Abstract:
An optical amplifier to reduce signal loss by reducing crosstalk, and method therefor. A demultiplexer isolates an optical signal into a first wavelength band and a second wavelength band. The first and second wavelength bands are separately amplified in first and second optical amplifiers, respectfully, first and second optical amplifies each including cutoff filters to cutoff the wavelength band not corresponding to the particular amplifier. Optical receiving elements monitor light input to the first and second optical amplifiers and receive crosstalk signals on an output side.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Japanese Application No. 11-295661, filed Oct. 18, 1999, in the Japanese Patent Office, the content of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an optical amplifier that reduces signal loss by reducing crosstalk. 
     With the rapid development of multimedia networks, demand for information is increasing remarkably and therefore there is a need for further improvement in capacity and network flexibility. These needs may be addressed by improving the main optical transmission system for concentrating information capacity. 
     The wavelength division multiplexing (WDM) system is currently the most effective system for meeting the above-described demand and it is now intended for commercial use mainly in North America. In the WDM transmission system, an optical fiber amplifier is an essential device. 
     The present invention is applied to a structure of an optical amplifier to amplify a plurality of multiplexed bands. More specifically, the present invention allows the band of an optical amplifier to be widened. 
     2. Description of the Related Art As a method of widening the bandwidth of a repeater utilizing an optical fiber amplifier, there is provided a structure in which a plurality of bands are respectively amplified with exclusive optical amplifiers. An optical multiplexer and an optical demultiplexer are respectively provided at the input and output ends for parallel amplification. 
     Referring to FIG. 1, which is a block diagram of a multiple-band-transmission system with a parallel amplifying structure, including a transmitting part  1 - 1  and a receiving part  1 - 2 , a demultiplex coupler  100  demultiplexes the C-band (1.53 to 1.56 μm) and the L-band (1.57 to 1.60 μm). A multiplex coupler  200  multiplexes the C-band and L-band. An optical amplifier  700  amplifies the C-band and an optical amplifier  800  amplifies the L-band. 
     The optical amplifying repeater of FIG. 1 has a multiple bandwidth parallel amplifying structure such that crosstalk exists in the demultiplex coupler  100  and the multiplex coupler  200 . Namely, in demultiplex coupler  100 , the C-band output is in the range of 1.53 to 1.56 μm. Ideally, this output is at the port on the C-band side of optical amplifier  700 , but when isolation for rejecting the L-band is set to a higher value, the insertion loss of demultiplex coupler  100  tends to become large and therefore loss of the signal transmitted (main signal light) becomes large, resulting in deterioration of an SN (signal to noise) ratio of the communication system as a whole. 
     Conversely, when isolation of the demultiplex coupler  100  is set to a relatively low value in order to maintain lower loss for the main signal, a problem arises in that the light of the L-band (1.57 to 1.60 μm) is output as the crosstalk light (leakage light), also at the port of the optical amplifier  700  side of the demultiplex coupler  100  which naturally has to output only 1.53 to 1.56 μm of the C-band. Similarly, a certain amount of C-band leakage light is also incident to the port on the optical amplifier  800  side on the L-band side of demultiplex coupler  100 . 
     If crosstalk light occurs, it will result in detection level error in an optical input monitor located within optical amplifier  700  on the C-band side and optical amplifier  800  on the L-band side. In particular, when the optical amplifiers  700 , 800  are controlled based on such a detection level (gain constant control or AGC control) and input off monitor (shut-down detection control) control error occurs because an L-band optical light leaks toward the C-band side and a C-band light leaks toward the L-band side. 
     For example, we will discuss the case of AGC control as applied to optical amplifier  700 , noting that the AGC control is performed in a similar manner at optical amplifier  800 . The optical power level of the exciting light source of optical amplifier  700  is controlled so that the gain becomes constant by detecting an input to output ratio. The input light monitor of optical amplifier  700  detects an input level in which a crosstalk light is added to the optical power of the wavelength band of the main element and the monitor of an output light of optical amplifier  700  can neglect the crosstalk light because optical amplifier  700  does not amplify the light outside the amplifying bandwidth. 
     Therefore, since the influence of the crosstalk light on the input monitor and output monitor is different, correct detection of gain, namely, AGC control, is not conducted for a light within the amplifying wavelength band of the optical amplifier  700 . 
     Moreover, when a shut-down detection is to be performed to prevent surge at the time of recovery by monitoring the input light and stopping pumping of optical amplifier  700  or fixing pumping to a particular value if there is no optical input, it may occur that the shut-down condition of the light cannot be detected due to the influence of the crosstalk light despite the fact that the light in the band of the optical amplifier  700  is shut-down. Moreover, such crosstalk may also occur in multiplex coupler  200 . 
     In the case when the output of optical amplifier  700  and the connectors of the transmission lines are disconnected, to assure operator safety, it is required that such a condition be automatically detected in order to stop or decrease the output of optical amplifiers  700 , 800 . 
     In general, the output of optical amplifier  700  is stopped or decreased by obtaining an amount of reflection with detection of the levels of the reflected light and output of light at the output of each optical amplifier  700 , 800  in order to determine a difference in reflection amount when the connectors are connected or when they are disconnected. 
     When the connectors are disconnected, the light, which is subjected to Fresnel reflection (−14 dB) at the connector, and loss equal to two times the loss of the multiplex coupler  200 , is returned to optical amplifiers  700 , 800 . Assuming the maximum multiplex filter loss, the connector opening detection threshold value is set to a value less than the amount of reflection. 
     Meanwhile, when the connectors are connected, considering two times the loss of multiplex coupler  200  as the amount of reflection (determined by reflection attenuation of the connector and Rayleigh scattering in the transmission line), the maximum amount of reflection is assumed and the threshold value of the connector connecting condition is set higher than the maximum reflection amount. 
     When isolation of optical multiplex coupler  200  is set to a relatively low value, the light of the other bands return to optical amplifiers  700 , 800  as the crosstalk light (leakage light). Therefore, a problem arises in that the amount of reflection when the connectors are connected increases, a difference of setting values of threshold values when the connectors are opened and connected becomes relatively small, and it becomes difficult to set the threshold value and thereby normal operation is lost. 
     SUMMARY OF THE INVENTION 
     In the present invention, in order to overcome the crosstalk of the filter for dividing the band and for amplifying the light in the multiple wavelength bandwidths during optical communication, the optical monitor for each band of the optical amplifier is given the characteristic of rejecting the crosstalk light. 
     Thereby, rejection of the crosstalk light can be realized without increasing the loss of the main signal and accordingly the structure of the optical amplifier/repeater, which maintains the controllability of the optical amplifier without determining the signal characteristics, is realized. 
     Additional objects and advantages of the invention will be set forth in part in the description which follows 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects and advantages of the invention will become apparent and more readily appreciated from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings of which: 
     FIG. 1 a block diagram of a multiple band transmission system with a parallel amplifying structure; 
     FIG. 2 is a block diagram of a multiple band optical amplifier according to a first embodiment of the present invention; 
     FIG. 3 is a wavelength characteristic of filters  771 ,  871 ,  772 ,  872  of FIG.  2 . 
     FIG. 4 is a block diagram of a multiple band optical amplifier according to a second embodiment of the present invention; 
     FIG. 5 is a block diagram of a multiple band optical amplifier according to a third embodiment of the present invention; 
     FIG. 6 is a wavelength characteristic of the third embodiment of the present invention; 
     FIG. 7 is a block diagram of a multiple band optical amplifier according to a fourth embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the present preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. 
     FIG. 2 illustrates a multiple-band optical amplifier  10  according to a first embodiment of the present invention. 
     In FIG. 2, band demultiplex coupler  1  demultiplexes C-band and L-band and band multiplex coupler  2  multiplexes C-band and L-band. First C-band optical amplifier  7  and first L-band optical amplifier  8  amplify C-band and L-band, respectively. L-band rejection filters  771 , 772  and C-band rejection filters  871 , 872  filter C-band and L-band, respectively. Optical receiving elements  791 ,  891 ,  792 ,  892 ,  793  and  893  monitor the optical powers, respectively. 
     An optically multiplexed signal from a transmission line (not shown) is input to band demultiplex coupler  1  and is demultiplexed to the 1.53 to 1.56 μm band of C-band and the 1.57 to 1.60 μm band of L-band and are respectively input to first C-band optical amplifier  7  and first L-band optical amplifier  8 . 
     In this case, in order to reduce loss of the main signal element, a bandwidth demultiplex coupler  1  having a relatively low isolation and large crosstalk between the bands is used. 
     The optical branching coupler  751  within first C-band optical amplifier  7  branches a predetermined amount of C-band output of the bandwidth demultiplex coupler  1 . The branched output is filtered with L-band rejecting filter  771  for the purpose of rejecting the wavelength element other than the C-band. Optical receiving element  791  (which may be a photodiode) monitors the C-band filtered light by detecting optical power of the C-band with rejection filter  771  and then inputs this optical power to control circuit  710 . 
     The control circuit  710  is operated by AGC (auto gain controlling) receives a light power from optical receiving element  792 (which may be a photodiode) that is amplified by Erbium-doped fiber (EDF) of C-band and output lights of lasers  731 , 732 ; outputs of the lasers  731 ,  732  are adjusted to obtain a constant value of the ratio of optical receiving elements  791 , 792  and a pumping light is pumped for the EDF  711  for C-band with the WDM (wavelength division multiplex) couplers  741 ,  742 . 
     In the present invention, even in the case of AGC control, since the crosstalk light does not enter the monitor of the optical amplifier  7  on the input side, correct AGC control is conducted for the light in the amplifying wavelength band of first optical amplifier  7 . 
     When shut-down is detected, outputs of lasers  731 ,  732  are controlled by control circuit  710  on the basis of the output received with the optical receiving element  791 . 
     Optical demultiplex coupler  755  demultiplexes the light reflected from bandwidth multiplex coupler  2  and the light is then filtered with L-band rejection filter  772  and is then input to optical receiving element  793  (which may be a photodiode). 
     Control circuit  710  detects an amount of reflection light from the transmission line and a repeater (not shown) in the subsequent stage based on the ratio of an output of optical receiving element  793  and output to the band multiplex coupler  2  from optical demultiplex coupler  755  in order to control the optical output of C-band optical amplifier  7 . The output to band multiplex coupler  2  from optical demultiplex coupler  755  is known from the value of optical receiving element  792 . 
     With the structure explained above, the crosstalk of the band multiplex coupler  2  is rejected with L-band rejection filter  772 . Therefore, even if crosstalk is generated in band multiplex coupler  2 , only the light of C-band is incident to optical receiving element  793  without influence of crosstalk. 
     Second C-band optical amplifier  78  and variable attenuator  76  are optionally provided. An output of the multiple band optical amplifier  10  is controlled to adjust (equalize) the gain for each wavelength under automatic level control (ALC). 
     Second C-band optical amplifier  78  usually performs the AGC control and the light is input to second C-band optical amplifier  78  in order to always make constant the output of second C-band optical amplifier  78  which is controlled with variable attenuator  76 . 
     The elements corresponding to the first C-band optical amplifier  7  in the first L-band optical amplifier  8  are designated with the same reference numbers except for the first digit and like elements perform like operations. In this embodiment, lasers  731 ,  732 ,  831 ,  832  produce a laser light having a wavelength of 0.98 μm or 1.48 μm. Moreover, operations are possible only with forward pumping of the lasers  731 ,  831  or alternatively with backward pumping of lasers  732 ,  832 . Control circuits  710 ,  810  have been explained with respect to AGC control, shut-down detection and backward monitoring. Moreover these control circuits may also be adapted to ALC control using an input monitor value. 
     In the first embodiment of the present invention, as illustrated in FIG. 3, band rejection filters  771 ,  871 ,  772 ,  872  are capable of transmitting one band and rejecting the other bandwidth. When this rejection characteristic is sufficient, the characteristic specification of crosstalk between bands of the band demultiplex coupler  1  is alleviated to reduce the loss of the main signal. Particularly, when each band is amplified with the optical amplifiers of two stages, if the gain is not corrected accurately in the preceding stage, an output deviation becomes large if gain tilt is generated after the output level becomes high in the subsequent stage. Therefore, it is necessary to accurately monitor the light in the band that is amplified with the optical amplifier. 
     When one isolation is neglected in the band demultiplex coupler  1 , it is enough for the respective band rejection filter to have only one band. For example, if isolation on the C-band side of the band demultiplex coupler  1  is raised, the L-band rejection filter  771  is no longer required and only the C-band rejection filter  871  is required. Moreover, if isolation on the C-band side of bandwidth multiplex coupler  2  is raised, L-band rejection filter  772  is no longer required and only C-band rejection filter  872  is required. However, at the time of amplification, it must be considered that if isolation is increased, loss becomes high. 
     FIG. 2 illustrates a structure which is applied to an inline amplifier. When this structure is used in a post-amplifier, band demultiplex coupler  1 , and rejection filters  771 ,  871  are omitted. When the structure is used as a pre-amplifier, the band multiplex coupler  2  and rejection filters  772 ,  872  are not used. This is also true of the second and fourth embodiments. 
     FIG. 4 illustrates a multiple band optical amplifier  20  according to a second embodiment of the present invention. Multiple band optical amplifier  20  differs from multiple band optical amplifier  10  of FIG. 2 in that the function of a band rejection filter (not shown) is provided inside the optical coupler and individual band rejection filters  771 ,  871  are therefore not needed. The other elements are identical to multiple band optical amplifier  10  of FIG.  2 . 
     Furthermore, in the second embodiment, WDM coupler  743  for demultipexing only a part of the C-band wavelength is provided on the output of the C-band side of the band demultiplex coupler  1 . In the same manner, WDM coupler  744  for multiplexing only a part of the C-band wavelength is also provided to the optical receiving element  791  for monitoring the emitted light in the C-band side of the band multiplex coupler  2 . Use of WDM coupler  744  which can realize the monitoring by rejecting the wavelength different from the amplifying band of the optical amplifier provides the advantage that characteristic specification of band demultiplex coupler  1  can be alleviated and a band reject filter can also be removed individually. 
     FIG. 5 illustrates a third embodiment of the present invention. In this third embodiment, particular attention is paid to the performance of band rejection filters  745 , 845  which perform a function similar to that of rejection filters  771 ,  772 ,  871 ,  872  and WDM couplers  743 ,  843 . Band rejection filters  745 ,  845  are designed to provide a smooth characteristic, as illustrated in FIG.  6 . 
     The amount of rejecting the crosstalk light through the band rejection filters  745 ,  845  is not required to be rejected perfectly in relation to the predetermined band, and is effective only by rejecting the predetermined amount of the total power. Thus, even if a band rejection type filter  771 ,  772 ,  871 ,  872  as illustrated in FIG. 3 is not required, any filter, which is designed to show such a smooth characteristic as illustrated in FIG. 6, sufficiently functions if it can reject the predetermined amount of total power in the predetermined band. 
     FIG. 7 illustrates a multiple band optical amplifier  30  according to a fourth embodiment of the present invention. The embodiment of FIG. 7 has a structure for electrically correcting a processor function that calculates, after detecting the receiving optical level in the optical receiving elements  791 ,  891 ,  793 ,  893 , respectively, the incident crosstalk light level from the assumed crosstalk of the band demultiplex coupler  1  and band multiplex coupler  2  and the optical receiving level of the other band, and then uses the optical receiving level after subtraction of such incident crosstalk light level as the optical input level. 
     For instance, when the receiving optical level on the L-band side is P inL , receiving optical level in the C-band side is P inC , crosstalk of L-band to the C-band side in the demultiplex coupler  1  is L Xt  (dB), crosstalk of C-band to the L-band side is C Xt  (dB), true L-band value is P L  (dBm), true C-band value is P C  (dBm), demultiplex loss of L-band in the bandwidth demultiplex coupler is L L  (dB), demultiplex loss of C-band in the bandwidth demultiplex coupler is L C  (dB), isolation for the C-band is I CL  (dB), isolation for the L-band is I LC  (dB) the following equations (1), (2) are defined, which describes that P inL  is the antilog sum of (P L −L L ) and (P C −I CL ), and that P inC  is the antilog sum of (P C −L C ) and (P L −I LC ). 
     
       
           P   inL   =P   L   −L   L   +C   Xt   =P   L   −L   L +( P   C   −I   CL )  (1) 
       
     
     
       
           P   inC   =P   C   −L   C   +L   Xt   =P   C   −L   C +( P   L   −I   LC )  (12) 
       
     
     Here, L L , L C , I CL , I LC , are determined uniquely depending on the bandwidth of demultiplex coupler  1  and the bandwidth of multiplex coupler  2 . Therefore, the true L-band value P L  and true C-band value P C  can be obtained by detecting P inL  and P inC  and then calculating these values by the equations (1) and (2). 
     In particular, processor circuit  91  satisfying the above equations is provided to conduct the calculation on the basis of the outputs of optical receiving elements  791 ,  891  and then the true L-band value P L  and true C-band value P C  which have been obtained by electrically correcting the error due to crosstalk of the band demultiplex coupler  1  are input to the control circuits  710  and  810 . Thereby, control circuits  710 ,  810  can execute the predetermined control explained in regard to the first embodiment. 
     Moreover, the control circuits  710 ,  810  can also execute the predetermined controls by providing processor circuit  92  satisfying the above equations to calculate outputs of optical receiving elements  793 ,  893  and then respectively inputting the true C-band value and L-band value to the control circuits  710  and  810  obtained by electrically correcting the error due to crosstalk data of the band multiplex coupler  2 . 
     The present invention can prevent control error due to the crosstalk between the bands occurring in the multiple bandwidth optical fiber amplifier without deterioration of characteristic of the main signal. 
     Although a few preferred embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes might be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.