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Xie YW, Hong SH, Yan H, Zhang CP, Zhang L et al. Low-loss chip-scale programmable silicon photonic processor. Opto-Electron Adv 6, 220030 (2023). doi: 10.29026/oea.2023.220030
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Abstract
Chip-scale programmable optical signal processors are often used to flexibly manipulate the optical signals for satisfying the demands in various applications, such as lidar, radar, and artificial intelligence. Silicon photonics has unique advantages of ultra-high integration density as well as CMOS compatibility, and thus makes it possible to develop large-scale programmable optical signal processors. The challenge is the high silicon waveguides propagation losses and the high calibration complexity for all tuning elements due to the random phase errors. In this paper, we propose and demonstrate a programmable silicon photonic processor for the first time by introducing low-loss multimode photonic waveguide spirals and low-random-phase-error Mach-Zehnder switches. The present chip-scale programmable silicon photonic processor comprises a 1×4 variable power splitter based on cascaded Mach-Zehnder couplers (MZCs), four Ge/Si photodetectors, four channels of thermally-tunable optical delaylines. Each channel consists of a continuously-tuning phase shifter based on a waveguide spiral with a micro-heater and a digitally-tuning delayline realized with cascaded waveguide-spiral delaylines and MZSs for 5.68 ps time-delay step. Particularly, these waveguide spirals used here are designed to be as wide as 2 µm, enabling an ultralow propagation loss of 0.28 dB/cm. Meanwhile, these MZCs and MZSs are designed with 2-µm-wide arm waveguides, and thus the random phase errors in the MZC/MZS arms are negligible, in which case the calibration for these MZSs/MZCs becomes easy and furthermore the power consumption for compensating the phase errors can be reduced greatly. Finally, this programmable silicon photonic processor is demonstrated successfully to verify a number of distinctively different functionalities, including tunable time-delay, microwave photonic beamforming, arbitrary optical signal filtering, and arbitrary waveform generation. -
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References
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Xie YW, Hong SH, Yan H, Zhang CP, Zhang L et al. Low-loss chip-scale programmable silicon photonic processor. Opto-Electron Adv 6, 220030 (2023). doi: 10.29026/oea.2023.220030
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Figure 1.
Schematic of an on-chip optical signal processor.
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Figure 2.
Schematic configuration of the present 2×2 thermo-optic MZS incorporating widened phase-shifter waveguides. (a) Top view. (b) Cross section of the MZS arm with a micro-heater.
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Figure 3.
(a) 3D-view of the low-loss waveguide spiral. (b) Calculated scattering loss as the waveguide core width increases at the wavelength of 1550 nm.
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Figure 4.
(a) Layout of the chip and the basic building blocks. (b) A photo of the fabricated chip. (c) A photo of the packaged chip.
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Figure 5.
Measurement results of a representative MZS device with 2-µm-wide phase shifters. (a) Optical microscope image. (b) Zoom-in picture of the heater on the MZS’s arm. (c) Transmissions at the cross and bar-ports for the central wavelength as a function of the heating power. (d) Transmission spectra for the off-state and the on-state. (e) The switching time of the MZS. (f) Normalized power consumption for compensating the phase error in MZSs for different channels.
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Figure 6.
(a) A single channel of 5-bit delay line. (b) Zoom in picture of the waveguide spiral for stage 5. (c) measured optical output waveforms of the 5-bit digitally-tuning delay line as the time delay varies from 5.68 ps (i.e., ∆t) to 90.88 ps (i.e., 24∆t).
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Figure 7.
(a) Waveguide spiral carpeted with a heater for continuously-tuning delay line. (b) Time-domain impulse responses for the cases with different heating powers for one heating spiral. (c) The delay time as the heating power increases for one heating spiral.
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Figure 8.
Ge/Si waveguide photodetector. (a) Chip picture. (b) Dark current. (c) Bandwidth. (d) Eye diagram at 40 Gbps at each channel.
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Figure 9.
(a) Realization of 4-channel OTTDL based photonic beamformer (Input from port I2, and output from four PDs). Here light goes along the blue paths, and there is no light in the grey paths. (b) Experimental setup. Measured phase response (c) and group delay response (d) of the microwave signals for all 32 delay states of the first channel of delay line. The delay variations (e) and the measured S21 (f) for all four channels at 18 GHz. PC: polarization controller, Mod: modulator, VNA: vector network analyzer.
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Figure 10.
Calculated beam patterns corresponding to the 11 steering angles enabled by the digitally-tuning delay line (f=18 GHz). (a) Four channels have uniform amplitudes. (b) Four channels have the optimized amplitudes.
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Figure 11.
(a) Calculated beam patterns corresponding to the 21 steering angles enabled with the fine-tuning delay. (b) Beam steering angle and extinction ratio as the delay difference between adjacent emitters increases from 0 to 56.8 ps.
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Figure 12.
(a) Calculated beam patterns when operating at different RF frequencies. (b) Calculated steering angle and extinction ratio as the steering angle varies. Here the frequency is from 12 GHz to 21 GHz.
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Figure 13.
(a) Principle of arbitrary filtering operation (Input from port I4, and output from port O3). Measurements of filter spectral responses: (b–d) demonstrations of the FSR tunability for the filter. (e) Demonstrations of passband shaping for the filter. (f) Notch-depth variation due to thermal crosstalk from neighboring heaters (e.g., V21 and V22 in channel #2). (g) Notch-depth variation due to thermal crosstalk from the neighboring channel (e.g., channel #2) when all the MZSs in channel #2 are turned on with the power of 40 mW. Inset table: filter configuration for achieving the desired FSRs by setting the power splitting ratios of the MZSs in channels #3 and #4.
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Figure 14.
(a) Arbitrary waveform generation configuration. Here light inputs from port I1, goes through channel #1, and finally outputs from port O1. The measured waveforms (see the blue solid line), which are normalized to the peak intensity. (b) Square waveform. (c) Gaussian waveform. (d) Rising ramp. (e) Falling ramp. The red dashed lines outline the envelopes of the ideal waveforms. Inset table shows the intensities (A1, A2, A3, and A4) for the four taps.
