Dual fiber optic amplifier with shared pump source

A dual fiber optic amplifier uses a single pump source for two or more optical power amplifiers. The dual fiber optic amplifier includes a pump source that emits light, a pump splitter, a first optical power amplifier and a second optical power amplifier. The pump splitter splits the light emitted by the pump source into two or more portions. The first optical power amplifier includes an optical fiber input, an optical fiber output, and a doped fiber portion, wherein the first portion of light from the splitter is coupled into the optical fiber input of the first optical power amplifier. The second optical power amplifier includes an optical fiber input, an optical fiber output, and a doped fiber portion, wherein the second portion of light emitted from the splitter is coupled into the optical fiber input of the second optical power amplifier.

The present invention relates generally to a system for pumping optical energy into doped fiber optical amplifiers, and particularly to a system using a single pump source to provide optical energy to more than one doped fiber optical amplifier.

BACKGROUND OF THE INVENTION

Fiber optic communication utilizes optical transmitters, optical receivers and optical fiber, among other components, to transmit light signals through the fiber. The transmitters and receivers are often integrated into a single component called a transceiver. Transmitters are light sources, such as lasers or light-emitting diodes. Receivers usually include a photo detector.

A signal being transmitted through a fiber optic system may suffer amplitude attenuation due to energy absorption, beam scattering and other processes during transmission. To compensate for such signal loss during transmission, optical amplification may be used to increase the amplitude of the signal leaving the transmitters or to increase the amplitude of a signal coming in to the receivers. An amplifier placed after the transmitter to boost an outgoing signal is often called a power amplifier, and an amplifier placed before the receiver to boost an incoming signal is often called a pre-amplifier. Passive optical amplifiers, such as erbium doped fiber amplifiers (EDFA), are often used for such amplification.

Doped fiber amplifiers typically include a length of optical fiber that has been doped with certain elements. Such amplifiers amplify a transmission signal when the doped fiber receives optical energy from a pump source. Such amplifiers produce amplification by stimulated emission—the dopants in the doped fiber are stimulated to a higher energy state by receiving pump power and may achieve a population inversion. As energy falls back to lower energy levels additional photons may be emitted. Usually the doped fiber responds most efficiently to one or more pumping wavelengths. In other words, the amplification imparted to the transmission signal may have a gain curve with one or more peaks corresponding to wavelengths specific to that amplifier. A pump source is often chosen based on the peak gain wavelength of the doped fiber amplifier.

Conventionally, the pump sources for the power amplifiers and pre-amplifiers are separate and individual. Thus, one separate pump source is used for the power amplifier, and one separate pump source is used for the pre-amplifier. Often, a pre-amplifier requires less pump power than a power amplifier, as the pre-amplifier is intended to produce less gain. The cost of the pump sources is often a significant part of the total cost of the amplifiers. For example, in a typical power amplifier or pre-amplifier, the cost for its associated pump source can be 60–80% of the total cost.

Both long distance and metro area telecommunications systems employ systems which include optical fiber, transceivers and amplifiers. However, metropolitan area networks (MANs) tend to have shorter distances between transceivers and are more cost sensitive than long distance telecommunications systems. As such, the considerable expense of multiple pump sources is relatively more significant for metropolitan area networks.

SUMMARY OF THE INVENTION

In summary, the present invention is a dual fiber optic amplifier using a single pump source to provide power to two or more optical power amplifiers. The dual fiber optic amplifier includes a pump source that emits light, a pump splitter, a first optical power amplifier and a second optical power amplifier. The pump splitter splits the light emitted by the pump source into two or more portions, with a first portion directed to the first optical power amplifier and a second portion directed in the second optical power amplifier. The first optical power amplifier includes an optical fiber input, an optical fiber output, and a doped fiber portion. The first portion of light emitted from the pump splitter is coupled into the optical fiber input of the first optical power amplifier. The second optical power amplifier includes an optical fiber input, an optical fiber output, and a doped fiber portion. The second portion of light emitted from the pump splitter is coupled into the optical fiber input of the second optical power amplifier.

A further embodiment of the present invention is an optoelectronic transceiver that includes an optical signal transmitter, an optical signal receiver, a pump source that emits light, a pump splitter, a first optical power amplifier, and a second optical power amplifier. The first optical power amplifier includes an optical fiber input coupled to an output of the optical signal transmitter, an optical fiber output, and a doped fiber portion. A first portion of the light emitted from the pump splitter is coupled into the optical fiber input of the first optical power amplifier. The second optical power amplifier includes an optical fiber input, an optical fiber output coupled to an input of the optical signal receiver, and a doped fiber portion. A second portion of light emitted from the pump splitter is coupled into the optical fiber input of the second optical power amplifier. In an embodiment comprising an integrated transceiver and dual fiber optic amplifier package, the optical power amplifier coupled to the transmitter output is a “power amplifier”, and the optical power amplifier coupled to the receiver input is a “pre-amplifier.”

In some embodiments, the pump splitter splits the pump source light in unequal portions, such that the first and second optical power amplifiers receive portions of pump light having different amplitudes.

By cutting the number of pump sources in half, the total cost of the power amplifier and pre-amplifier associated with a transceiver is significantly lowered. Replacing two pump sources with one also typically reduces the total power used by the amplifiers in the transceiver, and enables the transceiver to be placed in a smaller housing. Thus, it is highly desirable to use one pump source for both the power amplifier and pre-amplifier.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring toFIG. 1, there is shown an example of the conventional approach to multiple optical power amplifiers used in conjunction with transceivers or a set of transmitters and receivers that are relatively close to each other. Two separate pump sources118,146are used—one for each of the optical power amplifiers110,138. The two optical power amplifiers are a power amplifier110and a pre-amplifier138.

Each of the optical power amplifiers110,138is a doped fiber amplifier that includes a length of optical fiber that has been doped with certain elements. Such amplifiers amplify an optical signal, if the signal is within a particular wavelength range, when the doped fiber receives optical energy from a pump source. Such amplifiers produce amplification by stimulated emission—the dopants in the doped fiber are stimulated to a higher energy state by receiving pump power and may achieve a population inversion. As energy falls back to lower energy levels additional photons may be emitted.

In this example,FIG. 1shows a transmitter102transmitting an optical signal104into an optical fiber108. The optical fiber108is coupled to a power amplifier110to amplify the transmission signal before it is transmitted to an output fiber112. The power amplifier110receives optical pump energy (i.e., light) from the pump source118. The power amplifier110includes a doped fiber portion that amplifies light within a particular range of transmission wavelengths when the doped fiber is pumped by a pump source of light within another range of wavelengths. The pump source emits a beam of light116into an optical fiber114which is coupled into the power amplifier110.

In close proximity to the transmitter102and power amplifier110, a receiver130and pre-amplifier138are similarly equipped. Specifically, the receiver130receives an optical signal132from the end134of an optical fiber136. The optical fiber136is the output for a pre-amplifier138which amplifies an incoming signal from optical fiber140. The preamplifier138receives optical pump energy (i.e., light) from the pump source146. The pump source emits a beam of light144into an optical fiber142which is coupled into the power amplifier138.

In this way, both transmitted and received signals may be amplified by their respective amplifiers (power amplifiers or pre-amplifiers) and separate pump sources. The separate pump sources typically generate light at the same or similar wavelengths, especially where the optical power amplifiers110,138use doped fiber amplifiers with similar gain characteristics, such as erbium doped fibers. However, power amplifiers and pre-amplifiers often require different amounts of pump energy.

FIG. 2shows a conceptualized representation of a transceiver250that includes a dual fiber optic amplifier according to an embodiment of the present invention. In contrast to the conventional approach depicted inFIG. 1above (i.e., discrete and separate pump sources for each amplifier), one pump source222is used in an embodiment of the present invention to pump more than one optical power amplifier. As shown inFIG. 2, the two optical power amplifiers210,232are a power amplifier and a pre-amplifier, respectively. An optical splitter220divides the light output by the pump source222, with a first portion of the light being directed by the splitter220to the power amplifier210and a second portion of the light being directed to the pre-amplifier232. Different amounts of light may be directed by the splitter220to each of the optical amplifiers, with the ratio of light directed to each being set in accordance with the power needs of each of the amplifiers210,232.

The pump source and splitter arrangement ofFIG. 2may also be used with systems having multiple power amplifiers or multiple pre-amplifiers. Also, in some embodiments the dual doped fiber amplifier is a separate device from the transmitter202and receiver242, but coupled thereto as an add-on component.

The transceiver250usually includes a transmitter202and a receiver242packaged together with the pump222and splitter220. The transceiver has an electrical interface246to receive electrical power, control signals and an input data stream for the coupling to the transmitter202. The electrical interface246also conveys an output data stream from the receiver242to a host device (not shown). On optical interface244connects a pair of fiber optic cables to the transceiver. On the transmitter side, an optical transmission signal204is transmitted from the transmitter202into a fiber optic cable208. A power amplifier210amplifies the optical transmission signal. The power amplifier210is preferably a doped fiber amplifier, which requires an optical pump to produce the amplification of the optical transmission signal. The optical transmission signal204has a first wavelength (e.g., 1550 nm) that is within a predefined range of wavelengths (e.g., 1535 to 1560 nm) of light that are amplified (by stimulated emission) by the doped fiber amplifier210when the doped fiber amplifier is pumped with optical energy at a second wavelength to which the dopant in the doped fiber amplifier responds.

On the receiver side, an optical signal240being received from a fiber optic cable234/236is passed through a pre-amplifier232to amplify the incoming signal before receipt at the receiver242. The pre-amplifier232is also be a doped fiber amplifier requiring optical pumping to produce amplification of the received signal. The pre-amplifier232has a doped fiber portion that amplifies the received signal204when the pre-amplifier232is pumped with light at a wavelength (e.g., 980 nm) at which the dopant in the doped fiber responds with stimulated emission.

The optical pumping for both the power amplifier202and the power amplifier232is provided by a single optical pump source222. This is accomplished by splitting the pump beam224into two or more portions. An exemplary splitter220splits the pump beam224into two portions218and226. These portions may be of equal or unequal amplitudes. Typically, a smaller portion of the optical pump power will be directed to the pre-amplifier232than the portion directed to the power amplifier210. Each portion of light from the splitter220is coupled either directly or via a fiber optic cable into the power amplifier210and pre-amplifier232. Specifically, a first portion of light218may be coupled into a fiber214that in turn is coupled into the power amplifier210. This allows the first portion of light218to pump the power amplifier210and thereby amplify the transmission signal before it is transmitted into an optical system via fiber cable212and optical interface244. The second portion226of light from the splitter220may be coupled into a fiber230that in turn is coupled into the pre-amplifier232. In this way the second portion of light226is used to pump the pre-amplifier232and thereby amplify the signal being received by the receiver242.

The transceiver250preferably includes a housing (represented by the transceiver's outside border shown inFIG. 2) that provides electromagnetic shielding, to reduce EMI emissions from the transceiver250.

The necessity for amplification in optical transceiver systems can be seen, for example, when a 3 decibel (dB) coupler is used to multiplex different wavelengths into a single strand of an optical fiber. Loss caused by the 3 dB coupler will accumulate as the channel count increases. A power amplifier may then be needed to compensate these losses and to maintain adequate power into the transmission line. In addition, after transmission through the system, optical signals may be significantly weakened due to the transmission span loss. A pre-amplifier may then be employed to amplify the incoming signals to a level within the sensitivity of the receivers.

FIGS. 3A and 3Bshow gain curves for an exemplary pre-amplifier and an exemplary power amplifier. Both gain curves are for erbium doped fiber amplifiers, and show the gain in decibels (dB) as a function of wavelength in nanometers (nm). These gain curves are provided as examples of the gain characteristics and pump power requirements of typical preamplifiers and power amplifiers. To demonstrate an application of dual fiber optic amplifiers with a shared pump source, a simulation is shown inFIGS. 3A and 3Bof the performance of pre-amplifiers and power amplifiers for a 32-channel wave division multiplexing (WDM) system. A 40 milliwatt (mW) pump power is applied to a pre-amplifier, and a 120 mW pump power is applied to a power amplifier. This is done by splitting a 160 mW pump power utilizing a 25%/75% pump splitter, such that the pre-amplifier then receives 25% (or 40 mW) of the total pump power, while the power amplifier receives the remainder of the total (75% or 120 mW).

Power amplifiers and pre-amplifiers generally receive differing amounts of optical pump energy and are often designed differently. Power amplifiers often amplify signals at a higher signal level than pre-amplifiers. Stated another way, pre-amplifiers are often used to amplify much smaller signal levels than power amplifiers. Nevertheless, it is possible to use the same or similar designs for both types of amplifiers. When doped fiber amplifiers are used for either power amplifiers or pre-amplifiers, parameters for the design and use of such amplifiers include pump power, signal input power, doping levels and length of the doped fiber portions. These are the parameters that impact the gain curves shown inFIGS. 3Aand3B. While erbium doped fiber amplifiers are often used, other dopant elements may be used in other embodiments of the present invention.

Gain is defined here as follows:
Gain=10*log10(power out/power in)
where “power out” is a power measurement of the transmission power of a signal output by an amplifier, and “power in” is a power measurement of a signal input to that amplifier.

FIG. 3Ashows the gain curve of an erbium doped pre-amplifier receiving 40 mW of pump power. The gain curve is relatively flat for wavelengths between about 1535 nm and 1560 nm. This range of wavelengths coincides with the wavelengths at which optical telecommunications signals are often transmitted. The fact that the gain curve is relatively flat is important for systems in which optical signals are transmitted at several wavelengths, as in wave division multiplexing (WDM) systems. The gain curve is not exactly flat or uniform though, and thus different wavelengths may receive different gain amplitudes, which may require gain equalization (see discussion of Gain Equalization Filters (GEF) below).

FIG. 3Bshows the gain curve of an erbium doped power amplifier receiving 120 mW of pump power. The gain curve is also relatively flat in the range of approximately 1535 nm to 1560 nm. Again, optical telecommunication signals are often within this wavelength range. Pre-amplifiers may require gain equalization for reasons similar to those stated above for power amplifiers.

The results in this example show greater than 25 dB gain for the pre-amplifier (input signal −40 dBm/channel) and greater than 16 dB gain for the power amplifier (input signal −15 dBm/channel).

FIG. 4shows a first embodiment of the present invention.FIG. 4provides a more detailed view of an embodiment building on the conceptualized view ofFIG. 2.FIG. 4shows two fiber optic lines (straight line shown between Input1to Output1, and straight line shown between Input2to Output2, respectively). Each line has similar accompanying components, including one or more of each of the following: optical detectors, isolators, couplers, doped fiber amplifiers and gain equalization filters (GEF). A single pump source430plus a splitter434allows the single pump to pump two or more doped fiber amplifiers444,466to amplify the signals passing through the two fiber optic lines. It should be noted that the two lines, while shown as essentially identical inFIG. 4, do not have to be identical and may include different components.

Starting with Line1(i.e., represented by the straight line between Input1and Output1), fiber optic cable402carries one or more optical signals from Input1. These may be signals being transmitted from a transmitter or coming in to a receiver. A portion of the incoming signal may be split or tapped into fiber406. This splitting may be done by a beam splitter, thin film or other optical splitting mechanism. Fiber406is coupled to an optical detector410. This detector410is used to detect signals transmitted from Input1into the amplifier system via fiber408through to Output1. Upon detecting one or more signals, detector410provides an electrical signal to an electronics subsystem422. The electronics422includes logic to perform various operations on detected signals from one or more detectors in the system (see later discussion of various electronics configurations). The electronics422may be responsive to an external control426to control various electronic operations manually or automatically. One electronic operation is control of the electrical power supplied to the pump source430. The electrical power supplied to the pump source430may be varied to control the optical power emitted by the pump source430. The system may also operate absent such electronics422or external control426, by setting the power supplied to the pump source430to a fixed amount.

Line2(i.e., represented by the bold straight line between Input2and Output2) may include a similar detector configuration as that described above. Line2receives one or more signals at Input2via fiber optic cable404. A portion of this signal is split or tapped (via various beam splitting techniques) into fiber412to be transmitted to detector416. If a signal is present in fiber412, detector416detects it and signals such detection to the electronics422. The electronics422and logic therein use the detection signals from one or more detectors410,416(and/or others) separately or in combination to control the electrical power applied to the pump source430.

The portions of the input signals not diverted into fibers406and412continue into the amplification system. Specifically, the transmitted portion of Line1continues on fiber408, and the transmitted portion of Line2continues on fiber414. Each line preferably passes through an optical isolator in order to limit signals from being reflected back into the inputs. Specifically, isolator442blocks any light reflected from components downstream in the system of Line1. Similarly, isolator460blocks reflections from downstream components in Line2. Such isolators may include Faraday rotators, thin films or various polarization systems, among other optical isolators.

A pump beam is inserted into the same fiber444/442that carries the optical signal transmitted through isolator442. The pump beam is carried via fiber optic cable436from a splitter434. The single pump source430has its emitted light split by the splitter434into one or more portions of pump light. As discussed above, the resulting portions of the pump light may be of equal amplitude or unequal amplitude. In a preferred embodiment, the splitter434is configured to deliver about 75% of the pump light to coupler440via optical fiber436, and about 25% of the pump light to coupled458via line438. The splitter434may include thin films, diffraction gratings, polarizers or other light beam splitting mechanisms.

In a preferred embodiment, the couplers440and458are wave division multiplexing couplers. Each coupler may include thin films or fused-fibers, and is used for coupling two or more beams of light from several optic fibers. In this way the optical transmission signal from fiber444and the pump light from fiber436are combined into fiber442. Similarly, pump light is combined by coupler458with a signal transmitted through isolator460, down Line2and into fiber462. Usually the transmission signal will have a higher wavelength than that of the pump light from the pump source. More generally, the pump source is selected to have a wavelength that enables amplification of the transmission signal by the doped fiber amplifier. For example, a typical system incorporating the present invention uses a transmission signal in a range of about 1530 to 1560 nm, and pump light having a wavelength of about 980 nanometers.

The pump light coupled into lines1and2travels with the transmitted signals. The pump light then pumps the doped fiber amplifiers (444and466, respectively), thereby amplifying the respective transmission signals also traveling through the doped fiber amplifiers as discussed above in relation toFIGS. 1–3.

An isolator (448for Line1;470for Line2, respectively) may be included in a transmission line after the doped fiber amplifier. Such an isolator may be included in order to limit or block any reflected light from further downstream in the system from returning into the doped fiber amplifier. Such reflections may be detrimental to the operation of a doped fiber amplifier. For instance, reflected light allowed to bounce back and forth through the doped fiber amplifier may in effect cause the doped fiber to become a laser. Additionally, among other negative features, this may cause unwanted saturation of the doped fiber with a consequent reduction in the intended amplification of a transmission passing through the doped fiber amplifier.

In some embodiments, a gain equalization filter (GEF)452,474is included in one or both of the optical fiber lines (line1and line2, respectively) after the doped fiber amplifier. GEFs452,474may include thin film or diffraction grating mechanisms, among others, and are usually unidirectional. GEFs are used to equalize or flatten the gain curve over a predefined range of wavelengths. Equalizing optical power gain reduces differences in power gain across wavelengths that may occur in an optical amplifier without GEF. As shown by way of example inFIGS. 3A and 3Babove, the gain curve (as a function of wavelength) of a doped fiber pre- or power-amplifier is often not uniform. In order to get a more uniform gain across the intended transmission wavelengths, a GEF is employed to flatten the gain curve by attenuating the wavelenghts having the highest gain. This is especially important for use with pre-amplifiers in order to help improve operation of the corresponding receivers.

After passage through a doped fiber amplifier (444and446), with or without isolators (448and470, respectively) and GEFs (452and474, respectively), the now-amplified transmission signals continue through their corresponding lines (Line1and Line2, respectively) through Output1and Output2. In the embodiment shown inFIG. 4, transmission signals from Line1exit a GEF452via fiber cable454. A portion of the transmission signal may be tapped or split from the direct line (i.e. continuing from fiber454through fiber456to Output1). The split or tap may include a thin film or diffraction grating to divide portions of the transmission signal. A portion of the transmission signal may then be transmitted via fiber cable480to one or more detectors482. Similarly for Line2, a portion of the transmission signal exiting GEF474may be split into fiber486to be transmitted to one or more detectors486. These secondary detectors may be coupled to electronics422via electrical connection484.

The electronics422includes circuitry for performing predefined operations on detected signals from the detectors in the system. For example, electronics422are used in conjunction with detector482to measure the transmission signal level in Output1, or more simply to determine whether the signal doped fiber amplifier444on Line1is equal to or exceeds a predefined threshold level. The electronics422is preferably configured to control the amount of power supplied to pump source430in accordance with the measurements made by the signal detectors and electronics422. In some embodiments, electronics422measures the gain across each amplifier by comparing the measured input signal at detector410or416to the measured output signal at detector482or488. Electronics422may be configured to adjust the power to the pump430if the measured gain is lower than a first predefined threshold level or above a second predefined level. In other embodiments, electronics422is configured to perform additional control and monitoring functions.

In embodiments of the system depicted inFIG. 4, the pump source430may be employed with or without cooling. In other words, cooled or uncooled pumps may be used as the pump source430. The temperature of the pump source, especially for pump lasers, is important, as it may impact the operation of the pump source over time. It may also impact other nearby components. In either cooled or uncooled systems, the power supplied to the pump source430may be altered to control the optical pump power out of the pump source and the resultant amplification of the doped fiber amplifiers being pumped. For cooled pumps, additional electronics422may be used in order to control the cooling of the pump source—to hold the pump temperature at a set temperature or within a set range of temperatures.

FIG. 5shows a second embodiment of the present invention. This embodiment includes a more integrated approach to the system depicted inFIG. 4and described above. This second embodiment also uses a single pump source526to optically pump multiple doped fiber amplifiers for amplifying transmitted or received signals over one or more wavelengths. Only those apsects of the second embodiment that differ from the embodiment shown inFIG. 4will be described. Further, other embodiments may use some aspects of the embodiment ofFIG. 4and other aspects of the embodiment ofFIG. 5.

The system ofFIG. 5differs from that ofFIG. 4in its use of a number of dual-line components to reduce the number of components in the system and to reduce the size of the amplifier system. The dual-line components include dual-line optical isolators540,552, and a single, dual-line GEF564. These dual-line components have dual input pigtail connections and dual output pigtail connections. The two paths through these components may share optical sub-components. The embodiment shown inFIG. 5also differs from the first embodiment in its use of integrated detectors (514and578), each of which receives two input signals.

The dual-line GEF564may include thin film or diffraction grating mechanisms, among others. In this embodiment, GEF564receives signals from the outputs of both amplifiers (548and556). Similar to the first embodiment, separate couplers (536and538) and separate doped fiber portions (548and556) are be employed to maintain different pumping and amplification levels for the two Lines.

FIG. 6shows a third embodiment of the present invention. This embodiment shows a more integrated approach to the systems described above. A multiport isolator array648is the primary addition to the second embodiment shown inFIG. 5. A 4-way multiport isolator array648is employed to further reduce the number of components and the size of the amplifier system. The remainder of the third embodiment is essentially similar to that described above forFIG. 5. The multiple port optical isolator includes four input optical pigtails, four output optical pigtails and at least one passive optical isolator between the input optical pigtails and output optical pigtails. Two of the four input optical pigtails are coupled to the optical fiber outputs of the first and second optical power amplifiers, and two of the four output optical pigtails are coupled to the optical fiber inputs of the first and second optical power amplifiers.

While the present invention has been described with reference to a few specific embodiments, the description is illustrative of the invention and is not to be construed as limiting the invention. Various modifications may occur to those skilled in the art without departing from the true spirit and scope of the invention as defined by the appended claims. For example, the embodiments described above may be used with multiple wavelength or WDM systems; with pump lasers or other pump light sources; with cooled or uncooled pump lasers; with various combinations of power amplifiers and pre-amplifiers; with multiple transmitters and/or multiple receivers; integrated with a optoelectronic transceiver or as a separate component therefrom; with optical isolators or without; with or without gain equalization filters; with or without logic functions in electronics; and with or without external controls for electronics.