1. Introduction

Recently, the bandwidth density and power consumption limitations of traditional electrical interconnects have become apparent in rapid scaling computing systems such as data centers and supercomputers [1–3]. Silicon microring resonator-based WDM transmission systems are recognized as an attractive solution for next generation intra- and inter-chip optical interconnects, owing to their small footprint, low power consumption and inherent CMOS compatibility [3–6]. In the past decade, research effort has been focused on improving the device performance [5]. Major breakthrough has been made on the operation speed [7–10], modulation efficiency [11,12] and power consumption [13] of the ring modulators. However, relatively little attention has been paid to exploring the architectures of on-chip WDM transmitters. Published microring-based WDM transmitters have predominantly employed the simple “common-bus” architecture [14–16]. In this architecture, a pre-multiplexed or comb laser source is launched into a common bus waveguide, and each specific wavelength is selected and modulated by a ring modulator. Although the common-bus architecture has the advantage of low thermal tuning power due to the possibility of cyclic operation, it also has a few drawbacks. First, the need for a comb source substantially raises the cost of the system. Second, each ring modulator can see all the laser wavelengths, so there will be cross-modulation between the WDM channels. The cross-modulation will become stronger when the WDM channel spacing becomes smaller, therefore implementing dense WDM will be challenging. Third, the automated thermal stabilization is particularly difficult in the common-bus architecture, due to the fact that multiple wavelengths exist on the bus waveguide, but the monitor photodetectors are intrinsically insensitive to wavelength. Advanced algorithms need to be employed to lock each ring modulator to the optimized wavelength and ensure the cyclic operation. One possible walk around is to use the drop port monitor on the ring modulators. However, adding drop port filters is usually not preferred, as it will sacrifice the modulation performance by reducing both Q value and the tunability of the ring modulator.

In this work, we propose a new architecture for microring-based WDM transmitters, which we name Mod-MUX-Ring [17]. The concept of the Mod-MUX-Ring architecture is shown in Fig. 1(a). The transmitter consists of several independent branches. On each branch, a single wavelength source is first modulated by a ring modulator (Mod), and then multiplexed on to the bus waveguide by a ring based add/drop filter (MUX). Compared to the common-bus architecture, Mod-MUX-Ring exhibits a number of advantages. First, the requirement of comb sources is removed, not only reducing the overall cost of the system, but also making it more convenient for on-chip laser integration. Second, each ring modulator only sees one wavelength that is on its own branch, so the optical cross-modulation is eliminated. However, it is worth noting that if the WDM channel spacing becomes small, the followed ring filters will induce insertion losses, as some of the light will be coupled out. The third advantage of Mod-MUX-Ring is that the automated thermal stabilization is simpler. Each branch operates with only one wavelength, which enables stabilization by monitoring the average optical power [18]. Once all branches are aligned and stabilized, the whole system is automatically stabilized.

 

Fig. 1 (a) Sketch of Mod-MUX-Ring architecture. Individual single wavelength lasers are first modulated by ring modulators and then multiplexed on to a bus waveguide via ring add/drop filters. (b) Photograph of fabricated 4-channel Mod-MUX-Ring transmitter.

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In this paper, we demonstrate and fully characterize a prototype Mod-MUX-Ring transmitter that consists of 4 channels as shown in Fig. 1(b). All microrings are built with integrated thermal tuners. When correctly tuned, the channel spacing of the transmitter is 400 GHz, and 200 mW thermal tuning power is required. A 40 Gb/s channel-rate is achieved and an aggregate data rate of 160 Gb/s is expected. With 2.7V-V_pp driving voltage, error-free (BER < 10⁻¹²) operation is achieved on all channels with a typical extinction ratio of 3 dB and operation loss of 8 dB. This includes 7.2 dB bias loss on the modulation ring, and 0.8 dB insertion loss on the MUX ring. One can compare this value to the loss that would be seen for a typical LiNbO₃ modulator, combined with an optical MUX. Compared with a commercial LiNbO₃ modulator driven with an 8.8 dB extinction ratio, the Mod-MUX-Ring transmitter exhibits 2.2 dB power penalty, beyond the inherent 3.5 dB power penalty due to the lower extinction ratio. This additional 2.2dB penalty is likely caused by the bandwidth limitation as well as signal distortion from the rings.

2. Device design and fabrication

The transmitter chip was fabricated at the Institute of Microelectronics (IME)/A*STAR via an OpSIS multi-project-wafer run [19]. The process starts with an 8” Silicon-on-Insulator (SOI) wafer from SOITEC with 220 nm top silicon and 2 μm bottom oxide thickness. A high-resistivity handle silicon (750 Ω.cm) was used to ensure the RF performance. Grating couplers and silicon waveguides were formed by three dry etches. Six implantation steps were applied to silicon to form the pn junction and contact region. Two layers of aluminum were deposited for electrical interconnection.

Light is coupled on and off the chip via grating couplers. In order to obtain stable optical coupling, a polarization maintaining fiber array is attached to chip. Light is launched in the CW input port and the optical power is monitored from the bus waveguide output. The measured on-chip insertion loss is typically 6 dB at the pass wavelength of the add/drop filter. Majority of the insertion loss is due to the fact that part of the routing waveguide is covered by metal, which can be easily fixed in future designs.

During the test, the building blocks of the transmitter (ring modulator and add/drop filter) were first characterized individually. Then, the high-speed modulation performance of the whole transmitter was tested.

2.1 Ring modulator

The microring modulator in each channel is formed by a strip-loaded slab waveguide with 500 nm core width, 90 nm slab height and 8.0 μm radius, resulting in a FSR of 12 nm and Q of 5,200. Approximately 80% of the ring is loaded with a lateral pn junction targeting at 2x10¹⁸ doping level. The center of the pn junction is offset by 50 nm from the center of the waveguide to enable efficient electro-optical modulation. For thermal tuning, a silicon resistor was created at the coupling region of the ring resonator by implanting the waveguide and slab region with n type dopants. The static modulation performance of the pn junction is characterized by applying different bias voltage on the junction and recording the resonance peak shift. The modulation efficiency is measured to be 22.5 pm/V centered at 0 V. The thermal tuning performances of the integrated thermal tuner is tested by applying different DC voltage on the tuning pads and measuring the resonance peak shift as shown in Fig. 2(e), and a 190 pm/mW tuning efficiency is obtained by linear fit.

 

Fig. 2 (a) Photograph of fabricated ring modulator. (b) Transmission spectra of the ring modulator at different bias voltages applied to the pn junction. (c) Resonance wavelength shift as a function of bias voltage on the pn junction. (d) Measured ring modulator EO frequency response at 0 V bias. (e) Resonance peak shift as a function of thermal tuning power.

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The frequency response of the ring modulator is measured with a vector network analyzer and a photodetector with 70 GHz bandwidth. During the test, 0 V bias was applied on the pn junction. The measured EO frequency response (S₂₁) is plotted on Fig. 2(d), indicating a 3 dB bandwidth of 23.5 GHz, which is sufficient to generate 40 Gb/s non-return-to-zero (NRZ) modulation. Based on the phase and amplitude of the RF reflection (S₁₁), the capacitance and serial resistance of the ring modulator was estimated to be 28 fF and 70 Ω respectively.

2.2 Ring add-drop filter

For high-speed data transmission, the add/drop filters are desired to have wide passband and low insertion loss. In our system, the add/drop filters are also formed by 500 nm wide strip-loaded slab waveguides, having similar size and FSR to the modulators. In order to obtain low insertion loss on the drop port, the gap between the ring resonator and the bus waveguides is designed to be 200 nm, making the ring-bus coupling much larger than the round-trip loss. A full-width-half-magnitude (FWHM) of 100 GHz and insertion loss of 0.8 dB is measured on the drop port. We expect when the add/drop filter is used to multiplex a 40 Gb/s NRZ signal, the penalty should be minimal, since the passband of the filter exceeds the optical bandwidth of the target signal. Thermal tuning of the filter is achieved through a ~200 Ω resistor formed by n-type doped slab waveguide covering 60% circumference of ring. The measured thermal tuning efficiency is about 250 pm/mW as illustrated in Fig. 3.

 

Fig. 3 (a) Photograph of ring add/drop filter used in the Mod-MUX-Ring transmitter. (b) Through port and drop port spectra of the add/drop filter. (c) Thermal tuning efficiency of the add/drop filter.

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3. Modulation characterizations and discussion

The high-speed modulation performance of the whole transmitter is characterized after the individual components. The resonance wavelength of ring modulators and add filters need to be tuned to the proper position. We first tune the filters in channels so that their resonance peaks are evenly spaced within an FSR, resulting in a channel spacing of 3.2 nm (400 GHz). The ring modulators are then tuned so that their resonance wavelengths are aligned with the pass wavelengths of the associated filters. After this procedure, the transmission spectra on each channel were measured by sending a swept CW laser to the input of each channel, and monitoring the optical power on the bus output (see Fig. 4). On each channel, the Lorentzian shape envelope results from the add/drop filter, and the dip near center pass wavelength is due to the ring modulator. An 18 dB isolation is achieved between adjacent channels. The overall power consumption for thermal tuning is 200 mW.

 

Fig. 4 Spectra of the Mod-MUX-Ring transmitter before (dashed line) and after (solid line) thermal tuning.

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The high-speed transmission performance of the system was characterized by measuring the bit-error-ratio (BER) versus optical signal-to-noise ratio (OSNR) relationship. OSNR was chosen as a reference for BER so that the effect of photo receivers and fiber channels can be normalized, and different types of transmitters can be directly compared. The test setup is sketched in Fig. 5(a). During the test, a 40 Gb/s NRZ PRBS 2³¹-1 data stream was generated by electrically multiplexing 4 copies of de-correlated 10-Gb/s NRZ PRBS streams. The signal was then amplified and attenuated to a 2.7-V_pp signal with 1.3 V DC bias and launched to the ring modulators by a GS RF probe with inline 50 Ω termination. A tunable CW laser with 10 dBm output power was used to test the 4 WDM channels one by one. The modulated light was coupled out from the bus waveguide, combined with an amplified spontaneous emission (ASE) source, and amplified by Erbium doped fiber amplifier (EDFA). The amplified optical signal was split into two branches. One branch was directed to an optical spectrum analyzer (OSA) for OSNR monitoring and the other branch was directed to a u2t DPRV 2022A receiver with differential outputs, enabling simultaneous recording of eye-diagrams and BER. The received 40 Gb/s signal was connected to an SHF 34210A 1:4 demultiplexer followed by an SHF 58210A selector. A tunable optical delay line was employed to properly align the data and clock in the demultiplexer. The demultiplexed and selected tributary was sent to an Anritsu MU181040A error-detector for BER test. The reported BER is an average of all 4 tributaries.

 

Fig. 5 (a) Test setup for high-speed modulation performance. ASE: amplified spontaneous emission source. VOA: variable optical attenuator. TDL: tunable delay line. BPF: bandpass filter. OSA: optical spectrum analyzer. DSA: digital serial analyzer. (b) BER vs OSNR performance at 40 Gb/s for all channels in the Mod-MUX-Ring transmitter and a reference LiNbO₃ modulator. (c) Eye diagram for each channel under error free operation at 40-Gb/s. ER: extinction ratio. BL: bias loss.

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During the test, the tunable laser was first roughly aligned to the center pass wavelength of the add/drop filter. Then the ring modulator and add/drop filter was finely tuned to optimize the optical modulation amplitude (OMA) according to the optical eye diagram captured by the optical module on the digital serial analyzer (DSA), while the ASE source was disabled. After the optical modulation was optimized, the ASE source was enabled. The OSNR of the received signal was swept by adjusting the power of the ASE, and BER at each OSNR level was recorded.

The measured OSNR vs BER performance of all channels is presented in Fig. 3(a). For the same BER level, the variation of required OSNR is less than 2 dB among the channels. Error-free operation (BER < 10⁻¹²) was achieved on all channels for PRBS length of 2³¹-1. The eye diagrams at error-free operation are shown in Fig. 5(c). The extinction ratio and insertion loss are labeled below each eye diagram. The dynamic extinction ratio (ER) is defined by the extinction ratio between the ‘1’ and ‘0’ level $ER=10\log \left(〈1〉/〈0〉\right)$. The bias loss (BL) is defined as the loss of the average of the ‘1’ and ‘0’ level comparing to the off resonance transmission $BL=10\log \left(\left(〈1〉+〈0〉\right)/\left(2P_{off-res}\right)\right)$. With the driving condition mentioned in the earlier section (V_pp = 2.7 V, V_DC = 1.3 V) the typical ER and BL are 3 dB and 7 dB respectively. Combined with a measured capacitance of 28 fF, the dynamic energy consumption of each modulator is estimated to be 51 fJ/bit according to the expression $P=C{V}^2/4$ [20].

To evaluate the modulation performance of the Mod-MUX-Ring transmitter, a commercial LiNbO₃ modulator with 3-dB bandwidth rated to 30 GHz was tested with the same setup as a baseline for comparison. The LiNbO₃ was driven with nearly full Vπ (6V-V_pp), and the extinction ratio of the modulated signal was about 8.8 dB. As shown on Fig. 5(b), the Mod-MUX-Ring transmitter exhibits about 5.7 dB OSNR penalty at the BER level of 10⁻⁹, comparing with the LiNbO₃ modulator. The main source of the OSNR penalty is the reduced extinction ratio. The OSNR penalty due to a finite extinction ratio can be expressed as [21].

A remaining question is whether the ring add/drop filter has a wide enough passband to multiplex the modulated optical signal onto the bus waveguide without adding substantial penalty. To answer this question, we performed a control experiment in which we reversed the propagation direction of the light. In the control experiment, the ring add/drop filter and modulator are aligned as described before, but the CW laser is launched on the bus waveguide, dropped by the ring add/drop filter and modulated by the ring modulator. This way, the filtering effect imposed by the ring add/drop filter is removed. The OSNR vs. BER curve and eye-diagrams for forward and reverse light propagation are shown in Fig. 6. Comparing with the reverse direction, the forward direction exhibits about 0.1-dB OSNR penalty on the BER curve, which is within measurement error. From the eye diagrams, no noticeable degradation on the signal quality can be observed. Therefore, the penalty from the add/drop filter is proven to be minimal.

 

Fig. 6 (a) BER performance of Mod-MUX-Ring transmitter in forward and reverse light propagation at 40 Gb/s. (b) Eye diagram for forward propagation. (c). Eye diagram for reverse propagation.

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4. Conclusion

In conclusion, we propose and demonstrate a Mod-MUX-Ring transmitter with 4 WDM channels and 40 Gb/s data rate on each channel. Both 3 dB extinction ratio and 7 dB bias loss are achieved with 2.7V-V_pp driving voltage at 40-Gb/s/channel data rate. Under this condition, error-free operation is achieved on all four channels. The high-speed modulation performance of the Mod-MUX-Ring transmitter is characterized and analyzed. Compared to a reference LiNbO₃ modulator, the Mod-MUX-Ring transmitter exhibits 5.7 dB OSNR penalty, mainly due to the limited extinction ratio and bandwidth on the ring modulator. The modulation-multiplexing architecture imposed minimal penalty on the signal integrity. The proposed Mod-MUX-Ring architecture can be readily scaled to higher channel counts and integrate with on chip single wavelength laser source, making it a promising candidate for future WDM transmitters.