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An Si/III–V hybrid laser oscillating at a single wavelength was developed for use in a large-scale Si optical I/O chip. The laser had an InP-based reflective semiconductor optical amplifier (SOA) chip integrated with an Si wavelength-selection-mirror chip in a flip-chip configuration. A low coupling loss of 1.55 dB at the Si-SOA interface was accomplished by both mode-field-matching between Si-SOA waveguides and accurately controlling the bonding position. The fabricated Si hybrid laser exhibited a very low threshold current of 9.4 mA, a high output power of 15.0 mW, and a high wall-plug efficiency of 7.6% at 20 °C. Moreover, the device maintained a high output power of >10 mW up to 60°C due to the high thermal conductance between the SOA chip and Si substrate. The short cavity length of the flip-chip bonded laser expanded the longitudinal mode spacing. This resulted in temperature-stable single longitudinal mode lasing and a low RIN level of <−130 dB/Hz.
© 2012 Optical Society of America
1. Introduction
Si-based, large-scale photonic integrated circuits (PICs) represent a very promising technology to achieve high-capacity optical interconnections that are employed in high-performance computing systems and high-end servers [1, 2]. They have the potential to provide low-cost, compact optical I/O chips because of their compatibility with highly-scalable, mature fabrication technologies on Si substrates. The integration of high-performance light sources is a major challenge for Si-based optical I/O chips due to the inherent lack of light-emitting functions in Si crystal. There have been significant research efforts to attain Si-based light sources including Si Raman lasers [3] and strained Ge lasers [4]. However, a hybrid integration approach that combines III–V actives on Si waveguides presently offers a significant advantage in laser performance. Because this approach can utilize electrically-pumped strong optical gain coefficients in III–V actives, a number of Si/III–V hybrid lasers adopting different optical coupling schemes have demonstrated high output power operation [5–13]. Therefore, Si/III–V hybrid lasers are out-standing candidates for light sources in Si I/O chips especially for near-term development.
Si I/O chips are strongly expected to achieve both large transmission bandwidth and low energy cost (power consumption per bit). The use of ring resonator (RR)-based modulators is advantageous due to their high modulation efficiency to reduce the power consumption in Si I/O chips. In addition, the application of wavelength-division-multiplexing (WDM) technology is attractive to reduce the number of interfaces while maintaining large bandwidth. A single-wavelength light source that oscillates within the relatively narrow operation bandwidth of RR-based modulators is necessary to develop such high-performance Si I/O chips. Moreover, the lasing wavelength of lasers must be maintained at the operation window of RR-modulators even when the temperature of Si I/O chips changes. To date, some single-wavelength Si/III–V hybrid lasers have been reported with wafer-bonding [6, 11] and flip-chip bonding [9, 12] configurations. These devices have demonstrated lasing oscillation at a single wavelength determined by Bragg gratings or ring filters. However, it has been assumed to be difficult to align the lasing wavelength of these hybrid lasers to the operation window of RR-based modulators without wavelength tuning. However, the consecutive wavelength tuning of RR-based modulators or lasers consumes additional electric power that degrades the net energy efficiency of Si I/O chips. To overcome these difficulties, we previously proposed a novel Si transmitter that could achieve stable wavelength matching between lasers and modulators without additional wavelength tuning [14]. Our proposed Si transmitter (Fig. 1) was composed of a cascaded RR-loaded Si Mach-Zhender (MZ) modulator and an RR-loaded Si hybrid laser. The RR-loaded MZ modulator was designed to achieve high modulation efficiency due to accumulated phase-shift in cascaded-RRs. In addition, this modulator could be operated within an relatively wide operation window of 1 nm [14] because of the fair distribution in resonant wavelength of cascaded RRs. The Si hybrid laser was an external cavity laser that combined an InP-based semiconductor optical amplifier (SOA) and an Si wavelength filter chip. The Si wavelength filter contained an add-drop type RR that had a common structure with the modulator. Due to this, both RRs placed nearby were assumed to exhibit the same shift in the resonant peak wavelength even when the temperature of the transmitter chip changed. Therefore, the lasing wavelength of the Si hybrid laser could always be aligned within a 1-nm operation window of the modulator. We fabricated a proto-type of the Si hybrid laser using an Si wavelength filter chip and a fiber-connected SOA in a previous report [15]. The fabricated laser exhibited single wavelength lasing at a peak wavelength of RR. We also confirmed a comparable temperature shift of the lasing wavelength to that of the modulator for a temperature range of 25 to 55°C. However, the output power of the proto-type Si hybird laser was limited to ∼ 4 mW, and the wall-plug efficiency (WPE) was less than 1%.
This paper reports significant improvements to the performance of the previously proposed Si hybrid laser by adopting a flip-chip bonding configuration. We applied highly precise flip-chip bonding technology to build a low-loss Si-SOA interface. The flip-chip bonded Si hybrid laser exhibited a much higher output power of 15.0 mW at 200 mA drive current and WPE was remarkably improved to 7.6% at 20°C. The device maintained a high output power of >10 mW and WPE of >4.5% up to chip temperatures of 60°C.
2. Simulation of laser performance
We carried out numerical simulations to clarify the quantitative relations between laser performance and cavity parameters of the Si hybrid laser to evaluate the improved Si hybrid laser. There is a schematic model of our Si hybrid laser in Fig. 2(a). The device is composed of a reflective SOA and Si mirror chip. A Fabry-Perot laser cavity is defined between the high-reflection (HR) facet of the SOA and the distributed-Bragg-reflector (DBR) mirror formed on the Si chip. The output power is extracted from the cavity via the output directional coupler (DC) located between the SOA and RR, because this configuration allows us to obtain higher output power and spectral purity [16]. The lasing wavelength of the Si hybrid laser is determined by the reflection characteristics of the Si mirror that consists of the add-drop RR and DBR mirror. One of the multiple transmission peaks of RR is selected by using the stopband of the DBR. We established an on-chip Si mirror reflection loss of 3.5 dB excluding the splitting loss of the DC in our simulation from a measured characteristics of the Si mirror. As the SOA chip generated reflective gain between anti-reflection (AR) and HR (R=100%) facets, the threshold conditions of the laser were determined by bias conditions where the reflective gain of SOA was balanced with the roundtrip cavity loss including Si mirror reflection loss, twofolded splitting loss at the DC, and two-folded coupling loss between the SOA and Si mirror. The gain characteristics of the SOA were emulated from the measured gain characteristics of the fabricated device (similar to those in Fig. 8). We adopted a model for a multi-sectional rate equation for SOA that took the effect of spatial hole-burning into account.
Fig. 2 (a) schematic cavity structure of Si hybrid laser, (b) simulated LI characteristics for different coupling losses (C = 1.5,4.0, and 6.0 dB).
We investigated the influence of coupling loss between the SOA and Si mirror (C) by calculating the Light-Current (LI) characteristics for C = 1.5,4.0, and 6.0 dB. The simulated LI characteristics are plotted in Fig. 2(b). The output ratio of the DC was set to κ = 0.50 and the output power at the output Si waveguide was monitored. As expected, the results indicated that both threshold current and slope efficiency were improved by decreasing coupling loss, since the coupling loss affected both cavity loss and external quantum efficiency to the output waveguide. Our calculations predicted that we could obtain three-fold larger output power of >10 mW by reducing the coupling loss from 6.0 dB (previously reported fiber-connected cavity [15]) to 1.5 dB. Thus, improvements to Si-SOA coupling loss are essential to achieve a higher-performance Si hybrid lasers.
3. Low-loss optical coupling using precise flip-chip bonding
After the simulation results described in the previous section, we considered a specific configuration for the Si hybrid laser that could accomplish low-loss coupling in the Si-SOA interface. Optical butt-coupling using flip-chip bonding is a promising technology that has been successful in reports on Si/III–V integration [9, 10, 17]. Compared to wafer-bonding technology that has emerged as another major candidate for Si/III–V integration [6, 8], the main advantage of a flip-chip bonding configuration has been assumed to be its higher flexibility in the design of Si and SOA chips. In addition, since it can provide better thermal conductance between III–V active and the substrate, flip-chip bonding is suitable for the Si transmitter we propose that operates at high temperature.
In order to achieve low-loss optical coupling with a flip-chip bonding configuration, both mode-field matching at the interface and highly precise alignment of waveguides are crucial. There is a schematic of the structure of an Si-SOA interface in Fig. 3(a). We adopted spot-size-converters (SSCs) on both the Si and SOA interfaces. A rib-type Si waveguide was fabricated on a 250-nm thick SOI wafer having a 3 μm BOX layer. The width of the rib-waveguide was 480 nm and the slab thickness was designed to be 50 nm. At the interface of the Si mirror and SSC, the waveguide was abruptly converted to a channel structure with a horizontally tilted slab boundary. In the SSC region, the 300-μm length Si channel waveguide was linearly tapered to a tip width of 50 nm. A low-index (n = 1.50) SiON waveguide having a 3.0 × 3.0 μm core was formed to build an overclad-type SSC [18] to cover the Si tapered waveguide. We formed no other overcladding besides SiON core. The length of the SiON waveguide was designed to be 320 μm including an extended waveguide of 20 μm from the Si waveguide tip. The overclad-type SSC expanded the strongly confined mode-field of the Si waveguide to an isotropic mode-field of 3.0 × 3.4 μm as shown in Fig. 3(c). The reflective SOA chip had a buried-heterostructure waveguide with a uniform semi-insulating InP blocking layer [19]. This structure was advantageous in obtaining excellent current confinement. We obtained an isotropic mode-field at the interface facet using a simple 150-μm length SSC in which the waveguide width was linearly changed from 1.4 to 0.5 μm. The measured near field pattern (NFP) at the SOA facet (Fig. 3(b)) demonstrated a highly isotropic mode-field of 3.0 × 2.9 μm that provided outstanding mode-field matching at the Si-SOA interface.
Fig. 3 (a) schematic of structure of Si-SOA interface, (b) near field pattern (NFP) at SOA facet, (c) NFP at Si facet.
The reflective SOA chip was flip-chip bonded onto the Si terrace where the SOI and BOX layers were removed and AuSn solder was deposited. In the process of flip-chip bonding, each waveguide was aligned passively using multiple bonding markers formed on the Si and SOA chips. The flip-chip bonding process was carried out with a highly precise flip-chip bonder equipped with an advanced image recognition system (Toray Engineering Inc., OF2000). The influence of AuSn solder reflow to the position accuracy could be stably compensated by introducing a constant offset in bonding position. The distribution of horizontal misalignment during the flip-chip bonding process is plotted in Fig. 4(a). The mean misalignment for 17 test samples bonded under standard conditions was successfully controlled at 0.10 μm and the standard deviation was only σ = 0.37μm. We also plotted the measured coupling tolerance curve for the horizontal direction in the same graph. The relative coupling efficiency was scanned in the planar direction while the Si-SOA gap was maintained at about 7 μm which is consistent with a typical gap obtained in flip-chip bonded samples. We obtained a wide 1-dB tolerance of ±1.3μm that was larger than the ±3σ of the bonding position distribution due to the expanded mode-field at the interface. We basically adjusted the alignment of waveguide heights by finely controlling the thickness of each layer in the Si and SOA chips. We confirmed the height relation between Si and SOA waveguides by observing both cross-sections for a few bonded samples. The monitored relative misalignment ranged from −0.3 to +0.2 μm. They were within the measured 1-dB tolerance for the vertical misalignment of ±0.9μm as plotted in Fig. 4(b). In future study, the gap between Si-SOA can be further optimized by overcoming the position variance in SOA cleaved facet.
Fig. 4 (a) Distribution of horizontal misalignment and coupling tolerance, (b) Difference in waveguide heights and vertical coupling tolerance.
4. Device fabrication and its characteristics
A photograph of the top surface view of the fabricated Si hybrid laser is shown in Fig. 5. The waveguide pattern of the Si mirror chip was defined with electron-beam lithography and an 320-μm length SSC was integrated onto the interface of the Si chip. The area of the Si mirror waveguide was about 1.0 × 0.2 mm. The SOA chip was fabricated on an n-InP substrate using similar processes to those used in the reference [19], but the thicknesses of the p-InP cladding layer and anode electrode were carefully controlled for vertical alignment. The SOA chip was 600 μm long. The facets were coated with dielectric AR film for the Si-interface side and HR film for the opposite side. The reflective SOA chip was flip-chip bonded onto the Si mirror chip with a narrow waveguide gap of < 10μm. We newly adopted tilted SOA and Si waveguides at their interface to prevent unwanted reflections at the Si-SOA interface. The opposed waveguides in Si and SOA chips were angled at 15 and 7 degrees from the normal to the interface facet, respectively. This configuration improved the mode-stability of the Si hybrid laser from our previous design with a normal interface [20]. The gap was filled with UV epoxy resin after bonding, having a reflective index of n = 1.44. Figure 6 has the measured response of the Si mirror chip that had the same design as that in Fig. 5 except for some reference ports. We measured the transmission spectra at the output, thru, and drop port by inputting a signal light from the interface port to the SOA. The reflection spectrum of the DBR mirror was also measured at the drop port by using an optical circulator. Each spectra contained coupling losses of about 2 dB per facet between the Si waveguide and lensed fibers. The RR with a 7.2-μm radius exhibited a periodic response with an 11-nm free spectral range (FSR) and a 0.6-nm 3-dB-bandwidth as can be seen in the drop-port spectrum. The DBR mirror where the width of the Si waveguide was modulated at a 299-nm period selected one of the transmission peaks of RR with a stopband that was 5.6-nm wide. The transmission loss at RR and the reflection loss at the DBR mirror were estimated to be 1.1 dB and 0.6 dB, respectively. The output ratio, κ, at the DC output coupler was 0.44 at the selected RR peak wavelength, which could be increased in future optimizations.
The lasing characteristics of the Si hybrid laser were measured on a temperature-controlled stage where the temperature of the whole device including the Si chip and SOA was adjusted to a target temperature. The output lasing light from the output port was coupled to a fiber using a fiber-pigtailed lens module. We did not sum up the lasing power at the reflected output port, which was located near the main output port in Fig. 5, since the output from the reflected output port was much lower than that at the main output port. The Light-Current (LI) characteristics of the device are plotted in Fig. 7(a), where the coupling loss to the fiber (1.4 dB) is compensated on the output power. Therefore, the output power and WPE are defined with on-chip lasing power at the output waveguide. The flip-chip bonded Si hybrid laser exhibited a low threshold current of 9.4 mA at 20°C. The slope-efficiency was about 0.1 W/A around its threshold, and we obtained a high output power of 15.0 mW at 200 mA. There was no degradation involved with the flip-chip bonding process on the voltage-current curve plotted in Fig. 7(a). During operation up to 200 mA, the laser exhibited a single wavelength oscillation at a selected wavelength of the Si mirror. The dependence of the laser on temperature was characterized from 20 to 60°C. In Fig. 7(b), we plotted the temperature dependencies of the WPE at 70 mA and the threshold current. Although the threshold current gradually increased at higher temperature, the device maintained a low threshold current of 25 mA and a high-output power of 10.5 mW. As a result, we obtained excellent WPE that ranged from 7.6 to 4.5 % in this temperature range. We assume that further improvements in WPE over 10% would be possible by increasing the output ratio at the DC coupler.
Fig. 7 (a) Light-Current characteristics of Si hybrid laser (b) Temperature dependencies of wall-plug efficiency and threshold current.
We estimated the improvement in Si-SOA coupling loss by analyzing the threshold conditions of the Si hybrid laser. First, we evaluated the relation between on-chip reflective gain and drive current at lasing wavelength for a reflective SOA chip having the same length. The measured gain-current characteristics are plotted in Fig. 8. We looked at the threshold gain corresponding to the threshold current shown in Fig. 7(b) at each temperature. We confirmed that the temperature dependence of threshold current was consistent with that of the gain-current characteristics of SOA over the entire temperature range, thus, we obtained an estimated threshold gain of 11.7 dB, which is equal to the round-trip cavity loss of the fabricated Si hybrid laser. Table 1 summarizes the estimated breakdown for round-trip cavity loss. The losses at the output coupler, RR, and DBR mirror were estimated from the measured response of the Si mirror shown in Fig. 6. We assumed a loss of 0.6 dB at the Si waveguide for the whole round-trip waveguide length of 2.8 mm. The Si-SOA coupling loss was estimated to be 1.55 dB by subtracting these losses from the total round-trip cavity loss. From above analysis, we confirmed remarkable improvements to Si-SOA coupling by adopting a flip-chip bonding configuration.
Table 1. Estimated breakdown of roundtrip cavity loss
The lasing spectrum of the Si hybrid laser was evaluated with a grating-based optical spectrum analyzer (OSA) that had a highest resolution of 0.02 nm. The lasing spectrum measured for a wide wavelength range is shown in Fig. 9(a). The drive conditions were 70 mA and 20°C. The device exhibited a single-wavelength oscillation at a selected wavelength of the Si mirror. The unselected peaks of RR were successfully suppressed by the DBR mirror. We also did not observe Fabry-Perot lasing peaks at the SOA gain peak wavelength under any bias conditions due to adequate treatment of the Si-SOA gap that tended to generate unwanted reflections. In Fig. 9(b), we plotted the transition in the lasing spectrum during a temperature sweep of 20 to 60°C. The drive current was fixed at 70 mA during this temperature sweep. We observed stable lasing oscillation at a single longitudinal mode selected by the Si mirror over the entire temperature range. The lasing wavelength exhibited a constant red-shift of 0.0787 nm/°C which is determined by the temperature-induced wavelength shift in the longitudinal mode. We could observe neighboring longitudinal modes at higher temperatures having a mode spacing of 0.122 nm (15.3 GHz) at the longer wavelength side of the main mode. This mode spacing was slightly narrower than the expected mode spacing for the total cavity length of 2.0 mm with an Si waveguide group index of ng ∼ 4.0. The total cavity length includes the SOA length of 600 μm, SSC length of 300 μm, output coupler length of 50 μm, RR waveguide length of about 50 μm, DBR length of 120 μm, and the other Si waveguide length of about 800 μm. This is because the strong group index dispersion around the peak wavelength of RR modulated the uniform longitudinal mode spacing of the laser cavity. Although the Si mirror had a relatively wide 3-dB bandwidth of about 0.3 nm, it successfully suppressed multi-mode lasing between such closely spaced longitudinal modes. We assumed that strong 3-rd order nonlinear gain suppression would contribute to this stable single longitudinal mode oscillation [21]. Once a longitudinal mode which is closest to the peak wavelength of RR starts lasing, strong spectral hole burning effect suppresses the gain for the non-lasing longitudinal modes. In addition, since we experimentally confirmed that the temperature coefficient of the longitudinal modes in the Si hybrid laser cavity (0.0787 nm/°C) was very close to that in the peak wavelength of RR filter (0.076 nm/°C) [15], the RR could keep on selecting same longitudinal mode during temperature sweep. Meanwhile, the DBR mirror exhibited a temperature-induced wavelength shift in the same manner. The temperature coefficient of the DBR was close, but not same as that of RR because each component adopts a same Si waveguide structure, but different optical confinements. However, the difference in temperature coefficients of DBR and RR would not be critical since the DBR mirror has a broad reflection-bandwidth of 5.6 nm. As a result, the device maintained stable single longitudinal mode lasing with a Side Mode Suppression Ratio (SMSR) of > 40 dB for all temperatures. We also confirmed that the above temperature shift slope was comparable to that of the all-pass RR used in the modulator [15]; thus, the feasibility of operation that was free of temperature control in the proposed Si transmitter was demonstrated with the flip-chip bonded Si hybrid laser.
Fig. 9 (a) Lasing spectrum measured with high-resolution spectrum analyzer (b) temperature dependence of lasing spectrum at 70 mA.
We measured the relative intensity noise (RIN) characteristics of the Si hybrid laser to verify stable oscillation in single longitudinal mode. Figure 10 shows the RIN spectra of both flip-chip bonded and fiber-connected lasers. The conventional fiber-connected cavity has a narrow longitudinal mode spacing of several tens of MHz because of its long cavity length of a few meters. The fiber-connected laser exhibited strong mode-competition between a large number of longitudinal modes due to this narrow longitudinal mode spacing. It generated strong beat noise on the RIN spectrum that could be observed over the entire frequency range. The longitudinal mode spacing in the flip-chip bonded cavity, on the other hand, was significantly broadened to 15.3 GHz (Fig. 9(b)) because of its short cavity length of 2.0 mm. This change in the config-uration of the cavity permitted a very low noise floor of ∼ −145dB/Hz especially for the low frequency regime. We can see a small noise peak around 14 GHz that corresponds to its mode spacing especially at higher temperatures with lower SMSR. Nevertheless, the flip-chip bonded laser exhibited a low RIN level of < −130 dB/Hz over DC to 40 GHz frequency range for the entire temperature range. This noise would be equivalent to that of existing single wavelength lasers employed for telecommunication applications.
5. Conclusion
We described remarkable improvements to performance of an Si hybrid laser using a flip-chip bonding configuration. We quantitatively predicted, by utilizing a numerical simulation of our laser configuration, that the output power of the Si hybrid laser could be increased by developing an efficient structure for the Si-SOA interface. We integrated different types of SSCs on both Si and SOA chips to accomplish mode-field matching at about a radius of 3 μm. In combination with a precise flip-chip bonder and adequately designed Si and SOA chips, the SOA chip was precisely bonded onto the Si mirror chip with a small misalignment of 0.10 μm at μ = 0.37μm. The fabricated Si hybrid laser exhibited a low threshold current of 9.4 mA and high output power of 15.0 mW at 20°C. The Si-SOA coupling loss was estimated to be 1.55 dB by analyzing the threshold conditions. The device maintained a high output power of > 10 mW up to 60°C due to the excellent thermal conductivity of the flip-chip bonding configuration, which demonstrated a high WPE of 7.6 ∼ 4.5% for a temperature range from 20 to 60°C. The longitudinal mode spacing of the cavity was expanded to 14 GHz by adopting a flip-chip bonding configuration with a short cavity length of 2.0 mm. This resulted in temperature-stable single longitudinal mode lasing with an SMSR of > 40 dB and a low RIN level of < −130dB/Hz (DC to 40 GHz). The device exhibited a constant red-shift of lasing wavelength (0.0787nm/ °C) for the temperature range from 20 to 60°C that was almost identical to that of the modulator. These experimental results strongly support the compatibility of flip-chip bonded Si hybrid lasers with temperature-control-free Si transmitter. It would be possible to build large-capacity, highly-energy-efficient Si I/O chips that could be operated even under temperature-uncontrolled circumstances such as those inside CPU packages by integrating multiple temperature-control-free Si transmitters that incorporated flip-chip bonded Si hybrid lasers.
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