Erbium doped optical fiber amplifier for automatically tracing and filtering wavelength of transmitted light and its operation method

An erbium doped fiber amplifier, which is equipped with an optical filter at its output port to eliminate noise caused by properties of the amplifier, automatically traces and filters transmitted light signal wavelengths using a wavelength control unit for adjusting the central wavelength of the optical filter to correspond to the wavelength of the transmitted light signal after determining the wavelength of the transmitted light signal.

CLAIM OF PRIORITY 
This application makes reference to, incorporates the same herein, and 
claims all benefits accruing under 35 U.S.C. .sctn.119 from an application 
for ERBIUM DOPED OPTICAL FIBER AMPLIFIER FOR AUTOMATICALLY TRACING AND 
FILTERING WAVELENGTH OF TRANSMITTED LIGHT AND ITS OPERATION METHOD earlier 
filed in the Korean Industrial Property Office on the 1.sup.st day of Aug. 
1996 and there duly assigned Ser. No. 1996-32235, a copy of which 
application is annexed hereto. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an optical amplifier, and more 
particularly, it relates to an erbium doped fiber amplifier (EDFA) and its 
method of operation. The EDFA provided by this invention automatically 
traces transmitted light wavelengths and adjustably filters the 
wavelengths of light signals to be transmitted by adjusting the central 
wavelength of an optical filter installed at the output terminal of the 
amplifier, thereby preventing propagation of noise caused by the 
properties of the optical amplifier. 
2. Description of the Related Art 
When a transmission terminal in an optical communication network converts 
an electric signal into a light signal, and transmits it to a desired 
destination using optical fiber, EDFA are usually used to amplify the 
weakened light signals at predetermined distances along the transit route. 
This practice of periodic re-amplification ensures the transmission of 
stable signals. Such amplifiers are also typically installed in reception 
and transmission terminals to amplify electric power and perform 
pre-amplification. 
An EDFA commonly includes a tunable filter that removes noise introduced 
into the amplified light signal during the amplification process. Such a 
filter has a central wavelength at which a light signal passed through the 
filter receives the least attenuation. Signal components at different 
wavelengths receive greater attenuation the farther those wavelengths are 
from the filter's central wavelength. When the filter is tuned for a 
central wavelength equal to the nominal wavelength of an incoming light 
signal, noise introduced by the amplifier can be efficiently eliminated 
from the amplified signal. 
However, light signals received over optical communication networks do not 
always meet nominal parameters. In practice, such a signal will vary 
according to characteristics of the various components of the optical 
amplification device that generated or boosted it. Thus, transmitted light 
signal wavelengths may change as the optical amplification device operates 
over an extended period of time. They can also be influenced by ambient 
temperatures. To compensate for these effects, a wave-length-fixing type 
filter or a convertible-to-manual filter may be utilized. However, these 
approaches generally result in a loss of signal strength when 
instantaneous changes occur in the wavelength of the received signal. 
Also, they potentially create problems by decreasing the intensity of the 
light signals generated by the amplification device. 
The use of tunable filters to remove noise introduced into optical 
communications signals by optical amplifiers is well known in the 
literature. For example, U.S. Pat. No. 4,945,531 provides a system with 
several tunable filters interposed between an optical wavelength 
demultiplexer and a multiplexer to filter spontaneous emission noise from 
around several channels in a WDM communications signal. U.S. Pat. No. 
5,644,423 also discloses an amplifier for WDM signals that automatically 
adjusts a tunable filter central wavelength to trace the wavelength of a 
selected channel for gain control. This latter system constitutes a 
significant advance in EDFA filtering technology, but it requires a 
complicated wavelength feedback system that includes a reference 
oscillator and a synchronous detector to generate a wavelength error 
signal. 
U.S. Pat. No. 5,570,221, entitled "Light Amplification Device" and issued 
Oct. 29, 1996 to Fujita, the disclosure of which is incorporated herein by 
reference, provides another substantial advance in filtering control 
technology. This patent discloses an EDFA that automatically adjusts the 
central wavelength of a tunable filter, positioned downbeam from a doped 
fiber amplification element, to trace the wavelength of the input signal 
component of an amplified signal. 
The device provides many advantages, but it has a complex structure and 
relies upon specific features found in the output signals of typical 
current fiber amplifiers to achieve its objectives. In particular, it uses 
a two level wavelength sweep procedure: in a broad sweep, it locks onto a 
desired wavelength in the amplified signal by finding the wavelength where 
a negative peak occurs in the second derivative of the amplified signal 
intensity. In a narrow sweep, the intensity peak locked from the broad 
sweep is traced by repeatedly sweeping a narrow band to find the 
wavelength therein at which the first derivative of the intensity 
vanishes. This approach has undeniable elegance, but it requires both 
first and second order differentiators and also relies upon the amplified 
signal having a well-defined, single-peak feature corresponding to the 
input light signal. If the input signal is degraded, then the tests this 
system uses in its double sweep procedure may not provide reliable tracing 
results. 
U.S. Pat. No. 5,572,351, entitled "Optical Communications Systems" and 
issued Nov. 5, 1996 1996 to Hadjifotiou, the disclosure of which is 
incorporated herein by reference, also provides an advanced wavelength 
tracing system for an EDFA having a tunable filter. The disclosed system 
automatically adjusts the central wavelength of the tunable filter by 
adding a pilot signal to the data signal at a wavelength spaced apart 
(above or below) the band occupied by the data signal. The frequency of 
the pilot component of the received signal is detected by the EDFA and the 
central frequency of the data band in the received signal is deduced from 
the received pilot frequency. This ingenious system also has certain 
limitations, such as requiring use of a pilot signal and relying upon 
comparable dispersion in the pilot signal and the data signal. 
These systems, while providing significant advances, have certain 
limitations from which I have concluded that a wavelength-controlled EDFA 
with further improvements is needed. Such a system should provide robust 
wavelength tracing control without requiring special signal transmission 
formats. It should utilize only thoroughly proven control system 
components and should not rely upon specialized optical devices for 
operation. It also should not rely upon specific input signal features 
that may not be attainable in suboptimal operating situations. 
SUMMARY OF THE INVENTION 
An objective of the present invention is to provide an erbium doped fiber 
amplifier (EDFA) and its operation method for automatically tracing and 
filtering the transmitted light wavelengths in order to adjust the central 
wavelength of an optical filter, installed in its output stage, to the 
wavelength of the transmitted light, the amplifier being equipped with a 
microprocessor. 
To this and other objectives, the present invention provides in a first 
aspect an optical amplifier, comprising an optical amplifying unit having 
an input port and an output port, an optical filter coupled to the output 
port and having an adjustable central wavelength, and a wavelength control 
unit in communication with the optical filter. The optical amplifying unit 
receives an incoming light signal at its input port and emits an amplified 
outgoing light signal at its output port. The optical filter receives the 
outgoing light signal and removes from it a noise component introduced by 
the optical amplifying unit. 
The wavelength control unit includes an intensity detector for detecting an 
intensity of the outgoing light signal at each one of a plurality of 
discrete control levels within a control level range; an intensity 
comparator for comparing a first intensity, detected at a first one of the 
plurality of control levels, to a second intensity signal representative 
of a second intensity detected at a second one of the plurality of control 
levels; and a storage unit for storing a value representative of the first 
intensity when the first intensity is not less than the second intensity 
and storing a value representative of the second intensity when the first 
intensity is less than the second intensity. 
In a second aspect, the present invention provides a method for 
automatically adjusting a central wavelength of an optical filter to trace 
a peak intensity wavelength of an output light signal. The method 
comprises the step of adjusting the central wavelength in accordance with 
each one of a plurality of discrete control levels, with a next lower 
control level corresponding to each one of the plurality of control 
levels. It comprises a further step of measuring an intensity of the 
output light signal at each one of the plurality of control levels and 
storing a value representative of the intensity. It also includes the step 
of selecting as a maximum intensity control level one of the plurality of 
control levels corresponding to a maximum value of a plurality of values 
consisting of the value representative of the intensity at each one of the 
plurality of control levels. 
The method of the present invention further includes the step of adjusting 
the central wavelength in accordance with the next lower control level 
corresponding to the maximum intensity control level. It includes a step 
of measuring a next intensity of the output light signal at the next lower 
control level corresponding to the maximum intensity control level. It 
also includes the steps of comparing a value representative of the next 
intensity to the maximum value and generating a comparison result and 
adjusting the central wavelength in accordance with the comparison result.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 is a block diagram of an existing single pumped amplifier that 
provides an example of the general context of the present invention. An 
input connector connects an optical fiber, leading from outside the 
amplifier, to an internal optical fiber contained in the amplifier. A 
separation tap 2 splits the input light signal from the connected optical 
fiber into an incoming light signal and an input side monitoring light 
signal in accordance with a predetermined ratio and sends them to an 
optical isolator 4 and an input side photodiode 12, respectively. 
Photodiode 12 measures the intensity of the incoming light signal. 
Optical isolator 4, which has one input terminal and one output terminal, 
applies only minimal attenuation to light signals propagating from the 
input terminal to the output terminal, but it effectively interrupts light 
signals travelling toward the input terminal from the output terminal. 
Optical isolator 4 thus prevents distortion of the incoming light signal 
by interrupting feedback caused by amplified spontaneous emissions (ASEs) 
generated by amplification components of the amplifier (such as 
light-amplifying optical fibers). 
The incoming light signal proceeds from optical isolator 4 to a wavelength 
division multiplexer (WDM) 6. WDM 6 receives two different light signals 
at different wavelengths through its respective input terminals and 
combines them into a single multiplexed signal, which it sends out through 
an optical fiber terminal. The incoming (communication) light signal has a 
wavelength of 1,550 nm, whereas a pump laser diode, used as an excitation 
light source, provides a power signal with a wavelength of typically 980 
nm or 1,480 nm. WDM 6 sends the power signal (wavelength, e.g., 980 nm) 
and the communication signal (wavelength 1,550 nm) through an output 
terminal to an erbium doped amplification fiber (EDF) 16. 
EDF 16 is an optical fiber doped with the rare-earth metal erbium (element 
number 68), which provides the fiber with high absorption rates at 
specific wavelengths such as 800, 980, and 1,480 nm. This doped fiber 
absorbs the power signal and thereby amplifies the communication signal, 
which has a spectral bandwidth of about 60 nm centered at a predetermined 
wavelength (e.g., 1,550 nm). The output end of EDF 16 is connected to an 
optical isolator 8, which in turn is connected to a separation tap 10. 
Optical isolator 8 interrupts light signals reflecting back from 
separation tap 10 or other optical devices in the propagating light 
signal's downbeam path. Finally, separation tap 10 is connected to the 
output stage fiber by an output connector. 
Separation tap 10 receives an outgoing light signal from optical isolator 8 
and splits it into an output light signal, to be output to the downbeam 
fiber connected via the output connector, and an output side monitoring 
light signal for monitoring the output light signal. The output side 
monitoring light signal is received by an output side photodiode 14. 
Photodiode 12 generates an (electrical) input monitoring signal from the 
input side monitoring light signal, and photodiode 14 generates an 
(electrical) output monitoring signal. These two monitoring signals are 
amplified, respectively, by two analog amplifiers 20 and 22. An electronic 
controller 24 receives the amplified monitoring signals and, in accordance 
with them, controls the output of a pump laser diode 18. 
FIG. 2 illustrates an inherent problem that arises in EDFAs when the power 
signal generated by pump laser diode 18 is used with EDF 16 to amplify the 
incoming light signal. On the one hand, fiber optic communication over 
long distances requires periodic amplification of the communication 
signals, due to signal attenuation from the transmission fibers and other 
optical elements through which the signals propagate. On the other hand, 
to realize the potentially large data bandwidths afforded by fiber optic 
systems, received signals must have relatively narrow spectral bandwidths. 
The input light signal received by an EDFA may have a desirably narrow 
signal peak, such as peak 28 in FIG. 2. But the amplification process of 
EDF 16 introduces into the amplified signal a substantial noise component 
29. Accumulation of such noise components over several amplification 
stages would unacceptably deteriorate the communication signal peak. 
To address this problem, the EDFA includes a wavelength variation filter 26 
(equivalently, a wavelength fixing filter) that receives the output light 
signal from separation tap 10 filters out noise introduced during the 
amplification process. When the central wavelength of filter 26 is set at 
the central wavelength of the input light signal, e.g., at 1,550 nm, the 
noise component 29 can be efficiently eliminated from the amplified 
signal, leaving a sharpened signal 30. 
FIG. 3 is a block diagram of an EDFA in accordance with the present 
invention. This EDFA includes separation taps 202 and 214, optical 
isolators 204 and 208, a wavelength division multiplexer (WDM) 206, an EDF 
210, a tunable optical filter 212, photodiodes 216 and 220, and a pump 
laser diode 218 whose features and functions are the same as the 
corresponding components of the optical amplifier illustrated in FIG. 1. 
Detailed description of these components will therefore be omitted. 
In the present invention, a microprocessor 274 replaces electronic 
controller 24 of the existing optical amplifier for controlling the output 
of pump laser diode 18. A/D converters 222 and 228 are interposed between 
microprocessor 224 and photodiodes 216 and 220 and convert the analog 
monitoring signals generated by photodiodes 216 and 220 into corresponding 
digital monitoring signals. Microprocessor 224 receives measurements of 
the incoming and outgoing light signal intensities, as detected by 
photodiodes 216 and 220 and converted by A/D converters 222 and 228, and 
thereby controls the output power of pump laser diode 218. 
Microprocessor 224 serves in part as an intensity comparator of a 
wavelength control unit by receiving signals representing the measured 
intensity of the outgoing light signal and generating control signals to 
adjust the central wavelength of optical filter 212. The control signals 
from microprocessor 224 drive the central wavelength to a wavelength that 
induces a maximum intensity in the output light signal. Optical filter 212 
is installed between output side optical isolator 208 and separation tap 
214. D/A converter 226 is installed between optical filter 212 and 
microprocessor 224 to convert the digital signals from microprocessor 224 
into analog signals. 
FIG. 4 illustrates the attenuation of the output light signal due to filter 
212 as a function of the filter's central wavelength. When the output 
light signal has a peak intensity at 1,550 nm, its total light intensity 
is greatest when the central wavelength of the optical filter is adjusted 
to 1,550 nm, matching the signal's peak intensity wavelength. As the 
central wavelength of the filter increases or decreases, the total 
intensity of the output light signal decreases precipitously. For example, 
the graphs shown in FIG. 4 indicate that placing the central wavelength of 
the filter 0.5 nm below the output signal's peak intensity wavelength 
reduces the peak intensity by about 1 dBm and the total intensity of the 
output signal by about 5 dBm. Larger discrepancies between the central 
wavelength and the peak intensity wavelength result in greater 
attenuation, and the attenuation rate increases as the discrepancy 
increases. Thus, effective use of the available output of pump laser diode 
218 depends critically upon controlling the filter central wavelength to 
track the peak intensity wavelength of the output light signal. 
FIGS. 5a and 5b illustrate a method of operation for the device shown in 
FIG. 3. First, at step 502, microprocessor 224 initializes the system. 
This entails configuring various software registers, as will be apparent 
to persons skilled in the art, and specifying several operating parameters 
that will be described hereinbelow. Microprocessor 224 then sets a control 
level value (denoted Vhex) for optical filter 212 at step 504, based on 
the nominal wavelength of the input light signal. Vhex determines the 
central wavelength setting for optical filter 212. As the operation method 
proceeds, Vhex will be varied over a fixed range determined by the word 
length (number of bits) accommodated by D/A converter 226, as will be 
understood by persons of skill in the digital control art. In terms of 
wavelength ranges, the range of the control level value (Vhex) will 
usually be from 1,540 nm to 1,560 nm for a nominal wavelength of 1,550 nm. 
Microprocessor 224 controls the central wavelength of optical filter 212 at 
step 506 by successively setting the control level value at each of 
several levels into which the range for Vhex is divided, with the number 
of levels depending upon the tuning resolution of optical filter 212 and 
upon the word length of D/A converter 226. Starting with a first level 
Vhex+1, at step 508 microprocessor 224 measures and stores the intensity 
value of the output light signal. This value is stored in a storage device 
(not shown), which may be a RAM device and which may constitute a 
component part of microprocessor 224. The intensity of the output light is 
measured by output side photodiode 216, and sent to microprocessor 224 
through A/D converter 228. 
At the step 506 microprocessor 224 determines whether the current level is 
the last level in the range for Vhex, which is identified for future 
reference in the initialization step 502. If the current level is not the 
final level, then microprocessor 224 performs another iteration of steps 
506 and 508. If the current level is the last level, then at step 512 
microprocessor 224 determines whether the maximum value of the output 
light intensity is within fixed control level limits. The control level 
limits may be chosen within a range of from +3 dBm to -35 dBm and will 
depend upon the characteristics of the amplifier components. The control 
level limits are set at the initialization step 502. If the maximum 
measured intensity does not fall within the control level limits, then the 
process returns to step 504 to reset Vhex of optical filter 212 in 
accordance with the value held in A/D converter 228. 
If the maximum intensity value of the output light does fall within the 
control level limits, then at step 514 (FIG. 5B) microprocessor 224 enters 
a detailed trace mode for controlling the central wavelength of optical 
filter 212. At step 516 of the detailed trace mode, microprocessor 224 
adjusts the central wavelength of optical filter 212 to the control level 
that is one level below the control level at which the maximum intensity 
value of the output light signal was previously measured, and the 
intensity level is again measured. microprocessor 224 increases the 
control level of the central wavelength of optical filter 212 by one level 
at step 518 and once again measures the intensity of the output light 
signal. At step 520, microprocessor 224 compares the output light 
intensity measured at step 518 to the output light intensity measured at 
step 516. Thus, the intensity at the control level of the previously 
measured maximum intensity is compared with the intensity at a control 
level one level lower. 
If the output light intensity measured at step 518 is not less than the 
output light intensity measured at step 516, then the process returns to 
step 518 where the operations of increasing the control level and 
measuring the output light intensity are repeated. If the output light 
intensity measured at step 518 is less than the output light intensity 
measured at step 516, then at step 522 microprocessor 224 decreases the 
control level by one level and measures the intensity of the output light. 
Microprocessor 224 then determines at step 524 whether the output light 
intensity after decreasing the control level value by one level is less 
than the intensity before decreasing the control level. If the output 
light intensity before decreasing the level value is less than the 
intensity after decreasing the control level, then the process returns to 
the step 522 to repeat the operations of reducing the control level and 
measuring the output light intensity. 
If the output light intensity before decreasing the control level is 
greater than the intensity after decreasing the control level, then at 
step 526 microprocessor 224 determines whether a subtracted intensity 
value, obtained by subtracting the output light intensity measured at the 
current control level from the maximum output light intensity stored 
previously, exceeds an effective range. The effective range, set at the 
initialization step 502, is usually about 5 dBm and is chosen in 
accordance with the resolution provided by the word length of D/A 
converter 226. If the subtracted intensity value exceeds the effective 
range, then the process returns to step 504, where the control level value 
of optical filter 212 is reset in accordance with the wavelength of the 
transmitted light as determined by the value held by A/D converter 228, 
and the process repeats. If the subtracted intensity value is less than 
the effective range, then the central wavelength of optical filter 212 is 
adjusted to the current control level. The process then goes returns to 
the step 518 for continuing micro-control of the central wavelength of 
optical filter 212 in accordance with the wavelengths of subsequently 
received input light signals. 
As illustrated, the present invention provides a device and method for 
tracing the central wavelength of an amplified light signal by adjusting 
the central wavelength of an optical filter, installed in the output port, 
to a wavelength providing maximum output light signal intensity. The 
present invention therefore provides an optical amplifier with 
substantially improved reliability and efficiency. It should be noted that 
the device and method of the present invention achieve their advantageous 
results by providing an output light signal with high intensity, relative 
to the available power from the excitation light source, while maintaining 
the required sharpness of the output signal. 
It should be understood also that the present invention is limited neither 
to the particular embodiment disclosed herein as the best mode 
contemplated for carrying out the invention, nor to the specific 
embodiments described in this specification, but instead fully encompasses 
the scope of the invention as defined in the appended claims.