Abstract:
A laser system having an acoustic stimulator and amplifier section adjacent to the acoustic stimulator is disclosed. The stimulator is configured to apply acoustic energy to the amplifier section whereby luminescent output is produced in the amplifier section. This luminescent output may be concentrated to form a high intensity light output.

Description:
FIELD 
     The disclosure relates to a lasers, and more particularly to lasers using the principle of sonoluminescence. 
     BACKGROUND 
     High-powered lasers are an important tool in both the manufacturing and defense fields, having a variety of applications in each field. Such applications may include cutting and welding in the equipment manufacturing fields and in directed energy weapons in the defense field. 
     In the defense field, for example, high-powered lasers have been adapted to be directed against ballistic missiles. The success of the Boeing YAL-1 as a missile defense system has demonstrated that high-powered lasers may provide an effective defense against hostile, incoming ballistic missiles. 
     In the manufacturing industry, high powered lasers may improve manufacturing efficiency by reducing the time required to cut through an object, weld parts together or otherwise work a piece of material. 
     One type of high-power laser that is especially effective is a chemical laser, such as the COIL (Chemical Oxygen-Iodine Laser) or AGIL (All Gas-phase Iodine Laser), capable of producing relatively high power (potentially in the megawatt range) in the infrared spectrum. However, such lasers consume and produce a number of potentially toxic and hazardous chemicals and gases, including chlorine, iodine, hydrogen peroxide, potassium hydroxide, hydrazoic acid, and nitrogen trichloride. Because of their hazardous and toxic nature, such chemical lasers must be carefully contained. 
     There exists in the art a need for an environmentally friendly and non-toxic system capable of producing a high power laser. 
     Sonoluminescence is a phenomenon whereby a high-frequency oscillating pressure wave is applied to a liquid medium to generate gas-filled bubbles that expand and catastrophically collapse. As the bubbles collapse, the energy stored in the bubbles is released as electromagnetic energy. The released electromagnetic energy typically is in the form of visible light emitted in a spectrum that may be similar to black body radiation. The individual power of the emitted light may be low, on the order of a few watts per square centimeter. 
     A number of experiments (e.g., “Single Bubble Sonoluminescence from Noble Gas Mixtures,” J. da Graça and H. Kojima, Phys. Rev. E66, 006301) have been conducted involving single-bubble sonoluminescence through the use of standing waves to produce static regions where local pressure transitions between high and low values corresponding to the amplitude of the fluctuating pressure wave. As the local pressure oscillates between low and high values, the size of the bubbles will increase and decrease. These experiments have shown that at high pressures and frequencies these single bubbles collapse to provide a regular pulse of sonoluminescent light lasting for approximately 40-50 picoseconds (ps). The deviation between these pulses is accurate to within approximately 50 ps, providing a clock-like synchronicity. 
     Various color spikes within the sonoluminescence spectrum are present depending on the gas within the bubble. These spikes may color the output of the sonoluminescent reaction to anywhere within the visible light spectrum. Further, as the bubble collapses, the temperature and pressure inside the bubble increase dramatically, which may result in a variety of chemical reactions that may change the profile of the gas within the bubble, causing color differentiation. Noble gases, such as argon, neon, xenon and the like, may be used to control the output color of light produced by sonoluminescence and reduce the chance of chemical reaction between the gas and surrounding liquid. 
     SUMMARY 
     According to one aspect, a laser system may include an acoustic stimulator and a liquid-filled chamber operatively connected to the acoustic stimulator. The acoustic stimulator may apply acoustic energy to the liquid to stimulate luminescent output within the liquid. 
     The liquid may be water, dodecane or ethylene glycol and the may include a gas, such as a noble gas, dissolved in the liquid. 
     According to further variations of this first aspect, reflective surfaces, such as mirrors, may be disposed within the chamber to form a reflective path along which stimulated light is reflected and intensified. A partially reflective and transmissive surface, such as a partial mirror, may be disposed in the reflective path to transmit a portion of the luminescent output from the chamber as a laser. 
     According to a second aspect, a method of producing an amplified output from a laser input is disclosed. A chamber in communication with an acoustic stimulator may be provided, the chamber having a liquid contained therein. A laser is projected through the chamber and may be reflected in a path to repeatedly project the laser through the chamber. Acoustic energy is applied to the chamber thereby causing bubbles to form therein. The bubbles are stimulated to collapse and amplify the laser. A portion of the laser is projected out of said path, thereby forming a laser output. 
     The liquid within the chamber may contain a dissolved gas, such as an inert gas, that forms the bubbles when stimulated at an acoustic frequency. The acoustic frequency may, for example, be a resonant frequency of the chamber that causes the bubbles to collapse. Further, the laser may pass through one of the bubbles during collapse, causing the electromagnetic output to amplify the laser. 
     According to a second aspect of the invention, a method of creating stimulated emission of radiation is disclosed. The method includes the steps of applying acoustic energy to a liquid-filled chamber to produce bubbles, that collapse and produce luminescence, reflecting the light from reflective surfaces in the liquid-filled chamber and projecting a portion of the light out of the liquid chamber as a laser. 
     According to one variation, the liquid chamber may be stimulated at a predetermined acoustic frequency and the length of the reflective path between the reflective surfaces is selected to be inversely proportional to this frequency. 
     According to a third aspect, a laser welding device is disclosed. The laser welding device may include a chamber containing a liquid and a gas dissolved in the liquid, an acoustic stimulator for stimulating the liquid chamber, at least one mirror within the chamber for reflecting light along a path, and a partial mirror within the chamber for reflecting a portion of the light along said path and transmitting a portion of the light to form a laser. The path directs light from sonoluminescence through bubbles formed by stimulating the liquid chamber at an acoustic frequency to intensity the light. 
     According to a fourth aspect, an aircraft with a laser is disclosed. The aircraft laser device may include a liquid chamber having a liquid and dissolved gas, an acoustic stimulator for stimulating the liquid chamber, at least one mirror within the chamber for reflecting light along a path, and a partial mirror within the chamber for reflecting a portion of the light along said path and transmitting a portion of the light to form a laser. The path directs light from sonoluminescence through bubbles formed by stimulating the liquid chamber at an acoustic frequency to intensity the light. 
     According to a fifth aspect, a method for producing a high intensity light output is disclosed. This high intensity light output is produced by providing a chamber having a liquid and dissolved gas and a plurality of mirrors forming a reflective path; providing a stimulator in communication with the chamber; stimulating the chamber at an acoustic frequency to produce sonoluminescence within the chamber; reflecting light from the sonoluminescence about the reflective path; passing the light through another bubble as sonoluminescence occurs to intensify the light; and outputting from the chamber a portion of the light through a partial mirror in the reflective path. 
     The features, functions and advantages that have been discussed can be achieved independently in various embodiments of the present invention or may be combined in yet other embodiments, further details of which can be seen with reference to the following description and drawings. 
    
    
     
       DRAWINGS 
         FIG. 1  is a side view of an apparatus for producing a sonoluminescent laser. 
         FIG. 2  is a side view of a single amplifier section. 
         FIG. 3  is a graph illustrating various characteristics of the apparatus. 
         FIG. 4  is a graph illustrating various sonoluminescent spectral profiles. 
         FIG. 5  is a graph illustrating the absorption coefficient of water. 
         FIG. 6  is a graph illustrating the relationship between acoustic pressure, bubble radius and sonoluminescent pulses. 
         FIG. 7  is a graph illustrating sonoluminescent visible flux as a function of temperature for various fluids. 
     
    
    
     DESCRIPTION 
     The high degree of precision and predictability in single-bubble sonoluminescence lends itself to concentration by means of a repeating light wave as illustrated in  FIG. 1 , for example for use as a laser. As shown in  FIG. 1 , the laser  100  may include a liquid container  102  containing a plurality of amplifier sections  112  (shown in  FIG. 1  as  112 A-L, generally designated  112 ) and a plurality of pressure actuators  104  (shown in  FIG. 1  as  104 A-L, generally designated  104 ) that are capable of delivering a pressure wave (or acoustic wave) at a specified frequency to stimulate each of the amplifier sections  112  at a frequency. A limited number of amplifier sections  112  are shown in  FIG. 1  for purposes of scale. The liquid container  102  may be approximately 50 m long and the amplifier sections  112  may be approximately 20-30 cm wide. 
     Within the liquid container  102  may be a number of reflective surfaces (such as full mirrors)  106 A-D that reflect incident light and a partial mirror  108  that may reflect half of incident light and transmit the remainder. As shown in  FIG. 1 , mirrors  106 A-C and partial mirror  108  are positioned at approximately 90° angles to one another to reflect light travelling in a substantially clockwise direction in a semi-continuous path  110 , with a portion of the light being transmitted through the partial mirror  108 . A fourth full mirror  106 D is positioned to reflect light travelling in a substantially counter-clockwise direction to a clockwise direction. This mirror  106 D is shown positioned outside of the path  110  formed by the three full mirrors  106 A-C and partial mirror  108  such that counterclockwise travelling light reflected to a clockwise direction would be partially reflected and partially transmitted by the partial mirror  108 . 
     A single amplifier section is shown in enlarged view in  FIG. 2 . As shown, each amplifier section  112  preferably contains a liquid  114 , such as water, dodecane or ethylene glycol, and is enclosed in by a transparent barrier  116  to form an enclosure, which may be formed of glass, transparent plastic or other rigid material. The amplifier section  112  may also include a gas dissolved within the liquid  114 , the section  112  being pressurized so that the gas is fully dissolved within the liquid. 
     Associated with each amplifier section  112  is a pressure actuator  104  (acoustic stimulator) that is substantially adjacent the section  112  so as to excite the container  112  at a frequency, preferably a resonant frequency in the ultrasonic range. This ultrasonic frequency causes bubbles  118  to form within the liquid stored in the container  102 . By actuating the section  112  at a resonant frequency, a standing wave is formed within the section that forms bubbles  118  at fixed locations, causing each bubble  118  to function as a single bubble for purposes of the sonoluminescent process. The pressure actuator  104  may be any standard resonator, and may be an acoustic, electrical or mechanical device. According to one embodiment, the pressure actuator  104  is a piezoelectric resonator. 
     The operation of the above-described apparatus is described with reference to a single sonoluminescent bubble reaction in a single section  112  and a single oscillating electrical field (light wave), however it shall be appreciated that the reaction described may occur in a number of bubbles from a number of oscillating electrical fields. 
     An ultrasonic pressure wave is selected to have a pressure differential between peak and trough pressures close to but slightly below the pressure differential required for sonoluminescent excitation. The oscillating electrical field induces electrons moving in a plasma formed inside the collapsed bubble to emit light in phase with that electrical field, thereby contributing and amplifying the energy of the field. 
     Because the period between peaks of the standing wave for ultrasonic excitation is slow relative to the speed of the oscillating electrical field, the next pass of the oscillating field must be delayed until a bubble again forms. During this time, the oscillating electrical field may be reflected through the full mirrors  106 A-C and a portion of the energy may be transmitted through the partial mirror  108 . The distance around the path  110  defined by these mirrors  106 A-C,  108  is selected to be the distance travelled by an oscillating field in the time between peak amplitudes of the standing wave. 
     According to one embodiment, the pressure actuator  104  operates at an ultrasonic frequency of approximately 2 MHz, so that the period between peak amplitudes approximately 0.5 μs. In water, one type of preferred liquid, the speed of light is reduced to approximately 2.25×10 8  m/s, and therefore the oscillating electrical field will travel approximately 110 m before the bubble has recovered. The distance around the path  110  therefore must be selected to be equal to this distance so that the oscillating electrical field picks up the most amount of energy from the sonoluminescing bubbles. Accordingly, fine adjustment of the mirror positions and angles may be necessary in order to properly calibrate the device. 
     The amplifier sections  112  may contain a number of liquid and gas mixtures at a variety of pressures for maintaining solubility of the gas within the liquid. The liquid may be water, dodecane or ethylene glycol and the gas permeating the liquid may be any noble gas, such as argon, helium, krypton, neon or xenon. 
     The above-listed liquids are selected for their characteristics of being nonhazardous, eco-friendly and having a high degree of clarity, however, it will be appreciated that any liquid capable of providing sonoluminescence in a gas bubble may be substituted for these liquids. Other examples include, without limitation, dimethyl phthalate, O-xylene, isoamyl alcohol, chlorobenzene, n-butyl alcohol, isobutyl alcohol, toluene, sec butyl alcohol, n-propyl alcohol, isopropyl alcohol, ethyl alcohol, benzene and tert butyl alcohol. 
     The above-listed gases are also presented as exemplary, and are selected for their non-reactive character: these gases will not react with the liquid medium and therefore the system will be less likely to degrade. The gases also exhibit preferred spectrographic profiles when sonoluminescing and therefore are selected according to the preferred color of the laser and other technical considerations. However, other gases may be substituted in place of noble gases, as described in sonoluminescent literature. 
     The above method of operation has described the interaction of a single sonoluminescent wave and an oscillating electrical field. In order to increase the single-pass gain of the laser output (one cycle about the path  110 ), it is preferred to have a number of amplifier sections  112  wherein each section contributes to the oscillating electrical field. 
     According to this embodiment, the amplifier sections  112  may be in-line as illustrated in  FIG. 1  such that the path  110  passes through each section  112 . Because the oscillating electrical field takes time to pass from one section  112  to the next, each actuator  104  must be slightly out of phase with the next, such that the section  112  actuated by a specific actuator  104  sonoluminesces at the appropriate time as the field passes through that section  112 . It is undesirable to have the field pass through a bubble before collapse as a large bubble may scatter, diffract or otherwise distort the field. 
     As described above, each section  112  also may include a transparent barrier  116  that allows the oscillating field to pass through. This transparent barrier  116  is preferably selected to have the same or similar refractive index as the liquid  114  to avoid distortion or error due to slowing the oscillating field as it passes from one section  112  to the next. Further, the liquid container  102  may contain a liquid  114  of similar composition to the liquid  114  within each section  112  to preserve a consistent refractive index between sections. 
     The liquid container  102  may also contain a sound barrier  120  that may absorb and dissipate energy from the sections  112  so as to isolate each section  112  (or group of sections), thereby preventing them from influencing other sections  112 . The sound barrier  120  may be a porous plate or other sound-absorbing material. 
     It is also preferred that the barriers  116  and liquid container  102  are formed of a transparent material, such as glass or transparent plastic. When a bubble  118  sonoluminesces on its own, the light energy is dissipated away from an origin point in a number of directions. However, when an oscillating electrical field, such as a light wave, is in the region of the sonoluminescing bubble  118  the energy is dissipated in the same direction and phase as the oscillating field. The apparatus is therefore designed such that light adverse to the oscillating field is dissipated away from the concentrated laser. 
     The above-described high-intensity light source may be useful in a variety of applications, including but not limited to laser welding, laser cutting, and defense operations, such as anti-ballistic missile technology. The source may also be positioned on an aircraft or other vehicle, apparatus or structure to provide for various uses. 
     The above-described laser  100  may be operated in either an oscillator or amplifier mode. In an oscillator mode, the laser seed is created by sonoluminescence, and therefore would have a spectrum corresponding to the sonoluminescence. As the light is intensified by the energy release of the bubble collapse a dominant range of wavelengths will emerge. These wavelengths will be more likely to extract energy from the collapsing bubbles, thereby providing a concentrated single-color laser. 
     In an amplifier mode, a seed laser may be provided to the system that operates at a limited spectrographic range (for example, an infrared laser, or a laser having a specific visible light color). As light from the seed laser stimulates energy to be released from the collapsing bubbles (through stimulated emission), the energy released will be in a similar phase and wavelength to the seed laser. 
     Therefore, when operated in the oscillator mode the output from the laser may be limited to a wavelength influenced by the gas, liquid, and emerging dominant wavelength. When operated in an amplifier mode, the output from the laser will be generally the same as the seed laser as the energy released from the collapsing bubbles will be emitted at a similar phase, wavelength and direction as the seed. 
       FIG. 3  illustrates the relationship between the acoustic actuation frequency (kHz) versus length of the apparatus (feet), the power from a single pass (kW/steradian) and the weight of the apparatus (pounds). As shown, the power of the device increases approximately logarithmically as the frequency increases towards 2 MHz and the weight and length of the apparatus decrease. Therefore, it is preferable to have as high of an acoustic frequency as possible in order to maximize single-pass power while reducing the size and weight of the apparatus. 
       FIG. 4  illustrates the sonoluminescent intensity of single-bubble sonoluminescence in water and dodecane and multi-bubble sonoluminescence in water. As shown, single-bubble sonoluminescence peaks at a low wavelength in water (approximately 250 nm) while multiple-bubble sonoluminescence peaks at a higher wavelength in water (approximately 310 nm), both of which are ultraviolet. Dodecane realizes multiple peaks, particularly at 475 and 525 nm, representing blue-green light. 
       FIG. 5  illustrates the absorbtion coefficient of pure water as determined by a variety of sources. As best illustrated by the present work and Pope and Fry, the minimum absorption coefficient of water is for wavelengths in the 400-500 nm range, representing visible light colors violet to blue. Therefore, it may be preferable to select a gas that includes a sonoluminescent peak at or near this range. 
       FIG. 6  illustrates the acoustic pressure, bubble radius, and sonoluminescence as a function of time. As shown, the maximum bubble radius slightly lags the minimum driving pressure and sharply drops to a local minimum causing sonoluminescence shortly before the maximum driving pressure. Further, while the bubbles may enlarge and collapse slightly, no sonoluminescence is realized between the local minima of the driving pressure. 
       FIG. 7  illustrates the amount of sonoluminescent visible flux as a function of temperature for various fluids. As shown, the sonoluminescent visible flux is maximized at low temperatures and decreases as temperature increases. Ideal liquids appear to be Dimethyl Phthalate and Ethylene Glycol. 
     The above-disclosed apparatus has been described with respect to various embodiments, however those having skill in the art will appreciate that various modifications may be made to the apparatus without departing from the scope of the invention. The above-described method has also been described as having specific purposes, but those having skill in the art will appreciate that the apparatus may be used in a variety of ways without departing from the scope of the invention. The above description is intended to be exemplary and not limiting, any limitations will appear in the claims as allowed.