Tera-sample-per-second Real-time Waveform Digitizer

We demonstrate a real-time transient waveform digitizer with a record 1 TSa/s (TeraSample/sec) sampling rate. This is accomplished by using a photonic time stretch preprocessor which slows down the electrical waveform before it is captured by an electronic digitizer. * Present address: College of Optics and Photonics: CREOL & FPCE, University of Central Florida, 4000 Central Florida Blvd., Orlando, FL 32816 2 Tera-sample-per-second Real-time Waveform Digitizer Digital processing and storage of information is a ubiquitous function encountered in virtually every science and engineering discipline. Since signals originating in the physical world are analog, Analog-to-Digital Converters (ADC), otherwise known as digitizers, play a central role. The most challenging signals to digitize are ultrafast transients that are non-repetitive. These waveforms arise in a number of applications including (i) diagnostic tool in particle accelerators that probe fundamental building blocks of nature; (ii) X-ray free electron lasers; (iii) study of EMP (electromagnetic pulse) weapons 2 . Digitizers with real-time capability are required because the nonrepetitive nature of these signals renders traditional sampling oscilloscopes useless. The sampling rate of conventional electronic ADCs, such as ones used in high speed instruments, is limited by the speed of the electronic circuitry, and in the case where the interleaving architecture is used, by the mismatches in the multi-channel ADC array. While the performance of electronic ADCs continues to improve, the sampling rate of a state-of-the-art system is currently about 20 GSa/s with ~5 ENOB (Effective Number Of Bits). Achieving TSa/s performance is clearly beyond the reach of conventional approaches. One potential solution to overcome the electronic bottleneck is to use photonic pre-processing. In particular, the photonic time stretch approach has proven to be an effective way to extend the sampling rate and the bandwidth. Here, the high speed transient waveform is first slowed down and then captured by a conventional electronic digitizer. In this paper, we demonstrate such a system that achieves a sampling rate of 1 TSa/s. This system consists of a 50x time stretch pre-processor and a 20 GSa/s electronic 3 digitizer. To the best of our knowledge, this is the first time that the real-time digitization at 1 TSa/s has been achieved. The time stretch preprocessing, shown in Fig. 1, consists of three steps: time-towavelength transformation, wavelength domain processing, and wavelength-to-time transformation. Time-to-wavelength transformation occurs when the electrical signal modulates the intensity of a linearly-chirped optical pulse. At the output of modulator, the input signal’s time scale is linearly mapped onto the optical wavelength. The second and third steps occur simultaneously when the waveform is broadened as it travels through the second dispersive optical medium and is subsequently photodetected. The major obstacle to achieving a stretch factor of 50x is to overcome the frequency fading that is associated with dispersive propagation. This phenomenon has been described in details elsewhere. Briefly, it occurs due to the dispersion-induced interference between the two modulation-sidebands. We overcome this problem using the recently proposed phase diversity technique, where two stretched waveforms with complementary fading characteristics are realized and combined to eliminate the bandwidth limitation. Another practical concern is the large loss of dispersive fiber required to achieve such a large stretch factor. This problem is mitigated by the judicious use of optical amplification in such a manner as to optimize the overall signal to noise ratio while avoiding degradation from optical nonlinearity.