Multiple quantum laser
United States Patent [1 1 Hodgson et al.  
 [451 Apr. 29, 1975 MULTIPLE QUANTUM LASER [75] Inventors: Rodney Trevor Hodgson, Somers;  
 John Robert Lankard, Mahopac; Peter Pitirinovich Sorokin, White Plains, all of NY.  
 [73] Assignee: International Business Machines Corporation, Armonk, NY.  
 [22] Filed: Dec. 28, 1973 [21] App]. No.: 429,413  
 [52] U.S. Cl 307/88.3; 330/43 [51] Int. Cl. H031 7/04 [58] Field of Search 307/883; 331/945 N;  
 Primary E.\&#39;aminerRudolph V. Rolinec Assistant Examiner-Darwin R. Hostetter Attorney, Agent, or Firm-George Baron [57] ABSTRACT A double quantum laser employing volatile monoiodides as the active medium and using a combination of stimulated Raman emission and four wave parametric conversion to generate two pulses suitable for triggering the double quantum amplification process.  
 5 Claims, 3 Drawing Figures MULTIPLE QUANTUM LASER BACKGROUND AND BRIEF SUMMARY OF THE INVENTION A system has been devised for generating two frequencies of intense light from an optically pumped material, the sum of such frequencies being equal to the two photon frequency in that material (See discussion of a multiple quantum laser in US Pat. No. 3,483,486 that issued on Dec. 9, 1969 to P. P. Sorokin). The material chosen for the instant invention is a volatile monoiodide, i.e., CF 1 and C F l being exemplary. A very powerful laser, emitting at a frequency 14,, is injected into a quartz cell containing ordinary molecular iodine (I vapor and heated to a temperature 1200C. at the same time that the iodine vapor is being flash photolyzed to produce ground state iodine atoms. It is to be understood that ground state iodine can be obtained by heating alone, avoiding flash photolysis, if temperatures of 3000K. or higher can be obtained and tolerated by the container housing the iodine. The frequency 11,, generates, within the iodine atom-containing cell, an intense stimulated Raman light at a frequency v and another frequency 1 where 11,, v, 2 v Such two frequencies v and 11,, enter a volatile moniodide cell wherein the vapor is being photolyzed as the two frequencies enter. The photolyzed monoiodide produces amplified pulses of the two entering pulses 11 and BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic showing of a preferred embodiment of the double quantum laser forming this invention.  
  FIG. 2 is an energy level diagram of iodine atoms showing effects of being pumped with the output of a ruby laser.  
  FIG. 3 is an energy level diagram for iodine atoms pumped with the output of a neodymium laser.  
 DETAILED DESCRIPTION OF THE INVENTION In the basic patent on double quantum lasing (See US. Pat. No. 3,483,486 to P. P. Sorokin), a system was disclosed for producing laser emission so that an active ion could be made to emit two photons when it suffered a transition in a laser cavity between an excited state and an unexcited state. In normal transitions, E E, hv, where E,,, is the energy of the excited particle at the excited state m and E, is the energy of the particle at the unexcited state, h is Plancks constant and v is the frequency of the photons emitted during such transitions. In such Sorokin patent, two species of ions A and B were placed in a laser cavity and both were excited to have their respective populations inverted. The inverted population of the B-ions was prevented from lasing by means of a low cavity or by a choice of a very long spontaneous lifetime for such B-ions. The  
 cavity had a high Q for the inverted population of A-ions. Should the laser cavity be pumped with an initial intense light pulse of frequency 14., v,,/ 2, lasing commences at frequency 11,, and continues to emit strongly at this frequency, emitting two photons for each transition of a B-ion until the population of the B-ions is reduced to half its original value.  
  In the present example of a double quantum laser, two frequencies of intense light are generated in a material, the sum of such frequencies adding up to a two photon frequency in that material. The material chosen, namely, excited iodine atoms formed from the photodissociation of CF 1 or C F l and the like, offers possibilities for enormous energy storage and favorable double quantum selection rules. Instead of the use of two separate ions, as taught in the above noted Sorokin patent, this invention exploits stimulated Raman emission and four wave parametric conversion in atomic iodine gas for achieving double quantum lasing. How the latter is implemented is seen by looking at FIG. 1 in conjunction with FIGS. 2 and 3.  
  In FIG. 1, a laser 2 can be either a ruby or a neodymium glass laser capable of emitting picosecond pulses of one joule or more. The laser 12 will emit a beam 4 of light at its characteristic frequency D7,, which beam will impinge on a quartz cell 6 containing iodine vapor 8 maintained in the vapor state at a temperature of -l200C by a heater not shown. Surrounding such quartz cell 6 is a filter l0 and a xenon lamp l2, substantially concentric with cell 6, said filter 10 passing only those wavelengths of the xenon lamp 12 that are greater than 5000A. The pressure of the 1 vapor is approximately /2 atmosphere and] the output of the xenon flash lamp 12 is of the order of l kilojoule per microsecond or greater. The role of the flash lamp 12 is to convert or dissociate one molecule of I into two iodine atoms in the ground state. A conventional xenon flash lamp 12 lasts approximately 50 microseconds and the iodine molecules remain dissociated for at least 1 millisecond. Consequently, the ruby laser 2 can be turned on anytime within a millisecond after the flash of the lamp 12 has died down.  
  The ruby beam 4 has a frequency of 14,400 cm and it is sufficiently intense to interact with the ground state iodine atoms to cause a stimulated Raman effect so as to produce a Stokes frequency u For iodine, the excited upper state is 7598 cm. For the ruby laseriodine atoms combination, the stimulated Raman emission frequency 11,,- equals the ruby emission v minus the upper excited state frequency of iodine, or 14,400 cm 7598 cm. The latter difference is 6802 cm and represents the Stokes frequency v When using a ruby laser, there is a high conversion factor (about 20%) of ruby photons at a frequency of 14,400 cm being converted into Stokes photons at a frequency of 6802 cm.  
  As a result of the abovenoted dissociation of an iodine molecule into two iodine atoms in the ground state and subsequent pumping of said iodine atoms by an intense ruby beam 4, two beams 14 and 16 emanate from quartz cell 6, beam 14 being the original ruby laser beam of frequency 14,400 em but attenuated by -20% of its original energy and beam 16 being the Stokes frequency radiation of 6802 cm. The two beams impinge on a second quartz cell 18 which is transparent to these two frequencies. Iodine molecules are also dissociated into iodine atoms in cell 18, one iodine molecule dissociating into two iodine atoms, but the iodine molecules are maintained in this cell 18 at a pressure of 10 Torr, which is a much lower pressure than that at which the iodine molecules are maintained in cell 8. Additionally, such iodine gas molecules are maintained at about only C. A xenon lamp 20 and a filter 22 are employed to dissociate each iodine molecule in cell 18 into two iodine atoms. If desired, one lamp can be used to serve the same purpose as both lamps l2 and and one filter can be used to serve the same purpose as filters 10 and 22.  
  When the two beams 14 and 16 having, respectively, frequencies 11,, and v enter quartz cell 18 having dissociated iodine atoms therein, such frequencies are used for four wave parametric conversion, well known in the art of optics, for example, see Efficient Parametric Conversion in Cesium Vapor Irradiated by 3470-A Mode-Locked Pulses P. P. Sorokin et al. in the IEEE Journal of Quantum Electronics, Vol. QE 9, No. 2, Feb. 1973, pp. 227230 and Tunable Coherent [R Source Based Upon Four-Wave Parametric Conversion in Alkali Metal Vapors P. P. Sorokin et al. wherein one photon at a frequency of v, (see FIG. 2) is destroyed so that two photons at a frequency of v are l generated as well as a photon at a frequency of 11 wherein 11,, v, 211 As seen in FIG. 2, the energy sum of the two photons, of frequency u and another of frequency v,,, respectively, equals the excited energy state (P of the iodine atom. Thus, the two frequencies 1/ and v, that enter the left side of cell 18, by a process of efficient four wave parametric conversion, produce another frequency 11,; that equals 11 211 The window 24 of cell 18 permits the passage of all three frequencies 14, v and v Adjacent the output window 24 is located a filter 26 for substantially attenuating or blocking out the original ruby laser frequency 11 so that only the frequencies u and 1/ are available for entry into a third quartz cell 28. The filter 26 can be made of silicon or gaseous iodine. lnside cell 28 is placed a volatile monoiodide, for example, CF l. This iodide, when pumped with a xenon flash lamp 30, without any filter interposed between the cell 28 and lamp 30, will absorb the 2600 A. frequency of the pumping source 30. The CF 1, maintained at room temperature and at 50 200 Torr pressure, will dissociate into CF and l* (the excited state P of the iodine atom). In the dissociation of CF l to CE, 1*, there is unit quantum efficiency, the excited iodine atom is metastable, its radiative transition probability is equal to eight per second and collisions between residual iodide gas atoms and molecules do not quench lasing action. The two frequencies V and 11,, stimulate a double quantum transition 1 V Consequently, an output pulse 32 at frequency 11 appears that is an amplification of the input pulse at that frequency and a second output pulse 34, at the frequency V is an amplification of the input pulse of frequency FIG. 3 is the energy diagram that explains the double quantum laser operation when the pumping laser 2 is a pulsed neodymium glass laser emitting at a frequency I 9400 cm&#34;. The Stokes frequency u that is generated has a frequency of 1802 cm in that it is equal to thrust of the invention is to create two intense frequencies v and 11 such that hv 1111 E, where E is the,  
 energy of the material from its ground state to its excited state. An alternate way of practicing the invention is the employment of two independent stimulated Raman scatterers to achieve such frequencies. This invention is not precluded from using other techniques for achieving such two frequencies.  
 What is claimed is:  
 l. A system for generating stimulated two photon emission comprising volatile monoiodides in a transparent container,  
 flash photolysis means directed at said volatile monoiodides for achieving large inversions in atomic iodine to an excited state, and  
 means for generating two intense synchronous pulses v and v such that hil and hv equals the energy of the excited iodine atoms.  
 2. A system for generating stimulated two photon emission comprising atomic iodine in a transparent container,  
 an intense laser pulse of first frequency 11 directed at said atomic iodine to achieve by stimulated Raman scattering a second frequency v appearing with said first frequency 11 a second transparent container of atomic iodine excited by said frequencies v and 1 so as to produce by four wave parametric conversion a third frequency 11,, of light whereby 11,, l -zll means for filtering out 11 but transmitting v and 1 and means for amplifying said frequencies v and 1 comprising flash photolysis of a volatile monoiodide.  
 3. A system for achieving two stimulated emission pulses due to a double quantum transition comprising a laser emitting an intense beam at a frequency v iodine vapor in a cell that is excited by said beam of frequency v means for dissociating most molecules of said iodine vapor into iodine atoms while said vapor is excited by said laser beam so as to produce a Stokes frequency v equal to the frequency 11,, minus the frequency of the excited state of said iodine atoms,  
 a second cell in the path of said two frequencies 11,, and 11 and containing iodine molecules each of which has been dissociated into two iodine atoms, the interaction of said two frequencies with said iodine atoms producing a third frequency v, such that 11,, 11,, 211 which frequencies appear simultaneously as outputs from said second cell,  
 means for filtering out the frequency 1 so that only the frequencies v and 1 remain, and  
 a third cell containing excited iodine atoms in the path of said two frequencies u and 1 so as to produce amplified output signals corresponding to. frequencies I and 11 4. The system of claim 3 wherein said laser is a ruby laser.  
 5. The system of claim 3 wherein said laser is a neodymium glass laser.