Suppression of stimulated Brillouin scattering in higher-order-mode optical fiber amplifiers

Abstract

An HOM-based optical fiber amplifier is selectively doped within its core region to minimize the presence of dopants in those portions of the core where the unwanted lower-order modes (particularly, the fundamental mode) of the signal reside. The reduction (elimination) of the gain medium from these portions of the core minimizes (perhaps to the point of elimination) limits the amount of amplification impressed upon the backward-propagating Stokes wave. This minimization of amplification will, in turn, lead to a reduction in the growth of the Stokes power that is generated by the Brillouin gain, which results in increasing the amount of power present in the desired, forward-propagating HOM amplified optical signal output.

Claims

1. A higher-order mode gain (HOM) fiber for use in an optical fiber amplifier, the HOM gain fiber supporting the propagation of an optical signal at a selected HOM and comprising a core with a diameter greater than 80 m and an effective area of at least 1800 m.sup.2; and a cladding region disposed to surround the core, wherein the core is selectively doped with a gain dopant such that a gain dopant overlap integral associated with the selected HOM signal is increased and approaches unity, and a gain dopant overlap integral associated with backward-propagating unwanted Stokes signals at lower-order modes (LOM) signals is reduced and approaches zero.

2. The HOM gain fiber as defined in claim 1 wherein the unwanted LOM signals originate at imperfect splice locations.

3. The HOM gain fiber as defined in claim 1 wherein the unwanted LOM signals originate at imperfect mode conversion locations.

4. The HOM gain fiber as defined in claim 1 wherein the unwanted LOM signals originate from cross-mode coupling between the selected HOM signal and unwanted signals.

5. The HOM gain fiber as defined in claim 1 wherein the unwanted LOM signals originate from noise signals.

6. The HOM gain fiber as defined in claim 1 wherein the unwanted LOM signals originate from Stimulated Raman Scattering (SRS) along the HOM gain fiber.

7. The HOM gain fiber as defined in claim 1 wherein the core of the HOM gain fiber includes an inner core region and an outer core region and the gain dopant concentration is higher in the outer core region than in the inner core region.

8. The HOM gain fiber as defined in claim 7 wherein the selective doping of the core is controlled such that the inner region of the core remains undoped.

9. The HOM gain fiber as defined in claim 1 wherein the gain dopant comprises a rare earth material.

10. The HOM gain fiber as defined in claim 9 wherein the gain dopant comprises a material selected from the group consisting of: Er, Yb, Cr and Tm.

11. The HOM gain fiber as defined in claim 1 wherein the unwanted LOM signals include a fundamental LP.sub.01 mode signal.

12. The HOM gain fiber as defined in claim 1 wherein the selected higher-order mode comprises an LP.sub.0n mode, n8.

13. An optical fiber amplifier supporting the propagation and amplification of a selected higher-order mode (HOM) signal comprising an input mode converter for converting an input signal propagating in a fundamental mode into the selected higher order mode; and an optical gain fiber coupled to an output of the input mode converter for receiving the selected HOM signal and creating an amplified HOM signal, the optical gain fiber including a core with a diameter greater than 80 m and an effective area of at least 1800 m.sup.2, the core being selectively doped with a gain dopant such that the gain dopant concentration is higher in regions of the core where the selected HOM signal predominates such that an associated gain dopant overlap integral is increased and approaches unity, and the gain dopant concentration is lower in regions of the core where backward-propagating unwanted Stokes signals at lower-order modes (LOM) predominate such that an associated gain dopant overlap integral is reduced and approaches zero.

14. The optical fiber amplifier as defined in claim 13 wherein the core of the optical gain fiber includes an inner region and an outer region and the gain dopant concentration is higher in the outer region and lower in the inner region of the core of the optical gain fiber.

15. The optical fiber amplifier as defined in claim 14 wherein the selective doping of the core of the optical gain fiber is controlled such that the inner region of the core remains undoped.

16. The optical fiber amplifier as defined in claim 13 wherein the gain dopant comprises a rare earth material.

17. The optical fiber amplifier as defined in claim 13 wherein the unwanted LOM signals include a fundamental LP.sub.01 mode signal.

18. The optical fiber amplifier as defined in claim 13 wherein the unwanted LOM signals originate at imperfect splice locations.

19. The optical fiber amplifier as defined in claim 13 wherein the unwanted LOM signals originate at imperfect mode conversion locations.

20. The optical fiber amplifier as defined in claim 13 the unwanted LOM signals originate from cross-mode coupling between the selected HOM signal and unwanted signals.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Referring now to the drawings, where like numerals represent like parts in several views:

(2) FIG. 1 is a cross-sectional view of an exemplary section of conventional HOM fiber that may be used within a gain module of a fiber amplifier;

(3) FIG. 2 is a refractive index profile associated with the HOM fiber of FIG. 1;

(4) FIG. 3 is an enlarged view of the core region of the index profile of FIG. 2, indicating the overlap of both the desired LP.sub.08 forward-propagating signal and the undesired LP.sub.01 backward-propagating Stokes wave and the presence of uniform Yb dopant within the entire core region, as indicated by the shading;

(5) FIG. 4 is a dopant profile of a Yb-doped HOM fiber amplifier formed in accordance with the present invention to exclude the Yb dopant from the inner portion of the core region (where the LP.sub.01 mode resides) and reduce the presence of cross-modal SBS generation, the absence of shading in the inner portion indicating the absence of the Yb dopant; and

(6) FIG. 5 is graph of SBS power threshold, comparing a prior art HOM fiber to an HOM fiber amplifier formed in accordance with the present invention.

DETAILED DESCRIPTION

(7) Prior to describing the parameters of the present invention, the workings of a conventional HOM-based fiber amplifier will be briefly reviewed, in order to assist in the understanding of the improvements afforded by the arrangement of the present invention.

(8) FIG. 1 is a cross-sectional view of an exemplary section of HOM fiber 10 for use in a gain module of a fiber amplifier. While not illustrated in particular, it is to be understood that the actual gain module would also include input and mode converters to transition between the conventional fundamental mode signal propagating through the system and the selected HOM used by the gain module. As shown in particular in FIG. 1, HOM fiber 10 includes a core 11 comprised of a central (or inner) core region 12, and an outer core region 14, with core 11 surrounded by a lower index (down-doped) cladding region 16. FIG. 2 contains a refractive index profile for HOM fiber 10 of FIG. 1, showing the relative refractive index values of each region. Compared to conventional single mode fibers configured to support the propagation of only the fundamental LP.sub.01 mode, the diameter of an HOM fiber (in this case, the combination of inner region 12 and outer region 14 forming core 11) is relatively wide, on the order of 80 m or larger.

(9) To create a prior art fiber amplifier in HOM fiber, a gain medium dopant (such as, for example, Ytterbium (Yb)) is added to the entire core (in this case, therefore, both regions 12 and 14 forming core 11). The addition of the gain dopant increases the optical power present in the propagating higher-order mode signal traveling along the core. While the following discussion describes a specific embodiment where Yb is the dopant selected to provide gain, it is to be understood that various other rare earth materials may be used and also provide optical gain (e.g., erbium, neodymium, Cr, Tm, etc.).

(10) While it would seem that the utilization of higher-order modes (and the associated increased optical effective area) would increase the SBS power threshold and provide a significant improvement over previous designs based on using a fundamental mode signal, this has not been found to be the case. Indeed, it has been determined that there are at least two other considerations that may limit the amount of improvement in the SBS power threshold that an HOM fiber amplifier exhibits relative to a fundamental mode fiber amplifier. First, it has been found that the SBS power threshold is not determined by the ideal increased optical effective area of HOMs, but instead is determined by a relatively smaller cross-modal optical effective area, as will be described in detail below. Additionally, the LOMs populated by this cross-mode coupling may achieve significant ionic gain in a rare earth doped optical fiber amplifier as a result of the different modal overlap of the HOMs and LOM with the gain dopant.

(11) As a result of these discoveries, it has now been determined that there are several relevant optical effective areas corresponding to Stokes radiation in each of the propagating modes. Therefore, the idealized equation for effective area as shown above in equation (3) is more accurately reflected by the following expression, illustrating the multiplicity of optical effective areas relevant for determining the SBS threshold in an HOM fiber:

(12) $\begin{array}{cc}{A}_{0m0n}=\frac{{\left({f_{0m}\left(r\right)}^2\right)}_r{\left({f_{0n}\left(r\right)}^2\right)}_r}{{\left({f_{0m}\left(r\right)}^2{f_{0n}\left(r\right)}^2\right)}_r}& \left(4\right)\end{array}$

(13) where f.sub.0m is the electric field distribution for forward-directed light propagating in the LP.sub.0m mode and f.sub.0n is the electric field distribution for backward-directed Stokes light in the LP.sub.0n mode. Therefore, selected effective areas A.sub.0m0n as defined by equation (4) can be significantly smaller that the ideal A.sub.0808 value presumed for the original HOM fiber amplifier model, which accounted only for light propagating forward and backward in the LP.sub.08 mode and ignored the possibility of any cross-modal components.

(14) In one example, suppose it is desired to use the LP.sub.08 mode for the forward propagating signal to be amplified. In this case, A.sub.0801 (i.e., the cross-modal effective area between the LP.sub.08 mode and the fundamental LP.sub.01 mode) is estimated to be 700 m.sup.2, while A.sub.0808 is 1800 m.sup.2. Therefore, in actual design, the maximum power output of an HOM-based fiber amplifier will be limited by the cross-mode effective area A.sub.0801, not the single HOM mode optical effective area A.sub.0808.

(15) In accordance with the present invention, this problem of cross-modal effects is addressed by selectively doping regions in the fiber core so as to exclude dopants (i.e., gain medium) in regions where the cross-modal signal is high (ideally, where the cross-modal signal is greatest). This selective doping arrangement avoids the amplification of the noise signal present in the LOM signals. Stated another way, the fiber core is selectively doped such that the overlap integral between the gain dopant and the desired HOM mode, while minimizing the overlap integral of the gain dopant with the unwanted LOMs.

(16) FIG. 3 is an enlarged view of core 11 of the refractive index profile of FIG. 2, in this case indicating the presence of a Yb dopant as the gain medium. FIG. 3 illustrates a prior art arrangement where the gain medium is included within the entire expanse of core 11that is, within both inner region 12 and outer core regions 14. This conventional doping of both inner core region 12 and outer core region 14 is designated by the shaded areas in FIG. 3. In this case, the overlap integral .sub.sig of the LP.sub.08 signal with respect to the doped area is essentially equal to unity (i.e., a maximum value, since the presence of the dopant and the LP.sub.08 signal is essentially co-extensive within core 11). As shown, the overlap integral of the (unwanted) LP.sub.01 mode (denoted .sub.01) with the doped area is also close to unity (which is an undesirable result, since this overlap integral should be minimal). Indeed, this undesirable high overlap in the unwanted LP.sub.01 mode provides ionic gain to the backward-propagating Stokes light, increasing the backward-propagating Stokes power and reducing the SBS power threshold of the fiber amplifier. Note that while most of the LP.sub.08 mode propagates beyond inner core region 12, most of the LP.sub.01 mode resides within inner core region 12. Moreover, other mechanisms that also excite LOMs (such as imperfect splicing, SRS, etc.) may similarly impair amplifier performance.

(17) In accordance with the present invention, therefore, selective doping is used to introduce the gain dopant (in this case, Yb) in the core regions where the HOM signal (LP.sub.08, for example) predominantly resides, and exclude this gain dopant from core regions where a large fraction of the fundamental mode (or other unwanted LOMs) signals are found.

(18) FIG. 4 is a refractive index profile of a section of HOM fiber formed as a fiber amplifier, where the core region of the fiber is selectively doped to exclude the dopant from the region where the unwanted LOM signals are found. Referring to FIG. 4, both the desired HOM mode (LP.sub.08) signal and the unwanted LOM (the fundamental LP.sub.01 mode) are shown. While the diagram of FIG. 4 illustrates the use of LP.sub.08 mode as the HOM selected for use within the amplifier, it is to be understood that any other appropriate HOM signal may be selected and used as the forward-propagating signal mode within the fiber amplifier. In reviewing FIG. 4, it is clear that in this embodiment the unwanted fundamental mode predominantly resides within inner core region 12. In this particular example, therefore, the selective doping process is controlled such that the gain dopant (Yb in this case) is excluded from inner core region 12. By restructuring the dopant profile of the gain dopant in this manner, the overlap integral .sub.01 between the unwanted fundamental signal LP.sub.01 and the gain medium is substantially reduced. As a result, the ionic amplification of the backward-propagating Stokes light (or other mechanisms) minimized, the backward-propagating Stokes power and non-linear gain is significantly reduced and the SBS threshold power is increased accordingly.

(19) Therefore, by carefully controlling the specific regions of the HOM fiber core that are doped to correspond to those regions where the desired propagating mode signal is found (and thus leaving undoped those regions where the LOMs are greatest), the maximum output power of the HOM fiber amplifier is significantly increased, since it is no longer limited by the presence of a fundamental mode (and/or other LOM) signal that has been amplified by the gain medium. It is to be noted that while FIG. 4 shows the undoped region as being co-extensive with inner core region 12, the undoped region can extend into outer core region 14 or, alternatively, include only a portion of inner core region 12. At least one factor to consider in determining the extent of the doping is to maintain a large value of the ratio of the overlap integrals (i.e., the ratio of .sub.08 to .sub.01) such that the overlap integral of the gain dopant with the desired HOM signal is maximized and the overlap integral of the gain dopant with the unwanted LOMs is minimized.

(20) Indeed, the growth of the Stokes intensity I.sub.s(r,z) may be modeled by that of a combined Brillouin and ionic gain amplifier, and can be mathematically described by the following equation:

(21) $\begin{array}{cc}\frac{I_s\left(r,z\right)}{z}=-g_B.Math.I_s\left(r,z\right).Math.I_p\left(r,z\right)-\left[n_2\left(r,z\right).Math.{}_e-n_1\left(r,z\right).Math.{}_a\right].Math.I_s\left(r,z\right),& \left(5\right)\end{array}$
where I.sub.p is defined as the Brillouin pump intensity (which in this case is the amplifier signal intensity), I.sub.s is the backward-propagating Stokes intensity, .sub.e is the ionic emission cross section and .sub.a is the ionic absorption cross section. The Stokes power evolution is determined by a spatial integration across the cross section of the fiber and is given by:

(22) $\begin{array}{cc}\frac{P_s\left(z\right)}{z}=-\frac{g_B}{A_x}.Math.P_s\left(z\right).Math.P_p\left(z\right)-.Math.\left[n_2\left(z\right).Math.{}_e-n_1\left(z\right).Math.{}_a\right].Math.P_s\left(z\right),& \left(6\right)\end{array}$
where A.sub.x is defined as the cross-modal effective area given by equation (4) and defines the overlap integral of the Yb dopant with the Stokes signal. The growth of the backward-propagating Stokes wave of the combined Brillouin and Yb-dopant amplifier is expressed as:

(23) $P_s\left(0\right)=P_s\left(L\right).Math.\exp \left(\frac{g_B.Math.L.Math..Math.P_P\left(z\right).Math.}{A_x}\right).Math.\exp \left[.Math.L.Math.\left({}_e.Math..Math.n_2\left(z\right).Math.-{}_a.Math..Math.n_1\left(z\right).Math.\right)\right]=P_s\left(L\right).Math.G_B.Math.G_{\mathrm{Yb}},$
where < . . . > indicates an average along the fiber length. Thus, G.sub.BG.sub.Yb is the total gain of the backward propagating Stokes light. Thus, in accordance with the present invention, if the overlap of the Stokes light with the Yb dopant is minimal, the second exponential term goes to a unity value (that is, the value of G.sub.Yb is set equal to one) and the total amplification of the Stokes power is significantly reduced (i.e., is determined by only the nonlinear SBS gain G.sub.B without an ionic contribution G.sub.Yb), thereby increasing the SBS threshold power and the maximum output power of the fiber amplifier.

(24) FIG. 5 is a graph depicting the improvement in operation of an HOM fiber amplifier by selectively doping the fiber amplifier to eliminate the presence of dopant in the area where a large amount of backward-propagating Stokes signal is present. In particular, the dopant is eliminated from the region where the unwanted fundamental mode signal is propagating. Plot A is a diagram of a prior art arrangement with an essentially uniform gain dopant profile across the entire core area, illustrating the Stokes power as a function of fiber length. In this case, there is a large value in Stokes power in the core for this prior art arrangement where the Yb dopant is present within inner core region 12. Plot B is associated with an exemplary fiber amplifier of the present invention, where in this inner core region 12 remains undoped and the Yb dopant is introduced only within outer core region 14. As shown, the Stokes power is significantly reduced as a result of leaving this inner core region undoped. A reduction of 7.4 dB is associated with this particular arrangement.

(25) Although exemplary embodiments have been shown and described, it will be clear to those of ordinary skill in the art that a number of changes, modifications, or alterations to the disclosure as presented may be made. For example, while the LP.sub.08 mode as been described as the propagating higher-order mode, it is to be understood that other higher-order modes may be used in the design of a fiber amplifier. Similarly, unwanted Stokes signals may appear in modes other than the fundamental LP.sub.01 mode. Other gain dopant materials may be used in place of, or in combination with, Yb, and the areas where the dopant is both included and excluded may vary as a function of the modes desired to be supported and the modes desired to be eliminated. Similarly, the index profile may be more complex and consist of multiple regions of high and low index. Indeed, all such changes, modifications and alterations should be seen as within the scope of the disclosure and defined by the claims appended hereto.