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We propose to implement a 2-channel optical-time-division-multiplexed (OTDM) system for short-reach optical interconnects by using a sinusoidally modulated light source instead of a complicated mode-locked laser as an input pulse source. In this system, the OTDM signal is obtained by bit-interleaving two optical return-to-zero (RZ) signals generated by using the sinusoidally modulated light. We operate these RZ signals in the orthogonal in-phase and quadrature domains to avoid the unwanted beat components. After the transmission, the OTDM signal is detected by using single photodetector, and then processed by a 2 × 2 multiple-input multiple-output equalizer. For a demonstration, we generate 150-Gb/s OTDM signal operating in the 8-level pulse-amplitude modulated (PAM-8) format by using commercial LiNbO3 Mach-Zehnder modulators and transmit this OTDM signal over 1.9 km of the standard single-mode fiber (SSMF). In addition, we fabricate the proposed OTDM transmitter in an integrated silicon-photonics chip and use it to demonstrate the transmission of the 64-Gb/s OTDM PAM-4 signal over 2.2 km of SSMF.

We proposed to implement a 2-channel OTDM system by using a sinusoidally modulated light source instead of a mode-locked laser. For a demonstration, we transmit 150-Gb/s OTDM-PAM8 signal over 1.9-km long SSMF by using the proposed system.

We demonstrate the transmission of the 84-Gb/s DMT signal over 5 km of SSMF by using an injection-locked 10G-class 1.55-µm VCSEL having a 3-dB bandwidth of only 7.4 GHz.

We demonstrate the transmission of 2×28-Gb/s MGDM OOK signals operating at 852.6 nm over 2.2-km long SSMF. Only two photodetectors are used for the detection of these two MGDM signals with the help of a multiple-input single-output equalizer.

We report on the practical multiplexing techniques applicable to the next-generation ultrahigh-speed data center networks. In particular, we evaluate the feasibility of doubling the transmission capacity of the short-haul direct-detection system cost-effectively by utilizing the optical-time-division-multiplexing (OTDM) and mode-group-division-multiplexing (MGDM) techniques.

We experimentally demonstrate the transmission of 107-Gb/s PAM-4 signal over 25-km long standard single-mode fiber using a commercially available 1.31-μ m directly modulated laser. The limited bandwidth of laser is compensated bv using an electrical equalizer.

We demonstrate the transmission of 230-Gb/s DMT signal over 1.9 km of SSMF in the 1.5-μm IM/DD system by using a pre-chirp Mach-Zehnder modulator having a 3-dB bandwidth of only 22 GHz.

We design a heterogeneous 12-core 4-LP-mode fiber having two different types of cores and trench-assisted graded-index profiles. For the optimum design of the index profiles of these two heterogeneous cores, we first investigate the impacts of various geometric parameters on the effective refractive index of each LP mode, maximum differential modal delay (DMD), and bending losses of the guiding and higher order modes. We then design a heterogeneous 12-core 4-LP mode fiber having a cladding diameter of 242 μm. The numerical analyses based on the coupled-mode equation show that the inter-core crosstalk levels of the proposed fiber are lower than -55 dB/km, regardless of modes, as long as the fiber is not bent tightly with a bending radius of -3 μm-2 and 42.2, respectively.

We demonstrate the mitigation of MPI noises in the RoF-based mobile fronthaul network by using high-frequency phase dither. By using this technique, we could transport thirty-two 97.2-MHz OFDM signals in 64 QAM even when the SIR was only 20 dB.

We demonstrate a 4-channel mode-division-multiplexed (MDM) system by compensating the mode crosstalk using the digital optical phase conjugation technique.

We evaluate the requirements of the chirp parameters of the directly modulated lasers, which are used for the cost-effective implementation of the RoF-based mobile fronthaul network of the 5G system.

We demonstrate the transmission of twelve 360-MHz filtered-OFDM signals in 64-QAM format over 20 km of SSMF by using a 1.55-µm directly modulated laser and a DSP-based composite second-order cancellation technique.

We examine the feasibility of utilizing RoF-based fronthaul network for the next-generation mobile communication systems. For this purpose, we evaluate the effects of the dispersion-induced RF power fading, CSO distortions induced by the interplay between the laser chirp and fiber dispersion, and MPI noises induced by optical reflections, etc.

We propose a novel carrier phase estimation method for the high-speed coherent WDM PON based on the reflective semiconductor optical amplifier (RSOA). By using this method, we can utilize a cost-effective DFB laser as the seed light in such a network due to its fast trackability. We experimentally achieve an excellent receiver sensitivity of −30.8 dBm even when a DFB laser (linewidth: 3 MHz) is used as a seed light source in the 25.78-Gb/s upstream QPSK transmission over 20 km of SSMF.

We report on technical challenges associated with the use of directly modulated lasers in the mobile fronthaul networks based on radio-over-fiber (RoF) technology. In particular, the effects of dispersion-induced RF power fading and multi-path interference are investigated through theoretical and experimental analyses for the RoF-based transmission system implemented by using directly modulated lasers.

We design and fabricate a space-efficient single-mode fiber (SMF) having a diameter of only 82 µm. The results show that this fiber satisfies the optical and mechanical specifications of the ITU-T G.654 recommendation. In comparison to the conventional SMF having a diameter of 125 µm, this fiber can be used to double the spatial efficiency in the space-sensitive system.

We demonstrate a CPRI-based 25.2923-Gb/s optical link by using an electro-absorption modulated laser (EML) for the fronthaul network of LTE-A systems. We modulate the EML in OOK, electrical duobinary, and 4-PAM formats, and evaluate their performances for the fiber dispersion and optical reflections.