A signal may be split by a splitter into a plurality of output signals. Each of these output signals may then be amplified. Amplified spontaneous emission noise may be removed using a tunable filter for each of the signal outputs. As a result, an output signal may be provided with greater power so that, in some embodiments, a single split signal may be utilized to service more end users.

BACKGROUND

This invention relates generally to optical networks and, particularly, optical networks that use optical power splitters.

In an optical network, a signal may be transmitted over an optical fiber. The signal may include a plurality of channels, each of a different wavelength. In order to multiplex the different channels onto the fiber, a multiplexer may be used. A demultiplexer is used to separate the multiplexed channels at a destination.

A power splitter may divide a channel into a plurality of distinct outputs. A signal, containing a single channel or multiple channels, may be divided by a splitter and delivered to several different destinations.

Amplification is required to compensate for propagation losses and loss of power of the signals due to splitting. The amplification of the optical signal is usually provided by erbium-doped fiber amplifiers.

Currently, there is particular demand for optical splitter devices for use in fiber-to-the-curb (FTTC) and fiber-to-the-home (FTTH) communication networks. These splitter devices facilitate the distribution of a common signal to multiple customers. However, a conventional splitter severely limits the transmission link length and the number of customers due to the natural signal loss associated with every splitting function.

Erbium-doped amplifiers can be used to compensate for such losses, significantly increasing the number of customers that receive the same signal. However, erbium-doped amplifiers are too expensive for this low-cost application. Also the broadband amplified spontaneous emission (ASE) noise generated in the amplifier degrades the signal-to-noise ratio, posing a limit on the number of customers serviced by the split signal.

Thus, there is a need for better ways to provide amplified splitting in optical networks.

DETAILED DESCRIPTION

Referring toFIG. 1, a 1×N splitter12receives an input signal18which may be an optical multiplexed signal. The splitter12splits the input signal18into N output signals. For example, an input signal, as shown inFIG. 3, may be split to produce a plurality of output signals of the type shown inFIG. 4. Each split signal, as shown inFIG. 4, has the same peak wavelength as the input signal18, but the amplitude of that peak may be substantially diminished compared to the amplitude of the input signal18.

A variety of splitters12may be utilized, including a cascaded Y-junction splitter and a multi-mode interference splitter.

The split output signals from the splitter12are then amplified by the N-channel amplification gain block14. The amplification gain block14may use pump lasers and erbium-doped waveguides in one embodiment. The output from the gain block14in one hypothetical example is shown inFIG. 5. While the peak power is now higher, a noise floor has been created as a result of amplified spontaneous emission (ASE) from the amplifier.

The split signals from the gain block14may then be subjected to an N-channel, tunable filter16in accordance with one embodiment of the present invention. The filter16removes the noise floor resulting in the hypothetical output signal shown inFIG. 6. The filter16may, for example, be a thermo-optically tuned waveguide Bragg grating pair that is written using ultraviolet light on an integrated Michelson interferometer. In such case each of the Bragg gratings29is heated to a certain temperature to tune the reflected band of resonant wavelengths to correspond to the wavelength of the peak amplitude.

As another example, the reflected light from a single reflective Bragg grating can be separated from incident light using an optical circulator (not shown). The circulator passes the input light and outputs the light reflected by the tunable Bragg grating. As still another example, a single transmissive tunable Bragg grating may be used to pass the desired band of resonant wavelengths corresponding to the peak amplitude.

The structure shown inFIG. 1may be made using a monolithic integration approach with all three functional blocks,12,14, and16fabricated on a single planar waveguide optical chip. Alternatively, in the hybrid approach, functional blocks may be fabricated in separate chips and then directly attached in a multi-chip module format. In still another alternative, a fiber integration approach may be used in which the functional blocks are fabricated and packaged separately and then interconnected by way of optical fibers.

In some embodiments, the use of the tunable filter16may significantly increase the number of end points or customers accessible by a common network node. This may mitigate one of the most severe bottlenecks in FTTC/FTTH communication systems, namely, the restriction of the link length and the number of customers for a common signal due to losses associated with splitting. Furthermore, in some embodiments, the user may have less ASE noise, thereby improving the bit-error rate of the transmission system.

Referring toFIG. 2, an embodiment is illustrated in which the splitter12is implemented by a series of 1×2 Y-junction splitters24. An input pump22may be provided to each split signal from the splitter12as indicated at22a. The amplified signal line26exits the N-channel amplification gain block14and may go to a Michelson interferometer28in one embodiment. One of the arms of the interferometer28may provide the output signal20.

A thermally heated Bragg grating29may be provided in each of two arms of the Michelson interferometer28. These Bragg gratings29act as an optical filter to select one or more desired bands of wavelengths to form the output20. In one embodiment, the Bragg grating pairs29filter a desired band of resonant wavelengths by reflecting that band to become the output signal20. If the wavelength of the reflected band is tuned by heating, using the heaters40, to correspond to the peak amplitude (see the pass band A inFIG. 5), the noise floor (FIG. 5) may be removed (FIG. 6). In one embodiment, the heaters40may be micro-heaters that heat using electrical resistance.

Referring toFIG. 7, an optical system may include a multiplexer30that multiplexes a N number of channels1through N. Those signals may then be amplified by an amplifier32which may use erbium doping. A switch34may be used to switch different signals before demultiplexing at the demultiplexer36. Each demultiplexer signal may go to a desired destination38. Alternatively, the tunable splitter10may be utilized to further split the signal to increase the number of end users that can be serviced by the same demultiplexed output signal.