Mode scrambler

A mode scrambler comprises an optical fiber adapter having a diffuser disposed between optical fiber mating ends of the optical fiber adapter. When a single mode optical signal is launched in the mode scrambler, the mode scrambler converts the single mode optical signal to a multimode optical signal that has a substantially even intensity distribution across the modes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is directed to optical communication and, in particular, to an optical fiber mode scrambler.

2. Background Information

The maximum number of modes any optical fiber can propagate depends on the geometry/composition of the optical fiber and the wavelength of the optical source. The actual number of modes that do propagate depends on, among other things, the launch conditions from the optical source to the optical fiber.

There are two types of mode-distributions that have practical applications when working with multimode optical fiber. The first type of mode-distribution is “restricted launch”, where only a small sub-set of propagating modes is coupled. Restricted launch has the advantage of resulting in reduced differential mode delay and, hence, less optical fiber dispersion. The second type of mode distribution is called an “overfilled launch,” where optical power is coupled into as many possible propagating modes as is feasible.

There are advantages of an overfilled launch condition, whose product is referred to as a mode scrambler. For example, an overfilled launch condition may be used to characterizing multimode optical fiber components.

There are various techniques and devices for generating an overfilled launch condition in a multimode optical fiber. For example, one technique is to inject a single mode optical signal into several kilometers of multimode optical fiber. Micro-bending induced mode coupling along the optical fiber length eventually results in an optical signal that has stable equilibrium of optical power distributed among many modes (multimode optical signal). However, several kilometers of optical fiber are required for this mode transformation (from a single mode optical signal to a multimode optical signal). As a result, this approach is not practical, especially in a laboratory environment, which is where most testing occurs. Moreover, using several kilometers of optical fiber merely to test a multimode device is expensive and bulky.

Another technique for generating an overfilled launch condition in a multimode optical fiber when initially launching from a single mode optical fiber is to concatenate a short segment of graded index multimode optical fiber followed by a step index multimode optical fiber followed by another short segment of graded index optical fiber. The step index optical fiber effectively provides a launch condition that fills up the mode volume of the second graded index optical fiber, thus providing the desired overfilled launch condition.

Mechanical mode scramblers also have long been used to generate a multimode optical signal. A single mode optical signal is launched from a single mode optical fiber into a multimode optical fiber. The multimode optical fiber is placed in the mode scrambler, which has corrugated surfaces to provide micro-bends in the optical fiber and redistribute energy into all the modes in the multimode optical fiber, resulting in the desired overfilled launch condition. The mechanical mode scrambler physically bends the optical fiber such that the angle of reflection between the optical signal and the core/cladding interface will be altered as the single mode optical signal passes through the portion of the optical fiber being bent. In this way, the single mode launch optical signal will be coupled into many more modes to approximate an overfilled power distribution in the multimode optical fiber. One such mechanical mode scrambler is the FM-1 Mode Scrambler available from Newport Corporation in Irvine, Calif.

Despite the advantages, this type of mechanical mode scrambler imposes intolerable strain on the optical fiber when physically bending the optical fiber to alter the angle of reflection. Bending stretches one side of the optical fiber and compresses the other. Because most optical fibers are comprised of glass or plastic, any strain on the optical fibers increases the risk that they will break. Tight bends in optical fiber can cause cracks, which can affect the optical signal traveling through the optical fiber, and will eventually lead to breakage of the optical fiber. A broken or cracked optical fiber will not properly transmit an optical signal.

Additionally, to effectively approximate an overfilled power distribution in the optical fiber, the mode scrambler bends the optical fiber many times in alternating directions. This makes the mode scrambler difficult to use, and because the tests are not repeatable, the device cannot be properly characterized. The mode scrambler also must be physically large enough to accommodate multiple bends.

DETAILED DESCRIPTION

The present invention is directed to a mode scrambler. In the following description, numerous specific details are provided, such as particular processes, programming, components, etc., to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, etc. In other instances, well-known structures or operations are not shown or described in detail to avoid obscuring aspects of various embodiments of the invention.

Some parts of the description will be presented using terms such as optical fiber, multimode, single mode, optical signal, and so forth. These terms are commonly employed by those skilled in the art to convey the substance of their work to others skilled in the art.

Various operations will be described as multiple discrete steps, performed in turn, in a manner that is most helpful in understanding the invention. However, the order in which they are described should not be construed to imply that these operations are necessarily order dependent or that the operations be performed in the order in which the steps are presented.

According to aspects of the present invention, a single mode optical signal is applied to the input of the mode scrambler.FIG. 1illustrates an embodiment of the present invention, in which a single mode optical signal102is applied to an example mode scrambler100, which converts the single mode optical signal102to a multimode optical signal104. The single mode optical signal typically has a diameter of nine micrometers and a single Gaussian intensity distribution. The resulting multimode optical signal104has a diameter of fifty or 62.5 micrometers and a substantially uniform intensity distribution across modes. The mode scrambler100thereby simulates the effect of a single spatial mode optical beam having traveled through several kilometers of multimode optical fiber.

FIG. 2is a schematic diagram of one embodiment of the example mode scrambler100, which includes a diffuser202disposed in a gap204of an optical fiber adaptor206. A single mode optical fiber208is connected to one end210of the adaptor206and a multimode optical fiber212is connected to the other end214of the adaptor206.

In one embodiment, the diffuser202may be a thin-film diffuser, such as a piece of Scotch® tape, a thin piece of glass, a thin piece of plastic, a thin piece of acetate, a thin piece of acrylic, or the like.

In one embodiment, the gap204may be filled with air. In this embodiment, when a single mode optical signal is launched into the mode scrambler200, the diameter of the single mode optical signal expands after traveling through gap204. The diffuser202diffuses the optical signal to generate a multimode optical signal whose modal energy distribution is sufficiently homogenized.

With the diffuser202in place, the gap204is separated into a gap203and a gap205. In one embodiment, the gap203is filled with air. In this embodiment, the gap203allows a launched single mode optical signal to expand before encountering the diffuser202. When the gap204is filled with air, the gap205allows the multimode optical signal to expand further prior to being launched in the multimode optical fiber212.

The adaptor206can be any commercially available adapter that physically connects two optical fibers, such as well-known FC connectors, SC connectors, LC connectors, ST connectors, SMA connectors, and the like. For example, the adaptor206may be a fiber optic mating adapter F-MA-FC-FC, F-MA-SC-SC, F-MA-SC-FC, and the like, all available from Newport Corporation in Irvine, Calif.

FIG. 3is a schematic diagram of an example mode scrambler300according to an embodiment of the present invention. The mode scrambler300includes a gap304, an adapter306, a single mode optical fiber308in a ceramic ferrule housing342, and a multimode optical fiber312in a ceramic ferrule housing340. The multimode optical fiber312has a core314and cladding316. The single mode optical fiber308includes core318and a cladding320. The single mode optical fiber308in the ferrule342is connected to one end of the adaptor306and the multimode optical fiber312in the ferrule340is connected to the other end of the adaptor306. A dotted line330and a dotted line332represent the centerlines of the core314and the core318, respectively.

In one embodiment, a single mode optical signal is launched into the mode scrambler300and the gap304is filled with air. In this embodiment, the gap304allows the single mode optical signal to expand before encountering the core314of the multimode optical fiber312.

According to an embodiment, the gap304is etched into the multimode optical fiber312. A resulting roughened surface302serves as an equivalent diffuser. For example, the gap304may be formed using an etching compound, such as hydrofluoric acid (HF), e.g., screen etch, to remove the multimode optical fiber312, which leaves the ceramic ferrule340to mate with the ceramic ferrule342that houses the single mode optical fiber308. The ceramic ferrule340is dipped in the etching compound to remove the optical fiber312. According to an alternative embodiment, the gap304is formed by pulling the optical fiber312away from the mating end of the ceramic ferrule340, which leaves ferrule340to mate with the ferrule342on the single mode optical fiber side.

FIG. 4is a schematic diagram of an example mode scrambler400according to an embodiment of the present invention. The mode scrambler400includes a diffuser404, an adapter406, a single mode optical fiber408in a ceramic ferrule housing442, and a multimode optical fiber412in a ceramic ferrule housing440. The multimode optical fiber412has a core414and cladding416. The single mode optical fiber408includes core418and a cladding420. The single mode optical fiber408in the ferrule442is connected to one end of the adaptor406and the multimode optical fiber412in the ferrule440is connected to the other end of the adaptor406. A dotted line430and a dotted line432represent the centerlines of the core414and the core418, respectively.

The diffuser404is made of suitable particulate material, such as particulate450and452, suspended in a material having uniform index of refraction, such as epoxy, ultraviolet (UV) glue, or index matching gel. When a single mode optical signal is launched into the mode scrambler400, the diffuser404diffuses the single mode optical signal to generate a multimode optical signal.

The above description of illustrated embodiments of the invention is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. These modifications can be made to the invention in light of the above detailed description.