This invention relates to optical couplers, particularly tap couplers utilizing polarization maintaining (PM) optical fibers, and to a method of manufacturing such couplers.
Polarization maintaining fibers (PMF) are characterized in that they retain the polarization of the input signal within them throughout their length. Known PM fibers may be of various types e.g. Panda(trademark) or bow-tie (Tiger) type, varying in the shape of the so-called stress rods, or tensioning members, i.e. regions of different glass composition in the fiber cladding. Also known are PM fibers with an elliptical core shape. All these types of PM fibers are characterized by the presence of two principal polarization axes.
A tap coupler is designed such that the amount of optical energy coupled from one of the fibers into the other is substantially less than the 50% typical of a conventional 3 dB coupler.
Known tap couplers use either standard optical fibers (usually SM fibers) or PM fibers. It is known to produce such couplers by positioning two stripped fibers side-by side and either joining them (e.g. by fusing) in a parallel arrangement or twisting them before fusing. For tapping purposes, only three of the four ports of a coupler are used.
International Application WO 01.23939 published Apr. 5, 2001 discloses a fused optical coupler having a PM fiber and a standard (non-PM) fiber. The cross-section of the non-PM fiber is smaller than the cross section of the PM fiber in the area of fusion of the coupler.
U.S. Pat. No. 5,293,440 describes couplers made from PM fibers, SM fibers, multi-mode fibers (MMF) or their combination.
Polarization extinction ratio (PER) is an important parameter of a coupler employing a PM fiber. It is usually desired to keep PER at a relatively high level, preferably at least about 20 dB, as stated in the International Application, supra.
It is also desired to provide a coupler using a PM fiber and a non-PM fiber wherein the tap ratio is less than about 10% and preferably about 2% or lower.
It has been found unexpectedly that a tap coupler of the type defined above, with a combination of desired parameters, can be produced without reducing the diameter of one of the fibers of the coupler before the fusing and stretching of the fibers.
In accordance with one aspect of the invention, there is provided a fused optical coupler comprising
a PM fiber having two principal polarization axes, and
a standard (non-PM) fiber,
the PM and non-PM fibers having substantially equal clad diameters, the principal axes of the PM fiber arranged randomly relative to the non-PM fiber,
the fibers elongated to a degree such that the coupler exhibits a tap ratio less than about 10% and polarization extinction ratio (PER) higher than about 20 dB.
In accordance with another aspect of the invention, there is provided a method for producing a fused coupler, the method comprising:
providing a PM fiber and a non-PM fiber, the two fibers having a substantially equal clad diameter,
plaiting the two fibers together substantially without applying torsional force on either fiber with random orientation of the principal polarization axes of the PM fiber relative to the non-PM fiber,
tapering (stretching) and fusing the fibers together to a degree such that the tap ratio of the coupler is not more than about 10% and the PER of the coupler is at least about 20 dB.
The plaiting may be imparted on both of the fibers, whereby both fibers undergo bending, still substantially avoiding torsional force on either fiber. The plaiting results in a number, preferably from 2 to 10, more preferably 3-6, of so-called cross-overs of the fibers.
In one embodiment of the invention, the coupler is adapted to inputting optical energy into the PM fiber and tapping part of the energy into the non-PM fiber.