Gain-producing, large-mode-area, multimode, hybrid optical fibers and devices using same
A large mode area, gain-producing optical fiber is configured to support multiple transverse modes of signal radiation within its core region. The fiber is a hybrid design that includes at least two axial segments having different characteristics. In a first axial segment the transverse refractive index profile inside the core is not radially uniform being characterized by a radial dip in refractive index. The first segment supports more than one transverse mode. In a second axial segment the transverse refractive index profile inside the core is more uniform than that of the first segment. The two segments are adiabatically coupled to one another. Illustratively, the second segment is a terminal portion of the fiber which facilitates coupling to other components. In one embodiment, in the first segment M12>1.0, and in the second segment M22<<M12. In a preferred embodiment, M12>>1.0 and M22˜1.0.
This application claims priority from copending provisional application Ser. No. 60/750,967 filed on Dec. 16, 2005 and entitled “Rare-Earth-Doped, Large-Mode-Area, Multimode, Hybrid Optical Fibers and Devices Using Same.”
BACKGROUND OF THE INVENTION1. Field of the Invention
This invention relates to optical fibers and, more particularly, to gain-producing, large-mode-area, multimode optical fibers for high power optical amplifier or laser applications and improved coupling efficiency.
2. Discussion of the Related Art
Because of their high performance and cost effectiveness, rare-earth-doped fiber amplifiers (REDFAs), especially erbium-doped fiber amplifiers (EDFAs), are widely used in silica fiber-optic communication systems such as, for example, long-haul transport and CATV applications. Innovative design and optimization of rare-earth-doped fibers (REDFs), especially erbium-doped fibers (EDFs), have both played a critical role in these applications. In particular, designs that confine the optical mode field and control the erbium distribution enable efficient, low-noise amplification of light at low and medium optical power levels. On the other hand, for high power applications large-mode-area (LMA) fiber lowers the signal intensity, thereby reducing deleterious nonlinear effects, and also increases the pump absorption efficiency. High power REDFAs and rare-earth doped fiber lasers (REDFLs), especially those utilizing ytterbium-doped fibers (YDFs), also have many applications outside the traditional telecommunications industry. For example, high power, LMA, YDFs are used in welding and cutting, laser ranging and target designation, medical applications and pollution detection, and free space communications (e.g., between satellites).
The optical characteristics of a LMA fiber sensitively depend upon the details of its transverse refractive index profile. Conventional wisdom dictates that desirable LMA fibers have a fundamental mode with M2 very near to 1.0, meaning that the optical field of the fundamental transverse mode is very nearly Gaussian in shape because the transverse refractive index profile inside the core is essentially uniform; that is, the refractive index profile is essentially uniform within the transverse cross-section of the core. M2 measures the similarity between the mode field and a true Gaussian function. More specifically, M2=1.0 for a mode having a Gaussian shape, and M2>1.0 for all other mode field shapes. An M2 very near to 1.0 facilitates low loss optical coupling, and furthermore the beam emerging from the fiber may be efficiently collimated or tightly focused to a diffraction limited spot. However, fabricating an LMA fiber with an ideal fundamental mode (M2=1.0) and a uniform core refractive index profile can be difficult due to the tendency of the profile to exhibit a dip in refractive index near the longitudinal axis (also known as a center dip or burnoff). Moreover, LMA fibers with a fundamental transverse mode M2 very near to 1.0 exhibit smaller effective areas and hence lower thresholds for undesirable optical nonlinearities than the fundamental transverse modes of fibers with similar core diameters but pronounced center dips. Finally, when a LMA EDF's core transverse refractive index profile is essentially uniform and the fundamental mode's M2 is very near to 1.0, there is relatively little overlap between the fundamental mode and the outer region of the doped core. As a result, the fundamental mode may experience low amplification efficiency while high-order modes may experience undesirable amplification.
Thus, a need remains in the art for a LMA REDF with improved optical coupling efficiency.
There is also a need for such a LMA REDF that is suitable for high power optical fiber amplifier and laser applications.
BRIEF SUMMARY OF THE INVENTIONIn accordance with one aspect of our invention, a LMA gain-producing optical fiber is configured to support multiple transverse modes of signal radiation within its core region. Our fiber is a hybrid design that includes at least two axial segments having significantly different characteristics. In a first axial segment the transverse refractive index profile inside the core is not radially uniform, being characterized by a radial dip in refractive index. The first segment supports more than one transverse mode. In a second axial segment the transverse refractive index profile inside the core is more uniform than that of the first segment. The two segments are adiabatically coupled to one another. In one embodiment, the two segments are adiabatically coupled to one another by a third segment, which need not be (but may be) a gain-producing fiber. Illustratively, the second segment is a terminal portion of the fiber which facilitates coupling to other components.
In another embodiment of our invention, in the first segment M12>1.0, and in the second segment M22<<M12. In a preferred embodiment, M12>>1.0 and M22˜1.0.
Our invention, together with its various features and advantages, can be readily understood from the following more detailed description taken in conjunction with the accompanying drawing, in which:
A typical REDFA 10, as shown in
As is well known in the art, the pump signal generates a population inversion in the REDF 12, which amplifies the input signal from input source 16. The amplified input signal propagates along REDF 12 to utilization device 20. In high power applications the latter may include a myriad of well known devices or apparatuses; e.g., another REDFA, a beam collimator, a lens system, a work piece (e.g., for cutting or welding); whereas in telecommunications applications, utilization device 20 may include an optical receiver, an optical modulator, an optical coupler or splitter, or a piece of terminal equipment. Some of these may be coupled to the REDF 12 via a standard pigtail connector (not shown).
Illustratively, the input source 16 is a laser that generates a relatively low power optical input signal at wavelength in the amplification range of the rare earth species of REDF 12, whereas the pump source 18 is a semiconductor light emitting diode (LED) or an array of LEDs that generates a relatively high optical power (e.g., above about 150 mW) pump signal at a shorter center wavelength that produces the desired amplification of the input signal. Preferably, the REDF 12 is a ytterbium-doped fiber, the signal source 16 generates an input signal having a center wavelength of about 1080 nm, and the pump source 18 generates a pump signal at a center wavelength of about 915 nm, or alternatively at about 975 nm. We note here that a semiconductor laser may also be used as a pump source, but an LED, especially an array of LEDs, is preferred because more total light can be coupled into the fiber with an LED.
Although the REDFA of
In addition, when provided with a suitable, well-known optical resonator (e.g., a pair of spaced apart fiber gratings) the REDF may function as a laser.
Hybrid REDF DesignIn accordance with one aspect of our invention, as shown in
The refractive index of the core region 12.1 is higher than that of the cladding region 12.2, with the difference in index being designated Δn. Although not shown, it is well known that the cladding may include an inner (depressed) cladding region and an outer cladding region, with the refractive index of the outer cladding region being between that of the core and the inner cladding region.
In either case, the core and cladding regions are configured to support the propagation of multiple transverse modes of input signal radiation propagating therein from source 16 (
More specifically, in one sense the terminal and input segments have different characteristics in that they have different transverse refractive index profiles, as shown in
In addition, the input segment 12i is configured to support more than one transverse mode.
In designing the features of the pronounced transverse refractive index dip 12.1d we prefer that the magnitude of Δnd of the dip should be no greater than about 100% of Δn, the core-to-cladding index difference. The size of Δn depends on the rare earth dopant of the REDF as well as any index-altering dopants such as Ge, P, Al or F that might be added to the core and/or cladding regions; e.g., in Yb-doped fibers Δn ˜0.005, whereas in Er:Yb doped fibers Δn˜0.01 At the opposite extreme, the magnitude of the dip should not be smaller than about 5% of Δn. The lower end of the range is dictated primarily by the need to perturb sufficiently the transverse mode shape from pure Gaussian, as discussed below. On the other hand, the width or diameter dd of the dip should be larger than approximately the smallest wavelength of light used in the system (e.g., larger than the pump wavelength, which is typically shorter than the signal wavelength). At the opposite extreme, the maximum width dd of the dip may be equal to the diameter dc of the core region 12.1, but typically is about dc/3. The object of these conditions is that the light “see” the perturbation in refractive index produced by the dip. In addition, although the dip is depicted as being conical, other geometric shapes (e.g., cylindrical) as well as more complex shapes, may also be suitable.
In another sense, the terminal and input segments have different characteristics in that their M2 parameters are different from one another, where M2 defines the similarity that the fundamental transverse mode of the fiber has to an ideal Gaussian function, as described by P. A. Belanger, Optical Engineering, Vol. 32. No. 9, pp. 2107-2109 (1993), which is incorporated herein by reference. (Although this paper defines M2 for LP01 fundamental mode of a step-index optical fiber, the definition is valid for all optical fibers, including those with a center dip in the transverse refractive index profile of the type described herein.) In particular, the input segment 12i is characterized by a parameter M12, the terminal segment 12t is characterized by a parameter M22, and the following inequalities are satisfied: M12>1.0 and M22<<M12. In a preferred embodiment, M12>>1.0 and M22˜1.0. In theory M2 may be arbitrarily large, but in practice M2 for REDFs is typically in the range, 1<M2<10, approximately. Moreover, M2˜1.06 is typically considered to be small in the sense of M22˜1.0, for example, whereas M2˜1.3 is considered to be large in the sense of M12>>1.0, for example.
In addition the input segment 12i and the terminal segment 12t are coupled to one another adiabatically; for example, by means of a LMA adiabatic segment 12a, as shown in
(ii) controllably changing the amount of heat applied to the fiber in accordance with the longitudinal position of the torch along the fiber, so that the desired distribution of dopants is achieved. See, for example, H. Y. Tam, Electr. Lett., Vol. 27, No. 17, pp. 1597-1599 (1991), which is incorporated herein by reference.
The combination of the design of the M2 parameter of the segments and the use of an adiabatic transition between them improves the coupling of the fundamental transverse mode, and significantly decreases coupling to higher order transverse modes, from the input segment to the terminal segment.
Another advantage of our invention is that the foregoing principles can be applied even in the absence of a fusion splice (a typical prior art approach to coupling different single mode fibers), for example, when coupling between the terminal segment of an REDF and a bulk (non-fiber) optical element (e.g., a telescope) is achieved in free space with the use of a suitable lens or lens system.
In a typical silica-based REDF well known in the art for operation in the wavelength ranges discussed above, the core region 12.1 is doped with at least one rare earth element (e.g., Er, Yb, Th, Tm, Nd, and/or Pr) and one or more refractive-index-altering substances such as Ge, P or Al (to increase the index) or F (to decrease the index). The cladding region 12.2 may be pure silica, or it may also be doped. Illustratively the doping levels are chosen so that the index step μn between the core and cladding ranges from about 0.005 to 0.01 depending on the dopants used, as discussed previously, and the index dip Δnd in the input segment is about the same size as Δn.
Moreover, for the fiber to support multiple transverse modes the core diameter dc is illustratively about 20 μm. The outer diameter do of such fibers is typically in the approximate range of 125 μn to 600 μm. In addition, it is apparent that the input segment 12i is a major fraction of the total length of fiber 12′, whereas the terminal segment 12t is a relatively smaller fraction; e.g., the terminal segment is illustratively less than about 500 μm long, whereas the input segment is illustratively on the order of 1 m or 1 km long.
Fiber Termination TreatmentAs mentioned above, an elevated M2 optical fiber can be locally heated to induce dopant diffusion that locally decreases the fiber's M2. Heating a fiber to sufficient temperatures (for example, near or above fusion splicing temperatures of about 2000° C.) induces substantial diffusion of the index-altering dopants, thereby inducing significant changes in the fiber's transverse refractive index profile. Such dopant diffusion is employed to suppress center dips, ridges, or other refractive index profile features that increase the M2 of the fundamental LP01 mode. In many (but not all) implementations of our invention, the MFD (mode field diameter using the conventional “Petermann II” definition) of the fundamental transverse LP01 mode actually decreases following heat-induced diffusion.
In our invention, the drawn optical fiber is locally heated to a high temperature (>>1200° C.) to induce dopant diffusion that suppresses the center-dip or other features in the refractive index profile that elevate the M2 of the fiber's LP01 fundamental mode. Curve 5.1 of
For this particular example, it is important to note that for a given amount of power guided in the LP01 fundamental mode, the peak optical intensity of the as-drawn fiber is only about 37% of the peak optical intensity occurring in the heat-treated fiber. Therefore, if an optical fiber designed with the transverse refractive index profile depicted in
Fiber terminations can be heat treated using a conventional fusion splicer. If the fiber is cleaved or polished inside the heat-treated region, then free-space coupling (for example, with conventional bulk lenses) can be used to obtain efficient optical coupling to the LP01 fundamental transverse mode of a fiber whose as-drawn LP01 mode field shape is very non-Gaussian. Alternatively, heat treatment can be incorporated as part of a fusion splicing process. The predicted coupling loss between a Gaussian field matching the MFD of the as-drawn (unheated) fiber in
In order to ensure that energy is not lost from the LP01 fundamental mode in the transition region between the heat-treated and as-drawn regions of the fiber, the transition should be made gradual and adiabatic, as discussed previously. The change in the refractive index profile in the transition region must be very gradual along its length. When producing the transition region by heat-induced dopant diffusion, a gradual transition can be achieved by varying the amount of heat applied to the transition region along its length, for example by choosing a broad (i.e., fanned out) heat source or scanning a more focused heat source along the transition region. How gradually this change must occur depends upon the details of the index profiles and the operating wavelength, according to principles well known in the art. Numerical simulations based on refractive index profiles as well as empirical process optimization can be readily employed to find suitable heating conditions for which the transition losses are minimized.
Theory of OperationWhen the fundamental transverse mode of a LMA fiber has an M2>1.0, its coupling losses (free-space or fusion splice) are elevated, and the fundamental transverse mode input signal emerging from the fiber cannot be readily tightly focused down to a small spot size or readily collimated. However, there are certain advantages to having an elevated M2 (>1.0). In particular, fibers whose fundamental transverse mode fields have larger values for M2 exhibit larger effective mode areas and hence lower peak optical intensities than fibers with the same core diameters but lower M2. Consequently, fibers with elevated M2 exhibit higher thresholds for the onset of undesirable optical nonlinearities such as SBS (stimulated Brillouin scattering) and SRS (stimulated Raman scattering). In addition to this benefit, fibers with an elevated M2 (for example, due to a pronounced center-dip in the core region refractive index, as shown in
These advantages are evident in
More specifically, the as-drawn fiber, which corresponds, for example, to the input fiber segment 12i of
On the other hand, in the heat-treated fiber, which corresponds, for example, to the terminal fiber segment 12t of
Finally,
It is to be understood that the above-described arrangements are merely illustrative of the many possible specific embodiments that can be devised to represent application of the principles of the invention. Numerous and varied other arrangements can be devised in accordance with these principles by those skilled in the art without departing from the spirit and scope of the invention.
In particular, as shown in
In addition, although we have described our invention in the context of REDFA applications, those skilled in the art will readily recognize its application can be extended to any apparatus that requires coupling to an REDF (e.g., a fiber laser).
Moreover, the adiabatic coupling region, such as coupler 12a of
Finally, it will be apparent to those skilled in the art that optical fibers can be made to exhibit gain by doping them with species other than rare earth elements; for example, it is known that silica optical fibers doped with chromium (Cr) exhibit optical gain.
Claims
1. A multi-transverse-mode rare-earth-doped optical fiber comprising:
- a core region doped with at least one rare earth element, the cross-section of said core region having a transverse refractive index profile,
- a cladding region adjacent said core region,
- said core and cladding regions configured to support multiple transverse modes of optical signal radiation within said core region,
- said fiber including a first axial segment in which said profile is not radially uniform being characterized by a radial dip in refractive index, said first segment supporting more than one of said transverse modes,
- said fiber having a second axial segment optically coupled to said first segment, said profile of said second segment being more uniform than that of said first segment, and
- said segments being adiabatically coupled to one another.
2. The fiber of claim 1, wherein said first segment is characterized by a parameter M12 and said second segment is characterized by a parameter M22, where M2 defines the similarity that the fundamental transverse mode of said fiber has to an ideal Gaussian function, and wherein M12>1.0 and M22<<M12.
3. The fiber of claim 2, wherein M12>>1.0 and M22˜1.0.
4. The fiber of claim 1, wherein said first segment comprises a major portion of the length of said fiber, and said second segment comprises a terminal portion of said fiber.
5. The fiber of claim 4, wherein said fiber includes a third axial segment optically coupled to said first segment, said profile of said third segment being more uniform than that of said first segment and being adiabatically coupled to said first segment, said second segment being located at one end of said first segment and said third segment being located at the opposite end of said first segment.
6. The fiber of claim 1, wherein said profile of said core region exhibits a dip in refractive index of Δnd, which is approximately 5-100% of the difference Δn in transverse refractive index between said core region and said cladding region.
7. The fiber of claim 1, wherein fiber is configured to propagate said signal radiation in the fundamental transverse mode.
8. The fiber of claim 1, wherein said first segment comprises a major portion of the length of said fiber, and said second segment comprises an intermediate portion of said fiber.
9. The fiber of claim 1, wherein said core and cladding regions are configured to form a large mode area fiber.
10. An optical amplifier comprising:
- an optical fiber according to claim 1 for amplifying said signal radiation in response to optical pump energy applied thereto,
- a source of said pump energy, and
- a coupler for coupling said pump energy and said optical signal into said optical fiber.
11. The amplifier of claim 10, wherein said optical signal has a first center wavelength and said source of pump energy comprises a semiconductor light source for generating an optical pump signal having a second center wavelength.
12. A high power optical amplifier comprising:
- a multi-transverse-mode, large-mode-area hybrid optical fiber including a core region doped with at least one rare earth element, the cross-section of said core region having a transverse refractive index profile, said core region configured to amplify an optical input signal propagating therein in response to optical pump energy applied thereto, a cladding region adjacent said core region, said core and cladding regions configured to support multiple transverse modes of optical radiation within said core region, said fiber including a first axial segment in which said profile is not radially uniform being characterized by a radial dip in refractive index, said first segment supporting more than one of said transverse modes, said fiber having a second axial segment optically coupled to said first segment, said profile of said second segment being more uniform than that of said first segment, said segments being adiabatically coupled to one another so that energy propagating in particular transverse mode in said first segment is not significantly coupled into other transverse modes in said second segment; and said first segment being characterized by a parameter M12 and said second segment being characterized by a parameter M22, where M2 defines the similarity that the fundamental transverse mode of said fiber has to an ideal Gaussian function, and wherein M12>1.0 and M22<<M12, said second segment being located at either an input end of said first segment, at an output end of said first segment, or both,
- a LED for generating said optical pump energy at a center wavelength different from that of said optical signal, and
- a pump combiner for coupling said pump energy into said fiber.
13. The amplifier of claim 12, wherein M12>>1.0 and M22˜1.0.
14. The amplifier of claim 12, wherein said profile of said core region exhibits a dip in refractive index of Δnd, which is approximately 5-100% of the difference Δn in transverse refractive index between said core region and said cladding region
15. A multi-transverse-mode optical fiber comprising:
- first and second fiber segments each exhibiting optical gain, each segment having a core region and a cladding region adjacent said core region, the cross-section of each of said core regions having a transverse refractive index profile,
- said core and cladding regions configured to support multiple transverse modes of optical signal radiation within said core regions,
- said profile within said first axial segment not being radially uniform and being characterized by a radial dip in refractive index, said first segment supporting more than one of said transverse modes,
- said second axial segment optically coupled to said first segment, said profile of said second segment being more uniform than that of said first segment, and
- said segments being adiabatically coupled to one another.
16. The fiber of claim 15, further including a third segment axially disposed between said first and second segments, said third segment being configured to adiabatically couple said first and second segments to one another.
17. The fiber of claim 16, wherein said third segment does not exhibit optical gain.
18. The fiber of claim 17, wherein said first and second segments are rare-earth-doped and said third segment is not.
Type: Application
Filed: Dec 5, 2006
Publication Date: Jun 21, 2007
Inventors: Robert Scott Windeler (Clinton Township, NJ), Andrew Douglas Yablon (Livingston, NJ)
Application Number: 11/633,999
International Classification: G02B 6/02 (20060101); H01S 3/00 (20060101); H04B 10/12 (20060101);