15 Gb/s Transmission with Wide-FSR Carrier Injection Ring Modulator for Tb/s Optical Links M. Ashkan Seyedi 1 , Rui Wu 2 , Chin-Hui Chen 1 , Marco Fiorentino 1 , Ray G. Beausoleil 1 1 Hewlett Packard Labs, Hewlett Packard Enterprise, 3000 Hanover st, Palo Alto, CA 94304 2 University of California, Santa Barbara, Santa Barbara, CA 93106 [email protected] Abstract: A ring modulator with 22nm free spectral range is modulated and demonstrates an open eye diagram at 15Gb/s. This enables high channel counts towards Terabit per second bandwidth for optical data links using this device architecture. OCIS codes: (200.4650) Optical interconnects; (130.4110) Modulators; (230.5750) Resonators. 1. Introduction Cavity quality factor Q and free spectral range (FSR) determine the channel count, modulation data rate and the associated crosstalk of dense wavelength division multiplexing (DWDM) links. Carrier-depletion rings demonstrated 56Gb/s data rate for an FSR of 19.1nm and cavity Q of 3500 [1]. However, these devices have lower extinction ratio and higher insertion loss compared to carrier-injection modulators. And the channel spacing and modulation data rate tradeoff of DWDM ring modulators has been experimentally investigated previously [2]. At smaller (< 10μ m) diameters, thermal effects play an increasingly important role and athermal devices have been proposed [3] to help ameliorate these issues. The presented work demonstrates open eye diagrams for carrier-injection microring devices with FSR > 20nm at 10 - 15Gb/s aimed at achieving 1Tb/s overall bandwidth. 2. Experimental & Discussion The ring resonator uses a carrier-injection diode and was fabricated at CEA-Leti on a 250nm SOI wafer and forms a resonant ring cavity (5μ m diameter) with a 450nm wide waveguide to which light is coupled by a single bus waveguide that is 250nm away and is 300nm wide; there is no second waveguide for a drop port. The transmission spectra of the device covering two FSRs is shown in fig.1a. The output is normalized w.r.t. to the grating couplers, which have 8dB/unit insertion loss and a center wavelength of ˜1320nm. The variation in the extinction ratio of the resonant peaks is attributed to noise floor of the optical spectrum analyzer and high insertion loss of gratings at shorter wavelengths. The inset, fig.1b, shows a closer view of the resonance wavelength range used for the resulting data modulation with its associated cavity quality factor Q. A tunable laser injects -8dBm of optical power inside the bus waveguide. The optical output is detected by a commercial photodetector (Discover Semi.) with a -20dBm and -16dBm noise floor for error-free (defined as bit error ratio of less than 1 × 10 -9 threshold [4]) transmission at 10Gb/s and 15Gb/s, respectively. The non-inverting output of the photodetector’s limiting amplifier is used to capture eye diagrams and associated bit error ratio (BER) values. Due to the high thermal sensitivity of the compact ring, careful consideration must be given to the amount of optical power coupled into the cavity and the pre-emphasis voltage values driving the carrier-injection diode. A 10Gb/s pre- emphasis signal with 1.2V pp and 0.3V pp outer and inner eye amplitude, respectively, with a 0.2V DC offset is used, resulting in the output eye diagram from the detector, shown in Fig.1c. The input power to the detector is on the order of -18dBm and the associated BER value of this eye is error-free at 1.45 × 10 -11 . At higher data rates, we can expect a stronger thermal effect from carrier injection which heats the ring cavity. The data rate was increased to 15Gb/s with pre-emphasis voltages of 1.05V pp ,0.3V pp and 0.26V DC for outer, inner eye amplitude and offset, respectively. The optical input power to the ring and the output power to the detector remained unchanged. However, due to the increased noise floor of the detector at this data rate, a BER of 9.8 × 10 -6 was obtained for this increased data rate, shown in fig.1d. The microring modulator’s high speed behavior (ignoring thermal effects )with the pre-emphasis driver can be modeled using a small-signal circuit [5]. The model parameters are extracted by S 11 test of the microring modulator. The driving signal is applied at V in ; V out or the current through the diode resistance R D represent the optical