Apparatus comprising a waveguide magneto-optic isolator

The invention relates to optical systems comprising thin film optical waveguide isolators that are characterized by linear birefringence at least some wavelengths and temperatures. Disclosed is a method for using such a system at more than one wavelength and temperature.

TECHNICAL FIELD 
The invention pertains to optical systems, e.g., optical fiber 
communication systems and optical mass storage devices, which include thin 
film polarization rotators. 
BACKGROUND OF THE INVENTION 
The frequency and power intensity spectra of the light emitted by 
semiconductor lasers employed in optical systems can be altered when 
reflected light impinges upon the lasers. Such alterations are undesirable 
because they can lead to errors in the detected information. Thus, efforts 
have been made to develop devices, called optical isolators, for isolating 
the semiconductor lasers from reflected light. An optical isolator based 
on rotation of linearly polarized light is exemplified by a bulk magnetic 
garnet material, e.g., bulk single crystal yttrium iron garnet (Y.sub.3 
Fe.sub.5 O.sub.12, called YIG), positioned between a polarizer and an 
anlyzer. In operation, a magnet is employed to magnetize the YIG (in the 
direction of light propagation). Light emitted by a laser and linearly 
polarized after transmission through the polarizer is directed into the 
YIG material. Under the influence of the net magnetic moment within the 
(magnetized) material, the linearly polarized light experiences circular 
birefringence. As a consequence, the light remains linearly polarized, but 
the polarization direction is continuously rotated in either the clockwise 
or counterclockwise direction as the light traverses the material. If the 
material is of appropriate dimension, the polarization direction is 
rotated through, for example, 45.degree. and the light is thus 
transmitted by an analyzer set at 45.degree.. Reflected light transmitted 
by the analyzer also enters the YIG material and also undergoes a rotation 
of 45.degree. in the same direction as the light which originally 
traversed the material. Consequently, reflected light, after traversing 
the YIG material, is polarized at 90.degree. to the polarizer, and is thus 
precluded from impinging upon the laser. (The phenomenon by which a fixed 
length of magnetized material rotates both forward and backward 
propagating linearly polarized light by the same amount and in the same 
direction is denoted antireciprocal magneto-optical rotation. Devices 
which include such materials are referred to as antireciprocal devices. By 
contrast, an optical element that rotates oppositely propagating beams of 
light in the same direction, but not necessarily by the same amount, is 
said to be "non-reciprocal.") 
Although antireciprocal light-rotating devices based on bulk materials are 
useful, thin-film-waveguide light-rotating devices are advantageous where 
incorporation in miniaturized integrated optical devices is envisaged. For 
example, a thin film optical isolator using planar magnetization would 
readily permit the use of guided wave optics (and thus eliminate the need 
for focusing lenses) and could also serve as a building block for 
integrated optical devices. 
Thin film waveguiding devices employing planar magnetization have, in fact, 
been fabricated. Such devices have included, for example, a magnetized (in 
the plane of the film) layer of YIG epitaxially grown on a (closely 
lattice matched) substrate of, for example, gadolinium gallium garnet 
(Gd.sub.3 Ga.sub.5 O.sub.12, called GGG). While these devices are 
potentially attractive, they are, unfortunately, subject to linear 
birefringence. Linear birefringence means that the TE and TM components of 
the light see different refractive indices, resulting in one of these 
components propagating through the film at a faster speed than the other. 
Thus, when traversing a magnetized thin film, e.g., a magnetized layer of 
YIG, light is subjected to a birefringence which includes both a linear 
component and a circular component. As a consequence, reflected light is 
incompletely blocked. Thus, the effects of linear birefringence in thin 
film, magnetized, waveguiding devices have presented a serious obstacle to 
their advantageous use. 
The factors responsible for the linear birefringence found in thin films 
of, for example, YIG have been identified. One of these factors is what is 
here termed shape linear birefringence, which is due to the presence of 
discontinuities in refractive index at the film-air and film-substrate 
interfaces. A second factor responsible for linear birefringence, commonly 
termed stress-induced linear birefringence, is due to a lattice mismatch 
between the film and the substrate. This mismatch subjects the film to 
either a compressive or tensile stress in the plane of the film, which, 
like shape linear birefringence, has the effect of inducing a refractive 
index anisotropy in the film. A third factor responsible for linear 
birefringence, commonly termed growth-induced linear birefringence, is due 
to a non-random distribution of certain ions in the film crystal lattice, 
produced by the conventional techniques used to epitaxially grow films on 
substrates. In many cases, the sign of the stress-induced and/or 
growth-induced linear birefringence is opposite to that of the shape 
linear birefringence. Thus, these different sources of linear 
birefringence can be used to cancel each other to produce zero net linear 
birefringence. 
For example, R. Wolfe et al, "Thin-Film Garnet Materials with Zero Linear 
Birefringence for Magneto-Optic Waveguide Devices (Invited)", J. Applied 
Phys., Vol. 63, pp. 3099-3103, (1988) describes a method for fabricating a 
thin film, waveguiding, polarization rotator which achieves essentially 
zero net linear birefringence, i.e., achieves a value of the dimensionless 
ratio B/F less than or equal to about 0.1. The ratio B/F expresses the 
ratio of linear birefringence to Faraday rotation. B is equal to 
.DELTA..beta./2, where .DELTA..beta.=2.pi..DELTA.n/.lambda. and .DELTA.n 
denotes the difference in the refractive indices seen by the TE and TM 
components, while .lambda. denotes the wavelength of the light. 
Physically, .DELTA..beta. is the phase difference (induced by the net 
linear birefringence) between the TE and TM components per unit length of 
film, and has dimensions of, for example, radians per centimeter. In 
addition, F denotes the Faraday rotation per unit length of the film. F is 
expressed in the same units as .DELTA..beta., e.g., in radians per 
centimeter. 
Wolfe et al reported that in epitaxial garnet films, .DELTA.n can be 
reduced to small values by (1) growing single-mode multilayer films to 
minimize the shape effect, (2) growing the films in compression to control 
the stress-induced effect, and (3) annealing at high temperatures to 
eliminate the growth-induced effect. The remaining birefringence can be 
reduced to zero by growing the top active layer so thick that the shape 
effect is smaller in magnitude than the stress effect, and then thinning 
it by chemical etching until the effects exactly cancel each other at a 
particular wavelength and temperature. An alternative method is to begin 
with a thin top layer such that the magnitude of the shape effect is 
relatively large, and then to deposit a dielectric layer such as silicon 
nitride of the proper thickness to reduce the shape effect so that it 
exactly cancels the stress effect. 
By the method of Wolfe et al, linear birefringence at a given temperature 
and at a given wavelength can be essentially eliminated from a thin-film 
magnetic waveguide. If the waveguide is, moreover, composed of a 
non-reciprocal material, a useful non-reciprocal optical device such as an 
isolator is readily produced. 
However, it may be necessary, in practice, to operate the waveguide over a 
range of wavelengths and temperatures, and hence it may be necessary to 
tolerate a small but significant amount of linear birefringence in the 
waveguide. 
H. Dammann, et al, Abstract: "The 45.degree. Waveguide-Isolator," Journal 
of IOOC, July 1989, Kobe, Japan (to be published) have described a method 
of using a thin-film optical isolator that achieves useful optical 
isolation in the presence of linear, as well as magnetic circular, 
birefringence. Dammann, et al, observed that despite the presence of 
linear birefringence, light that enters such a waveguide in a linear 
polarization state will always exit in a linear polarization state, 
provided that at the midpoint of the waveguide, the major axis of the 
polarization ellipse is parallel or perpendicular to the major surface of 
the waveguide. (This condition is here referred to as the Dammann 
condition.) Significantly, magnetic materials having linear birefringence 
are not, in general, anti-reciprocal, although they may be non-reciprocal. 
As a consequence, although a non-reciprocal waveguide can readily be 
provided that satisfies the Dammann condition for light propagating in one 
direction, the forward beam and the reflected (reverse-propagating) beam 
will not, in general, simultaneously satisfy the condition. 
Thus, for example, an optical isolator using Dammann's principle is 
advantageously made by providing a non-reciprocal, 45.degree. optically 
rotating waveguide. For illustrative purposes, it is assumed that linearly 
polarized light enters such a waveguide through an input polarizer 
oriented at 67.5.degree. from the TE mode orientation (considered to 
correspond to 0.degree.) and exits through an output polarizer oriented at 
22.5.degree.. In a practical isolator, it is typically important to 
minimize the amount of reflected light that escapes in the reverse 
direction, even at the expense of suffering some loss in the forward 
direction. In order to assure that the reflected light is maximally 
blocked by the input polarizer (oriented at 67.5.degree. to the TE axis), 
the waveguide is designed such that the reflected light, rather than the 
forward-propagating light, satisfies the Dammann condition. 
That is, in general, the forward-propagating light arrives at the output 
polarizer in an elliptical polarization state. A portion of this light is 
resolved and transmitted by the output polarizer. Reverse-propagating 
light (i.e., light reflected by any discontinuity in the optical path) 
passes through the output polarizer and starts out with linear 
polarization at 22.5.degree.. This light satisfies the Dammann condition. 
That is, at the midpoint of the waveguide, the polarization ellipse has a 
major axis at 0.degree., and the light arrives at the entrance polarizer 
linearly polarized at -22.5.degree.. This light is completely blocked by 
the input polarizer, giving essentially perfect reverse extinction. 
Provided the amount of linear birefringence in the waveguide is relatively 
small, the forward-propagating light that is incident on the output 
polarizer will have a relatively large component transmissible by the 
output polarizer, and the waveguide will be useful as a practical optical 
isolator. However, the greater the linear birefringence, the greater the 
loss at the output polarizer is likely to be. 
Because linear birefringence is known to be sensitive to wavelength and 
temperature, it has until now been believed that the method of Dammann, et 
al, is useful only for operation at essentially a single optical 
wavelength, and within a narrow, carefully controlled temperature range. 
The effect of temperature on magnetic thin film optical isolators has been 
discussed, for example, by J. P. Castera et al, "Phase Matching in 
Magneto-Optic YIG Films by Waveguide Temperature Control," Electronics 
Lett., Vol. 25, p. 297 (1989). Castera reported the construction of a 
waveguide isolator that could be tuned to essentially zero linear 
birefringence by using temperature to alter the stress-induced component 
of the linear birefringence. Significantly, Castera reported that because 
of the sensitivity of the birefringence to temperature, such a tuned 
device would require temperature stabilization. For example, the 
temperature would have to be maintained within a 2.degree. C. range in 
order to achieve a stable isolation of 30 dB. 
The wavelength sensitivity of such isolators has been discussed, for 
example, by R. Wolfe, et al, "Etch-Tuned Ridged Waveguide Magneto-Optic 
Isolator," Appl. Phys. Lett., Vol. 56, p. 427 (1990). Wolfe reported that 
when pure TE light was injected into an etch-tuned waveguide isolator, the 
isolation ratio changed from a desirable value of -35 dB at the tuning 
wavelength of 1.545 .mu.m to a much less desirable value of -16 dB at 1.45 
.mu.m. Thus, because of the wavelength sensitivity of the birefringence, 
the isolation ratio fell in magnitude by 19 dB over a wavelength range of 
less than 0.1 .mu.m. 
SUMMARY OF THE INVENTION 
It has been discovered that when a thin-film magneto-optic isolator is 
operated according to the method of Dammann, et al, excellent reverse 
isolation and moderate forward loss can be obtained over an unexpectedly 
broad range of wavelengths and temperatures. 
Thus in one embodiment, the invention involves providing a thin-film 
magneto-optic isolator having an input polarizer oriented near 
67.5.degree. and an output polarizer oriented near 22.5.degree. to the TE 
or TM direction (or vice versa) and using the isolator at least two 
wavelengths, separated by at least about 0.05 .mu.m. Significantly, a 
wavelength range of at least 0.05 .mu.m is useful in connection with 
transmission of wavelength division multiplexed optical signals. 
In another embodiment, the invention comprises using such an isolator 
which, moreover, is tuned to have essentially zero linear birefringence at 
a first wavelength, near the low-temperature end of a temperature 
operating range spanning more than about 20.degree. C.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
The following discussion is simplified by reference to a visual aid, 
depicted in FIG. 1, known as the Poincare sphere. The Poincare sphere is a 
mapping of the polarization states of a light wave onto the surface of a 
sphere. The evolution of these states as the light wave propagates 
through, e.g., a non-absorbing, birefringent waveguide, is conveniently 
represented by trajectories on the surface of the sphere. 
On this sphere, points on the equator represent linearly polarized states, 
such that the point representing 0.degree. (pure TE) is diametrically 
opposite the point representing 90.degree. (pure TM). The poles represent 
circular polarization, and all other points correspond to elliptically 
polarized states. The point M.sup.+ corresponds to the polarization state 
of the faster of the two waveguide eigenmodes when the magnetization is 
parallel to the forward light propagation direction. Conversely, the point 
M.sup.- corresponds to the faster eigenmode when the magnetization is 
antiparallel to the forward propagation direction. (Thus, the point 
M.sup.+ would lie at the north pole if the material were characterized by 
a finite Faraday rotation, but zero linear birefringence.) Every 
trajectory representing the evolution of a polarization state (in a 
lossless waveguide) is an arc on the surface of the sphere of a clockwise 
rotation centered on M.sup.+ or M.sup.-. 
Thus, in the exemplary optical isolator discussed above, point A represents 
the input polarization of light propagating in the forward direction. Path 
AB represents the evolution of the light from the input to the output 
polarizers. The length of arc BC is indicative of the excess forward loss 
(i.e., forward loss due to polarization effects, and not attributable to 
inherent losses in the waveguide). Path CD represents the reverse 
propagating light. It should be noted, in particular, that the midpoint of 
path CD lies on the 0.degree. meridian, and thus the Dammann condition is 
satisfied. As a consequence, the reverse propagating light arrives at 
point D in a state of pure linear polarization that is completely blocked 
by the input polarizer. In this example, both path AB and path CD lie on 
clockwise rotations about point M.sup.+. 
Further properties of the optical isolator are made clear by reference to 
the Poincare sphere. For example, with reference to FIG. 2, a 22.5.degree. 
orientation of the output polarizer (point C), although optimal, is not 
unique. That is, a different orientation a few degrees away from 
22.5.degree. could be chosen. Such an orientation may be represented, for 
example, by point C'. In such a case, the Dammann condition is still 
satisfied, and essentially perfect reverse isolation is still achieved, 
provided point D, representing the state of polarization of the reverse 
propagating light when it arrives at the input polarizer, undergoes a 
symmetrical displacement to point D'. Because the input polarizer must be 
orthogonal to the polarization represented by point D', it is necessary to 
correspondingly shift point A to point A'. The result, in the example as 
depicted, is a total rotation in the isolator of somewhat less than 
45.degree., with input polarization angle somewhat greater than 
67.5.degree.. Because both the input state and the rotation are changed, 
the shift of point B (representing the fully rotated state of the 
forward-propagating light) to point B' has twice the angular size of the 
other corresponding shifts described here. The increased arc length B'C', 
relative to BC, represents the increased excess forward loss brought about 
by these changes. It is readily apparent from the figure that either a 
clockwise or a counterclockwise shift of point C will increase the excess 
forward loss. It is in this sense that the 22.5.degree. selection is 
optimal. Variations of up to about 5.degree. (in either the positive or 
negative direction) can be practiced without incurring prohibitive excess 
forward loss. 
The total length of waveguide required to produce a perfect 45.degree. 
rotation, corresponding to path CD, is here denoted L.sub.45, and is given 
theoretically by 
##EQU1## 
where, as noted previously, B=.DELTA..beta./2. 
Turning now to FIG. 3, a preferred embodiment of the invention involves an 
etch-tuned ridge waveguide isolator, as described, for example, in R. 
Wolfe, et al., J. Appl. Phys., Vol. 63, p. 3099 (1988), and in R. Wolfe, 
et al., Appl. Phys. Lett., Vol. 56, p. 427 (1990). The waveguide is a 
triple layer film of modified bismuth yttrium iron garnet (Bi-YIG) that 
has been designed to support a single TE mode and a single TM mode. The 
layers are grown, for example, by conventional methods of liquid phase 
epitaxy on a (111)-oriented GGG substrate 10. High optical loss is 
imparted to the bottom layer 20 to make it behave as a mode stripping 
layer (i.e., to make it absorb all modes except the lowest order TE and TM 
modes). Specifically, the bottom layer is doped with cobalt, and the 
yttrium is replaced with three rare earth elements having absorption peaks 
near the chosen operating wavelength, which is exemplarily about 1.5 
.mu.m. The three rare earth elements are praseodymium, samarium, and 
erbium. The nominal composition of this layer is (Bi.sub.0.5 Er.sub.1.4 
Sm.sub.0.7 Pr.sub.0.4)(Fe.sub.4.0 Co.sub.0.2 Al.sub.0.8)O.sub.12. 
The middle layer 30 and the top layer 40 are low loss layers, having 
nominal composition (Bi.sub.0.5 Y.sub.2.5)(Fe.sub.3.7 Ga.sub.1.3)O.sub.12. 
These layers are doped with calcium, which is partially compensated by 
oxygen vacancies, so that their growth-induced uniaxial anisotropy can be 
annealed out at a moderate temperature (e.g., about 1050.degree. C.). The 
concentration of bismuth in the top layer is greater than that in the 
middle layer, resulting in an increase of the refractive index of the top 
layer relative to the middle layer by about 0.5%. The top and middle 
layers are each about 3.4 .mu.m thick. 
The linear birefringence in the waveguide is reduced to zero at a 
wavelength of about 1.5 .mu.m by etching the top surface of the top layer 
in phosphoric acid. At least one ridge 50 is etched into the top surface 
of the waveguide to provide a guiding channel. The ridge is etched by the 
ion-implantation enhanced etching method. That is, the waveguide is 
implanted with silicon ions through a photoresist pattern. The photoresist 
is removed, and the waveguide is etched in phosphoric acid. The etch rate 
of the implanted regions is enhanced about 1000 times relative to the 
non-implanted regions. The phosphoric acid etch leaves at least one ridge, 
exemplarily 8 .mu.m wide and 0.5 .mu.m high. The ridge or ridges are 
aligned along a &lt;112&gt; direction, which is one of the easy magnetization 
directions in the (111) plane of the film. After the ridge or ridges are 
formed, the waveguide is retuned to zero linear birefringence at 1.545 
.mu.m by depositing a layer of Si.sub.3 N.sub.4, 700 .ANG. in thickness, 
on the top surface of the waveguide. 
The measured Faraday rotation of the exemplary waveguide, made as 
described, is about 133.degree./cm. To make a 45.degree. rotator, the 
waveguide is cut to a length of 3.4 mm, and the edges 60 and 70 are 
polished and anti-reflection coated. 
A useful input is provided by a polarization-maintaining optical fiber 80 
having a major axis that is rotated about 22.5.degree. from the TE or TM 
direction. The polarized light is end-fire coupled from the fiber into the 
waveguide. Alternatively, the fiber itself can be a polarizing fiber, 
oriented about 22.5.degree. from the TE or TM direction. 
Similarly, a useful output is provided by a polarization-maintaining 
optical fiber (not shown) having a major axis that is rotated 45.degree. 
(in the direction of optical rotation) from the major axis of the input 
fiber. (Significantly, if the major axis of the input fiber is oriented 
22.5.degree. from the TE direction, then the major axis of the output 
fiber is oriented 22.5.degree. from the TM direction, and vice versa.) 
Light exemplarily is directly coupled from the ridge waveguide into the 
fiber and an output polarizer is placed at the far end of the fiber. 
Alternatively, a polarizing fiber can be used. 
A magnetic field parallel to the propagation direction having sufficient 
strength to magnetically saturate the waveguide is applied using, for 
example, a permanent bar magnet or an electromagnetic coil. A field 
strength of 30 Oe is typically used. 
One useful figure of merit for the waveguide, here called the reverse 
isolation ratio (RIR), describes its effectiveness as an optical isolator. 
That is, let I.sub.rev.sup.out represent the intensity of reverse 
propagating light transmitted by the input polarizer. Then the RIR is the 
reverse attenuation, and it is equivalent to I.sub.rev.sup.out expressed 
as a fraction of the transmitted reverse-propagating light which is 
reflected into, or otherwise enters, the isolator. That is, 
EQU RIR=I.sub.rev.sup.out /I.sub.rev.sup.in, 
where I.sub.rev.sup.in represents the intensity of the light entering the 
isolator in the reverse direction. In the ideal case, no 
reverse-propagating light is transmitted. The corresponding value of the 
RIR is zero, or "negative infinity" decibels (dB). Experimentally measured 
values, expressed in dB, will be negative, and, for a useful waveguide, 
are desirably less than (i.e., more negative than) about -10 dB, and 
preferably less than about -20 dB. 
A second useful figure of merit for the waveguide, here called the forward 
extinction ratio (FER), is used here as a measure of the excess forward 
loss. That is, let I.sub.inh represent the intensity of a beam of light 
that exits the waveguide in the forward direction having suffered inherent 
losses in the waveguide (due, for example, to scatter and absorption), but 
that has not experienced any excess forward loss. Let I.sub.fwd represent 
the corresponding intensity when the same beam of light additionally 
suffers excess forward loss. Then the FER is the excess forward loss, 
expressed as a fraction of I.sub.inh. That is, 
EQU FER=(I.sub.inh -I.sub.fwd)/I.sub.inh. 
Ideally, there is only inherent loss in the forward-propagating beam, and 
the FER is consequently equal to zero (or "negative infinity" dB). In the 
worst possible case, there is no forward transmission at all, and the FER 
equals unity (or 0 dB). For a useful waveguide, the FER is desirably at 
most about -1 dB, and preferably less than about -3 dB. 
Both figures of merit are readily measured by a technique to be described 
below. 
It has been discovered that, contrary to the general prior art 
understanding, an optical isolator made as described above can exhibit 
values of the RIR less than -30 dB and values of the FER less than -10 dB 
over a spectral range of at least about 0.15 .mu.m, exemplarily extending 
from about 1.430 .mu.m (i.e., about 0.115 .mu.m below the exemplary tuning 
wavelength for B=0) to about 1.580 .mu.m (i.e., about 0.035 .mu.m above 
the exemplary tuning wavelength). It is believed that a useful range for 
the same isolator is at least 0.25 .mu.m in extent, and can include both 
1.3 .mu.m and 1.55 .mu.m, currently the two most important wavelengths for 
optical communication. A theoretical basis for predicting the useful range 
is discussed, for example, in R. Wolfe, et al., Appl. Phys. Lett., Vol. 
56, p. 427 (1990). 
Because L.sub.45 changes as the wavelength is varied, and is equal to the 
waveguide length only at the tuning wavelength, it is generally desirable 
to rotate the output polarizer each time the wavelength is changed in 
order to obtain the best value of the RIR at each wavelength. If the 
output polarizer is not rotated, then an inferior (although possibly 
acceptable) RIR is generally achieved. 
It should be noted that a similarly wide useful spectral range is expected 
even for at least some waveguide isolators that are not tuned to B=0 at 
any wavelength. That is, a useful range of at least about 0.2 .mu.m about 
a central wavelength is expected provided only that: (1) the waveguide has 
an input polarization of about 22.5.degree. relative to the TE or TM 
direction; (2) the waveguide satisfies the Dammann condition for 
reverse-propagating light at the central wavelength; and (3) the ratio B/F 
is less than about 1 at the central wavelength (for which the Dammann 
condition is exactly satisfied). 
Because the dependence of L.sub.45 on wavelength is subject to at least two 
separate effects that tend to partially compensate each other, there is a 
range of wavelengths over which the waveguide isolator can be used even 
without rotating the output polarizer. First, the Faraday coefficient F 
tends to decrease as the wavelength is increased. The result of this 
effect, alone, would be to increase L.sub.45 as the wavelength is 
increased. Second, L.sub.45 tends, at fixed values of F, to decrease as 
the linear birefringence B is increased. In a fixed-length waveguide, 
these effects can be made to at least partially cancel over some range of 
wavelengths by etch-tuning the waveguide to the low-wavelength end of the 
range, e.g., to the lowest wavelength of the range. As the wavelength is 
increased, F decreases and B increases, such that L.sub.45 remains close 
to the fixed length of the waveguide. Thus, not only can the same isolator 
be used, separately, at more than one wavelength, but furthermore, a 
signal or combination of signals comprising multiple wavelengths, 
simultaneously, can be usefully transmitted. As noted, a useful wavelength 
range for such transmission is at least about 0.05 .mu.m. Such 
multiple-wavelength transmission is especially useful in connection with 
wavelength division multiplexed signal transmission. 
Moreover, the linear birefringence varies not only with wavelength, but 
also with temperature. For example, if a waveguide isolator similar to 
that described here is either heated to 30.degree. C. above, or cooled to 
30.degree. C. below, the temperature at which it is tuned to B=0, B in an 
exemplary isolator increases from 0 (at the tuning wavelength) to about 
90.degree./cm, resulting in an excess forward loss of 1 dB. (See, for 
example, J. P. Castera, et al., "Phase Matching in Magneto-Optic YIG Films 
by Waveguide Temperature Control," Electronics Lett., Vol. 25, p. 297 
(1989).) Because 1 dB is generally an acceptable loss, the useful 
temperature range of the waveguide isolator can be at least 60.degree. C. 
wide. 
Good reverse isolation can be obtained over an even greater range of 
temperature by making use of the temperature dependence of F. That is, F 
generally decreases when the temperature is increased. Thus, if the 
waveguide is tuned at the low end of the temperature range over which the 
waveguide is to be used, any other temperature within that range will 
correspond to a smaller value of F and a greater value of B. Analogously 
to the wavelength compensation discussed above, these temperature effects 
will at least partially compensate to keep the value of L.sub.45 close to 
the fixed length of the waveguide over some temperature range. In the 
waveguide discussed here, such a temperature range is even greater than 
60.degree. C. In the absence of temperature stabilization, temperatures in 
optical isolators may fluctuate by, typically, more than about 20.degree. 
C. Thus, in particular, a useful temperature-compensated isolator is made 
by predetermining a temperature operating range spanning more than about 
20.degree. C., and tuning the waveguide at a temperature near the low end 
of the range, e.g., at a temperature differing from the lowest temperature 
of the range by less than about 10% of the range. 
Moreover, the temperature compensation effect permits operation with good 
reverse isolation over a moderate temperature range even without rotating 
the output polarizer. 
In summary, tuned waveguide isolators with input polarizations of 
22.5.degree. (relative to TE or TM) are useful even without perfect tuning 
of the linear birefringence to zero. As a consequence, a single substrate 
(e.g., a wafer of GGG) can be used to make isolators that can operate at 
different wavelengths over a substantial range, including, for example, 
both 1.3 .mu.m and 1.55 .mu.m. That is, with reference to FIG. 5 and FIG. 
6, a magnetic thin film optical waveguiding structure 100 can be formed on 
the substrate. The substrate can then be divided (e.g., into at least two 
essentially identical waveguide portions 110), and input polarizer 120 and 
output polarizer 130 added, to form at least two isolators. Each of the 
two isolators can be provided with a different light source 140, the 
sources providing, respectively, wavelengths differing by at least 0.05 
.mu.m (e.g., wavelengths of 1.3 .mu.m and 1.55 .mu.m, as noted). Although 
not essential, the relative orientations of the input and output 
polarizers can be adjusted to give the best reverse isolation at each of 
the respective wavelengths. Each such device can have good reverse 
isolation and moderate excess forward loss over a practical operating 
range of wavelength and temperature. In addition, the operating ranges of 
wavelength and temperature can be extended still further by designing the 
waveguides to make use of wavelength and temperature compensation 
phenomena. 
EXAMPLE 
An etch-tuned ridge waveguide isolator was made as described above. Light 
from a color-center laser, tunable from 1.43 .mu.m to 1.58 .mu.m, was 
end-fire coupled into the waveguide through a polarization-maintaining 
fiber. The major axis of the fiber was rotated by 22.5.degree. from the 
plane of the waveguide, such that linearly polarized light from the laser 
entered the waveguide in a state corresponding to point C on the Poincare 
sphere of FIG. 1. For purposes of the experimental demonstration, light 
was propagated through the waveguide only in the reverse direction. To 
simulate forward propagation, the applied magnetic field was periodically 
reversed. That is, with the magnetic field parallel to the propagation 
direction, the polarization state traced out path CD on the Poincare 
sphere. With the field antiparallel, the polarization state traced out 
path CE. 
Path CE is a clockwise rotation about point M.sup.-. It is apparent from 
symmetry considerations that path CE is congruent to path AB, which is a 
clockwise rotation about point M.sup.+. Therefore, arc BC is equal to arc 
EA, and the corresponding excess forward losses are equal. 
An output analyzer was provided, consisting of a Glan-Thompson prism 
polarizer. The output analyzer could be oriented, inter alia, in a range 
including -22.5.degree. and a range including 67.5.degree.. 
The RIR was measured by setting the output analyzer at 67.5.degree.. The 
respective transmitted light intensities with field parallel and 
antiparallel to the propagation direction were measured. The RIR was taken 
as the ratio of the parallel-field intensity (corresponding to point D on 
the Poincare sphere) to the antiparallel-field intensity (corresponding to 
point E). 
The FER was measured by setting the output analyzer at -22.5.degree.. The 
respective transmitted light intensities with field parallel and 
antiparallel to the propagation direction were again measured. The FER was 
taken as the ratio of the antiparallel-field intensity (point E) to the 
parallel-field intensity (point D). 
With reference to FIG. 4, the solid squares represent the measured RIR as a 
function of wavelength over the tunable range of the laser, and the open 
squares represent the measured FER over the same range. It will be noted 
that near the zero linear birefringence wavelength, the measured RIR was 
about -37 dB, and the measured FER was about -32 dB. At each wavelength, 
the output polarizer was rotated to optimize the RIR. The measured RIR was 
-32 dB or better over the entire accessible wavelength range. These 
measurements were limited by scattered light in the experiment and not by 
the inherent properties of the waveguide isolator. 
It should be noted that the light propagating in the input fiber was 
elliptically, rather than linearly, polarized because unintentional 
twisting of the fiber added a component of circular birefringence to the 
inherent linear birefringence of the fiber. As a consequence, the 
polarization state of the light entering the waveguide isolator varied 
periodically with wavelength. The light entering the waveguide isolator 
was linearly polarized at the wavelengths represented in FIG. 4 by data 
points, but between those points, the light entered the waveguide in a 
state of elliptical polarization. 
As is apparent from the figure, the FER deteriorated as the magnitude of 
the linear birefringence of the waveguide increased on either side of the 
tuned wavelength. However, the FER of -10 dB, at the shortest wavelength 
tested, corresponds to a maximum excess forward loss of only about 10%.