Large diameter optical waveguide, grating, and laser

CA 02394910 2010-12-03

Large Diameter Optical Waveguide, Grating, and Laser

TECHNICAL FIELD

This invention relates to optical waveguides, gratings and lasers and more particularly to optical waveguides, grating and laser having a large diameter.

BACKGROUND ART

It is known that optical fibers are stronger in compression than in tension, as described in U.S. Pat. No. 5,469,520, entitled Compression Tuned Fiber Grating to Morey, et al. It is also known that Bragg gratings embedded in the optical fiber may be used in compression to act as a tunable filter or tunable fiber laser, as is described in U.S. Pat. No. 5,469,520, entitled Compression Tuned Fiber Grating to Morey, et al and U.S. Pat. No. 5,691,999, entitled Compression Tuned Fiber Laser to Ball et al., respectively.

To avoid fiber buckling under compression, the technique described in the aforementioned U.S. Pat. Nos. 5,469,520 and 5,691,999 uses sliding ferrules around the fiber and grating and places the ferrules in a mechanical structure to guide, align and confine the ferrules and the fiber. However, it would be desirable to obtain a configuration that allows a fiber grating to be compressed without buckling and without sliding ferrules and without requiring such a mechanical structure.

Also, it is known to attach an optical fiber grating to within a glass tube to avoid buckling under compression for providing a wavelength-stable temperature compensated fiber Bragg grating as is described in U.S. Pat. No. 5,042,898, entitled Incorporated Bragg Filter Temperature Compensated Optical Waveguide Device, to

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Morey et al. However, such a technique exhibits creep between the fiber and the tube over time, or at high temperatures, or over large compression ranges.

It is also known that Bragg gratings in optical fibers may create unwanted coupling between the core and cladding modes of a fiber. The larger the mode field overlap between the two modes, the larger the coupling. Such coupling creates unwanted optical losses in the fiber.

It is also known that when writing Bragg gratings in optical fibers, if high power is used, the fiber may be damaged due to surface ablations on the surface of the fiber caused by the high optical power.

Summary of the Invention

Objects of the present invention include provision of a waveguide configuration that is suitable for compression without buckling and without requiring sliding ferrules or a mechanical supporting structure for the ferrules and/or that is suitable for reducing core to cladding coupling and/or that allow for increase optical power to be used when writing gratings.

According to the present invention, a large diameter optical waveguide, comprises an outer cladding having at least one inner core disposed therein which propagates light; said core propagating light in substantially a few spatial modes; and having an outer waveguide dimension of said waveguide being greater than about 0.3 mm.

According further to the present invention, said core has an outer core dimension of less than about 12.5 microns. According further to the present invention, said core propagates light in substantially a single spatial mode.

According further to the present invention, the waveguide has a cylindrical shape. According further to the present invention the core has a circular end crosssectional shape. According further to the present invention the outer dimension of the waveguide is greater than about 0.9 mm. According still further to the present invention a reflective element is embedded in the waveguide.

The present invention provides a significant improvement over prior art optical fibers by providing a glass (or silica-based) optical waveguide having a large

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PCT/US00/32934 diameter which allows for axial compression tuning without buckling the waveguide. The invention provides a surface to push against instead of having to bond, fuse or otherwise attach such surface to an optical fiber. The present invention also reduces coupling between the core and cladding modes due to the increased end crosssectional area between the core and cladding of the waveguide. Further, the invention allows for more optical power to be used when writing a grating in the waveguide without ablating the waveguide surface, due to the increased thickness of the cladding.

One or more gratings may be embedded (or imprinted) in the waveguide. The waveguide may be used for compressing a grating or laser to provide compression tuning in many applications. The waveguide may also be used for other applications that do not require a grating but require a compressible optical waveguide. Also, the large outer diameter allows the waveguide to resist damage from handling which typical bare optical fibers would exhibit.

The waveguide may be etched to provide alternative side (or axial) crosssectional geometries, e.g., a dogbone shape, that provides enhanced force to length change (or strain) sensitivity. Also, the waveguide may resemble a short “block” or a longer “cane” type, depending on the application and dimensions used. Further, the waveguide may be made in long lengths (on the order of inches, feet, or meters) then cut to size as needed for the desired application. Also, one or more gratings, fiber lasers, or a plurality of cores or concentric or ring cores, may be located within the waveguide cladding, thereby allowing for multiple waveguiding paths within the waveguide.

The foregoing and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof.

Brief Description of the Drawings

Fig. 1 is a side view of a large diameter optical waveguide, in accordance with the present invention.

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Fig. 2 is a side view of a large diameter optical waveguide with a grating embedded therein, in accordance with the present invention.

Fig. 3 is a side view of a large diameter waveguide having a pair of gratings embedded therein, in accordance with the present invention.

Fig. 4 is a side view of a large diameter waveguide having a DFB laser embedded therein, in accordance with the present invention.

Fig. 5 is an end view of a large diameter optical waveguide having two optical cores, in accordance with the present invention.

Fig. 6 is an end view of a large diameter optical waveguide having an elliptical core, in accordance with the present invention.

Fig. 7 is an end view of a large diameter optical waveguide having a plurality of concentric optical cores, in accordance with the present invention.

Fig. 8 is a graph of an optical transmission profile of a grating in a standard optical fiber showing cladding mode coupling, in accordance with the present invention.

Fig. 9 is a graph of an optical transmission profile of a grating in a large diameter optical waveguide showing reduced cladding mode coupling, in accordance with the present invention.

Best Mode for Carrying Out the Invention

Referring to Fig. 1, a large diameter optical waveguide 10, has at least one core 12 surrounded by a cladding 14. The waveguide 10 comprises silica glass (S1O2) based material having the appropriate dopants, as is known, to allow light 15 to propagate in either direction along the core 12 and/or within the waveguide 10. The core 12 has an outer dimension dl and the waveguide 10 has an outer dimension d2.

The cladding 14 has an outer dimension d2 of at least about 0.3 mm and the core 12 has an outer dimension dl such that it propagates only a few spatial modes (e.g., less than about 6). For example for single spatial mode propagation, the core 12 has a substantially circular transverse cross-sectional shape with a diameter dl less than about 12.5 microns, depending on the wavelength of light. One standard telecommunications nominal core diameter is 9 microns (and outer waveguide

-4CA 02394910 2011-06-30 diameter of 125 microns). The invention will also work with larger or non- circular cores that propagate a few (less than about 6) spatial modes, in one or more transverse directions.

Further, the optical waveguide 10 may be a biréfringent, polarization maintaining, polarizing, multi-core, or multi-cladding optical waveguide, or planar waveguide (where the waveguide is rectangular shaped), or other waveguides meeting the proper-ties described herein.

Also, other materials for the optical waveguide 10 may be used if desired. For example, the waveguide 10 may be made of any glass, e.g., silica, phosphate glass, or other glasses, or made of glass and plastic, or solely plastic. For high temperature applications, an optical waveguide made of a glass material is desirable.

In one embodiment, at least a portion of the optical waveguide 10 has a transverse cross-section that is continuous and comprises a substantially similar material.

In another embodiment, at least a portion of the optical waveguide 10 has a transverse crosssection that is continuous and comprises a substantially homogenous material.

Also, the waveguide 10 may have tapered (or beveled or angled) outer comers or edges 24 to provide a seat for the waveguide 10 to mate with another part (not shown) and/or to adjust the force angles on the waveguide 10, or for other reasons. The angle of the beveled comers 24 are set to achieve the desired function.

Also, the outer diameter d2 of the cladding 14 and the length L have values that will resist buckling when the waveguide 10 is placed in axial compression as indicated by the arrows 18. The length L of the waveguide 10 may be determined experimentally or analytically as discussed more hereinafter.

Referring to Fig. 2, the waveguide may have a Bragg grating 16 impressed (or embedded or imprinted) therein. A Bragg grating 16, as is known, is a periodic or aperiodic variation in the effective refractive index and/or effective optical absorption coefficient of an optical waveguide, such as that described in US Patent No. 4,725,110 and 4,807,950, entitled Method for Impressing Gratings Within Fiber Optics, to Glenn et al; and US Patent No. 5,3 8 8,173, entitled Method and Apparatus for Forming Aperiodic Gratings in Optical Fibers, to Glenn, which are hereby incorporated by reference to the extent necessary to understand the present invention. The grating 16 may be in the core 12 and/or in the cladding 14 (not shown). Any wavelength-tunable grating or reflective element embedded, etched, imprinted, or otherwise formed in the waveguide 10 may be used if desired. The waveguide 10 may be photosensitive if a grating 16 are to be written into the waveguide 10. As used

CA 02394910 2009-01-20 herein, the term “grating” means any of such reflective elements. Further, the reflective element (or grating) 16 may be used in reflection and/or transmission of light.

The grating 16 has a grating length Lg which is determined based on the application and may be any desired length. A typical grating 16 has a grating length Lg in the range of about 3-40 mm. Other sizes or ranges may be used if desired. The length Lg of the grating 16 may be shorter than or substantially the same length as the length L of the waveguide 10. Also, the core 12 need not be located in the center of the waveguide 10 but may be located anywhere in the waveguide 10.

Incoming light 27 may be launched into the waveguide 10 and/or the core 12 by splicing a suitable standard optical fiber 22 (having a cladding 26 and a core 25) to one or both axial ends 28 of the waveguide 10 using any known or yet to be discovered techniques for splicing fibers or coupling light from an optical fiber into a larger waveguide, that provides acceptable optical losses for the application. Alternatively, incident light 34 may be directly incident on the core or be focussed into the core 12 by a lens 32 which focuses input light 30.

If a grating 16 is in the waveguide 10, light 34 is incident on the grating 16 which reflects a portion thereof as indicated by a line 36 having a predetermined wavelength band of light, and passes the remaining wavelengths of the incident light 34 (within a predetermined wavelength range), as indicated by a line 38 (as is known).

It is known that for a mechanically unguided silica optical fiber under compression, the maximum allowable length Lcr of the waveguide 10 can be without buckling depends on the critical applied load Fcr, and can be analytically determined based on the buckling theory of columns, as described in A. Iocco, et al, “Bragg Grating Fast Tunable Filter for Wavelength Division Multiplexing”, IEEE, Journal of Lightwave Technology, Vol. 17, No. 7, July 1999, pp 1217-1221.

As discussed in the aforementioned article, the Euler buckling load formula for a column is:

<img file="CA2394910C_D0001.tif" />

-6CA 02394910 2011-06-30 where Fcr is the critical force, L is the pre- compressed unguided length, and a is the effective-length factor which is 0.5 for a column perfectly fixed (rigid) and straight at both ends, where and where I = π?/4 the inertia modulus for a circular cross section, and r is the radius of the column(or waveguide).

The relationship between the axial stress and axial strain for a material in the elastic domain and in the axial (or longitudinal) direction, is given by:

σ=Ε€ Eq. 2 where E is the Young modulus which is 7.25 x 10⁴ N/mm² for silica and € is the axial strain which is defined by AL/L, where AL is the axial displacement and L is axial length.

If a Bragg grating is formed in the waveguide, the relationship between axial strain and wavelength shift for the grating is as follows:

ΔΑ λ

Eq. 3 where Pe is the photo-elastic coefficient which is about 0.21 for silica λ is the initial wavelength and Δ λ is the wavelength shift. Substituting Eqs. 1,2,3 and taking into account that the axial stress σ is given by the applied force multiplied by the waveguide surface area, the critical length Lcr is calculated as follows:

™ U(l-p_e)

2α V ΔΛ

Eq. 4

For example, for a mechanically unguided standard telecommunications silica based optical fiber having an outer diameter of 125 microns (r =0.0625 mm in Eq. 1) and an initial wavelength λ of about 1550 nm, Lcr =1.08 mm before buckling occurs for a wavelength shift Δλ of 40 nanometers (nm). Accordingly, a standard optical fiber would need to be no longer than 1.08 min long to not buckle under an axial strain equivalent to shift a grating 40 nm. However, in practice, this length will be shorter than 1.08 mm because the end conditions are in general not perfectly rigid and straight. This may be accounted for by adjusting the value of a in Eq. 1 for the waveguide end conditions.

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We have found that the wavelength shift (or tuning range) Δλ in nm that can be achieved without buckling for a silica waveguide having a given diameter (d) and length (L) is as follows:

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PCT/US00/32934 _ΔΛ _ (^²^²)(1200)

167?a²(F5)

Eq.5 where a is a waveguide end condition scale factor or “fixity scale factor” (discussed hereinbefore with Eq. 1) and FS is a safety scale factor. A value of a= 4 for ideally fixed end conditions and a=l for ideal pinned end conditions. We have found that a value for a of about 0.6 is representative for a waveguide with coned seat end conditions, i.e., the tapered edges 24 (30 to 60 degrees from vertical; however, other angles may be used). Other values for a may be used depending on the geometry of the end conditions, the geometry of the waveguide, e.g., dogbone or straight, discussed hereinafter, and other factors. The value for FS is set based on dimensional tolerances, reliability, repeatability, statistical data variations, and other factors.

Accordingly, we have found that an outer diameter d2 of greater than about 400 microns (0.4 mm) provides acceptable results (without buckling) for a waveguide length L of 5 mm, over a grating wavelength tuning range of about 10 nm (with FS=1.3). For a given outer diameter d2, as the length L increases, the wavelength tuning range (without buckling) decreases. Other diameters d2 for the waveguide 10, may be used depending on the overall length L of the waveguide 10 and the desired amount of compression length change AL or wavelength shift Δλ. For example, for an outer diameter d2 of 1mm, having a length L of 5mm, the tuning range without buckling is about 64nm (with FS=1.3); and for an outer diameter of 1mm and a length L of about 20 mm, the tuning range without buckling is about 4nm (with FS=1.3). An outer diameter d2 as low as 0.3 mm will likely provide acceptable performance, depending on the grating length and desired tuning range.

The waveguide 10 may be made using fiber drawing techniques now known or later developed that provide the resultant desired dimensions for the core and the outer diameter discussed hereinbefore. As such, the external surface of the waveguide 10 will likely be optically flat, thereby allowing Bragg gratings to be written through

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PCT/US00/32934 the cladding similar to that which is done for conventional optical fiber. Because the waveguide 10 has a large outer diameter compared to that of a standard optical fiber (e.g., 125 microns), the waveguide 10 may not need to be coated with a buffer and then stripped to write the gratings, thereby requiring less steps than that needed for conventional optical fiber gratings. Also, the large outer diameter d2 of the waveguide 10, allows the waveguide to be ground, etched or machined while retaining the mechanical strength of the waveguide 10. Thus, the present invention is easily manufacturable and easy to handle. Also, the waveguide 10 may be made in long lengths (on the order of many inches, feet, or meters) then cut to size as needed for the desired application.

Also, the waveguide 10 does not exhibit mechanical degradation from surface ablation common with optical fibers under high laser fluence (or power or intensity) during grating exposure (or writing). In particular, the thickness of the cladding between the cladding outer diameter and the core outer diameter causes a reduced power level at the air-to-glass interface for a focussed writing beam.

Referring to Figs. 8,9, we have also found that the present invention also reduces coupling between the core and cladding modes due to the increased end cross-sectional area between the core and cladding of the waveguide. Thus, a grating 16 written in the core 12 of the waveguide 10 exhibits less optical transmission loss and a exhibits a cleaner optical profile than a conventional fiber grating because the large cladding region dissipates coupled cladding modes, thereby reducing the coupling of the core 12 to the cladding 14 modes. In general, the greater the difference in cross-sectional area between the core 12 and the cladding 14 the smaller the mode field overlap and the lower the coupling to the cladding modes. The thickness of the cladding 14 between the cladding outer diameter and the core outer diameter may be set to optimize this effect. Fig. 8 shows an optical transmission profile for a standard grating in an optical fiber having a 9 micron core diameter and 125 micron outer diameter. Such a grating exhibits coupling to the cladding modes as indicated by the spikes 100. Fig. 9 shows an optical transmission profile for a standard grating written in the waveguide 10 having a 9 micron core diameter and a 3 mm outer diameter which exhibits greatly reduced coupling to the cladding modes as

-9CA 02394910 2009-01-20 indicated by the lack of spikes on the profile. Other diameters of the core 12 and waveguide 10 may be used if desired such that the cladding modes are reduced to the desired levels.

Also, the waveguide 10 has a large end surface area to attach an optical fiber pigtail to the waveguide 10 or for attaching multiple optical fibers to multiple cores in the waveguide 10. Also, the size of the waveguide 10 has inherent mechanical rigidity which improves packaging options and reduces bend losses.

The side cross-section of the outer surface of the waveguide 10 may have a varying geometry, depending on the application. For example, the waveguide 10 may have a “dogbone” shape having a narrow central section 20 and larger outer sections 21. The dogbone shape may be used to provide increased sensitivity in converting axial force to length change AL and/or wavelength shift Δλ of the grating 16 and may be achieved by etching, grinding, machining, heating &amp; stretching, or other known techniques.

The waveguide 10 may have end cross-sectional shapes other than circular, such as square, rectangular, elliptical, clam-shell, octagonal, multi-sided, or any other desired shapes, discussed more hereinafter. Also, the waveguide may resemble a short “block” type or a longer “cane” type geometry, depending on the length of the waveguide and outer dimension of the waveguide.

Referring to Fig. 3, for any of the embodiments described herein, instead of a single grating encased within the waveguide 10, two or more gratings 50,52 may be embedded in the waveguide 10. The gratings 50,52 may have the same reflection wavelengths and/or profiles or different wavelengths and/or profiles. The multiple gratings 50,52 may be used individually in a known Fabry Perot arrangement.

Further, one or more fiber lasers, such as that described in US Patent No. 5,666,372, “Compression-Tuned Fiber Laser” may be in the waveguide 10 and may be compression-tuned. In that case, the gratings 50,52 form a cavity and the waveguide 10 at least between the gratings 50,52 (and may also include the gratings 50,52 and/or the waveguide 10 outside the gratings, if desired) at least a portion of which is doped with a rare earth dopant, e.g., erbium and/or yetterbium, etc.,

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PCT/US00/32934 and the lasing wavelength would be tuned accordingly as the force on the waveguide changes.

Referring to Fig. 4, another type of tunable fiber laser that may be used is a tunable distributed feedback (DFB) fiber laser, such as that described in V.C.

Lauridsen, et al, “Design of DFB Fibre Lasers”, Electronic Letters, Oct. 15, 1998, Vol.34, No. 21, pp 2028-2030; P. Vanning, et al, “Erbium Doped Fiber DGB Laser With Permanent π/2 Phase-Shift Induced by UV Post-Processing”, IOOC’95, Tech. Digest, Vol. 5, PD1-3, 1995; US Patent No. 5,771,251, “Optical Fibre Distributed Feedback Laser”, to Kringlebotn et al; or US Patent No. 5,511,083, “Polarized Fiber 10 Laser Source”, to D’Amato et al. In that case, a grating 84 is written in a rare-earth doped core and configured to have a phase shift of λ/2 (where λ is the lasing wavelength) at a predetermined location 56 near the center of the grating 16 which provides a well defined resonance condition that may be continuously tuned in single longitudinal mode operation without mode hopping, as is known. Alternatively, instead of a single grating, the two gratings 50,52 (Fig. 3) may be placed close enough to form a cavity having a length of (N + Ά)λ, where N is an integer (including 0) and the gratings 50,52 are in rare-earth doped fiber. The gratings 50,52 may have the same reflection wavelengths and/or profiles or different wavelengths and/or profiles.

Alternatively, the DFB laser 234 may be located on the fiber 10 between the pair of gratings 220,222 (Fig. 8) where the fiber 10 is doped with a rare-earth dopant along at least a portion of the distance between the gratings 220,222. Such configuration is referred to as an “interactive fiber laser”, as is described by J.J. Pan et al, “Interactive Fiber Lasers with Low Noise and Controlled Output Power”, E-tek Dynamics, Inc., San Jose, CA, internet web site www.e-tek.com/products/ whitepapers. Other single or multiple fiber laser configurations may be disposed on the fiber 10 if desired.

Alternatively, the DFB laser 84 may be located between a pair of gratings

90,92 where the core 12 is doped with a rare-earth dopant at least a portion of the distance between the gratings 90,92. Such configuration is referred to as an “interactive fiber laser”, such as is described by J.J. Pan et al, “Interactive Fiber

Lasers with Low Noise and Controlled Output Power”, E-tek Dynamics, Inc., San

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Jose CA, internet web site www.e-tek.com/products/whitepapers. Other single or multiple fiber laser configurations may be disposed in the waveguide 10 if desired.

Referring to Fig. 5, alternatively, two or more cores 60,62,63,65 may be located within the waveguide 10. The core 12 (Figs. 1-4) may be positioned axially anywhere within the waveguide 10 and need not be centered along the center line of the waveguide 10. Also, cores (Fig. 5) may be located close to each other (to be substantially touching or optically coupling to each other) as shown by the cores 63,65, and/or separated by any desired distance within the waveguide 10, as shown by cores 60,62. For multiple cores with different diameters, each of the cores should meet the requirements described herein for the core 12.

Referring to Fig. 6, alternatively, the core 12 may have an asymmetrical end cross-sectional shape such as elliptical shape 64 or a rectangular shape 66. For asymmetrical end cross-sectional shapes, the smaller dimension dl would be used for determining the maximum core dimension. In that case, the core may propagate only one mode in the direction of the dimension dl and propagate a few modes in the other direction. Also, the end cross-sectional shape of the core may have other shapes such as a square, clam-shell, octagonal, multi-sided, or any other desired shapes.

Also, the outer geometry may have end cross-sectional shapes, such as a rectangle shown by dashed lines 80 if desired. In that case, the larger dimension d2 would be used for determining the minimum outer dimension.

Referring to Fig. 7, alternatively, the waveguide may have multiple concentric cores 68,70 or a ring or annulus core 72. In that case, the dimension dl for calculating the minimum dimension for the core would be the core having the smallest outer dimension.

The dimensions and geometries for any of the embodiments described herein are merely for illustrative purposes and, as such, any other dimensions may be used if desired, depending on the application, size, performance, manufacturing requirements, or other factors, in view of the teachings herein.

It should be understood that, unless stated otherwise herein, any of the features, characteristics, alternatives or modifications described regarding a particular

- 12CA 02394910 2011-06-30 embodiment herein may also be applied, used, or incorporated with any other embodiment described herein. Also, the drawings herein are not drawn to scale.

Although the invention has been described and illustrated with respect to exemplary embodiments thereof, the foregoing and various other additions and omissions may be made therein and thereto without departing from the scope of the appended claims.