W4J.4.pdf OFC 2016 © OSA 2016

Ten-Channel Discrete Multi-Tone Modulation Using Silicon Microring Modulator Array

Po Dong1*, Jeffrey Lee2, Kwangwoong Kim1, Young-Kai Chen2, and Chengcheng Gui1 1Bell Labs, Alcatel-Lucent, 791 Holmdel Road, Holmdel, NJ 07733, USA

2 Bell Labs, Alcatel-Lucent, 600 Mountain Avenue, Murray Hill, NJ 07974, USA *E-mail address: po.dong@alcatel-lucent.com

Abstract: We report ten-channel discrete multi-tone generation using silicon microring modulator array, which simultaneously function as de-multiplexers, modulators, and multiplexers. An aggregated data rate of 0.88 Tb/s is achieved at a bit error ratio of 3.8x10-3. OCIS codes: (250.5300) Photonic integrated circuits; (130.3120) Integrated optical devices; (200.4650) Optical interconnects.

1. Introduction Recently, discrete multi-tone (DMT) modulation has attracted much attention as a 100G channel rate can be achieved even with 10-20G optical devices with direct detection techniques [1-9]. This is particularly promising to many short-distance communications for data centers, super computers and router systems where high-capacity, compact and low-cost optical transceivers are needed. The DMT technique utilizes many subcarriers and optimizes the modulation format for each one based on its achievable signal-to-noise ratio (SNR) in the optical link [1-2]. Previously, DMT has been demonstrated using directly modulated lasers (DMLs) [1], vertical cavity surface-emitting lasers (VCSELs) [2-3], discrete Mach-Zehnder modulators (MZMs) [4-6], and silicon MZMs [7-9]. In this paper we report high-capacity DMT modulation using silicon microring modulators for the first time to the best of our knowledge. The silicon microring modulator offers many benefits. By confining light in a small ring cavity, optical modulation with low drive voltage can be realized. Furthermore, the much-reduced area results in very small device capacitance, which significantly reduces power consumption of the electrical driver. The use of cascaded microring modulator array together with multi-wavelength sources is particularly attractive for wavelength-division mulitiplexing (WDM) transmitters [10-11], as the WDM architecture is greatly simplified by the microring which provide multiple functionalities as de-multiplexers, modulators and multipliers, simultaneously. Together with the efficient DMT scheme, here we report 0.88 Tb/s at a bit error ratio (BER) of 3.8x10-3 by implementing ten cascaded microring modulators on one chip. 2. Silicon PIC

Figure 1(a) shows the WDM transmitter architecture. A multi-wavelength source is coupled to a photonic chip which consists of one bus waveguide with microring modulator array coupled to it. The resonant wavelength of each ring can be programmable. Each ring is programmed to resonant with one of the input wavelengths and modulates this wavelength only. The microring is driven by DMT signal. The modulated optical signals are naturally coupled

Fig. 1: (a) Optical circuit of WDM DMT transmitter using multi-wavelength source and microring modulator array. (b) Picture of 10-channel silicon microring chip. The inset shows that the chip is packaged with fibers and a circuit board to control the resonant wavelengths of the rings.

Fig. 2: (a) Spectrum of the through port after ten rings are thermally tuned to 50-GHz spacing. (b) Ring spectra under different reverse biases.

DMT$data$

driver$

DMT$data$

driver$

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back to the bus waveguide. The output at the bus waveguide contains multiple wavelengths to provide high-capacity WDM signals. Figure 1(b) show the photograph of chip consisting of ten-channel microring modulator array, which is similar to previously reported silicon photonic networks-on-chip in [11]. The high-speed modulation is achieved by embedded reverse-biased pn junction in the ring waveguide. The silicon waveguides have a cross section of 0.5 µm x 0.22 µm and a slab thickness of 90 nm. The ring radius is 15 µm with a free-spectrum range (FSR) of 6.6 nm. On top of each ring, a metal resistive heater is employed to tune the resonant wavelength. The average tuning efficiency of the heaters is 0.081 nm/mW [11]. After fabrication, the resonant wavelengths have some random distribution due to fabrication tolerance. Using heaters, we can program the rings to match 50-GHz spacing, with a spectrum shown in Fig. 2(a). The extinction ratios are about 10 dB and the quality factors are ~19 k. Figure 2(b) shows the spectra for one of the rings under different reverse biases on the pn junction. The modulation efficiency decreases with larger bias. Under 5 V, the efficiency is ~8 pm/V. The extinction ratio also decreases with higher bias, indicating that this ring is over-coupled to the bus waveguide.

3. DMT modulation Figure 3 depicts the experimental setup of the DMT modulation with more details to be found in [9]. For the optical setup, a wavelength tunable laser with a power of 16 dBm is launched into the chip to test the rings one by one. The laser wavelength is tuned to each of the resonant wavelengths under test. The DMT electrical signal driving the ring has a peak-to-peak voltage (Vpp) of 2.5 V and is applied on the ring through a RF probe with a 50-ohm terminator. The ring is biased at 5 V through a RF bias tee and optically at -6 dB point from the maximum transmission. This optical bias point is optimized based on the Vpp, modulation extinction ratio and bandwidth. The output optical DMT signal is about -4 dBm, including 14 dB fiber-to-fiber packaging loss and 6 dB ring loss. The output signal is amplified by an erbium-doped fiber amplifier (EDFA) and transmitted through standard single-mode fibers. At the receiver, a commercial III-V-based receiver with an overall bandwidth >30 GHz is used in combination with a digital sampling scope as shown in Fig. 3.

3. Experimental results To optimize the capacity, the SNRs at each subcarrier of the received signal with a uniform loading are first measured. Figure 4(a) presents the SNRs versus carrier frequencies measured at 0 km, 2 km, and 4 km transmission distances. The SNR value is about 22 dB at its maximum, and drops to almost 10 dB at 15 GHz. Previously, we reported that the SNR drops to 10 dB at about 25 GHz in the DMT-modulated silicon MZMs [9]. This indicates the ring modulation bandwidth is much lower, mainly limited by the high quality factor of the microring. This shall not be a limiting factor, as microring modulators have been reported at 60 Gb/s [12]. After the channel SNRs are measured, the Chow bit loading algorithm is applied to optimize the bit and power allocation for each subcarrier for a targeted BER [2]. Then the DMT signal with the calculated bit and power allocation at each subcarrier is applied to the modulators, and the actual BER is measured. Figure 4(b) exhibits the superimposed ten channel DMT spectra, which are measured with an optical spectrum analyzer (OSA) with a resolution of 0.02 nm. Although the DMT is measured one by one, the superimposed spectra imply that the crosstalk from adjacent channels may be less than -50 dB for 50-GHz channel spacing. If this is not sufficient for a full WDM link which further suffers from de-mulitiplexing crosstalk at the receiver, one can increase the channel spacing to 100 GHz, which requires an FSR of 8 nm to accommodate ten channels. This FSR can be achieved for silicon microring modulators with a radius ~12 µm or smaller. The BER values are measured for each of the ten channels at different data rates over various fiber transmission lengths, shown in Fig. 5. Table 1 summarizes the achieved data rates at a BER of 3.8 x 10-3, the threshold for 7% overhead hard-decision forward error correction (HD-FEC). This BER threshold value was commonly used in

Fig. 3: DMT setup. P/S: parallel-to-serial converter; FFT: fast Fourier transform; CP: cyclic prefix; TS: time synchronization; DAC: digital to analog converter.

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Fig. 4: (a) Measured SNRs versus carrier frequencies. (b) Superimposed ten-channel DMT spectra.

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previous DMT experiments in [1-9]. The back-to-back (B2B) DMT system can achieve a bit rate of 85-92 Gb/s per channel for ten rings with an aggregate rate of 0.882 Tb/s. After 2-km and 4-km transmissions, the achievable data rates are 0.891 Tb/s and 0.857 Tb/s, respectively. The slight increase of data rates for 2-km transmission is within

the experimental tolerance. This indicates that 2-km transmission would not reduce the data rates. At 4-km transmission, the data rate drops ~3%.

4.Conclusion In summary, we have demonstrated the use of silicon microring modulator array to generate high-capacity DMT signals. The effective area of the ring array, excluding the heater metal wires and pads, is about 3 mm x 0.2 mm, resulting in a bandwidth density of 1.5 Tb/s/mm2. The performance can be further improved with smaller ring radius and higher modulation efficiency/bandwidth, which have been demonstrated in other silicon microring devices [12]. In addition, the current device has a fiber-to-fiber packaging loss of 14 dB, which can be reduced by 10 dB with optimized coupler design (~1 dB fiber coupling loss was demonstrated in [13]). In addition, the receiver used has the best performance at a receiving power of ~3 dBm, due to low-gain transimpedance amplifier (TIA with conversion gain of 150 V/W). The sensitivity can be improved by TIA with higher gain. With these optimization, we expect that 0-3 dBm input power without EDFA is feasible for 100G per channel. Silicon microring modulators, together with multi-wavelength sources and DMT formats, could lead to extremely compact and high-capacity transmitters with low power consumption. Acknowledgements: Part of this project is funded by Intelligence Advanced Research Projects Agency (IARPA) under the SPAWAR contract number N66001-12-C-2011. We acknowledge support of Drs. Dennis Polla and Carl McCants at IARPA, T.-Y. Liow and G.-Q. Lo of the Institute of Microelectronics, Singapore, and M. Zirngibl and S. Chandrasekhar at Bell Labs.

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Tu2H.2, Los Angles (2015). 4. T. Tanaka et al., “Experimental demonstration of 448-Gbps+ DMT transmission over 30km SMF,” Proc. OFC, M2I. 5, San Francisco (2014). 5. L. Zhang et al., "C-band Single Wavelength 100-Gb/s IM-DD Transmission over 80-km SMF without CD Compensation using SSB-DMT,"

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talk, 2015.

Table 1: Achieved data rates for ten channels and for different transmission distance.

Channel'#'Wavelength'

(nm)'Data'rate'at'BER'of'3.8e:3'(Gb/s)'0'km' 2'km' 4'km'

1" 1543.33" 86" 89" 87"2" 1543.73" 85" 90" 86"3" 1544.13" 86" 84" 84"4" 1544.53" 87" 88" 87"5" 1544.92" 90" 86" 84"6" 1545.32" 92" 93" 88"7" 1545.72" 88" 88" 84"8" 1546.12" 92" 90" 86"9" 1546.52" 86" 90" 83"10" 1546.92" 90" 93" 88"

Total'data'rate'(Gb/s)' 882' 891' 857'

Fig. 5: BERs as a function of data rates for ten channels.

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