A magneto-optical phase-modulating device comprising a magneto-optic layer (3) forming part of an optical stack and upon which, in use, light is incident and to which is applied a magnetic field, a layer of magnetic material (6) adjacent the magneto-optic layer (3), and at least two flat, low inductance conductors (9,12) connected in series and disposed one on either side of the layer of magnetic material (6), in use the conductors (9,12) having an electric current applied thereto to provide a magnetic field in the layer of magnetic material (6), which magnetic field is switchable between one direction and the opposite direction by reversing the current in the conductors (9,12) and is of a strength relative to that of the magnetic field applied to the magneto-optic layer (3) such that the latter field is also switched between said one direction and the opposite direction as the field associated with the layer of magnetic material (6) is switched.

This invention relates to magneto-optical phasemodulating devices operable 
to modulate light incident thereon. These devices may be employed in ring 
lasers and the invention will be discussed in the main with reference to 
ring lasers, and more specifically laser gyroscopes, but it is to be 
understood that it is not limited to this particular application. 
As is well known, a ring laser employs two beams of light propagated in 
opposite directions around the ring or so-called cavity. In an ideal ring 
laser, the frequency difference between the beams of light is zero when 
the ring is stationary but moves from zero when the ring is rotated about 
its axis, the frequency difference being proportional to the angular 
rotation rate of the cavity. Thus a ring laser is capable of functioning 
as a rate gyroscope. In practical ring lasers, however, there are many 
effects that degrade the performance, the majority of these are linked in 
some way to the amount of light that is lost in traversing the cavity. One 
of the most dominant, and hence troublesome, effect is lock-in which is 
caused by light scattered from each beam interacting with the opposite 
beam, suppressing the frequency difference at low rotation rates, and 
making the frequency difference non-linear at just above the lock-in 
frequency. 
When a ring laser is used as a gyroscope, the two output light beams are 
combined to provide interference fringes which may be counted by a 
photodetector. The fringe count is directly proportional to the total 
angle the ring laser has turned through provided the two beams of light 
are completely uncoupled. The ratio of the fringe count per unit angle of 
rotation is known as the scale factor. As a result of lock-in, no fringes 
will occur up to the lock-in threshold and the scale factor will be 
non-linear for a range of rotational rates above the lock-in threshold, 
both these phenomena seriously degrading the accuracy of the ring laser 
gyroscope. 
One method of avoiding the lock-in problem is to impart a bias to the ring 
laser such that a non-reciprocal phase shift is introduced to the 
contradirectional light beams. Various biasing techniques have been 
proposed ranging from a mechanical arrangement (known as "dither"), which 
oscillates the entire ring laser at a small amplitude, to magneto-optical 
arrangements. The magneto-optical arrangements fall in two categories, 
namely Faraday cells and magnetic bias mirrors. In the Faraday cell 
biasing devices, a paramagnetic or ferrimagnetic material, transparent to 
the laser wavelength, is inserted in the cavity in the paths of the two 
light beams. This arrangement suffers the disadvantage that high quality, 
and hence expensive, optical components have to be employed and 
furthermore, these components may give rise to increased light scatter 
which therefore adds to the lock-in problem. 
As regards the magnetic bias mirror, this replaces one of the usual three 
"corner" mirrors of the ring laser and an example is disclosed in British 
Patent Specification No. 1,406,730. In this example, the mirror comprises 
a ferromagnetic layer formed on a substrate and overcoated with layers of 
dielectric materials to give, among other things, the ferromagnetic layer 
sufficient reflectivity to produce a ring laser of a sufficient quality 
for gyroscopic purposes. In use, a magnetic field is applied to the 
ferromagnetic layer in the plane of the mirror and perpendicular to the 
plane of the laser cavity so as to exploit the transverse Kerr 
magneto-optic effect which results in a phase difference being imparted to 
the contradirectional light beams in addition to that created by any 
rotation of the ring laser, whereby the ring laser can be operated always 
with a linear scale factor even if the actual rate of rotation being 
sensed is below the lock-in threshold. As well as introducing the required 
phase difference, the transverse Kerr magneto-optic effect introduces an 
amplitude difference between two light beams by way of non-reciprocal 
reflectivity of the mirror. This has the detrimental effect on the 
performance of a laser gyroscope and needs to be minimised for optimum 
gyroscope performance. To this end, use is made of a layer of dielectric 
material immediately next to the ferromagnetic layer, the dielectric layer 
(termed the "control layer") being of a modified thickness compared with 
the adjacent dielectric layers which normally have a quarter-wave optical 
thickness. 
The advantages of the magnetic bias mirror are that it is non-mechanical, 
it can be subjected to switching as regards the magnetic field applied to 
the magneto-optical layer so as to reverse the bias as required, and the 
bias is defined by the saturation moment of the magnetic material as 
opposed to being defined by the magnitude of the magnetising drive current 
as it would be using paramagnetic materials. Switching also makes the bias 
independent of changes in saturation moment due to temperature drift. 
However, whilst switching of the magnetic field applied to the 
ferromagnetic layer can be effected relatively fast (of the order of one 
microsecond), this is not fast enough when turn rates of the order of 
400.degree./second have to be accommodated in missiles, for example. The 
required speed of switching to cope with this environment cannot be 
accomplished using known techniques in conjunction with the magnetising 
coils associated with the ferromagnetic layer of known magnetic bias 
mirrors. Clearly, the switch over time must be short compared with the 
desired resolution if fringe counts are not to be missed. 
It is the object of the present invention to provide a magneto-optical 
phase-modulating device utilizing a Faraday cell or a transverse Kerr 
effect device which enables switching of the field applied to the 
magneto-optical layer at a speed which is in excess of that presently 
attainable with known magnetic coil arrangements. 
According to the present invention there is provided a magneto-optical 
phase-modulating device comprising a magneto-optic layer forming part of 
an optical stack and upon which, in use, light is incident and to which is 
applied a reversible magnetic field, a layer of magnetic material adjacent 
the magneto-optic layer, and at least two flat, low inductance conductors 
connected in series and disposed one on either side of the layer of 
magnetic material, in use the conductors having an electric current 
applied thereto to magnetize in the layer of magnetic material, which 
megnetization is switchable between one direction and the opposite 
direction by reversing the current in the conductors and which induces an 
external field of a strength to magnetically affect the magneto-optic 
layer such that the consequent magnetization thereof is also switched 
between said one direction and the opposite direction as the field 
associated with the layer of magnetic material is switched. 
Preferably at least one of the conductors is a stripline conductor and 
desirably each conductor is of this type. The use of flat conductors gives 
rise to a low inductance arrangement because the magnetic field created by 
passing electrical current through the conductors is confined to the 
proximity of the conductors which means that very fast current pulses can 
be passed down the conductors with an attendant very fast switching of the 
magnetic field. 
The magneto-optic layer and magnetic layer, when provided, are preferably 
of the thin film type whereby they have a strong shape anisotropy which 
confines the magnetic moment to the plane of the film. Within the plane of 
the film or layer there is a small uniaxial anisotropy so that the 
magnetisation lies in one of two directions parallel to the so-called 
"easy" magnetic axis. Such films or layers can have applied magnetic 
fields switched between one sense and the opposite sense along the easy 
axis by relatively small applied fields. 
The direction of magnetisation in a thin film can be changed in two ways, 
namely by domain wall motion and by rotation, the latter being preferred 
since it results in faster switching. Accordingly, the field applied 
either directly or indirectly to the magneto-optic layer has at least a 
component in the direction of the hard axis and this can be accomplished 
either by applying the field at an angle to the easy or hard axis or by 
applying two orthogonal fields substantially along the easy and hard axes, 
respectively, and arranging for the field in the direction of the hard 
axis to be switched off prior to that in the direction of the easy axis. 
It is desirable to make the magnetic layer of a magnetically saturable 
material having a square loop hysteresis characteristic so that it will 
maintain the magneto-optic layer magnetically saturated even if that layer 
does not have a square loop hysteresis characteristic.

The two illustrated embodiments are in the form of magnetic bias mirrors 
for a ring laser gyroscope and that shown in FIGS. 1 and 2 compises a 
substrate 1 on which is formed an optical stack upon which, in use, two 
beams of light 2 and 2' are incident and from which the incident beams are 
reflected as indicated in FIG. 2. The optical stack comprises a 
magneto-optic layer 3 behind which, with respect to the incident light 2, 
2', is a highly reflective layer 4 in the form of a multilayer stack of 
two dielectric materials disposed alternately. In order to limit the 
amount of incident light 2, 2' reflected from the air/layer 3 interface, 
an anti-reflective layer 5 is provided on top of the magneto-optic layer 
3, the anti-reflective layer 5 also comprising a multilayer stack of two 
alternating dielectric materials. 
The dielectric materials used in the layers 4 and 5 may be magnesium 
fluoride and zinc sulphide and the magneto-optic layer may be composed of 
a ferromagnetic garnet, the general construction of the optical stack thus 
being similar to that disclosed in British Patent Specification No. 
2,006,456A. 
Between the substrate 1 and the reflective layer 4 there is provided a film 
of a saturable magnetic material 6 having a square loop hysteresis 
characteristic and a low in-plane anisotropy. A suitable magnetic material 
having these characteristics is a nickel-iron alloy (80% Nickel, 20% Iron) 
but others may be used. The film 6 is deposited by any conventional method 
and is typically between 1,000 and 2,000 Angstroms thick so as to have a 
strong shape anisotropy giving rise to easy and hard magnetic axes 7 and 8 
(FIG. 1) which are generally aligned with the respective magnetic axes of 
the magneto-optic layer 3. The magnetic film 6 is deposited on a ground 
conducting layer 9 of gold or other conductive material which in turn is 
deposited on the substrate 1 by any conventional method. An insulating 
layer 11 is provided over the magnetic film 6 and on top of the insulating 
layer there is provided a stripline conductor 12 connected electrically in 
series with the ground layer 9. The mirror has an overall diameter of 25 
mm with the magneto-optic layer 3 and magnetic layer 6 having a diameter 
of 15 mm. 
Contact pads 9' and 12' are provided for the ground layer 9 and stripline 
conductor 12, respectively, for application of electric current to the 
conductor 12 which is returned via the ground layer 9. The conductor 12 
and ground layer 9 are typically each of a thickness of 10,000 Angstroms 
and are spaced apart by about 3,000 Angstroms, whereby they are of very 
low inductance as the magnetic field created by electric current passing 
therethrough is confined to the proximity thereof. Accordingly, if very 
fast current pulses are propagated down the conductor 12 and ground layer 
9 (of one sense or another), the magnetic field thus applied to the 
magnetic film 6 will follow the sense of the pulses, whereby very fast 
switching of the magnetic film is obtained. 
It is required that the conductor 12 and the ground layer 9 be relatively 
closely spaced (S) compared with their widths (W), the relationship 
S.ltoreq.0.1W being satisfactory in this respect although it does not have 
to be applied rigidly. Also, the conductor 12 should have a length greater 
than its width, and a generally low electrical resistance. If the 
conductor 12 is of aluminium, then a length of 2 cm, a width of 1 mm and a 
thickness of 1 .mu.m gives a resistance of 0.6 ohms which is acceptable. 
The strength of the magnetization applied to the magnetic film 6, coupled 
with the close spacing of the film 6 and the magneto-optic layer 3, 
results in the former influencing the latter to the extent that the 
consequent magnetization of the magneto-optic layer 3 is switched in 
accordance with the switching of the magnetic field of the magnetic film 
6. More specifically, the magneto-optic layer 3 is switched by stray flux 
from the magnetic film 6 to form a closed flux situation which gives the 
lowest energy condition. As seen from FIG. 1, the stripline conductor 12 
is arranged at an angle, preferably between 20.degree. and 30.degree., to 
the hard magnetic axis 8, and hence at an angle to the easy magnetic axis 
7, so that components of the magnetic field created by current passing 
through the conductor lie in the directions of both the easy and hard 
magnetic axes of the magnetic film 6. In this way, the magnetic field is 
changed by rotation rather than by domain wall motion which is a slower 
process. 
Thus, very fast switching (of the order of nanoseconds) of the magnetic 
field of the magneto-optic layer 3 is achieved which means that any rates 
of turn of the gyroscope in which the magnetic mirror is fitted can be 
accommodated, even rates of turn of the order of 400.degree./second such 
as are experienced in missiles. This is because the gyroscope can be given 
a bias by the magnetic mirror such that it is always operating on (or 
close to) a linear part of the frequency difference (.DELTA.f) of the two 
beams 2, 2' versus rate of turn (.OMEGA.) characteristic as indicated in 
FIG. 6, the linear portion used depending on whether a positive or 
negative bias is applied. The essence of the present invention is to 
change quickly from one bias to the opposite bias when a given rate of 
turn would otherwise involve operating on an unacceptable non-linear 
portion of FIG. 6 or in the lock-in region indicated at L. The speed of 
switching the bias is so fast that fringe counts by a photodetector 
subjected to a combination of the light beams 2, 2' taken out of the ring 
laser are not lost which means that the output of the gyroscope is 
extremely accurate for all rates of turn, including very low rates which 
would normally not give rise to an output due to the problem of lock-in. 
The magnetically saturable film 6 is preferably of the thin film type as is 
the magneto-optic layer 3 with the two films having substantially the same 
product of cross-sectional area (A) and magnetic flux (M), whereby M.sub.1 
.times.A.sub.1 =M.sub.2 .times.A.sub.2. 
With the bias mirror of FIGS. 1 and 2 there arises a secondary advantage 
from the present invention which is that the magnetically saturable film 6 
maintains the magneto-optic layer 3 saturated. 
The embodiment of FIGS. 3 and 4 is very similar to that of FIGS. 1 and 2 
and like parts have similar reference numerals. The difference between 
these two embodiments is that the magnetic field applied to the magnetic 
film 6 in the first embodiment is at an angle to the hard and easy 
magnetic axes 7 and 8, whereas in the embodiment of FIGS. 1 and 2, two 
magnetic fields are applied to the film 6, one in the direction of the 
easy magnetic axis 7 and the other in the direction of the hard magnetic 
axis 8. This is achieved by providing a further insulating layer 13 on top 
of the stripline conductor 12, and a further stripline conductor 14 on top 
of the insulating layer 13. 
The stripline conductor 12 is oriented in the direction of the hard axis 8, 
and the conductor 14 in the direction of the easy axis 7. However, since 
the conductors produce a magnetic field at right angles to their length, 
then the conductor 12 provides an easy axis field and the conductor 14 a 
hard axis field. Contact pads 14' are provided for the further stripline 
conductor 14, the ground conducting layer 9 acting as the return path for 
both the conductors 12 and 14. In order to achieve switching of the 
magnetic field on the magnetic film 6 by rotation, as opposed to domain 
wall motion, it is necessary to ensure that the current applied to the 
conductor 14 (hard axis) is terminated before that applied to the 
conductor 12 (easy axis). 
FIGS. 2 and 4 show the bottom of the dielectric layer 4 spaced from the top 
of the uppermost conductor and this may or may not be necessary, depending 
on the specific design of mirror. If necessary, the space will be filled 
by a dielecric material chosen so that its thickness will create the 
correct phase relationship of light incident upon itself and other layers 
in the optical stack. 
The embodiments of FIGS. 1 and 3 employ single conductors 12 and 14 but 
these may be replaced by a multi-turn conductor or a plurality of 
conductors in order to increase the magnetic per unit current. A 
diagrammatic representation of the use of a multi-turn conductor 15 with 
the magnetic film 6 is shown in FIG. 5 of the drawings. 
The garnet magneto-optic layers 3 of the illustrated embodiments may be 
replaced by iron magneto-optic layers.