Abstract:
An optical transmission apparatus includes a splitter configured to split an input optical signal into a first optical signal and a second optical signal, a signal length determiner configured to determine a signal length of the first optical signal per unit time, an optical power detector configured to detect an optical power of the first optical signal per unit time, a delay unit configured to delay the second optical signal, an optical amplifier configured to amplify the second optical signal delayed by the delay unit, a first excitation light source configured to generate an excitation light to be supplied to the optical amplifier, and a first excitation light power adjustor configured to adjust an optical power of the excitation light to be supplied to the optical amplifier in accordance with the signal length of the first optical signal and the optical power of the first optical signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2012-134992, filed on Jun. 14, 2012, the entire contents of which are incorporated herein by reference. 
       FIELD 
       [0002]    The embodiments regarding technologies discussed herein are related to an optical transmission apparatus and an optical transmission method. 
       BACKGROUND 
       [0003]    An optical amplifier configured to amplify an optical burst signal is used in an optical packet transmission apparatus configured to use an optical burst signal. 
         [0004]    In the case where an optical burst signal is input to an optical amplifier, even though the optical amplifier is in a sufficiently excited state just after the optical burst signal is input, since the optical amplifier uses energy to amplify the input optical burst signal, the optical amplifier shifts from the excited state to a lower state as time goes by. As a result, the output power of an output signal from the optical amplifier is high just after the optical burst signal is input, and the output power decreases as time goes by. That is, an optical surge occurs just after the optical burst signal is input. A technology for suppressing this optical surge is proposed (see, for example, Japanese Laid-open Patent Publication No. 9-200145 and Japanese Laid-open Patent Publication No. 2001-352297). 
       SUMMARY 
       [0005]    According to an aspect of the invention, an optical transmission apparatus includes a splitter configured to split an input optical signal into a first optical signal and a second optical signal, a signal length determiner configured to determine a signal length of the first optical signal per unit time, an optical power detector configured to detect an optical power of the first optical signal per unit time, a delay unit configured to delay the second optical signal, an optical amplifier configured to amplify the second optical signal delayed by the delay unit, a first excitation light source configured to generate an excitation light to be supplied to the optical amplifier, and a first excitation light power adjustor configured to adjust an optical power of the excitation light to be supplied to the optical amplifier in accordance with the signal length of the first optical signal and the optical power of the first optical signal. 
         [0006]    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. 
         [0007]    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 DRAWINGS 
         [0008]      FIG. 1  is a schematic diagram that illustrates an optical transmission apparatus discussed herein; 
           [0009]      FIG. 2  is a schematic diagram that illustrates an optical transmission apparatus according to a first embodiment regarding a technology discussed herein; 
           [0010]      FIG. 3  is a schematic diagram that illustrates signals in the optical transmission apparatus according to the first embodiment regarding the technology discussed herein; 
           [0011]      FIG. 4  is a schematic diagram that illustrates signals in the optical transmission apparatus according to the first embodiment regarding the technology discussed herein; 
           [0012]      FIG. 5  is a schematic diagram that illustrates an optical transmission apparatus according to a second embodiment regarding a technology discussed herein; 
           [0013]      FIG. 6  is a schematic diagram that illustrates a modified example of the optical transmission apparatus according to the second embodiment regarding the technology discussed herein; 
           [0014]      FIG. 7  is a schematic diagram that illustrates an optical transmission apparatus according to a third embodiment regarding a technology discussed herein; and 
           [0015]      FIG. 8  is a schematic diagram that illustrates an optical transmission apparatus according to a fourth embodiment regarding a technology discussed herein. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0016]    With regard to an optical surge occurring in an optical packet transmission apparatus configured to use an optical burst signal, it becomes clear that the gain of an optical amplifier changes in accordance with the length of each of optical burst signals and the density of the optical burst signals (the proportion of optical burst signals per unit time) input to the optical amplifier, and consequently, the optical power of an output signal changes. 
         [0017]    Hereinafter, preferable embodiments for technologies regarding an optical transmission apparatus that may stabilize the gain of an optical amplifier even in the case where optical burst signals input to the optical amplifier change in terms of length and density will be described with reference to the figures. 
         [0018]      FIG. 1  is a schematic diagram that illustrates an optical transmission apparatus discussed herein. In the first to fourth embodiments described below, an optical packet transmission apparatus  1  configured to use optical burst signals includes an optical burst signal transmission device  120 , an optical amplification unit  200 , an optical packet switch  400 , an optical amplification unit  500 , and an optical burst signal receiving device  620 . The optical packet transmission apparatus  1  further includes a controller  700 . The optical burst signal transmission device  120 , the optical amplification unit  200 , the optical packet switch  400 , the optical amplification unit  500 , and the optical burst signal receiving device  620  are controlled by the controller  700 . The optical amplification unit  200  includes optical fiber amplifiers  211  and  212 . The optical amplification unit  500  includes optical amplifiers  510  and  520 . Into the optical packet transmission apparatus  1 , an optical burst signal is output from the optical burst signal transmission device  120  or from a wavelength separator (not illustrated) of a wavelength division multiplexing (WDM) transmission device  110  provided in a WDM network. An output optical burst signal is amplified by the optical fiber amplifier  212  or  211 , and subsequently a path for the amplified signal is selected by the optical packet switch  400 . The optical burst signal is subsequently amplified by the optical amplifier  520  or  510  and the loss caused by the optical packet switch  400  is compensated. Then, the optical burst signal is input to the optical burst signal receiving device  620  or a WDM transmission device  610  provided in the WDM network. 
       First Embodiment 
       [0019]      FIG. 2  is a schematic diagram that illustrates an optical transmission apparatus regarding a technology discussed herein. The optical amplification unit  200  includes an optical splitter  220 , an optical burst signal monitor  230 , a power detector  240 , an excitation laser output controller  250 , an excitation laser  253 , an optical delay unit  270 , and an optical fiber amplification unit  210 . 
         [0020]    The optical splitter  220  is provided between the optical burst signal transmission device  120  and the optical delay unit  270  and between the WDM transmission device  110  and the optical delay unit  270 , and connected to the optical burst signal monitor  230  and the power detector  240 . 
         [0021]    The optical burst signal monitor  230  includes a photo detector  231 , a differentiator  232 , a signal length determiner  233 , a signal density determiner  234 , and a supervisory timer  235 . The photo detector  231  is connected to the optical splitter  220 . The differentiator  232  is connected to the photo detector  231 . The signal length determiner  233  and the signal density determiner  234  are connected to the differentiator  232 . The supervisory timer  235  is connected to the signal length determiner  233  and the signal density determiner  234 . 
         [0022]    The power detector  240  includes a photo detector  241 , a comparator  242 , and a reference power-value generator  243 . The photo detector  241  is connected to the optical splitter  220  and the differentiator  232 . The comparator  242  is connected to the photo detector  241  and the reference power-value generator  243 . 
         [0023]    The excitation laser output controller  250  includes an optical switch  252  and an optical switch controller  251 . The optical switch controller  251  is connected to the signal length determiner  233 , the signal density determiner  234 , the comparator  242 , and the optical switch  252 . 
         [0024]    The optical delay unit  270  includes delay fibers  271  and  272 . The delay fibers  271  and  272  are each connected to the optical splitter  220 . 
         [0025]    The optical fiber amplification unit  210  includes the optical fiber amplifiers  211  and  212 . The optical fiber amplifiers  211  and  212  are connected to the delay fibers  271  and  272 , respectively. An erbium doped fiber (EDF) amplifier is used as each of the optical fiber amplifiers  211  and  212  in this embodiment. An excitation laser beam output from the excitation laser  253  enters each of the optical fiber amplifiers  211  and  212  via the optical switch  252 . 
         [0026]      FIG. 3  is a schematic diagram that illustrates signals in the optical transmission apparatus regarding the technology discussed herein. An operation of the optical amplification unit  200  will be described with reference to  FIG. 3 . 
         [0027]    The optical splitter  220  receives optical burst signals A (A 1 , A 2 , and A 3 ) output from the optical burst signal transmission device  120  or from the WDM transmission device  110 , and outputs optical burst signals B (B 1 , B 2 , and B 3 ) and optical burst signals C (C 1 , C 2 , and C 3 ). The optical burst signal A 1  is split into the optical burst signals B 1  and C 1 . The optical burst signal A 2  is split into the optical burst signal B 2  and C 2 . The optical burst signal A 3  is split into the optical burst signal B 3  and C 3 . The optical burst signals B, which are further split by the optical splitter  220 , are output to the photo detector  231  of the optical burst signal monitor  230  on one hand and to the photo detector  241  of the power detector  240  on the other hand. The optical burst signals C, which are split by the optical splitter  220 , are output to the optical delay unit  270 . More specifically, the optical burst signals A output from the optical burst signal transmission device  120  are input to the optical splitter  220  and the optical burst signals B and C are output from the optical splitter  220 . The optical burst signals B, which are output from the optical splitter  220 , are input to the photo detector  231  on one hand and to the photo detector  241  on the other hand. The optical burst signals C, which are output from the optical splitter  220 , are input to the delay fiber  272  of the optical delay unit  270 . The optical burst signals A output from the WDM transmission device  110  are input to the optical splitter  220  and the optical burst signals B and C are output from the optical splitter  220 . The optical burst signals B, which are output from the optical splitter  220 , are input to the photo detector  231  on one hand and to the photo detector  241  on the other hand. The optical burst signals C, which are output from the optical splitter  220 , are input to the delay fiber  271  of the optical delay unit  270 . 
         [0028]    The optical burst signal monitor  230 , to which the optical burst signals B output from the optical splitter  220  on one hand are input, performs a determination operation for determining the signal lengths, signal intervals, and signal densities (each signal density representing the proportion of optical burst signals each unit time) of the optical burst signals B, which are output from the optical splitter  220  on one hand. 
         [0029]    The optical burst signals B are input to the photo detector  231  in the optical burst signal monitor  230  receiving the optical burst signals B. The photo detector  231  detects the optical burst signals B. The detected optical burst signals B are input to the differentiator  232 . 
         [0030]    The differentiator  232  detects the start and the end of each of the optical burst signals B through detection of rising and falling edges of the optical burst signal B. Start detection signals D (D 1 , D 2 , and D 3 ) are generated by detecting the rising edges of the optical burst signals B (B 1 , B 2 , and B 3 ), and end detection signals E (E 1 , E 2 , and E 3 ) are generated by detecting the falling edges of the optical burst signals B (B 1 , B 2 , and B 3 ). The start detection signal D 1  is generated by detecting the rising edge of the optical burst signal B 1 , and the end detection signal E 1  is generated by detecting the falling edge of the optical burst signal B 1 . The start detection signal D 2  is generated by detecting the rising edge of the optical burst signal B 2 , and the end detection signal E 2  is generated by detecting the falling edge of the optical burst signal B 2 . The start detection signal D 3  is generated by detecting the rising edge of the optical burst signal B 3 , and the end detection signal E 3  is generated by detecting the falling edge of the optical burst signal B 3 . 
         [0031]    The signal length determiner  233  calculates signal lengths of optical burst signal lengths F (F 1 , F 2 , and F 3 ) by using the start detection signals D (D 1 , D 2 , and D 3 ) and the end detection signals E (E 1 , E 2 , and E 3 ). The signal lengths of optical burst signal length F 1  is calculated by using the start detection signal D 1  and the end detection signal E 1 . The signal lengths of optical burst signal length F 2  is calculated by using the start detection signal D 2  and the end detection signal E 2 . The signal lengths of optical burst signal length F 3  is calculated by using the start detection signal D 3  and the end detection signal E 3 . The signal length determiner  233  also calculates signal intervals of an optical burst signal by using the end detection signal E and the start detection signal D subsequent to the end detection signal E. For example, the signal interval between the optical burst signal B 1  and the optical burst signal B 2  is calculated by using the end detection signal E 1  and the start detection signal D 2 , which is subsequent to the end detection signal E 1 . For example, a counter is preferably used as the signal length determiner  233 . 
         [0032]    The signal density determiner  234  performs a determination operation for determining an optical burst signal density by using the number of times the start detection signals D (D 1 , D 2 , and D 3 ) are detected and the optical burst signal lengths F (F 1 , F 2 , and F 3 ) per unit time. 
         [0033]    Here, the unit time is a density monitor time period in which the optical burst signal density is monitored and which is determined by the supervisory timer  235 . In the case where the density monitor time period is too long, optical burst signal densities vary greatly from the average optical burst signal density. In the case where the density monitor time period is too short, output control of the excitation laser is not performed in time or the excitation laser is likely to oscillate because the density monitor time period is close to a time period in which the output control of the excitation laser is performed. Thus, the density monitor time period is set to a time period almost the same as a time period corresponding to the length of the longest optical burst signal (from about ten and several microseconds to about several tens of microseconds) and the output of the excitation laser is controlled in a stepwise manner in accordance with the optical burst signal density per density monitor time period. Here, since a desired performance and the amount of information handled in a transmission network vary from region to region, it is desirable that the density monitor time period is determined in accordance with the average of signal densities, the average of signal lengths, and a statistical distribution. 
         [0034]    As described above, the optical burst signal density is determined by using the number of times the start detection signals D are detected and the optical burst signal lengths F per unit time (the density monitor time period). Thus, a time period in which the signal length determiner  233  described above performs a determination operation for determining signal lengths is also determined by the supervisory timer  235 . Here, it is desirable that the optical burst signal density is obtained by using the number of optical burst signals per unit time (the density monitor time period) and the optical burst signal lengths F of the optical burst signals. Thus, in order to obtain the number of the optical burst signals, the number of times the end detection signals E are detected may be used instead of the number of times the start detection signals D are detected. The optical burst signal density is the proportion of optical burst signals each unit time (the density monitor time period). Determination of the density monitor time period and the determination operation for determining the optical burst signal density are performed by, for example, the controller  700 . 
         [0035]    With reference to  FIG. 3 , a density monitor time period  1  has the same time length as a density monitor time period  2 , and the density monitor time periods  1  and  2  are determined by the supervisory timer  235 . The density monitor time periods  1  and  2  are set to F 0 . The start detection signals D detected in the density monitor time period  1  are the start detection signals D 1  and D 2 , and the number of times the start detection signals D are detected is two. The lengths of the optical burst signals B (B 1  and B 2 ) determined in the density monitor time period  1  are F 1  and F 2 , respectively. Thus, the optical burst signal density is (F 1 +F 2 )/F 0  in the density monitor time period  1 . Moreover, the start detection signal D detected in the density monitor time period  2  is the start detection signal D 3 , and the number of times the start detection signals D are detected is one. The length of the optical burst signal B (B 3 ) detected in the density monitor time period  2  is F 3 . Thus, the optical burst signal density is F 3 /F 0  in the density monitor time period  2 . 
         [0036]    The power detector  240 , to which the optical burst signals B output from the optical splitter  220  on the other hand are input, performs a detection operation for detecting the optical power of only optical burst signals in accordance with burst monitor information supplied from the optical burst signal monitor  230 . The photo detector  241  of the power detector  240  receives the start detection signals D (D 1 , D 2 , and D 3 ) and end detection signals E (E 1 , E 2 , and E 3 ) of the optical burst signals B (B 1 , B 2 , and B 3 ) from the differentiator  232  of the optical burst signal monitor  230 . The photo detector  241  generates rectangular waves, each of which represents a time period in which an optical burst signal exists, by using the start detection signals D (D 1 , D 2 , and D 3 ) and the end detection signals E (E 1 , E 2 , and E 3 ), respectively. The photo detector  241  measures, for example, the optical power of the optical burst signal B 1  only in the time period in which the optical burst signal B 1  exists, the optical power of the optical burst signal B 2  only in the time period in which the optical burst signal B 2  exists, and the optical power of the optical burst signal B 3  only in the time period in which the optical burst signal B 3  exists, by using the generated rectangular waves. The comparator  242  compares the value of the measured optical power of each of the optical burst signals B 1 , B 2 , and B 3  with a reference power value generated by the reference power-value generator  243 , and outputs power information obtained as a result of comparison. 
         [0037]    The excitation laser output controller  250  performs control of a light beam output from the excitation laser  253  in accordance with monitor information of optical burst signals supplied from the optical burst signal monitor  230  and the power information supplied from the power detector  240 . The excitation laser  253  supplies excitation power to the optical fiber amplifiers  211  and  212  of the optical fiber amplification unit  210 . 
         [0038]    The excitation laser output controller  250  controls Off/On of the output of the excitation laser  253  by using the optical burst signal lengths F (F 1 , F 2 , and F 3 ) supplied from the signal density determiner  234  of the optical burst signal monitor  230  and intervals between the optical burst signals. Note that, in the case where the excitation laser  253  itself is turned off or on, the excitation laser does not stabilize until a certain time has passed. The excitation laser  253  remains turned on, and the optical switch controller  251  controls Off/On of the output of the excitation laser  253  from the optical switch  252  by controlling the optical switch  252 . 
         [0039]    An excitation laser output Off/On signal G used to control Off/On of the output of the excitation laser  253  and supplied from the optical switch controller  251  is basically generated in accordance with the optical burst signal lengths F (F 1 , F 2 , and F 3 ). In the case where the excitation laser output Off/On signal G is On (denoted by G 11  and G 13 ) as illustrated in an optical amplifier excitation state H, irradiation of the optical fiber amplifiers  211  and  212  with an excitation laser beam output from the excitation laser  253  starts. In the beginning, there are time periods (denoted by H 1  and H 3 ) in which excitation is insufficiently performed by the optical fiber amplifiers  211  and  212 . Moreover, there are time periods (denoted by H 2  and H 4 ) in which excitation is performed by fluorescence after the irradiation of the optical fiber amplifiers  211  and  212  with the excitation laser beam output from the excitation laser  253  finishes. In the case where there is a signal interval between the optical burst signals B that is shorter than a time period obtained by adding a time period (such as H 1  and H 3 ) in which excitation is insufficiently performed and a time period (such as H 2  and H 4 ) in which excitation is performed by fluorescence, the excitation laser output Off/On signal G remains On (denoted by G 4 ). 
         [0040]    The optical delay unit  270  delays the optical burst signals C output from the optical splitter  220 . The optical delay unit  270  includes the delay fibers  271  and  272 . The optical burst signals C (C 1 , C 2 , and C 3 ) supplied from the optical splitter  220  are delayed by the delay fibers  271  and  272  of the optical delay unit  270  so that the optical burst signals C (C 1 , C 2 , and C 3 ) enter the optical fiber amplification unit  210  after the excitation laser  253  starts to perform output. The delay fibers  271  and  272  include preferably a single mode fiber, a highly nonlinear fiber, or the like. 
         [0041]    The optical burst signals C (C 1 , C 2 , and C 3 ) delayed by the delay fibers  271  and  272  serve as optical amplifier input signals I (I 1 , I 2 , and I 3 ) and are input to the optical fiber amplifiers  211  and  212  of the optical fiber amplification unit  210 . In the case where irradiation of the optical fiber amplifiers  211  and  212  with an excitation laser beam output from the excitation laser  253  starts, in the beginning, there are time periods (denoted by H 1  and H 3 ) in which excitation is insufficiently performed by the optical fiber amplifiers  211  and  212 . Thus, the optical burst signals C are delayed so that the optical burst signal C 1  is input to the optical fiber amplifiers  211  and  212  after the time period (denoted by H 1 ) in which excitation is insufficiently performed by the optical fiber amplifiers  211  and  212  and so that the optical burst signal C 3  is input to the optical fiber amplifiers  211  and  212  after the time period (denoted by H 3 ) in which excitation is insufficiently performed by the optical fiber amplifiers  211  and  212 . 
         [0042]    The optical fiber amplifiers  211  and  212  of the optical fiber amplification unit  210  amplify the optical burst signals C (C 1 , C 2 , and C 3 ) delayed by the delay fibers  271  and  272 , that is, the optical amplifier input signals I (I 1 , I 2 , and I 3 ). The amplified optical amplifier input signals I (I 1 , I 2 , and I 3 ) are output as optical amplifier output signals  3  (J 1 , J 2 , and J 3 ) from the optical fiber amplifiers  211  and  212 . 
         [0043]    In the case where a signal is input to an optical amplifier such as the optical fiber amplifiers  211  and  212 , energy is used to amplify the signal. Therefore, the larger the optical power of the signal is and the higher the signal density of the signal is, the smaller the output of the optical amplifier becomes. As illustrated in  FIG. 3 , the signal density of the optical burst signals varies since the optical burst signals may exist in a dense manner or in a scattered manner in a transmission line. Moreover, the optical burst signals may be different in terms of optical power. Since the optical amplifier performs amplification in accordance with the average optical power of input signals, there is an optical power difference D between the input and the output of the optical amplifier as illustrated in  FIG. 4 . Here, it is assumed that all optical burst signals have the same optical power. That is, the optical amplifier input signals I (I 1 , I 2 , and I 3 ) have the same optical power. The signal density of the optical amplifier input signals I 1  and I 2  is high and the signal density of the optical amplifier input signal I 3  is low. Thus, in the case where the same optical output power of the excitation laser  253  is used for the optical amplifier input signals I (I 1 , I 2 , and I 3 ), the optical power of each of the optical amplifier output signals J 1  and J 2  is smaller than that of the optical amplifier output signal J 3 . For this reason, the optical power and signal densities of the optical burst signals are measured; in addition to the above-described controlling Off/On of the output of the excitation laser  253 , in the case where the signal density is high, control is performed by increasing the output of the excitation laser  253  so that the optical power difference D decreases and an amplification factor stabilizes. A larger output of the excitation laser  253  is used for the optical amplifier input signals I 1  and I 2 , which have a high signal density, than for the optical amplifier input signal I 3 , which has a low signal density, and the optical power of optical amplifier output signals J′ 1  and J′ 2  become the same as that of an optical amplifier output signal J′ 3 . Here, the optical power of a laser beam output from the excitation laser  253  does not change over time. 
         [0044]    More generally, the average optical power of optical burst signals input to an optical amplifier (hereinafter referred to as an “optical amplifier input power average”) is calculated by using a power detection result supplied from the power detector  240  and the signal lengths of the optical burst signals per unit time (the density monitor time period) supplied from the optical burst signal monitor  230 . The unit time (the density monitor time period) is set to F 0  as described above, and it is assumed that there are, for example, optical burst signals B, the number of which is n, (B 1 , B 2 , . . . , and Bn) per unit time (the density monitor time period). The optical burst signals B 1 , B 2 , . . . , and Bn have optical power P 1 , P 2 , . . . , and Pn, respectively. The optical burst signals B 1 , B 2 , . . . , and Bn have signal lengths F 1 , F 2 , . . . , and Fn, respectively. The optical amplifier input power average is (P 1 ·F 1 +P 2 ·F 2 + . . . +Pn·Fn)/F 0 . 
         [0045]    A reference value for the optical amplifier input power average is preset. In the case where the optical amplifier input power average is larger than this reference value, the optical switch controller  251  of the excitation laser output controller  250  controls the optical switch  252  so that the optical switch  252  opens, and the amount of the excitation laser beam to be input to the optical amplifier increases. In contrast, in the case where the optical amplifier input power average is smaller than this reference value, the optical switch controller  251  of the excitation laser output controller  250  controls the optical switch  252  so that the optical switch  252  closes, and the amount of the excitation laser beam to be input to the optical amplifier decreases. Such a control is performed by the controller  700 . 
         [0046]    Here, in the case where the optical burst signals B 1 , B 2 , . . . , and Bn have the same optical power (that is, the optical power P 1 , P 2 , . . . , and Pn are the same) and the optical power is set to P 0 , the optical amplifier input power average is P 0 (F 1 +F 2 + . . . +Fn)/F 0 . As described above, (F 1 +F 2 + . . . +Fn)/F 0  is the signal density (the proportion of the optical burst signals each unit time). Thus, in the case where the optical burst signals have the same optical power, a reference value for the average signal density is preset. In the case where the average signal density is larger than this reference value, the optical switch controller  251  of the excitation laser output controller  250  controls the optical switch  252  so that the optical switch  252  opens, and the amount of the excitation laser beam to be input to the optical amplifier increases. In contrast, in the case where the average signal density is smaller than this reference value, the optical switch controller  251  of the excitation laser output controller  250  controls the optical switch  252  so that the optical switch  252  closes, and the amount of the excitation laser beam to be input to the optical amplifier decreases. Such a control is performed by the controller  700 . 
         [0047]    An optical switch using a PLZT (Plumbum Lanthanum Zirconate Titanate) thin film or a Mach-Zehnder type optical switch is preferably used as the optical switch  252 . Such a switch may change arbitrarily the transmittance thereof in the case where the switch is off in accordance with the transmittance in the case where the switch is on. 
       Second Embodiment 
       [0048]      FIG. 5  is a schematic diagram that illustrates an optical transmission apparatus regarding a technology discussed herein. In this embodiment, an acousto-optical switch  254  is used instead of the optical switch  252  of the first embodiment as a switch configured to control the output of the excitation laser  253 . The rest of the structure is the same as that of the first embodiment. The acousto-optical switch  254  may change the transmittance thereof in a continuous manner by changing a voltage being applied or a current being supplied thereto. 
         [0049]    The amount of the excitation laser beam to be input to the optical fiber amplifiers  211  and  212  from the excitation laser  253  may be continuously changed by also using a digital to analog (DA) converter  259  or the like, in accordance with a signal supplied from the power detector  240  as illustrated in  FIG. 6 . As a result, the amount of the laser beam to be input to the optical fiber amplifiers  211  and  212  from the excitation laser  253  may be increased without stopping input of the excitation laser beam to the optical fiber amplifiers  211  and  212 . The DA converter  259  is inserted between the optical switch controller  251  and the acousto-optical switch  254 . 
       Third Embodiment 
       [0050]      FIG. 7  is a schematic diagram that illustrates an optical transmission apparatus according to a technology discussed herein. In this embodiment, an excitation laser  255 , an optical switch  256 , and an optical multiplexer  257  are used in addition to the excitation laser  253  and optical switch  252  of the first embodiment. The rest of the structure is the same as that of the first embodiment. The optical multiplexer  257  multiplexes an excitation laser beam output from the excitation laser  253  and an excitation laser beam output from the excitation laser  255  and supplies a resulting laser beam to the optical fiber amplifiers  211  and  212 . For example, an optical coupler is preferably used as the optical multiplexer  257 . In the case where a single excitation laser is used, when the density of optical burst signals becomes higher, there is a possibility that the optical output power of the excitation laser becomes insufficient. A sufficient optical output power may be achieved by providing a plurality of excitation lasers. Even in the case where the signal density of optical burst signals becomes 100%, a sufficient amplification factor may be obtained. 
       Fourth Embodiment 
       [0051]      FIG. 8  is a schematic diagram that illustrates an optical transmission apparatus according to a technology discussed herein. In this embodiment, an optical laser current/voltage controller  258  that controls a current to be supplied to and a voltage to be applied to the excitation laser  253  is used instead of the optical switch controller  251  and optical switch  252  of the first embodiment. The rest of the structure is the same as that of the first embodiment. In the first embodiment, the optical power of the laser beam output from the excitation laser  253  does not change over time, and the optical power of the excitation laser beam to be input to the optical fiber amplifiers  211  and  212  is controlled by controlling the transmittance of the optical switch  252 . In contrast, in this embodiment, increasing or decreasing of the optical power of an excitation laser beam is performed, in accordance with the signal densities and optical power of optical burst signals, by increasing or decreasing the optical power of a laser beam output from the excitation laser  253 . The optical power of the laser beam output from the excitation laser  253  is controlled by controlling a current to be supplied to or a voltage to be applied to the excitation laser  253  from the optical laser current/voltage controller  258 . In comparison with the first to third embodiments, cost reduction may be realized with the structure of this embodiment because the optical switches  252  and  256  and the acousto-optical switch  254  are not used. Note that, since there is a time lag after a current to be supplied or a voltage to be applied is changed until the output of the excitation laser  253  changes, the delay fibers  271  and  272  in the optical delay unit  270  are longer than those of the first to third embodiments. 
         [0052]    The optical amplification unit  200 , which includes the optical fiber amplifiers  211  and  212 , has been described in the first to fourth embodiments. The optical amplification unit  500 , which includes the optical amplifiers  510  and  520 , has the same structure as the optical amplification unit  200 . 
         [0053]    Structures obtained by combining some of the first to fourth embodiments may be used in the technologies discussed herein. 
         [0054]    In the first to fourth embodiments described above, gain control may be performed in accordance with the change in the optical power input to the optical amplification unit in the case where the density of packets changes. Thus, the optical power output from the optical amplification unit may be unlikely to change. As a result, even in the case where the density of optical packet signals becomes high, transmission performance improves since the optical signal-to-noise ratio (OSNR) is unlikely to decrease. Furthermore, the optical power output from the optical amplification unit is unlikely to change due to the change in the density of optical packet signals, and thus the range of the power input to the optical packet receiver is reduced. As a result, the optical packet signals may be farther transmitted. 
         [0055]    The embodiments, which are typical of the technologies discussed herein, are described above; however, the technologies discussed herein are not limited to the embodiments. 
         [0056]    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 embodiments of the present invention 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.