ELight: Toward Efficient and Aging-Resilient Photonic In-Memory Neurocomputing

Optical phase change material (PCM) has emerged promising to enable photonic in-memory neurocomputing in optical neural network (ONN) designs. However, massive photonic tensor core (PTC) reuse is required to implement large matrix multiplication due to the limited single-core scale. The resultant large number of PCM writes during inference incurs serious dynamic energy costs and overwhelms the fragile PCM with limited write endurance, causing the severe aging issue. Moreover, the aged PCM would distort the stored value and significantly degrade the reliability of PTC. In this work, we propose a holistic solution,
ELight
, to tackle both the aging issue and the post-aging reliability issue, where a proactive aging-aware optimization framework minimizes the overall PCM write cost and a post-aging tolerance scheme overcomes the effect of aged PCM. Specifically, in the aging-aware optimization part, we propose write-aware training to encourage the similarity among weight blocks and combine it with a post-training optimization technique to reduce programming efforts by eliminating redundant writes. Next, an efficient groupwise row-based weight-PTC remapping scheme is introduced to tolerate the reprogrammability degradation due to the aged PCM. Experiments show that
ELight
can achieve over
$20 \times $
reductions in the total number of write operations and dynamic energy cost with comparable accuracy. Moreover,
ELight
can guarantee significant accuracy recovery under the aged PCM within photonic memories. With our
ELight
, photonic in-memory neurocomputing will step forward toward practical applications in machine learning with order-of-magnitude longer lifetime, lower programming energy cost, and significant resilience against PCM aging effects.