Optical: systems and elements – Optical amplifier – Particular active medium
Reexamination Certificate
2001-10-15
2002-11-26
Hellner, Mark (Department: 3662)
Optical: systems and elements
Optical amplifier
Particular active medium
C359S341420
Reexamination Certificate
active
06487008
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a light amplifying device employing a semiconductor optical amplifier. More particularly, the present invention relates to a light amplifying device equipped with functions (ALC: Automatic Level Control, APC: Automatic Power Control), which control the level/power of output signal light to a constant value, and which can be used in a wavelength division multiplexed communication system.
Recently, development of a wavelength division multiplexed communication system, which will realize transmission through an optical fiber cable by multiplexing plural signal light of different wavelengths, has advanced to meet the increasing demands on communication systems. In a wavelength division multiplexed communication system, many optical components are used for light combination and light division, therefore, a light signal is attenuated by the losses of each optical component. To compensate for such losses, a light amplifying device is used.
Compared to the conventional optical fiber communication system, the wavelength division multiplexed communication system needs many more light amplifying devices, therefore, a light amplifying device must be compact and have low power consumption. Moreover, such a light amplifying device needs to have a large dynamic range to handle large variations in power level of the input signal light and functions (ALC/APC functions) to control the level/power of the output signal light to a constant value. Among various types of light amplifying devices, a semiconductor optical amplifier (SOA) is compact and has a low power consumption and, therefore, attracts interest as a light amplifying device to compensate for losses in the wavelength division multiplexed communication system.
To realize the ALC function in a light amplifying device employing a semiconductor optical amplifier, a method may be used in which electric current supplied to the semiconductor optical amplifier is changed to vary the gain of the semiconductor optical amplifier, but this method will bring about a problem in that the saturation level of the output light varies if the electric current supplied to the semiconductor optical amplifier is changed, and distortion of the signal occurs due to the pattern effect. Therefore, a structure is employed, in which the semiconductor optical amplifier is driven under a fixed condition almost near the upper limit so that the amplification factor is maintained constant, an attenuator to attenuate light input to the semiconductor optical amplifier is provided, and light is input to the semiconductor optical amplifier, after being attenuated by the attenuator, so as to have a constant power.
FIG. 1
is a diagram that shows the structure of an example conventional light amplifying device that employs a semiconductor optical amplifier and has an ALC function. As shown schematically, the output light of a modulator integrated DFB laser diode (MI-DFB-LD)
11
is modulated by the signal from a modulation signal source
12
. The light output from the MI-DFB-LD
11
is amplified to a fixed power by a light amplifying device
21
.
The optical amplifying device
21
comprises an attenuator (Att)
23
, which attenuates the input light and the attenuation factor of which can be changed, a divider
24
, which divides the output of the attenuator
23
in the ratio, for example, of 10:1, a power meter
25
, which detects the power of the light of lower strength divided by the divider
24
, a control unit
26
, which controls the attenuation of the attenuator
23
according to the light detected by the power meter
25
, and a semiconductor optical amplifier (SOA)
22
, which amplifies the light of larger strength divided by the divider
24
. The SOA
22
is driven under a fixed condition.
Next the ALC operation in the light amplifying device in
FIG. 1
is described with reference to FIG.
2
.
When modulated signal light is amplified in the SOA
22
, it is necessary to set the average light output power of the SOA
22
lower than the saturated light output by a few dB in order to avoid signal distortion due to the pattern effect based on the gain saturation, in which the output light is saturated. Here, for example, it is set 5 dB lower than the saturated light output PS (dBm). Therefore, the permissible maximum value PSM (dBm) of the average light output power of the SOA
22
is shown as follows.
PSM
(dBm)=
PS
(dBm)−5dB (1)
When the level of the output signal light of the light amplifying device is controlled to be constant by the ALC function, it is desirable that the target level is as large as possible, therefore, the target level is set to PSM (dBm).
As described above, the drive condition of the SOA
22
is fixed and, because the input light is amplified with a fixed gain Gs (dB), the permissible maximum value PSIM (dBm) of the average light input power to the SOA
22
is show as follows.
PSIM
(dBm)=
PSM
(dBm)−
Gs
(dB) (2)
Therefore, if the average light input power PSIM (dBm) to the SOA
22
is constantly adjusted so as to be PSIM (dBm) by the variable attenuator
23
, the level of the output signal light of the SOA
22
is constantly a fixed PSM (dBm).
The lower limit PIMIN (dBm) of the average light input power to the light amplifying device
21
, when no attenuation is carried out by the attenuator
23
, is shown by the following expression, the basic loss LA
1
of the attenuator
23
and the loss LD
1
of the optical divider
24
being taken into account.
PI
MIN (dBm)=
PSIM
(dBm)+
LA
1
+
LD
1
(3)
The dynamic range &Dgr;PIN (dB) of the light amplifying device
21
is determined by the maximum quantity of attenuation LATM (dB) of the attenuator
23
.
&Dgr;
PIN
(dB)=
LATM
(dB) (4)
Therefore, the upper limit PIMAX (dBm) of the average light input power of the light amplifying device
21
is determined by the following expression.
PI
MAX (dBm)=
PI
MIN (dBm)+&Dgr;
PIN
(dB) =
PSIM
(dBm)+
LA
1
+
LD
1
+
LATM
(dB) (5)
From the standpoint of generality, it is preferable that a light amplifying device can be used commonly for signal light of various wavelengths. For example, when combining plural types of signal light of different wavelengths transmitted from a transmitter, after each signal light is amplified to a fixed value, respectively, or when recombining plural types of signal light of different wavelengths received by a relay device, after each signal light is divided and amplified to a fixed value, individually, it is troublesome to use plural different light amplifying devices according to each wavelength, or to set different conditions even if a single light amplifying device is used.
The light amplifying device shown in
FIG. 1
can provide light output of a fixed level as long as the wavelength of the signal light is fixed. Because the gain of the SOA
22
has wavelength dependence, however, levels of the output signal light of the light amplifying device vary depending on the wavelength of signal light, even though the average light input power to the SOA
21
is controlled to be constant in the structure in FIG.
1
. Therefore, in the structure in
FIG. 1
, if only the average light input power of the signal light is monitored and the wavelength of the signal light is not monitored, the average light output power does not remain constant, and the average light output power of the signal light varies depending on the wavelength by the difference &Dgr;Gs (dB) between the maximum gain GsH (dB) and the minimum gain GsL (dB) of the SOA
22
in the range of the used wavelength, as shown below.
&Dgr;
Gs
(dB)=
GsH
(dB)−
GsL
(dB) (6)
In order to keep the average light output power constant even when the wavelength varies, a mechanism is needed, which detects the output of the SOA and attenuates the output of the SOA according to the detected value.
FIG. 3
shows an example of a structure, in which mechanisms that detect the level of the s
Fujitsu Limited
Hellner Mark
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