Sensors and Actuators A 108 (2003) 49–54

Single-/multi-mode tunable lasers using MEMS mirror and grating

A.Q. Liu a,∗, X.M. Zhanga, J. Lia, C. Lub

a School of Electrical & Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singaporeb Institute of High Performance Computing, 1 Science Park Road, # 01-01 The Capricorn, Singapore Science Park II, Singapore 117528, Singapore

Received 29 July 2002; received in revised form 25 March 2003; accepted 21 April 2003

Abstract

The microelectromechanical systems (MEMS) technology has been widely applied to develop the miniaturized external cavity tunablelasers. In this paper, two integrated MEMS tunable lasers using different configurations are presented and discussed. One uses a deep-etchedcircular mirror as the external converging reflector. It works in multiple longitudinal modes and has mode hopping. The other employsa deep-etched rotary blazed grating as the reflector and filter simultaneously. Single-longitudinal mode operation is achieved and modehopping can be avoided.© 2003 Elsevier B.V. All rights reserved.

Keywords: Tunable laser; Blazed grating; DWDM; MEMS; Optical MEMS

1. Introduction

Tunable lasers have wide applications in dense wave-length division multiplexing (DWDM) systems to save in-ventory cost and volume by replacing the wavelength-fixedlaser sources, and they can also greatly improve the func-tionality of optical network, for example, wavelength con-version, wavelength-based switching, routing and virtualnetworking. The microelectromechanical systems (MEMS)technology has shown strong promise to miniaturize the con-ventional mechanical tunable lasers with adding merits ofhigh compactness, high speed and batch production.

The physical concept of an external cavity tunable diodelaser is using an external reflector to feed back a portion ofoutput light into the lasing cavity. The phase and amplitudebalances inside the lasing cavity would be changed. Asa result, the wavelength varies. Although the wavelengthof diode lasers can be tuned by changing temperatureand injection current, the external cavity tunable laser hasattracted significant interests since it can provide largetuning range, continuous wave (CW) tuning, high outputpower and excellent wavelength accuracy while maintain-ing single-longitudinal mode. Also, the presence of externalcavity yields a number of performance enhancements suchas narrow linewidth (∼1 KHz) and high sidemode sup-pression ratio (>30 dB). The conventional opto-mechanical

∗ Corresponding author. Tel.:+65-790-4336; fax:+65-792-0415.E-mail address: eaqiu@ntu.edu.sg (A.Q. Liu).

external cavity tunable diode lasers have bulk sizes andslow speed, limiting their applications. The micromachinedtunable lasers have been developed using the microma-chined mirrors to form the external cavities[1–3]. The pre-cision and stable movement of the microactuators enablesfine-tuning of the wavelength. The small size of the micro-machined mirrors yields high tuning speed and also makesit feasible to form the extremely-short-external-cavity tun-able lasers. In addition, the micromachined tunable lasersyield high compactness, low fabrication cost, low powerconsumption and easy integration with IC control circuits.

A microfabricated tunable laser diode fabricated by nickelplating was demonstrated by Uenishi et al. and achieved awavelength tuning range of 20 nm[1]. Tunable lasers usingmicromachined Fabry–Perot (FP) etalons and micromir-rors were also reported[2,3]. An integrated tunable diodelaser using a surface-micromachined 3D mirror was alsoshown in our previous work[4]. However, they encounteredsome difficulties such as small wavelength tuning range,multiple longitudinal modes and mode hopping. Recently,it has attracted more and more interests to use gratingsas the external reflectors in MEMS tunable lasers sincethe gratings have very good filtering function to ensuresingle-longitudinal mode and narrow linewidth in very largetuning range. Berger and co-workers demonstrated a tunablelaser in which a grating was in Littman/Metcalf mountingand a rotary micromirror served as the reflector. It obtained+7 dBm output power, 55 dB sidemode suppression over40 nm tuning range, 2 MHz linewidth and 10 pm wavelength

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50 A.Q. Liu et al. / Sensors and Actuators A 108 (2003) 49–54

accuracy[5,6]. However, the grating was made separatelyand then be integrated with the MEMS actuators, it requiresaccurate alignment and also increases the total dimensionof the laser. Micromachined gratings fabricated along withthe MEMS structures would overcome this difficulty.

Deep etching technology is preferred to fabricate theMEMS structures for tunable laser rather than the surfacemicromachining technology since it can produce variousvertical optical components such as flat mirrors, circu-lar mirrors, gratings and microlenses as well as sometrenches/guides for optical alignment. In contrast, the surfacemicromachining is quite limited and it is difficult to obtainlarge mirror with high performance due to stress-inducedbending and the release holes, which are required to etchaway the sacrificial layers to free the mirror[8]. Thedeep-etched structures are ready to use once the fabricationis completed, whereas the surface-micromachined mirrorsneed to be assembled to form three-dimensional structures.The assembly is commonly done by hand and needs expe-rienced persons to operate very carefully for a long time,making it unsuitable for high volume industrial production.Besides, the deep-etched structures are strong, stable andreliable since their size in vertical direction is commonly30–75�m, while the surface-micromachined structures arevery easy to stick to substrate.

In this paper, two different configurations of MEMS tun-able lasers are demonstrated and characterized. The fabrica-tion uses silicon-on-insulator (SOI) wafers. Flat mirror, cir-cular mirror and rotary blazed grating are tried to serve asthe external reflectors for the tunable laser respectively.

2. Integrated MEMS tunable lasers

2.1. Schematic arrangements of tunable lasers

The schematic arrangements of MEMS tunable lasers areillustrated inFig. 1. Either a translating micromirror or a ro-tary blazed grating can act as the external reflector, as shownFig. 1(a) and (b)respectively. InFig. 1(a), the light beamemitted from one of the end facets of the laser diode is firstcollimated by a microlens and reflected by the micromir-ror, and is then coupled back into the lasing cavity of thelaser diode. With the translation of micromirror, the phaseof coupled light varies. As a result, the output wavelengthchanges. When the external cavity becomes very short, themicrolens is not necessary, simplifying the structure of tun-able laser. An alternative way is using a concave sphericalmirror rather than the flat micromirror. The concave mir-ror can reflect and focus the laser light without needing forany microlens. However, concave mirror is not easy to fab-ricate using MEMS technology. Therefore, a circular mirrorhas to be used. In this laser configuration, the wavelengthis mainly determined by the Fabry–Perot cavity formed bythe two facets of laser diode and the external mirror. Highcompactness and large tuning range can be achieved, how-

Laserdiode Microlens MicromirrorFiber

(a)

Laserdiode

Microlens Blazedgrating

Fiber

(b)

Fig. 1. Schematic configurations of the tunable lasers: (a) a translatingmicromirror tunable laser; (b) a rotary blazed grating tunable laser.

ever, the laser is generally operated in multiple longitudinalmodes, and the mode hopping is difficult to overcome.

In Fig. 1(b), a grating sits at the end of a cantilever beamand can be driven to rotate by a rotary comb drive. Thelaser light diffracted by the grating is also focused by themicrolens and enters the lasing cavity of the diode laser.The grating is arranged in Littrow mounting [7]. It actsnot only as an external reflector, but also as a wavelengthselective component to maintain the diode in single mode.Moreover, mode hopping can be avoided by carefully se-lecting the length of the cantilever beam. In this laser diodethe output wavelength is determined by the superimpositionof the gain bandwidth of the diode, the grating dispersionand the external cavity mode structure. When the externalcavity length changes, the possible wavelengths of the cav-ity modes change accordingly. To avoid mode hopping, thepassband of the grating should follow the change simul-taneously. This requirement can be approximately met by

Fig. 2. Tunable laser using a deep-etched circular mirror.

A.Q. Liu et al. / Sensors and Actuators A 108 (2003) 49–54 51

Fig. 3. Close-up of the deep-etched circular mirror.

selecting a proper length of cantilever beam. In both typesof tunable lasers, the laser light emitted from the other facetis butt-coupled by a single-mode fiber to form the output.

2.2. Tunable laser using translating mirror

Fig. 2 shows an integrated tunable laser that uses a cir-cular mirror as the external reflector, and Fig. 3 shows theclose-up of the circular mirror and its actuator. The MEMSstructures, including the circular mirror, the comb drive,and the trenches for laser diode and fiber, are fabricated ona SOI wafer using a proprietary process flow and recipe.The SOI wafer consists of a 75 �m-thick silicon layer anda 2 �m-thick silicon oxide layer stacked on a 450 �m-thicksilicon substrate. Key features of this fabrication are high

Fig. 4. Surface roughness of the circular mirror.

Fig. 5. Deep-etched flat mirror.

verticality and good coating of the sidewalls. The mirror isformed by a 0.3 �m-thick gold layer coated on a 2 �m-thicksilicon. The verticality of mirror can reach 90 ± 0.1◦ and itsreflectance measures larger than 97%. The surface rough-ness of the mirror is shown in Fig. 4. The root-mean-squareroughness is 19 nm. The mirror is driven by a combdriveactuator and has a resonant frequency measured to be about2.5 KHz. Deep-etched flat mirrors can also serve as theexternal reflector for the tunable laser as shown in Fig. 5.Since the light emitted from the laser diode diverges in largeangles, the microlenses should be engaged or the externalcavity should be very short. Otherwise, the wavelengthtunable range is very small. The circular mirror overcomesthis difficulty. Its mirror surface reflects the light while itscircular shape helps to focus. As a result, it greatly reduces

52 A.Q. Liu et al. / Sensors and Actuators A 108 (2003) 49–54

Fig. 6. Spectra of the tunable laser using the deep-etched circular mirror: (a) initial state; (b) actuated state.

the alignment difficulty when assembling the laser diodeonto the MEMS structures. In the assembly, the laser withthe contact on the top surface is placed with face downinto a trench in the silicon wafer (Fig. 2). The bottom ofthe trench is coated with gold and connected to the wafersurface, which is also gold coated. A layer of indium foilabout 45 �m thick is sandwiched between the laser and thetrench bottom. This foil serves two purposes. It bonds thelaser to the trench and provides better ohmic contact. And italso lifts up the optical axis of laser emission for fiber out-put coupling. In the experiment, two probes are employedto apply the injection current. One contacts with the wafersurface (i.e. connected to the contact surface of the laser)while the other contacts with the other laser surface.

Fig. 6 illustrates the spectra of the tunable laser using thecircular mirror in the initial state (0 V driving voltage) andone of the actuated states (5 V driving voltage). It is shownthat multiple longitudinal modes exist. When the circularmirror is actuated, the wavelengths of the different modesincrease while the power redistributes among the modes.The long wavelength modes get more power and the shortwavelength modes gradually disappear. Here define the cen-tral wavelength of a multi-mode laser to be the wavelengthof the mode that has the maximum power. It is observedin the experiment that the central wavelength can be tunedcontinuously within a small range as shown in Fig. 6, andthen it hops to the position of the next mode as the power

in the adjacent mode becomes dominant. From this point ofview, mode hopping is unavoidable.

2.3. Tunable laser using rotary blazed grating

A tunable laser using a deep-etched rotary grating is il-lustrated in Fig. 7. It implements the idea in Fig. 1(b). Theclose-ups of the grating and the rotary comb drive are shownin Fig. 8. The grating is formed by a 3 �m-thick silicon

Fig. 7. Tunable laser using a rotary blazed grating.

A.Q. Liu et al. / Sensors and Actuators A 108 (2003) 49–54 53

Fig. 8. Close-up of the rotary blazed grating.

layer coated with 0.3 �m-thick gold. One important featureof this tunable laser is that the coating area of the sidewallcan be controlled instead of uniformly coating. For example,the grating should be coated while the microlens is kept un-coated; otherwise the laser light is blocked by the microlenssidewalls. The grating has 3 �m pitch, 15◦ blazed angle andworks in the first diffraction order. The static actuation per-formance of the rotary grating is characterized in Fig. 9. Inthe loading process when the driving voltage increases from0 to 15 V, the rotation angle continuously goes to 1.6◦. Inthe unloading process, the actuation curve matches with that

Fig. 10. Spectra of the tunable laser using the deep-etched rotary blazed grating: (a) initial state; (b) actuated state.

0

0.3

0.6

0.9

1.2

1.5

1.8

0 2 4 6 8 10 12 14 16

Driving voltage (V)

Rot

atio

n an

gle

(deg

)

0-15 V

15-0 V

Fig. 9. Rotation angle vs. driving voltage for the rotary grating.

of the loading curve except for a 0.1◦ hysteresis in the mid-dle region. The angle is measured by digitally correlatingthe amplified images of the comb fingers taken by a chargecoupled detector (CCD) that is mounted on a microscope.The method can reach an accuracy of 0.05◦.

Two spectra of the tunable laser with the rotary mirrorare shown in Fig. 10 corresponding to the initial state (nodriving voltage) and the actuated state (5 V driving voltage).Compared with the laser using the circular mirror, the laser isoperated nearly in single-longitudinal mode, the side modesare always suppressed to low power level. The energy is

54 A.Q. Liu et al. / Sensors and Actuators A 108 (2003) 49–54

mainly concentrated on the single mode and varies onlyslightly, making it possible to avoid the mode hopping.

3. Conclusions

MEMS tunable lasers have also been developed by usinga deep-etched circular mirror and a rotary blazed grating asthe external reflectors. Both have the size of about 1.5 mm×1 mm. The circular mirror works in multi-longitudinal modesand the mode hopping phenomenon is observed, whereasthe rotary grating works in nearly single-longitudinal modeand the mode hopping can be overcome by selecting properparameters of the grating.

References

[1] Y. Uenishi, K. Honma, S. Nagaoka, Tunable laser diode using anickel micromachined external mirror, Electron. Lett. 32 (13) (1996)1207–1208.

[2] Y. Sidorin, M. Blomberg, P. Karioja, Demonstration of a tunablehybrid laser diode using an electrostatically tunable silicon micro-machined Fabry–Perot interferometer device, IEEE Photon. Technol.Lett. 11 (1) (1999) 18–20.

[3] M.-H. Kiang, O. Solgaard, R.S. Muller, K.Y. Lau,Silicon-micromachined micromirrors with integrated high-precisionactuators for external-cavity semiconductor lasers, IEEE Photon.Technol. Lett. 8 (1) (1996) 95–97.

[4] A.Q. Liu, X.M. Zhang, V.M. Murukeshan, Y.L. Lam, A novel in-tegrated micromachined tunable laser using polysilicon 3D mirror,IEEE Photon. Technol. Lett. 13 (5) (2001) 427–429.

[5] J.D. Berger, Y. Zhang, J.D. Grade, H. Lee, S. Hrinya, H. Jerman,Al Fennema, A. Tselikov, D. Anthon, Widely tunable external cav-ity diode laser using a MEMS electrostatic rotary actuator, in: Pro-ceedings of the Technical Digest LEOS Summer Topic Meeteeing,Copper Mountain, CO, USA, July 30, 2001.

[6] D. Anthon, J.D. Berger, J. Drake, J.D. Grade, S. Hrinya, F. Ilkov,H. Jerman, D. King, H. Lee, A. Tselikov, K. Yasumura, Externalcavity diode lasers tuned with silicon MEMS, Tech. Digest OFC2002,Anaheim, California, USA, March 17, 2002.

[7] M.C. Hutley. Diffraction Gratings, Academic Press, London, 1982.

[8] X.M. Zhang, A.Q. Liu, V.M. Murukeshan, F. Chollet, Integrated mi-cromachined tunable lasers for all optical network (AON) applica-tions, Sens. Actuators A 97–98 (2002) 54–60.

Biographies

A.Q. Liu received his PhD from National University of Singapore (NUS)in 1994. His MSE degree was in applied physics, and BEng degree wasin mechanical engineering from Xi’an Jiaotong University, in 1988 and1982, respectively. He started to explore MEMS technology in 1995 whenhe had worked in the DSO National Laboratory. In 1997, he joined In-stitute of Materials Research & Engineering (IMRE), National Universityof Singapore, as a Senior Research Fellow, to establish and drive theMEMS program. Currently, he is an associate professor of Division ofMicroelectronics, School of Electrical & Electronic Engineering, NanyangTechnological University (NTU). His research interest is MEMS tech-nology in infocomm applications, MEMS design and fabrication processintegration are also his major contribution areas.

X.M. Zhang received BEng degree in precision mechanical engineering in1994 from the University of Science & Technology of China, and MEngdegree in optical instrumentation in 1997 from Shanghai Institute of Optics& Fine Mechanics, the Chinese Academia of Science. Then he pursuedhis postgraduate degree in MEMS at the Department of MechanicalEngineering, National University of Singapore. Presently, he is a researchassociate at the School of Electrical & Electronic Engineering, NanyangTechnological University where he is pursuing his doctoral studies. Hisresearch interests include Optical MEMS, optical communication, opticalinstrumentation and optical measurement.

J. Li received BEng and MEng degree in microelectronics from the Xi’anJiaotong University of China in 1995 and 1998, respectively. From 1998to 2000, she worked in Huawei Technology Company in China. Currently,she is a PhD candidate at Nanyang Technological University of Singa-pore. Her current research interests are in the areas of Optical MEMS,optical-fiber components, optical measurement, and dense wavelength di-vision multiplexed (DWDM) telecommunication networks.

C. Lu is a member of technical staff in Institute of High PerformanceComputing, Singapore. He obtained his PhD from National University ofSingapore, ME from Beijing Institute of Technology and BSc from PekingUniversity. His research interests include computational solid mechanicsand MEMS.