Coherent optical transmission systems have been researched energetically since the 1980's. Many technologies and devices were developed such as heterodyne/homodyne detection, narrow linewidth lasers, LiNbO3 modulators, and twin-PIN photodetectors. CPFSK optical heterodyne detection systems for submarine communications were developed and tested for commercial use in 1990. However, the developed systems were not in use because optical amplifiers were developed and used instead of coherent systems. Recently, large capacity networks are required by increased internet traffic. Large capacity transmission systems are required without increasing the symbol rates. Therefore, coherent systems are attracting attention due to their potential for polarization multiplexing and multi-level modulation by combining digital processing technologies. Digital coherent communication systems, which have also been investigated, are indispensable for future networks. These technologies will play an important role for multi-core and few-mode fiber transmission systems.

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Abstracts: This paper reviews the recent progress in digital coherent optical communication technologies for core metro, and access transport network. Digital mitigations of various transmission impairment induced by Kerr -nonlinear effects in optical fibers are addressed.

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The expansion of digital coherent optical communication technologies to metro-area networks is accelerating the development of compact and energy efficient transceivers. To meet the strong demands for a smaller footprint and lower power consumption, such optical devices as tunable lasers, modulators, and coherent receivers, which consist of coherent transceivers, must also be more compact and efficient. This paper reviews the technologies of coherent transceivers and related optical devices by focusing on the development of optical devices using InP-based monolithic integration technologies. We also describe the current status of the development and the activities for the next generation networks with higher bit rates, such 400 Gbit/s and 1 Tbit/s.

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We describe narrow linewidth tunable lasers for digital coherent system. To realize high transmission capacity, and wider deployment to metro and data center network, packaging technology for small footprint, and μ-Integrable Tunable Laser Assembly (μ-ITLA) characteristics with DFB/DR laser array based tunable laser were reported. We realized the highly reliable packaging technique by applying an adhesive bonding technique, high performance fl exible grid μ-ITLA with less than 300kHz linewidth.

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We demonstrated Si-photonic hybrid ring external cavity wavelength tunable lasers by passive alignment techniques with over 100-mW fiber-coupled power along the whole C-band. A spectral linewidth of 100 kHz was successfully attained using Micro-ITLA with a low-noise driving circuit. We achieved these attractive performances due to very low loss Si-wire waveguides whose loss was lower than 0.5dB/cm. Our obtained results show the excellent features of Si-photonics toward commercial products.

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In recent years, with the aim of keeping up with the rapid increase in transmission capacity, coherent optical transmission employing multi-level modulation formats has been intensively studied. In particular, quadrature amplitude modulation (QAM), in which information is encoded on both the amplitude and phase of the optical carrier, has attracted a lot of attention because of its ability to achieve higher spectral efficiency (SE) than other formats. Indispensable to the realization of ultra-multilevel QAM coherent optical transmission with an extremely high SE are narrow linewidth lasers and a precise optical carrier phase synchronization scheme between the transmitted data and the local oscillator. In this paper, we summarize our recent progress on ultra-multilevel QAM coherent transmission using a homodyne detection method by employing a frequency-stabilized fiber laser as a coherent light source, an optical phase-locked loop circuit, and an injection locking scheme for carrier phase synchronization at the receiver.

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We review high-speed multilevel optical modulators using a hybrid configuration of silica planar lightwave circuits and LiNbO3 (LN) phase modulators. The hybrid configuration is highly scalable for advanced modulation formats and functions. We also show the configuration of an optical time-division multiplexing (OTDM) modulator and results of a large-capacity signal-generation experiment.

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The growth of terahertz technology has opened up the strong potential of various attractive applications in the terahertz waveband. However, conventional optical elements such as collimated lenses are too large for the wavelength, and terahertz devices demand compact and thin optical elements for industrial applications. We propose and design an ultrathin gradient lens with unprecedented high refractive indices and extremely low refl ection controlled by paired metal cut wires in the 0.3-THz band. We used a unit model with periodic boundary walls and designed a refractive index neff = 7.06 + j0.330, refl ection power of 1.0%, and transmission power of 82.0% for the center of the lens. We also designed a refractive index of neff = 3.55 + j0.0582, reflection power of 15.2%, and transmission power of 81.9% for the periphery. A full model analysis also verifi es a focusing effect at 4.69 mm (4.69λ 0) from a lens. The proposed ultrathin gradient lens with high refractive indices and extremely low refl ection will provide valuable solutions for a wide range of applications.

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Stabilizing the resonance frequency of an optical cavity was achieved to obtain a reference signal for the 399-nm light source used for collimating a Yb atomic beam. We attempted cavity control to compensate for laser power by dividing the transmitted light intensity of the cavity by the monitor signal intensity of an incident optical power. As a result, the stability of the resonance frequency for 780-nm light was almost 0.5 MHz, which continued for more than 2 hours. This stabilization was maintained until the incident optical power was decreased to 10% of the initial power. With our technique, highly stable 399- nm light is expected to be generated which enables us to achieve high-performance atomic nanofabrication using Yb atoms.

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