According to research conducted by T. W. Hughes, M. Minkov, Y. Shi, and S. Fan, artificial neural networks can be directly trained on an optical chip.
The research, titled “Training of photonic neural networks through in situ backpropagation and gradient measurement” demonstrates that an optical circuit has all the capabilities to perform the critical functions of an electronics-based artificial neural network. This makes performing complex tasks like speech or image recognition less expensive, faster and more energy efficient.
According to research team leader, Shanhui Fan of Stanford University “Using an optical chip to perform neural network computations more efficiently than is possible with digital computers could allow more complex problems to be solved”.
During the research, the training step on optical ANNs was performed using a traditional digital computer. The final settings were then imported into the optical circuit. But, according to Optica (the Optical Society journal for high impact research at Stanford),. there is a more direct method for training these networks. This involves making use of an optical analog within the ‘backpropagation’ algorithm.
Tyler W. Hughes, the first author of the research paper, states that “using a physical device rather than a computer model for training makes the process more accurate”. He also mentions that “because the training step is a very computationally expensive part of the implementation of the neural network, performing this step optically is key to improving the computational efficiency, speed and power consumption of artificial networks.”
Neural network processing is usually performed with the help of a traditional computer. But now, for neural network computing, researchers are interested in Optics-based devices as computations performed on these devices use much less energy compared to electronic devices.
In New York researchers designed an optical chip that imitates the way, conventional computers train neural networks. This then provides a way of implementing an all-optical neural network.
According to Hughes, the ANN is like a black box with a number of knobs. During the training stage, each knob is turned ever so slightly so the system can be tested to see how the algorithm’s performance changes. He says, “Our method not only helps predict which direction to turn the knobs but also how much you should turn each knob to get you closer to the desired performance”.
How does the new training protocol work?
This new training method uses optical circuits which have tunable beam splitters. You can adjust these spitters by altering the settings of optical phase shifters.
First, you feed a laser which is encoded with information that needs to be processed through the optical circuit. Once the laser exits the device, the difference against the expected outcome is calculated. This information that is collected then generates a new light signal through the optical network in the opposite direction.
Researchers also showed that neural network performance changes with respect to each beam splitter’s setting. You can also change the phase shifter settings based on this information. The whole process is repeated until the desired outcome is produced by the neural network.
This training technique has been further tested by researchers using optical simulations. In these tests, the optical implementation performed similarly to a conventional computer.
The researchers are planning to further optimize the system in order to come out with a practical application using a neural network.