• Chinese Optics Letters
  • Vol. 19, Issue 7, 071301 (2021)
Hui Ma1, Haotian Yang1, Bo Tang2, Maoliang Wei1, Junying Li1, Jianghong Wu3、4, Peng Zhang2, Chunlei Sun3、4, Lan Li3、4, and Hongtao Lin1、*
Author Affiliations
  • 1State Key Laboratory of Modern Optical Instrumentation, College of Information Science and Electronic Engineering, Zhejiang University, Hangzhou 310007, China
  • 2Institute of Microelectronics, Chinese Academic Society, Beijing 100029, China
  • 3Key Laboratory of 3D Micro/Nano Fabrication and Characterization of Zhejiang Province, School of Engineering, Westlake University, Hangzhou 310007, China
  • 4Institute of Advanced Technology, Westlake Institute for Advanced Study, Hangzhou 310023, China
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    DOI: 10.3788/COL202119.071301 Cite this Article Set citation alerts
    Hui Ma, Haotian Yang, Bo Tang, Maoliang Wei, Junying Li, Jianghong Wu, Peng Zhang, Chunlei Sun, Lan Li, Hongtao Lin. Passive devices at 2 µm wavelength on 200 mm CMOS-compatible silicon photonics platform [Invited][J]. Chinese Optics Letters, 2021, 19(7): 071301 Copy Citation Text show less

    Abstract

    As a promising spectral window for optical communication and sensing, it is of great significance to realize on-chip devices at the 2 µm waveband. The development of the 2 µm silicon photonic platform mainly depends on the performance of passive devices. In this work, the passive devices were fabricated in the silicon photonic multi-project wafer process. The designed micro-ring resonator with a 0.6 µm wide silicon ridge waveguide based on a 220 nm silicon-on-insulator platform achieves a high intrinsic quality factor of 3.0×105. The propagation loss is calculated as 1.62 dB/cm. In addition, the waveguide crossing, multimode interferometer, and Mach–Zehnder interferometer were demonstrated at 2 µm with good performances.

    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[1] and large-bandwidth thulium-doped fiber amplifier (TDFA)[2], the 2 µm wavelength band is becoming an emerging communication window[3]. Besides, it is also emerging as a promising spectral window for chemical sensing, since the molecular fingerprint regions of many gases (e.g.,  CO2,H2O, and CH4)[4] and the absorption bands of biomolecules like blood glucose fall in this wavelength range[5]. Thus, arguably, the 2 µm waveband has shown great potential in next-generation optical communication and sensing.

    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[6,7], modulators[8,9], photodetectors[10], on-chip spectrometers[11], and nonlinear optical devices[12,13], have been demonstrated on the silicon-on-insulator (SOI) platform. Meanwhile, as essential components of the on-chip photonic system[14], passive devices with different functions operating in the 2 µm band have also been demonstrated, including the low-loss single-mode waveguide[15], grating coupler (GC)[15,16], micro-ring resonator (MRR)[17], multimode interferometer (MMI)[18], arrayed waveguide grating[19], and mode demultiplexer[20,21]. Despite the significant progress that promotes the development and practical implementation of silicon photonics at the 2 µm waveband, some essential components, such as waveguide crossings, are still missing. Low-loss passive photonic components are the basis of high-performance optical communication and sensing systems.

    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 3.0×105. 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.

    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 SiO2 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.

    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.

    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 neff 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.

    (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.

    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 p-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[22]. The loaded quality factor (Qload) of the MRR without doping is 1.7×104. The inferred propagation loss of the ring waveguide is about 1.62 dB/cm, and its intrinsic quality factor (Qin) is as high as 3×105. On the other hand, the Qload and Qin of the MRR with light p-type doping (boron ion implantation with a doping concentration of 7×1017cm3) are 1.1×104 and 3.5×104, respectively, and the inferred propagation loss is 13.7 dB/cm. The p-type doping in the MRR could have led to this excess loss. The low-loss ridge waveguide supports the applications in the 2 µm waveband.

    (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.

    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: Λ=λneffn1sinθ,where Λ is the grating period, λ is the central wavelength, neff is the effective refractive index of the waveguide, n1 is the index of cladding, and θ is the incident angle. The GC’s optimized constant duty cycle was obtained by monitoring its coupling coefficiency in 3D finite difference time domain (FDTD) simulation while sweeping the grating period around the initial estimated value. The highest coupling efficiency is 6dB for a grating period of 0.985 µm. As shown in Figure 4(a), the fabricated GC presents a minimal coupling loss of 7.9 dB at 2020 nm. The etching depth offset in the grating may have mainly resulted in the difference between the simulated and measured coupling efficiency.

    (a) Simulated and measured coupling efficiency of the grating coupler. (b) Top-view SEM image of the fabricated GC.

    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[2325]. We designed and fabricated the crossing operated at 2 µm with low insertion loss and crosstalk for the first time, to the best of our knowledge. The crossing is composed of two orthogonal 1×1 MMIs with a total size of 18.08µm×18.08µm (length×width), and the main sizes of l1,l2, and w1 are 3.5 µm, 11.08 µm, and 2.36 µm, which are shown in Fig. 5(b-3). The transmission spectra and spatial mode field distribution are shown in Fig. 5(a). By optimizing the taper region’s width and length, the light beam is focused at the intersection region with a small amount of light leakage to the cross ports. To accurately characterize the insertion loss of waveguide crossings, cut-back measurement was performed via cascading 15, 30, and 45 waveguide crossings, as shown in Fig. 5(b-1). The zoom-in view of crossing is shown in Fig. 5(b-3). We fitted the transmission loss of these cascaded structures, as shown in Fig. 5(c), and achieved an insertion loss of 0.08±0.011dB for a single waveguide crossing. Based on the device structure shown in Fig. 5(b-2), the transmission spectra were measured from port 1 to ports 2 and 3, as depicted in Fig. 5(d). The crosstalk of the crossing is less than 34dB.

    (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.

    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, 1×2 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 1×2 MMI with the total size of 72.5µm×4.4µm (length×width) and main sizes of l1,w1,w2,w3, 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 3.05dB, 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 1×2 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 3.03±0.43dB per port as shown in Fig. 6(d). The discrepancy between the simulation and measured results is due to the dimension offset.

    (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.

    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[26,27], detectors[28], optical switches[29], filters[30], and sensors. In general, the MZI is mainly composed of two 3 dB couplers and phase-shifting arms. The MMI and directional coupler are the commonly used 3 dB couplers. With higher fabrication tolerance, polarization independence, and wavelength independence, the MMI was used in our MZI structure. The radius of the bent waveguide inside the MZI was set to be 50 µm to avoid the bending loss. Here, we designed and fabricated the imbalanced MZI [Fig. 7(a)] with the 3 dB 1×2 MMI as discussed above. The length difference between the two asymmetric arms is 280 µm. Afterward, we measured the transmission spectra from port 1 to port 2. The insertion loss was less than 1 dB, and the extinction ratios of dips are larger than 20 dB, as shown in Fig. 7(b), which indicates the low insertion loss and balanced power splitting ratio of the MMI. The FSR of the MZI is 4.0 nm, and the corresponding group index is 3.66, which agrees well with the simulation results.

    (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.

    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 3.0×105. The GC’s coupling efficiency is about 7.9dB. 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 3.03±0.43dB 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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    Hui Ma, Haotian Yang, Bo Tang, Maoliang Wei, Junying Li, Jianghong Wu, Peng Zhang, Chunlei Sun, Lan Li, Hongtao Lin. Passive devices at 2 µm wavelength on 200 mm CMOS-compatible silicon photonics platform [Invited][J]. Chinese Optics Letters, 2021, 19(7): 071301
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