Abstract:
A laser apparatus and bio-imaging apparatus are provided, which include: a mode-lock laser unit including a saturable absorber section that applies a bias voltage, a gain section that feeds a gain current, a semiconductor laser that emits laser light, and an external resonator; an optical modulation unit performing amplification modulation on the laser light emitted from the mode-lock laser unit; a reference signal generation unit generating a master clock signal and supplying a signal synchronized with the master clock signal to the gain section of the semiconductor laser; and a driving circuit generating a driving pulse used to drive the optical modulation unit based on the reference signal.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. application Ser. No. 13/204,217, filed Aug. 5, 2011, and which claims priority to Japanese Priority Patent Application JP 2010-184594 filed in the Japan Patent Office on Aug. 20, 2010, the entire contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND 
     The present application relates to a recording apparatus that uses an MOPA (Master Oscillator Power Amplifier) in which a mode-lock oscillation type laser and an optical amplifier are combined as a recording optical source. 
     High-peak power laser light, particularly, ultra-short pulse light, is very effective for realizing the procedure of nonlinear multi-photon absorption. 
     This procedure of the nonlinear multi-photon absorption is expected to be applied to three-dimensional optical recording, ultra-fine processing, non-destructive bio-imaging, or the like. 
     For example, there has been reported a method of realizing multi-layer recording by emitting high-power laser light to a transparent bulk material having a nonlinear effect (see ISOM2009 Digest Th-1-01, 2009 by Seiji Kobayashi, Kimihiro Saito, Takashi Iwamura, Hisayuki Yamatsu, Toshihiro Horigome, Mitsuaki Oyamada, Kunihiko Hayashi, Daisuke Ueda, Norihiro Tanabe and Hirotaka Miyamoto). 
     According to this method, recording on a large-capacity recording medium can be realized more cheaply than a lamination type disc according to the related art. 
     A mode-lock type titanium-sapphire laser has been used as an optical source that outputs high-power laser light. Even in the example of ISOM2009 Digest Th-1-01, 2009 by Seiji Kobayashi, Kimihiro Saito, Takashi Iwamura, Hisayuki Yamatsu, Toshihiro Horigome, Mitsuaki Oyamada, Kunihiko Hayashi, Daisuke Ueda, Norihiro Tanabe and Hirotaka Miyamoto, emission light of a titanium-sapphire laser with 810 nm is converted into light with a 405 nm wavelength by an SHG (Second Harmonic Generator) and is used in a short-wavelength recording optical source advantageous for high-density recording. 
     Such a large expensive solid-state laser is limited to applications to experiments in laboratories (for example, see Spectra-Physics Inc. Online (searched in Aug. 6, 2010), Internet i_Series_Data_Sheet.pdf&gt;). 
     For this reason, many researchers have tried to develop a small-sized and stable pulse optical source as a semiconductor base to put to practical use. 
     In the next-generation optical recording, as in the above-mentioned method, a blue-purple laser optical source advantageous for the high-density recording of all semiconductors is strongly preferred. 
     For example, in a gain switching type laser, it has been reported that when strongly excited driving is repeatedly performed at 1 MHz, the peak power of 55 W has been realized (see Appl. Phys. Lett. 96, 051102, 2010 by M. Kuramoto, T. Oki, T. Sugahara, S. Kono, M. Ikeda, and H. Yokoyama). 
     However, a higher repetition frequency is necessary even in a data recording optical source according to a request for a high data transmission rate in the market. 
     In recent years, it has been reported that an optical source of 100 W has been realized at a repetition frequency of 1 GHz by a blue laser effective for high-density recording (for example, see APPLIED PHYSICS LETTERS 97, 021101, 2010 by Rintaro Koda, Tomoyuki Oki, Takao Miyajima, Hideki Watanabe, Masaru Kuramoto, Masao Ikeda, and Hiroyuki Yokoyama). 
     This optical source has a configuration called an MOPA (Master Oscillator Power Amplifier) in which a semiconductor mode-lock laser and a semiconductor optical amplifier are combined. 
     A recording reproduction apparatus has to record data used in recording at an arbitrarily position based on a wobble signal read from an optical recording medium. 
     At this time, it is necessary to modulate the recording data in synchronization with an oscillation pulse. 
     SUMMARY 
     When the MOPA using the mode-lock oscillation type laser is applied to a recording reproduction apparatus, it is possible to modulate the recording data by outside driving of an optical amplifier which is a power amplifier. 
     However, since the optical oscillation frequency of the mode-lock oscillation type laser is generally determined as a unique frequency by the resonant length of a spatial resonator, the optical oscillation frequency is not synchronized with the outside driving of the optical amplifier. Even when the optical oscillation frequency matches with the frequency of a master clock of a recording system used for the outside driving by adjusting the resonant length, phase synchronization may not be achieved. 
     For this reason, the following problem may arise. 
     For example, it is supposed that the optical amplifier is driven with the master clock and a pulse for the recording data corresponding to a 5T mark is amplified. When the pulse light from the mode-lock oscillation type laser incident on the optical amplifier is synchronized with the driving of the optical amplifier in the optimum phase, a pulse signal is accurately generated. 
     However, when the pulse light incident on the optical amplifier is not synchronized with the master clock or the phase is not optimum, the pulse via the optical amplifier may become a signal corresponding to a 4T mark from the signal corresponding to the original 5T mark (see  FIG. 5 ). For this reason, the signal may not be accurately generated. 
     It is desirable to provide a recording apparatus capable of easily obtaining synchronization of modulation of an optical pulse of laser light and the laser light with a simple configuration. 
     According to an embodiment, a recording apparatus is a recording apparatus that records information on an optical recording medium. 
     The recording apparatus includes a mode-lock laser unit including a saturable absorber section that applies a bias voltage, a gain section that feeds a gain current, a semiconductor laser that emits laser light used to record the information on the optical recording medium, and an external resonator. 
     The recording apparatus further includes an optical modulation unit performing amplification modulation on the laser light emitted from the mode-lock laser unit and a reference signal generation unit generating a master clock signal and supplying a signal synchronized with the master clock signal to the gain section of the semiconductor laser. 
     The recording apparatus further includes a recording signal generation unit generating a recoding signal based on the master clock signal and a driving circuit generating a driving pulse used to drive the optical modulation unit based on the recording signal. 
     In the recording apparatus according to the embodiment, the signal synchronized with the master clock signal is supplied from the reference signal generation unit to the gain section of the semiconductor laser. Thus, the optical pulse of the laser light emitted from the mode-lock laser unit including the semiconductor laser and the master clock signal can be synchronized with each other. 
     The recording signal generation unit generates the driving pulse based on the master clock signal and drives the optical modulation unit modulating the laser light based on the driving pulse. Thus, since the optical modulation unit is driven to be turned on or off in synchronization with the master clock signal, the laser light is modulated in synchronization with the master clock signal. 
     Due to the synchronization with the master clock signal, the optical pulse of the laser light and the modulation of the laser light can be synchronized with each other. 
     According to the above-described embodiment, since the optical pulse of the laser light and the modulation of the laser light can be synchronized with each other, the optical pulse of the laser light and the modulation of the laser light can easily be synchronized with each other even when the laser light has a very high pulse optical frequency. 
     Accordingly, the recording can be accurately realized with a high density and at a high speed. 
     Additional features and advantages are described herein, and will be apparent from the following Detailed Description and the figures. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a schematic diagram illustrating the configuration of a recording apparatus according to a first embodiment; 
         FIGS. 2A and 2B  are schematic diagrams illustrating an example of a semiconductor laser in  FIG. 1 ; 
         FIG. 3  is a schematic diagram illustrating an example of a semiconductor optical amplifier in  FIG. 1 ; 
         FIGS. 4A and 4B  are diagram illustrating input light and output light of the semiconductor optical amplifier in  FIG. 1 ; 
         FIGS. 5A to 5D  are diagrams illustrating a problem at a rising point/falling point of a driving pulse of the semiconductor optical amplifier; 
         FIG. 6  is a schematic diagram illustrating the configuration of a recording apparatus according to a second embodiment; 
         FIGS. 7A to 7E  are diagrams illustrating phase adjustment performed by a phase adjustment circuit in  FIG. 6 ; 
         FIG. 8  is a diagram illustrating a case where zoning is performed in an optical recording medium; 
         FIG. 9  is a diagram illustrating a relationship between a clock frequency and a clock density of each zone in an example of Table 1 and an example of Table 2; and 
         FIG. 10  is a schematic diagram illustrating the configuration of a recording apparatus according to a third embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present application will be described below in detail with reference to the drawings. 
     The description thereof will be made in the following order. 
     1. First Embodiment 
     2. Second Embodiment 
     3. Third Embodiment 
     1. First Embodiment 
       FIG. 1  is a schematic diagram illustrating the configuration of a recording apparatus according to a first embodiment. 
     A recoding apparatus  200  shown in  FIG. 1  includes a mode-lock laser unit  210 , an optical isolator unit  220 , an optical amplifier unit (SOA unit)  230 , and a beam shaping unit  240 . 
     The recording apparatus  200  further includes a bias Tee formed by a variable voltage source  12 , a coil  13 , and a capacitor  14 , a reference signal generation unit  15 , a data generation unit (recording signal generation unit)  16 , an SOA drive circuit  17 , a spindle circuit  18 , and a spindle motor  20 . 
     Further, the recording apparatus  200  includes various kinds of circuits or various kinds of optical components (none of which are shown). 
     The mode-lock laser unit  210  includes a semiconductor laser  1  and optical components such as a lens  3 , a band-pass filter  4 , and a minor  5  passing through laser light emitted from the semiconductor laser  1 . The band-pass filter  4  passes through light with a given wavelength range and does not pass through light with a wavelength out of the wavelength range. 
     An external resonator (spatial resonator) is disposed between a minor of the rear end surface of the semiconductor laser  1  and the mirror  5 . The frequency of the laser light emitted from the mode-lock laser unit  210  is determined in accordance with the path length of the external resonator. Thus, since the frequency of the laser light can be forcibly locked to a specific frequency, the mode of the laser light can be locked. 
     An exemplary configuration of the semiconductor laser  1  shown in  FIG. 1  is schematically illustrated in  FIGS. 2A and 2B .  FIG. 2A  is a perspective view of the semiconductor laser and  FIG. 2B  is a schematic view of the laser light emitted from the semiconductor laser. 
     The semiconductor laser  1  includes a gain section  116  and a saturable absorber section  117 , as shown in  FIGS. 2A and 2B . That is, the semiconductor laser  1  is a BS (bisectional) type semiconductor laser. 
     When the saturable absorber section  117  is provided, an absorption rate is lowered with an increase in the intensity of light incident on the absorber section. Accordingly, since only a pulse with a large intensity can be transmitted, a narrower pulse can be obtained. 
     Further, a gain current is fed to the gain section  116 . 
     An n-type GaN layer  103 , an n-type clad layer  104 , an active layer  105 , a p-type electron barrier layer  106 , and a p-type clad layer  107  are laminated on an n-type GaN substrate  102 . 
     As shown in  FIG. 2A , a ridge structure is formed in the middle of the p-type clad layer  107 . A SiO2 layer  108  and a Si layer  109  are formed on the ridge side surfaces or a portion where the ridge structure of the p-type clad layer  107  is not formed. 
     On the p-type clad layer  107  and the Si layer  109 , p-type electrodes  113  and  114  are formed by an ohmic contact. 
     That is, a main electrode  113  is formed on the gain section  116  and a sub-electrode  114  is formed on the saturable absorber section  117 . Since the electrodes  113  and  114  are separated by a separation section  115  formed with a groove with a width of, for example, 20 μm, the electrodes  113  and  114  are electrically isolated from each other. The lengths of the main electrode  113  and the sub-electrode  114  are, for example, 520 μm and 60 μm, respectively. 
     A lower electrode  101  is formed by an ohmic contact on the lower surface of the n-type GaN substrate  102 . 
     An anti-reflection film (see  FIG. 2B )  118  is coated on the cleaved plane of the front surface of the gain section  116 . A high reflection film  119  (see  FIG. 2B ) is coated on the cleaved plane of the rear surface of the saturable absorber section  117 . 
     In the semiconductor laser  1 , as shown in  FIG. 2B , a reverse bias voltage Vsa is applied to the saturable absorber section  117  by the sub-electrode  114 . At this time, by feeding a current I from the main electrode  113  to the gain section  116 , the laser light is emitted in a direction indicated by an arrow A 1 . 
     In the recording apparatus  200  according to this embodiment, the configuration of the used semiconductor laser  1  is not limited to the configuration of the semiconductor laser  1  shown in  FIGS. 2A and 2B , but a semiconductor laser with another configuration can be used. 
     The semiconductor material of the semiconductor laser  1  is selected depending on the wavelength of the laser light used for recording information in the recording apparatus  200 . 
     The optical isolator unit  220  is disposed on the rear stage of the mode-lock laser unit  210  with a lens  6  interposed therebetween. 
     The optical isolator unit  220  includes an optical isolator  7  and a mirror  8 . 
     The optical isolator  7  has a function of preventing light reflected from optical components or the like on the rear stage from being incident on the semiconductor laser  1 . 
     The optical amplifier unit (SOA unit)  230  serves as an optical modulation unit that performs amplification modulation on the laser light emitted from the semiconductor laser  1 . The optical amplifier unit  230  is disposed on the rear stage of the optical isolator unit  220  with a lens  9  interposed therebetween. 
     The optical amplifier unit (SOA unit)  230  is configured by an SOA (Semiconductor Optical Amp), that is, a semiconductor optical amplifier  2 . 
     The semiconductor optical amplifier  2  is a small-sized and cheap optical amplifier and can be used as an optical gate or a light switch that turns light on or off. 
     In this embodiment, the laser light emitted from the semiconductor laser  1  is modulated when the semiconductor optical amplifier  2  is turned on or off. 
     An exemplary configuration of the semiconductor optical amplifier  2  shown in  FIG. 1  is schematically illustrated in  FIG. 3 . 
     In a general semiconductor laser, light is confined in a resonator that is configured by minors on the both end surfaces and laser light is oscillated by optical gain due to inter-band transition. 
     Meanwhile, as shown in  FIG. 3 , instead of the mirrors, anti-reflection films  25  are disposed on the both end surfaces of the semiconductor optical amplifier  2  to suppress the oscillation of the laser light, so that the semiconductor optical amplifier  2  is operated as an amplifier corresponding to one pass. 
     In the semiconductor optical amplifier  2 , a semiconductor layer including an active layer  24  is configured to be laminated, as in the semiconductor laser. 
     In the semiconductor optical amplifier  2 , an upper electrode  22  is formed on the upper surface and a lower electrode  23  is formed on the lower surface. The upper electrode  22  is connected to a current source  26  and the lower electrode  23  is a connected to a ground potential. 
     When a driving current is fed from the current source  26  to the upper electrode  22  and the laser light is incident from the incident end surface on which the anti-reflection film  25  is formed, the laser light is amplified by induced emission during wave-guiding of the laser light in the active layer  24 . 
     By controlling the amount of driving current fed to the semiconductor optical amplifier  2 , the amplification amount of the laser light can be controlled. 
     However, the laser light incident to the semiconductor optical amplifier  2  may not be necessarily amplified. When a sufficient laser light power can be obtained, the gain of the semiconductor optical amplifier  2  may be set to 1. 
     In the semiconductor optical amplifier  2  with the above-described configuration, since a carrier lifespan is short, high-speed response is made for a variation in the current or the optical intensity. Accordingly, as for continuous pulse light input from the semiconductor laser  1 , as shown in  FIG. 4A , for example, pulse light with a waveform shown in  FIG. 4B  can be obtained as light output from the semiconductor optical amplifier  2 . 
     That is, since ON and OFF can be controlled by the signal of the driving current, the semiconductor optical amplifier  2  can be used as a switch of high-speed light so as to correspond to even the pulse light frequency of the semiconductor laser  1 . 
     When the recording apparatus  200  has a configuration in which laser light with, for example, a 407 nm wavelength is emitted from the semiconductor laser  1 , the semiconductor optical amplifier  2  is also made of the same material as that of a blue-purple semiconductor laser that emits light with the same wavelength as that of the active layer  24 , a guide layer, a clad layer, or the like. 
     The beam shaping unit  240  is disposed on the rear stage of the optical amplifier unit  230 . 
     The beam shaping unit  240  includes a lens  10  and a prism (for example, an anamorphic prism)  11  that shapes the beam of the laser light. 
     The laser light shaped by the beam shaping unit  240  is emitted to an optical recording medium  21  via an optical pickup (not shown). 
     The variable voltage source  12  supplies the bias voltage Vsa to the sub-electrode  114  of the saturable absorber section  117  of the semiconductor laser  1  of the mode-lock laser unit  210 . 
     The bias Tee configured by the coil  13  and the capacitor  14  supplies a gain current Ig to the main electrode  113  of the gain section  116  of the semiconductor laser  1  of the mode-lock laser unit  210 . An AC component and a DC component of the gain current Ig are supplied from the reference signal generation unit  15  to the capacitor  14  and the coil  13 , respectively. 
     The reference signal generation unit  15  generates the master clock signal. The master clock signal generated by the reference signal generation unit  15  is transmitted to the data generation unit (recording signal generation unit)  16 . 
     The reference signal generation unit  15  supplies, to the capacitor  14  of the bias Tee, a signal synchronized with the master clock signal as the AC component of the gain current Ig for the semiconductor laser  1 . 
     The data generation unit (recording signal generation unit)  16  generates a data pulse by loading recording data in synchronization with the master clock signal. The generated data pulse is transmitted to the SOA drive circuit  17 . 
     The SOA drive circuit  17  generates a driving current of the semiconductor optical amplifier (SOA)  2  based on the data pulse. The driving current is supplied to the semiconductor optical amplifier  2  of the optical amplifier unit (SOA unit)  230 . 
     The signal synchronized with the master clock signal is transmitted from the reference signal generation unit  15  to the spindle circuit  18 . 
     The driving current is supplied from the spindle circuit  18  to the spindle motor  20 , so that the rotation speed of the disc-shaped optical recording medium  21  is controlled by the driving current of the spindle motor  20 . 
     The signal synchronized with the master clock signal is supplied as the AC component of the gain current Ig for the semiconductor laser  1 , the optical pulse of the laser light emitted from the semiconductor laser  1  can be synchronized with the master clock signal. 
     Further, since the driving current of the semiconductor optical amplifier (SOA)  2  is generated based on the data pulse generated by loading the recording data in synchronization with the master clock signal, the semiconductor optical amplifier  2  is driven in synchronization with the master clock signal. In this way, the modulation (ON or OFF) of the laser light by the semiconductor optical amplifier  2  can be synchronized with the master clock signal. 
     Furthermore, since the signal synchronized with the master clock signal is transmitted to the spindle circuit  18  that supplies the driving current to the spindle motor  20 , the rotation driving of the optical recording medium by the spindle motor  20  can be synchronized with the master clock signal. 
     In the recording apparatus  200  according to the above-described embodiment, the reference signal generation unit  15  supplies the signal synchronized with the master clock signal to the gain section  116  of the semiconductor laser  1 . Thus, the optical pulse of the laser light emitted from the semiconductor laser  1  can be synchronized with the master clock signal. 
     The data generation unit  16  generates the data pulse by loading the recording data based on the master clock signal and the SOA drive circuit  17  generates the current pulse of the driving current of the semiconductor optical amplifier  2  based on the data pulse. 
     Since the semiconductor optical amplifier  2  is driven by the current pulse of the driving current, the semiconductor optical amplifier  2  is driven to be turned on or off in synchronization with the master clock signal, so that the laser light is modulated in synchronization with the master clock signal. 
     Due to the synchronization with the master clock signal, the optical pulse of the laser light and the modulation of the laser light can be synchronized with each other. 
     Thus, since the optical pulse of the laser light and the modulation of the laser light can be synchronized with each other, the optical pulse of the laser light and the modulation of the laser light can easily be synchronized with each other even when the laser light has a very high pulse optical frequency. 
     Accordingly, in the recording apparatus, the recording can be accurately realized with a high density and at a high speed. 
     Due to the synchronization with the master clock signal, the modulation of the laser light by the semiconductor optical amplifier  2  and the rotation of the optical recording medium  21  can be synchronized with each other. 
     Thus, since information can be recorded with an appropriate density at a desired position on the disc-shaped optical recording medium  21 , the recording can be accurately realized with a high density and at a high speed. 
     2. Second Embodiment 
     However, even when the oscillation pulse of the semiconductor laser  1  and the data pulse are synchronized with each other, the amplitude of the output light may become unstable due to the change in the time axis in the rising point and the falling point of the pulse of the driving current of the semiconductor optical amplifier  2 . 
     That is, the amount of current is considerably changed in transition regions (a portion indicated by diagonal lines) near the rising point and the falling point of the pulse of the driving current of the semiconductor optical amplifier  2  shown in  FIG. 5B , which is generated based on the data pulse shown in  FIG. 5A . Therefore, the current amount is not sufficient. 
     For this reason, in output light obtained from input light shown in  FIG. 5C , as shown in  FIG. 5D , its amplitude of the portions (portions indicated by diagonal lines) corresponding to the transition regions near the rising point and the falling point of the pulse of the driving current of the semiconductor optical amplifier  2  may decrease. 
     An embodiment of a recording apparatus configured to resolve such a problem will be described below. 
     The configuration of the recording apparatus according to a second embodiment is schematically illustrated in  FIG. 6 . 
     A recording apparatus  300  shown in  FIG. 6  includes a phase adjustment circuit  19  disposed between the data generation unit (the recording signal generation unit)  16  and the SOA drive circuit  17 . 
     The phase adjustment circuit  19  adjusts the phase of the data pulse generated by the data generation unit (the recording signal generation unit)  16  and then supplies the data pulse to the SOA drive circuit  17 . 
     Thus, the influence of the rising point and the falling point of the pulse of the driving current of the semiconductor optical amplifier  2  on the output light is reduced. 
     The adjustment of the phase of the driving current by the phase adjustment circuit  19  will be described with reference to  FIGS. 7A to 7D . A data pulse shown in  FIG. 7A  is adjusted by the phase adjustment circuit  19  so as to be shifted backward by a period corresponding to a part of the pulse width, as shown in  FIG. 7B . The data pulse is preferably shifted by about the half of a period of laser light input to the semiconductor optical amplifier  2  shown in  FIG. 7D . 
     Thus, in the vicinity (transition regions) of the rising point and the falling point of the pulse of the SOA driving current shown in  FIG. 7C , the intensity of the light input to the semiconductor optical amplifier  2  shown in  FIG. 7D  decreases. 
     Accordingly, as the waveform of the light output from the semiconductor optical amplifier  2  is shown in  FIG. 7E , the optical pulse with sufficient amplitude can be obtained from the output light without the influence of the rising point and the falling point of the driving pulse on the output light. 
     The remaining configuration of the recording apparatus  300  shown in  FIG. 6  is the same as that of the recording apparatus  200  shown in  FIG. 1  according to the first embodiment. The same reference numerals are given and the description thereof will not be repeated. 
     In the recording apparatus  300  according to the above-described second embodiment, the optical pulse of the laser light and the modulation of the laser light can easily be synchronized with each other, as in the recording apparatus  200  according to the first embodiment. 
     Thus, the optical pulse of the laser light and the modulation of the laser light can be synchronized with each other, even when the laser light has a very high pulse optical frequency. 
     Accordingly, in the recording apparatus, the recording can be accurately realized with a high density and at a high speed. 
     As in the recording apparatus  200  according to the first embodiment, the modulation of the laser light by the semiconductor optical amplifier  2  and the rotation of the optical recording medium  21  can be synchronized with each other. 
     Thus, since information can be recorded with an appropriate density at a desired position on the disc-shaped optical recording medium  21 , the recording can be accurately realized with a high density and at a high speed. 
     In the recording apparatus  300  according to the second embodiment, the phase adjustment circuit  19  is provided between the data generation unit  16  and the SOA drive circuit  17 . The phase adjustment circuit  19  adjusts the phase of the data pulse generated by the data generation unit  16  and then supplies the data pulse to the SOA drive circuit  17 . 
     Further, the phase adjustment circuit  19  adjusts the phase of the recording signal so as to prevent the light emission pulse of the laser light from overlapping with the rise and fall transition regions of the driving pulse of the semiconductor optical amplifier  2 . Thus, since the influence of the transition regions of the driving pulse on the modulated laser light can be eliminated, it is possible to obtain the optical pulse with sufficient amplitude. 
     3. Third Embodiment 
     As a method of improving the transmission rate of the optical recording medium, there is a method of providing a plurality of optical pickups and configuring a plurality of laser beams to be emitted to the optical recording medium in order to increase data channels. 
     For example, the recording region of the optical recording medium is divided into several zones (zoning) and assigning the zones to the plurality of optical pickups. 
     A case where the zoning is performed on the optical recoding medium  21  is shown in  FIG. 8 . 
     As shown in  FIG. 8 , the optical recording medium  21  is divided into the plurality of zones  21 A,  21 B, and  21 C with a doughnut shape in a radius range from the center of the optical recording medium  21  and assigning the zones  21 A,  21 B, and  21 C to two optical pickups  31  and  32 . 
     With such a configuration, when the rotation number of the optical recording medium  21  is set to be constant, it is necessary to set each clock of the inner circumference and the outer circumference of each zone of the optical recording medium  21 , so that the recording density is constant, in order to realize a linear density in all the zones. 
     For example, when the frequency of the master clock is 1713.60 MHz, the minimum clock density is 66 nm/clock, and the rotation number of the optical recording medium is 3600 rpm, a setting example of each clock of the inner circumference and the outer circumference of each zone is shown in Table 1. In Table 1, R denotes a radius from the center, Zone denotes a zone number, V 1  denotes a linear velocity, Clk denotes a clock frequency, Inner denotes the clock density of the inner circumference of the zone, and Outer denotes the clock density of the outer circumference of the zone. Further, Ratio denotes a ratio at which the frequency of the master clock is eliminated by the clock frequency of each zone. 
     
       
         
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 R (mm) 
                 Zone 
                 VI (m/s) 
                 Clk (MHz) 
                 Inner 
                 Outer 
                 Ratio 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 30 
                 0 
                 11.31 
                 171.36 
                 66.00 
                 70.40 
                 10.00 
               
               
                 32 
                 1 
                 12.06 
                 182.78 
                 66.00 
                 70.13 
                 9.38 
               
               
                 34 
                 2 
                 12.82 
                 194.21 
                 66.00 
                 69.88 
                 8.82 
               
               
                 36 
                 3 
                 13.57 
                 205.63 
                 66.00 
                 69.67 
                 8.33 
               
               
                 38 
                 4 
                 14.33 
                 217.06 
                 66.00 
                 69.47 
                 7.89 
               
               
                 40 
                 5 
                 15.08 
                 228.48 
                 66.00 
                 69.30 
                 7.50 
               
               
                 42 
                 6 
                 15.83 
                 239.90 
                 66.00 
                 69.14 
                 7.14 
               
               
                 44 
                 7 
                 16.59 
                 251.33 
                 66.00 
                 69.00 
                 6.82 
               
               
                 46 
                 8 
                 17.34 
                 262.75 
                 66.00 
                 68.87 
                 6.52 
               
               
                 48 
                 9 
                 18.10 
                 274.18 
                 66.00 
                 68.75 
                 6.25 
               
               
                 50 
                 10 
                 18.85 
                 285.60 
                 66.00 
                 68.64 
                 6.00 
               
               
                 52 
                 11 
                 19.60 
                 297.02 
                 66.00 
                 68.54 
                 5.77 
               
               
                 54 
                 12 
                 20.36 
                 308.45 
                 66.00 
                 68.44 
                 5.56 
               
               
                 56 
                 13 
                 21.11 
                 319.87 
                 66.00 
                 68.36 
                 5.36 
               
               
                 58 
                 14 
                 21.87 
                 331.30 
                 66.00 
                 68.28 
                 5.17 
               
               
                 60 
                 15 
                 22.62 
                 342.72 
                 66.00 
                 68.20 
                 5.00 
               
               
                   
               
             
          
         
       
     
     When the clock Clk of each zone is set, as in Table 1, the clock density of the inner circumference of each zone is constantly set to 66 (nm/clock) and the recording density of the inner circumference where the recording density is the highest in the zone can be made to be constant. 
     In a case of the MOPA optical source using the mode-lock laser, the clock frequency is determined by the length of a resonator of the mode-lock laser. Accordingly, as in the example shown in Table 1, it is difficult to minutely optimize the clock frequency of each zone. 
     In order to resolve this problem, the clock frequency of each zone is set at an integer ratio so as to be easily generated from the same master clock by signal processing or the like. 
     Further, the clock frequency of each zone is set at an integer ratio within a range in which the recording density of the zone does not exceed the limit so that the clock frequency is slightly high. 
     A setting example in which the clock frequency is set in this manner is shown in Table 2. 
     
       
         
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                   
                 Integer 
                   
                   
                   
               
               
                 R (mm) 
                 Zone 
                 VI (m/s) 
                 Ratio 
                 Clk (MHz) 
                 Inner 
                 Outer 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 30 
                 0 
                 11.31 
                 10 
                 171.36 
                 66.00 
                 70.40 
               
               
                 32 
                 1 
                 12.06 
                 10 
                 171.36 
                 70.40 
                 74.80 
               
               
                 34 
                 2 
                 12.82 
                 9 
                 190.40 
                 67.32 
                 71.28 
               
               
                 36 
                 3 
                 13.57 
                 9 
                 190.40 
                 71.28 
                 75.24 
               
               
                 38 
                 4 
                 14.33 
                 8 
                 214.20 
                 66.88 
                 70.40 
               
               
                 40 
                 5 
                 15.08 
                 8 
                 214.20 
                 70.40 
                 73.92 
               
               
                 42 
                 6 
                 15.83 
                 8 
                 214.20 
                 73.92 
                 77.44 
               
               
                 44 
                 7 
                 16.59 
                 7 
                 244.80 
                 67.76 
                 70.84 
               
               
                 46 
                 8 
                 17.34 
                 7 
                 244.80 
                 70.84 
                 73.92 
               
               
                 48 
                 9 
                 18.10 
                 7 
                 244.80 
                 73.92 
                 77.00 
               
               
                 50 
                 10 
                 18.85 
                 6 
                 285.60 
                 66.00 
                 68.64 
               
               
                 52 
                 11 
                 19.60 
                 6 
                 285.60 
                 68.64 
                 71.28 
               
               
                 54 
                 12 
                 20.36 
                 6 
                 285.60 
                 71.28 
                 73.92 
               
               
                 56 
                 13 
                 21.11 
                 6 
                 285.60 
                 73.92 
                 76.56 
               
               
                 58 
                 14 
                 21.87 
                 6 
                 285.60 
                 76.56 
                 79.20 
               
               
                 60 
                 15 
                 22.62 
                 5 
                 342.72 
                 66.00 
                 68.20 
               
               
                   
               
             
          
         
       
     
     In the example shown in Table 2, the zones of the optical recording medium  21  are set by integers and the clock frequency of each zone is set at an integer ratio. That is, on the assumption that the integer ratios are n (integers from 5 to 10), a relationship of “the clock frequency=1713.6/n” is satisfied. 
     Thus, when the clock frequency is set at the integer ratio, the clock frequency can easily be generated from the same master clock by signal processing or the like. 
     The clocks and the clock densities are compared in the inner circumference and the outer circumference of the zone in the examples of Table 1 and Table 2 and the result is shown in  FIG. 9 . In  FIG. 9  a dashed line indicates the case of Table 1 and polygonal lines indicated by signs •, O, and Δ indicate the case of the Table 2. 
     An embodiment of a recording apparatus with such a configuration will be described below. 
     The configuration of a recording apparatus according to a third embodiment is shown in  FIG. 10 . 
     A recording apparatus  400  shown in  FIG. 10  includes a zone frequency-divider circuit  35  disposed between the reference signal generation unit  15  and the data generation unit (the recording signal generation unit)  16 . 
     The zone frequency-divider circuit  35  generates a clock signal of each zone of the optical recording medium  21  having the relationship of an integer ratio between the frequency of the master clock signal and the frequency of the clock signal using the master clock signal generated by the reference signal generation unit  15 . Then, the clock signal of each zone generated by the zone frequency-divider circuit  35  is supplied to the data generation unit (the recording signal generation unit)  16 . 
     The clock signal of each zone generated in this way is reflected on the data pulse or the driving current of the semiconductor optical amplifier (SOA)  2 . When the semiconductor optical amplifier  2  is turned on or off, the laser light from the semiconductor laser  1  is supplied to the optical pickup  31  in synchronization with the clock signal of each zone. Thus, information can be recorded at an appropriate clock in each zone of the optical recording medium  21 . 
     In  FIG. 10 , only one optical pickup  31  is illustrated. However, as shown in  FIG. 8 , the plurality of optical pickups  31  and  32  are assigned to the zones. 
     The remaining configuration of the recording apparatus  400  shown in  FIG. 10  is the same as that of the recording apparatus  300  shown in  FIG. 6  according to the second embodiment. The same reference numerals are given and the description thereof will not be repeated. 
     In the recording apparatus  400  according to the above-described third embodiment, the optical pulse of the laser light and the modulation of the laser light can easily be synchronized with each other, as in the recording apparatuses  200  and  300  according to the first and second embodiments. 
     Thus, the optical pulse of the laser light and the modulation of the laser light can be synchronized with each other, even when the laser light has a very high pulse optical frequency. 
     Accordingly, in the recording apparatus, the recording can be accurately realized with a high density and at a high speed. 
     As in the recording apparatuses  200  and  300  according to the first and second embodiments, the modulation of the laser light by the semiconductor optical amplifier  2  and the rotation of the optical recording medium  21  can be synchronized with each other. 
     Thus, since information can be recorded with an appropriate density at a desired position on the disc-shaped optical recording medium  21 , the recording can be accurately realized with a high density and at a high speed. 
     As in the recording apparatus  300  according to the second embodiment, the phase adjustment circuit  19  adjusts the phase of the recording signal so as to prevent the light emission pulse of the laser light from overlapping with the rise and fall transition regions of the driving pulse of the semiconductor optical amplifier  2 . 
     Thus, since the influence of the transition regions of the driving pulse on the modulated laser light can be eliminated, it is possible to obtain the optical pulse with a sufficient amplitude. 
     Further, in the recording apparatus  400  according to the third embodiment, the zone frequency-divider circuit  35  is disposed between the reference signal generation unit  15  and the data generation unit  16 . The zone frequency-divider circuit  35  generates the clock signal of each zone of the optical recording medium  21  having the relationship of the integer ratio between the frequency of the master clock signal and the frequency of the clock signal. 
     Accordingly, the clock signal of each zone can easily be generated. Therefore, even when the mode-lock oscillation type laser is used as an optical source, the same linear density can be set in the respective zones under the constant rotation number of the optical recording medium  21  by setting the recording density of each zone to be as high as possible. 
     In the above-described third embodiment, the zone frequency-divider circuit  35  is further provided in the configuration of the second embodiment in which the phase adjustment circuit  19  is included. The zone frequency-divider circuit may be provided in the configuration of the first embodiment in which no phase adjustment circuit is included. 
     In the recording apparatus according to each embodiment described embodiment, the optical recording medium  21  to be used has a dick shape and information is recorded by rotating the optical recording medium  21  by the spindle motor  20 . However, the embodiments are applicable to a recording apparatus with other configurations. 
     For example, information may be recorded in an optical recording medium with a card shape by scanning laser light to the optical recording medium. 
     It should be understood that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope and without diminishing its intended advantages. It is therefore intended that such changes and modifications be covered by the appended claims.