Abstract:
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.

Description:
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 λ1, λ3 and down signal light wavelengths λ2, λ4 is multiplexed and transmitted. 
     In an up optical transmission end office 1a, signal lights 6 with wavelengths λ1, λ3 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 λ1, λ3, and being received by optical receivers 11. The down optical transmission system where signal lights with wavelengths λ2, λ4 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 λ1, λ3 or λ2, λ4 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 λ2 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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be explained in more detail in conjunction with the appended drawings, wherein: 
     FIG. 1 is a block diagram showing a conventional optical wavelength multiplexing transmission apparatus, 
     FIG. 2 shows spectra of signal lights and wavelength properties of the optical fiber transmission line system in the apparatus shown in FIG. 1, 
     FIG. 3 is a block diagram showing an optical wavelength multiplexing transmission apparatus in a preferred embodiment according to the invention, 
     FIG. 4 shows spectra of signal lights and wavelength properties of the optical fiber transmission line system in the apparatus shown in FIG. 3, 
     FIG. 5 is a block diagram showing an example of an optical power supervisory circuit in FIG. 3, 
     FIG. 6 is a block diagram showing an example of an optical power transmission control circuit in FIG. 3, 
     FIG. 7 is a block diagram showing an example of an optical amplifier repeater in FIG. 3, 
     FIG. 8 is a block diagram showing another example of an optical transmission end office in FIG. 3, 
     FIG. 9 is a block diagram showing further another example of an optical transmission end office in FIG. 3, and 
     FIG. 10 is a block diagram showing another example of an optical power supervisory circuit in FIG. 3. 
    
    
     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 λ1, λ3 and down signal light wavelengths λ2, λ4 is multiplexed and transmitted. 
     In an up optical transmission end office 1a, signal lights 6 with wavelengths λ1, λ3 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 λ1, λ3, and being received by optical receivers 11. The down optical transmission system where signal lights with wavelengths λ2, λ4 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(λ1, λ2, λ3, λ4) 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 λ1, λ3 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 λ2, λ4 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 λ2, λ4 in the spectrum has an optical-power-to-wavelength inclination indicated by a dotted line B. The optical power of the up signal light(λ1 and λ3) 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 λ2, λ4 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 λ1, λ3 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(λ2 and λ4) 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 λ1, λ2, λ3 and λ4 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.