1. Introduction

Silicon photonics has emerged as a promising and commercially-viable solution for high speed intra/inter-system optical interconnection with low energy consumption, low latency and scalability [1,2]. Thanks to progress in CMOS technology, silicon photonics platforms have become available in multiple commercial foundries which have lead to both high repeatability and low-cost production at higher resolutions than typically available with III-V foundry services. Silicon-on-insulator (SOI) technology provides an excellent passive optical device platform for silicon photonics circuitry including low-loss waveguides, directional couplers, splitters, grating couplers, filters, polarization converters, (de)multiplexers and so on [3,4]. Active optical devices such as modulators, photodetectors, and micro-heaters have been successfully demonstrated in CMOS platforms with superior optical/electrical performance [5,6].

However, the light source for a silicon photonics platform cannot be easily realized monolithically in a CMOS process and is therefore still an active area of development. Several approaches have been demonstrated for small foot-print and low energy consumption laser sources that can be also integrated with silicon photonics circuits while providing sufficient power to sustain a photonic link [7–15]. So far, the most practical approach has been the hybrid integration of III-V with SOI. One promising approach is III-V die to Si wafer bonded heterogeneous integration [7,8]. This can be done entirely with wafer level SOI and III-V processing with a relaxed alignment precision at the point of bonding but a complicated integration of III-V and Si processes. Another approach is the hybrid integration between two separate SOI and III-V chips based on optical coupling through facets. Although this integration approach requires two chips to be aligned, it has the advantage of allowing independent optimization of the two chips and their respective fabrication processes. Edge-coupling and vertical-coupling approaches have been demonstrated with high power conversion efficiencies [9–15].

One of the key metrics for an integrated hybrid laser is its waveguide-coupled wall-plug-efficiency (wcWPE) which is defined as the ratio of laser optical power available in the silicon waveguide to applied electrical power. For silicon photonics links note that the usable power is the optical power coupled into the Si waveguide and hence our objective is to maximize wcWPE. In a recent study, a minimum 10% wcWPE was established to accomplish sub-pJ/bit operation, representing a regime where optical interconnects can viably replace chip-to-chip copper based electrical interconnects [16]. Hybrid integration based on an edge-coupling configuration has been previously reported with a high wcWPE resulting from a repeatable spot-size converter (SSC) process technique that enabled low-loss and low polarization dependent optical coupling. A 9.6% wcWPE was reported with a DBR reflector on a “thick” (3μm) SOI platform [9]; a 7.6% wcWPE was achieved with a flip-chip bonded hybrid approach [10]; and a 7.8% wcWPE was implemented with a micro-ring based 0.3μm SOI platform with wavelength tunability [12].

In this paper, we demonstrate a record-high wcWPE of 12.2% at room temperature for an external-cavity C-band hybrid laser that also provides wavelength tunability and single-mode continuous-wave (CW) operation. We describe a device optimization approach based on gain measurements and modeling of the effective Si mirror in the external-cavity configuration of the hybrid laser. Calculated laser characterization results are compared to measured performance of our laser. Furthermore, we present a novel way to stabilize the wavelength of the hybrid laser by an active feedback loop and provide preliminary results to validate our laser stabilization approach.

2. Device design and characterization

2.1 SOI reflector

An external-cavity laser is formed by optical coupling between an SOI chip containing a ring reflector and a III-V chip providing the optical gain. Wavelength selectivity and tunability are provided by a micro-ring filter in SOI chip while the optical gain is generated by InP based multiple quantum wells (MQW) in the gain chip. Details of the SOI device designs are described elsewhere [12]. The schematic configuration of our external-cavity hybrid laser is shown in Fig. 1. The loop type micro-ring reflector is composed of a Y-junction and a 5μm radius ring which enables wavelength-selective feedback with a free spectral range (FSR) of ~20nm. The quality (Q) factor of the micro-ring, based on symmetric coupling gaps to the ring, defines the reflection bandwidth and is designed to ensure single-mode operation while minimizing optical loss. This symmetrical design also makes the device intolerant to fabrication process variations. Based on simulation results, we chose a ring-bus waveguide (WG) gap of 150nm which results in a filter full-width half-maximum (FWHM) of 1 nm with a coupling coefficient of 20% [16]. A 3dB Si directional output coupler extracts the laser output from the cavity. This cross-coupling ratio dominates the effective mirror loss and primarily determines the slope efficiency and the threshold of the laser, both of which are critical parameters for optimizing the laser WPE. All output ports are connected to their respective grating couplers (GCs). Their output is collected with a fiber array. A reference GC loop, as labeled in Fig. 1, connects GCs on either side of the output array and is used for chip-to-chip alignment and accurate calibration of the coupling losses. A SiN_x spot-size converter (SSC) is incorporated at the optical waveguide facet as described in [12]. A 200 μm long Si inverse taper with an 80 nm tip is embedded in a SiN_x WG with a cross-section of 2μm (height) × 4μm (width) which is designed to match the optical mode size of the gain chip. The cleaved optical facet was coated with MgF₂ to suppress unwanted back-reflections at the interface. The coupling loss of 2.2dB was measured between a reflective semiconductor optical amplifier (RSOA) with a mode size of 1.3μm(H) by 3.2μm(W) and a SiN_x SSC supporting a fundamental mode of 1.5μm(H) by 3.5μm(W). A top-view micrograph of the edge-coupled hybrid laser is shown in the inset of Fig. 1.

 

Fig. 1 Schematic of an external-cavity hybrid laser configuration. The RSOA and SOI chip are coupled through a SiN_x SSC. The upper-left photograph shows the region where the SOI and RSOA chip are coupled. All output ports are connected to GCs inside a reference GC loop for precise GC coupling loss calibration.

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2.2 Gain chip design and optimization

The gain chip is an RSOA based on an InP ridge waveguide MQW structure. The gain material was characterized through cutback measurements by analyzing optical power versus applied current of the cleaved Fabry-Perot lasers [17], and by studying the spectrum of an amplified spontaneous emission (ASE) using the method presented by Cassidy [18]. Figure 2 shows the modal gain from these analyses plotted as a function of the current density assuming a uniform 2 μm wide current path to match the ridge width of the gain chip. The gain derived from the ASE analysis is deduced from different length cleaved devices below lasing threshold. A single facet AR coating with a characterized reflectivity of 0.06% was used to allow below threshold measurement at higher current densities. The data is in strong agreement over the range of operation of a Si/III-V hybrid laser. Additionally, transparent current density (J_tr) measurements that have been collected using the method presented by Andrekson and Olsson [19] are plotted for device lengths of 350-610µm at a wavelength of ~1550nm. J_tr vs. temperature data is shown in the inset in Fig. 2 with J_tr at room temperature around 800 A/cm². The material properties extracted from this analysis (injection efficiency η_i = 0.65, J_tr ≈800A/cm², internal loss α_i = 6.5cm⁻¹, modal differential gain Γg₀ = 45cm⁻¹) were then used to numerically simulate the external cavity laser with an effective mirror model detailed in [16]. Figure 3 presents the estimated peak WPE values with different RSOA lengths and various coupling losses for this external-cavity hybrid laser structure. The effective mirror reflectivity from the Si cavity takes into account the Si/III-V coupling loss of the device at each pass and an additional 1dB of loss in the Si. This assumption is a combination of waveguide and ring loss and scattering loss of the directional couplers and Y-junction splitter. As shown, the wcWPE reaches its peak value for RSOA lengths in the range of ~300-400µm when the coupling loss varies from 1 to 3dB. For 1dB coupling loss, a ~20% wcWPE is achievable with a 300μm long gain section and ~10-15% wcWPE for 2-3 dB coupling loss in the hybrid laser cavity configuration. Bias current for maximum wcWPE increases with gain section length when RSOAs are longer than ~200μm. Figure 3(b) shows the wcWPE vs. bias current relation based on optimized laser cavity parameters assuming a 2.2dB coupling loss as measured for fabricated SiNx SSCs. The wcWPE reaches its peak value of 13.5% at bias current of 63mA and then decreases with higher bias current. This analysis confirms that the current hybrid laser cavity structure can achieve higher than 10% wcWPE with optimized RSOA designs. Additional optimization of the laser cavity can be made by increasing the output coupler strength at the expense of reducing the effective reflection from the Si, making the device more sensitive to parasitic reflections which could lead to unstable laser operation.

 

Fig. 2 Calculated modal gain vs. current density. Inset shows measured J_tr at different temperatures.

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Fig. 3 (a) Maximum wcWPE and bias current at that operating point for various RSOA lengths. (b) wcWPE vs. RSOA bias current for RSOA lengths of 370µm with coupling loss of 2.2dB

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3. External-cavity laser characterization

To characterize the hybrid laser performance, the SOI reflector and gain chip are actively aligned using 6-axis piezo stages while monitoring and maximizing the output optical power. The laser outputs were collected using a fiber array through GCs as bias current was applied to the gain chip. The GC coupling loss (5.4dB @ 1550nm) is calibrated out using the reference loop. The laser operates at room temperature, uncooled and continuous-wave (CW).

3.1 L-I-V and WPE characterization

In section 2.2, based on the current gain chip configuration and Si cavity loss, we showed that a ~370μm length gain chip was optimal to achieve the most efficient laser operation. In the actual laser experiment, a 375μm long RSOA chip and a SOI reflector chip were actively aligned to each other and the laser characterization was performed by monitoring the output power and spectrum during the bias scan. Figure 4(a) presents measured L-I-V data of this hybrid laser. The laser threshold current was 15mA with a slope efficiency of 0.2W/A. The laser output power which is the sum of the two main output port powers (shown in Fig. 1) reaches 10mW when the bias current is greater than 80mA. The series resistance was 6Ω, extracted from the I-V curve. Kinks in the L-I curve stemmed from mode hopping mainly caused by mismatch of the respective thermal optic coefficients (TOC) of the Si and III-V chips. As previously discussed, wcWPE is defined by the ratio of laser optical power in the Si WG and electrical RSOA bias power. Based on the L-I-V data, the wcWPE was calculated and the result shown in Fig. 4(b). The maximum wcWPE reaches 12.2% at a bias current of 52mA, which is a record high wcWPE for single mode C-band hybrid laser to the best of our knowledge. The laser output power is 7mW at the same bias. A wcWPE of over 10% can be achieved with a bias range of 30~60mA. The wcWPE drops quickly when the bias current increases above 60mA, which we believe is attributable to thermal roll-over and mode instability. The maximum wcWPE result matches well with our laser modeling presented in section 2.2. The deviation in wcWPE at higher bias current is likely due to thermal effects and mode hopping, not included in our numerical calculations.

 

Fig. 4 External-cavity hybrid laser characterization results with 375µm long RSOA chip and SOI ring reflector: (a) L-I-V curve in which blue graph represents L-I measurement and red graph is for I-V measurement; (b) wcWPE plot calculated from L-I-V data.

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We also built an external-cavity hybrid laser with a longer (600μm) gain chip. The SOI reflector was the same chip used for the previous measurement and all material and structure parameters of the gain chip were identical except for the length. From the measured L-I-V curve shown in Fig. 5(a), the threshold is 19mA with slope efficiency of 0.16W/A. The output power rises above 14mW as the bias current is exceeds 100mA. A series resistance of 4 Ω was measured for the 600µm RSOA chip. This L-I curve is much smoother than the previous case which we believe is due to lower resistance and lower thermal impedance from the longer gain length which causes a small thermal drift with bias current [20]. This combination resulted in less mode-hopping for the hybrid laser. The wcWPE calculated from the L-I-V data is shown in Fig. 5(b). The maximum wcWPE reaches 10.6% at 100mA which is slightly lower than the previous case. This is expected from our laser modeling in section 2.2 which showed decreased WPE for a longer gain chip length compared to the optimum 370µm long gain chip. None-the-less, greater than 10% wcWPE was achieved for a wide range of bias currents (50mA to 110mA) due to low thermal degradation from longer gain chip. Thermal roll-over was observed when the bias current is greater than 110mA where the wcWPE starts to drop significantly. A shorter gain length (375μm) helps to achieve a higher wcWPE at low bias currents regime but suffers from early thermal roll-over and multiple mode-hops. A longer gain section (600μm) helps to increase the laser output power and maintain the stable laser operation with slightly reduced wcWPE. Therefore, the hybrid laser can be optimized to meet specific requirements such as wcWPE and optical power which are both critical for low energy and high-density silicon photonics links.

 

Fig. 5 Hybrid laser characterization results with 600µm long RSOA chip and SOI reflector. (a) L-I-V curve in which blue graph represents L-I measurement and red graph is for I-V measurement (b) wcWPE data from L-I-V curve

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3.3 Laser linewidth and wavelength tuning

The laser linewidth is measured with an APEX high resolution optical spectrum analyzer (OSA) with a 0.16pm optical resolution. The measured laser spectrum is shown in Fig. 6(a) and the estimated laser linewidth is 0.22pm (27 MHz) when the bias is 40mA. The splitting at the laser peak wavelength indicates that the linewidth measurement is limited by the OSA resolution and, therefore, the linewidth is likely smaller. This is constituent with studies of linewidth in lasers with ring-based mirrors that exhibit linewidths of ~100-1000kHz [15] depending on cavity loss and length, the Q of the ring filter and an additional linewidth reduction factor [21,22]. Because the measured linewidth was sufficiently narrow for high data rate WDM communication over fiber spans of many km without requiring dispersion compensation, alternative linewidth measurements on these devices were not conducted.

 

Fig. 6 Laser spectrum measured by high-resolution OSA: (a) linewidth measurements with the bias current of 40mA, single mode operation with SMSR > 40dB; (b) laser tuning results with different tuning powers. Note: splitting of the lasing peak in (a) is an artifact of the heterodyne measurement in the OSA due to the limited optical resolution.

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The laser operates with a single longitudinal mode with more than 40dB side-mode suppression ratio (SMSR). An ohm-shaped metal heater was integrated on top of the Si micro-ring for thermal tuning of the lasing wavelength. The heater is made with 100nm thick NiCr resulting in ~120Ω of overall resistance. We monitored the laser wavelength shifting as shown in Fig. 6(b) while applying electrical power to the heater. Without applied tuning power, the laser operates at 1548nm, where the ring resonance peak is located. Wavelength tuning occurs as the ring resonance peaks red-shifts by the thermo-optic effect. When the laser wavelength is red-shifted beyond 1552nm, the laser mode jumps to the next ring FSR mode, located at 1534nm, and then continues to red-shift. A total wavelength tuning range of 8nm (1548~1551nm and 1533~1538nm) with a tuning efficiency of 0.15nm/mW was demonstrated in our external-cavity laser device. These measurement results were limited by the current capacity and efficiency of the metal-heater could be readily increased. We have previously demonstrated the laser wavelength tuning range and tuning efficiency can be significantly improved (up to 2.2nm/mW) by a localized back-side substrate removal process [13].

4. Laser wavelength stabilization

One remaining challenge for the external-cavity hybrid laser is laser mode stabilization. As observed from the L-I curves presented earlier, the lasers exhibited mode-hoping as bias current was swept. Such mode hopping is problematic for a data link and should be eliminated for efficient high-speed data transfer. The main cause of mode hopping is a mismatch of the thermal drift between the SOI wavelength filter and the cavity modes of the coupled III-V and Si cavities –exacerbated by even very small reflections between them. Therefore, we implement an active monitoring and feedback control system to synchronize the drifts. A few approaches have been adopted to accomplish the wavelength stabilization of the external-cavity laser [23,24]. One of the advantages of a Si ring reflector structure is that its through-port can be utilized as a monitoring port (as shown in Fig. 1). This allows a simple implementation without any degradation of laser performance while achieving a minimal foot-print. The output from the monitoring port can serve as an indicator to the relative alignment between the laser cavity mode and ring filter resonance peak by simply measuring its optical power. When the laser cavity mode starts to deviate from the ring filter resonance peak due to the temperature variation, more light escapes from the through port and is measured at the monitoring port. Therefore, the feedback loop is designed to minimize the optical power level at the monitoring port by tuning the ring filter resonance with changes in the integrated metal-heater tuning power. This algorithm can be easily implemented in a VLSI circuit which can be packaged with this hybrid laser.

For a proof-of-concept experiment, we built a simple packaged external-cavity hybrid laser structure in which the gain chip and SOI reflector chip were aligned and then bonded together using glass supporting bars on both sides of the chips with epoxy. This was then permanently mounted onto a common Si carrier for handling. A photograph of the packaged hybrid laser is shown in Fig. 7(a). The RSOA chip used in this package was a commercially available off-the-shelf device that was not optimized for high wcWPE.

 

Fig. 7 Hybrid laser mode stabilization experiment: (a) a photograph of the packaged external-cavity hybrid laser, inset shows the alignment of SOI reflector and III-V RSOA chip; (b) spectral-L-I data without feedback loop; (c) spectral-L-I data with feedback loop. Optical power is color-coded based on power level (dBm). Inset shows corresponding L-I curve for each laser measurement.

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The feedback loop incorporated a minimization algorithm that was implemented using an off-chip controller. In the experiment, the feedback loop consistently tuned the ring resonance peak wavelength by controlling the metal-heater and forced it to stay at the optimum heating condition where the optical power from the monitoring port was the lowest during the each point of the RSOA bias current scan. Figure 7(b) and 7(c) present the spectral-L-I data collected during this experiment. Without the feedback loop, the hybrid laser exhibited several mode hops that included hops to neighboring cavity modes (at 90mA and 124mA) as well as hops to the next ring resonance mode ~18nm away (at 102mA) as shown in Fig. 7(b). Those mode hops were also observed in L-I curve (inset in Fig. 7(b)). With the feedback loop on, we observed significantly improved mode stability showing a clear region free of mode hopping for a bias current scan from 70mA to 130mA (Fig. 7(c)). The laser wavelength drifts with the increased bias current as expected since the specific lasing cavity mode moves when the gain chip heats up. The implemented feedback loop did not completely remove mode hopping. A hop occurred at 130mA, possibly due to a combination of parasitic reflections inside cavity and the relatively broad ring filter bandwidth (~1nm), both of which make the cavity more susceptible to mode-hopping. The mode-hop-free region can be further improved with revised cavity designs by using a narrower ring filter bandwidth and by minimizing back-reflections at the coupling interface. This demonstration clearly proves the effectiveness of the proposed laser stabilization approach required for practical applications of an external cavity hybrid laser in a system.

6. Conclusion

We demonstrated a record-high wcWPE in a C-band single-mode Si/III-V laser based on an external cavity configuration, wherein a SOI micro-ring reflector provided efficient wavelength selective feedback and the optical coupling was supported by a SSC implemented on SOI. We built an effective mirror model to study our SOI reflector and a simple gain model for our RSOA chip to optimize the laser design. We experimentally demonstrated 12.2% WPE from the external-cavity Si/III-V hybrid laser. The laser operated uncooled on a single mode in the C-band with >40dB SMSR. The laser was also capable of wavelength tuning and exhibited a linewidth below 0.22pm. The peak laser output power exceeded 14mW. In addition, we proposed and demonstrated a laser mode stabilization technique by a feedback loop using the through port of the ring reflector. The optical power from the ring monitoring port operated as a simple, yet sensitive indicator for the laser stability and allowed the ring heater to be controlled by a feedback loop. In preliminary experiments, we demonstrated mode-hop free operation of a hybrid external-cavity laser with the simple feedback loop as bias current was varied by over 60mA.