A novel hybrid III—V/silicon deformed micro-disk single-mode laser

    Corresponding author: Wanhua Zheng, whzheng@semi.ac.cn
  • 1. State Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China
  • 2. Laboratory of Solid-State Optoelectronics Information Technology, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China

Key words: hybrid laserdeformed micro-disksingle longitudinal mode

Abstract: A novel hybrid III—V/silicon deformed micro-disk single-mode laser connecting to a Si output waveguide is designed, and fabricated through BCB bonding technology and standard i-line photolithography. Compared to a traditional circular micro-disk in multi-longitudinal-mode operation, unidirectional emission and single longitudinal-mode output from a Si waveguide are realized. In the experiments, an output power of 0.31 mW and a side-mode suppression ratio of 27 dB in the continuous-wave regime are obtained.

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1.   Introduction
  • In recent years, the hybrid silicon platform (HSP) has emerged as an important platform for the realization of large-scale photonic integrated circuits (PICs), which is attributed to the transparency of silicon at the telecom wavelength and the mature complementary metal oxide semiconductor (CMOS) technology that can be used for fabricating photonic devices with sub-micron features. Currently, state-of-the-art passive waveguide circuits[1], low power consumption resonant modulators[2], and high-speed photodetectors[3] are available on this platform. However, the integration of a coherent light source on the silicon platform remains an issue, due to the indirect band gap of silicon.

    For an on-chip light source in silicon, many approaches to light emission have been demonstrated, including Raman lasers[4], the nano-patterning laser[5], and Ge-on-Si laser[6]. Currently, heterogeneous integration of III-V materials and SOI waveguides (including molecular bonding and adhesive bonding) is a principal approach to obtain an on-chip light source in silicon, owing to the high-density integration and avoiding costly active alignment required in the case of packaged lasers. With the use of bonding technology, a number of hybrid lasers have been successfully demonstrated, like the Fabry-Perot (FP) laser[7, 8], the racetrack laser[9, 10], the distributed feedback (DFB) laser[11], the distributed Bragg reflector (DBR) laser[12] and the micro-cavity laser[13, 14, 15, 16]. However, these lasers require defining a resonant cavity by cleaving the Si wafer, except for the racetrack laser, DBR laser and micro-disk laser[10], which makes it a difficulty for an on-chip light source in silicon. Although the hybrid laser by etching DBR on Si can obtain a single-mode light source in silicon, its large size and mode instability block its further development[12]. Besides, an on-chip racetrack resonator laser without facet polishing and dicing for defining the laser cavity has been demonstrated[9]. However, it was a pity that only multimode lasing was obtained. In order to overcome this problem and obtain single longitudinal mode operation, our team has designed and demonstrated the single mode laser (SMSR of larger than 20~dB in the CW regime) through integrating a racetrack ring with slots to build a mode-selection mechanism in III-V/SOI hybrid architectures[10]. Despite the realization of single-mode lasing, the footprint of this laser was relatively large, resulting in the high threshold current and high power consumption. Therefore, how to reduce the footprint of a racetrack ring has been an attractive subject for improving the characteristics of the laser.

    Diode lasers with ring or disk resonator geometries are one of the most attractive on-chip light sources for PICs, because of their low threshold current and low power consumption[13, 14]. In the past few decades, the development of hybrid III-V/silicon micro-cavity lasers (ring or disk) have acquired remarkable achievements[13, 14, 15, 16, 17]. Electrically pumped hybrid micro-disk[13] and micro-ring lasers[14] were realized separately, which all demonstrated low threshold current and power consumption. However, the single mode operation and directional emission in micro-disk lasers were greatly limited by the symmetry of the micro-disk, and much work was focused to get directional emission microlasers[15, 16]. Currently, one of the most popular approaches is to add an integrated optical reflector, such as a teardrop reflector[15] }$or DBR[16], at one end of the bus waveguide. These approaches have been shown to obtain directional output[15, 16]. However, stable directional operation requires strongly high requirements for photolithography and etching, in order to reduce the sidewall roughness and minimize back-reflection[17]. In addition, the SMSR of these structures was less than 20 dB[16], which showed a bad single mode output. Another approach is to break the rotational symmetry by using deformed microcavities[18] or connecting an output waveguide[19] to increase the directionality of emission and power collection efficiency. In fact, a deformed micro-disk laser with an output waveguide is an excellent choice to realize the single mode lasing and directional emission. An AlGaInAs/InP octagonal resonator microlaser connected to an output waveguide has been designed and demonstrated, which obtained the SMSR of 24 dB[20].

    In this letter, an electrically-pumped hybrid III-V/silicon deformed micro-disk laser connected to a Si output waveguide has been designed and demonstrated. In order to ensure the light lasing from the III-V active area efficiently couples to the silicon waveguide, a taper mode converter structure is introduced. Furthermore, the deformed design of the micro-disk laser has realized unidirectional emission and single-longitudinal-mode operation from a Si waveguide, which can be utilized as an effective light source for silicon-based optical interconnects. In the experiments, an output power of 0.31 mW and an SMSR of 27 dB in the CW regime is obtained.

2.   Design and simulation
  • The schematic structure of a hybrid III-V/silicon deformed micro-disk laser with an output waveguide is shown in Figure 1. The III-V epitaxial structure of the laser is shown in Reference [8]. The active layer consists of eight AlGaInAs quantum wells which have an emission peak around 1535 nm at room temperature. The SOI material with a 340-nm-thick top silicon and 2-$\mu $m-thick buried oxide (BOX) is utilized. As shown in Figures 1(a) and 1(b), the hybrid laser consists of a deformed micro-disk and an output waveguide. The deformed micro-disk in our design consists of three parts: an arched micro-disk (with a diameter $D$ $=$ 20 $\mu $m), a rectangular micro-disk and a tapered micro-disk, which can also be considered a combination of circular and hexagonal micro-disks. In such a structure, a Si deformed micro-disk with the same diameter in the SOI is carried to ensure the strongly optical field confinement in the gain medium and the bonding.

    The above design not only breaks the circular symmetry and achieves unidirectional emission[18, 19], but also eliminates multi-longitudinal modes and realizes single-longitudinal mode lasing[21, 22]. Besides, the output waveguide is a III-V/silicon hybrid waveguide; to achieve index matching between the two waveguides, a III-V mode converter is adopted. As shown in Figure 1(b), a tapered structure is applied to allow the efficient coupling of the optical mode from the III-V waveguide to the silicon waveguide.

    Under the effective index approximation, the three-dimensional (3D) deformed micro-disk is converted to a two-dimensional (2D) structure with an effective refractive index of 3.178 surrounded by air. In order to prove that the deformed micro-disk achieves a better characteristic of single-mode choice, the mode characteristics are simulated for the 2D circular resonator and 2D deformed micro-disk with an output waveguide by the FDTD technique, respectively.

    The obtained intensity spectra for TE modes of 2D deformed micro-disk ($D$ $=$ 20 $\mu $m) with a 2-$\mu $m-wide output waveguide are plotted in Figure 2(a). The intensity spectrum for TE modes in the corresponding circular micro-disk with the output waveguide is calculated and plotted in Figure 2(b) with the spectrum from 1450 to 1550 nm. By contrast, we find that the deformation of the micro-disk achieves better single-mode selectivity, and the peak wavelength of the spectrum is near 1514 nm, which is in good agreement with the experimental results. In addition, connecting an output waveguide to the micro-disk obtains stable unidirectional emission, which has also been verified in our experiment.

3.   Fabrication and results
  • The hybrid III-V/silicon deformed micro-disk laser is fabricated using an AlGaInAs quantum well epitaxial structure, as shown in Reference [8], which is bonded to an SOI micro-disk with an output waveguide. The top silicon micro-disk and waveguide is formed on the (100) surface of a SOI substrate using standard projection photolithography and plasma reactive ion etching[22]. The diameter of the top silicon deformed micro-disk is 20 $\mu $m, and the width of the silicon waveguide is 2 $\mu $m. The top silicon micro-disk and waveguide structure is fabricated with an etch depth of 0.34 $\mu $m. This III-V structure is then transferred to the patterned SOI wafer through BCB adhesive bonding. The bonding process starts with the cleaning of the SOI substrate and the III-V die.

    After the BCB: mesitylene solution is spin-coated onto the III-V die. The III-V die is then baked and cooled down to room temperature. Subsequently, the SOI substrate and the III-V die are aligned and annealed at 350 C. After InP substrate removal, the deformed micro-disk ($D=$ 20 $\mu $m) connecting to a tapered output waveguide is defined by wet etching. Then a SiO$_{2}$ layer with a thickness of 200 nm is deposited on the wafer for electrical insulation, which is also thick enough to reduce the absorption loss of the optical field from the top contact. After removing the SiO$_{2}$ on the waveguides, Ti/Au is deposited onto the whole wafer. Through photolithography and wet etching, the p-type and the n-type electrode pads are formed. Finally, the wafer is diced without any polishing and single deformed micro-disk lasers are fabricated.

    A cross-sectional SEM (scanning electron micrograph) image of the final fabricated hybrid laser is shown in Figure 3(a). The thickness of the BCB bonding layer is near 56 nm, which is thin enough to ignore its impact on the device characteristics. In experiments, bonding layer thicknesses down to 100 nm can be reproducibly achieved, and a nice bonding between the SOI with the waveguide and the III-V material can be found from Figure 3(a). As shown in Figure 3(b), the output light spot has been focused in the silicon waveguide, which further confirms the output of light in the silicon waveguide. The electrode pad distribution is shown in Figure 3(c).

    The output power of the laser is measured at one facet of the device using a large-area photodetector positioned in close proximity to the facet. The device is mounted on a copper plate. The sample is put into the low temperature Dewar equipment. The following results are measured under cooling conditions. As shown in Figure 4, for a 20-$\mu $m-diameter deformed micro-disk laser with a 2-$\mu $m-wide output waveguide, the threshold current is 20 mA, and the maximum optical power is 0.31 mW while the slope efficiency is 4.86 mW/A.

    The external differential quantum efficiency of the device is 0.59 % derived from the slope efficiency, which is improved ten times with respect to the value of the AlGaInAs-InP micro-disk lasers with the same footprint[19]. This is due to the benefit from the stronger vertical optical confinement supported by the deformed Si micro-disk. At the 120 mA current, the laser reaches saturation.

    To measure the optical spectrum of the device, we coupled the deformed micro-disk hybrid laser output into a 62.5/125~$\mu $m fiber connected to an ADVANTEST Q8384 optical spectrum analyzer with a resolution of 0.01 nm. The typical optical spectrum of the laser in the CW regime is given in Figure 5. At currents of 60 mA, the optical spectrum is presented and the lasing wavelength is 1514 nm. A SMSR of 27 dB is obtained at the current of 60 mA. The corresponding 3 dB width is 0.355 nm, larger than the value of AlGaInAs-InP micro-disk lasers with the same footprint[19], which is due to the reduction of the $Q$ factor derived from the inevitable non-radiation loss in the bonding surface and the phase noise caused by an imperfect wet etching process. From the experimental results, we find that the deformed hybrid micro-disk can affect the mode competition strongly and obtain single mode operation. The lasing wavelength agrees well with that of the calculation, as shown in Figure 2.

    However, for obtaining higher quantum efficiency and lower threshold current, some work has to be done to reduce the internal loss and improve the mode gain from both structural optimization and process improvement. To balance the modal gain of the III-V layer and coupling efficiency from the III-V layer to the Si waveguide, the deformed micro-disk and tapered waveguide structure of the III-V/Si hybrid waveguide need to have optimized width, length (or diameter) and shape. Besides, the isotropic wet etching process brings an excessive loss of the cavity face, which causes a larger line width (3 dB width) and a larger threshold current. To improve these characteristics, the effective association of dry etching and wet etching will be employed in the next process.

    Moreover, the SiO$_{2}$ layer of SOI and the bonding layer (BCB) lead to a bad thermal conductivity of the laser, which hinder the room temperature operation of a deformed micro-disk III-V/Si hybrid laser. Next, more work will focus on further reducing these problems by optimizing the structure and the technological process.

4.   Conclusion
  • In conclusion, a deformed micro-disk hybrid laser with a silicon output waveguide coupling is designed and fabricated. An output power of 0.31 mW and an SMSR of 27 dB are obtained from experiments. The results presented show that this deformed micro-disk design in a hybrid laser realizes unidirectional emission and single mode lasing from a Si waveguide. Moreover, this hybrid laser only needs low cost standard photolithography in the whole technological process. Overall, the deformed micro-disk III-V/SOI hybrid laser we have designed provides an attractive selection for on-chip light sources in silicon.

Figure (5)  Reference (23) Relative (20)

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