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Fabrication of an optical fiber reflective notch coupler

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Fabrication of an optical fiber reflective notch coupler Cathy M. Rushworth, Dean James, Charlie J. V. Jones, and Claire Vallance* Department of Physical and Theoretical Chemistry, University of Oxford, South Parks Road, Oxford OX1 3QZ, UK *Corresponding author: [email protected] Received June 13, 2011; revised July 7, 2011; accepted July 7, 2011; posted July 8, 2011 (Doc. ID 148956); published August 1, 2011 A method of fabricating a reflective notch coupler in an optical fiber has been developed. The coupler consists of a 45° microprism that penetrates into the core of a multimode optical fiber. One face, at 90° to the fiber axis, is non- reflective, and one face, at 45° to the fiber axis, is reflective. Our method of fabricating a notch and selectively mir- roring only the 45° face is low-cost, precise, and easily scalable. The coupler allows near-100% coupling of light into an optical fiber from the side, while allowing coupling of any desired fraction of light out from the core at a 90° angle on the opposite side of the fiber. © 2011 Optical Society of America OCIS codes: 060.2340, 220.4000, 220.5450, 230.1360, 230.4040, 140.3560. Side-coupling of light into an optical fiber is important for a variety of applications. It has been used most exten- sively in fiber lasers, where side-coupling of pump light into the inner cladding of a double clad optical fiber is advantageous for (a) increasing the maximum power that can be coupled into an optical fiber and (b) increasing the number of pump light sources that can be coupled into an optical fiber [1]. Additionally, side-coupling may be necessary because of the absence of free optical fiber ends through which to introduce light, for example, in fiber ring lasers [2] and in fiber-loop cavity-based spec- trometers [3]. In both of these cases, the two ends of one length of optical fiber are joined together to form an op- tical fiber loop that acts as a cavity. Elements sensitive to changes in both physical and chemical parameters can be incorporated into fiber-loop cavities, generating interest in their use for a range of sensing applications [4]. High- efficiency side-coupling of light into and out of the cavity, while minimizing the loss per pass within the cavity itself is, in these cases, a requirement. A variety of approaches to side-coupling via a micro- prism arrangement are presented in Fig. 1. By far the most commonly used technique is that developed by Ripin and Goldberg, who cut a 90° v-groove, shown in Fig. 1(a), into the inner cladding of a double clad fiber and obtained a single-mode laser diode coupling effi- ciency of 96%, taking advantage of the total internal re- flection (TIR) at the uncoated groove [1]. For low-loss, unidirectional coupling of light (for example, in a ring fiber cavity), the fabrication of a 45°, notch as shown in Fig. 1(b) is preferable to a v-groove. By mirroring the 45° angle surface, the notch coupler can be used as both an input and an output coupler to a fiber-loop cavity. This is represented in Fig. 1(c). The fabrication and selective mirroring of such a notch in the side of an optical fiber is difficult, and we therefore use a slightly different ap- proach to simplify the problem, as illustrated in Fig. 1(d). The reflective notch is fabricated in the side of an optical fiber end, and a second, nonmirrored fiber end is aligned with the notched end to create the input/output coupler. Refractive-index matching across the gap using either a suitable liquid or adhesive effectively splices the two fiber ends together, resulting in minimal insertion losses for the device. The notch fabrication method has been developed using a 365 μm core diameter, 400 μm cladding diameter optical fiber (BFH22-365, Thorlabs), although our strat- egy can easily be extended to other fiber types and sizes. The first step is to strip the soft jacket from the optical fiber using a fiber stripping tool (T18S31, Thorlabs) and then to cleave the optical fiber using a Shortix capillary cutter (21386-U, Sigma Aldrich). The cutter has a rotating diamond blade and is designed for cutting 0:30:78 mm fused silica capillary tubing at a 90° angle to the axis. The initial cleave ensures that the fiber end is cut at a 90° angle; however, the end face quality resulting from this cleave is poor, and the fiber end face must be po- lished to ensure optical quality. We use a home-built polishing wheel to polish our optical fiber, with the fiber clamped in a home-built perspex mount, similar to that of Figure 3 in [5]. The optical fiber is polished on 0:1 μm diamond lapping film (693118SO, Buehler). After polishing, the end face is inspected under a microscope, and polishing is re- peated if necessary until the end face is flat and defect free [Fig. 2(a)]. This is the most critical stage of fiber preparation; if the fiber end is of poor quality, or not polished at 90° to the fiber axis, this will translate into significant losses in the finished coupler. (a) (b) (c) Input Input Input Output d p Cladding Core (d) Input Output Fig. 1. Approaches to side-coupling into optical fibers. (a) Em- bedded 90° v-groove, (b) embedded 45° notch, (c) reflective 45° notch, (d) simple reflective 45° notch fabrication in a fiber end. Inset: end view of (d), where d p is the penetration depth of the notch. 2952 OPTICS LETTERS / Vol. 36, No. 15 / August 1, 2011 0146-9592/11/152952-03$15.00/0 © 2011 Optical Society of America
Transcript

Fabrication of an optical fiber reflectivenotch coupler

Cathy M. Rushworth, Dean James, Charlie J. V. Jones, and Claire Vallance*Department of Physical and Theoretical Chemistry, University of Oxford,

South Parks Road, Oxford OX1 3QZ, UK*Corresponding author: [email protected]

Received June 13, 2011; revised July 7, 2011; accepted July 7, 2011;posted July 8, 2011 (Doc. ID 148956); published August 1, 2011

A method of fabricating a reflective notch coupler in an optical fiber has been developed. The coupler consists of a45° microprism that penetrates into the core of a multimode optical fiber. One face, at 90° to the fiber axis, is non-reflective, and one face, at 45° to the fiber axis, is reflective. Our method of fabricating a notch and selectively mir-roring only the 45° face is low-cost, precise, and easily scalable. The coupler allows near-100% coupling of light intoan optical fiber from the side, while allowing coupling of any desired fraction of light out from the core at a 90° angleon the opposite side of the fiber. © 2011 Optical Society of AmericaOCIS codes: 060.2340, 220.4000, 220.5450, 230.1360, 230.4040, 140.3560.

Side-coupling of light into an optical fiber is important fora variety of applications. It has been used most exten-sively in fiber lasers, where side-coupling of pump lightinto the inner cladding of a double clad optical fiber isadvantageous for (a) increasing the maximum power thatcan be coupled into an optical fiber and (b) increasingthe number of pump light sources that can be coupledinto an optical fiber [1]. Additionally, side-coupling maybe necessary because of the absence of free optical fiberends through which to introduce light, for example, infiber ring lasers [2] and in fiber-loop cavity-based spec-trometers [3]. In both of these cases, the two ends of onelength of optical fiber are joined together to form an op-tical fiber loop that acts as a cavity. Elements sensitive tochanges in both physical and chemical parameters can beincorporated into fiber-loop cavities, generating interestin their use for a range of sensing applications [4]. High-efficiency side-coupling of light into and out of the cavity,while minimizing the loss per pass within the cavity itselfis, in these cases, a requirement.A variety of approaches to side-coupling via a micro-

prism arrangement are presented in Fig. 1. By far themost commonly used technique is that developed byRipin and Goldberg, who cut a 90° v-groove, shown inFig. 1(a), into the inner cladding of a double clad fiberand obtained a single-mode laser diode coupling effi-ciency of 96%, taking advantage of the total internal re-flection (TIR) at the uncoated groove [1]. For low-loss,unidirectional coupling of light (for example, in a ringfiber cavity), the fabrication of a 45°, notch as shownin Fig. 1(b) is preferable to a v-groove. By mirroring the45° angle surface, the notch coupler can be used as bothan input and an output coupler to a fiber-loop cavity. Thisis represented in Fig. 1(c). The fabrication and selectivemirroring of such a notch in the side of an optical fiberis difficult, and we therefore use a slightly different ap-proach to simplify the problem, as illustrated in Fig. 1(d).The reflective notch is fabricated in the side of an opticalfiber end, and a second, nonmirrored fiber end is alignedwith the notched end to create the input/output coupler.Refractive-index matching across the gap using eithera suitable liquid or adhesive effectively splices the two

fiber ends together, resulting in minimal insertion lossesfor the device.

The notch fabrication method has been developedusing a 365 μm core diameter, 400 μm cladding diameteroptical fiber (BFH22-365, Thorlabs), although our strat-egy can easily be extended to other fiber types and sizes.The first step is to strip the soft jacket from the opticalfiber using a fiber stripping tool (T18S31, Thorlabs) andthen to cleave the optical fiber using a Shortix capillarycutter (21386-U, Sigma Aldrich). The cutter has a rotatingdiamond blade and is designed for cutting 0:3–0:78mmfused silica capillary tubing at a 90° angle to the axis.The initial cleave ensures that the fiber end is cut at a90° angle; however, the end face quality resulting fromthis cleave is poor, and the fiber end face must be po-lished to ensure optical quality. We use a home-builtpolishing wheel to polish our optical fiber, with the fiberclamped in a home-built perspex mount, similar to that ofFigure 3 in [5].

The optical fiber is polished on 0:1 μmdiamond lappingfilm (693118SO, Buehler). After polishing, the end faceis inspected under a microscope, and polishing is re-peated if necessary until the end face is flat and defectfree [Fig. 2(a)]. This is the most critical stage of fiberpreparation; if the fiber end is of poor quality, or notpolished at 90° to the fiber axis, this will translate intosignificant losses in the finished coupler.

(a) (b)

(c)

Input Input

Input

Output

dp

Cladding

Core

(d)

Input

Output

Fig. 1. Approaches to side-coupling into optical fibers. (a) Em-bedded 90° v-groove, (b) embedded 45° notch, (c) reflective 45°notch, (d) simple reflective 45° notch fabrication in a fiber end.Inset: end view of (d), where dp is the penetration depth of thenotch.

2952 OPTICS LETTERS / Vol. 36, No. 15 / August 1, 2011

0146-9592/11/152952-03$15.00/0 © 2011 Optical Society of America

Once the fiber face has been prepared, the fiber isclamped in a second perspex mount at 45° to the polish-ing wheel and polished until the desired penetrationdepth dp is achieved. Typically, this takes less than 30 s,and the fiber end is inspected frequently during this pol-ishing step [Fig. 2(b)]. A deeper notch than required isfabricated, as the penetration depth of the notch can bereduced at any point (typically the last step) simply byrepolishing the fiber end face.The simple nonreflective notch that has been fabri-

cated at this point may be used as a TIR input coupler,by directing light toward the notch at a near-normal angleto the fiber axis from the opposite side of the fiber to thenotch [Fig. 1(b)]. TIR occurs above a critical angle of43:2° at the glass–air boundary (the refractive indices offused silica and air at 532 nm are 1.461 and 1.000, respec-tively). However, while the bare notch functions well asan input coupler, it does not perform well as an outputcoupler; rather than the light incident on the notch beingreflected out of the fiber, the majority of the light incidenton the nonreflective notch refracts into higher order coremodes or cladding modes. This may be overcome bycoating the notch to form a reflective surface so that alllight incident on the notch is coupled out of the fiber anddetected.Techniques for bonding metal directly to glass have

been discussed elsewhere [6]. Often, a chemical linkingreagent is used. Such reagents typically contain a silanegroup at one end, which bonds to glass, and a nucleophi-lic group at the opposite end, which bonds to metal.In order to fabricate a reflective coating on the angledsurface of our coupler, the notched fiber end was coatedwith VECTABOND (SP1800, Vector Laboratories) ac-cording to the manufacturer’s instructions. VECTABONDis a proprietary silane-based reagent containing a pri-mary amine group. After coating, the flat fiber end faceis repolished briefly, so that VECTABOND only remainsbonded to the angled notch face, and not to the flatend face.A metallic mirror coating is deposited using a simple

chemical reaction (Tollens’ reagent), which involvesthe reduction of silver nitrate [7]. Concentrated ammo-nium hydroxide (255210025, Acros Organics) is addeddropwise to 15mL of 0:1MAgNO3 solution (38310, SigmaAldrich) until the brown precipitate formed on firstaddition has dissolved. 7:5mL of 0:8M NaOH (BPE359,Fisher Scientific) is added to the solution, and additionalammonium hydroxide is added if the brown precipitatereforms. 1:5mL of saturated glucose solution (G8270,Sigma Aldrich) is then added. The fiber end is held sub-merged in the solution for 3–5 minutes, followed by rin-sing in deionized water and air drying. Figure 2(c) showsan image of the silvered fiber end.The silver coating adheres weakly to the flat fiber face

and strongly to the VECTABOND-coated notch. The flatface is repolished to remove the weakly bound silvercoating [Fig. 2(d)], and further polishing is carried out

if required to achieve the desired penetration depth ofthe notch [Fig. 2(e)].

After fabrication, the notched fiber end is aligned in afiber holder (MDE710, Elliot Scientific) fixed on a micro-meter-precision three-axis translation stage (MDE510,Elliot Scientific). The notched fiber end is clamped onthe stationary side of the translation stage, so that theinput light beam and output light collector can be alignedrelative to a static fiber end. The notched fiber is rotatedto the desired angle by hand and then clamped. The sec-ond fiber end is polished flat using high-grade diamondlapping film and aligned with the notched fiber end toproduce the completed coupler. In order to further re-duce the insertion losses of the coupler, the air gapbetween the two fiber ends is minimized by bringingthe fiber ends into close contact, without stressing thefibres. Fresnel reflections are reduced by refractive-index-matching the air gap with water or high-indexoil. It is also possible to splice the fiber ends togetherby placing a drop of refractive-index-matched UV-curingoptical adhesive (Norland Optical Adhesive 84, Norland)on the gap and then rapidly curing the adhesive under aUV lamp (SB-100P/FB 110W spot lamp, RSL NDT) whilethe fiber ends are well-aligned. This does not reduce thesignal, although realignment of the input beam and out-put collector may be required after liquid has been addedto the fiber ends.

The new coupler extends the use of prism-based side-coupling into optical fibers from double clad fibers tosingle clad fibers. Crucially, for fiber-loop-based devicesin particular, the reflective notch coupler can functionas a spatially separating input and output coupler. Lightis coupled into the core from one side of the fiber, anda fraction of light already propagating in the core iscoupled out in the opposite direction from that coupledin. The outcoupling is completely controlled simply bychoosing the penetration depth of the notch to achievethe desired coupling ratio, defined using simple geometryand assuming a uniform distribution of light in thefiber core.

For a perfect notch, the output coupling efficiencyis simply given by the fractional area of the core takenup by the notch. Straightforward trigonometry shows thisto be

L ¼ 100π

hcos−1ð1 − f Þ − ð1 − f Þff ð2 − f Þg1

2

i; ð1Þ

(a) (b) (c) (d) (e)

Fig. 2. Selected reflective notch fabrication steps.

Fig. 3. (a) Zemax simulations of light coupling out from thenotch, assuming multimode propagation of light in the core,(b) percentage of light coupled out from the fiber core as a func-tion of percentage penetration into the core. Solid points are theresults of Zemax ray tracing simulations [Eq. (2)], and the solidcurve is a plot of Eq. (1).

August 1, 2011 / Vol. 36, No. 15 / OPTICS LETTERS 2953

where f ¼ dpr , with r the radius of the fiber core and dp the

penetration distance of the notch into the core.We have carried out simulations of the notch coupler

acting as a fiber beam splitter using the Zemax opticalmodelling software suite. Zemax is an optical ray-tracingprogram that is commonly used for optical design andtolerance analyses [8]. In the simulations, 400,000 analy-sis rays are launched from a fiber-coupled divergentsource, whose angle of divergence (9°) matches the max-imum acceptance angle of our optical fiber. This providesa fairly realistic mode picture of the fiber, so that therange of propagation angles of light escaping from thenotch is modeled correctly.The detectors labeled 1, 2, and 3 in Fig. 3(a) are used in

the software for the quantitative determination of outputcoupling efficiency, plotted in Fig. 3(b) as a function ofnotch penetration depth. Detector 1 measures theamount of light lost at the notch, Detector 2 measuresthe amount of light remaining in the fiber core after thenotch, and Detector 3 measures the small amount of lightthat travels in the cladding. The percentage of propagat-ing light L that is coupled out of the fiber is determinedfrom the signals D1, D2, and D3 recorded at the threedetectors:

L ¼ D1

D1 þ D2 þ D3× 100: ð2Þ

As shown in Fig. 3(b), the results of the Zemaxsimulations are in almost perfect agreement with thepredictions of Eq. (1).As an experimental demonstration of the coupler in

operation, a 20 μm deep reflective notch was fabricatedin a 3:91m loop of 365 μm core diameter optical fiber andused as an input and output coupler. A short (0:9 ns)pulse of 532 nm light from a frequency-doubled Nd:YAGlaser (NP-10620-100, Teem Photonics) was coupled intothe loop from the top side of the notch. Light outputfrom the coupler was collected using a 3mm liquid light-guide (NT53-328, Edmund Optics) aligned on the bottomside of the notch, then directed to a photomultipliertube (PMT, H10304-06-NF, Hamamatsu). A diagram of

the setup is presented in Fig. 4. The detected signal ispresented in Fig. 5.

The round-trip time of the light pulse in the fiber loop(about 19 ns) was short enough to resolve the pulse pas-sing the coupler on each circuit, resulting in a series ofpeaks decaying exponentially. The detected signal is pro-portional to the amount of light remaining in the loop,and the ratio of consecutive peak amplitudes gives theoutcoupling ratio. From this, the loss from the couplerscan be calculated to be only 2.5% (with an additional lossof 1.5% from fiber absorptions). As noted previously, dif-ferent outcoupling ratios may be achieved by controllingthe penetration depth of the notch into the fiber core.

In summary, we have developed a straightforwardmethod of fabricating a reflective notch coupler in an op-tical fiber, which allows for near-100% coupling of lightinto an optical fiber from the side, while allowing cou-pling of a tuneable proportion of light out from the coreat a 90° angle on the opposite side of the fiber.

This work was supported by the United KingdomEngineering and Physical Sciences Research Council,through grant code EP/G027838/1, and is subject to aBritish patent application from ISIS Innovation Ltd.(application No. 1108557.8). We would particularly liketo thank the Physical and Theoretical Laboratory work-shops for fabricating the polishing equipment.

References

1. D. J. Ripin and L. Goldberg, Electron. Lett. 31, 2204 (1995).2. A. Hideur, T. Chartier, C. Özkul, and F. Sanchez, Opt. Lett.

26, 1054 (2001).3. R. S. Brown, I. Kozin, Z. Tong, R. D. Oleschuk, and H.-P.

Loock, J. Chem. Phys. 117, 10444 (2002).4. C. Wang, Sensors 9, 7595 (2009).5. V. Allen, T. J. H. Essex, and A. L. McKenzie, Phys. Med. Biol.

34, 927 (1989).6. C. A. Goss, D. H. Charych, and M. Majda, Anal. Chem. 63,

85 (1991).7. S. Sandlin, T. Kinnunen, J. Rämö, and M. Sillanpää, Surf.

Coat. Technol. 201, 3061 (2006).8. www.zemax.com.

Fig. 4. Schematic of experimental demonstration.Fig. 5. Experimental demonstration of using a reflective notchas both an input and an output coupler.

2954 OPTICS LETTERS / Vol. 36, No. 15 / August 1, 2011


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