All-Optical Discrete Fourier Transform Based On Multimode Interference Couplers

In this letter, an all-optical discrete Fourier transform (DFT) device based on multimode interference (MMI) couplers is proposed. The DFT device is composed of a discrete sine transform device and a discrete cosine transform device, both of which are based on MMI couplers. The main advantage of this DFT device is that it is composed of two compact MMI couplers, which greatly reduces the fabrication effort on planar lightwave circuits.

I. Introduction

Orthogonal frequency-division multiplexing (OFDM) has emerged as a promising technology in communication. Recently, studies have been focused on the application of this novel technology in optical communication systems [1]–[3]. One of the limitations of its application in broadband optical transmission is the difficulty to process the OFDM signals in the optical domain. The discrete Fourier transform (DFT) is the key procedure of signal processing in OFDM systems, and there has been literature on realizing all-optical DFT on a planar lightwave circuit (PLC) [4]. However, due to the nature of DFT, it requires very complicated PLC structures. An alternative to reduce the fabrication effort is to use the concept of the fast Fourier transform (FFT). This was first proposed by Marhic [5] in the optical domain and realized on a PLC chip later by Koichi Takiguchi et al. [6], but it is still composed of a tremendous number of Mach–Zehnder interferometers.