WDM optical communication method with pre-emphasis technique and an apparatus therefor

A wavelength division multiplexed (WDM) optical communication method and apparatus uses a pre-emphasis technique to adjust the attenuation or amplification of a particular optical channel at a transmitter terminal to produce identical signal-to-noise ratios for all of the optical channels at a receiver terminal. The pre-emphasis adjustments to the transmitted signals are made on the basis of signal-to-noise ratio measurements performed at the receiver terminal. The signal-to-noise ratio values for each channel are transmitted through a facing line that is also used to transmit data along optical communication lines from the receiver terminal back to the transmitter terminal. The present invention also provides a method and apparatus for monitoring optical transmission paths for the deterioration of optical amplifier repeaters or an optical fiber. The optical signals are then adjusted to account for the location of the particular amplifier or fiber that has deteriorated.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a WDM (Wavelength Division Multiplexed) 
optical communication method with pre-emphasis technique, and to an 
apparatus therefor; and in particular to a WDM optical communication 
method with pre-emphasis technique, which employs a WDM optical signal and 
enables to increase the transmission capacity of an optical fiber 
communication system, and an apparatus therefor. 
2. Description of the Related Arts 
Since optical fiber communication systems that utilize WDM optical signals 
can increase their transmission capacities without altering their 
transmission paths, the employment of such systems for trunk lines of 
optical fiber communication systems can be expected in the future. For a 
WDM optical signal transmission path along which multiple optical 
amplifiers are used to relay signals, the signal-to-noise ratios of the 
individual signal wavelengths of a WDM optical signal differ from each 
other due to the wavelength-gain responses of the optical amplifier 
repeaters. Specifically, there is an increased signal-to-noise ratio for a 
wavelength of a higher gain. As a result, the performance of the WDM 
optical signal is not uniform; and although a preferable performance is 
obtained with a wavelength having a high gain, the performance is reduced 
for a wavelength having a low gain. 
A pre-emphasis technique at an optical transmitting terminal is well known 
as a method for acquiring uniform and preferable performance of a 
multiplexed signal for all the wavelengths. A well known example 
publication is "One-third Terabit/s transmission through 150 km of 
dispersion managed fiber", R. W. Tkach et al, ECOC '95, Post-deadline 
paper, pp.45-48. In this publication a technique is disclosed that uses 
the pre-emphasis technique to provide almost equal signal-to-noise ratios 
for all the channels at an optical receiving terminal. 
The manner in which the pre-emphasis technique is employed for controlling 
pre-emphasis values depends on the experience and intuition of an 
operator. The operator observes in real time a signal-to-noise ratio at a 
receiving terminal by using a measurement device, such as an optical 
spectrum analyzer, while at the same time varying pre-emphasis attenuation 
or gain at the transmitting terminal to equalize the signal-to-noise 
ratios of all the signal wavelengths. 
Further, as a result of study by the present inventors, it was found that 
with the above conventional technique a serious problem arises when a 
failure occurs in an optical transmission path. In a graph in FIG. 10, the 
difference of the reduction of the Q value due to the repeater failure is 
shown for both the presence or the absence of the pre-emphasis technique. 
In this experiment, four wavelengths were multiplexed and transmitted 
through eleven optical amplifier repeaters, and the repeater failure was 
caused by the degradation of the excitation light power by half. The 
pre-emphasis is performed by reducing the power for the transmission 
signals for channels 2 and 3 (the same method as is described in reference 
material "One-third terabit/s transmission through 150 km of dispersion 
managed fiber", W. Tkach, et al., ECOC '95, Post-deadline paper, pp. 
45-48). In other words, by this method, while the power for the 
transmission signals on channels 1 and 4 is not changed, the power for the 
transmission signals on channels 2 and 3 is reduced. It is apparent from 
the graph in FIG. 10 that, when the pre-emphasis technique is employed, 
the degradation of the signal-to-noise ratio due to a failure, which 
occurred at a repeater close to a transmitting terminal, is drastically 
increased the penalty for channel 2, while there is almost no degradation 
of the signal-to-noise ratio for channel 4. 
This can be explained as follows. When an optical amplifier along an 
optical transmission path fails, the output signal power of a repeater is 
reduced. Thus, the signal-to-noise ratios for all the WDM optical signals 
are degraded. When the pre-emphasis technique at the optical transmitting 
terminal is employed, the power of a signal wavelength having a high gain 
is lower than that of the signal wavelengths having a low gain, i.e. the 
signal power of channels 2 and 3 is lower than that of channels 1 and 4. 
As a result, the signal-to-noise ratio for the signal wavelengths of the 
channels 2 and 3 is greatly degraded compared with that for the other 
signal wavelengths, i.e., channels 1 and 4. Therefore, in a system wherein 
both a wavelength having a high gain and a wavelength having a low gain 
should originally have the same property, the property of the wavelength 
having a high gain is considerably deteriorated compared with that of the 
other wavelength. 
The difference between the power values of WDM optical signals that are 
input to an optical repeater that is remote from the optical transmitting 
terminal is not so great; even when a failure has occurred at the 
repeater, the performances for the individual signal wavelengths when 
pre-emphasis is employed do not differ very much. 
SUMMARY OF THE INVENTION 
It is, therefore, one object of the present invention to provide a WDM 
optical communication method with a pre-emphasis technique, by which 
optimal pre-emphasis attenuation and/or gain can be automatically set 
without depending on the experience and the intuition of an operator, and 
by which a difference between the performances of WDM optical signals can 
be reduced, even when a failure occurs at an optical transmission path. 
To achieve the above object, according to the present invention, provided 
are a method and an apparatus, whereby and wherewith: all WDM optical 
signals emitted by a WDM optical transmitting terminal are output with an 
equal transmission signal power; a signal-to-noise ratio measurement is 
performed for each of the WDM optical signals at a WDM optical receiving 
terminal; information obtained by each signal-to-noise ratio measurement 
is superimposed on an optical signal along a facing line by an information 
transfer circuit, and resultant information is fed back to a pre-emphasis 
control means of the WDM optical transmitting terminal; and a pre-emphasis 
value at the WDM optical transmitting terminal is automatically set by the 
pre-emphasis control means, thus providing a constant performance by each 
of the WDM optical signals at the WDM optical receiving terminal. 
Further, according to the present invention, provided are a method and an 
apparatus, whereby and wherewith: when during monitoring of an optical 
transmission path deterioration of either an optical amplifier repeater or 
an optical fiber is detected, pre-emphasis control means automatically 
calculates a pre-emphasis value at the WDM optical transmitting terminal 
that is in consonance with information that indicates a location of the 
deteriorated optical amplifier repeater or the deteriorated optical fiber 
relative to the WDM optical transmitting terminal, so that the 
pre-emphasis control means reduces the degree of attenuation for an 
attenuated signal wavelength and the degree of amplification for an 
amplified signal wavelength, and steadily maintains total power for all 
optical signals that are output by the WDM optical transmitting terminal. 
According to the present invention, a signal-to-noise ratio for all WDM 
optical signals, which are output with equal transmission signal power by 
a WDM optical transmitting terminal, is measured by a WDM optical 
receiving terminal. The measured result is superimposed on an optical 
signal on a facing line by an information transfer circuit, and the 
resultant information is returned to the WDM optical transmitting 
terminal. The pre-emphasis control means at the WDM optical transmitting 
terminal sets a pre-emphasis value based on the received information. As a 
result, a pre-emphasis value at the WDM optical transmitting terminal can 
be automatically set, so that the performances of the individual WDM 
optical signals are constant at the WDM optical receiving terminal. 
In addition, according to the present invention, the line monitoring means 
detects deterioration of the optical amplifier repeater or the optical 
fiber. Upon receipt of the information from the line monitoring means, the 
pre-emphasis control means automatically calculates a pre-emphasis value 
at the WDM optical transmitting terminal so as to reduce the degree of 
attenuation of an attenuated signal wavelength and the degree of 
amplification of an amplified signal wavelength, and to maintain a 
constant sum of the power levels for optical signals, which are output at 
the WDM optical transmitting terminal. Therefore, even when there is 
deterioration of an optical amplification repeater or an optical fiber, 
the performance of each WDM optical signal at the WDM optical receiving 
terminal can be preferably maintained.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention will now be described in detail while referring the 
accompanying drawings. FIG. 1 is a schematic block diagram illustrating 
the arrangement of the present invention. A WDM optical transmitting 
terminal 1A along an upstream line comprises optical transmitters (XMTRs) 
1 through 4 for channels 1 through 4 for an upstream line; a pre-emphasis 
controller 5 for an upstream line; pre-emphasis attenuation and/or gain 
controllers (CONTs) 6 though 9 for an upstream line; and optical 
multiplexer 10 for channels 1 through 4 for an upstream line. One end of a 
transmission optical fiber 12 for an upstream line is connected to the 
optical multiplexer 10 and the other end is connected to an optical 
amplifier 13. A WDM optical receiving terminal 1B for an upstream line is 
connected to the last transmission optical fiber 12 across multiple 
optical fibers 12 and optical amplifiers 13. 
The WDM optical receiving terminal 1B along the upstream line comprises 
optical demultiplexer 15 for channels 1 through 4 for the upstream line; 
optical receivers (RECs) 16 through 19 for channels 1 through 4 for the 
upstream line; and a signal-to-noise ratio measurement unit 20. A line 
monitoring unit 42 superimposes a signal-to-noise ratio of each channel 1 
to 4, which is measured by the signal-to-noise measurement unit 20, on an 
optical signal at one arbitrary channel from among channels 1 through 4 
for a downstream line, and transmits the resultant signal to a WDM optical 
receiving terminal 2B along the downstream line. The line monitoring unit 
42 also monitors the condition of the downstream line that is constituted 
by transmission fibers 32 and optical amplifiers 33 by using a line 
monitoring signal, which is returned across a return circuit that will be 
described later. 
A WDM optical transmitting terminal 2A along the downstream line comprises: 
optical transmitters 22 through 25 for channels 1 through 4 for the 
downstream line; a pre-emphasis controller 26 for the downstream line; 
pre-emphasis attenuation and/or gain controllers 27 through 30 for the 
downstream line; and an optical multiplexer 31 for channels 1 through 4 
for the downstream line. One end of the transmission optical fiber 32 for 
the downstream line is connected to the optical multiplexer 31, and the 
other end is connected to the optical amplifier 33. The WDM optical 
receiving terminal 2B along the downstream line is connected to the last 
transmission optical fiber 32 across multiple transmission optical fibers 
32 and the optical amplifiers 33. 
The WDM optical receiving terminal 2B along the downstream line comprises 
an optical demultiplexer 35 for channels 1 through 4 for the downstream 
line; optical receivers 36 through 39 for channels 1 through 4 for the 
downstream line; and a signal-to-noise measurement unit 40. A line 
monitoring unit 41 monitors the condition of the upstream line that is 
constituted by the transmission fibers 12 and the optical amplifiers 13 by 
using a line monitoring signal, which is returned across a return circuit 
that will be described later. In addition, the line monitoring unit 41 
superimposes a signal-to-noise ratio for each channel 1 to 4, which is 
measured by the signal-to-noise measurement unit 40, on an optical signal 
for the upstream line, and transmits the resultant signal to the WDM 
optical receiving terminal 1B along the upstream line. 
Optical amplifier repeaters 43, which are inserted into the upstream and 
downstream lines at proper intervals, each comprise: an optical amplifier 
13 for the upstream line; an optical amplifier 33 for the downstream line; 
a return circuit 44 for returning a line monitoring signal from the 
upstream line to the downstream line; and a return circuit 45 for 
returning a line monitoring signal from the downstream line to the 
upstream line. 
One specific example arrangement of the pre-emphasis attenuation and/or 
gain controller 6 through 9 and 27 through 30 will now be explained while 
referring to FIG. 2. Although the pre-emphasis attenuation and/or gain 
controller 6 is employed for explanation, the other pre-emphasis 
attenuation and/or gain controllers 7 through 9 and 27 through 30 have the 
same structure. The pre-emphasis attenuation and/or gain controller 6 
includes an optical amplifier (AMPL) 6a for amplifying an optical signal 
that is received from the optical transmitter 1; an optical signal branch 
circuit 6b; and an automatic gain control circuit 6c. The optical signal 
branch circuit 6b outputs to the optical multiplexer 10 an optical signal 
from the optical amplifier 6a, and also dispatches one part of the optical 
signal to the automatic gain control circuit 6c. Upon receipt of the part 
of the optical signal and a control signal from the pre-emphasis 
controller 5, the automatic gain control circuit 6c sets a gain for the 
optical amplifier 6a, i.e., a pre-emphasis value. A variable optical 
attenuator, which incorporates a GP-IB (computer interface) sold by HP 
Corp., can be employed as another example for the pre-emphasis attenuation 
and/or gain controller 6. 
The process for setting a pre-emphasis value for the pre-emphasis 
attenuation and/or gain controllers 6 though 9 will now be explained while 
referring to FIG. 3. At step S1, the pre-emphasis controller 5 resets the 
pre-emphasis attenuation and/or gain controllers 6 through 9. By this 
resetting, at step S2 the power values of the wavelengths for channels 1 
through 4 that are input across the light transmission path 12, i.e., the 
power values at the output points of the WDM optical transmitting terminal 
1A, are equal. At step S3, the WDM optical receiving terminal 1B measures 
the signal-to-noise ratio of each signal wavelength. The process at step 
S3 will be described in detail later while referring to FIG. 5. 
At step S4, the WDM optical receiving terminal 1B returns the acquired 
signal-to-noise ratio data from the WDM optical transmitting terminal 2A 
across the downstream line to the WDM optical receiving terminal 2B. For 
example, the acquired signal-to-noise ratio data are transmitted from the 
optical transmitter 22 of the WDM optical transmitting terminal 2A along 
the downstream line, and are received by an optical receiver 36 of the WDM 
optical receiving terminal 2B. A method shown in FIG. 4A or 4B, for 
example, can be employed for returning the signal-to-noise ratio data. In 
FIG. 4A is shown an example wherein signal-to-noise ratio data a2, which 
is a tone signal having a low frequency, is superimposed on a main line 
signal a1, which is a high speed digital signal. In FIG. 4B is shown an 
example wherein bit b2 that is included in a header portion b1 of the main 
line signal is used as data for returning the signal-to-noise ratio data. 
At step S5, the pre-emphasis controller 5 reads the signal-to-noise data 
that are received by the WDM optical receiving terminal 2B along the 
downstream line e.g., that are received by the optical receiver 36, and 
calculates a pre-emphasis value. At this time, the pre-emphasis controller 
5 regards, as a pre-emphasis value for each signal wavelength, a relative 
value between the signal-to-noise ratio, of the signal wavelength, that is 
the most preferable and the signal-to-noise ratio of each signal 
wavelength. More specifically, the pre-emphasis controller 5 performs a 
calculation to ensure that the transmission power from the WDM optical 
transmitting terminal is increased for the wavelengths that have a lower 
signal-to-noise ratio, and to ensure that all the optical power values of 
the WDM optical signals at the WDM optical transmitting terminal are 
maintained steady before and after the pre-emphasis calculation is 
performed. At step S6, the pre-emphasis controller 5 sets the pre-emphasis 
value for the individual pre-emphasis attenuation and/or gain controllers 
6 through 9. 
The process at step S3, i.e., the processing performed by the WDM optical 
receiving terminal 1B for measuring the signal-to-noise ratio of each 
signal wavelength, will now be explained in detail. A WDM optical signal 
is received via the transmission optical fiber 12 at the signal-to-noise 
ratio measurement unit 20 of the WDM optical receiving terminal 1B. At 
step S11, the signal-to-noise ratio measurement unit 20 performs an 
optical spectrum measurement, as is shown in FIG. 6A. At step S12, a peak 
for each signal wavelength is searched for. In an example shown in FIG. 
6A, the heights of four signal peaks are measured. At step S13, masking is 
performed in front and in back of each signal wavelength, and a curve 
(hereinafter referred to as a "fitted curve") that is closest to all the 
remaining points is drawn. Hatched portions in FIG. 6B indicate those 
areas that are masked, and a curve indicated by the solid line is the 
curve for which fitting was performed. At step S14, as is shown in FIG. 
6C, the noise levels at the portions that correspond to the signal peaks 
are acquired from values at points c1, c2, c3 and c4 at which fitting was 
performed. Then, at step S15, signal-to-noise ratios d1, d2, d3 and d4 are 
obtained from differences between the acquired noise levels and the signal 
peak power levels. 
Thus, the acquired signal-to-noise ratio data are prepared so that they are 
correlated with channel numbers, as is shown in FIG. 7, and are returned 
with the signal form in FIG. 4 to the WDM optical receiving terminal 2B. 
As is apparent from the above description, according to this embodiment, 
the pre-emphasis value of the pre-emphasis attenuation and/or gain 
controllers 6 through 9 can be automatically set without depending on the 
experience and intuition of an operator. As a result, the performances of 
the WDM optical signals, which are received by the WDM optical receiving 
terminal 1B, are uniform, and preferable performances can be provided 
automatically. 
The operation of the WDM optical communication apparatus with the 
pre-emphasis technique in this embodiment will now be described. The line 
monitoring unit 41 for the upstream line employs an optical transmitter 
for an upstream line, e.g., the optical transmitter 1, for superimposing 
on a transmission signal an amplitude modulation signal having a low 
frequency. The degree of modulation in this amplitude modulation is small, 
several % or lower (e.g., 1 to 2%), so as not to degrade the performance 
of the transmission signal. An optical signal that is emitted by the 
optical transmitter 1 is forwarded through the pre-emphasis attenuation 
and/or gain controller 6 and the optical multiplexer 10 to the upstream 
line and via the optical amplifier repeaters 43 to the WDM optical 
receiving terminal 1B for the upstream line. One part of the optical 
signal is returned to the downstream transmission path by the return 
circuit 44 that is provided in each optical amplifier repeater and that 
has a loss. The loss at the return circuit 44 is set so that a signal 
along the downstream path is not degraded by a signal returned along the 
upstream path, and is about 45 dB. 
A signal, which is returned by the return circuit 44 and is attenuated to 
about 45 dB relative to the output level of the optical amplifier along 
the upstream line, is transmitted through the transmission optical fiber 
32 and the optical amplifier repeater 33 along the downstream line to the 
optical receiver for the downstream line, i.e., the optical receiver 36. 
The line monitoring unit 41 receives the signal that was input at the 
optical receiver 36, cancels a signal along the downstream path, and 
demodulates the amplitude modulation signal having a low frequency that 
was previously superimposed. 
The optical amplifier relative to the transmission side for which output is 
deteriorated can be detected as follows. The line monitoring unit 41 for 
the upstream line measures and stores in advance data concerning the 
amount of level variation of the light that is returned by each of the 
optical amplifiers when these optical amplifiers are operated normally. 
Then, when practical use of the transmission path is being made, the line 
monitoring unit 41 acquires an output level for the optical amplifier of 
the upstream path by using a time delay value and a time correlation value 
between a transmission line monitoring signal and a reception line 
monitoring signal. The line monitoring unit 41 compares the output level 
change with the above described data that are stored in advance, and 
determines whether or not each of the optical amplifiers is operating 
normally. 
FIG. 9 is a graph showing an example for measuring a fluctuation level for 
each optical amplifier repeater, and a measurement example wherein the 
output of the optical amplifier repeater 55 is degraded. Points "X" in the 
graph indicate an measurement example wherein all the optical amplifier 
repeaters are normal, and points "O" indicate an example wherein a failure 
has occurred at the optical amplifier repeater 55. During normal 
operation, since fluctuation in the output level is the same as the 
fluctuation data that are stored in advance, a relative loop gain is 0. 
However once a failure has occurred at one of the optical amplifiers, the 
fluctuation of its output level is reduced greatly. In the example shown 
in FIG. 9, since the level of a return signal output from the optical 
amplifier repeater 55 is greatly reduced, it is understood that some 
failure has occurred at the optical amplifier repeater 55. 
Therefore, when a failure has occurred at an optical amplifier repeater 43 
for the upstream line and the property is degraded, at which optical 
amplifier repeater relative to the transmission side a failure has 
occurred can be detected. An example of the technique concerning the 
monitoring of the optical amplifier repeaters is disclosed in detail in 
Japanese Unexamined Patent Publication No. Hei 5-344067, which was 
submitted by the present applicant. 
The pre-emphasis control, which is applied when some deterioration occurs 
at an optical amplifier repeater along the transmission path that includes 
the optical fibers 12 and the optical amplifiers 13, will be explained 
while referring to the flowchart in FIG. 8. 
Suppose that the total number of the optical amplifiers that are located 
along the transmission path is n. At step S21, the line monitoring unit 41 
determines at which optical amplifier 13 relative to the transmission side 
has deterioration of output occurred, and transmits this information to 
the pre-emphasis controller 5. In this embodiment, it is assumed that the 
deterioration of output has occurred at the m-th optical amplifier 
relative to the transmission side (m is a positive integer). 
At step S22, the pre-emphasis controller 5 compares half of the total n of 
the optical amplifiers with m. If m.ltoreq.n/2, program control advances 
to step S23, and the pre-emphasis value for each signal wavelength is 
recalculated using the following expression: 
EQU y=1/log n.multidot.{log n-log(n/2-m+1)}.multidot.x. 
x(dB) denotes the initial value of a pre-emphasis value and y(dB) denotes a 
pre-emphasis value after conversion. The above expression was obtained 
through various experiments, and is thus an empirical expression. That is, 
when a failure is caused in optical amplifiers at different locations for 
simulation, and the fluctuation values of the pre-emphasis value at that 
time are represented on a graph, the results that can be obtained 
approximate those that can be obtained by using the above expression. 
At step S24, the value acquired by re-calculation is transmitted to the 
pre-emphasis attenuation and/or gain controllers 6 through 9, and is set 
again. For example, if the initial value x is employed as the pre-emphasis 
value for channels 2 and 3, the pre-emphasis value for channels 2 and 3 
are again set to y(dB). With this setup, the performances of the 
wavelengths for channels 1 through 4 that are to be received by the WDM 
optical receiving terminal 1B can be almost equalized with this control, 
the pre-emphasis value at the WDM optical transmitting terminal is so 
controlled that the degree of attenuation is reduced for the attenuated 
signal wavelength and the degree of amplification is reduced for the 
amplified signal wavelength, and that the power for all the optical 
signals that are output by the WDM optical transmitting terminal is 
constant. If the decision at step S22 is m&gt;n/2, program control goes to 
step S25 and the process is thereafter terminated without performing any 
processing. 
As is described above, according to this embodiment, even when a failure 
has occurred in the optical amplifier repeater along the transmission path 
and its transmission property has been deteriorated, a pre-emphasis value 
can be controlled so that there is no difference between the performances 
of the WDM optical signals. 
As is apparent from the above description, according to the present 
invention, an optimal pre-emphasis value for the pre-emphasis attenuation 
and/or gain controllers can be set automatically without depending on the 
experience and the intuition of an operator. Further, according to the 
present invention, even when there has been deterioration of an optical 
amplifier repeater or an optical fiber along the optical transmission path 
while working, a difference between the performances of the WDM optical 
signals can be reduced. In addition, according to the present invention, 
the setting of a pre-emphasis value for the WDM optical communication 
apparatus, and the transmission property when a failure occurs are 
substantially improved, and the present invention provides great results 
when employed for the construction of a WDM optical communication system.