Optical waveguides – With optical coupler – Switch
Reexamination Certificate
2001-12-12
2004-11-16
Font, Frank G. (Department: 2883)
Optical waveguides
With optical coupler
Switch
C385S017000
Reexamination Certificate
active
06819815
ABSTRACT:
TECHNICAL FIELD
The invention described herein relates to adjusting light beams in optical switches. In particular, methods and apparatus for achieving desired reflector positioning in optical switches through the indirect measurement of light beams.
BACKGROUND
As is well known, fiber optic technology is a rapidly growing field with vastly expanding commercial applicability. As with all technologies, fiber optic technology is faced with certain practical difficulties. In particular, implementation of efficient coupling between an input optical elements and output optical elements in the optical switching elements of an optical network is a significant consideration of designers, manufacturers, and users of optical systems. Optical systems use light beams, usually laser-generated, to carry various types of information. Commonly, these light beams travel through optical fibers or through other optical elements such as optical switches. In optical systems, light beams are directed through complex optical paths with the assistance of optical switching elements. As it happens, losses of optical power in switching elements are a significant concern.
Fiber-to-fiber coupling in an optical switch should be efficient to avoid unnecessary losses in optical power. Coupling efficiency is especially important in optical systems where light beams are subject to reflection as part of the optical switching process. If too much light is lost due to alignment and reflection errors in the switch, the light output from the switch might be insufficient for its intended purpose.
When efficiently coupling, a light beam travelling through an optical switch enters an output optical fiber so that the amount of light transmitted through the fiber is maximized. The most efficient coupling between an optical beam and a fiber occurs when the light beam is centrally positioned on the core of the fiber (on the fiber center) and when the beam enters the fiber at an acceptable angle of entry. Such an acceptable angle of entry is dependent on fiber characteristics, such as, fiber type, size, and cladding. When the light beam enters an output fiber at an acceptable angle of entry and at a central position, an optimal amount of light is transmitted through the output fiber.
However, once positioned on the fiber center at an acceptable angle, the light beam does not always remain in place. Operating conditions which may cause the system to suffer a shock or vibration, for example, can cause the physical components of the optical system to shift, causing the light beam to be offset from the fiber center. Other factors may also cause the light beam position and angle to shift. Changing environmental conditions may result in beam variance from the original position. For example, thermal expansion of a fiber may result in a shift in beam position. Thermal effects may also cause subtle distortion of switch components (like reflector surfaces) resulting in changes in beam angle and position. These and other effects can result in reduced coupling efficiency between the light beam and output fiber. A system and method for efficient coupling must be able to correct offsets due to vibration, thermal expansion, or other causes. Moreover, there can be occasions when it is desirable to intentionally reduce the coupling efficiency between the light beam and output fiber. For example, coupling efficiency can be reduced in order to attenuate optical power in a light beam. The system and method for efficient coupling must be able to accommodate these needs as well.
Previous attempts to solve the foregoing problems have met with mixed success.
FIG. 1
shows the inner workings of a conventional optical switching array
1
. Briefly, the components include an array of input optical fibers
10
, which are positioned and aligned using an input block
11
. The light beams exiting the optical fibers
10
are directed through an input lens array
12
. The lens array
12
, collimates and focuses the optical beams (shown here by a single example optical beam B) such that they are directed onto a first movable reflector array
13
, which directs the beams onto a second movable mirror array
14
such that the beams are directed through an output lens array
15
into output channels which correspond to optical fibers
20
, which are aligned and positioned in an output fiber block
16
. The reflectors of the movable reflector arrays
13
,
14
are oriented to direct the optical beams from selected input fibers into selected output fibers. By correctly orienting the mirrors, beams are switched from input fiber to output fiber in order to accomplish the switching function of the switch. The orientation of the reflectors of the movable mirror arrays
13
,
14
is controlled by control circuitry (not shown), which moves the individual reflectors of the reflector arrays
13
,
14
to accomplish the switching function of the optical switches discussed herein.
A typical example of a movable reflector array
13
,
14
is a Micro Electro-Mechanical System (MEMS) reflector array constructed of a plurality of micro-scale movable reflectors formed on a monolithic silicon substrate. Such devices are manufactured by, for example, Analog Devices of Cambridge, Mass., or MCNC of Research Triangle Park, North Carolina.
FIG. 2
is a block diagram illustrating one implementation used to optimize mirror orientation in an optical switch to obtain maximum beam power in an output light beam. The collimator and reflector elements depicted in
FIG. 1
are schematically depicted as the switch
17
. Optical beams are input into the switch
17
through the input fibers
10
. The output optical beams are received by the output fibers
20
. Each input fiber
10
is equipped with a detector element
21
that monitors optical power. Similarly, each output fiber
20
includes a similar detector element
22
. The outputs from the input detectors
21
and output detectors
22
directly measure optical power in the light beams to position the switch reflectors in order to optimize power. The light detectors
21
,
22
directly measure input power and output power and uses this information to adjust the reflectors of the switch path in accordance with power optimization algorithms to maximize the fiber coupled output power. Examples of such power optimization techniques using directly measured light beams is described in detail in the U.S. Patent Application entitled: “Feedback Stabilization of a Loss Optimized Switch”, filed on Apr. 30, 2000, Ser. No. 09/548,587, which is hereby incorporated by reference. Although such systems are satisfactory for their intended purpose, improvements can be made.
A disadvantage of such conventional direct measurement devices is that each fiber
10
,
20
requires a detector element (e.g.,
21
,
22
) so that input power can be directly compared to output power. Consequently, in a switch having, for example, 256 input and output fibers, 512 such detectors are required (one for each input fiber and each output fiber). Still other approaches use pairs of quadrature detectors for each fiber. Because each quadrature detector comprises four photodetectors, these solutions require eight photodetectors and their supporting circuitry (including amplifiers) per light beam. In addition to the large number of detectors needed by such implementations, the detectors themselves can be quite large, thereby substantially increasing the size of such switches. Also, each splitter/tap is an expensive component requiring individual alignment during manufacture. These factors can significantly increase the cost of such switches. Also, existing switches use a test light beam which is propagated in the direction opposite that of an output beam. This counter propagating light beam is used to align and adjust the beams of the switch and also prevent “false positive” readings generated by stray light in the switch. The need for a test beam increases cost and complicates the system. As a result, it is desirable to develop methods and apparatus for optimizing light beam power
Bowers John E.
Corbalis Charles
Helkey Roger J.
Welsh David
Yuan Shifu
Beyer Weaver & Thomas LLP
Calient Networks
Kianni K. Cyrus
LandOfFree
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