Introduction
Nowadays, on-chip optical interconnects have drawn a great research attentions for their valuable advantages over electrical interconnects, such as wide bandwidth and low-power consumption. Silicon photonics is widely considered as a promising platform for realizing on-chip optical interconnects. Recent advances in silicon photonics, such as low loss waveguides [1], narrowband filters [2], optical demultiplexer [3], fast modulators [4] and Ge-on-Si photodetectors [5], have been already demonstrated. However, bulk silicon has very low efficiency of light emission due to its indirect bandgap. Therefore, achieving practical laser sources has been one of most challenging goals in silicon photonics.
In order to realize efficient silicon based laser sources, silicon/III-V hybrid lasers have been recently demonstrated, including III-V microdisk lasers [6,7] and microring lasers [8] vertically coupled to a silicon-on-insulator (SOI) bus-waveguide. One shortcoming of the demonstrated hybrid lasers is the bi-directional laser emissions, which imposes additional structures in order to re-route or combine the laser emissions.
Recently, we have proposed and demonstrated III-V microdisk lasers with an output waveguide for realizing unidirectional emission laser sources [9,10]. And in this study, we propose a new silicon/III-V laser structure, where an III-V microdisk connected with an output waveguide is used to provide the gain and then the laser emission in the III-V output waveguide is vertically coupled to a SOI waveguide. The proposed unidirectional emission hybrid laser would be more compact and more favorable for on-chip optical interconnects.
Simulation and analysis
Layout of the proposed unidirectional silicon/III-V hybrid laser is shown in Fig. 1. The proposed laser comprises mainly two parts, one part is an III-V microdisk connected directly to an output waveguide, and the other part is an SOI waveguide. And two parts of the hybrid laser are integrated together through benzocyclobutene (BCB) bonding [11]. For the III-V microdisk structure, it comprises a gain material (quantum wells) layer sandwiched between two layers of InP. The thickness of the gain material layer and the upper InP layer are tg and tu, respectively. The bottom InP layer is etched incompletely, leaving a thin lateral bottom contact layer with thickness tl, and the thickness of the etched bottom InP layer is tb. The III-V microdisk has a diameter D and the output waveguide has a width w1. For the SOI waveguide, it has a width w2 and a height h. The III-V output waveguide is laterally aligned to the SOI waveguide, and the III-V microdisk with the output waveguide is embedded in a thick BCB layer which is not drawn on Fig. 1.
In the following, the characteristics of the III-V microdisk with an output waveguide and the mode coupling between the III-V output waveguide and the SOI waveguide are respectively investigated by using three-dimensional (3D) finite-difference time-domain (FDTD) method.
When a microdisk is connected to an output waveguide, the output waveguide will destroy the symmetry of the microdisk and result in the mode coupling of two whispering gallery modes (WGMs) in the corresponding perfect microdisk [9,12]. If a coupled mode corresponds to two high-Q modes in a perfect microdisk with very small wavelength difference, the Q factor of the coupled mode can be very high. Figure 2 shows the intensity spectrum for a microdisk with an output waveguide, which is obtained by FDTD simulation and Padé approximation [13]. The intensity spectrum is calculated from the time transience of the Hz component recorded at an arbitrarily selected point inside the microdisk. By fitting the peaks of the intensity spectrum with Lorentzian functions, we can obtain the mode frequency and Q factor from the peak frequency and the ratio of the peak frequency to the corresponding 3 dB bandwidth. As is shown in Fig. 2, there are three modes in the calculated wavelength range, which are denoted as modes A, B and C, respectively. Simulation results show that the Q factors of the three modes are 8.5×104, 1.5×104 and 6.4×103, and the corresponding mode wavelength are 1525.8, 1556.2 and 1587.6 nm, respectively. Parameters used in the simulation are as follows, refractive index of BCB nBCB = 1.54, refractive index of gain material ng = 3.40, refractive index of InP nInP = 3.17, D = 7.5 μm, w1 = 500 nm, tg = 300 nm, tu = 150 nm, tb = 100 nm, tl = 50 nm. The three modes in Fig. 2 are all coupled modes, and each of them is formed by coupling of two resonant modes in a corresponding perfect microdisk without an output waveguide. Figures 3(a)–3(c) show the two resonant modes in the perfect microdisk, which are related to modes A, B and C in Fig. 2, respectively. The differences of wavelength between the two resonant modes in Figs. 3(a)–3(c) are 0.2, 1.8 and 3.3 nm, respectively. Because the two resonant modes corresponding to mode A have the smallest wavelength difference, the Q factor of mode A is the highest among that of those three modes in Fig. 2. Figures 4(a) and 4(b) show the magnetic filed component Hz distribution of mode A in Fig. 2 at the center plane of the gain material layer and at x-O-z plane respectively, the field in the output waveguide is magnified 30 times. As is shown in Fig. 4, most field of this mode is well confined in the gain material layer of the microdisk.
As for the mode coupling between the III-V output waveguide and the SOI waveguide, if we choose a proper size of the SOI waveguide after determining the size of III-V waveguide, the coupling efficiency between those two waveguides can be close to 100%. Figure 5 shows the effective index of the fundament mode as a function of the mode wavelength for an III-V waveguide and three SOI waveguides with different widths. Parameters of the III-V waveguide in Fig. 5 are the same as that in Fig. 2. And parameters of the SOI waveguide other than the width used in Fig. 5 are as follows, h = 340 nm, refractive index of silicon nSi = 3.48 and refractive index of the buried oxide layer of SOI . As is shown in Fig. 5, when w2 = 565 nm, the modes of the III-V waveguide and the SOI waveguide have almost the same effective index N around the wavelength of 1525.8 nm (resonant wavelength of mode A in Fig. 2). The dispersion curves in Fig. 5 are calculated by 3D FDTD method with periodic boundary condition [14] in x direction. Figures 6(a) and 6(b) show the magnetic filed component Hz distributions of the fundament mode at the cross section (y-O-z plane) of the III-V waveguide and SOI waveguide with w2 = 565 nm, other parameters used in Fig. 6 are the same as in Fig. 5.
When an III-V waveguide is put on top of a SOI waveguide, the guided mode in the III-V waveguide can be coupled efficiently to the SOI waveguide if those two waveguides have similar effective index. Figure 7(a) shows the steady-state distribution of Hz at x-O-z plane of the coupled III-V waveguide and SOI waveguide. Figures 7(b) and 7(c) show the Hz distributions at the center plane of III-V waveguide and SOI waveguide, respectively. A Gaussian beam at wavelength of 1525.8 nm is injected from the left port of the III-V waveguide in Fig. 7, and the light is coupled from the III-V waveguide to the underlying SOI waveguide in the coupling region. After the coupling region, the light is guided by the SOI waveguide. Waveguide parameters used in Fig. 7 are the same as that in Fig. 6, and the BCB thickness between the III-V waveguide and the SOI waveguide is 100 nm, and the length of the coupling region is 9.4 µm. Simulation result shows that coupling efficiency, which is defined as the ratio of the energy flux confined in the SOI waveguide after the coupling region to the energy flux confined in the III-V waveguide before the coupling region, is over 96%. If the effective indexes of the III-V waveguide and the SOI waveguide are mismatched, the coupling efficiency will decrease. For example, if we choose w2 = 500 nm, the highest coupling efficiency of those two waveguides will decrease to 73%.
Conclusions
We proposed a unidirectional emission silicon/III-V hybrid laser by using BCB bonding of an III-V quantum wells microdisk with an output waveguide on top of a SOI waveguide. A high Q mode in the microdisk with an output waveguide and high coupling efficiency between the III-V waveguide and SOI waveguide have been identified by 3D FDTD simulations. The proposed hybrid laser would be of useful application in on-chip interconnects for its uni-port emission, small size, high quality factor and high coupling efficiency.