Apparatus for measuring optical transmission line property and optical wavelength multiplexing transmission apparatus

Disclosed is an apparatus for measuring an optical transmission line property, which has: a pair of up and down optical fiber transmission apparatuses, each of which includes an optical transmission end office which includes a plurality of signal light sources for outputting signal lights with different wavelengths and an optical multiplexer for multiplexing the signal lights, an optical fiber transmission line and an optical reception end office which includes an optical divider for dividing the signal lights multiplexed and an optical receiver corresponding to each of the signal lights divided with different wavelengths; an optical turn circuit which includes an optical multiplexer and an optical divider and leads a part of light transmitting through the optical fiber transmission line to another optical fiber transmission line; and an optical power measuring circuit for measuring optical power distributions of a signal light propagating on the optical fiber transmission line and another signal light led to the optical fiber transmission line from another optical fiber transmission line.

FIELD OF THE INVENTION 
This invention relates to an apparatus for measuring optical transmission 
line property, a method for measuring an optical transmission line 
property, an optical wavelength multiplexing transmission apparatus and an 
optical wavelength multiplexing transmission method. 
BACKGROUND OF THE INVENTION 
FIG. 1 shows a conventional optical wavelength multiplexing transmission 
system. This system is an example of two-wavelength-multiplexing optical 
signal transmission system, where each of up signal light wavelengths 
.lambda.1, .lambda.3 and down signal light wavelengths .lambda.2, 
.lambda.4 is multiplexed and transmitted. 
In an up optical transmission end office 1a, signal lights 6 with 
wavelengths .lambda.1, .lambda.3 output from a signal light source 5 are 
multiplexed by an optical multiplexer 7 and then amplified by an optical 
amplifier 8, thereafter being transmitted through an optical fiber 
transmission line 3 and an optical amplifier repeater 4 to an up optical 
reception end office 2a. In the up optical reception end office 2a, the 
signal lights 6 are again amplified by an optical amplifier 8, being 
divided by an optical divider 9, being subject to the wavelength selection 
by optical filters 10 which correspond to the respective wavelengths 
.lambda.1, .lambda.3, and being received by optical receivers 11. The down 
optical transmission system where signal lights with wavelengths 
.lambda.2, .lambda.4 are transmitted has a like composition. In general, 
as the optical multiplexer 7 and optical divider 9, a wavelength 
multiplexing coupler, a fiber coupler or the like is used. 
In the optical wavelength multiplexing transmission system, the signal 
lights with the wavelengths .lambda.1, .lambda.3 or .lambda.2, .lambda.4 
which are output from the signal light sources 5 will have different gains 
and losses to the respective wavelengths due to the optical amplifier 8, 
optical fiber transmission line 3, optical coupler 7, 9 for coupling or 
dividing etc. This is because the gain or loss property in the above 
components composing the optical transmission line depends on wavelengths, 
particularly the optical amplifier 8 having a significant 
wavelength-dependency for gain. Here, erbium-doped optical fiber 
amplifiers, which are at present most generally used for the optical 
communication, also have the wavelength-dependency for gain. Therefore, 
research and development are being made such that they have a constant 
gain, i.e., a leveled gain, to different wavelengths of signal light to be 
suitable for the wavelength multiplexing transmission. Other than fiber 
amplifiers such as the erbium-doped optical amplifier, a semiconductor 
amplifier to which a semiconductor laser is applied can be used, but it 
also has the wavelength-dependency for gain. 
Meanwhile, in the respective signal light sources 5 of the transmission end 
office, feedback control(output power control) is in general conducted to 
control the optical power(optical electric power) of the output signal 
light to be always constant while monitoring a part of the output signal 
light. For example, there is a method that a light-receiving 
photodiode(PD) detects the power of back emitting light of a laser 
diode(LD) as a signal light source to control the drive current of the 
laser diode. 
At present, as described above, improvements of the wavelength property in 
such optical amplifiers and control techniques of the optical power of 
signal light sources are used in the development of optical wavelength 
multiplexing transmission system. 
However, when an optical wavelength multiplexing transmission system is 
actually constructed and operated, the property change with time in the 
optical amplifier, optical fiber transmission line, optical couplers for 
multiplexing and dividing etc. may cause the change of gain or loss in 
signal lights with different wavelengths. Particularly in the optical 
amplifier, since the wavelength property for gain changes depending on the 
power(electric power) of the input signal light, the power of the is 
signal light input to the optical amplifier is reduced due to the increase 
in loss of the optical fiber transmission line with time, therefore 
damaging the levelness of gain of optical amplifier. Thus, in the signal 
light with a wavelength greatly affected by this, the power(electric 
power) of the signal light may be weakened when received by the optical 
reception end office, therefore not giving a sufficient S/N ratio. 
This situation is explained with reference to (a) to (f) in FIG. 2. The 
situation of the up optical transmission line is shown by (a), (b) and (c) 
in FIG. 2, the situation of the down optical transmission line is shown by 
(d), (e) and (f) in FIG. 2. FIG. 2 shows the case that the up optical 
transmission line is normally operated and the wavelength property in the 
down optical transmission line is not normal. In FIG. 2, (a) and (d) show 
outputs of the signal lights 6 emitted from the signal light sources 5 of 
up and down optical transmission end offices 1a, 1b, respectively, (b) and 
(e) show the wavelength properties of up and down optical transmission 
systems, respectively, (c) and (f) show inputs of the signal lights 6 to 
the up and down optical reception end offices 2a, 2b, respectively. 
If the wavelength property of the optical transmission line is, as shown in 
FIG. 2(e), not normal, the power of the signal light entering to the down 
optical reception end office 2b is affected by this, and particularly in 
the signal light(with the wavelength .lambda.2 shown in FIG. 2(f)) having 
much loss in the corresponding optical transmission line the power of 
receiving light is reduced, therefore deteriorating the S/N ratio. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the invention to provide an method and 
apparatus which are provided with a means for determining and monitoring 
the situation of the wavelength properties of the entire optical 
transmission system. 
According to the invention, a method for measuring an optical transmission 
line property, wherein a property of an optical transmission line where a 
wavelength-multiplexed signal light propagates is measured, comprises the 
steps of: 
leading a part of the signal light propagating on the optical transmission 
line to another optical transmission line; and 
measuring optical power distributions of up and down signal lights with 
different wavelengths. 
According to another aspect of the invention, an apparatus for measuring an 
optical transmission line property, comprises: 
a pair of up and down optical fiber transmission apparatuses, each of which 
includes an optical transmission end office which includes a plurality of 
signal light sources for outputting signal lights with different 
wavelengths and an optical multiplexer for multiplexing the signal lights, 
an optical fiber transmission line and an optical reception end office 
which includes an optical divider for dividing the signal lights 
multiplexed and an optical receiver corresponding to each of the signal 
lights divided with different wavelengths; 
an optical turn circuit which includes an optical multiplexer and an 
optical divider and leads a part of light transmitting through the optical 
fiber transmission line to another optical fiber transmission line; and 
an optical power measuring circuit for measuring optical power 
distributions of a signal light propagating on the optical fiber 
transmission line and another signal light led to the optical fiber 
transmission line from another optical fiber transmission line. 
According to further aspect of the invention, an optical wavelength 
multiplexing transmission method, comprises the steps of: 
leading a part of signal light which propagates on one of up and down 
optical transmission lines to another optical transmission line; 
measuring optical power distributions of the up and down signal lights with 
different wavelengths; and 
controlling an optical transmission power of a signal light source on the 
basis of the distributions. 
According to still further aspect of the invention, an optical wavelength 
multiplexing transmission apparatus, comprises: 
a pair of up and down optical fiber transmission apparatuses, each of which 
includes an optical transmission end office which includes a plurality of 
signal light sources for outputting signal lights with different 
wavelengths and an optical multiplexer for multiplexing the signal lights, 
an optical fiber transmission line and an optical reception end office 
which includes an optical divider for dividing the signal lights 
multiplexed and an optical receiver corresponding to each of the signal 
lights divided with different wavelengths; 
an optical turn circuit which includes an optical multiplexer and an 
optical divider and leads a part of light transmitting through the optical 
fiber transmission line to another optical fiber transmission line; 
an optical power measuring circuit for measuring optical power 
distributions of a signal light propagating on the optical fiber 
transmission line and another signal light led to the optical fiber 
transmission line from another optical fiber transmission line; and 
an optical transmission power control circuit for controlling an optical 
transmission power of the signal light source on the basis of the 
distributions from the optical power measuring circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An apparatus for measuring optical transmission line property and an 
optical wavelength multiplexing transmission apparatus in the preferred 
embodiment will be explained in FIG. 3, wherein like parts are indicated 
by like reference numerals as used in FIG. 1. Similarly to the example as 
shown in FIG. 1, this is an example of two-wavelength-multiplexing 
transmission, where each of up signal light wavelengths .lambda.1, 
.lambda.3 and down signal light wavelengths .lambda.2, .lambda.4 is 
multiplexed and transmitted. 
In an up optical transmission end office 1a, signal lights 6 with 
wavelengths .lambda.1, .lambda.3 output from a signal light source 5 are 
multiplexed by an optical multiplexer 7 and then amplified by an optical 
amplifier 8, thereafter being transmitted through an optical fiber 
transmission line 3 and an optical amplifier repeater 4 to an up optical 
reception end office 2a. In the up optical reception end office 2a, the 
signal lights 6 are again amplified by an optical amplifier 8, being 
divided by an optical divider 9, being subject to the wavelength selection 
by optical filters 10 which correspond to the respective wavelengths 
.lambda.1, .lambda.3, and being received by optical receivers 11. The down 
optical transmission system where signal lights with wavelengths 
.lambda.2, .lambda.4 are transmitted has a like composition. In general, 
as the optical multiplexer 7 and optical divider 9, a wavelength 
multiplexing coupler, a fiber coupler or the like is used. 
In the composition as shown in FIG. 3, there are provided optical turn 
circuits 12 for connecting between the optical transmission end office 1a 
and optical reception end office 2b and between the optical transmission 
end office 1b and optical reception end office 2a, respectively, whereby a 
part of the up signal light can be led turned to the down optical 
transmission line and a part of the down signal light can be led turned to 
the up optical transmission line. In the optical turn circuit 12, an 
optical divider 13 and an optical multiplexer 14 which are composed of an 
optical fiber coupler, optical wavelength multiplexing coupler or the like 
are used, where the dividing/coupling ratios of the optical divider 13 and 
optical multiplexer 14 are set such that the signal light to be turned has 
an optical power(electric power) that it does not affect the transmitting 
signal light 6. Optionally, the power(electric power) of the signal light 
to be turned may be optimized by inserting an optical attenuator 15 in the 
optical turn circuit 12. 
As a means for monitoring the signal light 6, an optical divider 16 and an 
optical power supervisory circuit 17 are provided. The optical divider 16, 
which is inserted in the optical turn circuit 12 in FIG. 3, may be located 
at any place where the signal light to be transmitted through the optical 
transmission line can be divided, for example, between the optical divider 
13 and the optical divider 9. Also, it may be inserted in the optical 
transmission end office. The optical power(electric power) supervisory 
circuit 17 has a function that it receives the light divided by the 
optical divider 16 and detects the optical power of the received signal 
light at each wavelength, and it further has an operational function that 
it arithmetically calculates the wavelength property of the entire optical 
transmission system from the optical power of the detected signal light at 
each wavelength. 
If the wavelength property of the optical transmission system obtained by 
the optical power(electric power) supervisory circuit 17 is beyond a 
permitted range, the optical transmission condition may be modified to 
control it. For example, the power of the light source may be controlled 
based on the wavelength property of the optical transmission line. In FIG. 
3, an optical transmission power(electric power) control circuit 24 is 
provided which controls the optical output power(optical transmission 
electric power) of the signal light source at each wavelength, based on 
the wavelength property of the optical transmission system which is output 
as an operational result from the optical power supervisory circuit 17. 
With reference to (a) to (f) in FIG. 4, operations of the optical power 
supervisory circuit 17 will be explained. The situation of the up optical 
transmission line is shown by (a), (b) and (c) in FIG. 4, the situation of 
the down optical transmission line is shown by (d), (e) and (f) in FIG. 4. 
FIG. 4 shows the case that the up optical transmission line is normally 
operated and the wavelength property in the down optical transmission line 
is not normal. In FIG. 4, (a) and (d) show outputs of the signal lights 6 
emitted from the signal light sources 5 of up and down optical 
transmission end offices 1a, 1b, respectively, (b) and (e) show the 
wavelength properties of up and down optical transmission systems, 
respectively, and (c) and (f) show inputs of the signal lights 6 to the up 
and down optical reception end offices 2a, 2b, respectively. 
The spectra of the signal lights(.lambda.1, .lambda.2, .lambda.3, 
.lambda.4) which are output from the optical amplifiers 8 of the optical 
transmission end offices 1a, 1b are, as shown by (a), (d) in FIG. 4, 
controlled to be constant both at up and down wavelengths. 
The signal light with wavelengths .lambda.1, .lambda.3 output from the up 
optical transmission end office 1a transmits through the up optical 
transmission line to be received by the up optical reception end office 
2a. On the other hand, the signal light with wavelengths .lambda.2, 
.lambda.4 output from the down optical transmission end office 1b 
transmits through the down optical transmission line, being led to the up 
optical transmission line through the optical turn circuit 12, 
transmitting through the up optical transmission line to be received by 
the up optical reception end office 2a. In this case, the spectrum of the 
signal light turned and input into the optical power supervisory circuit 
17 is as shown by (c) in FIG. 4. The signal light with wavelengths 
.lambda.2, .lambda.4 in the spectrum has an optical-power-to-wavelength 
inclination indicated by a dotted line B. The optical power of the up 
signal light(.lambda.1 and .lambda.3) shows an inclination indicated by a 
dotted line A. The difference between the inclinations of the dotted lines 
A and B is equal to the property as shown by (e) in FIG. 4, i.e., the 
wavelength property of the entire down optical fiber transmission system. 
Similarly, the signal light with wavelengths .lambda.2, .lambda.4 output 
from the down optical transmission end office 1b transmits through the 
down optical transmission line to be received by the down optical 
reception end office 2b. On the other hand, the signal light with 
wavelengths .lambda.1, .lambda.3 output from the up optical transmission 
end office 1b transmits through the up optical transmission line, being 
led to the down optical transmission line through the optical turn circuit 
12, transmitting through the down optical transmission line to be received 
by the down optical reception end office 2b. In this case, the spectrum of 
the signal light turned and input into the optical power supervisory 
circuit 17 is as shown by (f) in FIG. 4. The signal light with wavelengths 
A1, A3 in the spectrum has an optical-power-to-wavelength inclination 
indicated by a dotted line D. The optical power of the down signal 
light(.lambda.2 and .lambda.4) shows an inclination indicated by a dotted 
line C. Calculating the difference between the inclinations of dotted 
lines C and D, the property as shown by (b) in FIG. 4, i.e., the 
wavelength property of the entire up optical fiber transmission system, 
can be determined. 
Here, the wavelengths .lambda.1, .lambda.2, .lambda.3 and .lambda.4 of the 
signal lights are 1554 nm, 1556 nm, 1558 nm and 1560 nm, respectively. 
Based on these wavelength properties, the control circuit 24 can control 
the power of the light source in the optical transmission end office to 
respond to the changes in gain or loss with time in the optical 
transmission system. 
An example of the optical power supervisory circuit 17 will be explained in 
FIG. 5. A control circuit 21 sets a wavelength of signal light to be 
extracted by a wavelength tunable extraction filter 18, and the power of 
the signal light which is converted into electricity by-an optoelectric 
converter 19 is sampled by a sampling circuit 20, and the power value 23 
of the signal light at each wavelength is led into a calculator 22. When 
the control circuit 21 sequentially selects the wavelength of the signal 
light to be measured and the power values 23 of the signal lights at 
different wavelengths are input to the calculator 22, the calculator 22 
calculates the wavelength property regarding the gain/loss of the entire 
optical transmission system and outputs the result of the calculation. 
An example of the optical transmission power(electric power) control 
circuit 24 will be explained in FIG. 6. The signal light source 5 
modulates the light of a laser diode 25 by an optical intensity modulator 
27 to superpose the data. Here, the optical intensity modulator 27 may 
include a lithium niobate modulator, EA(electro-absorption) modulator and 
the like. The optical transmission power(electric power) control circuit 
24, when it inputs the measurement result output from the optical power 
supervisory circuit 17, orders LD current control circuits 29 to change 
the bias current of each laser diode 25 in order to change the power of 
the signal light corresponding to a wavelength where the loss value in the 
optical fiber transmission line system is varied, based on the resultant 
information. For example, LD current is increased in the case that the 
loss value of the optical transmission line to a wavelength is increased. 
Thus, the optical transmission power(electric power) 24 always monitors 
the transmission optical power(electric power) at each wavelength and 
controls it such that the correction power value obtained from the 
measurement result is added or subtracted to or from the current power 
value. 
Here, used is a method for monitoring the transmission optical power, where 
the back output power of the laser diode 25 is detected by a photodiode 
26, then converting into electric information at a LD power monitoring 
circuit 28. In another way, it may be monitored such that the output light 
is divided by an optical coupler and then is photoelectrically detected. 
FIG. 7 shows an example of the optical amplifier repeater 4 which is 
provided with optical turn circuits 12. In the embodiment as shown in FIG. 
3, there are provided the optical turn circuits 12 in the optical 
transmission end office and optical reception end office. However, as 
shown in FIG. 7, when the optical turn circuit 12 is provided with the 
optical amplifier repeater 4 which is inserted in the optical fiber 
transmission line, the transmission line property at each repeater section 
can be monitored. Herein, if a plurality of such optical amplifier 
repeaters 4 are inserted, optical signals from the respective repeaters 
will be simultaneously turned. Therefore, to separately analyze them, the 
sampling time of each signal is needed to be adjusted by a certain signal 
sampling method. 
FIG. 8 shows an example of the optical transmission end office which is 
provided with supervisory light sources 30. In addition to the example as 
shown in FIG. 3, the supervisory light for monitoring the optical fiber 
transmission line property other than the signal light can be inserted. As 
shown in FIG. 8, a plurality of supervisory light sources 30, where even 
one light source can be effective, different from the signal light source 
5 are provided, their lights being output multiplexed with the signal 
lights. Though the optical power supervisory circuit 17 may be the same as 
that in FIG. 5, it needs to be set to also detect the optical power at the 
wavelengths of the supervisory lights. In this case, the wavelength 
property of the entire optical transmission system can be calculated only 
by the supervisory light or by the signal light and supervisory light. The 
merit of this manner is in that the optical fiber transmission line 
property can be in detail measured by optionally increasing the number of 
wavelengths of the supervisory light. The wavelengths of the supervisory 
light, corresponding to the wavelengths of the signal light described 
above, may be located between the signal light wavelengths or apart from 
the signal light wavelengths by a few nanometers, for example, 1553 nm, 
1555 nm, 1557 nm, 1559 nm and 1561 nm. 
A method for superposing an auxiliary signal with a frequency proper to the 
signal light source will be explained in FIG. 9. In the above-mentioned 
embodiments, the power(electric power) of the signal light at each 
wavelength or the supervisory light is directly measured. However, in this 
embodiment, the auxiliary signal with a proper frequency (or a proper 
signal manner) is superposed on the signal light at each wavelength and 
then is transmitted, and the optical power supervisory circuit 17 detects 
the intensity of the auxiliary signal to measure the wavelength property 
regarding the gain/loss of the optical transmission system. In this case, 
in an example of the optical transmission end office, the bias current to 
the laser diode 25 is intensity-modulated by a frequency oscillator 31 
with a proper supervisory frequency to modulate the intensity of the 
signal light to output the auxiliary signal. Alternatively, the auxiliary 
signal can be superposed by way of the frequency modulation or phase 
modulation. 
In the case of using the above method, the optical power supervisory 
circuit 17, as shown in FIG. 10, first converts the signal light into an 
electric signal through the optoelectric converter 19, then selecting 
signals through a frequency tunable extraction filter 32 for electric 
signal by order of a control circuit 21, thereafter monitoring the 
intensity of the auxiliary signal with the proper supervisory frequency. 
In this case, the frequencies of the auxiliary signals to be superposed 
are, for example, 10 MHz, 10.1 MHz and 10.2 MHz. 
Meanwhile, this measuring method that proper auxiliary signals are 
superposed can be also applied to the above-mentioned method of using the 
supervisory light source. 
Although the invention has been described with respect to specific 
embodiment for complete and clear disclosure, the appended claims are not 
to be thus limited but are to be is construed as embodying all 
modification and alternative constructions that may be occurred to one 
skilled in the art which fairly fall within the basic teaching here is set 
forth.