JPH0546507B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <<Industrial Application Fields>) The present invention relates to an improvement of a highly sensitive SQUID magnetometer that utilizes the Josephson effect.

≪Prior art)) Conventionally known RF, SQUID magnetometers (RF: Radio
Frequency) operates in liquid helium
It consists of a SQUID probe, an amplifier section and a controller that operate at room temperature, and the SQUID probe in liquid helium and the room temperature amplifier are connected by a coaxial cable. Such SQUID magnetometers have very high sensitivity in magnetic flux resolution, so
The drawback was that it was susceptible to external noise and induced noise.

Countermeasures have been taken to prevent induced noise, such as battery-driving the amplifier system at room temperature, but if the external induced noise is large, such as radio waves, television waves, high-frequency sputtering equipment, lightning, etc., operation may be affected. It may be temporarily interrupted. In such cases, continuous recording of magnetic measurements was impossible.

The SQUID contains a Josephson junction,
Its response reaches even DC to GHz to THz, and while it has the advantage of a wide response range, it also responds with high sensitivity to noise at all frequencies, and when considering practical use, it is difficult to make it resistant to noise. That becomes a big problem.

In order to solve these problems, a special patent application was filed in 1983.
The 190583 SQUID magnetometer uses optical fiber to transmit data between the low-temperature part and the room temperature part, making it more resistant to noise. There was a problem that vibration and bending caused unstable operation and noise. Also
Three optical fibers were required to perform SQUID operation, and the configuration was complicated. For applications such as magnetocardial measurement, it is desirable to have a single fiber with a long cable length and easy cable connection.

≪Problems to be Solved by the Invention) The present invention has been made to solve the above problems.
The aim is to realize a SQUID magnetometer.

≪Means for Solving the Problems)) The SQUID magnetometer of the present invention includes a SQUID ring including a Josephson junction, and an LC coupled to this ring.
a resonant circuit; a first circuit that amplifies the SQUID signal from the LC resonant circuit and outputs an optical signal related to the SQUID signal; and a first circuit that receives a feedback signal transmitted as an optical signal and receives an electric signal related to the feedback signal. A cryostat section comprising an average value output of the signal and a second circuit that applies a PWM clock signal separated from the electrical signal to the LC resonant circuit or a coil provided close to the SQUID ring; output from the cryostat section; A lock-in detection circuit that synchronously detects an electrical signal related to the optical signal at a specific reference frequency, and detects a signal related to the output of the lock-in detection circuit at the reference frequency. Convert to PWM signal with the same clock frequency as
A PWM circuit, and a controller that outputs an optical signal corresponding to the PWM signal as the feedback signal to the cryostat unit via an optical fiber.

≪Operation)) By using the SQUID magnetometer with the above configuration, the feedback signal is transmitted as a PWM modulated optical signal, and the reference frequency can also be separated from this optical signal, so it is possible to eliminate noise caused by induced noise or deformation of the optical fiber. It can be made resistant to noise, etc.

≪Example)) The present invention will be explained in detail below using the drawings.

Figure 1 is a configuration block diagram showing an embodiment of the SQUID flux meter according to the present invention, and Figure 2 is a block diagram of this example.
FIG. 2 is a diagram illustrating a configuration example when a SQUID magnetometer is applied to actual biomagnetism measurement. is a cryostat section (low-temperature operation section), and is a controller (room temperature section), which are connected to the cryostat section by an optical fiber.

1 operates at 4.2〓 in a cryostat.
In the SQUID sensor, this SQUID sensor 1,
11 is a SQUID ring, and 12 is a pickup coil that detects the magnetic signal to be measured.
An input coil 13 for introducing the detection signal into the SQUID ring 11 is an LC resonant circuit magnetically coupled to the SQUID ring 11.
2 applies an RF bias to this LC resonant circuit 13,
It is an oscillator with a frequency f ₁ of 20-30MHz. In the first circuit 100, 3 is an RF amplifier for amplifying the SQUID signal appearing at both ends of the resonant circuit 13, 4 is a detection circuit for detecting the SQUID signal from this RF amplifier 3, and 5 is a detection circuit for detecting the SQUID signal from this RF amplifier 3. It is an electro-optical converter that converts an output signal into an optical signal. In the second circuit 200, 7 is an opto-electrical converter that converts the transmitted feedback optical signal into an electrical signal;
8 is the PWM (pulse amplifier) of this photoelectric converter
An averaging circuit consisting of a low-pass filter that calculates the average value of the signal output, 9 is from the PWM signal output
A separation circuit that separates the PWM clock signal and obtains a reference signal for the flux lock loop, _R1 is connected to the current output of the averaging circuit 8, and is used for feedback so that the magnetic flux in the SQUID ring is kept constant. The feedback resistor _R2 is connected to the current output of the separation circuit 8, and the reference signal for synchronous detection is connected to the SQUID.
This is a resistance for input to. The outside portions of these circuits surrounded by broken lines are installed in and above the cryostat containing liquid helium, and constitute the cryostat section.

In the controller, 16 is the optical fiber 1
4 is a photoelectric converter that converts the optical signal (SQUID signal) transmitted from the cryostat side into an electrical signal, 17 is a buffer amplifier, and 18 is a frequency for generating a reference signal for synchronous detection and a clock signal for PWM. _AF (Audio
19 is a phase shift circuit for compensating circuit delay and optimally adjusting the phase of synchronous detection; 20 is a phase shift circuit for adjusting the phase of synchronous detection;
A multiplier forming a lock-in amplifier for synchronously detecting the SQUID signal with a reference signal from the phase shift circuit 19; 21 an integrator into which the signal from the multiplier 20 is input; 22 an output terminal of the integrator 21; Output terminal of this SQUID magnetometer to be connected, 23
is the output of the integrator 21 with a clock of frequency f ₂
PWM circuit that converts to PWM signal, 24 is this
The electro-optic converter 15 converts the output signal of the PWM circuit 23 into an optical signal, and is an optical fiber that transmits the optical signal from the electro-optic converter 24 to the cryostat section.

In the apparatus configured in this manner, the SQUID signal modulated with the reference signal of frequency _f2 is amplified, electro-optically converted, and transmitted from the cryostat section to the controller. In the controller,
After photoelectric conversion, this optical signal is synchronously detected at frequency _f2 . The output of the integrator 21 is pulse width modulated in a PWM circuit 23 with a clock signal having the same frequency f ₂ as the reference signal. This PWM signal is converted into an optical signal by the electro-optic converter 24 and then transmitted to the cryostat section. In the cryostat section, the photoelectric conversion circuit 7 converts the signal into an electric signal, and the averaging circuit 8 and the separation circuit 9 separate the signal into a low frequency signal and a clock signal of frequency _f2 , which are then converted into a reference signal and a feedback signal. It is added to the resonant circuit 13.

The operation of the SQUID magnetometer is similar to that of a conventional flux lock loop, so a description thereof will be omitted.

If you use a SQUID magnetometer with this configuration,
An analog output proportional to the external magnetic flux to be measured can be obtained from the output terminal 22.

Furthermore, since optical fiber is used for signal transmission, it is possible to create a SQUID magnetometer that is resistant to externally induced noise.

Furthermore, since the optical feedback circuit uses a PWM method, it can operate stably even when the optical fiber is vibrated or bent.

Furthermore, the reference signal for synchronous detection and the PWM clock are synchronized to produce the same frequency signal, which simplifies the configuration.

Note that although the above embodiment shows a case using an rf SQUID, it can be similarly applied to a magnetometer using a DC SQUID. However, when using a DC SQUID, it is necessary to apply the feedback signal from the averaging circuit and the AF clock signal from the separation circuit to a coil installed close to the SQUID ring (Japanese Patent Application No.
(See 190583 “SQUID Magnetometer”).

Also, although two optical fibers are used in the above embodiment, it can also be operated with a single optical fiber by performing bidirectional transmission using two optical signals with different (light) wavelengths. . In particular, by using the same frequency as the reference frequency for synchronous detection and the PWM clock, it is possible to operate with a single optical fiber even with a simple configuration.

<<Effects of the Invention>) As described above, according to the present invention, a SQUID magnetometer that is resistant to externally induced noise and noise caused by bending or vibration of an optical fiber can be realized with a simple configuration.

[Brief explanation of drawings]

Figure 1 is a configuration block diagram showing an embodiment of the SQUID magnetometer according to the present invention, and Figure 2 is the same as that of Figure 1.
FIG. 2 is an explanatory diagram showing a configuration example when applying a SQUID magnetometer to actual biomagnetism measurement. 11...SQUID ring, 13...LC resonant circuit, 2
0...Lock-in detection circuit, 23...PWM circuit, 1
00...first circuit, 200...second circuit,...cryostat section,...controller,...optical fiber, _f2 ...reference frequency.

Claims (1)

[Claims] 1. A SQUID magnetometer characterized by comprising (a) and (b). (a) A SQUID ring containing a Josephson junction;
LC resonant circuit coupled to this ring and this LC
a first circuit that amplifies the SQUID signal from the resonant circuit and outputs an optical signal related to the SQUID signal; a first circuit that receives a feedback signal transmitted as an optical signal; The PWM clock signal separated from this electrical signal is transferred to the LC.
A cryostat section comprising a resonant circuit or a second circuit coupled to a coil located close to the SQUID ring. (b) a lock-in detection circuit that transmits the optical signal output from the cryostat section via an optical fiber and synchronously detects an electrical signal related to the optical signal at a specific reference frequency; a PWM circuit that converts a signal related to the output into a PWM signal at the same clock frequency as the reference frequency;
A controller that outputs an optical signal corresponding to the signal as the feedback signal to the cryostat section via an optical fiber. 2. The SQUID magnetometer according to claim 1, wherein the optical signal output from the cryostat section and the optical signal output from the controller are transmitted at different wavelengths through the same optical fiber.