1. Introduction

Silicon photonics offers promising low-power solutions to meet the growing bandwidth demands of optical interconnects and communications. The technical merit of silicon photonics relies on high-index-contrast waveguide technology, which offers many compact and energy efficient optical elements. One of the critical devices is the microring modulator, where optical modulation with low drive voltage can be realized by confining light in a small cavity. The use of a microring modulator array together with multi-wavelength sources is particularly attractive for wavelength-division multiplexing (WDM) transmitters [1–4], as the WDM architecture is greatly simplified by the microring, which provides multiple functionalities such as de-multiplexers, modulators and multipliers.

Nevertheless, silicon microrings are highly sensitive to fabrication tolerance and environmental temperature, requiring active tuning of the resonance. Fortunately, the thermal tuning power can be as low as 2.4 mW per free spectral range (FSR) tuning for a micro-heater close to the rings [5]. The next challenge would be closed-loop locking of the resonance to the desired wavelength in which the control requires a monitoring signal. Such wavelength tuning and stabilization of passive ring filters have been intensively investigated using various monitoring signals and heater elements [6–12]. For active microring modulators, previous reports have demonstrated wavelength locking with the optical power detection in the through or drop ports and setting up the power level to a prescribed level, or using more complicated detection schemes such as homodyne detection, bit error ratio (BER) optimization, or photo current induced by two-photon absorption (TPA) [13–21]. Extremely low power (~0.2 mW for the closed-loop control) can be achieved with CMOS control circuits [20]. However, these reported techniques require at least one monitoring signal for each ring and can only realize individual ring control. A more simple and universal control method for simultaneous wavelength locking of a microring modulator array is still missing. In this paper, we propose and demonstrate that by detecting the RF components of the optical signal in the through port, simultaneous locking of multiple ring modulators is feasible, together with automatically finding the best heater power bias to achieve maximum modulation.

2. Locking mechanism

The key idea here is to use an RF power detector to measure the modulated non-DC components of the optical fields in the through port and feed back this monitoring signal to control all the microrings’ heaters, as shown in Fig. 1. As multiple-wavelength continuous-wave (CW) lasers are launched into the bus waveguide, the RF power detector measures zero power if all the rings are off resonance, which pertains to no modulation. On the other hand, it detects maximum power level if all the rings are modulated and tuned to individual wavelengths. By locking the ring heating powers to the maximum RF power, all the ring modulators can be wavelength-locked. The maximum RF power corresponds to the cases where each ring modulates a different wavelength. This is guaranteed by three conditions: (1) the monitor photodetector (MPD) and the following RF detector cannot detect the high frequency of beating between two different wavelengths; (2) the data applied to different ring modulators are uncorrelated; and (3) the RF power drops if two rings simultaneously modulate the same wavelength due to the modulation loss. The first two conditions are easily satisfied since the channel spacing between two WDM channels is typically more than 50 GHz and the data in different channels are independent in real applications.

 

Fig. 1 Proposed closed-loop control of a modulator array with the detection of RF power in the through port. MPD: monitor photo detector. In this paper, we use an external MPD and RF power detector.

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There are multiple advantages of this proposed locking mechanism. The RF power of an on-off keying (OOK) signal is proportional to the square of the peak-to-peak voltage of the signal. The peak-to-peak output voltage of an optical detector (more precisely, the voltage signal converted from the photocurrent) is linearly proportional to the optical modulation amplitude (OMA). Therefore, the largest RF power corresponds to the largest OMA in OOK modulation. By locking to the maximum, the ring resonance is set at the best point by the heater under certain driving conditions (voltage swing and bias). Therefore, this by-product advantage solves the challenge to find best heating power setting for ring modulators. Secondly, as the RF power detector only provides a monitoring signal output rather than full data decoding, the MPD and its amplifier and the RF power detector itself do not need to have high bandwidth. In a non-return-to-zero OOK signal, the integrated RF power below the frequency of 20% of the inverse of the bit period would occupy around 40% of the total RF power. Therefore, a bandwidth in the range of 10%-50% of the inverse of the bit period shall be sufficient for most cases. Due to this low bandwidth requirement, the RF power detector can be made in CMOS circuitry with simple components such as rectifiers, which cost very low power. The third but the most important advantage is the ability to simultaneously wavelength lock multiple rings due to the incoherent summation of the RF power from different wavelength channels. The use of a single monitor for multiple rings can save power consumption of the transmission systems.

3. Experimental results

3.1 Silicon photonic devices and testing setup

To experimentally demonstrate the proposed locking method, we use a silicon photonic chip with 20 ring modulators coupled to a bus waveguide (only 3 of them will be used in the following experiments). Each ring employs a p-n junction across the waveguide for high-speed modulation and a resistive heater on top of the ring cladding for thermal tuning [4]. The ring spectra are shown with a substrate temperature of 25 °C and 70 °C in Fig. 2. A wavelength red shift of 3.8 nm is observed, corresponding well with the theoretical wavelength shift of ~0.08 nm/°C. For high-speed testing, three CW wavelength-tunable lasers are multiplexed and launched into the chip each with a power of ~3 dBm. The output of the chip is power-divided into two branches, with the monitor branch tapping 20% of the output. This branch (with a power of about −15 dBm when all three rings are modulated) is measured by an external 10-Gb/s optical receiver with an optical bandwidth from 1200 nm to 1600 nm. The electrical output of the receiver is measured by a RF power detector (Mini-Circuits, ZX47-60-S + ) with a detection frequency range of 10 MHz to 8 GHz. The RF power detector converts the RF power into a voltage signal which is read by an analog-to-digital converter (ADC) in a microcontroller with a clock speed of 41.78 MHz. The microcontroller also has multiple 12-bit digital-to-analog converters (DACs), and three of them are used to control the ring’s heaters. The other branch of the output optical signal is amplified by an erbium-doped fiber amplifier (EDFA) and filtered by a tunable filter with a 3dB-bandwidth of ~0.5 nm to select the channel for optical eye diagram acquisition or BER testing. The drive signal is a 20-Gb/s pseudorandom binary sequence (PRBS) with a length of 2¹⁵-1, a voltage swing of 5 V and a DC bias of 3 V.

 

Fig. 2 Spectra of the silicon photonic chip under test with a temperature of 25 °C and 70 °C. The inset shows a picture of the device packaged with RF and DC boards.

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3.2 3x20 Gb/s OOK locking

We first verify that the measured RF power signal can accurately indicate the magnitude of OMA and the incoherent summation of RF power occurring within WDM channels. Figures 3(a)-3(d) illustrate the optical eye diagrams for one of the channels with different RF power levels. With increasing RF power, the eye opening widens and the maximum RF power indicates the ring is modulated at the best heater bias. The incoherent RF power summation is proven in Fig. 3(e), where the RF power is measured versus the number of rings which are tuned to the best modulation bias. The RF power is observed as linearly proportional to the number of rings locked. Figure 3(f) depicts the corresponding change in RF power versus each ring’s heater bias. Troughs in the RF power depict the points when several rings simultaneously modulate the same wavelength channel, thus resulting in modulation loss. Note that for a microring resonator, modulation can occur on either side of its spectral resonance, hence the existence of another peak close the maximum RF power.

 

Fig. 3 (a-d) Optical eye diagrams with increasing monitor RF power. (e) Detected RF power as a function of number of rings locked to show incoherent summation of RF power among different channels. (f) Change in RF power for each ring as a function of their respective heater power biases.

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Next, we demonstrate active locking of the ring resonances while the temperature of the device cycles between ~27 °C and ~70 °C set by a temperature controller. The measured temperature as a function of time is shown in Fig. 4(a) and all the following experiments are carried out while the device undergoes the same temperature cycle. We program the microcontroller with an optimization algorithm based on the gradient method [22,23] to maximize the incoherent RF power. At initialization, each ring is consecutively tuned first by an exhaustive search until there is a notable gradient in the RF power. At that point, the gradient method takes over for quick convergence to the best heater bias. During this process, each ring is locked to a desired wavelength. After the final ring is tuned, the algorithm uses the gradient method endlessly over each ring to ensure tracking with temperature drift. Note that once a ring is tuned to a wavelength, the other rings will not compete for it as the incoherent RF power decreases when other rings try to tune towards the same wavelength.

 

Fig. 4 (a) Temperature cycle during active locking. (b)-(d) Eye diagrams of three channels during the temperature cycle shown in (a), while the rings are simultaneously locked to the input wavelengths. (e) Locked modulation spectrum.

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Figures 4(b)-4(d) illustrate the optical eye diagrams of the three modulated rings, depicting that all the eye diagrams have a decent opening over a temperature change of more than 40 °C. Figure 4(e) shows the spectrum of the modulated optical WDM signals. We further characterize the BER of the three rings with different launching power to a 40-Gb/s optical receiver and a BER tester, with the results shown in Figs. 5(a)-5(c). The BERs are reasonably stable during the entirety of the temperature cycle. With enough launching power (>-8 dBm, limited by the optical receiver), all three rings can maintain a BER < 10⁻⁹ within a temperature range of >40 °C, demonstrating the robust locking of the current method. Further increase of launching power (>-6 dBm), zero BER are observed for all the rings during the temperature cycling period (~500 seconds).

 

Fig. 5 BERs during the temperature cycles for (a) ring 1, (b) ring 2, and (c) ring 3. Different curves represent the cases with different launching power to the optical receiver before BER tester. The launching powers for ring 1 are −11 dBm (red), −10 dBm (green), and −8 dBm (blue). For ring 2 they are −12 dBm (red), −10 dBm (green), and −9 dBm (blue). For ring 3 they are −13 dBm (red), −11.5 dBm (green), and −10.5 dBm (blue).

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3.3 3x75 Gb/s DMT locking

The same locking technique can be applied to other advanced intensity modulation such as discrete multiple-tone modulation (DMT). Recently, DMT modulation has attracted much attention as a 100 Gb/s channel rate can be achieved even with 10-20G optical devices with direct detection techniques. 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. In [4], high-capacity DMT modulation using silicon microring modulators was demonstrated for the first time with a channel rate of about 90 Gb/s. Further improvement of channel rate to 128 Gb/s was demonstrated in [24].

Here we use the same setup in [4] to generate and detect DMT signals with three ring modulators locked. The input laser power is set as 10 dBm per wavelength. Figure 6 demonstrates the BERs for three rings with modulation rates of 76, 68 and 82 Gb/s, respectively. During temperature cycling with a temperature change >40 °C, all the BER can be below 3.8 x 10⁻³, which is the hard-decision forward error correction (FEC) limit with 7% overhead. This demonstration proves that the proposed locking technique is robust and applicable in various modulation formats.

 

Fig. 6 BERs for DMT modulation while three rings are wavelength-locked.

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4. Discussions and conclusions

In summary, we have proposed and demonstrated a novel technique for simultaneous wavelength locking of multiple ring modulators to generate WDM signals. The monitoring signal for the closed-loop control is derived from the RF power detection of the modulated components. Maximizing this signal not only enables simultaneously tracking multiple rings to the input laser wavelengths but also allows maximizing the OMA. The number of rings which can be simultaneously locked mainly depends on the bit number of the ADC that reads the output of the RF power detector. Assuming a 10-bit ADC’s dynamic range is fully utilized and each ring requires a 10-dB dynamic range for locking, the number of rings that can be locked becomes about 100. However, this number may drop because of the finite resolutions of DACs which control the ring heaters. We note that the heater power is proportional to the square of DAC output voltage, and therefore the tracking resolution decreases with voltage. This causes concern when the heaters are tuning at high voltage, resulting in some instability in the observed eye diagrams, but this can be easily improved with higher bit DACs. As the thermal tuning speed of silicon photonic devices is typically on the order of 10 μs [5], the tracking speed would be mainly limited by DACs, RF power detector, and ADCs used in the experiment. The electrical bandwidth of our implemented feedback loop is estimated to be on the order of 120 kHz and tracking is maintained with thermal fluctuations as high as 0.64 °C/second. Performance can still be improved with faster control circuitry and more efficient tracking algorithms. The additional RF power detector, compared with low-power CMOS control circuit in [20], can be built on CMOS as well, which costs very little power. In addition, an analog proportional–integral–derivative controller (PID) [21] or a comparator circuit [20] can be used for closed-loop control, instead of a microcontroller used here.