Best Paper Award

Demultiplexing Method of Variable Capacity Optical OFDM Signal Using Time Lens-Based Optical Fourier Transform

Koichi TAKIGUCHI, Takaaki NAKAGAWA, Takaaki MIWA


  OFDM is a multiplexing scheme that realizes highly spectral-efficient communication by utilizing sub-carrier channels whose baud rates are equal to their frequency spacing. It is important for optical communication, in which further capacity increase is required, as well as wireless communication. Time Fourier transform is necessary for demultiplexing channels of the OFDM signal. Optical OFDM is useful for not only fixed capacity optical communication but also next-generation variable capacity optical networks, which adaptively allocate the required bandwidth, in terms of the decrease of their guard bands. In next-generation optical communication, each channel speed exceeds several tens of Gbaud.
  In this paper, the time lens method was focused on and investigated with a view to realizing high-speed and low-power-consumption optical Fourier transformations that can demultiplex the variable capacity optical OFDM signal directly in the optical domain. The method realizes optical time Fourier transformation by adding periodic linear frequency chirping, namely periodic quadratic phase modulation and chromatic dispersion to the signal. The time lens method was named after a lens because both optical pulse broadening due to the group velocity dispersion and optical spatial pattern broadening due to Fresnel diffraction are described by numerically equivalent equations. In this paper, an LN phase modulator and a grating-type tunable chromatic dispersion compensator were adopted for generating chirping and chromatic dispersion, respectively, in order to realize an integrated-photonic time lens-based optical Fourier transformation circuit in the future and demultiplexing a wide capacity range of optical OFDM signals precisely. After explaining the operating principle of the proposed method, the operating conditions and achievable characteristics (OFDM signal demultiplexing up to about 200 Gbaud) using the existing devices performance were investigated and then numerically clarified. The investigation included the relationship between driving voltage to the phase modulator and requisite chromatic dispersion value, and the relationship between the frequency spacing of channels and the maximum number of demultiplexed channels. Then, in this paper, demultiplexing experiments of 32 to 40 Gbaud variable capacity optical OFDM signals were carried out based on the numerical investigation, and the operating principle of the proposed method was validated. It is difficult to generate a periodic quadratic waveform voltage in the order of ten gigahertz, which is required for driving the phase modulator. However, in this paper, it was also experimentally clarified that the OFDM signal can be demultiplexed by driving a modulator with the approximate quadratic waveform, which can easily be synthesized with two synchronized oscillators for producing fundamental and second order harmonic sinusoidal waves.
  To sum up, in this paper, time lens-type optical Fourier transformation was proposed as a means for demultiplexing the variable capacity optical OFDM signals, which are promising in next-generation optical communication. Also in this paper, a configuration suitable for realizing the proposed optical Fourier transformation method with an integrated-photonic circuit was advanced, the characteristics of the proposed method were clarified, and an experiment to validate the operating principle of the proposed method was carried out. Thus this paper is suitable for the IEICE best paper award because its content can contribute to the future progress of the optical OFDM.