Abstract
Keywords
1. Introduction
With the development of information technology, the channel capacity of current communication systems based on telecom-wavelength bands is gradually approaching their theoretical limits. The development of new technology will be the key to meeting the increasing demand for high-speed data transmission. Due to the recent advancement of the low-loss hollow-core photonic bandgap fiber[
Complementary metal-oxide-semiconductor (CMOS)-compatible silicon photonic technology offers unique advantages for integrated devices at 2 µm in terms of high performance, large-scale fabrication, and low cost. Recently, some critical devices for this band, including lasers[
In this paper, passive photonic devices operating at 2 µm were fabricated by using the silicon photonic multi-project wafer (MPW) process. A propagation loss of 1.62 dB/cm for ridge waveguides was calculated from MRRs with an intrinsic quality factor around . Waveguide crossing at 2 µm was designed and measured for the first time, to the best of our knowledge, with an insertion loss of less than 0.08 dB/crossing, and the coupling loss of the GC is optimized to be 7.9 dB/facet. Other passive devices, including the MMI and Mach–Zehnder interferometer (MZI), have been simultaneously designed, fabricated, and characterized at 2 µm.
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2. Fabrication and Measurement System
The passive devices were fabricated with the 180 nm silicon photonic MPW process in the Institute of Microelectronics of Chinese Academy of Sciences (IMECAS). The ridge waveguide was etched by 150 nm and cladded by with 1 µm thickness. To characterize the fabricated 2 µm passive devices, we set up the measurement system shown in Fig. 1. A tunable laser (New Focus TLB-6700) was used as the light source. The light launched from the tunable laser was adjusted by the polarization rotator (PR) and coupled into the chip through a vertical coupling system. The transmission responses from the device-under-test (DUT) were monitored by an InGaAs PIN photodetector. The photocurrent from the diode was converted into a voltage signal through a trans-impedance amplifier (TIA) (Thorlabs PDA200 C) and then collected by a National Instruments (NI) data acquisition card (NI USB-6212). By sweeping the wavelength of the light source, high-resolution transmission spectra of the fabricated passive devices could be obtained.
Figure 1.Schematic diagram of the measurement system. PR, polarization rotator; DUT, device-under-test; PD, photodetector; TIA, trans-impedance amplifier; DAQ, data acquisition card; PC, personal computer.
3. SOI Passive Devices
3.1. Waveguide and micro-ring resonator
The cross section of the waveguide is shown in Fig. 2(a). To determine the single-mode condition and avoid high-order modes, we simulated the effective refractive index of the supported modes in the waveguides with different widths by finite differential element (FDE) simulation, as shown in Fig. 2(b). The designed waveguide was etched by 150 nm on a 220 nm silicon film, and the modal profile was shown in the inset of Fig. 2(b). A second-order mode appeared when the waveguide width is larger than 0.6 µm, as shown in the red effective index curve. To achieve better confinement while maintaining the single-mode condition, we chose the waveguide width as 0.6 µm.
Figure 2.(a) Cross-section diagram of the SOI ridge waveguide. (b) Calculated effective refractive index of the fundamental (black) and first-order (red) TE modes as a function of the ridge waveguide width at 2025 nm. Inset: the spatial distribution of the fundamental and second-order polarized optical modes into the SOI ridge waveguide with a width of 0.6 µm and 1.4 µm at 2025 nm, respectively.
MRRs were implemented to characterize the propagation loss of the waveguide. The inset in Fig. 3(a) is the SEM image of the MRR with a diameter of 80 µm. Transmission spectra of MRRs without and with light -type doping are shown in Figs. 3(a) and 3(b), respectively. The gap between the coupling waveguide and the ring resonators of the two types of rings is 400 nm, and their extinction ratios are different due to the different propagation losses. Both rings were operated in the under-coupling regime, and their free spectral range (FSR) is about 4.5 nm. The corresponding group index is 3.58, which agrees well with our finite element simulation. Transmission spectra were fitted by using the coupled-mode theory[
Figure 3.(a) Measured spectral response of an MRR without doping. Inset: enlarged view of the measured resonance peak obtained by Lorentzian fitting and top-view SEM image of the MRR. (b) The measured spectral response of an MRR with light p-type doping.
3.2. Grating coupler
Due to the compact footprint and the convenience of alignment, GCs attracted extensive attention in fiber-to-waveguide couplings. Here, we designed and fabricated a 150 nm etched GC (period 0.985 µm, duty cycle 0.5) with a coupling angle of 8 deg. Based on the Bragg equation, the period of the GC is estimated as below:
Figure 4.(a) Simulated and measured coupling efficiency of the grating coupler. (b) Top-view SEM image of the fabricated GC.
3.3. Waveguide crossing
Waveguide crossings are basic components for large-scale on-chip optical interconnection. According to the self-imaging theory, many high-performance crossings have been demonstrated in communication wavebands[
Figure 5.(a) Simulated transmission spectra of the crossing from port 1 to ports 2 and 3 [the port numbers are shown in (b-2)]. Inset: electric field distribution at 2025 nm. (b) Microscope and SEM images of the cascaded and single crossing. (b-1) Microscope view of cascaded crossing with numbers 15, 30, and 45, and the device structure for crosstalk test; (b-2) enlarged view of the structure for crosstalk test; (b-3) enlarged view of a single crossing. (c) Cut-back measurements for characterizing the insertion loss of crossings. (d) Measurements of the device crosstalk.
3.4. Multimode interferometer
In the on-chip photonic system, MMI is one of the fundamental components for optical signal splitting and combining. Here, MMIs were fabricated and characterized. Based on the self-imaging theory, the width and length of the MMIs were tuned to achieve a small footprint while maintaining low crosstalk between the two output waveguides. The 3D FDTD simulations were applied for the optimization. Here, we designed a MMI with the total size of () and main sizes of , and taper lengths were 12.5 µm, 1.9 µm, 4.4 µm, 1.1 µm, and 30 µm, which are shown in Fig. 6(b-2). The designed insertion loss was , and its transmission spectrum is shown in Fig. 6(a). To characterize the insertion loss of the designed MMI, a cascade structure shown in Fig. 6(b-1) was fabricated through the same MPW run. We measured the transmission spectra from port 0 to ports 1–7 [the number of ports is shown in Fig. 6(b-1)] of cascaded MMIs in the 2005–2035 nm wavelength range, as depicted in Fig. 6(c). A linear fit of the transmission yields an insertion loss of per port as shown in Fig. 6(d). The discrepancy between the simulation and measured results is due to the dimension offset.
Figure 6.(a) Simulated transmission spectrum of the 1 × 2 MMI at the wavelength of 2005–2035 nm. Inset: electric field distribution at the wavelength of 2020 nm. (b-1) Microscope view of cascaded 1 × 2 MMIs; the white numbers 0–7 represent the port number; (b-2) zoom-in SEM image of 1 × 2 MMI. (c) Measured transmission spectra of the 1 × 2 MMI at the wavelength of 2025 nm. (d) Total insertion losses as a function of the number of cascaded 1 × 2 MMIs at the wavelength range of 2020–2030 nm. The port numbers shown in (c) correspond to the number marked in (b-1), for example, the curve 0-1 shows the transmission spectrum from port 0 to 1.
3.5. Mach–Zehnder interferometer
MZI is a prominent constituent of modulators[
Figure 7.(a) Optical image of the fabricated MZI. Ports 1 and 2 represent the input and output ports of the MZI. (b) Measured transmission spectrum of the MZI.
4. Conclusion
In summary, we have designed, fabricated, and characterized the fundamental passive silicon photonic components operating in the 2 µm waveband. All devices were etched by 150 nm on a 220 nm thick silicon layer in a silicon photonic MPW process. The propagation loss of the waveguide was 1.62 dB/cm, which was inferred from a ring resonator with an intrinsic quality factor as high as . The GC’s coupling efficiency is about . Waveguide crossings at this wavelength range were also fabricated for the first time, to the best of our knowledge, with an insertion loss of less than 0.08 dB. Finally, the MMI with a low insertion loss of was demonstrated in an imbalanced MZI with an extinction ratio larger than 20 dB. These low-loss passive devices could be critical functional units for on-chip optical interconnect and sensing applications at the 2 µm waveband.
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