OFC/NFOEC 2008 a2048_1.pdf JThA39.pdf

A Monolithic InP-based Photonic Integrated Circuit for Optical Arbitrary Waveform Generation W. Jiang1, F. M. Soares1, S. W. Seo1, J. H. Baek1, N. K. Fontaine1, R. G. Broeke1, J. Cao1, J. Yan1, K. Okamoto1, F. Olsson2, S. Lourdudoss2, A. Pham1, and S. J. Ben Yoo1 Department of Electrical and Computer Engineering1, University of California, Davis, 95616 Department of Microelectronics and Information Technology2, Royal Institute of Technology, Sweden email: [email protected] Abstract: We demonstrate a compact monolithically-integrated InP optical arbitrary waveform generator, consisting of an arrayed waveguide grating pair with 10 GHz channel spacing, 10 high-speed optical amplitude modulators, and 10 high-speed optical phase modulators. ©2008 Optical Society of America OCIS codes: (130.3120) Integrated optics devices; (320.5540) Pulse shaping; (250.4745) Optical processing devices.

1. Introduction Line-by-line pulse shaping [1], in which intensity and phase manipulation of individual optical spectral components, allows optical arbitrary waveform generation (OAWG), which leads to diverse promising applications in optical communications [2], RF photonics [3], terahertz radiation [4], and LIDARs [5]. While the feasibility of OAWG has been demonstrated in a free-space optical platform [1], monolithic chip-scale integration is critically important for reliable, low-cost, and large-scale deployment of the OAWG technology. The arrayed waveguide grating (AWG) is an attractive dispersive element for the spectrum synthesis in OAWG, due to its scalability to a large number of channels with high spectral resolution. Thanks to the significant progress in high-precision InP fabrication technologies, InP-based AWGs with high-spectral resolution has recently been demonstrated [6][7]. This paper presents an InP-based photonic integrated circuit (PIC) for OAWG, which includes an AWG pair with 10 GHz channel spacing and an array of 10-channel high-speed amplitude and phase modulators, all monolithically integrated on an InP substrate. 14.6 mm CPW 10.7 mm

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(a) (b) Figure 1: (a) Design layout of the InP-based OAWG chip (CPW denotes coplanar waveguide), and (b) a picture of the fabricated device.

2. Operation Principle, Design and Fabrication Figure 1(a) shows a design layout of the monolithically-integrated InP chip for OAWG. It consists of two 10-GHzchannel-spacing AWGs with 10 channel waveguides in between. Each channel waveguide has both a traveling-wave electro-absorption-based (EA) amplitude modulator and a traveling-wave electro-optic-based (EO) phase modulator. The operation principle of the integrated OAWG device is as follows: The OAWG device receives signals from a coherent optical-comb source (e.g. a mode-locked laser), whose spectral components (comb lines) are demultiplexed into the 10 channel waveguides by the first (left) AWG. Then the amplitude and phase of each spectral component will be controlled individually by the amplitude and phase modulators. Finally, the manipulated spectral components are synthesized into the user-specified waveform by the right AWG. The 10-GHz-channel-spacing InP AWG [7] is significantly more advanced than the 20-GHz AWG [6], and Fig 5 shows the transmission spectrum. To achieve effective high-speed amplitude and phase modulations, coplanar waveguides (shown in Fig. 2(a)) are carefully designed and fabricated with two key requirements: (1) velocity matching between optical group-velocity

OFC/NFOEC 2008 a2048_1.pdf JThA39.pdf

and RF phase-velocity, and (2) transmission-line-impedance matching (at ~50 ohms). Fig. 2(b) shows RF insertion loss of the planar waveguide by HFSS simulation and by RF measurements. Figure 1(b) shows a photograph of the fabricated InP OAWG chip. The total chip size is about 15 mm × 11 mm. The epitaxial wafer was grown on a semi-insulating InP substrate by metal-organic-vapour-phase-epitaxy (MOVPE). The waveguide structure consists of a 500-nm i-InP buffer layer, a 50-nm Q(1.25) Etch stop layer, a 600-nm n-InP spacer layer, a 100-nm n-doped InGaAs contact layer, a 2-µm n-doped InP lower-cladding layer, a 500-nm Q(1.15) waveguide core layer, a 2-µm p-doped InP top cladding layer, and a 100-nm p-doped InGaAs layer. After photolithographic patterning, the 4-µm-wide waveguides were etched in a Br2/N2 reactive-ion-etcher using a 550-nm SiO2 layer as mask. Subsequently, the SiO2 mask was selectively removed in a buffered hydrofluoric-acid solution. Then, Fe-doped semi-insulating InP was regrown by low pressure hydride vapour phase epitaxy (HVPE). Afterwards, the Fe-doped InP (next to the modulators) was etched down to the bottom InGaAs layers to access the n-contacts. Then, we isolated the structure with Benzocyclobutene (BCB) and patterned Ti\Pt\Au of coplanarwaveguide metal lines. Finally, photo-resist patterning and He-implantation on the waveguide were applied to achieve impedance-matching and to block axial current propagation for reduced RF losses. 3. OAWG Measurement Results Fig. 3(a) shows a block diagram of the experimental setup for OAWG measurement. Fig. 3(b) shows the chip packaged on a high-speed RF-Optical modulation board made by Inphi Corp. An optical frequency comb generator [8] with wavelength and comb spacing tunability was used as a light source. As Fig. 4 shows, the comb generator was configured to generate optical combs with 10 GHz (or 0.08 nm) spacing centered on around 1550.7 nm, so that they aligned with the 10 wavelength channels of the AWG (Fig. 5). To preliminarily demonstrate the OAWG, a 2.5 Gbps RF signal from a pattern generator shown in Fig. 6(a) was applied to four of the ten phase modulators. The temporal arbitrary waveform was monitored by a 65-GHz digital communications analyzer (DCA) after it was amplified by an Erbium Doped Fiber Amplifier (EDFA). Fig. 6(b) shows the 2.5 Gbps modulated arbitrary waveform, and the waveform change (A-B waveform shown in Fig. 6(c) and (d) at 2.5 Gbps can be observed. However, the modulation effect is not as pronounced. This is mainly because of the mismatch of the transmission spectra of the two high-precision 10 GHz channel spacing AWGs causing degradations in the waveform fidelity and in the effective modulation bandwidth. Insertion Loss (dB)

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(a) Figure 3: (a) A block diagram of the OAWG experiment setup, and (b) a photograph of a RF Packaged OAWG Chip.

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OFC/NFOEC 2008 a2048_1.pdf JThA39.pdf

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Figure 6. (a) Waveform of 2.5 Gbps RF signal, (b) Generated arbitrary waveform by modulation, (c) Waveform B (zoom in), and (d) Waveform A (zoom in).

4. Conclusion We fabricated a compact monolithically-integrated InP-based chip for OAWG. The preliminary operation of 2.5Gbps modulated OAWG was experimentally investigated, and the A-B waveform change was observed. In the near future, we will employ several techniques to improve the OAWG performance, such as vernier AWG [9] to match the transmission spectra of two AWGs or a loopback geometry to achieve intrinsic alignment using a single AWG. 5. References [1] Z. Jiang and et al., "Spectral line-by-line pulse shaping," Opt. Lett. 30, 1557-1559 (2005). [2] B. Jalali and et al., "Photonic arbitrary waveform generator," in Proc. IEEE LEOS 2001 Annual Meeting, pp. 253-254, 2001. [3] J. D. Mckinney and et al., "Millimeter-wave arbitrary waveform generation with a direct space-to-time pulse shaper," Opt. Lett. 27, 13451347 (2002). [4] Y. Liu and et al., "Enhancement of narrow-band terahertz radiation from photoconducting antennas by optical pulse shaping," Opt. Lett. 21, 1762-1764 (1996). [5] L. Mullen and et al., "Application of RADAR technology to aerial LIDAR systems for enhancement of shallow underwater target detection," IEEE Transaction on Microwave Theory and Techniques 43, 2370-2377 (1995). [6] F. M. Soares and et al., "20 GHz channel spacing InP-based Arrayed Waveguide Grating," in the Technical Digest of ECOC 2007, paper no. MO02, 2007. [7] F. M. Soares and et al., "InP-based Arrayed Waveguide Grating with a channel spacing of 10 GHz," submitted to OFC 2007. [8] R. P. Scott and et al., "3.5-THz wide, 175 mode optical comb source," in Technical Digest of OFC, paper no. OWJ3, 2007. [9] H. Uetsuka and et al., "Novel 1×N guidedwave multi/demultiplexer for FDM," in the Technical Digest of OFC, paper no. TU07, 1995.

This work was supported in part by DARPA/DSO OAWG under agreement number HR0011-05-C-0155, and by DARPA/SPAWAR under agreement number N66001-02-1-8937.