GoKawiilDesign and fabrication
Photonic device and circuit-level simulations were performed using ANSYS Lumerical tools, whereas the integrated electronics followed a design flow using Cadence Virtuoso.
The finalized design was verified against the design rules using Cadence’s Physical Verification System (PVS). The demonstrated integrated monostatic FPA was fabricated using GlobalFoundries’ 45SPCLO 300-mm silicon photonics platform, which enables monolithic integration of photonic devices with 45-nm silicon-on-insulator RF CMOS electronics. Most of the photonic devices in the demonstrated FPA were based on the foundry’s standard process development kit but were further miniaturized to meet stringent footprint requirements, allowing the integration of 61,952 pixels. Several dies from different wafers were tested and no inoperative thermo-optic switches or dead pixels were observed; however, a mean of 42 out of 61,952 pixels showed noise greater than twice the mean over the entire array, leading to reduced SNR.
Loss budget
The FPA is supplied with FMCW light through 16 optical channels by means of a fibre ribbon. Each channel passes through a switch network before reaching its designated pixel row. This switch network consists of cascaded 1:2 thermo-optic switches, with the first five switching layers located outside the pixel array and an extra four layers integrated within each pixel block. The switch architecture introduces approximately 0.4 dB of loss per layer outside the array and 0.5 dB per layer within the pixel block, resulting in a total switching loss of around 4 dB. An extra insertion loss around 0.7 dB occurs at the V-grooves36, in which optical fibres are coupled to the chip.
A mean value of 426 μW per pixel is measured when 32 mW is delivered per optical channel. Nonlinear effects such as two-photon absorption and free-carrier absorption37 limit the power in silicon waveguides to approximately 16 mW. The extra 4-dB loss is attributed to a combination of two-photon absorption/free-carrier absorption and routing. Nonlinear losses can be eliminated by the use of advanced architectures combining efficient distribution of power in silicon waveguides with silicon nitride components and efficient routing.
Experimental set-up
The experimental set-up used for the measurement in Fig. 3 is presented in Fig. 4e. Frequency-modulated light at 1,310 nm is generated from a fixed frequency butterfly-packaged single-mode DFB laser (Innolume DFB-1310-PM-50-NL) with a linewidth of approximately 100 kHz. The infrared light from the seed laser is modulated using a silicon photonics IQ modulator. The modulated output then undergoes two-stage amplification using booster optical amplifiers (BOA-1310-50-PM-200mW).
To enable simultaneous operation of different sections of the 4D imaging sensor, the FMCW light is split into 16 fibres and coupled into the FPA through V-groove inputs. To ensure stability and optimal performance, all 16 polarization-maintaining optical fibres required for the full array must be precisely aligned and epoxied into the V-grooves38,39.
The lens systems used for imaging in Fig. 3 are the commercial lenses VS-3514H1-SWIR and VS-5018H1-SWIR from VS Technology with focal lengths f of 35 mm and 50 mm, respectively. They are mechanically attached to the imaging FPA by means of a 3D-printed adaptor, designed to place the lens at the proper working distance. The adaptor is mechanically screwed onto the carrier board. This configuration eliminates relative movements between the imaging FPA and the lens, therefore decreasing the sensitivity of the system to mechanical vibrations.
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