Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
2000-02-29
2003-06-17
Cherry, Euncha (Department: 2872)
Optical waveguides
Optical fiber waveguide with cladding
C065S403000
Reexamination Certificate
active
06580860
ABSTRACT:
TECHNICAL FIELD
This invention relates to the field of fiber optics. More particularly, this invention relates to shaped highly birefringent optical fibers and methods for their manufacture.
BACKGROUND
Optical fibers that maintain a polarized signal in an optical fiber, referred to as polarization maintaining (PM) fibers, are described, for example, in U.S. Pat. No. 4,896,942. Optical fibers that polarize light from a non-polarized or partially polarized light source, referred to as polarizing (PZ) optical fibers, are described, for example, in U.S. Pat. No. 5,656,888. PM and PZ fibers are used in many different applications, such as sensors, inline fiber devices, Raman lasers, and the like. To polarize or maintain a polarized signal in an optical fiber, the light guiding properties of the core of the optical fiber must be highly birefringent. An elliptical core may cause the anisotropic fiber geometry responsible for this high birefringence. However, this anisotropy is more commonly achieved by depositing or locating adjacent the core diametrically opposed sections of cladding material(s) with substantially higher or lower thermal coefficients of expansion than the outer fiber regions. The diametrically opposed regions define one of the highly birefringent fiber's two transverse orthogonal polarization axes and decouple the components of the wave traveling along the fiber. In a polarizing fiber, one of the decoupled components is leaked to the cladding and completely attenuated, leaving a single linearly polarized wave. In contrast, a polarization maintaining fiber retains both of the orthogonal signal components with virtually no cross-coupling or loss of signal strength.
Typical highly birefringent fiber designs have two perpendicular planes of symmetry. One plane of symmetry passes through the center of the fiber core and its two diametrically opposed cladding regions. The second plane of symmetry, which is normal to the first plane of symmetry, also passes through the center of the fiber core.
Referring to FIGS.
1
(
a
)-(
c
), a conventional modified chemical vapor deposition (MCVD) process is shown that may be used to make a collapsed optical fiber preform to be drawn into a PM or PZ optical fiber. Referring to FIG.
1
(
a
), a starting preform
10
includes a fused silica support tube
12
with a known refractive index. An optional outer cladding region
14
made of materials with a refractive index either less than or equal to the refractive index of the support tube
12
is deposited on the inside of the tube
12
. The outer cladding region
14
is typically a relatively pure deposition region that prevents migration of contaminants from the support tube
12
into the interior regions of the optical fiber. Inside the outer cladding region
14
is a stress region
16
formed by layers of glass with a high thermal coefficient of expansion. The stress region
16
has an index of refraction that approximately matches the index of refraction of the cladding region
14
. In longer wavelength PM designs, an optional inner cladding (Iclad) region
18
may be incorporated between the stress region
16
and a core region
20
. The inner cladding region
18
has an index of refraction that is closely matched to the index of refraction of the outer cladding
14
in these PM designs. In PZ designs, the inner cladding
18
is normally a narrow depressed index region. The core region
20
has an index of refraction sufficiently higher than the index of refraction of the surrounding regions to ultimately create a waveguiding region
21
needed for single mode operation at the design wavelength. The waveguiding region
21
typically includes the core
20
and the region immediately adjacent the core, but FIG.
1
(
a
) illustrates a more general case in which the waveguiding region
21
includes the core
20
and at least one other region between the core
20
and the support tube
12
.
Referring to FIG.
1
(
b
), the starting preform
10
of FIG.
1
(
a
), which has a substantially circular cross-section, is then ground equally on opposite sides
24
,
26
to form a ground preform
22
with a non-circular outer periphery, also referred to herein as a non-circular cross-sectional geometry. In this grinding step a substantial amount of the wall thickness of the support tube
12
is removed, and, in some instances, even the outer cladding region
14
may be ground away. The exact amount of material removed in the grind will affect the cutoff wavelength characteristics and the polarizing holding properties of the fiber that is ultimately drawn from the ground preform
22
, and as such is a carefully controlled parameter in the fiber manufacturing process.
Referring to FIG.
1
(
c
), the ground preform
22
of FIG.
1
(
b
) is drawn at high temperature (typically, about 21000 C. to about 2200° C.), which causes the ground sides of preform
22
to “circularize” into an optical fiber
30
with a substantially circular cross-section. The circularized optical fiber
30
has an outer cladding
34
and a stress region
36
, each with a substantially elliptical cross-section, surrounding an inner cladding region
38
and a core region
40
, each with a substantially circular cross-section. Normally, the stress region
36
is made of low melting temperature materials that become fluid during the draw process. This allows the relatively soft outer cladding
34
and the fluid stress region
36
to assume an elliptical cross-sectional shape as the outer fiber region made up of the fused quartz support tube
32
circularizes due to surface tension effects. The inner cladding region
38
, if present, retains its substantially circular cross-section, as does the core region
40
, to provide, along with the elliptical outer cladding and stress regions, a waveguiding region
31
.
The waveguiding region of the PM or PZ optical fiber may also have a core region with a non-circular cross section, such as an ellipse or a rectangle. However, a fiber with a non-circular core design is difficult to splice or connect to conventional round core fibers and generally does not develop sufficient birefringence for more demanding applications.
To maintain or preserve the polarization properties of a signal in an optical fiber, the optical properties of the PM or PZ fiber must be anisotropic. The differing cross-sectional profiles of the layers of the waveguiding region formed by the cladding and core regions in the fiber define two transverse orthogonal axes, which permit the de-coupling of waves polarized along those axes. If a signal launched into these fibers has its polarization aligned with one of these transverse axes, the polarization tends to remain aligned with that axis as the signals are propagated though the fiber. This preserves the polarization of the signal.
PM and PZ fibers often require precise alignment of their transverse orthogonal axes when they are joined to other similar fibers or interfaced to other polarized sources or detectors. For example, to join a PZ fiber with a polarized light source having a known polarization orientation, a polarizer is used to launch light into the fiber, and either the fiber or the polarizer is rotated to identify the axes of maximum and minimum light transmission. The axis of maximum transmission is then aligned with the known polarization orientation of the source. The ratio between the maximum light transmission and the minimum light transmission is referred to as the extinction ratio. To join a PM fiber with another PM fiber, a polarized source or a detector, a similar procedure is used, which requires a polarizer at the fiber input and an analyzer at the fiber output. In this process both the analyzer and the polarizer are rotated to locate the maximum and minimum transmitted power. Both of these procedures require time, optical sources, detectors, lenses, translation stages etc. to identify the axes. Lens tracing techniques can also be used in which light is injected through the side of the fiber and the intensity pattern is scanned on the opposite side
3M Innovative Properties Company
Cherry Euncha
Gwin, Jr. H. Sanders
Ho Nestor F.
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