1. Introduction

According to International Technology Roadmap for Semiconductors (ITRS), in order to realize optical interconnects that are competitive with the electrical interconnects in the 2020 – 2025 timeframe, the energy budget of optical devices should be less than 2 ~10 fJ/bit for on-chip interconnects and 10 ~20 fJ/bit for off-chip interconnects [1]. While conventional photonic devices are often large in physical dimensions and consume a large amount of energy, photonic crystal (PhC) has been well recognized as a strong candidate for building highly integrated photonic network-on-chips (NoCs) to overcome the scaling limits of CMOS technology [2]. The ultra-compactness and low-power consumption because of strong light-matter interaction (small mode volume and high Q-factor) in PhC nanocavities have therefore created new opportunities for applications in power-efficient high-speed optical interconnects. Figure 1 shows an example of photonic NoC configuration that consists of a photonic plane overlaid on a CMOS electronic plane.

 

Fig. 1 Photonic NoC with a photonic plane integrated with a CMOS plane of a many-core processor. Inset shows the proposed three-terminal output device.

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Generally speaking, there are two schemes to convert electrical signals to optical signals in a photonic NoC: (1) on-chip directly modulated lasers and (2) optical external modulators with off-chip light sources. Compared to the latter case, the on-chip directly modulated laser offers several advantages including compact size, high optical power efficiency, and low energy consumption. The compact size can be realized not only by the wavelength-scale nanocavity of lasers but also by eliminating the massively distributed waveguides in chip packaging, which are frequently used in the scheme of off-chip light sources. Without the additional loss due to fiber coupling and long-distance propagation in the distributed waveguides, the optical power can therefore be used more efficiently with on-chip directly modulated lasers. On the other hand, when we take the energy consumption of the transmitter driver circuits into account, a well-designed driver circuit for on-chip directly modulated lasers has comparable operating energy with the on-chip laser itself, while for the scheme of using an external modulator, the overall energy consumption is usually dominated by the driver circuit for the modulator, which is usually in the order of tens of pJ/bit [3]. In order to meet the on-chip interconnect requirements in both energy and data rate, we have previously demonstrated a high-speed ultra-compact directly modulated PhC laser with a low energy consumption of 8.76 fJ/bit at 20-Gb/s [4]. In parallel to the effort towards on-chip interconnects, we also explore the potentials of this directly modulated PhC laser to be used for prospective off-chip interconnects.

The bandwidth requirement of optical output devices for off-chip interconnects is usually more stringent than their on-chip counterparts, although the energy requirement can be slightly relaxed. While we are pursuing higher chip performance with higher data rates, it becomes more and more difficult for the off-chip I/O traffic to keep up with it. In order to alleviate the pin-count limitations, optical output devices with high off-chip clock rate and wavelength-division multiplexing (WDM) technology have emerged as promising means. In practical use, however, the operational wavelength of optical devices may drift with environment factors, which inevitably limits the minimal channel spacing in WDM. An optical output device that can be operated at the high data rate with low energy consumption is therefore highly desirable to reduce the total number of channels and to enable less sophisticated transceiver designs with lower cost.

One of the major challenges for off-chip output devices is the direct modulation bandwidth (BW) of a typical semiconductor laser, which is ultimately limited by its relaxation resonance frequency (f_r) and considered to be insufficient for future off-chip clock rate requirements. Moreover, an adequate optical output power level of the output devices is required in order to compensate for optical loss due to propagation and coupling to meet the requirements of photodetector sensitivity at the receiver. While nanolasers are advantageous for low power consumption, its sub-milliwatt (mW) output power may hinder its off-chip applications.

Optical injection-locking (OIL), on the other hand, is known to be an effective technique to enhance both the f_r and the modulation BW. The output power level and wavelength of an injection-locked slave laser are mainly determined by the master laser at strong injection, and therefore an output power at mW level can be easily obtained. Enhancement of f_r to beyond 100 GHz has been demonstrated in VCSELs and DFB lasers [5]; however, reports on large-signal modulation above 10 Gb/s are still missing. This is likely due to strong injection power requirements together with practical limitations such as heating and gain compression at high power level, which in turn limits the performance of large-signal modulation.

In this paper, we propose a three-terminal (input/output/injection) optical output device as shown in the inset of Fig. 1. By utilizing a PhC laser with an external light injection operated within the injection-locking range, we can realize low-power and high-speed off-chip interconnects. Experimental results of small-signal modulation frequency response and optical spectra under various injection conditions are reported. By optimizing the injection power and wavelength detuning, we have successfully demonstrated flat broadband frequency response and 40 Gb/s large-signal operation with an energy cost of 6.6 fJ/bit. The main focus of the experiment was on modulation speed enhancement with a low energy cost operation.

Assuming the power dissipated by a properly designed electronic driver circuit for our on-chip directly modulated laser is comparable to the power consumed by the laser itself as suggested in [3], we can then estimate the total power of the transmitter with the proposed BH-PhC laser to be less than 20 fJ/bit at 40Gbps. Compared with applying an off-chip modulator that requires a high power (~25 pJ/bit) [3] wideband driver to deliver large voltage swings, our directly modulated BH-PhC laser can provide a power efficient solution for next-generation NoCs.

2. Buried heterostructure photonic crystal laser (BH-PhC laser)

The PhC laser contains an ultra-small buried heterostructure (BH) active region embedded in a PhC air-bridge structure as shown in Fig. 2(a) . Compared with a typical PhC configuration formed in a thin membrane of gain material (usually InGaAsP), the introduction of the BH region greatly improves the thermal conductivity as well as the confinements of both carriers and photons in the cavity [2]. The BH active region with a size of 4 × 0.3 × 0.16 μm³ is placed within a line-defect PhC waveguide (WG) in an InP slab and consists of three InGaAs quantum wells with a 1.55 μm photoluminescence (PL) peak, which is sandwiched between InGaAsP barrier layers with a 1.35 μm PL peak.

 

Fig. 2 (a) Scanning electron microscope (SEM) images of the top view and (b) cross-sectional view of the fabricated BH-PhC laser. (c) The FDTD mode profile of the PhC cavity calculated without output/injection waveguide. (d) Light-in-light-out curve (L-L) of the laser. A threshold is observed at 11 μW pump power.

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Due to the high thermal conductivity of the surrounding InP that is more than ten times higher than that of InGaAsP, the generated heat can easily escape from the cavity. An index-modulated mode-gap cavity with ultrahigh-Q and small mode volume [6] is formed by the BH region in the line-defect PhC waveguide as a result of the higher refractive index of the active region than the neighboring InP. The BH region also effectively confines the generated carriers, and therefore a better overlap between carriers and photons provides efficient pumping.

The line-defect PhC waveguide also serves as an input waveguide as shown in Fig. 2(b) because InP is transparent to the 1.3 μm pumping light and thus undesired absorption outside the cavity is ignorable. Figure 2(c) shows the FDTD-calculated mode profile without an output waveguide. The output waveguide, as shown in Fig. 2(b), is placed at an offset position with respect to the cavity, which enables optimized coupling between the cavity and itself [4]. This output waveguide is also used for external light injection for later experiments.

The light-in-light-out (L-L) curve of this laser is shown in Fig. 2(d). A threshold power of the BH-PhC laser is observed at 11 μW incident pump power, and the maximal output power is around 50 μW. It has been known [7] that f_r of a semiconductor laser at free-running depends on the number of photons in the cavity. And thus, as the number of photons increases with the pump power for the laser, the f_r continues to increase until practical factors such as gain compression and heating take effects. Moreover, higher damping at higher frequencies also limits the maximum BW. As a result, the maximal 3-dB BW is 15.7 GHz at a pump power of 169 μW (not shown). All the power levels in this paper refer to the power within the PhC waveguides, unless otherwise stated. All the measurements were carried out at room-temperature.

3. BH-PhC laser + Optical injection-locking (OIL)

The frequency response of the BH-PhC laser under injection-locking was determined by measuring the S21 parameter with a network analyzer (NA) using the apparatus shown in Fig. 3 . The frequency response at free-running was also measured with the same apparatus except for the light injection. The PhC laser was optically modulated by an electrical signal from port 1 of the NA via a LiNbO₃ modulator (LN-Mod) to modulate the pump light. A CW light from a tunable laser was coupled to the PhC laser cavity through the output/injection waveguide with an optical circulator. The injected wavelength was detuned from the PhC lasing peak at free-running by an amount of δ, and the injection power was adjusted by a digital variable optical attenuator (VOA). The output of the PhC laser was taken after the circulator and a 90/10 coupler, and fed into an optical spectrum analyzer (OSA) and port 2 of the NA through an erbium-doped fiber amplifier (EDFA) followed by an optical bandpass filter. An automated computer program was used to precisely control the injection conditions, both the power level and wavelength detuning value, and acquire data from the NA and OSA at each injection condition.

 

Fig. 3 Schematic diagram of the apparatus used for the frequency response measurement. VOA: variable optical attenuator. BPF: optical bandpass filter. LN-Mod: lithium-niobate modulator. EDFA: erbium-doped fiber amplifier. OSA: optical spectrum analyzer.

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Figures 4(a) –4(c) show the optical spectra and Figs. 4(d)–4(f) show the frequency responses of the PhC laser biased at a pump power of 113 μW (~10 × P_th) with various injection conditions as indicated on the figure (−15.07, −12.07, −9.07, and −6.07 dBm). When the PhC laser is at free-running, the coupling between carriers and photons determines its f_r. The 3-dB BW of the PhC laser at a pump power of 113 μW is around 11 GHz as shown by the black lines in all subplots of Fig. 4.

 

Fig. 4 (a)–(c) Optical spectra and (d)–(f) frequency responses of OIL BH-PhC laser with injection conditions indicated. Black lines in all figures represent the PhC laser at free-running with a pump power of 113 μW. The injection wavelength (λ_inj) and the shifted cavity modes (λ_cav) are also shown in the figures.

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When the PhC laser is under light injection, the injected light boosts the stimulated emission process and depletes more carriers (ΔN < 0) in the PhC cavity, which results in a reduction of the gain for the slave PhC laser. Even though the gain is lower than its threshold value, externally stimulated emission from the master laser compensates for this reduction, and thus the slave laser under injection-locking is lasing at the wavelength of the master laser (λ_inj). Other than the main locked mode lasing at λ_inj, a shifted cavity mode (λ_cav) on its longer wavelength side can also be easily seen in the optical spectrum. It is known that by analyzing the nonlinear dynamics with the commonly used Lang-Kobayashi rate equations and solving for the steady-state solutions, the amount of cavity shift can be obtained through the linewidth enhancement factor (α): Δλ_cav = - (λ₀²/2πc)∙α∙g∙ΔN/2, where g is the differential gain, ΔN is the change of carrier density, λ₀ is the wavelength of the master laser, and c is the speed of light [8]. The cavity mode is red-shifted since ΔN is always a negative number.

Figures 4(d)–4(f) show the corresponding frequency responses over the instrument-limited 20 GHz range. The frequency responses have been calibrated to the back-to-back configuration, and also normalized to their DC values to find their 3-dB modulation BWs. Apparent enhancement of the f_r can be observed with higher injection power, while the response is damped to a greater extent and a low-frequency roll-off becomes obvious for higher injection power in each subfigure. The resonance frequency (f_r’) at strong OIL is dominated by the interaction between the photons from the injection light and from the shifted cavity mode, and the frequency difference between these two modes matches to the f_r’ shown in the frequency responses [9]. A larger damping in the frequency response curves is found to occur at larger δ for the same injection power. As δ increases, the shifted cavity mode is also suppressed further on the optical spectra. With a subtle balance among f_r’, damping factor, and a low frequency roll-off [10,11], a broadband operation can be attained.

4. Direct modulation of OIL-PhC lasers at 40-Gb/s

Figure 5 shows the eye diagrams of directly modulated 40-Gb/s nonreturn-to-zero (NRZ) signals (a) at the input, (b) at the output of the free-running PhC laser, and (c) at the output of the injection-locked PhC laser. Since the maximal 3-dB BW of the free-running PhC laser was only 15.7 GHz, it results in zero eye-opening when directly modulated at 40 Gb/s regardless of the pump power. On the other hand, a clear 40-Gb/s eye opening under injection-locking can be observed in Fig. 5(c) with an average pump power of 50 μW and an injection power of 163 μW at δ ~ + 0.1nm. The corresponding extinction ratio of the input eye was 4.5 dB, and that of the output eye under injection-locking was 3.2 dB.

 

Fig. 5 Eye diagram for 40-Gb/s direct modulation. (a) Input signal. (b) PhC laser at free-running with pump power of 50 μW. (c) OIL-PhC laser with pump power of 50 μW and injection power of 163 μW in the PhC waveguide.

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In the case of Fig. 5(c), the energy of the BH-PhC laser under injection-locking was estimated to be 6.6 fJ at 40-Gb/s. Here a peak value of the pump power was used instead of the average value. The total power consumption was then calculated as the sum of the pump power (100 μW) and the injection power (163 μW). The energy efficiency of 6.6 fJ/bit is considered to be small enough to meet the energy efficiency requirement in later years. It is also the smallest number, to our best knowledge, among those ever reported for any type of semiconductor lasers. We can see that the price of the additional injection power can be regained by the enhanced data rate in terms of the energy cost per bit. The present demonstration is focused on modulation speed enhancement with a low energy cost and is limited by the available power in our setup. It is believed that further improvements of the signal quality and modulation speed can be realized with higher pump power and injection power.

The coupling loss in the pump side and in the output side of the device can be estimated to be 10 dB and 8.5 dB, respectively. This results in a great rise of the required external power to be around 53.8 fJ/bit in the current apparatus. However, in the future development we may incorporate properly designed spot-size converters (loss < 1dB possible) [12,13] so that the fiber coupling loss and required external power can be kept minimal, and the total power consumption can still be within < 10 fJ/bit regime.

5. Conclusion

In this paper, we proposed and demonstrated a low-power and high-speed three-terminal device based on an OIL-PhC laser for the next-generation photonic NoCs. Enhancement of both f_r and 3-dB BW of more than 20 GHz are presented with a flat broadband frequency response by OIL. Large-signal direct modulation at 40 Gb/s have been successfully demonstrated with an ultralow energy cost of 6.6fJ/bit, which is the smallest number ever reported for semiconductor lasers. The OIL-PhC laser is therefore proven to be an effective solution to meet the future needs of photonic NoCs especially for the stringent off-chip requirements.