Abstract:
An apparatus including a semiconductor optical amplifier configured to amplify an input optical signal, and a controller configured to supply preheat current to the semiconductor optical amplifier when the input optical signal is not input to the semiconductor optical amplifier.

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. 2009-30993 filed on Feb. 13, 2009, the entire contents of which are incorporated herein by reference. 
       FIELD 
       [0002]    Embodiments discussed herein are related to a semiconductor optical amplifier apparatus. 
       BACKGROUND 
       [0003]    Semiconductor optical amplifiers (SOAs) typically have optical amplification factors that vary under different conditions. For example, the optical amplification factor of a semiconductor optical amplifier has a correlation with temperature. More specifically, the optical amplification factor of a semiconductor optical amplifier increases with a decrease in the temperature of an active layer of the semiconductor optical amplifier and decreases with an increase in the temperature of the active layer. Therefore, the temperature of the semiconductor optical amplifier is controlled using a thermo-electric cooler (TEC). 
         [0004]    Furthermore, the optical amplification factor of a semiconductor optical amplifier has a correlation with driving current supplied to an active layer thereof. More specifically, the optical amplification factor of a semiconductor optical amplifier increases with an increase in driving current and decreases with a decrease in driving current. Accordingly, a technology for controlling the optical amplification factor of a semiconductor optical amplifier by using driving current is disclosed (for example, “The Institute of Electronics, Information and Communication Engineers (IEICE) Technical Report, CPM, electronic parts and materials (CPM 2001-125),” Vol. 101, No. 517 (20011214) pp. 23-28 (The Institute of Electronics, Information and Communication Engineers ISSN: 09135685)). 
       SUMMARY 
       [0005]    According to an aspect of an embodiment, an apparatus includes a semiconductor optical amplifier configured to amplify an input optical signal, and a controller configured to supply preheat current to the semiconductor optical amplifier when the input optical signal is not input to the semiconductor optical amplifier. 
         [0006]    It is to be understood that both the foregoing summary description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0007]      FIG. 1  is a schematic view of a PON system to which a semiconductor optical amplifier device according to a first embodiment of the present invention is applied; 
           [0008]      FIG. 2A  is a schematic view illustrating the configuration of an entire semiconductor optical amplifier device according to an embodiment; 
           [0009]      FIG. 2B  illustrates an optical burst signal; 
           [0010]      FIGS. 3A ,  3 B,  3 C,  3 D,  3 E, and  3 F illustrate the relationship between the temperature of a semiconductor optical amplifier and the gain thereof; 
           [0011]      FIG. 4  illustrates output light in a case where a typical optical burst signal is input to a semiconductor optical amplifier; 
           [0012]      FIGS. 5A ,  5 B,  5 C,  5 D,  5 E, and  5 F illustrate the relationship between the temperature of a semiconductor optical amplifier and the gain thereof in a case where preheat current is supplied to the semiconductor optical amplifier; 
           [0013]      FIG. 6  illustrates output light in a case where an optical burst signal is input to a semiconductor optical amplifier as in  FIG. 4 ; 
           [0014]      FIG. 7  is a schematic view illustrating the configuration of an entire semiconductor optical amplifier device according to a second embodiment of the present invention; 
           [0015]      FIG. 8  is a schematic view illustrating the configuration of an entire semiconductor optical amplifier device according to a third embodiment of the present invention; and 
           [0016]      FIG. 9  illustrates output light in a case where an optical burst signal is input to a semiconductor optical amplifier as in  FIG. 4 . 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0017]      FIG. 1  is a schematic view of a passive optical network (PON) system to which a semiconductor optical amplifier device  100  according to a first embodiment of the present invention is applied. As illustrated in  FIG. 1 , in the downward direction in which an optical signal is transmitted from an optical line terminal (OLT) of an Internet station to each optical network unit (ONU), an optical signal of substantially the same optical intensity is continuously transmitted. Therefore, variations in the light intensity do not occur in each optical signal. 
         [0018]    On the other hand, in the forward direction in which an optical signal is transmitted from each ONU to the OLT, since each optical signal is combined in an optical coupler, an optical signal is non-continuously transmitted, and variations in the light intensity occur in each optical signal. Therefore, in the forward direction, variations occur in the light intensity of each optical signal, and a case in which an optical signal exists and a case in which an optical signal does not exist. This optical signal may be referred to as an optical burst signal. The semiconductor optical amplifier device  100  according to the present embodiment is disposed in the OLT and amplifies an optical burst signal input in the forward direction. 
         [0019]      FIG. 2A  is a schematic view illustrating the configuration of the semiconductor optical amplifier device  100  according to an embodiment.  FIG. 2B  illustrates an optical burst signal. 
         [0020]    As illustrated in  FIG. 2A , the semiconductor optical amplifier device  100  includes a temperature control circuit  10 , a thermo-electric cooler  20 , a semiconductor optical amplifier  30 , a beam splitter  40 , a delay unit  50 , a power monitor  60 , a controller  70 , and an optical filter  80 . As illustrated in  FIG. 2B , an optical burst signal has a state in which an optical signal exists and a state in which an optical signal does not exist. 
         [0021]    The temperature control circuit  10  supplies TEC driving current to the thermo-electric cooler  20  so that a temperature of the surface on which the elements of the thermo-electric cooler  20  are mounted is maintained substantially constant. Examples of the thermo-electric cooler  20  include a Peltier device. The semiconductor optical amplifier  30  is mounted on the surface on which the elements of the thermo-electric cooler  20  are mounted. Since the temperature of the surface on which the elements of the thermo-electric cooler  20  are mounted is maintained substantially constant, the temperature of the semiconductor optical amplifier  30  is controlled. 
         [0022]    The beam splitter  40  splits an input optical burst signal in two directions. One of the split optical burst signals passes through the delay line  50  and is input to the semiconductor optical amplifier  30 . The delay unit  50  is a delay line that provides a delay of the optical burst signal by a given time period. In the semiconductor optical amplifier  30 , the optical burst signal is amplified in accordance with the driving current supplied to the active layer. The other split optical burst signal is input to the power monitor  60 . The power monitor  60  detects the light intensity of the input optical burst signal and supplies the detection result to the controller  70 . 
         [0023]    The controller  70  includes a central processing unit (CPU), a non-volatile memory (read only memory (ROM)), a volatile memory (random access memory (RAM)), and the like. The controller  70  has a correspondence table  71  indicating the correspondence between light intensities of an optical burst signal input to the semiconductor optical amplifier  30  and driving currents stored therein in advance in the non-volatile memory. The driving currents stored in the correspondence table  71  are electrical currents that are set in such a manner that the light intensity of each optical signal output from the semiconductor optical amplifier  30  is amplified to a fixed value. 
         [0024]    By referring to the correspondence table  71 , the controller  70  supplies the driving current corresponding to the detection result of the power monitor  60  to the active layer of the semiconductor optical amplifier  30 . In this case, the semiconductor optical amplifier  30  is feed-forward-controlled. The amount of delay in the delay line  50  is set in such a manner so as to correspond to the amount of delay that occurs when the controller  70  supplies the driving current to the active layer of the semiconductor optical amplifier  30  in response to the input of an optical burst signal to the power monitor  60 . As a result, the time required for control by the controller  70  is cancelled out. The optical filter  80  substantially cuts off noise light (amplified spontaneous emission (ASE)) that is generated in the semiconductor optical amplifier  30 . 
         [0025]      FIGS. 3A to 3F  illustrate the relationship between the temperature of the semiconductor optical amplifier  30  and the gain thereof. 
         [0026]    As illustrated in  FIG. 3B , in a case where an optical signal is not input to the semiconductor optical amplifier  30 , the case that no driving current is supplied as illustrated in  FIG. 3A  will be considered. In this case, in the semiconductor optical amplifier  30 , heat resulting from the driving current is not generated. As a result, when the state in which an optical signal is not input continues, the temperature of the semiconductor optical amplifier  30  becomes approximately equal to the temperature (for example, 25° C.) of the surface on which the elements of the thermo-electric cooler  20  are mounted. 
         [0027]    In a case where an optical signal is input after the temperature of the semiconductor optical amplifier  30  becomes approximately equal to the temperature of the element mounted surface of the thermo-electric cooler  20 , the optical signal is amplified as a result of the driving current being supplied from the controller  70 . In this case, in the semiconductor optical amplifier  30 , heat is generated resulting from the driving current ( FIG. 3C ). As a result, the temperature of the active layer of the semiconductor optical amplifier  30  increases. In consequence, the gain of the semiconductor optical amplifier  30  decreases. As a result, as illustrated in  FIG. 3D , the output light intensity from the semiconductor optical amplifier  30  is decreased. 
         [0028]    When the state in which the driving current is supplied to the active layer of the semiconductor optical amplifier  30  continues, as illustrated in  FIG. 3E , a given temperature slope is formed in the semiconductor optical amplifier  30 . In this case, the temperature change of the active layer of the semiconductor optical amplifier  30  is suppressed. As a result, as illustrated in  FIG. 3F , the variations in the gain of the semiconductor optical amplifier  30  are suppressed, and the output light intensity is converged. 
         [0029]      FIG. 4  illustrates output light in a case where a typical optical burst signal is input to the semiconductor optical amplifier  30 . In a case where an optical signal  1  and an optical signal  2  having different light intensities are input at intervals that are not large, the light intensities are amplified to an approximately fixed light intensity in the semiconductor optical amplifier  30 . However, it is assumed that, for example, 10 microseconds pass until an optical signal  3  is input after the optical signal  2 . In this case, as illustrated in  FIGS. 3C to 3F , an overshoot excessive gain occurs. 
         [0030]    Therefore, in one embodiment, in a case where an optical signal is not input, preheat current is supplied to the active layer of the semiconductor optical amplifier  30 . The controller  70  causes preheat current in the case of no input optical to be stored in the correspondence table  71 . The magnitude of the preheat current is, for example, approximately equal to that of the electrical current that flows as a result of the diode threshold voltage of the semiconductor optical amplifier  30  being applied. A description will be given below, with reference to  FIGS. 5A to 5F , of the relationship between the temperature of the semiconductor optical amplifier  30  and the gain thereof in a case where preheat current is supplied to the active layer of the semiconductor optical amplifier  30 . 
         [0031]    In a case where no optical signal is input to the semiconductor optical amplifier  30  as illustrated in  FIG. 5B , predetermined preheat current is supplied to the active layer of the semiconductor optical amplifier  30  as illustrated in  FIG. 5A . In this case, in the semiconductor optical amplifier  30 , heat is generated resulting from the preheat current. As a result, a given temperature slope is formed in the semiconductor optical amplifier  30 , and the temperature of the active layer of the semiconductor optical amplifier  30  becomes higher than the temperature of the element mounted surface of the thermo-electric cooler  20 . 
         [0032]    Next, in a case where an optical signal is input as illustrated in  FIG. 5D , driving current is supplied to the active layer of the semiconductor optical amplifier  30  in accordance with the correspondence table  71 . As a result, in the semiconductor optical amplifier  30 , the optical signal is amplified. In this case, since a temperature slope has already been formed in the semiconductor optical amplifier  30 , the temperature change in the active layer of the semiconductor optical amplifier  30  is decreased. As a result, the change in the output light intensity of the semiconductor optical amplifier  30  is decreased. 
         [0033]    Even if the state in which driving current is supplied to the active layer of the semiconductor optical amplifier  30  continues, as illustrated in  FIG. 5E , a temperature slope similar to that of  FIG. 4A  is formed in the semiconductor optical amplifier  30 . In consequence, temperature changes in the active layer of the semiconductor optical amplifier  30  are suppressed. As a result, as illustrated in  FIG. 5F , changes in the output light intensity of the semiconductor optical amplifier  30  are decreased. 
         [0034]      FIG. 6  illustrates output light in a case where an optical burst signal is input to the semiconductor optical amplifier  30  as in  FIG. 4 . In a case where an optical signal  1  and an optical signal  2  having different light intensities are input at intervals that are not large, the light intensities are amplified to an approximately fixed light intensity in the semiconductor optical amplifier  30 . At intervals of each optical signal, preheat current is supplied to the active layer of the semiconductor optical amplifier  30 . In consequence, the semiconductor optical amplifier  30  is preheated. As a result, even if the period in which an optical signal  3  is input after the optical signal  2  is increased, an overshoot excessive gain is suppressed. 
         [0035]    As described above, by supplying preheat current to the active layer of the semiconductor optical amplifier  30  in a case where no optical signal is input, variations in the gain of the semiconductor optical amplifier to which the optical burst signal is input may be suppressed. As a result, it is possible to suppress an overshoot excessive gain. Noise generated in the semiconductor optical amplifier  30  resulting from the preheat current is substantially cut off by the optical filter  80 . 
         [0036]      FIG. 7  is a schematic view illustrating the configuration of an entire semiconductor optical amplifier device  100   a  according to a second embodiment of the present invention. As illustrated in  FIG. 7 , the differences of the semiconductor optical amplifier device  100   a  from the semiconductor optical amplifier device  100  of  FIG. 2  are that the power monitor  60  is not provided and a correspondence table  71   a  in place of the correspondence table  71  is stored in the controller  70 . In the correspondence table  71   a,  the correspondence among signal transmission sources in the forward direction, time slots, and driving currents are stored. The preheat current values are stored so as to cope with a case in which a signal transmission source does not exist. 
         [0037]    In this embodiment, the OLT supplies the transmission source of an optical signal in the forward direction and a time slot to the controller  70 . The controller  70  obtains the driving current from the correspondence table based on the transmission source and the time slot supplied from the OLT. As a result, the gain of the semiconductor optical amplifier  30  is controlled based on the transmission source. With respect to a time slot in which there is no transmission source (input optical signal does not exist), preheat current is supplied to the active layer of the semiconductor optical amplifier  30 . As a result, variations in the gain in the semiconductor optical amplifier  30  are suppressed. As a result, an overshoot excessive gain is suppressed. Furthermore, a power monitor and a beam splitter may not be provided. Therefore, it is possible to reduce costs. 
         [0038]      FIG. 8  is a schematic view illustrating the configuration of an entire semiconductor optical amplifier device  100   b  according to a third embodiment of the present invention. As illustrated in  FIG. 8 , the difference of the semiconductor optical amplifier device  100   b  from the semiconductor optical amplifier device  100  of  FIG. 2  is that a controller  70   b  is provided in place of the controller  70 . The controller  70   b  includes a rising edge detector  72  and a current value holding circuit  73 . 
         [0039]    In this embodiment, the rising edge detector  72  detects the rising edge of the input optical signal in response to the detection result of the power monitor  60 . Furthermore, the controller  70  supplies driving current corresponding to the detection result of the power monitor  60  to the active layer of the semiconductor optical amplifier  30  by referring to the correspondence table  71 . The current value holding circuit  73  holds the driving current until the rising edge detector  72  detects the next rising edge. In this case, even in a case where an optical signal is no longer input, electrical current is supplied to the active layer of the semiconductor optical amplifier  30 . As a result, variations in the gain in the semiconductor optical amplifier  30  are suppressed. 
         [0040]      FIG. 9  illustrates output light in a case where an optical burst signal is input to the semiconductor optical amplifier  30  as in  FIG. 4 . In a case where an optical signal  1  is input, the rising edge detector  72  detects a rising edge. In response, the current value holding circuit  73  holds the driving current value held in the correspondence table  71  until an optical signal  2  is input. When the optical signal  2  is input next, the rising edge detector  72  detects a rising edge. In response, the current value holding circuit  73  holds the driving current value held in the correspondence table  71  until an optical signal  3  is input. 
         [0041]    Therefore, even if the interval between the optical signal  2  and the optical signal  3  is increased, temperature changes of the semiconductor optical amplifier  30  are suppressed. In consequence, variations in the gain of the semiconductor optical amplifier  30  are suppressed. As a result, an overshoot excessive gain is suppressed. 
         [0042]    According to various embodiments of the semiconductor optical amplifier device as disclosed herein, it is possible to suppress variations in the gain of a semiconductor optical amplifier to which an optical burst signal is input. 
         [0043]    All examples and conditional language recited herein are intended for pedagogical objects 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. 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.