OSA / OFC/NFOEC 2010 a1880_1.pdf OThS1.pdf
Monolithically Integrated InP Wafer-Scale 100-Channel × 10-GHz AWG and Michelson Interferometers for 1-THzBandwidth Optical Arbitrary Waveform Generation F. M. Soares1, J. H. Baek1, N. K. Fontaine1, X. Zhou1, Y. Wang1, R. P. Scott1, J.P. Heritage1, C. Junesand2, S. Lourdudoss2, K.Y. Liou3, R. A. Hamm3, W. Wang3, B. Patel3, S. Vatanapradit3, L. A. Gruezke3, W. T. Tsang3, and S. J. B. Yoo1 1
Department of Electrical and Computer Engineering1, University of California, Davis, 95616 Department of Microelectronics and Information Technology2, Royal Institute of Technology, Sweden 3 Multiplex, Inc., 5000 Hadley Road, South Plainfield, New Jersey 07080 USA email:
[email protected]
2
Abstract: We discuss monolithic integration of a 100-channel AWG with a 10-GHz channel spacing with 100 Michelson-interferometer-based phase- and amplitude-modulators. The AWG showed approximately 10 dB crosstalk, and the twin-integrated devices comprise a 2” InP wafer. ©2009 Optical Society of America OCIS codes: (130.3120) Integrated optics devices; (130.5990) Semiconductors.
1. Introduction Optical arbitrary waveform generation (OAWG) has potential applications such as: ultra-wideband wireless communications, pulsed radar, RF/THz remote sensing, and LIDAR [1]. Integrated silica-based OAWGs have been demonstrated that can perform line-by-line Fourier spectral shaping to create arbitrary waveforms with a temporal resolution up to 5.1 THz [2]. However, these OAWGs have a much slower waveform update rate than can be achieved when using InP-based electro-optic- or quantum-well-based phase modulators. Recently, we have reported a 10-channel x 20-GHz InP-based OAWG chip with a waveform bandwidth of 200 GHz [3]. In this work, we discuss the monolithically integrated InP OAWG chip with 100 channels spaced at 10 GHz for generating waveforms with a bandwidth of 1 THz. InP Optical Arbitrary Waveform Generator 1 THz
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Figure 1 Schematic of the 100-channel × 10-GHz OAWG device showing the operating principle.
2. 1-THz Optical-Arbitrary-Waveform-Generation Concept and InP-Based Implementation Figure 1 shows the schematic of the 100-channel × 10-GHz OAWG chip. The input is a coherent optical frequency comb of 100 wavelengths spaced at 10 GHz covering a 1-THz-bandwidth. This 1-THz pulse is coupled into the InP OAWG chip, after which it passes through a 2×1 input multi-mode-interference (MMI) coupler. Then, the 100channel x 10-GHz AWG demultiplexes the frequency comb onto 100 different outputs and couples each comb line into a Michelson interferometer. The Michelson interferometer consists of a 2x1 MMI splitter/combiner with each arm containing a 2-mm-long quantum-well (QW) phase modulator and a 80% HR-coated cleaved-facet mirror. After the reflection the signals propagate back and re-combine at the start of the Michelson. Thus, ‘push-pull’ modulation of the phase of the two arms will yield amplitude modulation and ‘push-push’ will result in phase modulation of
OSA / OFC/NFOEC 2010 a1880_1.pdf OThS1.pdf
each comb line. The QW phase modulators have a photoluminescence peak at 1490nm, and our epitaxial layer stack has been designed such that we can achieve modulation in two distinct ways: either by applying a reverse bias to the QW modulator and exploiting the electro-optic effect, or by injecting a 1310-nm control signal at the HR-facet side (see Figure 1) and inducing cross-phase modulation. input MMI coupler
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Figure 2 (a) Mask layout of the 100-channel x 10-GHz OAWG device, and (b) a photograph of a fabricated wafer which fits two overlaid devices. There are a total of 1200 devices integrated on this wafer-scale chip.
After the modulation, the comb lines are multiplexed back onto the original input of the AWG (brown arrows on Figure 1) and coupled out at the input facet. The 2×1 MMI coupler allows separation of the reflected waveformshaped signal from the background reflection at the input facet. Figure 2a shows the mask layout of the OAWG device. The device is 2.9 cm × 3.5 cm. We have designed the AWG to have a flat-top transmission by adding a parabolic taper at the AWG inputs and a narrow waveguide at the AWG outputs [4]. The AWG contains 400 arrayed waveguides (AWs), and the slab regions are roughly 1.7cm long. Highresolution AWGs are extremely sensitive to phase perturbations in the AWs, and therefore we have added an electro-optic phase modulator on all 400 AWs to compensate for these phase errors [5]. 3. Fabrication The fabrication process contains three epitaxial growth steps. The first epitaxial step defines a 150nm p-doped InP layer, a 212-nm 12 QW layer, a 0.5-µm n-doped Q(1.15) waveguide core layer, and a 2-µm n-doped buffer layer. The 12 QW layers consist of 12 quaternary wells, and 13 quaternary barriers with a combined photoluminescence peak around 1490 nm wavelength. Subsequently, we pattern the QW layers in the modulator regions shown in Figure 2 by using lithography and wet-chemical etching. The second epitaxial growth grows a 2-µm p-doped InP top-cladding layer followed by a 100-nm p-doped InGaAs layer. After photo lithographically defining the waveguides, they are etched in a Br2/N2 reactive-ion-etcher using a 550-nm SiO2 layer as mask. The third growth step grows Fe-doped semi-insulating InP laterally by low pressure hydride vapor phase epitaxy (HVPE) on the side of the etched waveguides. Afterwards, we deposit a layer of 350-nm of SiO2 and define via contact openings to the QW phase modulators and the phase-error-correction modulators. Finally, we pattern the metal layer on the top-sideand the back-side of the wafer using lithography and lift-off. Figure 2b shows a photograph of a fabricated 2” InP wafer which includes two identical OAWG devices by overlaying the slab regions of the AWGs in mirror image. The wafer-scale twin OAWG chip includes 1200 independent optoelectronic components monolithically integrated. 4. Results and Discussion Characterization of the 100-channel × 10-GHz OAWG device included optical vector network analyzer (OVNA) [6]. The advantages of using the OVNA for characterization are: a very high wavelength resolution (50 MHz, in our case), ability to measure both the amplitude- and phase response of the device (which is essential for phase error correction of the AWG), and a very high dynamic range due to balanced detection. At first we performed corrected for the phase errors on the AWs of the AWG to improve the AWG transmission. Figure 3a shows the measured AWG transmission that shows that we were able to achieve approximately -10 dB adjacent-channel crosstalk
OSA / OFC/NFOEC 2010 a1880_1.pdf OThS1.pdf
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throughout all the 100 AWG outputs. Afterwards, we proceeded to measuring the transmission spectrum of the OAWG device. Figure 3b shows the measured OAWG-device transmission where we can distinguish all the 100channels. Subsequently, we measured the transmission of the OAWG device while electrically biasing the QW phase modulator of channel # 92. Figure 3c shows the transmission for three different applied voltages, where a 20 dB amplitude modulation has been achieved, without affecting the transmission of the adjacent channels. Finally, we injected a 1310-nm optical control signal into this same channel to observe cross-phase modulation effect from the 1310nm signal to the 1550-nm wavelength comb. Figure 3d shows the transmission after applying 0 mW and 2.5 mW of 1310-nm optical phase-control signal under an applied reverse bias of -3V. This plot reveals the same 20-dB amplitude modulation depth as in the case of electrically reverse biasing the QW modulators.
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c) d) Figure 3 Normalized AWG transmission after phase error correction a), and the measured transmission of the OAWG device under the following conditions: without applying any bias or optical phase-control signal to the phase modulators b), while electrically reverse-biasing the QW modulator of channel # 92 c), and while injecting 1310-nm optical phase-control signal to channel # 92 at a reverse bias of -3V d).
4. Summary We realized a monolithically integrated, InP-based 1-THz-bandwidth OAWG device consisting of a 100-channel × 10-GHz AWG with 200 QW phase modulators and 400 electro-optic phase modulators in twin configurations on the 2 inch wafer. We observed 20-dB modulation of the 100-channels individually in two regimes: the electro-optic regime by reverse-biasing the QW phase modulators in each channel, and the cross-phase modulation regime by injecting an optical phase-control signal in the QW modulators. Experiments are in progress to address all 100 channels and to achieve full modulation of the 1-THz OAWG device. 5. References [1] B. Jalali, P. Kelkar, and V. Saxena, "Photonic arbitrary waveform generator," in Lasers and Electro-Optics Society, 2001. LEOS 2001. The 14th Annual Meeting of the IEEE, 2001, pp. 253-254 vol.1. [2] N. K. Fontaine, R. P. Scott, C. Yang, D. J. Geisler, K. Okamoto, J. P. Heritage, and S. J. B. Yoo, "Integrated, Ultrahigh-Fidelity 17x40 GHz OAWG," in Conference on Lasers and Electro-Optics/Quantum Electronics and Laser Science Conference and Photonic Applications Systems Technologies, 2008, p. CTuA5. [3] S. W. Seo, F. M. Soares, J. H. Baek, W. Jiang, N. K. Fontaine, R. P. Scott, C. Yang, D. J. Geisler, J. Yan, and R. G. Broeke, "Monolithically Integrated InP Photonic Micro Systems on a chip for O-CDMA and OAWG applications," Photonics in Switching, 2007, pp. 97-98, 2007. [4] K. Okamoto and A. Sugita, "Flat spectral response arrayed-waveguide grating multiplexer with parabolic waveguide horns," Electronics Letters, vol. 32, p. 1661, 1996. [5] N. K. Fontaine, J. Wei, F. M. Soares, R. G. Broeke, S. W. Seo, J. H. Baek, J. Cao, K. Okamoto, S. J. B. Yoo, and F. Olsson, "Determination of 20 GHz InP AWG Phase Errors by Measurement of AWG Pulse Train," Lasers and Electro-Optics Society, 2007. LEOS 2007. The 20th Annual Meeting of the IEEE, pp. 725-726, 2007. [6] D. K. Gifford, B. J. Soller, M. S. Wolfe, and M. E. Froggatt, "Optical vector network analyzer for single-scan measurements of loss, group delay, and polarization mode dispersion," Applied Optics, vol. 44, pp. 7282-7286, 2005.
This work was supported in part by DARPA/SPAWAR under agreement number N66001-02-1-8937, and by DARPA/DSO OAWG under agreement number HR0011-05-C-0155.
