Increasing demand for efficient, robust, and reliable light sources for datacom, 5G, sensing, and metrology applications has triggered tremendous interest in quantum-dot (QD) lasers due to multiple intrinsic QD material advantages over their conventional quantum-well (QW) counterparts. A nano-scale 3D dot configuration endows devices for low-threshold operation, superior thermal stability, excellent tolerance to defects, low mode partition noise, a small linewidth enhancement factor, etc. All of these material advantages are ideal for building QD lasers on silicon (Si) through monolithic [1] or heterogeneous integration [2]. QW-based distributed feedback (DFB) lasers on both monolithic III-V and heterogeneous Si substrate have been the primary single-$\lambda$ source choice in main-stream applications for their exceptional spectral purity and stability. Research on Si-based QD-DFB lasers is gaining momentum due to their promise of becoming an isolator-free, robust, single-$\lambda$ source integrated with Si photonics for pluggable transceivers, co-packaged optics, and beyond. Here, we report a heterogeneous Si-based QD DFB laser showing significantly improved efficiency, multiple QD material-resulted record performances, and encouraging modulation capability.

Figures 1(a)–1(c) show, respectively, the device 3D schematic and 2D cross sections in the $x {-} z$ and $y {-} z$ planes. The 1.25-µm-wide, 300-nm-thick Si rib waveguide with 170 nm rib etch depth contains a 700-µm-long first-order grating with a pitch of 197 nm and 10-nm-deep 50% duty-cycled surface corrugations. It results in a III-V-on-Si grating coupling coefficient ${\sim}{{150}}\;{\rm{c}}{{\rm{m}}^{- 1}}$. A 1/4 wavelength shift in the center was designed for single-mode lasing at 1310 nm. A thin layer of ${{\rm{Al}}_2}{{\rm{O}}_3}$ was sandwiched between the top Si layer and III-V sample during the molecular wafer bonding step [Fig. 1(e)]. This realized a heterogeneous GaAs/oxide/Si metal-oxide semiconductor capacitor (MOSCAP) configuration, which enables plasma dispersion effects for ultra-low energy phase tuning and high-speed modulation in lasers and modulators on the same chip [3,4]. III-V processing is largely identical to that of previous comb lasers [2] to form the III-V mesa and diode terminals of the anode (P1) and cathode (N). An additional terminal P3 was formed on a p++ Si region outside the III-V section to bias the MOSCAP [Fig. 1(b)]. The SEM image in Fig. 1(d) shows the 4-µm-wide III-V mesa above the 6-µm-wide active region with a proprietary design of 5 layers of InAs/GaAs QDs. Finally, the benzocyclobutene (BCB) polymer encapsulated the entire 800-µm-long III-V mesa plus two 26-µm-long adiabatic tapers for mode transition to a Si waveguide with two grating couplers (GCs).

 

Fig. 1. Device schematic in (a) 3D and (b), (c) 2D cross-sectional configurations; (d) device cross-sectional SEM image and (e) transmission electron microscopy image of the MOSCAP.

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The finished chip shows mode hop-free cw lasing on a temperature-controlled stage in Fig. 2(a). At 20°C, the device started lasing at 6.7 mA and generated over 7 mW power in the Si output waveguide under 50 mA injection after considering the symmetric laser output and Si GC loss (${-}{7.5}\;{\rm{dB}}$). The resulted threshold current density and wall-plug efficiency are ${{134}}\;{\rm{A}}/{{\rm{cm}}^2}$ and 9.4%, respectively. The good adiabatic taper (${\lt}- {0.4}\;{\rm{dB}}$ loss, ${\sim}- {{38}}\;{\rm{dB}}$ reflection), 60% of the lasing TE0 mode confined in the active region, and high-quality QD epitaxial material contributed to the excellent threshold and efficiency results. The device could still produce 2 mW output at 60°C and maintained cw lasing to 70°C. Single-mode lasing around 1297 nm over a 100 nm range and up to 61 dB side-mode suppression ratio (SMSR) is exhibited in the inset of Fig. 2(a). Detailed spectral maps in Figs. 2(b) and 2(c) show excellent SMSR of at least 40 dB at 20°C and 35 dB at 60°C from 3 mA above the respective threshold until measurement stop of 50 mA. Small wavelength shifts of 68 pm/°C and 10.1–11.6 pm/mW in 20–60°C were extracted. Higher-temperature operation to 100°C and lasing close to 1330 nm are likely if grating design and fabrication control are improved to position the Bragg wavelength to the gain peak at a target temperature like a prior work in [5]. Thicker metal pads and implementation of a previously demonstrated thermal shunt [6] will facilitate effective thermal dissipation as well. In comparison to the typical solitary QW laser linewidth of several megahertz, this laser also exhibited a narrow Lorentzian linewidth $\Delta \nu$ in a range of 211–273 KHz in 30–50 mA at 20°C in Fig. 2(d). We attribute the narrow linewidth to small linewidth enhancement factor ${\alpha _H}\sim{0.9}$ in our QD material versus ${\alpha _H} \gt {{3}}$ in QW in general, since frequency noise is a function of $\alpha _H^2$. All of these static characteristics are ideal for the majority of single-$\lambda$ laser applications.

 

Fig. 2. (a) Temperature-dependent cw light–current–voltage characteristic, inset: spectrum at 72 mA and 20°C; spectral map vs. current at (b) 20°C and (c) 60°C; (d) device frequency noise characteristic.

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Due to finite intra-band relaxation time and gain saturation effect in QDs, QD lasers are known for small direct modulation bandwidth with 13 GHz being the record for InAs/GaAs QDs and below 10 GHz in our lab. We managed to obtain an open on–off key (OOK, PRBS15) eye diagram at 12.5 Gb/s with 50 mA injection and 0.9 V voltage swing at 20°C. No optical amplification or isolation was implemented in the link. A 4.3 dB extinction ratio (ER) was measured in Fig. 3(a) despite ${\sim}- {{15}}\;{\rm{dB}}$ reflection from the output GC. Alternatively, a MOSCAP-induced plasma dispersion effect offers a new mechanism for potentially faster modulation [3,4]. We did observe a blue shift of the lasing wavelength with increasing bias on MOSCAP, which demonstrates the MOSCAP-induced phase tuning in a grating structure for the first time, to the best of our knowledge. Unfortunately, the wavelength shift was too small for high-speed characterization. Simulations indicate that a modified design with larger optical confinement to the MOSCAP and a higher MOS capacitance can lead to a sufficient wavelength shift for frequency modulation [7]. We also performed a transmitter experiment by launching the DFB laser output into our heterogeneous MOSCAP microring modulator on another wafer, which was fabricated at the same time [4]. Identical (III-V and Si) material stack and fabrication processes were used for lasers and modulators, i.e., they share the same $n \text{-} {\rm{GaAs}}/{\rm{A}}{{\rm{l}}_2}{{\rm{O}}_3}/{\rm{p}} \text{-} {\rm{Si}}$ MOSCAP structure, as in the inset of Fig. 3(b). The transmitter link diagram in Fig. 3(b) indicates no optical isolator as well, but a praseodymium-doped fiber amplifier (PDFA) was necessary to overcome ${\gt} 20 \;{\rm dB}$ additional loss, primarily from two GCs on a modulator and tunable filter. The laser and modulator wafers were fixed at respective stage temperatures of 22°C and 10°C to roughly align the lasing wavelength with a resonance of this 10 µm in radius microring modulator. Fine tuning through current bias (68 mA) of the laser and voltage bias (${{\rm{V}}_{\rm{DC}}} = {{3}}\;{\rm{V}}$, ${{\rm{V}}_{\rm{pp}}} = {{4}}\;{\rm{V}}$) of the modulator resulted in a 25 Gb/s open eye diagram (PRBS15) at 1297.8 nm in Fig. 3(c). ER and relative intensity noise (RIN) referring to the optical modulation amplitude (OMA) of this transmitter link are 5.8 dB and ${-}{{125}}\;{\rm{dBc/Hz}}$, respectively. Cable and RF amplifier losses were calibrated by the arbitrary wave generator (AWG) but no pre-emphasized signaling was used. While PDFA includes an internal optical isolator, measured total reflection of the modulator chip from two GCs is ${-}{{20}}\;{\rm{dB}}$. For comparison, we replaced it with a commercial Santec tunable laser (TSL-510) with output power equal to fiber-coupled DFB laser power and the same wavelength. The obtained eye diagram in Fig. 3(d) shows a similar ER of 5.8 dB and RIN (OMA) of ${-}{{126}}\;{\rm{dBc/Hz}}$, indicating that our solitary QD laser without optical isolation delivers comparable performance to a high-end commercial external cavity QW lasers with a packaged isolator.

 

Fig. 3. (a) Eye diagram of direct modulation at 12.5 Gb/s; (b) external modulation link diagram and 25 Gb/s eye diagrams using (c) DFB laser and (d) Santec laser as sources for the same MOSCAP modulator.

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Table 1. cw Operation of O-band Grating Lasers on Si

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Table 1 summarizes room-temperature cw operation of several reported pioneering works of O-band QD DFB/DBR lasers on Si [5,8–10]. With this critical addition to our QD-on-Si laser family, we now have all of the building blocks for DFB laser-based coarse and comb laser or microring laser-based dense wavelength division multiplexing transceivers. It also shows great versatility of this heterogeneous integration platform for communications, sensing, computing, and many other applications.