Abstract:
An optical module includes a semiconductor laser for output light with a wavelength, a temperature stabilization unit arranged for adjusting temperature of the semiconductor laser, and a controller for controlling a current injected to the semiconductor leaser by the use of a first function in accordance with changing of the wavelength on the bases of heat at the time of changing of the wavelength of the outputted light of the semiconductor leaser in a predetermined first period, and controlling the current injected to the semiconductor leaser by the use of a second function in accordance with changing of the wavelength on the bases of the temperature stabilization unit in a predetermined second period after the first period.

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. 2008-074955, filed on Mar. 24, 2008, the entire contents of which are incorporated herein by reference. 
       FIELD 
       [0002]    The embodiments discussed herein are related to an optical module including a semiconductor laser. 
       BACKGROUND 
       [0003]    As data traffic is increased in recent years, long-distance, high-speed, large-capacity communications are required. There have been constructed DWDM (Dense Wavelength Division Multiplexing) networks that are one of communication technologies using an optical fiber and in which an optical fiber is used in a multiplexed manner by using multiple optical signals of different wavelengths simultaneously. Toward the realization of larger-capacity transmission, it is desired to construct a next-generation photonic network for performing dynamic wavelength switching or wavelength routing. 
         [0004]    In order to realize such a network, it is necessary to develop a wavelength tunable light source that is allowed to tune a variation in a wavelength at a high speed. A semiconductor laser (laser diode: LD) is typically used as a wavelength tunable light source. 
         [0005]    While a temperature control-type wavelength tunable light source that changes the oscillation wavelength by controlling the temperature or a mechanical control-type wavelength tunable light source that changes the oscillation wavelength mechanically have relatively low response speeds, e.g., on the order of ms (millimeter sec), a current injection-type wavelength tunable light source that changes the oscillation wavelength by injecting a current has a response speed of the order of ns (nanometer sec) in principle. Therefore, a current injection-type wavelength tunable light source is preferably used as a wavelength tunable light source. 
         [0006]    In particular, a TDA-DFB-LD (tunable distributed amplification distributed feed back laser diode) is a current injection-type wavelength tunable light source that illustrates excellent operations such as simplified wavelength control using a single injection current and no mode-hops (mode-hop-free). For example, see Japanese Laid-open Patent Publication No. 2006-295102. 
         [0007]    However, in the case of a TDA-DFB-LD, the temperature of the LD is changed by heat caused due to the injection of a wavelength control current at the time of wavelength switching. As a result, a wavelength drift occurs. 
         [0008]      FIGS. 20A-20C  include graphs illustrating a cause of occurrence of a drift. 
         [0009]    As illustrated in  FIG. 20A , the current value of the injection current is changed from a current value ILD 91  to a current value ILD 92  at time t 90 . 
         [0010]    As illustrated in  FIG. 20B , a drift occurs due to heat caused by an increase in current value of the injection current. Thus, between time t 90  and time t 91 , a temperature TLD of the wavelength tunable light source is changed from a temperature value TLD 91  to a temperature value TLD 92 . Subsequently, between time t 91  and time t 92 , the heat is reduced by the TEC and the temperature moves toward the stabilization. As illustrated in  FIG. 20C , these have an influence on the drift of the wavelength. 
         [0011]    A system having short wavelength intervals, such as DWDM, has a problem that this wavelength drift has a nonnegligible influence on an adjacent channel. 
         [0012]    With regard to this problem, a technology is known for, in a wavelength tunable light source for feedback-controlling the LD current using a wavelength monitor, starting feedback control after the temperature is stabilized after a current is injected at the time of wavelength switching. For example, see Laid-open Patent Publication No. 2005-64300. 
         [0013]    However, this method that waits for the temperature to be stabilized has a problem that it is difficult to perform wavelength switching at a high speed. 
       SUMMARY 
       [0014]    According to an aspect of the invention, an optical module includes a semiconductor laser for output light with a wavelength, a temperature stabilization unit arranged for adjusting temperature of the semiconductor laser, and a controller for controlling a current injected to the semiconductor leaser by the use of a first function in accordance with changing of the wavelength on the bases of heat at the time of changing of the wavelength of the outputted light of the semiconductor leaser in a predetermined first period, and controlling the current injected to the semiconductor leaser by the use of a second function in accordance with changing of the wavelength on the bases of the temperature stabilization unit in a predetermined second period after the first period. 
         [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. 
         [0016]    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 
         [0017]      FIG. 1  is a graph illustrating an outline of the present invention. 
           [0018]      FIG. 2  is a block diagram illustrating functions of an optical module. 
           [0019]      FIG. 3  is a configuration of a TDA-DFB-LD. 
           [0020]      FIG. 4  is a plan view illustrating a configuration of the TDA-DFB-LD. 
           [0021]      FIGS. 5A and 5B  include graphs illustrating a variation in a wavelength caused by a wavelength control current. 
           [0022]      FIG. 6  is a flowchart illustrating a function calculation process in a first control method. 
           [0023]      FIG. 7  is a flowchart illustrating a wavelength tuning process in the first control method. 
           [0024]      FIGS. 8A-8D  include graphs schematically illustrating a result of control performed using the first control method. 
           [0025]      FIG. 9  is a drawing illustrating a table stored in a memory. 
           [0026]      FIG. 10  is a graph illustrating a variation in a wavelength caused by the temperature of a gain control current. 
           [0027]      FIG. 11  is a flowchart illustrating a function calculation process in a second control method. 
           [0028]      FIG. 12  is a flowchart illustrating a wavelength tuning process in the second control method. 
           [0029]      FIGS. 13A-13D  include graphs schematically illustrating a result of control performed using the second control method. 
           [0030]      FIG. 14  is a block diagram illustrating functions of a second embodiment. 
           [0031]      FIG. 15  is a drawing illustrating a specific example of a shutter. 
           [0032]      FIG. 16  is a flowchart illustrating a wavelength tuning process in a first control method according to the second embodiment. 
           [0033]      FIGS. 17A-17C  include graphs schematically illustrating a result of control performed using the first control method according to the second embodiment. 
           [0034]      FIG. 18  is a flowchart illustrating a wavelength tuning process in a second control method according to the second embodiment. 
           [0035]      FIGS. 19A-19D  include graphs schematically illustrating a result of control performed using the second control method according to the second embodiment. 
           [0036]      FIGS. 20A-20C  include graphs illustrating a cause of occurrence of a drift. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0037]    Now, embodiments will be described with reference to the accompanying drawings. 
         [0038]      FIG. 1  is a graph illustrating an embodiment. When the wavelength of a semiconductor laser is switched from λα to λβ, a variation in the wavelength due to heat caused by the wavelength switching is controlled during a predetermined first interval, using a first function for determining a current to be injected to the semiconductor laser. 
         [0039]    During a predetermined second interval, a variation in the wavelength caused by a temperature-control element for controlling the temperature of the semiconductor laser is controlled using a second function for determining a current to be injected to the semiconductor laser. 
         [0040]    By using the above-mentioned semiconductor laser control method, the temperature of the semiconductor laser is changed by heat caused by the injection of a wavelength control current at the time of wavelength switching. This suppresses occurrence of a wavelength drift. 
         [0041]    Hereafter, the embodiments will be described. 
         [0042]      FIG. 2  is a block diagram illustrating functions of an optical module. An optical module  10  includes an LD  11 , a control unit  12 , a memory  13 , and a PD (photo diode)  14 . 
         [0043]    The LD  11  includes a TDA-DFB-LD  110  as the wavelength tunable light source. The LD  11  is placed on a temperature stabilization unit  150  including a temperature-control element (e.g. thermoelectric cooler). 
         [0044]    The control unit  12  includes a CPU (central processing unit). The control unit  12  has a timer function and outputs a wavelength control current I tune  (hereafter simply referred to as the “current I tune ”) and a gain control current I act  (hereafter simply referred to as the “current I act ”) to the LD  11  using a function (to be described later) in predetermined cycles so as to control the LD  11 . 
         [0045]    The memory  13  includes a ROM (read only memory). The memory  13  is storing various types of data necessary when the control unit  12  performs control. 
         [0046]    A PD  14  detects an optical signal inputted from the outside and converts the optical signal into an electric signal. 
         [0047]      FIG. 3  is a sectional view illustrating a configuration of the TDA-DFB-LD and  FIG. 4  is a plan view illustrating a configuration of the TDA-DFB-LD. 
         [0048]    The TDA-DFB-LD  110  includes an optical waveguide (optical waveguide layer)  111  including a gain waveguide part (active waveguide unit)  111   a  that generates a gain due to the injection of the current I act  and a wavelength control waveguide part  111   b  that controls the oscillation wavelength using a variation in the index of refraction due to the injection of the current I tune . The TDA-DFB-LD  110  also includes a diffraction grating (diffraction grating layer)  112  provided near the optical waveguide  111 . 
         [0049]    When the current I act  is injected into the gain waveguide part  111   a,  the TDA-DFB-LD  110  oscillates with a wavelength corresponding to the cycle of the diffraction grating  112 . Also, when the current I tune  is injected into the wavelength control waveguide part  111   b,  the TDA-DFB-LD  110  controls the oscillation wavelength. 
         [0050]    The optical waveguide  111  has a configuration in which gain waveguide units  111   a  and wavelength control waveguide units  111   b  are alternately provided. That is, the optical waveguide  111  includes multiple gain waveguide units  111   a  and multiple wavelength control waveguide units  111   b  and has a configuration in which the gain waveguide units  111   a  and wavelength control waveguide units  111   b  are alternately disposed on the same plane in series in cycles. 
         [0051]    The diffraction grating  112  is provided below the optical waveguide  111  throughout the length of the optical waveguide  111  in parallel with the optical waveguide  111 . In other words, the diffraction grating  112  is continuously formed in positions associated with the gain waveguide units  111   a  and in positions associated with the wavelength control waveguide units  111   b.  The diffraction grating  112  formed in the positions associated with the gain waveguide units  111   a  is referred to as a gain diffraction grating  112   a.  In addition, the diffraction grating  112  formed in the positions corresponding to the wavelength control waveguide units  111   b  is referred to as a wavelength control diffraction grating  112   b.    
         [0052]    Since the TDA-DFB-LD  110  is one type of DFB laser, it does not need to perform phase control when performing wavelength change control, unlike a DBR laser. Accordingly, the TDA-DFB-LD  110  is allowed to perform simple wavelength control using only the current I tune . Since the diffraction grating  112  is provided throughout the length of the optical waveguide  111  in the TDA-DFB-LD  110 , the TDA-DFB-LD  110  also does not need to perform initial phase control. 
         [0053]    In the TDA-DFB-LD  110 , the gain waveguide parts  111   a  of the optical waveguide  111  and wavelength control waveguide parts  111   b  thereof are independently provided with gain electrodes  113   a  forming P-side electrodes and wavelength control electrodes  113   b  forming P-side electrodes, respectively, so that currents are independently injected into the gain waveguide parts  111   a  and wavelength control waveguide parts  111   b.    
         [0054]    Specifically, a gain electrode  113   a  is formed above the upper surfaces of the gain waveguide parts  111   a  of the optical waveguide  111  with a contact layer  118   a  therebetween. A common electrode  113   c  forming an N-side electrode is formed below the gain waveguide parts  111   a.  Thus, the current I act  is injected into active layers (gain layers or waveguide core layers)  116  of the gain waveguide parts  111   a.  Also, a wavelength control electrode  113   b  is formed above the upper surfaces of the wavelength control waveguide parts  111   b  of the optical waveguide  111  with a contact layer  118   b  therebetween. The common electrode  113   c  is formed below the wavelength control waveguide parts  111   b.  Thus, the current I tune  is injected into wavelength control layers  119  of the wavelength control waveguide parts  111   b.    
         [0055]    As illustrated in  FIG. 4 , the gain electrode  113   a  and wavelength control electrode  113   b  are each formed as a comb-shaped electrode. 
         [0056]    An area made up of each gain waveguide part  111   a,  gain diffraction grating  112   a,  gain electrode  113   a,  and common electrode  113   c  is referred to as a gain area  11   a.  An area made up of each wavelength control waveguide part  111   b,  wavelength control diffraction grating  112   b,  wavelength control electrode  113   b,  and common electrode  113   c  is referred to as a wavelength control area  11   b.    
         [0057]    As is understood from the above description, each gain area  11   a  has a layer structure in which an n-InP layer  114 , the diffraction grating  112 , an n-type InP layer  115 , each active layer  116 , a p-InP layer  117 , and the contact layer  118   a  are sequentially stacked in layers. 
         [0058]    Also, each wavelength control area  11   b  has a layer structure in which the n-InP layer  114 , diffraction grating  112 , n-InP layer  115 , wavelength control layer  119 , p-InP layer  117 , and contact layer  118   a  are sequentially stacked in layers. 
         [0059]    A SiO2 film (Passivation Film)  1100  is formed in an area in which none of the contact layers  118   a  and  118   b,  wavelength control electrode  113   b,  and gain electrode  113   a  is formed. Specifically, by forming the contact layers  118   a  and  118   b,  then forming the SiO2 film  1100  on all surfaces of these layers, and then eliminating only the SiO2 film  1100  formed on these layers so as to form the gain electrode  113   a  and wavelength control electrode  113   b  on the contact layers  118   a  and  118   b,  the SiO2 film  1100  is formed in an area in which none of the gain electrode  113   a  and wavelength control electrode  113   b  is formed. 
         [0060]    In particular, as illustrated in  FIGS. 3 and 4 , in order to electrically separate the gain areas  11   a  and wavelength control areas  11   b,  separation areas  11   c  are provided between the gain electrode  113   a  and wavelength control electrode  113   b.  That is, by avoiding formation of the wavelength control electrode  113   b,  gain electrode  113   a,  and contact layers  118   a  and  118   b  in an area above the vicinity of the bonding interface between each gain area  11   a  and wavelength control area  11   b,  each separation area  11   c  is formed. 
         [0061]    First Control Method: 
         [0062]    Hereafter, a first method for controlling the optical module  10  will be described. 
         [0063]    The first control method is a method in which the drift of the wavelength due to an increase in temperature of the LD 11  is suppressed by temporally controlling the current I tune  using the control unit  12  after the injection of the current I tune  when the LD 11  performs wavelength switching (at the time of wavelength switching). 
         [0064]      FIGS. 5A and 5B  include graphs illustrating a variation in the wavelength caused by a wavelength control current.  FIG. 5A  is a graph illustrating a variation in the wavelength due to a carrier plasma effect of a wavelength control current and  FIG. 5B  is a graph illustrating a variation in the wavelength caused by the temperature of a wavelength control current. 
         [0065]    As illustrated in  FIG. 5A , a variation value h of the wavelength due to a carrier plasma effect of the current I tune  is on the order of −100 pm/mA in an area whose inclination is approximately constant. 
         [0066]    In addition, as illustrated in  FIG. 5B , a variation value d 1  of the wavelength caused by the current value of the current I tune  is on the order of several pm/mA. This is a variation of the temperature caused by an increase or a decrease in the current value. 
         [0067]    Next, a function calculation process performed by the control unit  12  in given cycles when performing control using the first control method will be described. 
         [0068]      FIG. 6  is a flowchart illustrating a function calculation process in the first control method. 
         [0069]    First, times t 1  and t 2  are calculated from a thermal response characteristic demonstrated when the current I tune  is injected, and the calculated times t 1  and t 2  are stored in the memory  13  (step S 1 ). Time t 2  is set to, for example, the order of seconds so that a response is made to a heat reduction by the TEC. 
         [0070]    Next, a first current I tune  determination function for determining the current value of the current I tune  between times t 0  and t 1  and a second current I tune  determination function for determining the current value of the current I tune  between times t 1  and t 2  are determined using times t 1  and t 2 , the variation value d 1  and a variation value f 1 , and a difference value (I t2 −I t1 ) between current values I t2  and I t1  indicating injection amounts of the current I tune  (step S 2 ). 
         [0071]    The first current I tune  determination function is represented by Formula 1 below and the second current I tune  determination function is represented by Formula 2 below. 
         [0000]        I   tune   =−d   1 ×( I   t2   −I   t1 )/( f   1   ×   t1 )× t+I   t2    (1) 
         [0000]        I   tune   =d   1 ×( I   t2   −I   t1 )/( f   1 ×( t   2   −t   1 ))× t+I   t2   −dI×t   2 ( I   t2   −I   t1 )/( f   1 ×( t   2   −t   1 ))   (2) 
         [0072]    As is understood from the above description, the first current I tune  determination function and second current I tune  determination function are a function taking into account a variation due to a carrier plasma effect of the current I tune  and a function taking into account a variation due to the temperature of the current I tune , respectively. 
         [0073]    This completes the function calculation process in the first control method. 
         [0074]    Next, a wavelength tuning process in the first control method will be described. 
         [0075]      FIG. 7  is a flowchart illustrating the wavelength tuning process in the first control method. 
         [0076]    The current I tune  is controlled from the current value I t1  to the current value I t2  so as to change the wavelength (step S 11 ). 
         [0077]    Next, the current I tune  is controlled using the first current I tune  determination function calculated in step S 2  of  FIG. 6  (step S 12 ). 
         [0078]    Next, whether time to has elapsed is determined (step S 13 ). 
         [0079]    If time t 1  has not elapsed (No in step S 13 ), the wavelength tuning process moves to step S 12  and the process in step S 12  is performed again. 
         [0080]    On the other hand, if t 1  has elapsed (Yes in step  13 ), the current I tune  is controlled using the second current I tune  determination function calculated in step S 2  of  FIG. 6  (step S 14 ). 
         [0081]    Next, whether time t 2  has elapsed is determined (step S 15 ). 
         [0082]    If time t 2  has not elapsed (No in step S 15 ), the wavelength tuning process moves to step S 14  and the process in step S 14  is performed again. 
         [0083]    On the other hand, if t 2  has elapsed (Yes in step  15 ), the wavelength tuning process is completed. 
         [0084]      FIGS. 8A-8D  include graphs schematically illustrating a result of control performed using the first control method. 
         [0085]    As illustrated in  FIG. 8A , when the wavelength is changed, the control unit  12  performs control at time t 0  so that the current value of the current I tune  is changed from the current value I t1  to the current value I t2 , which is larger than the current value I t1 . 
         [0086]    Between time t 0  and time t 1 , the control unit  12  performs control so that the current I tune  is changed from the current value I t2  to the current value I t3  at the maximum using the first current I tune  determination function. 
         [0087]    Subsequently, between time t 1  and time t 2 , the control unit  12  performs control so that the current I tune  is changed from the current value I t3  to the current value I t2  using the second current I tune  determination function. Note that, as illustrated in  FIG. 8B , the current I act  is kept constant at a current value I a1  in the first control method. 
         [0088]    As illustrated in  FIG. 8C , between time t 0  and time t 1 , a drift occurs due to heat caused by an increase in the current value of the current I tune . Thus, the temperature TLD of the LD 11  is changed from a temperature value T LD1  to a temperature value T LD2 . Subsequently, between time t 1  and time t 2 , the heat is reduced by the TEC and the temperature moves toward the stabilization. These have an influence on the drift of the wavelength. 
         [0089]    As a result, as illustrated in  FIG. 8D , compensation taking into account a variation caused by a carrier plasma effect is made for a drift caused by a variation in the temperature by using the first current I tune  determination function between time t 0  and time t 1 . Thus, a wavelength λ2 is kept constant. Also, between time t 1  and time t 2 , compensation taking into account a heat reduction caused by the TEC is made for the drift by using the second current I tune  determination function. Thus, the wavelength λ2 is kept constant. After time t 2  elapses, the compensation using the second current I tune  determination function is cancelled. 
         [0090]      FIG. 9  is a drawing illustrating a table stored in a memory. 
         [0091]    In each of steps  12  and  14  of the wavelength tuning process in this control method, the current value of the current I tune  is calculated on the basis of the function calculated in the function calculation process; however, the relations between the times and the current values of the current I tune  may be stored in the form of a table in the memory  13  and the values may be read out. 
         [0092]    Second Control Method: 
         [0093]    Hereafter, a second method for controlling the optical module  10  will be described. 
         [0094]    The second control method is a method of keeping the temperature TLD constant and thus suppressing the drift of the wavelength by controlling the current I act  after the injection of the current I tune  at the time of wavelength switching so as to keep constant the total calorie of the current I tune  and current I act . 
         [0095]      FIG. 10  is a graph illustrating a variation in the wavelength caused by the temperature of a gain control current. 
         [0096]    A variation value d 2  of the wavelength caused by the current value of the current I act  is on the order of several pm/mA. 
         [0097]    While a function is calculated by performing a function calculation process also in the second control method, the formula for the calculation is different from that in the first control method. 
         [0098]      FIG. 11  is a flowchart illustrating a function calculation process in the second control method. 
         [0099]    First, like in the first control method, times t 1  and t 2  are calculated from a thermal response characteristic demonstrated when the current I tune  is injected, and the calculated times t 1  and t 2  are stored in the memory  13  (step S 21 ). 
         [0100]    Next, a first current I act  determination function for determining the current value of the current I act  between times t 0  and t 1  and a second current I act  determination function for determining the current value of the current I act  between times t 1  and t 2  and are determined using times t 1  and t 2 , the variation values d 1  and d 2 , the current value I a1  of the current I act  before the wavelength switching, and the difference value (I t2 −I t1 ) between the current values I t2  and I t1  indicating injection amounts of the current I tune  (step S 22 ). The first current I act  determination function is represented by Formula 3 below and the second current I act  determination function is represented by Formula 4 below. 
         [0000]        I   act   =I   a1   −d   1 ×( I   t2   −I   t1 )/ d   2    (3) 
         [0000]        I   act   =d   1 ×( I   t2   −I   t1 )/( d   2 ×( t   2   −t   1 ))× t+I   a1   −d   1   ×t   2 ( I   t2   −I   t1 )/( d   2 ×( t   2   −t   1 ))   (4) 
         [0101]    As is understood from the above description, the first current I act  determination function and second current I act  determination function are a function taking account a variation due to the temperature of the current I act  and a function taking into account a variation due to the temperature of the current I tune , respectively. 
         [0102]    This completes the function calculation process in the second control method. 
         [0103]    Next, a wavelength tuning process in the second control method will be described. 
         [0104]      FIG. 12  is a flowchart illustrating the wavelength tuning process in the second control method. 
         [0105]    First, the current I tune  is controlled from the current value I t1  to the current value I t2  to change the wavelength (step S 31 ). 
         [0106]    Next, the current I act  is controlled using the first current I act  determination function calculated in step S 22  (step S 32 ). 
         [0107]    Next, whether time to has elapsed is determined (step S 33 ). 
         [0108]    If time t 1  has not elapsed (No in step S 33 ), the wavelength tuning process moves to step S 32  and the process in step S 32  is performed again. 
         [0109]    On the other hand, if time t 1  has elapsed (Yes in step  33 ), the current I act  is controlled using the second current I act  determination function calculated in step S 22  (step S 34 ). 
         [0110]    Next, whether time t 2  has elapsed is determined (step S 35 ). 
         [0111]    If time t 2  has not elapsed (No in step S 35 ), the wavelength tuning process moves to step S 34  and the process in step S 34  is performed again. 
         [0112]    On the other hand, if time t 2  has elapsed (Yes in step  35 ), the wavelength tuning process is completed. 
         [0113]      FIGS. 13A-13D  include graphs schematically illustrating a result of control performed using the second control method. 
         [0114]    As illustrated in  FIG. 13A , the control unit  12  performs control at time t 0  so that the current value of the current I tune  is changed from the current value I t1  to the current value I t2 . 
         [0115]    As illustrated in  FIG. 13B , between time t 0  and time t 1 , the control unit  12  performs control using the first current I act  determination function so that the current I act  is changed from the current value I a1  to a current value I a1a . 
         [0116]    Subsequently, between time t 1  and time t 2 , the control unit  12  performs control using the second current I act  determination function so that the current I act  is changed from the current value I a1a  to the current value I a1 . 
         [0117]    As illustrated in  FIG. 13C , between time t 0  and time t 1 , a drift caused by the heat of the current I tune  and an increase in temperature of the LD  11  from the temperature value T LD1  are compensated for by performing control using the first current I act  determination function, that is, by performing control so that a drift occurs due to the heat of the current I act  and the temperature of the LD 11  is lowered from the temperature value T LD1 . Subsequently, between time t 1  and time t 2 , compensation is made with respect to an area influenced by a heat reduction caused by the TEC, by performing control using the second current I act  determination function. 
         [0118]    As a result, the temperature TLD is kept at the temperature value T LD1  and, as illustrated in  FIG. 13D , the wavelength λ2 is kept constant. 
         [0119]    Also, in the wavelength tuning process in this control method, the relations between the times and the current values of the current I act  may be stored in the form of a table in the memory  13  and the values may be read out, like in the first control method. 
         [0120]    As described above, if the optical module  10  is used, the drift of the wavelength due to the temperature of the LD  11  is suppressed by temporally controlling the current I tune  or the current I act  after the injection of the current I tune  at the time of wavelength switching. As a result, switching is performed at a high speed. 
         [0121]    Next, an optical module according to the second embodiment will be described. 
         [0122]    Hereafter, the optical module according to the second embodiment will be described while focusing on differences between the optical module according to the second embodiment and the optical module  10  according to the first embodiment and same items will not be described. 
         [0123]      FIG. 14  is a block diagram illustrating functions of the second embodiment. 
         [0124]    An LD  11   a  of an optical module  10   a  according to the second embodiment illustrated in  FIG. 14  includes a shutter  120  having a function of shutting off an optical signal outputted from the TDA-DFB-LD  110 . 
         [0125]      FIG. 15  is a drawing illustrating a specific example of a shutter. 
         [0126]    The shutter  120  includes a SOA (semiconductor optical amplifier)  121  and an EA (elector absorption) modulator  122 . 
         [0127]    An integrated circuit of the TDA-DFB-LD  110 , SOA  121 , and EA modulator  122  constitutes the main part of a TDA-EML (tunable distributed amplification electro absorption modulated laser). By configuring a TDA-EML as described above, the optical module  10 a is downsized. 
         [0128]    The SOA  121  includes an amplification layer  121   a  for amplifying an optical signal outputted from the TDA-DFB-LD  110  when a current I soa  is injected. 
         [0129]    The SOA  121  serves as a shutter for shutting off the output of an optical signal outputted from the TDA-DFB-LD  110  when a SOA voltage (Vsoa) is set to 0V and outputting an optical signal when the current I soa  is added to the SOA  121 . 
         [0130]    The EA modulator  122  includes an absorption layer  122   a  for absorbing an optical signal outputted from the SOA  121  when a modulation signal Vp-p is applied. The EA modulator  122  is provided with a power supply for applying a bias voltage VEA and a capacitor C 1  and an inductor L 1  for preventing entry of a modulation signal to the power supply. 
         [0131]    The EA modulator  122  performs as a shutter for shutting off the output of an optical signal outputted from the TDA-DFB-LD  110  when the bias voltage VEA voltage is applied for outputting an optical signal and when the applied the bias voltage VEA voltage is cancelled for shut off the optical signal. 
         [0132]    The time during which the shutter shuts off the output of an optical signal is on the order of several ns. 
         [0133]    Shutter control is realized, for example, by making an interrupt when the CPU included in the control unit  12  is performing processing. 
         [0134]    First Control Method: 
         [0135]    Hereafter, a first method for controlling the optical module  10   a  will be described. 
         [0136]    A function calculation process in the first method for controlling the optical module  10   a  is similar to the function calculation process in the first control method according to the first embodiment. 
         [0137]      FIG. 16  is a flowchart illustrating a wavelength tuning process in the first method for controlling an optical module according to the second embodiment. 
         [0138]    First, the output of an optical signal to the outside is shut off by controlling the shutter  120  (step S 41 ). 
         [0139]    The current I tune  is controlled from the current value I t1  to the current value I t2  to change the wavelength (step S 42 ). 
         [0140]    Next, the current I tune  is controlled using the first current I tune  determination function calculated in step S 2  of  FIG. 6  (step S 43 ). Immediately after that (e.g., after approximately several ns has elapsed), the light shutoff by the shutter  120  is cancelled and an optical signal is outputted to the outside (step S 44 ). 
         [0141]    Next, whether time t 1  has elapsed is determined (step S 45 ). 
         [0142]    If time t 1  has not elapsed (No in step S 45 ), the wavelength tuning process moves to step S 43  and the process in step S 43  is performed again. 
         [0143]    On the other hand, if time t 1  has elapsed (Yes in step  45 ), the current I tune  is controlled using the second current I tune  determination function calculated in step S 2  of  FIG. 6  (step S 46 ). 
         [0144]    Next, whether time t 2  has elapsed is determined (step S 47 ). 
         [0145]    If time t 2  has not elapsed (No in step S 47 ), the wavelength tuning process moves to step S 46  and the process in step S 46  is performed again. 
         [0146]    On the other hand, if time t 2  has elapsed (Yes in step  47 ), the wavelength tuning process is completed. 
         [0147]      FIGS. 17A-17C  include graphs schematically illustrating a result of control performed using the first control method according to the second embodiment. 
         [0148]    If shutoff is performed using the SOA  121 , the voltage Vsoa to be provided to the SOA  121  is set to 0V as illustrated in  FIG. 17B  before the control unit  12  performs control at time t 0  so that the current value of the current I tune  is changed from the current value I t1  to the current value I t2  as illustrated in  FIG. 17A . 
         [0149]    After the current value of the current I tune  is changed from the current value I t1  to the current value I t2 , the current value of the current I soa  to be provided to the SOA  121  is set to a current value I S2 , which is larger than a current value I S1 . 
         [0150]    Thus, as illustrated in  FIG. 17C , the output of an optical signal is shut off during a time when the voltage Vsoa is 0. 
         [0151]    A variation in the temperature TLD and a variation in the wavelength in  FIGS. 17A-17C  are similar to those in the first control- method according to the first embodiment illustrated in  FIGS. 8A-8D  and are not illustrated. 
         [0152]    Second Control Method 
         [0153]    Next, a second method for controlling the optical module  10   a  will be described. 
         [0154]    A function calculation process in the second method for controlling the optical module  10   a  is similar to the function calculation process in the second control method according to the first embodiment. 
         [0155]      FIG. 18  is a flowchart illustrating a wavelength tuning process in the second method for controlling an optical module according to the second embodiment. 
         [0156]    First, the control unit  12  controls the shutter  120  to shut off the output of an optical signal to the outside (step S 51 ). 
         [0157]    Next, the current I tune  is controlled from the current value I t1  to the current value I t2  to change the wavelength (step S 52 ). 
         [0158]    Next, the current I act  is controlled using the first current I act  determination function calculated in step S 22  of  FIG. 11  (step S 53 ). Immediately after that (e.g., after approximately several ns has elapsed), the light shutoff by the shutter  120  is cancelled and an optical signal is outputted to the outside (step S 54 ). 
         [0159]    Next, whether time t 1  has elapsed is determined (step S 55 ). 
         [0160]    If time t 1  has not elapsed (No in step S 55 ), the wavelength tuning process moves to step S 53  and the process in step S 53  is performed again. 
         [0161]    On the other hand, if time t 1  has elapsed (Yes in step  55 ), the current I act  is controlled using the second current I act  determination function calculated in step S 22  of  FIG. 11  (step S 56 ). 
         [0162]    Next, whether time t 2  has elapsed is determined (step S 57 ). 
         [0163]    If time t 2  has not elapsed (No in step S 57 ), the wavelength tuning process moves to step S 56  and the process in step S 56  is performed again. 
         [0164]    On the other hand, if time t 2  has elapsed (Yes in step  57 ), the wavelength tuning process is completed. 
         [0165]    If the shutter  120  shuts off light using the SOA  121 , a variation in power caused when the current I act  is controlled is compensated for using the current I soa . 
         [0166]    Specifically, if the proportionality factor of the current I act  with respect to power is represented by “a” and the proportionality factor of the current I soa  with respect to power is represented by “b,” a relation illustrated in Formula 5 below exists. 
         [0000]        I   S3   −I   S2   =a/b ( I   a1   −I   a1a )   Formula 5 
         [0167]    Therefore, a variation in power caused when the current I act  is controlled is compensated for using the current I soa  by previously calculate a/b and controlling a current I S3    50  that Formula 5 is met. 
         [0168]      FIGS. 19A-19D  include graphs schematically illustrating a result of control performed using the second control method according to the second embodiment. 
         [0169]    The control of the current I tune  illustrated in  FIG. 19A  and the control of the current I act  illustrated in  FIG. 19B  is similar to that in the second control method according to the first embodiment illustrated in  FIGS. 13A-13D . 
         [0170]    In the case of the second control method according to this embodiment, if shutoff is performed using the SOA  121 , the current value of a current to be provided to the SOA  121  is set to a current value I S3  larger than the current value I S2  so that Formula 5 is met, as illustrated in  FIG. 19C  after the current value of the current I tune  is changed from the current value I t1  to the current value I t2 . Thus, a reduction in current value of the current I act  is compensated for. 
         [0171]    In  FIG. 19A-19D , a variation in the temperature TLD and a variation in the wavelength are similar to those in the second method according to the first embodiment illustrated in  FIGS. 13A-13D  and are not illustrated. 
         [0172]    By adopting the optical module  10   a  according to the second embodiment, an advantage similar to that of the optical module  10  according to the first embodiment is obtained. 
         [0173]    Also, by adopting the optical module  10   a  according to the second embodiment, wavelength switching is performed without affecting other channels in operation. 
         [0174]    While the semiconductor laser control method and semiconductor laser control apparatus according to the present invention have been described on the basis of the illustrated embodiments, the invention is not limited thereto. Each component can be replaced with an arbitrary component having a similar function. Also, other arbitrary components or steps may be added to the present invention. 
         [0175]    Also, the present invention may be combinations of arbitrary two or more components (features) of the above-mentioned embodiments. 
         [0176]    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 illustrating of the superiority and inferiority of the invention. Although the embodiments 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.