JPS63158480A - Optical magnetic resonance magnetometer - Google Patents

Abstract

PURPOSE:To reduce the change in brightness generated in a light source, by a method wherein a high frequency magnetic field is applied to an absorption cell for a definite period or is not applied thereto to alternately perform non- magnetic resonance operation and magnetic resonance operation and the measuring signals of both operations are set off by a differential amplifier. CONSTITUTION:The second sample holding circuit 17 almost continuously measures the change in the brightness of a lamp or absorbing cell when no magnetic resonance is generated and the first sample holding circuit 16 almost continuously measures the absorption signal at the time of magnetic resonance and the change in the brightness of a light source appearing in a state superposed to said signal. A differential amplifier 18 respectively inputs the output signals SIG, NOIS of the above mentioned first and second sample holding circuits, 16, 17 at the positive and negative input terminals thereof and sets off the detect signals observed at the times of magnetic resonance operation and non-magnetic resonance operation to reduce the change in the brightness of the light source superposed to the absorption signal at the time of magnetic resonance.

Description

[Detailed Description of the Invention] [Field of Industrial Application] This invention is an optical system that optically detects magnetic resonance absorption of atoms and measures magnetic fields by utilizing the fact that the magnetic resonance frequency is proportional to the strength of the magnetic field. This paper concerns improvements to magnetic resonance magnetometers.

[Conventional technology]

Atoms of certain substances such as cesium and helium undergo a change in energy that is proportional to the strength of the surrounding magnetic field, so this energy change is detected using specific light and a high-frequency magnetic field.
There is a magnetometer that tracks the frequency of a high-frequency magnetic field so that the change in light intensity is maximized, but here, as an example, we will introduce a conventional helium frequency tracking type using helium atoms as shown in Figures 4 and 5. The optical magnetic resonance magnetometer will be briefly explained. Figure 4 shows an example of a conventional optical magnetic resonance magnetometer, in which (1) shows a helium lamp;
(2) is the lamp excitation electrode, (3I is the lens, (4) is the circularly polarizing plate, (5) is the absorption cell, (6) is the absorption cell excitation electrode, (71ti photodetector, (8) is the amplifier , (9)
is a phase detector, αat-S voltage controlled oscillator, and aυ is a buffer resistor.

α2 is an RF coil (I3 is a high frequency oscillator).

Note that components not related to this invention are omitted in the main text.

In this optical magnetic resonance magnetometer, a helium lamp (1
) is discharged by a high-frequency voltage of several tens of MH2 applied from a high-frequency oscillator α3 via the lamp excitation electrode (2), and generates light with a wavelength of 1.08μ, which is unique to helium atoms.

This light is made into parallel light by the lens (3), changed into circularly polarized light by the circularly polarizing plate (4), and irradiated onto the absorption cell (5).
A high frequency voltage of 0 MH2 is applied via the absorption cell excitation electrode +6) to create a glow discharge state. The light transmitted through the absorption cell (5) is converted into an electrical signal by a photodetector (7), then amplified by an amplifier (8), and then phase detected by a 1-phase detector (9) to generate an error signal. The oscillation frequency of the voltage controlled oscillator αG is controlled by this error signal, the output flows to the RF coil via the buffer resistor 0υ, and a high frequency magnetic field H1 is generated and applied to the absorption cell (5).

Here, it is assumed that an absorption cell (51K fl) encapsulates He atoms having a very short lifetime in an excited state.

This motion of helium atoms will be explained using the related energy level diagram in FIG. First, the absorption cell (5) K is glow-discharged in advance by a high frequency voltage of several tens of MH2, and the energy of helium atoms is in a metastable state of 25S1. Helium atoms in this metastable state are exposed to helium lamps (
Since the light (Do~D2) with a wavelength of 1.08μ from 1) is irradiated, it is absorbed and the excited state 2'P0.1.
However, the lifetime of the excited state is short, and it loses energy in about 10 seconds and returns to the metastable state of 23S1. Furthermore, when the system shown in Fig. 4 is in a static magnetic field, the helium atoms in the absorption cell (5) undergo Larmor's one-difference motion around the static magnetic field due to the force of the static magnetic field. Since it performs a rotational movement called ``energy,'' it causes a displacement in energy, and multiple Zeeman sublevels (Zeeman 5ublevels) shown in Figure 5 are generated.
occurs. Such a change in atomic energy due to a static magnetic field is called the Zeeman effect, and the frequency of one difference in motion of the magnetic moment of an atom is called the Larmor frequency.

Both are proportional to the strength of the static magnetic field.

Therefore, when the helium atoms in the static magnetic field are irradiated with 1.08μ light emitted from the helium lamp (1) from a direction parallel to the static magnetic field and converted into circularly polarized light by the circular polarizer (4) K, the helium atoms absorb the light and become excited. State 2 po, it will have an energy of 1.2, but at this time, the effect of circular polarization selects the Zeeman sublevel within the excited state, and it will have the energy of a certain Zeeman sublevel. . After this, it loses energy in a short time and enters a metastable state 2.
81 energy, but in this case, the selectivity of the Zeeman sublevel is preserved, and a biased distribution in which the number of atoms differs for each Zeeman sublevel is created within the Zeeman sublevel of 2S1. When an electromagnetic wave with an energy equal to the energy difference between the Zeeman sublevels of 281, that is, a high-frequency magnetic field at the Larmor frequency, is applied to this unevenly distributed state in a direction perpendicular to the static magnetic field, magnetic resonance occurs between the high-frequency magnetic field and the magnetic moment of the atoms. As a result, energy conversion occurs, and the above-mentioned uneven distribution is eliminated. The helium atoms return to the initial state in which nearly equal numbers of atoms are distributed in each of the 28103 Zeeman sublevels in a metastable state.

The above process, i.e. 238. →2Po
, 1.2-+281 energy changes are repeated every time the frequency of the high-frequency magnetic field matches the Larmor frequency because the 1.08 μO light is continuously irradiated, but the system in Figure 4 is parallel to the static magnetic field. The frequency of the high-frequency magnetic field is always controlled to match the Larmor frequency by taking advantage of the fact that light in the same direction is absorbed during the above process, and as a result, the light transmitted through the absorption cell is reduced. At this time, the relational expression (1) is established between the high-frequency magnetic field, the atomic constant, and the static magnetic field.

ω=ωO= rH(3...0・(11ω) Angular frequency of two high-frequency magnetic fields ω0: Larmor frequency of the atom γ: Magnetic rotation ratio of the atom (constant) Ho: Strength of the silent magnetic field In this way The system in Figure 4 locks on to the Larmor frequency, which is proportional to the static magnetic field strength HO.
Since the frequency of the high-frequency magnetic field at this time, that is, the oscillation frequency of the voltage controlled oscillator aQ, matches the Larmor frequency, by measuring this, the strength H of the silent magnetic field can be accurately measured.

[Problem that the invention seeks to solve]

In a conventional optical magnetic resonance magnetometer, when the brightness of the helium lamp (1) and the absorption cell (5) fluctuate, the fluctuation appears on the photodetector (7) K, and the signal-to-noise ratio of the absorption signal decreases. The drawback was that the accuracy deteriorated.

The present invention was made to solve these problems, and aims to improve measurement accuracy by reducing noise caused by changes in brightness of a light source.

[Means for solving problems]

A conventional magneto-optical resonance magnetometer has an analog switch that cuts off the high-frequency current to apply or not apply a high-frequency magnetic field to the absorption cell, and a sampling hold that alternately synchronizes with the opening/closing operation of this analog switch. A switching signal generator for controlling the two sampling holds and the above analog switch, a switching signal generator for controlling the two sampling holds, and a differential amplifier for canceling the outputs of these two sampling holds are added, and the above absorption cell is activated at regular intervals. A high-frequency magnetic field is applied to the lamp and absorption cells to alternately perform non-magnetic resonance operation and magnetic resonance operation, and in synchronization with this, the output of the photodetector is alternately sampled and held in the two sample holds described above. The brightness change of the light source and the magnetic resonance absorption signal and the brightness change of the light source that appears superimposed thereon are almost continuously measured. Next, by canceling these two measurement signals using the differential amplifier, changes in brightness of the dip and absorption cell superimposed on the absorption signal are removed, and noise accompanying changes in brightness of the light source is reduced.

[Effect]

Conventionally, changes in brightness caused by lamps and absorption cells appeared as noise components, but in this invention, by applying a high-frequency magnetic field to the absorption cell at regular intervals, non-magnetic resonance operation and magnetic resonance operation can be performed alternately. In synchronization with this, the output of the photodetector is alternately sampled and held by two sample holds, thereby producing changes in the brightness of the lamp and absorption cell, as well as changes in the brightness of the magnetic resonance absorption signal and the light source superimposed thereon. is measured almost continuously. Therefore, since changes in the brightness of the light source appear in the outputs of the two sample holds mentioned above under almost the same conditions, by canceling these signals using a differential amplifier, it is possible to cancel out the changes in the brightness of the light source. , noise associated with changes in brightness of the light source can be significantly reduced.

〔Example〕

FIG. 1 shows one embodiment of this invention, and FIG.
The figure shows the waveform of the switching signal of the switching signal generator of the present invention. In Fig. 1, aat: switching signal generator, a
! 9 is an analog switch, αet-! First sample hold, Q? lt: Second sample hold.

αI is a differential amplifier, SIG, NOIS are sample hold on (171 output signal, 5NVi differential amplifier (II output signal). In such a configuration, the switching signal generator (141 is shown in FIG. 2) The periods of logic level High and Low are respectively TH.

The period of the pulse signal T1 of TL (TH sa 10・TL) and the logic level high and low is TL, respectively.

A pulse signal T2 whose logic level is exactly opposite to that of the pulse signal T1 is generated. Note that the above pulse signal TI, T2 0 period T (=TL+
TH) is set at a time interval value such that it is possible to accurately capture the waveform information of the magnetic resonance absorption signal and the noise component due to the luminance change of the light source.

Further, the analog switch α is controlled by the pulse signal T1 of the switching signal generator I, and the logic level is High.
The logic level of the pulse signal T1 is set to be on and off when h and low, respectively. When gh, the buffer resistor αD and the RF coil α2 are electrically connected and a high frequency magnetic field H1 is applied to the absorption cell (51) to cause the absorption cell (5) to perform magnetic resonance operation, but the logic level of the pulse signal T1 is Lo
When w, the buffer resistor I and the RF coil are electrically disconnected, and the absorption cell (5) is brought into a non-magnetic resonance operation (no magnetic resonance occurs) without applying the high-frequency magnetic field H1. In FIG. 3, (- indicates this situation.

The photodetector (7) has a first sample hold a.
The second sample hold ano is connected,
It is controlled by the pulse signals TI and T2 of the switching signal generator α gradient. Second sample hold (
u9 (lη is the pulse signal TI, respectively. When the logic level of T2 is High, it becomes a sampling mode and outputs the detection signal of the photodetector (7) as it is, and when the logic level is Low, it switches to t:m Low. Since the previous detection signal is held in the zero-order state and output, the first sample hold aQ and the second sample hold αη can almost continuously measure the detection signal observed during magnetic resonance operation and non-magnetic resonance operation, respectively. That is, the second sample hold α'r) almost continuously measures the luminance changes of the lamp or absorption cell when no magnetic resonance is occurring.

The first sample and hold sensor almost continuously measures the absorption signal during magnetic resonance and the luminance change of the light source that appears superimposed on the absorption signal.

The 10,000 differential amplifier is the first one mentioned above. The output signals SIG and N0IS of the second sample hold αe0 are input to the positive input and negative input, respectively, and the output signals SIG and N0IS of the second sample hold αe0 are inputted to the
The output signal N0I8 of the second sample hold αD is subtracted from the output signal SIG of the second sample hold αD and outputted as a signal SN, but such a subtraction function cancels out the detection signal observed during magnetic resonance operation and non-magnetic resonance operation. It works to reduce the brightness change of the light source that is superimposed on the absorption signal during magnetic resonance.

Figure 3 (bl (cl (d)) shows this situation.

In the end, in the optical magnetic resonance magnetometer configured in this way, the analog switch is controlled on and off by the pulse signal from the switching signal generator I, and as a result, a high frequency magnetic field H1 is applied to the absorption layer (5) at regular intervals. The magnetic resonance operation and the non-magnetic resonance operation are performed alternately, and in synchronization with this, the output of the photodetector (7) is changed to the above-mentioned first. By alternately sampling and holding at the second sample hold αG, the second sample hold aη almost continuously measures the luminance change of the lamp or absorption cell during non-magnetic resonance operation, and the second sample hold αG almost continuously measures the absorption signal during magnetic resonance operation and the luminance change of the light source that appears superimposed on the absorption signal.

Next, the above 1. By canceling the output of the second sample hold tieaη by the differential amplifier α ladder, the brightness changes of the lamp and absorption cell superimposed on the absorption signal are removed, thereby significantly reducing the noise caused by the brightness changes of these light sources. be able to.

As described above, the optical magnetic resonance magnetometer of the present invention detects changes in brightness caused by light sources such as lamps and absorption cells, and
Since this is significantly reduced, it is possible to provide an apparatus that measures magnetic fields without impairing the high sensitivity of optical magnetic resonance.

〔Effect of the invention〕

As explained above, this invention adds an analog switch, a switching signal generator, and two sample holds to a conventional magneto-optical resonance magnetometer, and makes it possible to apply a high-frequency magnetic field to the absorption cell for a certain period of time or not. By alternately performing non-magnetic resonance operation and magnetic resonance operation, and synchronously switching and inputting the detection signal of the photodetector to two sample holds, measurements can be made during non-magnetic resonance operation and magnetic resonance operation, respectively. The brightness change of the light source, the absorption signal and the brightness change of the light source superimposed thereon are almost continuously measured at each sample hold.

Since these two electrical signals are canceled out by a differential amplifier,
It is possible to significantly reduce brightness changes caused by light sources such as lamps and absorption cells, and it is possible to measure magnetic fields without impairing the high sensitivity of optical magnetic resonance.

In addition, in order to detect changes in the luminance of the light source, it is only necessary to add relatively simple and inexpensive electronic circuits such as two sample holds, an analog switch, and a switching signal generator, which is advantageous in terms of cost and miniaturization. It has the advantage of being superior.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the present invention, and FIG. 2 is a diagram showing the waveform of a switching signal of a switching signal generator. (at (b) (c) (d) in Figure 3 are the pulse signal T1 of the switching signal generator and the high frequency magnetic field applied to the absorption cell, respectively. The output signal SIG of the first sample hold, the output signal SIG of the second sample hold Figure 4 is a diagram showing the configuration of a conventional optical magnetic resonance magnetometer, and Figure 5 is a diagram showing the energy level of helium atoms. In the figure, (1) is a helium lamp, +21H lamp excitation electrode, (3) is a lens, [41Fi circularly polarizing plate, (5) is an absorption cell, and (61
is an absorption cell excitation electrode, (7) is a photodetector, (8) is an amplifier, (9) is a phase detector, aQ is a voltage controlled oscillator,
I is a buffer resistor, a3 is an RF coil. 03 is a high frequency oscillator, (141 is a switching signal generator, α
5 is an analog switch, and ae is the first sample hold. αη is the second sample hold, (IS is the differential amplifier. SIG and N0IS are the output signals of the first and second sample holds ae and ση, respectively. SN is the output signal of the differential amplifier α barrel. Identical or equivalent parts are indicated by the same reference numerals.

Claims (1)

[Claims] an absorption cell containing a lamp that generates a light beam used for optical detection of optical magnetic resonance and a substance that generates magnetic resonance;
a high-frequency oscillator that causes the lamp and absorption cell to discharge and emit light; an RF coil that applies a high-frequency magnetic field to the absorption cell to generate magnetic resonance; A photodetector that converts the electrical signal into a signal, an amplifier that amplifies the electrical signal of the photodetector, a phase detector that detects the phase of the output of the amplifier and generates an error signal, and a Larmor that controls the oscillation frequency with the error signal. A voltage controlled oscillator that generates a high frequency voltage with a frequency equal to the frequency, and a voltage controlled oscillator that converts the high frequency voltage into a current and generates an
In a magneto-optical resonance magnetometer, which consists of a buffer resistor that is applied to a coil to generate a high-frequency magnetic field, an analog switch that electrically opens and closes between the buffer resistor and the RF coil, and an analog switch that inputs the output of the photodetector and generates a high-frequency magnetic field. Two sample holds that sample and hold each other in synchronization with the opening and closing of the switch, and a switching signal generator that controls the operation of the analog switch and the two sample holds are added to the absorption cell at regular intervals. A high-frequency magnetic field is applied to cause magnetic resonance operation and non-magnetic resonance operation to occur alternately,
By sampling and holding the magnetic resonance absorption signal measured at this time and the brightness change of the lamp and absorption cell in respective sample holds, the magnetic resonance absorption signal and the brightness change are almost continuously measured, and An optical magnetic resonance magnetometer characterized in that noise accompanying brightness changes of the lamp and absorption cell is canceled by subtracting the outputs of these two sample holds.