A.S. Bhushan

Photonic time stretch and its application to analog-to-digital conversion

We demonstrate a new concept for analog-to-digital (A/D) conversion based on photonic time stretch. The analog electrical signal is intensity modulated on a chirp optical waveform generated by dispersing an ultrashort pulse. The modulated chirped waveform is dispersed in an optical fiber, leading to the stretching of its envelope. We have derived analytical expressions for the stretch factor and the resolution of the system. An analog-to-digital converter (ADC) consisting of the photonic time-stretch preprocessor and a 1-Gsample/s electronic ADC is demonstrated. This technique is promising for A/D conversion of ultrafast signals and, hence, for realization of the digital receiver.

Fast wavelength-hopping time-spreading encoding/decoding for optical CDMA

We demonstrate a new technique for implementation of fast wavelength-hopping incoherent optical code-division multiple-access (CDMA). The output pulse from a mode locked laser is spectrally broadened through supercontinuum generation. This then encoded into fast wavelength-hopped time-spread waveforms through a wavelength-selective time-delay device. At the receiver, matched-filter decoding is used to recover data. We present a 1-Gb/s digital transmission experiment through a 15-km dispersion-shifted (DS) single-mode fiber link. This technique avoids the need for a fast wavelength tunable optical source.

130-GSa/s photonic analog-to-digital converter with time stretch preprocessor

In this letter, we demonstrate a photonic analog-to-digital converter with time stretch (TS) preprocessor that has a sampling rate of 130 GSa/s. The system has a signal-to-noise ratio (SNR) exceeding seven effective number of bits over a 1-GHz bandwidth at 18 GHz. We present an analytical model of the SNR in the TS preprocessor which shows that over the specified bandwidth, the SNR is limited by the amplified spontaneous emission beat noise.

Nondispersive wavelength-division sampling

We propose and demonstrate a new wavelength-division-sampling technique with high temporal resolution. A discrete-time true-time delay generates multiwavelength near-transform-limited pulses from a supercontinuum source. Pulses sample the analog signal in an electro-optic modulator and are subsequently demultiplexed in a wavelength-division-multiplexing filter. A 100-Gsample/s experimental demonstration of this concept is presented.

30 Gsample/s 4-bit time-stretch analog-to-digital converter

Summary form only given.Photonic time stretch preprocessing (TSP) has been proposed as a means to enhance the performance of electronic ADCs. This concept is based on the premise that if the analog signal can be stretched in time free of distortion, then the effective sampling rate and the input bandwidth of the ADC is increased. The technique is best suited for time-limited analog signals, such as those encountered in pulsed radars. In this paper we demonstrate complete time-stretch ADC with 30 Gsample/s sampling rate, and 4 effective number of bits resolution realized over a 4 GHz bandwidth. The system consists of a photonic TSP and a commercially available electronic digitizer with 8 Gsample/s sampling rate and 1.5 GHz bandwidth.

Effect of Mach-Zehnder modulator's spectral response on the time-stretch analog-to-digital converter

Summary form only given. Photonic Time Stretch (TS) preprocessing is a promising technique for enhancing the performance of electronic Analog-to-Digital Converters (ADC). In the TS technique, broadband (10-30 nm) nearly-transform-limited pulses are dispersed through a length (L/sub 1/) of a single mode fiber (SMF-28) creating a linear chirp. The RF signal is then imposed onto the chirped carrier in a MZ intensity modulator biased at quadrature point. A second fiber (L/sub 2/) is then used to stretch the modulation envelope. A major difference between the TS and a conventional analog link is the broad bandwidth of the source in the TS system. Because the modulators transfer function depends on wavelength, this can distort RF signals. In this paper, we report, for the first time, the impact of the modulators wavelength dependence on the achievable dynamic range in a TS ADC system.

Time domain optical sensing

We have demonstrated a simple yet effective spectrometer. It uses chirped supercontinuum pulses as a broadband source and a sampling oscilloscope or A/D converter for spectral analysis. Distinct advantages for this system include the absence of an optical spectrometer, the broad spectral bandwidth available from the supercontinuum source, and the possibility of real time spectral analysis. A passively mode locked erbium doped fiber laser with a pulse compression stage was used as the supercontinuum source.

12 Gsamples/s real-time analog-to-digital converter based on wavelength division sampling

In this paper we present a 12 Gsample/s continuous-time wavelength division ADC system. The system uses the recirculating photonic filter true-time-delay (TTD) for dispersion-free chirp generation. It can digitize continuous-time analog signals at 12 Gsample/s using three wavelength channels at 4Gsample/s and three real-time electronic digitizers operating at 4Gsample/s.

Time-stretched analog-to-digital conversion (TSADC)

Summary form only given. The ability to perform ultrafast A/D and D/A conversion is critical for high performance communication and radar systems. Challenges with traditional approaches have led us to propose the concept of time-stretch preprocessing as a possible solution to the ultrafast A/D conversion. In Nyquist type A/D converters (ADCs) time stretching increases the effective input bandwidth and sampling rate of the ADC. In /spl Sigma/-/spl Delta/ type converters, it has the additional benefit of reducing the quantization noise within the signal bandwidth. In this paper, we will present the experimental data and theoretical analysis for time-manipulation of electrical signal using optoelectronic techniques. We also introduce a new discrete-time rendition of this technique with near transform-limited temporal resolution. Analytical and numerical calculation of the fundamental limits of this approach will be presented. We will conclude by presenting an overview of the COAST (Consortium for Optical A/D System Technology), a new DARPA funded UCLA-lead consortium consisting of 8 Universities, industry and national laboratories dedicated to research in optically assisted A/D conversion.

130 Gsample/s photonic analog to digital converter

Digital processing of the received signals in radar systems leads to higher performance and the ability to rapidly reconfigure such systems. Typically, the analog-to-digital converter (ADC) is the major bottleneck in realizing the digital receiver. Time stretch (TS) preprocessing is a new photonic based approach that can potentially boost the capability of the available electronic ADCs (Coppinger et al, 1999; Bhushan et al, 2000). In this approach, the analog signal is imposed on a chirped optical carrier using an electro-optic modulator. This time to wavelength mapped signal is then dispersed and hence stretched in time prior to digitization by the electronic ADC. Time stretching increases the effective sampling rate and input bandwidth of the ADC. We demonstrate a photonic ADC with a time stretch preprocessor. The system has a sampling rate of 130 GSa/s and a signal-to-noise ratio exceeding 7 effective number of bits.