Dielectric multilayer reflector, and laser system based thereon
BACKGROUND OF THE INVENTION 
     The present invention relates to laser technology, and more particularly to a dielectric multilayer reflector and a laser system using such reflectors. 
     As resonators for a laser system, fully reflective mirrors, which completely reflect laser beams, and output reflective mirrors, which selectively take out a part of laser beams, are commonly used. These laser mirrors are given their desired reflectivity or transmissivity by coating multiple dielectric layers over a glass or quartz substrate. A dielectric multilayer reflector is usually produced by alternately vapor-depositing films of high and low refractive indices, each layer having a thickness of λ/4, and the materials to be vapor-deposited, design of the number of layers and vapor-depositing method are well known to those skilled in the art (for instance the U.S. Pat. No. 4,099,840). Since a reflector of this kind, including both its substrate and coated films, consists of insulative materials, its film surfaces have high resistivities both thereupon and in the vertical direction thereof, and therefore are readily electrically chargeable. 
     If a reflector coated in this manner is used in the path of a laser beam, the part of the reflector irradiated by the beam will be electrically charged in proportion to the duration of irradiation. The charged part will gradually attract dust and vapor present nearby or in the air, and therefore become unable to maintain its prescribed reflectivity or transmissivity. At the same time, the dust and vapor which have adhered to the reflector will absorb the laser beam and eventually destroy the vapor-deposited films. This phenomenon is particularly conspicuous where the reflector is used as a resonator because of the greatness of the laser power within the resonator. 
     To make up for this disadvantage, it has been necessary, where a laser mirror is used, to isolate either the laser-irradiated part or the whole surface of the mirror from the external air and to keep it dry, as well as to fill the path of the laser beam with dust-free clean air. However, even if this precaution is taken, it is impossible to perfectly shut out dust and water vapor, and therefore the laser mirror has to be cleaned from time to time while in use. 
     SUMMARY OF THE INVENTION 
     An object of the present invention, therefore, is to obviate the aforementioned disadvantage and to provide a laser mirror so structured as not to be electrically charged even when irradiated with a laser beam, together with a laser system using such laser mirrors, whose laser power does not weaken with the lapse of time. 
     According to the invention, there is provided a laser mirror comprising a transparent substrate, dielectric layers formed over the surface of said transparent substrate and a conductive layer formed over the top surface of the dielectric layers, wherein a laser beam is incident on the conductive layer. 
     A laser system according to the invention uses dielectric multilayer reflectors of the above described composition as laser resonators. 
     The conductive outermost coated layer does not remain locally charged, and uniformly distributes the electric charge. Further, by grounding the conductive layer, the charge can be caused to leave the reflectors. Accordingly, the surface potentials of the reflectors can be reduced to zero. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and advantages of the present invention will be more apparent from the detailed description hereunder taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 shows a cross section of a preferred embodiment of the laser mirror according to the invention; 
     FIG. 2 is a block diagram showing a schematic view of a preferred embodiment of the laser system according to the invention; 
     FIG. 3 illustrates a second embodiment of the invention in which both the faces of the substrate are coated with dielectric films and a conductive layer. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a reflector consists of a glass substrate 20, dielectric multilayer films 30 formed over the substrate 20, and a conductive film or layer 40 formed over the uppermost of the dielectric multilayer films 30. The multilayer films consist of 17 dielectric films or layers, which are high refractive index layers 31 and low refractive index layers 32 vapor-deposited one over another. The bottom and top are high refractive index layers. The number of layers is determined according to the desired reflectivity. In this preferred embodiment, the high refractive index layers 31 are made of titanium oxide (TiO 2 ), whose refractive index is 2.25, and the low refractive index layers 32, of silicon oxide (SiO 2 ), whose refractive index is 1.45. The method of their vapor deposition is described, for instance, in the U.S. Pat. No. 4,099,840 referred to earlier. 
     The conductive layer 40 is formed after the dielectric multilayer films 30 have been formed and their spectrotransmissivity has been measured. To form this layer, first ITO (a mixture of indium oxide [In 2  O 3  ] and tin [Sn]), which is to be vapor-deposited, is put in a melting crucible, to whose holder is attached a monitor glass for controlling the film thickness, and the glass substrate over which the dielectric multilayer films are formed is fitted to the holder. Then the substrate is heated to 100° C. with a heater, and the inside of the vapor-depositing apparatus is evacuated to an air pressure of about 1×10 -5  Torr. The ITO which is to be vapor-deposited is evaporated by raising its temperature to about 1000° C. with an electron beam, and the conductive layer 40 is thereby deposited on the top surface of the dielectric multilayer films. 
     The conductivity of the conductive layer 40 is about 1 KΩ/cm 2 , and its thickness is not greater than 1/4 of the wavelength of the laser beam to be used. This thickness is selected to be thin enough so as not to have adverse effect on reflectivity and transmissivity of the reflector. The laser beam passes the conductive layer 40, and is either reflected or transmitted by the dielectric multilayer films 30. Since this reflector has a conductive film over the top surface of the dielectric films, its irradiated face will never remain locally charged even if it is placed in the laser beam path, but the charge is uniformly distributed by the conductive layer 40, and the surface potential on the reflector is reduced. 
     As the laser beam-irradiated part of the reflector does not remain charged, it will attract no dust or water vapor in the air. Further, the reflection or transmission by the irradiated part is reduced to eliminate the danger of destruction by the absorption of the laser beam. 
     If the conductive layer 40 is grounded through a grounding feeler 50, the potential of the conductive coating layer 40 will be always kept at zero, the reflector will never be charged. 
     Whereas the conductivity of the conductive layer 40 is selected at 1 KΩ/cm 2  for this preferred embodiment, this level of conductivity is sufficient to disperse the electrons gathering on the top surface of the dielectric multilayer films 30. 
     If both faces of the substrate 20 are coated with dielectric multilayer films 30, it is desirable to coat the top surface of the layers 30 on each side with a conductive layer 40 as illustrated in FIG. 3. 
     FIG. 2 illustrates a Q-switched solid state laser system using a reflector which has a conductive film formed over the top surface of dielectric multilayer films. 
     In the figure, a solid state laser material 1, consisting of YAG crystal rods, is optically-pumped by a flash lamp 2. The laser material 1 and the flash lamp 2 are housed in a cavity 3, which is so shaped as to enable a light beam from the flash lamp 2 to be efficiently focused on the laser material 1. The electrodes of the flash lamp 2 are connected to a high power source 7 to be applied with a high D.C. voltage, and a thin wire is wound round the lamp over its full length. A trigger voltage supplied from a trigger circuit 8 to this wire causes the flash lamp 2 to emit light. 
     On the axis of the laser beam are arranged reflectors 4 and 5 constituting resonators, a Pockels cell 6 and a polarizing element 60, and the laser beam is outputted from the reflector 4. A driving circuit 9, driving the Pockels cell 6, generates repetitive two-level pulse signals. When the signal level is zero, the loss of the Pockels cell 6 with respect to a light beam polarized by the polarizing element 60 in a specific direction is virtually eliminated, and giant pulses are generated from the laser material which has been pumped so far. The reflectors used in this laser system are multilayer reflectors having conductive films of the kind described earlier, and the reflector 5 is a totally reflective mirror. 
     In this laser system, the optical energy over the surfaces of the reflectors 4 and 5 is greater in an over two-digit order than that outside the resonators, but the presence of conductive films over the reflector surfaces prevents the reflectivity of the reflectors from being deteriorated by aging, and thereby makes it possible to provide a stable laser output. 
     Although a Pockels cell is used as Q-switching element in the circuitry of FIG. 2, a Kerr cell or a Faraday cell may be substituted therefor.