Optical: systems and elements – Optical amplifier – Optical fiber
Reexamination Certificate
2001-09-07
2003-03-25
Hellner, Mark (Department: 3663)
Optical: systems and elements
Optical amplifier
Optical fiber
C372S006000
Reexamination Certificate
active
06538807
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an amplification optical fiber for amplifying signals of a plurality of channels, a fiber optic amplifier including the amplification optical fiber, an optical transmitter including the fiber optic amplifier, and an optical communication system including the optical transmitter.
2. Related Background Art
A fiber optic amplifier is an optical device for amplifying multiplexed signals (WDM signals) of a plurality of channels with different wavelengths at once through an amplification optical fiber to which pumping light is supplied in WDM (Wavelength Division Multiplexing) transmission, and is provided in an optical transmitter, relay device, or optical receiver in an optical communication system. As a general amplification optical fiber, a silica-based optical fiber (EDF: Erbium-Doped Fiber) having a core region doped with Er is known. A fiber optic amplifier (EDFA: Erbium-Doped Fiber optic Amplifier) using such an EDF as an amplification optical fiber amplifies signals in the C band (wavelength band: 1,530 to 1,560 nm) or L band (wavelength band: 1,570 to 1,600 nm) using pumping light having a wavelength of 980 nm or 1,480 nm.
An EDFA for amplifying C-band signals is described in, e.g., reference 1 “Tashiro et al., “High-Power constant polarization mode dispersion-shifted Er-doped optical fiber amplifier”, 1996, IEICE General Conference, C-330” or reference 2 “Izoe et al., “980 nm Pumping Polarization Holding Er-Doped Optical Fiber”, 2000, IEICE General Conference, C-3-26”. Each of the EDFAs described in these references uses, as an amplification optical fiber, an EDF having a polarization maintaining structure for maintaining the polarized state of propagated light and amplifies C-band signals without changing the polarized state. The core region of the EDF described in reference
1
is doped not only with erbium (Er) at a concentration of 244 ppm but also with aluminum (Al) at a concentration of 600 ppm. The core region of the EDF described in reference
2
is doped not only with Er at a concentration of 1,210 wt.ppm but also with Al at a concentration of 11,600 wt.ppm.
Such an EDFA for amplifying signals while keeping the polarized state unchanged is arranged in, e.g., an optical transmitter which sets and outputs WDM signals of a plurality of channels in one of two polarized states that are orthogonal to each other. In this case, the EDFA is used to compensate for a transmission loss in a dispersion compensator in the optical transmitter, and the necessary gain is generally relatively as small as about 15 dB. Even when the EDFA has wavelength dependence of the gain (gain uniformity), the levels of WDM signals output from the optical transmitter are adjusted to a predetermined level by adjusting the output levels of light sources for outputting the signals of the channels. Hence, a gain equalizer is not always needed.
SUMMARY OF THE INVENTION
The present inventors examined the above-described prior arts and found the following problem. In the conventional optical transmitter, the output level of a general DFB-LD (Distributed FeedBack Laser Diode) used as a light source is adjusted by adjusting the supplied current value, though the output waveform changes simultaneously. In D-WDM (Dense Wavelength Division Multiplexing) transmission, the allowable variation width of a signal wavelength per channel is generally 0.1 nm, and a corresponding adjustable width of the output level of a DFB-LD is 2 dB.
Hence, in such D-WDM transmission, an EDFA installed in an optical transmitter preferably has a small gain uniformity (a sufficient gain spectrum flatness is ensured in the use wavelength band) and more preferably has a gain uniformity of 2 dB or less. In this case, a gain equalizer must be arranged in the EDFA. However, a gain equalizer having a polarization maintaining function is expensive and also degrades the polarization extinction ratio.
The present invention has been made to solve the above-described problem, and has as its object to provide an amplification optical fiber (EDF) which requires no gain equalizer and has a small gain uniformity (flat gain spectrum) between signal channels and high polarization extinction ratio, a fiber optic amplifier (EDFA) including the amplification optical fiber, an optical transmitter including the fiber optic amplifier, and an optical communication system including the optical transmitter.
An amplification optical fiber according to the present invention is a silica-based optical fiber for amplifying WDM signals of a plurality of channels with different wavelengths at once upon receiving pumping light. The amplification optical fiber has a polarization maintaining structure for maintaining the polarized state of signals. In addition, erbium (Er) and aluminum (Al) at a concentration of 4 wt % or more are doped into at least part of the light propagation region through which the signals propagate. A fiber optic amplifier according to he present invention comprises the amplification optical fiber (amplification optical fiber according to the present invention) for amplifying signals upon receiving pumping light, and a pumping light source for supplying the pumping light to the amplification optical fiber. When pumping light (the wavelength is, e.g., 1,480 or 980 nm) is supplied to the amplification optical fiber, signals (e.g., C band or L band) input to the amplification optical fiber are amplified while keeping their polarized state.
The amplification optical fiber applied to the fiber optic amplifier according to the present invention has a polarization maintaining structure and also a structure in which Er and Al at a concentration of 4 wt % or more are doped into at least part of the core region included in the light propagation region. For this reason, a flat gain spectrum and high polarization extinction ratio can be obtained without using a gain equalizer. In addition, since this amplification optical fiber requires no gain equalizer, the manufacturing cost can be reduced.
The amplification optical fiber according to the present invention preferably has a cutoff wavelength of 1.15 &mgr;m or more. The fiber optic amplifier according to the present invention preferably amplifies signals with wavelengths included in the L band by the amplification optical fiber. In this case, even when the amplification optical fiber is wound in a coil shape and accommodated, the bending loss of the amplification optical fiber falls within the allowable range. Hence, even when the amplification optical fiber becomes long to amplify L-band signals, the fiber optic amplifier can be made compact, and any increase in bending loss can be effectively suppressed.
In the fiber optic amplifier according to the present invention, the polarization extinction ratio at a wavelength of 1.60 &mgr;m (1,600 nm) is preferably 20 dB or more, and more preferably, 25 dB or more.
An optical transmitter according to the present invention comprises a first light source system for outputting signals of a plurality of channels with different wavelengths, each signal being set in a first polarized state, and a second light source system for outputting signals of a plurality of channels with wavelengths different from those of the signals output from the first light source system, each signal being set in a second polarized state different from the first polarized state. Especially, the signal channels output from the optical transmitter are sequentially alternately assigned to the first and second light source systems such that the polarized state alternately changes from the short wavelength side to the long wavelength side so as to effectively suppress any nonlinear optical phenomenon such as four wave mixing in the transmission path. The optical transmitter has a multiplexer for multiplexing the signals output from the first and second light source systems while keeping their polarized states unchanged. The optical transmitter also has a fiber optic amplifier having the above-described struc
Kakui Motoki
Takagi Masahiro
Hellner Mark
McDermott & Will & Emery
Sumitomo Electric Industries Ltd.
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