Patent Publication Number: US-2022214212-A1

Title: Optical measurement apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority to Japanese Patent Application No. 2020-215721 filed on Dec. 24, 2020, the entire contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to an optical measurement apparatus. 
     BACKGROUND 
     Apparatuses for measuring the light intensity of light to be analyzed are conventionally known (see PTL 1, for example). The photoelectric conversion circuit disclosed in PTL 1 includes an amplifier having a negative feedback path for logarithmic amplification loaded with a logarithmic conversion element and a negative feedback path for linear amplification loaded with a resistor connected in parallel. 
     CITATION LIST 
     Patent Literature 
     PTL 1: JP H02-090025 A 
     SUMMARY 
     An optical measurement apparatus according to some embodiments includes: a light receiving element capable of converting a light intensity of light to be analyzed into an electrical signal; an input terminal to which the electrical signal is input; a first amplifier and a nonlinear element configuring a logarithmic amplifier, an inverting input terminal of the first amplifier being electrically connected to the input terminal; a plurality of offset resistors having resistance values different from each other; a switch unit capable of switching an offset resistor to be electrically connected between a voltage source and the input terminal, of the plurality of offset resistors; and a controller, wherein an offset current is input to the input terminal by the offset resistor electrically connected between the voltage source and the input terminal, and the controller measures the light intensity based on an output voltage value of the logarithmic amplifier. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a block diagram of an optical measurement apparatus according to a first embodiment of the present disclosure; 
         FIG. 2  is a block diagram of a logarithmic amplifier circuit illustrated in  FIG. 1 ; 
         FIG. 3  is a diagram illustrating an example of settings of the measurement sensitivities; 
         FIG. 4  is a diagram illustrating waveforms of optical spectra; 
         FIG. 5  is a flowchart illustrating an example of an optical measurement method by the optical measurement apparatus illustrated in  FIG. 1 ; 
         FIG. 6  illustrates an optical measurement apparatus provided with a linear amplifier according to a first comparative example; 
         FIG. 7  is a diagram illustrating an example of settings of the measurement sensitivities according to the first comparative example; 
         FIG. 8  illustrates an optical measurement apparatus provided with a logarithmic amplifier according to a second comparative example; 
         FIG. 9  illustrates waveforms of optical spectra according to the first and second comparative examples; 
         FIG. 10  is a block diagram of an optical measurement apparatus according to a second embodiment of the present disclosure; 
         FIG. 11  is a graph indicating the relationship between output voltage values versus current values; 
         FIG. 12  illustrates an example of a table indicating current values versus output voltage values; 
         FIG. 13  is a flowchart illustrating an example of an optical measurement method by the optical measurement apparatus illustrated in  FIG. 10  (part  1 ); 
         FIG. 14  is a flowchart illustrating the example of the optical measurement method by the optical measurement apparatus illustrated in  FIG. 10  (part  2 ); 
         FIG. 15  is a block diagram of an optical measurement apparatus according to a third embodiment of the present disclosure; and 
         FIG. 16  is a diagram illustrating waveforms of output voltage values of an amplified logarithmic circuit illustrated in  FIG. 15 . 
     
    
    
     DETAILED DESCRIPTION 
     In an apparatus for measuring the light intensity of light to be analyzed, an improvement in the performance of detection of the light intensity of the light to be analyzed is desired. 
     The present disclosure has been conceived of in view of the aforementioned, and it is an aim of the present disclosure to provide an optical measurement apparatus having an improved light intensity detection performance of light to be analyzed. 
     An optical measurement apparatus according to some embodiments includes: a light receiving element capable of converting a light intensity of light to be analyzed into an electrical signal; an input terminal to which the electrical signal is input; a first amplifier and a nonlinear element configuring a logarithmic amplifier, an inverting input terminal of the first amplifier being electrically connected to the input terminal; a plurality of offset resistors having resistance values different from each other; a switch unit capable of switching an offset resistor to be electrically connected between a voltage source and the input terminal, of the plurality of offset resistors; and a controller, wherein an offset current is input to the input terminal by the offset resistor electrically connected between the voltage source and the input terminal, and the controller measures the light intensity based on an output voltage value of the logarithmic amplifier. Such a configuration can provide a fast measurement of the light intensity of light to be analyzed without any distortions. Accordingly, an optical measurement apparatus which has an improved light intensity detection performance is provided. 
     In the optical measurement apparatus according to one embodiment, the controller measures a current value of the offset current based on the output voltage value of the logarithmic amplifier when the light receiving element is shaded. Such a configuration enables the optical measurement apparatus to measure the current value of the offset current. 
     In the optical measurement apparatus according to one embodiment, the controller calculates the light intensity of the light to be analyzed by subtracting a current value of the offset current calculated based on the output voltage value of the logarithmic amplifier when the light receiving element is shaded, from a current value calculated based on the output voltage value of the logarithmic amplifier when the light to be analyzed is incident on the light receiving element. Such a configuration enables the light intensity of light to be analyzed across a wide range. 
     The optical measurement apparatus according to one embodiment further includes: a second amplifier having a non-inverting input terminal electrically connected to the voltage source; a first switch capable of switching whether or not the light receiving element and the input terminal are electrically connected; a second switch capable of switching whether or not the light receiving element and an inverting input terminal of the second amplifier are electrically connected; a third switch capable of switching whether or not the inverting input terminal of the second amplifier and an output terminal of the second amplifier are electrically connected; and a fourth switch capable of switching whether or not the non-inverting input terminal of the second amplifier is electrically connected to a reference potential, wherein the plurality of offset resistors are provided between the light receiving element and the output terminal of the second amplifier. Such a configuration enables the optical measurement apparatus to use the offset resistors as a feedback resistor of the second amplifier as well as using it for adjusting the current value of the offset current. As a result, there is provided an optical measurement apparatus that configures both a logarithmic amplifier and a linear amplifier while achieving a reduction in costs and reduction in the footprint. 
     The optical measurement apparatus according the first embodiment has a first mode for measuring the light intensity based on the output voltage value of the logarithmic amplifier and a second mode for measuring the light intensity based on an output voltage value of the second amplifier, in the first mode, the light receiving element and the input terminal are electrically connected by the first switch, the light receiving element and the inverting input terminal of the second amplifier are electrically disconnected by the second switch, the inverting input terminal of the second amplifier and the output terminal of the second amplifier are electrically connected by the third switch, and the non-inverting input terminal of the second amplifier is electrically disconnected from the reference potential by the fourth switch, and in the first mode, the offset current is input to the input terminal via the offset resistor electrically connected between the output terminal of the second amplifier and the input terminal. Such a configuration allows the user to appropriately switch the mode of the optical measurement apparatus between the first mode and the second mode depending on the light to be analyzed, for example. 
     In the optical measurement apparatus according to one embodiment, the voltage source is a digital-to-analog converter. Such a configuration enables the optical measurement apparatus to input a voltage according to the first mode or the second mode to the non-inverting input terminal of the second amplifier, by the digital-to-analog converter. 
     In the optical measurement apparatus according to one embodiment, the controller measures the current value of the offset current based on the output voltage value of the second amplifier when the light receiving element is shaded in the first mode. Such a configuration enables the optical measurement apparatus to measure the current value of the offset current. 
     In the optical measurement apparatus according to one embodiment, the controller: measures the output voltage value of the logarithmic amplifier and the output voltage value of the second amplifier when the light receiving element is shaded for each current value of the offset current while switching the switch unit, and generates a table by associating the measured output voltage value of the logarithmic amplifier with a current value of the offset current calculated based on the measured output voltage value of the second amplifier, and measures the light intensity of the light to be analyzed based on the output voltage value of the logarithmic amplifier when the light to be analyzed is incident on the light receiving element and the table. Such a configuration can provide an accurate measurement of the light intensity of the light to be analyzed in the first mode. 
     The optical measurement apparatus according to one embodiment further includes a transistor being a depletion type N-channel field effect transistor, wherein the light receiving element is a photodiode, an anode of the photodiode is electrically connected to the first switch and the second switch, and a gate of the transistor is electrically connected to the input terminal, a source of the transistor is electrically connected to a cathode of the photodiode, and a voltage having a positive voltage value is input to a drain of the transistor. Such a configuration can provide an accurate measurement of the light intensity of the light to be analyzed in the first mode. 
     The optical measurement apparatus according to one embodiment further includes a fifth switch capable of switching whether the cathode of the photodiode is electrically connected to the reference potential or to the source of the transistor, in the second mode, the cathode of the photodiode is electrically connected to the reference potential by the fifth switch. Such a configuration can provide an accurate measurement of the light intensity of the light to be analyzed in the second mode. 
     According to the present disclosure, there is provided an optical measurement apparatus which has an improved light intensity detection performance. 
     As used in the present disclosure, the term “linear amplifier” refers to an amplifier including a fixed resistor to be used in the feedback section of the amplifier. The fixed resistor used in the feedback section is also referred to as “feedback resistor”. A linear amplifier can be used in a transimpedance circuit. 
     As used in the present disclosure, the term “logarithmic amplifier” refers to an amplifier including a nonlinear element to be used in the feedback section of the amplifier. The nonlinear element is, for example, a transistor or a diode. The nonlinear element used in the logarithmic amplifier converts a current value which is input to the nonlinear element into a voltage value proportional to the logarithm of the current value. Such a nonlinear element is also referred to as “logarithmic conversion element”. A logarithmic amplifier can be used in a transimpedance circuit. 
     First Embodiment 
     An optical measurement apparatus  1  is applicable to a wide variety of applications where the light intensity of light to be analyzed is measured. As will be described below, the light measurement apparatus  1  can measure a light intensity across a wide range. The optical measurement apparatus  1  is applicable to an optical spectrum analyzer, an optical power meter, or the like, in which a light intensity needs to be measured across a wide range. For example, an optical spectrum analyzer needs to measure a light intensity in a wide range from +10 dBm to −90 dBm. Hereinafter, the optical measurement apparatus  1  will be described assuming that it is applied to an optical spectrum analyzer. 
     In an optical spectrum analyzer, light to be analyzed is split by a monochromator such as a diffraction grating. The light to be analyzed which has been split is input to the optical measurement apparatus  1 , as illustrated in  FIG. 1 . 
     Referring to  FIG. 1 , the optical measurement apparatus  1  includes a photodiode  10  (light receiving element), a logarithmic amplification circuit  20 , a resistor R 5 , a switch unit  30 , offset resistors R 30 - 1  to R 30 -N, an analog-to-digital (AD) converter  40 , and a processing unit  2 . The processing unit  2  includes a storage  50 , an input unit  51 , and a controller  52 . However, the light measurement apparatus  1  may include a light receiving element other than the photodiode  10 , as long as the light receiving element is capable of converting a light intensity of light to be analyzed into an electrical signal. The logarithmic amplifier circuit  20  has an input terminal P 1 , an input terminal P 2 , and an output terminal P 3 . 
     Hereinafter, when the offset resistors R 30 - 1  to R 30 -N are not specifically distinguished from each other, they may also be collectively referred to as “offset resistors R 30 ”. 
     The anode of the photodiode  10  is electrically connected to the input terminal P 1  of the logarithmic amplifier circuit  20 . The cathode of the photodiode  10  is electrically connected to the reference potential. Light to be analyzed is incident on the photodiode  10 . The photodiode  10  converts the light intensity of the light to be analyzed into a photocurrent ip by means of the photovoltaic effect. The photocurrent ip is input to the input terminal P 1  of the logarithmic amplifier circuit  20 . The current value of the photocurrent ip is also referred to as “photocurrent value Ip”. 
     The photocurrent value Ip is converted into a voltage value by a transimpedance circuit. In the present embodiment, the transimpedance circuit is a logarithmic amplifier as will be described below. The optical measurement apparatus  1  calculates the light intensity of the light to be analyzed based on the output voltage value of this logarithmic amplifier. 
     As illustrated in  FIG. 2 , the logarithmic amplifier circuit  20  includes an amplifier  21  (first amplifier), an amplifier  22 , an amplifier  23 , a transistor T 1 , a transistor T 2 , a resistor R 1 , a resistor R 2 , a resistor R 3 , and a resistor R 4 . The transistor T 1  and the transistor T 2  are, for example, bipolar transistors. 
     The amplifier  21  and the transistor T 1  configure a logarithmic amplifier. For example, the emitter of the transistor T 1  is electrically connected to the output terminal of the amplifier  21 , and the collector of the transistor T 1  is electrically connected to the inverting input terminal of the amplifier  21 . Further, the transistor T 1  is configured as a base-grounded bipolar transistor. For example, the base of the transistor T 1  is electrically connected to the reference potential. Further, the amplifier  21  is configured so that the inverting input terminal and the non-inverting input terminal are virtually short-circuited. For example, the non-inverting input terminal of the amplifier  21  is electrically connected to the reference potential. 
     The inverting input terminal of the amplifier  21  is electrically connected to the input terminal P 1  of the logarithmic amplifier circuit  20 . The current that is input to the input terminal P 1  is also referred to as “current i 1 ”. Further, the current value of the current i 1  is also referred to as “current value I 1 ”. 
     The relationship between the current value I 1  and the output voltage value Va 1  of the amplifier  21  is expressed by the equation (1) due to the characteristics of the bipolar transistor. 
     
       
         
           
             
               
                 
                   
                     Va 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   = 
                   
                     
                       - 
                       k 
                     
                     ⁢ 
                     
                       T 
                       / 
                       q 
                     
                     × 
                     
                       ln 
                       ⁡ 
                       
                         ( 
                         
                           I 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             1 
                             / 
                             Is 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     1 
                     ) 
                   
                 
               
             
           
         
       
     
     In the equation (1), the constant k is Boltzmann&#39;s constant. The constant k is, for example, 1.38×10 −23  J/K. The temperature T is the absolute temperature of the transistor T 1 . The charge amount q is the amount of charge per electron. The charge amount q is, for example, 1.602×10 −19  C. The current value Is is the current value of the reverse direction saturation current of the transistor T 1 . 
     In the present embodiment, the amplifier  22 , the amplifier  23 , the transistor T 2 , the voltage source Vref, and the resistor R 5  are used to cancel out the current value Is in the equation (1), as will be described below. 
     The amplifier  22  is a replica circuit of the amplifier  21 . The amplifier  22  has the same electrical characteristics as the amplifier  21 . The transistor T 2  is a replica circuit of the transistor T 1 . The transistor T 2  has the same electrical characteristics as the transistor T 1 . The transistor T 2  is in a thermal contact with the transistor T 1 . The transistor T 2  operates at the same temperature as the transistor T 1 . 
     The amplifier  22  and the transistor T 2  configure a logarithmic amplifier. For example, the emitter of the transistor T 2  is electrically connected to the output terminal of the amplifier  22 , and the collector of the transistor T 2  is electrically connected to the inverting input terminal of the amplifier  22 . Further, the transistor T 2  is configured as a base-grounded bipolar transistor. For example, the base of the transistor T 2  is electrically connected to the reference potential. Further, the amplifier  22  is configured so that the inverting input terminal and the non-inverting input terminal are virtually short-circuited. For example, the non-inverting input terminal of the amplifier  22  is electrically connected to the reference potential. 
     The inverting input terminal of the amplifier  22  is electrically connected to the input terminal P 2  of the logarithmic amplifier circuit  20 . The voltage from the voltage source Vref is input to the input terminal P 2  via the resistor R 5 . The current i 2  is input to the input terminal P 2 . The current value of the current i 2  is also referred to as “current value I 2 ”. 
     The current value I 2  is set by the voltage value VREF of the voltage source Vref and the resistor R 5 . This makes the current value I 2  known. The resistor R 5  is configured to include a fixed resistor. The resistor R 5  has two terminals. One terminal of the resistor R 5  is electrically connected to the input terminal P 2 . The other terminal of the resistor R 5  is electrically connected to the voltage source Vref. 
     The relationship between the current value I 2  and the output voltage value Va 2  of the amplifier  22  is expressed by the equation (2) similarly to the equation (1) described above. 
     
       
         
           
             
               
                 
                   Va 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   2 
                   ⁢ 
                   
                     = 
                     
                       
                         - 
                         k 
                       
                       ⁢ 
                       
                         T 
                         / 
                         q 
                       
                       × 
                       
                         ln 
                         ⁡ 
                         
                           ( 
                           
                             I 
                             ⁢ 
                             
                               2 
                               / 
                               I 
                             
                             ⁢ 
                             s 
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     2 
                     ) 
                   
                 
               
             
           
         
       
     
     The inverting input terminal of the amplifier  23  is electrically connected to the output terminal of the amplifier  21  via the resistor R 1 . The inverting input terminal of the amplifier  23  is electrically connected to the output terminal of the amplifier  23  via the resistor R 2 . Further, the output terminal of the amplifier  22  is electrically connected to the non-inverting input terminal of the amplifier  23  via the resistor R 3 . The non-inverting input terminal of the amplifier  23  is electrically connected to the reference potential via the resistor R 4 . 
     The resistor R 1  and the resistor R 3  are each configured to have a temperature measuring resistor having a positive temperature coefficient. The resistor R 2  and the resistor R 4  are each configured to have a fixed resistor. 
     Each of the resistor R 1  and the resistor R 2  has two terminals. One terminal of the resistor R 1  is electrically connected to the output terminal of the amplifier  21 . The other terminal of the resistor R 1  is connected to one terminal of the resistor R 2  and to the inverting input terminal of the amplifier  23 . The other terminal of the resistor R 2  is connected to the output terminal of the amplifier  23 . Each of the resistor R 3  and the resistor R 4  has two terminals. One terminal of the resistor R 3  is electrically connected to the output terminal of the amplifier  22 . The other terminal of the resistor R 3  is electrically connected to one terminal of the resistor R 4  and to the non-inverting input terminal of the amplifier  23 . The other terminal of the resistor R 4  is electrically connected to the reference potential. 
     As will be described below, the light intensity of the light to be analyzed is calculated based on the output voltage value of the logarithmic amplifier circuit  20 . The output voltage value of the amplifier  23 , i.e., the output voltage value Vo 1  of the logarithmic amplifier circuit  20 , is expressed by the equation (3). 
     
       
         
           
             
               
                 
                   
                     Vo 
                     ⁢ 
                     1 
                   
                   = 
                   
                     
                       G 
                       × 
                       
                         ( 
                         
                           
                             Va 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             2 
                           
                           - 
                           
                             Va 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             1 
                           
                         
                         ) 
                       
                     
                     = 
                     
                       Gk 
                       ⁢ 
                       
                         T 
                         / 
                         q 
                       
                       × 
                       
                         ln 
                         ⁡ 
                         
                           ( 
                           
                             I 
                             ⁢ 
                             
                               1 
                               / 
                               I 
                             
                             ⁢ 
                             2 
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     3 
                     ) 
                   
                 
               
             
           
         
       
     
     In the equation (3), G=r 2 /r 1 =r 4 /r 3 . The resistance value r 1  is the resistance value of the resistor R 1 . The resistance value r 2  is the resistance value of the resistor R 2 . The resistance value r 3  is the resistance value of the resistor R 3 . The resistance value r 4  is the resistance value of the resistor R 4 . 
     The equation (3) indicates that the output voltage value Vo 1  has a temperature dependence. For example, the output voltage value Vo 1  is proportional to the temperature T. By using temperature measuring resistors having a positive temperature coefficient as the resistors R 1  and R 3  of the resistors R 1  to R 4 , the temperature dependence of the output voltage value Vo 1  is reduced. 
     Here, when the photodiode  10  is shaded, the photocurrent value Ip becomes 0 A or less. When the photocurrent value Ip falls to 0 A and the current value I 1  thus falls to 0 A or less, only a very small current flows from the emitter to the collector of the transistor T 1 . As a result, the output voltage value Va 1  of the amplifier  21  saturates to a positive voltage value. In other words, the output voltage value Va 1  of the amplifier  21  saturates to the positive voltage value, resulting in latch-up of the amplifier  21 . For preventing latch-up of the amplifier  21 , an offset current ioff is input to the input terminal P 1  via a switch unit  30  and an offset resistor R 30  in the present embodiment. The current value of an offset current ioff is also referred to as “offset current value Ioff”. In this configuration, the current value I 1  is the sum of the offset current value Ioff and the photocurrent value Ip. Using the sum of the offset current value Ioff and the photocurrent value Ip as the current value I 1  prevents the current value I 1  from falling to 0 A or less, which prevents latch-up of the amplifier  21 . 
     Each offset resistor R 30  has two terminals. Each offset resistors R 30  is configured to include a fixed resistor. The offset resistors R 30 - 1  to R 30 -N have resistance values that are different from each other. For example, the resistance values of the offset resistors R 30  increase by a factor of 10 from the offset resistor R 30 - 1  to the offset resistor R 30 -N. 
     The switch unit  30  is capable of switching an offset resistor R 30  to be electrically connected between the voltage source Vb and the input terminal P 1 , of the offset resistors R 30 - 1  to R 30 -N. The switch unit  30  includes changeover switches SW 30 - 1  to SW 30 -N. 
     Hereinafter, when the changeover switches SW 30 - 1  to SW 30 -N are not specifically distinguished from each other, they are also referred to as “changeover switches SW 30 ”. 
     Each changeover switch SW 30  has two terminals. Each changeover switch SW  30  is configured to include a mechanical relay, a photoMOS relay, an analog switch, or the like. Alternatively, the switch unit  30  may be configured as an analog multiplexer. In the case where the switch unit  30  is configured as an analog multiplexer, either terminals of the changeover switches SW 30 - 1  to SW 30 -N can be unified. 
     Hereinafter, the offset resistor R 30  corresponding to a changeover switch SW 30 - i  (i is an integer from 1 to N) is also referred to as “offset resistor R 30 - i”.    
     One terminal of the changeover switch SW 30 - i  is electrically connected to the voltage source Vb. The other terminal of the changeover switch SW 30 - i  is electrically connected to one terminal of the offset resistor R 30 - i.  The other terminal of the offset resistor R 30 - i  is electrically connected to the input terminal P 1 . Alternatively, one terminal of the offset resistor R 30 - i  may be electrically connected to the voltage source Vb, and the other terminal of the offset resistor R 30 - i  may be connected to one terminal of the changeover switch SW 30 - i.  In such a case, the other terminal of the changeover switch SW 30 - i  is electrically connected to the input terminal P 1 . 
     A control signal from the controller  52  is output to a changeover switch SW  30 . The changeover switch SW 30  is turned on (changes to the conductive state) or turned off (changes to the non-conductive state) according to the control signal from the controller  52 . When the changeover switch SW 30 - i  is turned on, the offset resistor R 30 - i  is electrically connected between the voltage source Vb and the input terminal P 1 . Or, when the changeover switch SW 30 - i  is turned off, the offset resistor R 30 - i  is electrically disconnected from the voltage source Vb and the input terminal P 1 . 
     The AD converter  40  is an analog-to-digital converter. The AD converter  40  is electrically connected to the output terminal of the amplifier  23 . The output voltage value Vo 1  of the logarithmic amplifier circuit  20  is input to the AD converter  40 . The AD converter  40  converts the output voltage value Vo 1  in the form of an analog signal into a digital signal. The AD converter  40  outputs the digital signal to the controller  52 . 
     The storage  50  is, for example, a semiconductor memory, a magnetic memory, an optical memory, or the like. The storage  50 , however, is not limited to these. The storage  50  may function as, for example, a main storage device, an auxiliary storage device, or a cache memory. The storage  50  stores certain information used for operations of the optical measurement apparatus  1 . For example, the storage  50  may store various types of information and the like, such as a system program and an application program. 
     The input unit  51  includes an input interface for receiving an input from a user. The input interface can be physical keys, capacitive keys, a touch screen, or a microphone for receiving voice inputs. The input interface, however, is not limited to these. 
     The controller  52  includes at least one processor, at least one dedicated circuit, or a combination thereof. The processor is, for example, a general-purpose processor such as a central processing unit (CPU) or a graphics processing unit (GPU), or a dedicated processor adapted to particular processing. The dedicated circuit is, for example, a field-programmable gate array (FPGA) or an application specific integrated circuit (ASIC). The controller  52  executes processing pertaining to operations of the optical measurement apparatus  1  while controlling the components of the optical measurement apparatus  1 . 
     &lt;Measurement Sensitivity Setting Processing&gt; 
     Before carrying out an analysis of an optical spectrum, for example, the controller  52  receives an input of the measurement sensitivity to be described below via the input unit  51 . This input is provided by a user via the input unit  51 . 
     The measurement sensitivity is a metric of an allowable noise level upon measuring the light intensity of light to be analyzed. The measurement sensitivity is defined by the sensitivity of the optical measurement apparatus  1 , for example. As the sensitivity of the optical measurement apparatus  1  increases, the noise level decreases. As an example, as exemplified in  FIG. 3 , the measurement sensitivities of the sensitivities A, B, C, and D are set. The noise level decreases when the sensitivity changes in the order of the sensitivities A, B, C, and D. The noise levels of the optical measurement apparatus  1  corresponding to the sensitivities A, B, C, and D are −50 dBm, −60 dBm, −70 dBm and −80 dBm, respectively. 
     As the feedback resistance value of the feedback section of the amplifier  21 , i.e., the resistance value Rt of the transistor T 1 , increases, the sensitivity of the output voltage value Vo 1  of the logarithmic amplifier circuit  20  to the current value I 1  increases. In other words, as the resistance value Rt of the transistor T 1  increases, the sensitivity of the logarithmic amplifier circuit  20  increases and the noise level of the optical measurement apparatus  1  decreases. However, as the resistance value Rt of the transistor T 1  increases, the response speed of the amplifier  21  decreases and the measurement speed of the optical measurement apparatus  1  decreases accordingly. In other words, as the resistance value Rt of the transistor T 1  increases, the noise level of the optical measurement apparatus  1  decreases but the measurement speed of the optical measurement apparatus  1  decreases. For example, the noise level decreases when the sensitivity decreases when the sensitivity changes in the order of the sensitivities A, B, C, and D. However, the measurement speed decreases by a factor of  10  when the sensitivity changes in the order of the sensitivities A, B, C, and D. 
     Here, the resistance value Rt of the transistor T 1  is expressed by the equation (4). The equation (4) is derived by differentiating the equation (1). Further, the cutoff frequency fc of the amplifier  21  is expressed by the equation (5). 
     
       
         
           
             
               
                 
                   Rt 
                   = 
                   
                     kT 
                     / 
                     
                       ( 
                       
                         q 
                         × 
                         I 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         1 
                       
                       ) 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     4 
                     ) 
                   
                 
               
             
             
               
                 
                   fc 
                   = 
                   
                     
                       1 
                       / 
                       
                         ( 
                         
                           2 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           π 
                           × 
                           Rt 
                           × 
                           Cj 
                         
                         ) 
                       
                     
                     = 
                     
                       
                         ( 
                         
                           q 
                           × 
                           I 
                           ⁢ 
                           1 
                         
                         ) 
                       
                       / 
                       
                         ( 
                         
                           2 
                           ⁢ 
                           π 
                           × 
                           kT 
                           × 
                           Cj 
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     5 
                     ) 
                   
                 
               
             
           
         
       
     
     In the equation (5), the capacitance value Cj is the junction capacitance value of the transistor T 1 . For example, in the case where the capacitance value Cj is 1 pF and the current value I 1  is 100 pA, the cutoff frequency fc is 600 Hz. Or, in the case where the capacitance value Cj is 1 pF and the current value I 1  is 1 nA, the cutoff frequency fc is 6 kHz. 
     The equation (4) indicates that the resistance value Rt increases as the current value I 1  decreases. As described above, as the resistance value Rt of the transistor T 1  increases, the measurement sensitivity of the optical measurement apparatus  1  increases but the measurement speed of the optical measurement apparatus  1  decreases. As a result, as the current value I 1  decreases, the resistance value Rt increases and the measurement sensitivity of the optical measurement apparatus  1  increases but the measurement speed of the optical measurement apparatus  1  decreases. Further, the equation (5) indicates that the cutoff frequency fc is proportional to the current value I 1 . In other words, the cutoff frequency fc increases in proportion to the current value I 1  up to an amplification band of the amplifier  21  of about several megahertz (MHz). Accordingly, as the current value I 1  increases, the cutoff frequency fc increases and the frequency band of the amplifier  21  widens. 
     In the present embodiment, as described above, the current value I 1  is the sum of the offset current value Ioff and the photocurrent value Ip. In the present embodiment, the offset current value Ioff can be adjusted by switching an offset resistor R 30  to be electrically connected between the voltage source Vb and the input terminal P 1  by the switch unit  30 . In other words, in the present embodiment, the measurement sensitivity of the optical measurement apparatus  1 , the measurement speed of the optical measurement apparatus  1 , and the frequency band of the amplifier  21  can be adjusted through an adjustment of the offset current value Ioff to thereby adjust the current value I 1 . 
     For example, when the light intensity of light to be analyzed is low, an offset resistor R 30  having a large resistance value is caused to be electrically connected between the voltage source Vb and the input terminal P 1  by the switch unit  30 . Such a configuration reduces the offset current value Ioff to thereby reduce the current value I 1 , leading to an increased measurement sensitivity of the optical measurement apparatus  1 . 
     For example, when the light intensity of light to be analyzed is high, an offset resistor R 30  having a small resistance value is caused to be electrically connected between the voltage source Vb and the input terminal P 1  by the switch unit  30 . Such a configuration increases the offset current value Ioff to thereby increase the current value I 1 , leading to an increased measurement speed of the optical measurement apparatus  1 . 
     The sensitivities A, B, C and D as exemplified in  FIG. 3  are set by setting the offset current value Ioff to 200 nA, 20 nA, 2 nA, and 200 pA, respectively. In this case, the cutoff frequencies fc for the sensitivities A, B, C, and D are set to 1 MHz, 100 kHz, 10 kHz, and 1 kHz, respectively. In  FIG. 3 , the resistance value Rs 30  is the resistance value of an offset resistor R 30  which is electrically connected between the input terminal P 1  and the voltage source Vb by the switch unit  30 . When the voltage value VB of the voltage source Vb is 0.2 V, the measurement sensitivities are set to the sensitivities A, B, C, and D by setting the resistance values Rs 30  to 1 MΩ, 10 MΩ, 100 MΩ, and 1 GΩ, respectively. 
     Hereinafter, the resistance values of the offset resistors R 30 - 1 , R 30 - 2 , R 30 - 3 , and R 30 - 4  are assumed to be 1 M Ω, 10 MΩ, 100 MΩ, and 1 GΩ, respectively. When one of the offset resistors R 30 - 1  to R 30 - 4  is electrically connected between the input terminal P 1  and the voltage source Vb by the switch unit  30 , the measurement sensitivity of the optical measurement apparatus  1  is set to the corresponding one of the sensitivities A to D. In other words, one of the sensitivities A to D is set by turning on the corresponding one of the changeover switch SW 30 - 1  to SW 30 - 4  and turning off the changeover switches SW 30  other than that one of the changeover switches SW 30 - 1  to SW 30 - 4 . 
     The measurement sensitivity and switching information for the switch unit  30  for setting to that measurement sensitivity are stored in the storage  50 , while being associated with each other. The switching information for the switch unit  30  includes information of the changeover switch SW  30  to be turned on and information of the changeover switches SW  30  to be turned off. For example, in the case where the measurement sensitivity is the sensitivity A as exemplified in  FIG. 3 , the switching information for the switch unit includes information of the changeover switch SW 30 - 1  to be turned on and information of the changeover switches SW 30  to be turned off other than SW 30 - 1 . 
     &lt;Offset Current Value Measurement Processing&gt; 
     The controller  52  can measure the offset current value Ioff by measuring the output voltage value Vo 1  of the logarithmic amplifier circuit  20  by the AD converter  40  when the photodiode  10  is shaded. In other words, the controller  52  can calculate the offset current value Ioff based on the output voltage value Vo 1 . The offset current value Ioff is used for a calculation of the light intensity of light to be analyzed, as will be described below. The actual offset current value Ioff may deviate from the offset current value Ioff as exemplified in  FIG. 3  due to manufacturing errors of the optical measurement apparatus  1 . By measuring the offset current value Ioff, the light intensity of light to be analyzed can be measured more accurately. 
     The controller  52  calculates the offset current value Ioff 1  based on the output voltage value Vo 1  and the equation (6). The offset current value Ioff 1  is a current value that is calculated from the output voltage value Vo 1  as the offset current value. 
     
       
         
           
             
               
                 
                   Ioff 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   1 
                   ⁢ 
                   
                     = 
                     
                       I 
                       ⁢ 
                       2 
                       × 
                       
                         exp 
                         ⁡ 
                         
                           ( 
                           
                             Vo 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               1 
                               / 
                               K 
                             
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     6 
                     ) 
                   
                 
               
             
           
         
       
     
     In the equation (6), K=GkT/q. 
     The equation (6) indicates that the offset current value Ioff 1  calculated based on the output voltage value Vo 1  is dependent on the temperature T. More specifically, even when the same offset resistor R 30  is connected between the voltage source Vb and the input terminal P 1 , the offset current value Ioff 1  varies if the temperature T upon an optical measurement by the optical measurement apparatus  1  varies. 
     To address this issue, the controller  52  measures the offset current value Ioff 1  before carrying out light intensity measurement processing or periodically, for example. The controller  52  may measure an offset current value Ioff 1  for each offset resistor R 30 , i.e., for each measurement sensitivity. In this case, the controller  52  measures output voltage value Vo 1  of the logarithmic amplifier circuit  20  when the photodiode  10  is shaded while switching among the offset resistors R 30  to be electrically connected between the voltage source Vb and the input terminal P 1  by the switch unit  30 . The controller  52  calculates the offset current value Ioff 1  for each offset resistor R 30  based on the output voltage value Vo 1  and the equation (6). The controller  52  stores the offset current value Ioff 1  associated with the offset resistance R 30 , i.e., the measurement sensitivity, in the storage  50 . 
     &lt;Light Intensity Measurement Processing&gt; 
     The controller  52  measures the output voltage value Vo 1  of the logarithmic amplifier circuit  20  by the AD converter  40  when light to be analyzed is incident on the photodiode  10 . The controller  52  calculates the current value I 1  based on the measured output voltage value Vo 1  and the equation (7). Note that the equation (7) is derived from the equation (3). Further, the current value I 2  in the equation (7) is known as described above. The controller  52  obtains the offset current value Ioff 1  associated with the measurement sensitivity from the storage  50 . The controller  52  calculates the photocurrent value Ip by subtracting the obtained offset current value Ioff 1  from the current value I 1  according to the equation (8). The controller  52  calculates the light intensity Pin based on the photocurrent value Ip and the equation (9). 
     
       
         
           
             
               
                 
                   
                     I 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   = 
                   
                     I 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     2 
                     × 
                     
                       exp 
                       ⁡ 
                       
                         ( 
                         
                           Vo 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             1 
                             / 
                             K 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     7 
                     ) 
                   
                 
               
             
             
               
                 
                   Ip 
                   = 
                   
                     
                       I 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       1 
                     
                     - 
                     
                       Ioff 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       1 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     8 
                     ) 
                   
                 
               
             
             
               
                 
                   Pin 
                   = 
                   
                     I 
                     ⁢ 
                     
                       p 
                       / 
                       S 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     9 
                     ) 
                   
                 
               
             
           
         
       
     
     In the equation (7), K=GkT/q. Further, in the equation (9), the photosensitivity S is the photosensitivity of the photodiode  10 . 
       FIG. 4  is a diagram illustrating waveforms of optical spectra. In  FIG. 4 , the horizontal axis represents the wavelength of light (nm). The vertical axis represents the light intensity (dBm). An optical spectrum analyzer analyzes an optical spectrum by continuously measuring the light intensity across light wavelengths. In other words, when the optical measurement apparatus  1  is applied to an optical spectrum analyzer, the light intensity is measured while light wavelengths are swept in the optical measurement apparatus  1 . 
     The waveform W 1  indicates the light intensity calculated from the equation (9) assuming Ip=I 1  without subtracting the offset current value Ioff 1  from the current value I 1  according to the equation (8). The waveform W 2  is obtained based on the photocurrent value Ip and the equation (9), wherein the photocurrent value Ip is calculated by subtracting the offset current value Ioff 1  from the current value I 1  according to the equation (8). 
     As described above, in the optical measurement apparatus  1 , the controller  52  calculates the photocurrent value Ip by subtracting the offset current value Ioff 1  from the current value I 1  according to the equation (8). Such a configuration enables the light intensity of light to be analyzed across a wide range. 
     &lt;Operation of Optical Measurement System&gt; 
       FIG. 5  is a flowchart illustrating an example of an optical measurement method by the optical measurement apparatus  1  illustrated in  FIG. 1 . The optical measurement method may be implemented as an optical measurement program which is executed by a processor such as the controller  52 . The optical measurement program may be stored on a non-transitory computer-readable medium. In response to the controller  52  detecting an input of a measurement sensitivity by the input unit  51 , the controller  52  starts the processing of Step S 10  as illustrated in  FIG. 5 . 
     In the processing of Step S 10 , the controller  52  receives the input of the measurement sensitivity via the input unit  51 . The user enters one of the sensitivities A to D as exemplified in  FIG. 3  according to the light intensity of light to be analyzed, via the input unit  51 . For example, when the light intensity of light to be analyzed is low, the user enters the sensitivity D via the input unit  51 . Or, when the light intensity of light to be analyzed is high and the user wishes to increase the measurement speed, the user enters the sensitivity A via the input unit  51 . 
     Before the processing of Step S 11  is carried out, the photodiode  10  is shaded. In the processing of Step S 11 , the controller  52  measures output voltage values Vo 1  of the logarithmic amplifier circuit  20  when the photodiode  10  is shaded while switching among the offset resistors R 30  to be electrically connected between the voltage source Vb and the input terminal P 1  by the switch unit  30 . 
     In the processing of Step S 12 , the controller  52  calculates the offset current value Ioff 1  for each offset resistor R 30  based on the output voltage value Vo 1  and the equation (6). The controller  52  stores the offset current value Ioff 1  associated with the offset resistance R 30 , i.e., the measurement sensitivity, in the storage  50 . 
     In the processing of Step S 13 , the controller  52  obtains, from the storage  50 , the switching information for the switch unit  30  for setting to the measurement sensitivity received in the processing of Step S 10 . The controller  52  controls the switch unit  30  based on the obtained switching information for the switch unit  30 . For example, in the case where the measurement sensitivity received in the processing of Step S 10  is sensitivity A as exemplified in  FIG. 3 , the controller  52  turns on the changeover switch SW 30 - 1  and turns off the changeover switches SW 30  other than the changeover switch SW 30 - 1 . 
     Before the processing of Step S 14  is carried out, the photodiode  10  is set such that light to be analyzed can pass through the photodiode  10 . In the processing of Step S 14 , light to be analyzed is input to the photodiode  10 . In the processing of Step S 14 , the controller  52  measures, by the AD converter  40 , the output voltage value Vo 1  of the logarithmic amplifier circuit  20  when the light to be analyzed is incident on the photodiode  10 . 
     In the processing of Step S 15 , the controller  52  obtains, from the storage  50 , the offset current value Ioff 1  associated with the measurement sensitivity received in the processing of Step S 10 . In the processing of Step S 15 , the controller  52  calculates the light intensity of the light to be analyzed based on the output voltage value Vo 1  measured in the processing of Step S 14 , the offset current value Ioff 1  obtained from the storage  50 , and the equations (7) to (9). 
     Note that the controller  52  may not carry out the processing of Steps S 11  and S 12  if the processing of Steps S 11  and S 12  has been carried out in advance, for example. 
     Further, the controller  52  may periodically carry out the processing of Steps S 11  and S 12  at any timing. In such a case, the user may manually shade the photodiode  10  when the processing of Step S 11  is carried out. Alternatively, the optical measurement apparatus  1  may be configured so that the photodiode  10  is automatically shaded when the processing of Step S 11  is carried out. 
     The effects of the optical measurement apparatus  1  according to the present embodiment will be described by making comparisons against an optical measurement apparatus  301  according to a first comparative example and an optical measurement apparatus  401  according to a second comparative example. 
     FIRST COMPARATIVE EXAMPLE 
       FIG. 6  illustrates an optical measurement apparatus  301  including a linear amplifier according to a first comparative example. An optical measurement apparatus  301  includes a photodiode  10 , an AD converter  40 , an amplifier  302 , feedback sections  303 - 1  to  303 -N, a resistor R 304 , a resistor R 305 , and a digital-to-analog (DA) converter  306 . The resistors R 304  and R 305  are used similarly to the resistors R 6  and R 7  as illustrated in  FIG. 10  which will be described below. Further, the DA converter  306  is used similarly to the DA converter  70  as illustrated in  FIG. 10  which will be described below. 
     The anode of the photodiode  10  is electrically connected to the inverting input terminal of the amplifier  302 . In response to light to be analyzed being incident on the photodiode  10 , a photocurrent ip flows from the photodiode  10  toward the inverting input terminal of the amplifier  302 . 
     The amplifier  302  and the feedback sections  303 - 1  to  303 -N configure a linear amplifier. For example, the feedback sections  303 - 1  to  303 -N are electrically connected between the output terminal of the amplifier  302  and the inverting input terminal of the amplifier  302 . 
     The feedback sections  303 - 1  to  303 -N are electrically connected in parallel. The feedback sections  303 - 1  to  303 -N include capacitors C 303 - 1  to C 303 -N, resistors R 303 - 1  to R 303 -N, and switches SW 303 - 1  to SW 303 -N, respectively. 
     Hereinafter, when the feedback sections  303 - 1  to  303 -N are not specifically distinguished from each other, they may also be collectively referred to as “feedback sections  303 ”. Further, when the capacitors C 303 - 1  to C 303 -N are not specifically distinguished from each other, they may also be collectively referred to as “capacitors C 303 ”. Further, when the resistors R 303 - 1  to R 303 -N are not specifically distinguished from each other, they may also be collectively referred to as “resistors R 303 ”. Further, when the switches SW 303 - 1  to SW 303 -N are not specifically distinguished from each other, they may also be collectively referred to as “switches SW 303 ”. 
     Each capacitor C 303  has two terminals. Each resistor R 303  has two terminals. Each switch SW 303  has two terminals. 
     Hereinafter, the capacitor C 303 , the resistor R 303 , and the switch SW 303  included in the feedback section  303 - i  (i is an integer from 1 to N) are also referred to as “capacitor C 303 - i”,  “resistor R 303 - i”,  and “switch SW  303 - i”,  respectively. 
     One terminal of the capacitor C 303 - i  is electrically connected to one terminal of the resistor R 303 - i.  The other terminal of the capacitor C 303 - i  is electrically connected to the other terminal of the resistor R 303 - i.  One terminal of the resistor R 303 - i  is electrically connected to the inverting input terminal of the amplifier  302 . The other terminal of the resistor R 303 - i  is electrically connected to one terminal of the switch SW 303 - i.  The other terminal of the switch SW 303 - i  is electrically connected to the output of the amplifier  302 . 
     In the first comparative example, the AD converter  40  is electrically connected to the output terminal of the amplifier  302 . The output voltage value Vo 302  of the amplifier  302  is input to the AD converter  40 . In the first comparative example, the light intensity of light to be analyzed is measured based on the output voltage value Vo 302  of the amplifier  302 . 
     In the optical measurement apparatus  301  according to the first comparative example, the measurement sensitivity is set by the resistance value of a resistor R 303 , i.e., a feedback resistor, connected between the output terminal of the amplifier  302  and the inverting input terminal. As the resistance value of the feedback resistor increases, the sensitivity of the optical measurement apparatus  301  increases and the noise level of the optical measurement apparatus  301  decreases. For example, as exemplified in  FIG. 7 , the measurement sensitivity of the optical measurement apparatus  301  is set to one of a high sensitivity, a medium sensitivity, and a low sensitivity. The resistance value of the feedback resistor increases and the noise level of the optical measurement apparatus  301  decreases when the sensitivity changes in the order of the low sensitivity, the medium sensitivity, and the high sensitivity. However, as the resistance value of the feedback resistor increases, the response speed of the amplifier  302  decreases and the measurement speed of the optical measurement apparatus  301  decreases accordingly. In other words, when the resistance value of the feedback resistor increases, the noise level of the optical measurement apparatus  301  decreases but the measurement speed of the optical measurement apparatus  301  decreases. For example, as exemplified in  FIG. 7 , the measurement speed of the optical measurement apparatus  301  decreases from fast to moderate, then to slow when the sensitivity changes in the order of the low sensitivity, the medium sensitivity, and the high sensitivity. 
     In the first comparative example, the cutoff frequency fc of the amplifier  302  is represented by the equation (10). 
     
       
         
           
             
               
                 
                   fc 
                   = 
                   
                     1 
                     / 
                     
                       ( 
                       
                         2 
                         ⁢ 
                         π 
                         × 
                         Rf 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         303 
                         × 
                         Cf 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         303 
                       
                       ) 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     10 
                     ) 
                   
                 
               
             
           
         
       
     
     In the equation (10), the feedback capacitance value Cf 303  is a capacitance value of the capacitor C 303 , i.e., the feedback capacitance, electrically connected between the output terminal of the amplifier  302  and the inverting input terminal. Further, the feedback resistance value Rf 303  is the resistance value of a resistor R 303 , i.e., the feedback resistor, electrically connected between the output terminal of the amplifier  302  and the inverting input terminal. For example, in the case where the feedback capacitance value Cf 303  is 1 pF and the feedback resistance value Rf 303  is 100 MΩ, the cutoff frequency becomes 1.6 kHz. 
     The equation (10) indicates that the cutoff frequency fc increases and the frequency band of the amplifier  302  widens as the feedback resistance value Rf 303  decreases. Further, as described above, as the feedback resistance value Rf 303  decreases, the measurement sensitivity of the optical measurement apparatus  301  decreases. In other words, as the feedback resistance value Rf 303  decreases, the measurement sensitivity of the optical measurement apparatus  301  decreases but the frequency band of the amplifier  302  widens. For example, as exemplified in  FIG. 7 , the cutoff frequency widens from the low band to the medium band, then to the high band, when the sensitivity changes in the order of the high sensitivity, the medium sensitivity, and the low sensitivity. 
     Note that the equation (10) indicates that the cutoff frequency fc increases and the frequency band of the amplifier  302  widens as the feedback capacitance value Cf 303  decreases. However, as the feedback capacitance value Cf 303  decreases, the high-frequency noises of the amplifier  302  increase. As a result, the feedback capacitance value Cf 303  is limited to a certain value, and the cutoff frequency fc and the like are adjusted by the feedback resistance value Rf 303 . 
     Here, in the optical measurement apparatus  301  of the first comparative example, measurable light intensities of light to be analyzed are limited by the power supply voltage of the linear amplifier configured from the amplifier  302  and the feedback section  303 . In the first comparative example, because measurable light intensities of light to be analyzed are limited by the power supply voltage of the linear amplifier, the feedback resistance value Rf 303  needs to be decreased when light to be analyzed has a high light intensity. Further, in the first comparative example, when light to be analyzed has a low light intensity, the feedback resistance value Rf 303  needs to be increased for increasing the measurement sensitivity of the optical measurement apparatus  301 . In other words, for applying the optical measurement apparatus  301  to an optical spectrum analyzer, the feedback resistance value Rf 303  needs to be decreased when light to be analyzed has a high light intensity, whereas the feedback resistance value Rf 303  needs be increased when light to be analyzed has a low light intensity, in the optical measurement apparatus  301 . Due to the configuration thereof, in the first comparative example, the switches SW 303  need to be switched for connecting an appropriate resistor R 303  between the output terminal of the amplifier  302  and the inverting input terminal according to the light intensity of light to be analyzed. Accordingly, in the first comparative example, operation of switching the switches SW 303  according to the light intensity of light to be analyzed is required while the light wavelengths are swept during an analysis of an optical spectrum. Because switching operations of the SW 303  are required in the first comparative example, the time to analyze an optical spectrum increases. Further, in the optical measurement apparatus  301 , an analysis of the optical spectrum needs to be suspended until the output voltage value Vo 302  of the amplifier  302  stabilizes after the switches SW 303  are switched. In the optical measurement apparatus  301 , because an analysis of an optical spectrum is suspended until the output voltage value Vo 302  of the amplifier  302  stabilizes, the time to analyze an optical spectrum is increased. 
     In contrast to the first comparative example, in the optical measurement apparatus  1  according to the present embodiment, the output voltage value Vo 1  is the logarithm of the photocurrent value Ip, as indicated by the equations (7) and (8). Because the output voltage value Vo 1  is the logarithm of the photocurrent value Ip, measurable light intensities of light to be analyzed are not limited by the power supply voltage of the logarithmic amplifier circuit  20  in the optical measurement apparatus  1 . Unlike the first comparative example, this configuration eliminates switching operations of the switches SW 303  during an analysis of an optical spectrum in the optical measurement apparatus  1 . Further, unlike the first comparative example, suspension of an analysis of an optical spectrum until the output voltage value Vo 302  of the amplifier  302  stabilizes is not required in the optical measurement apparatus  1 . Accordingly, in the optical measurement apparatus  1  according to the present embodiment, the time to analyze an optical spectrum is reduced as compared with the optical measurement apparatus  301  according to the first comparative example. 
     SECOND COMPARATIVE EXAMPLE 
       FIG. 8  illustrates an optical measurement apparatus  401  including a logarithmic amplifier for a second comparative example. An optical measurement apparatus  401  includes a photodiode  10 , a logarithmic amplifier circuit  20 , and a resistor R 5 . The optical measurement apparatus  401  has a resistor R 403  in place of the switch unit  30  and the offset resistor R 30  in the present embodiment. 
     In the second comparative example, a bias current ib is used to prevent latch-up of the amplifier  21  when the photodiode  10  is shaded. The current value of bias current ib is also referred to as “bias current value Ib”. The bias current ib is input to the input terminal P 1  in the same manner as the offset current ioff illustrated in  FIG. 1 . The bias current value Ib is set by the voltage of the voltage source V 402  and the resistance value of the resistor R 403 . The resistor R 403  is configured to include a fixed resistor. The resistor R 403  has two terminals. One terminal of the resistor R 403  is electrically connected to the input terminal P 1 . The other terminal of the resistor R 403  is electrically connected to the voltage source  402 . 
     Also in the second comparative example, the output voltage value Vo 1  of the logarithmic amplifier circuit  20  is measured by the AD converter  40 . However, unlike the present embodiment, the offset current value Ioff 1  is not measured based on the output voltage value Vo 1  in the second comparative example. In the second comparative example, the current value I 1  calculated based on the output voltage value Vo 1 , the current value I 2 , and the equation (7) are substituted into the photocurrent value Ip in the equation (9) to measure the light intensity Pin. In other words, in the second comparative example, the light intensity Pin is measured based on the equation (9) assuming I 1 =Ip without subtracting the offset current value Ioff 1  from the current value I 1  according to the equation (8). 
     In the second comparative example, the current value I 1  is the sum of the photocurrent value Ip and the bias current value Ib. Also in the second comparative example, as described above with reference to the equation (4), the resistance value Rt increases and the measurement sensitivity of the optical measurement apparatus  401  increases as the current value I 1  decreases. Further, as described above with reference to the equation (5), as the current value I 1  increases, the cutoff frequency fc increases and the frequency band of the amplifier  21  widens. With such a configuration, in the second comparative example, the measurement sensitivity of the optical measurement apparatus  401 , the measurement speed of the optical measurement apparatus  401 , and the frequency band of the amplifier  21  are adjusted by adjusting the bias current value Ib to thereby adjust the current value I 1 . 
     In the second comparative example, for adjusting the bias current value Ib, for example, a digital-to-analog converter may be employed as the voltage source V 402  so that the voltage value of the voltage source V 402  is made variable. Here, the voltage range of the voltage source V 402  is limited by the power supply voltage of the optical measurement apparatus  401 . As a result, for setting the bias current value Ib across a wide range, the resistance value of the resistor R 403  needs to be set low. Setting the resistance value of the resistor R 403  low permits the current value Ib to set high. 
     However, when the resistance value of the resistor R 403  is set low, the gain relative to the non-inverting input terminal side of the amplifier  21 , i.e., the noise gain, increases. Accordingly, even when the voltage value of the voltage source V 402  is reduced to increase the measurement sensitivity, the noise level is not reduced. In the second comparative example, the resistance value of the resistor R 403  needs to be increased to some extent for preventing an increase in the noise gain of the amplifier  21 . For example, the resistance value of the resistor R 403  is set to several gigaohms (GΩ). 
     As set forth above, in the second comparative example, the resistance value of the resistor R 403  needs be increased to some extent for preventing an increase in the noise gain of the amplifier  21 . As a result, the bias current value Ib cannot be set high in the second comparative example. Because the bias current value Ib cannot be set high in the second comparative example, the frequency band of the amplifier  21  cannot be widened. When the frequency band of the amplifier  21  cannot be widened, the waveform distorts at light wavelengths where the light intensity of light to be analyzed is reduced in the optical measurement apparatus  401 , as will be explained below with reference to  FIG. 9 . 
       FIG. 9  is a diagram illustrating waveforms of optical spectra according to the first and second comparative examples. In  FIG. 9 , the horizontal axis represents the wavelength of light (nm). The vertical axis represents the light intensity (dBm). The waveform W 3  is a waveform of an optical spectrum analyzed by the optical measurement apparatus  301  according to the first comparative example. The waveform W 4  is a waveform of an optical spectrum analyzed by the optical measurement apparatus  401  according to the second comparative example. When the waveform W 4  is compared with the waveform W 3 , distortions arise in the light wavelength range where the light intensity is reduced. 
     In contrast to the second comparative example, in the optical measurement apparatus  1  according to the present embodiment, as illustrated in  FIG. 1 , a variety of offset current values Ioff can be set by the offset resistors R 30  having resistance values different from each other. In the optical measurement apparatus  1 , an increase in the noise gain of the amplifier  21  can be prevented by electrically connecting an offset resistor R 30  having a high resistance value between the voltage source Vb and the input terminal P 1 , to set the offset current value Ioff low, without reducing the voltage value VB of the voltage source Vb. Further, in the optical measurement apparatus  1 , the offset current value Ioff can be set high by electrically connecting an offset resistor R 30  having a low resistance value between the voltage source Vb and the input terminal P 1 . As a result, the frequency band of the amplifier  21  can be widened by setting the offset current value Ioff high while preventing an increase in the noise gain of the amplifier  21 . Such a configuration reduces possible distortions of the waveform in light wavelength ranges where the light intensity is low, as illustrated in  FIG. 9 , in the optical measurement apparatus  1 . 
     In the second comparative example, as described above, the offset current value Ioff 1  is not measured based on the output voltage value Vo 1 . In other words, unlike the present embodiment, in the second comparative example, the light intensity Pin is measured assuming I 1 =Ip in the equation (9) without subtracting the offset current value Ioff 1  from the current value I 1  according to the equation (8). As a result, in the second comparative example, a photocurrent value Ip smaller than the bias current value Ib cannot be measured. 
     In contrast to the second comparative example, in the optical measurement apparatus  1  according to the first embodiment, the controller  52  calculates the photocurrent value Ip by subtracting the offset current value Ioff 1  from the current value I 1  according to the equation (8). Further, the controller  52  calculates the light intensity Pin based on the calculated photocurrent value Ip and the equation (9). In this manner, by calculating the photocurrent value Ip by subtracting the offset current value Ioff 1  from the current value I 1  according to the equation (8), the offset current value Ioff 1  can be made greater than the bias current value Ib. By increasing the offset current value Ioff 1 , the measurement speed of the optical measurement apparatus  1  can be increased. 
     As described above, the performance of detection of the light intensity of light to be analyzed is improved in the optical measurement apparatus  1  according to the first embodiment. 
     Second Embodiment 
     Referring to  FIG. 10 , an optical measurement apparatus  101  includes a photodiode  10 , a logarithmic amplifier circuit  20 , a resistor R 5 , an AD converter  40 , and an processing unit  2 . The optical measurement apparatus  101  includes an amplifier  60  (second amplifier), a resistor R 6 , a resistor R 7 , a DA converter  70 , a switch SW 1  (first switch), a switch SW 2  (second switch), a switch SW 3  (third switch), a switch SW 4  (fourth switch), and a switch SW 5 . The optical measurement apparatus  101  includes a switch unit  130 , offset resistors R 130 - 1  to R 130 -N, and capacitors C 130 - 1  to C 130 -N. 
     Hereinafter, when the offset resistors R 130 - 1  to R 130 -N are not specifically distinguished from each other, they may also be collectively referred to as “offset resistors R 130 ”. Further, when the capacitors C 130 - 1  to C 130 -N are not specifically distinguished from each other, they may also be collectively referred to as “capacitor C 130 ”. 
     Each of the switches SW 1  to SW 5  is configured to include a mechanical relay, a photoMOS relay, an analog switch, or the like. The switches SW 1  to SW 5  may be configured as an analog multiplexer. Although the switch SW 1  and the switch SW 2  are illustrated as independent switches in  FIG. 10 , the switch SW 1  and the switch SW 2  may be configured as a single switch. In this case, the switches SW 1  and SW 2  are a switch for switching among the connections to the anode of the photodiode  10 , and may be configured similarly to the switch SW 5 , for example. Further, although the switch SW 2  and the switch SW 3  are illustrated as independent switches in  FIG. 10 , the switch SW 2  and the switch SW 3  may be configured as a single switch. In this case, the switch SW 2  and the switch  3  are a switch for switching among the connections to the inverting input terminal of the amplifier  60 , and may be configured similarly to the switch SW 5 , for example. 
     The switch SW 1  is capable of switching whether or not the input terminal P 1  and the anode of the photodiode  10  are electrically connected. For example, the switch SW 1  has two terminals. One terminal of the switch SW 1  is electrically connected to the input terminal P 1 . The other terminal of the switch SW 1  is electrically connected to the anode of the photodiode  10 . The switch SW 1  is turned on or off according to a control signal from the controller  52 . When the switch SW 1  is turned on, the input terminal P 1  and the photodiode  10  are electrically connected. Or, when the switch SW 1  is turned off, the input terminal P 1  and the photodiode  10  are electrically disconnected. 
     The switch SW 2  is capable of switching whether or not the anode of the photodiode  10  and the inverting input terminal of the amplifier  60  are electrically connected. For example, the switch SW 2  has two terminals. One terminal of the switch SW 2  is electrically connected to the anode of the photodiode  10 . The other terminal of the switch SW 2  is electrically connected to the inverting input terminal of the amplifier  60 . The switch SW 2  is turned on or off according to a control signal from the controller  52 . When the switch SW 2  is turned on, the anode of the photodiode  10  and the inverting input terminal of the amplifier  60  are electrically connected. When the switch SW 2  is turned off, the anode of the photodiode  10  and the inverting input terminal of the amplifier  60  are electrically disconnected. 
     The switch SW 3  is capable of switching whether or not the inverting input terminal of the amplifier  60  and the output terminal of the amplifier  60  are electrically connected. For example, the switch SW 3  has two terminals. One terminal of the switch SW 3  is electrically connected to the inverting input terminal of the amplifier  60 . The other terminal of the switch SW 3  is electrically connected to the output terminal of the amplifier  60 . The switch SW 3  is turned on or off according to a control signal from the controller  52 . When the switch SW 3  is turned on, the inverting input terminal of the amplifier  60  and the output terminal of the amplifier  60  are electrically connected. When the switch SW 3  is turned off, the inverting input terminal of the amplifier  60  and the output terminal of the amplifier  60  are electrically disconnected. 
     The switch SW 4  is capable of switching whether or not the non-inverting input terminal of the amplifier  60  is electrically connected to the reference potential. For example, the switch SW 4  has two terminals. One terminal of the switch SW 4  is electrically connected to the non-inverting input terminal of the amplifier  60  via the resistor R 6 . The other terminal of the switch SW 4  is electrically connected to the reference potential. The switch SW 4  is turned on or off according to a control signal from the controller  52 . When the switch SW 4  is turned on, the non-inverting input terminal of the amplifier  60  is electrically connected to the reference potential. When the switch SW 4  is turned off, the non-inverting input terminal of the amplifier  60  is electrically disconnected from the reference potential. 
     The switch SW 5  is capable of switching whether the output terminal of the amplifier  60  is electrically connected to the AD converter  40  or the output terminal P 3  of the logarithmic amplifier circuit  20  is electrically connected to the AD converter  40 . For example, the switch SW 5  has three terminals. One of the terminals of the switch SW 5  is electrically connected to the output terminal of the amplifier  60 . One of the terminals of the switch SW 5  is electrically connected to the output terminal P 3  of the logarithmic amplifier circuit  20 . One of the terminals of the switch SW 5  is electrically connected to the AD converter  40 . The switch SW 5  switches whether the output terminal of the amplifier  60  is electrically connected to the AD converter  40  or the output terminal P 3  of the logarithmic amplifier circuit  20  is electrically connected to the AD converter  40 , according to a control signal from the controller  52 . 
     The output terminal of the amplifier  60  is electrically connected to the switch SW 5 . The inverting input terminal of the amplifier  60  is electrically connected to the switch SW 2  and the switch SW 3 . The non-inverting input terminal of the amplifier  60  is electrically connected to the resistor R 6  and the resistor R 7 . 
     Each of the resistor R 6  and the resistor R 7  has two terminals. Each of the resistor R 6  and resistor  7  is configured to include a fixed resistor. One terminal of the resistor R 6  is electrically connected to the non-inverting input terminal of the amplifier  60 . The other terminal of the resistor R 6  is electrically connected to the switch SW 4 . One terminal of the resistor R 7  is electrically connected to the non-inverting input terminal of the amplifier  60 . The other terminal of the resistor R 7  is electrically connected to the DA converter  70 . 
     The DA converter  70  is a digital-to-analog converter. The DA converter  70  is electrically connected to the non-inverting input terminal of the amplifier  60  via the resistor R 7 . A control signal from the controller  52  in the form of a digital signal is input to the DA converter  70 . The DA converter  70  converts the input digital signal into an analog signal. The DA converter  70  outputs the analog signal to the non-inverting input terminal of the amplifier  60  via the resistor R 7 . This DA converter  70  inputs a voltage according to a first mode or a second mode, which will be described below, to the non-inverting input terminal of the amplifier  60 . 
     Each offset resistor R 130  has two terminals. Each offset resistor R 130  is configured to include a fixed resistor. The offset resistors R 130 - 1  to R 130 -N have resistance values that are different from each other. For example, the resistance values of the offset resistors R 130  increase by a factor of 10 from the offset resistor R 130 - 1  to the offset resistor R 130 -N. 
     Each capacitor C 130  has two terminals. Each capacitor C 130  is electrically connected in parallel to the corresponding offset resistor R 130 . For example, one terminal of a capacitor C 130  is electrically connected to one terminal of the offset resistor R 130 . The other terminal of the capacitor C 130  is electrically connected to the other terminal of the offset resistor R 130 . 
     The capacitors C 130  function as a feedback capacitance of the amplifier  60  in the second mode to be described below. The capacitance values of the capacitors C 130  may be set appropriately based on the response characteristics of the amplifier  60  in the second mode to be described below. 
     The switch unit  130  is capable of switching an offset resistor R 130  to be electrically connected between the output terminal of the amplifier  60  and the photodiode  10 , of the offset resistors R 130 - 1  to R 130 -N. The switch unit  130  is also capable of switching a capacitor C 130  to be electrically connected between the output terminal of the amplifier  60  and the photodiode  10 , of the capacitors C 130 - 1  to C 130 -N. The switch unit  130  includes the changeover switches SW 130 - 1  to S 130 -N. 
     Hereinafter, when the changeover switches SW 130 - 1  to SW 130 -N are not specifically distinguished from each other, they are also referred to as “changeover switches SW 130 ”. 
     Each changeover switch SW 130  has two terminals. Each changeover switch SW  130  is configured to include a mechanical relay, a photoMOS relay, an analog switch, or the like. Alternatively, the switch unit  130  may be configured as an analog multiplexer. In the case where the switch unit  130  is configured as an analog multiplexer, either terminals of the changeover switches SW 130  to S 130 -N can be unified. 
     Hereinafter, the offset resistor R 130  and the capacitor C 130  corresponding to a changeover switch SW 130 - i  (i is an integer from 1 to N) are also referred to as “offset resistor R 130 - i”  and “capacitor C 130 - i”,  respectively. 
     One terminal of the changeover switch SW 130 - i  is electrically connected to the output terminal of the amplifier  60 . The other terminal of the changeover switch SW 130 - i  is electrically connected to one terminal of the offset resistor R 130 - i  and one terminal of the capacitor C 130 - i.  The other terminal of the offset resistor R 130 - i  and the other terminal of the capacitor C 130 - i  are electrically connected to the anode of the photodiode  10 . Alternatively, one terminal of the changeover switch SW 130 - i  may be electrically connected to the anode of the photodiode  10 . In this case, the other terminal of the changeover switch SW 130 - i  is electrically connected to one terminal of the offset resistor R 130 - i  and one terminal of the capacitor C 130 - i.  Further, the other terminal of the offset resistor R 130 - i  and the other terminal of the capacitor C 130 - i  are electrically connected to the output terminal of the amplifier  60 . 
     A control signal from the controller  52  is output to a changeover switch SW  130 . The changeover switch SW 130  is turned on or off according to the control signal from the controller  52 . When the changeover switch SW 130 - i  is turned on, the offset resistor R 130 - i  and the capacitor C 130 - i  are connected between the output terminal of the amplifier  60  and the anode of the photodiode  10 . Or, when the changeover switch SW 130 - i  is turned off, the offset resistor R 130 - i  and the capacitor C 130 - i  are electrically disconnected from the output terminal of the amplifier  60  and the anode of the photodiode  10 . 
     The optical measurement apparatus  101  has a first mode and a second mode. The first mode is a mode in which the light intensity of light to be analyzed is measured based on the output voltage value Vo 1  of the logarithmic amplifier circuit  20  as illustrated in  FIG. 2 , similarly to the first embodiment. The second mode is a mode in which the light intensity of light to be analyzed is measured based on the output voltage value Vo 2  of the amplifier  60 , as will be described below. 
     &lt;First Mode&gt; 
     The controller  52  receives an input for switching a mode of the optical measurement apparatus  101  via the input unit  51 . This input is provided by the user via the input unit  51 . In response to the controller  52  receiving this input via the input unit  51 , the controller  52  switches the mode of the optical measurement apparatus  101  to the first mode. 
     The controller  52  outputs control signals to the switches SW 1  to SW 4  as appropriate to switch the connection states in the optical measurement apparatus  101  to the connection states corresponding to the first mode. In the first mode, the controller  52  turns on the switch SW 1 , turns off the switch SW 2 , turns on the switch SW 3 , and turns off the switch SW 4 . 
     In the first mode, the anode of the photodiode  10  and the input terminal P 1  are electrically connected by the switch SW 1 , and the anode of the photodiode  10  and the inverting input terminal of the amplifier  60  are electrically disconnected by the switch SW 2 . Further, in the first mode, the inverting input terminal of the amplifier  60  and the output terminal of the amplifier  60  are electrically connected by the switch SW 3 , and the non-inverting input terminal of the amplifier  60  is electrically disconnected from the reference potential by the switch SW 4 . 
     The controller  52  outputs a control signal to the switch SW 5  upon measuring the output voltage value Vo 1  of the logarithmic amplifier circuit  20  in the first mode. The controller  52  outputs the control signal to the switch SW 5  to cause the switch SW 5  to electrically disconnect the output terminal of the amplifier  60  from the AD converter  40 , and to electrically connect the output terminal P 3  of the logarithmic amplifier circuit  20  to the AD converter  40 . Such a configuration enables the controller  52  to measure the output voltage value Vo 1  by the AD converter  40 . 
     The controller  52  outputs a control signal to the switch SW 5  when measuring the output voltage value Vo 2  of the amplifier  60  in the first mode. The controller  52  outputs the control signal to the switch SW 5  to cause the switch SW 5  to electrically connect the output terminal of the amplifier  60  to the AD converter  40 , and to electrically disconnect the output terminal P 3  of the logarithmic amplifier circuit  20  from the AD converter  40 . Such a configuration enables the controller  52  to measure the output voltage value Vo 2  of the amplifier  60  by the AD converter  40 . 
     In the first mode, the inverting input terminal of the amplifier  60  and the output terminal of the amplifier  60  are electrically connected by the switch SW 3 , so that the amplifier  60  functions as a voltage follower. As a result of the amplifier  60  functioning as the voltage follower, the output voltage value Vo 2  of the amplifier  60  becomes equal to the voltage at the non-inverting input terminal of the amplifier  60 . The voltage to be input to the non-inverting input terminal of the amplifier  60  is set by the DA converter  70 , the resistor R 6 , and the resistor R 7 . In the first mode, the DA converter  70  functions as a voltage source Vb as illustrated in  FIG. 1 . As a result of the amplifier  60  functioning as the voltage follower, the output voltage value Vo 2  of the amplifier  60  becomes equivalent to the voltage value VB of the voltage source Vb as illustrated in  FIG. 1 . 
     In the first mode, the switch unit  130  is capable of switching an offset resistor R 130  to be electrically connected between the output terminal of the amplifier  60  and the input terminal P 1 , of the offset resistors R 130 - 1  to R 130 -N. In the first mode, the offset resistor R 130  electrically connected between the output terminal of the amplifier  60  and the input terminal P 1  by the switch  130  causes the offset current ioff to be input to the input terminal P 1  via the switch SW 1 . 
     &lt;First Mode: Measurement Sensitivity Setting Processing&gt; 
     The controller  52  receives an input of the measurement sensitivity for the first mode via the input unit  51  before processing of analyzing an optical spectrum, for example. The measurement sensitivity for the first mode is the measurement sensitivity of the optical measurement apparatus  101  to be set in the first mode. The measurement sensitivity for the first mode may be the one described above with reference to  FIG. 3 . 
     In the second embodiment, the storage  50  stores the measurement sensitivity for the first mode, switching information for the switch unit  130  for setting to that measurement sensitivity for the first mode, and information of the digital signal to be input to the DA converter unit  70  for setting to that measurement sensitivity for the first mode, in the manner that they are associated with each other. The switching information for the switch unit  130  includes information of the changeover switches SW  130  to be turned on and information of the changeover switches SW  130  to be turned off. 
     In response to the controller  52  receiving the measurement sensitivity for the first mode via the input unit  51 , the controller  52  obtains, from the storage  50 , the switching information of the switch unit  130  associated with the measurement sensitivity for the first mode and the information of the digital signal to be input to the DA converter unit  70 . The controller  52  sets the output voltage value of the DA converter  70  by outputting a digital signal to the DA converter  70  based on the information of the obtained digital signal. Further, the controller  52  controls the switch unit  130  based on the obtained switching information for the switch unit  130  in the same manner as the switch unit  30  of the first embodiment. 
     &lt;First Mode: Table Generation Processing&gt; 
     In the meantime, the actual relationship between the output voltage values Vo 1  and the current values I 1  may deviate from the relationship represented by the equation (7).  FIG. 11  is a graph indicating the relationship between the output voltage value Vo 1  and the current value I 1 . In  FIG. 11 , the horizontal axis represents the output voltage value Vo 1  (V). Further, the vertical axis represents the current value I 1  (A). In  FIG. 11 , the dashed line indicates the relationship between the output voltage value Vo 1  and the current value I 1  calculated according to the equation (7). The solid line indicates the relationship between the actual output voltage value Vo 1  and the current value I 1 . As exemplified in  FIG. 7 , there is a deviation between the solid line and the dashed line. The amount of this deviation varies depending on the temperature. 
     Here, when the photodiode  10  is shaded, the photocurrent value Ip becomes 0 A (Ip=0) and the current value I 1  becomes equal to the offset current value Ioff (I 1 =Ioff). Further, in the first mode, as described above, the output voltage value Vo 2  of the amplifier  60  becomes equivalent to the voltage value VB of the voltage source Vb as illustrated in  FIG. 1 . Accordingly, in the second embodiment, the controller  52  can measure the current value I 1  by measuring the output voltage value Vo 2  of the amplifier  60  by the AD converter  40  when the photodiode  10  is shaded. 
     The controller  52  sets the offset current value Ioff, i.e., the current value I 1 , by changing the output voltage value of the DA converter unit  70  and by switching the switch unit  130 . The controller  52  measures, by the AD converter  40 , an output voltage value Vo 1  and an output voltage value Vo 2  when the photodiode  10  is shaded for each offset current value Ioff which has been set by appropriately switching the switch SW 5 . For example, the controller  52  outputs a control signal to the switch SW 5  for each offset current value Ioff which has been set, to cause the switch SW 5  to electrically connect the output terminal of the amplifier  60  to the AD converter  40 , and to electrically disconnect the output terminal P 3  of the logarithmic amplifier circuit  20  from the AD converter  40 . The controller  52  measures, by the AD converter  40 , the output voltage value Vo 2  of the amplifier  60  for each offset current value Ioff which has been set. Further, the controller  52  outputs a control signal to the switch SW 5  for each offset current value Ioff which has been set, to cause the switch SW 5  to electrically disconnect the output terminal of the amplifier  60  from the AD converter  40 , and to electrically connect the output terminal P 3  of the logarithmic amplifier circuit  20  to the AD converter  40 . The controller  52  measures, by the AD converter  40 , the output voltage value Vo 1  of the logarithmic amplifier circuit  20  for each offset current value Ioff which has been set. 
     The controller  52  generates a table as exemplified in  FIG. 12  by associating, for each offset current value Ioff which has been set, the measured output voltage value Vo 1  with a current value I 1  calculated based on the measured output voltage value Vo 2 . The controller  52  calculates the offset current value Ioff, i.e., the current value I 1 , based on the measured output voltage value Vo 2  and the equation (11). 
     
       
         
           
             
               
                 
                   Ioff 
                   ⁢ 
                   
                     = 
                     
                       V 
                       ⁢ 
                       o 
                       ⁢ 
                       
                         2 
                         / 
                         R 
                       
                       ⁢ 
                       s 
                       ⁢ 
                       1 
                       ⁢ 
                       3 
                       ⁢ 
                       0 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     11 
                     ) 
                   
                 
               
             
           
         
       
     
     In the equation (11), the resistance value Rs 130  is the resistance value of an offset resistor R 130  electrically connected between the output terminal of the amplifier  60  and the input terminal P 1  by the switch unit  130 . 
     In a table as exemplified in  FIG. 12 , the output voltage values Vo 1  in the form of digital data measured by the AD converter  40  are used. In  FIG. 12 , the output voltage values Vo 1 , i.e., AAA, BBB, and CCC, in the form of digital data are associated with the current values I 1 , i.e., aaa, bbb, and ccc, respectively. The controller  52  stores the generated table in the storage  50 . 
     Here, the range of the offset current value Ioff that can be set by changing the output voltage value of the DA converter  70  and switching the switch unit  130  is the same as the range of the photocurrent value Ip corresponding to a wide range from +10 dBm to −90 dBm. The range of light intensity from +10 dBm to −90 dBm is similar to the measurement range of the second mode to be described below. The controller  52  can set a plurality of offset current values Ioff in a wide range, and can generate a table from the output voltage values Vo 1  and Vo 2  measured for the respective offset current values Ioff which have been set. 
     The controller  52  can calculate the light intensity of light to be analyzed by using the generated table in place of the equation (7). The current value I 1  calculated according to the equation (7) deviates from the actual current value I 1  as described above. By using the table, the light intensity of light to be analyzed is calculated based on the actual relationship between the output voltage values Vo 1  and the current values I 1 . Such a configuration can provide a more accurate measurement of the light intensity of light to be analyzed. 
     Here, when all changeover switches SW 130 - 1  to SW 130 -N are turned off, the offset current value Ioff becomes 0 A (Ioff=0) and the current value I 1  becomes equal to the photocurrent value Ip (I 1 =Ip). The table can also be generated by inputting light to be analyzed with known light intensities to the photodiode  10  and measuring the output voltage values Vo 1  of the logarithmic amplifier circuit  20  corresponding to the photocurrent values Ip calculated from the known light intensities of the light to be analyzed while changing the light intensities of the light to be analyzed. However, inputting the light to be analyzed with known light intensities to the photodiode  10  is cumbersome. Further, a temperature-induced change in the deviation amount between the current value I 1  calculated according to the equation (7) and the actual current value I 1  cannot be addressed without providing separate light to be analyzed. In contrast to such a configuration, in the present embodiment, Ioff can be set in a wide range by changing the output voltage value of the DA converter  70  and switching the switch unit  130  when the photodiode  10  is shaded, without inputting light to be analyzed with known light intensities to the photodiode  10 . Further, in the present embodiment, the table can be generated by measuring the output voltage values Vo 1  and the output voltage values Vo 2  while switching the switch SW 5 . Accordingly, the table can be easily generated in the present embodiment. Hence, in the present embodiment, it is possible to address to a temperature-induced change in the deviation amount between the current value I 1  calculated according to the equation (7) and the actual current value I 1  by generating the table before carrying out light intensity measurement processing or periodically, for example. 
     It may be time-consuming to set a plurality of offset current values Ioff in a wide range and to measure the output voltage values Vo 1  of the logarithmic amplifier circuit  20  and the output voltage values Vo 2  of the amplifiers  60  for the respective offset current value Ioff which have been set for generating the table. Here, the temperature-induced change in the deviation amount between the current value I 1  calculated according to the equation (7) and the actual current value I 1  is dominated by a drift of the output voltage value Vo 1 . Accordingly, the controller  52  may measure the output voltage value Vo 1  of the logarithmic amplifier circuit  20  and the output voltage value Vo 2  of the amplifier  60  for one offset current value Ioff setting. The controller  52  then calculates, from the table, the output voltage value Vo 1  corresponding to the offset current value Ioff calculated based on the measured output voltage value Vo 2  and the equation (11). The controller  52  calculates the difference between the calculated output voltage value Vo 1  and the measured output voltage value Vo 1 , and adds the difference to every output voltage value Vo 1  in the table. By adding the difference to every output voltage value Vo 1  in the table, the drift-induced change in the deviation amount can be reflected to the table. This configuration can reduce the time to generate the table. Here, the output voltage value of the DA converter  70  and the offset current value Ioff set by the switch unit  130  may equal as the current value I 2 . 
     &lt;First Mode: Offset Current Value Measurement Processing&gt; 
     Similarly to the first embodiment, the controller  52  measures the offset current value Ioff 1  before carrying out light intensity measurement processing or periodically. In the second embodiment, the controller  52  measures the output voltage value Vo 1  of the logarithmic amplifier circuit  20  by the AD converter  40  when the photodiode  10  is shaded by outputting a voltage value corresponding to each measurement sensitivity by the DA converter  70 , and causing the switch unit  130  to set the offset current value Ioff corresponding to the each measurement sensitivity. The controller  52  sets the offset current value Ioff corresponding to the each measurement sensitivity by switching an offset resistor R 130  to be electrically connected between the output terminal of the amplifier  60  and the input terminal P 1 . The controller  52  calculates the offset current value Ioff 1  for the each measurement sensitivity based on the output voltage value Vo 1  and the above-mentioned table. The controller  52  stores the offset current value Ioff 1  associated with the measurement sensitivity in the storage  50 . Instead, the controller  52  may calculate the offset current value Ioff 1  based on the output voltage value Vo 1  and the equation (6), similarly to the first embodiment. 
     &lt;First Mode: Light Intensity Measurement Processing&gt; 
     Before carrying out the light intensity measurement processing, the controller  52  outputs a control signal to the switch SW 5 . The controller  52  outputs the control signal to the switch SW 5  to cause the switch SW 5  to electrically disconnect the output terminal of the amplifier  60  from the AD converter  40 , and to electrically connect the output terminal P 3  of the logarithmic amplifier circuit  20  to the AD converter  40 . 
     The controller  52  measures the output voltage value Vo 1  of the logarithmic amplifier circuit  20  by the AD converter  40  when light to be analyzed is incident on the photodiode  10 . The controller  52  calculates the light intensity of the light to be analyzed based on the measured output voltage value Vo 1 , the table as exemplified in  FIG. 12  stored in the storage  50 , the offset current value Ioff 1  associated with the measurement sensitivity, and the equations (8) and (9). In other words, the controller  52  calculates the light intensity of the light to be analyzed using the table as exemplified in  FIG. 12  instead of the equation (7). Such a configuration can provide an accurate measurement of the light intensity of light to be analyzed as described above. Instead, the controller  52  may calculate the light intensity of the light to be analyzed based on the output voltage value Vo 1  and the equations (7) to (9), similarly to the first embodiment. 
     &lt;Second Mode&gt; 
     The controller  52  receives an input for switching the mode of the optical measurement apparatus  101  to the second mode via the input unit  51 . This input is provided by the user via the input unit  51 . In response to the controller  52  receiving this input via the input unit  51 , the controller  52  switches the mode of the optical measurement apparatus  101  to the second mode. 
     The controller  52  outputs control signals to the switches SW 1  to SW 4  as appropriate to switch the connection states in the optical measurement apparatus  101  to the connection states corresponding to the second mode. In the second mode, the controller  52  turns off the switch SW 1 , turns on the switch SW 2 , turns off the switch SW 3 , and turns on the switch SW 4 . 
     In the second mode, the anode of the photodiode  10  and the input terminal P 1  are electrically disconnected by the switch SW 1 , and the anode of the photodiode  10  and the inverting input terminal of the amplifier  60  are electrically connected by the switch SW 2 . Further, in the second mode, the inverting input terminal of the amplifier  60  and the output terminal of the amplifier  60  are electrically disconnected by the switch SW 3 , and the non-inverting input terminal of the amplifier  60  is electrically connected to the reference potential by the switch SW 4 . 
     The controller  52  outputs a control signal to the switch SW 5  upon measuring the output voltage value Vo 2  of the amplifier  60  in the second mode. The controller  52  outputs the control signal to the switch SW 5  to cause the switch SW 5  to electrically connect the output terminal of the amplifier  60  to the AD converter  40 , and to electrically disconnect the output terminal P 3  of the logarithmic amplifier circuit  20  from the AD converter  40 . Such a configuration enables the controller  52  to measure the output voltage value Vo 2  of the amplifier  60  by the AD converter  40 . 
     In the second mode, the switch unit  130  is capable of switching an offset resistor R 130  to be electrically connected between the output terminal of the amplifier  60  and the inverting input terminal of the amplifier  60 , of the offset resistors R 130 - 1  to R 130 -N. An offset resistor R 130  electrically connected between the output terminal of the amplifier  60  and the inverting input terminal, and the amplifier  60  configure a linear amplifier. In other words, an offset resistor R 130  electrically connected between the output terminal of the amplifier  60 , and the inverting input terminal function as a feedback resistor of the amplifier  60 . Hereinafter, the resistance value of an offset resistor R 130  electrically connected between the output terminal of the amplifier  60  and the inverting input terminal in the second mode is also referred to as “feedback resistance value Rf 130 ”. 
     In the second mode, the switch unit  130  is capable of switching a capacitor C 130  to be electrically connected between the output terminal of the amplifier  60  and the inverting input terminal of the amplifier  60 , of the capacitors C 130 - 1  to C 130 -N. A capacitor C 130  electrically connected between the output terminal of the amplifier  60  and the inverting input terminal functions as a feedback capacitance of the amplifier  60 . Hereinafter, the capacitance value of a capacitor C 130  electrically connected between the output terminal of the amplifier  60  and the inverting input terminal in the second mode is also referred to as “feedback capacitance value Cf 130 ”. 
     &lt;Second Mode: Measurement Sensitivity Setting Processing&gt; 
     Before carrying out an analysis of an optical spectrum, for example, the controller  52  receives an input of the measurement sensitivity for the second mode via the input unit  51 . The measurement sensitivity for the second mode is the measurement sensitivity of the optical measurement apparatus  101  to be set in the second mode. 
     In the second mode, the measurement sensitivity of the optical measurement apparatus  101  changes according to the resistance value of an offset resistor R 130  electrically connected between the output terminal of the amplifier  60  and the inverting input terminal, i.e., the feedback resistance value Rf 130 . As the feedback resistance value Rf  130  increases, the measurement sensitivity of the optical measurement apparatus  101  increases and the noise level of the optical measurement apparatus  101  decreases. However, as the feedback resistance value Rf 130  increases, the response speed of the amplifier  60  decreases and the measurement speed of the optical measurement apparatus  101  decreases accordingly. The cutoff frequency fc of the amplifier  60  is expressed by the equation (12). 
     
       
         
           
             
               
                 
                   fc 
                   ⁢ 
                   
                     = 
                     
                       1 
                       / 
                       
                         ( 
                         
                           2 
                           ⁢ 
                           π 
                           × 
                           Rf 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           130 
                           × 
                           Cf 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           130 
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     12 
                     ) 
                   
                 
               
             
           
         
       
     
     In the equation (12), the feedback capacitance value Cf 130  is the capacitance value of a capacitor C 130  electrically connected between the output terminal of the amplifier  60  and the inverting input terminal, i.e., the feedback capacitance, as described above. 
     The equation (12) indicates that the cutoff frequency fc increases and the frequency band of the amplifier  60  widens as the feedback resistance value Rf 130  decreases. It also indicates that the cutoff frequency fc increases and the frequency band of the amplifier  60  widens as the feedback capacitance value Cf 130  decreases. However, as the feedback capacitance value Cf 130  decreases, the high-frequency noise of the amplifier  60  increases. Hence, in the present embodiment, the feedback capacitance value Cf 130  is limited to a certain value, and the cutoff frequency fc is adjusted by the feedback resistance value Rf 130 . 
     In summary, in the second mode, as the feedback resistance value Rf 130  increases, the measurement sensitivity for the second mode increases. As the measurement sensitivity for the second mode increases, the frequency band of the amplifier  60  narrows and the measurement speed of the optical measurement apparatus  101  decreases. Further, in the second mode, as feedback resistance value Rf decreases, the measurement sensitivity for the second mode decreases. As the measurement sensitivity for the second mode decreases, the frequency band of the amplifier  60  widens and the measurement speed of the optical measurement apparatus  101  increases. 
     For the above reasons, in the second mode, the maximum feedback resistance value Rf 130  is set to each measurement sensitivity for the second mode received via the input unit  51 . The feedback resistance value Rf 130  is adjusted based on the output voltage value Vo 2  of the amplifier  60  so as not to exceed this maximum feedback resistance value Rf 130 . Note that data of the maximum feedback resistance value Rf 130  may be stored in the storage  50  while being associated with the measurement sensitivity for the second mode. 
     Here, in the second mode, the light intensity of light to be analyzed is calculated based on the output voltage value Vo 2  of the amplifier  60 , as will be described below, in the light intensity measurement processing. This output voltage value Vo 2  increases as the light intensity of light to be analyzed increases. When the output voltage value Vo 2  increases to some extent, an accuracy of the light intensity of light to be analyzed based on this output voltage value Vo 2  may not be guaranteed because the supply voltage of the linear amplifier is limited. In other words, in the second mode, when the light intensity of light to be analyzed increases to some extent, it is desirable to reduce the feedback resistance value Rf 130  to thereby reduce the output voltage value Vo 2 . 
     As an example, the controller  52  measures the output voltage value Vo 2  of the amplifier  60  by the AD converter  40  in the light intensity measurement processing to be described below. In this case, the controller  52  determines whether or not the absolute value of the output voltage value Vo 2  is within a predetermined range. The upper limit of the predetermined range may be set based on the maximum rated voltage value of the output voltage value Vo 2  of the amplifier  60 , or based on the maximum value of the absolute value of the output voltage value Vo 2  that guarantees the measurement accuracy of the light intensity. The lower limit of the predetermined range may be set based on the feedback resistance value Rf 130  that is one step higher than the present feedback resistance value Rf 130 , and the upper limit of the predetermined range, or based on the minimum value of the absolute value of the output voltage value Vo 2  that guarantees the measurement accuracy of the light intensity. 
     If the controller  52  determines that the absolute value of the output voltage value Vo 2  is within the predetermined range, the controller  52  measures the light intensity of the light to be analyzed based on the output voltage value Vo 2 , as will be described below in the description of the light intensity measurement processing. 
     Otherwise, if the controller  52  determines that the absolute value of the output voltage value Vo 2  exceeds the predetermined range, the controller  52  controls the switch unit  130  to switch the present feedback resistance value Rf 130  to the feedback resistance value Rf 130  which is one step lower. When the absolute value of the output voltage value Vo 2  exceeds the predetermined range, it is highly likely that the light intensity of the light to be analyzed cannot be calculated accurately based on the output voltage value Vo 2  because the supply voltage of the linear amplifier is limited. When the absolute value of the output voltage value Vo 2  exceeds the predetermined range, the present feedback resistance value Rf 130  is switched to a feedback resistance value Rf 130  that is one step lower. As a result, the output voltage value Vo 2  can be reduced so as to be lower than the present voltage value. Such a configuration can provide an accurate calculation of the light intensity of the light to be analyzed based on the output voltage value Vo 2  after the feedback resistance value Rf 130  is switched. 
     Otherwise, if the controller  52  determines that the absolute value of the output voltage value Vo 2  is below the predetermined range, the controller  52  controls the switch unit  130  to switch the present feedback resistance value Rf 130  to a feedback resistance value Rf 130  that is one step higher. When the absolute value of the output voltage value Vo 2  is below the predetermined range, it is highly likely that the light intensity of the light to be analyzed cannot be calculated accurately based on the output voltage value Vo 2  due to noises or other factors. When the absolute value of the output voltage value Vo 2  is below the predetermined range, the present feedback resistance value Rf 130  is switched to a feedback resistance value Rf 130  that is one step higher than the present measurement sensitivity Rf 130 . As a result, the measurement sensitivity of the optical measurement apparatus  101  is increased so as to be higher than the present measurement sensitivity. Such a configuration can provide an accurate calculation of the light intensity of the light to be analyzed based on the output voltage value Vo 2  after the feedback resistance value Rf 130  is switched. 
     &lt;Second Mode: Light Intensity Measurement Processing&gt; 
     Before carrying out the light intensity measurement processing, the controller  52  outputs a control signal to the switch SW 5 . The controller  52  outputs the control signal to the switch SW 5  to cause the switch SW 5  to electrically connect the output terminal of the amplifier  60  to the AD converter  40 , and to electrically disconnect the output terminal P 3  of the logarithmic amplifier circuit  20  from the AD converter  40 . 
     The controller  52  measures, by the AD converter  40 , the output voltage value Vo 2  of the amplifier  60  when the light to be analyzed is incident on the photodiode  10 . The relationship between the output voltage value Vo 2  and the photocurrent value Ip is expressed by the equation (13). 
     
       
         
           
             
               
                 
                   
                     Vo 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     2 
                   
                   = 
                   
                     
                       
                         - 
                         Rf 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       130 
                       × 
                       Ip 
                     
                     + 
                     Voff 
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     13 
                     ) 
                   
                 
               
             
           
         
       
     
     In the equation (13), the feedback resistance value Rf 130  is the resistance value of an offset resistor R 130  electrically connected between the output terminal of the amplifier  60  and the inverting input terminal of the amplifier  60 , as described above. The voltage value Voff is the output voltage value Vo 2  of the amplifier  60  when the potential difference between the inverting input terminal and the inverting input terminal of the amplifier  60  is 0 V, in other words, the output offset voltage value of the amplifier  60 . 
     For reducing the voltage value Voff in the equation (13), in the configuration illustrated in  FIG. 10 , a voltage obtained by dividing the voltage output from the DA converter  70  between the resistors R 6  and R 7  is input to the non-inverting input terminal of the amplifier  60 . The voltage value Voff is about several millivolts (mV). The resistance value of the resistor R 6  is set, for example, to several ohms (Ω), and the resistance value of the resistor R 7  is set sufficiently higher than the resistance value of the resistor R 6 . The voltage value Voff is stored in the storage  50  while being associated with the feedback resistance value Rf 130 , as will be described below. 
     The controller  52  calculates the photocurrent value Ip based on the voltage value Voff obtained from the storage  50 , the measured output voltage value Vo 2 , and the equation (13). The controller  52  calculates the light intensity Pin of the light to be analyzed based on the calculated photocurrent value Ip and the equation (9). 
     Here, if the user wishes to measure the light intensity of pulsed light, the user can switch the mode of the optical measurement apparatus  101  to the second mode by inputting, via the input unit  51 , an input to switch the mode of the optical measurement apparatus  101  to the second mode. The linear amplifier in the second mode can measure a time-averaged value of the light intensities of pulsed light more easily than the logarithmic amplifier, for example. For example, a time-averaged value of the light intensities of pulsed light can be measured by the linear amplifier by increasing the feedback capacitance value Cf 130  of the linear amplifier. In such a case, in the second mode, the controller  52  may cause the switch unit  130  to electrically connect a capacitor C 130  having a feedback capacitance value Cf 130  corresponding to the period of the pulsed light, between the output terminal of the amplifier  60  and the inverting input terminal of the amplifier  60 . Further, a low-pass filter may be provided between the output terminal of the amplifier  60  and the AD converter  40 . The time-averaged value of the light intensities of pulsed light can be measured by measuring the output voltage value of the amplifier  60  by the AD converter  40  through the low-pass filter. 
     The 1/f noise of the logarithmic amplifier in the first mode may exceed the 1/f noise of the linear amplifier in the second mode. As a result, the measurement sensitivity of the optical measurement apparatus  101  may not be set as high in the first mode as in the second mode. In such a case, if the light intensity of light to be analyzed is low and the user thus wishes to set the measurement sensitivity of the optical measurement apparatus  101  higher, the user can switch the mode of the optical measurement apparatus  101  to the second mode. 
     &lt;Second Mode: Adjustment Processing of Voltage Value Voe and Measurement Processing of Voltage Value Voff&gt; 
     When the photodiode  10  is shaded, the equation (13) is expressed by the equation (14). 
     
       
         
           
             
               
                 
                   
                     Vo 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     2 
                   
                   = 
                   Voff 
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     14 
                     ) 
                   
                 
               
             
           
         
       
     
     The equation (14) indicates that the voltage value Voff can be measured by measuring the output voltage value Vo 2  of the amplifier  60  when the photodiode  10  is shaded. 
     Here, the voltage value Voff is expressed by the equation (15). 
         V off=(1+ Rf 130/ Rpd )×( Voe+r 6/( r 6+ r 7)× Vo 3)   Equation (15)
 
     In the equation (15), the voltage value Voe is the input offset voltage value of the amplifier  60 . The resistance value Rpd is the parallel resistance value of the photodiode  10 . The resistance value r 6  is the resistance value of the resistor R 6 . The resistance value r 7  is the resistance value of the resistor R 7 . The output voltage value Vo 3  is the output voltage value that is output by the DA converter  70 . 
     The controller  52  may adjust the output voltage value of the DA converter  70  for cancelling out the voltage value Voe. For example, when the feedback resistance value Rf 130  equals the minimum value thereof, the effects of the noise gain of the linear amplifier is reduced and the relationship between the voltage value Voff and the voltage value Voe can thus be expressed by the equation (16). 
     
       
         
           
             
               
                 
                   Voff 
                   ≅ 
                   
                     Voe 
                     + 
                     
                       r 
                       ⁢ 
                       
                         6 
                         / 
                         
                           ( 
                           
                             
                               r 
                               ⁢ 
                               6 
                             
                             + 
                             
                               r 
                               ⁢ 
                               7 
                             
                           
                           ) 
                         
                       
                       × 
                       V 
                       ⁢ 
                       o 
                       ⁢ 
                       3 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     16 
                     ) 
                   
                 
               
             
           
         
       
     
     As indicated by the equation (16), when the feedback resistance value Rf 130  equals the minimum value thereof, the output voltage value Vo 3  for cancelling out the voltage value Voe can be determined by measuring the voltage value Voff. 
     The controller  52  controls the switch unit  130  to set the feedback resistance value Rf  130  to the minimum value, and measures the output voltage value Vo 2  of the amplifier  60  when the photodiode  10  is shaded, i.e., the voltage value Voff, by the AD converter  40 . The controller  52  calculates the output voltage value Vo 3  of the DA converter  70  for setting the voltage value Voff to 0 V according to the equation (16). The controller  52  identifies a digital signal to be input to the DA converter  70  to cause the DA converter  70  to output the calculated output voltage value Vo 3 . The controller  52  inputs this digital signal to the DA converter  70  to cancel out the voltage value Voe. The controller  52  stores the information of this digital signal in the storage  50 . 
     Such a configuration can reduce the error of the voltage value Voff from 0 V when the feedback resistance value Rf 130  equals the minimum value thereof. However, when the feedback resistance value Rf 130  has a value other than the minimum value, the error of the voltage value Voff from 0 V increases because the error is amplified by the noise gain. 
     For this reason, the controller  52  measures the output voltage value Vo 2  of the amplifier  60  when the photodiode  10  is shaded, i.e., the voltage values Voff, for each feedback resistance value Rf  130  by the AD converter  40  while switching the switch unit  130 . The controller  52  stores, in the storage  50 , the measured voltage value Voff and the feedback resistance value Rf 130  at the time when the voltage value Voff is measured in the manner that they are associated with each other. The controller  52  stores, in the storage  50 , the measured voltage value Voff and the switching information for the switch unit  130  for setting to the feedback resistance value Rf 130  at the time when the voltage value Voff is measured in the manner that they are associated with each other. 
     The controller  52  may not adjust the output voltage value Vo 3  of the DA converter  70  for cancelling out the voltage value Voe. In this case, the controller  52  measures the voltage value Voff for each feedback resistance value Rf 130  as described above, and stores, in the storage  50 , the measured voltage value Voff and the feedback resistance value Rf 130  at the time when the voltage value Voff is measured in the manner that they are associated with each other. 
     The controller  52  may periodically adjust the output voltage value Vo 3  of the DA converter  70  for cancelling out the voltage value Voe. Further, the controller  52  may periodically measure the voltage value Voff for each feedback resistance value Rf 130 . The voltage value Voe varies as the temperature of the optical measurement apparatus  101  changes. Periodical adjustments of the output voltage value Vo 3  of the DA converter  70  for cancelling out the voltage value Voe can provide an accurate measurement of the light intensity of light to be analyzed in the second mode. Further, periodical measurements of the voltage value Voff for each feedback resistance value Rf 130  can provide a more accurate measurement of the light intensity of light to be analyzed in the second mode. 
     &lt;Operation of Optical Measurement System&gt; 
       FIGS. 13 and 14  are flowcharts illustrating an example of an optical measurement method by the optical measurement apparatus  101  illustrated in  FIG. 10 . The optical measurement method may be implemented as an optical measurement program which is executed by a processor such as the controller  52 . The optical measurement program may be stored on a non-transitory computer-readable medium. In response to the controller  52  detecting an input for switching the mode of the optical measurement apparatus  101  by the input unit  51 , the controller  52  starts the processing of Step S 20  as illustrated in  FIG. 13 . 
     In the process of step  20 , the controller  52  receives, via the input unit  51 , the input for switching the mode of the optical measurement apparatus  101 . The controller  52  determines whether or not this input is an input for switching to the first mode (Step S 21 ). If the controller  52  determines that this input is an input for switching to the first mode (Yes in Step S 21 ), it proceeds to the processing of Step S 22 . Otherwise, if the controller  52  determines that this input is an input for switching to the second mode (No in Step S 21 ), it proceeds to the processing of Step S 32 . 
     In the processing of Step S 22 , the controller  52  outputs control signals to the switches SW 1  to SW 4  as appropriate to switch the connection states in the optical measurement apparatus  101  to the connection states corresponding to the first mode. 
     In the processing of Step S 23 , the controller  52  receives an input of the measurement sensitivity for the first mode via the input unit  51 . 
     Before the processing of Step S 24  is carried out, the photodiode  10  is shaded. In the processing of Step S 24 , the controller  52  sets offset current values Ioff, i.e., the current values I 1 , by changing the output voltage value of the DA converter  70  and by switching the switch unit  130 . The controller  52  measures, by the AD converter  40 , an output voltage value Vo 1  and an output voltage value Vo 2  when the photodiode  10  is shaded for each offset current value Ioff which has been set by appropriately switching the switch SW 5 . 
     In the processing of Step S 25 , the controller  52  generates a table as exemplified in  FIG. 12  by associating, for each offset current value Ioff which has been set, the output voltage value Vo 1  measured in the processing of Step S 24  with the current value I 1  calculated based on the output voltage value Vo 2  measured in the processing of Step S 24 . The controller  52  calculates the offset current value Ioff, i.e., the current value I 1 , based on the measured output voltage value Vo 2  and the equation (11). 
     In the processing of Step S 26 , the controller  52  outputs a control signal to the switch SW 5  to cause the switch SW 5  to electrically disconnect the output terminal of the amplifier  60  from the AD converter  40  and to electrically connect the output terminal P 3  of the logarithmic amplifier circuit  20  to the AD converter  40 . 
     Before the processing of Step S 27  is carried out, the photodiode  10  is shaded. In the processing of Step S 27 , the controller  52  obtains, from the storage  50 , the switching information for the switch unit  130  for setting to each measurement sensitivity, and the information of the digital signal to be input to the DA converter unit  70  for setting to the each measurement sensitivity. Based on the obtained information, the controller  52  measures, by the AD converter  40 , the output voltage value Vo 1  of the logarithmic amplifier circuit  20  while outputting the output voltage value corresponding to the each measurement sensitivity by the DA converter  70  and setting the offset current value Ioff corresponding to the each measurement sensitivity by the switch unit  130 . 
     In the processing of Step S 28 , the controller  52  calculates the offset current value Ioff 1  based on the output voltage value Vo 1  measured in the processing of Step S 27  and the table generated in the processing of Step S 25 . The controller  52  stores, in the storage  50 , the offset current value Ioff 1  associated with each measurement sensitivity for the first mode. 
     In the processing of Step S 29 , the controller  52  obtains, from the storage  50 , the switching information for the switch unit  130  for setting to the measurement sensitivity for the first mode received in the processing of Step S 23  and the information of the digital signal to be input to the DA converter unit  70  for setting to that measurement sensitivity for the first mode. The controller  52  controls the DA converter  70  and the switch unit  130  based on the obtained information. 
     Before the processing of Step S 30  is carried out, the photodiode  10  is set such that light to be analyzed can pass through the photodiode  10 . In the processing of Step S 30 , light to be analyzed is input to the photodiode  10  while the light wavelengths are swept. In the processing of Step S 30 , the controller  52  measures, by the AD converter  40 , the output voltage value Vo 1  of the logarithmic amplifier circuit  20  when the light to be analyzed is incident on the photodiode  10 . 
     In the processing of Step S 31 , the controller  52  calculates the light intensity of the light to be analyzed based on the output voltage value Vo 1  measured in the processing of Step S 30 , the table generated in the processing of Step S 25 , and the equations (8) and (9). 
     In the processing of Step S 32 , the controller  52  outputs control signals to the switches SW 1  to SW 4  as appropriate to switch the connection states in the optical measurement apparatus  101  to the connection states corresponding to the second mode. 
     In the processing of Step S 33 , the controller  52  receives an input of the measurement sensitivity for the second mode via the input unit  51 . 
     In the processing of Step S 34 , the controller  52  outputs a control signal to the switch SW 5  to cause the switch SW 5  to electrically connect the output terminal of the amplifier  60  to the AD converter  40  and to electrically disconnect the output terminal P 3  of the logarithmic amplifier circuit  20  from the AD converter  40 . 
     Before the processing of Step S 35  is carried out, the photodiode  10  is shaded. In the processing of Step S 35 , the controller  52  controls the switch unit  130  to set the feedback resistance value Rf 130  to the minimum value, and measures the output voltage value Vo 2  of the amplifier  60  when the photodiode  10  is shaded, i.e., the voltage value Voff, by the AD converter  40 . In the processing of Step S 35 , the controller  52  identifies a digital signal to be input to the DA converter  70  to set the voltage value Voff to 0 V. The controller  52  inputs the identified digital signal to the DA converter  70  to cancel out the voltage value Voe. 
     Before the processing of Step S 36  is carried out, the photodiode  10  is shaded. In the processing of Step S 36 , the controller  52  measures, by the AD converter  40 , the output voltage value Vo 2  of the amplifier  60  when the photodiode  10  is shaded, i.e., the voltage value Voff, for each feedback resistance value Rf 130  while switching the switch unit  130 . 
     In the processing of Step S 37 , the controller  52  obtains, from the storage  50 , the data of the maximum feedback resistance value Rf 130  associated with the measurement sensitivity for the second mode received in the processing of Step S 33 . The controller  52  controls the switch unit  130  to set the feedback resistance value Rf 130  to an intermediate value between the minimum feedback resistance value Rf 130  and the obtained maximum feedback resistance value Rf 130 . This minimum feedback resistance value Rf 130  is the smallest feedback resistance value Rf 130  of the feedback resistance values Rf 130  that can be set by the offset resistor R 130 . 
     Before the processing of Step S 38  is carried out, the photodiode  10  is set such that light to be analyzed can pass through the photodiode  10 . Further, before the processing of Step S 38  is carried out, the controller  52  outputs the digital signal identified in the processing of Step S 35  to the DA converter  70 . In the processing of Step S 38 , light to be analyzed is input to the photodiode  10 . In the processing of Step S 38 , the controller  52  measures the output voltage value Vo 2  of the amplifier  60  by the AD converter  40  when the light to be analyzed is incident on the photodiode  10 . 
     After the processing of Step S 38  is carried out, the controller  52  proceeds to the processing of Step S 39  as illustrated in  FIG. 14 . 
     In the processing of Step S 39 , the controller  52  determines whether or not the absolute value of the output voltage value Vo 2  measured in the processing of Step S 38  is within a predetermined range. If the controller  52  determines that the absolute value of the output voltage value Vo 2  is within the predetermined range (Yes in Step S 39 ), it proceeds to the processing of Step S 40 . Otherwise, if the controller  39  determines that the absolute value of the output voltage value Vo 2  is out of the predetermined range (No in Step S 39 ), it proceeds to the processing of Step S 41 . 
     In the processing of Step S 40 , the controller  52  calculates the photocurrent value Ip based on the voltage value Voff measured in the processing of Step S 36 , the measured output voltage value Vo 2 , and the equation (13). In the processing of Step S 40 , the controller  52  calculates the light intensity Pin of the light to be analyzed based on the photocurrent value Ip and the equation (9). 
     In the processing of Step S 41 , the controller  52  determines whether or not the absolute value of the output voltage value Vo 2  measured in the processing of Step S 38  exceeds the predetermined range. If the controller  52  determines that the absolute value of the output voltage value Vo 2  exceeds the predetermined range (Yes in Step S 41 ), it proceeds to the processing of Step S 42 . Otherwise, if the controller  52  does not determine that the absolute value of the output voltage value Vo 2  exceeds the predetermined range (No in Step S 41 ), in other words, if the absolute value of the output voltage value Vo 2  is below the predetermined range, the controller  52  proceeds to the processing of Step S 44 . 
     In the processing of Step S 42 , the controller  52  determines whether or not the present feedback resistance value Rf 130  equals the minimum feedback resistance value Rf 130 . If the controller  52  determines that the present feedback resistance value Rf 130  equals the minimum feedback resistance value Rf 130  (Yes in Step S 42 ), it proceeds to the processing of Step S 40 . Otherwise, if the controller  52  determines that the present feedback resistance value Rf 130  does not equal the minimum feedback resistance value Rf 130  (No in Step S 42 ), it proceeds to the processing of Step S 43 . 
     In the processing of Step S 43 , the controller  52  controls the switch unit  130  to switch the present feedback resistance value Rf 130  to a feedback resistance value Rf 130  that is one step lower. After the processing of Step S 43  is carried out, the controller  52  returns to the processing of Step S 38 . 
     In the processing of Step S 44 , the controller  52  determines whether or not the present feedback resistance value Rf 130  equals the maximum feedback resistance value Rf 130  obtained in the processing of Step S 37 . If the controller  52  determines that the present feedback resistance value Rf 130  equals the maximum feedback resistance value Rf 130  (Yes in Step S 44 ), it proceeds to the processing of Step S 40 . Otherwise, if the controller  52  determines that the present feedback resistance value Rf 130  does not equal the maximum feedback resistance value Rf 130  (No in Step S 44 ), it proceeds to the processing of Step S 45 . 
     In the processing of Step S 45 , the controller  52  controls the switch unit  130  to switch the present feedback resistance value Rf 130  to a feedback resistance value Rf 130  that is one step higher. After the processing of Step S 45  is carried out, the controller  52  returns to the processing of Step S 38 . 
     The controller  52  may not carry out the processing of Steps S 24  to S 25 , S 26  to S 28 , S 35 , and S 36  if the controller  52  carries out the processing of Steps S 24  to S 25 , S 26  to S 28 , S 35 , and S 36  in advance, for example. 
     Further, the controller  52  may periodically carry out the processing of Steps S 24  to S 25 , S 26  to S 28 , S 35 , and S 36  at any timing. When Steps S 24  to S 25 , S 26  to S 28 , S 35 , and S 36  are carried out, the user may manually shade the photodiode  10  or the optical measurement apparatus  101  may be configured so that the photodiode  10  is automatically shaded, as described above in the first embodiment. 
     As described above, in the second embodiment, the offset resistors R 130  can be used as a feedback resistor of the amplifier  60 , as well as being used for adjusting the offset current value Ioff. Such a configuration can provide an optical measurement apparatus  101  that configures both a logarithmic amplifier and a linear amplifier, while achieving a reduction in costs and reduction in the footprint. Additionally, because the optical measurement apparatus  101  has the first mode and the second mode, the user can appropriately switch the mode of the optical measurement apparatus  101  to the first mode or the second mode depending on the light to be analyzed, for example. 
     Other configurations and effects of the optical measurement apparatus  101  according to the second embodiment are the same as those of the optical measurement apparatus  1  according to the first embodiment. 
     Third Embodiment 
     Referring to  FIG. 15 , an optical measurement apparatus  201  includes a photodiode  10 , a logarithmic amplifier circuit  20 , a resistor R 5 , a resistor R 6 , a resistor R 7 , an AD converter  40 , an processing unit  2 , an amplifier  60 , a DA converter  70 , a switch SW 1 , a switch SW 2 , a switch SW 3 , a switch SW 4 , a switch SW 5 , a switch unit  130 , offset resistors R 130 - 1  to R 130 -N, and capacitors C 130 - 1  to C 130 -N. The optical measurement apparatus  201  includes a transistor T 3 , a resistor R 8 , and a switch SW 6  (fifth switch). 
     The transistor T 3  is a depletion type N-channel field effect transistor (FET). The transistor T 3  is, for example, a depletion-type N-channel junction field effect transistor (JFET). Instead, the transistor T 3  may be a depletion-type N-channel metal-oxide semiconductor field-effect transistor (MOSFET). 
     The gate of the transistor T 3  is electrically connected to the input terminal P 1  of the logarithmic amplifier circuit  20 . The drain of the transistor T 3  is electrically connected to the voltage source Vcc. The voltage source Vcc supplies a voltage value having a voltage value VCC. The voltage value VCC is a positive voltage value. The voltage value being a voltage value VCC is input to the drain of the transistor T 3 . The source of the transistor T 3  is electrically connected to the voltage source Vee via the resistor R 8 . The voltage source Vee supplies a voltage having a voltage value VEE. The voltage value VEE is a negative voltage value. The voltage having a voltage value VEE is input to the source of the transistor T 3  via the resistor R 8 . In transistor T 3 , the drain and source are functionally indistinguishable. Therefore, in the description of the present embodiment, even if the drain and the source of the transistor T 3  are exchanged, the transistor T 3  functions similarly regardless of whether they are exchanged or not. 
     The resistor R 8  has two terminals. The resistor R 8  is configured to include a fixed resistor. One terminal of the resistor  8  is electrically connected to the source of the transistor T 3 . The other terminal of the resistor R 8  is electrically connected to the voltage source Vee. The resistance value of the resistor R 8  is set appropriately based on the desired source voltage value of the transistor T 3 . 
     The switch SW 6  is configured to include a mechanical relay, a photoMOS relay, an analog switch, or the like. The switch SW 6  may be configured as an analog multiplexer. 
     The switch SW 6  is capable of switching whether the cathode of the photodiode  10  is electrically connected to the reference potential or to the source of the transistor T 3 . For example, the switch SW 6  has three terminals. One of the terminals of the switch SW 6  is electrically connected to the cathode of the photodiode  10 . One of the terminals of the switch SW 6  is electrically connected to the source of the transistor T 3 . One of the terminals of the switch SW 6  is electrically connected to the reference potential. The switch SW 6  switches whether the cathode of the photodiode  10  is electrically connected to the reference potential or to the source of the transistor T 3  according to a control signal from the controller  52 . 
     Similarly to the second embodiment, the optical measurement apparatus  201  has a first mode and a second mode. 
     &lt;First Mode&gt; 
     Similarly to the second embodiment, the controller  52  receives an input for switching the mode of the optical measurement apparatus  201  to the first mode via the input unit  51 . In response to receiving this input, the controller  52  outputs control signals to the switches SW 1  to SW 4  as appropriate to switch the connection states in the optical measurement apparatus  201  to the connection states corresponding to the first mode, similarly to the second embodiment. 
     Similarly to the second embodiment, the controller  52  outputs a control signal to the switch SW 5  upon measuring the output voltage value Vo 1  of the logarithmic amplifier circuit  20  in the first mode. The controller  52  outputs the control signal to the switch SW 5  to cause the switch SW 5  to electrically disconnect the output terminal of the amplifier  60  from the AD converter  40 , and to electrically connect the output terminal P 3  of the logarithmic amplifier circuit  20  to the AD converter  40 . 
     Similarly to the second embodiment, the controller  52  outputs a control signal to the switch SW 5  upon measuring the output voltage value Vo 2  of the amplifier  60  in the first mode. The controller  52  outputs the control signal to the switch SW 5  to cause the switch SW 5  to electrically connect the output terminal of the amplifier  60  to the AD converter  40 , and to electrically disconnect the output terminal P 3  of the logarithmic amplifier circuit  20  from the AD converter  40 . 
     &lt;First Mode: Light Intensity Measurement Processing&gt; 
     Similarly to the second embodiment, before carrying out light intensity measurement processing, the controller  52  outputs a control signal to the switch SW 5 . The controller  52  outputs the control signal to the switch SW 5  to cause the switch SW 5  to electrically disconnect the output terminal of the amplifier  60  from the AD converter  40 , and to electrically connect the output terminal P 3  of the logarithmic amplifier circuit  20  to the AD converter  40 . 
     In the third embodiment, the controller  52  outputs a control signal to the switch SW 6  before carrying out the light intensity measurement processing. The controller  52  outputs a control signal to the switch SW 6  to cause the switch SW 6  to electrically disconnect the cathode of the photodiode  10  from the reference potential, and to electrically connect the cathode of the photodiode  10  to the source of the transistor T 3 . 
     The gate of the transistor T 3  is electrically connected to the anode of the photodiode  10  via the switch SW 1 . The source of the transistor T 3  is electrically connected to the cathode of the photodiode  10  via the switch SW 6 . The voltage value of the anode relative to the cathode of the photodiode  10  becomes equal to the voltage value of the gate to the source of the transistor T 3 . Here, because the transistor T 3  is a depletion type transistor, the transistor T 3  can be turned on even when the voltage value of the gate of the transistor T 3  is a negative voltage value relative to the source of the transistor T 3 . Such a configuration permits a reverse bias to be input to the photodiode  10 . Further, the reverse bias that is input to the photodiode  10  is maintained at the voltage value of the gate relative to the source of the transistor T 3 . In other words, the transistor T 3  functions as a bootstrap circuit. 
     Because the reverse bias that is input to the photodiode  10  is maintained to the voltage value of the gate relative to the source of the transistor T 3 , the capacitance value of the photodiode  10  relative to the input terminal P 1  of the logarithmic amplifier circuit  20  is equivalently reduced. Equivalently reducing the capacitance value of the photodiode  10  can reduce the recovery time, as will be described below with reference to  FIG. 16 . 
       FIG. 16  illustrates waveforms of the output voltage value Vo 1  of the logarithmic amplifier circuit  20  illustrated in  FIG. 15 . In  FIG. 16 , the horizontal axis represents time (msec). The vertical axis represents the output voltage value Vo 1  (V). At time of 0 msec, the current value I 1  drops abruptly from 10 mA to 10 nA. 
     The waveform W 5  is a waveform of the output voltage value Vo 1  in an optical measurement apparatus without a transistor T 3 . The waveform W 6  is a waveform of an output voltage value Vo 1  in the optical measurement apparatus  201  provided with the transistor T 3 . 
     When the current value I 1  drops abruptly, the amplifier  21  as illustrated in  FIG. 2  may not be able to respond to the abrupt change in the current value I 1  and a current may continue to flow from the collector to the emitter of the transistor T 1  as illustrated in  FIG. 2 . When the current continues to flow from the collector to the emitter of the transistor T 1 , the voltage value that is input to the input terminal P 1  becomes a negative value. When the voltage value that is input to the input terminal P 1  becomes a negative value, the current flowing from the emitter to the collector of the transistor T 1  becomes very small and the output voltage value Va 1  of the amplifier  21  thus saturates to a positive voltage value. When the output voltage value Va 1  of the amplifier  21  saturates to the positive voltage value, the output voltage value Vo 1  of the logarithmic amplifier circuit  20  saturates to a negative voltage value. 
     When the output voltage value Vo 1  saturates to the negative voltage value, as indicated by the waveform W 5 , the optical measurement apparatus without the transistor T 3  requires recovery time until the output voltage value Vo 1  recovers to a voltage value corresponding to the current value I 1 . The recovery time of the waveform W 5  is about 0.2 msec. 
     In contrast, in the present embodiment, because the capacitance value of the photodiode  10  can be equivalently reduced by the transistor T 3 , the recovery time is reduced as indicated by the waveform W 6 . 
     Here, the recovery time could be reduced by increasing the offset current value Ioff. However, when the offset current value Ioff is increased, noises of the logarithmic amplifier circuit  20  may be increased. The present embodiment can reduce the recovery time through provision of the transistor T 3  without increasing the offset current value Ioff. Such a configuration can reduce the recovery time while reducing an increase in noises of the logarithmic amplifier circuit  20 . 
     The recovery time could also be reduced by reducing the light-receiving area of the photodiode  10  to thereby reduce the capacitance value of the photodiode  10 . However, if the photosensitive area of photodiode  10  is reduced, the efficiency of coupling of light to be analyzed to the photodiode  10  is reduced. The present embodiment can reduce the recovery time through provision of the transistor T 3  without reducing the photosensitive area of the photodiode  10 . Such a configuration can reduce the recovery time while preventing a reduction in the efficiency of coupling of light to be analyzed to the photodiode  10 . 
     &lt;First Mode: Measurement Sensitivity Setting Processing&gt; 
     The source voltage value of the transistor T 3  is determined by the current characteristic of the transistor T 3  and the resistance value of the resistor R 8 . For example, the source voltage value of the transistor T 3  is about 1 V. When the source voltage value of the transistor T 3  becomes 0 V or higher, thereby causing the source of the transistor T 3  to be electrically connected to the cathode of photodiode  10 , a dark current having a current value of several nanoampere (nA) to several tens picoampere (pA) may be generated. 
     For this reason, in the third embodiment, the controller  52  adjusts the output voltage value of the DA converter  70  as will be described below. Information of the digital signal to be input to the DA converter  70  to cause the DA converter  70  to output the adjusted output voltage value is stored in the storage  50  for each measurement sensitivity. 
     Similarly to the second embodiment, the controller  52  receives an input of the measurement sensitivity for the first mode via the input unit  51  before processing of analyzing an optical spectrum, for example. In response to receiving this input, the controller  52  obtains information of the digital signal for adjusting the output voltage value of the DA converter  70  associated with the measurement sensitivity for the first mode. The controller  52  outputs a digital signal to the DA converter  70  based on the obtained information. Similarly to the second embodiment, the controller  52  obtains, from the storage  50 , the switching information for the switch unit  130  associated with the measurement sensitivity for the first mode. The controller  52  controls the switch unit  130  based on the obtained switching information for the switch unit  130 , similarly to the second embodiment. 
     &lt;First Mode: Table Generation Processing&gt; 
     The controller  52  outputs a control signal to the switch SW 6  upon generating a table as exemplified in  FIG. 12 . The controller  52  outputs the control signal to the switch SW 6  to cause the switch SW 6  to electrically connect the cathode of the photodiode  10  to the reference potential, and to electrically disconnect the cathode of the photodiode  10  from the source of the transistor T 3 . Such a configuration enables generation of a table illustrated in  FIG. 12  without being affected by a dark current. 
     &lt;First Mode: Offset Current Value Measurement Processing&gt; 
     Similarly to the second embodiment, the controller  52  measures the offset current value Ioff 1  before carrying out light intensity measurement processing or periodically, for example. 
     In the third embodiment, the controller  52  outputs a control signal to the switch SW 6  before carrying out the measurement processing of the offset current value. The controller  52  causes the switch SW 6  to electrically disconnect the cathode of the photodiode  10  from the reference potential, and to electrically connect the cathode of the photodiode  10  to the source of the transistor T 3 . 
     Similarly to the second embodiment, the controller  52  measures the output voltage value Vo 1  of the logarithmic amplifier circuit  20  by the AD converter  40  when the photodiode  10  is shaded by outputting a voltage value corresponding to each measurement sensitivity by the DA converter  70 , and causing the switch unit  130  to set the offset current value Ioff corresponding to the each measurement sensitivity. Further, the controller  52  calculates the offset current value Ioff 1  for the each measurement sensitivity based on the output voltage value Vo 1  and the above-mentioned table. 
     The controller  52  stores the offset current value Ioff 1  associated with the measurement sensitivity in the storage  50 . Instead, the controller  52  may calculate the offset current value Ioff 1  based on the output voltage value Vo 1  and the equation (6), similarly to the first embodiment. 
     Here, in the third embodiment, the offset current value Ioff 1  may differ significantly from the offset current value Ioff for each sensitivity setting as exemplified in  FIG. 3  due to the effects of a dark current. The offset current value Ioff 1  can vary with the output voltage value of the DA converter  70 . For this reason, the controller  52  identifies a digital signal to be input to the DA converter  70  when the offset current value Ioff 1  becomes equal to the offset current value as exemplified in  FIG. 3 . For example, when the measurement sensitivity for the first mode is the sensitivity A as exemplified in  FIG. 3 , the resistance value of the offset resistor R 130  to be electrically connected between the output terminal of the amplifier  60  and the input terminal P 1  is set to 1 MΩ, which is the same as the resistance value Rs 30  as exemplified in  FIG. 3 . When the measurement sensitivity for the first mode is the sensitivity A, the controller  52  identifies a digital signal to be input to the DA converter  70  when the offset current value Ioff 1  becomes 200 nA. The controller  52  stores information of the identified digital signal in the storage  50  while associating it with the measurement sensitivity for the first mode. 
     Other processing in the first mode is the same as the processing in the second embodiment. 
     &lt;Second Mode&gt; 
     Similarly to the second embodiment, the controller  52  receives an input for switching the mode of the optical measurement apparatus  201  to the second mode via the input unit  51 . In response to receiving this input, the controller  52  outputs control signals to the switches SW 1  to SW 4  as appropriate to switch the connection states in the optical measurement apparatus  201  to the connection states corresponding to the second mode, similarly to the second embodiment. 
     Similarly to the second embodiment, the controller  52  outputs a control signal to the switch SW 5  upon measuring the output voltage value Vo 2  of the amplifier  60  in the second mode. The controller  52  outputs the control signal to the switch SW 5  to cause the switch SW 5  to electrically connect the output terminal of the amplifier  60  to the AD converter  40 , and to electrically disconnect the output terminal P 3  of the logarithmic amplifier circuit  20  from the AD converter  40 . 
     &lt;Second Mode: Light Intensity Measurement Processing&gt; 
     Similarly to the second embodiment, before carrying out light intensity measurement processing, the controller  52  outputs a control signal to the switch SW 5 . The controller  52  outputs the control signal to the switch SW 5  to cause the switch SW 5  to electrically connect the output terminal of the amplifier  60  to the AD converter  40 , and to electrically disconnect the output terminal P 3  of the logarithmic amplifier circuit  20  from the AD converter  40 . 
     Here, as described above, when the source voltage value of the transistor T 3  becomes 0 V or higher, a dark current having a current value of several nanoampere (nA) to several tens picoampere (pA) may be generated. 
     For this reason, the controller  52  outputs a control signal to the switch SW 6  before carrying out the light intensity measurement processing. The controller  52  outputs a control signal to the switch SW 6  to cause the switch SW 6  to electrically connect the cathode of the photodiode  10  to the reference potential, and to electrically disconnect the cathode of the photodiode  10  from the source of the transistor T 3 . Such a configuration reduces generation of a dark current described above in the optical measurement apparatus  201 , to thereby reduce an increase in noises of the optical measurement apparatus  201 . 
     Other processing in the second mode is the same as the processing in the second embodiment. 
     &lt;Operation of Optical Measurement System&gt; 
     An optical measurement method by the optical measurement apparatus  201  of the third embodiment may be carried out as described above with reference to  FIGS. 13 and 14 . 
     Note that, in the third embodiment, in the processing of Step S 24 , the controller  52  outputs a control signal to the switch SW 6  before measurements of the output voltage value Vo 1  and the output voltage value Vo 2 . The controller  52  outputs the control signal to the switch SW 6  to cause the switch SW 6  to electrically connect the cathode of the photodiode  10  to the reference potential, and to electrically disconnect the cathode of the photodiode  10  from the source of the transistor T 3 . 
     Further, in the third embodiment, in the processing of Step S 26 , the controller  52  controls the switch SW 6  in addition to the control on the switch SW 5 . In the processing of Step S 26 , the controller  52  outputs a control signal to the switch SW 6 . The controller  52  outputs the control signal to the switch SW 6  to cause the switch SW 6  to electrically disconnect the cathode of the photodiode  10  from the reference potential, and to electrically connect the cathode of the photodiode  10  to the source of the transistor T 3 . 
     Further, in the third embodiment, in the processing of Step S 34 , the controller  52  controls the switch SW 6  in addition to the control on the switch SW 5 . In the processing of Step S 34 , the controller  52  outputs a control signal to the switch SW 6  to cause the switch SW 6  to electrically connect the cathode of the photodiode  10  to the reference potential, and to electrically disconnect the cathode of the photodiode  10  from the source of the transistor T 3 . 
     Other configurations and effects of the optical measurement apparatus  201  according to the third embodiment are the same as those of the optical measurement apparatus  1  according to the first embodiment or the optical measurement apparatus  101  according to the second embodiment. 
     Although the embodiments of the present disclosure have been described with reference to the drawings and examples, it is to be noted that a person skilled in the art can easily make a wide variety of variations or modifications based on the present disclosure. Accordingly, it is noted that such variations or modifications are encompassed within the scope of the present disclosure. For example, functions included in the components and the steps can be rearranged unless they are logically inconsistent, or multiple components or steps can be combined into one or a single component or step may be divided. 
     For example, in the above-described embodiments, the transistor T 1  as a nonlinear element has been described as being electrically connected between the output terminal of the amplifier  21  and a non-inverting input terminal. Instead, a nonlinear element other than the transistor T 1  may be electrically connected between the output terminal of the amplifier  21  and the non-inverting input terminal. 
     For example, an analog-to-digital converter may be employed as the voltage source Vb as illustrated in  FIG. 1 . By employing an analog-to-digital converter as the voltage source Vb, the voltage value VB can be made variable. 
     For example, the optical measurement apparatus  1  as illustrated in  FIG. 1  may include a transistor T 3 , a resistor R 8 , and a switch SW 6  as illustrated in  FIG. 15 . 
     For example, the optical measurement apparatus  1  as illustrated in  FIG. 1  may generate a table as exemplified in  FIG. 12 , and may measure the light intensity of light to be analyzed using the table instead of the equation (7). Upon generating the table in the optical measurement apparatus  1 , light to be analyzed with a known light intensity is input to the photodiode  10  while the offset current value Ioff is set to 0 A. In other words, the current value I 1  becomes equal to a known photocurrent value Ip. The controller  52  measures the output voltage value Vo 1  of the logarithmic amplifier circuit  20  by the AD converter  40  while varying the known photocurrent value Ip, i.e., the current value I 1 . The controller  52  generates a table by associating the photocurrent value Ip with the output voltage value Vo 1 .