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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 11, NO. 4, APRIL 1999 439

Microlens for Efficient CouplingBetween LED and Optical FiberEun-Hyun Park, Moon-Jung Kim, and Young-Se Kwon, Member, IEEE

AbstractA new fabrication method of a microlens is proposedthat can be easily applied to optical devices and microlenssystems. The proposed microlens is formed by self-surface tensionand cohesion of UV curing material. Since the microlens ishardened by short time UV exposure, the fabrication processis very simple. Integration with surface emitting-light emittingdiode (SE-LED) results in enhanced coupling to optical fiber withcoupling efficiency larger than the conventional case by 1.5 times.We also made a hemispheric microlensed fiber using this method.Compared with a typical arc-lensed fiber and a flat-end fiber, thecoupling efficiency is improved to 18% and 40%, respectively.

Index Terms Coupling, lenses, light-emitting diodes, opticalfibers.

I. INTRODUCTION

MICROLENSES can be applied to optical devices such asemitters and detectors to boost the optical efficiency [1].Also, a microlens system using MEMS technology is used to

change the optical path [2] and a microlens arrays are utilized

for surface-normal optical interconnection [3]. Up to now,

many fabrication methods for a microlens have been developed

including surface micromachining [4], Ar ion-beam etching

technique [5], and photoresist reflow method [6]. But these

methods suffer from complicated steps like precise etching

process control and difficulty in adjusting the microlens radius.

New proposed method is very simple, and we can make good

quality microlenses with radius ranging from a few tens of

microns to a few hundreds of micrometers.

In this letter, we propose a new fabrication technique of

microlens with UV curing material. To demonstrate its appli-

cation, the new microlens is applied to surface-emittinglight-

emitting diode (SE-LED) and multimode optical fiber (MMF).

II. MICROLENS FABRICATION

For the fabrication of the proposed microlens, we used

a viscous liquid material that could be hardened by UV

exposure. When a viscous liquid material was dropped on

a prepared circular pedestal, a hemispheric microlens wasformed by self-surface tension and cohesion. Because the

microlens is formed by surface tension, it can be applied to a

variety of substrates including semiconductor and glass.

Step 1) Formation of Circular Pedestals: There are three

main purposes for the circular pedestal. First, it prevents

Manuscript received October 26, 1998; revised December 17, 1998.The authors are with the Department of Electrical Engineering, Korea

Advanced Institute of Science and Technology (KAIST), 373-1, Kusong-dong,Yusong-gu, Taejon 305-701, Korea.

Publisher Item Identifier S 1041-1135(99)02523-9.

(a) (b)

(c) (d)

Fig. 1. Microlens pedestals. (a) Circular pillar shape by substrate etching. (b)Doughnut shape by substrate etching. (c) Circular pillar shape by photoresist.

(d) Doughnut shape by photoresist.

the spreading of liquid material and defines the position

of the microlens. Second, its radius defines the microlens

radius. Third, its height determines the distance between the

substrate and the microlens. In Fig. 1, we proposed several

simple methods for the formation of circular pedestals. We

can make pedestals by substrate etching [Fig. 1(a), (b)] or by

photoresist patterning [Fig. 1(c), (d)]. In the second method,

we can obtain an undercut shape of the circular pedestal by

using the image reversal process of photoresist. After the

image reversal process (PR: AZ5214, soft-baking (90 C): 4

min, first UV exposure (10 mW/Cm ): 1 sec, Second-backing

(120 C): 60 s, Second UV exposure: 15 s), the thickness

of the pedestal [Fig. 1(c), (d)] is about 2 m. This shape is

very useful for stopping the spreading and the overflowing of

injected liquid material.

Step 2) Formation of Microlens: When UV curing liquid

material (NOA77, viscosity: 5500 cps, refractive index: 1.51,

optical transmission ( m): 100%) is dropped on the

circular pedestal using a micro-injector or tapered optical fiber,

the microlens is formed by self-surface tension and cohesion.

A micro-injector is used for a large microlens with a radius

of a few hundred micrometers and the tapered optical fiber

is used for a small-size microlens the radius of which is a

few tens of micrometers. Watching the shape of the microlenswith a monitoring system, we control the injection quantity of

the liquid material. When using the tapered fiber for injection,

the quantity of a one time drop is determined by the size of

the tapered fiber tip. The dropped microlens is hardened by

shot time UV exposure (10 W, 2 s). The volumetric shrinkage

when curing is 4%, and any deformation isnt observed at

high temperature (120 C).

The microlens size is mainly determined by the radius of

the circular pedestal and controlled by the quantity of injected

liquid material. Because the minimization of surface energy

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440 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 11, NO. 4, APRIL 1999

(a)

(b)

Fig. 2. SEM pictures of (a) a microlens array and (b) a magnified microlens(lens height = 4 0 m, pedestal radius = 7 0 m, lens, radius = 8 1 m).

leads to a spherical shape, we can calculate the microlens

radius from the following equations:

where is the height of the microlens, is the radius of

the circular pedestal and is the quantity of injected liquid

material as shown in Fig. 1. Fig. 2 shows SEM pictures of the

microlens array fabricated by this way on InP substrate. The

microlens radius is 81 m where m, m

and ml. From the SEM picture, we can see

that the fabricated microlens has a very smooth surface and

good truncated spherical shape. The surface roughness of the

microlens measured with Atomic Force Microscope (AFM) is

within 20 A. We think that the good surface quality resultsfrom self-forming characteristic by surface tension.

III. INTEGRATION WITH SURFACE-EMITTING

LED AND OPTICAL FIBER

To demonstrate its application, we integrated a microlens

with a surface-emitting LED ( m) and an opti-

cal fiber. Fig. 3(a) and (b) shows a schematic structure and

photograph of the fabricated lensed LED. The diameter of the

p-ohmic contact window is 40 m and the wafer thickness

is 100 m. The circular pedestal (height m, radius

m) is formed by the photoresist image reversal process

(a)

(b)

(c)

Fig. 3. (a) Schematic structure of the SE-LED [(c

= 1 : 5 5 m]. (b) Thephotograph of the fabricated SE-LED. (c) The photograph of the fabricated

microlensed optical fiber (MMF).

[Fig. 1(d)]. From the near-field intensity pattern of the surface-

emitting LED, we concluded that the magnification of the

microlens was 1.5. Also, the output power of the lensed LED

increased 1.6 times because of enhanced directivity. From the

photograph of the new microlensed fiber in Fig. 3(c), we can

see that the microlens has a good hemispheric shape (

m). Because the shape of the optical fiber end is the same as

in Fig. 1(a), the pedestal is not needed.

Using these optical components, we measured the coupling

efficiency between the SE-LED and on MMF (core diameter

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PARK et al.: MICROLENS FOR EFFICIENT COUPLING BETWEEN LED AND OPTICAL FIBER 441

Fig. 4. Coupling results between SE-LEDs (with and without microlens)and optical fibers (flat end, typical arc-lensed and new proposed lensed).

m, graded index type). Fig. 4 represents coupling

results between SE-LEDs (with and without microlens) and

three kinds of fibers [normal flat end, typical arc-lensed (

m), and new lensed ( m)]. The fiber couplingefficiency of the lensed LED is increased by approximately 1.5

times for three kinds of optical fibers. This result of increased

coupling efficiency agrees well with the calculated result [7].

Also the new microlensed fiber shows the highest coupling

efficiency among the three kinds of fibers. Compared with

the typical arc-lensed fiber and the flat-end fiber, the coupling

efficiency of the new microlensed fiber is improved to roughly

18% and 40%, respectively.

IV. CONCLUSION

In this letter, we developed a new microlens fabrication

process using UV curing method. Because the fabrication

process is very simple, it is very easy to integrate with optical

devices. The microlens radius is easily controlled by varying

the radius of the pedestal and the quantity of injected liquid

material. Also, the distance between the lens and the substrate

can be controlled. To show the application of the microlens,

we made a microlensed SE-LED and a hemispheric lensed

fiber. The fabricated LED shows the output power increased

to 1.6 times and the coupling efficiency ratio is 1.5

where and , are coupling efficiencies of lensed and flat

surface-emitting LEDs, respectively. Compared with typical

arc-lensed fiber and flat-end fiber, the coupling efficiency of

the new proposed microlensed fiber is improved by 18% and

40%, respectively. We believe that this microlens can have

wide applications for optical devices.

ACKNOWLEDGMENT

The authors would like to thank J. S. Park for AFM and

SEM measurements.

REFERENCES

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[2] M. C. Wu, L. Fan, and S.-S. Lee, Optical MEMS: Huge possibilitiesfor lilliputian-sized devices, Optics Photon. News., vol. 9, no. 6, pp.2529, 1998.

[3] S. Tang, T. Li, F. Li, C. Zhou, and R. T. Chen, A holographicwaveguide microlens array for surface-normal optical interconnects,

IEEE Photon. Technol. Lett., vol. 8, pp. 14981500, Nov. 1996.[4] C. R. King, L. Y. Lin, and M. C. Wu, Out-of-plane refractive microlens

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