Abstract:
An input monitoring system is provided in an optical amplifying repeater to monitor a level of an optical input signal. A monitored level of the optical input signal is used to detect a fault on an optical transmission path or to control a bias current for a laser diode which emits and supplies an exciting signal for exciting an optical fiber amplifier. To accurately monitor a level of the optical input signal, the input monitoring system comprises an input terminal supplied with the optical input signal, an optical fiber amplifier for amplifying the optical input signal, a first photo diode for detecting and outputting an electric signal corresponding a level of the optical input signal, and an optical switch operatively connected to the input terminal, the optical fiber amplifier and first photo diode for switching the optical input signal alternatively into the optical fiber amplifier and the first photo diode.

Description:
This is a continuation of application Ser. No. 08/541,788, filed Oct. 10, 1995, now U.S. Pat. No. 5,838,476. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an input monitoring system for an optical amplifying repeater, and more particularly to an input monitoring system for an optical amplifying repeater, which includes an optical fiber amplifier for amplifying an optical signal. 
   2. Description of the Related Art 
     FIG. 14  shows a basic structure of an optical amplifying repeater having a function of ALC (Automatic Level Control), for which an object of the present invention is present. 
   In  FIG. 14 , reference numeral “ 1 ” is an optical fiber transmission path. An optical fiber amplifier  2  is formed of an optical fiber, which is doped with rare-earth material, such as Erbium (Er). 
   In this example, a principle of the optical fiber amplifier  2  is the same as that of a laser. The Er doped fiber generates a predetermined Fermi energy level, at which strong adsorption occurs. Accordingly, when electrons in an optical fiber are excited to the high Fermi energy level and thus the energy level of Er is reversed, a light having a wave-length corresponding to the difference between the high Fermi energy level and the low Fermi level is inputted and causes induced scattering in the optical fiber amplifier  2 . As the result, an amplified optical signal is output from the optical fiber amplifier  2 . 
   In  FIG. 14 , reference numeral “ 3 ” is a coupler. For exciting electrons in an optical fiber as described above, an exciting light outputted from a laser diode  4  is supplied to an optical fiber amplifier  2  via the coupler  3 . 
   The amplified optical signal outputted from the optical fiber amplifier  2  branches in a coupler  5 , and the signal branching is inputted to a photo diode unit  6 . A photo diode provided in the photo diode unit  6  converts the amplified optical signal to an corresponding electrical signal. In the photo diode unit  6 , the electrical signal outputted from the photo diode is further compared with a predetermined reference signal, and the difference between the electrical signal and the predetermined reference signal is supplied to an amplifier  7  to control a bias current for the laser diode  4 . Thus, the output of the laser diode  4  is automatically level-controlled. 
   In this example, ALC (Automatic Level Control) means to control the optical output signal sent from the laser diode  4  for exciting the optical fiber amplifier  2  according to the input signal level, and control so as to keep the level of the output signal to constant by changing an amplifying rate of the optical fiber amplifier  2 . 
   On the other hand, the repeater employs a structure for monitoring input signal levels to detect faults generated on the optical fiber transmission paths.  FIG. 15  shows a conventional structural example to monitor the input signal level. In  FIG. 15 , a coupler  10  is provided on input side of the optical fiber amplifier  2 . An optical input signal branches at the coupler  10 . Then, the signal branching is inputted to a photo diode unit  11  for monitoring the input level. 
   The photo diode unit  11  for monitoring input level converts the optical input signal, which branched in the coupler  10 , to an electrical signal to judge the input level of the signal. However, in the structure, when the level of the optical input signal is low, the level of the optical signal inputted to the photo diode  11  becomes more lower level, according to the branching in the coupler  10 . 
   Accordingly, errors become larger on detecting level of the optical input signal, which is converted to an electrical signal in the photo diode unit  11  and monitored. Reversely, if a branching ratio to the photo diode unit  11  becomes larger, a signal component, which is inputted to the optical fiber amplifier  2 , becomes smaller, thus making deterioration of SN ratio (Signal-to-Noise ratio) larger. A branching ratio, such as 20:1, is selected as usual. 
   Further, an optical fiber for transmitting the signal sent from a signal light source is connected to the input side of the optical fiber amplifier  2  shown in  FIG. 15 . The optical fiber has the property, of reflecting slightly an incoming optical signal to the reversed direction in an optical signal path, which is called so as Rayleigh scattering. The residual excited light, which is not used to amplify, leaks at the input side of the optical amplifier. Then, the leaked light goes back to the optical amplifier according to the Rayleigh scattering inside of the optical fiber for transmission, and enters to the photo diode for monitoring the input level with the signal light. Therefore, the input monitor value is not equal to an input power of the signal light, as shown in  FIG. 17 . Accordingly, the input power cannot be normally measured. 
   When the exciting light is supplied in the same direction as that of the signal light, namely by a forward excitation, the residual exciting light, which is not used to amplify, leaks at the output side of the optical fiber amplifier  2 . Therefore, the leaked light is delivered to the next optical amplifier via an optical fiber for transmission. The residual excited light enters to the photo diode for monitoring the input level of the next amplifier with the signal light. Accordingly, it is also impossible to normally measure the input power at the next optical amplifier. 
     FIG. 16  shows the other conventional structural example for monitoring the input signal level. As described above, the automatic level controller (ALC) controls the optical output signal of the laser diode  4  to change according to the input signal level. The photo diode  13  monitors the optical output of the laser diode  4 , so that it becomes possible to equivalently find an optical input signal level. 
   However, in this structure, if the level of the optical input signal is large, light-emission of the laser diode  4  becomes small to reduce an amplifying ratio of the optical fiber amplifier  2 . 
   Accordingly, in a structure shown in  FIG. 16 , the level of the exciting light, which is inputted to the photo diode  13  for monitoring, and branches at the coupler  3 , becomes smaller, whereas error due to the result of monitoring becomes larger. 
   As described above, in the conventional input monitoring system of the optical amplifying repeater, either of cases where the level of the optical input signal is low, that is, as shown in  FIG. 15 , and the level of the optical input signal is high, that is, as shown in  FIG. 9 , makes error in monitoring larger. 
   Therefore, if the optical input signal level varies in a wide range, it is impossible to accurately monitor the levels. 
   SUMMARY OF THE INVENTION 
   Accordingly, it is an object of the present invention to provide an input monitoring system of an optical amplifying repeater, which is very accurate in monitoring the optical input signal, when the optical input signal level varies in a wide range. 
   An input monitoring system is provided in an optical amplifying repeater to monitor a level of an optical input signal. A monitored level of the optical input signal is used to detect a fault on an optical transmission path or to control a bias current for a laser diode which emits and supplies an exciting signal for exciting an optical fiber amplifier. To accurately monitor a level of the optical input signal, the input monitoring system according to the present invention, comprises an input terminal supplied with the optical input signal, an optical fiber amplifier for amplifying the optical input signal, a first photo diode for detecting and outputting an electric signal corresponding a level of the optical input signal, and an optical switch operatively connected to the input terminal, the optical fiber amplifier and first photo diode for switching the optical input signal alternatively into the optical fiber amplifier and the first photo diode. 
   Further, other objects of the present invention become clearly by the description for explaining, embodiments according to the attached drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A through 1C  show a first embodiment of the present invention. 
       FIG. 2  is an operational explanatory diagram of the present invention. 
       FIG. 3  is a diagram showing a second embodiment of the present invention. 
       FIGS. 4A and 4B  show a third embodiment of the present invention. 
       FIGS. 5A and 5B  show a fourth embodiment of the present invention. 
       FIGS. 6A through 6C  show a fifth embodiment of the present invention. 
       FIG. 7  shows a sixth embodiment of the present invention. 
       FIG. 8  shows a diagram indicating an input spectrum to an optical filter. 
       FIG. 9  shows a diagram indicating a characteristic of the optical filter. 
       FIG. 10  shows a diagram indicating an output spectrum of the optical filter. 
       FIG. 11  shows a diagram indicating a characteristic of a conventional input power monitor. 
       FIG. 12  shows a diagram indicating a characteristic of an input power monitor according to the present invention. 
       FIG. 13  shows a diagram indicating an error of an input power monitor. 
       FIG. 14  shows a diagram indicating a basic structure of an optical amplifying repeater for an automatic level controller (ALC). 
       FIG. 15  shows an example of a prior art. 
       FIG. 16  shows another example of prior art. 
       FIG. 17  is a diagram explaining obstruction of an input level monitor. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Throughout the following descriptions, the same reference numerals are used to denote and identify corresponding or identical components. 
     FIGS. 1A through 1C  show a first embodiment of an input monitoring system for optical amplifying repeaters according to the present invention. In  FIG. 1A , the system includes an optical switch  12  on the input side of an optical fiber amplifier  2 . It is possible to input all amount of the optical input signal to an photo diode  11  for monitoring by switching an optical switch  12 . 
   The optical switch  12  is the same as those employed in other embodiments, which are described later. Various types of optical switches, which have been developed in recent years, can be used as well as a mechanical switch, for the optical switch  12 . 
   Consequently, it is possible to input an optical input signal to the photo diode  11  for monitoring, and reduce errors generated according to the result of monitoring, even if the level of the optical input signal is small, without further attenuating. 
   The structure shown in  FIG. 1B  further includes a photo diode  13 . A exciting light output of the laser diode  4  branches into the photo diode  13  and is monitored therein, so that level of the optical input signal can be equivalently detected. 
   Features of the level of an optical signal inputted to the photo diode  11 , which is provided on the input side of the optical fiber amplifier  2 , and of the level of an optical signal inputted to the photo diode  13 , which monitors the branched, excited light of the laser diode  4  have been explained above, regarding the prior arts.  FIG. 2  shows the features of the input level of optical signals. 
   In  FIG. 2 , an axis of abscissas shows a power in an optical input signal (P in dBm). Further, V 1  indicates output voltages converted by the photodiode  11  from optical input signals. V 2  also indicates voltages converted by the photo diode  12  for monitoring from the exciting light emitted in the laser diode  4 . 
   Positions for measuring the voltages V 1  and V 2  are as shown in  FIG. 3 . The voltage V 1  generates on a resistor  16  by current flowing to the photo diode  11  for monitoring according to an optical input signal. 
   The voltage V 2  is a voltage generated on the resistor  17  by flowing a current converted by the photo diode  13  from a light output emitted by the laser diode  4 , to which the photo diode  13  is coupled as a pair. 
   Returning now back to  FIG. 2 , the voltage V 1 , which indicates an output of the photo diode  11 , becomes smaller, as the optical input signal level becomes smaller. The minimum value is near 100 MV, which means that it is difficult to detect the level of the optical input signal in the photo diode  13  for monitoring. 
   Reversely, the level of the voltage V 2  that indicates converted outputs by the photo diode  13  for monitoring the emitted light from the laser diode  4  becomes larger, as the level of the optical input signal becomes smaller, according to the reason described above. 
   Accordingly, in the present embodiment shown in  FIG. 1B , to employ the features of voltages V 1  and V 2  efficiently, when an optical input signal becomes larger, the output from the photo diode  11  for monitoring on the input side is monitored, while the optical input signal becomes smaller, the switch is controlled to monitor output voltage V 2  of the photo diode  13 , which detects an exciting light of the laser diode  4 . 
   Therefore, in comparison with the conventional system, it becomes possible to monitor the input level in a wider dynamic range of an optical input signal. 
     FIG. 1C  shows a structure, in which power of the laser diode  4  in a backward direction is monitored by the photo diode  14 . The explanations relating to  FIGS. 2 and 3  are commonly applied to the structure to overcome the conventional drawbacks. 
     FIGS. 4A and 4B  further show an embodiment according to the present invention. Structures shown in  FIGS. 4A and 4B , as a basic structure, are the same as those shown in  FIGS. 1A  to  1 C. However, it is a different point that an avalanche photo diode  15 , of which amplifying rate is more than “1”, is employed instead of the photo diode  11  for monitoring, which is located on the input side of the optical fiber amplifier  2 . By that, it is possible to monitor the level of an optical input signal with a low error rate, even if the level of the optical input signal is small. 
   Other structures and functions are the same as those explained in  FIG. 1 , so that the explanations will be now abbreviated for simplicity. 
     FIGS. 5A and 5B  show another embodiment of the present invention. The embodiment shown in  FIGS. 5A and 5B  is different from the above-described embodiments, in that, a coupler  10  is provided instead of the optical switch  12 , which is located on the input side of the optical fiber amplifier  2 , on the structure. 
   The embodiment shown in  FIGS. 5A and 5B , to become possible to more accurately monitor the level of an optical input signal, a photodiode  13 , which monitors a light output emitted by the laser diode  4  for excision, or a photodiode  14 , which monitors the light output of the laser diode  4  in a backward direction, in correspondence with  FIGS. 1B and 1C . The reason is, therefore, the same as that explained with the use of  FIGS. 2 and 3 . 
     FIGS. 6A through 6C  show a fifth embodiment, which is further modified on the embodiment of  FIGS. 5A and 5B . That is, a coupler  10  is provided on the input side of the optical fiber amplifier  2 , and a photo diode  11  for monitoring an input signal level is placed to an avalanche photo diode  15 . 
   Further, the avalanche photo diode  15 , of which amplifying rate is more than “1”, is employed. Thereby, it becomes possible to effectively monitor the level of an optical input signal without errors, even if the level of the optical input signal is small. 
   Accordingly, as shown in the example of  FIG. 6A , when it is enough to monitor the optical input signal by the avalanche photo diode  15 , it is possible to omit a photo diode  12 , which monitors a light output emitted by the laser diode  4  for excision, or a photo diode  13 , which monitors the backward output of the laser diode  4  for excision on the output side of the optical fiber amplifier  2 . 
     FIGS. 6B and 6C  are respectively corresponding to  FIGS. 5A and 5B , in which the photo diode  11  is replaced to an avalanche photo diode  15 . Thereby, it becomes possible to more accurately monitor the optical input signal. 
     FIG. 7  further shows the other embodiment of the present invention. In the diagram shown in  FIG. 7 , an optical filter  20 , which removes residual excited lights, is provided in the previous stage of a photo diode  11  for monitoring the input level. 
   A part of an optical input signal a of 1.55 μm, which is sent from the optical fiber transmission path  1  branches at the coupler  10 . The branching signal is inputted to a photo diode  11  to monitor the input level of the optical input signal. The exciting light b of 1.48 μm, which is emitted by the laser diode  4 , is inputted to an optical fiber amplifier  2 , via a wave-length multiplying coupler  3 . However, a part of the light is leaked to the optical fiber transmission path  1  as a residual exciting light. Further, a part of the residual exciting light returns as a Rayleigh scattered light, but it is blocked by the optical filter  20 , and is not inputted to the photo diode for monitoring the input level. 
   As shown in  FIG. 8 , both the residual exciting light b besides the signal light a input to the optical fiber  20 . However, the optical filter  20  has a transmission characteristic, as shown in  FIG. 9 . Therefore, the light, of which wave-length is less than 1.50 μm, is removed, thus removing the residual exciting light, as shown in  FIG. 10 . Accordingly, it becomes possible to monitor the level of an optical input signal accurately. 
   A long wave-length pass filter, a band-stop filter for the 1.48 μm wave-band, a band-pass filter for 1.55 μm wave-band or an optical blanching device, which can pass the light of 1.55 μm, can be employed as an optical filter. Further, a bulk type filter, which employs a dielectric multi-layered film, or a fiber type filter, which has a filter characteristic due to cutting or fusing a fiber, can be used as an optical filter. 
   Furthermore, it is possible to accurately monitor the level of an optical input signal by providing the optical filter, even when it is the case of forward exciting. 
     FIG. 11  shows a characteristic diagram of an input power monitor for an optical direct amplifier corresponding to  FIG. 8 . 
   In  FIG. 11 , square marks (□) show data measured in the case of that an optical fiber for transmission, of which a length is several meters to several hundred meters, is not connected. Plus marks (+) show data measured in the case of that the optical fiber for transmission is connected. 
   When the optical fiber for transmission is connected, the residual exciting light goes back to the optical amplifier through the optical fiber, because of Rayleigh scattering. In this case, the characteristic of the input power monitor indicates a rising portion on a characteristic line, resulting in that it is difficult to accurately measure an input power level, and thus as shown in  FIG. 11 , values less than the rising portion cannot be measured at all. 
   If the optical fiber is not connected, no problem is brought for monitoring an input power, but the optical direct amplifier is provided to transmit an optical signal for a long distance, an therefore, an optical fiber for transmission is indispensable for transmitting the optical signal. 
     FIG. 12  shows a characteristic of the input power monitor when removing the influence of the residual exciting light by the system according to the present invention. In  FIG. 12 , a solid line shows data that are measured on the status where an optical amplifier having a structure shown in  FIG. 11  is employed and the optical fiber for transmission is connected. Plus marks (+) show data that are measured on the status where the optical fiber for transmission is connected. In  FIG. 12 , there is almost no difference between two cases. It is apparent from  FIG. 12  that the influence of the residual exciting light can be removed. 
     FIG. 13  represents graphically errors of measurement due to the influence of the residual exciting light of the input level monitor shown in  FIGS. 11 and 12 . Square marks (□) show an influence of the residual exciting light, when there is no filter on a PD. The input level, which is less than 17 dBm, becomes infinite, thus it is impossible to measure. Reversely, when a filter is connected to the PD, which is indicated by a plus mark (+), the input level, which is −20 dBm, can be within 1 dB of errors. Further, the data, which is less than 25 dBm, can be measured. 
   As explained according to the embodiments, the present invention makes it possible to monitor an optical input signal adaptively, even if the level of the optical input signal level is small. 
   Therefore, it is possible to monitor the optical input signal without errors in a wider dynamic range.