Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2000-01-18
2002-03-12
Lee, John D. (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
C359S710000
Reexamination Certificate
active
06356682
ABSTRACT:
The invention relates to a device for making grid structures in optical fibers by local laser radiation, with a laser emitter installed on the surface enveloping the optical fiber, as well as beam converting facilities-inserted between the laser emitter and the optical fiber.
Optical gratings are made perpendicular to the laser's longitudinal direction in the material on the boundary layer between the optical fiber's core and cladding or on the cladding's exterior, in order to correct dispersion in the optical fibers and to compensate for a given amplitude-frequency curve associated with the light source being used. Here, the order of magnitude of the grating constants is either approximately 0.5&mgr; for a narrow-band amplification for carrying out wavelength division multiplexing, such as in erbium amplifiers, or in the order of magnitude of several hundred p to compensate for different amplitude-frequency characteristics of the emitter being used. Grids with different grating constants are sometimes combined with one another in optical filter structures for the above applications. The grids are usually equidistant, but non-equidistant grids are also used for special applications.
The grid structures are made by exposing the optical fiber to a linear laser beam of sufficient intensity, which penetrates the cladding from the outside. Energy absorption around the boundary causes a local structural change in the material, resulting in a permanent change in the index of refraction at these points. Such structures are generated at a distance corresponding to the desired grating constant to produce a diffraction grating.
It is decidedly time-consuming and consequently uneconomical to produce an entire grid structure by sequential exposure using a single focused laser beam. It is also possible for inaccuracies to occur from the frequent relative motions of the fiber and optical system to one another. Several well-known methods exist that produce the desired grid structure through simultaneous exposure of the fiber. One known option, for example, consists of drawing the master diffraction grating toward the light source. The intensity of the individual diffraction maxima are indeed relatively small in this case, and the total efficiency is relatively small, so that only maxima of low order can be utilized. Another known possibility provides for exposure through a slot mask. The disadvantages of this method are that the efficiency is very low, since most of the incident light is absorbed by the mask, and that the masks are difficult and expensive to manufacture. In addition, the width of the structures that can be produced is limited by the diffraction effects at the mask's apertures. Not least, this method is relatively inflexible, since the mask itself generally becomes largely unusable after the exposure process.
Proceeding from these difficulties, it is the basic objective of the invention to make available an exposure device for optical fibers that enables improved utilization of the light, i.e. has better efficiency, and is more flexible in its application than existing known systems.
To solve this problem, the invention proposes that the beam converting facilities should have at least one fan-shaped cylindrical lens array aligned perpendicular to the optical fiber, with the optical fiber being placed in the focusing plane, which can also be displaced relative to said optical fiber.
According to the invention, a cylindrical lens array is illuminated with a collimated, line-shaped laser beam longitudinal with respect to the fiber to be exposed, that illuminates the entire width of the array. The laser beam is focused into the fiber by each separate cylindrical lens, i.e. at the boundary between the core and the cladding, or within the cladding, so that the lumination needed for structural conversion exists at these points. The grating period results from the relative distance between the cylindrical lenses, which are aligned perpendicular to the optical fiber.
One advantage results immediately from the use of a cylindrical lens array. It ensures that practically the entire incident laser light is used for exposure. The efficiency is consequently correspondingly high and is nearly 1. Additionally, the grid structures can be manufactured quickly and economically.
The special flexibility of the device according to the invention is a consequence of the fan-shaped arrangement of the cylindrical lenses within the array. This arrangement comes from the fact that the cylindrical lenses widen in the longitudinal direction, from an initial width A to A+dx for example, while the focal length remains constant. As a result, the distances of the focal points or lines, of the cylindrical lenses that are adjacent to each other within the array but not parallel, will also spread apart from each other, such as from an initial distance p up,to a distance p+dx at the other end of the array.
This produces the invention's special advantage that, for an illumination device located above, an optical fiber placed underneath the cylindrical lens array can be exposed with a grating period having any continuous value between p and p+dx, merely by displacing the array relative to the optical fiber in the lenses' longitudinal direction. To change the desired grating period, it is therefore only necessary to displace the array.
In addition to the possibility that has been presented for increasing the grating period by displacing the array perpendicular to the optical fiber, it is also conceivable that a combination of different grating constants could be produced by moving the entire array along the length of the fiber in discrete steps, and performing the exposure each time. If for example, displacements at a distance of dy were sequentially repeated, then several gratings with this period would be generated at a distance of p+dx from each other.
Appropriately, the cylindrical lens array is arranged on guides to carry out its movements. Depending on the desired direction of motion, the array can run on high-precision linear guides that are aligned perpendicular to and/or along the fiber. Precise and reproducible position control is ensured in this manner.
It is preferable for the above guides to be equipped with motor-operated actuators. Actuators of this kind, such as piezoelectric drives, stepping motors, or other drives, allow high-precision and largely automated displacements to be carried out. Connection of the actuators to computer-aided control systems allows efficient automated manufacture and program-controlled generation of different kinds of grids.
An advantageous intrinsic extension of the invention's possible applications results from making the individual cylindrical lenses in the array mobile with respect to one another. This produces the additional option of allowing the distances between the cylindrical lenses within the array to vary freely so that the grating periods produced will also vary freely to a large extent. This allows practically any grid structures, even non-equidistant grid structures, to be generated particularly efficiently. This provides the flexibility for special adaptation to different applications.
Appropriately, the individual cylindrical lenses in the above embodiment are likewise attached to high-precision guides, which are likewise preferably equipped with motorized actuators. In practice, this configuration then makes extensively automated computer-aided production of programmed grid structures possible.
In an advantageous extension of the invention, several cylindrical lens arrays according to the invention, each of which can swing between the laser emitter and optical fiber, are fastened to a movable support. For ease of handling and manufacturing, a limited region should be covered by each individual cylindrical lens array. A larger region can be covered, in automated manufacturing plants for example, by integrating the arrays corresponding to adjacent regions within one magazine-like movable support. Furthe
Hentze Joachim
Lissotschenko Vitalij
Hoffman Wasson & Gitler
Lee John D.
Song Sarah U.
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