Fault tolerant optical amplifier configuration using pump feedthrough

A method and apparatus for amplifying optical transmission signals is described. A bi-directional amplifier utilizes a pump feed-through signal from one of the optical pumps used to pump a first amplifying fiber to provide pump power to a second amplifying fiber. If an optical pump within the amplifier fails, this feed-through signal is used to pump the amplifying fiber directly pumped by the failed pump source.

FIELD OF THE INVENTION

The present invention relates generally to optical fiber amplifiers and more particularly to optical fiber amplifiers having an optical pump fault tolerant capability using an optical pump feed-through configuration.

BACKGROUND OF THE INVENTION

Optical fiber amplifiers have been employed in telecommunication systems to amplify light transmission signals. Optical amplifiers, and more particularly erbium doped fiber amplifiers, are attractive for telecommunications systems for a number of reasons including wavelength independence and bit rate transparency.

Optical amplifiers are disposed along the transmission path of an optical communication system. These amplifiers include at least an amplifying fiber, a coupler, and a pump source used to excite the amplifying fiber. The amplifying fiber is doped with a rare earth element (e.g. erbium) that is excited by light emitted from the pump source to amplify the signals propagating through the amplifying fiber. The pump light usually has a wavelength of 980 or 1480 nm. When a transmission signal, using having a wavelength in the 1550 nm range, propagates through the amplifying fiber, this light stimulates the erbium atoms to release their stored energy as additional 1550 nm light waves which continues as the transmission signals propagates through the amplifying fiber.

Optical amplifiers are attractive because they exhibit low noise, provide a relatively large bandwidth, which is not polarization dependent, and provide low insertion loses at the transmission signal operating wavelengths in the 1550 nm range. The pump light used to excite the amplifying fiber can be configured to co-propagate or counter-propagate with respect to the direction of propagation of the transmission signal. The couplers used to provide the pump light to the amplifying fiber have a high coupling ratio at the pump wavelength and a low coupling ratio at the signal wavelength.

Within an optical communication system, amplifiers are normally configured in pairs, since the optical transmission signals are bi-directional. Because of this, the pump sources are shared among the amplifiers in both directions. These pump sources are the only active component in the amplifier and are usually the most expensive. In telecommunication systems where high reliability is a requirement, for example in undersea systems, redundant pump configurations are necessary to compensate for pump failures. However, some redundant configurations employ additional pump sources that increase costs and power requirements.

Accordingly, there is a need for a fault tolerant amplifier configuration that employs optical pump sources capable of compensating for failed pumps in either signal transmission direction.

SUMMARY OF THE INVENTION

One embodiment of the invention comprises a method and apparatus that provides a fault tolerant pump source configuration in an optical amplifier. A first pump source supplies pump power to a first amplifying fiber segment for amplifying transmission signals propagating through the first fiber segment in a first direction. A second pump source supplies pump power to a second amplifying fiber segment for amplifying transmission signals propagating through the second fiber segment in a second direction. A portion of the pump signal supplied to the first amplifying fiber segment remains after propagation through the first fiber segment. This remaining feed-through signal is supplied to a second amplifying fiber segment and amplifies the transmission signals propagating through the second amplifying fiber. Likewise, a portion of the pump signal supplied to the second amplifying fiber segment remains after propagation through the second fiber segment. This remaining feed-through signal is supplied to the first amplifying fiber segment and amplifies transmission signals propagating through the first amplifying fiber. When a pump source fails, the remaining operating pump directly pumps its associated amplifying fiber segment and its feed-through signal pumps the other amplifying fiber segment associated with the failed pump source. This feed-through architecture alleviates the need for additional pump sources for pump redundancy purposes in high reliability communication systems.

With these and other advantages and features of the invention that will become hereinafter apparent, the nature of the invention may be more clearly understood by reference to the following detailed description of the invention, the appended claims and to the several drawings attached herein.

DETAILED DESCRIPTION

FIG. 1schematically illustrates an exemplary configuration for amplifier pair110for use within an optical communication system or network. Amplifier pair110may be one of a plurality of amplifiers disposed along a transmission path. The amplifier pair is configured to amplify transmission signals carrying communications traffic in two directions; from input port180to output port185and from input port190to output port195(e.g. East to West and West to East). A first amplifier comprises a light pump source120, wavelength discriminating coupler140(WDM coupler) and amplifying fiber segment160. A second amplifier includes light pump source130, wavelength discriminating coupler150and amplifying fiber segment170.

The first amplifier amplifies light entering port180and outputted via port185and the second amplifier amplifies light entering port190and outputted via port195. Fiber segments160and170are doped with a rare earth element, such as erbium, that, when excited by light emitted from pump source120or130amplifies transmission signals entering each amplifier via ports180and190respectively. Pump sources120and130can be, for example, laser light sources providing light having wavelengths of 980 or 1480 nm. In the exemplary embodiment shown inFIG. 1, the light signal from pump120is supplied to fiber segment160via directional coupler140in a counter propagating manner with respect to the propagation of the transmission signals as indicated by directional arrows161and the light from pump source130is supplied to fiber segment170via directional coupler150in a co-propagating manner as indicated by the directional arrows162. Coupler140is disposed between pump source120and amplifying fiber segment160and is configured to have an input port that receives the pump signal from pump120and supplies it to amplifying fiber160. Likewise, coupler150is disposed between pump source130and amplifying fiber segment170and is configured to have an input port that receives the pump signal from pump130and outputs it to amplifying fiber170. Couplers140and150have high coupling ratios at the pump wavelength and low coupling ratios at the transmission signal wavelength. In other words, the couplers140and150couple light from the pump sources120and130(e.g. having wavelengths in the 980 nm range or 1480 nm range) to the respective amplifying fibers160and170, while supplying the transmission signals (e.g. having wavelengths in the 1550 nm range) from amplifying fiber160to output port185and from amplifying fiber170to output port190. Accordingly, each pump120and130provides light at a particular wavelength sufficient to excite the rare earth elements in fiber segments160and170. In this manner, the amplifier pair is configured to amplify signals traversing a communication system bi-directionally; i.e. East to West via ports180and185; and West to East via ports190and195.

After the light from pump120propagates through amplifying fiber segment160, a portion of the pump light signal still remains (“pump feed-through signal”). An additional directional or WDM coupler115disposed between input port180and amplifying fiber segment160is used to couple the pump feed-through signal to WDM coupler125disposed between output port195and amplifying fiber170via a bi-directional connection135which may be, for example, a fiber jumper. Again, coupler125has a high coupling ratio at the pump wavelength and a low coupling ratio at the signal wavelength. This pump feed-through signal supplied to fiber segment170via coupler125pumps amplifying fiber segment170in a counter-propagating manner as shown by directional arrow161. Thus, amplifying fiber segment170is pumped twice, once from its primary pump130and once from the pump feed-through signal from fiber segment160and pump120.

Similarly, after the light from pump130propagates through amplifying fiber segment170, a portion of the pump light signal still remains as a pump feed-through signal. This pump feed-through signal is supplied in a co-propagating manner to fiber segment160via WDM couplers125,115and bi-directional connection135. Thus, amplifying fiber segment160is pumped twice, once from its primary pump120in a counter-propagating direction and once from the pump feed-through signal from fiber segment170and pump130in a co-propagating direction.

If a pump120or130should fail, one of the amplifying fiber segments160,170would lose its primary pump, but would still receive pump light from the remaining functioning pump via the feed-through signal. For example, if pump130fails, amplifying fiber segment160is pumped by pump120directly in a counter-propagating direction, and fiber segment170is pumped by pump120by the feed-through signal from pump120via couplers115,125and bi-directional connection135also in a counter-propagating direction. Similarly, if pump120fails, amplifying fiber segment170is pumped directly by pump130in a co-propagating direction and fiber segment160is pumped by the remaining feed-through signal from pump130via couplers125,115and bi-directional connection135in a co-propagating direction Obviously, the pumps can be configured such that the direction of the primary pump light is either co-propagating or counter propagating and the feed-through signal is either counter propagating or co-propagating. This choice may also be dependent on the desired gain profile of the respective amplifier or amplifier pair within the transmission system. In addition, if this amplifier configuration is used as a pre-amplifier, each amplifying fiber segment is not highly saturated and therefore relatively strong pump feed-through signal will be present sufficient to pump the remaining amplifying fiber segment.

FIG. 2illustrates an alternative configuration for amplifier pair210for use within an optical communication system. Transmission signals enter input port280propagate through amplifying fiber segment260and exit via output port285. Likewise, transmission signals enter input port290, propagate through amplifying fiber segment270and exit via output port295. A first pump source220supplies pump light in a co-propagating manner to amplifying fiber segment260via WDM coupler240. Pump source230supplies pump light in a counter-propagating manner to fiber segment270via WDM coupler250.

After the light from pump220propagates through amplifying fiber segment260, a portion of the pump light signal still remains (“pump feed-through signal”). Similarly, after the light from pump230propagates through amplifying fiber segment270, a portion of the pump light signal still remains. WDM coupler215is disposed between amplifying segment260and output port285. WDM coupler225is disposed between input port290and amplifying fiber segment270. The pump feed-through signal from pump source220is supplied to fiber segment270via couplers215,225and bi-directional connection235. This feed-through signal pumps amplifying fiber segment270in a co-propagating manner as shown by directional arrow261. Thus, amplifying fiber segment270is pumped twice, once from its primary pump230in a counter propagating direction and once from the pump feed-through signal from pump source220via fiber segment260and couplers215and225.

Similarly, the pump feed-through signal from pump source230is supplied to fiber segment260via couplers225,215and bi-directional connection235. This feed-through signal pumps amplifying fiber segment260in a counter-propagating manner as shown by directional arrow262. Thus, amplifying fiber segment260is pumped twice, once from its primary pump220in a co-propagating direction and once from the pump feed-through signal from pump source230via fiber segment270and couplers225and215in a counter propagating direction.

If one of the pump sources220or230should fail, each fiber segment260and270would still receive pump light from the remaining functioning pump either directly or from a pump feed-through signal. For example, if pump230fails, amplifying fiber segment260is pumped by pump220directly, and fiber segment270is pumped by the feed-through signal from pump source220via couplers215,225and bi-directional connection235. Similarly, if pump220fails, amplifying fiber segment270is pumped directly by pump230and fiber segment260is pumped by the remaining feed-through signal from pump230via couplers225,215and bi-directional connection235.

When this configuration is used in post-amplifier applications, the amplifiers are not highly inverted and less pump feed-through light is available that can be used to pump the second fiber segment in case the first fiber segment pump fails. For example, if pump220fails, pump230will pump fiber segment270and the feed-through light will pump segment260. However, because in post amplifier applications, the amplifiers are not highly inverted, less pump feed-through light from segment270is available to pump segment260. In this situation, higher pump feed-through light needs to be obtained. This may be done by using shorter lengths of fiber segments260,270so sufficient pump feed-through light is present, using higher pump powers to increase feed-through or employing feedback circuitry to increase the power of a remaining operating pump source to compensate for a failed pump.