US3609570A

US3609570A - Light excited maser

Info

Publication number: US3609570A
Application number: US751017*A
Authority: US
Inventors: Gordon Gould
Original assignee: Control Data Corp
Current assignee: Control Data Corp
Priority date: 1968-06-17
Filing date: 1968-06-17
Publication date: 1971-09-28
Anticipated expiration: 1988-09-28

Abstract

1. Apparatus for amplifying or generating microwave frequency oscillations through amplification by stimulated emission of radiation, the energy for such amplification being supplied by substantially unpolarized light or nonuniform spectral density, said apparatus comprising a radiofrequency resonant structure formed at least in part of conductive material, an enclosure at least partially coextensive with said structure, a gaseous working medium in said enclosure, said medium having atoms, ions, or molecules with a pair of energy levels separated by a value of energy substantially corresponding to a resonant frequency of said resonant structure, a source of light having spectral components exciting said pair of energy levels to higher energy levels, means causing the excitation by said light source of atoms, ions, or molecules existing in the lower of the said pair of energy levels to be greater than the excitation of those existing at the higher of said pair of levels, means for ingress of light from said source into said enclosure and means for transmitting radiofrequency energy from said resonant structure to an exterior point of utilization.

Description

United States Patent Davidovits et al., pp. 155-170 330-4 Gordon Gould Bronx, N.Y.

Proceedings of the IEEE," Feb. 1966,

[72] Inventor Wittke, Proceedings of the IRE," March 1957, pages 291-316.

[54] LIGHT EXCITED MASER 18 Claims, 5 Drawing Figs.

Primary ExaminerRodney D. Bennett, Jr. Assistant Examiner-Daniel C. Kaufman Attorney-Darby & Darby CLAIM: 1. Apparatus for amplifying or generating microwave frequency oscillations through amplification by stimulated emission of radiation, the energy for such amplification being supplied by substantially unpolarized light or nonuniform spectral density, said apparatus comprising a radiofrequency resonant structure formed at least in part of conductive material, an enclosure at least partially coextensive with said structure, a gaseous working medium in said enclosure, said medium having atoms, ions, or molecules with a pair of energy levels separated by a value of energy substantially corresponding to a resonant frequency of said resonant structure, a source of light having spectral components exciting said pair of energy levels to higher energy levels, means causing the ex- 5 6 6 44443444 .9 5 40H0 WWWONWOO 9 3 33333333 Q 1 33333333 31 2 3 30 3 H In 1 w 3 m m6 2 S "u mmmmm m W T mm "0 u u 2 N H mm m m WW E mm 0 u m m d T m m m m m u MM U" "hm" i U" a a m m c s mi" m m m m m m E a m m m w m m m M m n n bu m m eeeeeew n e kkkkkka m s .wmmmwwmm m m R w DDDDDDsA m 68889900 r N55555566 w 99999999 S HHHHHHH .1 95994430 0 o 1 d L d 12323422 n i 72059226 I F 26112495 .1. .1. 1 63558825 1 O 6 ,3 ,3 ,9 D U 22222222 citation by said light source of atoms, ions, or molecules existing in the lower of the said pair of ene than the excitation of those existin of levels, means for ingress of lig hdy. w s o a S .me t fdO a g mm z S.1 m bf ct nu 0 f tf e B O t. mm ehwcmo Y r m m m to r a e enclosure and means for transmittin from said resonant structure to an e tt PATENTEDS281Q7| INVENTOR.

fiawo/v 60010 wag 602 T ATTORNEY? sum 3 or 3 MAM Fm LIGHT EXCITED MASER This application is a continuation of Ser. No. 682,678, filed Nov. 13, 1967 and now abandoned which was a continuation of Ser. No. 837,471 filed Sept. 1, 1959 and now abandoned.

The present invention relates to apparatus of the type commonly known as MASERS and more particularly to MASER- type oscillators or amplifiers wherein atoms are excited by unpolarized light in order to bring about a condition where a microwave radiofrequency signal will be amplified or generated as a result of emissions of the atoms.

It has been demonstrated that molecules or atoms in a resonant cavity will, under the right conditions, generate an electrical oscillation or will amplify an electrical signal in the cavity. Apparatus for amplifying or generating a signal by the use of this phenomenon has come to be known as a MASER (See the article by J. P. Gordon, H. J. Zeiger and C. H. Townes, Phys. Rev., Vol. No. 95, page 282L (1954). See also James P. Wittke, Proc. Inst. Rad. Eng. 45, 291 (I957). I

The conditions necessary for the operation of a MASER oscillator are (I) the atom or molecules used shall possess two energy levels closely spaced so that the difference between the levels corresponds to a convenient microwave frequency, (2) the higher energy level shall have a higher population that the lower energy level, (3) there shall be sufficient number of molecules or atoms of the upper energy level available for transition such that the emitted power will exceed the cavity loss thus sustaining oscillation.

Of the above-stated conditions the second one is particularly important. Various methods have been proposed to achieve condition (2) above but previous methods have been of limited utility for various reasons.

The present invention utilizes optical excitation by unpolarized light of nonuniform spectral density in order to obtain condition (2 described above which is necessary for the operation of an atomic, ionic, or molecular cavity oscillator of the MASER type. The technique of the present invention provides an exceptionally practical oscillator in that it does not require the maintenance of exceptionally low temperatures nor does it require gas pumping apparatus to maintain a vacuum and there is no necessity for the production of an atomic beam in a device according to the present invention.

It should be mentioned that a most important application of the present invention would be as a frequency standard, since an oscillator constructed according to the present invention has a frequency which will remain constant with a high degree of accuracy. The accuracy of a frequency standard utilizing the present invention may be of the order of one part in Although it is contemplated that the primary use for the present invention will be as a frequency standard it will be understood that the apparatus is susceptible of being used as an oscillator or amplifier in other applications as well.

Although the general nature of the invention has been set forth above, it will be understood that the invention also includes various features of the apparatus which serve to make it more practical and efficient and may or may not be necessary to its operation in a particular application.

Accordingly in addition to the features and advantages set out above it is an object of the present invention to provide a MASER-type oscillator or amplifier which is optically excited by unpolarized light.

It is another object of the present invention to provide a MASER-type apparatus which is excited by unpolarized light of nonuniform spectral density.

It is another object of the present invention to provide a light-excited MASER utilizing an optical filter of the same element as the atoms which are excited in the MASER.

It is another object of the present invention to provide a light-excited MASER utilizing rubidium as the excited element.

It is another object of the present invention to provide a light-excited MASER-type apparatus wherein means are provided for the prevention of reemission of light which would adversely affect the operation of the apparatus.

It is another object of the present invention to provide a light-excited MASER utilizing a gas-discharge light source wherein the gas utilized includes the same element as that of the atoms excited in the MASER.

It is still another object of the present invention to provide a MASER-type apparatus utilizing a microwave cavity oscillating in a high-order mode.

It is still another object of the present invention to provide a light-excited MASER wherein the light is introduced into a resonant cavity through circumferential slots at a nodal point for electrical oscillations in the cavity wall.

It is a further object of the present invention to provide a light-excited MASER utilizing a microwave frequency-energized gas-discharge lamp wherein the gas-discharge tube is a center conductor in a coaxial-type microwave transmission line.

It is a further object of the present invention to provide an optically excited MASER wherein the exciting light is introduced to the capacity through a waveguide of dimensions below cutoff for the resonant microwave frequency.

It is a still further object of the present invention to provide an optically excited MASER wherein excited atoms are primarily of one isotope of an element and an optical filter is provided containing another isotope of the same element.

Further objects and advantages will be apparent from a consideration of the following description in conjunction with the appended drawings, in which:

FIG. 1 is a fragmentary isometric view partially schematic in form of a light-excited MASER according to the present invention;

FIG. 2 is a diagram of the ground energy level substates of an alkali atom with nuclear spin, l--3/2(h/21r) to said in the explanation of the invention.

FIG. 3 is a diagram showing the effect of optical pumping on the state distribution of atoms with various kinds of illumination, presented to aid in the explanation of the invention;

FIG. 4 is an isometric fragmentary partially schematic illustration of an alternative form of light-excited MASER according to the present invention;

FIG. 5 is a chart showing a plot of the absorption coefficient of Rb near 4,200 4 presented to aid in the explanation of the apparatus of FIG. 4.

Before beginning to consider the particular apparatus illustrated to practice the present invention it will be desirable to review the general principles involved.

It is known that atoms and molecules exist in nature in certain discrete energy states. Since atomic particles are utilized in the illustrated forms of the present invention, the particles involved will be referred to as atoms, although it will be understood that much of what is said will be applicable to molecules also.

The energy state of atoms may be changed with an accompanying absorption or emission of energy. For example, in an ordinary gasdischarge lamp energy is supplied in electrical form which raises atoms in the gas-discharge lamp from a lower energy level to a higher energy level. At the same time atoms in the lamp are decaying with a resulting emission of energy in the form of light and possibly in the form of other frequencies of electromagnetic radiation.

When an atom is changed from one discrete energy level to another there is associated with such a change a particular frequency of electromagnetic radiation. For example, if an atom decays from a relatively high energy level to a relatively lower energy level, electromagnetic radiation of a particular frequency is emitted which frequency is related to the energy difference between these two levels. On the other hand if electromagnetic radiation is absorbed thereby raising the energy level of an atom from one state to another, there is a similar correspondence between the difference in energy 0 the two states and the frequency of the absorbed electromagnetic radiation.

In the usual case the energy difference between energy states of an atom is such that a transition from one state to another produces electromagnetic radiation in the region of the light portion of the spectrum. However, there are some energy states which are very closely spaced in terms of energy level. These states will be referred to as hyperfine states or substates. A transition from the upper to the lower one of these closely spaced states results in the emission of electromagnetic radiation in the microwave portion of the frequency spectrum. In the case of one set of hyperfine states of rubidium which is utilized in the present invention, the energy difference is that corresponding to a frequency of approximately 6834.68 megacycles per second. It is a particularly useful characteristic of the radiation resulting from a transition between such states that it is highly uniform in frequency and is not strongly affected by changes in temperatures, pressures, etc. This characteristic of the electromagnetic radiation renders it very useful for use as a frequency standard.

Coherent radiation in the microwave frequency range does not occur from atoms in useful quantities under ordinary conditions. However, it is known that if one were to excite a quantity of atoms in such a way that the population of atoms in a lower one of two hyperfine energy states were reduced substantially below the population of the upper energy state, then the atoms would tend to resume their normal distribution (with approximately equal population in each of the hyperfine states). Obviously as atoms decay from the upper energy level to the closely spaced lower energy level, microwave energy would be emitted and if the process reaches a sufficiently high efficiency it will be possible to capture the radiation, in a microwave frequency resonant cavity, for example, and to utilize the microwave frequency oscillation of the cavity in a frequency standard or for some other purpose.

In a light-excited MASER the greater population in the higher energy state may be achieved by reducing the population of the lower energy state by optical pumping, that is by differentially exciting the atoms in the lower energy state at a higher rate than those of the upper state.

If atoms are excited by radiation having (I) constant spectral density, (2) isotropic angular intensity, (3) no polarization, then all substates of the ground level remain or become equally populated. However, if any of the above conditions are violated, population difference may arise.

In order to visualize this process it will be useful to refer to FIG. 2 which shows a diagram of the ground level substates of an alkali atom with nuclear spin. I=3/2(H/21-r). The spacing of the levels is not drawn to scale.

From FIG. 2 it will also be seen that higher energy levels exist which are labeled 5 6S and 6 There are substates associated with these levels which are not shown and in addition there are further states which have been omitted to simplify the presentation.

It will be noted that the horizontal axis of the chart of FIG. 2 is marked off in units of the component of angular momentum, Mp, along an arbitrary axis in units of 1/2 h/21r. It will be further noted that there are several states indicated by dots at the F=l level and also at the F=2 level. At each level the states have different positions along the horizontal axis or in other words, have a different component of angular momentum, M

FIG. 2 represents the condition which exists for a zero magnetic field. However, if there is a nonzero magnetic field then the different M states will not have the same energy level within a particular hyperfine state but will rather be displaced from one another and will have different energy levels. The zero Mp states are only very slightly affected by the presence of a magnetic field and it is accordingly desired to operate the apparatus utilizing the M, =0 states. Otherwise variations in magnetic field intensity would cause variations in the resonant frequency of the frequency standard and would accordingly adversely affect the accuracy of the standard. In order to operate the apparatus utilizing the Mp=O states without interference from the other M, states, a small magnetic field is provided in the apparatus so that the nonzero M F states are displaced to different energy levels where their energy and accordingly the frequency associated therewith are not such that they will interfere with the operation of the apparatus. At the same time the magnetic field should be kept to a low value so that variations in the magnetic field do not adversely affect the accuracy of the frequency standard. Although the zero M,- states are only slightly affected by variations in the magnetic field they are somewhat affected and large variations in magnetic field are preferably avoided. By way of explanation it may be said that the magnetic field will generally be less than the earths magnetic field and may be as small as 1 percent of the earth s magnetic field.

Obviously in view of the fact that the magnetic field within the apparatus is to be maintained lower than the earth's magnetic field it will be desirable to provide magnetic shielding to substantially eliminate the influence of the earths magnetic field from the apparatus.

From the foregoing explanation and from FIG. 2 it will be seen that if it were possible to depopulate the F=l level so that its population was substantially less than that of the F=2 level, transitions would occur from the F=2 level to the F=l level tending to restore the population of these two levels to their equilibrium condition. These transitions would result in the emission of electromagnetic energy of approximately 6,834 megacycles per second frequency. One way to obtain the desired differential population of the hyperfine states is to excite atoms in the hyperfine states to a higher energy level in such a way that a greater number of atoms in the F=l state are excited than are excited from the F=2 state. In this fashion the population of the F=l state will be diminished by a greater amount than is the population of the F=2 state thereby producing the desired differential population.

The result of various techniques for obtaining differential population in the hyperfine states is illustrated in FIG. 3. It should be noted in FIG. 3 that the various states are identified in terms of two parameters, F and M The F parameter indicates whether the state is an upper or a lower one of a pair of hyperfine states. This parameter is associated with the total angular momentum of the atom. The M,- parameter is associated with the orientation of the angular momentum of the atom in space. It will be observed that this parameter was plotted as the abscissa of the chart of FIG. 2.

FIG. 3 shows the zero-field ground state distribution after each atom in an initially uniform distribution has absorbed and reemitted one light photon. These distributions approximate those which would obtain if the thermal relaxation rate of the system were equal to the optical pumping rate.

Case A shows the resulting population distribution for excitation with unpolarized light with nonuniform spectral density. It is assumed that only those light frequencies are present which will excite atoms in the F=1 hyperfine level. It will be noted in case A that a high population differential is created between the F=2 states and the F=1 states with the higher population being in the F=2 states as is desired to produce a microwave frequency oscillation.

Case B shows the results of excitation by an unpolarized beam of light with uniform spectral density where the axis of quantization is parallel to the beam. It will be obvious that this type of excitation is not nearly so effective in producing a population differential as is case A.

Case C-P shows results of excitation by radial illumination in a plane with plane polarized light. The axis of polarization and quantization is normal to the plane of illuminization. Some optical pumping is produced in this situation, however, it will be noted that this technique is far inferior to that illustrated in case A.

Case C-C of FIG. 3 shows the results of excitation by a circularly polarized beam and it will be seen that this technique is likewise not advantageous for producing a preponderance of population in the F=2 hyperfine states.

From the foregoing explanation it will be seen that it is possible to efficiently produce optical pumping at a differential rate between a pair of hyperfine states most efficiently by excitation with light having a particular nonuniform spectral distribution. The spectral distribution required is one in which a relatively large portion of the light is of a frequency which will excite atoms in the F=l hyperfine state and wherein a relatively small portion of the light is of a frequency which will excite atoms in the F=2 hyperfine state. Unfortunately there is no natural source of light readily available having this spectrum distribution.

A possible source of light for the light excitation is a gas discharge lamp containing the same element as the atoms which are to be excited. A well-designed spectral lamp produces light having resonance lines in the visible region of a width of approximately 500 to 1,000 megacycles. This is true of both emission and absorption lines. If the hyperfine state separations are greater than this amount, the spectral lines corresponding to hyperfine states are resolved.

The intensities of spectral line components corresponding to transitions to various hyperfine states are not exactly equal. Accordingly excitation by light from a gas-discharge lamp containing the same element as the atoms to be excited provides a possible source for producing a differential population in the hyperfine states.

However, if the ground level J=1/2 (the usual case for atoms and ions having otherwise convenient properties), and if the magnetic moment of the nucleus is negative (i.e. g, is equal to l d, (opposite to that of the electron and the usual case), then the hyperfine state level of greater state multiplicity has higher energy. In this case the natural spectral distribution is such that the higher energy level of the hyperfine states will be excited to the greatest extent and a population distribution between the hyperfine states exactly opposite to that desired will be produced.

From the above explanation it will be seen that if light from a discharge lamp is to be utilized to excite the atoms in a MASER, it must be modified to produce the desired spectral distribution. In apparatus according to the present invention this is done by the use of a filter consisting of the same element as that used in the discharge lamp and the same element as the excited atoms. Just as the spectral distribution for convenient elements is such that light of a frequency associated with the upper energy level of hyperfine states is more intense than that of the lower level, it is also true that for such elements the absorption of light by these elements is greater for light corresponding to higher energy levels of hyperfine states than the for light corresponding to lower energy levels of these states. Although the difference in the absorption of the light associated with the two energy levels of the hyperfine state is small it can be shown that if the amount of absorption is large enough a substantial difference in intensity can be produced as between the spectral lines capable of exciting respective ones of the energy levels of hyperfine states. Thus, with the sacrifice of considerable light intensity by filtering, it is possible to produce light having the properties necessary to produce optical pumping by reason of nonuniform spectral density and further more that this nonuniformity of spectral density is in the proper sense to produce differential populations as between the levels of hyperfine states so that emission of electromagnetic energy of a microwave frequency is produced.

FIG. 1 shows apparatus suitable for practicing the previously explained technique as will now be explained. A microwave resonant cavity is shown at 12 having an output 14 which is illustrated in the form of a circular waveguide but may be alternatively any suitable microwave transmission line output. A seal 16 is provided in the output 14 to physically isolate the contents of the cavity 12 while allowing transmission of microwave energy through the output 14. An iris is provided in output 14 to limit the coupling of the cavity to the output waveguide thus permitting a high cavity Q.

The cavity 12 is preferably a high Q cavity in order that cavity losses may be minimized and oscillation may be obtained with a minimum power output from atomic emission. It will be understood that although the cavity 12 is shown to be of cylindrical form, other forms of resonant cavities may be utilized and in general the cavity will be designed in accordance with customary procedure.

The electrically conductive lower wall of the cavity 12 is formed by a screen 18 of conductive material. The screen 18 should be of sufficiently fine gauge so that it presents effectively a continuous electrical surface for the frequencies involved. However, due to the apertures in the screen 18 it will be capable of passing light energy into the cavity for the purpose of exciting the atoms within the cavity.

the bottom wall of the cavity 12 is physically closed off by a glass wall 20 so that the interior of the cavity 12 is physically enclosed and surrounded by a wall of conductive material yet at the same time the bottom wall of the cavity is transparent to allow light energy to be directed into the interior of the cavity.

A second glass wall 22 is provided which is separated from glass wall 20 to provide a space therebetween. Enclosure 26 is therefor formed between the

glass walls

20 and 22. This enclosure is filled at least partially with atoms of the element from which microwave emission is to be obtained; this element will generally be in gaseous fonn. A gas-filled enclosure 26 forms an optical filter which will selectively absorb certain frequencies in the spectrum of the element involved to a greater degree than other portions of the spectrum as will later be more fully explained.

Below the

glass walls

20 and 22 of the cavity there is placed a gas discharge lamp 24. The gas-discharge lamp 24 is at least partially filled with the element which is too be optically excited to produce microwave emission in the MASER apparatus.

As indicated in FIG. 1, one element suitable for use in the light-excited MASER apparatus is rubidium; in its broader aspects, however, the invention is not limited solely to the use of rubidium as the excited element and other elements could be utilized.

The interior 28 of the cavity 12 is also filled in part with rubidium in gaseous form. It will be understood that conventional heating and control apparatus may be utilized to maintain the rubidium in this gaseous form and at the proper temperature and pressure.

For reasons which have previously been explained it is desirable to maintain a low, constant, intensity magnetic field within the cavity 12. Accordingly a conventional magnetic shield 29 is provided around the cavity to shield the cavity from the earths magnetic field and the fluctuations of the intensity thereof.

Within the magnetic shield 29 there is provided a coil 30 or other suitable source of magnetic field, to provide a constant magnetic field of low intensity within the cavity 12. As previously explained, the intensity of this field should be less than the earths magnetic field and the particular intensity utilized will vary with circumstances but may readily be determined from considerations previously explained. The current required in the coil 30 will be supplied from any suitable source (not shown). Alternatively a permanent magnet may be used to provide a magnetic field.

The operation of the light-excited MASER of FIG. 1 will now be explained with reference to the principles of the emission of microwave frequency energy by light-excited atoms explained herein before.

The rubidium atoms contained within the interior 28 of the cavity 12 have energies corresponding to the number of discrete levels. Among these discrete energy levels are levels of hyperfine states which are separated by an energy corresponding to a microwave frequency. When transitions take place from an upper level of hyperfine states to a lower level of hyperfine states, electromagnetic radiation of microwave frequency is produced. The apparatus of FIG. 1 is designed to produce such transitions in sufficient quantity to produce a useful output of microwave frequency energy for use as a frequency standard for example. The transition which is utilized is the transition from the F=2 state to the F=l state as illustrated in H6. 2.

To produce these microwave transitions from the F=2 to the F=l state the population of the F=l state relative to that of the F=2 state is reduced. This is accomplished by producing transitions from the F=l state to higher energy states, such as the 5 F state shown in FIG. 2. When transitions from the F=l state are produced there will generally be transitions produced also from the F=2 state to higher energy levels. However, if the number of transitions from the F=l state sufficiently exceed the number of transitions from the F=2 state, a net relative reduction in the population of the F=l state as compared to the F=2 state will still be produced. This means that the light which excites the atoms within the cavity 12 must be such that light which excites the F=l state have a substantially greater intensity than the light which excites the F=2 state.

The original source of light represented by the lamp 24 is a gas-discharge lamp containing rubidium and accordingly emitting light having the characteristic spectrum of rubidium. In this spectrum the spectral lines of a frequency which excite the F=2 state are more intense than those lines which excite the F=l state, just the opposite condition from that which is necessary for the proper operation of the light-excited MASER.

In order to correct this condition the optical filter comprising the

glass walls

20, 22 and the enclosure 26 containing rubidium is interposed between the lamp 24 and the cavity interior 28. The absorption spectrum of rubidium is such that spectral lines which excite the F=2 state are more strongly absorbed than are spectral lines which excite the F=l state. Although the difference in the absorption coefficients of these two lines is not very great, due to the exponential character of the absorption of light in an optical filter it is possible to produce, by filtration, light which has a substantially stronger intensity for the F=l exciting spectral lines than for the F=2 exciting spectral lines. Thus the combination of the rubidiumdischarge lamp 24 with the rubidium filter consisting of glass walls and 22 and the rubidium-filled enclosure 26 produces light having the necessary spectral characteristics to excite the rubidium in the enclosure 28 so that transitions are produced from the F=2 state to the F=l state with an ensuing emission of microwave frequency energy.

It should be noted here that although it is contemplated that the apparatus of FIG. 1 would be utilized in such a way that the emission of electromagnetic radiation of microwave frequency would be sufficient to maintain a sustained oscillation in the cavity 12, it is possible to utilize the same technique to produce atomic emission of microwave frequency which is of insufficient magnitude to produce a sustained oscillation but is rather utilized to amplify a microwave signal of the same frequency introduced into the cavity. Such an apparatus could be utilized as a passive frequency standard rather than an active frequency standard by well-known techniques. For example, a microwave signal of approximately the frequency of the rubidium transition could be introduced into the cavity 12. This signal could be varied cyclically over a small frequency range including the frequency of the rubidium transition. Amplification of the signal would be produced only in the frequency range very close to the frequency of the rubidium transition. This amplification could be detected and utilized to control the center frequency of the introduced microwave signal so that it remains equal to the rubidium transition frequency to a high degree of accuracy and thereby provides a signal capable of being utilized as a frequency standard.

From the foregoing explanation it will be understood that the light-excited MASER apparatus described is not limited to use as an oscillator but also may be utilized as an amplifier. In addition, the apparatus is not restricted to use as a frequency standard where its high-frequency stability makes it exceedingly desirable, but it may also be used in other applications where other characteristics of the apparatus may be useful such as its low noise figure.

Fig. 4 which shows an alternative MASER-type apparatus operating on the same basic principles of FIG. 1, but incorporating various improvements adapted to improve the efficiency and practicality of the apparatus. A microwave resonant cavity is shown at 32 having an output 34 with a seal 36 therein to physically isolate the interior of the cavity 32. An iris 10 is provided in output 14 to limit the coupling of the cavity to the output waveguide thus permitting a high cavity Q.

The design of the cavity 32 departs to a considerable extent from the normal design of microwave frequency cavities. Where energy is introduced at an input into a microwave frequency cavity from an external source, the practical size of the microwave cavity is generally limited by the problem of coupling to parasitic modes. That is when a large cavity is used, and accordingly the mode of oscillation is of a high order, then there is a great increase in the number of nearby parasitic modes which exist. This problem is so serious that it is the general practice in the design of microwave cavities for externally supplied excitation to make the cavity as small as conveniently possible to provide a low-order mode of oscillation and thus eliminate the virtually insurmountable problem of coupling to parasitic modes. This practice is followed notwithstanding the fact that it is known that increasing the size of the cavity increases the Q of the cavity since the Q depends in part on the ratio of the volume to the surface area of the cavity.

Notwithstanding the previous teaching of the art of construction of microwave cavities, the microwave cavity for the apparatus of FIG. 4 is constructed to oscillate in the TE mode as indicated by the dotted field lines in FIG. 4. This particular mode of oscillation is illustrated as a mode which is deemed to be particularly convenient for various reasons as will be more fully explained hereinafter. The design of the cavity to utilize a high-order mode in disregard of the previous practice in the art as a result of the discovery that the lightexcited MASER apparatus of the present invention is such that the problem of parasitic modes in the resonant cavity will not arise as it would in more conventional externally excited cavity apparatus.

The distinction in the present apparatus lies in the fact that the energy bringing about the activity oscillation is not introduced at a limited area of the cavity such as is the case when microwave frequency energy is introduced through an input to the cavity from an external source. On the other hand, the energy in the present case is introduced from the atoms filling the interior of the cavity in a substantially uniform manner throughout the volume of the cavity. It will therefore be seen that a distinctly different situation exists in the present invention than in the customary resonant cavity which is excited by microwave energy from an external source. As a result of this different situation, the conditions for infinite gain by MASER action will be met only for the designed mode. Although the reasons for this are somewhat complex it may be explained briefly by pointing out that the microwave radiation of the atoms is coherent with the inducing field and hence has the same field configuration as the designed resonant mode (the atoms do not diffuse very far in the cavity). But all modes are orthogonal to each other, and therefore no other modes can be excited by the atoms themselves. That is Low-Q parasitic modes can be coupled to the desired mode of oscillation only by the perturbations of the cavity posts and these perturbations may be made very small. This principle not only allows the use of the particular mode shown in FIG. 4, namely the TE mode, but also if desired the cavity dimension could be made still larger, perhaps as large as 3 feet if necessary and thereby obtain a still larger O which could exceed 500,000.

In addition to the advantages of a larger volume cavity and a consequent increase in Q which is derived by the use of the high-order TE mode as explained above, this particular mode also provides other advantages. It will be noted that a coil 38 is provided which produces a magnetic field extending in a direction parallel to the axis of the cylindrical cavity. The effect of this magnetic field is generally the same as that described in the explanation of FIG. 1 and the preceding general explanation. It should be noted however that only the RF field components in the direction of the DC field is effective in inducing the desired hyperfine state transition. For the TE mode utilized in the embodiment illustrated in FIG. 4 it will be noted that the magnetic field loops are stretched in the Z direction and consequently the ratio of the average total magnetic field to the average magnetic field in the Z direction is only slightly greater than one. Thus a high degree of interaction between the RF field and the atoms is assured thus increasing the efficiency of the system.

In FIG. 4 slits 40 are cut in the walls of the resonant cavity and are located circumferentially around the cylindrical cavity at its midpoint. This location for the illuminating slits provides a substantially uniform illumination of the interior of the cavity particularly when allowance is made for reflection from the cavity walls. A further advantage accrues from the use of the TE mode in that both the filed and the wall currents pass through zero in the midsection of the cavity and hence the loss due to the presence of the slits in the cavity wall is minimized.

Alternatively the light may be introduced through an opening in a different part of the cavity. The dimensions of any opening in either case should be those of a waveguide below cutoff" for the microwave frequency. Thus no microwave power will escape through the light entrance.

Flanges 42 extend around the exterior of the cavity structure 32 on either side of the position of the slots 40. A window 44 is mounted between the flanges 42 so that light may be directed through the slots 40 from the exterior of the cavity 32 yet at the same time the cavity 32 may be hermetically sealed. The window 44 may be made of glass or any other suitable material transparent to the light which is utilized to excite the atoms in the interior of the cavity 32.

In the apparatus of FIG. 4 a light source energized by radiofrequency energy is utilized. A radiofrequency transmission line comprising an annular rectangular tube 46 and having centrally located therein a gas-discharge tube 48 is placed around the microwave cavity 32 at the midsection thereof.

The tube 46 is not completely closed on its inner side and a wire grid 50 is placed across this side of the tube 46 to provide a completely closed structure for electrical purposes. The wire grid 50 allows light emanating from the tube 48 to pass from the rectangular tube 46 through the window 44 and into this microwave cavity 32 by way of the slots 40. Radiofrequency energy may be supplied to the annular rectangular tube 46 by means of the coaxial input 52 or by any other suitable means.

The purpose for providing radiofrequency excitation for the gas-discharge lamp 48 is to provide greater efficiency since generally the efficiency of such a lamp increases with the frequency of the exciting energy. It is preferred to utilize a radiofrequency power supply for the gas-discharge lamp of a frequency of approximately 100 megacycles, since this provides substantial increase in efficiency over that of a lowfrequency power source and at the same time this frequency is not so high as to present great difficulty in the generation of power at this frequency.

A filter 54 is placed between the gas-discharge lamp 48 and the slots in the microwave resonant cavity. This filter comprises a container of transparent material having a filter material in gaseous form in the enclosure 56 therein.

A substantial improvement is obtained by reason of the different composition of the gases within the discharge lamp, the filter 54 and the resonant cavity 32 as compared with the composition of gases described in FIG. 1. The composition of the gas within the resonant cavity 32 has been altered by the addition of a reemission-inhibiting gas. Before explaining the exact purpose and function of this reemission inhibitor, it is necessary to review to a limited extent the operation of the apparatus.

Referring to FIG. 2 it has been explained that an excess population of the F=2 state as compared with the F==l state is obtained by exciting a substantial number of atoms in the F=l state to higher energy levels while at the same time assuring that a substantially lesser number of atoms of the F=2 state are excited to higher energy levels.

Normally there will be a tendency for the atoms excited to higher energy levels in the foregoing process to decay to lower energy states with a consequent emission of light energy. Light energy produced as a result of decay from an upper energy level to the F=2 state will be of such a frequency that it will be capable of producing the secondary effect of being absorbed by an atom in the F=2 state thus raising that atom to a higher energy level. In other words reemission of light within the cavity causes the emission of light which is of a frequency which it is desired to eliminate from the cavity in so far as possible. Although the filter previously described is efi'ective for minimizing the amount of light of this frequency entering the cavity it is obviously ineffective as regards any light generated by reemission within the cavity. Since this light of a frequency capable of exciting F=2 state atoms has the effect of reducing the population of F=2 state atoms and thereby counteracts the optical pumping effect upon which the operation of the system depends, this reemission is highly undesirable and should be eliminated.

In order to minimize detrimental reemission within the cavity a quantity of polyatomic gas such as N or H is introduced into the cavity together with the gaseous element which constitutes the working medium. By the introduction of this polyatomic gas as a reemission inhibitor the energy which would have been transformed into reemitted light is largely taken up as a vibrational energy of the molecule. The particular type and quantities of such gases to be used will vary under different circumstances and may be determined for an individual case by application of known principles. A treatment of this matter may be found for example in Resonance Radiation and Excited Atoms by A. C. G. Mitchell and M. W. Zemansky, (Cambridge University Press, 1934.)

In the formulation of the theory governing the operation of this apparatus it has always been assumed that the illumination of the cavity would be substantially uniform and it is presumed that this is a desirable if not necessary condition for the operation of the apparatus. In order to obtain substantially uniform illumination of the cavity configuration shown in FIG. 4, it is necessary that the exciting light be transmitted back and forth across the cavity a number of times with multiple reflections. In order for this to occur the absorption of exciting light must not be so great as to substantially absorb the exciting light before multiple reflections occur. It has been determined that under typical operating conditions utilizing Rb at a pressure of 2X10 mm. Hg the absorption coefficient for light inducing the (5 P 5 8 transition in rubidium is K =0.07l centimeters Thus this light would be substantially absorbed in crossing the cavity once, the cavity would not be uniformly illuminated in contradiction to the basic assumptions in developing the apparatus and the apparatus would be rendered unworkable, or, at best, the required Q of the cavity would be raised by a factor greater than 2.

Accordingly referring to FIG. 2 rather than utilizing the transition from 5 8 to 5 F which is associated with a frequency of 3.8Xl0 megacycles per second as shown in FIG. 2, the transition from 5 8 to 6 which is associated with a frequency of 7.1Xll) megacycles per second will be utilized instead. Light of this frequency has an absorption coefficient K =0.00 52 cm. under the same conditions. A photon will thus be reflected about 10 times before being absorbed and thus the desirable condition of substantially uniform illumination will be achieved.

To be utilized to excite the atoms in a MASER, the light must be modified to produce the desired spectral distribution. This may be done by the use of a filter consisting of the same element as that used in the discharge lamp and the same element as the excited atom.

The efficiency of the apparatus of FIG. 4 can further be improved by making use of the fact that there are two different isotopes of rubidium with slightly different spectral emission and absorption characteristics. Due to the slightly different characteristics as regards the spectral emission and absorption of rubidium as compared with rubidium 87, the former may be utilized to provide a filter of remarkable selectivity for eliminating the component hyperfine line which excites rubidium 87 atoms from the F=2 level. Rb may be utilized as a filter by filling the interior 56 of the filter 54 in FIG. 4 with a gas mixture comprising primarily Rb while the mixture in the interior 58 of the cavity 32 is primarily Rb".

The utility of the optical filter containing Rb gas is greatly enhanced by the addition of a polyatomic buffer gas of approximately 5 cm. Hg pressure. The mode of operation of the Rb filter will be understood by reference to FIG. 5, which is a plot of the absorption coefficient of Rb near 4,200 The absorption line is Lorentz-broadened to about the Doppler width, (1,100 me. see. by collisions in the polyatomic bufier gas forming a part of the gaseous mixture in the filter.

The spectral lines of Rb'" are shown in dashed lines, and it will be observed that the absorption line of Rb, marked F=2, substantially overlaps the spectral line of F=2 of Rb. It will be appreciated that the degree of overlap is substantially increased by the broadening of the Rb F=2 line through the addition of the polyatomic bufi'er gas in the filter. Accordingly, the absorption of the F=2 line of Rb by the Rb filter is substantially increased, thus eliminating the necessity for an unduly large filter or other disadvantages which would result when the filter material had a low coefficient of absorption for the spectral line which it was desired to eliminate.

Referring now to the right-hand side of Fig. 5, the absorption line of Rb marked F=l is spaced somewhat farther from the spectral line of Rh", marked F=l falls farther out on the wing of the absorption line of Rb, and the absorption coefficient is accordingly much lower. It may be calculated that at a pressure of Rb of approximately mm. Hg, the light transmission of the Rb" components through a 1 cm. filter are, for the F=2 hyperfine component 0.0025 and for the F=l hyperfine component 37. Thus virtually none of the F=2 hyperfine component is transmitted, while a substantial amount of the F=l component is transmitted through the filter and is available for transmission into the cavity 32. It will be appreciated that the isotopes or rubidium are separated with difficulty and accordingly some allowance should be made as a practical matter for residual concentration of the Rb in the filter. In the above computation a residual concentration of 5 percent of Rb" is assumed.

When the Rb filter is utilized it is preferable that the interior of the cavity 32 be filled primarily with Rb" (together with a reemission-inhibiting buffer gas, if desired.)

From the foregoing explanation it will be understood that the present invention provides a light-excited amplifier or oscillator operating on a principle of amplification by stimulated emission of radiation (MASER), which is particularly simple in construction and operation. Various modifications and variation of the apparatus will be apparent to those of ordinary skill in the art, in addition to those shown and suggested herein above. Accordingly the scope of the invention is not to be construed to be limited to only those embodiments shown or suggested, but is to be limited solely by the appended claims.

What is claimed is:

1. Apparatus for amplifying or generating microwave frequency oscillations through amplification by stimulated emission of radiation, the energy for such amplification being supplied by substantially unpolarized light of nonuniform spectral density, said apparatus comprising a radiofrequency resonant structure formed at least in part of conductive material, an enclosure at least partially coextensive with said structure, a gaseous working medium in said enclosure, said medium having atoms, ions, or molecules with a pair of energy levels separated by a value of energy substantially corresponding to a resonant frequency of said resonant structure, a source of light having spectral components exciting said pair of energy levels to higher energy levels, means causing the excitation by said light source of atoms, ions, or molecules existing in the lower of the said pair of energy levels to be greater than the excitation of those existing at the higher of said pair of levels, means for ingress of light from said source into said enclosure and means for transmitting radiofrequency energy from said resonant structure to an exterior point of utilization.

2. Apparatus as claimed in claim 1 further including means for maintaininga low-level substantially constant magnetic field in said resonant structure.

3. Apparatus as claimed in claim 1 wherein said gaseous medium comprises rubidium.

4. Apparatus for amplifying or generating microwave frequency oscillations by means of amplification by stimulated emission of radiation, the energy for such amplification being supplied by substantially unpolarized light of nonuniform spectral density, said apparatus comprising a microwave frequency cavity resonator, an enclosure substantially coextensive with said resonator, a gaseous working medium in said enclosure, said medium having atoms, ions, or molecules with a pair of energy levels separated by a value of energy corresponding to a microwave radiofrequency, a gaseousdischarge lamp light source having spectral components exciting said pair of energy levels to higher energy levels, means causing the excitation by said light source of atoms, ions, or molecules existing in the lower of the said pair of energy levels to be greater than the excitation of those existing at the higher of said pair of levels, means for ingress of light from said source into said enclosure and means for transmitting radiofrequency energy from said resonator to an exterior point of utilization.

5. Apparatus as claimed in claim 1 further including means for maintaining a low-level substantially constant magnetic field in said resonator.

6. Apparatus as claimed in claim 4 wherein the amplification of said apparatus is of magnitude great enough to cause self sustained radiofrequency oscillations to be initiated from random emission and maintained by stimulated emission.

7. Apparatus as claimed in claim 4 wherein said gaseous medium comprises rubidium.

8. Apparatus for amplifying or generating microwave frequency oscillations through amplification by stimulated emission of radiation, the energy for such amplification being supplied by substantially unpolarized light of nonuniform spectral density, said apparatus comprising a radiofrequency resonant structure, and enclosure at least partially coextensive with said structure, a gaseous working medium comprising rubidium with a predominance of the isotope of rubidium of atomic weight 87 in said enclosure, said element having atoms, ions, or molecules with a pair of energy levels separated by a value of energy corresponding to a radiofrequency, a source of light having spectral components exciting said pair of energy levels to higher energy levels, means causing the excitation by said light source of atoms, ions, or molecules existing in the lower of the said pair of energy levels to be greater than the excitation of those existing at the higher of said pair of levels, said means comprising an optical filter including in substantial proportion the isotope of rubidium of atomic weight 85, said optical filter further including a polyatomic buffer gas at a partial pressure of approximately 5 cm. of mercury to cause the absorption coefficient of the spectral components exciting said upper one of said pair of energy levels to be increased to a substantial value, means for ingress of light from said source into said enclosure and means for transmitting radiofrequency energy from said resonant structure to an exterior point of utilization.

9. Apparatus for amplifying or generating microwave frequency oscillations through amplification by stimulated emission of radiation, the energy for such amplification being supplied by substantially unpolarized light of nonuniform spectral density, said apparatus comprising a radiofrequency resonant structure, an enclosure at least partially coextensive with said structure, a gaseous working medium comprising a multiisotope chemical element with a predominance of one isotope of said element in said enclosure said element having atoms, ions, or molecules with a pair of energy levels separated by a value of energy corresponding to a radiofrequency, a source of light having spectral components exciting said pair of energy levels to higher energy levels, means causing the excitation by said light source of atoms, ions, or molecules existing in the lower of the said pair of energy levels to be greater than the excitation of those existing at the higher of said pair of levels, said means comprising an optical filter including said chemical element and including in substantial proportion another isotope of said element different from said one isotope, said optical filter further including a polyatomic buffer gas to cause the absorption coefficient of the spectral components exciting said upper one of said pair of energy levels to be increased to a substantial value, means for ingress of light from said source into said enclosure and means for transmitting radiofrequency energy from said resonant structure to an exterior point of utilization.

10. Apparatus for amplifying or generating microwave frequency oscillations through amplification by stimulated emission of radiation, said apparatus comprising radiofrequency resonant structure, an enclosure at least partially coextensive with said structure, a gaseous working medium comprising a multiisotope chemical element with a predominance of one isotope of said element having atoms, ions, or molecules with a pair of energy levels separated by a value of energy corresponding to a radiofrequency, a source of light having spectral components exciting said pair of energy levels to higher energy levels, means causing the excitation by said light source of atoms, ions, or molecules existing in the lower of the said pair of energy levels to be greater than the excitation of those existing at the higher of said pair of levels, said means comprising an optical filter including said chemical element and including in substantial proportion another isotope of said element different from said one isotope, means for ingress of light from said source into said enclosure and means for transmitting radiofrequency energy from said resonant structure to an exterior point of utilization.

1 1. Apparatus as claimed in claim wherein said chemical element is rubidium.

12. Apparatus for amplifying or generating microwave frequency oscillations through amplification by stimulated emission of radiation, said apparatus comprising a microwave frequency cavity resonator, resonant in a high-order mode, the magnetic field loops of said mode being elongated in a direction parallel to the magnetic field in said cavity resonator, the average magnetic field in said resonator in a direction parallel to said magnetic field being a substantial portion of the average total magnetic field, an enclosure at least partially coextensive with such structure, a gaseous working medium in said enclosure, said medium having atoms, ions, or molecules with a pair of energy levels separated by a value of energy corresponding to a radiofrequency, a polyatomic reemission-inhibiting gas in said enclosure for reducing reemission of light photons from said working medium, a source of light having spectral components exciting said pair of energy levels to higher energy levels, said light source comprising a radiofrequency-energized gaseous-discharge lamp substantially encircling said cavity resonator at the midportion thereof, said lamp comprising an annular conductive tube and a gas-discharge tube centrally located therein, said annular tube and said gas-discharge tube forming a radiofrequency coaxial line, means causing excitation by said light source of atoms, ions, or molecules existing in the lower of the said pair of energy levels to be greater than the excitation of those existing at the higher of said pair of levels, means for ingress of light from said source into said enclosure, the last said means comprising slots placed circumferentially around the wall of said cavity resonator at a nodal point for the resonant mode of said cavity resonator and means for transmitting radiofrequency energy from said cavity resonator to an exterior point of utilization.

13. Apparatus for amplifying or generating microwave frequency oscillations through amplification by stimulated emission of radiation, said apparatus comprising a microwave frequency cavity resonator, resonant in .a high-order mode,

the magnetic field loops of said mode being elongated in a direction parallel to the magnetic field in said cavity resonator, the average magnetic field in said resonator in a direction parallel to said magnetic field being a substantial portion of the average total magnetic field, an enclosure at least partially coextensive with such structure, a gaseous working medium in said enclosure, said medium having atoms, ions, or molecules with a pair of energy levels separated by a value of energy corresponding to a radiofrequency, a source of light having spectral components exciting said pair of energy levels to higher energy levels, means causing excitation by said light source of atoms, ions, or molecules existing in the lower of the said pair of energy levels to be greater than the excitation of those existing at the higher of said pair of levels, means for ingress of light from said source into said enclosure, and means for transmitting radiofrequency energy from said cavity resonator to an exterior point of utilization.

14. Apparatus for amplifying or generating microwave radiofrequency oscillations through amplification by stimulated emission of radiation, the energy for such amplification being supplied by substantially unpolarized light of nonuniform spectral density, said apparatus comprising a microwave frequency cavity resonator, and enclosure at least partially coextensive with such structure, a gaseous working medium in said enclosure, said medium having atoms, ions, or molecules with a pair of energy levels separated by a value of energy corresponding to a radiofrequency, a polyatomic reemission-inhibiting gas in said enclosure for reducing reemission of light photon from said working medium, a source of light having spectral components exciting said pair of energy levels to higher energy levels, means causing excitation by said light source of atoms, ions, or molecules existing in the lower of the said pair of energy levels to be greater than the excitation of those existing at the higher of said pair of levels, means for ingress of light from said source into said enclosure, and means for transmitting radiofrequency energy from said cavity resonator to an exterior point of utilization.

15. Apparatus for amplifying or generating microwave frequency oscillations through amplification by stimulated emission of radiation, the energy for such amplification being supplied by substantially unpolarized light of nonuniform spectral density, said apparatus comprising a radiofrequency resonant structure formed at least in part of conductive material, an enclosure at least partially coextensive with said structure, a gaseous working medium in said enclosure, said medium having atoms, ions, or molecules with a pair of energy levels separated by a value of energy corresponding to a radiofrequency, a source of light having spectral components exciting said pair of energy levels to higher energy levels, and comprising a gas-discharge lamp utilizing the same chemical element as said working medium, means causing the excitation by said light source of atoms, ions, or molecules existing in the lower of the said pair of energy levels to be greater than the excitation of those existing at the higher of said pair of levels, said means comprising an optical filter including the same chemical element as said working medium, means for ingress of light from said source into said enclosure and means for transmitting radiofrequency energy from said resonant structure to an exterior point of utilization.

16. Apparatus for amplifying or generating microwave frequency oscillations through amplification by stimulated emission or radiation, the energy for such amplification being supplied by substantially unpolarized light of nonuniform spectral density, said apparatus comprising a radiofrequency resonant structure formed at least in part of conductive material, an enclosure at least partially coextensive with said structure, a gaseous working medium in said enclosure, said medium having atoms, ions, or molecules with a pair of energy levels separated by a value of energy corresponding to a microwave radiofrequency, a gaseous-discharge lamp light source having spectral components exciting said pair of energy levels to higher energy levels, means causing the excitation by said light source of atoms, ions, or molecules existing in the lower of the said pair of energy levels to be greater than the excitation of those existing at the higher of said pair of levels, means for ingress of light from said source into said enclosure and means for transmitting radiofrequency energy from said resonant structure to an exterior point of utilization.

17. Apparatus for amplifying or generating microwave frequency oscillations through amplification by stimulated emission or radiation, the stimulation being produced by substantially unpolarized light of nonuniform spectral density, said apparatus comprising a microwave frequency cavity resonator, an enclosure at least partially coextensive with such structure, a gaseous working medium in said enclosure, said medium having atoms, ions, or molecules with a pair of energy levels separated by a valve of energy corresponding to a radiofrequency, a source of light having spectral components exciting said pair of energy levels to higher energy levels, means causing excitation by said light source of atoms, ions, or molecules existing in the lower of the said pair of energy levels to be greater than the excitation of those existing at the higher of said pair of levels, means allowing ingress of light from said source into said enclosure, the last said means comprising at least one opening in the wall of said cavity resonator at a nodal point for the resonant node of said cavity resonator, and means for transmitting radiofrequency energy from said cavity resonator to an exterior point of utilization.

18. Apparatus for generating radiofrequency oscillations comprising a radiofrequency resonant structure for sustaining a radiofrequency field in a field space, a gaseous working medium having a pair of energy levels separated by a value of energy corresponding to a radiofrequency and at least partially coextensive with said field space, and a source of light directed into said medium and having stronger spectral components for exciting the lower of said pair of levels than for exciting the upper of said pair of levels.

Claims

2. Apparatus as claimed in claim 1 further Including means for maintaining a low-level substantially constant magnetic field in said resonant structure.

4. Apparatus for amplifying or generating microwave frequency oscillations by means of amplification by stimulated emission of radiation, the energy for such amplification being supplied by substantially unpolarized light of nonuniform spectral density, said apparatus comprising a microwave frequency cavity resonator, an enclosure substantially coextensive with said resonator, a gaseous working medium in said enclosure, said medium having atoms, ions, or molecules with a pair of energy levels separated by a value of energy corresponding to a microwave radiofrequency, a gaseous-discharge lamp light source having spectral components exciting said pair of energy levels to higher energy levels, means causing the excitation by said light source of atoms, ions, or molecules existing in the lower of the said pair of energy levels to be greater than the excitation of those existing at the higher of said pair of levels, means for ingress of light from said source into said enclosure and means for transmitting radiofrequency energy from said resonator to an exterior point of utilization.

9. Apparatus for amplifying or generating microwave frequency oscillations through amplification by stimulated emission of radiation, the energy for such amplification being supplied by substantially unpolarized light of nonuniform spectral density, said apparatus comprising a radiofrequency resonant structure, an enclosure at least partially coextensive with said structure, a gaseous working medium comprising a multiisotope chemical element with a predominance of one isotope of said element in said enclosure, said element having atoms, ions, or molecules with a pair of energy levels separated by a value of energy corresponding to a radiofrequency, a source of light having spectral components Exciting said pair of energy levels to higher energy levels, means causing the excitation by said light source of atoms, ions, or molecules existing in the lower of the said pair of energy levels to be greater than the excitation of those existing at the higher of said pair of levels, said means comprising an optical filter including said chemical element and including in substantial proportion another isotope of said element different from said one isotope, said optical filter further including a polyatomic buffer gas to cause the absorption coefficient of the spectral components exciting said upper one of said pair of energy levels to be increased to a substantial value, means for ingress of light from said source into said enclosure and means for transmitting radiofrequency energy from said resonant structure to an exterior point of utilization.

10. Apparatus for amplifying or generating microwave frequency oscillations through amplification by stimulated emission of radiation, said apparatus comprising a radiofrequency resonant structure, an enclosure at least partially coextensive with said structure, a gaseous working medium comprising a multiisotope chemical element with a predominance of one isotope of said element having atoms, ions, or molecules with a pair of energy levels separated by a value of energy corresponding to a radiofrequency, a source of light having spectral components exciting said pair of energy levels to higher energy levels, means causing the excitation by said light source of atoms, ions, or molecules existing in the lower of the said pair of energy levels to be greater than the excitation of those existing at the higher of said pair of levels, said means comprising an optical filter including said chemical element and including in substantial proportion another isotope of said element different from said one isotope, means for ingress of light from said source into said enclosure and means for transmitting radiofrequency energy from said resonant structure to an exterior point of utilization.

11. Apparatus as claimed in claim 10 wherein said chemical element is rubidium.

13. Apparatus for amplifying or generating microwave frequency oscillations through amplification by stimulated emission of radiation, said apparatus comprising a microwave frequency cavity resonator, resonant in a high-order mode, the magnetic field loops of said mode being elongated in a direction parallel to the magnetic field in said cavity resonator, the average magnetic field in said resonator in a direction parallel to said magnetic field being a substantial portion of the average total magnetic field, an enclosure at least partially coextensive with such structure, a gaseous working medium in said enclosure, said medium having atoms, ions, or molecules with a pair of energy levels separated by a value of energy corresponding to a radiofrequency, a source of light having spectral components exciting said pair of energy levels to higher energy levels, means causing excitation by said light source of atoms, ions, or molecules existing in the lower of the said pair of energy levels to be greater than the excitation of those existing at the higher of said pair of levels, means for ingress of light from said source into said enclosure, and means for transmitting radiofrequency energy from said cavity resonator to an exterior point of utilization.