BRIEF NOTE
Japanese Journal of Applied Physics 53, 038001 (2014) http://dx.doi.org/10.7567/JJAP.53.038001
Slow-light Bragg reflector waveguide array for two-dimensional beam steering Kensuke Nakamura1, Akihiro Matsutani2, Moustafa Ahmed3, Ahmed Bakry3, and Fumio Koyama1,3* 1
Photonics Integration System Research Center, Tokyo Institute of Technology, Yokohama 226-8503, Japan Semiconductor and MEMS Processing Center, Tokyo Institute of Technology, Yokohama 226-8503, Japan 3 Department of Physics, Faculty of Science, King Abdulaziz University, 80203 Jeddah 21589, Saudi Arabia E-mail:
[email protected] 2
Received November 19, 2013; accepted January 10, 2014; published online January 31, 2014 We propose and demonstrate a slow-light Bragg reflector waveguide array for two-dimensional (2D) beam steering. The device consists of a highly dispersive Bragg reflector waveguide array with a quarter-wavelength stack semiconductor mirrors and a Y-branch 1 ' 4 splitter with a branching angle of 30°. The beam steering characteristics in the two orthogonal directions are clarified by tuning the input wavelength. Beam steering was demonstrated using wavelength tuning with the assistance of delay sections. The results show the possibility of 2D beam steering using the Bragg reflector waveguide array by combining wavelength tuning and thermo-optic phase tuning. © 2014 The Japan Society of Applied Physics
A beam scanner is a key element for various optical sensing and imaging applications, such as projection displays, laser printers and light detection and ranging (LIDAR) systems. Mechanical beam scanners such as polygonal mirror scanners are popular because they are capable of performing wide deflection angle operations with high-resolutions.1–3) However, mechanical scanners are bulky, and their beam-steering speed is limited by their moving parts. For various applications in which the beam direction changes rapidly to random locations, different non-mechanical beam steering approaches have been reported.4–10) The beam steering performance can be determined using the number of resolutionpoints N, which is defined as distinguishable spot counts in the far-field patterns (FFPs). Generally, it is difficult to obtain over 100 resolution points for non-mechanical beam steering techniques. We proposed and demonstrated a high-resolution beam steering device based on a slow-light Bragg waveguide.11–14) Although we were able to demonstrate a highresolution beam steering capable of obtaining over 1000 resolution points for a few mm long devices, only onedimensional beam steering was possible. In this paper, we propose and demonstrate a slow-light Bragg reflector waveguide array for two-dimensional (2D) beam steering. The beam steering in the orthogonal two directions is clarified by tuning the input wavelength. Figure 1 shows the schematic diagram of a 2D beam scanner based on a slow-light Bragg reflector waveguide array. The device consists of three parts: a splitter, phaseshifter array, and slow-light Bragg reflector array. The working principle for beam steering in the propagation direction is the same as that of our previously reported 980nm-band-beam-steering devices.12,13) The slow-light propagates in a zigzag route with a traveling angle, which is strongly dependent on the wavelength; therefore, the slowlight is highly dispersive.15) Because the top-mirror has a lower reflectivity than the bottom-mirror, radiation occurs at the waveguide surface. By tuning the input wavelength, we are able to steer the radiation beam. In fact, the angular dispersion is larger than 1°/nm, which is 10 times larger than that of conventional diffraction gratings.14) Thus, wavelength tuning of only a few tens of nanometers yields a large beam steering angle of over 30°.12) In order to steer the beam in the lateral direction, an optical phase-array approach4) is used. The tunable phase shifters generate the desired phases; hence, the beam is deflected to the desired angle in the lateral direction of the waveguide array. In a separate experiment,
Fig. 1. (Color online) Schematic diagram of a 2D beam scanner based on the slow-light Bragg reflector waveguide array.
we demonstrated a thermo-optic phase shifter with the same waveguide structure, which exhibited a 2³-phase shift below 50 mW of electrical power consumption.16) Based on the phased array principle, the lateral out-coupling angle v of a phased array with a pitch d of the waveguide is given by4) sin v ¼
; 2d
ð1Þ
where ¦º is the uniform phase difference between the neighboring waveguides and is the free-space wavelength. The maximum steering angle occurs at ¦º = 2³, which is the spacing ¦ª between the FFP lobes. For example, a phased array of d = 21 and 2 µm offers a maximum steering angle of 2.7 and 29°, respectively. Therefore, a higher density array produces a larger steering angle. A phase-shifter array shown in Fig. 1 is required for lateral beam steering. Alternatively, an optical delay line is inserted between the neighboring waveguides of the array to implement the phase difference ¦º, which can be varied by tuning the wavelength. Although the beam steering in the two orthogonal directions is dependent of each other, we are able to evaluate the feasibility of 2D beam steering. Figures 2(a) and 2(b) show the schematic structure and cross-section of the fabricated Bragg reflector waveguide array, respectively. The fabricated waveguide has a -thick GaAs waveguide core sandwiched between 40-pair Al0.16Ga0.84As/Al0.96Ga0.04As bottom distributed Bragg reflector (DBR) and 28pair top Al0.16Ga0.84As/Al0.96Ga0.04As DBR. The fabrication
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© 2014 The Japan Society of Applied Physics
Jpn. J. Appl. Phys. 53, 038001 (2014)
K. Nakamura et al.
(a) (a)
(b) (b) Fig. 3. (Color online) (a) Top view of the fabricated Bragg reflector waveguide array and (b) measured near-field pattern with an input wavelength of 988.7 nm.
6° (c) Fig. 2. (Color online) (a) Schematic structure and (b) cross-section of the fabricated Bragg reflector waveguide array as well as (c) optical coupling setup with a tilted lensed fiber.
process includes two iterations of inductive coupled plasma reactive ion etching, which form waveguide mesas and a coupling region. The external tunable light source is linearly polarized and then coupled into the waveguide through a lensed fiber as shown in Fig. 2(c). Figure 3(a) shows the top view of the fabricated Bragg reflector waveguide array, which has a Y-branch 1 © 4 splitter with a branching angle of 30°. The array number, array pitch, and width of the high-mesa waveguide were 4, 21, and 7 µm, respectively. We measured the near-filed pattern (NFP) and FFP using a NFP/FFP measurement system (Optosystems M-Scope type D). The loaded imaging setup, which is comprised of a lens system with a half-mirror beam splitter and two charge coupled device (CCD) cameras, enabled us to capture the NFP and FFP simultaneously. Figure 3(b) shows the measured NFP with an input wavelength of 988.7 nm in the TE mode, which exhibits that the light is divided into the four slow-light waveguide arrays. Figure 4 shows the superimposed image of the measured FFPs while the input wavelength was varied between 988.0 and 989.4 nm in 0.2 nm steps. Because of the large angular dispersion of the Bragg reflector waveguide, a continuous shift in the
Fig. 4. (Color online) Superimposed image of the measured far-field patterns while the input wavelength is varied between 988.0 and 989.4 nm in 0.2 nm steps.
propagation (ªl) direction is observed from a smaller to larger angle. In the lateral (v ) direction, periodic radiation peaks can be observed at a pitch of 2.7°, which is the spacing ¦ª of the FFP lobes. This is in good agreement with the calculation determined using the pitch of the waveguide array given by Eq. (1).4) It is also noted that the peak position is shifted in the v direction. Figure 5 shows the measured angular shifts as a function of input wavelength for the two orthogonal directions. Form Eq. (1), the angular shift v in the v direction can be written as
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Lng v ; ¼ d
ð2Þ
© 2014 The Japan Society of Applied Physics
Jpn. J. Appl. Phys. 53, 038001 (2014)
K. Nakamura et al.
was demonstrated by wavelength tuning with the assistance of optical delay sections. The results show the possibility of 2D beam steering using the Bragg reflector waveguide array by combining wavelength tuning and thermo-optic phase tuning. High-resolution and non-mechanical beam steering can be expected because of the giant angular dispersion12) of the Bragg reflector waveguide. Acknowledgement This work was supported by JSPS KAKENHI Grant Number S22226008 and was funded by the Deanship of Scientific Research (DSR), King Abdulaziz University (KAU) under Grant No. 23/34/RG. The authors, therefore, acknowledge with thanks DSR technical and financial support.
Fig. 5. Measured angular shifts as a function of input wavelength in the two orthogonal directions.
where ¦L is the length of a delay line, which is 5.6 µm, for the device shown in Fig. 3(a); ng is the group index; and ¦ is the wavelength tuning range. The measured lateral angular shift v of 0.6°/nm is in agreement with Eq. (2) if we assume that ng = 40. Moreover, the angular shift ¦ªl in the propagation direction can be as large as 3.8°/nm since the wavelength is near the cut-off wavelength, which is also in agreement with the calculation.14) In this preliminary experiment, the beam steering in the two directions was not independent when a single steering parameter of the wavelength was used. However, if we integrate a thermo-optic phase-shifter array, 2D beam steering can be performed independently in the ªl direction by tuning the wavelength and in the v direction by thermo-optic phase tuning. In addition, by decreasing the array pitch, we are able to increase the angular shift as well as the beam steering range. In conclusion, we proposed and demonstrated a slow-light Bragg reflector waveguide array for 2D beam steering. The beam steering characteristics in the two orthogonal directions were clarified by tuning the input wavelength. Beam steering
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J. C. Wyant, Appl. Opt. 14, 1057 (1975). T. Matsuda, F. Abe, and H. Takahashi, Appl. Opt. 17, 878 (1978). H. H. Refai, M. P. Tull, and J. J. Sluss, Jr., Opt. Eng. 46, 085401 (2007). P. F. McManamon, T. A. Dorschner, D. L. Corkum, L. J. Friedman, D. S. Hobbs, M. Holz, S. Liberman, H. Q. Nguyen, D. P. Resler, R. C. Sharp, and E. A. Watson, Proc. IEEE 84, 268 (1996). K. Nakamura, J. Miyazu, M. Sasaura, and K. Fujiura, Appl. Phys. Lett. 89, 131115 (2006). S. Mukai, Y. Kaneko, M. Watanabe, H. Itoh, and H. Yajima, Appl. Phys. Lett. 51, 2091 (1987). N. W. Carlson, G. A. Evans, R. Amantea, S. L. Palfrey, J. M. Hammer, M. Lurie, L. A. Carr, F. Z. Hawrylo, E. A. James, C. J. Kaiser, J. B. Kirk, and W. F. Reichert, Appl. Phys. Lett. 53, 2275 (1988). Y. Kurosaka, S. Iwahashi, Y. Liang, K. Sakai, E. Miyai, W. Kunishi, D. Ohnishi, and S. Noda, Nat. Photonics 4, 447 (2010). K. Van Acoleyen, W. Bogaerts, J. Jágerská, N. Le Thomas, R. Houdré, and R. Baets, Opt. Lett. 34, 1477 (2009). J. K. Doylend, M. J. R. Heck, J. T. Bovington, J. D. Peters, L. A. Coldren, and J. E. Bowers, Opt. Express 19, 21595 (2011). X. Gu, T. Shimada, and F. Koyama, Opt. Express 19, 22675 (2011). X. Gu, T. Shimada, A. Matsutani, and F. Koyama, IEEE Photonics J. 4, 1712 (2012). X. Gu, T. Shimada, A. Fuchida, A. Matsutani, A. Imamura, and F. Koyama, Jpn. J. Appl. Phys. 51, 020206 (2012). F. Koyama and X. Gu, IEEE J. Sel. Top. Quantum Electron. 19, 1701510 (2013). P. Yeh, A. Yariv, and E. Marom, J. Opt. Soc. Am. 68, 1196 (1978). A. Fuchida, A. Matsutani, M. Ahmed, A. Bakry, and F. Koyama, Jpn. J. Appl. Phys. 53, 010306 (2014).
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