1. Introduction

Photonics will play quite important roles in coping with the increasing traffic in the datacenters [1,2]. Today, vertical-cavity surface-emitting lasers (VCSELs) dominate short-reach links because of their good cost performance and low-power consumption; the VCSELs can be directly modulated at 25 Gbit/s with an energy cost of less than 100 fJ/bit [3,4]. However, there remains a problem when the data-transmission distance becomes more than 100 m because VCSELs use a multimode-fiber link. In addition, VCSELs have little leverage in wavelength-division multiplexing (WDM) technology though the WDM is desired to increase the transmission capacity.

To overcome these hurdles and meet the requirements towards a new era of datacenter networks, we demonstrate membrane distributed-feedback (DFB) lasers with lateral current-injection structures on an SiO₂/Si substrate by focusing two features [5–7]. First, the DFB laser is a key device to realize a WDM lightsource. To date the DFB lasers fabricated by InP-based materials have been used commonly for long-reach telecom applications with WDM technology. Second, the membrane structure on SiO₂ attributes to increase optical confinement within the active region in comparison with the conventional DFB lasers, which leads to reduce power consumption [8–10]. In fact, the membrane DFB lasers can reduce the energy cost to 171 fJ/bit with 25.8-Gbit/s non-return-to-zero (NRZ) direct modulation [7], which is a comparable value with that of commercial VCSELs and the lowest energy cost among all DFB lasers to the best of our knowledge. In addition, we demonstrate 40-Gbit/s direct modulation at room temperature and stable single-mode lasing up to 100 °C [6]. However, further increase of bandwidth and wavelength channels with lowering power consumption are important to meet the future criteria.

Moreover, integration of the membrane DFB lasers with Si photonics devices is another important research topic because it provides high functionality. Particulary, integration of a spot-size converter (SSC) based on Si photonics technology [11] could reduce fiber coupling loss and drastically change optical assembly methods for the lasers because it enables us to connect a fiber block by using adhesion in a similar way with silica planar-lightwave-circuits (PLCs) industries, which decrease a packaging cost. In addition, integration with the other Si photonics devices on a Si platform potentially provides dense integrated photonic circuits with high-perfomance passive devices and electronics [12,13]. However, the integration of the InP-based membrane devices and Si photonics devices is a remaining challenge.

In this paper, for the first time, we demonstrate monolithic integration of a membrane InP distributed-reflector (DR) laser and a silicon-oxide (SiO_x)-based SSC on the SiO₂/Si substrate. The membrane DR laser consists of a DFB active section with a rear distributed Braggreflector (DBR) and front output waveguide, and enables us to shorten the cavity length that helps to reduce power consumption and achieve stable single-mode lasing with asymmetric output [14]. The DR laser with 50-μm cavity length exhibits a high modulation efficiency of 9.4 GHz/mA^0.5 and an energy cost of 132 fJ/bit when modulated with 25.8-Gbit/s NRZ signal. In addition, the SSC provides 2.2-dB butt-coupling loss with a single-mode high-numerical-aperture fiber (HNAF), which is a decrease of about 6 dB from previous reported device using a lensed fiber [7].

2. Device structure and fabrication

Figure 1 shows a schematic diagram and fabrication procedure of the membrane DR laser and SiO_x-based SSC. The first step is fabrication of the active structure in a similar way with our previous reports [5–7]. The device was fabricated on the Si substrate with a top SiO₂ thickness of 2 μm. The active layers was 150-nm thick InGaAsP-based six-quantum-well layers sandwiched by top and bottom 50-nm InP layers (total thickness was 250 nm), which was grown on the other InP support substrate Then, epitaxial layers were directly bonded with SiO₂/Si substrate. After removing InP support substrate, the active region was buried with 250-nm-thick regrown InP. The two types of active region with different lengths (50 and 80 μm) and same width (0.8 μm) were made. The regrown InP was used to make both the rear DBR and the front output waveguide with a width of 1.5 μm. The surface grating for the DFB and DBR sections were simultaneoulsly formed on the top InP layer by dry etching. The length of DFB section (L_DFB) was consistent with the embedded active region length. To form a lateral current injection structure, we employed Si-ion implantation for n-type and Zn thermal diffusion for p-type doping of 2 × 10¹⁸ and 1 × 10¹⁸ cm⁻³ concentration, respectively.The DFB section has uniform grating which contributes to suppress spatial hole burning in comparison with a λ/4-shift grating even though the DFB section has relatively high coupling coefficient (κ) to achieve low-threshold-current operation. Figures 2(a) and 2(b) are simulated modefields for DFB and DBR sections, respectively. Reflectance at the boundary between DFB and DBR section is estimated −32 dB thanks to their similar mode fields. Figure 2(c) shows calculated coupling coefficients of both the DBR and DFB sections as a function of the InP etching depth (d). For these calculation, we used a commercial software [16]. Thanks to the high refractive index difference between InP core (n = 3.169) and SiO_x cladding (n = 1.505), we can obtain high κ easily. For selecting single lasing mode at the stop-band edge, we integrated the rear DBR. For the rear DBR grating, we set its Bragg wavelength (λ_{B_DBR}) different from that of DFB grating (λ_{B_DFB}) with the wavelength shift of Δλ_B ( = λ_{B_DBR}-λ_{B_DFB}) to select the single lasing mode at the stop-band edge of the DFB section. For the practical fabrication, we set d to 30 nm which makes κ_DBR and κ_DFB about 900 and 1000 cm⁻¹, respectively. In addition, λ_{B_DFB} and λ_{B_DBR} were set to 1532 and 1547 nm, respectively. Here, the stop-band overlap of the DFB and DBR section was about 5 nm. A calculated spectrum output from the front waveguide is shown in Fig. 2(d). Large bandwidth of DFB and DBR sections increase the fabrication tolerance to achieve single mode lasing.

 

Fig. 1 Schematic of the DR laser integratd with the SiO_x-based SSC, and its fabrication procedure.

Download Full Size | PDF

 

Fig. 2 Simulated modefields for (a) the DFB and (b) the DBR section. (c) Estimated κ for the DFB and DBR section as a function of etching depth for the top InP layer. (d) Calculated emission spectrum output from the front waveguide.

Download Full Size | PDF

In order to constitute the SSC, the front output InP waveguide is laterally tapered and reduced its width from 1.5 μm to 0.1 μm with length of 300 μm. This 250-nm-heights InP taper is embedded in the SiO_x core with a 3-μm-square cross section. As we mentioned above, this SiO_x layer is functionalized as a cladding of the DFB and DBR in a similar way for Si pin devices [16]. In addition, the SiO_x core and cladding are covered with SiO₂ overcladding so that a relative refractive-index difference (Δ) becomes about 3%. Figure 3 shows simulated images of mode-field conversion at the SSC. The SSC loss is calculated as 1.7 dB by an eigenmode-expansion method [15]. It should be noted that we can decrease the SSC loss to 0.1 dB ideally by the optimization of the taper structure [17]. We deposited the SiO_x core layer by using an electron-cyclotron-resonance (ECR) PECVD method at less than 200°C, which prevents the DR laser from thermal damage during deposition. After that, we partly removed the SiO_x by reactive ion etching and defined the SiO_x core, followed by deposition of 4-μm-thick SiO₂ overcladding layer. Finally, we opened contact holes for probing by dry etching of SiO₂ and SiO_x. Before obtaining the cleaved facets for fiber butt coupling, we performed mechanical polish of the fabricated wafer from the back side.

 

Fig. 3 Calculated images of the SSC characteristics. (a) and (b) show mode fields at the DFB section in the DR laser and at the SiO_x waveguide, respectively. (c) and (d) show side-view and top-view images of mode conversion from the DR laser to SiO_x waveguide via the SSC.

Download Full Size | PDF

3. Device characteristics

First, we evaluated static characteristics. Figure 4(a) shows relationships between light output power and injection current. For device characterization, we used a single-mode high-numerical-aperture fiber (HNAF) with a mode-field diameter of 4.1 μm. As shown in inset of Fig. 4(a), the cleaved HNAF was physically contacted to the SiO_x waveguide facet. A curve measured by a photodetector (PD) placed in front of the SiO_x waveguide facet shows some kinks. This is due to the reflection at the SiO_x waveguide facet due to the refractive index mismatch between air and the SiO_x waveguide. In comparison, a curve measured by the HNAF exhibits no kink thanks to the suppression of the reflection at the boundary between the SiO_x waveguide facet and the HNAF. We confirmed lasing at up to 80°C though the threshold current increases as temperature increases. At 20°C, the threshold current and the maximum fiber output power are 0.8 mA and 0.43 mW, respectively. The fiber coupling loss was 2.2 dB, which reduced about 6 dB from a coupling loss in previous device without SSC by using a lensed fiber [7]. The additional loss of 0.5 dB in comparison with calcucation occurred due to increase of the scattering loss. The lasing spectrum with the current of 6 mA is shown in Fig. 5(b). We achieved stable single-mode lasing at the longer stop-band edge up to 80°C. We estimated the κ_DFB was about 1500 cm⁻¹ from the DFB stop bandwidth of about 30 nm, which is higher than our designed value due to the fabrication error. Reducing κ_DFB with presice process control and a shallower etching depth, we can reduce the κ_DFB and increase the output power. At 20°C, the side mode suppression ratio was 42 dB. We estimated a thermal impedance to be about 1900 K/W by measuring lasing-wavelength shifts with various temperature and input power. The thermal impedance is higher than our previous device [7], which may restrict high-temperature operation and output power. We think the thermal impedance will be reduced by adding some heat-sink structures fabricated by semiconductors or metals.

 

Fig. 4 Static Characteristics of of a fabricated device; (a) I-L characteristics obtained by a PD and a HNAF at from 20°C to 80°C CW operation. The inset shows a top view of experimental setup by HNAF butt coupling to the laser chip. (b) Lasing spectra at from 20°C to 80°C CW operation. The bias current was 6 mA.

Download Full Size | PDF

 

Fig. 5 Dynamic characteristics of the DR laser; (a) f_r estimated from small-signal frequency response as a function of square root of bias current above threshold. (b) Relationship between the calculated modulation speed and energy cost. (c) and (d) shows an eye diagram for 25.8-Gbit/s NRZ direct modulation with a bias current of 2.5 mA and 5.0 mA, respectively.

Download Full Size | PDF

Next, we evaluated dynamic caracteristics. Figure 5(a) shows relaxation oscillation frequency f_r as a function of the square root of bias current above threshold. The f_r was measured from small-signal frequency response obtained at 25°C. For comparison, we draw results for DR lasers with a L_DFB of 50 and 80 μm. The modulation efficiency were set to 9.4 and 8.4 GHz/mA^0.5 for a L_DFB of 50 and 80 μm, respectively. We successfully increase the modulation efficiency, which is the almost same with the highest efficiency among the DFB lasers [18], by decreasing L_DFB. Figure 5(b) shows a relationship between the calculated modulation speed and energy cost for the device with a L_DFB of 50 μm. The modulation speed can be calculated as to be 1.3 × 1.55 × f_r [19,20] and the energy cost is defined as the product of bias voltage and current divided by modulation speed. Estimated energy cost was less than 100 fJ/bit when the modulation speed was up to 35 Gbit/s. Finally, we measured eye diagrams using non-return-to-zero (NRZ) modulation signals with a pseudo-random bit sequence (PRBS) of 2³¹-1. Here, we also used the device with a L_DFB of 50 μm. Figures 5(c) and 5(d) was an eye diagram obtained with a bias current and modulation voltage of 2.5 mA and 5.0 mA, which results in an energy cost of 132 and 185 fJ/bit, respectively. The rise and fall times are seems to be fast enough, which is consistent with the experimental results in Fig. 5(a). The modulation voltage for each was 0.80 V_pp and 1.52 V_pp, and the extinction ratio estimated from centers fo zero and one level was over 5.5 dB for both current conditions. However, the eye-opening is not so clear due to the noise. We seem that the noise is caused by back reflection from the abruptly ended rear DBR section. In future, we can employ the tapered termination of the rear DBR then reduce the back reflection and noises.

4. Conclusion

We have successfully demonstrated the integration of the DR laser and SiO_x-based SSC on the SiO₂/Si substrate. The device exhibits stable single-mode lasing up to 80°C, quite high modulation efficiency of 9.4 GHz/mA^0.5, and clear eye openings for 25.8-Gbit/s NRZ direct modulation. These results clearly show that our device is quite suitable for employing the optical interconnect application in datacenters. In addition, the integration of the SiO_x-based SSC enables us to achieve 2.2-dB fiber-butt-coupling loss. Adding to contribution for the reduction of fiber-coupling loss and assembly cost, the integrated SiO_x waveguide can be a interconnecting part from the lasers to the Si-, SiN-, and SiO_xN_y-waveguide devices. Thus, the Si photonics devices can equip high-speed, low-cost, and low-power-consumption lasers by employing our technlogy.