Planar lightwave circuit dispersion compensator using a compact arrowhead arrayed-waveguide grating

Transcription

1 Planar lightwave circuit dispersion compensator using a compact arrowhead arrayed-waveguide grating Takanori Suzuki 1a), Kenichi Masuda 1, Hiroshi Ishikawa 2, Yukio Abe 2, Seiichi Kashimura 2, Hisato Uetsuka 2, and Hiroyuki Tsuda 1 1 Graduate School of Science and Technology, Keio University Hiyoshi, Kouhoku-ku, Yokohama-shi, Kanagawa, , Japan 2 Hitachi Cable Ltd Hidaka-cho, Hitachi-shi, Ibaraki, , Japan a) suzutaka@tsud.elec.keio.ac.jp Abstract: An ultra-compact chromatic dispersion compensator, based on a compact arrowhead arrayed-waveguide grating (AWG), has been proposed and fabricated. A dispersion compensating mirror, monolithically integrated into the second slab of the AWG, modulates the phase of each spectral component of the input light. The use of the compact arrowhead structure provides a dispersion compensator with a small footprint. The chromatic dispersion, the bandwidth and the insertion loss of the dispersion compensator based on the 8ch, 12.5 GHz-spacing high-resolution arrowhead AWG are 123 ps/nm, 70 GHz and 7.5 db, respectively. Dispersion compensation experiments for 40 Gbit/s, NRZ and RZ signals have been successfully demonstrated. Keywords: dispersion, waveguide arrays, optical planar waveguides, integrated optics, mirrors Classification: Photonics devices, circuits, and systems References [1] K. Takiguchi, K. Okamoto, Y. Inoue, M. Ishii, K. Morikawa, and S. Ando, Planar lightwave circuit dispersion equalizer module with polarization insensitive properties, Electron. Lett., vol. 31, no. 1, pp , [2] C. K. Madsen, S. Chandrasekhar, E. J. Laskowski, K. Bogart, M. A. Cappuzzo, A. Paunescu, L. W. Stulz, and L. T. Gomez, Compact integrated tunable chromatic dispersion compensator with a 4000 ps/nm tuning range, Proc. OFC 2001, Anaheim, 2001, PD [3] C. R. Doerr, L. W. Stulz, S. Chandrasekhar, and R. Pafchek, Colorless tunable dispersion compensator with 400-ps/nm range integrated with tunable noise filter, IEEE Photon. Technol. Lett., vol. 15, no. 9, pp , [4] H. Tsuda, T. Ishii, K. Naganuma, H. Takenouchi, K. Okamoto, Y. Inoue, and T. Kurokawa, Second- and Third- order dispersion compensator usc IEICE

2 ing a high-resolution arrayed-waveguide grating, IEEE Photon. Technol. Lett., vol. 11, no. 5, pp , [5] H. Tsuda, H. Takenouchi, A. Hirano, T. Kurokawa, and K. Okamoto, Performance analysis of a dispersion compensator using arrayedwaveguide gratings, IEEE J. Lightwave Technol., vol. 18, no. 8, pp , [6] T. Suzuki and H. Tsuda, Ultrasmall arrowhead arrayed waveguide grating using v-shaped bend waveguides, IEEE Photon. Technol. Lett., vol. 17, no. 4, pp , [7] T. Suzuki, Y. Shibata, and H. Tsuda, Small v-bend silica waveguide using an elliptic mirror for miniaturization of planar lightwave circuits, IEEE J. Lightwave Technol., vol. 23, no. 2, pp , [8] T. Suzuki and H. Tsuda, 16ch, 100 GHz-spacing compact arrayedwaveguide grating using a double bending structure, Proc. Pacific Rim Conference on Lasers and Electro-Optics 2005, Tokyo, Ctuk3 3, 2005/07. 1 Introduction The optical pulse distortion due to the chromatic dispersion in the transmission fibers is one of the serious problems to be solved for a wavelength division multiplexing (WDM) technology in a high speed and a large capacity communication photonic network. Although a dispersion compensating fiber (DCF) or a fiber grating (FG) is generally used for a dispersion compensation, planar lightwave circuit (PLC) dispersion compensators [1] [5] are being widely studied because of their features of low loss, high reliability, high stability and compactness. This paper addresses the proposal and the fabrication of PLC type dispersion compensator using an ultra-compact high resolution arrowhead AWG [6] with monolithically integrated dispersion compensating mirror. The size of the AWG can be significantly reduced by using the arrowhead structure; therefore, the dispersion compensator with high compensating performance can be realized for a given space of the wafer. The experimental results for 40 Gbit/s, nonreturn-to-zero (NRZ) signal and return-to-zero (RZ) signal are also described. 2 Dispersion compensator using an arrowhead AWG The arrowhead AWG, as shown in Fig. 1, has v-bend optical waveguides using elliptic integrated mirrors [7] in each array waveguide. The schematic configuration of the v-bend waveguide is depicted in the inset (a) of Fig. 1. The size of the arrowhead AWG is much smaller than a conventional AWG. The maximum compensatable dispersion is determined by the resolution of the AWG; which is proportional to the delay between the longest waveguide and the shortest waveguide in the arrayed-waveguide. Therefore, the dispersion compensator with large dispersion would be configured using a multiple arrowhead structure to fold a long waveguide in a given substrate [8]. The 573

3 detailed designs of the arrowhead AWG and the v-bend optical waveguide are summarized in Refs. [6] and [7], respectively. The dispersion compensator has the monolithically integrated dispersion compensating quadratic-shaped mirror near the focal plane in the second slab region. The input signal light is spectrally decomposed in the second slab and reflected at the integrated mirror, where the phase of each spectral component is modulated. The phase shift of each spectral component is determined by the form of the quadratic-shaped dispersion compensating mirror (an enlarged view is shown in the inset (b) of Fig. 1). The reflected light is retransmitted and synthesized in the arrowhead AWG. Fig. 1. Schematic configuration of the dispersion compensator based on an arrowhead AWG and a dispersion compensating mirror. The inset (a) is an enlarged view of the v-bend optical waveguide applied in each array waveguide and (b) is an enlarged view of the dispersion compensating mirror, integrated in the second slab waveguide. The modified optical path length L( ω) of each spectral component, controlled by the dispersion compensating mirror, is obtained as follows; the phase shift φ( ω) for a given frequency is determined in order to compensate the phase distortion due to the second order dispersion of the transmission fiber and it is described as iφ( ω) =i 2πn sl( ω) = i λ 2 β 2z ω 2, (1) where ω is the spectral displacement from the center frequency, β 2 is the group velocity dispersion (GVD) coefficient, z is the propagation distance of the light in the transmission fiber, n 2 is the effective index of the slab waveguide and λ is the wavelength of the light. Combining Eq. (1) with the relation of both ω = 2π ν and D = 2πcβ 2 /λ 2,(whereD is the dispersion parameter and c is the velocity of the light), the optical path length to compensate the GVD is given by L( ν) = λβ 2z 4πn s ω 2 = λ3 Dz 8π 2 cn s ω 2 = λ3 Dz 2cn s ν 2. (2) 574

4 Using the spectral-spatial conversion property of the AWG, ν = x (ν FSR / X FSR ), Eq. (2) is rewritten as a function of the focal position L( x), L( x) = λ3 DzνFSR 2 2cn s XFSR 2 x 2. (3) Where, x is the focal position of each component of the spectrum, X FSR is the free spatial range and ν FSR is the free spectral range. The shape of the dispersion compensating mirror must be designed to have L( x)/2 because of the use of reflective optics, as shown in the inset (b) of Fig Fabrication and experimental results The dispersion compensator, based on the 8ch, 12.5 GHz-spacing highresolution arrowhead AWG (free spectral range: 100 GHz) and the dispersion compensating mirror, was fabricated in silica with the refractive index difference of 0.75 %. The core size and the minimum radius of the curvature were 6 6 µm 2 and 5 mm, respectively. The size of the AWG was only mm 2. The integrated waveguide mirror was fabricated by reactive ion etching (RIE) to form a deep trench and the deposition of silver onto the side walls. The dispersion parameter was 100 ps/nm and the length of the dispersion compensating mirror along x axis was set to the free spatial range of 160 µm, so as to eliminate the interference with the light of the neighboring grating order. Fig. 2 shows the characteristics of the dispersion compensator. The black line and the gray line indicate a group delay and an insertion loss, respectively. In spite of three-fold mirror reflections in the AWG, the minimum insertion loss was only 7.5 db and the polarization dependent loss (PDL) was under 0.1 db. The measured second-order chromatic dispersion parameter was 123 ps/nm with a 3-dB down bandwidth of 70 GHz. Furthermore, the maximum group delay ripple within the bandwidth was 5.8 ps. The difference between the theoretical dispersion parameter (100 ps/nm) and the measured dispersion parameter (123 ps/nm) was probably caused by the mirror position misalignment due to the fabrication process. In order to confirm the dynamic dispersion compensation characteristics of the compensator, 40 Gbit/s NRZ and RZ pseudo random bit stream (PRBS) with optical signal lengths of were used. The eye diagrams for the NRZ signal experiment of the back-to-back, after the transmission through the optical fiber with a dispersion of 123 ps/nm, and after the dispersion compensating AWG are shown in Fig. 3 (a), (b) and (c). The power penalty for the bit error rate (BER) of 10 9 was about 4 db. On the other hand, the eye diagrams for the RZ signal experiment are shown in Fig. 3 (d), (e) and (f). The power penalty for the BER of 10 9 was about 3 db. Clear eye openings were observed and the dispersion compensation characteristics were confirmed. The power penalties were mainly caused by the insufficient bandwidth of the dispersion compensator and the group delay ripple. 575

5 Fig. 2. Characteristics of the dispersion compensator. The black and the red lines indicate the group delay and the insertion loss, respectively. (a) (b) (c) (d) (e) (f) Fig. 3. Eye diagrams for 40 Gbit/s, NRZ signals. (a) Back-to-back, (b) after the optical fiber with a dispersion of 123 ps/nm and (c) after the optical fiber with a dispersion of 123 ps/nm and the dispersion compensating AWG. Eye diagrams for 40 Gbit/s, RZ signals. (d) Back-to-back, (e) after the optical fiber with a dispersion of 123 ps/nm and (f) after the optical fiber with a dispersion of 123 ps/nm and the dispersion compensating AWG. 4 Conclusion We have proposed and fabricated the ultra-compact PLC chromatic dispersion compensator using a compact arrowhead AWG and an integrated dispersion compensating mirror. The size of the device based on an 8ch, 576

6 12.5 GHz-spacing high-resolution arrowhead AWG is only mm 2.The dispersion, the insertion loss and the bandwidth were 123 ps/nm, 7.5 db and 70 GHz, respectively. The dispersion compensation was successfully demonstrated with a power penalty of 4 db at a BER of 10 9 for 40 Gbit/s, NRZ signal and with a power penalty of 3 db at the BER of 10 9 for 40 Gbit/s, RZ signal. Acknowledgments Authors are grateful to Dr. Ryo Inohara and Dr. Hidenori Takahashi of KDDI R&D Laboratories Inc. for 40 Gbit/s dispersion compensation experiments. This study was supported by the grant from Ministry of Public Management, Home Affairs, Posts and Telecommunications, and also supported by the grant from Japan Science and Technology Corporation. 577