Abstract:
The present invention is a wavelength conversion laser apparatus for converting wavelength by utilizing a nonlinear optical crystal and capable of safely using it over a long time. For this end, the fundamental wave laser light emitted from the pumping chamber unit  3  which is a solid state laser light source is converted into the second harmonic wave by the first nonlinear optical crystal unit  20 . And the second harmonic wave is, then, converted into the third (or fourth) harmonic wave by the second nonlinear optical crystal unit  30 . Filled in the container  24, 35  for each of the nonlinear optical crystal units  20, 30  is a dry inert gas. A humidity sensor  51  detects humidity inside the container. The output from the humidity sensor  51  is supplied to the laser control portion  58 . When the detected humidity exceeds a predetermined value, the laser oscillation is interrupted, thereby preventing the nonlinear optical crystal from being damaged in case of any defect in the sealing structure of the container.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a wavelength conversion laser apparatus for converting wavelength by utilizing a nonlinear optical crystal, more specifically to a wavelength conversion laser apparatus in which the nonlinear optical crystal can be easily managed. 
   2. Description of Related Art 
   In short wavelength laser apparatus, nonlinear optical crystals are generally utilized for wavelength conversion. Ultra violet solid state laser devices among such laser apparatus are generally constructed as follows. A fundamental wave laser light of 1064 nm in wavelength is oscillated by a Nd:YAG laser, Nd:YVO 4  laser or the like. The nonlinear optical crystal generates a second harmonic wave of the fundamental wave laser light. Furthermore, third and fourth harmonic waves are also generated by nonlinear optical crystals. Crystals to be utilized for generating the second harmonic wave include an LBO crystal or a KTP crystal. Crystals to be utilized for generating the third harmonic wave include an LBO crystal, a BBO crystal or GdYCOB crystal. Crystals to be utilized for generating the fourth harmonic wave include a BBO crystal, a CLBO crystal or the like. Most of such nonlinear optical crystals are deliquescent. In order to prevent them from degrading by moisture absorption, it is required to pay special attention to atmosphere where the crystals are disposed, especially humidity where they are used. 
   Particularly, a CLBO crystal which is the nonlinear optical crystal to be utilized for generating the fourth harmonic wave exhibits significant crystal degradation in 30% or higher relative humidity. A crystal cell model no. 10031 commercially available from Crystal Association Inc. employs a method for filling a dry gas in the cell. Alternatively, there is an instance where a nonlinear optical crystal is used in dipping into oil. In Japanese patent non-examined publication no. 9-292638 (or JP-A-292638/94) entitled “High Output Ultra Violet Laser Light Generation Apparatus”, a deliquescent-free protection film is formed on the ultra violet light output end surface of the nonlinear optical crystal. 
   However, one problem associated with such conventional technique of simply filling a dry gas in a crystal cell is a limited lifetime. In other words, if any defect may occur in the sealing structure during a long time use, humidity in the sealed container tends to change. If the laser is continuously used in high humidity condition inside the cell, the nonlinear optical crystal is increasingly degraded and damaged. This accompanies with significant decrease in wavelength conversion efficiency and thus significant decrease in the laser output. 
   SUMMARY OF THE INVENTION 
   It is, therefore, an object of the present invention to provide a wavelength conversion laser apparatus that can be safely used for a long time by controlling the laser light generation operation by accurately detecting any humidity change in atmosphere of the nonlinear optical crystal. 
   In order to achieve the above object, the wavelength conversion laser apparatus according to the present invention includes harmonic wave generation means for receiving the laser light of a predetermined wavelength and generating the harmonic wave thereof, and the harmonic wave generation means comprises a sealed container formed with a through-hole through which the laser light propagates and windows at the laser light incident and output sides of the through-hole, a nonlinear optical crystal disposed in the through-hole and a humidity sensor disposed in the through-hole. In this arrangement, humidity change in atmosphere of the nonlinear optical crystal can be accurately detected. 
   Additionally, a control portion for controlling the laser light source is provided. When humidity in atmosphere of the nonlinear optical crystal is increased, the laser light source is interrupted to prevent the nonlinear optical crystal from being damaged. 
   In the wavelength conversion laser apparatus which is constructed as described hereinabove, the humidity sensor continuously and accurately monitors humidity in the cell in which the nonlinear optical crystal is accommodated, thereby enabling the user to use the wavelength conversion laser apparatus without any fear of damaging the nonlinear optical crystal. It is to be noted that the units for generating the harmonic wave are connected in series to generate sequentially higher order of harmonic waves. 
   Additionally, the provision of the control portion for controlling the laser light source by receiving the signal from the humidity sensor enables to stop the laser light source and prevent the nonlinear optical crystal from being damaged in case when humidity in atmosphere of the nonlinear optical crystal is increased. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a simplified side cross sectional view of one embodiment of the wavelength conversion laser apparatus according to the present invention; 
       FIG. 2  is a simplified cross section view of a nonlinear optical crystal cell in the embodiment of the wavelength conversion laser apparatus; 
       FIG. 3  is a simplified cross section view of a cover member for the nonlinear optical crystal cell of the embodiment of the wavelength conversion laser apparatus; and 
       FIG. 4  is a simplified block diagram of a humidity monitoring apparatus of the embodiment of the wavelength conversion laser apparatus. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Now, an embodiment of the present invention will be described in detail hereunder by reference to the accompanying drawings, namely  FIG. 1  through  FIG. 4 . 
   An embodiment of the present invention is the wavelength conversion laser apparatus in which a humidity sensor for detecting humidity in a sealed container filled with a dry inert gas and for accommodating a nonlinear optical crystal which generates harmonic waves upon receiving a laser light having a predetermined wavelength, and the humidity sensor is connected to a laser control portion. 
     FIG. 1  illustrates a simplified side cross sectional view of one embodiment of the wavelength conversion apparatus according to the present invention. In  FIG. 1 , mirrors  1  and  2  are reflection mirrors which constitute a resonator for the fundamental wave laser light. A pumping chamber unit  3  is a unit including a solid state laser medium (such as Nd:YAG, Nd:YVO 4 , etc.) to be excited by a semiconductor laser. A Q switch  4 , a Brewster plate  5 , a shutter  6  and a first focusing lens  7  are basic elements constituting a solid state laser. Heaters  8   a ,  8   b ,  8   c  are heating means for maintaining the solid state laser at constant temperature. A temperature sensor  9  is means for detecting temperature of the solid state laser. A case  11  is a case to constitute a container of the fundamental wave unit (A). The case  11  is U-shape in cross section and has a window for outputting the fundamental wave laser light. A cover member  12  is a cover of the container of the fundamental wave unit (A) for hermetically covering the upper face of the case  11 . The container  13  is a container for accomodating the fundamental wave unit (A) and comprises the case  11  and the cover member  12 . 
   A first nonlinear optical crystal unit  20  is a unit which contains an LBO crystal, a KTP crystal or the like for converting the fundamental wave laser light having the predetermined wavelength (angular oscillation frequency ω) into a second harmonic wave (angular oscillation frequency 2 ω). A second focusing lens  21  is a lens which focuses the second harmonic wave. A separation mirror  22  is a half mirror which separates the fundamental wave from the second harmonic wave. An output window  23  is a window through which the second harmonic wave is outputted. A unit case  24  is a case which accommodates a first wavelength conversion unit (B). 
   A second nonlinear optical crystal unit  30  is a unit which converts the laser light from the first wavelength conversion unit (B) into a third harmonic wave (or a fourth harmonic wave). A collimation lens  31  is a lens which converts the third harmonic wave (or fourth harmonic wave) into a parallel light. A separation mirror  32  is a half mirror which separates the second harmonic wave from the third (or fourth) harmonic wave. A power meter  33  is means which measures the output of the third or fourth harmonic wave. An output window  34  is a window through which the third (or fourth) harmonic wave is outputted outside the unit. A unit case  35  is a container which accommodates the second wavelength conversion unit (C). 
   The wavelength conversion laser apparatus as illustrated in  FIG. 1  comprises the fundamental wave unit (A), the first wavelength conversion unit (B) and the second wavelength conversion unit (C). The fundamental wave unit (A) comprises basic optical units such as the mirrors  1  and  2 , the pumping chamber unit  3 , the Q switch  4 , the Brewster plate  5 , the shutter  6 , the first focusing lens  7 , etc. The elements constituting the fundamental wave unit (A) are accommodated in the container  13 . Filled in the container  13  is an inert gas such as nitrogen or the like. The heaters  8   a ,  8   b ,  8   c  are suitably buried in the bottom portion  11   a  of the case  11 . The temperature sensor  9  for monitoring temperature in the container  13  is disposed near the pumping chamber unit  3 . 
   The first wavelength conversion unit (B) comprises the first nonlinear optical crystal unit  20 , the focusing lens  21  and the separation mirror  22 . The first nonlinear optical crystal unit  20  includes an LBO crystal, a KTP crystal or the like which converts the fundamental wave laser light which is focused by the focusing lens  7  and propagates through the window  14  into the second harmonic wave. The separation mirror  22  is a half mirror which separates the fundamental wave from the second harmonic wave. These optical devices are accommodated in the unit case  24  which is provided with the output window  23 . 
   On the other hand, the second wavelength conversion unit (C) comprises the second nonlinear optical crystal unit  30 , the collimation lens  31 , the separation mirror  32  and the power meter  33 . The optical devices such as the second nonlinear optical crystal unit  30 , the collimation lens  31 , the separation mirror  32  and the power meter  33  are accommodated in the unit case  35  which is provided with the output window  34 . The above described construction is the same as the conventional wavelength conversion laser apparatus. 
     FIG. 2  is a simplified cross section view of the nonlinear optical crystal cell of the embodiment of the wavelength conversion laser apparatus according to the present invention. In  FIG. 2 , the cell main body  40  corresponds to the first nonlinear optical crystal unit  20  or the second nonlinear optical crystal unit  30 . A foundation table  41  is a table on which the nonlinear optical crystal is placed. A through-hole  42  is a path through which the laser light propagates. A nonlinear optical crystal  43  is an LBO crystal or the like. A nonlinear optical crystal holder  44  is a member which holds down the nonlinear optical crystal on the table  41 . A heater  45  is heating means which keeps the cell at constant temperature. 
   An opening portion  46  is an opening in the cell main body  40 . A cell cover member  50  is a cover of the cell. A humidity sensor  51  is means which measures humidity of the cell. A hermetic seal terminal  52  is a pick-up port for a lead. A humidity detection circuit board  53  is a circuit board for a hygrometer. A recessed portion  54  is a portion in which the humidity sensor  51  is accommodated in the cell cover member  50 . A humidity detection amplifier circuit  55  as shown in  FIG. 4  is a circuit of the hygrometer. Cell windows  60   a ,  60   b  are input and output ports for the laser light. Nonlinear optical crystal cell holders  70   a ,  70   b  are members which hold down the windows  60   a ,  60   b . O-rings  80   a ,  80   b ,  80   c ,  80   d ,  80   e  are hermetic sealing members. 
   Now, the first or second nonlinear optical crystal unit  20 ,  30  is collectively referred to as a nonlinear optical crystal unit (NLU) hereinafter. The nonlinear optical crystal unit (NLU) comprises the cell main body  40  of U-shape in cross section and having the through-hole through which the laser light propagates in the horizontal direction and the cell cover member  50  which hermetically covers the opening portion  46  of the cell main body  40 . The foundation table  41  is provided at the center portion of the cell main body  40  which defines the through-hole  42 . Disposed on the foundation table  41  is the nonlinear optical crystal  43  which generates the harmonic wave. The nonlinear optical crystal  43  is secured on the cell main body  40  by the crystal holder  44 . 
   The recessed portion  54  is formed in the bottom surface of the cell cover member  50 , i.e., the surface which faces the through-hole  42 . The humidity sensor  51  is mounted inside the recessed portion  54  and at a location remote from the through-hole  42  so that the laser light is not interfered. In order to achieve electrical insulation, the humidity sensor  51  is connected through the hermetic seal terminal  52  to the humidity detection circuit board  53  which is provided outside the cell main body  40  (i.e., on the cell cover member  50 ). 
   Disposed at both left and right ends of the through-hole  42  are the cell windows  60   a ,  60   b  which hermetically seal the nonlinear optical crystal unit (NLU), the window holders  70   a ,  70   b  which secure the cell windows  60   a ,  60   b  onto the cell main body  40  by interposing the O-rings  80   a ,  80   b ,  80   c ,  80   d ,  80   e  between the cell main body  40  and the cell cover member  50 . An inert gas such as Ar, N 2  or the like is filled in the sealed unit (NLU) which has the above construction. The heater  45  is buried in the cell main body  40  so as to maintain temperature of the unit constant. 
   The O-rings  80   a ,  80   b ,  80   c ,  80   d ,  80   e  used herein are resistant to high temperature. Preferably, carlet material which is commercially available from DuPontDow Elastomer Inc. is used as the O-rings  80  because it has very low gas emission and gas permeability and yet provides a long sealing lifetime. Used as the cell windows  60   a ,  60   b  which are disposed in the propagation path of the laser light is synthetic quartz glass or CaF 2  which can resist high output laser. 
   Used as the humidity sensor  51  is an electrical capacity type humidity sensing device which is formed by vapor deposition on a glass substrate and emits no gas from the sensor portion. The humidity sensor  51  is highly durable. In order to detect humidity in the through-hole  42 , the humidity sensing device is mounted on one side surface portion of the cover member at the location which does not interrupt the through-hole  42 . Since the humidity sensor  51  is disposed in the medium which does not interrupt the propagation path of the laser light, it causes no adverse effect on the use of the laser apparatus. The humidity sensing device is connected to the humidity detection circuit board  53  which is disposed on the other side surface of the cover member  50  through the hermetic seal terminal  52  in order to electrically isolate the cell cover member  50 . 
   Now, illustrated in  FIG. 3  is a simplified cross section view of the cover member of the nonlinear optical crystal cell which constitutes the embodiment of the wavelength conversion laser crystal cell. On the other hand, illustrated in  FIG. 4  is a simplified block diagram of the humidity monitoring apparatus of the embodiment of the wavelength conversion laser apparatus according to the present invention. In  FIG. 3  and  FIG. 4 , the hygrometer  56  is means which measures humidity. A switch  57  is one which turns the laser light on or off. A laser control portion  58  is an apparatus for controlling the laser light source. The output from the humidity sensor  51  in the unit (NLU) is supplied to the hygrometer  56  by way of the humidity detection amplifier circuit  55  which is provided on the humidity detection circuit board  53 . The output of the humidity detection amplifier circuit  55  is supplied by way of the switch  57  to the laser control portion  58  which shuts off the operation of the laser apparatus. 
   Now, operation of the embodiment of the wavelength conversion laser apparatus according to the present invention which has the abovementioned construction will be described hereunder. Firstly, the outline function of the wavelength conversion laser apparatus will be described by reference to  FIG. 1 . A solid state laser medium is utilized as the laser light source. The fundamental wave laser light of 1064 nm emitted from the pumping chamber unit  3  which is the fundamental laser light is focused by the focusing lens  7  and propagates into the first nonlinear optical crystal unit  20  through the window  14 . The first nonlinear optical crystal unit  20  is an LBO crystal, a KTP crystal or the like which converts the fundamental wave laser light which is focused by the focusing lens  7  and propagates through the window  14  into the second harmonic wave, i.e., the laser light of 532 nm in wavelength. A part of the fundamental wave laser light which is an incident light to the first optical crystal unit  20  is converted into the second harmonic wave by the first optical crystal unit  20  and is outputted from the nonlinear optical crystal. The separation mirror  22  separates the fundamental wave from the second harmonic wave. 
   Furthermore, the incident fundamental wave laser light and the second harmonic wave which are propagated through the focusing lens  21  and the output window  23  are inputted to the second nonlinear optical crystal unit  30 . The second nonlinear optical crystal unit  30  converts the laser light from the first wavelength conversion unit (B) into the third harmonic wave of 355 nm in wavelength (or the fourth harmonic wave of 266 nm in wavelength). The nonlinear optical crystal converts the laser light from the first wavelength conversion unit (B) into the third or fourth harmonic wave before being outputted from the output window  34 . The separation mirror  32  separates the second harmonic wave from the third (or fourth) harmonic wave. An LBO crystal, a BBO crystal or GdYCOB crystal may be utilized as the nonlinear optical crystal which generates the third harmonic wave. On the other hand, a BBO crystal or a CLBO crystal may be utilized as the nonlinear optical crystal which generates the fourth harmonic wave. By controlling temperature of the heaters  8   a ,  8   b ,  8   c  by way of a temperature controlling apparatus (not shown) based on the output from the temperature sensor  9 , temperature inside the container is always maintained at desired value. What is described hereinabove is the same as the conventional wavelength conversion laser apparatus. 
   Now, the operation of the nonlinear optical crystal unit (NLU) will be described hereunder by reference to  FIG. 2  and  FIG. 3 . The incident laser light to the nonlinear optical crystal unit is converted into the harmonic wave and outputted from the nonlinear optical crystal. The heater  45  which is buried in the cell main body  40  is utilized to maintain temperature of the unit constant. The humidity sensor  51  detects humidity inside the through-hole  42 . It is possible to dispose the humidity sensor  51  in the cell main body  40  through the opening portion  46  in the cell main body  40 . Because the humidity sensor  51  and the humidity detection circuit board  53  are disposed on the cell cover member  50 , it is possible to mount the humidity sensor  51  at the appropriate location of the nonlinear optical crystal unit when the cover member  50  is assembled. It is also easy to inspect these devices. 
   Now, the operation of the humidity monitoring apparatus will be described by reference to  FIG. 4 . The output from the humidity sensor  51  in the unit (NLU) is supplied to the hygrometer  56  by way of the humidity detection amplifier circuit  55  which is provided in the humidity detection circuit board  53 . The output of the humidity detection amplifier circuit  55  is supplied by way of the switch  57  to the laser control portion  58  which interrupts the operation of the laser apparatus. Provision of the humidity sensor  51  in the nonlinear optical crystal unit (NLU) enables the user of the laser apparatus to monitor humidity inside the nonlinear optical crystal unit (NLU) by the hygrometer  56 . As a result, it is possible to manually (or automatically) turn off the switch  57  and to interrupt the laser oscillation when humidity exceeds a predetermined value. 
   As understood from the above description, the embodiment of the wavelength conversion laser apparatus according to the present invention features in that a dry inert gas is filled in the sealed container accommodating the nonlinear optical crystal which generates the harmonic wave of the laser light of a predetermined wavelength and that the humidity sensor which detects humidity inside the sealed container is provided and connected to the laser control portion. Accordingly, if the humidity sensor detects any humidity increase, the laser is immediately controlled to stop oscillation, thereby preventing the nonlinear optical crystal from being damaged.