On-chip photonic-neural-network processors have potential benefits in both speed and energy efficiency but have not yet reached the scale to compete with electronic processors. The dominant paradigm is to build integrated-photonic processors using relatively bulky discrete components connected by single-mode waveguides. A far more compact alternative is to avoid explicitly defining any components and instead sculpt the continuous substrate of the photonic processor to directly perform the computation using waves freely propagating in two dimensions. In this talk, I will present our recent work [1] on experimentally realizing this approach with a device whose refractive index as a function of space, n(x,z), can be rapidly reprogrammed. This device combines photoconductive gain with the electro-optic effect in a lithium niobate slab waveguide. Using this device, we performed neural-network inference with up to 49-dimensional input vectors in a single pass.
[1]: T. Onodera*, M.M. Stein*, et al. arXiv:2402.17750 (2024)
On-chip photonic-neural-network processors promise benefits in both speed and energy efficiency but have not yet reached the scale to compete with electronic processors. The dominant paradigm is to build integrated-photonic processors using discrete components connected by single-mode waveguides. A far more compact alternative is to avoid discrete components and instead sculpt a complex and continuous microphotonic medium in which computations are performed by multimode waves controllably propagating in two dimensions. We show our realization of this approach with a device whose refractive index as a function of space can be rapidly reprogrammed. We demonstrate optical computations much larger and more error-resilient than previous photonic chips relying on discrete components. We argue that beyond photonic-neural-network processors, devices with such arbitrarily programmable index distributions enable the realization of a wide range of photonic functionality.
We report on the realization of an on-chip waveguide platform capable of creating arbitrary two-dimensional refractive index profiles in situ and in real-time. The device exhibits complex multimode dynamics which we train to perform machine learning. We tune the refractive index profile in situ using a backpropagation algorithm to perform audio and image classification with up to 50-dimensional inputs. The two-dimensional programmability is realized by sandwiching a photoconductive film and a lithium niobate slab waveguide between two flat electrodes. While applying voltage between the electrodes, we program the effective index of the waveguide by projecting different light patterns onto the photoconductive film. The effective index increases by 10^-3 in illuminated regions via the electro-optic effect, free from any measurable memory effects or cyclic degradation. In conclusion, we developed a photonics platform with versatile spatial programmability that opens new avenues for optical computing and photonic inverse-design.
We demonstrate both second harmonic generation (with a normalized efficiency of 0.20 %W−1 cm−2 ) and, to our knowledge, the first degenerate χ (2) optical parametric amplifier (with an estimated normalized gain of 0.6 dBW−1/2 cm−1 ) using silicon-on-insulator waveguides fabricated in a CMOS-compatible commercial foundry.
KEYWORDS: Sum-frequency generation, Frequency conversion, Current controlled current source, Optical parametric oscillators, Mid-IR, Frequency combs, Femtosecond phenomena, Nonlinear dynamics, Harmonic generation, Femtosecond frequency combs
Half-harmonic generation is the reverse of second harmonic generation that happens in optical parametric oscillators (OPOs) at degeneracy. It is an intrinsically phase-locked down-conversion process, which can be used to efficiently transfer well-developed near-IR frequency combs to the mid-IR.
We overview recent experimental progress in cascading multiple stages of half-harmonic generation of femtosecond frequency combs starting from a 1-μm pump. We have achieved stable operation with efficiencies as high as ~64%, pulses as short as three optical cycles at 4 μm, and output powers as high as 2.6 W at 2 μm. Our recent numerical and analytical studies of nonlinear dynamics and different operation regimes of femtosecond OPOs indicate a path toward achieving even higher efficiencies and shorter pulses.
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