Abstract:
Beth of highly stable light having the same optical performance and fast-modulated light are emitted simply and inexpensively. A light source apparatus is provided, which includes semiconductor laser elements that emit laser light according to an inputted current signal, a light receiving element that receives the laser light emitted from the semiconductor laser element, and a controller that controls light emission of the semiconductor laser element, wherein the controller includes a first semiconductor laser element drive circuit that outputs a current signal to the semiconductor laser elements according to an instruction signal, a second semiconductor laser element drive circuit that adjusts the current signal on the basis of light quantity of the laser light received by the light receiving element, and outputs the adjusted current signal to the semiconductor laser element, and a circuit switching section that switches between the first and the second semiconductor laser element drive circuits according to an instruction signal.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a light source apparatus and a laser scanning microscope. 
         [0003]    This application is based on Japanese Patent Applications, Nos. 2007-304896 and 2008-266551, the content of which is incorporated herein by reference. 
         [0004]    2. Description of Related Art 
         [0005]    A light source apparatus has been known in the past, which includes a laser diode generating repetitive pulse light (for example, refer to a specification of the Publication of Japanese Patent NO. 2777268). 
         [0006]    The light source apparatus includes a lock-in amplifier, and is intended to provide a high speed pulse with low noise by providing a peak in frequency of a reference signal for the lock-in amplifier. 
         [0007]    However, when the lock-in amplifier is in use, the reference signal needs to have a frequency, and therefore when light having certain light quantity is continuously emitted, there is a problem in that the quantity of light cannot be stabilized. Moreover, when the lock-in amplifier is in use, there is a problem in that a light source apparatus becomes expensive. 
         [0008]    On the other hand, in some method, feedback of light is made using a detector such as photodiode in order to stabilize the quantity of light. However, when light modulated at high frequency is detected and fed back, there is a problem in that response speed of a feedback circuit cannot follow modulation speed of the light due to floating capacitance or high feedback gain of the feedback circuit. 
         [0009]    Therefore, when both of highly stable light and fast-modulated light are emitted, even if light having the same wavelength are emitted, separate laser diodes are required. 
         [0010]    When a laser diode is in use at a low power, a photodiode incorporated in the laser diode may be reduced in response speed (much time may be taken before the photodiode is stabilized) in some photodiode type. Therefore, when a feedback circuit is configured, there is a problem in that actual output value overshoots a target output value given by a lighting instruction signal. 
         [0011]    Therefore, even when laser light is modulated at low frequency, when feedback of light is made using a detector-such as photodiode, stability of the light may be reduced in some lighting power. 
       BRIEF SUMMARY OF THE INVENTION 
       [0012]    The present invention provides a light source apparatus and a laser scanning microscope, which can simply and inexpensively emit both of highly stable light having the same optical performance and fast-modulated light. 
         [0013]    A first aspect of the present invention provides a light source apparatus includes a light source apparatus having laser diode elements that emit laser light according to an inputted current signal; a light receiving element that receives the laser light emitted from the laser diode element; and a controller that controls light emission of the laser diode element, wherein the controller includes, a first laser diode drive circuit that outputs a current signal to the laser diode elements according to an instruction signal; a second laser diode drive circuit that adjusts the current signal on the basis of on the light quantity of the laser light received by the light receiving element, and outputs the adjusted current signal to the laser diode element; and a circuit switching section that switches between the first laser diode drive circuit and the second laser diode drive circuit according to an instruction signal. 
         [0014]    According to the first aspect of the present invention, when fast-modulated laser light is required, the controller actuates the circuit switching section to select the first laser diode drive circuit, so that the current signal is outputted to the laser diode elements according to the instruction signal. Thus, fast-modulated laser light according to a high-speed instruction signal can be emitted from the laser diode element. On the other hand, when highly stable laser light is required, the controller actuates the circuit switching section to select the second laser diode drive circuit, so that emitted laser light is received by the light receiving element, and consequently a current signal fed back with the light quantity of the received laser light can be inputted into the laser diode element. Thus, highly stable laser light can be emitted from the laser diode element. 
         [0015]    That is, according to the present invention, both of fast-modulated laser light and highly stable laser light can be emitted from the same laser diode elements with simply configuration and at a low cost. 
         [0016]    A second aspect of the present invention is a light source apparatus of the first aspect, wherein the first laser diode drive circuit outputs a current signal to allow the laser diode elements to emit pulsed laser light, and the second laser diode drive circuit is used when a current signal is outputted to allow the laser diode elements to emit continuous laser light. 
         [0017]    By configuring in this way, fast-modulated pulsed laser light and continuous laser light can be emitted from the same laser diode element. 
         [0018]    In the second aspect, the light source apparatus may include a current detection section that detects a current signal value to be outputted to the laser diode elements when the second laser diode drive circuit is selected; a storage section that stores the current signal value detected by the current detection section and the light quantity of laser light detected by the light receiving element while relating the signal value and the light quantity to each other; and an instruction signal correction section that corrects an instruction signal to be inputted into the first laser diode drive circuit on the basis of the current signal value and the light quantity stored in the storage section when the first laser diode drive circuit is selected. 
         [0019]    By configuring in this way, when the second laser diode drive circuit is selected, and the light quantity of laser light detected by the light receiving element is fed back so that highly stable laser light is emitted, the current detection section defects the current signal value to be outputted to the laser diode element, thereby even if the laser diode elements are degraded, and thereby a current signal value corresponding to the same instruction signal increases, laser light having the stable light quantity can be emitted from the laser diode elements when the first laser diode drive circuit is selected. 
         [0020]    A third aspect of the present invention is a light source apparatus of the first aspect, in which the first laser diode drive circuit is used when output power of the laser diode elements is low, and the second laser diode drive circuit is used when output power of the laser diode elements is high. 
         [0021]    By configuring in this way, stable light can be obtained while eliminating influence of a response characteristic, being reduced in low output power, of a photodiode incorporated in each laser diode, and consequently a sample can be observed more accurately. 
         [0022]    In the third aspect, the light source apparatus may include a current detection section that detects a current signal value to be outputted to the laser diode elements when the second laser diode drive circuit is selected; a storage section that stores the current signal value detected by the current detection section and the light quantity of laser light detected by the light receiving element while relating the signal value and the light quantity to each other; and an instruction signal correction section that corrects an instruction signal to be inputted into the first laser diode drive circuit on the basis of the current signal value and the light quantity stored in the storage section when the first laser diode drive circuit is selected. 
         [0023]    By configuring in this way, when the second laser diode drive circuit is selected, and the light quantity of laser light detected by the light receiving element is fed back so that highly stable laser light is emitted, the current detection section detects the current signal value to be outputted to the laser diode element, thereby even if output of the laser diode elements is reduced, and thereby a current signal value corresponding to the same instruction signal increases, laser light having the stable light quantity can be emitted from the laser diode elements when the first laser diode drive circuit is selected. 
         [0024]    A fourth aspect of the present invention is a laser scanning microscope including the light source apparatus of one of the first to third aspects, a scan section that two-dimensionally scans laser light emitted from the light source apparatus; an objective lens that irradiates the laser light scanned by one scan section to a sample, and collects returning light returning from the sample; and a light detection section that detects the returning light being collected by the objective lens, and returning via the scan section. 
         [0025]    According to the fourth aspect of the present invention, the laser light emitted from the light source apparatus is two-dimensionally scanned by the scan section, and irradiated to the sample by the objective lens, thereby returning light from the sample is collected by the objective lens, and detected by the light detection section. The light source apparatus may emit highly stable laser light and fast-modulated laser light while switching between them, and thus may observe a sample while irradiating laser light having the same optical characteristics such as the same wavelength, the same beam diameter, or the same beam divergence angle. 
         [0026]    A fifth aspect of the present invention is a laser scanning microscope including the light source apparatus of the second aspect, a scan section that two-dimensionally scans laser light emitted from the light source apparatus; an objective lens that irradiates the laser light scanned by the scan section to a sample, and collects returning light returning from the sample; a light detector that detects the returning light being collected by the objective lens, and returning via the scan section; and a timing switching section that switches detection timing of the light detector depending on frequency of pulsed laser light to be emitted when the circuit switching section switches a circuit to the first laser diode drive circuit. 
         [0027]    According to the fifth aspect of the present invention, when the circuit switching section of the light source apparatus selects the first laser diode drive circuit, pulsed laser light is emitted from the light source, and the timing switching section operates to switch detection timing of the light detector depending on frequency of the pulsed laser light. Thus, the pulsed laser light emitted from the light source is irradiated to a sample, thereby returning light from the sample can be more securely detected. 
         [0028]    A sixth aspect of the present invention is a laser scanning microscope including the light source apparatus of the second aspect, a scan section that two-dimensionally scans laser light emitted from the light source apparatus; an objective lens that irradiates the laser light scanned by the scan section to a sample, and collects returning light returning from the sample; a plurality of light detectors having different detection timing, which detect the returning light being collected by the objective lens, and returning via the scan section; and a detector switching section that switches between the light detectors when the circuit switching section switches between the first and second laser diode drive circuits. 
         [0029]    According to the sixth aspect of the present invention, when the circuit switching section of the light source apparatus switches between the first and second laser diode drive circuits, the detector switching section switches between the light detectors having different detection timing depending on a type of laser light emitted from the light source. This enables more secure detection of returning light from the sample caused by irradiating each type of laser light depending on a type of laser light emitted from the light source. 
         [0030]    A seventh aspect of the present invention is a laser scanning microscope including the light source apparatus of the second aspect, a scan section that two-dimensionally scans laser light emitted from the light source apparatus; an objective lens that irradiates the laser light scanned by the scan section to a sample, and collects returning light returning from the sample; a light detector that detects the returning light being collected by the objective lens, and returning via the scan section; and a timing switching section that switches at least one of detection timing of the light detector and a detection circuit when the circuit switching section switches between the first and second laser diode drive circuits. 
         [0031]    The invention exhibits an advantage that both of highly stable light having the same optical performance and fast-modulated light can be emitted simply and inexpensively. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0032]      FIG. 1  shows a general block diagram showing a laser scanning microscope according to a first embodiment of the present invention; 
           [0033]      FIG. 2  shows a block diagram showing a light source apparatus according to the embodiment for use in the laser scanning microscope of  FIG. 1 ; 
           [0034]      FIG. 3  shows a block diagram showing a first modification of the light source apparatus of  FIG. 2 ; 
           [0035]      FIG. 4  shows a block diagram showing a second modification of the light source apparatus of  FIG. 2 ; 
           [0036]      FIG. 5  shows a block diagram showing a light source apparatus according to a second embodiment of the present invention; 
           [0037]      FIG. 6  shows a graph showing temporal change of a drive instruction signal, a current signal, and light quantity of laser light in a high resolution mode when laser diode elements are degraded in the light source apparatus of  FIG. 5 ; 
           [0038]      FIG. 7  shows a graph showing temporal change of a drive instruction signal, a current signal, and light quantity of laser light in a high speed mode when laser diode elements are degraded in the light source apparatus of  FIG. 2 ; 
           [0039]      FIG. 8  shows a graph showing temporal change of a drive instruction signal, a current signal, and light quantity of laser light in the high speed mode when laser diode elements are degraded in the light source apparatus of  FIG. 5 ; 
           [0040]      FIG. 9  shows a diagram showing a timing chart for achieving the light quantity of laser light in the high speed mode in  FIG. 8 ; 
           [0041]      FIG. 10  shows a general block diagram showing a modification of the laser scanning microscope of  FIG. 1 ; 
           [0042]      FIG. 11  shows a block diagram showing an internal structure example of a detection circuit of the laser scanning microscope of  FIG. 10 ; 
           [0043]      FIG. 12  snows a graph showing temporal change (target value) of the light quantity of laser light given by a lighting instruction signal in the case that a feedback circuit is configured in the light source apparatus of  FIG. 2 ; 
           [0044]      FIG. 13  shows a graph showing temporal change of light quantity of laser light compared with the target value when the light quantity overshoots in low power in the case that the feedback circuit is configured in the light source apparatus of  FIG. 2 ; and 
           [0045]      FIG. 14  shows a graph showing temporal change of light quantity of actual laser light at low response speed of a photodiode incorporated in a laser diode in the case that the feedback circuit is configured in the light source apparatus of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     First Embodiment 
       [0046]    Hereinafter, a light source apparatus  1  and a laser scanning microscope  2  according to a first embodiment of the present invention will be described with reference to  FIGS. 1 and 2 . 
         [0047]    As shown in  FIG. 1 , the laser scanning microscope  2  according to the embodiment includes the light source apparatus  1  that emits laser light; a scanner (scan section)  3  that two-dimensionally scans the laser light emitted from the light source apparatus  1 ; a pupil projection lens  4  and an imaging lens  5  that condense the laser light scanned by the scanner  3 ; an objective lens  6  that irradiates the condensed laser light to a sample A, and collects returning light returning from the sample A; a light detection section  7  that detects the returning light being collected by the objective lens  6 , and returning via the imaging lens  5 , the pupil projection lens  4 , and the scanner  3 ; and a controller  8  for controlling these. In the figure a symbol  9  shows a mirror. 
         [0048]    The light source apparatus  1  includes a plurality of laser diode elements  10  that emit laser light having different wavelengths, a mirror  11  and a dichroic mirror  12  that allow laser light from the laser diode elements  10  to join into the same optical path, a beam splitter  13  that branches part of the laser light emitted from the laser diode elements  10 , and a photodiode  14  that detects the branched laser light. 
         [0049]    The light detection section  7  includes a dichroic mirror that branches the returning light returning via the scanner  3  from an optical path of the laser light, a confocal lens  16 , a confocal pin hole  17 , a condensing lens  18 , mirrors  19  and a beam splitter  20 , barrier filters  21 , and light detectors  22  and  23 . The light detectors  22  and  23  include a first light detector (APD: Avalanche photodiode)  23  that detects fast-modulated, pulsed returning light, and a second light detector (PMT: photomultiplier)  22  that detects stably continued. continuous returning light. The PMT  22  and APD  23  are connected to detection circuits  221  and  231  respectively, each detection circuit integrating an output signal and performing A/D conversion of the integrated signal. Sampling timing (speed) of the detection circuits  221  and  231  are set by the controller  8  in accordance with characteristics of the detectors  22  and  23  respectively. Since the APD  23  may detect light at high speed compared with the PMT  22 , sampling timing of the detection circuit  231  for the APD  23  is set short compared with sampling timing of the detection circuit  221  for the PMT  22 . 
         [0050]    As shown in  FIG. 2 , the controller  8  includes an input section  24  to be inputted with a dimming value, an observation mode, and modulation timing, a drive command  25  that outputs a drive instruction signal for the laser diode elements  10  on the basis of the dimming value and the observation mode inputted from the input section  24 , and a laser diode drive circuit  26  that generates a current signal to be inputted into the laser diode elements  10  according to the drive instruction signal from the drive command  25 . 
         [0051]    The observation mode includes a high resolution mode such as a mode in long time-lapse observation or fluorescence correlation spectroscopy (FCS) observation, and a high speed mode such as a mode in fluorescence lifetime measurement, fluorescence frequency separation, or pulse width modulation (PWM) in a low power region. In the high resolution mode, continuous laser light being stable for a long time is desired to be irradiated, and in the high speed mode, fast-modulated laser light, for example, pulsed laser light is desired to be irradiated. 
         [0052]    The laser diode drive circuit  26  includes a first laser diode drive circuit  27  that outputs a current signal according to the instruction signal from the drive command  25 , and a second laser diode drive circuit  28  that includes the first laser diode drive circuit  27 , and controls a current signal to be inputted into the laser diode elements  10  on the basis of the light quantity of laser light detected by the photodiode  14 , and a changeover switch  29  that changes between the laser diode drive circuits  27  and  28 . 
         [0053]    The drive command  25  has a memory  25   a  that stores offset and gain for finely adjusting a drive instruction signal being changed when the observation mode is changed. The offset and the gain are used to adjust the drive instruction signals outputted from the two laser diode drive circuits  27  and  28  respectively such that a level of either signal perfectly corresponds to one target value. That is, when the high resolution mode is assumed to be inputted as an observation mode, V slow  is outputted as the drive instruction signal, and when the high speed mode is inputted as an observation mode, V fast =V slow *gain+offset is assumed to be outputted as the drive instruction signal. Obviously, such signal levels may be converted with V fast  as a reference, or a different, absolute reference value may be set. The drive command  25  turns on or off the changeover switch  29  depending on the observation mode to be inputted, and when the high speed mode is inputted, the section  25  reads offset and gain from the memory  25   a  to calculate a drive instruction signal. In the figure, a symbol  25   b  shows CPU that performs the calculation. 
         [0054]    The first laser diode drive circuit  27  includes a D/A converter  27   a  that converts a drive instruction signal including a digital signal from the drive command  25  into a voltage signal, an adder  27   b,  and a V/I converter  27   c  that converts a voltage signal into a current signal. 
         [0055]    The second laser diode drive circuit  28  further includes an I/V converter  28   a  that converts a detection signal including a current signal from the photodiode  14  into a voltage signal, and a comparator  28   b  that compares between the detection signal converted into the voltage signal by the I/V converter  28   a  and the drive instruction signal converted into the voltage signal by the D/A converter  27   a  of the first laser diode drive circuit  27 . 
         [0056]    When the high resolution mode is selected as the observation mode, the changeover switch  29  is turned on, thereby the switch  29  connects the comparator  28   b  in the second laser diode drive circuit  28  to the adder  27   b  in the first laser diode drive circuit  27  so that an output signal from the comparator  28   b  is inputted into the adder  27   b.  When the high speed mode is selected as the observation mode, the changeover switch  29  is turned off so that the comparator  28   b  is disconnected from the adder  27   b.    
         [0057]    The controller  8  switches between the light detectors  22  and  23  on the basis of on the observation mode inputted from the input section  24 . That is, when the high resolution mode is inputted to perform high resolution observation, the PMT (photomultiplier)  22  is selected as the light detector, and when the high speed mode is inputted to perform high speed observation, the APD (avalanche photodiode)  23  is selected as the light detector. Switching between the light detectors  22  and  23  is performed by, for example, switching the beam splitter  20  so as to switch a detection light path, as shown in  FIG. 1 . 
         [0058]    Operation of the laser scanning microscope  2  according to the embodiment configured in this way is described below. 
         [0059]    When high resolution observation of the sample A is performed using the laser scanning microscope  2  according to the embodiment, a dimming value and the high resolution mode are inputted from the input section  24 . 
         [0060]    The drive command  25  generates the drive instruction signal V slow  on the basis of the inputted dimming value and outputs the signal V slow  while switching the changeover switch  29  to be on. Thus, a current signal according to the drive instruction signal V slow  is outputted to the laser diode elements  10 , and laser light having the light quantity according to the drive instruction signal V slow  is emitted from the laser diode elements  10 . 
         [0061]    Laser light is partially detected by the photodiode  14  and thus fed back via a feedback circuit configured by the comparator  28   b  and the adder  27   b.  Thus, laser light is emitted from laser diode elements  10  while stability of its light quantity is kept high such that the light quantity hardly varies. 
         [0062]    When the high resolution mode is inputted from the input section  24 , the controller  8  switches the beam splitter  20  so that the PMT  22  detects returning light. Thus, the PMT  22  detects continuous returning light obtained by irradiating laser light having the stable light quantity, and consequently high resolution observation can be made. 
         [0063]    Next, the dimming value, the high speed mode, and the modulation timing are inputted from the input section  24  to make high speed observation of the sample A by means of the laser scanning microscope  1  according to the embodiment. 
         [0064]    The drive command  25  generates the drive instruction signal V fast  on the basis of the inputted dimming value and modulation timing, and outputs the signal V fast  while switching the changeover switch  29  to be off. Thus, a current signal according to the drive instruction signal V fast  is outputted to the laser diode elements  10 , and laser light having the light quantity according to the drive instruction signal V fast  is emitted from the laser diode elements  10 . 
         [0065]    At that time, since the changeover switch  29  disconnects the feedback circuit, fast-modulated laser light, having the light quantity determined by only the drive instruction signal V fast  from the drive command  25 , is emitted. 
         [0066]    When the high speed mode is inputted from the input section  24 , the controller  8  switches the beam, splitter  20  so that the APD  23  detects returning light. Thus, the returning light can be detected at fast detection timing following fast-modulated laser light emitted from the laser diode elements  10 , and consequently quick motion of the sample A can be observed more accurately. 
         [0067]    In this way, according to the light source apparatus  1  and the laser scanning microscope  2  according to the embodiment, when the high resolution mode is selected, the laser light having the stable light quantity is continuously irradiated so that high resolution observation can be made. When high speed observation is made, since the laser light modulated at high frequency is not fed back, which eliminates the difficulty of stray capacitance or response speed of the feedback circuit, and consequently observation can be made more accurately. 
         [0068]    In addition, according to the light source apparatus  1  and the laser scanning microscope  2  according to the embodiment, the laser diode elements  10  generating laser light having the same wavelength can emit both of the highly stable laser light in the high resolution mode, and fast-modulated laser light in the high speed mode, leading to an advantage that an installation space is reduced so that cost is suppressed, and an advantage that beam diameter and a beam divergence angle are made equal in either mode so that the same optical resolution can be achieved in either mode. 
         [0069]    In the light source apparatus  1  according to the embodiment, the second laser diode drive circuit  28  for high resolution observation is formed by adding the feedback circuit to the first laser diode drive circuit  27 . However, in place of this, the first laser diode drive circuit  27  and the second laser diode drive circuit  28  may be separately provided so that the changeover switch  29  switches between the separated laser diode drive circuits  27  and  28 , as shown in  FIG. 3 . 
         [0070]    In this case, the second laser diode drive circuit  28  needs to have a D/A converter  28   c,  an adder  28   d,  and a V/I converter  28   e.    
         [0071]    By configuring in this way, as the two laser diode drive circuits  27  and  28 , a drive circuit that can be modulated at high speed, and a drive circuit having a current stabilizing circuit, an offset adjusting mechanism, and a gain adjusting mechanism, which are suitable for the observation modes respectively, can be further advantageously used. 
         [0072]    In the embodiment, the first laser diode drive circuit  27  has the adder  27   b,  and a deviation signal between the detection signal from, the photodiode  14  and the drive instruction signal is returned to the adder  27   b,  so that the feedback circuit is configured. However, in place of this, as shown in  FIG. 4 , a feedback circuit may be configured by returning the detection signal from the photodiode  14  to the drive command  25 . In this case, the second laser diode drive circuit  28  needs to have an A/D converter  28   f.  Even in this case, drive circuits suitable for respective observation modes may be used as the two laser diode drive circuits  27  and  28 . 
         [0073]    In the embodiment, laser light emitted from the laser diode elements  10  is partially branched and detected by the photodiode  14 . However, in place of this, laser diode elements  10  with a photodiode  14  may be used. 
       Second Embodiment  
       [0074]    Next, a light source apparatus and a laser scanning microscope according to a second embodiment are described below with reference to  FIGS. 5 and 9 . 
         [0075]    In description of the embodiment, a portion having a configuration common to a portion in the light source apparatus  1  according to the first embodiment is marked with the same symbol, and description of the portion is omitted. 
         [0076]    As shown in  FIG. 5 , the light source apparatus according to the embodiment is different from the light source apparatus  1  according to the first embodiment in that a galvanometer  27   d  for detecting a current signal for driving the laser diode elements  10  is provided in a first drive circuit  27 ′, a detection signal given by the galvanometer  27   c  is inputted into the drive command  25 , the drive command  25  stores a drive instruction signal and the detection signal by the galvanometer  27   d  while relating the signals to each other, and the CPU  25   b  generates a drive instruction signal on the basis of the stored relation between the drive instruction signal and the detection signal. 
         [0077]    More specifically, when the high resolution mode is selected as the observation mode, laser light having a certain light quantity is stably emitted from the laser diode elements  10  according to the drive instruction signal. If the laser diode elements  10  are degraded, a current signal, being inputted into the laser diode elements  10  to allow the elements  10  to emit laser light having the same light quantity, automatically increases irrespective of the drive instruction signal as shown in  FIG. 6 . 
         [0078]    In this case, in the case of the first embodiment, when the high speed mode is selected as the observation mode, a current signal according to the drive instruction signal is inputted into the laser diode elements  10  irrespective of degradation of the laser diode elements  10 , therefore the light quantity of laser light emitted from the laser diode 
         [0079]    elements  10  decreases as shown in  FIG. 7 . 
         [0080]    On the other hand, in the light source apparatus  1 ′ according to the embodiment, using a fact that a current signal is automatically adjusted by the feedback circuit in the high resolution mode, the memory  25   a  stores a relationship between the current signal and the drive instruction signal in the high resolution mode, and in the high speed mode, a drive instruction signal for giving desired quantity of light can be generated based of the stored relationship between the current signal and the drive instruction signal. This leads to an advantage that laser light having stable quantity of light can be emitted not only in the high resolution mode but also in the high speed mode irrespective of degradation of the laser diode elements  10  as shown in  FIG. 8 . 
         [0081]    In this case, a current signal detected in the high resolution mode is used in the high speed mode. Therefore, the current signal needs to be detected regularly or as needed in the high resolution mode prior to the high speed mode. 
         [0082]    In the laser scanning microscope according to the embodiment, for example, in a blanking period of the scanner  3  during observation in the high speed mode, a mode is changed into the high resolution mode and a current signal is detected, and a result of such detection is reflected to observation in the high speed mode, consequently a drive instruction signal can be effectively generated depending on a preceding scare of the laser diode elements  10 . 
         [0083]    In the embodiment, as shown in  FIGS. 10 and 11 , two systems of switchable circuits  221 A and  221 B may be provided in the detection circuit  221 , so that a switch  221 C in the detection circuit  221  may be switched in synchronization with switching between the laser diode drive circuits  27  and  28 . 
         [0084]    The circuit  221 A includes an integrating circuit and an A/D converter, and selected in the high resolution mode. The circuit  221 B includes only an A/D converter, and selected in the high speed mode. 
         [0085]    The detection circuit  221  may be configured by only one system of the circuit  221 A without providing the switch  221 C therein. In such a case, integration time of the integrating circuit or a sampling rate of the A/D converter can be changed in synchronization with switching between the laser diode drive circuits  27  and  28 . 
         [0086]    Even in the case that switching is performed between the two systems of circuits  221 A and  221 B, integration time of the integrating circuit or a sampling rate of the A/D converter may be changed in synchronization with switching between the laser diode drive circuits  27  and  28 . 
         [0087]    When the high speed mode is selected and the laser diode drive circuit is switched to the first laser diode drive circuit  27 , the integration time or the sampling rate may be changed depending on frequency of the pulse laser light. 
       Third Embodiment 
       [0088]    A light source apparatus  1  according to a third embodiment is described below with reference to  FIGS. 12 to 14 . 
         [0089]    In the past, in the case that modulation is performed at low frequency, when feedback is made using a detector such as photodiode, response speed varies depending on lighting power, and therefore stability of light has been reduced. This is because when lighting power is low, some kind of photodiode incorporated in a laser diode is low in response speed (requires much time before stabilizing) as shown in  FIG. 14 , therefore when a feedback circuit is configured, although a lighting instruction signal gives a target value as shown in  FIG. 12 , actual output Inconveniently overshoots the target value as shown in  FIG. 13 . Therefore, an external photodiode has been usually additionally needed for feedback, leading to increase in size of an optical system, and increase in cost. 
         [0090]    Thus, the light source apparatus  1  of the embodiment is configured such that in the light source apparatus  1  according to the first embodiment, the first laser diode drive circuit  27  is used when output power of the laser diode elements  10  is low, and the second laser diode drive circuit  28  is used when output power of the laser diode elements  10  is high. 
         [0091]    In the embodiment, feedback is stopped when lighting power is low, which may prevent the phenomenon that some kind of photodiode incorporated in a laser diode is reduced in response speed (requires much time before stabilizing), leading to overshoot of accrual output. Therefore, even if modulation is performed at low frequency, an external photodiode needs not be additionally provided, consequently an optical system is not increased in size, and stable light can be obtained at a low cost. 
         [0092]    The embodiment in the low power may be achieved by modifying the first embodiment. That is, “high speed mode” in the first embodiment is changed for a low power mode, and “high resolution mode” is changed for a high power mode. By modifying in this way, a mode and an instruction signal are modified as follows respectively. 
         [0093]    When output intensity of laser is inputted from the input section  24 , and set in the controller, the controller  8  (CPU  25   b ) determines whether a mode is the low power mode or the high power mode depending on the set intensity. As a criterion, for example, 1% of maximum output of a laser diode is used. 
         [0094]    When an instruction signal in the low power mode is assumed as V slow , and an instruction signal in the high power mode is assumed as V high , V slow −V high *gain+offset is outputted. When the low power mode is inputted, the drive command  25  reads offset and gain from the memory  25   a  and thus calculates a drive instruction signal. After that, the same operation is performed as in the first embodiment, therefore description of the operation is omitted. 
       Fourth Embodiment 
       [0095]    Next, a fourth embodiment of the invention is described below. 
         [0096]    A light source apparatus  1  of the embodiment is configured such that the light source apparatus  1  according to the third embodiment further includes a current detection section  27   d  that detects a current signal value to be outputted to the laser diode elements  10  when the second laser diode drive circuit  28  is selected; a storage section  25  that stores the current signal value detected by the current detection section  27   d  and light quantity of laser light detected by a light receiving element while relating the signal value and the light quantity to each other; and an instruction signal correction section  25   b  that corrects an instruction signal to be inputted into the first laser diode drive circuit  27  on the basis of the current signal value and the light quantity stored in the storage section  25  when the first laser diode drive circuit  27  is selected. 
         [0097]    In the light source apparatus  1  according to the embodiment, using a fact that a current signal is automatically adjusted by the feedback circuit in the high power mode, a relationship between a current signal and a drive instruction signal in such automatic adjustment is stored in the memory  25   a,  and in the low power mode, a drive instruction signal for giving desired light quantity can be generated on the basis of the stored relationship between the current signal and the drive instruction signal. Thus, laser light having the stable light quantity can be emitted even in the low power mode. 
         [0098]    The embodiment may be combined with the second embodiment of the invention and may be carried out at the same time. In such a case, when an observation mode is the high resolution mode and a setting power mode is the high power mode, feedback control is performed, and in other cases (a case of the high speed mode, and a case of the high resolution mode and the low power mode), feedback is stopped. Feedback is stopped in the low power mode, which eliminates influence of a bad response characteristic of a photodiode incorporated in each laser diode in low power mode, consequently stable light can be obtained, and observation can be made more accurately. 
         [0099]    The laser diode is a type of semiconductor laser.