Optical: systems and elements – Optical amplifier – Correction of deleterious effects
Reexamination Certificate
2001-11-28
2003-08-12
Black, Thomas G. (Department: 3663)
Optical: systems and elements
Optical amplifier
Correction of deleterious effects
C359S341400, C359S337000, C359S341300
Reexamination Certificate
active
06606190
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an inhomogeneity tunable erbium-doped fiber amplifier with a long wavelength gain band and method of blocking the propagation of a backward amplified spontaneous light emission in the same, and more particularly to an inhomogeneity tunable erbium-doped fiber amplifier with a long wavelength gain band and method of blocking the propagation of a backward amplified spontaneous light emission in the amplifier, which is capable of amplifying optical signals of a 1565 to 1605 nm wavelength band transmitted from a transmission line of a WDM (Wavelength Division Multiplexing) fiber transmission system, and tuning backward amplified spontaneous light emissions, thereby improving its output characteristics, such as gain inhomogeneity and a noise figure.
2. Description of the Prior Art
In order to obtain a long wavelength gain band using erbium-doped optical fiber for C-band (Conventional band) whose gain characteristic is optimized at a 1520 to 1560 nm wavelength, 80 m or more of optical fiber should be utilized. However, in this case, there occur problems that the in/out power conversion efficiency of a long wavelength band is decreased to 35% or less, and a noise figure is increased due to low population inversion.
In addition, when an optical amplifier with a long wavelength gain band is designed to amplify multi-wavelength optical signals at a long wavelength band at the same time, the gain spectrum of the erbium-doped fiber cannot be kept the same with regard to the wavelengths of multi-wavelength optical signals, and gain distortion with regard to the wavelengths of optical signals occurs due to the inhomogeneity characteristics of the erbium-doped optical fiber.
In order to overcome the above problems, D. J. DiGiovanni proposed a two-stage optical amplifier with a long wavelength band. This optical amplifier with a long wavelength band, as shown
FIG. 1
, is comprised of a first stage of a forward stimulation structure using a 980 nm stimulation laser diode, and a second stage of a backward stimulation structure using a 1480 nm stimulation laser diode (see U.S. Pat. No. 5,430,572, filed on Jul. 4, 1995). An optical isolator is positioned between the first stage and the second stage so as to completely block (or isolate) backward amplified spontaneous light emissions that is generated at the second stage and is being propagated to the first stage. The first amplification stage performs amplification using a comparatively short length of an erbium-doped optical fiber, and the second amplification stage performs additional amplification.
An optical amplifier suggested by H. Ono et al. in “Journal of Lightwave Technology, vol. 17”, as shown in
FIG. 1
, has a two-stage amplification arrangement of a hybrid type. The first amplification stage is stimulated using a 980 nm wavelength laser diode, and the second amplification stage is stimulated using a 1480 nm wavelength laser diode. At the first amplification stage, a short length of erbium-doped optical fiber is stimulated by the 980 nm wavelength laser diode so as to decrease a noise figure, while at the second amplification stage, a longer length of erbium-doped optical fiber is stimulated by the 1480 nm wavelength laser diode so as to increase efficiency.
With reference to
FIG. 1
, a conventional erbium-doped optical fiber amplifier with a long wavelength gain band is described in detail. This conventional amplifier comprises a first stage of a forward stimulation structure using the 980 nm stimulation laser diode
106
for the generation of stimulation light and a second stage of a backward stimulation structure using the 1480 nm stimulation laser diode
108
for the generation of stimulation light. An input optical signal
101
generated at the first stage and stimulation light emitted from the 980 nm laser diode
106
are coupled to each other through a light-isolation type wavelength multiplex optical coupler (IWDM coupler)
105
having a function of isolating light. In order to separate an optical signal output
104
and stimulation light emitted from the 1480 nm laser diode
108
at the second stage and to block backward optical signals being propagated from an output end to the second stage, a wavelength multiplex optical coupler (IWDM coupler)
109
having a function of isolating light is also employed. In this case, the wavelength multiplex optical coupler
109
of the second stage has an insertion loss of 0.3 dB, thereby limiting the increase of a noise figure due to an insertion loss resulting from a front insertion of the wavelength multiplex optical coupler
105
at the first stage.
The optical isolator
107
positioned between the first amplification stage and the second amplification stage blocks a backward amplified spontaneous light emission, which is generated at the second stage and is being propagated to the first stage. That is, the optical isolator
107
allows light being propagated from an output point
102
of the first stage to an input point
103
of the second stage to pass therethrough, and blocks light being propagated from the input point
103
of the second stage to the output point
102
of the first stage. An erbium-doped optical fiber
110
having a cut-off wavelength of 895 nm, which is effective for 980 nm stimulation light, is employed as a gain medium at the first amplification stage. Additionally, an erbium-doped optical fiber
111
, which has a cut-off wavelength of 1310 nm and an erbium-doped density of 1000 ppm, is effective for the 1480 nm stimulation light and has high in/output conversion efficiency of 38 to 50% at a long wavelength band, is employed as a gain medium at the second amplification stage.
The 980 nm stimulation light is completely absorbed into the erbium-doped optical fiber
110
at the first amplification stage, so the stimulation light does not exist at the output end
102
of the first stage. The backward amplified spontaneous light emission generated from the second amplification stage should includes a 90% backward amplified spontaneous light emission of a 1560 to 1600 nm band in order that a ratio of 1560 to 1600 band intensity and 1520 to 1560 nm band intensity of the emission is 90:10.
In the optical amplifier with a long wavelength gain band shown in
FIG. 1
, the spontaneous light emission backwardly propagated from the second amplification stage to the first stage is blocked by the optical isolator
107
, so the noise figure and inhomogeneity characteristics of the optical amplifier with a long wavelength gain band are improved and high power output and high efficiency are achieved.
FIGS. 2
to
5
are graphs showing the variations of characteristics in the cases where the backward amplified spontaneous light emission is blocked by the optical isolator in the two-stage optical amplifier with a long wavelength gain band constructed as shown in
FIG. 1
, and it is not blocked.
FIG. 2
is a graph showing in/out power conversion efficiency according to an input light signal. A solid line in
FIG. 2
represents the case where the backward-propagating spontaneous light emission is blocked, and a dotted line in
FIG. 2
represents the case where the backward-propagating spontaneous light emission is not blocked.
Referring to
FIG. 2
, when the intensity of an optical signal input is low, the in/out power conversion efficiency is increased in the case where the backward amplified spontaneous emission is blocked; whereas when the intensity of an optical signal input is increased, in/out power conversion efficiency is gradually decreased in the case where the backward amplified spontaneous light emission is blocked, and the efficiency is gradually increased in the case where the backward amplified spontaneous light emission is unblocked. When the backward amplified spontaneous emission is blocked, it has a problem that the in/out power conversion efficiency is lower as the intensity of optical signal input is higher.
FIG. 3
is a graph showing the variation of a noise figure accordin
Chu Moo Jung
Kim Kwang Joon
Lee Jong Hyun
Lee Jyung Chan
Black Thomas G.
Blakely & Sokoloff, Taylor & Zafman
Korea Electronics & Telecommunications Research Institute
Sommer Andrew R.
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