Photonic Analog-to-Digital Converters (PADC)

Analog-to digital converters (ADC) play a very important role of converting the real life analog signals to digital signals in modern signal processing systems. They are widely used as principle parts for high resolution test instruments, radar systems and medical imaging systems etc. Originally, analog to-digital conversion is carried out by electronic means and involves two functions: 1) Sampling and 2) Quantization. Over the years, considerable strides have been made in the field of electronic ADCs (E-ADC). The overall performance of the E-ADCs is basically limited by the timing jitter of the clock source and power consumption at higher bandwidths and sampling rates. So, the developments in terms of operating speed and resolution of E-ADCs are relatively slower as compared to high speed communication systems, so they are viewed as “bottlenecks”. Thus, requirement of newer technology capable of digitizing inputted high bandwidth information at faster rates as compared to electronic ADCs, gave birth to the idea of photonic ADC (P-ADC). A number of P-ADCs strategies such as a) Photonic assisted ADCs, b) Photonic sampled ADCs, c) Photonic quantized ADCs and d) Photonic sampled and quantized ADCs have been proposed over the years by several research groups. In most of P-ADCs reported till date, either sampling, quantization or both operations are performed in optical domain. However, still most of P-ADCs technologies are presently in development phase and require considerable research efforts. One of the most promising P-ADC technology which has enormous commercial potential and viability is time-stretched photonic ADCs (TS-PADC). In TS-PADC, architecture, the electrical signal to be digitized if first imprinted on an optical pulse train, and the optical pulse is time-stretched by passing it through a dispersive medium (DM). The effective bandwidth of the signal to be sampled is reduced as much as the stretch factor, thus enabling sampling and quantization at lower bandwidths using E-ADCs. Single shot ADCs allow the capture of sporadic events with high bandwidth while continuous ADCs enables RF signals such as in radar systems- to be captured continuously. Another interesting PADC variant is under-sampled photonic analog-to-digital converter, which has the capability to digitize a small bandwidth around high frequency carrier. This under-sampled ADC, when it is deployed inside RADAR receivers, it has the ability to bypass following two tasks in same step: 1) Frequency down-conversion through multiple stages of mixing and 2) analog-to-digital conversion