Patent Application
20090181842
The invention relates to a polarization-maintaining optical fiber and a method for manufacturing the same. More particularly, it relates to a polarization-maintaining optical fiber having a large doped inner-region suitable for high power fiber lasers and amplifiers.
Optical fibers are used in a variety of applications such as telecommunications, illumination, fiber lasers, laser machining and welding, sensors, medical diagnostics and surgery.
A typical standard optical fiber is made of transparent material. It is uniform along its length, and has a cross-section of varying refractive index. For example, the transparent material in the central region, i.e. the core, may have a higher refractive index than the transparent material in the outer region, i.e. the cladding. Light is confined in or near the core and guided along the length of the optical fiber by the principle of total internal reflection at the interface between the core and cladding.
In general, an optical fiber may be multi-mode or single-mode. A multi-mode fiber allows for more than one mode of the light wave, each mode travelling at a different phase velocity, to be confined to the core and guided along the fiber. A single-mode fiber supports only one transverse spatial mode at a frequency of interest. Given a sufficiently small core or a sufficiently small numerical aperture (defined as NA=√{square root over (ncore2−ncladding2)}, where ncore and ncladding represent the refractive index of the core and the cladding, respectively) it is possible to confine a single mode, the fundamental mode, to the core. Single-mode fibers are preferred for many applications because the problem of intermodal dispersion encountered by multi-mode fibers is avoided, and the intensity distribution of the light wave emerging from the fiber is unchanged regardless of launch conditions and any disturbances of the fiber.
For some applications, it is advantageous to carry as much optical power as possible. However, if the light intensity within the fiber exceeds a certain threshold, the material from which the fiber is made will suffer irreversible damage. Increasing the diameter size of the core of the fiber reduces the intensity of the light for a given power and allows a greater power to be carried without damage. Using a larger core fiber also helps to reduce the non-linear effects that appear at high power. For example, in the field of high-power lasers and amplifiers, the onset of adverse non-linear effects can severely degrade the spectral content and limit the power output of the laser source. Using a single-mode large-mode-area active fiber as the amplifying medium is a relatively easy solution to the problem of non-linear effects which can be detrimental to the operation of the laser.
For applications in high-power lasers and amplifiers, it is therefore desirable to use an active multi-clad polarization-maintaining (PM) optical fiber having a large doped inner-region. This doped inner region of the fiber has a central core region surrounded by a cladding region. The central core may be composed of silica-based material containing a certain concentration of an active ion (e.g.: ytterbium, neodymium, . . . ), along with an appropriate concentration of one or more of the usual co-dopants typically found in active optical fibers, such as aluminum, germanium, phosphorous, boron, fluorine, etc. This central core is surrounded by a first cladding composed of doped-silica having a refractive index lower than that of the central core but higher than that of an outer second cladding made of pure silica. For example, this first cladding may be composed of germanium-doped silica. In the situation where this inner cladding is surrounded by a second cladding of lower refractive index (e.g.: pure silica), which is also surrounded by an even lower refractive index polymer, the result is a “triple-clad optical fiber” [Reference: U.S. Pat. No. 6,941,053 (Lauzon et al)]. One or more supplemental claddings may be added between any two of the above-mentioned claddings to generally form a “multi-clad optical fiber” [reference: U.S. Pat. No. 7,068,900 (Croteau et al)].
The purpose of the raised-index first cladding is to allow for high pump power to be injected into a larger numerical aperture while providing a highly doped core having a numerical aperture small enough to have single-mode or quasi-single-mode operation. Some benefits attributable to the ability to dope the central core with a significant concentration of certain specific dopants include:
A design parameter to consider is the thickness of the raised-index first cladding. In the case of a fiber having a thin first cladding, the fiber can be very resistant to bend losses, which is useful for a purely single-mode fiber, but can prohibit higher-order mode filtering through fiber bending. For very high-power application, which commands a large core, it can become difficult to fabricate an optical fiber having a core numerical aperture sufficiently small to ensure a purely single-mode operation of the fiber. For those cases, a large-dimensioned first cladding can permit higher-order mode filtering through bend losses and effective single-mode operation of the fiber. A large first cladding allows the use of the differential bending losses to achieve enhanced mode quality in terms of M2, astigmatism and modal roundness in a triple-clad/multi-clad large-mode-area fiber design.
A principal difficulty associated with a large first-cladding multi-clad fiber design in which the first cladding is composed of doped silica is the fabrication of a polarization-maintaining version of such a fiber. Typically, the fabrication of a polarization-maintaining optical fiber involves drilling a hole on each side of the core in the mother preform for insertion of two stress-applying parts (SAP) and then drawing the preform into a polarization-maintaining (PM) fiber. The most common implementations of such PM fibers are the Panda double-clad or triple-clad fibers, where the SAPs are inserted into a second cladding consisting of pure silica.
Typically, such a drilling operation is extremely difficult and in some cases almost impossible, especially in the case where the concentration of dopants in the cladding region to be drilled is substantial.
In accordance with a first aspect of the present invention, there is provided a method for manufacturing a polarization-maintaining optical fiber, the method including:
At least one of the inner filler rods may have a different diameter.
The plurality of inner filler rods may include primary inner filler rods that have a diameter substantially equal to a diameter of the at least one stress-applying part, and secondary inner filler rods that have a diameter smaller than a diameter of the primary inner filler rods and are arranged in gaps between the core rod, the at least one stress-applying part and the primary inner filler rods.
The plurality of outer filler rods may have substantially a same refractive index as the over-cladding tube and/or substantially a same coefficient of thermal expansion as the cladding of the core rod.
In accordance with a second aspect of the present invention, there is provided a method for manufacturing a polarization-maintaining optical fiber, the method including:
The cladding of the core rod may include two or more cladding layers surrounding the inner core.
The primary inner filler rods may have a diameter substantially equal to a diameter of the at least one stress-applying part. In general, the primary inner filler rods may have different diameters for space-filling arrangement about the core rod.
The primary inner filler rods may have substantially a same refractive index as the secondary inner filler rods. Additionally or alternatively, they may have substantially a same coefficient of thermal expansion as the secondary inner filler rods.
The plurality of inner filler rods may have substantially a same coefficient of thermal expansion as the cladding of the core rod.
The step of making a fiber preform may include providing in the over-cladding tube a plurality of outer filler rods arranged adjacent the over-cladding tube between the over-cladding tube and the inner filler rods, the at least one stress-applying part and the core rod, the outer filler rods consisting of a same material as the over-cladding tube.
In accordance with a third aspect of the present invention, a polarization-maintaining optical fiber obtained according to the method(s) described above is provided.
In accordance with a fourth aspect of the present invention, use of the polarization-maintaining optical fiber, obtained according to the method(s) described above, in a fiber amplifier or fiber laser is provided.
In accordance with a fifth aspect of the present invention, there is provided a fiber preform for making a polarization-maintaining optical fiber which includes:
The fiber preform may include a plurality of outer filler rods arranged within the over-cladding tube, adjacent the over-cladding tube, between the over-cladding tube and the inner filler rods, the at least one stress-applying part and the core rod, the outer filler rods consisting of a same material as the over-cladding tube.
At least one of the inner filler rods may have a different diameter. The plurality of inner filler rods may include a plurality of primary inner filler rods and a plurality of secondary inner filler rods, the secondary inner filler rods having a diameter smaller than a diameter of the primary inner filler rods and being arranged in gaps between the core rod, the at least one stress-applying part and the primary inner filler rods.
The objects, advantages and other features of the present invention will become more apparent and be better understood upon reading of the following non-restrictive description of the preferred embodiments of the invention, given with reference to the accompanying drawings. The accompanying drawings are given purely for illustrative purposes and should not in any way be interpreted as limiting the scope of the invention.
The aspects of the present invention will be described more fully hereinafter with reference to
The polarization of light travelling in a standard optical fiber changes in an uncontrolled, wavelength-dependent manner because optical fibers generally exhibit some degree of birefringence owing to mechanical stress, for example mechanical stress arising from any bending of the fiber or changes in temperature of the fiber. However, fluctuations or uncontrolled changes in the polarization state of light are highly detrimental in many high-power applications. For applications where the polarization state cannot be allowed to drift, polarization-maintaining (PM) fibers having a strong built-in birefringence that favourably preserves a given polarization state are preferably used. As mentioned previously, large-mode-area (LMA) double-clad fibers are typically used for high-power pulsed-laser applications. As such, the large diameter of the active core-cladding region must be taken into account when introducing a controlled stress-induced birefringence by incorporating one or more stress-applying parts (SAPs).
The present invention aims to provide a polarization-maintaining optical fiber 10 (for example, a PM fiber as shown in
Small mode-area single-mode fibers need not have a multi-clad design as they can be designed to be intrinsically single-moded even if they have a large numerical aperture.
As it has been mentioned above, a PM large-mode-area multi-clad fiber suitable for high power applications can be extremely difficult to fabricate through the conventional drilling procedure due to the risk of cracking the preform while drilling in the doped cladding region.
Hence, in accordance with aspects of the present invention, there is provided a method for manufacturing a polarization-maintaining optical fiber—one that avoids such a drilling process—and a polarization-maintaining optical fiber obtained therefrom, as described in more detail hereinbelow.
In general, as depicted in
As shown in
To start, an over-cladding tube 14 in which the separate elements of the fiber preform 12 may be assembled is provided (104). The over-cladding tube 14 may be made of silica glass, preferably pure undoped silica glass, but of course it may be made of any appropriate material.
The fiber preform 12 is made (102) by generally providing in the over-cladding tube 14 the following elements: a core rod, at least one stress-applying part (SAP), inner filler rods and/or outer filler rods (106, 108, 110 and/or 112 respectively depicted in
As shown in the exemplary fiber preforms 12 of
Advantageously for an active PM optical fiber, the inner core 18 and the cladding 20 may be composed of doped silica glass, for example silica-based material containing a certain concentration of an active ion (e.g. ytterbium, neodymium, etc.) along with an appropriate concentration of one or more co-dopants typically found in active optical fibers (e.g. aluminum, germanium, phosphorous, boron, fluorine, etc.). In order to optimize the size of the doped core region of the resulting PM fiber, as is needed for high power applications, the cladding 20—or in the case of a multi-layer cladding at least the innermost cladding layer 20A—may have a refractive index lower than that of the inner core 18. However, as mentioned above, the cladding may have any appropriate refractive index profile, for example a depressed cladding profile, a pedestal cladding profile, etc.
At least one stress-applying part (SAP) 24 is provided (108) and disposed adjacent to the core rod 16 along an outer periphery of the cladding 20 thereof. The stress-applying part 24 includes material having a coefficient of thermal expansion that is different from that of the cladding 20 of the core rod 16. Due to the difference in coefficients of thermal expansion, a mechanical stress is applied to the inner core-cladding region 16A of the PM fiber 10 shown in
Since the SAP 24 is disposed adjacent to the core rod 16 along an outer periphery of the cladding 20 of the core rod 16, the refractive index of the SAP 24 may be lower than, substantially the same as or as close as possible to the refractive index of the cladding 20, or at least the outer cladding layer 20B in the case of a multi-layer cladding. It is actually advantageous to have SAPs with a refractive index lower than that of the cladding to help mode mixing and also to enhance the power density in the cladding, which may be preferable for short fibers. SAPs may have a refractive index substantially the same as that of an outer layer of the cladding so as to prevent diffusion of light that is travelling through the cladding 20 to the SAP 24. Of course, the SAP(s) may have a refractive index slightly higher than that of the cladding without causing any detrimental effects to the operation of the resultant PM fiber.
As seen in the exemplary fiber preforms of
The doped silica core 26 of the SAP 24 may contain germanium oxide, boron oxide, fluorine, phosphorous oxide, lead oxide, aluminum oxide, zirconium oxide, or any combination thereof so as to exhibit a coefficient of thermal expansion that is greater than that of pure silica glass. Titanium oxide may be used to decrease the coefficient of thermal expansion. Now, among the above-mentioned compounds, germanium oxide, phosphorous oxide, titanium oxide, lead oxide, aluminum oxide, and zirconium oxide may be introduced into silica glass to increase the refractive index of silica glass while boron oxide and fluorine may be introduced into silica glass to reduce the refractive index of silica glass.
One or more SAPs 24 may be used to create the stress-induced birefringence in the resultant PM fiber 10. The SAPs 24 may be disposed at discrete intervals along the outer periphery of the cladding 20 of the core rod 16. In one embodiment, two SAPs 24 may be diametrically opposed along the outer periphery of the cladding 20. In another embodiment, two or more SAPs may be disposed symmetrically, with respect to an axis of the core rod 16, along the outer periphery of the core rod 16. It should be noted that although two diametrically opposed SAPs are depicted in the exemplary fiber preforms of
In order to create a thick doped cladding region 38 of the PM fiber 10 as shown in
The inner filler rods are made of a material having a coefficient of thermal expansion different from that of the stress-applying part (SAP) 24, but preferably similar to that of the cladding 20 and inner core 18 of the core rod 16 so as not to negate the stress-induced birefringence created by the SAP 24.
The inner filler rods may have a refractive index the same as, higher than or lower than the cladding 20. Not all of the inner filler rods need have the same refractive index. Generally, the inner filler rods have a refractive index consistent with the desired refractive index profile of the core-cladding region and the optical properties of the resultant PM fiber.
In accordance with an aspect of the present invention, a plurality of outer filler rods 36 consisting of a same material as the over-cladding tube 14 may be provided (112) and arranged adjacent the over-cladding tube 14, between the over-cladding tube 14 and the inner filler rods, the SAP 24 and the core rod 16, to enhance mode-mixing. The role of the outer filler rods 36 is to break the circular symmetry of the fiber to ensure a good energy transfer from the helicoidal modes (propagating in the cladding) exhibiting no overlap with the fiber core to the modes that do cross the core. That way, the absorption of pump light propagating in the cladding by the central active core is enhanced. For a perfectly circularly symmetric fiber, the power contained in the helicoidal modes would not be absorbed by the core of the fiber and would not be available for amplification. The outer filler rods 36 can be seen in
The outer filler rods 36 preferably have substantially a same refractive index as the over-cladding tube 14 and substantially a same coefficient of thermal expansion as the over-cladding tube 14.
For the exemplary fiber preform 12 shown in
With the elements of the fiber preform 12 thus assembled, the fiber preform is drawn (116) into an elongated fiber, using for example a fiber drawing tower, thus obtaining the PM optical fiber. As seen from the exemplary PM fiber found in
Therefore, in accordance with an aspect of the present invention, there is provided a PM fiber obtained from the above-described method, for use in a fiber amplifier or fiber laser, more preferably a high power fiber amplifier or fiber laser with mode-filtering possibilities as explained previously hereinabove.
In accordance with yet another aspect of the present invention, there is also provided the fiber preform described hereinabove and shown in
Summarily, the fiber preform 12 includes:
The fiber preform may include several outer filler rods 36 arranged within the over-cladding tube 14, adjacent the over-cladding tube 14, between the over-cladding tube 14 and the inner filler rods, the stress-applying part 24 and the core rod 16, the outer filler rods 36 consisting of a same material as the over-cladding tube 14.
The inner filler rods 30 may have various diameters as seen in
In one particular implementation of the design of the PM fiber 10, the following different elemental-preform types may be needed to fabricate the fiber preform 14:
To realize the fiber preform assembly, it may be necessary to draw or stretch the above-mentioned elemental preforms in order to obtain canes of suitable dimension to be used in the assembly. Then, those canes are preferably chemically etched in order to remove the unwanted outer region composed, for example, of pure silica. In order to make the large thick cladding region of the PM optical fiber, it is important to etch the canes to remove the unwanted outer layer of pure silica and obtain the wanted inner doped region. However, it may be desirable to leave a thin layer 28 (e.g. made of pure silica) outside the doped core 26 of the SAP 24. For example, in the case of a boron-doped core of the SAP, the thin cladding layer 28 serves to prevent contact of the boron-doped core with the acid used in the etching process because boron reacts strongly with the acid. The presence of the thin cladding layer poses no optical problem for the SAP which preferably has a refractive index lower than that of the cladding of the core rod and hence the cladding of the resultant PM fiber.
In view of the above description, the present invention is also directed to the polarization-maintaining optical fiber obtained according to the method of the present invention. The polarization-maintaining optical fiber, obtained according to the method of the present invention, may be advantageously used in a fiber amplifier, fiber laser or in any other appropriate optical device.
Numerous modifications could be made to any of the embodiments described above without departing from the scope of the present invention as defined in the appended claims.
This application claims benefit of Ser. No. 61/019,698, filed Jan. 8, 2008 in the United States and which application is incorporated herein by reference. To the extent appropriate, a claim of priority is made to the above disclosed application.
| Number | Date | Country | |
|---|---|---|---|
| 61019698 | Jan 2008 | US |
