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.

We experimentally investigated the impact of the mode filtering technique on the performances of pulse amplification in a fiber with a large core diameter. The technique was applied to a femtosecond pulse amplifier, and was based on a large area double-clad Yb-doped fiber. The mode filtering enabled selective excitation of the lowest transverse mode with minimal contamination of higher order modes. The output pulses with 110 fs duration, > 30 nJ pulse energy (> 3 W average power), and clean spatial/temporal profiles were successfully generated. Benefits of this technique are also discussed.

To obtain an ultra-short high-intensity pulse source, we investigated the amplification characteristics of two types of pulses (dissipative soliton and stretched pulses) produced by our Yb-doped fiber laser oscillator. Our results show that the dissipative soliton pulse can be amplified with less deterioration than the stretched pulse.

We propose a new configuration for a parallel fiber amplifier that can amplify both the C- and L-bands simultaneously by employing bundled Er3+-doped fiber (EDF). The bundled EDF is a candidate amplification medium for multi-core optical fiber amplifiers. Our parallel fiber amplifier is another application of the multi-core amplification medium. The amplifier achieves almost the same signal gain of 20 dB for both the C- and L-bands by using a bundled EDF, which is realized by bundling seven identical single-core EDFs.

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.

This paper describes a low-loss submarine optical fiber cable for a long-distance submarine repeaterless transmission system that employs remote pumping. The features of this system are that it has an increased signal power budget and is cost effective and easy to maintain. First, we investigated the relationship between the signal and pump losses and the Raman gain efficiency of optical fiber needed to achieve a submarine repeaterless transmission system operating at 2.5 Gbps and over a distance exceeding 370 km. We manufactured a submarine optical fiber cable based on the results and confirmed that it had low-loss characteristics. Second, we evaluated the long-term loss stability of the optical fiber with a high-power continuous wave (CW) laser light as the pump source. We confirmed that the loss remained unchanged after 1900 hours of exposure to 8 W CW laser light at a wavelength of 1.48 µm. This submarine optical fiber cable is being employed in a commercial submarine repeaterless transmission system between Okinawa and Miyakojima.

We design and demonstrate a high repetition-rate similariton generation system using normal dispersion fiber amplifiers (NDFA's). We numerically calculate the pulse evolution in NDFA's and clarify the condition to generate similariton pulses in a finite-length NDFA. Then we design the similariton generation system in consideration of the use of Erbium-doped fibers (EDF's) and show that a km-long fiber amplifier with low normal dispersion can generate a high repetition-rate similariton train from practical pico-second pulse sources. In the experiment, we demonstrate a 10-GHz similariton source using a 1.2-km-long EDF. For application to multi-wavelength light sources, we measure the bit-error rate of the spectrally sliced similariton, and show that it exhibits low-noise performance, which is attributed to the spectral flatness.

In this paper, a detailed model of a hybrid dual-stage Raman/erbium-doped fiber (EDF) amplifier is presented. This model takes into account the impact of double Rayleigh backscattering (DRB) noise, amplified spontaneous emission (ASE) noise and Kerr-nonlinearity induced impairments in the amplification process. Using this model, we present a comprehensive analysis of the operation of hybrid dual-stage Raman/EDF amplifiers under above impairments. We show that under fixed total gain conditions for the amplifier module, high Raman gain causes the introduction of increased DRB noise to the amplified signals whereas low Raman gain causes the introduction of high ASE noise power through EDF amplifier. Therefore a balance between the Raman amplifier gain and EDF amplifier gain is required for optimal operation. These observations are then combined to show an optimization process, which could be applied to improve the design of hybrid dual-stage Raman/EDF amplifiers.

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 proposed a simple technique for measuring the effective zero-dispersion wavelength. In this study, we measured the effective zero-dispersion wavelength of a 25-km-long dispersion-shifted fiber (DSF) using the four-wave mixing (FWM) of a spectrum-sliced fiber amplifier light source, and then compared our results with other conventional techniques to confirm the validity of our method.

An improvement of the fiber-optic transceiver having both transmitter and receiver functions of optical time-domain reflectometers is examined. The improvement is achieved by introducing an external optical amplifier without changing the previously reported configuration. The characteristics of the transmitted Q-switched pulse and the receiver gain is studied theoretically and experimentally to estimate the performance improvement. It is found that the introduction of the external optical amplifier is a simple and effective way to the performance improvement.

A novel 1470-nm-band (S+ band) wavelength-division multiplexing (WDM) transmission system is described. The first advantage of S+-band transmission is suppression of degradation caused by four-wave mixing (FWM), which has been the dominant impairment factor in WDM transmission systems on dispersion-shifted fibers (DSFs). FWM suppression by using the S+ band instead of the conventional 1550-nm-band (M band) is successfully demonstrated. The second advantage is expansion of the usable bandwidth by using the S+ band together with other wavelength bands. A triple-wavelength-band WDM repeaterless transmission experiment using the S+ band, the M band and the L band (1580-nm-band) is conducted over DSF, and it is shown that degradation due to inter-wavelength-band nonlinear interactions is negligible in the transmission. Moreover, the transmission performance of an S+-band linear repeating system is estimated by computer simulation, and compared with that of other wavelength-band systems. In the experiments, thulium-doped fiber amplifiers (TDFAs) are used for amplification of signals in the S+ band.

A novel 1470-nm-band (S+ band) wavelength-division multiplexing (WDM) transmission system is described. The first advantage of S+-band transmission is suppression of degradation caused by four-wave mixing (FWM), which has been the dominant impairment factor in WDM transmission systems on dispersion-shifted fibers (DSFs). FWM suppression by using the S+ band instead of the conventional 1550-nm-band (M band) is successfully demonstrated. The second advantage is expansion of the usable bandwidth by using the S+ band together with other wavelength bands. A triple-wavelength-band WDM repeaterless transmission experiment using the S+ band, the M band and the L band (1580-nm-band) is conducted over DSF, and it is shown that degradation due to inter-wavelength-band nonlinear interactions is negligible in the transmission. Moreover, the transmission performance of an S+-band linear repeating system is estimated by computer simulation, and compared with that of other wavelength-band systems. In the experiments, thulium-doped fiber amplifiers (TDFAs) are used for amplification of signals in the S+ band.

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.

We have investigated the gain and noise figure characteristics of a Nd-doped silica single-mode fiber amplifier (NDFA) at 1.06 µm which is applicable to various systems using a Nd: YAG laser light source at 1.06 µm, such as free-space laser communications, a fiber sensing system, and a lidar system. A fluorescence spectrum observation of the Nd-doped fibers with various co-dopants shows that the Nd-A1 co-doped fiber is suitable for realizing a high-gain amplifier for the 1.06-µm wavelength region. The pump wavelength tolerance at around 0.81 µm , the gain bandwidth and the sufficient value of the Nd concentration and length product for achieving maximum small signal gain are clarified. A noise figure of almost 3 dB and small signal gain of more than 30 dB are attained by 50-mW pump power. The unique four-level system characteristics, even in low pumppower conditions, provide low noise amplification in the NDFA. These gain and noise characteristics are well described by a simple theoretical model. We also demonstrated high-power operation of the NDFA with four pump LD modules adoptinga polarization-multiplexing technique. More than 100-mW signal output power is available for 1-mW signal input power at 200mW launched pump power. These features of the NDFA as a compact, polarization-independent, spatial-beam -distortion-free amplifier, will allow it to replace the solid sate laser in various applications using a Nd: YAG laser light source at 1.06µm.

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.

Fundamentals and development of PDFAs are described. Spectroscopic data of Pr3+ in a fluoride glass are presented with a view to understanding the performance of PDFA. An amplification mechanism model which explains PDFA performance is established. On the basis of the model pump schemes which efficiently extract the potential in Pr3+-doped fluoride fiber are discussed in order to construct amplifier modules. Gain characteristics of Pr3+-doped fluoride fibers are clarified. Codoping effect on pump wavelength extension is investigated. LD-pumped PDFA construction and performance are described. PDFAs are shown to be attractive device to upgrade the performance of optical systems at 1.3µm. Furthermore future approaches to PDFA research are discussed.

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.

This letter reports in detail on the temperature-dependent signal gain characteristics of Er3+-doped optical fiber amplifiers at signal wavelengths of 1.536µm and 1.552µm. The amplifiers were pumped at 0.825µm in a temperature range of 40 to 200. The signal gain for optimum length at both wavelengths stops increasing and begins to decrease at about 80. In the temperature region below 80, both signal gains increase with fiber temperature for fibers of optimum length or less. A temperature independent length aroud the optimum length is observed from 80 to 200 for both signal wavelengths. Theoretically, the temperature dependence of the signal gain characteristics rerults from the changes in fluorescence, absorption, GSA and ESA cross sections.

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.