US3177445A

US3177445A - Harmonic generator using equally spaced energy levels

Info

Publication number: US3177445A
Authority: US
Publication date: 1965-04-06
Anticipated expiration: 1982-04-06

Description

April 1965 c. M. KELLINGTON HARMONIC GENERATOR USING EQUALLY SPACED ENERGY LEVELS Filed June 28, 1963 FIELD 0mmzw V INVENTOR, CHARLES M- KELLINGTON United States Patent 3,177,445 HONIC GENERATOR USING EQUALLY SPAQED ENERGY LEVELS Charles M. Kellington, Point Pleasant Beach, N .J., assignor to the United States of America as represented by the Secretary of the Army a Filed June 28, 1963, Ser. No. 292,21

3 Claims. (Cl. 331-94) (Granted under Title 35, US. Code (1952), see. 266) The invention described herein may be manufactured and used by or for the Government for governmental purposes, without the payment of any royalty thereon.

The present invention relates to a harmonic generator whereinthe harmonic frequencies are generated by the interaction of a quantum-mechanical system and an electromagnetic field.

In the field of frequency generation of millimeter and submillimeter Wavelengths, it has been the general practice to employ frequency doublers or harmonic generators for increasing the frequency of the available energy. The most recent advances include the varactor and the crystal diode. Although such devices have served the purpose, considerable difficulty has been experienced in obtaining a substantially stable, noise free signal as a useful power output. Such devices drastically fall-off in efficiency with the increase in frequency.

The recent development of themaser has demonstrated that microwave. energy may be amplified efliciently, to a useful power level without the introduction of noise and instabilities, by the stimulated emission of radiation in solid state devices.

The operation of a solid state maser is dependent upon the fact that in some materials atoms and molecules exist in certain quantized energy levels or states. These states arise from the interaction of the magnetic moments of the electrons with internal or external fields. The electrons possess magnetic moments due to the spins of the electrons in their orbits. A material having a plurality of available energy levels will have, under ordinary conditions, more atoms at the-lower energy states than in the upper. To obtain maser action'this situation is reversed by absorption of energy at the correctfrequency i.e. the upper state is made more populated; after which radiation is then emitted by those atoms when they return to the lower or natural states.

The instant invention, utilizing this ability of a solid state crystal to absorb and emit energy, pumps a crystal at one frequency while permitting the crystal to emit at a frequency which is the harmonic of the pump frequency.

It is an object of the present invention to provide a quantum mechanical harmonic generator.

A further object of the invention is to provide a harmonic generator the efiiciency of which will not falloif with an increase in the operating frequency.

Still another object of this invention is to provide a device which can produce harmonic generation in the millimeter and submillimeter range.

Yet another object of the invention is to provide a harmonic generator which is inherently stable and noise free. i

The exact nature of this invention as well as other objects and advantages thereof will be readily apparent from consideration of the following specification relating to q the annexed drawing in which:

3,177,445 Patented Apr. 6, 1965 FIGURE 3 is an illustration of one embodiment of the invention.

Referring now to the drawings, there is shown in FIG- URE 1 a graph, the abscissa of which represents the energy possessed by atoms in a paramagnetic crystal, while the ordinate represents the value of an external D.-C. magnetic field applied to the crystal. Four curves, designated E E E and B are plotted on the graph. These curves represent the available energy levels for the atoms' of a four-level paramagnetic crystal. The exact curves shown in FIGURE 1 approximately represent the available spin energy levels for the Cr ion in the Al O Cr O (ruby) crystal While under the influence of a D.-C. magnetic field which is oriented at approximately an angle of 65 with respect to the C or optical axis of the crystal. As shown in the graph of FIGURE 1, the energy levels are two-fold degenerate in a zero magnetic field. However, these levels, when under the influence of an external magnetic field, lose their degeneracy with respect to the spin as represented by the curves E1, E2, E3, and E4.

The spacing between these levels varies with the amount of the external magnetic field. However, for some particular values of the magnetic field the spacing between some of the levels will be equal to each other. As shown in FIGURE 1, at the value H, for the magnetic field, the distance from E to E is equal to the distance from E to E For the particular crystal mentioned above, i.e. ruby, and for the conditions outlined, the application of substantially 3430 gauss will result in an equal spacing between levels E and E and levels E and E These levels, which are shown in FIGURE 2, may be used to produce harmonic generation.

Under ordinary conditions the distribution of the atoms in the energy levels is such that the lower energy levels are more densely populated than the higher levels. The densities may be altered by applying electromagnetic energy of the proper frequency to the crystal such that absorption will take place in the crystal. Once the upper levels are populated above normal, emission of radiation will occur. when these atoms return to their normal or ordinary states.

It has been determined, subject to quantum selection rules, that atoms being pumped to various energy states absorb energy according to the following relationship:

where h is Planks constant, v is the frequency of the absorbed energy and E is the increase in energy of the atom. The reverse process is also true; that is, the atom will give ofi energy of the frequency v when the atom decreases in energy of the amount AE. This value, AB, is a quantized value as shown in FIGURE 2 thereby permitting only certain values of AE to exist for a particular condition.

If the spacings between energy levels are equal, harmonic generation may be produced by pumping, with energy at a single frequency v corresponding to the energy level spacing AE, the atoms in a multi-step process and then extracting energy from the system while permitting the atoms to return to the initial energy state in a single step. This principle is illustrated in FIGURE 2 which shows an arrow E which represents a rise in energy of an atom from state E to state E and an arrow E which represents a rise in the energy of an atom from state E to state E Energy will be emitted when the atoms return to level E from level E Since the frequency v of the emitted radiation is proportional to the change in energy E and since the spacing of the energy levels are equal i.e. E =E then the frequency of the emitted radiation will be equal to twice the frequency of the pumping energy.

For the ruby crystal mentioned above and for the conditions outlined in connection with this crystal, the spacings E and E 4 correspond substantially to a frequency of 9425 me. Emission will take place at the second harmonic or substantially 18,850 me. which is the sum of E and E and corresponds to E FIGURE 3 which illustrates one embodiment of the invention shows a pump using a waveguide 11 to feed energy to a cavity 12. The cavity 12 is so designed as to be resonant at both the pump frequency and the particular harmonic desired. Located in the cavity 12 is a paramagnetic crystal 13, the C-axis of which is oriented with respect to a magnetic field produced by

magnetic poles

14 and 15. The output of cavity 12 is connected to a waveguide 16 which feeds a load 17.

The magnetic field produced by poles 14 and will be adjusted to a value corresponding to the value H of FIGURE 1. Of course, any value of H may be picked so long as the energy level spacings through which the atoms are to bepumped are equal.

The cavity or the crystal will operate at room temperature; however, if the temperature of the crystal is lowered to obtain a greater population at the lower levels, the efficiency of the device will generally increase. The crystal will also be shielded from thermal noise when refrigerated. However, it has long been recognized that the emission of radiation in paramagnetic materials gencrates a very small amount of noise at room temperature.

The operating range of such devices is limited only by the strength of the available magnetic field and the energy level characteristics of the available crystals. Of course, present crystals are capable of emitting radiation in the microwave and infrared regions. Obviously, the range of frequencies may be extended as desired by using more than three levels thereby producing larger harmonies and higher frequencies.

Stability is inherent since the crystal will resonate only at the frequency represented by the quantized energy levels.

It should be understood, of course, that the foregoing disclosure relates to only a preferred embodiment of the invention and that numerous modifications or alterations may be made therein without departing from the spirit and scope of the invention as set forth in the appended claims.

What is claimed is:

1. A generator for producing second harmonic frequencies comprising a paramagnetic crystal, means applying a fixed direct current magnetic field to said crystal, said magnetic field providing first, second, and third energy levels in said crystal, the separation between said first and said second energy levels being equal to the separation between said second and said third energy levels, means for applying to said crystal electromagnetic energy of the frequency corresponding to each said equal separations, and means extracting from said crystal energy of the frequency corresponding to the sum of said equal separations.

2. A harmonic generator comprising a paramagnetic crystal, means applying a fixed direct current magnetic field to said crystal, said magnetic field providing a plurality of distinct and consecutive energy level separations, said separations being equal to each other, means for applying to said crystal electromagnetic energy of the frequency corresponding to each said equal separations, and means extracting from said crystal energy of the frequency corresponding to the sum of said equal separations.

3. A harmonic generator comprising a cavity resonant at a first frequency and a harmonic of said first frequency, a paramagnetic crystal located in said cavity, means applying a fixed direct current magnetic field to said crystal, said magnetic field providing a plurality of distinct and consecutive energy level separations equal in number to said harmonic, each of said energy level separations being equal and corresponding to said first frequency, means for applying to said cavity electromagnetic energy at said first frequency, and means for extracting from said cavity energy at said harmonic frequency.

References Cited by the Examiner UNITED STATES PATENTS 2,909,654 10/59 Bloembergen 3304 3,001,142 9/61 Mims 330-4 OTHER REFERENCES Geusic: Physical Review, April 1960, pages 129-130.

Arams: Proceedings of the IRE, January 1960, page 108.

Advances in Quantum Electronics, edited by Singer, Columbia University Press, 1961, pages 612-618 relied on (Fontana et al.).

Fontana et a1.: Proceedings of the IRE, April 1962, pages 469-470.

ROY LAKE, Primary Examiner.

Claims

1. A GENERATOR FOR PRODUCING SECOND HARMONIC FREQUENCIES COMPRISING A PARAMAGNETIC CRYSTAL, MEANS APPLYING A FIXED DIRECT CURRENT MAGNETIC FIELD TO SAID CRYSTAL, SAID MAGNETIC FIELD PROVIDING FIRST,SECOND, AND THIRD ENERGY LEVELS IN SAID CRYSTAL, THE SEPARATION BETWEEN SAID FIRST AND SAID SECOND ENERGY LEVELS BEILNG EQUAL TO THE SEPARATION BETWEEN SAID SECOND AND SAID THIRD ENERGY LEVELS, MEANS FOR APPLYING TO SAID CRYSTAL ELECTROMAGNETIC ENERGY OF THE FREQUENCY CORRESPONDILNG TO EACJH SAID EQUAL SEPARATIONS, AND MEANS EXTRACTING FROM SAID CRYSTAL ENERGY OF THE FREQUENCY CORRESPONDING TO THE SUM OF SAID EQUAL SEPARATIONS.