Advanced optical transmission fibers have enabled 40-Gb/s transmission over distances of up to 5200km with 100-km amplified spans. This paper will discuss a number of the enabling fiber properties including dispersion, dispersion slope, Raman gain efficiency, and polarization mode dispersion.

High-capacity multiwavelength ring networks with bidirectional WDM add/drop multiplexer (WADM) having built-in EDFAs is analyzed and demonstrated. All WDM channels can be added/dropped independently in each direction. The capacity of a bidirectional ring is found to be approximately twice that of an unidirectional ring. An eight-wavelength WADM is demonstrated for a data rate of 10 Gb/s per channel, providing an overall capacity of 80 Gb/s. The performance of the add/drop multiplexer is not degraded by backward backscattering light. The same WADM is also demonstrated to be able to serve as a bidirectional in-line optical amplifier.

Wavelength-division multiplexing (WDM) technique employing broadband erbium-doped fiber amplifiers (EDFAs) is considered to be the most effective solution to respond to the increasing demand for transmission capacity. As a means to extend the optical bandwidth outside the conventional band (C-band) ranging from 1530 to 1565 nm, silica-based EDFAs (EDSFAs) operating within the long-wavelength band (L-band) ranging form 1570 to 1600 nm seem to be the most attractive candidate because they can be composed of the same material as C-band EDSFAs, i. e. silica-based Al codoped EDF. However, there exist several discrepancies between C-band and L-band EDSFAs which originate inevitably from the difference in the inversion level and the band location. This paper reviews the basic characteristics of L-band EDSFAs, which have been a controversial issue for practical application of the L-band EDSFAs, such as required EDSF lengths, power conversion efficiency, noise performances, and optical bandwidth. We will also describe L-band EDSFAs' behavior under circumstantial changes, such as the variation of the span-loss, the temperature of the EDSF, and the number of wavelengths, which are expected in the field WDM systems. The dynamic-gain-tilt and temperature-induced change in the gain spectra of L-band EDSFAs are more significant than those of C-band EDSFAs are. Moreover, L-band EDSFAs exhibit a greater apparent inhomogeneous broadening effect, which may hinder the precise gain control when the number of wavelengths is dynamically changed. All of these characteristics must be considered for future designs of broadband WDM networks.

For measuring high frequency ultrasonic fields which are often spatially distributed and transient, an array probe with small element sensors is highly required. In this paper, we propose a fiber-optic micro-probe array which is based on wavelength-division-multiplexing technique. The element sensor consists of a micro optical cavity of 100 µm long made at the end of optical fiber. Optical path length of the cavity is changed by the applied acoustic field, and the modulation of output light intensity is monitored at another end of the fiber for the information of the acoustic field. Array of sensor elements and a light source as well as a photo detector are connected together by an optical star coupler. The Fabry-Perot resonance wavelength of each sensor element is designed different one another, and the outputs from the sensors are discriminated by sweeping the wavelength of light source with the use of a tunable semiconductor laser. In this paper, the performance of the micro-probe array is discussed experimentally.

Wavelength-division multiplexing (WDM) transmission systems have been intensely researched in order to increase the transmission capacity. One of the most important key devices for this use is erbium-doped fiber amplifiers (EDFAs) which feature a flattened gain, a high pumping efficiency and a low noise figure (NF), simultaneously. To fulfill these requirements, hybrid silica-based EDFAs (EDSFAs) composed of Al codoped and P/Al codoped EDSFs have been proposed so far. They are also attractive from the viewpoint of productivity, reliability, and cost-effectiveness. On the other hand, the optical bandwidth has been around 15 nm at most. In this paper, we have proposed newly designed hybrid EDSFAs for more than 25 nm optical bandwidth. The gain peak around 1. 53 µm can be suppressed through the saturation degree control in both EDSFs. The remaining obstacle is the diparound 1. 54 µm, which results in the relative gain non-uniformity of 10. 7% over the wavelength range from 1535 to 1560 nm. Owing to the glass composition optimization, the relative gain non-uniformity has been reduced to 5.8% without gain equalizers(GEQs), which is comparable to that of EDFFAs. As another solution, the hybrid EDSFA including two-stage Fabry Perot etalons as the GEQ has been proposed. In this configuration, the hybrid EDSFA has been designed to exhibit the gain profile similar to the summation of two sinusoidal curves, and the relative gain non-uniformity has been reduced to 3. 7%, which is almost equal to that of the hybrid EDFAs composed of EDSF and EDFF. Moreover, it has been demonstrated that newly developed hybrid EDSFAs exhibit a higher pumping efficiency and a lower NF than EDFFAs and hybrid EDSF/EDFFAs.

To expand signal wavelength bandwidth in long-haul, large-capacity WDM transmission systems, we investigated gain-equalizers (GEQs) for Erbium doped fiber amplifiers (EDFAs). We applied GEQs using Mach-Zehnder type filters with two different free-spectral-ranges (FSRs) to accurately compensate for the EDFAs gain-wavelength characteristics. The 1st GEQ with a longer FSR was the main GEQ to compensate for the overall gain-wavelength characteristics, and the 2nd GEQ with a shorter FSR was the secondary GEQ to compensate for the resultant gain undulation after the 1st GEQ. The 2nd GEQ had low maximum loss and long period of equalization-spacing compared to the 1st GEQ. We designed that the FSR for the 1st GEQ was twice the signal wavelength bandwidth, and the FSR for the 2nd GEQ was two thirds of the signal wavelength bandwidth. To compensate for the asymmetry in the EDFAs gain-wavelength characteristics, we designed that the 2nd GEQ minimum-loss wavelength was shorter than the 1st GEQ maximum-loss wavelength. Using a circulating loop with a 21-EDFA chain, we confirmed the signal wavelength bandwidth expanded by the above GEQs. We also investigated the trade-off relationship between the signal wavelength bandwidth and the optical signal-to-noise ratio, as the parameter of the number of the 1st GEQ inserted in the EDFAs chain. The achieved signal wavelength bandwidth after 10,000-km transmission was 12 nm. We successfully transmitted 170 Gbit/s (325. 332 Gbit/s) WDM signals over 9,879 km employing high alumina codoped EDFAs and Mach-Zehnder type filters with long FSRs.

With the foreseen growth of communication capacity, further capacity and flexibility enhancements are required for future transport networks. Photonic switching is expected to be a key technology to solve the potential bottleneck, which could be found in transport network nodes. This paper first explains the "Optical Fiber Freeway" concept, as an example of future transport networks. Following this, the possible optical transport network structure using photonic switching technologies, for realizing the Optical Fiber Freeway concept, is explained. An Optical CrossConnect (OXC) and optical Add/Drop Multiplexer (ADM) are key components. Examples of recent development of photonic switching systems toward these targets are also reviewed. An OXC using photonic Space-Division (SD) switching technology has been proposed and demonstrated. This type of OXC will realize flexible reconfiguration and optical hitless switching, and it can meet the introduction of Wavelength Division Multiplexing (WDM) technique. Line failure restoration operation at 2.4Gb/s has been successfully demonstrated. An optical packet network with a slotted ring/bus structure using a wavelength address technique has been proposed as a packet/cell based optical ADM. The experimental system employs a practical media access control system as well as a fast-wavelength switched transmitter suppressing thermally induced wavelength drift. Cell communication at 622Mb/s has been demonstrated with the experimental system. These results show that hardware technologies have been developed steadily. With a future study on an all optical network management scheme, a high capacity and flexible optical network would be realized.