Fault Tolerance and Noise Immunity in Freespace Diffractive Optical Neural Networks

Free-space diffractive optical networks are a class of trainable optical media that are currently being explored as a novel hardware platform for neural engines. The training phase of such systems is usually performed in a computer and the learned weights are then transferred onto optical hardware ("ex-situ training"). Although this process of weight transfer has many practical advantages, it is often accompanied by performance degrading faults in the fabricated hardware. Being analog systems, these engines are also subject to performance degradation due to noises in the inputs and during optoelectronic conversion. Considering diffractive optical networks (DON) trained for image classification tasks on standard datasets, we numerically study the performance degradation arising out of weight faults and injected noises and methods to ameliorate these effects. Training regimens based on intentional fault and noise injection during the training phase are only found marginally successful at imparting fault tolerance or noise immunity. We propose an alternative training regimen using gradient based regularization terms in the training objective that are found to impart some degree of fault tolerance and noise immunity in comparison to injection based training regimen.

A Numerical Method for Pulse Propagation in Nonlinear Dispersive Optical Media

In this work, the pulse propagation in a nonlinear dispersive optical medium is numerically investigated. The finite difference time-domain scheme of third order and periodic boundary conditions are used to solve generalized nonlinear Schr¨odinger equation governing the propagation of the pulse. As a result a discrete system of ordinary differerential equations is obtained and solved numerically by fourth order Runge-Kutta algorithm. Varied input ultrashort laser pulses are used. Accurate results of the solutions are obtained and the comparison with other results is excellent.

Quantum Probes for the Characterization of Nonlinear Media

Active optical media leading to interaction Hamiltonians of the form H=λ˜(a+a†)ζ represent a crucial resource for quantum optical technology. In this paper, we address the characterization of those nonlinear media using quantum probes, as opposed to semiclassical ones. In particular, we investigate how squeezed probes may improve individual and joint estimation of the nonlinear coupling λ˜ and of the nonlinearity order ζ. Upon using tools from quantum estimation, we show that: (i) the two parameters are compatible, i.e., the may be jointly estimated without additional quantum noise; (ii) the use of squeezed probes improves precision at fixed overall energy of the probe; (iii) for low energy probes, squeezed vacuum represent the most convenient choice, whereas for increasing energy an optimal squeezing fraction may be determined; (iv) using optimized quantum probes, the scaling of the corresponding precision with energy improves, both for individual and joint estimation of the two parameters, compared to semiclassical coherent probes. We conclude that quantum probes represent a resource to enhance precision in the characterization of nonlinear media, and foresee potential applications with current technology.

Results of cataract surgery in children with retinoblastoma

Purpose. To present results of cataract surgery in children with retinoblastoma. Material and methods. Within the period from 2012 to 2020, cataract surgery was performed in 21 children (22 eyes) with retinoblastoma aged 28 to 155 months (average – 65 months). Tumors of group B occurred in 2 cases, group C – in 4, group D – in 14, group E – in 1. In 8 cases, cataract occurred in a single eye. Posterior capsular cataract was observed in 18 patients, total – in 3. In all cases, the presence of cataract impeded the control of the tumor. Before cataract surgery all children underwent systemic chemotherapy, some of them also underwent intra-arterial and/or intravitreal chemotherapy, external beam radiotherapy, Gamma Knife stereotactic radiosurgery and local treatment modalities (episcleral plaque brachytherapy, thermotherapy, cryotherapy). There were no signs of tumor progression in all cases at the time of cataract surgery. The mean interval between complete tumor regression and cataract surgery was 23 months (range 6–75 months). Results. Full transparency of optical media was achieved in all eyes after surgery. No intraoperative and postoperative complications occurred. The final visual acuity was improved in 12 eyes, in the rest visual acuity could not be determined due to age. There were no signs of tumor recurrence or metastases in mean follow-up after surgery of 29 months (range 3–60). Conclusion. Our experience of cataract surgery in children with complete regression of retinoblastoma was proved to be a safe and effective way to improve visual functions and achieve transparency of optical media. Key words: retinoblastoma, cataract surgery, ophtalmooncology, functional results

The Clinical Use of Vernier Acuity: Resolution of the Visual Cortex Is More Than Meets the Eye

Vernier acuity measures the ability to detect a misalignment or positional offset between visual stimuli, for example between two vertical lines when reading a vernier scale. It is considered a form of visual hyperacuity due to its detectable thresholds being considerably smaller than the diameter of a foveal cone receptor, which limits the spatial resolution of classical visual acuity. Vernier acuity relies heavily on cortical processing and is minimally affected by optical media factors, making it a useful indicator of cortical visual function. Vernier acuity can be measured, usually in seconds of arc, by freely available automated online tools as well as via analysis of steady state visual-evoked potentials, which allows measurement in non- or pre-verbal subjects such as infants. Although not routinely measured in clinical practice, vernier acuity is known to be reduced in amblyopia, glaucoma and retinitis pigmentosa, and has been explored as a measure of retinal or neural visual function in the presence of optical media opacities. Current clinical utility includes a home-based vernier acuity tool, preferential hyperacuity perimetry, which is used for screening for choroidal neovascularisation in age-related macular degeneration. This review will discuss the measurement of vernier acuity, provide a current understanding of its neuro-ophthalmic mechanisms, and finally explore its utility through a clinical lens, along with our recommendations for best practice.