Abstract:
A harmonic Q switch laser device and its control method for obtaining a stable laser pulse while protecting a nonlinear optical crystal used for a wavelength conversion are presented. In this laser device, in a pause period of a laser train, a Q switch which is turned on makes the laser oscillates spuriously and continuously, and prevents a gain from accumulating in a gain medium Before a generation of a laser pulse, the Q switch is turned off for a specified period to raise the laser gain, so that a pulse of a specified magnitude is generated from the first shot when turning on the Q switch.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a laser device with a Q switch for generating a higher harmonics, a method of controlling the device, and a laser processing machine comprising the device.  
         BACKGROUND OF THE INVENTION  
         [0002]    A conventional Q switch laser device will be explained. FIG. 7 shows a configuration of a conventional a harmonic-generating Q switch laser device. The device comprises a laser head  11 , a Q switch  12 , an excitation light source  13 , a radio frequency (RF) driver  14  for the Q switch, a controller  15 , an interface  16 , a power source  17 , and an operation unit  18 .  
           [0003]    The operation of the device will be explained with referring to FIG. 7. An oscillation condition instruction set through the operation unit  18  such as a personal computer (PC) is sent to the control circuit  15  through the interface  16 . The controller  15  interprets the transmitted oscillation condition instruction, generates a control signal according to the instruction, transmits the condition to the excitation light source  13  and RF driver  14  for the Q switch, and determines the oscillation condition of the laser head. At the same time, the controller  15  also controls the alarm from the laser head  11  and a temperature of a nonlinear optical crystal.  
           [0004]    An optical operation of the harmonic-generating Q switch laser  
           [0005]    An optical operation of the harmonic-generating Q switch laser device will be explained with referring to FIG. 8. FIG. 8 shows an internal structure of the laser head  11  of the harmonic-generating Q switch laser device shown in FIG. 7. The head  11  comprises a reflecting mirror  21 , a Q switch element  22 , a gain medium  23 , an output mirror  24 , a condenser lens  25 , a nonlinear optical crystal  26 , an optical lens  27 , a narrow band filter or dichroic mirror  28 , and two lenses  25  and  27 . The lenses  25 ,  27  function as a collimator. The device where the nonlinear optical crystal  26  is disposed outside of the mirrors  21 ,  24  is called an extra cavity system.  
           [0006]    The optical operation of this harmonic-generating Q switch laser head will be explained with referring to FIG. 8. When an excitation light enters the gain medium  23 , an optical resonation occurs between the reflector mirror  21  and output mirror  24 . In this case, when the Q switch element  22  inserted between the mirrors  21  and  24  is turned on, the optical path opens, and the laser oscillates. When the element is turned off, the optical path closes, and the oscillation stops. Thus, a pulse laser oscillation is enabled. The Q switch element  22  is turned on and off by the RF driver  14  for the Q switch, and enables the Q switch laser head to pulse-oscillate. The laser light issued from the output mirror  24  is condensed by the condenser lens  25 , and emitted to the nonlinear optical crystal  26 . A harmonic laser is generated by the nonlinear optical crystal  26 , is collimated by the optical lens  27 , and is separated into an IR laser, a fundamental wave and harmonic laser by the narrow band filter or dichroic mirror  28 . The harmonic laser is used for a processing machine.  
           [0007]    For example, the power P SHG  of a harmonic laser of second harmonic generation (SHG) is expressed in the following formula.  
             P   SHG   =d   eff   ·I   eff ·(P IR ) 2   /A    
           [0008]    where d eff  is a nonlinear constant, a constant determined by the type of the nonlinear optical crystal  26 , the larger constant, a higher conversion efficiency; l eff  is an effective length, a length in which the nonlinear optical crystal  26  has a nonlinear effect; P IR  is the power of the fundamental wave; and A is a laser beam area.  
           [0009]    Under this relationship, reducing the beam area A and reinforcing the fundamental wave power P IR  into the nonlinear optical crystal  26  increases the harmonic laser output. In order to increase the power, the beam put into the nonlinear optical crystal 26  is condensed.  
           [0010]    However, the nonlinear optical crystal  26  has a limited light strength, i.e., the crystal is broken when a light exceeding a damage threshold is input. Even if the light is not reaching the damage threshold, a light approaching the damage threshold affects a life of the nonlinear optical crystal  26 .  
           [0011]    The optical characteristic of the harmonic-generating Q switch laser and a breakdown of the nonlinear optical crystal will be explained with referring to FIG. 9. FIG. 9 shows the oscillation characteristic of the Q switch laser. In a simplest method of operating the Q switch laser device, an arc current of a lamp or current of a laser diode (LD) is maintained in constant, and accordingly, the excitation light power is kept constant, and the Q switch is turned on and off. However, since the excitation light enters the gain medium  23  for a long period while the Q switch closes before an oscillation is started, a gain accumulated in the gain medium  23  excesses a specified value. Therefore, when a gate signal for starting an oscillation is turned on, a giant pulse is generated at a first shot or several shots depending on cases. This giant pulse may destroy the nonlinear optical crystal  26  or shorten the life of the crystal.  
           [0012]    To avoid generating such a giant pulse, the controller of the Q switch laser device may have a first pulse suppression (FPS) function.  
           [0013]    Referring to FIG. 10, the FPS function of the Q switch laser device will be explained. In order to suppress the giant pulse in the first shot or several shots when the gate signal is turned on upon starting the oscillation, the excitation light into the gain medium  23  is weakened to such an extent as to maintain a specified gain while the Q switch is closed for a long time.  
           [0014]    In this method, the laser gain accumulated in the gain medium  23  is prevented from getting excessive. Since the maximum excitation light power is limited, the gradient of building-up the laser gain is limited. The period until the specified laser gain is accumulated depends on the interval of the pulse train following the first pulse, that is, a pulse frequency. Therefore, whenever the pulse frequency changes, the condition of the FPS function such as the laser gain retention current ΔI and retention period ΔT must be adjusted.  
           [0015]    In other method of realizing the FPS function, the rise time of the Q switch may be delayed, but it is similarly needed to set the condition of the FPS function for each pulse frequency.  
           [0016]    [0016]FIG. 11 shows an optical system of a laser processing machine having the harmonic-generating Q switch laser device. The processing machine comprises a harmonic-generating Q switch laser device  31 , a collimator lens  32 , a mask changer  33 , a bend mirror  34 , a galvanoscanner  35 , a scanner lens  36 , and a working table  37 .  
           [0017]    The laser emitted from the harmonic-generating Q switch laser device  31  have the beam diameter optimized by the collimator  32 , and is emitted to the mask on the mask changer  33 . A portion of the emitted laser passes through the mask, and is condensed at a specified position through the scanner lens  36  by the galvanoscanner  35  through the bend mirror  34 , and processes the work fixed on the table  37 .  
           [0018]    While processing the work, generally, a pulse train of a specified frequency is needed, a long pause period is necessary in order to convey the work, and the pulse oscillation and pause period are repeated.  
           [0019]    [0019]FIG. 12 shows an output characteristic of the harmonic-generating Q switch laser device. A low pulse frequency increases the pulse energy, and increases a possibility of damaging the nonlinear optical crystal  26  or shortening of the life of the crystal.  
           [0020]    Therefore, a long pause period longer in order to convey the work makes the gain be accumulated excessively in the gain medium  23 , and generates the giant pulse, which damages the nonlinear optical crystal  26  or shorten the life of the crystal, and thereby reduces the reliability.  
           [0021]    To avoid such circumstances, the harmonic-generating Q switch laser device  31  generally incorporated in the laser processing machine has the FPS function as mentioned above.  
           [0022]    Particularly a fine processing, the pulse frequency is required to change on the way. In this case, the FPS function is suspended, or even if the FPS function is employed for safety, the period for sending the condition setting instruction of the FPS function is required, and makes an entire processing time longer.  
           [0023]    Besides, an unstable pulse processes the work uniformly.  
           [0024]    Emitting the giant pulse or first pulse to a dummy target other than the work. the galvanoscanner  35  installed in the processing machine. may eliminates an affect of the pulse to the work. That, however, does not protect the nonlinear optical crystal  26 , and the reliability of the device is still not improved.  
         SUMMARY OF THE INVENTION  
         [0025]    A laser device and a method of controlling the device are provided for protecting a nonlinear optical crystal without a loss of a cycle time, and for assuring a reliability of a laser processing machine. And a laser processing machine having the device and a processing method using the machine are provided.  
           [0026]    The laser device generates a laser pulse train formed of a sequence of laser pulses. The laser device comprises an output mirror, a reflector mirror, a gain medium disposed between the output mirror and reflector mirror for accumulating the laser gain, a Q switch disposed between the output mirror and the reflector mirror for turning on and off a laser oscillation by the output mirror, reflector mirror and gain medium, a nonlinear optical crystal for generating a harmonic laser by recieving a fundamental wave laser by the laser oscillation. During a first pause period before the laser pulse train, the laser oscillation is turned on by the Q switch, and during a second pause period before each laser pulse, the laser oscillation is turned off by the Q switch. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]    [0027]FIG. 1 is a diagram for explaining an operation of a Q switch laser device according to a first exemplary embodiment of the present invention.  
         [0028]    [0028]FIG. 2 is a diagram for explaining an operation of other Q switch laser device according to the first embodiment.  
         [0029]    [0029]FIG. 3 is a diagram for explaining an operation of a Q switch laser device according to a second exemplary embodiment of the present invention.  
         [0030]    [0030]FIG. 4 is a diagram for explaining an the operation of other Q switch laser device according to the second embodiment.  
         [0031]    [0031]FIG. 5 shows a configuration of a laser head of a harmonic-generating Q switch laser device according to the embodiment.  
         [0032]    [0032]FIG. 6 shows an output characteristic of harmonic generating Q switch laser device according to the embodiment.  
         [0033]    [0033]FIG. 7 shows a configuration of the harmonic-generating Q switch laser device.  
         [0034]    [0034]FIG. 8 shows a configuration of a laser head of the harmonic-generating Q switch laser device.  
         [0035]    [0035]FIG. 9 shows a control of a conventional harmonic-generating Q switch laser oscillator.  
         [0036]    [0036]FIG. 10 shows a control of a conventional harmonic-generating Q switch laser oscillator.  
         [0037]    [0037]FIG. 11 shows a laser processing machine using a harmonic-generating Q switch laser device.  
         [0038]    [0038]FIG. 12 shows an output characteristic of a conventional harmonic-generating Q switch laser device. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0039]    (First Exemplary Embodiment)  
         [0040]    The operation of a laser device according to a first exemplary embodiment of the present invention will be explained with referring to FIG. 1. The harmonic-generating Q switch laser device according to the first embodiment is same as shown in FIG. 8, and a control of the device will be explained.  
         [0041]    In this device, an arc current of a lamp emitting excitation light or a current of a laser diode (LD) is maintained in constant, and the excitation light power is kept constant, and the Q switch is turned on and off to control a laser oscillation.  
         [0042]    As shown in FIG. 1, during a first pause period T 1  before an oscillation of a harmonic laser, the Q switch which is turned on sets the device to a continuous oscillation mode for oscillating the laser continuously. During a specified second pause period T 2 A before a harmonic laser pulse is generated, the Q switch which is turned off makes a laser power be accumulated in the laser medium irradiated with an excitation light. The second pause period T 2 A is equal to the duration of a pulse period T 0  of the laser pulse train extracted by a pulse width TW. The pulse width TW is so small to be negligible for the pulse period T 0 , and hence the pause period T 2 A substantially coincides nearly with the pulse period T 0 .  
         [0043]    By turning on the Q switch  22  during the first pause period T 1  before the oscillation of the harmonic laser to set the device to the continuous oscillation mode, a laser gain is prevented from being accumulated in the gain medium. A specified second pause period T 2 A nearly coinciding with the pulse period T 0  of the pulse train which is preset before a generation of the pulse makes the laser gain before the oscillation of the harmonic pulse be always kept at a specified value.  
         [0044]    [0044]FIG. 6 shows a laser output characteristic of the laser device according to the embodiment. As clear from the diagram, the pulse energy at a low frequency where the second pause period T 2 A is long is prevented from rising excessively, and the nonlinear optical crystal is protected owing to the characteristic. This control, therefore, makes a harmonic pulse be oscillated stably, and eliminates a damage or a shortening of life of the nonlinear optical crystal. Further, the control prevents a giant pulse from being generated without the FPS function.  
         [0045]    In the laser processing machine having such a laser device, a stable harmonic pulse is obtained even if the pulse train is generated randomly. The stable pulse improves the quality of the laser processing and generates no loss time even when the pulse oscillation condition changes, so that a laser processing machine of high productivity may be provided.  
         [0046]    Moreover, a constant arc current of a lamp emitting excitation light or a current of an LD laser effectively prevents the life of the lamp or the LD from being shortened due to a heat cycle and fluctuations.  
         [0047]    A harmonic-generating Q switch laser device of an intra cavity system according to the first embodiment is shown in FIG. 5. The nonlinear optical crystal  26  is positioned differently from the device shown in FIG. 8, and is located between the mirrors  21  and  24 , and hence it is called the intra cavity system. This device includes a harmonic dispensing device  51  like an optical modulator such as an acousto-optical modulator (AOM). Other components are the same as in the extra cavity system shown in FIG. 8 although the order is different.  
         [0048]    The nonlinear optical crystal  26  generates higher harmonics while the incident beam diameter is wide. Therefore, a weak harmonic laser may be issued with an output infrared ray (IR) during the first pause period T 1  of the laser pulse, i.e., a continuous wave (CW) of a fundamental wave laser. This device installed in the laser processing machine may generate the weak harmonic laser exceeding a processing threshold. The harmonic pulse is dispensed for extracting only harmonic pulse as shown in FIG. 2 by the harmonic dispensing device  51  to which a dispensing signal is input. The device  51  is disposed in the harmonic output path as shown in FIG. 5. As a result, the laser device of the intra cavity system has the same effect as the extra cavity system.  
         [0049]    (Second Exemplary Embodiment)  
         [0050]    An operation of a laser device according to a second exemplary embodiment of the present invention will be explained with referring to FIG. 3. The configuration of a harmonic-generating Q switch laser device according to the second embodiment is the same as shown in FIG. 8.  
         [0051]    The gain medium  23  of this device receives an excitation light having a constant power as the same as in the first embodiment. During a first pause period T 1  before an oscillation of a harmonic laser, the Q switch  22  which is turned on sets the device to a continuous oscillation mode. During a specified second pause period T 2 B before a generation of the harmonic laser pulse, the Q switch  22  which is turned off makes more gain accumulate in the gain medium  23  by the excitation light. The pause period T 2 B is shorter than the pulse period T 0  of pulse train. During the pause period T 2 B, the gain accumulated in the medium  23  is controlled, and the pulse output is thus controlled.  
         [0052]    The Q switch  22  which is turned on during the first pause period T 1  before the oscillation of the harmonic laser sets the device to the continuous oscillation mode, and prevents the laser gain for excessively accumulating in the gain medium. The pause period T 2 B, which is shorter than the pulse period, controls the laser gain before the oscillation of the harmonic pulse stably.  
         [0053]    [0053]FIG. 6 shows a laser output characteristic according to the embodiments. As clear from the diagram, the pulse energy at low frequency, where the second pause period T 2 B is long, is prevented from rising excessively, and the nonlinear optical crystal is protected from damage or shortening of life owing to this characteristic. The shorter the pause period TB 2 , the much smaller the pulse energy, and the nonlinear optical crystal is protected more securely. Therefore, the harmonic pulse always oscillates stably, and the laser output is controlled by the pause period T 2 B. Besides, without the FPS function, the giant pulse is not generated.  
         [0054]    In the laser processing machine having such laser device, not only a stable harmonic pulse is obtained even if the pulse train is generated randomly, but also the laser output can be controlled precisely through the pause period control, so that a laser processing quality may be excellent. Further, a loss time does not occur even when the pulse oscillation condition changes, so that a high productivity may be presented.  
         [0055]    Moreover, a constant arc current of the lamp emitting an excitation light or a current of an LD effectively prevents the life of the lamp or LD from being shortened due to fluctuations.  
         [0056]    A harmonic-generating Q switch laser device of intra cavity system according to the second embodiment is shown in FIG. 5. As the same as in the first embodiment, the nonlinear optical crystal  26  is positioned differently from the device shown in FIG. 8, and is located between the mirrors  21  and  24 . The control signal is shown in FIG. 4.  
         [0057]    In FIG. 5, a harmonic dispensing device  51  is an optical modulator such as acousto-optical modulator (AOM). Other components are same as in the extra cavity system shown in FIG. 8 although the order is different.  
         [0058]    The nonlinear optical crystal  26  generates a higher harmonics while the incident beam diameter is wide. Therefore, a weak harmonic laser may be issued with an infrared ray (IR) during the first pause period T 1  of the laser pulse, that is, a continuous wave (CW) of a fundamental wave laser. In this device installed in the laser processing machine, even if the weak harmonic laser exceeds the processing threshold, the harmonic pulse is dispensed by the harmonic dispensing device  51 . The device  51 , a control signal for dispensing only harmonic pulse as shown in FIG. 4 is input, and is disposed in the harmonic output path as shown in FIG. 5.  
         [0059]    Further, after generation of pulse, a weak harmonic laser may be generated also in a period T 3  when the Q switch is turned on. But it is shielded by the harmonic dispensing device  51  and does not come out. As a result, the laser device of the intra cavity system has the same effect as the extra cavity system.