Abstract
An analog comparator is one of the core units in all-optical analog-to-digital conversion (AO-ADC) systems, which digitizes different amplitude levels into two levels of logical ‘1’ or ‘0’ by comparing with a defined decision threshold. Although various outstanding photonic ADC approaches have been reported, almost all of them necessitate an electrical comparator to carry out this binarization. The use of an electrical comparator is in contradiction to the aim of developing all-optical devices. In this work, we propose a new concept of an all-optical analog comparator and numerically demonstrate an implementation based on a quarter-wavelength-shifted distributed feedback laser diode (QWS DFB-LD) with multiple quantum well (MQW) structures. Our results show that the all-optical comparator is very well suited for true AO-ADCs, enabling the whole digital conversion from an analog optical signal (continuous-time signal or discrete pulse signal) to a binary representation totally in the optical domain. In particular, this all-optical analog comparator possesses a low threshold power (several mW), high extinction ratio (up to 40 dB), fast operation rate (of the order of tens of Gb/s) and a step-like transfer function.
Highlights
An analog comparator is one of the core units in all-optical analog-to-digital conversion (AO-ADC) systems, which digitizes different amplitude levels into two levels of logical ‘1’ or ‘0’ by comparing with a defined decision threshold
We propose a new concept of an all-optical analog comparator and numerically demonstrate an implementation based on a quarter-wavelength-shifted distributed feedback laser diode (QWS DFB-LD) with multiple quantum well (MQW) structures
Our results show that the all-optical comparator is very well suited for true AO-ADCs, enabling the whole digital conversion from an analog optical signal to a binary representation totally in the optical domain
Summary
We observed the carrier density distributions along the chip length when our comparator works at different input power levels around the transition region between the logical ‘1’ and logical ‘0’ region. Input power reaches above 1.640 mW, the laser will abruptly switch into the non-lasing state [logical ‘0’ region] At this time, we can see that the associated carrier density becomes extremely non-uniform. The carrier density monotonically decreases along the laser length and the laser cannot remain in a lasing state This demonstrates an extremely vertical transition region – the output ‘jumps’ between ‘0’ and ‘1’ when the input changes by as little as 0.001 mW(=1.640 − 1.639 mW). 3 all-optical analog comparators (#1, #2 and #3) are needed to set 3 different threshold levels, which divide the whole input power scope to 4 equal parts. By controlling the power of the CW output and tuning the variable optical attenuators (VOA), one can accurately set the three different biases (#1, #2 and #3) at 0.41 (=1 .64 × 1/4) mW, 0.82 (=1 .64 × 2/4) mW and 1.23 (=1 .64 × 3/4) mW, respectively
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