Nobuyuki Kagi

Temperature dependence of the gain in erbium-doped fibers

The temperature dependence of the gain in erbium-doped fibers between -20 and +85 degrees C is discussed. The characteristics of three types of erbium-doped fibers with different erbium concentrations and glass compositions are investigated. The gain decreased with increasing temperature at the rate of -0.07 dB/ degrees C when pumped at 1.48 mu m. The change in the gain was small when pumped at 0.98 mu m. By measuring the change in the absorption and fluorescence spectra with temperature, it is deduced that the change in the gain is mainly due to the change in the saturated population inversion. >

Efficient optical amplifier using a low-concentration erbium-doped fiber

A gain coefficient of 3.8 dB/mW was achieved for an erbium-doped fiber amplifier pumped by a 1.48 mu m laser diode. The main reasons for the improvement are high NA (0.23) and low concentration (43 p.p.m.). Pump-to-signal energy conversion efficiency was 18% at 3 dB gain compression. A decrease in saturation power with increasing erbium concentration was also demonstrated. In high-concentration fiber, fluorescence at 0.98 mu m due to cooperative upconversion was detected. These results indicate that several kilometers of distributed fiber amplifier with high gain and high output saturation power could be possible, because the absorption coefficient at 1.48 mu m is still two orders higher than the background loss in the 43 p.p.m. fiber.<<ETX>>

Optical burst amplification using EDFA with fast feedback control

We show that optical bursts can be amplified with suppressed gain transients using an erbium-doped fiber amplifier controlled by a fast electrical feedback loop. Transmission without power penalty is demonstrated for various burst lengths.

622 Mb/s, 144 km transmission using a bidirectional fiber amplifier repeater

A successful 144.2-km transmission of a 622-Mb/s signal through standard single-mode fibers using a bidirectional fiber amplifier repeater is demonstrated. The bit-error-rate measurement showed neither sensitivity degradation nor BER floor in the bidirectional transmission, suggesting a great potential of system improvement and capacity increase of the existing system by the use of the repeater.<<ETX>>

Gain characteristics of ER/sup 3+/ doped fiber with a quasi-confined structure

The fiber-structure dependence of the gain characteristics of Er/sup 3+/ doped fibers pumped at 1.48 mu m is analyzed. The optimum V value is derived theoretically and experimentally. For step-index fibers, the optimum V value is 1.6, which is smaller than that needed to minimize spot size. The fiber with a small V value enjoys a large Er/sup 3+/ confinement effect. For laser diode pumping, an efficiency of 1.7 dB/mW is achieved at 1.536 mu m. The bending characteristics are also described. >

WDM burst mode signal amplification by cascaded EDFAs with transient control

We demonstrate that WDM burst signals are amplified with suppressed gain transients in cascaded erbium-doped fiber amplifiers (EDFAs) with fast electrical feedback control. In a recirculating loop experiment, the gain excursion is suppressed to 5.1 dBp-p after 45 EDFAs while the excursion is 28 dBp-p after 5 EDFAs without the gain control. Spectral hole burning is a main limiting factor for the transient suppression. The effect of the gain control is also verified in a transmission experiment at 10 Gb/s.

Tunable laser source for fast wavelength switching using a short-cavity DBR laser packaged with wavelength locker

Simple switching scheme of frequency with precise feedback control of robust wavelength locker has been presented for a short-cavity DBR laser array. Switching time was less than 13 musec within +/- 2.5GHz from ITU grid.

Gain competition in multichannel optical communication systems using erbium-doped fiber amplifiers

Abstract Gain competitions in saturated erbium-doped fiber amplifiers are investigated experimentally in a two-channel system. The experimental results show that, unlike the case of single-carrier amplification, in case of multichannel amplification, large gain compression appears at short wavelength region of the gain profile due to the energy conversion, and that a flat gain profile normally cannot be obtained for saturated amplifiers.

Widely tunable lasers based on mode-hop-free semiconductor laser array

Integration of mode-hop-free tunable laser array and a semiconductor optical amplifier is most reliable approach to realize widely tunable lasers. We have developed two types of tunable lasers, one is a thermally tunable DFB laser array for DWDM tunable transponders, which has shown high power and wide tunability covering Cband or L-band, housing in compact butterfly packages with robust wavelength locker. Another is a short-cavity DBR laser array for optical burst switching, whose lasing frequency can be monotonously tuned and locked on the ITU grid within 5 microseconds. Both lasers have demonstrated superior performances in system experiments.

Programmably repetition tunable low pedestal femtosecond pulse train generation using comb-like profiled fiber

We demonstrated programmably repetition tunability from 5 MHz to 500 MHz on low pedestal 300 fs pulse train generation through the compression of gain-switched laser diode pulse by comb-like profiled fiber with normal-dispersion Er doped fiber.