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

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Sensors and Actuators A 108 (2003) 49 54 Single-/multi-mode tunable lasers using MEMS mirror and grating A.Q. Liu a,, X.M. Zhang a,j.li a,c.

1 Sensors and Actuators A 108 (2003) Single-/multi-mode tunable lasers using MEMS mirror and grating A.Q. Liu a,, X.M. Zhang a,j.li a,c.lu b a School of Electrical & Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Singapore , Singapore b Institute of High Performance Computing, 1 Science Park Road, # The Capricorn, Singapore Science Park II, Singapore , 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 tunable lasers. In this paper, two integrated MEMS tunable lasers using different configurations are presented and discussed. One uses a deep-etched circular mirror as the external converging reflector. It works in multiple longitudinal modes and has mode hopping. The other employs a deep-etched rotary blazed grating as the reflector and filter simultaneously. Single-longitudinal mode operation is achieved and mode hopping can be avoided Elsevier B.V. All rights reserved. Keywords: Tunable laser; Blazed grating; DWDM; MEMS; Optical MEMS 1. Introduction Tunable lasers have wide applications in dense wavelength division multiplexing (DWDM) systems to save inventory cost and volume by replacing the wavelength-fixed laser sources, and they can also greatly improve the functionality of optical network, for example, wavelength conversion, wavelength-based switching, routing and virtual networking. The microelectromechanical systems (MEMS) technology has shown strong promise to miniaturize the conventional mechanical tunable lasers with adding merits of high compactness, high speed and batch production. The physical concept of an external cavity tunable diode laser is using an external reflector to feed back a portion of output light into the lasing cavity. The phase and amplitude balances inside the lasing cavity would be changed. As a result, the wavelength varies. Although the wavelength of diode lasers can be tuned by changing temperature and injection current, the external cavity tunable laser has attracted significant interests since it can provide large tuning range, continuous wave (CW) tuning, high output power and excellent wavelength accuracy while maintaining single-longitudinal mode. Also, the presence of external cavity yields a number of performance enhancements such as narrow linewidth ( 1 KHz) and high sidemode suppression ratio (>30 db). The conventional opto-mechanical Corresponding author. Tel.: ; fax: address: eaqiu@ntu.edu.sg (A.Q. Liu). external cavity tunable diode lasers have bulk sizes and slow speed, limiting their applications. The micromachined tunable lasers have been developed using the micromachined mirrors to form the external cavities [1 3]. The precision and stable movement of the microactuators enables fine-tuning of the wavelength. The small size of the micromachined mirrors yields high tuning speed and also makes it feasible to form the extremely-short-external-cavity tunable lasers. In addition, the micromachined tunable lasers yield high compactness, low fabrication cost, low power consumption and easy integration with IC control circuits. A microfabricated tunable laser diode fabricated by nickel plating was demonstrated by Uenishi et al. and achieved a wavelength tuning range of 20 nm [1]. Tunable lasers using micromachined Fabry Perot (FP) etalons and micromirrors were also reported [2,3]. An integrated tunable diode laser using a surface-micromachined 3D mirror was also shown in our previous work [4]. However, they encountered some difficulties such as small wavelength tuning range, multiple longitudinal modes and mode hopping. Recently, it has attracted more and more interests to use gratings as the external reflectors in MEMS tunable lasers since the gratings have very good filtering function to ensure single-longitudinal mode and narrow linewidth in very large tuning range. Berger and co-workers demonstrated a tunable laser in which a grating was in Littman/Metcalf mounting and a rotary micromirror served as the reflector. It obtained +7 dbm output power, 55 db sidemode suppression over 40 nm tuning range, 2 MHz linewidth and 10 pm wavelength /$ see front matter 2003 Elsevier B.V. All rights reserved. doi: /s (03)

50 A.Q. Liu et al. / Sensors and Actuators A 108 (2003) 49 54 accuracy [5,6].

2 50 A.Q. Liu et al. / Sensors and Actuators A 108 (2003) accuracy [5,6]. However, the grating was made separately and then be integrated with the MEMS actuators, it requires accurate alignment and also increases the total dimension of the laser. Micromachined gratings fabricated along with the MEMS structures would overcome this difficulty. Deep etching technology is preferred to fabricate the MEMS structures for tunable laser rather than the surface micromachining technology since it can produce various vertical optical components such as flat mirrors, circular mirrors, gratings and microlenses as well as some trenches/guides for optical alignment. In contrast, the surface micromachining is quite limited and it is difficult to obtain large mirror with high performance due to stress-induced bending and the release holes, which are required to etch away the sacrificial layers to free the mirror [8]. The deep-etched structures are ready to use once the fabrication is completed, whereas the surface-micromachined mirrors need to be assembled to form three-dimensional structures. The assembly is commonly done by hand and needs experienced 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 and reliable since their size in vertical direction is commonly m, while the surface-micromachined structures are very easy to stick to substrate. In this paper, two different configurations of MEMS tunable lasers are demonstrated and characterized. The fabrication uses silicon-on-insulator (SOI) wafers. Flat mirror, circular mirror and rotary blazed grating are tried to serve as the 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 are illustrated in Fig. 1. Either a translating micromirror or a rotary blazed grating can act as the external reflector, as shown Fig. 1(a) and (b) respectively. In Fig. 1(a), the light beam emitted from one of the end facets of the laser diode is first collimated by a microlens and reflected by the micromirror, and is then coupled back into the lasing cavity of the laser diode. With the translation of micromirror, the phase of coupled light varies. As a result, the output wavelength changes. When the external cavity becomes very short, the microlens is not necessary, simplifying the structure of tunable laser. An alternative way is using a concave spherical mirror rather than the flat micromirror. The concave mirror can reflect and focus the laser light without needing for any microlens. However, concave mirror is not easy to fabricate using MEMS technology. Therefore, a circular mirror has to be used. In this laser configuration, the wavelength is mainly determined by the Fabry Perot cavity formed by the two facets of laser diode and the external mirror. High compactness and large tuning range can be achieved, how- (a) (b) Fiber Fiber Laserdiode Microlens Micromirror Laser diode Microlens Blazed grating Fig. 1. Schematic configurations of the tunable lasers: (a) a translating micromirror tunable laser; (b) a rotary blazed grating tunable laser. ever, the laser is generally operated in multiple longitudinal modes, and the mode hopping is difficult to overcome. In Fig. 1(b), a grating sits at the end of a cantilever beam and can be driven to rotate by a rotary comb drive. The laser light diffracted by the grating is also focused by the microlens and enters the lasing cavity of the diode laser. The grating is arranged in Littrow mounting [7]. It acts not only as an external reflector, but also as a wavelength selective component to maintain the diode in single mode. Moreover, mode hopping can be avoided by carefully selecting the length of the cantilever beam. In this laser diode the output wavelength is determined by the superimposition of the gain bandwidth of the diode, the grating dispersion and the external cavity mode structure. When the external cavity length changes, the possible wavelengths of the cavity modes change accordingly. To avoid mode hopping, the passband of the grating should follow the change simultaneously. 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.

2 shows an integrated tunable laser that uses a circular mirror as the external reflector, and Fig. 3 shows the close-up of the circular mirror and its actuator.

The SOI wafer consists of a 75 m-thick silicon layer and a2 m-thick silicon oxide layer stacked on a 450 m-thick silicon substrate. Key features of this fabrication are high Fig. 5.

3 A.Q. Liu et al. / Sensors and Actuators A 108 (2003) Fig. 3. Close-up of the deep-etched circular mirror. selecting a proper length of cantilever beam. In both types of tunable lasers, the laser light emitted from the other facet is butt-coupled by a single-mode fiber to form the output Tunable laser using translating mirror Fig. 2 shows an integrated tunable laser that uses a circular mirror as the external reflector, and Fig. 3 shows the close-up of the circular mirror and its actuator. The MEMS structures, including the circular mirror, the comb drive, and the trenches for laser diode and fiber, are fabricated on a SOI wafer using a proprietary process flow and recipe. The SOI wafer consists of a 75 m-thick silicon layer and a2 m-thick silicon oxide layer stacked on a 450 m-thick silicon substrate. Key features of this fabrication are high Fig. 5. Deep-etched flat mirror. verticality and good coating of the sidewalls. The mirror is formed by a 0.3 m-thick gold layer coated on a 2 m-thick silicon. The verticality of mirror can reach 90 ± 0.1 and its reflectance measures larger than 97%. The surface roughness of the mirror is shown in Fig. 4. The root-mean-square roughness is 19 nm. The mirror is driven by a combdrive actuator and has a resonant frequency measured to be about 2.5 KHz. Deep-etched flat mirrors can also serve as the external reflector for the tunable laser as shown in Fig. 5. Since the light emitted from the laser diode diverges in large angles, the microlenses should be engaged or the external cavity should be very short. Otherwise, the wavelength tunable range is very small. The circular mirror overcomes this difficulty. Its mirror surface reflects the light while its circular shape helps to focus. As a result, it greatly reduces Fig. 4. Surface roughness of the circular mirror.

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.

In the assembly, the laser with the contact on the top surface is placed with face down into a trench in the silicon wafer (Fig. 2).

4 52 A.Q. Liu et al. / Sensors and Actuators A 108 (2003) 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 diode onto the MEMS structures. In the assembly, the laser with the contact on the top surface is placed with face down into a trench in the silicon wafer (Fig. 2). The bottom of the trench is coated with gold and connected to the wafer surface, which is also gold coated. A layer of indium foil about 45 m thick is sandwiched between the laser and the trench bottom. This foil serves two purposes. It bonds the laser to the trench and provides better ohmic contact. And it also lifts up the optical axis of laser emission for fiber output coupling. In the experiment, two probes are employed to apply the injection current. One contacts with the wafer surface (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 the circular mirror in the initial state (0 V driving voltage) and one of the actuated states (5 V driving voltage). It is shown that multiple longitudinal modes exist. When the circular mirror is actuated, the wavelengths of the different modes increase while the power redistributes among the modes. The long wavelength modes get more power and the short wavelength modes gradually disappear. Here define the central wavelength of a multi-mode laser to be the wavelength of the mode that has the maximum power. It is observed in the experiment that the central wavelength can be tuned continuously within a small range as shown in Fig. 6, and then it hops to the position of the next mode as the power in the adjacent mode becomes dominant. From this point of view, mode hopping is unavoidable Tunable laser using rotary blazed grating A tunable laser using a deep-etched rotary grating is illustrated in Fig. 7. It implements the idea in Fig. 1(b). The close-ups of the grating and the rotary comb drive are shown in 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 1.8 Rotation angle (deg) 1.5 1.2 0.9 0.6 0.3 0-15 V 15-0 V 0 0 2 4 6 8 10 12 14 16 Driving voltage (V) Fig. 8. Close-up of the rotary blazed grating.

One important feature of this tunable laser is that the coating area of the sidewall can be controlled instead of uniformly coating.

5 A.Q. Liu et al. / Sensors and Actuators A 108 (2003) Rotation angle (deg) V 15-0 V Driving voltage (V) Fig. 8. Close-up of the rotary blazed grating. Fig. 9. Rotation angle vs. driving voltage for the rotary grating. layer coated with 0.3 m-thick gold. One important feature of this tunable laser is that the coating area of the sidewall can be controlled instead of uniformly coating. For example, the grating should be coated while the microlens is kept uncoated; otherwise the laser light is blocked by the microlens sidewalls. The grating has 3 m pitch, 15 blazed angle and works in the first diffraction order. The static actuation performance of the rotary grating is characterized in Fig. 9. In the loading process when the driving voltage increases from 0 to 15 V, the rotation angle continuously goes to 1.6.In the unloading process, the actuation curve matches with that of the loading curve except for a 0.1 hysteresis in the middle region. The angle is measured by digitally correlating the amplified images of the comb fingers taken by a charge coupled detector (CCD) that is mounted on a microscope. The method can reach an accuracy of Two spectra of the tunable laser with the rotary mirror are shown in Fig. 10 corresponding to the initial state (no driving voltage) and the actuated state (5 V driving voltage). Compared with the laser using the circular mirror, the laser is operated nearly in single-longitudinal mode, the side modes are always suppressed to low power level. The energy is Fig. 10. Spectra of the tunable laser using the deep-etched rotary blazed grating: (a) initial state; (b) actuated state.

6 54 A.Q. Liu et al. / Sensors and Actuators A 108 (2003) mainly concentrated on the single mode and varies only slightly, making it possible to avoid the mode hopping. [8] X.M. Zhang, A.Q. Liu, V.M. Murukeshan, F. Chollet, Integrated micromachined tunable lasers for all optical network (AON) applications, Sens. Actuators A (2002) Conclusions MEMS tunable lasers have also been developed by using a deep-etched circular mirror and a rotary blazed grating as the external reflectors. Both have the size of about 1.5mm 1 mm. The circular mirror works in multi-longitudinal modes and the mode hopping phenomenon is observed, whereas the rotary grating works in nearly single-longitudinal mode and the mode hopping can be overcome by selecting proper parameters of the grating. References [1] Y. Uenishi, K. Honma, S. Nagaoka, Tunable laser diode using a nickel micromachined external mirror, Electron. Lett. 32 (13) (1996) [2] Y. Sidorin, M. Blomberg, P. Karioja, Demonstration of a tunable hybrid laser diode using an electrostatically tunable silicon micromachined Fabry Perot interferometer device, IEEE Photon. Technol. Lett. 11 (1) (1999) [3] M.-H. Kiang, O. Solgaard, R.S. Muller, K.Y. Lau, Silicon-micromachined micromirrors with integrated high-precision actuators for external-cavity semiconductor lasers, IEEE Photon. Technol. Lett. 8 (1) (1996) [4] A.Q. Liu, X.M. Zhang, V.M. Murukeshan, Y.L. Lam, A novel integrated micromachined tunable laser using polysilicon 3D mirror, IEEE Photon. Technol. Lett. 13 (5) (2001) [5] J.D. Berger, Y. Zhang, J.D. Grade, H. Lee, S. Hrinya, H. Jerman, Al Fennema, A. Tselikov, D. Anthon, Widely tunable external cavity diode laser using a MEMS electrostatic rotary actuator, in: Proceedings of the Technical Digest LEOS Summer Topic Meeteeing, Copper Mountain, CO, USA, July 30, [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, External cavity diode lasers tuned with silicon MEMS, Tech. Digest OFC2002, Anaheim, California, USA, March 17, [7] M.C. Hutley. Diffraction Gratings, Academic Press, London, Biographies A.Q. Liu received his PhD from National University of Singapore (NUS) in His MSE degree was in applied physics, and BEng degree was in mechanical engineering from Xi an Jiaotong University, in 1988 and 1982, respectively. He started to explore MEMS technology in 1995 when he had worked in the DSO National Laboratory. In 1997, he joined Institute of Materials Research & Engineering (IMRE), National University of Singapore, as a Senior Research Fellow, to establish and drive the MEMS program. Currently, he is an associate professor of Division of Microelectronics, School of Electrical & Electronic Engineering, Nanyang Technological University (NTU). His research interest is MEMS technology in infocomm applications, MEMS design and fabrication process integration are also his major contribution areas. X.M. Zhang received BEng degree in precision mechanical engineering in 1994 from the University of Science & Technology of China, and MEng degree in optical instrumentation in 1997 from Shanghai Institute of Optics & Fine Mechanics, the Chinese Academia of Science. Then he pursued his postgraduate degree in MEMS at the Department of Mechanical Engineering, National University of Singapore. Presently, he is a research associate at the School of Electrical & Electronic Engineering, Nanyang Technological University where he is pursuing his doctoral studies. His research interests include Optical MEMS, optical communication, optical instrumentation and optical measurement. J. Li received BEng and MEng degree in microelectronics from the Xi an Jiaotong University of China in 1995 and 1998, respectively. From 1998 to 2000, she worked in Huawei Technology Company in China. Currently, she is a PhD candidate at Nanyang Technological University of Singapore. Her current research interests are in the areas of Optical MEMS, optical-fiber components, optical measurement, and dense wavelength division multiplexed (DWDM) telecommunication networks. C. Lu is a member of technical staff in Institute of High Performance Computing, Singapore. He obtained his PhD from National University of Singapore, ME from Beijing Institute of Technology and BSc from Peking University. His research interests include computational solid mechanics and MEMS.

A novel tunable diode laser using volume holographic gratings

A novel tunable diode laser using volume holographic gratings Christophe Moser *, Lawrence Ho and Frank Havermeyer Ondax, Inc. 85 E. Duarte Road, Monrovia, CA 9116, USA ABSTRACT We have developed a self-aligned