Patent Publication Number: US-2009225392-A1

Title: Optical Modulator and Method of Providing the Same

Description:
TECHNICAL FIELD 
     The present invention relates generally to optics, and more particularly to an optical modulator and a method of providing the same. 
     BACKGROUND 
     A pyroelectric crystal is a crystal that experiences a pyroelectric effect due to changes in temperature. The pyroelectric effect changes the net dipole moment peer unit volume (i.e., the polarization ) of a pyroelectric material as temperature changes. The dipole moment of the unit cell is changed by thermally exciting the main donar atom in each unit cell to spatially displace itself relative to other atoms. The net result is a change in the bound surface charge along a particular crystal axis. In a pyroelectric crystal this is referred to as the pyroelectric axis, which is along the same path as the propagation path of a laser beam when the pyroelectric crystal is employed, for example, as a Q-switch optical modulator. The charge buildup on opposing end faces of the pyroelectric crystal along the pyroelectric axis builds up opposite polarity charges based on increases and/or decreases in temperature. The surface charge on the respective opposing faces can be neutralized by binding surface charge of opposite polarity to each respective opposing face employing a charge dissipating device. The charge dissipation should be done fairly uniformly over the faces of pyroelectric crystal so as to neutralize the internal field gradients within the pyroelectric crystal. 
     Q-switched laser systems typically need to operate over large temperature ranges, in spite of the fact that the electro-optical material of choice, lithium niobate, is highly pyroelectric. Significant temperature excursions with a pyroelectric Q-switch configuration typically lead to a loss in cavity holdoff performance, which is a condition known throughout the laser industry as “prelasing”. Because the pyroelectric effect is caused by a change in bulk polarization of the electro-optic material, the means of neutralizing it quickly is by covering the dielectric surface with a sufficient amount of (opposite) free charge. In the past, the most effective way of accomplishing this has been to use a radioactive substance to emit alpha particles, which then serve to ionize the air surrounding the lithium niobate crystal. The ionized air then serves as a constant source of both positive and negative airborne charges for the polarized dielectric surfaces to attract for neutralization. The use of radioactive materials, however, has fallen out of favor in the user community due to the licensing issues associated with the use of radioactive materials, the amount of paperwork, monitoring and disposal issues that follows products which contain radioactive materials. 
     There are currently several other proposed methods for neutralizing pyroelectric charge in the quest for prelaser suppression without the use of radioactive materials. All of these methods rely on the transfer of free charge onto the pyroelectric surfaces of the dielectric, which provides a means to neutralize the excess static charge resulting from any temperature change. Various sources of this free charge have been proposed, including high-voltage proximity electrodes, optically-transparent conductive coatings, and mechanically active conductive wipers. High-voltage needle electrodes placed within the proximity of the pyroelectric surface may accomplish the task of ionizing the surrounding air, but require an electrical driver. Mechanical systems have general difficulty of meeting many performance requirements and conditions associated with laser systems. An optically-transparent conductive coating laid over the pyroelectric surface would effectively neutralize the static charge, but the absorptive properties intrinsic to these conductors provide a cavity loss mechanism which can easily damage such coatings at higher optical power levels. 
     SUMMARY 
     In one aspect of the invention, an optical modulator is provided. The optical modulator comprises a Q-switch comprising a first pyroelectric crystal having opposing end faces disposed along a first pyroelectric axis, and a charge dissipating device comprising a second pyroelectric crystal having opposing end faces disposed along a second pyroelectric axis. The charge dissipating device is configured to provide pyroelectric charge from the opposing end faces of the second pyroelectric crystal to neutralize pyroelectric charge on opposing end faces of the first pyroelectric crystal. 
     In yet another aspect of the invention, an optical modulator is provided. The optical modulator comprises a Q-switch comprising a first pyroelectric crystal having opposing end faces disposed along a first pyroelectric axis, and a second pyroelectric crystal having opposing end faces with conductive surfaces disposed along a second pyroelectric axis that is orthogonal to the first pyroelectric axis. The optical modulator further comprises conductors extending from conductive surfaces of each end of each opposing end face of the second pyroelectric crystal in proximity of each end face of the first pyroelectric crystal, such that pryoelectric charge of both positive and negative polarity from the second pyroelectric crystal are provided in proximity of each opposing end face of the first pyroelectric crystal. 
     In yet a further aspect of the invention, a method is provided for providing an optical modulator. The method comprises providing a Q-switch comprising a first pyroelectric crystal having opposing end faces disposed along a first pyroelectric axis and providing a second pyroelectric crystal having opposing end faces disposed along a second pyroelectric axis. The method further comprises configuring the second pyroelectric crystal to provide pyroelectric charge from the opposing end faces of the second pyroelectric crystal to neutralize pyroelectric charge on opposing end faces of the first pyroelectric crystal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a perspective view of an optical modulator in accordance with an aspect of the present invention. 
         FIG. 2  illustrates a plan view of the optical modulator of  FIG. 1  in accordance with an aspect of the present invention. 
         FIG. 3  illustrates a cross-sectional view of the optical modulator of  FIG. 1  taken along the lines A-A in accordance with an aspect of the present invention. 
         FIG. 4  illustrates a schematic block diagram of a laser system in accordance with another aspect of the present invention. 
         FIG. 5  illustrates a methodology for providing an optical modulator in accordance with an aspect of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     An optical modulator is provided that includes a Q-switch formed of a first pyroelectric crystal and a charge dissipating device formed of a second pyroelectric crystal that is cut and oriented with its pyroelectric axis orthogonal (e.g., 90°) relative to the first pyroelectric crystal. For example, the second pyroelectric crystal can be cut and oriented with its pyroelectric axis and opposing end faces in the y-direction relative to the first pyroeletric crystal that is cut and oriented with its pyroelectric axis and opposing end faces in the z-direction. The second pyroelectric crystal is configured and oriented in a manner to provide pyroelectric charge near the opposing end faces of the first pyroelectric crystal to counterbalance the pyroelectric charge produced on the opposing end faces of the first pyroelectric crystal. Pryroelectric charge is the surface charge buildup on a pyroelectric crystal due to the pyroelectric effect. 
     In one aspect of the invention, the charge dissipating device includes conductive surfaces disposed on the opposing end faces of the second pyroelectric crystal and conductors that extend from opposing ends of each of the conductive surfaces of the opposing end faces of the second pyroelectric crystal. The conductors are configured to extend orthogonal (e.g., 90°) to the second pyroelectric axis (e.g., the x-direction) near opposing end faces of the first pyroelectric crystal. The conductors are employed to concentrate the pyroelectric field and pyroelectric charge of the second pyroelectric crystal in order to ionize the air in the proximity of the faces of the first pyroelectric crystal. The ionized air serves as a source of free charge (positively charged ions and negatively charged electrons) that the pyroelectric field of the first pyroelectric crystal can attract to the dielectric faces of the first pyroelectric crystal to neutralize the bound charge induced by the pyroelectric effect of the first pyroelectric crystal. Neutralization of the bound charge of the first pyroelectric crystal mitigates preleasing and holdoff problems when the first pyroelectric crystal is employed in a Q-switch optical modulator of a laser system. 
     The second pyroelectric crystal serves to generate enough ions in the surrounding air to neutralize the pyroelectric field of the first pyroelectric crystal despite significant shifts in system operating temperature. Furthermore, the second pyroelectrical crystal is a passive device and does not require electrical or mechanical power to produce the desired ions. Furthermore, the optical modulator of the present invention provides for a compact, passive Q-switch package that is similar in size and price to a radioactive source solution without the deleterious effects of employing radioactive materials. 
       FIG. 1  illustrates a perspective view of an optical modulator  10  in accordance with an aspect of the present invention. The optical modulator  10  comprises a Q-switch  12  formed of a first pyroelectric crystal  14  shaped into a block that is fastened into an electrically insulated housing  18 . The first pyroelectric crystal  14  is cut to have opposing end faces  16  configured to pass radiation from a laser along a propagation path that is the same as a first pyroelectric axis (e.g., z-direction) of the first pyroelectric crystal  14 . Static pyroelectric charge builds up on the end faces  16  due to the pyroelectric effect. The amount and polarity of charge on each opposing end face  16  depends on the amount of increase or decrease in the system temperature, such that the charge build up on opposing end faces is of opposite polarity to one another. The first pyroelectric crystal  14  may be formed of Lithium Niobate (LiNbO 3 ), which has a pyroelectric coefficient of −83 μC/m 2 /K. Alternatively, the first pyroelectric crystal may be formed of Lithium tantalate (LiTaO 3 ) with a pyroelectric coefficient of −176 μC/m 2 /K or barium borate (BBO) with a pyroelectric coefficient of −14.5 μC/m 2 /K, such that the charge induced per unit area per unit temperature change increases with a decrease in the pyroelectric coefficient. Other pyroelectric electro-optical material may be employed to form the Q-switch  12 , such as other tantalates, Rubidium Titanyle Phosphate (RTP) (RbTiOPO 4 ) and other titanyl phosphates. 
     The optical modulator  10  further comprises a charge dissipating device  20  disposed adjacent the Q-switch  12 . The charge dissipation device  20  is formed of a second pyroelectric crystal  22  that is cut and oriented with a second pyroelectric axis and opposing end faces  24  orthogonal (e.g., y-direction) to the first pyroelectric axis of the first pyroelectric crystal  14 . The opposing end faces  24  of the second pyroelectric crystal  22  are conductively plated to form conductive end surfaces  26 . The conductive plating can be gold or some other suitable plating material. The charge dissipating device  20  further comprises a set of four conductors  28  configured to extend orthogonal (e.g., x-direction) to the second pyroelectric axis to provide conductors from both opposing end faces  24  of the second pyroelectric conductor  22  in proximity of each opposing end face  16  of the first pyroelectric crystal  14 , such that pryoelectric charge of both positive and negative polarity from the second pyroelectric crystal are provided in proximity of each opposing end face of the first pyroelectric crystal. The conductors  28  concentrate the field strength of the conductive end surfaces of the opposing end faces  24  of the second pyroelectric crystal  22  at tips of the conductors  28  in order to concentrate a coronal discharge about the opposing end faces  16  of the first pyroelectric crystal  14 . 
     Each conductor  28  can be formed of gold plated tungsten needles, which represents a conductively-plated system that can attain extremely small point dimensions, but other alternatives to tungsten and gold may be employed. A given conductor is bonded on each end of the conductive surface of each opposing face of the second pyroelectric crystal. It is to be appreciated that a given conductor can be replaced by a plurality of conductors disposed at each end of the opposing end faces of the second pyroelectric crystal. The conductors  28  can be bonded to the conductive end surfaces, for example, by a conductive epoxy  30 . Other mechanisms to conductively bond the conductors  28  to the conductive ends surfaces  26  of the opposing end faces  24  of the second pyroelectric crystal  22  can be employed, such as a low temperature solder or other method that does not subject the conductive end surfaces  26  to highly localized heat. 
     It is to be appreciated that the charge dissipating device  20  can be oriented and configured in a variety of different ways as long as pryoelectric charge of both positive and negative polarity from the second pyroelectric crystal are provided in proximity of each opposing end face of the first pyroelectric crystal. 
       FIG. 2  illustrates a top view of the optical modulator  10  of  FIG. 1 . As illustrated in  FIG. 2 , a conductor  28  is disposed at each end of both opposing end faces  24  of the second pyroelectric crystal  22 , such that conductors of oppositely charged polarities are provided at both opposing end faces  16  of first pyroelectric crystal  14 . The oppositely charged conductors  28  can provide both positively charged ions and negatively charged electrons at each opposing end face  16  of the first pyroelectric crystal  14 . Therefore, it does not matter which of the opposing end faces of the first or second pyroelectric crystals  14  and  22  are positively charged and which of the opposing end faces of the first and second pyroelectric crystals  14  and  22  are negatively charged since the charge dissipating device  20  provides charges of both polarities at both opposing end faces  16  of the first pyroelectric crystal  14 . 
       FIG. 3  illustrates a cross-sectional view of the optical modulator of  FIG. 1  along the lines A-A. As illustrated in  FIG. 3 , the conductors  28  concentrate the pyroelectric field and pyroelectric charge of the second pyroelectric crystal  22  at end points of the conductors  28  in order to ionize the air in the proximity of the opposing end faces  16  of the first pyroelectric crystal  14 . The ionized air serves as a source of free charge that the pyroelectric field at the opposing end faces  1   6  of the first pyroelectric crystal  14  can attract to its end face surfaces to neutralize the bound charge induced by the pyroelectric effect of the first pyroelectric crystal  14 . The ionization only occurs when the ambient temperature changes since both the first and second pyroelectric crystals  14  and  22  are pyroelectric. As illustrated in  FIG. 3 , the negatively charged electrons are attracted to the end face being positively charged and the positively charged ions are attracted to the end face being negatively charged. 
       FIG. 4  illustrates a laser system  50  in accordance with an aspect of the present invention. The laser system  50  of  FIG. 4  employs the optical modulator  10  of  FIGS. 1-3 . The laser system  50  can be employed in a laser range finder, a laser designator or a variety of other laser applications. The laser system  50  includes a laser rod  54  whose axial stimulated emission passes through both the aperture of the Q-switch  12  of the optical modulator  10 , as well as a polarizer  56 . The laser system  50  also includes two prisms  52  and  62  that define the optical cavity of the laser system  50 . The Q-switch  12  of the optical modulator  10  is provided with two electrodes  58  connected to a modulating power supply  60 . The optical components are arranged on an optical axis  64 , with the electrodes  58  being arranged adjacent opposite ends of the first pyroelectric crystal of the Q-switch  12  on opposite sides of the optical axis  64 . The optical axis  64  is also the propagation path of the laser light from the laser pump and Q-switch in addition to being along the same direction as the pyroelectric axis of the first pyroelectric crystal. 
     In view of the foregoing structural and functional features described above, a methodology in accordance with various aspects of the present invention will be better appreciated with reference to  FIG. 5 . While, for purposes of simplicity of explanation, the methodologies of  FIG. 5  are shown and described as executing serially, it is to be understood and appreciated that the present invention is not limited by the illustrated order, as some aspects could, in accordance with the present invention, occur in different orders and/or concurrently with other aspects from that shown and described herein. Moreover, not all illustrated features may be required to implement a methodology in accordance with an aspect of the present invention. 
       FIG. 5  illustrates a methodology for providing an optical modulator in accordance with an aspect of the present invention. The methodology begins at  100  where a Q-switch is provided having a first pyroelectric crystal with a first pyroelectric axis. At  102 , a second pyroelectric crystal is provided with a second pyroelectric axis. At  104 , opposing end faces of the second pyroelectric crystal disposed along the second pyroelectric axis are conductively coated to provide conductive end surfaces. At  106 , conductors are coupled to opposing ends of both opposing end faces of the second pyroelectric crystal and are configured to extend orthogonal to the second pyroelectric axis. At  108 , the second pyroelectric crystal is located adjacent the Q-switch to locate a conductor from each end face of the second pyroelectric crystal in proximity with each opposing end face of the first pyroelectric crystal to form an optical modulator, such that pryoelectric charge of both positive and negative polarity from the second pyroelectric crystal are provided in proximity of each opposing end face of the first pyroelectric crystal. Furthermore, the first pyroelectric axis of the first pyroelectric crystal is aligned orthogonal to the second pyroelectric axis of the second pyroelectric crystal. At  110 , the optical modulator is provided in a laser system, such that the first pyroelectric axis of the first pyroelectric crystal is aligned along the propagation path of the laser system. 
     What have been described above are examples of the present invention. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the present invention, but one of ordinary skill in the art will recognize that many further combinations and permutations of the present invention are possible. Accordingly, the present invention is intended to embrace all such alterations, modifications and variations that fall within the scope of the appended claims.