Optical detector for magnetic fields employing feedback circuitry

An optical detector for magnetic fields detects rotations in the polarization of a beam of polarized light caused by the magnetic fields. A modulator responsive to a fixed frequency signal introduces a fixed frequency of oscillation in the polarization of the light beam. The output signal of a polarization detector is filtered by a band pass filter and is fed, along with the fixed frequency signal, to a phase detector. The output of the phase detector is inverted and used as a feed-back signal to the modulator to cancel the rotation in polarization caused by the magnetic field. This feedback signal is also the final output signal of the detector.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to optical apparatus for detecting magnetic fields 
where such detection is based on magnetic rotations to a polarized light 
beam. 
2. Description Relative to the Prior Art 
It is well known to detect magnetic fields based on Kerr or Faraday effect 
rotations to a polarized inspection beam. Unfortunately, such optical 
detection of magnetic fields, generally recordings, tends to yield an 
undesirably low signal-to-noise ratio. This is because of the small 
magnetic rotation angles which are produced and the numerous possibilities 
for introducing noise when attempting to detect such angles. 
In the prior art, regarding optical detection of magnetic recordings, 
various approaches have been taken to reduce noise levels. It is known, 
for example, to utilize signal differencing techniques for cancelling 
certain types of noise. With such techniques the polarized inspection beam 
is split into two parts and two slightly displaced beam analyzers are 
employed for detecting rotation angles. By differencing the analyzer 
outputs, certain of the noise components are caused to cancel whereas a 
signal representative of beam rotation is retained. 
It is also known to oscillate the polarizaton direction of the inspection 
beam to provide a reference frequency for use in detecting magnetic 
rotations. U.S. Pat. No. 3,947,890 describes such a technique wherein the 
degree of assymetry introduced in an output waveform by magnetic rotations 
serves to indicate the nature of magnetic records. In a preferred 
implementation of that technique, frequencies around the beam oscillation 
frequency and the second harmonic thereof are isolated for detecting 
magnetic rotations. 
Notwithstanding these noise reduction techniques it would be desirable to 
have other approaches for increasing signal-to-noise ratios in an optical 
detection apparatus. 
SUMMARY OF THE INVENTION 
An optical detector for a magnetic field, such as the field produced by a 
magnetic recording, relies upon Kerr or Faraday effect rotations for 
detection, but in doing so utilizes a feedback circuit for nullifying such 
rotations. A feedback signal activates a modulator, which is located in 
the beam path, to produce a cancelling rotation, i.e., a rotation opposing 
the magnetic rotation, and such feedback signal then serves to indicate 
the nature of the field which produced the magnetic rotation. 
In a presently preferred implementation, the modulator also introduces a 
fixed frequency oscillation in the direction of beam polarization. An 
analyzer is arranged to receive the inspection beam and for the presently 
preferred implementation has its direction of polarizaton at substantially 
right angles to the polarization direction of the inspection beam. 
As is discussed more fully below, it is recognized in the preferred 
implementations for the invention that the mere presence of a component in 
the output of the analyzer at the oscillation frequency indicates a 
magnetic rotation (the periodic beam oscillation itself produces a 
component in the analyzer output at double the oscillation frequency). By 
isolating the analyzer output in a narrow frequency band around the 
oscillation frequency and detecting the phase thereof to indicate the 
magnetic rotation direction, a feedback signal is produced for nulling 
such rotation. Such filtering around the oscillator frequency works to 
remove noise components to a significant degree and has the effect of 
appreciably enhancing the ratio of signal to noise. As was mentioned 
above, with such an arrangement according to the invention, it is the 
feedback signal that then serves to indicate the nature of, say, a 
magnetic recording on a medium which is passed through the inspection beam 
.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Preferred detection apparatus includes a light source 1 cooperating with a 
polarizer 2 to produce a polarized inspection beam. Such beam propagates 
along a beam path and is acted upon by an electro-optical modulator 3, a 
lens 20, a medium such as a magnetic tape 5, a lens 22, an analyzer 7, 
which is arranged with its polarization direction relative to the beam 
path being substantially perpendicular to that of the polarizer 2, and a 
photosensitive receiver 8. The receiver 8 produces an output signal 
related to the intensity of the beam component transmitted by the analyzer 
7 and sends such signal to a feedback circuit 9 which is connected to 
provide control inputs to the modulator 3. 
For the illustrated apparatus arrangement, the tape 5 is substantially 
transparent and Faraday effect rotations are detected. It will be 
appreciated, however, that Kerr effect rotations could instead be detected 
if a reflecting tape were employed. To utilize the Kerr effect the lens 
22, the analyzer 7, and the receiver 8 would, of course, be arranged to 
influence a reflected beam component. Also, while a tape has been 
indicated as a magnetic medium, other sources of a magnetic field may be 
employed such as a magnetic disc. 
Means is preferably provided for advancing the magnetic tape via guide 
rollers 6 and 6' through the beam path, and between, for example, supply 
and takeup reels 4 and 4'. The orientation of the tape in the beam path is 
selected so that the magnetic records on the tape have a magnetization 
component in the direction of the polarized inspection beam. Such 
component them influences the beam by introducing Kerr or Faraday effect 
rotations. 
Now considering the feedback circuit 9 in further detail, an oscillator 30 
supplies a signal of a preselected reference frequency (N) to the modular 
3. Preferably the frequency N is chosen to be around ten times the highest 
frequency introduced by the influence of the magnetic medium. In response 
to such reference frequency signal, the modulator 3 produces an 
oscillation in the polarization direction of the inspection beam. As the 
magnetic records on the tape 5 pass through the inspection beam, they have 
the effect of either retarding or advancing the phase position of the 
oscillations in the polarization direction produced by the reference 
frequency signal. Thus, the reference frequency is analogous to a "carrier 
frequency" and the frequency of rotation introduced by the magnetic medium 
is analogous to the "modulating frequency" in a phase-modulated type 
communication system. In such a system, the information represented by the 
modulating signal appears in side bands about the carrier frequency. For a 
background discussion of phase modulation, see Terman, F. E. Electronic 
and Radio Engineering N.Y. McGraw-Hill, 1955, Fourth Edition, pp 592-594. 
The circuit 9 as implemented, includes a current-to-voltage converter 10 
which receives the output (denoted i) of the photosensitive receiver 8 and 
provides a voltage signal (denoted V) to a selective-bandpass amplifier 
11, which isolates a signal component (denoted V') having frequencies in a 
band around the frequency N. Such a signal component represents magnetic 
rotation of the polarization direction of the beam. By filtering out 
frequencies above and below the pass-band, fluctuations in the intensity 
of the inspection beam due to causes other than rotation of the 
polarization direction of the beam, such as variations in the 
transmittance or reflectivity of the magnetic medium 5, or variations in 
the emission of the beam source 1, can be strongly suppressed. This 
greatly enhances the signal/noise ratio when detecting small magnetic 
rotations of the polarized beam. The width of the passband is 
predetermined to include the sideband frequencies introduced by magnetic 
rotations of the polarization direction caused by the magnetic records on 
the tape 5. After such bandpass filtering by amplifier 11, the isolated 
signal component is applied to a phase detector 12 which also receives, as 
a reference, the output signal of the oscillator 30. The polarity and 
amplitude of the output signal (denoted V.sub.1) from the phase detector 
12 changes according to phase relationship of the reference signal to the 
signal component from selective bandpass amplifier 11. The signal V.sub.1 
is amplified by an amplifier 13 to produce an output feedback signal 
(denoted V.sub.2) which is applied to the modulator 3. Such feedback 
signal augments the signal from the reference oscillator 30 and has a 
polarity (negative feedback) to cause the modulator 3 to drive the 
polarization direction of the inspection beam toward an orientation for 
nulling the output of the phase detector 12. The use of a negative 
feedback loop further suppresses any noise that may be introduced into the 
output signal by components of the device and thereby further enhances the 
signal/noise ratio of the output signal. For a background discussion of 
the use of feedback to suppress output disturbances caused by noise in 
internal components of a device, see James, Nicholas, and Phillips Theory 
of Servomechanisms N.Y. McGraw-Hill, 1947, p 145. 
The output signal of the amplifier 13 is indicative of the rotation 
produced by the magnetic record and provides an electrical signal 
representation of the magnetic record on the tape 5. Such signal by virtue 
of the selective bandpass filtering at amplifier 11 and of the cancelling 
of the beam rotation has desirable signal-to-noise characteristics. 
Now considering particularly a presently preferred embodiment for the 
invention, the light source 1 is a helium-neon laser and the inspection 
beam is polarized substantially linearly by the polarizer 2, which is a 
Glazebrook prism. The modulator 3 consists of a unit formed of a 
quarter-wave plate (one of the neutral lines of which is parallel to the 
direction of polarizer 2) and of a Pockels-effect electro-optical 
modulator, for example, a modulator such as Isomet, Model EOLM, type 400, 
manufactured by the Isomet Corporation, 103 Bauer Drive, Oakland, New 
Jersey, U.S.A. The neutral lines of the modulator are shifted 45.degree. 
from the direction of prism 2. The amplitude of the oscillations produced 
by the modulator, is preferably on the order of one degree. 
The inspection beam is focused by the lens 20 which defines a reading 
"spot" on the tape 5. A driving mechanism (not shown) moves the magnetic 
tape through the beam path . . . the tape 5 unwinds from the spool 4 and 
is rewound onto the spool 4' after having encircled guide rollers 6 and 6' 
which orient the surface of the tape at an angle to the axis of the beam. 
The magnetization vector of the magnetic tape (along the longitudinal tape 
axis) consequently has a component along the axis of the beam, and this 
component causes a rotation, as a result of the Faraday effect, in the 
orientation of the plane of polarization of the beam. 
The beam after being influenced by the magnetic tape 5 is directed by the 
lens 22 onto the analyzer 7, preferably a Glazebrook prism and a 
transmitted component thereof activates the photosensitive receiver 8, a 
silicon photodiode. The polarization direction of the analyzer 7 is shited 
90.degree. from the polarization direction of the polarizer 2. 
As was mentioned above, the operating frequency N of oscillator 30, is 
preferably selected to be at least 10 times greater than the frequency of 
the recorded signal. By so selecting the oscillator frequency, the 
passband of the amplifier 11 may be relatively narrow. For example, in 
reading magnetic tape at say a rate of 19 centimeters per second, where 
the tape has a numerical recording of density 3200 bits per inch, the 
frequency N is preferably at least: 
EQU 10 .times. 3200 .times. (19/2.54) = 240,000 Hz 
In the presently preferred embodiment of the invention, an oscillator 
frequency of 1.25 MHz is employed. 
The output voltage of amplifier 13 is representative of the magnetic 
recording and can feed a loud-speaker (not shown) or some other type of 
utilization device. 
The invention has been described in detail with reference to a presently 
preferred implementation thereof; however, it will be appreciated that 
various modifications are possible within the spirit and scope of the 
invention. For example, while preferred application of this invention is 
in the contactless reading of, say iron oxide-, MnBi-, ferrite-, or 
garnet-based transparent magnetic tapes, it is also useful in the 
contactless reading, by means of reflection (Kerr effect), of magnetic 
recording tapes having a degree of specular reflectance (such as supports 
coated with a cobalt-phosphor magnetic layer obtained by autocatalytic 
deposit). Also, the invention may be employed in measuring the beam 
rotating effect of a test sample of any optically active material.