A single-mode, single-polarization optical fiber ("PZ fiber") can have a large single-polarization wavelength bandwidth when .eta. (as herein defined) for one symmetry axis is positive when calculated from the refractive index profile determined with one of two orthogonal orientations of polarized light and is negative when calculated from the refractive index profile determined with the other orientation, and for each other symmetry axis of the novel optical fiber .eta. is positive for both orientations of plane polarized light. Preferably the absolute values for .eta. when positive and .eta. when negative are about equal for said one symmetry axis. A preferred PZ fiber can be formed by depositing siliceous layers onto the interior surface of a hollow substrate tube of quartz to provide a preform. After forming two parallel flat faces in its outer surface, the preform is pulled to form a PZ fiber having an ellipitcal stress-applying region.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention concerns an optical fiber that propagates only one 
polarization state of the fundamental mode and so can be used to polarize 
light or to propagate polarized light over long distances. 
2. Description of the Related Art 
At the present time, when it is necessary to polarize light for 
transmission through single-mode optical fibers, this is usually done with 
either a bulk optic polarizer or an integrated optical polarizer, both of 
which are quite expensive and undesirably large for many optical fiber 
applications. 
In the mid 1980's, York Technologies introduced a polarizer based on a 
single-mode, single-polarization optical fiber that has been tightly 
coiled and supplied in a sealed housing. The coiling results in a bending 
loss which, in combination with stress-induced birefringence, causes one 
polarization to be attenuated while the orthogonal state propagates down 
the fiber core. Because the York device has a rather narrow bandwidth, it 
must be matched with any single-mode optical fiber with which it is used. 
A York brochure marked 6/87 (apparently June 1987) reports a bandwidth of 
at least 20 nm. A publication by the developers of the York device 
indicates that the optical fiber of the York polarizer has an asymmetric 
stress-applying region of a cross-sectional geometry resembling a bow-tie 
and "a depressed index giving a W index profile in one direction", i.e., 
an index of refraction profile in the shape of a W. See Varnham et al.: 
"Single Polarization Operation of Highly Birefringent Bow-Tie Optical 
Fibres," Elec. Lett., Vol. 19, pp. 679-680 (1983). 
FIGS. 1, 3, and 5 of U.S. Pat. No. 4,515,436 (Howard et al.) showing three 
single-mode, single-polarization optical fibers that also have a depressed 
index or W index profile, and more specifically "the refractive index of 
an outer cladding region is greater than the refractive index of an inner 
cladding region but less than that of the core region" (Abstract). 
Operation at an intermediate wavelength allows the fiber to act as a 
polarizer. Howard says in connection with FIGS. 1 and 2: 
"The birefringence decreases with distance from core 10 so that maximum 
bandwidth requires a large inner cladding 12. However, this inner cladding 
region 12 should be narrow for rapid tunneling loss of the undesired 
polarization. Therefore, a trade-off exists between providing rapid 
tunneling loss and providing a large bandwidth" (col. 4, lines 18-25). 
Although the Howard optical fiber can act as a polarizer without being 
coiled or otherwise bent, the embodiment of FIG. 5 was bent into four 
turns with a 1.5 cm bending radius to obtain the transmission data of FIG. 
6 which indicates a bandwidth of about 25 nm at 570 nm. 
Simpson et al.: "A Single-Polarization Fiber, J. of Lightwave Tech, Vol. 
LT-1, No. 2 (1983) describes 
"A single-polarization fiber which uses a combination of high 
stress-induced birefringence combined with a depressed or W-type cladding 
structure. . . . The depressed cladding provides a tunneling loss which 
increases rapidly with wavelength. The anisotropic stress created by a 
highly-doped elliptical cladding splits the mode-effective indexes so that 
the cutoff wavelength differs for the two polarizations. Bandwidths of 8 
percent are achieved for fibers with core sizes and refractive indexes 
typical of single-mode transmission fibers. Extinction ratios of more than 
30 dB with less than 1-dB insertion loss have been obtained with fiber 
lengths on the order of 1 m. The wavelength of useful operation can be 
tuned by bending the fiber" (page 370). 
See also Simpson et al.: "Properties of Rectangular Polarizing and 
Polarization Maintaining Fiber," Proc. SPIE, Vol. 719, pp. 220-225 (1986) 
which says that a polarizing fiber of substantially rectangular shape with 
a W-type index profile had provided greater than 30 dB of extinction ratio 
at a length of 5 cm, with a modal birefringence which separates the 
orthogonal polarization mode cutoff wavelengths by 0.1 um. 
A "W-tunneling fiber polarizer" is held straight for testing in Stolen et 
al.: "Short W-Tunneling Fibre Polarizers," Elec. Lett., Vol. 24, pp. 
524-525 (1988) which indicates a bandwidth of about 25 nm at 633 nm to 
achieve 39 dB polarization. Unfortunately, the bandwidth of about 25 nm of 
Stolen and some of the other above-discussed publications, leaves little 
margin for manufacturing error. For example, it is difficult to build a 
narrow-bandwidth semiconductor light source (e.g., a laser) to a precise 
operating wavelength. Also, shifts in bandwidths could be expected in a 
series of optical fibers made to duplicate the Stolen or other prior 
optical fiber polarizers. Furthermore, variations in conditions of use 
could result in occasional mismatching of wavelengths even if a laser and 
a polarizer had been initially matched. 
Okamoto et al.: "High-Birefringence Polarizing Fiber with Flat Cladding," 
J. of Lightwave Tech., Vol. LT-3, No. 4, pp. 758-762 (1985) remarks that 
the Varnham publication shows "that the polarizing effect can be enhanced 
when the birefringent fiber is bent with the fast axis oriented parallel 
to the plane of the fiber coil." Okamoto's fiber, which has a depressed or 
W index/profile along the x-axis (slow axis) has flats that are parallel 
to its slow axis. When the fiber is coiled, the flats keep the slow axis 
parallel, and this produces substantially broader bandwidths. FIG. 9 of 
Okamoto shows that a bandwidth of 390 nm can be attained at a bending 
diameter of 4.5 cm. See also Okamoto et al.: "Single-Polarization 
Operation of Highly Birefringent Optical Fibres," Applied Optics, Vol. 23, 
No. 15, pp. 2638-2641 (1984) and U.S. Pat. No. 4,480,897 (Okamoto et al.). 
SUMMARY OF THE INVENTION 
The invention provides what is believed to be the first single-mode, 
single-polarization optical fiber which (without being bent) has a 
sufficiently large single-polarization wavelength bandwidth to permit it 
to be manufactured without the need for careful matching to specific 
narrow-bandwidth light sources. In other key respects, the single-mode, 
single-polarization optical fiber of the invention is at least equal to 
prior single-polarization optical fibers, namely, in satisfactorily high 
extinction coefficient of the tunneling polarization state and in minimal 
attenuation of the propagating polarization state. 
Briefly, the single-mode, single-polarization optical fiber of the 
invention, like those of several of the above-cited publications, 
incorporates an asymmetric stress-applying region which causes the fiber 
to be birefringent. Also like those prior fibers, that of the invention 
has along at least one of its axes of symmetry a depressed refractive 
index profile, that is, the core is separated from the cladding by an 
intermediate region whose refractive index is less than that of either the 
core or the cladding. 
To see how the novel single-polarization optical fiber (sometimes here 
called "PZ fiber") of the invention differs from prior single-polarization 
fibers, consider a pair of refractive index profiles from a single 
symmetry axis of the optical fiber, measured with plane polarized light 
that is aligned to be either parallel to, or perpendicular to, the chosen 
symmetry axis. The parameter .eta. is calculated from each such profile by 
the expression 
##EQU1## 
where n(r) represents the refractive index at the radial position r 
measured along said symmetry axis and with one of the two said 
orientations of polarized light; n.sub.c1 is the average refractive index 
of the cladding over a distance from 4 to 7 core radii from the core 
center, and is considered to extend to infinity; and .alpha. has a value 
between 0 and 1, with its precise value determined by the shape of the 
core and the cladding. When both the core and cladding are circular, 
.alpha. is 1; when the core and cladding are so elongated that the fiber 
behaves essentially as a planar waveguide, .alpha. approaches 0; and for 
intermediate shapes, .alpha. is between 0 and 1. 
The novel single-polarization optical fiber of the invention differs from 
prior single-polarization fibers in that .eta. for one symmetry axis is 
positive (greater than 0) when it is calculated from the refractive index 
profile determined with one of the two said orientations of polarized 
light and is negative (less than 0) when it is calculated from the 
refractive index profile determined with the orthogonal orientation of 
polarized light. The parameter .eta. for each other symmetry axis of the 
novel optical fiber is positive for both orientations of plane polarized 
light. 
The polarization state that results in a positive value of .eta., this 
value subsequently referred to as n.sub.g, is guided for all finite 
wavelengths of light neglecting losses caused by bending and absorption. 
The polarization state that results in a negative value of .eta., this 
value subsequently referred to as n.sub.t, tunnels out of the fiber 
waveguide at all wavelengths of light longer than some finite wavelength. 
For fibers of the invention, then, the wavelength bandwidth over which one 
and only one polarization state of light propagates is large, neglecting 
losses of the guided state caused by bending and absorption. The tunneling 
criterion is derived for an analogous problem in Simon, "The Bound State 
of Weakly Coupled Schroedinger Operators in One and Two Dimensions," 
Annals of Physics, 97, pp 279-288 (1976). 
Preferably the positive and negative values of .eta. for said one symmetry 
axis have about equal absolute values, because this should maximize the 
manufacturing tolerance of the novel single-polarization fiber. 
Another test for determining whether a single-mode optical fiber is useful 
in the present invention employs the parameter .chi. as given by the 
expression 
##EQU2## 
For an optical fiber of the invention, .chi. is between -1 and +1 for one 
symmetry axis with the preferred value .chi.=0. For each other symmetry 
axis .chi. is outside said range. The value of .chi. decreases as the 
birefringence increases. Hence, a large birefringence increases 
manufacturing tolerances. The birefringence should also be large to lessen 
the effect of environmentally-induced birefringence on the fiber's 
performance. 
DETAILED DISCLOSURE 
For a given symmetry axis, we find that the birefringence is essentially 
constant over the regions interior to the stress-applying region. The fact 
that the birefringence is not confined to the fiber core appears to have 
no deleterious effect on the fiber's performance; the criteria listed in 
the invention summary can still be met, and we find that the wavelength 
bandwidth over which the fibers polarize is large. 
To increase the attenuation rate of the tunneling state in a step index, 
W-type fiber, the ratio of the diameter of the intermediate region to the 
diameter of the core should be made as small as possible. As this ratio is 
made small, either the index difference between the intermediate region 
and the cladding must be increased, or the index difference between the 
core and the cladding must be decreased, in order to maintain said desired 
balance between .eta..sub.g and .eta..sub.t. The index difference between 
the core and the cladding must also be chosen to achieve the desired 
cutoff wavelength of the tunneling state, and this cutoff wavelength can 
be calculated for a circularly symmetric fiber using well known 
relationships (see, for example, M. Monerie, "Propagation in Doubly Clad 
Single-Mode Fibers," IEEE J. Quantum Electron., Vol. QE-18, no. 4, p. 535 
(1982)). 
It may be feasible to optimize the dispersion or the bend tolerance of the 
novel optical fiber by employing two or more materials of differing index 
of refraction in the intermediate region, and the index of refraction of 
one or more of those materials can exceed n.sub.c1. 
A preferred PZ fiber of the invention can be formed by depositing siliceous 
layers onto the interior surface of a hollow substrate tube of quartz. 
Even though quartz is silica, commercially available hollow substrate 
tubes contain impurities that would make it difficult to deposit heavily 
doped silica. Hence, it is desirable for the first siliceous layer to be 
pure silica or lightly doped silica to provide what is here sometimes 
called "an outer barrier" and its refractive is n.sub.c1. 
When the first layer is pure or lightly doped silica, the next layer to be 
deposited can form a stress-applying region and preferably is doped to 
have a refractive index less than n.sub.c1. Over this is deposited a layer 
that can form what is here called "an intermediate region" and preferably 
is doped to have a refractive index about equal to that of the 
stress-applying region, thus assuring that the guided state will not be 
cut off at too short a wavelength. The final layer can form the core of 
the PZ fiber. 
After collapsing the coated substrate tube to provide a preform, two 
diametrically opposed parallel flat faces are ground into the outer 
surface of the preform. Upon pulling the ground preform to form the PZ 
fiber, the outer surface of the PZ fiber becomes cylindrical, and the 
stress-applying region becomes elliptical. 
Instead of grinding flat faces into the preform, the preform can be 
flattened and drawn while so controlling the temperature to produce a PZ 
fiber of the invention, the outer surface of which is substantially 
elliptical like that of FIG. 5 of Howard. The PZ fiber of the invention 
can have other forms, e.g., a PANDA configuration like that illustrated in 
the above-cited Okamoto publication or a bow-tie configuration. 
The stress-applying region of the novel PZ fiber preferably is doped to 
have the same index of refraction as that of the intermediate region, thus 
minimizing any tunneling of the guided state. The index of refraction of 
the stress-applying region can be matched to that of the intermediate 
region by co-doping B.sub.2 O.sub.3 doped silica with GeO.sub.2. By also 
doping it with P.sub.2 O.sub.5, the temperature at which the 
stress-applying region is deposited can be lowered, but the presence of 
phosporous sometimes causes fibers to be degraded upon exposure to 
ionizing radiation. 
Preferably, the core of the novel PZ fiber is silica doped with germanium 
oxide, and the intermediate region is silica doped with fluorine. By also 
including P.sub.2 O.sub.5 in the intermediate region, the temperature at 
which it is deposited can be lowered, again at the possible expense of 
degradation upon exposure to ionizing radiation. 
When the novel PZ fiber contains phosphorous, it may be desirable for its 
outer barrier to be silica doped with both P.sub.2 O.sub.5 and F, the 
former dopant lowering its deposition temperature, and the latter 
offsetting the change in index of refraction that otherwise would result 
from the presence of the phosphorous. 
When the novel PZ fiber has an elliptical stress-applying region, its minor 
diameter preferably is from 20% to 40% of its major diameter. At greater 
than 40%, the stress-applying region might not produce the desired degree 
of stress on the core, whereas at less than 20%, the major diameter of the 
stress-applying region would necessarily be quite large to permit the 
inner cladding to have adequate thickness. 
To be compatible with sensor fibers now on the market, the novel PZ fiber 
produced by the above-outlined method can be drawn to any diameter, e.g., 
80 .mu.m for operation at 850 nm and 125 um for operation at 1300 nm.

The optical fiber 10 of FIG. 1 has been made using a hollow substrate tube 
by the above-outlined method. The deposited layers have produced a core 12 
of circular cross section, an intermediate region 14, and an outer 
cladding region that includes an elliptical stress-applying region 15 and 
an elliptical outer barrier 16. The outer cladding region also includes a 
jacket 18 that has been provided by the hollow substrate tube. 
The curves 20 and 21 of FIGS. 2a and 2b respectively are plots of 
refractive index versus radial position for the example fiber, measured 
with plane polarized light oriented along the major symmetry axis of the 
optical fiber. The plots are determined from the minor and major symmetry 
axes, respectively. The phantom lines 22 and 23 show the refractive index 
of the cladding. 
The curves 30 and 31 of FIGS. 3a and 3b respectively are plots of 
refractive index as in FIG. 2, except with plane polarized light oriented 
along the minor symmetry axis of the optical fiber. The phantom lines 32 
and 33 show the refractive index of the cladding. 
The curves 40 and 41 of FIG. 4 are plots of attenuation versus wavelength, 
measured respectively with the tunneling polarization state and the guided 
polarization state, on a 42 m length of fiber. The operating bandwidth, 
42, is the wavelength band over which the tunneling state is attenuated by 
at least 30 dB, and the guided state suffers less than 1 dB/km of 
additional attenuation. 
EXAMPLE 1 
making a polarization-maintaining 
optical fiber of the invention 
A. Preform Fabrication: 
The preform in this example was fabricated by the modified chemical vapor 
deposition process (MCVD). In this process, glass of controlled 
composition and thickness is deposited on the inside of a fused silica 
tube by the chemical reaction of oxygen with metal chlorides or bromides. 
A more complete description of the process may be found in U.S. Pat. No. 
4,217,027 (MacChesney et al.) 
A fused silica tube (General Electric #982) with an inside diameter of 
nominally 17.0 mm and an outside diameter of nominally 20.0 mm was 
inserted into a deposition apparatus (preform lathe, gas flow system, 
hydrogen torch). The inside wall of the tube was first etched with 
fluorine to produce an uncontaminated surface for deposition. Four layers 
of glass were then deposited onto the inside wall of the tube. The 
functions and compositions of the four layers are described below. 
______________________________________ 
Function Composition 
______________________________________ 
Layer-1 Outer barrier 
SiO.sub.2 /P.sub.2 O.sub.5 /F 
Layer-2 Stress-applying region 
SiO.sub.2 /B.sub.2 O.sub.3 /GeO.sub.2 /P.sub.2 
O.sub.5 
Layer-3 Intermediate region 
SiO.sub.2 /P.sub.2 O.sub.5 /F 
Layer-4 Core SiO.sub.2 /GeO.sub.2 
______________________________________ 
Stepwise conditions are listed in Table I. Temperatures reported in Table I 
are pyrometer readings of the external surface of the fused silica tube. 
TABLE I 
__________________________________________________________________________ 
Vapor Flow* 
(cm.sup.3 /min) No. of 
Temp 
Speed 
Step SiCl.sub.4 
GeCl.sub.4 
POCl.sub.3 
BBr.sub.3 
Freon 
O.sub.2 
Passes 
(.degree.C.) 
(mm/min) 
__________________________________________________________________________ 
Etch 200 1000 
2 1750 
150 
Clear 1000 
1 1580 
150 
Layer-1 
300 83.5 4.5 1000 
5 1580 
150 
Clear 1000 
1 1580 
150 
Layer-2 
300 
515 75 1140 2000 
15 1725 
200 
Clear 2000 
4 1725 
200 
Layer-3 
300 46 15 1000 
3 1700 
150 
Clear 1000 
1 1700 
150 
Layer-4 
50 
29 1000 
3 1750 
200 
Clear 1000 
1 1750 
200 
__________________________________________________________________________ 
*Vapor flow indicates flow of carrier gas (O.sub.2 for the SiCl.sub.4, 
GeCl.sub.4, and POCl.sub.3, and Ar for the BB or direct flow of Freon and 
O.sub. 2. Spindle rotation speed is 50 rpm throughout. 
After completion of the deposition process, the annular tube with inner 
deposited layers was collapsed to a non-hollow preform by standard 
techniques. 
B. Preform Shaping: 
Two diametrically opposed flat surfaces were ground onto the initially 
cylindrical preform with a conventional surface grinding machine and a 
diamond grinding wheel, removing at each flat 2.54 mm radially. Then the 
preform was thoroughly cleaned to remove any particulate contamination 
that might result from the grinding procedure. 
C. Fiber Draw: 
Using a zirconia induction furnace, the preform was drawn into a fiber 
having a diameter of 75 .mu.m while maintaining a temperature sufficiently 
high to give the fiber a circular cross-section. The temperature read by 
the pyrometer that monitored the furnace was 2125.degree. C. As it was 
drawn, the fiber was coated with two separate acrylate layers that were 
individually cured with ultra-violet light. The first acrylate coating 
applied was D1-11 from DeSoto Co., the second was D-100, also from DeSoto 
Co. The coated fiber was spooled onto a reel. 
D. Fiber Properties: 
The PZ optical fiber resulting from steps A through C has the following 
mechanical and optical properties. 
______________________________________ 
Mechanical properties: 
______________________________________ 
Core diameter 4.9 .mu.m 
Intermediate region diameter 
12 .mu.m 
Elliptical stress applying region 
major diameter 59 .mu.m 
minor diameter 15 .mu.m 
Outer barrier 
major diameter 65 .mu.m 
minor diameter 17 .mu.m 
Cladding diameter 76 .mu.m 
Acrylate coating diameter 
216 .mu.m 
______________________________________ 
______________________________________ 
Optical properties: 
______________________________________ 
Refractive index, measured with circularly polarized 
light, of 
core 1.462 
inner barrier 1.457 
stress-applying region 
along major axis 1.457 
along minor axis 1.4595 
outer barrier 1.459 
substrate tube 1.459 
Loss at 850 nm, guided state 
4.7 dB/km 
Mode field diameter at 850 nm 
5.8 .mu.m 
LP-11 cutoff wavelength &lt;550 nm 
LP-01 cutoff wavelength, 780 nm 
tunneling state 
Wavelength of 1 dB/km increase in 
890 nm 
attenuation, guided state 
Polarizing bandwidth 110 nm 
Birefringence at 633 nm 5 .times. 10.sup.-4 
______________________________________ 
Because the stress-applying region is asymmetric, the cladding of the PZ 
fiber is not circular even though the core is circular. Hence, .alpha. is 
between 0 and 1. However, assuming .alpha. to be either 0 or 1, from the 
minor axis 
______________________________________ 
.alpha. 
.eta..sub.t .eta..sub.g 
.chi. 
______________________________________ 
0 -0.006 .mu.m 0.010 .mu.m 
0.20 
1 .sup. -0.056 .mu.m.sup.2 
.sup. 0.006 .mu.m.sup.2 
-0.80 
______________________________________ 
and from the major axis 
______________________________________ 
.alpha. 
.eta..sub.t .eta..sub.g 
.chi. 
______________________________________ 
0 0.006 .mu.m 0.007 .mu.m 
10 
1 -0.008 .mu.m.sup.2 
-0.009 .mu.m.sup.2 
12 
______________________________________ 
Although the novel single-polarization optical fiber has excellent 
bandwidth when straight, bending or coiling can permit a shorter length of 
the fiber to polarize light. In any sensor that includes a polarizer with 
polarization-maintaining optical fiber, those elements should be 
replacable by the novel single-polarization optical fiber, e.g., in fiber 
gyroscopes, magnetometers, and current sensors.