Carrier conversion from terahertz wave to dual-wavelength near-infrared light for photonic terahertz detection in wireless communication.

THz waves are promising wireless carriers for next-generation wireless communications, where a seamless connection from wireless to optical communication is required. In this study, we demonstrate carrier conversion from THz waves to dual-wavelength NIR light injection-locking to an optical frequency comb using asynchronous nonpolarimetric electro-optic downconversion with an electro-optic polymer modulator. THz wave in the W band was detected as a stable photonic RF beat signal of 1 GHz with a signal-to-noise ratio of 20 dB via the proposed THz-to-NIR carrier conversion. In addition, the results imply the potential of the photonic detection of THz waves for wireless-to-optical seamless communication.

Waveband-shift-free optical phase conjugator based on difference-frequency generation.

In this paper, a waveband-shift-free optical phase conjugator based on difference-frequency generation (DFG-OPC) that enables broadband operation is proposed and theoretically investigated. First, the principle of phase-conjugated (PC) wave generation with the DFG-OPC is mathematically described. Using a Sagnac loop interferometer with a χ(2) optical nonlinear material and two dispersive elements (DEs), a PC wave with the same wavelength as a signal can be generated. Subsequently, the required DE length difference for the PC wave generation is theoretically calculated. The calculation results indicate the minimal DE length difference is 20.0 µm, and this is because the DFG-OPC enables broadband operation. Second, the wavelength characteristics of the DFG-OPC are investigated through numerical simulation. The operation bandwidth of the DFG-OPC depends on the DE length difference, and an operating bandwidth of the DFG-OPC of 54.5 nm can be achieved when the DE length difference is less than 0.01 m. Finally, the influence of the splitting ratio of an optical 3-dB coupler in the DFG-OPC is numerically studied. The results indicate that tolerance of the optical coupler splitting ratio is equal to or less than ±6% for the DFG-OPC.

Optical pump phase locking to a carrier wave extracted from phase-conjugated twin waves for phase-sensitive optical amplifier repeaters.

In this paper, an optical phase-locked loop assisted by sum-frequency and second-harmonic generation (SS-OPLL) for frequency nondegenerate optical parametric phase-sensitive amplifier repeaters is experimentally demonstrated. First, theoretical derivations show that carrier extraction from phase-conjugated twin waves (PCTWs) and reference light generation are achieved by sum-frequency generation; therefore, the SS-OPLL circuit enables optical phase locking between PCTWs and a pump wave by a simple architecture based on a balanced OPLL. Then, optical phase locking between 20-Gbit/s quadrature phase-shift keying PCTWs and an individual pump source is experimentally demonstrated. Experimental results indicate that phase errors were reduced during the SS-OPLL operation.

Simultaneous detection of 10-Gbit/s QPSK × 2-ch. Fourier-encoded synchronous OCDM signals with digital coherent receiver.

We experimentally demonstrate the simultaneous detection of 10-Gbit/s quadrature phase shift keying (QPSK) × 2-channel Fourier-encoded synchronous optical code division multiplexing (FE-SOCDM) signals using a digital coherent receiver, for the first time. First, we analytically verify that simultaneous detection can be achieved with an N-point discrete Fourier transform (DFT) using digital signal processing (DSP) because the N-channel Fourier encoding corresponds to an N × N inverse DFT, then the operation is experimentally confirmed. Simultaneous detection of 10-Gbit/s QPSK × 2-channel FE-SOCDM signals is evaluated. The proposed scheme dramatically expands the capability of OCDM systems.