With the development of micro-optics and all-fiber technologies mainly driven by the telecommunication industry, there is an increased need for miniaturized optics and especially miniaturized optical devices that could be simply assembled with optical waveguides in general and optical fibers in particular. Among those devices, microlenses are of particular interest. The field of microlenses combined with optical fibers is quite extensive and comprises many applications such as: coupling into optical fiber light sources such as laser (Cohen L. G. and Schneider M. V. Appl. Opt. (1974) Vol. 13 n°1, pp. 89 94 “Microlenses for coupling junction lasers to optical fibers”; Ghafoori-Shiraz H. and Asano T. Opt. Lett. (1986) Vol. 11 n°8, pp. 537 539 “Microlens for coupling a semiconductor laser to a single-mode fiber”; Lin G.-R. IEEE Phot. Tech. Lett. (2003) Vol. 15 n°9, pp. 1201 1203 “Improving the quantum efficiency of erbium-doped fiber laser by using a low-cost tipped fiber splicing process”; and Ozyazici M. S. Opt. Appl. (2004) Vol. XXXIV n°2, pp. 185 202 “Increasing semiconductor laser-optical fiber coupling efficiency by introducing microlens”) or light emitting diodes (Park E.-H. et al. IEEE Phot. Tech. Lett. (1999) Vol. 11 n°4, pp. 439 441 “Microlens for efficient coupling between LED and optical fiber”), coupling light detectors with optical fibers, coupling between identical or different types of optical fibers, coupling optical fibers with planar waveguides, coupling optical fibers with other photonic components, various sensor applications, applications in micro-optics etc.
A Variety of microlens designs used in combination with an optical fiber are known in the art. Microlenses are often used to separate components positioned in the vicinity of an optical fiber tip and to perform various functions such as, most often, collimation or focusing of light that enters or exits single-mode or multimode optical fibers. It is known in the art that due to the size and to the shape of such devices, the process of microlens alignment in front of an optical fiber is very complex, always expensive and in many cases an impractical task. Some solutions involving the use of arrays have however been proposed.
Once alignment is performed, microlenses are usually bonded to the optical fibers typically using various specialty adhesives, such as polymeric adhesives. Beside the constraints for selecting such adhesive in relation to their adhesion and optical properties, the main problem of this approach is the mechanical instability of the adhesives with time and temperature, which is unacceptable as microlenses are usually used in devices where positioning is critical. The use of polymeric adhesives is therefore problematic since they have increased temperature dependence and are also susceptible to other environmental factors such as the presence of organic vapors that could make them swell or shrink or modify their optical properties, usually by reducing their light transmission, not mentioning the fact that they are often aging with time and are also frequently degraded by high light power (usually in the short energetic wavelength range) and by high temperature. Some inorganic adhesives such as low melting point solder glasses could alternatively be used for demanding applications, but they have also their limitations and create problems associated with residual thermal stresses. For applications where high light intensity and high temperature could be generated, such as in applications with fiber high power solid state lasers, the use of adhesives for microlens assembly could be in some cases problematic.
Quite few methods described in the literature allow the formation of a microlens at the tip of an optical fiber with a simple and precise positioning of the microlens. Some of them use of a transparent polymer to create a microlens. The polymeric microlens could be for instance shaped by laser beam melting (such as described in U.S. Pat. No. 4,380,365), by photolithography techniques (Minh P. N. et al Opt. Rev. (2003) Vol. 10 n°3, pp. 150 154 “Batch fabrication of microlens at the end of optical fiber using self-photolithography and etching techniques”), by dry resist process involving polymerization induced by high energy electrons (Babin S. et al. J. Vac. Sci. Technol. B (1996) Vol. 14 n°6, pp. 4076 4079 “Fabrication of a refractive microlens integrated onto the monomode fiber”) or more simply by surface tension (Kim K. R. et al. IEEE Phot. Tech. Lett. (2003) Vol. 15 n°8, pp. 1100 1102 “Refractive microlens on fiber using UV-curable fluorinated acrylate polymer by surface-tension”) or by hydrophobic effects (Hartmann D. M. et al. IEEE Phot. Techn. Lett. (2001) Vol. 13 n°10, pp. 1088 1090 “Microlenses self-aligned to optical fibers fabricated using the hydrophobic effect”). Although those polymeric microlenses may have interesting optical properties and advantages, such as for some techniques the possibility of custom surface shaping, their long term use and their temperature resistance is compromised due to the polymeric material used in their design.
A better stability could be obtained if the microlens is composed of glass instead of polymer molecules. Several approaches involving glass microlenses manufacturing have been proposed so far. In some reported methods such as described in patent EP 1 298 460 A1 or in published papers by Modavis R. A. and Webb T. W. IEEE Phot. Techn. Lett. (1995) Vol. 7 n°7, pp. 798 800 “Anamorphic microlens for laser diode to single-mode fiber coupling” and Yeh Z.-M. et al. J. Lightwave Tech. (2004) Vol. 22 n°5, pp. 1374 1379 “A novel scheme of lensed fiber employing a quadrangular-pyramid-shaped fiber end face for coupling between high-power laser diodes and single-mode fibers”, a precise polishing of the end of an optical fiber is performed to create a microlens. Some other reported manufacturing techniques involve laser micromachining of the tip of an optical fiber such as reported in patent EP 0 430 532 or in the published paper by Presby H. M. and Edwards C. A. Electron. Lett. (1992) Vol. 28 n°6, pp. 582 584 “Near 100% efficient fibre microlens”. Although a precise micromachining allows the shaping of interesting microlens profiles such as hyperbolic shapes, such techniques are complicated and not well suited for low-cost and high volume microlens manufacturing. Another approach described in the literature consists of shaping by heat melting the end of an optical fiber, generally using an arc-discharge fiber splicer, (such as for example described in U.S. Pat. No. 5,563,969 or in published papers by Shiraishi K. et al. J. Lightwave Tech. (1995) Vol. 13 n°8, pp. 1736 1744 “A fiber lens with a long working distance for integrated coupling between laser diodes and single-mode fibers”; and Shiraishi K. et al. IEEE J. Lightwave Tech. (1997) Vol. 15 n°2, pp. 358 364 “A lensed-fiber coupling scheme utilizing a graded-index fiber and a hemispherically ended coreless fiber tip”). An alternate method using also arc-discharge heating is described in U.S. Pat. No. 5,551,968 where a microlens at the tip of a fiber is formed by jerking apart two fused fibers. For all methods involving shaping or creating the glass microlens with heat, the size of the microlens could in some cases exceed the diameter of the optical fiber and the shape of the microlens is often difficult to control accurately. The main drawbacks of these methods are limited repeatability and time-consuming fabrication process that requires individual machining of each produced microlens.
The use of chemical etching to help creating microstructures is the basis of some of the most important technologies used in the semiconductor industry, such as for instance photolithography. However there are only few examples in the literature applying this concept for shaping glass microlenses at the tip of an optical fiber. For instance interesting papers were published describing how to produce chalcogenide-glass microlenses attached to optical fibers (Saitoh A. et al. Opt. Lett. (2000) Vol. 25 n°24, pp. 1759 1761 “Chalcogenide-glass microlenses attached to optical-fiber end surfaces”; and Saitoh A. et al. J. Non-cryst. Solids (2002) Vol. 299 302, pp. 983 987 “Chalcogenide-glass microlenses for optical fibers”). In those papers an As2S3 film, which is first deposited under vacuum at the tip of an optical fiber, is allowed to cross-link under illumination coming from the optical fiber so that the cross-linked area is centered with the fiber core. The film is then etched away using a basic solution with controlled conditions in order to produce a plano-convex chalcogenide-glass microlens. U.S. Pat. Nos. 4,469,554 and 5,800,666 disclose a microlens fabrication method also using chemical etching. The optical fiber, reshaped with chemical etching, has a conical shape with a rounded tip. The fabrication process consists of a precise controlled pulling of an optical fiber out of an etching solution in order to achieve the desired shape of the optical fiber tip. A microlens fiber fabricated by direct etching of a single-mode fiber and then by melting the tip of the fiber was also published (Kawashi M. and Edahiro T. Electron. Lett. (1982) Vol. 18 n°2, pp. 71 72 “Microlens formation on VAD single-mode fibre ends”; Barnard C. W. and Lit J. W. Y. Appl. Opt. (1991) Vol. 30 n°15, pp. 1958 1962 “Single-mode fiber microlens with controllable spot size”; Lay T.-S. et al. Jpn. J. Appl. Phys. (2003) Vol. 42, pp. 453 455 “1.55-μm fiber grating laser utilizing an uncoated tapered hemispherical-end fiber microlens”).
In the literature, most of the microlenses are directly assembled at the tip of an optical fiber. However there are few examples where a spacer is used to optimize the optical properties of the microlens. In one paper (Kalonji N. and Semo J. Electron. Lett. (1994) Vol. 30 n°11, pp. 892 894 “High efficiency, long working distance laser diode to single mode fibre coupling arrangement”) a section of graded index multimode fiber (GRIN MMF) is fused to a single-mode fiber and a suitable amount of glass is heat deposited on the spacer and is finally heat shaped into a microlens. In another paper (Kim K.-R. et al. IEEE Phot. Tech. Lett. (2003) Vol. 15 n°8, pp. 1100 1102 “Refractive microlens on fiber using UV-curable fluorinated acrylate polymer by surface-tension”) a coreless silica fiber (CSF) is used as a spacer between the single-mode fiber and the polymeric microlens.
A different chemical etching technique is disclosed in a paper of G. Eisenstein and D. Vitello (Applied Optics (1982) Vol. 21 n°19, “Chemically etched conical microlenses for coupling single-mode lasers into single-mode fibers”) where a selective etching of the optical fiber tip is used. The HF acid buffered with NH4F is used as etchant where the etching rate of GeO2 doped core is lower than the etching rate of pure silica cladding. Different etching rates result in formation of a conical pike at the tip of an optical fiber that acts as a microlens. The shape and radius of microlens is therefore determined by profile of the single-mode lead optical fiber. The authors mentioned that the cone could be reshaped to a hemispherical microlens by fire polishing or arc melting. They also showed a slight increase of coupling efficiency between a fiber with a microlens and a laser diode, but the technique remained undeveloped and it does not allow for the realization of a microlens with arbitrary size and appropriate quality.
In view of the above, there is still a need for a versatile, efficient and commercially viable technique for providing a microlens at the extremity of a lead fiber or other waveguide.
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