translated into the photonic domain since it can be easily realized using a variety of basic photonic structures. The simplicity in the implementation makes the allpass structure ideal for serving as the basic building block for complex photonic integrated systems. The increasingly congested frequency spectrum has emphasized the necessity for Radio Frequency (RF) photonics, where the photonic allpass filters can serve as the foundation to all-optical signal processing backends. This paper reviews the concepts of allpass filter design, and discusses its role in photonic systems.