ULTRA-HIGH COUPLING EFFICIENCY OF MEMS TUNABLE LASER VIA 3-DIMENSIONAL MICRO-OPTICAL COUPLING SYSTEM

J. F. Tao1, 2, A. B. Yu3, H. Cai3, W. M. Zhu1, Q. X. Zhang3, J. Wu2, K. Xu2, J. T. Lin2, A. Q. Liu1† 1School of EEE, Nanyang Technological University, SINGAPORE, 639798

2Beijing University of Posts and Telecommunications, CHINA, 100876

3Institute of Microelectronics, 11 Science Park Road, SINGAPORE, 117685 (†E-mail: eaqliu@ntu.edu.sg; Tel: +65 6790-4336; Fax: +65 6793-3318)

ABSTRACT

This paper reports a 3-dimensional (3D) micro-optical coupling system for improving coupling efficiency in the Littrow configured micro-electro-mechanical system (MEMS) tunable lasers. In the coupling system, an optical fiber acts as a rod lens for light convergence in the vertical plane, while a deep-etched silicon parabolic mirror confines the light in the horizontal plane. Compared with previous MEMS lasers without any light focusing or only one-directional focusing mechanism, the proposed 3D micro-optical system allows longer external cavity length and provides higher coupling efficiency. A prototype is fabricated on a SOI-wafer with an etching depth of 100 µm. The laser obtains a coupling efficiency as high as 76.5%, which is much higher than typical value of 3% – 50% in previous designs. The laser has dimensions as small as 3 mm × 3.2 mm in single-chip integration. It achieves large tuning range of 48.3 nm within 1 ms tuning speed. INTRODUCTION

Wavelength tunable lasers have attracted considerable research interest for some time. Among them, the miniaturization of external cavity tunable lasers using microelectromechanical system (MEMS) technology has attracted attention and some have been lead to commercial success [1 – 10]. With the merits of MEMS technology, these lasers have compact size, good mechanical stability, large tuning range, and fast tuning speed. Among many developed MEMS tunable lasers, the typical configurations can be classified as mirror scheme [1, 2], Littrow scheme [5-8], Littman scheme [9], and other non-standard schemes [10, 11].

Particularly, the Littrow configured tunable laser is one of the most important and high-gain commercial potential approaches due to its simple tuning structure, wide tuning range and high single-mode suppression ratio. However, the challenge remains in obtaining a real single-chip integrated laser due to the limitations of low coupling efficiency within the external cavity and low diffractive efficiency of the silicon gratings. Although by integrating micro-ball lens and other commercialized micro-gratings into the system can solve the above problems [7, 8], problems such as high costs and time-consuming in active alignment remain unsolved.

In order to overcome these problems, we present a 3D micro-optical coupling system for MEMS tunable lasers. This coupling system consists of a single mode fiber, a parabolic mirror, and a deep-etched silicon blazed grating. Besides the high coupling efficiency, this 3D micro-optical

coupling system also features simple packaging since passive alignment approach can be used to assembling [12, 13].

The organization of this paper is as follows. The micro-optical design and analysis are given in Section 2. Then the experimental results and discussions are shown in Section 3. Finally the conclusions are given. DESIGN AND ANALYSIS

Figure 1: Schematic of the proposed MEMS Littrow tunable laser. The fiber, the parabolic mirror and the blazed-grating construct a 3D optical system for improving optical coupling efficiency within the external cavity.

The working principle of the proposed MEMS tunable laser is illustrated in Fig. 1. The laser consists of an AR-coated gain chip, an optical fiber, a 90° off-axis parabolic mirror and a rotational grating. The gain chip is located at the focusing point of the fiber-rod-lens and the parabolic mirror, so that the light from the gain chip can become parallel light after 3D collimating by the optical fiber and the parabolic mirror. After collimation, the pattern becomes a rectangle pattern which is able to cover more pitches of the grating. Therefore, the cavity has improved diffraction efficiency and higher wavelength selective resolution. Finally, the selected wavelength is diffracted back by grating and feedbacks into the gain chip. With the merits of 3D coupling at the feedback trip, the optical power loss

Rotation

Gain chip Fiber rod lens

External cavity

Off-axis parabolic mirror

Grating

978-1-4244-9634-1/11/$26.00 ©2011 IEEE 13 MEMS 2011, Cancun, MEXICO, January 23-27, 2011

induced by light divergence is greatly reduced in both directions.

For a stable single wavelength lasing, the wavelength must satisfies both the laser cavity resonance condition and the grating diffraction condition [8], as given by

)sin(2][2

00

0

θλϕλ

pMLnN

g

extLDLDm

=+=

where λm and λg represent the nominal wavelengths determined by the laser cavity and the grating, respectively. N0 is the laser mode number, M0 is the grating diffraction order, nLD is the refractive index of the gain chip, and LLD is the length of the gain chip in the free space (internal cavity), φext is the phase shift after round trip in the external cavity. p0 is the grating period, and θ stands for the diffraction angle of the grating. The actual output wavelength is affected by both grating and the laser cavity. When λm = λg, a stable single wavelength output is obtained. By rotating the grating, both cavity length and diffraction conditions are changed, and another single-wavelength lasing can be achieved only when the above two conditions are satisfied again. The wavelength tuning range mostly depends on the design of the pivot structure and its position of the grating [6]. With the grating rotation, a wavelength difference between λm and λg is introduced. In such case, λm may become the output wavelength over one certain range, while λg rises to dominance over another range. The laser output shifts between λm and λg. Therefore, we use an optimized pivot design [6] in order to obtain a large wavelength tuning range. The employed double-clamped beam approach enables 50 nm continuous tuning range of the proposed design.

Figure 2: Comparison of beam patterns at the different facets. (a) Original laser emission; (b) expended beam without any focusing; (c) focused beam only by a fiber lens; (d) focused beam by the 3D micro-optical system.

The design of the 3D coupling system for the MEMS tunable laser mainly focuses on the grating design and

proper choice of the parameters, including the distance from the laser back facet to the fiber (L1), the p-parameter of the parabolic mirror (p), and the distance from the fiber to the mirror (L2). With such design parameters, the external cavity is able to maintain high coupling efficiency (> 90%), while providing a large tuning range. Based on the above considerations, the optimized parameter settings are L1 = 35.5 μm, p = 800 μm, L2 = 582 μm.

The simulated field patterns of the beams are compared as shown in Fig. 2. The original beam emitted from the laser diode has an elliptical shape (beam radii of 1.5 μm × 1.5 μm) as shown in Fig. 2(a). When there is no optical confine within the external cavity, the light divergences very fast along its propagation. The beam size is expanded to 265.5 μm × 265.5 μm after propagation of 800-μm distance in free space as shown in Fig 2(b). Fig. 2(c) shows the focused beam pattern by only the fiber rod lens. Both patterns Fig. 2(b) and Fig. 2(c) are different from the initial beam size, resulting in low coupling efficiency. In contrast, when the 3D micro-optical coupling system (including the rod-lens and the parabolic mirror) is employed, the reshaped beam size is 21.2 μm × 215.3 μm (Fig. 2(d)) before hitting the grating, which can be totally covered by the deep-etched grating. In the feedback trip, the light is re-focused back to the size of 1.45 μm × 1.55 μm, which is comparable to its original beam size. Therefore, after the 3D coupling system, the feedback beam can be effectively coupled back to the gain chip internal cavity. FABRICATION AND INTEGRATION

The scanning electron micrographs (SEMs) of an integrated MEMS tunable laser and the close-up of the blazed grating are shown in Fig. 3(a) and (b), respectively. The MEMS structures, including the parabolic mirror, the blazed grating, the rotary actuators and the fiber grooves are fabricated by the deep reactive ion etching (DRIE) process on the 8-inch silicon-on-insulator wafer. The wafer has a structural silicon layer of 100 µm and a buried oxide buffer layer of 2 µm. The fabrication processes include: aluminum bonding pads pattern; DRIE for laser bonding cavity; DRIE for MEMS structures (e.g. grating, parabolic mirror, fiber groove and rotary comb-drive); release and solder films deposition. In order to release the movable parts, the dry release method is implemented, which can ensure a clean and dry environment, as well as avoid damaging the structures due to the surface tension of water. After all MEMS structures are formed, it is ready for MEMS tunable laser integration.

(a) (b)

(c) (d)

(1)

(2)

14

(a)

(b)

Figure 3: SEM of the fabricated tunable laser on the SOI-wafer. (a)Overview of the MEMS integrated tunable laser; (b) close-up of the deep etched Si blazed grating and rotary combdriver on 100 um thick structure layer. The grating has a period of 3.2 μm and a blazed angle of 45°.

Before the solder film (Au/Sn: 80%/20%) deposition, a thick layer of Cu film is deposited onto the under-bump-metallurgy layer to compensate the inevitable over-etch in DRIE process for gain chip bonding holder structure. With the merits of passive alignment consideration, the packaging process can be simplified into 4 steps. First, the gain chip is bonded to the substrate using flip-chip bonding; the second step is assembling device into the butterfly box and passively insert the fiber; the third step is wire-bonding; and the final step is sealing the butterfly box in the nitrogen filled chamber.

In the 3D coupling system, the parabolic mirrors have been designed with p = 800 μm, and an open angle of 80° relative to the laser emitting point so as to handle more than 99% of the power. The traveling distance (free space) of the beam consists of the distance between the laser diode to the grating (742 μm) and the distance between parabolic mirror and grating (350 μm). The beam size expanded to about 265.5 μm with the divergence angle of 18.85 degree in the horizontal plane once it reaches the mirror surface. In order

to reduce the divergence of the beam in the vertical direction, a normal single mode fiber (diameter 125 μm) is used as the rod lens. Moreover, the rotary actuator follows the design to produce a rotation angle of 2 degree under a driving voltage of 18 V. EXPERIMENTAL RESULTS

The effectiveness of the proposed 3D micro-optical coupling system is measured by capturing the transformed beam patterns using an infrared CCD camera. The results are shown in Fig. 4. As it is difficult to directly measure the initial beam pattern of the gain chip, a FP laser chip with same cavity length and same gain material is used for measurement. Fig. 4(a) is the initial beam pattern. After passing through the fiber lens and the parabolic mirror, the beam becomes an enlarged elliptical pattern (Fig. 4(b)), where the beam is confined in vertical direction but expanded in horizontal direction. Such elliptical beam can be later on totally confined by the planar devices, but also improve the grating resolution. When the light is reflected back by the grating and the mirror, the beam becomes a focused beam again (Fig. 4(c)). Therefore, after passing the 3D coupling system, the feedback beam is well confined along the feedback trip.

Figure 4: Beam patterns captured by infrared camera. (a) Initial laser emission; (b) Shaped beam by 3D micro-optical coupling system, which is also the received beam by the grating; (c) Focused beam when the feedback light through the 3D micro-optical coupling system again.

0 5 10 150

0.2

0.4

0.6

0.8

Cavity length increase (um)

Cou

plin

g ef

fici

ency

(n.

u.)

3D Littrow 3D MirrorFlat Mirror

Figure 5: The measured coupling efficiency of the optical power with the change of external cavity length. The 3D coupling components in this MEMS Littrow tunable laser are effective and are insensitive to the cavity changing.

(a) (c)(b)

Output

Gain chip

Fiber rod lens

Parabolic mirror Grating

15

0 0.5 1 1.5 21.53

1.55

1.57

1.59

Grating rotation angle (deg)

Wav

elen

gth

(um

)

Figure 6: The output wavelength with the increase of grating angle. The inset exemplifies the measured output spectrum of the device, showing a large side-mode-suppress ratio of ~ 22 dB.

The coupling efficiency of the 3D micro-optical system is measured as shown in Fig. 5. The 3D coupling system provides a stable coupling efficiency of 76.5% for this Littrow-configured MEMS tunable laser. The efficiency is much higher than that of the flat mirror tunable laser (~ 3%) [2] and the 3D mirror tunable laser (~ 46%) [4]. More importantly, the coupling efficiency has less significant drop during the external cavity change, while it drops about 70% in the 3D mirror tunable laser.

Benefit from the high coupling efficiency, a large wavelength tuning is obtained, from 1542.2 nm to 1587.5 nm (tuning range of 45.3 nm) when the grating is rotated within 2°. The stable output power remains at 2.5 mW at the injected current of 30 mA. The inset examplifies the measured laser output spectrum, showing a high side-mode-suppress ratio of ~ 22 dB and the linewidth of ~ 0.01 nm. CONCLUSIONS

In conclusion, a 3D micro-coupling system used for MEMS Littrow tunable lasers has been demonstrated. With a high coupling efficiency of 76.5%, the MEMS laser has a large tuning range of 45.3 nm. Compared with conventional laser optical systems, which employ external ball-lenses and other optical components, our demonstrated MEMS tunable laser possesses the advantages of simple packaging, large alignment tolerance and easy mass production. It has potential applications in fiber optical communications and laser sensing systems. ACKNOWLEDEGEMENT

This work was supported by the National Research Foundation of Singapore (Grant No. NRF2009NRF-POC001-021).

REFERENCES [1] Y. Uenishi, K. Honma, and S. Nagaoka, “Tunable

laser diode using a nickel micromachined external mirror,” Elect. Lett., vo. 32, no. 13, pp. 1207-1208, 1996

[2] X. M. Zhang, A. Q. Liu, D. Y. Tang, and C. Lu, “Discrete wavelength tunable laser using microelectromechanical systems technology,” Appl. Phys. Lett., vol. 84, no. 3, pp. 329-331, 2004

[3] H. Cai, A. Q. Liu, X. M. Zhang, J. Tamil, D. Y. Tang, Q. X. Zhang and C. Lu, “A miniature tunable coupled-cavity laser constructed by micromachining technology,” Appl. Phys. Lett., vol. 92, 031105, 2008

[4] X. M. Zhang, H. Cai, C. Lu, C. K. Chen and A. Q. Liu, “Design and experiment of 3-dimensional micro-optical system for MEMS tunable lasers,” MEMS2006 Conference, Istanbul, Turkey, pp. 830-833, 2006

[5] A. Lohmann and R. R. A. Syms, “External cavity laser with a vertically etched silicon blazed grating,” Phot. Tech. Lett., vol. 15, no. 1, 2003

[6] X. M. Zhang, A. Q. Liu, C. Lu, and D. Y. Tang, “Continuous wavelength tuning in micromachined littrow external-cavity lasers,” J. Quan. Elect., vol. 41, no. 2, pp. 187-197, 2005

[7] E. Geerling, M. Rattunde, J. Schmitz, G. Kaufel, J. Wagner, D. Kallweit and H. Zappe, “Micro-mechanical external-cavity laser with wide tuning range,” MEMS 2007 Conference, Hyogo, Japan, pp. 731-734, 2007

[8] R. A. Syms, and A. Lohmann, “MOEMS Tuning Element for a Littrow External Cavity Laser,” J. Microelectromech. Syst., vol. 12, no. 6, pp. 921-928, 2003

[9] D. Anthon, J. D. Berger, J. Drake, S. Dutta, A. Fennema, et al., “External cavity diode lasers tuned with silicon MEMS,” OFC2002 Conference, San Diego, CA, pp. 97-98

[10] W. M. Wang, K. T. V. Gratten, A. W. Palmer, and W. J. O. Boyle, “Self-mixing interference inside a single-mode diode laser for optical sensing application,” J. Light. Tech., vol. 12, no. 9, pp. 1577-1587, 1994

[11] M. Bouda, M. Matsuda, K. Morito, S. Hara, T. Watanabe, T. Fujii, and Y. Kotaki, “Compact high-power wavelength selectable lasers for WDM applications,” OFC2000 Conference, Baltimore, MD vol. 1, pp. 178-180, 2000

[12] A. Q. Liu, X. M. Zhang, H. Cai, A. B. Yu, and C. Lu, “Retro-axial VOA using parabolic mirror pair,” Phot. Tech. Lett., vol. 19, pp. 692-694, 2007

[13] H, Cai, B. Liu, X. M. Zhang, W. M. Zhu, J. Tamil, W. Zhang, Q. X. Zhang and A. Q. Liu, “A micromachined thermo-optic tunable laser,” MEMS2009 Conference, Sorrento, Italy, pp. 1027-1032, 2009

22 dB

22dB

16