Abstract:
A plasma lamp is provided having an integrated dielectric waveguide structure having a body, a gas housing formed within the body and having an aperture formed at a first outer surface of the body, a fill mixture disposed within the gas housing, and a probe operatively coupled to the body so that microwave energy supplied to the fill mixture forms a plasma that emits high intensity light.

Description:
RELATED U.S. APPLICATION DATA 
       [0001]    This application is a continuation application of U.S. patent application Ser. No. 09/818,092, filed Mar. 26, 2001, which claims the benefit of the following U.S. Provisional Applications: U.S. Provisional Applications Nos. 60/192,731 filed Mar. 27, 2000; 60/224,059 filed Aug. 9, 2000; 60/224,060 filed Aug. 9, 2000; 60/224,061 filed Aug. 9, 2000; 60/224,298 filed Aug. 10, 2000; 60/224,503 filed Aug. 10, 2000; 60/224,290 filed Aug. 10, 2000; 60/224,291 filed Aug. 10, 2000; 60/224,257 filed Aug. 10, 2000; 60/224,289 filed Aug. 10, 2000; 60/224,866 filed Aug. 11, 2000; 60/224,961 filed Aug. 11, 2000; 60/224,617 filed Aug. 11, 2000; and 60/234,415 filed Sep. 21, 2000; 60/241,198 filed Oct. 17, 2000; 60/246,662 filed Nov. 7, 2000; 60/253,261 filed Nov. 27, 2000; 60/254,727 filed Dec. 11, 2000; 60/262,537 filed Jan. 17, 2001; 60/262,536 filed Jan. 17, 2001; 60/262,538 filed Jan. 17, 2001; 60/265,945 filed Feb. 1, 2001 and 60/270,857 filed Feb. 21, 2001. 
         [0002]    Each of U.S. Provisional Application Nos. 60/192,731 filed Mar. 27, 2000; 60/224,059 filed Aug. 9, 2000; 60/224,298 filed Aug. 10, 2000; 60/224,290 filed Aug. 10, 2000; 60/224,291 filed Aug. 10, 2000; 60/224,257 filed Aug. 10, 2000; 60/224,289 filed Aug. 10, 2000; 60/224,866 filed Aug. 11, 2000; and 60/234,415 filed Sep. 21, 2000 are hereby incorporated by reference in their entireties. 
     
    
     FIELD OF THE INVENTION 
       [0003]    The present invention is directed generally to high intensity light sources and more particularly to plasma light sources. 
       BACKGROUND OF THE INVENTION 
       [0004]    There is a continuing need for long-lived, efficient, compact, and high intensity white light sources for applications such as projection-based televisions, optical fiber light sources, and general lighting sources, as well as a need for ultra violet light sources for water treatment, spot curing of organic materials, and other specialized applications. The various kinds of light sources which have been used previously include short arc lamps, arc lamps, high intensity discharge bulbs, Halogen bulbs and electrode-less plasma bulbs. Although a short arc lamp produces an intense light by maintaining an electric arc between two closely spaced electrodes, short arc lamps have not tended to be long-lived for at least two reasons. First, the electrodes between which the arc is formed inevitably deteriorate and erode during operation, and ultimately this erosion leads to bulb failure as the bulb approaches and finally exceeds half brightness, an industry standard for the end of life of the bulb. Second, short arc lamps conventionally employ an envelope or bulb made from a transparent material in order to contain the gas fill of the lamp. Quartz has conventionally been used for such bulbs. 
         [0005]    Quartz bulbs, however, have several disadvantages that materially affect the life of the bulb. Because quartz devitrifies at elevated temperatures, particularly when exposed to moisture, oils from finger prints, and other contaminates that are ever present in air pollution, quartz bulbs do not endure well when exposed to repeated heating and cooling inherent in lamp operation. The result of exposure to these elements is that they tend to eventually discolor or crack causing lamp failure and limiting the useful life span of the lamp. In addition, because quartz has a low thermal conductivity, the use of the quartz bulb limits the maximum operating temperature of the lamp, and, therefore, the maximum obtainable brightness from the short arc created between the electrodes. Furthermore, quartz is partially permeable so that certain gases contained within the bulb gas mixture tend to slowly diffuse out of the bulb envelope during operation of the bulb, and other gases tend to slowly diffuse into the bulb during cool-down, thus diluting the gas constituents and changing the bulb operating properties. Ultimately, this diffusion causes the lamp to fail. 
         [0006]    Unlike arc lamps, RF plasma lamps do not rely on electrodes, but rather produce light by creating a plasma discharge in a gas contained in a bulb by exposing the lamp gas to intense radio wave or radio frequency radiation. (As used herein, the phrase “radio frequency radiation”, as well as the acronym “RF”, is intended to encompass electromagnetic radiation frequencies in either the conventional radio frequency range or in the conventional microwave frequency range.) Although there are no electrodes to fail in the case of a plasma lamp, the transparent bulb that is conventionally used to contain the gas is also typically made of quartz and has similar disadvantages discussed above in connection with the arc lamp because of the high operating temperatures involved. 
         [0007]    In order to mitigate the bulb failure problem, various mechanical cooling arrangements have been developed, e.g., to rotate the bulb and/or propel cooling air onto the outer surface of the bulb during lamp operation. However, such mechanical arrangements are complex, expensive, have moving parts that fail, and occupy space which is often a scarce resource in the intended application for the lamp. In addition, the presence of these mechanical arrangements tends to compromise the ability to collect the light generated by the lamp, thereby reducing efficiency. 
         [0008]    RF plasma lamps also conventionally require a separate mechanism to couple the radio wave radiation generated by the radiation source to the bulb filled with the plasma discharge-forming medium. The need for such a separate coupling mechanism is another problem with the RF plasma lamp because inefficiency of the coupling correspondingly constrains the overall efficiency of the plasma lamp itself. One conventional approach to such coupling is to mount the bulb within or near a separate air-filled RF structure, such as a waveguide, that receives the radio wave radiation from the radiation source and transmits the radiation into the bulb. In practice this approach may lead to a power loss of as high as 60% because of coupling inefficiencies. In addition, the resulting lamp structure is not physically compact because the RF structure is separate from the bulb. 
         [0009]    Alternatively, it is known to mount the quartz bulb inside a separate structure and to place coils near to the bulb in order to inductively transfer radio wave radiation energy to the gas in the bulb. Again, however, the resulting structure lacks integration and compactness because the RF structure is separate from the bulb. 
         [0010]    It is desirable to provide improved light sources that avoid these and other known problems, and it is to these ends that the present invention is directed. 
       SUMMARY OF THE INVENTION 
       [0011]    According to one aspect of the invention, a plasma lamp is provided that comprises a gas housing containing a plasma discharge forming medium, and a source of radio frequency energy coupled to the plasma discharge medium forming usable light. The gas housing is constructed from one or more suitable ceramic materials and has a window transparent to visible light. 
         [0012]    In more specific aspects, the window may be a sapphire window. The invention greatly extends the operating life expectancy of the plasma lamp as compared with the prior art lamps which use quartz because the problems of quartz devitrification at high temperature and quartz gas permeability are eliminated. 
         [0013]    According to another aspect of the present invention, the RF structure used for the radio wave radiation and the envelope used to house the gas fill are formed so as to constitute a single, integrated ceramic structure. 
         [0014]    According to another aspect of the present invention, solid material such as ceramic rather than air is used for the dielectric and the gas fill is contained by a combination of solid ceramic and a sapphire window. In this way the separate gas envelope and air-filled waveguide structure employed in the prior art are replaced by a single, integrated structure. 
         [0015]    Because the integration of the RF structure and the gas envelope permits the quartz bulb to be done away with entirely, plasma lamps according to the present invention enjoy an unprecedented operating life expectancy as compared with the prior art. This is so in part because the problems associated with the inability of the quartz bulb to withstand repeated heating and cooling are eliminated. 
         [0016]    In addition, the integrated design of the present invention enables a much higher proportion of the radio wave radiation energy to be focused onto the gas fill. As a result, the plasma lamp according to the present invention is made much more efficient. 
         [0017]    The present invention enables these and many other benefits to be obtained. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0018]      FIG. 1  is a side cross-sectional view of a gas housing for a plasma lamp according to a first embodiment of the invention. 
           [0019]      FIG. 2  is a side cross-sectional view of a plasma lamp according to a second embodiment of the invention. 
           [0020]      FIG. 3  is a side cross-sectional view of a plasma lamp according to a third embodiment of the invention in which the gas housing is integral with a waveguide comprising a solid dielectric material. 
           [0021]      FIG. 4A  is an end view of a plasma lamp according to a fourth embodiment of the invention in which the gas housing is integral with a waveguide comprising a solid dielectric material while  FIG. 4B  is a side cross-sectional view of the same plasma lamp. 
           [0022]      FIG. 5  is a side cross-sectional view of a plasma lamp according to a fifth embodiment of the invention in which the gas housing is also integral with a waveguide comprising a solid dielectric material. 
           [0023]      FIG. 6  shows a process suitable for sealing a gas housing according to the present invention. 
           [0024]      FIG. 7  is a side cross-sectional view of an alternative embodiment of the plasma lamp of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0025]      FIG. 1  shows a first embodiment of an improved light source in accordance with the invention. The light source may be a plasma lamp comprising a gas housing  20  preferably formed from a ceramic material  22 , as will be described below, with an interior cavity or chamber  24  for containing gas. The housing may generally be rectilinear or cubic, and the chamber may be spherical. A channel  30  may connect the chamber to an exterior surface  32  of the housing. The channel  30  may be made of light transmissive material, preferably of sapphire in order to form a window  34  for emitting visible light from the chamber. The window preferably has a generally tapered, conical shape; i.e., a frusto-conical shape. The sapphire window seals the chamber to contain the gas, while affording an exit for the light produced by the plasma discharge. 
         [0026]    Sapphire is preferred for the window since it is less gas permeable than quartz, for example, and better withstands the heat cyclings and high temperatures associated with lamp operation. Furthermore, the gas housing  20  is preferably made from a ceramic material, as described below, since ceramics are much more durable under heating than other materials such as quartz. As a result, the ceramic housing affords a much longer life expectancy for the plasma lamp than the conventional quartz bulb of the prior art. In addition, the ceramic housing advantageously enables the plasma lamp to be operated at a much higher maximum temperature than the quartz bulb, because it avoids the lower softening temperature point and low thermal conductivity limitations of quartz. 
         [0027]    The sapphire window  34  may function as a “light integrator” for transmitting the light of the plasma lamp from the chamber, for example, to application-specific optics. The tapered, conical sapphire window  34  may be sealed against the surrounding ceramic material forming the channel  30  by coating the outside edges of the sapphire window with a material such as a glass containing MgO, or, alternatively, with SiO 3  or SiO 2 . Next the mating surfaces of both the window and the ceramic channel may each be coated with a thin layer of metallic material, such as copper, a copper alloy, or platinum. Then a piece of preferably pure platinum wire may be placed between the two thin film layers. Finally, a laser is used to heat the wire, and thereby melt the metallic material and bond the layers together. 
         [0028]    Alternatively, the coated sapphire window  34  may be sealed to the ceramic housing by heating a glass frit. In yet another alternative, the ceramic housing may be “shrunk down” onto the sapphire window during high temperature firing. 
         [0029]    The gas fill in the plasma lamp according to the first embodiment of the invention can be coupled to a source of electromagnetic energy, such as radio wave radiation in any of a variety of ways in order to create a plasma discharge within chamber  24 . Preferably this should be done so that the RF structure that is active with the radio wave radiation energy is integrated with the gas housing  20 , as will be described. 
         [0030]    The gas fill may appropriately be a combination of a metal compound and a carrier gas. The metal compound may preferably be a metal halide such as indium bromide. Other examples of suitable metal compounds are praseodymium and mercury. Preferred gases for the carrier gas are xenon, neon, argon, or krypton. 
         [0031]      FIG. 2  shows a second embodiment of a lamp in accordance with the invention which is somewhat similar to  FIG. 1  except that the gas housing has an integrated RF energy structure. In  FIG. 2 , the elements are designated similarly to  FIG. 1 , using like reference numerals for like elements. The gas fill chamber  24  may be housed in a gas housing  20  preferably comprising a ceramic material  22  and provided with a light transmissive window  34 , preferably of a tapered rod of sapphire and a fill plug  38  as previously described. In this embodiment, an RF energy structure such as one or more coils  36  may be formed within the ceramic housing. The coils  36  function to inductively couple radio wave radiation energy to the gas fill in chamber  24  in order to create the plasma discharge. In this way, the RF structure of the plasma lamp that is active with radio wave energy is integral with the ceramic housing  20  that contains the plasma gas fill. This integration of the RF structure of the plasma lamp and the gas housing into a single structure, as shown, improves the coupling of RF energy to the gas, and allows significant gains in lamp efficiency and compactness. 
         [0032]    The second embodiment may also comprise segments of ferrite material  41  placed adjacent the coils  36  in order to help concentrate the magnetic field associated with the coils  36  on the gas fill. An illustration of this embodiment is shown in  FIG. 7 . 
         [0033]      FIG. 3  shows a third embodiment of a lamp in accordance with the invention which integrates both the gas housing and an RF energy source within the same structure. A gas housing  50  for the gas fill may be formed so as to be integral with a waveguide  52  which preferably comprises a ceramic structure having a substantially rectangular cross-section. Because no separate bulb is used, the housing  50  and waveguide  52  comprise a single, integrated structure. A source of radio wave radiation  54  may be disposed within the ceramic structure, for example, near one end of the waveguide. The RF source  54  may be an RF antenna, a probe, or the like for introducing RF energy into the waveguide. The gas housing  50  may be located near the other end of the waveguide, for example. As shown, the gas housing may further include a light transmissive window  56  connected to the end wall of the housing. The window is preferably made from sapphire. 
         [0034]    The dimensions of the waveguide and the locations of the RF source and gas housing preferably are chosen so that the electromagnetic field produced by the radio wave radiation in the waveguide exhibits a maximum in intensity at or near to the location of the housing in order to optimize the energy coupling to the gas. The waveguide may form a resonant structure having a resonant mode at the frequency of the radiation from the RF source  54 . The necessary relationship among the waveguide dimensions, dielectric constant, and RF frequency can be determined in a well-known way using electromagnetic waveguide theory. For example, it is well-known that for a rectangular waveguide cavity containing a dielectric with permeability and permittivity constants μ and ε, and having width, depth and length with dimensions a, b, and d respectively, and metal boundaries, the frequencies w(m,n,p) for the resonant modes are given by the following equation: 
         [0000]        w ( m,n,p )=(2π) −1 (με) −1/2 ( m   2 π 2   /a   2   +n   2 π 2   /b   2   +p   2 π 2   /d   2 ) −1/2    
         [0000]    where m, n, and p are integers. 
         [0035]    Furthermore, because the dimensions of the waveguide scale with the square root of the dielectric constant of the dielectric, use of a solid dielectric material instead of an air dielectric permits a dramatic reduction in waveguide size, particularly if a ceramic material with an appropriately high dielectric constant is chosen. The waveguide is preferably made from a solid ceramic material with a high dielectric constant (higher than air or greater than 1), such as titanium dioxide (TiO 2 ) or barium neodymium titinate. In practice, it is found that materials that exhibit a suitably high dielectric constant are typically porous and unable to provide the required hermicity to contain the gas fill. Accordingly, as shown in  FIG. 3 , a liner  58  of a better hermetic ceramic, such as alumina (Al 2 O 3 ), is preferably deposited along the inner boundary of the ceramic material that forms the gas housing. This liner  58  improves the sealing of the gas fill. 
         [0036]      FIGS. 4A and 4B  show a fourth embodiment of a light source in accordance with the invention. A gas housing  60  for the gas fill is formed so as to be integral with a cylindrical resonant waveguide structure  62  comprising ceramic material. Because a separate bulb is not used, the gas housing  60  and waveguide  62  comprise a single, integrated structure. A source of radio wave radiation  64  may be disposed near one end of the waveguide, while the gas housing is formed at an opposite end. The gas housing  60  may include a window  66  preferably made from sapphire. 
         [0037]    As with the embodiment of  FIG. 3 , the dimensions of the waveguide structure, the locations of the RF source and gas housing, and the frequency of the radio wave radiation source may be chosen so as to support resonant modes which optimize the RF energy coupling from the RF source to the gas housing. The gas housing  60  may, therefore, be appropriately located so that the housing receives a high level of radio wave radiation energy from the source  64 . 
         [0038]      FIG. 5  shows a fifth embodiment of the present invention. In this case the waveguide  72  may have a cross-section with a varying dimension, such as a varying profile rather than a rectangular cross-section in order to improve the matching of the impedance of the waveguide to that of a gas housing  70  in the waveguide. In turn, this improved impedance matching broadens somewhat the range of frequencies over which the waveguide forms a resonant structure so as to efficiently deliver power to the gas housing. As with the first embodiment, however, a separate bulb is not used so that the gas housing  70 , waveguide  72 , and radio wave radiation source  74  comprise a single, integrated structure. The dimensions of the waveguide and the locations of the radio wave radiation source and housing, may appropriately be chosen to produce a resonant mode that maximizes the energy coupled from the source to the gas housing for the operating frequency band of the source. 
         [0039]    In other embodiments of the invention, the interior of the gas housing may be coated with a thin film of protective material such as MgO. The MgO will protect the inner surface of the gas housing from the spontaneous conversion of ceramic to elemental metal that sometimes occurs in the presence of a partial vacuum and high temperature. This effect is not desirable and may cause failure of the bulb. Because the film of MgO acts as a secondary electron emitter, the film can also add to the brightness of the plasma lamp. 
         [0040]    In alternative embodiments of the invention, a bulb made from quartz or another suitable material may be retained as a structure which houses the gas fill, but the quartz structure is sized so as to fill the interior space in the ceramic gas housing, which ceramic gas housing may be integrated into a ceramic waveguide as described above. This variation can be utilized in conjunction with any of the embodiments of the invention shown in  FIGS. 1-5  by expanding the bulb into the interior of the ceramic gas housing with a heating process. One possible heating process is to electrically overdrive the bulb. Alternatively, the outer surface of the quartz bulb may be ground so as to fit closely into the ceramic gas housing or integrated ceramic gas housing and waveguide structure. 
         [0041]    An example of a waveguide structure according to these alternative embodiments is a rectangular waveguide structure having dimensions of a=34.72 mm, b=17.37 mm and d=48.84 mm and composed of alumina (Al 2 O 3 ) ceramic. For such a waveguide, the RF structure, e.g., antenna, may appropriately be driven at a frequency of 2.4 gigahertz (GHz) in order to efficiently couple radio wave radiation of that frequency to the gas fill in the quartz bulb within the waveguide. 
         [0042]    When the plasma lamp is constructed in such a way, the heat produced by the bulb operated in the normal drive mode will be dissipated more uniformly and rapidly than in the prior art because of the tight fit between the quartz bulb and the surrounding ceramic. In this way the ceramic encasing the quartz bulb acts as a heat sink and ameliorates the problems associated with the heating of a quartz material. 
         [0043]    These alternative embodiments having a quartz bulb can be improved by depositing a thin, non-conductive reflective coating on either the inside or outside walls of the quartz bulb. The reflective coating can be deposited by evaporation, spraying, painting or other method and should cover the bulb apart from an “exit” window for the light. The material used may be liquid bright platinum or a similar reflective material. The function of the coating is to improve upon the reflectance of the ceramic and thereby increase the brightness yielded by the lamp. 
         [0044]    In other embodiments of the invention, the bulb for containing the gas fill may be made entirely from sapphire rather than quartz. Sapphire is transparent to visible light and can better withstand high temperatures than quartz. Sapphire is also less permeable than quartz. Accordingly, the use of sapphire for the bulb can significantly improve the performance of the plasma lamp as compared with the prior art quartz bulb lamp. 
         [0045]    A method for constructing a representative embodiment of the ceramic gas housing for the fill gas of the plasma lamp will now be described with reference to  FIG. 6 . The first step in this method is to fabricate the housing  80  as by pressing ceramic into a mold. A small fill hole  40  may be left in one end of the housing. A sapphire window  84  is then sealed to the other end of the housing. The ceramic housing may then be placed in a vacuum chamber. An appropriate metal halide material may then be put into the enclosure through the fill hole  40 . Next, the vacuum chamber can be pumped down. After the proper subatmospheric pressure is reached, the chamber can then be backfilled with an excitation gas. 
         [0046]    The excitation gas is allowed to backfill until the chamber and, hence, the ceramic housing reaches the desired pressure. A ceramic plug  85  may then be used to seal the fill hole in a manner discussed more fully below in connection with  FIG. 6 . After the fill hole is sealed in such a manner, the lamp is then removed from the vacuum system and tested. 
         [0047]      FIG. 6  illustrates an improved sealing procedure that is useful for making plasma lamp gas housings according to the present invention. In particular, it has been found that a tapered fill hole  40  and a matchingly tapered plug  85  provide a stronger seal than a straight-edged fill hole and matching plug. The actual seal between the hole and the plug is made with a glass frit or a ceramic material  82 . The seal is formed by suitably heating the fill hole region such as by using laser light  86 . The use of laser light is advantageous because it allows the sealing process to be conveniently accomplished while the plasma gas housing is still in the vacuum chamber immediately after the fill material has been added. Furthermore, lasers are especially well suited for this application which requires the quick heating of a small region to a high temperature. 
         [0048]    The scope of the present invention is meant to be that set forth in the claims that follow and equivalents thereof, and is not limited to any of the specific embodiments described above.