Th1F.003

A MEMS DIGITAL MIRROR FOR TUNABLE LASER WAVELENGTH SELECTION

W. M. Zhu1, 2, W. Zhang1, H. Cai1, J. Tamil1, B. Liu1, T. Bourouina2 and A. Q. Liu1 1 School of Electrical & Electronic Engineering, Nanyang Technological University, Singapore 639798 2 ESIEE, University of Paris-Est, Paris, France, 93162

ABSTRACT wavelength of the MEMS tunable laser can be finely tuned This paper presents a digital mirror for tunable laser without increasing the internal cavity length or introducing wavelength selection using microelectromechanical complex design to the mechanical part. systems (MEMS) technology. The digital mirror which has This paper is organized as follows. First, the design of discrete reflection spectrum is totally different compared to the digital MEMS mirror with MEMS tunable lasers is the current broadband mirrors MEMS tunable lasers [1-3]. described. Then, the theoretical results are analyzed. Finally, The reflection of the digital mirror can be tuned by digital the experimental results and conclusions are presented. pumping signals which promise fast output wavelength switching. The experimental results show that the output DESIGN OF DIGITAL MIRROR wavelength of the MEMS tunable laser using the digital Figure 1 shows the schematic of the MEMS digital mirror has high tuning resolution without increasing the mirror tunable laser. The tunable laser consists of three physical length of the internal cavity of the tunable laser. parts: a single mode fiber with a facet coated with gold, a The output wavelength of tuning resolution can reach 0.2 laser diode and a group of silicon slabs as a digital mirror. nm and the bandwidth is around 0.04 nm. The laser diode works as the gain medium. The internal cavity of the MEMS tunable laser is formed by the coated KEYWORDS fiber facet on the left side and the digital mirror on the right Photonic MEMS, Digital mirror, Tunable laser, side. The digital mirror consists of ten silicon slabs. The Thermo-optical effect. working wavelength is smaller than the thickness of silicon slabs which work as resonant reflectors [11, 12]. Therefore INTRODUCTION the digital mirror processes two merits: Firstly, the digital MEMS tunable laser has promising potential in many mirror reflects discrete wavelengths which are further applications, such as the coherent light source, biological selected by the resonant conditions of the internal cavity of detector within highly integrated chips and terahertz (THz) the MEMS tunable laser. Secondly, the reflected wave generators [4-5]. The resolution of wavelength tuning wavelength is more dependent on the refractive index (RI) is an important criterion for tunable lasers. However, the of one silicon slab which makes it possible to tune the wavelength resolution is limited by the internal cavity output wavelength of the MEMS tunable laser through the lengths of the typical tunable external cavity lasers which RI change of the silicon slab of the digital mirror. suffer from the mode hopping effect [6]. It is not a major problem for the bulk tunable lasers with large cavities. But the beam divergence becomes one of the main reasons for the cavity loss when the tunable laser is minimized to micrometer scale. For example, if the wavelength resolution is required to be 0.1 nm, the internal cavity has to be 3.5 mm which is more than ten times of the typical internal cavity length of the MEMS tunable laser thus increases the cavity loss by hundreds of times. Therefore, there are always trade-offs between the loss and the wavelength tuning resolutions. One solution for fine Figure 1: The schematics of the MEMS coded laser. wavelength tuning is to tune the internal cavities together with the external cavities [7-9] which complicates the Light reflected by the digital mirror is selected by the mechanical design and fabrication processes of the MEMS resonant modes of Fabry-Perot (F-P) cavity between the tunable lasers. digital mirror and the coated fiber facet and thus achieving In this paper, a digital mirror is designed for the MEMS the single wavelength output through Vernier effect. The tunable laser. This is totally different compared to the refractive index of each silicon slab is controlled by an traditional MEMS tunable lasers which are tuned by using electrical signal individually through thermo-optical effect. movable mirrors [10]. The reflection spectrum of the The change in the refractive index of any slab introduces a designed digital mirror is tuned together with the internal defect to the digital mirror and changes the output of the cavity of the MEMS tunable laser. Therefore, the output digital laser. When the refractive index of any silicon slab

978-1-4244-4193-8/09/$25.00 ©2009 IEEE 2206 Transducers 2009, Denver, CO, USA, June 21-25, 2009 is changed, the resonant condition of the digital mirror is changed and the output wavelength of the MEMS tunable laser is tuned.

NUMERICAL ANALYSE The effects on the digital mirror reflectivity of single silicon slab RI change are simulated by using Finite difference time domain (FDTD) method. The incident light is inputted form the left side to the right side as shown in Fig. 2. The width of the waveguide within the laser diode is 2 µm and the width of each silicon slab is 3 µm. In order to achieve the best coupling efficiency, air spacing between the laser diode and the digital mirror is optimized. The numbers of silicon slabs are sequenced from 1st to 10th.

(a)

Figure 2: FDTD simulation results of the incident light at 1571 nm are coupled into the digital mirror when 2nd silicon slab is heated. (a) before and (b) after. (b) The incident light wavelength (1571 nm) is chosen to have Figure. 3: Numerical results showing the single mode output high transmission coefficient (> 99.5%) at initial state. The achieved by using the digital mirror. (a) The normalized optical incident light is propagated through the digital mirror by the power as the function of wavelength for the reflection of the coupling of the adjacent F-P cavities that is formed by the reflector and the resonant mode in the F-P cavity. (b) The normalized laser output due to the Vernier effect of the reflector silicon slabs as shown in Fig. 2 (a). The coefficient of and the F-P cavity. silicon RI change at room temperature is approximately 1.74 × 104 per Kelvin. When the temperature of the 2nd Since the mechanism of the F-P cavities can support many silicon slab is increased by 60º approximately, the RI of the modes within the gain region, one of the modes is selected silicon slab is changed around 0.01. This is how the defects in order to achieve single mode output. The digital mirror are introduced to the digital mirror through the can choose a narrow band reflection region that is thermo-optical effect. When the 2nd silicon slab breaks the dependent on how to pick one lasing mode. The conditions coupling between the adjacent F-P cavities, the incident are as follows. Firstly, the relatively higher reflection light is reflected back to the waveguide of the laser diode. coefficient of the digital mirror ensures the interaction The reflection coefficient is above 90% as shown in Fig. 2 between the gain medium and simulated light. Secondly, (b). It is pointed out that the defect is located at different the resonant conditions between the coated fiber facet and sequence of the silicon slab and the actual length of the the digital mirror are satisfied. The transfer matrix method internal cavity is also different. is applied to analyze the mode competition mechanism of the MEMS tunable laser. The peak position of the reflection can be expressed as follow [13]:

2N 2N 2 Rmax = {[1-(1/n) ]/[1+(1/n) ]} (1)

2207 applied to monitor the alignment of the device setup. The where n is the refractive index of the silicon slabs and N is nano-probes are used to pump the laser diode and heat up the number of the silicon slabs. The reflection versus the the silicon slabs. The laser diode is lasing under the incident light wavelength for the digital mirror is shown in pumping voltage of around 6.9 V. The laser output is Fig. 3 (a) by the dark line. The resonant modes of the F-P monitored by an optical spectrum analyzer (OSA) when cavity between the digital mirror and the coded fiber facet different silicon slab is heated up. are shown by the gray line. The overlap between the reflection peak and the resonant modes stands for the high resonant wavelength in the MEMS tunable laser as shown in Fig. 3 (b). As a result, the tunable laser has single mode output within the gain region through design of the digital mirror. The F-P cavity length of the MEMS tunable laser is not the physical distance between the first silicon slab of the digital mirror and the coated fiber facet. As the distributed feed back digital mirror, the cavity length also depends on how the silicon slabs is penetrated by the incident light. In this way, the cavity length change depends on which silicon slab is heated in the digital mirror as shown in Fig. 2. The reflection spectrum is tuned along with the internal cavity length by introducing the defects to the digital mirror at different locations. It takes over the mode hopping effect of Figure 5: Experimental results of the transmission spectrum after the MEMS tunable laser and achieves fine tuning resolution heating on each silicon slab. Output wavelength as the function of without further increasing the external cavity length. the slab index is shown. The output wavelength after heating different silicon slab of EXPERIMENTAL RESULTS the digital mirror is shown in Fig. 5. The bandwidth of the The MEMS tunable laser is fabricated on a output laser is around 0.04 nm that is narrower compared to silicon-on-insulator wafer with the structure layer thickness the simulated results (in Fig. 3). This is because the mode of 75 µm. The width of each silicon slab is 3 µm. A laser competition further narrows down the bandwidth within the diode with the scale of 250 × 250 µm2 is coated with gain region. The MEMS tunable laser has five different anti-reflection (AR) film to serve as the gain medium. modes within 1 nm region when heating the corresponding silicon slab. The wavelength tuning resolution is 0.2 nm approximately. The insert shows the output wavelength versus the sequence number of the silicon slab. Therefore, it is noted that the output wavelength of the MEMS tunable laser is dependent on the tuning of the digital mirror.

CONCLUSIONS A digital mirror for MEMS tunable laser is designed, fabricated and experimented. The digital mirror provides wavelength selection for the MEMS tunable laser and the reflection spectrum of the digital mirror can be tuned along with the internal cavity length of the MEMS tunable laser through the change in the refractive index of the silicon Figure. 4: SEM photograph of the MEMS coded laser. slabs. Based on the experimental results, the wavelength tuning resolution is 0.2-nm which is ten times higher than The output laser is coupled to a single mode fiber which is the traditional MEMS external cavity tunable laser. passively aligned to a laser diode by a fiber groove. The facet of the fiber is coated with gold as a broadband mirror. The MEMS tunable laser is tested using fiber end-fire alignment system. A micro-scope and a CCD camera are

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