Abstract:
An optical amplifier for amplifying light includes a light source for emitting pump light in accordance with current amount; a rear earth element doped optical fiber doped rear earth element, the rear earth element doped optical fiber pumped by the pump light from the light source; a detector for detecting upconversion light leaked from the rear earth element doped optical fiber; a memory for storing correspondence relationship data of the current amount for the light source and an intensity of the upconversion light in normal state of the light source; a difference calculator for calculating a difference between the intensity of the upconversion light being detected by the detector and a converted amount being converted the current amount for the light source by the use of the correspondence relationship data; and a discriminator for discriminating whether the difference calculated at the difference calculator exceeds a predetermined value.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2008-045198, filed on Feb. 26, 2008, the entire contents of which are incorporated herein by reference. 
       FIELD 
       [0002]    The embodiments discussed herein are related to an optical amplifier for amplifying light. 
       BACKGROUND 
       [0003]    In optical communications, signal light is transmitted via an optical fiber. Since power of the signal light drops when the signal light travels along the optical fiber by a long distance, an optical fiber amplifier is used to amplify the power. More specifically, the optical fiber amplifier amplifies signal light power by the use of the stimulated emission process using excited state rare-earth ions so as to input pump light into an optical fiber doped a rare-earth element (amplifier medium). 
         [0004]    Currently, optical fiber amplifiers having the amplification medium doped with erbium are gaining popularity. Such an amplification medium and such an optical fiber amplifier are respectively referred to as EDF (Erbium Doped Fiber) and EDFA (Erbium Doped Fiber Amplifier). 
         [0005]    Japanese Laid-open Patent Publication No. 08-248455 discloses an optical fiber amplifier in which two stages of EDF are arranged with a variable optical attenuator inserted therebetween. Such a two-staged EDF optical fiber amplifier design is a widely used structure. 
         [0006]    The above-described two-staged EDF of optical fiber amplifier performs an efficient amplification operation so as to be inputted different pump light to each of stages, respectively. 
         [0007]    More specifically, a first-stage EDF (at a signal input side) receives an pump light failing within a 0.98 μm band that is excellent in noise characteristics but low in amplification efficiency, and a second-stage EDF (at a signal output side) receives an pump light falling within a 1.48 μm band that has a high amplification efficiency. 
         [0008]    Such pump light are generated by a pump light source such as an LD (Laser Diode). Such a pump light source continuously outputs a pump light at a substantially high power within a range of 100 mW—several hundreds mW in response to a current flowing therethrough. The pump light source fatigues more quickly than other optical components. Output power of the pump light source drops gradually. 
         [0009]    The EDFA largely changes in characteristics as an optical amplifier depending on the output power of the pump light incident on the EDF. When the output power drops below a lower limit thereof, replacing steps are preferably taken, for example, by replacing the light excitation source with a new one, or by replacing one component. For example, the output power of the pump light may be initially 300 mW with a current of certain value flowing through the pump light source. If the output power starts to drop later on and only an output power lower than 150 mW is obtained even with the same current flowing (also in the case of a trouble in which the output power suddenly drops to zero mW), it is determined that the pump light source suffers from a performance deterioration. 
         [0010]    If the output power of the pump light within the 1.48 μm band directed to the second EDF drops in the two-staged EDFA, the output level of the signal light also drops. Therefore, the output level of the second stage signal light is monitored, and when the output level drops below a predetermined lower limit, it is determined that the pump light source suffers from a performance deterioration. 
         [0011]    In contrast, if a drop takes place in the output power of the pump light within the 0.98 μm band directed to the first EDF, noise characteristics as an optical amplifier are deteriorated, but the output level of the signal light output from the first stage is not so largely affected. For this reason, even if the output level of the signal light from the first stage is monitored, it is difficult to determine whether the pump light source is degraded. 
         [0012]    Available as another determination method is the use of back power monitor provided by the LD. However, currently, particularly, the accuracy of the back monitor of the LD with an FBG (Fiber Bragg Grating) is low, and unstable depending a polarized wave state of the pump light, etc. Because of this, there are times when the amplifier is determined to be degraded even though the LD is not actually degraded. 
         [0013]    In another method, branching part of the pump light, such as a beam splitter, is arranged immediately subsequent to the output of the LD, and the output power of the branched pump light is directly monitored. With this method, the deterioration determination is performed. However, costs for mounting the branching means are required, and the LD needs to output extra power for branching the pump light. Costs for installing such an LD are additionally needed. 
       SUMMARY 
       [0014]    According to an aspect of the invention, an optical amplifier includes a light source for emitting pump light in accordance with current amount; a rear earth element doped optical fiber doped rear earth element, the rear earth element doped optical fiber pumped by the pump light from the light source; a detector for detecting upconversion light leaked from the rear earth element doped optical fiber; a memory for storing correspondence relationship data of the current amount for the light source and an intensity of the upconversion light in normal state of the light source; a difference calculator for calculating a difference between the intensity of the upconversion light being detected by the detector and a converted amount being converted the current amount for the light source by the use of the correspondence relationship data; and a discriminator for discriminating whether the difference calculated at the difference calculator exceeds a predetermined value. 
         [0015]    The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  illustrates a determination method of a deterioration of a pump light source. 
           [0017]      FIG. 2  illustrates a method of picking up up-conversion light with an optical fiber. 
           [0018]      FIG. 3  diagrammatically illustrates an optical fiber amplifier. 
           [0019]      FIG. 4  illustrates an example of information registered in a mapping table. 
           [0020]      FIG. 5  is a flowchart illustrating the flow of a process of a pump light LD deterioration detector  200 . 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0021]    An aspect of embodiments is to provide an optical amplifier to determine the deterioration of the pump light source mounted at a stage prior to a stage from which the signal light is output. 
         [0022]    Embodiments of an optical fiber amplifier of the present invention are described below in detail with reference to the drawings. 
         [0023]    Referring to  FIG. 1 , the principle of an optical fiber amplifier is described first. Wrapped round a bobbin  300  is an optical fiber (hereinafter referred to as EDF (Erbium Doped Fiber)) doped with erbium ions. Signal light (I 1 ) to be amplified and 0.98 μm pump light (I 2 ) generated by a pump light LD (Laser Diode)  320  are multiplexed by an optical coupler  330  and then incident on the EDF  310 . In the EDF  310 , erbium ions are excited by the 0.98 μm pump light, and the light signal is then incident on the excited erbium ions, thereby causing stimulated emission. As a result, the signal light is amplified. 
         [0024]    When the signal light is amplified as illustrated, green light called up-conversion light  340  is observed in the EDF  310 . This light is light emitted when some erbium ions excited at two stages by the pump light return to ground level. 
         [0025]    The intensity of the up-conversion light  340  is proportional to the intensity of the pump light. If the pump light weakens, the number of erbium ions excited at two stages decreases, and the intensity of the up-conversion light  340  weakens. 
         [0026]    The pump light LD  320  operates by a driving current flowing from an LD driver circuit  350 , and emits pump light responsive to the driving current. Given the same driving current, the pump light LD  320 , if degraded, can no longer emit the pump light at the same level. 
         [0027]    If the intensity of the up-conversion light  340  responsive to the pump light emitted from the pump light LD  320  not yet degraded is stored, the degree of deterioration of the pump light LD  320  is determined with reference to the stored intensity. In accordance with the present embodiment, the deterioration of the pump light LD  320  is detected by monitoring the up-conversion light  340 . 
         [0028]    The deterioration detection method of the pump light LD  320  is continuously discussed. First, the pump light LD  320  is driven by the LD driver circuit  350  in no deterioration state before an actual service phase. With the driving current then changed, a plurality of driving current values are extracted as data. 
         [0029]    Also, the intensity of the up-conversion light  340  leaking out of the EDF  310  is measured by a photo-detector  210 . The measured values are then taken and stored in a mapping table  220  with the above described driving current values mapped thereto. In addition to the up-conversion light  340 , the light leaking from the EDF  310  includes ASE (Amplified Spontaneous Emission) in a 1.55 μm band and the 0.98 μm pump light. A filter is preferably mounted to filter out these light rays in order allow only the up-conversion light  340  to pass therethrough. 
         [0030]    With the pump light LD  320  in service, the driving current flowing from the LD driver circuit  350  and the measurement values measured by the photo-detector  210  are constantly acquired. These two pieces of data acquired are input to a calculation circuit  250 , which in turn calculates a difference between the measurement value mapped to the input driving current value in the mapping table and the input measurement value. If the calculated difference exceeds above a predetermined value, an pump light LD deterioration alarm unit  260  emits an alarm sound, thereby alerting a person to the deterioration of the pump light LD  320 . 
         [0031]    The up-conversion light  340  is equally observed as long as optical fibers doped with a rare-earth element are used. The present invention is not limited to erbium ions. The present invention is applicable to any optical fiber amplifier having as an amplification medium an optical fiber doped with a rare-earth element. 
         [0032]    The up-conversion light  340  leaks out from the side of the EDF  310  wrapped around the bobbin  300  and then scattered in all directions. The up-conversion light  340  is thus preferably collected using a lens rather than directly collected. Furthermore, as illustrated in  FIG. 2 , another optical fiber  360  is routed along the side of the EDF, and leaked light is allowed to be picked up by the optical fiber  360  and then similarly directed to the photo-detector  210  (the other end is terminated). If the routed optical fiber  360  is deviated in position, for example, due to vibration, the intensity of the up-conversion light measured by the photo-detector  210  varies. The optical fiber  360  is thus preferably secured with an adhesive agent such as a UV (Ultraviolet rays) resin. 
         [0033]    The notification of the deterioration of the pump light LD  320  is not limited to the alarm sound. Any type of notification is acceptable as long as a person in charge is notified. For example, information regarding the deterioration detection may be transmitted a center. 
         [0034]    The determination of the intensity of the up-conversion light  340  measured prior to the actual service of the pump light LD  320  is not limited to the determination based on the mapping relationship stored in the mapping table  220 . An approximation equation is beforehand determined from the mapping relationship, and the intensity of the up-conversion light  340  is thus determined by substituting the driving current value into the approximation equation. 
         [0035]    A general structure of the optical fiber amplifier including a pump light LD deterioration detector detecting a deterioration of the pump light LD through the above-described deterioration detection method is described below with reference to  FIG. 3 . 
         [0036]      FIG. 3  illustrates the general structure of the optical fiber amplifier. As shown in  FIG. 3 , an optical fiber amplifier  10  has an optical connector  20  on the left-hand side of the page serving as an input terminal of the signal light and an optical connector  150  on the right-hand side of the page serving as an output terminal of the signal light. The optical fiber amplifier  10  includes, in succession to the optical connector  20  as the input terminal, an optical isolator  30  for suppressing oscillation, an optical coupler  40  for multiplexing the signal light and the pump light, an EDF  50  for amplifying the signal light, and an optical isolator  60  for suppressing oscillation, and further an pump light LD  70  for inputting the pump light to the optical coupler  40 , and an LD driver circuit  80  for driving the pump light LD  70 . 
         [0037]    These elements form a front stage of the optical amplifier, and 0.98 μm pump light serves as the pump light. This arrangement improves noise characteristics and causes the signal light to be amplified at the front stage, and generally controls a deterioration in the noise characteristics of the whole optical fiber amplifier  10  (due to a loss in a subsequent gain equalizer  90  and variable optical attenuator  100 ). 
         [0038]    In succession to the front stage, the optical fiber amplifier  10  includes the gain equalizer  90  for flattening multi-wave signal gain characteristics of the optical amplifier and the variable optical attenuator  100  for keeping multi-wave signal gain flat against input level variations of the signal light. Another successive stage of amplifier is arranged to compensate for the level drop of the signal light caused by the gain equalizer  90  and the variable optical attenuator  100 . 
         [0039]    More specifically, as in the front stage, the optical fiber amplifier  10  includes in succession to the variable optical attenuator  100  an optical isolator  110  for suppressing oscillation, an optical coupler  120  for multiplexing the signal light and the pump light, an EDF  130  for amplifying the signal light, and an optical isolator  140 , connected to the optical connector  150 . Further included are a pump light LD  160  for inputting the pump light to the optical coupler  120  and a LD driver circuit  170  for driving the pump light LD  160 . 
         [0040]    These elements form a back stage, and 1.48 μm pump light is used as the pump light. Since a high signal output power is typically required of the back stage, the 1.48 μm pump light is used. However, to improve noise characteristics, the 0.98 μm pump light may also be used. 
         [0041]    The optical fiber amplifier  10  includes a pump light LD deterioration detector  200  for detecting a deterioration in the pump light LD  70  at the front stage. 
         [0042]    The pump light LD deterioration detector  200  includes the photo-detector  210 , the mapping table  220 , a current detector  230 , a current detector  240 , the calculation circuit  250 , and the pump light LD deterioration alarm unit  260 . 
         [0043]    The photo-detector  210  detects the up-conversion light leaking out of the EDF as a physical quantity. More specifically, the photo-detector  210 , including an optical filter, blocks light having wavelengths other than the up-conversion light of 0.5 μm-0.6 μm with the optical filter, and detects the up-conversion light with a photo-diode. In this way, only the up-conversion light is incident on the photo-diode, causing a current responsive to the intensity of the light to flow through the photo-diode. 
         [0044]    The mapping table  220  stores as a plurality of different current value pairs the correspondence relationship between the value of the current flowing to operate the pump light LD in an undegraded state prior to actual service and the intensity of the up-conversion light emitted from the EDF in response to the pump light of the pump light LD supplied with the current. More specifically, as illustrated in  FIG. 4 , the mapping table  220  stores as the plurality of different current value pairs the correspondence relationship between the value of the current being an LD driving current value of the LD driver circuit and the light intensity being a photo-diode output current value. 
         [0045]    The mapping table  220  is produced prior to the actual service of the optical fiber amplifier  10 . The current the LD driver circuit supplies to the pump light LD is changed in intensity prior to the actual service of the optical fiber amplifier  10 . 
         [0046]    The current detector  230  acquires a plurality of LD driving current values from the LD driver circuit and registers the acquired LD driving current values in the mapping table  220 . 
         [0047]    The up-conversion light leaks out of the EDF in response to the pump light of the pump light LD, and a current flows through the photo-diode of the photo-detector  210  in response to the intensity of the light. The current detector  240  acquires a plurality of photo-diode output current values from the photo-detector  210  at the timing the current detector  230  acquires the LD driving current values, and then registers the acquired photo-diode output current values in the mapping table  220 . 
         [0048]    When the actual service of the optical fiber amplifier  10  starts, the deterioration detection of the pump light LD by the pump light LD deterioration detector  200  starts. 
         [0049]    With the optical fiber amplifier  10  in service, the calculation circuit  250  calculates a difference between the photo-diode output current value corresponding to the value of the current actually flowing from the LD driver circuit to the pump light LD according to the mapping table  220  and the value of the current actually flowing through the photo-diode. 
         [0050]    The pump light LD deterioration alarm unit  260  monitors the difference calculated by the calculation circuit  250 , and emits an alarm sound if a predetermined value is exceeded. In this way, the maintenance person is alerted to a deterioration of the pump light LD  70 . 
         [0051]    Furthermore, the deterioration detection target of the pump light LD deterioration detector  200  is not limited to the pump light LD at the front stage in the two-staged optical fiber amplifier  10  illustrated in  FIG. 3 . In the case of a three-staged or higher-staged optical fiber amplifiers, pump light LDs at all the stages prior to the final stage from which the signal light is output may be deterioration detection targets. 
         [0052]    Lastly, with reference to  FIG. 5 , the flow of the process of the pump light LD deterioration detector  200  is described.  FIG. 5  is a flowchart illustrating the flow of the process of the pump light LD deterioration detector  200 . 
         [0053]    If the pump light LD is mounted (yes in step S 110 ), the current detector  230  and the current detector  240  register the correspondence relationship between the LD driving current value and the photo-diode output current value into the mapping table  220  (step S 120 ). 
         [0054]    When the actual service of the optical fiber amplifier  10  starts, the pump light LD deterioration detector  200  starts monitoring a deterioration of the pump light LD (step S 130 ). 
         [0055]    With the optical fiber amplifier  10  in service, the calculation circuit  250  calculates a difference between the value of the current actually flowing through the photo-diode and the photo-diode output current value corresponding to the value of the current actually flowing from the LD driver circuit to the pump light LD according to the mapping table  220 . (step S 140 ) 
         [0056]    The pump light LD deterioration alarm unit  260  determines whether the difference exceeds the predetermined value (step S 150 ). If it is determined that the difference exceeds the predetermined value (yes in step S 150 ), the pump light LD deterioration alarm unit  260  emits an alarm sound (step S 160 ). Processing thus ends. 
         [0057]    In accordance with the above-described embodiments, the deterioration of the pump light source is detected. Also, cost reduction can be achieved in comparison with the method of detecting the deterioration by directly monitoring part of the pump light. Furthermore, relatively accurate detection is achieved in comparison with the method of monitoring the back power of the pump light source. 
         [0058]    In the disclosed device, an intensity of up-conversion light drops if an output power of pump light drops. The intensity of the light of an pump light source is monitored during an operation thereof, and is compared with a light intensity that is obtained by measuring the output power of the pump light output by the pump light source that is not yet degraded in order to determine whether the pump light source is degraded. Even with multi-staged amplification media, it is determined whether the pump light source mounted at a stage prior to a stage from which the signal light is output is degraded or not. 
         [0059]    All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiment(s) of the present inventions have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.