15 Gb/s Transmission with Wide-FSR CarrierInjection Ring Modulator for Tb/s Optical Links

M. Ashkan Seyedi1, Rui Wu2, Chin-Hui Chen1, Marco Fiorentino1, Ray G. Beausoleil11Hewlett Packard Labs, Hewlett Packard Enterprise, 3000 Hanover st, Palo Alto, CA 94304

2University of California, Santa Barbara, Santa Barbara, CA 93106ashkan.seyedi@hpe.com

Abstract: A ring modulator with 22nm free spectral range is modulated and demonstratesan open eye diagram at 15Gb/s. This enables high channel counts towards Terabit per secondbandwidth 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 theassociated crosstalk of dense wavelength division multiplexing (DWDM) links. Carrier-depletion rings demonstrated56Gb/s data rate for an FSR of 19.1nm and cavity Q of 3500 [1]. However, these devices have lower extinction ratioand higher insertion loss compared to carrier-injection modulators. And the channel spacing and modulation datarate tradeoff of DWDM ring modulators has been experimentally investigated previously [2]. At smaller (< 10µm)diameters, thermal effects play an increasingly important role and athermal devices have been proposed [3] to helpameliorate these issues. The presented work demonstrates open eye diagrams for carrier-injection microring deviceswith FSR > 20nm at 10−15Gb/s aimed at achieving 1T b/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 aresonant ring cavity (5µm diameter) with a 450nm wide waveguide to which light is coupled by a single bus waveguidethat is 250nm away and is 300nm wide; there is no second waveguide for a drop port. The transmission spectra of thedevice covering two FSRs is shown in fig.1a. The output is normalized w.r.t. to the grating couplers, which have8dB/unit insertion loss and a center wavelength of ˜1320nm. The variation in the extinction ratio of the resonant peaksis 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 withits associated cavity quality factor Q.

A tunable laser injects −8dBm of optical power inside the bus waveguide. The optical output is detected by acommercial photodetector (Discover Semi.) with a −20dBm and −16dBm noise floor for error-free (defined as biterror ratio of less than 1× 10−9 threshold [4]) transmission at 10Gb/s and 15Gb/s, respectively. The non-invertingoutput 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 opticalpower coupled into the cavity and the pre-emphasis voltage values driving the carrier-injection diode. A 10Gb/s pre-emphasis signal with 1.2Vpp and 0.3Vpp outer and inner eye amplitude, respectively, with a 0.2VDC 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 orderof −18dBm and the associated BER value of this eye is error-free at 1.45×10−11. At higher data rates, we can expecta stronger thermal effect from carrier injection which heats the ring cavity. The data rate was increased to 15Gb/swith pre-emphasis voltages of 1.05Vpp, 0.3Vpp and 0.26VDC 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 theincreased 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-emphasisdriver can be modeled using a small-signal circuit [5]. The model parameters are extracted by S11 test of the microringmodulator. The driving signal is applied at Vin; Vout or the current through the diode resistance RD represent the optical

Fig. 1. Transmission spectra of the device showing a) two FSR and b) narrow view of the high-Qresonance point used for data modulation. Experimental eye diagrams are shown for c) 10Gb/s andd) 15Gb/s. A compact model is used to show theoretical output eye diagrams at e) 15Gb/s and f)25Gb/s using the g) circuit of the ring diode.

output. The model is used to simulate optical output at 15Gb/s and 25Gb/s, as shown in fig.1e and 1f, respectively. Wecan see good agreement between the experimental and theoretical eye at 15Gb/s and can expect operation at 25Gb/sif thermal issues can be decreased or eliminated.

The decreased pre-emphasis voltages at 10 versus 15Gb/s show the impact of driving voltage and data rate, asexpected from the aforementioned thermal effects. An increase of the injected optical power is not directly possibleas a means to overcome the detector’s noise floor since it will start to introduce thermal bi-stability issues due to highcavity Q. Furthermore, fig.1c and 1d do not have characteristics that the modulator is bandwidth-limited, showingpromise for performance at even higher data rates for these modulators, as predicted by our model.

3. Conclusion

Small ring diameters enable higher channel counts for DWDM links. However, compact volume and high Q factorincrease thermal sensitivity. Careful control of the input optical power and pre-emphasis drive voltages allow error-free data transmission by these devices. The presented device enables channel counts in excess of 60, nearing 1T b/soverall bandwidth and offer a power saving in thermal tuning and can achieve a larger absolute resonance wavelengthtuning due to larger FSR. Further reduction of the grating coupler insertion loss will allow for even lower input powerinto the bus waveguide, reducing ring self-heating due to high internal optical fields.

References

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2. M. A. Seyedi, C.-H. Chen, M. Fiorentino, R. Beausoleil, ”Error-free DWDM transmission and crosstalk analysisfor a silicon photonics transmitter,” Optics Express, 23, 32968–32976 (2015).

3. K. Padmaraju, K. Bergman, ”Resolving the thermal challenges for silicon microring resonator devices,”Nanophotonics, 3 269–281 (2014).

4. S. Haykin, An Introduction to Analog and Digital Communication (Wiley & Sons, 1989)5. R. Wu, C.-H. Chen, T.-C. Huang, K. Cheng, R. Beausoleil, ”20 Gb/s carrier-injection silicon microcing mod-

ulator with SPICE-compatible dynamic model,” presented at Photonics in Switching Post-Deadline Papers,Florence, Italy, 304–306 Sept 2015.