Analysis of replacing an optical fibercore with polymer

Kevin H. Smith, Richard H. Selfridge, Stephen M. Schultz, Douglas J.Markos, and Benjamin L. Ipson

Department of Electrical Engineering, Brigham Young University, Provo, Utah 84602

khs9@ee.byu.edu

Abstract: This paper presents the analysis of a 2 cm long in-fiber polymerwaveguide formed on the platform of a D-shaped optical fiber. Numericalsimulations provide an understanding of the major loss mechanisms for fea-sible in-fiber polymer waveguide geometries. The primary loss mechanismis determined to be excitation of slab modes on the flat surface of the fiberwith transition geometry being the next major contribution to loss.

© 2004 Optical Society of America

OCIS codes:(060.2310) Fiber optics; (130.2790) Guided waves; (130.3120) Integrated opticsdevices; (060.5060) Phase modulation

References and links1. R. Mears, L. Reekie, I. Jauncey, and D. Payne, “Low noise erbium-doped fiber amplifier aperating at 1.54µm,”

Electron. Lett.23,1026–1028, (1987).2. K. O. Hill and G. Meltz, “Fiber Bragg grating technology fundamentals and overview,” IEEE J. Lightwave

Technol.15,1263–1276 (1997).3. S. Tseng and C. Chen, “Side-polished fibers,” Appl. Opt.31,3438–3447, (1992).4. D. J. Welker, J. Tostenrude, D. W. Garvey, B. K. Canfield, and M. G. Kuzyk, ”Fabrication and characterization

of single-mode electro-optic polymer optical fiber,” Opt. Lett.23, 1826–1828 (1998).5. D. J. Markos, B. L. Ipson, K. H. Smith, S. M. Schultz, R. H. Selfridge, T. D. Monte, R. B. Dyott, and G. Miller,

“Controlled core removal from a D-shaped optical fiber,” Appl. Opt.42,7121–7125 (2003).6. K. H. Smith, D. J. Markos, B. L. Ipson, S. M. Schultz, R. H. Selfridge, J. P. Barber, K. J. Campbell, T. D. Monte,

and R. B. Dyott, “Fabrication and analysis of a low-loss in-fiber active polymer waveguide,” Appl. Opt.43,933–939 (2004).

7. S. Mononobe and M. Ohtsu, “Fabrication of a pencil-shaped fiber probe for near-field optics by selective chemicaletching,” J. Lightwave Technol.14,2231–2235 (1996).

8. S. Garner, “Three dimensional integration of passive and active polymer waveguide devices,” Ph. D. dissertation,Dept. Elect. Eng., Univ. of Southern California, Los Angeles, CA, 1998.

9. BeamPROP™User’s Guide, RSoft Inc., 200 Executive Blvd., Ossining, NY 10562.10. J. D. Love, W. M. Henry, W. J. Stewart, R. J. Black, S. Lacroix, and F. Gonthier, “Tapered single-mode fibres and

devices,” IEE Proc. J.138,343–354, Oct. 1991.

1. Introduction

In-fiber devices have received attention because they allow optical signals to be generated andmanipulated entirely in the optical fiber domain. Existing in-fiber devices such as erbium-dopedfiber amplifiers and Bragg gratings have demonstrated these advantages [1, 2]. Side-polishedfibers are another variety of in-fiber device used in some applications [3], although they limitdevice length and only permit interaction with the evanescent field of the fiber. In-fiber deviceshave also been constructed using fiber made completely out of polymer [4].

(C) 2004 OSA 9 February 2004 / Vol. 12, No. 3 / OPTICS EXPRESS 354#2985 - $15.00 US Received 4 September 2003; revised 3 November 2003; accepted 4 November 2003

Prior work in our laboratory [5, 6] demonstrated how to etch an arbitrary length of D-fiberto remove the core of the fiber and then replace that section of the core with another opti-cal material. Possible applications for such in-fiber waveguides include amplitude and phasemodulation, fiber sensors, tunable filters, frequency converters, etc.

Previous papers [5, 6] focused on the experimental techniques used to produce such devices.However, there is a need to examine the major loss mechanisms associated with this new in-fiber polymer waveguide section. This paper provides a detailed investigation into the majorloss mechanisms and exploits the graphical capabilities of this journal. Section 2 provides anoverview of the fabrication of in-fiber polymer waveguides and Section 3 presents the analysisof primary loss mechanisms in these waveguides.

2. Background

2.1. D-fiber platform

Figure 1 is an illustration of a D-shaped optical fiber composed of a germania-doped core, afluorine-doped cladding, and an undoped supercladding. The protective jacket is not shown inthe figure. The core of the fiber is approximately 2 x 4µm in size and is located roughly 10µmfrom the flat side of the D-fiber. The fiber manufacturing process results in a small undopedsection in the center of the core.

Undoped silica, nsi

Germania doped core, ncore

Fluorine doped cladding, nclad

Fig. 1. The elliptical germania doped D-fiber core is surrounded by a lower index fluorinedoped cladding region.

The differential doping of the fiber allows for selective chemical etching with hydrofluoric(HF) acid [7]. In a previous paper [5] we show how to use this selective chemical etchingto controllably remove a portion of the core of the D-fiber while leaving the fluorine dopedcladding intact, preserving the structural integrity of the fiber. Figure 2 shows a movie of thecore etch, demonstrating the differential etch rates of the core and cladding. The etch depthis chosen to allow for the formation of a single mode polymer waveguide near the center ofthe fiber. Figure 3 is a cross-sectional scanning electron microscope (SEM) image of an etchedD-fiber with a groove along the flat side of the fiber into which polymer is deposited.

2.2. Polymer waveguide

The polymer is deposited by spin casting and forms a waveguide that is contiguous with the coreof the unetched portion of the fiber. In this research we use polymethyl methacrylate (PMMA)as the host polymer with DR1-azo dye as the guest chromophore. After the polymer is depositedon the fiber, the fiber is spun in a standard commercial spinner, forming a polymer waveguidein the groove left by the wet etch.

(C) 2004 OSA 9 February 2004 / Vol. 12, No. 3 / OPTICS EXPRESS 355#2985 - $15.00 US Received 4 September 2003; revised 3 November 2003; accepted 4 November 2003

Fig. 2. (452 KB) Movie of the cross-section of the fiber as it is etched in HF acid.

Fig. 3. SEM images of a D-fiber and a typical etch profile. The cladding has been etchedonly slightly while about half of the core has been removed.

As demonstrated in an earlier paper [6], polymer waveguide thickness can be changed byvarying the polymer viscosity. Figure 4 shows cross-sectional SEM images of polymer waveg-uide sections with increasing viscosities. These images show that a layer of polymer in thegroove is accompanied by a layer of polymer on the flat surface. Figure 4(a) shows that thepolymer in the groove is substantially thicker than the polymer layer on the flat surface whena low viscosity polymer is applied. Figures 4(b) and 4(c) show that once polymer viscosity isabove a certain point, further increases in viscosity cause the thickness of the polymer in thegroove and on the flat to increase at the same rate.

The transmission loss of the approximately 2 cm long polymer waveguides shown in Figs.(a) - (c) were measured at a wavelength of 1550 nm to be respectively 1.6 dB, 36 dB, and toohigh to measure. Experiments demonstrated that waveguides with a thick layer of polymer onthe flat surface of the fiber have high loss. Of the three samples shown in Fig. 4, only sample(a) is suitable for many applications because of the high loss of the other two.

3. Loss analysis

Several factors affect transmission loss as light travels through the polymer waveguide section.First, the electro-optic polymer has a higher bulk absorption coefficient than the germania-doped glass. In particular, DR1-azo dye doped PMMA has a bulk absorption coefficient ofabout 1 dB/cm at a wavelength of 1300 nm [8]. Light scattering along the length of the polymerwaveguide is another source of loss. This source of loss can be avoided by keeping fibers free

(C) 2004 OSA 9 February 2004 / Vol. 12, No. 3 / OPTICS EXPRESS 356#2985 - $15.00 US Received 4 September 2003; revised 3 November 2003; accepted 4 November 2003

1131nm

1165nm

(c)

709nm

177nm

(a)

1060nm

695nm

(b)

Fig. 4. Cross-sectional SEM images of polymer waveguides with polymer viscosities in-creasing from (a) to (c). The white lines were added to show the interface between the glassand polymer.

of contamination between the core removal step and the polymer application step.As noted above there is a strong relationship between the polymer thickness on the flat sur-

face of the fiber and transmission loss. This loss is attributed to the excitation of slab modesin the polymer on the flat side of the fiber. Also, there is transmission loss associated with themismatch between the mode supported by the unetched fiber section and that supported by thepolymer waveguide. Such losses can be reduced by forming a gradual transition to and fromthe polymer waveguide section. We performed numerical simulations to analyze these sourcesof loss.

All numerical simulations of waveguides were performed at a wavelength of 1550 nm us-ing the beam propagation method (BPM) in the commercial software package BeamPROP™.BPM is based on the paraxial approximation to the Helmholtz equation [9]. To simulate three-dimensional propagation, the fundamental mode of the unetched fiber is computed via the imag-inary distance beam propagation method and then launched into the fiber. BeamPROP™thensimulates the propagation of light through the polymer waveguide. The computed electric fielddistribution at regular intervals along the propagation axis is stored and used to calculate lossin the waveguide as a function of propagation distance.

3.1. Slab modes

Experiments consistently demonstrate that waveguides with thick polymer layers on the flatsurface of the fiber have high transmission loss. These thick polymer layers on the flat result inhigh loss because they behave like slab waveguides, allowing light to couple out of the polymerwaveguide in the core region and into slab modes on the flat side of the fiber.

nclad=1.441

ncore=1.476

npolymer=1.54

1.0 µm0.2 µm

(a) thin (b) thick

Fig. 5. The cross-sections of the two polymer waveguides that were analyzed.

BeamPROP™’s three-dimensional simulation capabilities model light propagation throughthe transition from the unetched fiber core to the polymer waveguide. Figure 5(a) shows a

(C) 2004 OSA 9 February 2004 / Vol. 12, No. 3 / OPTICS EXPRESS 357#2985 - $15.00 US Received 4 September 2003; revised 3 November 2003; accepted 4 November 2003

model of a waveguide with a thin layer of polymer on the flat surface of the fiber, similar tothe cross-sectional image shown in Fig. 4(a). Figure 5(b) shows a waveguide with a thick layerof polymer similar to the cross-section shown in Fig. 4(c). All indices of refraction are givenat a wavelength of 1550 nm and absorption loss in the polymer has been neglected in thesemodels. If thick enough, polymer layers on the flat surface of the fibers behave as asymmetricslab waveguides. The polymer layer on the flat of the fiber in Fig. 5(a) is too thin to support aslab mode, but that in Fig. 5(b) does support a slab mode [6]. The full three-dimensional modelassumes a butt-coupled transition between the unetched section of optical fiber and the polymerwaveguide sections illustrated in Fig. 5.

Fig. 6. Top view of the three-dimensional simulation of light propagating from unetchedfiber into polymer waveguide for thin (a) and thick (b) polymer layers.

Figure 6 shows the top view of the simulations of light propagating from the unetched coreinto the polymer waveguide section. The abrupt transitions from unetched core to polymerwaveguide occur at the longitudinal position ofz= 10µm. At the transition, any power that isnot coupled into the polymer layer quickly radiates away from the core. In addition to this lossfrom mode mismatch, a thick layer of polymer also results in continuous radiation of light awayfrom the core region along the length of the fiber. Even though more power is coupled from theunetched fiber into the polymer waveguide when the polymer is thicker, the total transmissionloss is substantially higher because the power that couples into slab modes does not coupleback into the optical fiber core. Figure 7 shows full simulations of light propagating throughthe abrupt transition from unetched fiber to polymer waveguide. The green ellipse in the figureshows the location of the elliptical core (before etching). The top half of Fig. 7 is a simulation ofthe thin-polymer waveguide in Fig. 4(a) and the bottom half is a simulation of a thick-polymerwaveguide as in Fig. 4(c). Power continuously couples out of the core region along the length ofthe waveguide in the simulation of the thick polymer waveguide. The simulated thick polymerwaveguide loses about 1.5 dB/mm to slab modes, whereas the thin polymer waveguide loses nopower to slab modes.

The cutoff thickness for different polymer indices gives an estimate of the upper bound on thethickness of the polymer on the flat side of the optical fiber. The cutoff thickness is defined asthe point where the effective index of refraction of the fundamental mode falls below the indexof the substrate. To ensure that no slab modes were supported, the thickness of the polymer onthe flat needs to be kept below 0.4µm [6].

3.2. Transition loss

The mode supported by the unetched optical fiber is different than the mode supported by thepolymer waveguide. This mode mismatch contributes to the transmission loss of the device.

(C) 2004 OSA 9 February 2004 / Vol. 12, No. 3 / OPTICS EXPRESS 358#2985 - $15.00 US Received 4 September 2003; revised 3 November 2003; accepted 4 November 2003

Fig. 7. (1.27 MB) Simulations of light propagating through a transition from unetched fiberto thin (top) and thick (bottom) polymer waveguides.

Butt-coupled transitions can result in a total transmission loss of about 3 dB [6]. The lossassociated with mode mismatch is decreased by a gradual transition between the unetched coreand the polymer waveguide [10].

Light propagation through transitions from fiber core to polymer waveguide and back intothe fiber core was modeled using BeamPROP™. The transition length was varied in order toquantify its effect on transmission loss. The model consists of a short unetched length of fiberfollowed by a transition region, a 2500µm section of polymer waveguide, another transition,and a final length of unetched fiber 2500µm long. Figure 8 is a movie of a cross-sectionalview of the fiber along the z-direction. This movie shows the transitional geometry from theunetched fiber into the polymer waveguide.

Fig. 8. (322 KB) Video of the index profile of the fiber with 110µm long transition regionsas a function of z.

Figure 9 shows the relationship between loss and propagation distance for waveguides withdifferent transition lengths. The 1100µm transition fiber performs best, having only 0.5 dB ofloss. The plot shows that the overall loss is approximately evenly divided between the transi-tions to and from the polymer waveguide.

0 2000 4000 60000

1

2

3

z-Position (microns)

Loss

in d

B

Transition Length

110 microns 560 microns 1100 microns

Fig. 9. Plot of loss as light propagates through transitions of different lengths.

(C) 2004 OSA 9 February 2004 / Vol. 12, No. 3 / OPTICS EXPRESS 359#2985 - $15.00 US Received 4 September 2003; revised 3 November 2003; accepted 4 November 2003

4. Summary and conclusions

Insertion of a polymer that is contiguous with an optical fiber presents a new means of launchinginto and guiding light in a variety of materials. Using experiment and numerical simulations,we have designed and analyzed a low-loss in-fiber polymer waveguide.

The simulations and experiments discussed above provide bounds on the waveguide dimen-sions in order to achieve low transmission loss. An in-fiber polymer waveguide must have avery thin layer of polymer on the flat side of the fiber for the waveguide to have low loss. Nu-merical results indicate that a polymer waveguide of index 1.54 must have a flat thickness lessthan 0.4µm to avoid excitation of slab modes. The transition length from unetched fiber topolymer waveguide should also be greater than 1 mm to ensure low loss.

(C) 2004 OSA 9 February 2004 / Vol. 12, No. 3 / OPTICS EXPRESS 360#2985 - $15.00 US Received 4 September 2003; revised 3 November 2003; accepted 4 November 2003