Dynamic Distortion Characteristics of Silicon Evanescent Detectors and Phase Modulators Nobuhiro Nunoya, Anand Ramaswamy, Hui-Wen Chen, Hyundai Park1 and John E. Bowers Department of Electrical and Computer Engineering, University of California Santa Barbara, CA 93106, USA 1 Currently working for Intel Corp. E-mail: [email protected] Abstract: The linearity of silicon evanescent modulators and quantum well detectors was measured for the first time. An output IP3 of 21 dBm for detectors and a peak phase input IP3 of 2.6π for reverse biased phase modulators was achieved at 500 MHz. ©2008 Optical Society of America OCIS codes: 250.0040 Detectors; 250.4110 Modulators 1. Introduction Silicon photonics has been receiving a lot of interest because it opens up the potential to integrate photonics devices with CMOS electronics resulting in complex system functionality at a low cost. A technique that bonds III-V active material on a Si substrate is one way of realizing photonic devices in silicon [1]. Based on this bonding technique, a wide range of devices, including modulators and detectors, have been demonstrated [2,3]. Detailed characterization of these devices is essential in determining their feasibility in digital and analog communications systems. In this work, we focus on the linearity of 1) hybrid silicon evanescent detectors and 2) hybrid silicon evanescent phase modulators. Using a conventional two tone measurement technique for the detectors as well as a relatively new dynamic characterization technique for the phase modulators [4], the third-order intercept point (IP3) is determined. Further, output IP3s (OIP3) for the detectors and input IP3s (IIP3) for the phase modulators are compared under various operating conditions including as a function of input optical power and applied bias voltage. 2. Linearity of hybrid silicon evanescent detectors A cross sectional structure of the hybrid silicon evanescent photodetector and calculated band diagram at reverse bias of 8V are indicated in Fig 1. The active region consists of eight strain-compensated InGaAlAs quantum wells (QWs). The thickness and strain are 7 nm and +0.85% for active layers, and 10 nm and -0.55% for barrier layers, respectively. The 0.25-μm-thick p-AlGaInAs separate confinement heterostructure (SCH) layer, 1.5-μm-thick p-InP cladding layer and 0.1-μm-thick p-contact layer are stacked on the QWs. The 0.11-μm-thick n-type InP contact layer and superlattice layers for the bonding exist under the QWs[2]. The third order intermodulation distortion (IMD3) of the detector was characterized using a conventional two tone measurement technique. The experimental setup consists of two DFB semiconductor lasers with slightly differing lasing wavelength (1549.82 nm, 1549.36 nm). The output of the two lasers are modulated separately by two commercial intensity modulators at f1=480 MHz and f2=500 MHz. The modulated optical signals are combined and then amplified before being coupled into the device under test (DUT). The two tone measurement result for a reverse bias Vb of 4.5 V and a photo current Ipd of 10 mA can be seen in Fig. 2(a). Fig. 2(b) shows OIP3 for various reverse bias conditions as a function of photocurrent. Higher reverse bias results in higher OIP3 because the increased field overcomes the space charge. In addition, there is a tendency that Air gapp-InGaAsp-InP claddingp-InGaAlAs SCHAlGaInAs MQWsn-InP/InGaAsPn-InPIII-VRegionSOIRegion(a)(b) Fig. 1. (a) Cross sectional structure of hybrid silicon-evanescent devices with a narrow III-V mesa for detectors (b) Calculated band diagram at reverse bias voltage of 8 V. a1415_1.pdf OMR8.pdf © 2009 OSA/OFC/NFOEC 2009 OMR8.pdfthe higher photocurrent improves OIP3 even if a measurement error of a few dBm is considered. This can be explained from the band diagram in Fig. 1(b) by noting that the band curvature in the lightly doped p-type SCH layer suggests that some charge compensation is occurring. The effect of this at higher photocurrent values is similar to the charge compensation effect in modified uni-traveling-carrier photodiodes (UTC-PD) [5]. Consequently, the field is more uniform resulting in improved linearity at higher photocurrent values. In order to achieve higher linearity, bulk p-i-n diodes or even UTC-PD specially designed for linear applications [6] can be incorporated on the silicon evanescent platform. -20 0 20 40 60 80-100-80-60-40-200204060Input Modulation Power (dBm)Output Power(dBm)FundamentalIMD3Output IP3Vb=4.5 V Ipd=10 mA0 2 4 6 8 10 120510152025Photocurrent (mA)OIP 3(dBm)Vb=4.5VVb=3.5VVb=2.5V(a)(b) Fig. 2. (a) The result of two tone measurement Photocurrent and (b) bias voltage dependence of OIP3 3. Linearity of hybrid silicon evanescent phase modulators The device structure and fabrication process of the hybrid silicon evanescent phase modulator can be seen in Ref. [3]. This modulator has 15 InGaAlAs QWs consisting of 8-nm-thick compressive-strained (+0.3%) active layers and 5-nm-thick tensile strained (-0.41%) barrier layers. These QWs and upper SCH layer are lightly n doped for carrier depletion. In this work, a 1mm long phase modulator with a 1.5 μm wide silicon waveguide was measured. Fig. 3 shows the measurement system to determine the IMD3 contribution of a phase modulator. Light from a 1550-nm source is split into the two arms of a MZI by a polarized beam splitter (PBS), in which the splitting ratio can be changed by means of the polarization controller. In the upper arm of the interferometer, it can be seen that the electrical signal (f1: 504.9 MHz, f2: 505.1 MHz) from two microwave synthesizers is combined and then split before being applied to a LiNbO3 reference phase modulator (PM1) and the hybrid silicon phase modulator (DUT). The variable attenuators in the electrical path ensure that the drive signal to the two modulators produces the same phase swing in each device. However, the electrical path length between the 1:2 splitter and the DUT is adjusted to achieve a phase delay of π radians. The phase modulator (PM2) in the lower arm of the MZI forms part of a slow feedback circuit that is used to stabilize the MZI about the quadrature point. At the output of the interferometer, the two light waves are combined, and the interfered signals are detected by a balanced PD made with commercial CATV linear detectors (OIP3 20 dBm at a photocurrent of 5 mA). Theoretically, the phase detection process
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