GB1595991A

GB1595991A - Nuclear magnetic resonance arrangements

Info

Publication number: GB1595991A
Application number: GB5103177A
Authority: GB
Original assignee: Litton Systems Inc
Current assignee: Northrop Grumman Guidance and Electronics Co Inc
Priority date: 1977-12-07
Filing date: 1977-12-07
Publication date: 1981-08-19

Description

(54) NUCLEAR MAGNETIC RESONANCE ARRANGEMENTS (71) We, LITTON SYSTEMS, INC., a corporation organised and existing under the laws of the State of Delaware, United States of America, having an office at 360 North Crescent Drive, Beverly Hills, California 90210, 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:- This invention relates to a nuclear magnetic resonance arrangement and is particularly applicable to a nuclear magnetic resonance gyroscope.

A number of approaches have been suggested in the prior art for implementing the basic concept of a nuclear magnetic resonance NMR gyroscope. In general, they utilize a nuclear magnetic resonance controlled oscillator and derive rotational information from the phases of the nuclear moment Larmor precession signals by means of suitable phase comparison and magnetic field control circuitry.

According to the present invention, there is provided a nuclear magnetic resonance arrangement comprising: a nuclear magnetic resonance cell containing a gas or vapour comprising an optically pumpable substance that possesses a magnetic moment, and at least one nuclear moment gas, the or each nuclear moment gas having nuclear magnetic moments; means for applying a steady magnetic field to the cell; means for applying an alternating current carrier magnetic field to the cell; first means for illuminating the cell with light of a wavelength which can be absorbed by the optically pumpable substance to align at least partly the magnetic moments of the optically pumpable substance in one direction; means for at least partly aligning the nuclear magnetic moments of the or each nuclear moment gas and for causing the aligned nuclear magnetic moments of the or each nuclear moment gas to precess about the direction of the steady magnetic field at substantially the respective Larmor precession frequency of the or each gas; second means for illuminating the cell with light of a wavelength substantially equal to a wavelength which can be absorbed by the optically pumpable substance, which light has a component of illumination orthogonal to the direction of the alternating current carrier magnetic field so as to produce modulations to the intensity of the transmitted part of the light at substantially the frequency of at least one harmonic, including the fundamental, of the alternating current carrier magnetic field; means for detecting at least one of the modulations in the intensity of the transmitted light; and apparatus for electrically demodulating the detected light intensity modulations to obtain a signal whose frequency is representative of the Larmor precession frequency of the or each nuclear moment gas and whose amplitude is proportional to the degree of alignment of the nuclear magnetic moments of the or each gas.

In accordance with one embodiment of the invention a nuclear magnetic resonance (hereinafter referred to as "NMR") gyroscope is disclosed hereinafter that operates on the principle of sensing inertial angular rotation rate or angular displacement about a sensitive axis of the device as a shift in the Larmor precession frequency or phase, respectively, of one or more isotopes that possess nuclear magnetic moments. The gyroscope is composed of an angular rotation sensor and associated electronics. The principal elements of the sensor are a light source, an NMR cell, a photodetector, a set of magnetic shields and a set of magnetic field coils.The principal elements of the electronics are signal processing circuits for extracting the Larmor precession frequency and phase information as well as circuits for generating and controlling various magnetic fields, both steady and varying sinusoidally with time, that are required for the desired operation of the device.

The NMR cell is mounted within a set of magnetic shields in order to attenuate external magnetic fields to acceptably low levels. Magnetic field coils are used to apply very uniform magnetic fields to the NMR cell. Both a steady field and an AC carrier field are applied along the sensitive axis of the device and AC feedback fields are applied along one of the transverse axes. The DC magnetic fields along both transverse axes are controlled to be virtually zero. The NMR cell contains an alkali metal vapor, such as rubidium, together with two isotopes of one or more noble gases, such as krypton-83 and xenon-129. A buffer gas such as helium may also be contained in the cell.

The NMR cell is illuminated by a beam of circularly polarized light that originates from a source such as a rubidium lamp and which passes through the cell at an angle with respect to the steady magnetic field Absorption of some of this light causes the atomic magnetic moments of the rubidium atoms to be partially aligned in the direction of the steady magnetic field. This alignment is partially transferred to the nuclear magnetic moments of the noble gases and these moments are caused to precess about the direction of the steady magnetic field, which in turn creates magnetic fields that rotate at the respective Larmor precession frequencies of the two noble gases.These rotating fields modulate the precessional motions of the rubidium magnetic moments, which in turn produces corresponding modulations of the transmitted light, thereby making it possible to optically detect the Larmor precession frequencies of the two noble gases.

The modulations of the light intensity are converted into electrical signals by a photodetector and these signals are then electronically demodulated and filtered to provide signals at the Larmor precession frequencies pf the two noble gases. The difference between the two precession frequencies is used to accurately control the steady magnetic field so that it is constant. One of the noble gas precession frequencies is compared to a precision reference frequency and the resulting difference frequency is the angular rotation rate of the gyroscope.

The two detected noble gas precession signals are also used to generate two AC feedback magnetic fields at the Larmor precession frequencies of the noble gases, and these are responsible for sustaining the precession of the nuclear magnetic moments of the noble gases. The use of an AC carrier magnetic field facilitates the optical detection of the precessing noble gas moments as well as providing means for controlling the DC magnetic fields along the two transverse axes of the gyroscope.

Summarising that embodiment, the NMR gyroscope includes means for the simultaneous alignment of the nuclear magnetic moments of at least two nuclear moment gases, thereby constituting a nuclear magnetic moment alignment device; means for achieving sustained precession of these moments, thereby constituting a nuclear magnetic resonance oscillator capable of sustained oscillations; means for the optical detection of these precessing nuclear moments, thereby constituting a nuclear magnetic resonance detection device; means for accurately controlling the internal magnetic field of the device; and means for the accurate measurement of the frequency or phase of the detected nuclear moment precession signal of at least one of the nuclear moment gases to provide a measurement of the angular rotation rate or angular displacement, respectively, of the device with respect to inertial space, thereby constituting an NMR gyroscope.

More particularly, a steady magnetic field is applied to an NMR cell which is substantially shielded from other steady magnetic fields. The NMR cell contains a gas or vapor of a substance that possesses a magnetic moment that can be aligned by optical pumping together with one or more additional gases, each of which possesses a nuclear magnetic moment. The NMR cell is illuminated by optical pumping light which has a directional component that is parallel to the direction of the steady magnetic field and which has the proper wavelength to be absorbed by the optically umpable substance and partially align the magnetic moments of that substance. The nuclear moments of the nuclear moment gases are caused to become aligned and are caused to precess at their respective Larmor precession frequencies about the direction of the steady magnetic field. An AC magnetic field at a suitable carrier frequency is also applied to the NMR cell and the cell is illuminated by detection light which has a directional component that is orthogonal to the direction of the AC carrier magnetic field and which has a wavelength that is essentially the same as that of the optical pumping light. The intensity ot that part of the detection light that is transmitted by the cell is modulated in accordance with the totality of the magnetic fields present in the cell, including the magnetic fields that are generated by the precessing nuclear magnetic moments. These modulations of the transmitted light intensity are detected by a photodetector, after which they are electronically demodulated to obtain signals at the Larmor precession frequencies of the nuclear moment gases.

In one embodiment, the alignment of the nuclear magnetic moments of each nuclear moment gas is accomplished by collisional interactions between the atoms of the optically pumpable substance and the atoms of the nuclear moment gas or gases. Sustained precession of the nuclear magnetic moments of each nuclear moment gas is accomplished by the application of an AC feedback magnetic field at the Larmor precession frequency of the nuclear moment gas in a direction that is orthogonal to the direction of the steady magnetic field.The AC carrier magnetic field is applied at substantially the Larmor precession frequency of the optically pumpable substance and in a direction that is substantially parallel to the direction of the steady magnetic field, thereby permitting the device to be operated at higher values of the steady magnetic field strength and with correspondingly higher Larmor recession frequencies for the nuclear moment gases.

In the preferred embodiment, an optically pumpable substance such as an alkali metal vapor is placed in an NMR cell together with two noble gases and the nuclear magnetic moments of both noble gases are aligned simultaneously by collisional interactions between the atoms of the alkali metal atoms and the atoms of the two noble gases. In this preferred embodiment of the invention, the alkali metal is rubidium and the noble gases are krypton-83 and xenon-129.

Another preferred feature involves the use of at least one buffer gas in substantial quantities in the NMR cell.

In accordance with still another preferred feature, the magnitude of the steady magnetic field is caused to remain constant by feedback control of the field in such a way that the difference between the Larmor precession frequencies of the two noble gases in the NMR cell is caused to be equal to a predetermined constant value.

In accordance with yet another preferred feature, one of the Larmor precession frequencies is compared to a precision reference frequency and the resulting difference frequency is utilized to provide a measurement of angular displacement or angular rate of the device about the direction of the steady magnetic field.

These and - other features of the invention will be explained further with reference to the following description of operation of the NMR gyroscope outlined above.

The NMR gyroscope operates on the principle of sensing angular rotation rate as a shift in the Larmor precession frequency of one or more nuclear species that possess nuclear magnetic moments.

Many atomic isotopes (usually those with odd atomic mass number) possess an inherent angular momentum (spin) associated with the nucleus. Always coexisting with such nuclear angular momentum is a magnetic moment parallel with it. The ratio between the nuclear magnetic and the nuclear angular momentum is a constant, y, called the gyromagnetic ratio, which has a particular value for each type of isotope.

If a nuclear magnetic moment is placed in a magnetic field, with any orientation other than being parallel to the direction of the field, then the magnetic moment will precess about the direction of the field with an angular frequency, o,, called the Larmor precession frequency, which is equal to: c),=yH (1) where y is the gyromagnetic ratio and H is the magnetic field strength. Each isotope, therefore, has a characteristic Larmor precession frequency in a given magnetic field.

If a system containing atoms that collectively have a precessing magnetic moment is itself rotating at an angular rate, " about the direction of H, then the observed precession frequency will be shifted by an amount equal to that rotation rate so that the observed Larmor precession frequency will become: w=yH-cor (2) Thus, a measurement of the observed Larmor frequency, no, can be used as a measure of this rotation rate is both v and H are known.

If the Larmor precession frequencies of two isotopes, each having a different value of y, are measured in the same magnetic field, then the rotation rate can be measured without direct knowledge of the value of the magnetic field. The equations for the two isotopes are: no,=yaH-nor (3) #b=γbH-#r where noa and Wb are the observed Larmor frequencies of the two isotopes having gyromagnetic ratios Ya and Yb respectively.Solving these equations for either H or o, gives the following expressions: H=(#a-#b)/(γa-γb) (4) which is independent of the angular rotation rate,eor, and γb#a-γa#b Ur= (5) Ya Yb which is independent of the magnetic field strength, H.

In one of the embodiments of this invention, the magnetic field strength is caused to be constant by controlling the field in such a way that the frequency difference, #a-#b, between the two observed Larmor precession frequencies is always equal to a constant. Specifically, two precision reference frequencies, noa and rsb, which are derived from a very stable, common frequency source, are chosen such that G9á is approximately equal to γaH1 and nobr IS approximately equal to YhH, and their ratio accurately satisfies the following relationship:: #a'/#b'=γa/γb (6) The magnetic field strength is then servo-controlled in such a way that the measured frequency difference between the two observed Larmor precession frequencies is always caused to be equal to the frequency difference between the two precision reference frequencies, namely: noa-nob=noa'-nob' (7) As a consequence of imposing the two conditions defined by equations (6) and (7), it follows that the magnetic field strength is equal to:

which is a constant, and that the angular rotation rate is equal to: #r=#a'-#a=#b'-#b (9) and can therefore be readily obtained by measuring the difference between either one or the other of the observed Larmor precession frequencies and its associated precision reference frequency.

In addition to the basic phenomenon of magnetic moment precession and the mathematical basis for the signal processing mechanization which permits angular rotation rate information to be measured, as described above, there are several other physical phenomena involved in the implementation of a practical nuclear magnetic resonance gyroscope. Those that will be described are the alignment of nuclear magnetic moments, the achievement of sustained precession of these moments, and the optical detection of the precessing moments to provide a signal from which the Larmor precession frequency can be determined.

The magnitude of an individual nuclear magnetic moment is extremely small and the natural equilibrium condition is one in which a nearly random orientation of moments exists in an ensemble of atoms. Techniques must be used to orient a significant fraction of these magnetic moments in a single direction so that a macroscopic magnetic moment, and consequently a measureable signal, will be produced.

The technique that is used for aligning nuclear magnetic moments, in this embodiment of the invention, is a two-stage process and will be referred to as pumping". The two nuclear magnetic moment gases, which are noble gases in the preferred embodiment of this invention, are combined with an alkali metal vapor in a single, optically transparent cell. This cell is illuminated by a spectrally filtered, circularly polarized, beam of light which is emitted bv an alkali metal vapor electric discharge lamp. A steady magnetic field is applied in such a direction that a significant component of this field is parallel to the direction of the light that is incident on the cell.

The first stage of pumping is an optical pumping process in which the alkali metal vapor atoms are optically pumped by absorption of some of the incident light. This results in the alignment of a significant fraction of the atomic magnetic moments of the alkali atoms in a direction that is parallel to that of the applied steady magnetic field.

The second stage of pumping is a spin exchange pumping process in which some of the alignment of the atomic magnetic moments of the alkali atoms is transferred to the nuclear magnetic moments of the noble gas atoms by spin exchange interactions during collisions between the alkali atoms and noble gas atoms. This results in the alignment of a significant fraction of the nuclear magnetic moments of the noble gas atoms in a direction that is parallel to that of the steady magnetic field. This spin exchange pumping technique is an extension of the techniques of Bouchiat, Carver, and Varnum (Phys. Review Letters 5, Page 373, [1960]). In particular, in this embodiment it has been extended to include the simultaneous alignment of the nuclear magnetic moments of two different noble gas isotopes contained in the same cell.

The aligned magnetic moments of the alkali system and of both noble gas systems of atoms are subject to relaxation mechanisms which cause their alignments to decay exponentially with time towards their natural equilibrium condition of a random orientation. Each system of moments is characterized by a relaxation time constant which depends upon the kinds and quantities of all other constituents and upon the total environment in the NMR cell. The steady state fractional alignment of each system of moments is a function of both the pumping rate and the relaxation time for that system, with larger fractional alignments, hence larger signal amplitudes, being achieved when the relaxation times are also long.Accordingly, in order to achieve the longest possible relaxation times, a suitable amount of a buffer gas such as helium or nitrogen is also contained in the cell in order to reduce the relaxation effects due to interactions of the magnetic moments with the walls of the cell. Furthermore, particular isotopes of particular noble gases have been chosen as the nuclear magnetic moment gases specifically because of their long relaxation times.

Precession of the two systems of noble gas magnetic moments is started and sustained by applying two AC magnetic fields in a direction which is orthogonal to that of the applied steady magnetic field. These fields have frequencies that are equal to the respective Larmor precession frequencies of the two noble gases and are referred to as the AC feedback magnetic fields in as much as they provide the feedback signal function that is necessary in any oscillator in order to achieve sustained oscillations. These feedback fields cause each individual system of noble gas magnetic moments to be torqued coherently away from the direction of their initial alignment, which is parallel to that of the steady magnetic field, into a plane which is orthogonal to the direction of the steady magnetic field.The magnetic moments of each system precess continuously in this plane, thereby creating two macroscopic magnetic moments throughout the volume of the NMR cell, hence two magnetic fields, which rotate in this plane at the respective Larmor precession frequencies of the two noble gases. The physics associated with the torquing of spinning bodies requires that the phases of the applied feedback fields be in quadrature with the respective phases of the precessing nuclear magnetic moments.

These precessing nuclear magnetic moments are optically detected using an approach which is adapted from a magnetometer technique that was first developed in France by C. Cohen Tannoudji, J. Dupont-Roc, S. Haroche, and F. Value (Rev. de Phys. Appl. 5, 95 [1970]). This magnetometer technique works on the principle that the degree of absorption of optical pumping light by the alkali atoms in the NMR cell depends upon the directional orientation of the magnetic moments of the individual alkali atoms with respect to the direction of the incident light.Both of the two rotating magnetic fields, which are created by the two systems of precessing noble gas nuclear magnetic moments, individually and simultaneously exert torques on the precessing alkali mgnetic moments, thereby imparting nutational motions to the precessing alkali moments which in turn, modulate the intensity of the transmitted light. The mathematical description and the salient characteristics of this optical detection process can be summarized very briefly as follows: A sinusoidal AC magnetic field, H, cos cost, which will be referred to as the carrier magnetic field, is applied to the cell and the direction of this carrier magnetic field is used to define the z-axis. A steady magnetic field is also applied to the cell essentially in the direction of the z-axis.The components of all magnetic fields, excluding the carrier magnetic field, are denoted as Ha, Hy, and HZ. The optical pumping light is incident on the cell in the x-z plane and has components and Iz, which produce alkali magnetization components Mx and Mz.

It can be shown that under the following magnetic field conditions: IHxI l/Yt IHyl 1 and (10) lH2(nwc/Y)I 1/Y# where y is the gyromagnetic ratio for the alkali atom, T is the total alkali relaxation time under the influence of the light absorption and relaxation processes, #c is the frequency of the carrier magnetic field, and n is an integer, that the x-component of the transmitted light intensity, Itx, is described (excluding a constant term) by the relation:

where k is a constant and J+ is defined as:: J#=Jn+p(H1/#c)#Jn-p(γH1/#c) (12) and where Jn and Jn+p are Bessel functions of order n and N+p, respectively, with the same argument~yHl/c, and H1 and coc are the amplitude and frequency, respectively, of the carrier magnetic field.

We note several aspects of equation (11) which are relevant: (a) The x-component of the transmitted light intensity, Itx, consists of a sum of harmonics of the carrier frequency, no.

(b) The in-phase (cos pno0t) reponse is linear in the field Hy for small values of Hy.

(c) The quadrature-phase (sin pw,t) response is linear in the field Ha for small values of Ha.

(d) The x-component of the transmitted light intensity can be made to be linear in either Hx alone or in Hy alone by choosing a particular amplitude for the carrier magnetic field, H1, such that either J+ or J~, respectively, equals zero.

(e) The response of the x-component of the transmitted light intensity to either the Ha or the Hy magnetic field components is proportional to the product of the x-component of the incident light and the z-component of the magnetization, IXMze The incident light beam must therefore have components in both the x-direction and the z-direction.

(f) Due to the conditions imposed on the magnetic fields, as defined by equation (10) for the case n+0, the steady magnetic field must be applied essentially in the z-direction and the precession of the nuclear moments must occur essentially in the x-y plane. In particular, these precessing moments create a macroscopic magnetic field that rotates at the Larmor precession frequency and which has an amplitude that is proportional to the fractional alignment of the nuclear magnetic moments.This rotating magnetic field is responsible for a term in the x-component of the transmitted light intensity that is due to the y-axis component of this field, namely: I,-h,(cos no8t) (cos pa)ct) (15) where ha is the amplitude of this rotating magnetic field and no8 is the Larmor precession frequency of the nuclear moment gas. It is this term which is utilized for extracting the nuclear Larmor precession frequencies in the embodiment of this invention. The preceding analysis is valid for steady magnetic fields and also for slowly varying fields, including the above rotating magnetic field, in particular, provided that the condition no t l is satisfied.

(g) The effects of steady magnetic field components of Hx and Hy can be separately determined from the light intensity modulations and this makes it possible to independently measure or control these field components.

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a sectional drawing showing the physical arrangement of components of an NMR gyro sensor assembly; Figures 2A and 2B are perspective drawings illustrating the configuration of the magnetic field coils that are a part of Figure 1; Figure 3 is a conceptual diagram illustrating the processes of optical pumping and of modulation of the intensity of the light that is transmitted by the NMR cell; Figure 4 is a block diagram indicating the functional mechanization of the electronics of an NMR gyro; and Figure 5 is a conceptual diagram showing an alternative configuration of an NMR gyro sensor assembly. Figure 5 also serves to illustrate the configuration of a research apparatus.

With reference to Figure 1, which is a sectional drawing showing the physical arrangement of components of an NMR gyro assembly, a rubidium vapor lamp 10, which is excited by a high frequency power source, is used to emit light containing the spectral lines of rubidium. This lamp is similar in design to that described by Bell, Bloom and Lynch (Rev. Sci. Instr. 32, 688 [1961]). The lamp 10 is housed in an enclosure 12 which is used to maintain the lamp at an elevated temperature suitable for maximum light emission. The light passes through a glass condenser lens 14 and through a plastics fresnel collimating lens 16 before passing through an optical interference filter 18.This filter is designed to transmit most of the 794.7 nanometer wavelength light from one spectral line of the rubidium while blocking most of the 78.0 nanometer wavelengthlight from an adjacent spectral line. The filtered light passes through a second fresnel collimating lens 20, is reflected in a prism 22 to change its direction and converges on the end of an input fiber optics bundle 24.

This fiber optics bundle then transmits the light towards the center of the device and makes a bend so that the light leaves the end 25 of the bundle 24 with a mean angle of about 45 degrees relative to the vertical as shown in the drawing. The vertical axis as shown in the drawing is designated as the z-axis. The x-axis is defined to be pointing to the left in the drawing. Thus, Figure 1 is a sectional drawing in the x-z plane. The light leaving the bundle passes through a circular polarizer 26, and enters the NMR cell 28.

The NMR cell 28 is a sealed, optically transparent, glass cylindrical enclosure containing a small quantity of isotopically enriched rubidium-87 metal, approximately 0.5 torr of isotopically enriched xenon 29 gas, approximately 20 torr of isotopically enriched krypton-83 gas, and a buffer gas consisting of either about 400 torr of helium-4 or about 100 torr of nitrogen. These are introduced into the cell in the order stated while the cell is attached to a vacuum filling station and the cell is then sealed off.

The cell 28 is mounted in a temperature controlled alumina oven 30 which is heated and controlled by a resistance band heater 32 that uses a high frequency power source. The oven is maintained at a temperature of about 65"C, at which temperature approximately one-half of the light entering the cell 28 is absorbed.

Most of the light that is not absorbed in the cell 28 enters an output fiber optics bundle 36 and passes through a lens 38 to a silicon photodetector 40. Other components shown in this drawing are a magnetic field coil structure 34, which will be described in more detail below, (Figures 2A and 2B) a set of several layers of magnetic shielding 42 designed to attenuate the influence of external magnetic fields, and a supporting structure 44.

The magnetic field coil structure 34 consists of a machinable glass (Corning "Macor") cylindrical coil form, on the outer surface of which grooves have been cut and then wires laid in the grooves to form the-magnetic field coils.

Figures 2A and 2B are perspective drawings illustrating the configuration of the magnetic field coils that are a part of Figure 1. Figure 2A shows the coil form 34' and the main solenoid coil windings 50 that produce a magnetic field that is parallel to the axis of the cylinder, which is designated as the z-axis. Additional coil windings 52 at the ends of the coil form are employed to improve the spatial uniformity of the magnetic field. The coil windings 52 are commingled with the coil windings 50. The combination of 50 and 52 will be referred to as the z-axis field coils.

Figure 2B shows the same coil form 34' and two additional coil pairs that provide magnetic fields along two axes that are mutually orthogonal to each other and to the axis of the cylinder. Coil pair 54 provides a magnetic field along the xaxis and coil pair 56, only one member of which is visible in the drawing, provides a magnetic field along the y-axis.

Figure 3 is a conceptual diagram illustrating for each of the noble gases the processes of optical pumping and of modulation of the intensity of the light that is transmitted through the NMR cell. Because these processes are so similar for the two noble gases, they are illustrated and described for only one of the two noble gases. In particular, they apply for the case n=l, where n is as used in equations (11) and (12). The circularly polarized light which enters the NMR cell 28' has a component 64 along the z-axis, which is referred to as optical pumping light, and a component 66 along the x-axis, which is referred to as detection light. Through the interactions of the optical pumping light 64 and the steady magnetic field 68 the rubidium atoms 60 have their magnetic moments aligned preferentially in the zdirection.By interatomic collisions, this magnetic moment alignment is transferred from the rubidium atoms 60 to the noble gas nuclei 62.

A sinusoidal AC feedback magnetic field 70 that is matched in frequency and phase to the Larmor precession frequency of the collective magnetic moment of the noble gas nuclei 62 is applied in the x-direction and serves to torque the magnetic moment of these nuclei into the x-y plane. This component of noble gas nuclear magnetic moment then precesses in the x-y plane at the noble gas Larmor precession frequency, nova, about the steady magnetic field 68. This precessing nuclear magnetic moment component creates a nuclear precession magnetic field of strength, has that rotates in the x-y plane and which therefore has a component in the y-direction that is equal to (ha cos noat).

The detection light 66 interacts with the rubidium atoms 60, which are under the influence of the steady magnetic field 68, a superimposed AC carrier magnetic field 69, and the y-component of the nuclear precession field, ha. This interaction causes the intensity of the x-component of the transmitted light 72 to be modulated at the carrier frequency, w,, with a modulation envelope 74 at the nuclear precession frequency, nova. These light modulations are then converted into electrical signals by the silicon photodetector 40'.

With reference to Figure 4, which is a block diagram indicating the functional mechanization of the electronics of an NMR gyro, light from the light source 10 enters the device through the input optics 82 and passes through the NMR cell 28.

Input optics 82 comprises items 14 through 26 as discussed above. The light that is not absorbed and which is modulated in intensity, as described above (Figure 3), is transmitted by means of the output optics 86 to the photodetector 40 where the light intensity modulations are converted into an electrical signal 89. Output optics 86 comprises items 36 and 38 as discussed above. The signal 89 is first amplified and then synchronously demodulated in two separate operations in a carrier signal detector 90 in order to generate control signals for the x-axis and y-axis magnetic fields.

A DC signal 93 for controlling the y-axis DC magnetic field is generated by synchronously demodulating the signal 89 using a sinusoidal reference signal having a frequency, fcb that is derived from a crystal controlled precision reference frequency source 92. The frequency and phase of the sinusoidal signal from source 92 are the same as those of the applied AC carrier magnetic field. The amplitude of the DC control signal 93 is proportional to the amplitude of that component of the light intensity modulations at the carrier frequency that is in phase with the applied AC carrier magnetic field. By reference to equation 1), this DC signal 93 is also proportional to the value of the y-axis magnetic field.The DC control signal 93 is summed at point 95 with an additional constant DC signal 94 that is generated in the DC power supplies 96 and the resultant is used to supply the total DC current to the y-axis magnetic field coil 56. The y-axis DC magnetic field is thereby controlled in such a way that the amplitude of the DC signal 93 remains close to zero, which results in a suppressed carrier mode of operation. In this manner, changes in the yaxis magnetic field are sensed and corrected to maintain carrier suppression.

In a similar manner, a DC signal 104 for controlling the DC component of the x-axis magnetic field is generated by synchronously demodulating the signal 89 using a sinusoidal reference signal having a frequency, 2f', that is derived from a crystal controlled precision reference frequency source 102. By setting Jo(ylti/noc)=J2(YHi/noj), for n=p=l, the coefficient of the sin w,t term becomes zero, and a higher harmonic is then used.The reference signal at 2fC generated by source 102 is timed relative to the reference signal at fc such that when the fc signal is represented as cos 't, the 2fuzz signal is represented as sin 2cost. The amplitude of the DC control signal 104 is proportional to the value of the x-axis magnetic field.

The DC control signal 104 is summed at point 107 with an additional constant DC signal 106 that is generated in the DC power supplies 96 and the resultant is used to supply the total DC current to the x-axis magnetic field coil 54. In this manner, the value of the DC component of the x-axis magnetic field is controlled to be essentially equal to zero.

In addition to the DC signal 93 resulting from the synchronous demodulation at the frequency, c' in the carrier signal detector 90 there are AC signals 109 which are proportional to the AC components of the y-axis magnetic field. Of particular interest, are the modulations at the nuclear Larmor precession frequencies. These signals are separated and filtered in a nuclear precession signal separator 110 to yield a signal 112 at the xenon 29 precession frequency, f,, of about 135 hertz, a signal 114 at the krypton-83 precession frequency, fb, of about 19 hertz, and a signal 116 at their difference frequency, fafb, of about 116 hertz. These stated values for the nuclear Larmor precession frequencies are for a z-axis steady magnetic field value of 0.114 gauss which is used in the preferred embodiment.

A DC signal 122 for controlling the DC component of the z-axis magnetic field is generated by comparing the precession difference frequency, f,-f,, 116 in a frequency comparator 118 to a reference frequency, fá-fb, that is generated by the crystal controlled precision reference frequency source 120. Any phase difference between the signals 116 and 120 creates a DC control signal 122 which is summed at point 123 with an additional constant DC signal 126 that is generated in the DC power supplies 96 and the resultant signal 125 is used to supply the total DC current to the z-axis magnetic field coil 124 which comprises coils 50 and 52. In this manner, the value of the DC component of the z-axis magnetic field is controlled to be equal to a specific constant value as given by equation (8).

A sinusoidal AC current 128, that is generated by the carrier field supply 130, is also applied to the z-axis magnetic field coil 124 to produce an AC carrier magnetic field. The AC carrier current 128 is summed at point 127 with the DC currents 125 and the resultant comprises the total current supplied to the z-axis magnetic field coil 124. The sinusoidal AC carrier current 128 has a frequency, f', that is generated by the crystal controlled precision reference frequency source 92*which is the same as the signal used as a reference signal for the carrier signal detector 90.The carrier frequency, f, is about 80,000 hertz, which is equal to the Larmor precession frequency of rubidium-87 for a z-axis steady magnetic field value of 0.114 gauss which is used in the preferred embodiment.

The amplitude of the AC carrier current 128 is selected to have a specific value such that the amplitude of the sinusoidal AC carrier magnetic field is equal to a particular factor times the DC component of the z-axis magnetic field which is produced by the DC current 125. In the preferred embodiment, this factor has a value of 1.84 and the amplitude of the AC carrier magnetic field is made equal to 0.210 gauss. In this manner, the amplitude of the component of the signal 89 at the carrier frequency, fc, is made to be insensitive to x-axis magnetic fields. The mathematical basis for this preferred relationship between the two fields is contained in equations (11) and (12) for the case n=l and p=l.

Two feedback magnetic fields are created along the x-axis in order to achieve sustained precession of the nuclear magnetic moments of xenon 29 and krypton83. The xenon-129 signal 112 is used in an AC feedback magnetic field generator 144 to generate a sinusoidal AC feedback signal 148 which has a constant amplitude and a frequency and phase that are identical with those of the xenon-129 signal 112. The signal 148 is summed with a similarly generated sinusoidal AC feedback signal 146 that is derived from the krypton-83 signal 114. The sum 150 of the two AC feedback currents 146 and 148 is further summed at point 107 with the DC currents 104 and 106 and the resultant comprises the total current that is supplied to the x-axis magnetic field coil 54.The function of the AC feedback magnetic fields is to continuously torque the xenon and krypton nuclear magnetic moments, that have been newly aligned along the z-axis, into the x-y precession plane to replenish those moments that have been lost through nuclear magnetic moment relaxation processes. In this manner, the sustained precession of xenon and krypton magnetic moments creates two steady state magnetic fields that rotate in the x-y plane and which consequently produce steady state light intensity modulations at the Larmor precession frequencies, fa and fb.

The angular rotation rate of the gyro is obtained by comparing the Larmor precession frequency, fa, of the xenon-129 signal 112 in a frequency comparator 134 with a reference frequency fá, that is derived from a crystal controlled precision reference frequency source 136. The resultant difference frequency f-f,, is equal to the angular rotation frequency, f,, of the gyro, in accordance with equation (9), and this data 138 is sent to a computer for further processing. The gyro angular rotation rate data 138 contains both frequency information and phase information and therefore contains both angular rate information and angular displacement information, respectively.

All precision reference frequency sources 92, 102, 120 and 136 are driven by a common crystal controlled master oscillator 152 by digital multiplication and division techniques. The frequency of the master oscillator 152 is denoted as frn in Figure 4. The angular rotation rate data 138 is, to first order, independent of the frequency stability of the master oscillator 152.

With reference to Figure 5, which is a conceptual diagram showing an alternative embodiment of an NMR gyro sensor assembly, items identified with primed numerals are functionally similar to the corresponing unprimed items.

Rubidium lamp 10' supplies optical pumping light through the input light pipe 24' to the NMR cell 28'. The lamp 10' also supplies detection light to the NMR cell 28' through a second channel which includes input light pipe 154 and input prism 155.

The detection light that is transmitted by the NMR cell 28' passes through the output prism 158 and output light pipes 156 and 160 to the photodetector 40'.

Suitable magnetic fields are applied to the NMR cell through the three-axis Helmoltz coil assembly 161, 162 and 163, which in this arrangement are the field coils for the z-axis, y-xis, and x-axis, respectively. The direction of the input light through the light pipe 24' is here defined to be along the z-axis, the x-axis is up in the drawing, and the y-axis is out of the paper.

The arrangement shown in Figure 5 is an alternative to that of Figure 1 which serves to emphasize that optical detection should be accomplished in a direction that is transverse to that of the steady magnetic field which is along the z-axis. This may be accomplished either as shown in Figure 1 using a 45 degree or other similar angle between the direction of the light beam through the NMR cell relative to the direction of the steady magnetic field, or as shown in Figure 5 using two separate light paths with the pumping light being parallel with the direction of the steady magnetic field and the detection light being transverse to the direction of the field.

This arrangement also includes the possibility that the pumping and detection light beams could originate from separate light sources and that they could also have different spectral or polarization characteristics.

With certain modifications, Figure 5 can also be used to illustrate the configuration of a research apparatus that is especially useful for performing experimental investigations of the properties of noble gas-alkali vapor systems.

The modifications consist of deleting the detection light path 154, 155, 156, 158, 160 and 40', and adding the output light path 174 and 175. For this application, which corresponds to the case of n=0 as used in equations (11) and (12), the coordinate axes are relabelled, with the x-axis and z-axis being interchanged from before, so that the direction of the input light through the light pipe 24' is redefined as being along the x-axis and the z-axis is up in the drawing. The input light passes through the cell 28' and into an output light pipe 174 which transmits the light to the photodetector 175. The AC carrier magnetic field is applied using the z-axis field coil 163 and a small DC field of approximately 100 microgauss is applied using the y-axis field coil 162.During operation, a larger DC field of approximately 10 milligauss is applied by the x-axis field coil 161 during the initial nuclear magnetic moment spin exchange pumping time. At the conclusion- of the pumping time, which is typically a few minutes, this field is quickly turned off leaving the aligned nuclear magnetic moments to precess in the x-z plane, which is in the plane of the paper. The z-axis component of the precessing nuclear magnetic field produces light intensity modulations analogous to the ones described above. This mode of operation is quite similar to that described by Cohen-Tannoudji, et al. (ibid) except that in this alternative embodiment the rubidium magnetic moments that are used for detection and the noble gas nuclear magnetic moments that are used for nuclear Larmor precession are located in the same cell 28'.The close association of the rubidium atoms during collisions with the noble gas atoms causes the rubidium atoms to sense a much larger average magnetic field from the noble gas nuclei. This proximity effect results in signals that are much larger than might otherwise be detectable. This apparatus is therefore especially useful for research studies on the properties of the noble gas-alkali vapor system.

In conclusion, the present invention has been described in terms of particular elements and particular physical arrangements, but it is clear that reasonable alternatives, such as the use of different optical paths accomplishing the same results, or the use of different combinations of the noble gases or the use of a different substance than rubidium, or the use of other values for the frequencies or magnetic fields mentioned in the foregoing specification, may all be within the scope of the present invention.

WHAT WE CLAIM IS: 1. A nuclear magnetic resonance arrangement comprising: a nuclear magnetic resonance cell containing a gas or vapor comprising an optically pumpable substance that possesses a magnetic moment, and at least one nuclear moment gas, the or each nuclear moment gas having nuclear magnetic moments; means for applying a steady magnetic field to the cell; means for applying an alternating current carrier magnetic field to the cell; first means for illuminating the cell with light of a wavelength which can be absorbed by the optically pumpable substance to align at least partly the magnetic moments of the optically pumpable substance in one direction; means for at least partly aligning the nuclear magnetic moments of the or each nuclear moment gas and for causing the aligned nuclear magnetic moments of the or each nuclear moment gas to precess about the direction of the steady magnetic field at substantially the respective Larmor precession frequency of the or each gas; second means for illuminating the cell with light of a wavelength substantially equal to a wavelength which can be absorbed by the optically pumpable substance, which light has a component of illumination orthogonal to the direction of the alternating current carrier magnetic field so as to produce modulations to the intensity of the transmitted part of the light at substantially the frequency of at least one harmonic, including the fundamental, of the alternating current carrier magnetic field: means for detecting at least one of the modulations in the intensity of the transmitted light; and apparatus for electrically demodulating the detected light intensity modulations to obtain a signal whose frequency is representative of the Larmor precession frequency of the or each nuclear moment gas and whose amplitude is proportional to the degree of alignment of the nuclear magnetic moments of the or each gas.

2. An arangement according to claim 1, wherein the illuminating means is arranged so that the illuminating light has a component in the direction of the steady magnetic field so that it is operable to partially align the magnetic moments of the optically pumpable substance in a direction parallel to that of the steady magnetic field.

3. An arrangement according to claim 1 or 2, wherein there are two nuclear moment gases.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. The modifications consist of deleting the detection light path 154, 155, 156, 158, 160 and 40', and adding the output light path 174 and 175. For this application, which corresponds to the case of n=0 as used in equations (11) and (12), the coordinate axes are relabelled, with the x-axis and z-axis being interchanged from before, so that the direction of the input light through the light pipe 24' is redefined as being along the x-axis and the z-axis is up in the drawing. The input light passes through the cell 28' and into an output light pipe 174 which transmits the light to the photodetector 175. The AC carrier magnetic field is applied using the z-axis field coil 163 and a small DC field of approximately 100 microgauss is applied using the y-axis field coil 162.During operation, a larger DC field of approximately 10 milligauss is applied by the x-axis field coil 161 during the initial nuclear magnetic moment spin exchange pumping time. At the conclusion- of the pumping time, which is typically a few minutes, this field is quickly turned off leaving the aligned nuclear magnetic moments to precess in the x-z plane, which is in the plane of the paper. The z-axis component of the precessing nuclear magnetic field produces light intensity modulations analogous to the ones described above. This mode of operation is quite similar to that described by Cohen-Tannoudji, et al. (ibid) except that in this alternative embodiment the rubidium magnetic moments that are used for detection and the noble gas nuclear magnetic moments that are used for nuclear Larmor precession are located in the same cell 28'.The close association of the rubidium atoms during collisions with the noble gas atoms causes the rubidium atoms to sense a much larger average magnetic field from the noble gas nuclei. This proximity effect results in signals that are much larger than might otherwise be detectable. This apparatus is therefore especially useful for research studies on the properties of the noble gas-alkali vapor system. In conclusion, the present invention has been described in terms of particular elements and particular physical arrangements, but it is clear that reasonable alternatives, such as the use of different optical paths accomplishing the same results, or the use of different combinations of the noble gases or the use of a different substance than rubidium, or the use of other values for the frequencies or magnetic fields mentioned in the foregoing specification, may all be within the scope of the present invention. WHAT WE CLAIM IS:

1. A nuclear magnetic resonance arrangement comprising: a nuclear magnetic resonance cell containing a gas or vapor comprising an optically pumpable substance that possesses a magnetic moment, and at least one nuclear moment gas, the or each nuclear moment gas having nuclear magnetic moments; means for applying a steady magnetic field to the cell; means for applying an alternating current carrier magnetic field to the cell; first means for illuminating the cell with light of a wavelength which can be absorbed by the optically pumpable substance to align at least partly the magnetic moments of the optically pumpable substance in one direction; means for at least partly aligning the nuclear magnetic moments of the or each nuclear moment gas and for causing the aligned nuclear magnetic moments of the or each nuclear moment gas to precess about the direction of the steady magnetic field at substantially the respective Larmor precession frequency of the or each gas; second means for illuminating the cell with light of a wavelength substantially equal to a wavelength which can be absorbed by the optically pumpable substance, which light has a component of illumination orthogonal to the direction of the alternating current carrier magnetic field so as to produce modulations to the intensity of the transmitted part of the light at substantially the frequency of at least one harmonic, including the fundamental, of the alternating current carrier magnetic field: means for detecting at least one of the modulations in the intensity of the transmitted light; and apparatus for electrically demodulating the detected light intensity modulations to obtain a signal whose frequency is representative of the Larmor precession frequency of the or each nuclear moment gas and whose amplitude is proportional to the degree of alignment of the nuclear magnetic moments of the or each gas.

4. An arrangement according to claim 3, wherein the nuclear moment gases

are noble gas isotopes.

5. An arrangement according to claim 4, wherein the two noble gas isotopes are different isotopes of the same gas.

6. An arrangement according to any one of the preceding claims wherein the or each nuclear moment gas is chosen from the class consisting of those isotopes of xenon and krypton which have a nuclear magnetic moment.

7. An arrangement according to any one of the preceding claims. arranged so that collisions of atoms of the aligned optically pumpable substance with the or each nuclear moment gas comprise the mechanism whereby the nuclear magnetic moments of the or each nuclear moment gas are partially aligned in the direction of the steady magnetic field.

8. An arrangement according to any one of the preceding claims, wherein the optically pumpable substance is an alkali metal.

9. An arrangement according to claim 8, wherein the optically pumpable substance is rubidium.

10. An arrangement according to ano one of the preceding claims and further comprising apparatus which is operable to accurately measure the magnitude and the direction of the steady magnetic field.

11. An arrangement according to claim 10 and further comprising means which is operable accurately to control the magnitude and the direction of the steady magnetic field.

12. An arrangement according to any one of the preceding claims, wherein said precession causing means comprises: apparatus operable to apply an alternating current feedback magnetic field which has a frequency substantially equal to the detected Larmor precession frequency of the or each nuclear moment gas in the direction orthogonal to the direction of the steady magnetic field: apparatus for detecting the phase of the or each Larmor precession frequency and means, sensitive to the or each detected phase, to control the respective phase of the alternating current feedback magnetic field to be substantially in quadrature with the phase of the precessing nuclear magnetic moment of the or each nuclear moment gas to sustain precession of the moment of the or each nuclear moment gas.

13. An arrangement according to any one of the preceding claims comprising apparatus which is operable to sense angular displacements of a nuclear magnetic resonance device, in which the arrangement is being used, about the direction of the steady magnetic field by sensing changes in the phase of the Larmor precession signal of at least one of the nuclear moment gases.

14. An arrangement according to any one of the preceding claims, wherein said steady magnetic field applying means is operable to generate a steady magnetic field with a magnitude sufficient to cause precession of the magnetic moment of the optically pumpable substance to occur substantially at a harmonic, including the fundamental, of the frequency of the applied alternating current carrier magnetic field.

15. An arrangment according to any one of the preceding claims, wherein said illuminating means comprises a single source operable to generate the light.

16. An arrangement according to any one of claims 1 to 14, wherein said illuminating means comprises two light sources, one of said light sources being operable to generate light propagating substantially parallel to the steady magnetic field and the other light source being operable to generate light propagating substantially orthogonal to the alternating current carrier magnetic field.

17. An arrangement according to any one of the preceding claims, comprising apparatus which is operable to electrically demodulate detected light intensity modulations and to generate control signals having amplitudes proportional to magnetic field components transverse to the alternating current carrier magnetic field, the arrangement further comprising means for measuring and controlling said transverse field components.

18. An arrangement according to any one of the preceding claims comprising means operable to sense angular displacements or angular rates of the arrangement about the direction of the steady magnetic field by sensing changes in phase or changes in frequency, respectively, of the Larmor precession frequencies of at least one of the nuclear moment gases.

19. An arrangement according to any one of the preceding claims, wherein the cell also contains a buffer gas.

20. An arrangement according to claim 19, wherein the buffer gas is helium.

21. An arrangement according to claim 19, wherein the buffer gas is nitrogen.

22. An arrangement according to any one of the preceding claims and having means arranged to provide said steady magnetic field with a value exceeding 0.01 gauss.

23. A nuclear magnetic resonance arrangement substantially as hereinbefore described with reference to Figures 1 to 4 or to Figure 5 of the accompanying drawings.