This paper reviews long optical reach and large capacity transmission which has become possible because of the application of wide-band and low-noise optical fiber amplifiers and digital coherent signal processing. The device structure and mechanism together with their significance are discussed.

A bidirectional amplification module has been proposed for use in IP-over-CWDM networks. The module is based on a bidirectional erbium-doped fiber amplifier. The loss compensation characteristics of the module obtained in a bidirectional IP transmission experiment confirmed that the losses of the optical node and the transmission fiber in the network can be compensated for effectively by the module making it possible to increase the number of nodes and the total fiber length of the network.

The oscillation characteristics of a 40 GHz, 1-3 ps regeneratively and harmonically mode-locked erbium-doped fiber laser have been investigated in detail with respect to stability, linewidth, and mode hopping. We show that because the Q value of the microwave filter in the feedback loop is limited to around 1000, which is almost the same as that in a 10 GHz laser, the cavity length should not be greatly increased as this would result in as much as a fourfold increase in the number of longitudinal beat signals. We undertook a detailed stability analysis by using three cavity lengths, 60, 80, and 230 m. The 80 m long cavity greatly improved the long-term stability of the laser because the supermode noise was suppressed and there were not too many longitudinal modes. We measured the linewidth of the longitudinal mode of the laser using a heterodyne method, and it was less than 1 kHz. We also point out that there is a longitudinal mode hopping effect with time that is induced by very small changes in temperature.

We propose and demonstrate a novel method to measure the polarization mode dispersion (PMD) of optical devices. The device under test (DUT) is installed in a fiber laser cavity which can operate at multiwavelength. PMD can be evaluated by the wavelength spacing of the multiwavelength laser output spectrum. In our method, the maximum extrema wavelength is easier to be identified than in the conventional fixed-analyzer (FA) method. We measure the PMD of polarization maintaining fibers (PMFs) and the ITU-T round robin KDD samples.

We describe the width conversion of an optical signal by using an erbium-doped fiber and an asymmetric optical circuit. The width of an optical signal was measured to be a respective 350 nsec and 200 nsec for a 70 m and 40 m fiber (Lf). The width of the pumping pulse was 5 nsec and the length of erbium-doped fiber was 3 m. We also extended the optical signals to a respective 300 nsec and 150 nsec wide at a pumping pulse 10 nsec by inserting a 60 m and a 30 m fiber (Lf) inside a circuit.

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.

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.

The dependence of the output characteristics of a regeneratively and harmonically FM mode-locked erbium-doped fiber laser on intracavity dispersion have been investigated by changing the group velocity dispersion (GVD) of the fiber. It is shown that a stable pulse train can be obtained only when the GVD of the cavity is anomalous in the presence of self-phase modulation (SPM). The shortest pulse obtained was 2. 0 ps at a repetition rate of 10 GHz.

Optical transmission systems with large capacity employing wavelength-division multiplexing (WDM) techniques are now widely under development. Optical amplifiers, especially Erbium-Doped Fiber Amplifiers (EDFA's), are vital components for such transmission systems. Optical amplifiers in WDM systems are employed as common amplifiers for all WDM'ed optical carriers, therefore, change in power of a specific carrier gives rise to gain fluctuation of the remaining carriers. In this paper, we discuss about automatic gain control (AGC) of EDFA for WDM'ed optical carriers under transient gain saturation. Two methods have been reported to perform AGC, i.e., pump feedback control method and compensation light feedback control method. Theory and experimental results have been already reported on pump feedback control method. Here, theory has been generalized to be applicable for compensation light feedback method including schematics with amplified spontaneous emission (ASE) as a probe light to measure the gain of EDFA. Experimental results have confirmed the analysis. Good performance has been obtained for both methods with simple electronic circuits and ASE has been found to work as an excellent probe light source.

Experimental optical gain characteristics of an erbium-doped fiber amplifier have not been explained well by conventional laser schemes in the case of two-channel amplification. Modified simple laser schemes including cross relaxation among degenerate levels were valid for the explanation of the optical gain dependence on input signal power and on the erbium-doped fiber length.

Optical amplifier structures suitable for a 622Mbit/s repeater in an optical communication system containing one in-line amplifier have been investigated. Two wavelengths of 1.533µm and 1.549µm are considered for two cases, i.e., single-channel transmission and two-channel wavelength division multiplexing transmission. The basic amplifier structure is of a two-stage type where forward pumped and backward pumped erbium-doped fibers are connected with each other through intermediate optical filters and an optical isolator. First, the effect of the intermediate optical filters was clarified in optical gain and bit error rate characteristics. Then, the erbium-doped fiber length was optimized on the basis of the allowable optical loss of the optical system which was operated at a bit error rate of 10-9. As a result, the appropriate length of the forward pumped erbium-doped fiber was found to be about 20m for both cases of single-channel and two-channel wavelength multiplexing amplifiers. With the designed amplifier used in the system, the calculated allowable optical line loss was more than 90dB for both the cases.

By optimizing the structure of erbium-doped fibers, high efficiency such as a gain coefficient of 6.3dB/mW, or a slope efficiency of 92.6% have been realized with very flat wavelength dependence. Though the optimized structure has high NA, the splice loss with standard fibers can be lowered by the additional arc technique. The carbon coated fiber with a fatigue parameter over 150 guarantees the reliability, even when wounded on a small coil. In-line isolators and WDM couplers have been also developed. An amplifier module has been assembled, resulting in an output power more than +16dBm owing to the high performance of each component.

Noise properties in cascaded erbium-doped fiber amplifiers used in subcarrier multiplexed analog video distribution systems are experimentally examined. The noise dependency on signal wavelength is measured for a 4 stage EDFA cascade, and it is shown that an optical narrow bandpass filter is not necessary after each fiber amplifier for signal wavelength of 1.5511.560µm and that optical bandpass filters are necessary for shorter wavelength than 1.551µm to avoid the noise degradation by spontaneous-spontaneous beat noise. Finally, the attainable distribution loss is estimated for AM and FM video distribution systems.