WO2003044914A1

WO2003044914A1 - Multi-wavelength raman fiber laser

Info

Publication number: WO2003044914A1
Application number: PCT/US2002/036871
Authority: WO
Inventors: Andrey A. Demidov; Andrei N. Starodoumov; Hong Po; Xiaojun Li
Original assignee: Optical Power Systems Inc.
Priority date: 2001-11-16
Filing date: 2002-11-18
Publication date: 2003-05-30
Also published as: AU2002366156A1; WO2003044914A8

Abstract

Raman fiber lasers (100) and systems using Raman fiber lasers are disclosed.

Description

MULTI-WAVELENGTH RAMAN FIBER LASER

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Provisional Patent Application No. 60/333,813, entitled "MULTIWAVELENGTH RAMAN FIBER LASER," filed 5 November 16, 2001.

BACKGROUND

This invention relates to optical fibers and systems containing optical fibers. Certain optical fibers can be used as fiber amplifiers or fiber lasers. Fiber amplifiers are typically used to amplify an input signal. Often, the input o signal and a pump signal are combined and passed through the fiber amplifier to amplify the signal at the input wavelength. The amplified signal at the input wavelength can then be isolated from the signal at undesired wavelengths.

Raman fiber lasers can be used, for example, as energy sources. In general, Raman fiber lasers include a pump source coupled to a fiber, such as an optical fiber, 5 having a gain medium with a Raman active material. Energy emitted from the pump source at a certain wavelength λ_p, commonly referred to as the pump energy, is coupled into the fiber. As the pump energy interacts with the Raman active material in the gain medium of the fiber, one or more Raman Stokes transitions can occur within the fiber, resulting in the formation of energy within the fiber at wavelengths corresponding to 0 the Raman Stokes shifts that occur (e.g., λ_si, λg₂, λ_s3, λg₄, etc.).

The Raman active material in the gain medium of a Raman fiber laser may have a broad Raman gain spectrum. Usually, conversion efficiency varies for different frequencies within the Raman gain spectrum and many Raman active materials exhibit a peak in their gain spectrum, corresponding to the frequency with highest conversion 5 efficiency. Additionally, the gain spectrum for different Raman active materials may be substantially different, partially overlapping, or of different conversion efficiency. Typically, a Raman fiber laser is designed so that the energy formed at one or more Raman Stokes shifts is substantially confined within the fiber. This can enhance the formation of energy within the fiber at one or more higher order Raman Stokes 0 shifts. Often, the fiber is also designed so that at least a portion of the energy at wavelengths corresponding to predetermined, higher order Raman Stokes shifts (e.g., λ_sx where x is equal to or greater than one) is allowed to exit the fiber.

Raman fiber lasers are used, among other things, for amplification in optical fiber communication systems. Signals transmitted through optical fibers are attenuated by absorption and scattering. As a result, optical fiber communication systems require periodic amplification. One approach that has been used for such amplification is Raman amplification, in which light traveling within an optical fiber is amplified by lower wavelength pump light traveling in the same fiber. Raman fiber lasers have been used as a source of pump light for such Raman amplification systems. In order to achieve the necessary flat gain profile in such amplification systems, however, it is desirable for the pump source to produce light at several different wavelengths. Commonly, such multi-wavelength pump sources have been created by multiplexing several single wavelength Raman fiber lasers or semiconductor laser diodes together. This approach can be costly and complex.

SUMMARY

In general, in a first aspect, the invention features a Raman fiber laser system. The Raman fiber laser system includes an energy source configured to emit energy at a wavelength λ_p and an optical fiber containing a gain medium having a Raman active material, the optical fiber being configured so that energy emitted by the energy source at λ_p can be coupled into the optical fiber. The Raman fiber laser system also includes a plurality of reflectors and output couplers disposed in the optical fiber, at least some of the plurality of reflectors and output couplers forming optical cavities in the optical fiber, each optical cavity having a resonance wavelength, the resonance wavelength of each optical cavity being different than the resonance wavelength of any other cavity, and at least two of the optical cavities do not overlap with each other in the optical fiber. Furthermore, the plurality of reflectors and output couplers are configured so that when the optical fiber receives energy at λ_p, the optical fiber generates energy at a first wavelength, λi, corresponding to the resonance wavelength of a first of the optical cavities in the optical fiber, and optical fiber generates output energy at three different wavelengths, λ₂, λ₃, and λ₄, wherein λ₂, λ₃, and λ₄ correspond to resonant wavelengths of other optical cavities in the optical fiber Embodiments of the invention may include one or more of the following features.

The optical fiber can include a first pair of overlapping optical cavities and a second pair of overlapping optical cavities. In some embodiments, the first optical cavity pair does not overlap with the second optical cavity pair. λ₂ can correspond to a resonant wavelength of a second optical cavity in the optical fiber, and the second optical cavity can overlap with the first optical cavity. The first optical cavity can be nested within the second optical cavity. Alternatively, the second optical cavity can be nested within the first optical cavity. λ₃ can correspond to a resonant wavelength of a third optical cavity in the optical fiber, and λ can correspond to a resonant wavelength of a fourth optical cavity in the optical fiber. The third optical cavity can overlap with the fourth optical cavity (e.g., the third optical cavity can be nested within the fourth optical cavity, or the fourth optical cavity can be nested within the third optical cavity). The first and second optical cavities can be in a portion of the optical fiber different from the third and fourth optical cavities.

The plurality of reflectors can include a pump wavelength reflector configured to reflect substantially all energy impinging thereon at λ_p. The pump wavelength reflector can be disposed in the optical fiber between the first and second optical cavities and the third and fourth optical cavities. Alternatively, the third and fourth optical cavities can be disposed in the optical fiber between the pump wavelength reflector and the first and second optical cavities.

The plurality of reflectors can include a first wavelength reflector configured to reflect substantially all energy impinging thereon at λ_\. The third and fourth optical cavities can be disposed in the optical fiber between the first and second optical cavity and the first wavelength reflector.

In some embodiments, at least one of the output couplers is a variable output coupler.

The following relationships can exist between the resonant wavelengths of different cavities in the optical fiber and between the resonant wavelengths and the pump wavelength: λ₂ > λ_\ and/or λ₂ > λ₃ and/or λ₂ > λ₄ and/or λ_ > λ_p. Alternatively, or additionally, the pump and/or resonant wavelengths can be within the following ranges: 1200 nm < λ_p < 1300 nm (e.g., λ_p = 1277 nm); and or 1300 nm < λ. ≤ 1400 nm (e.g., λi = 1353 nm); and/or 1400 nm < λ- < 1500 nm (e.g., λ₂ = 1463 nm); and/or 1400 nm < ^ < 1500 nm (e.g., λ₃ = 1425 nm); and/or 1400 nm < λ₄ < 1500 nm (λ₄ = 1454 nm). hi some embodiments, λ^ = λ_p ^'x - λ^' _p ^x , where {c/λ_rp\) is a frequency within the

Raman gain spectrum of the Raman active material contained in the gain medium and c is the speed of light. Alternatively, or additionally λ~^l =

, where {clλ_rU ) is a frequency within the Raman gain spectrum of the Raman active material contained in the gain medium and c is the speed of light, hi some embodiments,

, where {c/λ_m) is a frequency within the Raman gain spectrum of the Raman active material contained in the gain medium and c is the speed of light. In some embodiments, λ^¹ = λr¹ - λ^₄ , where {c/λ_{rl .}) is a frequency within the Raman gain spectrum of the Raman active material contained in the gain medium and c is the speed of light. Alternatively, or additionally, λ^¹ = λ^¹ - λ^₄ , where {c/λ_r_ _.) is a frequency within the Raman gain spectrum of the Raman active material contained in the gain medium and c is the speed of light.

The Raman fiber laser system can include a power splitter (e.g., a WDM) disposed in the optical fiber and configured to couple power at λ_p out of the optical fiber. The power splitter can be disposed in the optical fiber between the first optical cavity and an input end of the optical fiber, the input end being where energy emitted by the energy source at λ_p is coupled into the optical fiber.

The Raman fiber laser system can include a power splitter (e.g., a WDM) disposed in the optical fiber and configured to couple power at λi out of the optical fiber. The power splitter can be disposed in the optical fiber between the first optical cavity and the third optical cavity.

Embodiments can include a fiber amplifier, wherein the fiber amplifier includes the Raman fiber laser system described in regard to aspects of the invention, and a second optical fiber configured to waveguide an optical signal and to receive energy from the Raman fiber laser system, wherein during operation of the fiber amplifier, energy from the Raman fiber laser amplifies the intensity of the optical signal waveguided by the second optical fiber. Embodiments can also include a communication system, which includes the Raman fiber laser system described in regard to the aspects of the invention, and includes an optical fiber span. The optical fiber span is configured to waveguide an optical signal from a source location to a destination location, and configured to receive energy from the Raman fiber laser system.

In a second aspect, the invention features a Raman fiber laser configured to receive energy at a pump wavelength and to output energy at multiple wavelengths. The Raman fiber laser includes a first stage containing a first gain medium having a first Raman active material and including first and second optical cavities disposed within a first length of optical fiber. The Raman fiber laser also includes a second stage optically coupled to the first stage, the second stage containing a second gain medium having a second Raman active material and including third and fourth optical cavities disposed within a second length of optical fiber. During operation of the Raman fiber laser, the first stage of the optical fiber generates energy at a first wavelength, λi, corresponding to a resonance wavelength of the first optical cavity in the optical fiber, and optical fiber generates output energy at three different wavelengths, λ₂, λ₃, and λ , wherein λ₂, λ₃, and λ₄ correspond to resonant wavelengths of the second, third, and fourth optical cavities respectively.

Embodiments of the Raman fiber laser can include features described in regard to other aspects of the invention. Alternatively, or additionally, embodiments of the invention can include one or more of the following features.

The Raman fiber laser can include a source configured to direct energy at the pump wavelength into the first stage of the Raman fiber laser.

The first optical cavity can overlap with the second optical cavity. For example, the first optical cavity can be nested within the second optical cavity. The third optical cavity can overlap with the fourth optical cavity. For example, the third optical cavity can be nested within the fourth optical cavity.

The first gain medium can the same as or different from the second gain medium. At least the first or second optical cavity can include a variable output coupler.

Alternatively, or additionally, at least the third or fourth optical cavity can include a variable output coupler. In a third aspect, the invention features a Raman fiber laser system, including an energy source configured to emit energy at a wavelength λp, and an optical fiber containing a gain medium having a Raman active material, the optical fiber being configured so that energy emitted by the energy source at λ_p can be coupled into the optical fiber. The Raman fiber laser system also includes a plurality of reflectors and output couplers disposed in the optical fiber, at least some of the plurality of reflectors and output couplers forming optical cavities in the optical fiber, each optical cavity having a resonance wavelength, the resonance wavelength of each optical cavity being different than the resonance wavelength of any other cavity. The plurality of reflectors and output couplers are configured so that when the optical fiber receives energy at λ_p, the optical fiber generates energy at a first wavelength, λ_1} corresponding to the resonance wavelength of a first of the optical cavities in the optical fiber, and optical fiber generates output energy at three different wavelengths, λ₂, λ₃, and λ₄, wherein λ₂, λ₃, and λ₄ correspond to resonant wavelengths of other optical cavities in the optical fiber, and at least two of the optical cavities do not overlap with each other, and a first stage of the optical fiber includes at least two overlapping cavities and a second stage of the optical fiber includes at least two overlapping cavities, wherein the first stage is different from the second stage.

Embodiments of the Raman fiber laser system can include one or more of the features described in regard to other aspects of the invention.

In a further aspect, the invention features a Raman fiber laser including a first stage of an optical fiber, the first stage including first and second optical cavities, and a second stage of the optical fiber, optically coupled to, but distinct from, the first stage. The second stage includes third and fourth optical cavities. Each stage of the optical fiber contains a gain medium having a Raman active material, and the first, second, third, and fourth optical cavities are configured so that when the optical fiber receives energy at a pump wavelength, λ_p, the optical fiber generates energy at a first wavelength, λi, corresponding to the resonance wavelength of the first optical cavity in the optical fiber, and optical fiber generates output energy at three different wavelengths, λ₂, λ₃, and λ₄, wherein λ₂, λ₃, and λ₄ correspond to resonant wavelengths of other second, third, and fourth optical cavities, respectively. Embodiments of the Raman fiber laser can include one or more of the following features.

Embodiments of the Raman fiber laser can also include features described in regard to other aspects of the invention.

Embodiments of the invention may include one or more of the following advantages. Embodiments include Raman fiber lasers with multiple (e.g., more than two) output wavelengths. These Raman fiber lasers can have high total conversion efficiency. They may be easily adapted to provide a desired set of output wavelengths and output power spectrum (e.g., the ratio of power in different output wavelengths). Raman fiber lasers can have more than one output waveguides for coupling output energy out of the Raman fiber laser. This can allow for flexible connections in optical communication systems, and can reduce the amount of output power in each individual waveguide. Raman fiber lasers may be used in fiber amplifiers to provide uniform gain over a range of wavelengths.

Raman fiber lasers can allow easy and efficient control of the intensity of individually generated wavelengths over a large dynamic range.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control.

The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of a Raman fiber laser.

FIG. 2 is a plot of the Raman gain spectrum of an GeO₂ fiber. FIG. 3 is a plot of an attenuation spectrum in an optical fiber.

FIG. 4 is a plot of the power of Stokes waves in a Raman fiber laser as a function of pump power.

FIG. 5 is a plot of the power of Stokes waves in a first portion of a Raman fiber laser as a function of pump power.

FIG. 6 is a plot of the power of Stokes waves in a portion of a Raman fiber laser as a function of reflectivity of a first output coupler.

FIG. 7 is a plot of the power of Stokes waves in a portion of a Raman fiber laser as a function of reflectivity of a second output coupler. FIG. 8 is a plot of the power of Stokes waves in a portion of a Raman fiber laser as a function of reflectivity of a third output coupler.

FIG. 9 is a plot of the power of Stokes waves in a portion of a Raman fiber laser as a function of reflectivity of a fourth output coupler.

FIG. 10 is a plot of the power of output Stokes waves of a Raman fiber laser as a function of reflectivity of the first output coupler.

FIG. 11 is a plot of the power of output Stokes waves of a Raman fiber laser as a function of reflectivity of the second output coupler.

FIG. 12 is a schematic diagram of another Raman fiber laser.

FIG. 13 is a schematic diagram of a communication system including two Raman fiber lasers.

FIG. 14 is a schematic diagram of another communication system including a Raman fiber laser.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION Referring to FIG. 1, a Raman fiber laser system includes Raman fiber laser 100.

Raman fiber laser 100 includes an optical fiber 110 having a gain medium. A pump source 160 is configured so that energy emitted by pump source 160 at a pump wavelength, λ_p, is coupled into optical fiber 110. Optical fiber 110 is configured to waveguide energy within a wavelength range (e.g., between 400 nm and 2500 nm, such as between 900 nm and 1600 nm). Typically, λp will be within this wavelength range. The gain medium of optical fiber 110 contains an active material. Examples of active materials include GeO₂, P₂O₅, SiO₂, B₂O₃, SiO_xF_y, and the like. Energy in optical fiber at the pump wavelength interacts with the gain medium to produce energy at other wavelengths inside optical fiber 110.

In some embodiments, optical fiber 110 can include more than one portion of optical fiber, each portion having similar or different Raman gain medium and/or similar or different geometrical and/or optical parameters. For example, a first portion of Raman fiber laser 100 may have a different gain medium from the gain medium used in a second portion of Raman fiber laser 100.

Optical fiber 110 includes a first reflector 122 (e.g., a fiber Bragg grating) and a first output coupler 124. Reflector 122 is designed to reflect substantially all (e.g., more than about 95 percent, more than 96 percent, more than 98 percent more than 99 percent, such as about 100 percent) waveguided energy impinging thereon at a first wavelength, λ_\. Here, λ_\ is related to λ_p by λ ^x = λ_p ^' - λ_r ^' _pl , where {c/λ_rp\) is a frequency in the Raman gain spectrum of the active material in the gain medium of optical fiber 110, and c is the speed to light. Output coupler 124 is designed to reflect a portion (e.g., less than 100 percent) of waveguided energy impinging thereon at λ_\. Reflector 122 and output coupler 124 define an optical cavity 121.

Optical fiber 110 also includes a second reflector 132 (e.g., a fiber Bragg grating) and a second output coupler 134. Reflector 132 is designed to reflect substantially all (e.g., more than about 95 percent, more than 96 percent, more than 98 percent more than 99 percent, such as about 100 percent) waveguided energy impinging thereon at a second wavelength, L₂. Here, λ is related to λi by

, where {cl λ_rU ) is a frequency in the Raman gain spectrum of the active material in the gain medium of optical fiber 110. Output coupler 134 is designed to reflect a portion (e.g., less than 100 percent) of waveguided energy impinging thereon at λ₂. Reflector 132 and output coupler 134 define a second optical cavity 131. Optical cavity 131 is nested with optical cavity 121. As used herein, a nested cavity refers to an optical cavity that is confined within another optical cavity. In the described embodiment, optical cavity 121 is nested within optical cavity 131. Nested optical cavities use the same Raman gain medium. Together, optical cavity 121 and optical cavity 131 form an optical cavity pair 120. Optical cavity pairl20 form the first stage of Raman fiber laser 100.

In the described embodiment, the reflectors and output couplers in optical cavity pair 120 are ordered 132, 122, 124, 134 from left to right in FIG. 1, and optical cavity 121 is nested within optical cavity 131. In other embodiments, the reflectors and output couplers in optical cavity pair may be order differently. For example, in some embodiments, the reflectors and output couplers maybe ordered 122, 132, 134, 124 or 132, 122, 134, 124 or 122, 132, 124, 134. Optical cavity 131 can be nested within optical cavity 121, or optical cavities 121 and 131 can overlap one another without one optical cavity being nested within the other optical cavity.

Optical fiber 110 further includes a second optical cavity pair 140, which forms the second stage of Raman fiber laser 100. Optical cavity pair 140 includes first optical cavity 141 and a second optical cavity 151 nested with optical cavity 141. Optical cavity 141 is defined by a reflector 142 (e.g., a fiber Bragg grating) and an output coupler 144. Reflector 142 is designed to reflect substantially all (e.g., more than about 95 percent, more than 96 percent, more than 98 percent, more than 99 percent, such as about 100 percent) of waveguided energy impinging thereon at a third wavelength, λ_, while output coupler 144 is designed to reflect a portion (e.g., less than 100 percent) of waveguided energy impinging thereon at this wavelength. Similarly, optical cavity 151 is defined by a reflector 152 (e.g., a fiber Bragg grating) and an output coupler 154. Reflector 152 is designed to reflect substantially all (e.g., more than about 95 percent, more than 96 percent, more than 98 percent, more than 99 percent, such as about 100 percent) of waveguided energy impinging thereon at a fourth wavelength, λ_., while output coupler 154 is designed to reflect a portion (e.g., less than 100 percent) of waveguided energy impinging thereon at this wavelength. Here, λ_ is related to λ_\ by Λ₃ ^"1 = λ~^Λ - X_λ ^x _Z , and λ_. is related to

and to λ_ by λ₄ =

where (clλ ₂) , {c I λ_rU) and (clλ_r34) are frequencies in the Raman gain spectrum of the active material in the gain medium. In preferred embodiments, l₂ is related to λ_ and ₄ by A ¹ =

and

A-¹ = , respectively, (c / λ_r32 ) and (c / λ_r42 ) are frequencies in the Raman gain spectrum of the active material in the gain medium.

In the described embodiment, the reflectors and output couplers in optical cavity pair 140 are ordered 152, 142, 144, 154 from left to right in FIG. 1, and optical cavity 141 is nested within optical cavity 151. In other embodiments, the reflectors and output couplers in optical cavity pair may be order differently. For example, in some embodiments, the reflectors and output couplers maybe order 142, 152, 144, 154 or 152, 142, 154, 144 or 142, 152, 154, 144. Optical cavity 151 can be nested within optical cavity 141, or optical cavities 141 and 151 can overlap one another without one optical cavity being nested within the other optical cavity.

In the described embodiment, optical cavity pair 120 and optical cavity pair 140 are included in a single portion of optical fiber. However, in some embodiments, optical cavity pair 120 can be in a different portion of optical fiber than optical cavity pair 140. For example, optical cavity pair 120 can be in a portion of optical fiber having a first gain medium (e.g., having a first active material), while optical cavity pair 140 can be in a second portion of optical fiber having a second gain medium (e.g., containing a second active material). The first and second portions of optical fiber may be spliced together, or may be coupled together using, e.g., free space coupling optics.

In the described embodiment, optical cavity pair 120 does not overlap with optical cavity pair 140. In general, at least one of the optical cavities in optical cavity pair 120 does not overlap with at least one of the optical cavities in optical cavity pair 140. In some embodiments, optical fiber 110 may include additional cavities that can overlap with optical cavity pair 120 and optical cavity pair 140.

Although the described embodiment includes two stages, each having a pair of optical cavities, the invention is not so limited. For example, in some embodiments, the Raman fiber laser can include additional stages, the additional stages including one or more optical cavities.

As described above, output couplers 124, 134, 144, and 154 are configured to reflect a portion of waveguided energy impinging thereon at λ_\, λ_∑, λ_, and λ__\, respectively. In general, an output coupler is configured to reflected less than 100 percent of waveguided energy impinging thereon at the cavity's resonant wavelength (e.g., less than 99 percent, less than 95 percent, less than 90 percent, less than 85 percent, less than 80 percent, less than 75 percent, less than 70 percent, less than 65 percent, less than 60 percent, less than 55 percent, less than 50 percent, less than 45 percent, less than 40 percent, less than 35 percent, less than 30 percent, less than 25 percent, less than 20 percent, less than 15 percent, such as 10 percent or less). In each case, the output coupler couples a portion of waveguided energy at the respective wavelength impinging thereon out of the cavity. For example, one or more of the output couplers can couple at least 1 percent of the energy at the resonant wavelength impinging thereon out of the cavity (e.g., at least 2 percent, at least 5 percent, at least 10 percent, at least 15 percent, at least 20 percent, at least 25 percent, at least 30 percent, at least 35 percent, at least 40 percent, at least 45 percent, at least 50 percent, at least 55 percent, at least 60 percent, at least 65 percent, at least 70 percent, at least 75 percent, at least 80 percent, at least 85 percent, at least 90 percent, at least 95 percent). One example of an output coupler is a fiber Bragg grating configured to reflect less than 100 percent of waveguided energy impinging thereon at the cavity's resonant wavelength. In some embodiments, one or more of the cavities can include variable output couplers. Examples of variable output couplers are described in PCT Patent Application No. PCT/US02/19420, entitled "APPARATUS AND METHOD FOR VARIABLE OPTICAL OUTPUT CONTROL," and filed June 19, 2002.

Optical fiber 110 also includes a reflector 164 (e.g., a fiber Bragg grating) designed to reflect substantially all (e.g., more than about 95 percent, more than 96 percent, more than 98 percent, more than 99 percent, such as about 100 percent) energy propagating in optical fiber 110 at λ_p. In the described embodiment, reflector 164 is placed after cavity pair 120. Reflector 164 facilitates double pass of pump energy in cavity pair 120, by reflecting energy at the pump wavelength propagating from left to right in optical fiber 110 back towards optical cavity pair 120. In some embodiments, reflector 164 can be placed between cavity pair 140 and output end 112 of optical fiber 110. hi such configurations, energy at the pump wavelength can contribute in cascaded Raman processes in the cavity pair 140 as well.

Optical fiber 110 further includes a reflector 126 (e.g., a fiber Bragg grating) designed to reflect substantially all (e.g., more than about 95 percent, more than 96 percent, more than 98 percent, more than 99 percent, such as about 100 percent) energy propagating in optical fiber 110 at λ_. Reflector 126 substantially reduces (e.g., eliminates) the propagation of energy at λ_\ from optical cavity pair 140 out of an output end 112 of optical fiber 110. Reflector 126 also facilitates double pass of the energy at λ_\ in the cavity pair 140.

With this arrangement, as energy at wavelength λ_p enters optical fiber 110, the energy propagates along optical fiber 110 until it impinges upon reflector 164, where it is reflected and propagates through optical fiber 110 in the reverse direction. As energy at λ_p propagates through optical fiber 110 in the forward and reverse directions, it interacts with the active material and generates energy at wavelength λ_\. Energy at wavelength λ_\ created within cavity 121 propagating in the reverse direction propagates until it impinges on reflector 122, where it is reflected. Energy at λ_\ within optical cavity 121 propagating in the forward direction propagates until it impinges on output coupler 124, where a portion of this energy is reflected. A portion of this energy is transmitted by output coupler 124 and continues to propagate in the forward direction. As energy at λ_\ propagates through optical fiber 110, it interacts with the active material and generates energy at λ_∑, λ_, and A . Energy at λ_\ propagates along optical fiber 110 until it impinges upon reflector 126, where it is reflected and propagates through optical fiber 110 in the reverse direction. Energy at 2₂ generated within optical cavity 131 propagates back and forth between reflector 132 and output coupler 134, amplifying energy intensity at λ within optical cavity 131. Whenever energy at i propagating in the forward direction impinges on output coupler 134, a portion of this energy is transmitted by output coupler 134 and continues to propagate in the forward direction. Similarly, energy at λ_ generated within optical cavity 141 propagates back and forth between reflector 142 and output coupler 144, amplifying energy intensity at λ_ within optical cavity 141. Whenever energy at λ_ propagating in the forward direction impinges on output coupler 144, a portion of this energy is transmitted by output coupler 144 and continues to propagate in the forward direction. Energy at both λ_\ and λ_ propagating through optical fiber 110 interacts with the active material in optical fiber 110 to generate energy at λt_..

Furthermore, energy at λt_. generated within optical cavity 151 propagates back and forth between reflector 152 and output coupler 154. Whenever energy at λ_. propagating in the forward direction impinges on output coupler 154, a portion of this energy is transmitted by output coupler 154 and continues to propagate in the forward direction.

Energy at λ_. makes a single pass through the cavity pair 140 and interacts with energy at λ_\, λ_ and λt_.. Accordingly, the majority of energy exiting the first stage (optical cavity pair 120) is usually energy at λ_\, which is used to generate waves λ_ and λt_. in the second stage (optical cavity pair 140).

Accordingly, output energy 199 including energy at i, λ_, and λ_. exits optical fiber 110 through output end 112. The absolute and relative intensity of energy at λ%, λ_, and λt_. in output energy 199 can be varied by varying the pump power, and/or the reflectance of output couplers 124, 134, 144, and 154.

Pump power at the pump wavelength can be varied as desired. In general, power at the pump wavelength is selected to provide a desired total output power and power spectrum. For example, power at the pump wavelength can be selected to be above a threshold for output at a first output wavelength, but below a threshold for output at a second output wavelength. Accordingly, the output spectrum will include energy at the first output wavelength, but will include substantially no energy at the second output wavelength. In embodiments, power at the pump wavelength can be more than about 100 mW (e.g., more than about 200 mW, more than about 300 mW, more than about 400 mW, more than about 500 mW, more than about 600 mW, more than about 700 mW, more than about 800 mW, more than about 900 mW, more than about 1000 mW, more than about 1100 mW, more than about 1200 mW, more than about 1300 mW, more than about 1400 mW, more than about 1500 mW, more than about 1600 mW, more than about 1700 mW, more than about 1800 mW, more than about 1900 mW, more than about 2000 mW, more than about 2500 mW, such as 3000 mW or more).

In some embodiments, the output power in one or more of the output wavelengths can be more than about 5 mW (e.g., more than about 10 mW, more than about 25 mW, more than about 50 mW, more than about 100 mW, more than about 150 raW, more than about 200 mW, more than about 250 mW, more than about 300 mW, more than about 350 mW, more than about 400 mW, more than about 450 mW, more than about 500 mW, more than about 600 mW, more than about 700 mW, more than about 800 mW, more than about 900 mW, more than about 1000 mW, more than about 1200 mW, more than about 1400 mW, more than about 1500 mW, more than about 1600 mW, more than 1800 mW, such as 2000 mW or more).

Raman fiber laser 100 can have high total conversion efficiency. As used herein, total conversion efficient refers to the ratio of total output power to total input power. Total conversion efficiency, for example, can be 30 percent or more (e.g., more than 40 percent, more than 50 percent, more than 60 percent, more than 70 percent, such as 80 percent or more). The output power at each output wavelength can be the same or different. In some embodiments, one of the output wavelengths can include more than 33 percent of the total output energy (e.g., more than 40 percent, more than 50 percent, more than 60 percent, more than 70 percent, more than 80 percent, such as 90 percent or more). In some embodiments, one of the output wavelengths can include less than 33 percent of the total output energy (e.g., less than 30 percent, less than 20 percent, such as 10 percent or less).

In some embodiments, for a given pump power, the rate at which energy at λχ and λ_\ exits the first stage can be regulated by the reflectivity of output coupler 124. For example, A₂ can increase and λ_\ decrease (e.g., monotonically) with increasing reflectivity for at least a range of reflectivity (e.g., from about 25 percent to 50 percent or more) for output coupler 124 at λ_\.

Alternatively, or additionally, for a given pump power, the rate at which energy at i and λ_\ exits the first stage can be regulated by the reflectivity of output coupler 134. For example, ₂ can increase and λ_\ decrease (e.g., monotonically) with increasing reflectivity for at least a range of reflectivity (e.g., from about 40 percent to 65 percent or more) for output coupler 134 at λ .

In some embodiments, for a given pump power, the output power at λ_ and λt_. can be regulated by the reflectivity of output coupler 144. For example, λ_ can decrease (e.g., monotonically) with increasing reflectivity for at least a range of reflectivity (e.g., from about 50 percent to 85 percent or more) for output coupler 144 at λ_. The output power at λ_. can also depend on the reflectivity of output coupler 144. For example, λ_. can increase (e.g., monotonically) with increasing reflectivity for at least a range of reflectivity (e.g., from about 50 percent to 85 percent or more) for output coupler 144 at λ_. In some cases, the output power at /L₂ does not substantially change (e.g., changes by less than about 20 percent, by less than about 15 percent, by less than about 10 percent, by less than about 5 percent) with the change of reflectivity for at least a range of reflectivity (e.g., from about 50 percent to 85 percent or more) for output coupler 144 at λ_. In addition, in some embodiments, the total output power does not substantially change (e.g., changes by less than about 20 percent, by less than about 15 percent, by less than about 10 percent, by less than about 5 percent) with increasing reflectivity for at least a range of reflectivity (e.g., from about 50 percent to 85 percent or more) for output coupler 144 at λ_.

For a given pump power, the output power at λ_ can also depend on the reflectivity of output coupler 154. For example, λ_ can decrease (e.g., monotonically) with increasing reflectivity for at least a range of reflectivity (e.g., from about 25 percent to 50 percent or more) for output coupler 154 at λ_.. The output power at λt_. can also depend on the reflectivity of output coupler 154. For example, in some embodiments, λt_. can increase (e.g., monotonically) with increasing reflectivity for at least a range of reflectivity (e.g., from about 25 percent to 50 percent or more) for output coupler 154 at λ_.. In some cases, the output power at A₂ does not substantially change (e.g., changes by less than about 20 percent, by less than about 15 percent, by less than about 10 percent, by less than about 5 percent) with the change of reflectivity for at least a range of reflectivity (e.g., from about 25 percent to 50 percent or more) for output coupler 154 at λ_.. In addition, in some embodiments, the total output power does not substantially change (e.g., changes by less than about 20 percent, by less than about 15 percent, by less than about 10 percent, by less than about 5 percent) with the change of reflectivity for at least a range of reflectivity (e.g., from about 25 percent to 50 percent or more) for output coupler 154 at /t₄.

Accordingly, in some embodiments, the total output power of Raman fiber laser 100 can be set by selecting the pump power, while the power distribution between i, λ_, and λ_. in the output spectrum can be set by selecting the appropriate reflectivity of output couplers 124, 134, 144, and/or 154. The power distribution can be varied by varying the reflectivity of one or more of these output couplers. For example, to increase the proportion of output energy at λ , one can increase the reflectivity of output coupler 124 at λ_\ and/or the reflectivity of output coupler 134 at 2₂- To increase the ratio of energy distribution between λ_ to λ_. in the laser output, one can decrease the reflectivity of output coupler 154 at λt_. and/or the reflectivity of output coupler 144 at λ_.

In an exemplary embodiment, the output wavelengths, λ₂, λ_, and λ_, are selected to be 1463 nm, 1425 nm, and 1454 nm, respectively. The pump wavelength, λ_p, is 1277 nm, and λ is 1353 nm. The active material in the gain medium is GeO₂. Referring to FIG. 2, in a GeO₂ doped fiber and using the aforementioned wavelengths, the relative Raman gain is highest for generating λ_\ from λ_p. For the transition generating λ₂ from λ , the gain is about 35 percent of the maximum. The gain for other transitions is shown in FIG. 2.

The output, pump, and intermediate wavelengths can be selected according to the absorption spectrum of the optical fiber. Referring to FIG. 3, output, pump, and intermediate wavelengths are selected according to energy absorption in the typical GeO₂-doped fiber . In particular, the selected wavelengths in the present example avoid the water peak absorption peak at about 1390 nm.

Table 1. Propagation losses in GeO₂-doped fiber.

Table 2. Raman gain in GeO₂ doped fiber. Data represents pumped Stokes, (i.e., in equation for a pumping signal the gain will be higher by a factor of wavelength ratio (pumped/pumping). Performance of the Raman fiber laser was studied by computer simulation using the theory of stimulated Raman scattering in fibers (see, e.g., J. AuYeung and A. Yariv, "Spontaneous and stimulated Raman scattering in long low loss fibers," IEEE Journal of Quantum Electronics, v. QE-14, pp. 347-352 (1978) J. AuYeung and A. Yariv, "Theory of cw Raman oscillation in optical fibers," Journal of the Optical Society of America, v.69, pp. 803-807 (1979)).

Table 1 lists propagation losses used in computer simulations of the three wavelength Raman laser of the present invention while Table 2 lists Raman gains.

In the simulation, the length of the optical fiber in each cavity pair was 200 m. Reflectors 122, 132, 142, and 152 have 97 percent reflectivity at λ_\, λ_, λ_, and λt_., respectively. Nominal values for the output couplers are as follows: 30 percent reflectivity at λ_\ for output coupler 124, 48 percent reflectivity at λ₂ for output coupler 134; 75 percent reflectivity at λ_ for output coupler 144; and 33 percent reflectivity at λ_. for output coupler 154. During the simulations, to study the effect of output coupler reflectivity on the output energy spectrum, the reflectivity of one of the output couplers was varied while the others were kept constant.

One control that affects the total power in the three output wavelengths is the pump power at 1277 nm. Referring to FIG. 4, the total output power of the Raman fiber laser as a function of pump power for the present example is indicated by curve 410. Output power at 1425 nm is indicated by curve 420, output power at 1454 nm is indicated by curve 430, and output power at 1463 nm is indicated by curve 440. Power at 1353 nm exiting optical cavity 121 is indicated by curve 450, and power at 1463 mn exiting optical cavity 131 is indicated by curve 460. Furthermore, the sum of output power at 1425 nm and 1454 nm is indicated by curve 470. Each output wavelength has a threshold pump power. The threshold for output energy at 1425 nm is about 0.6 W, the threshold for output energy at 1454 nm is about LO W, and the threshold for 1463 nm is about 1.4 W. In other words, as pump power is increased, the Raman fiber laser first produces output energy at 1425 nm alone, then at 1425 nm and 1454 nm, and finally at 1425 nm, 1454 m, and 1463 nm for the given set of output couplers.

The sequence of threshold powers for the output wavelengths can depend on the selection of wavelengths and their positions relative to the Raman gain curve (see FIG. 2), as well as wavelength allocation between optical cavities 120 and 140. The sequence of threshold powers, and the value of the threshold powers can also depend on the reflectivity of the output couplers.

As illustrated by FIG.2, energy at 1463 nm has a relatively weak interaction with the other wavelengths (i.e., 1353 nm, 1425 nm and 1454 nm). Thus, the energy at 1463 nm should interact minimally with the energy at 1425 nm and the energy at 1454 nm during its single pass propagation through optical cavity 140.

Referring to FIG. 5, the behavior of optical cavity pair 120 is shown in addition to the laser output. The total power of energy at 1353 nm and energy at 1463 nm in optical cavity pair 120 is indicated by curve 510. The threshold for energy at 1353 nm is about 0.4 W and its slope efficiency close to 94 percent in proximity of the threshold for energy at 1463 nm (i.e., about 1.4 W). At higher pump power than the threshold for energy at 1463 nm, the increase of energy at 1353 nm as a function of pump power is slowed and flattened (clamped). In addition, the total conversion slope efficiency of pump energy to energy at 1353 mn and 1463 nm is decreased (e.g., to about 56 percent). Highest conversion efficiency occurs when the pump power is a little greater than the threshold for energy atl463 nm (e.g., when the pump power is about 1.6 W). At this condition, the total conversion efficiency of pump power to energy at 1353 nm and 1463 nm is about 79 percent. Total conversion efficiency of pump power to energy at 1353 nm and 1463 nm decreases above a pump power of about 1.6 W. At a fixed pump power, for example 1.6 W, the power distribution between energy at 1353 nm and energy at 1463 nm in optical cavity pair 120 can depend on the reflectivity of output coupler 124 and output coupler 134. FIG. 6 and FIG. 7 show the dependence of the output powers on these output couplers. For each of these simulations, the reflectivity of the reflectors and non- variable output couplers are set to according to the nominal values above. FIG. 6 shows the power of energy at different wavelengths in optical cavity pair 120 as a function of reflectivity of output coupler 124 at 1353 nm. The total power (power at 1353 nm plus power at 1463 nm) is indicated by curve 610. Power at 1353 nm is indicated by curve 620, and power at 1463 nm is indicated by curve 630. Similarly, FIG. 7 shows the power of energy at different wavelengths in optical cavity pair 120 as a function of reflectivity of output coupler 134 at 1463 nm. The total power (power at 1353 nm plus power at 1463 nm) is indicated by curve 710. Power at 1353 nm is indicated by curve 720, and power at 1463 nm is indicated by curve 730. FIG. 6 and FIG. 7 show similar behavior. Accordingly, either output coupler can be set to control the power of energy in optical cavity pair 120. However, there is a slight difference in the dynamic range of the output coupler 124 compared to output coupler 134. In particular, the dynamic range of output coupler 134 is smaller than output coupler 124. Dynamic range refers to the total variation of power at 1353 nm and 1463 nm as a function of output coupler reflectivity. Thus, for certain applications, control of output coupler 134 (e.g., by using a variable output coupler) may be preferred because it is less sensitive to small variations and/or fluctuations of output coupler reflectivity than output coupler 124. Moreover, the reflectivity of output coupler 134 can be varied to control power distribution between energy at 1353 nm and energy at 1463 nm.

FIG. 8 and FIG. 9 show the effect of reflectivity of output couplers 144 and 154 on the output power spectrum, respectively. For each of these figures, the reflectivity of the reflectors and non- variable output couplers are set to according to the nominal values above. FIG. 8 shows the dependence of the output power spectrum on the reflectivity of output coupler 144. The output power at 1425 mn is indicated by curve 910, the output power at 1454 nm is indicated by curve 920, and the output power at 1463 nm is indicated by curve 930. Total output power is indicated by curve 940. At about 50 percent reflectivity output power at 1454 nm is low (e.g., about 20 mW), while output power at 1425 nm is significantly higher (e.g., about 740 mW). As reflectivity increases, output power at 1454 nm increases while output power at 1425 nm decreases. Output power at 1425 nm and 1454 nm are approximately equal at about 76 percent reflectivity. Above about 76 percent reflectivity, the output power is greater at 1454 nm than at 1425 nm. Output power at 1463 nm is not significantly affected by the reflectivity of output coupler 144 for this configuration. Total output power is also not significantly affected by the reflectivity of output coupler 144 for this configuration.

FIG. 9 shows the dependence of the output power spectrum on the reflectivity of output coupler 154. The output power at 1425 nm is indicated by curve 1010, the output power at 1454 nm is indicated by curve 1020, and the output power at 1463 nm is indicated by curve 1030. Total output power is indicated by curve 1040. At low reflectivity (e.g., about 25 percent), output power at 1454 nm is low (e.g., about 50 mW), while output power at 1425 nm is significantly higher (e.g., about 600 mW). As reflectivity of output coupler 154 increases, output power at 1454 nm increases while output power at 1425 nm decreases. Output power at 1425 nm and 1454 nm are approximately equal at about 37 percent reflectivity. Above about 37 percent reflectivity, the output power is greater at 1454 mn than at 1425 nm. Above about 52 percent reflectivity output power at 1425 nm falls to substantially zero mW. Output power at 1463 nm falls monotonically from about 200 mW at 20 percent reflectivity to about 120 mW at 55 percent reflectivity. Total output power varies minimally as a function of the reflectivity of output coupler 144 for this configuration. The total conversion efficiency of pump power to output power is about 57 percent. Reflectivity of output couplers 124 and/or 134 can also affect the output power spectrum. FIG. 10, for example, shows the dependence of the output power spectrum on the reflectivity of output coupler 124. For this example, the reflectivity of the reflectors and non-variable output couplers are set to according to the nominal values above. At low reflectivity (e.g., about 20 percent), there is no power at 1463 nm (curve 1120), approximately 400 mW at 1425 nm (curve 1110), and approximately 600 mW at 1454 nm (curve 1130). The threshold for output power at 1463 nm is about 25 percent reflectivity. Output power at 1463 nm increases with increased reflectivity to about 500 mW at 50 percent reflectivity. Output power at 1454 nm decreases to zero mW at about 40 percent reflectivity, while output power at 1425 nm peaks at about 420 mW (at about 38 percent reflectivity). Total output power is approximately constant as function of reflectivity between 20 percent and 50 percent. The power distribution between energy at 1463 nm and the sum of energy at 1425 nm and energy at 1454 nm has a behavior similar to the power distribution between energy at 1463 nm and energy at 1353 nm as described above. FIG. 11 shows the dependence of the output power spectrum on the reflectivity of output coupler 134. In this example, the reflectivity of the reflectors and non- variable output couplers are set to according to the nominal values above. At low reflectivity (e.g., about 30 percent), there is no power at 1463 nm (curve 1230), approximately 400 mW at 1425 nm (curve 1210), and approximately 620 mW at 1454 nm (curve 1220). The threshold for output power at 1463 nm is about 40 percent reflectivity. Output power at 1463 nm increases with increased reflectivity to about 300 mW at 60 percent reflectivity. Output power at 1454 nm decreases to zero mW at about 60 percent reflectivity, while output power at 1425 nm peaks at about 420 mW (at about 57 percent reflectivity). Total output power (curve 1240) decreases slightly (from about 1000 mW to about 900 mW) as function of reflectivity between 40 percent and 60 percent. Accordingly, output coupler 134 can be used to regulate the power split between output energy at 1463 nm wave and output energy at 1425 nm and 1454 nm.

In preferred embodiments, three controls are used to regulate power distribution between the three output wavelengths 1425 nm, 1454 nm and 1463 nm. hi particular, pump power at 1277 nm controls the total output power in all three output wavelengths; variable output coupler 134 controls the power distribution between output energy at 1463 nm and the sum of output energy at 1425 nm and output energy at 1454 nm; and, variable output coupler 154 controls the power distribution between output energy at 1425 nm and output energy at 1454 nm.

Although the foregoing example refers to a specific active material and to specific input and output wavelengths, Stokes shifts and wavelength allocation between optical cavity pairs 120 and 140 of the Raman fiber laser 100, other wavelengths, active materials, pump energy, etc can be used. In general, while the specifics of each system will depend on the end use application and desired performance of the system, the principles embodied by the foregoing example can also apply to other implementations. In some embodiments, Raman fiber laser 100 can be adapted to provide output energy at five wavelengths. Referring to FIG. 12, a Raman fiber laser 1300 includes power splitters 1310 and 1320 in optical fiber 110.

Power splitter 1310 is located in optical fiber 110 between input end 111 and optical cavity pair 120. Power splitter 1310 is configured to couple energy at λ_p out of optical fiber 110 and into an optical fiber 1312. This energy at λ_p exits Raman fiber laser 1300 from output 1314.

Power splitter 1310 may be configured to couple as much energy at λ_p out of optical fiber 110 as desired. In general, power splitter 1310 will be configured to couple sufficient energy at λ_p out of optical fiber 110 necessary for its end use application without depleting the output energy 199 below a useable level. In embodiments, power splitter 1310 may be configured to couple more than one percent of the energy at λ_p out of optical fiber 110 (e.g., more than about five percent, more than about eight percent, more than about 10 percent, more than about 20 percent, more than about 30 percent, more than about 40 percent, more than about 50 percent, more than about 60 percent, more than about 70 percent, more than about 80 percent, such as 90 percent or more).

Power splitter 1310 can be a variable power splitter. In particular, the amount of energy at λ_p couple from optical fiber 110 into optical fiber 1312 can be variably controlled.

Power splitter 1320 is located in optical fiber 110 between optical cavity pair 120 and optical cavity pair 140. Power splitter 1320 is configured to couple energy at _\ out of optical fiber 110 and into an optical fiber 1322. This energy at λ_\ exits Raman fiber laser 1300 from output 1324. Power splitter 1320 may be configured to couple as much energy at λ_\ out of optical fiber 110 as desired. In general, power splitter 1320 will be configured to couple sufficient energy at λ_\ out of optical fiber 110 necessary for its end use application without depleting the output energy 199 below a useable level. In embodiments, power splitter 1320 may be configured to couple more than one percent of the energy at λ_\ out of optical fiber 110 (e.g., more than about five percent, more than about eight percent, more than about 10 percent, more than about 20 percent, more than about 30 percent, more than about 40 percent, more than about 50 percent, more than about 60 percent, more than about 70 percent, more than about 80 percent, such as 90 percent or more). Power splitter 1320 can be a variable power splitter. In particular, the amount of energy at λ_\ couple from optical fiber 110 into optical fiber 1322 can be variably controlled. In preferred embodiments, power splitter 1320 the amount of energy at λ₂ coupled from optical fiber 110 into optical fiber 1322 is negligible.

One or both of power splitters 1310 and 1320 may be a simple power splitter having no spectral selectivity. Alternatively, one or both of power splitters 1310 and 1320 can be spectrally selective power splitters, such as a tunable wavelength division multiplexer (WDM).

If desired, output from one or both of optical fibers 1312 and 1322 can be combined with output energy exiting Raman fiber laser 100 from output end 112 (e.g., coupled into a common waveguide). Alternatively, output from optical fibers 1312 and 1322 may be combined, but kept separate from output energy from output end 112. In some embodiments, output energy from output end 112, optical fiberl312 and optical fiber 1322 can be kept separate.

The Raman fiber lasers described above can be used to amplify signals in an optical communication system. For example, referring to FIG. 13, an optical communication system includes two Raman fiber lasers 1450 and 1460. Both Raman fiber lasers have three output fibers. Raman fiber laser 1450 provides energy at λ₂, λ_, and λ_. through optical fiber 1451 to span 1410. Raman fiber laser also provides energy at λ_p through optical fiber 1452 to span 1410, and provides energy at λ_\ to span 1420 through optical fiber 1453. Spans carry optical signal between hubs. For example, span 1420 carries optical signals between a first hub 1430 and a second hub 1440. These optical signals can be in the C-band of the optical signal communication spectrum (e.g., between about 1530 nm and 1570 nm).

Raman fiber laser 1460 is similar to Raman fiber laser 1450 and supplies energy in the span 1420 at λ_p through optical fiber 1462 and at λ₂, λ_, and λ_. through optical fiber 1461. As a result, the communication signal co-propagates with the wave λ_\ and counter-propagates with the waves λ , λ_, λ_. and λ_p in fiber span 1420. Such a configuration may be advantageous for a high order Raman pump amplification (e.g., the co-propagating energy from the Raman fiber lasers can amplify the communication signal propagating in the span). In some embodiments either λ_p or λ_\ can be omitted. Spans 1410 and 1425 have amplification scheme similar to span 1420.

Referring to FIG. 14, another embodiment of an optical communication system 1500 includes a Raman fiber laser 1510, spans 1520 and 1530, and hub 1540. Raman fiber laser 1510 provides energy at five output wavelengths through optical fibers 1560, 1562, and 1564 to span 1520. Optical energy from Raman fiber laser 1510 propagates counter to optical signals propagating in span 1520.

In the simulated embodiment of Raman fiber laser 100 wavelengths λ₂, λ_, and λ_. are in the 1400 nm - 1500 nm wavelength range. In general, Raman fiber laser's can be configured to generate output energy at other wavelengths (e.g., from about 400 nm to about 2500 nm)

A number of embodiments have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, higher order output wavelengths can be produced by including additional optical cavities in the Raman fiber laser or more than one output wavelength can be generated in one or more optical cavity. Accordingly, other embodiments are within the scope of the following claims.

Claims

WHAT IS CLAIMED IS:

1. A Raman fiber laser system, comprising: an energy source configured to emit energy at a wavelength λ_p; an optical fiber containing a gain medium having a Raman active material, the optical fiber being configured so that energy emitted by the energy source at λ_p can be coupled into the optical fiber; and a plurality of reflectors and output couplers disposed in the optical fiber, at least some of the plurality of reflectors and output couplers forming optical cavities in the optical fiber, each optical cavity having a resonance wavelength, the resonance wavelength of each optical cavity being different than the resonance wavelength of any other cavity, and at least two of the optical cavities do not overlap with each other in the optical fiber; wherein the plurality of reflectors and output couplers are configured so that when the optical fiber receives energy at λ_p, the optical fiber generates energy at a first wavelength, λi, corresponding to the resonance wavelength of a first of the optical cavities in the optical fiber, and optical fiber generates output energy at three different wavelengths, λ₂, λ₃, and λ , wherein λ₂, λ₃, and λ₄ correspond to resonant wavelengths of other optical cavities in the optical fiber

2. The Raman fiber laser of claim 1, wherein the optical fiber comprises a first pair of overlapping optical cavities and a second pair of overlapping optical cavities.

3. The Raman fiber laser system of claim 2, wherein the first optical cavity pair does not overlap with the second optical cavity pair.

4. The Raman fiber laser system of claim I, wherein λ₂ corresponds to a resonant wavelength of a second optical cavity in the optical fiber, and the second optical cavity overlaps with the first optical cavity.

5. The Raman fiber laser system of claim 4, wherein the first optical cavity is nested within the second optical cavity.

6. The Raman fiber laser system of claim 4, wherein the second optical cavity is nested within the first optical cavity.

7. The Raman fiber laser system of claim 4, wherein λ₃ corresponds to a resonant wavelength of a third optical cavity in the optical fiber, and λ₄ corresponds to a resonant wavelength of a fourth optical cavity in the optical fiber.

8. The Raman fiber laser system of claim 7, wherein the third optical cavity overlaps with the fourth optical cavity.

9. The Raman fiber laser system of claim 8, wherein the third optical cavity is nested within the fourth optical cavity.

10. The Raman fiber laser system of claim 8, wherein the fourth optical cavity is nested within the third optical cavity.

11. The Raman fiber laser of claim 7, wherein the first and second optical cavities are in a portion of the optical fiber different from the third and fourth optical cavities.

12. The Raman fiber laser of claim 7, wherein the plurality of reflectors includes a pump wavelength reflector configured to reflect substantially all energy impinging thereon at λ_p.

13. The Raman fiber laser system of claim 12, wherein the pump wavelength reflector is disposed in the optical fiber between the first and second optical cavities and the third and fourth optical cavities.

14. The Raman fiber laser system of claim 12, wherein the third and fourth optical cavities are disposed in the optical fiber between the pump wavelength reflector and the first and second optical cavities.

15. The Raman fiber laser system of claim 7, wherein the plurality of reflectors includes a first wavelength reflector configured to reflect substantially all energy impinging thereon at λi.

16. The Raman fiber laser system of claim 15 , wherein the third and fourth optical cavities are disposed in the optical fiber between the first and second optical cavity and the first wavelength reflector.

17. The Raman fiber laser system of claim 1, wherein at least one of the output couplers is a variable output coupler.

18. The Raman fiber laser system of claim 1 , wherein λ₂ > λi .

19. The Raman fiber laser system of claim 11 , wherein λ > λ₃.

20. The Raman fiber laser system of claim 12, wherein λ₂ > λ₄.

21. The Raman fiber laser system of claim 18, wherein λi > λ_p.

22. The Raman fiber laser system of claim 1, wherein -,^'1 = λ^'x

{c/λrpϊ) is a frequency within the Raman gain spectrum of the Raman active material contained in the gain medium and c is the speed of light.

23. The Raman fiber laser system of claim 1, wherein

, where ( c I λ_rl2 ) is a frequency within the Raman gain spectrum of the Raman active material contained in the gain medium and c is the speed of light.

24. The Raman fiber laser system of claim 1, wherein ₃ ^"1 = A,^"1 - λ^' _l ^x ₃ , where

{c/λ_n_) is a frequency within the Raman gain spectrum of the Raman active material contained in the gain medium and c is the speed of light.

25. The Raman fiber laser system of claim 1, wherein λ^¹ = λ^¹ - λ^~ _u ^] , where {c/λ_n.) is a frequency within the Raman gain spectrum of the Raman active material contained in the gain medium and c is the speed of light.

26. The Raman fiber laser system of claim 1, wherein λ^¹ = ^~K₃ ^~ - X_r ^~\ , where {c/λ_r_ _.) is a frequency within the Raman gain spectrum of the Raman active material contained in the gain medium and c is the speed of light.

27. The Raman fiber laser system of claim 7, further comprising a power splitter disposed in the optical fiber and configured to couple power at λ_p out of the optical fiber.

28. The Raman fiber laser system of claim 27, wherein the power splitter is disposed in the optical fiber between the first optical cavity and an input end of the optical fiber, the input end being where energy emitted by the energy source at λ_p is coupled into the optical fiber.

29. The Raman fiber laser system of claim 7, further comprising a power splitter disposed in the optical fiber and configured to couple power at λi out of the optical fiber.

30. The Raman fiber laser system of claim 29, wherein the power splitter is disposed in the optical fiber between the first optical cavity and the third optical cavity.

31. A fiber amplifier, comprising: the Raman fiber laser system of claim 1; and a second optical fiber configured to waveguide an optical signal and to receive energy from the Raman fiber laser system, wherein during operation of the fiber amplifier, energy from the Raman fiber laser amplifies the intensity of the optical signal waveguided by the second optical fiber.

32. A communication system, comprising: the Raman fiber laser system of claim 1; and an optical fiber span, configured to waveguide an optical signal from a source location to a destination location, and configured to receive energy from the Raman fiber laser system.

33. The Raman fiber laser system of claim 1 , wherein 1200 nm < λ_p ≤ 1300 nm .

34. The Raman fiber laser of claim 33, wherein λ_p = 1277 nm.

35. The Raman fiber laser system of claim 1, wherein lSOO nm≤ _j ≤MOO nm .

36. The Raman fiber laser system of claim 35, wherein λi = 1353 nm.

37. The Raman fiber laser system of claim 1 , wherein 1400 nm < λ₂ ≤ \ 500 nm .

38. The Raman fiber laser system of claim 37, wherein λ₂ = 1463 nm.

39. The Raman fiber laser system of claim 1 , wherein 1400 nm ≤ Λ, < 1500 nm.

40. The Raman fiber laser system of claim 39, wherein λ = 1425 nm.

41. The Raman fiber laser system of claim 1 , wherein 1400 nm < A₄ < 1500 nm .

42. The Raman fiber laser system of claim 40, wherein λ₄ = 1454 nm.

43. A Raman fiber laser configured to receive energy at a pump wavelength and to output energy at a plurality of wavelengths different from the pump wavelength, the Raman fiber laser comprising: a first stage containing a first gain medium having a first Raman active material and including first and second optical cavities disposed within a first length of optical fiber; and a second stage optically coupled to the first stage, the second stage containing a second gain medium having a second Raman active material and including third and fourth optical cavities disposed within a second length of optical fiber, wherein during operation of the Raman fiber laser, the first stage of the optical fiber generates energy at a first wavelength, λi, corresponding to a resonance wavelength of the first optical cavity in the optical fiber, and optical fiber generates output energy at three different wavelengths, λ , λ , and λ₄, wherein λ₂, λ , and λ₄ correspond to resonant wavelengths of the second, third, and fourth optical cavities respectively.

44. The Raman fiber laser of claim 43, further comprising a source configured to direct energy at the pump wavelength into the first stage of the Raman fiber laser.

45. The Raman fiber laser of claim 43, wherein the first optical cavity overlaps with the second optical cavity.

46. The Raman fiber laser of claim 45, wherein the first optical cavity is nested within the second optical cavity.

47. The Raman fiber laser of claim 43, wherein the third optical cavity overlaps with the fourth optical cavity.

48. The Raman fiber laser of claim 47, wherein the third optical cavity is nested within the fourth optical cavity.

49. The Raman fiber laser of claim 43, wherein the first gain medium is different from the second gain medium.

50. The Raman fiber laser of claim 43, wherein the first gain medium is the same as the second gain medium.

51. The Raman fiber laser of claim 43, wherein at least the first or second optical cavity comprises a variable output coupler.

52. The Raman fiber laser of claim 43, wherein at least the third or fourth optical cavity comprises a variable output coupler.

53. A Raman fiber laser system, comprising: an energy source configured to emit energy at a wavelength λ_p; an optical fiber containing a gain medium having a Raman active material, the optical fiber being configured so that energy emitted by the energy source at λ_p can be coupled into the optical fiber; and a plurality of reflectors and output couplers disposed in the optical fiber, at least some of the plurality of reflectors and output couplers forming optical cavities in the optical fiber, each optical cavity having a resonance wavelength, the resonance wavelength of each optical cavity being different than the resonance wavelength of any other cavity, wherein the plurality of reflectors and output couplers are configured so that when the optical fiber receives energy at λ_p, the optical fiber generates energy at a first wavelength, λi, corresponding to the resonance wavelength of a first of the optical cavities in the optical fiber, and optical fiber generates output energy at three different wavelengths, λ₂, λ₃, and λ₄, wherein λ₂, λ₃, and λ₄ coπespond to resonant wavelengths of other optical cavities in the optical fiber, and at least two of the optical cavities do not overlap with each other, and a first stage of the optical fiber includes at least two overlapping cavities and a second stage of the optical fiber includes at least two overlapping cavities, wherein the first stage is different from the second stage.

55. A Raman fiber laser, comprising: a first stage of an optical fiber, the first stage including first and second optical cavities; and a second stage of the optical fiber, optically coupled to the first stage, the second stage including third and fourth optical cavities, wherein each stage of the optical fiber contains a gain medium having a Raman active material, and the first, second, third, and fourth optical cavities are configured so that when the optical fiber receives energy at a pump wavelength, λ_p, the optical fiber generates energy at a first wavelength, λi, corresponding to the resonance wavelength of the first optical cavity in the optical fiber, and optical fiber generates output energy at three different wavelengths, λ₂, λ₃, and λ₄, wherein λ₂, λ₃, and λ₄ coπespond to resonant wavelengths of other second, third, and fourth optical cavities, respectively.

56. The Raman fiber laser of claim 55, wherein the first optical cavity overlaps with the second optical cavity.

57. The Raman fiber laser of claim 56, wherein the first optical cavity is nested within the second optical cavity.

58. The Raman fiber laser of claim 55, wherein the third optical cavity overlaps with the fourth optical cavity.

59. The Raman fiber laser of claim 58, wherein the third optical cavity is nested within the fourth optical cavity.