Optical waveguides – Accessories – External retainer/clamp
Reexamination Certificate
1998-09-25
2001-06-05
Lee, John D. (Department: 2874)
Optical waveguides
Accessories
External retainer/clamp
C385S039000, C385S136000
Reexamination Certificate
active
06243528
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the mounting of optical fibers, and more particularly to a method for minimizing the stresses on an optical fiber which is mounted between fixed points.
BACKGROUND OF THE INVENTION
In fiber optic apparatus it may be necessary to anchor one end of an optical fiber in a fixed position. For example, the end of an optical fiber would have to be set in a fixed position at the output of a laser or other light projecting or light sensing apparatus. To anchor a fiber in a fixed position, it is generally necessary to lock the fiber at two spaced points, as depicted schematically in FIG.
1
. In this depiction, laser module
10
comprises a laser
12
which projects light through fiber
14
. Fiber
14
passes through lock units
16
and
18
, which are anchored to case
11
of laser module
10
. These locks hold the fiber in place at locking points
17
and
19
, respectively. In general, the purpose of the first lock
16
is to hold fiber
14
in alignment with laser
12
, while the second lock
18
is provided so that any stress on the loose “pigtail” portion
20
of the fiber will not be transmitted to lock
16
, or so that the entire laser unit
10
can be hermetically sealed. Locking is achieved at the lock points by securing the fiber, such as by clamping, soldering or gluing, as is well known in the art. In the usual configuration, the locks are collinear and the fiber is locked from all movement.
When the fiber is locked in place, stress is almost invariably imposed on the fiber either initially during the manufacturing process or subsequently during use because of thermal expansion mismatch between the fiber and the laser module support structure over the range of operating temperatures. During manufacture and subsequent use, the fiber and laser module are subject to a range of operating temperatures caused by such factors as changes in ambient temperature, manufacturing processes such as soldering, and heat generated by operation of the laser during use. Such changes in temperature cause the fiber to expand and contract in accordance with its coefficient of thermal expansion, while causing the locks to move together or apart in accordance with the thermal expansion coefficient of the components of the laser module to which the locks are attached. The difference in the expansion coefficients causes the fiber and locks to expand and contract with respect to each other, thus causing stress on the fiber.
For example, in the simplified depiction of
FIG. 1
fiber
14
is mounted in a straight line between locking points
17
and
19
, which are separated by a distance L. Initially both the length of the fiber and the distance between the locks is L. As the temperature changes, stress on the fiber can be quantified by saying that the distance L+dL between the locks differs from the length L that the fiber would assume were the fiber free at the same temperature. If dL is greater than zero, then the fiber is in tension and subject to immediate or eventual fracture. If dL is less than zero, and the fiber is free to flex between the locks, then the fiber tends to assume a sinusoidal shape, as depicted in FIG.
2
.
In
FIG. 2
, the maximum points of stress M are at the locking points
17
and
19
, as well as at the center of the sine wave. The minimum stress is at intermediate inflection points I. The bending of the fiber at the locks can damage the fiber and eventually cause it to break. As an analogy, to break a stick, one could extend part of the stick past a table edge (the lock) and bend down on the cantilever so as to concentrate the stress at the table edge, causing the stick to bend and eventually break. Likewise, bending of the optical fiber at the rigid lock point can damage or break the fiber. Further, inhomogeneities in the solder or glue can enhance the stress concentration at the lock.
Such bending stresses are well known, and various techniques have been used to reduce these stresses. One technique to reduce stress is to use materials for case
11
which match the expansion coefficient to that of the glass optical fiber. However, such materials may be quite expensive and often have poor physical properties over the range of operating temperatures and conditions, and, furthermore, the expansion match is usually imperfect.
Another technique used is to provide a slack in the fiber to allow for expansion and contraction. When the locks are aligned as in
FIG. 2
, the slack would assume the same sinusoidal shape as results from fiber expansion. As discussed above, this results in maximum bending stress at points M, which is particularly damaging at the lock points. In an attempt to alleviate this stress,
FIG. 3
depicts a technique in which the second lock
18
is set at an angle to reduce the stress on fiber
14
at lockpoint
19
. However, as the fiber assumes its natural sinusoidal bend, it was found that such a configuration still causes undesirable bending stress on the fiber at lockpoints
17
and
19
.
Accordingly, there is a need for a method of mounting a fiber between lockpoints to minimize the stresses to which the fiber is exposed during operational temperature cycling.
SUMMARY OF THE INVENTION
In accordance with the present invention, a system is provided for mounting a fiber between locks by placing the locks at or near the natural inflection points of a sinusoidally curved fiber. This is accomplished by angularly offsetting the fiber exiting the locks at equal angles from an imaginary center line drawn between the locks, and by providing enough slack in the fiber between the locks to cause the inflection points to be positioned at or near the locks.
The fiber optic assembly of the present invention comprises an optical fiber extending between first and second locks for holding the fiber in a fixed position, wherein the fiber extending between the locks exits each lock at approximately the same angle G from a straight line of length L defined by the two points at which the fiber exits the locks, wherein the length of the fiber is greater than L. The fiber can exit the locks on the same side of the straight line, in which case it preferably lies in a single curve in which the optimum value b of the greatest distance of the curve from the straight line is defined by the equation b=(L/&pgr;) * tan G, and wherein enough fiber is provided between said locks such that said greatest distance at a predetermined temperature is within about 20% of b. Alternatively, the fiber can exit the locks on opposite sides of the straight line, in which case it preferably lies in a double curve in which the optimum value b′ of the greatest distance of the curve from the straight line is defined by the equation b′=(L/2&pgr;) * tan G, and wherein enough fiber is provided between said locks such that said greatest distance at a predetermined temperature is within about 20% of b′.
REFERENCES:
patent: 4645294 (1987-02-01), Oguey et al
patent: 4803361 (1989-02-01), Aiki et al.
patent: 4832442 (1989-05-01), Pappas
patent: 5185835 (1993-02-01), Vial et al.
patent: 5692086 (1997-11-01), Beranek et al.
patent: 5789701 (1998-08-01), Wettengel et al.
patent: 5914972 (1999-06-01), Siala et al.
patent: 6016377 (2000-01-01), Suhir
“Spring Constant in the Buckling of Dual-Coated Optical Fibers;” E. Suhir; fromJournal of Lightwave Technology; vol. 6, No. 7; 1240-1244; Jul. 1988.
“Effect of Initial Curvature on Low Temperature Microbending in Optical Fibers;” E. Suhir; fromJournal of Lightwave Technology; col. 6, No. 8; 1321-1327; Aug. 1988.
“Input/Output Fiber Configuration in a Laser Package Design;” E. Suhir, C. Paola, and W.M. MacDonald; fromJournal of Lightwave Technology, vol. 11, No. 12; 2087-2092; Dec. 1993.
Anthony Philip J.
Joyce William B.
Moyer Ralph S.
Agere Systems Optoelectronics Guardian Corp.
Lee John D.
Synnestvedt & Lechner
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