Ultra-wide bandwidth optical amplifier

A broadband optical amplifier is disclosed. The amplifier includes an input having a plurality of optical wavelengths, including optical wavelengths between 1610 and 1620 nanometers, and a first optical splitter optically connected to the input. The first optical splitter splits the input into a first band signal portion, a second band signal portion, and a third band signal portion. An amplifying portion is optically disposed along each of the first, second, and third band signal portions optically downstream from the first optical splitter. A first optical combiner is optically connected to the first, second, and third band signal portions to form an output. A method of amplifying a broadband optical signal is also disclosed.

FIELD OF THE INVENTION

The present application relates to optical amplifiers that amplify optical signals over a broad bandwidth.

BACKGROUND OF THE INVENTION

Conventional erbium doped fiber amplifiers (EDFA) have been extensively used in optical telecommunications as means to amplify weak optical signals in the third telecommunication window (near 1550 nm) between telecommunication links. Much work has been done on the design of these amplifiers to provide efficient performance, such as high optical gain and low noise figure. However, with the recent enormous growth of data traffic in telecommunications, owing to the Internet, intranets, and e-commerce, new optical transmission bandwidths are required to provide increased transmission capacity in dense wavelength division multiplexing (DWDM) systems.

There are a few solutions to this demand. One proposed solution is to utilize new materials compositions as a host for the fiber gain medium (instead of silica), such as telluride, which may provide broader amplification bandwidth (up to 80 nm). However, the non-uniform gain shape and poor mechanical properties of telluride glass make these amplifiers difficult to implement in telecommunication systems. Also, Raman amplifiers can be considered as an alternative solution to high bandwidth demand, since these amplifiers are capable of providing flexible amplification wavelength with a broad bandwidth. However, these amplifiers place restrictions on optical system architectures because of their required designs for efficient performance, such as long fiber length (>5 km), high pump power (>500 mW) and co-pumping configurations. On the other hand, relatively long erbium doped fibers (EDFs) may also provide amplification in the long wavelength range (1565-1620 nm) when they are used with high power pump sources. This range is commonly called “L band”, which can be further subdivided in a 1565-1605 nm range and a 1605 nm and greater range, which is referred to as “ultra-L band”. The conventional range, currently being used for most commercial applications, also known as “C band”, is in the wavelength range between 1520-1565 nm.

With the need to increase transmission capacity to accommodate the rapid growth of optical telecommunications, the industry is looking to L band and possibly ultra-L band as solutions to this need. However, in order to amplify C band, L band, and ultra-L band signals, multiple amplifiers are currently required. It would be beneficial to provide a single optical amplifier that amplifies a light signal over a large bandwidth encompassing C band, L band, and ultra-L band light.

BRIEF SUMMARY OF THE INVENTION

Briefly, the present invention provides a broadband optical amplifier comprising an input having a plurality of optical wavelengths, including optical wavelengths between 1610 and 1620 nanometers and a first optical splitter optically connected to the input. The first optical splitter splits the input into a first band signal portion, a second band signal portion, and a third band signal portion. The amplifier further comprises an amplifying portion optically disposed along each of the first, second, and third band signal portions optically downstream from the first optical splitter and a first optical combiner optically connected to the first, second, and third band signal portions to form an output.

Additionally, the present invention provides a method of amplifying a broadband signal comprising providing a broadband optical signal having a plurality of optical wavelengths, including optical wavelengths between 1610 and 1620 nanometers; splitting the broadband optical signal into first, second, and third optical signals; separately amplifying each of the first, second, and third optical signals; and combining the first, second, and third optical signals into an amplified broadband optical signal.

DETAILED DESCRIPTION OF THE INVENTION

In the drawings, like numerals indicate like elements throughout. Reference is made to U.S. patent application Ser. No. 09/888,880 and U.S. patent application Ser. No. 09/888,881, both of which were filed on Jun. 25, 2001, are owned by the assignee of the present invention, and are incorporated herein by reference in their entireties. As used herein, when two or more elements are “optically connected”, light can be transmitted between the elements. Further, a second element is “optically downstream” of a first element when a light being transmitted through the first and second elements encounters the first element prior to encountering the second element. Also, “backward” is defined to mean a direction optically from a receiver toward a transmission source and “forward” is defined to mean a direction optically from the transmission source toward the receiver.

Referring toFIG. 1, an ultra-wide bandwidth optical amplifier100is disclosed. The optical amplifier100amplifies a broadband signal light λSthat extends across at least 100 nanometers, and preferably approximately 110 nanometers, preferably from between approximately 1510 to 1620 nanometers, although those skilled in the art will recognize that the band can be greater than 110 nanometers wide and can begin at less than 1510 nanometers and/or extend to greater than 1620 nanometers. Preferably, the signal light λSincludes C band signal light λC(between approximately 1510-1565 nanometers), L band signal light λL(between approximately 1565 and 1605 nanometers) and ultra-L band signal light λLL(between approximately 1605 and 1620 nanometers).

The amplifier100includes an input102where the signal light λSenters the amplifier100from a transmission source (not shown). A first optical isolator110is optically connected to the input102optically downstream from the transmission source. The first optical isolator110prevents optical noise from traveling backwards from the amplifier100toward the transmission source. A first optical splitter120, preferably a wavelength division multiplexer (WDM) is optically connected to the first optical isolator110.

Preferably, the first optical splitter120splits the input102into two lines, a first signal line122and a second signal line124. The first signal line122is optically connected to a first amplifying portion130. Preferably, the first amplifying portion130is a rare earth doped medium, such as a fiber or a planar waveguide. Also preferably, the first amplifying portion130is approximately fifteen meters long. A first optical multiplexer132optically connects an amplifying power source, preferably a first pump laser134, to the first amplifying portion130via a pump line136. Preferably, the first pump laser134is a 980 nanometer pump and has a power of approximately 90 mW, although those skilled in the art will recognize that the first pump laser134can be other than 980 nanometers and have a power of other than 90 mW.

The second signal line124is optically connected to a second WDM140, which splits the second signal line124into a third signal line142and a fourth signal line144. The third signal line142is optically connected to a second amplifying portion150. Preferably, the second amplifying portion150is a rare earth doped medium, such as a fiber or a planar waveguide. Also preferably, the second amplifying portion150is approximately sixty meters long. A second optical multiplexer152optically connects an amplifying power source, preferably a second pump laser154, to the second amplifying portion150via a pump line156. Preferably, the second pump laser154is a 980 nanometer pump and has a power of approximately 180 mW, although those skilled in the art will recognize that the second pump laser154can be other than 980 nanometers and have a power of other than 180 mW. Also preferably, a reflector146is optically disposed in the third signal line142. The reflector146can be a fiber grating or other reflector designed to reflect predetermined wavelengths. In a preferred embodiment, the reflector146reflects light having a wavelength of approximately 1560 nanometers, although those skilled in the art will recognize that the reflector146can reflect light having other wavelengths.

The fourth signal line144is optically connected to a third amplifying portion160. Preferably, the third amplifying portion160is a rare earth doped medium, such as a fiber or a planar waveguide. Also preferably, the third amplifying portion160is approximately one hundred and twenty meters long. A third optical multiplexer162optically connects an amplifying power source, preferably a third pump laser164, to the third amplifying portion160via a pump line166. Preferably, the third pump laser164is a 980 nanometer pump and has a power of approximately 200 mW, although those skilled in the art will recognize that the third pump laser164can be other than 980 nanometers and have a power of other than 200 mW. Also preferably, a reflector148is optically disposed in the signal line144. The reflector148can be a fiber grating or other reflector designed to reflect predetermined wavelengths. In a preferred embodiment, the reflector148reflects light having a wavelength of approximately 1560 nanometers, although those skilled in the art will recognize that the reflector148can reflect light having other wavelengths.

Also, preferably, the third amplifying portion160can be comprised of a plurality of amplifying sections160a,160b, with at least one reflector168optically disposed between the amplifying sections160a,160b. An auxiliary power source in the form of a fourth pump laser174is optically connected to a fourth optical multiplexer172optically downstream of the third amplifying portion160via a pump line176. Preferably, fourth pump laser174is a 980 nanometer pump and has a power of approximately 200 mW, and is disposed to provide counter-pumping for the amplifying portion160, although those skilled in the art will recognize that the fourth pump laser174can be other than 980 nanometers and have a power of other than 200 mW. Also preferably, a reflector170is optically disposed in the signal line144optically downstream of the third amplifying portion160. The reflector170can be a fiber grating or other reflector designed to reflect predetermined wavelengths. In a preferred embodiment, the reflector170reflects light having a wavelength of approximately 1555 nanometers, although those skilled in the art will recognize that the reflector148can reflect light having other wavelengths.

A first optical combiner180is disposed optically downstream of the second and third amplifying portions150,160and combines the third signal line142and the fourth signal line144to form an intermediate signal line182. A second optical combiner190is disposed optically downstream from the amplifying portion130and the first optical combiner180and combines the intermediate signal line182and the first signal line122to form the amplifier output192, disposed optically downstream of the second optical combiner190. A second optical isolator194is optically disposed along the amplifier output192. The second optical isolator192prevents optical noise from traveling backwards to the amplifier100from a receiver (not shown) disposed optically downstream of the amplifier100.

A second embodiment optical amplifier200of the present invention is shown schematically in FIG.2. The second embodiment is generally identical to the first embodiment, with the exception that the first optical combiner180is disposed optically downstream of the first and second amplifying portions130,150and combines the first signal line122and the third signal line142to form an intermediate signal line182′. The second optical combiner190is disposed optically downstream from the third amplifying portion160and the first optical combiner180and combines the intermediate signal line182′ and the fourth signal line144to form the amplifier output192, disposed optically downstream of the second optical combiner190.

Operation of the first embodiment amplifier100will now be described. The broadband signal light λS, having a spectrum of approximately between 1520 and 1620 nm, is provided to the input102from the transmission source (not shown). The signal light λStravels through the optical isolator110and to the first optical splitter120. The first optical splitter120splits the signal light λSinto the C band signal light λCand an intermediate band signal light λI. The C band signal light λCis transmitted along the signal line122to the first optical multiplexer132, where first pump light λP1, generated by the first pump laser134and transmitted along the pump line136, joins the C band signal light λC.

The combined C band signal light λCand first pump light λP1are transmitted along the first amplifying portion130where the C band signal light λCis amplified. As light amplification by means of pumping a signal in a rare earth doped medium is well known to those skilled in the art, a detailed description of the physics of light amplification by this method will be omitted.

The intermediate band signal light λIis transmitted along the signal line124to the second optical splitter140. The second optical splitter140splits the intermediate band signal light λIinto the L band signal light λLand the ultra-L band signal light λLL. The L band signal light λLis transmitted along the signal line142to the second optical multiplexer152, where second pump light λP2, generated by the second pump laser154and transmitted along the pump line156, joins the L band signal light λL.

The combined L band signal light λLand second pump light λP2are transmitted along the second amplifying portion150where the L band signal light λLis amplified. Backward ASE, generated during amplification of the L band signal light λL, is transmitted from the second amplifying portion150along the signal line142optically toward the second optical splitter140, but is reflected by the reflector146back into the second amplifying portion150. The ASE acts as a seed to supplement the pump power of the second pump laser154, increasing the amplification of the L band signal light λL.

After being split by the second optical splitter140, the ultra-L band signal light λLLis transmitted along the signal line144to the third optical multiplexer162, where third pump light λP3, generated by the third pump laser164and transmitted along the pump line166, joins the ultra-L band signal light λLL.

The combined ultra-L band signal light λLLand third pump light λP3are transmitted along the third amplifying portion160where the ultra-L band signal light λLLis amplified. Backward ASE, generated during amplification of the ultra-L band signal light λLL, is transmitted from the third amplifying portion160along the signal line142optically toward the second optical splitter140, but is reflected by the reflector148back into the third amplifying portion160. The ASE acts as a seed to supplement the pump power of the third pump laser164, increasing the amplification of the ultra-L band signal light λLL.

The fourth pump laser174is connected to the fourth signal line144by the pump line176and the fourth optical multiplexer172, which is optically disposed in the fourth signal line144optically downstream of the amplifying portion160. The fourth pump laser174provides a fourth pump light λP4to counter pump the fourth amplifying portion160. Forward ASE, generated during amplification of the ultra-L band signal light λLLby the counter-pumping, is transmitted from the third amplifying portion160along the signal line144optically toward the first optical combiner180, but is reflected by the reflector170back into the third amplifying portion160. The ASE acts as a seed to supplement the pump power of the fourth pump laser174, increasing the amplification of the ultra-L band signal light λLL. The grating168suppresses further ASE in the signal line144.

After amplification, the L band signal light λLand the ultra-L band signal light λLLare combined by the first optical combiner180to form amplified intermediate signal light λI, which is transmitted along the intermediate signal line182. The intermediate signal light λIand the C band signal light λCare then combined by the second combiner190to reform the signal light λS, now amplified, which is transmitted along the amplifier output192, the second optical isolator194, and out of the amplifier100.

Operation of the second embodiment of the amplifier200is similar to the operation of the first embodiment of the amplifier100with the exception that, after amplification, the C band signal light λCis combined with the L band signal light λLat the first optical combiner180to form an intermediate band signal light λI′. The intermediate band signal light λI′is then combined with the ultra-L band signal light λLLat the second optical combiner190to reform the signal light λS, now amplified.

AlthoughFIGS. 1 and 2show separate optical splitters120,140and separate optical combiners180,190, those skilled in the art will recognize that alternative means for splitting the signal light λScan be used. For example, referring toFIG. 2A, an optical amplifier100′ uses an arrayed waveguide grating (AWG)196in the place of the optical splitters120,140and an AWG198in place of the optical combiners180,190. Whereas the optical splitter120shown inFIGS. 1 and 2splits the signal light λSinto the C band signal light λCand the intermediate band signal light λI, and the optical splitter140subsequently splits the intermediate band signal light λI, into the L band signal light λLand the ultra-L band signal light λLL, the AWG196generally simultaneously splits the signal light λSinto the C band signal light λC, the L band signal light λL, and the ultra-L band signal light λLL, eliminating the need for the intermediate band signal light λI. Similarly, the AWG198generally simultaneously combines the C band signal light λC, the L band signal light λL, and the ultra-L band signal light λLL, also eliminating the need for the intermediate band signal light λI.

A third embodiment optical amplifier300of the present invention is shown schematically in FIG.3. The amplifier300is similar to the amplifier100, with the exception that, instead of optical splitters120,140and optical combiners180,190, optical circulators are used in conjunction with reflectors.

A first optical circulator320replaces the first optical splitter120and is disposed optically downstream of the first optical isolator110. The first optical circulator320has an input320aoptically connected to the input102, an input/output320boptically connected to the signal line124, and an output320coptically connected to the signal line122. A plurality of reflectors322,324,326are disposed optically downstream of the input/output320b.

A second optical circulator340replaces the second optical splitter130and is disposed optically downstream of the input/output320bof the first optical isolator320. The second optical circulator340has an input320aoptically connected to the input/output320b, an input/output340boptically connected to the third amplifying portion160, and an output340coptically connected to the amplifying portion150. A plurality of reflectors342,344,346are disposed optically downstream of the input/output340b.

A third optical circulator380replaces the first optical combiner180and is disposed optically downstream of the second and third amplifying portions150,160. The third optical circulator380has an input380aoptically connected to the second amplifying portion150, an input/output380boptically connected to the third amplifying portion160, and an output portion380coptically connected to the intermediate signal line182. A plurality of reflectors382,384,386are disposed optically downstream of the input/output380b.

A fourth optical circulator390replaces the second optical combiner190and is disposed optically downstream of the first amplifying portion130and the third optical circulator380. The fourth optical circulator390has an input390aoptically connected to the first amplifying portion130, an input/output390boptically connected to the intermediate signal line182, and an output portion390coptically connected to the output192. A plurality of reflectors392,394,396are disposed optically downstream of the input/output390b.

A fourth embodiment optical amplifier400of the present invention is shown schematically in FIG.4. The fourth embodiment is generally identical to the third embodiment, with the exception that the third optical circulator380is disposed optically downstream of the first and second amplifying portions130,150and combines the first signal line122and the third signal line142to form an intermediate signal line182′. The fourth optical circulator390is disposed optically downstream from the third amplifying portion160and the third optical circulator380and combines the intermediate signal line182′ and the fourth signal line144.

Operation of the third and fourth embodiments300,400are as follows. The broadband signal light λSis provided to the amplifier300from the transmission source. The signal light λSenters the input102and is transmitted through the first optical isolator110. The signal light λSenters the first optical circulator320at the input320aand exits at the input/output320b. The signal light λSencounters the reflectors322,324,326, where predetermined wavelengths, particularly C band signal light λC, are reflected by the reflectors322,324,326back into the first optical circulator320at the input/output320b, through the first optical circulator320and out through the output320c. The C band signal light λCis transmitted along the signal line122where the C band signal light λCis amplified as described above.

The remaining intermediate band signal light λInot reflected by the reflectors322,324,326, particularly L band signal light λLand ultra-L band signal light λLL, is transmitted along the signal line124to the second optical circulator340. The intermediate band signal light λIenters the second optical circulator340at the input340aand exits at the input/output340b. The intermediate band signal light λIencounters the reflectors342,344,346, where predetermined wavelengths, particularly L band signal light λL, are reflected by the reflectors342,344,346back into the second optical circulator340at the input/output340b, through the second optical circulator340and out through the output340c. The L band signal light λLis transmitted along the signal line142where the L band signal light λLis amplified as described above. The ultra-L band signal light λLLis transmitted along the signal line144where the ultra-L band signal light λLLis amplified as described above.

In the amplifier300, the L band signal light λL, now amplified, enters the third optical circulator380at the input380aand exits at the input/output380b. The L band signal light λLencounters the reflectors382,384,386, where the L band signal light λLis reflected by the reflectors382,384,386back into the third optical circulator380at the input/output380b, through the third optical circulator380and out through the output380c. The ultra-L band signal light λLL, now amplified, enters the third optical circulator at the input/output380band exits through the output380c, combined with the L band signal light λLas the amplified intermediate band signal light λI.

The amplified intermediate band signal light λIenters the fourth optical circulator390at the input/output390band exits through the output390c. The C band signal light λC, now amplified, enters the fourth optical circulator390at the input390aand exits at the input/output390b. The C band signal light λCencounters the reflectors392,394,396, where the C band signal light λCis reflected by the reflectors392,394,396back into the fourth optical circulator390at the input/output390b, through the fourth optical circulator390and out through the output390c, where the C band signal light λCcombines with the intermediate band signal light λIas the amplified signal light λS.

In the amplifier400, shown inFIG. 4, the C band signal light λC, having been amplified in the manner described above, enters the third optical circulator380at the input380aand exits at the input/output380b. The C band signal light λCencounters the reflectors382,384,386, where the C band signal light λCis reflected by the reflectors382,384,386back into the third optical circulator380at the input/output380b, through the third optical circulator380and out through the output380c. The L band signal light λL, having been amplified in the manner described above, enters the third optical circulator at the input/output380band exits through the output380c, combined with the C band signal light λCas the amplified intermediate band signal light λI′.

The ultra-L band light λLL, having been amplified in the manner described above, enters the fourth optical circulator at the input/output390band exits through the output390c. The amplified intermediate band signal light λI′enters the fourth optical circulator390at the input390aand exits at the input/output390b. The intermediate band signal light λI′encounters the reflectors392,394,396, where the intermediate band signal light λI′is reflected by the reflectors392,394,396back into the fourth optical circulator390at the input/output390b, through the fourth optical circulator390and out through the output390c, where the intermediate band signal light λI′combines with the ultra-L band signal light λLLas the amplified signal light λS.

Those skilled in the art will recognize that, although three reflectors are shown for each circulator320,340,380,390, more or less than three reflectors can be used.

FIG. 5shows a graph of amplification of a broadband signal light λShaving wavelength between 1520 nm and 1620 nm using the amplifier100according to the first embodiment of the present invention having a fifteen meter long erbium doped fiber as the first amplifier portion130, a sixty-two meter long erbium doped fiber as the second amplifier portion150, a forty meter long erbium doped fiber as the amplifier section160a, and an eighty meter long erbium doped fiber as the amplifier section160b.FIG. 5shows that significant gains of over 25 dB, with noise figures of less than 7 dB, can be achieved with the present invention.

AlthoughFIGS. 1-4illustrate splitting a signal light λSinto three separate bands, λC, λL, and λLL, with each band being optically amplified separately, those skilled in the art will recognize that the signal light λScan be split into more than three separate bands, with each band being optically amplified separately. For example, the signal light λShaving wavelengths between 1510 and 1620 nanometers can be split into four separate bands having wavelengths of, 1510-1560 nanometers, 1560-1580 nanometers, 1580-1605 nanometers, and 1605-1620 nanometers, without departing from the spirit and scope of the present invention.