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An optical neural network (ONN) is a physical implementation of an artificial neural network with optical components.

Biological neural networks are electrochemical, while optical neural networks use electromagnetic waves. Optical interfaces to biological neural networks can be created with optogenetics. Biological neural networks use multiple mechanisms to change the state of the neurons. These include short term and long term synaptic plasticity. Synaptic plasticity is among the electrophysiological phenomena used to control the efficiency of synaptic transmission, long term for learning and memory, and short term for transient changes in synaptic transmission efficiency. Implementing this ideally requires advanced photonic materials. Desirable properties in photonic materials for optical neural networks include the ability to change their efficiency of transmitting light, based on the intensity of incoming light.

Early ONNs used a photorefractive volume hologram to interconnect arrays of input neurons to arrays of output with synaptic weights in proportion to the multiplexed hologram's strength. Volume holograms were further multiplexed using spectral hole burning to add one dimension of wavelength to space to achieve four dimensional interconnects of two dimensional arrays of neural inputs and outputs. This led to research on alternative methods using the strength of the optical interconnect to implement neuronal communications.

Silicon photonics offers superior speed, but lacks the massive parallelism that free space optics can deliver.

Free space optics offers substantial parallelism. One implementation used phase masks for a handwritten digit classifier. Light passing through stacked 3D printed phase masks can be read by a photodetector array of ten detectors, each representing a digit class (0 through 9). Although this network can achieve terahertz speeds, it lacks flexibility, as the phase masks are fabricated for a specific task and cannot be trained. An alternative method employs a 4F convolutional system. This system uses two lenses to execute the convolution Fourier transforms, enabling passive conversion into the Fourier domain without power consumption or latency. The convolution kernels are task specific fixed phase masks. Another technique used kernel tiling to access the parallelism of the 4F system while using a digital micromirror device (DMD) instead of a phase mask. Kernels can be entered into the 4F system and conduct inference. Typical neural networks are not designed for the 4F systems, using a lower resolution and more channels in their feature maps.

The Programmable Optical Array/Analogic Computer (POAC) was implemented in 2000 based on a modified joint Fourier transform correlator (JTC) and bacteriorhodopsin (BR) as a holographic optical memory. The system offered full parallelism, large array size and the speed of light in an optical convolutional neural network. POAC is a general purpose and programmable array computer that has a wide range of applications including: image processing, pattern recognition, target tracking, real time video processing, document security, and optical switching.

Taichi is a hybrid ONN that combines the power efficiency and parallelism of optical diffraction and the configurability of optical interference. Taichi offers 13.96 million parameters. Taichi avoids the high error rates that afflict deep (multi layer) networks by combining clusters of fewer layer diffractive units with arrays of interferometers for reconfigurable computation. Its encoding protocol divides large network models into sub models that can be distributed across multiple chiplets in parallel. Taichi achieved 91.89% accuracy in tests with the Omniglot database. It was also used to generate music Bach and generate images the styles of Van Gogh and Munch. and the Kohonen self organizing map with liquid crystal spatial light modulators. Neuromorphic engineering can be used to create neuromorphic photonic systems. Typically, these systems encode information in the networks using spikes, mimicking the functionality of spiking neural networks in optical and photonic hardware. Other photonic devices have demonstrated neuromorphic functionalities, including vertical cavity surface emitting lasers, integrated photonic modulators, optoelectronic systems based on superconducting Josephson junctions or systems based on resonant tunnelling diodes.