Hybrid fiber amplifier using dispersion compensating raman amplifier

The invention relates to a hybrid fiber amplifier in which a dispersion compensating Raman amplifier is associated with an erbium doped fiber amplifier to enhance the amplifier efficiency. It is an object of the invention to provide a dispersion compensating Raman amplifier(DCRA) by inducing Raman pump light into a dispersion compensating fiber to obtain a Raman gain in which a depolarizer is used to eliminate the pump light polarization dependent Raman gain, and a hybrid fiber amplifier in which the DCRA is associated with an erbium doped fiber amplifier(EDFA) to enhance the efficiency. The hybrid fiber amplifier using a dispersion compensating amplifier comprises dispersion compensating Raman amplifier unit for performing a dispersion compensating amplification to an incident optical signal by launching Raman pump light, which is depolarized by a depolarizer, backwardly; and fiber amplifier unit for receiving and amplifying again the optical signal amplified via the dispersion compensating Raman amplifier unit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Hereinafter, preferred embodiments of the invention will be described in detail in reference to the appended drawings. FIGS. 2A to 2 B show the structures of hybrid fiber amplifiers according to embodiments of the invention. FIG. 2A is a hybrid fiber amplifier of the invention, which is a simple structure comprised of a DCRA 210 and an EDFA 220 only. As shown in FIG. 2 A, the hybrid fiber amplifier of the invention has the DCRA 210 for performing a dispersion compensating amplification by using Raman pump light which is depolarized by a depolarizer, and the EDFA 220 for amplifying again the light signal from the DCRA 210 . Here, the EDFA 220 associated with the DCRA 210 in use is structured to pump in forward direction by a laser diode of 980 mm wavelength. The DCRA 210 is arranged in front of the EDFA module 220 to construct the hybrid fiber amplifier for measuring the gain and noise figure (refer to FIG. 4 ), and used as an in-line amplifier to perform a 160 km transmission experiment also (refer to FIG. 5 and FIG. 6 ). In the foregoing experiment, 16 channels having 0.8 nm spacing, which are used for a 160 Gb/s optical transmission system, are used as signal optical sources, the intensity of the inputted optical signal is −4.5 dBm(−16.5 dBm/ch) in the middle of −7 to −2 dBm which is the range of the in-line amplifier inputting optical intensity in the 160 Gb/s optical transmission system. In FIG. 2B, a short length erbium doped fiber (hereinafter referred to as EDF) 230 of about 3 m is connected to the leading end of the amplifier to enhance the noise figure. In other words, as shown in FIG. 2 B, the hybrid fiber amplifier of the invention has the EDF 230 for obtaining a high population inversion at the leading end of the optical amplifier, a DCRA 210 for obtaining a dispersion compensating amplification to a signal from the EDF 230 by using Raman pump light which is depolarized by a depolarizer, and an EDFA 220 for amplifying again the optical signal from the DCRA 210 . As can be seen in FIG. 4 and FIG. 6 , the amplifier in FIG. 2B shows a gain similar to that of the amplifier in FIG. 2 A, and has the noise figure improved about 0.7 to 1.0 dB, and it can be seen that the BER is almost same as in FIG. 2A . FIG. 3 shows the structure of a DCRA 310 using a depolarizer according to an embodiment of the invention. As shown in FIG. 3 , the DCRA 310 using the depolarizer of the invention has the depolarizer 313 for eliminating polarization dependence of incident pump light for a Raman gain, a coupler 312 for inputting the Raman pumped optical signal in backward direction from the depolarizer 313 , and a dispersion compensating fiber module or DCFM 311 for amplifying the inputted optical signal. The DCRA 310 is comprised of the DCFM 311 actually used in a WDM optical transmission system, and one 1480 nm laser diode used as a Raman pumping light source. In the invention, to obtain a gain about a signal in 1550 nm regime, the temperature of the pumping laser diode is adjusted so that the center wavelength is adjusted toward the short wavelength of 1465 to 1470 nm. Also, to eliminate the pump light polarization dependence of the Raman gain, a Lyot type fiber depolarizer 313 is used in the invention, and the pump light is incident in backward direction to eliminate the gain fluctuation according to the pump light intensity fluctuation. Meanwhile, for the incident optical signal, dispersion compensation is carried out in the DCFM 311 , where the pump light 314 for the Raman gain which is incident backwardly via the coupler 312 is coupled for amplification. However, such a Raman amplifier of the related art has an unstable gain due to the polarization dependence of the incident Raman pump light 314 . Therefore, for the stable gain of the amplifier, the Raman pump light 314 is conducted to pass through the depolarizer before passing through the coupler 312 in the invention so that stable amplified signal can be obtained. FIG. 4 A and FIG. 4B show measured results of the gains and the noise figures of the hybrid fiber amplifiers according to the embodiments of the invention. As shown in FIG. 4 A and FIG. 4 B, in comparing the gains and noise figures of the amplifiers in FIGS. 2A and 2B of the invention with those of a two stage amplifier of the related art which is actually used in the 160 Gb/s optical transmission system, the gains of the invention have almost the same amount and flatness as those of the related art, and the noise figures of the invention are about 7 dB, which are larger about 1.5 to 2 dB than that of the related art but within the range desired in the transmission system. FIG. 5 is a view for showing an experimental schematic for measuring the BER of the hybrid fiber amplifier of the invention. In the 160 km transmission experiment as shown in FIG. 5 , each of amplifiers used at the position of an in-line amplifier for the BER measurement for the 16 signal channels. Measured results of the BER by the above experiment are shown in FIGS. 6A and 6B . FIGS. 6A and 6B show the results of the BER measurement in a channel obtained by the 160 km optical transmission experiment by using 160 Gb/s WDM optical signal to the fiber amplifier of the invention. In the transmission experiment, graphs shown in FIGS. 6 A and 6 B are measured BER values of the corresponding amplifiers for any two channels of the 16 channels used in the experiment, in which the hybrid fiber amplifier in FIGS. 2A or 2 B of the invention show the BER that is almost the same or slightly enhanced compared with the two stage EDFA of the related art. In other words, in comparing with previous two stage EDFA, the hybrid fiber amplifiers of the invention have the simpler structure in which the EDFA in the leading end is omitted while the performance thereof is almost the same as that of the two stage EDFA. While the invention has been described in reference to the preferred embodiments and the appended drawings, it is apparent to those skilled in the art that various modifications, changes and equivalents can be made within the scope of the invention.