Self-aligning method of fiber-to-waveguide pigtailing
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Self-aligning method of fiber-to-waveguide pigtailing Brant C. Gibson and Shane T. Huntington NANO-MRNF, School of Physics, University of Melbourne, Parkville, Victoria 3010, Australia John D. Love Applied Photonics Group, Research School of Physical Sciences and Engineering, Australian National University, Canberra, Australian Capital Territory 0200, Australia Received May 18, 2005; revised manuscript received July 11, 2005; accepted July 17, 2005 An innovative self-aligning technique for the pigtailing of optical fibers to buried channel planar waveguides is presented, based on selective etching. This technique utilizes a plug-and-socket mechanism that is intrin- sically self-aligning and mechanically stable. The processes involved have been specifically designed to fa- cilitate the bulk manufacture of pigtailed single or multiple fibers and waveguides. An optimized alignment geometry for the physical connection of fibers to waveguides is presented. © 2005 Optical Society of America OCIS codes: 220.1140, 220.4610, 230.7380. Scalable, mechanically stable pigtailing techniques are of paramount importance for photonics applications. 1,2 In most cases, optical fibers need to be connected to planar waveguide structures in such a way as to allow efficient signal transmission across the connection. This task is challenging due to the in- trinsic differences in the geometry of circularly sym- metric fiber cores and nominally square-core planar waveguides. 3 The connection of optical fibers to pla- nar waveguides can be achieved using a number of existing techniques. The most popular utilize V grooves and gluing, with active or passive alignment of the cores. 4,5 Other available techniques involve metallization and soldering, 6 fusion technology, 7 or clip technology. 8 In addition to these methods, match- ing the fundamental mode of a waveguide and that of an attached optical fiber has been attempted through judicious choice of waveguide dimensions and refrac- tive indices. 9 Satisfactory fiber-to-waveguide connec- tions can be achieved using each of these techniques; however, the alignment in each case is necessarily in- volved, and both the fiber and the waveguide end faces ideally need to be polished to guarantee a high- quality join. In this Letter a new technique is demonstrated that utilizes a self-aligning technique based on the exact positioning of the doped guiding core region of the waveguide and the convection-controlled etching of the circularly symmetric fiber. In this case, the op- tical fiber and the planar waveguide are modified via chemical etching to produce a plug-and-socket ar- rangement. The core of the fiber essentially plugs di- rectly into the socket defined by the guiding region of the waveguide, thus producing a perfectly aligned, scalable connection. A pure-silica cladding-based buried channel wave- guide must possess a doped guiding core region, typi- cally with germanium, to create the necessary wave- guide parameters for the propagation of light. It has been observed that the etch rates of doped core glasses generally increase with increasing dopant ox- ide concentration. 10,11 If the planar structure is im- mersed in hydrofluoric (HF) acid, as shown schemati- cally in Fig. 1(a), the doped core region is found to etch at a greater rate because of the increase in solu- bility compared with that of the surrounding pure- silica cladding region. This has the effect of creating a recess in the end face of the planar structure di- rectly at the end of the guiding region. This selective etching thus forms the socket that is schematically depicted in Fig. 1(b). Initially, the end face of a buried channel wave- guide, with a germanium-doped core and a pure- silica cladding, was cleaved by a standard mechani- cal technique. A backilluminated optical image of the end face is shown in Fig. 2(a), which clearly shows the core, cladding, and substrate regions. The end face of the waveguide was then submersed in 48% HF acid to a depth of approximately 1 mm. Etching was carried out in a Teflon vessel at room temperature for 15 min to etch a 20 m socket with a mean width of 10 m. The exact etching time depends on the dopant concentration of the core and the desired shape (depth) of the socket. An optical image of the end face can be observed in Fig. 2(b), which shows the etched core region, and Fig. 2(c) clearly depicts this socket from above. This socket forms one half of this intrin- sically aligned pigtailing technique. Fig. 1. Schematic representation of (a) the immersion of the buried channel waveguide into a bath of HF acid to (b) selectively etch the waveguide core to form a socket. 2858 OPTICS LETTERS / Vol. 30, No. 21 / November 1, 2005 0146-9592/05/212858-3/$15.00 © 2005 Optical Society of America
Transcript of Self-aligning method of fiber-to-waveguide pigtailing
2858 OPTICS LETTERS / Vol. 30, No. 21 / November 1, 2005
Self-aligning method of fiber-to-waveguidepigtailing
Brant C. Gibson and Shane T. HuntingtonNANO-MRNF, School of Physics, University of Melbourne, Parkville, Victoria 3010, Australia
John D. LoveApplied Photonics Group, Research School of Physical Sciences and Engineering, Australian National University,
Canberra, Australian Capital Territory 0200, Australia
Received May 18, 2005; revised manuscript received July 11, 2005; accepted July 17, 2005
An innovative self-aligning technique for the pigtailing of optical fibers to buried channel planar waveguidesis presented, based on selective etching. This technique utilizes a plug-and-socket mechanism that is intrin-sically self-aligning and mechanically stable. The processes involved have been specifically designed to fa-cilitate the bulk manufacture of pigtailed single or multiple fibers and waveguides. An optimized alignmentgeometry for the physical connection of fibers to waveguides is presented. © 2005 Optical Society of America
OCIS codes: 220.1140, 220.4610, 230.7380.
Scalable, mechanically stable pigtailing techniquesare of paramount importance for photonicsapplications.1,2 In most cases, optical fibers need tobe connected to planar waveguide structures in sucha way as to allow efficient signal transmission acrossthe connection. This task is challenging due to the in-trinsic differences in the geometry of circularly sym-metric fiber cores and nominally square-core planarwaveguides.3 The connection of optical fibers to pla-nar waveguides can be achieved using a number ofexisting techniques. The most popular utilize Vgrooves and gluing, with active or passive alignmentof the cores.4,5 Other available techniques involvemetallization and soldering,6 fusion technology,7 orclip technology.8 In addition to these methods, match-ing the fundamental mode of a waveguide and that ofan attached optical fiber has been attempted throughjudicious choice of waveguide dimensions and refrac-tive indices.9 Satisfactory fiber-to-waveguide connec-tions can be achieved using each of these techniques;however, the alignment in each case is necessarily in-volved, and both the fiber and the waveguide endfaces ideally need to be polished to guarantee a high-quality join.
In this Letter a new technique is demonstratedthat utilizes a self-aligning technique based on theexact positioning of the doped guiding core region ofthe waveguide and the convection-controlled etchingof the circularly symmetric fiber. In this case, the op-tical fiber and the planar waveguide are modified viachemical etching to produce a plug-and-socket ar-rangement. The core of the fiber essentially plugs di-rectly into the socket defined by the guiding region ofthe waveguide, thus producing a perfectly aligned,scalable connection.
A pure-silica cladding-based buried channel wave-guide must possess a doped guiding core region, typi-cally with germanium, to create the necessary wave-guide parameters for the propagation of light. It hasbeen observed that the etch rates of doped coreglasses generally increase with increasing dopant ox-
10,11
ide concentration. If the planar structure is im-0146-9592/05/212858-3/$15.00 ©
mersed in hydrofluoric (HF) acid, as shown schemati-cally in Fig. 1(a), the doped core region is found toetch at a greater rate because of the increase in solu-bility compared with that of the surrounding pure-silica cladding region. This has the effect of creatinga recess in the end face of the planar structure di-rectly at the end of the guiding region. This selectiveetching thus forms the socket that is schematicallydepicted in Fig. 1(b).
Initially, the end face of a buried channel wave-guide, with a germanium-doped core and a pure-silica cladding, was cleaved by a standard mechani-cal technique. A backilluminated optical image of theend face is shown in Fig. 2(a), which clearly showsthe core, cladding, and substrate regions. The endface of the waveguide was then submersed in 48% HFacid to a depth of approximately 1 mm. Etching wascarried out in a Teflon vessel at room temperature for15 min to etch a 20 �m socket with a mean width of10 �m. The exact etching time depends on the dopantconcentration of the core and the desired shape(depth) of the socket. An optical image of the end facecan be observed in Fig. 2(b), which shows the etchedcore region, and Fig. 2(c) clearly depicts this socketfrom above. This socket forms one half of this intrin-sically aligned pigtailing technique.
Fig. 1. Schematic representation of (a) the immersion ofthe buried channel waveguide into a bath of HF acid to (b)
selectively etch the waveguide core to form a socket.2005 Optical Society of America
November 1, 2005 / Vol. 30, No. 21 / OPTICS LETTERS 2859
To create an optical fiber plug, a fiber tip must beaccurately modified to fit into the waveguide recessshown in Figs. 2(b) and 2(c). This is readily achievedby use of the tube etching technique that was previ-ously employed for fabrication of near-fieldprobes.12,13 A schematic diagram of this etching tech-nique is shown in Fig. 3. This technique relies on thechemical etching of a fiber through its permeableacrylate coating, as opposed to the typical etching ofbare fibers with their coatings removed. Initially, anacrylate-coated fiber is dipped into a bath of HF acid,as shown schematically in Fig. 3(a). The acid does notdissolve the acrylate coating, and it diffuses throughto etch only the silica fiber. Figure 3(b) shows thethinning of the fiber as a result of this HF diffusionthrough the coating and the shortening of the fiberowing to direct etching at the end of the fiber. A pre-liminary taper is formed at the meniscus region be-cause of a gradient in the lateral diffusion within thisregion. The plug that is formed at this stage of etch-ing is optimal for the successful implementation ofthis new pigtailing technique. As soon as the taperbegins to form, convection begins to deliver the acidto the upper region of the conically etched fiber, asshown schematically in Fig. 3(c). Concentration gra-dients, which result from the etching process, and thegravitational removal of the reaction products propel
Fig. 2. Optical image of the waveguide end face (a) beforeand (b) after etching. (c) Socket as viewed from above.
this convection to ultimately form a self-terminating
cone-shaped tip. Figure 3(c) shows an undesirable tipshape for optimal coupling of the fiber plug to thewaveguide socket.
It should be noted that any type of acrylate-coatedsilica-based optical fiber can be used to produce plugsthat are suitable for insertion into the etched socket.The choice of optical fiber core dimensions can bemade with fiber-to-waveguide mode matching inmind. The fiber used in the research described in thisLetter was chosen primarily to demonstrate the me-chanical aspects of the self-aligning pigtailing tech-nique and is not mode matched to the waveguide.
A single-mode optical fiber at 633 nm (3M, FS-SN-3224), with a core diameter of 3.4 �m, was used dur-ing the fabrication of the fiber plug. Initially the fiberwas stripped of 5 cm of its acrylate coating and thencleaved by an optical microscope and a SiemensS46999-M9-A8 fiber cleaver such that 300 �m of thebare fiber was protruding from the coating. To pro-duce a fiber plug such as that shown in Fig. 3(b), witha tip diameter of 9 �m, the cleaved fiber was then im-mersed in 48% HF acid at room temperature for 3 hand 55 min. The resulting etched fiber plug is shownbelow an unetched optical fiber in Fig. 4.
Having successfully fabricated both the waveguidesocket and the fiber plug, we can now readily positionthe plug into the socket. Three successive images ofthe insertion of the plug into the socket can be ob-served from Figs. 5(a)–5(c). And Fig. 5(d) shows thefully self-aligned pigtailed device; the plug has beeninserted into the socket to a depth of approximately20 �m.
The most important feature of this innovative tech-nique for pigtailing is that, when the waveguide andthe fiber are connected, they are also intrinsicallyaligned. This occurs because the socket is preciselydefined by the preferential etching of the core of thewaveguide itself. Furthermore, the fiber is symmetri-cally etched about the core; hence the center of thecore region will coincide with the very center of thewaveguide. No further positioning or alignment isnecessary, as the plugged-in position represents theoptimal alignment of the system. Investigations indi-cate that the process is directly scalable. Thereforethis method not only is functional for single fibersand waveguides but also is potentially invaluable forthe alignment of ribbon fibers (multiple fibers) with
Fig. 3. Schematic representation of the etching of a per-meable acrylate-coated fiber as a function of time, showing(a) initial immersion of the fiber into a bath of HF acid, (b)plug formation by HF acid diffusing through the coatingand from the end, and (c) convection-controlled etching to
form a tapered probe.
reference.
2860 OPTICS LETTERS / Vol. 30, No. 21 / November 1, 2005
corresponding planar arrays of waveguide cores. Inaddition, this technique can be utilized for fiber–fiberconnections. This procedure also removes the nor-mally critical requirement for a smooth polishedwaveguide and fiber end faces.
A new technique for the pigtailing of optical fibersto waveguides has been developed that is intrinsi-cally self-aligning. The simple and rapid fabricationof fiber plugs and waveguide sockets removes theneed for fiber and waveguide polishing and makessingle or arrayed pigtailing easily achievable.
Financial support from the Australian ResearchCouncil is acknowledged. The authors also acknowl-edge useful discussions with Sue Law and DavidThorncraft. B. Gibson’s e-mail address [email protected].
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Fig. 4. Optical image of the chemically etched fiber plug,which is shown below an unetched optical fiber for size
Fig. 5. (a)–(d) Optical images showing successive inser-tion of an etched optical fiber that ultimately forms a self-aligning pigtail to a buried channel planar waveguide. Thearrow in each image shows the position of the fiber tip.
