The present invention relates to communication using optical amplifiers with multiple cores.
FIG. 1A shows an example of a single-core fiber where the optical light is guided inside the core. FIG. 1B shows an example of a multi-core fiber that has multiple cores inside the same fiber therefore capable of guiding multiple signals simultaneously in parallel. The cores in a multi-core fiber can be arranged in many different ways. In general, the cores have a higher refractive index than the surrounding cladding so that the light can be guided inside the cores with minimal loss.
In current transmission systems only single-core fibers are used for transmission. After transmission, the loss in the fiber is compensated for by using optical amplifiers. In general, these amplifiers are called fiber amplifiers as the amplification occurs in fibers. Commonly, these fiber amplifiers are doped fiber amplifiers, and most commonly such amplifiers are doped with erbium. Without loss of generality, the discussion below is directed at erbium doped fiber amplifiers (EDFAs) even though the system would work with other types of fiber amplifiers.
FIG. 2 shows an exemplary optical transmission system where the transmitter converts data to optical signals that transmit over optical fiber spans and the loss of the spans are compensated by optical amplifiers. The receiver converts the optical signals back into the transmitted data.
FIG. 3 shows an exemplary optical fiber amplifier set up with an Erbium-doped fiber (EDF), a Wavelength division multiplexing (WDM) coupler and a laser diode (LD). The WDM coupler combines the pump laser with the signal so they can co-propagate in the amplifying medium which is the EDF. Inside the EDF pump power is transferred to the signal and the signal comes out with more power than it entered the amplifier. In current systems, almost all of the amplifiers have single cores. Moreover, in current systems, the transmission signal is always in a single mode, therefore the EDFs also support only a single mode. As a result, in current systems, the pump lasers also produce single-mode lasers. The reason for using single-mode pump is that, if the pump is multimoded (multimode pump), then the multimode light generated by the pump cannot be efficiently launched into an EDF that supports only a single mode. The power produced by a multimode pump is distributed over many modes. A single-mode EDF accepts only a single mode and all the power in the rest of the pump laser modes that cannot be launched into the single mode of the EDF would be wasted.
Nevertheless, if the light from the multimode laser could be delivered efficiently to the EDF there would be several significant advantages to using a multimode pump instead of a single mode pump. First, single-mode pumps cannot generate very high power. Typically output from a single-mode pump is below 1 W, sometimes it can go up to 2 W. Multimode pumps can easily generate more than 10 W. Second, a single-mode pump is more expensive than a multimode pump. Third, multimode pumps are more reliable than single-mode pumps because they typically have a lower resistivity than single-mode pumps. Fourth, multimode pumps are more efficient in converting electrical power to optical power. This can be a significant advantage especially in submarine type optical transmission system where the electrical power has to be supplied to the pumps inside the amplifier from ends of a cable that can span thousands of km. This IR pertains to a method of efficiently replacing multiple single-mode pump lasers with a single multimode laser.
It has been shown that one system application for replacing single-mode pumps with multimode pumps is the case of multicore fiber amplifiers. The motivation behind using multicore fiber amplifiers instead of single mode amplifiers is to reduce the cost and packaging size. In typical transmission systems, multiple transmission systems are placed together in parallel. For instance, in the case of submarine transmission a cable may contain multitude of fibers each carrying a signal in parallel with its own dedicated amplifiers.
FIG. 4 shows replacing a transmission system that has multiple transmission fibers and amplifiers with single multicore fibers and multicore amplifiers. The fan in (FI) device would take the signal from each transmitter and launches into individual cores of a multicore fiber (MCF). A plurality of multicore amplifiers (MCAs) would amplify signal from individual cores of the MCF individually in the cores of the MCA. The fan out (FO) device would take the signal coming from each core and deliver them to individual receivers. It is possible not to replace the single core fibers by MCFs, but still use MCAs to amplify the signal from multiple single core fibers inside individual cores of an MCA.
FIG. 5 shows an example where multiple single core amplifiers can be replaced by a single MCA. One advantage of this arrangement is the reduction in cost and packaging size. Multiple components on the left are replaced by single components on the right. Such integration can reduce both cost and size. Another advantage is that instead of using multiple single-mode pumps one can use a single multi-mode pump. In this case, note that a multi-mode pump can deliver as much power as multiple single mode pumps. Moreover, it is possible to direct different modes of a multimode pump to different cores of the MCA even if the cores of the MCA support only a single or a few modes only. This allows for a more efficient use of the power from multi-mode pump which is distributed into multitudes of modes as opposed to trying to launch many modes of the multi-mode pump into a single core which can accept only one of the modes, now it is possible to direct different modes of the multi-mode pump into individual cores of an MCA. State of the art systems operate with the pump power launched into the cladding of the MCA and little of the pump power actually overlaps with the signals which are confined to the cores. Signal can only derive power from the portion of the pump that it coincides with physically inside the amplifier. In general when the pump is launched into the entire fiber cladding, the portion of the pump that remains inside the cores is very little. Under such circumstances, first, most of the pump power would go to waste. Second, quality of the signal suffers as the pump intensity that overlaps with the signal cannot create enough so called upper state population. When upper state population is not high enough amplifiers add large amount of noise to the signal.
To mitigate this impact a pump waveguide is added to the multicore EDF that surrounds the cores of the amplifier and boosts the level of pump inside that pump waveguide around the cores. Other solutions include:
(1) Adding a cladding layer to concentrate the pump power around the cores that carry the signal.
(2) Using multicore amplifiers with a hollow center.
FIG. 6 shows a multicore EDF example including a pump waveguide drawn as a ring surrounding the 4 cores carrying the signal. In such a design, the ring area that is designed to concentrate the pump power has a higher refractive index than the surrounding cladding area so that it can guide the pump but it has a lower refractive index than the cores so that the signal would not leak out into the pump waveguide. In this approach, the pump was coupled into the fiber through cladding, in other words from the side of the fiber. With this kind of launch, the pump would just pass through the pump ring and still disperse all around the entire fiber. In order to make sure pump remains only inside the pump waveguide area, the pump has to be launched only into the waveguide from the ends of the fiber (not from the sides of the fiber). In such a case it is not possible to make sure that the pump remains inside the waveguide area and it still disperses all around the fiber with a negligible increase in the pump power inside the waveguide area.
FIG. 7 shows an exemplary hollow center multi-core EDF. The fiber is a glass ring with multiple cores in it where signals are carried. The center of the fiber is hollow. In this case, the pump can still be combined through the sides of the fiber, but because there is nowhere else for the pump to go, the pump would be concentrated around the cores in a narrow ring. However, this method has various disadvantages. First, it is difficult to fabricate high quality fibers with hollow centers. Second, such fibers lack strength because of the hollow center. Third, such fibers are susceptible to dirt and other contaminants that can be almost impossible to clean once they go into the hollow center. Fourth, these fibers are more difficult to splice to standard fibers because of the hollow core. Fifth, these fibers would have large loss for the pump because the fibers have to be jacketed for easy manipulation, handling and protection. Once they are jacketed, the pump light would come into contact with the jacket and pump would experience large loss.
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