GB1572490A - Source of tunable intense coherent radiation in the range of 620 cm-1 to 632 cm-1 - Google Patents

Description

(54) A SOURCE OF TUNABLE INTENSE COHERENT RADIATION IN THE RANGE OF 620 CM-'TO 632 CM1 (71) We, INTERNATIONAL BUSI NESS MACHINES CORPORATION, a Corporation organized and existing under the laws of the State of New York in the United States of America, of Armonk, New York 10504, United States of America do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:: The present invention relates to sources of tunable intense coherent radiation in the range of 620 cm-' ' to 632 cm For a number of years, there has been interest in achieving a tunable intense source of coherent radiation in the vicinity of 628 cm-'. This interest arises from a number of schemes which have been proposed to separate various isotopes of uranium in a multipste- process, one of whose steps requires radiation at approximately 628 cm . In this regard, see U.S. Patents 3,443,087 and 3,937,956, as well as "Photochemical Isotope Separation as Applied to Uranium" (Union Carbide Corporation, Nuclear Division, Oak Ridge Gaseous Diffusion Plant, March 15, 1972, K-L-3054, Revision 1, page 29).

The prior art is replete with techniques for producing intense coherent radiation at a variety of wavelengths, and further illustrates examples of apparatus that can employed to tune such radiation to specific wavelengths. There are even examples of lasers which emit in the vicinity of 628 cm For example, Lyon mentions, in U.S. Patent 3,937,956, a hydrogen fluoride laser emit ting at 629.16 cm~ l, but that laser cannot be tuned to another frequency. For one reason or another the prior art techniques have not achieved what is required for the infra red radiation step in the processes illustrated by the references cited above.

According to the present invention, a source of tunable intense coherent radiation in the range of 620 cm-' l to 632 cm-' ' com- prises a pumping means, including at least one optical pumping source for directing energy at a cavity containing a vapour medium capable of stimulated emission in response to optical pumping, and having (i) at least three atomic energy levels with at least first and second atomic energy levels separated by a particular energy quantum in a range of 620 cm~ l to 632 cm;)atrans- ition from the first to the second atomic energy levels favoured over all other possible transitions from the first atomic energy level; and (iii) a third atomic energy level being one, from which atoms can be pumped to the first atomic energy level.

Such a source is capable of being used in one or more of the isotope separation processes disclosed in the references cited above.

In all the embodiments of this present invention described, there is a cavity, and pumping means including at least one optical pumping source (for example a laser) for directing energy at the cavity. Material in the vapour state is included within the cavity capable of stimulated emission at operating temperature and pressure in response to the pumping. The material has at least three atomic energy levels with at least a first and second atomic energy level separated by a particular energy quantum approximately equal to 628 cam~'. The transition in such material from the first to second energy levels is favoured over all other possible transitions from the first atomic energy level. The third atomic energy level is one from which atoms can be pumped to the first atomic energy level in response to the pumping means.Finally, the material has a vapour density sufficient for lasing in response to the pumping. In some of the embodiments of the invention described, classical laser techniques are employed whereby a population inversion is established in the first atomic energy level which, upon depletion, results in intense coherent radiation. With such embodiments tunability is achieved by employing either the Zeeman or Stark effects. Other embodiments of the invention described rely upon the stimulated Ram an scattering to produce the intense coherent radiation. For these embodiments tunability is achieved by tuning the pumping means.

Specific examples of lasing material are potassium and strontium in the vapour phase.

The scope of the invention is defined by the appended claims; and how it can be carried into effect is hereinafter particularly described with reference to the accompanying drawings, in which: Figure 1A is an energy level diagram of the material; Figure 1B is a schematic diagram of apparatus embodying the invention; Figure 2A is a specific energy level diagram for atomic potassium; Figures 2B, 3, 4, 5A and SB are block diagram of apparatus embodying the invention; Figures 6A and 6B are energy level diagrams of stimulated Raman scattering processes embodying the invention; Figure 7A is an energy level diagram of atomic strontium; and Figure 7B is a schematic diagram of apparatus embodying the invention.

Classical laser operation requires a population inversion in which a higher energy level is more populated than a lower energy level.

Laser action was first established in an optical cavity which allowed photons to be reflected back and forth so as to build up the intensity of the radiation. Another type of laser does not require such an optical cavity as the photon amplification is so large that sufficient intensity is produced without the necessity of mirrors; this type of laser is referred to as "super-radiant".

A further technique for producing intense tunable coherent radiation is stimulated Raman scattering (hereinafter SRS). In employing SRS, an intense laser beam is converted into a beam of another frequency by coherent stimulation of a Raman or two (or more) step scattering processes.

Tuning a classical or super-radiant laser can be achieved with application of electric or magnetic fields to the lasing medium.

Tuning an SRS device can be achieved by tuning the intense laser beam.

The invention can employ either classical laser techniques (including super-radiant) or SRS to produce the desired result. Either technique requires optical pumping of a suitable medium. Due to the transparency of vapours (mediums in the gaseous phase), vapour media are used.

Figure 1A illustrates, in schematic form, the essential atomic energy levels needed for materials used in applying the principles of this invention to emit at the desired wavelength. More particularly, Figure 1A illustrates three of the atomic energy levels employed in the system. Those skilled in the art will understand that many other atomic energy levels may be available, lying either between the levels L3-L1, or outside these levels or both. These other levels have been omitted from Figure 1A for ease of illustra tion. As shown in Figure 1A, level L3 is the lowest energy level. Although this may, in fact, be the ground state for this atom, that is not necessarily the case. Rather, L3 may also comprise some intermediate excited level to which atoms must be pumped from the ground state.As is illustrated in Figure 1, the essential transition of the process is the transition T2 from L1 to L2. Emission of radiation associated with this transition is indicated by wave E. In some embodiments of the invention it is necessary to establish a population inversion at L1, and for that reason, for those embodiments, L1 should be an energy level in which establishment of such a population inversion is possible. The transition from L3 to Li is via absorption of photons from some pumping source. Typi cally, the source includes a laser. Stimulated emission occurs in transition T2 to produce the desired intense coherent radiation.

Transition T3 is desirable so that after the population inversion has been depleted (in those embodiments which establish such inversion), that is after the transition T2, the atoms can again be pumped in transition T1 back up to the excited level L1. Thus, one desirable requirement for the material emp loyed is that there be no terminal energy levels between L2 and L3. The transition T3, or the multiple transitions which make up T3, may also result in either spontaneous or stimulated emission. Depending on a var iety of factors, the desired radiation may be continuous wave or pulsed and, if pulsed, of varying pulse widths. Clearly, if transition T3 involves self-trapping levels some means must be provided to overcome this difficulty if the apparatus is to provide long pulse widths or continuous wave output.It is preferable that the transition T3 not involve self-trapping levels, although it will be understood that this is not essential.

In summary, the present invention is predicated on choosing material having at least' three atomic energy levels, L1, L2 and L3 illustrated in Figure 1A. To ensure produce tion of the desired radiation the transition T2 from L1 to L2 should be the favoured transition, among all possible transitions, from energy level L1. The energy quantum released upon stimulated emission from level Li to L2 should be approximately that of the desired radiation, that is, approximately 628 cam~ 1. The transition T3 from L2 to L3 may actually comprise one or more transitions.Similarly, the transition T1 may comprise multiple transitions so long as the net result is a strongly favoured transition from level L3 to L. If L3 is not the ground state of the atom, it should be a relatively long lifetime in comparison with the other steps in the process.

Figure 1B illustrates apparatus for producing the atomic transitions illustrated in Figure 1A to thereby produce the desired coherent radiation. More particularly, a laser cavity 10 is filled with a suitable vapour medium and maintained by means well known to the art at suitable operating temperatures and pressures. A pumping source 15 provides energy, in the correct form, for exciting the medium contained in cavity 10.

The pumping is schematically illustrated by the arrowed line 16, although those skilled in the art will understand that the pumping can be carried out with a variety of forms of energy, in addition to light coherent or incoherent), such as electricity and/or magnetism. The arrows 11 associated with laser cavity 10 are intended to represent the emission of intense coherent radiation of the desired wavelength. In addition to the foregoing apparatus, Figure 1B illustrates a tuning means which is connected, via dotted lines 21 and 22 to, respectively, laser cavity 10 and pump 15. Specifically, this portion of Figure 1B illustrates that tuning means 20 is capable of tuning the desired wavelength by affecting the laser cavity 10 and/or the pump 15.

Pump 15 is intended to represent one or more pumping devices. Specifically, it was mentioned with respect to Figure 1A, that atomic energy level L3 need not be the ground state for the medium. If it is, then the pumping device included within the pump 15 is arranged so as to produce the transition T1, that is, to raise the atomic energy level from L3 to L1. If atomic energy level 3 were not the ground state, it is also necessary to include an additional pumping device, in the pump 15, so as to excite the medium up to the level L3.

In one form of the invention, classical laser operation is employed, that is, a popu lation inversion is established at the atomic energy level L1 in a laser cavity 10. With this type of operation the output wavelength is independent of the tuning of the pump 15, and therefore, the laser cavity itself is tuned by employing the Zeeman and/or Stark effect; that is, application of magnetic or electric fields to the laser cavity 10. In other embodiments of the invention, a population inversion is not established, and radiation is produced by stimulated Raman scattering.

In this class of operation, tuning is effected by tuning one or more of the pumping devices included within the pump 15.

The vapour medium included in the laser cavity 10 may be chosen from a wide variety of materials. The embodiments disclosed below rely on potassium or strontium in the gaseous phase. In this application where wavelengths are referred to, they are in air.

Embodiment 1 Figures 2A and 2B are repsectively an energy level diagram and a block diagram ofa physical arrangement employing the principles of the present invention with a potassium vapour cell to produce intense tunable coherent radiation at 628 cm-l. S, P, D and F are the quantum mechanical designations of the orbital angular momentum of the quantum number of an atom.

The 1/2 and 3/2 suffixed are quantum numbers describing the total angular momentum of an atom.

In particular; as shown in Figure 2A, potassium is raised from its ground state (4S112) to the 4P1,2 level by a suitable pumping device, which will be disclosed hereinafter. With number densities for the potassium vapour on the order of 1016 atom/cm3, severe radiative trapping will occur and the effective lifetime of the excitation in the 4P1/2 level can be lengthened from 30 nsec to the order of 1 msec. These number densities can be achieved, for example, at a pressure of 1 Torr and a temperature of 350"C. These values are exemplary and wide variations can be accommodated if changes in input power levels are made. For instance, a cw laser pumping beam tuned to the transition of 12985.2 cm~l (7699 ) can be employed.With a power of 260 mW it can easily maintain 10 n atoms in the excited state. A second pumping device optically pumps the potassium (in pulse fashion) with power resonant with the 4Pl2e 6D3,2 transition, i.e., the frequency corresponding to 18710.6 cm~l (5343.1 P;). With this pumping we can saturate the 6D3,2 level, i.e., a number density of about 0.5 x 1015 atoms/cm3. With this material the branch- ing ratio is such that the highest gain is for the 6D3X2 I 7P1/2 transition and a single frequency output is expected.

For tuning purposes, the Stark effect is employed and application of a relatively strong electric field to the potassium vapour should allow for tuning several cm in a continuous fashion. Alternatively, the Zeeman effect would also allow for continuous tuning of the output by several cm~l (e.g., i 6cm).

It should be noted that the pulsed pumping of the 4P1,2 to 6D3/2 transition allows relatively higher electric fields to be applied without ionizing the vapour and thus tuning due to Stark effect can be very extensive.

Employing the Stark effect requires some means to subject the vapour medium in the laser cavity 10 to an electric field. Therefore, tuning means 20 (Figure 1B), for this embodiment, includes electric field generating means such as a pair of plates preferably mounted within the cavity 10 to subject the medium therein to the electric field. Suitable power supplies synchronized with the optical pumping energizes the electrodes to produce the necessary electric field. In like fashion, employment of the Zeeman effect necessitates subjecting the vapour medium in the laser cavity 10 to a magnetic field, and thus tuning means 20 (Figure 1B), for this embodiment, includes a means for generating such a magnetic field, such as a coil, which may be mounted outside the cavity 10.

A typical apparatus embodying the example referred to above is illustrated in Figure 2B. In this Figure, a laser cavity 30 is associated with a pair of mirrors 31, 32.

Mirror 32 is the output mirror. Laser cavity 30 is pumped by a 7699.or CW beam and a pulsed 5343.1 A beam through a Dichroic mirror 31. Mirror 31 has reflectance at 16L and is transmitting at 7699.0 and 5343.1 . Also included within the laser cavity 30 are electrodes 35 which are coupled to electric field generator 20' for tuning purposes.

As explained above, the electric field generator 20' is synchronized with t.le pulsed 5343.1 A beam to allow tuning with the Stark effect. Finally, potassium in the vapour phase is also included within the laser cavity 30.

In operation, potassium atoms within cavity 30 are excited to the 4P1i2 level by the 7699.0 A beam. From this level the atoms are excited to 6D312 in response to the 5343.1 A pulsed pumping. The long lifetime at 4P1i2 allows a population inversion to be established at 6D3,2. The transition from 6do/2 to 7P1/2 is the most favoured transition from 6D3,2 and will produce radiation at 625.8 cm-'. The tuning means, electrical field generator 20' or a magnetic field generator, should allow tuning within a range 620 cm~l to 632 cm~l and more particularly to the desired 628 cm A different pumping scheme employing the same transitions is illustrated in Figure 3. Figure 3 illustrates much of the same apparatus as Figure 2 except that the mirror 31 has been replaced by a mirror 37 which need only have reflectance at 16cm, and need not be transmitting to the c.w. or pulsed pumping beams. Rather, these beams are directed to a KC1 prism 36 where they are properly directed into the laser cavity 30 to be co-linear with the 16eel laser beam. In operation, the apparatus of Figure 3 employs the same transitions to produce the 16, output as does the apparatus of Figure 2.

As a further alternative pumping scheme, a pair of dye lasers can be employed, one producing the 7699.0 A energy and the sec ond producing the 5343.1 A energy. The outputs from the dye cells can be pulsed and can be synchronously produced if each of the dye cells is itself pumped by a common nitrogen laser, as illustrated in Figure 1 of "A Tunable Infrared Coherent source for the 2 to 25L Region and Beyond" by Wynne, Sorokin and Lankard, at pages 103-111 of the book, Laser Spectroscopy, edited by Brewer and Mooradian (Plenum Publishing Corp., New York, New York 1974). Thus, as shown in Figure 4, the nit rogen laser 40 produces a suitable output at wavelength 3371 A, which is divided by beam splitter 41.Associated with dye laser 42 is a beam expander 43 and grating 44 allowing a spectrally narrow (about 0.1 cm1) tunable output. The other portion of the nitrogen laser beam is reflected by the reflector 47 to pump a different dye cell 48.

Associated with dye cell 48 is a beam expander 49 and grating 50. The pulsed outputs of both dye cells 42 and 48 are combined in the Glan prism 52 for purposes of pumping the laser cavity 10. For instance, dye cell 48 can produce the 7699.0 A beam and dye cell 42 can produce the 5343.1 A beam. With this apparatus the K atoms are excited to 6D3,2 and then decay to the 7D1,2 level with the emission at 625.8 cm-'. Tuning should allow this to be shifted in the range 620 cm-'to l to 632 cm-'.

As still a further pumping alternative to produce 16 radiation employing the same transitions, an incoherent light pump from a coaxial discharge in K vapour is employed to ensure high population density in the 4P1,2 (as well as the 4P3/2) states. Interior of the coaxial discharge tube is a central column of K vapour. The vapour pressure in the outer tube is adjusted so that the emission width of the K resonance lines equals the resonance broadened absorpotion width of the atoms in the inner tube.

Figure 5A is a cross section of apparatus to perform such pumping. More particularly, the outer tube wall 50 enclosed an interior tube comprising a wall 51 of a material transparent to the radiation emitted by the potassium vapour in the outer tube, i.e., 7699.0 . Enclosed within the walls 51 of the inner tube is additional potassium vapour which is subjected to 5343.1 A radiation from an external source. The arrow 52 illustrates the spontaneous emission due to the electrically excited potassium atoms in the outer tube. This latter radiation pumps the potassium in the inner tube to the 4P1,2 state. The 5343.1 A radiation incident on the potassium vapour in the inner tube pumps the excited atoms up to the 6D3/2 state.As is mentioned above the most favoured transition from the 6D3,2 is to the 7P1,2 producing 625.8 cm~l radiation, the desired output.

For tuning purposes, with this pumping scheme, the Stark or Zeeman effects are employed with apparatus such as is illustrated in Figures 2 and 3. This should allow tuning in the range 620 cm~ l to 632 cm As still a further pumping alternative and one which we have employed, to produce 16 y laser radiation employing the same transitions, a heat pipe discharge tube may be used to ensure a high population density in the 4P1,2 and 4P3,2 states. Such a discharge tube is disclosed, for instance, in U.S. Patent 3,654,567 issued April 4 1972, or "Emission Spectra of Alkali Metal Molecules Observed with a Heat Pipe Discharge Tube", by P.P. Sorokin and J. Lankard in the Journal of Chemical Physics, Vol. 55, No. 8, October 15, 1971, pages 3810-3813.

The heat pipe discharge tube is shown in cross section in Figure 5B with a central column of K vapour. The vapour pressure is adjusted so that a glow discharge is supported in the K vapour and a sufficient density of K atoms in the 4P1i2 and 4P3/2 states results. The outer tube wall 100 encloses the vapour. A current is passed through electrodes 101 to excite atoms to the 4P1/2 and 4P3,2 state. These states are subjected to 5343.1 radiation from an external source. The 5343.1 A radiation pumps the excited atoms in the 4P 112 state to up to the D3,2 state. As mentioned above, the most favoured transition from the 6D3,2 state is to the 7P1,2 state producing 625.8 cm1 ' radiation, the desired output.

Embodiment 2 As a further example, instead of employing potassium vapour, strontium vapour can be employed.

Figure 7A and 7B are, respectively, an energy level diagram and a block diagram of a physical arrangement employing the principles of the present invention with a strontium vapour cell to produce intense tunable coherent radiation at 628 cam 1 More particularly, strontium is excited from the ground (5s2'S) state to the 5swPo level. This pumping can be provided by an electrical discharge such as that employed in Figure 5B. An optical pump (for example, a laser) pulses the excited atoms at 13029.0 cm (7677.3 ) to populate the 5s5d'D state.In this fashion, an inverted population density is established with respect to the 6s6ptPo state resulting in lasing at 629 cm 1 Tuning to the desired wavelength is accomplished with an electric or magnetic field, as previously disclosed.

Figure 7B shows, schematically, a heat pipe discharge tube 100 (similar to Figure 5B) enclosing strontium vapour subjected to an electrical discharge as a result of a potential difference applied between terminals + and - connected to electrodes 101. An optical pump provides 7677.3 radiation to excite the atoms to the 5s5d'D state. The transition from this state to 5s6p' PO is most favoured and lasing at 629 cm~l results.

Although specific tuning apparatus is not illustrated, the Stark or Zeeman effects will allow tuning of appropximately + 6 cm-'to to achieve the desired 628 cm-l l output.

Although only a single pumping arrangement for strontium is shown herein, those skilled in the art will be able to readily adapt the specific pumping arrangements of Figures 2B, 3, 4, 5A and 5B to the strontium vapour embodiment.

Embodiment 3 The previous examples, employing potassium and strontium for producing 16 CL intense tunable coherent radiation, have been based upon classical laser techniques in which an inverted population is achieved from which the most favoured transition produces the desired output with tuning.

The invention, however, also produced the desired radiation, using similar materials and transition levels, but it need not establish the inverted population and may, instead, rely upon stimulated Raman scattering (hereinafter SRS). In this process, tuning is achieved by varying the wavelength of the pumping source rather than tuning the cavity or the medium therein. Thus, the SRS process corresponds to employing a tuning means 20 to tune the pump 15 rather than the laser cavity 10 (see Figure 1B).

For example, Figure 6A shows certain of the relevant energy levels of potassium, the 4Sl,2 or the ground state, and intermediate excited level 4Pl,2, a further excited state 6D3/2 and another excited state, 7Pv2 along with the wave numbers associated with those states. In employing SRS, the 4P1/2 level is first populated by using any of the apparatus previously referred to. For example, an intense laser pulse near 7699.0 A or by means of the heat pipe discharge or a nitrogen laser pumped dye laser. In the second step an intense narrow band laser pulse of wavelength #p is employed which is slightly different from that necessary to populate the 6D3,2 level.Of course, the second pumping beam must be applied before the atoms pumped to the 4P1,2 state have had a chance to decay back to the ground state, but this is a requirement in common with the examples in the preceding discussion.

A 16y output is generated as a so-called Stokes wave based upon the simultaneous occurrence of the following events: (1) a pump photon at #p (wave number Up) is absorbed; (2) a Stokes photon at h (wave number Us) is emitted; and (3) the atom interacting with the radiation fields, in this case an atom in the potas sium 4P1i2 state makes an energy conserving transition by jumping to another state, here the 7P112. Thus, Up = Us + 31070.0 cm~l 12985.2 cm~l. For example, if the exact value desired for Us is 628 cm1 , Up should be 18712.8 cmT1, which corresponds to 5342.6 A. The normal wavelength for resonance between the 4Pl,2 and 6D3,2 states is 5343.1 A so that it is clear that the laser pumping of the second step must be tuned to a slightly shorter wavelength (or a greater wave number for energy of photons) than that of the transition employed to reach the 6D3,2 state.In Figure 6A this slight energy difference is represented by 8 , and here 8 is 2.2cm~l.

It should be apparent from the foregoing example, however, that any specific wavelength near 16,u can be produced since the second pumping step can employ a tun able dye laser to produce Up. However, for any given value of 8 the SRS process has a threshold, that is, there is a minimum value of intensity of the beam Up which must be supplied before the process produces the desired output. This threshold varies as the square of delta and depends also on the combined radiative strengths of the atomic transitions from which the cross section of the Raman scattering process is derived. In this case, 4P-6D and 6D-7P.

The SRS radiation has been observed in potassium, although at energy levels differ ent from those employed in this invention, see "Resonance Raman Effect in Free Atoms of Potassium" by Rokni and Yatsiv, appearing in Physics Letters, Vol. 24A, No 5 (February 27, 1967) pages 277-78. These reported results serve to establish that a threshold level for the process illustrated in Figure 6A is practicable and attainable.

That is, the value of 8 employed in the referenced article is four times bigger than that employed in the process of Figure 6A.

Therefore, the threshold for SRS in the case reported in this article should be some 16 times that for the process of Figure 6A. The fact that SRS was observed in the reported case for the modest intensity for the beam at Up serves as evidence that the threshold for the process of Figure 6A is indeed practical.

Thus, it should be apparent that the example given herein allows employing a potassium vapour to produce intense coherent radiation in the vicinity of 1 6jt, moreover which radiation is tunable by varying the pumping frequency Up . With a dye laser to produce Up it would appear simple to obtain the necessary tuning.

The preceding example of the SRS process can be characterized as a two photon process (Up, and #s). This is not essential and this will be demonstrated by describing a three photon process (Upl, #p2 and o.) for producing tunable intense coherent radiation at 16,u.

Figure 6B illustrates the three photon process which operates on K atoms in the ground state. Here two laser beams provide pumping energy, one at Upl and the second at op2. Coherent stimulation occurs when the beam intensities are sufficiently high. The process relies upon the following simultaneous events: (1) a pump photon at o pl, is absorbed; (2) a pump photon at Up2, is absorbed; (3) a Stokes photon at Us is emitted; and (4) an atom in the ground state 4S interacting with the radiation fields makes an energy conserving jump to 7P1/2 state.

In the process the 4P1/2 state is not directly involved, except in the subsequent decay to the ground state. The 4P1,2 state does however, serve as a close-lying state required to give the three photon Raman scattering process a reasonably large possibility of occurrence or, alternatively expressed, a relatively large "cross section". This probability is proportional to (1/ A 2) (1/ 8 2 ).

In general, the laser beams #pl, Up2 must be tunable around the frequencies of the 4S1/2 to 4Pl,2 resonance line and the 4P1,2to 5D3,2 transition, respectively. There is, of course, an exact requirement that Upl + Up2-Us = 31070.0 cm . Thus, for example, if it is desired that Us = 628.0 cmi, the sum of Upl and Up2 must be 31698 cm . (The quantity 8 of Figure 6B would be the same as the quantity 8 of Figure 6A, in other words.) Otherwise, there is some flexibility in the choice of the input beam frequencies.

Whereas 6 (in Figure 6B) will be = 2.2 cm-' for #s = 628 cm-, a typical value of # will be 10 100 cm~l, so as not to have too much of the primary beam at Upl actually absorbed.

Although potassium is described in the example of the SRS process to produce 16 radiation, this process can also employ strontium.

Claims (46)

WHAT WE CLAIM IS:

1. A source of tunable intense coherent radiation in the range of 620 cm-l to 632 cm~ l, comprising a pumping means, including at least one optical pumping source for directing energy at a cavity containing a vapour medium capable of stimulated emission in response to optical pumping, and having (i) at least three atomic energy levels with at least first and second atomic energy levels separated by a particular energy quantum in a range of 620 cm~l to 632 cm1; (ii) a transition from the first to the second atomic energy levels favoured over all other possible transitions from the first atomic energy level; and (iii) a third atomic energy level being one, from which atoms can be pumped to the first atomic energy level.

2. A source according to claim 1, in which pumping establishes an inverted population density at the first atomic energy level with respect to the second level.

3. A source according to claim 1 or 2, in which the vapour medium comprises potassium.

4. A source according to claim 3, in which the optical pumping source comprises a laser emitting at 5343.1 A.

5. A source according to claim 4, in which the pumping means further comprises a discharge tube, surrounding the medium and electrically pumped to emit radiation at 7699.0 .

6. A source according to claim 5, in which the discharge tube is a heat pipe discharge tube.

7. A source according to claim 5 or 6, in which the discharge tube includes an inner tube coaxial with the tube and in which the 5343.1 A laser output is directed at said inner tube.

8. A source according to claim 4, in which the pumping means includes a dye laser emitting at 7699.0 .

9. A source according to claim 3, in which the pumping means includes a pair of dye lasers.

10. A source according to claim 2 or any claim appendant to claim 2, including tuning means for affecting the medium so as to emit at 628 cm~l in response to the pumping.

11. A source according to claim 10, in which the tuning means applies an electric field to the medium.

12. A source according to claim 10, in which the tuning means applies a magnetic field to the medium.

13. A source according to claim 1 or 2, in which the vapour medium comprises strontium.

14. A source according to claim 13, in which the optical pumping source comprises a laser emitting at 7677.3-A.

15. A source according to claim 14, in which the pumping means further comprises a discharge tube surrounding said medium and electrically pumped.

16. A source according to claim 13, in which the pumping means includes a pair of dye lasers, one tuned to the 5s2' S - SsSp'Po transition and a second tuned to the 5s5p'Po - 5s5d' D transition.

17. A source according to claim 1, which employs stimulated Raman scattering and in which the pumping means includes a laser emitting at Up which is different by 8 cm l from that required to excite the medium to the first level.

18. A source according to claim 1, which employs stimulated Raman scattering and in which the pumping means includes a pair of lasers emitting at Upl and Up2 respec tively, the sum of Upl and Up2 different, by 8 cm from that required to excite the medium to the first level.

19. A source according to claim 17 or 18 in which the vapour medium includes potassium.

20. A source according to claim 17 or 18, in which the vapour medium includes strontium.

21. A source according to claim 3, in which the pumping means is such as to pump atomic potassium to the 6D3/2 level.

22. A source according to claim 21, including a further pumping means which is such as to pump atomic potassium to 4P1,2 level.

23. A source according to claim 22, in which the pumping means comprises a pair of dye lasers, one emitting radiation for pumping to the 4P1,2 level, and the other emitting radiation for pumping from the 4P1,2 level to the 6D3,2 level to the 6D3,2 level.

24. A source according to claim 23, in which each of the dye lasers is synchronously pumped by a single nitrogen laser.

25. A source according to claim 22, in which the pumping means includes a discharge tube which emits radiation at 7699.0 when electrically pumped.

26. A source according to claim 21, which further includes a tuning means for precisely tuning the transition from the 6D3,2 level.

27. A source according to claim 26, in which the tuning means includes means for subjecting the cavity to an electrical field.

28. A source according to claim 26, in which the tuning means includes means for subjecting the cavity to a magnetic field.

29. A source according to claim 13, in which the pumping means is such as to pump atomic strontium to a Ss5d ' D level.

30. A source according to claim 29, in which the pumping means includes a further pumping means.

31. A source according to claim 30, in which the pumping means comprises two dye lasers.

32. A source according to claim 30, in which the further pumping means comprises a heat pipe discharge tube which is for pumping to the 5s5ptPo level when electrically pumped.

33. A source according to claim 29, which further includes tuning means for precisely tuning the transition from the 5d'D level.

34. A source according to claim 33, in which the tuning means includes means for subjecting the cavity to an electric field.

35. A source according to claim 33, in which the tuning means includes means for subjecting the cavity to a magnetic field.

36. A source according to claim 21, in which the pumping means is for pumping the cavity at Up, where Up is different by #cm-, from that necessary to excite the potassium to the 6D3,2 level.

37. A source according to claim 36, in which the difference ô = 2.2 cm

38. A source according to claim 22, in which the pumping means emits Upl and Dp2, where Upl is different, by A cm~l from that necessary to excite potassium to the 4P1,2 level, and Upl + Up2 is different by 8 cal from that necessary to excite potassium to the 6D3,2 level.

39. A source according to claim 38, which A is much larger than #.

40. A source according to claim 38 or 39 in which 10 cm-l < A < 100 cm~l.

41. A source according to claim 29, in which the pumping means is for pumping the cavity at Up, where Up is different by acm-1, from that necessary to excite the strontium to the 5s5d' D level.

42. A source according to claim 30, in which the pumping means emits at Upl and Up2, where Upl is different, by A cm-l, from that necessary to excite strontium to the 5s5p 'PO level, and Upl + Up2 is different, by 8 cm-' from that necessary to excite strontium to a 5s5d' D level.

43. A source according to claim 42, in which A is much larger than X.

44. A source of tunable intense coherent radiation in the range of 620 cm~l to 632 cm1, substantially as hereinbefore particularly described as embodiment 1, with reference to Figures 1A, 1B, 2A, 2B, 3, 4, 5A and 5B of the accompanying drawings.

45. A source of tunable intense coherent radiation in the range of 620 cm~l to 632 cm1, substantially as hereinbefore particularly described as embodiment 2, with reference to Figures 7A and 7B.

46. A source of tunable intense coherent radiation in the range of 620 cm~l to 632 cm , substantially as hereinbefore particularly described as embodiment 3, with reference to Figures 6A and 6B.