In this paper, a reconfigurable optical network intermediate node scheme with format conversion capacity and full-quadrature regeneration functionality is proposed. With a degraded input quadrature phase-shift keying (QPSK), the proposed network node can be dynamically reconfigured based on the network situation to output a QPSK or a four-level pulse amplitude modulation (PAM4), both full-quadrature regenerated. The input QPSK is first converted into the two full-quadrature regenerated binary phase-shift keying tributaries after phase sensitive amplifier-based quadrature de-multiplexing and selfphase modulation-based amplitude regeneration. Then the output regenerated QPSK or PAM4 are generated through vector addition by tuning the relative phase and amplitude between two tributaries. For an input 20 Gb/s QPSK with optical signal-to-noise ratio (OSNR) of 15 dB, the output regenerated QPSK gets 10.4% decrease in error vector magnitude (EVM), 2 dB receiver OSNR improvement at the bit error rate of 10 -3 , and regeneration factors of 6.4 dB and 3.7 dB in amplitude and phase, respectively. In the case with regenerated PAM4 as the output, compared with the input QPSK, it has 10.4% improvement in EVM and regeneration factors of 7.1 dB and 2.5 dB in amplitude and phase, respectively. The numerical simulation results reveal that the system effectively regenerates input signal in full-quadrature with format conversion capacity. The proposed node scheme can be applied as a fullquadrature QPSK regenerator between optical networks, which both employ QPSK to increase the transmission distance, or an alloptical interconnection node between different optical networks where different modulation formats are used individually.
We present a newly developed two-dimensional photodetector array (2D-PDA) device and its application for short-range free-space optical (FSO) communication using a wavelength-division multiplexed beam. The 2D-PDA enables direct optical coupling to the FSO beam, simplifying the optical alignment process needed for 10-μm single-mode fiber coupling and reducing the need for beam-tracking function. In the wavelength-division multiplexing (WDM) FSO communication demonstration, a four-wavelength 25-Gbaud parallel beam filtered by a 4-WDM filter through free space was successfully received by 4-pixels in the 2D-PDA. This paper discusses the design and fabrication in the 2D-PDA device, as well as the FSO communication demonstration using four combined WDM filters.
In our former report, we have proposed a resonant-electrode-type optoelectronic oscillator (RE-OEO). An optical modulator used in the RE-OEO has a resonant electrode, over which standing wave arises around the frequency of the resonance. The frequency components out of the resonant frequency are highly suppressed, so that single-mode oscillation in microwave frequency is easily achieved. In addition, the RE-OEO oscillates with lower input threshold power since modulation efficiency can be improved at the resonant condition. We experimentally investigate, in this paper, the characteristics of injection locking for the RE-OEO when microwave near the oscillation frequency is launched on it.
We experimentally demonstrate generation of polarization-multiplexed phase-conjugated twin signals by using oppositely-biased dual-polarization IQ modulators, compensating for nonlinear signal distortion in fiber transmission. Bias condition is optimized investigating nonlinearity tolerance of the generated signals transmitted over SMF.
We fabricated an integrated reciprocating optical modulator consisting of a phase-shifted fiber Bragg grating and an optical high-speed phase modulator. High-order double sideband components were effectively generated by reciprocating modulation process. We also demonstrated photonic millimeter-wave generation by feeding the double-sideband components to a high-speed photodetector.
We experimentally demonstrate a novel nonlinearity-mitigation scheme based on a complex-valued neural network (CVNN) which is constructed by artificial neurons with complex-valued input and output. The in-phase (I) and quadrature (Q) components of optical signal are operated as complex values in the CVNN. A 40-Gbit/s optical 16QAM signal distorted by SPM was successfully compensated, improving error vector magnitude (EVM) by about 15%. The learning speed of the nonlinear equalizer was improved by using the CVNN, compared with conventional real-valued neural network (RVNN). Furthermore, the study show that CVNN has the potential to improve the computational complexity of RVNN.
One of polarization diversity techniques in offset-frequency-spaced two-tone optical coherent transmission of radio-over-fiber signal is investigated. As the polarization diversity, successful combination of quadrature-phase-shift-keying constellations recovered from two orthogonal polarization components after a 20-km fiber-optic transmission is experimentally demonstrated.
For simultaneous stabilization of a set of cw-THz source devices oscillating at different frequencies, a number of frequency reference sources are required. In this paper, we propose a multi-frequency reference source based on Mach-Zehnder-modulator-based flat comb generator (MZ-FCG) combined with a programmable optical filter (POF). An optical comb generated by the MZ-FCG was spectrally-synthesized by the POF. By photomixing of the synthesized comb modes using a high-speed photodiode, multi-frequency THz waves are generated. As a preliminary experiment, generation of multi-frequency THz wave in the W-band has been demonstrated.
We propose a method of the precise frequency tuning in millimeter wave (MMW) generation using a Mach-Zehnder-modulator-based flat comb generator (MZ-FCG). The MZ-FCG generates a flat comb signal where the comb spacing is exactly the same as the frequency of a radio-frequency signal driving the MZ-FCG. Two modes are extracted from the comb signal by using optical filters. One of them was modulated by a phase modulator, creating precisely frequency-controllable sidebands. In the experiment, typical phase modulation was used. By photomixing of the extracted two modes using a high-speed photodiode, MMW signals with precisely frequency-controllable sidebands are generated. By changing the modulation frequency, the frequency of MMW signals can be continuously tuned. In this scheme, there are two methods for the frequency tuning of MMW signals; one is a coarse adjustment which corresponds to the comb spacing, and the other is fine tuning by the phase-modulation. It was demonstrated that the intensity fluctuation of the upper sideband of the modulated MMW signal was less than 1dB, and the frequency fluctuation was less than the measurement resolution (300Hz).
