Dan Sadot

Tunable optical filters for dense WDM networks

WDM is currently taking over as the leading technology in point-to-point transmission links. For optical implementation of WDM networks, logical functionalities such as wavelength (channel) selection should be carried out in the wavelength domain; thus, the development of dynamic optical devices is required. One key device is a tunable optical filter. Important features of such a filter include low insertion loss, narrow bandwidth, high sidelobe suppression, large dynamic range, fast tuning speed, a simple control mechanism, small size, and cost effectiveness. Here, an extensive overview of the different technologies used to produce tunable optical filters is presented. Among them, fiber filters such as fiber Bragg gratings and fiber Fabry Perot are the most commercialized, yet inherently limited in their dynamic speeds. For high demanding dynamics, micro-machined and acousto-optic filters can offer a good solution for microsecond tuning speeds. Faster tunable devices, in nanosecond tuning speeds, might emerge out of microresonators, electrooptic filters, and active DBR filters.

Free-space optical cross-connect switch by use of electroholography

An electrically controlled holographic switch is proposed as a building block for a free-space optical interconnection network. The switch is based on the voltage-controlled photorefractive effect in KLTN crystals at the paraelectric phase. It is built of electrically controlled Bragg gratings stored in the volume of the crystal. A compact switch that connects four high-speed fiber-optic communication channels with high efficiency is demonstrated experimentally. The switch performance is investigated and optimized. This switch is extremely attractive for cascaded switching arrays such as those found in multistage interconnect networks.

Implementation of STARNET: a WDM computer communications network

STARNET is a broadband backbone optical wavelength-division multiplexing (WDM) local area network (LAN). Based on a physical passive star topology, STARNET offers all users two logical subnetworks: a high-speed reconfigurable packet-switched data subnetwork and a moderate-speed fix-tuned packet-switched control subnetwork. Thus, STARNET supports traffic with a wide range of speed and continuity characteristics. We report the analysis and implementation of an entire STARNET two-node network, from the optical to the computer layer, at the Optical Communications Research Laboratory (OCRL) of Stanford University. To implement the two logical subnetworks, we designed and implemented two different techniques: combined modulation and multichannel subcarrier multiplexing (MSCM). OCRL has already demonstrated several combined modulation techniques such as phase shift-keyed and amplitude shift-keyed (PSK/ASK), and differential phase shift-keyed and amplitude shift-keyed (DPSK/ASK), yielding combined ASK/DPSK modulation receiver sensitivities better than -32 dBm. OCRL has designed and implemented a high-speed high-performance packet-switched STARNET computer interface which enables high-throughput transfer to/from host computer, low latency switching, traffic prioritization, and capability of multicasting and broadcasting. With this interface board, OCRL has achieved average transmit and receive throughputs of 685 Mb/s and 571 Mb/s, respectively, out of the 800 Mb/s theoretical maximum of the host computer bus. The incurred packet latency due to the interface for a specified multihop network configuration has been simulated to be 24 /spl mu/s. Using simulation and experimental results, it is shown that STARNET is highly suitable for high-speed multimedia network applications.

Forecasting optical turbulence strength on the basis of macroscale meteorology and aerosols: models and validation

Although optical turbulence is usually modeled with micrometeorology, it is shown here that this can be done successfully too with macrometeorology using meteorological parameters measured with standard weather stations and predicted in standard weather forecasts. This makes it possible to predict C2n according to weather forecast. Two experimentally derived models are developed-one for practical use and the other for scientific understanding. Correlation of prediction with measurement is on the order of 90% or more, over large dynamic ranges of meteorological parameters. One interesting aspect of these measurements is the statistical evidence that scintillations are affected by aerosols, particularly under conditions of high total aerosol cross-sectional area. Various explanations for effects of aerosols on C2n and its measurement are suggested. In addition, validity of the models was examined, and experimental comparisons in two very different climates and surface conditions are presented. High correlation is found in both cases between prediction and measurement.

Single channel 112Gbit/sec PAM4 at 56Gbaud with digital signal processing for data centers applications.

112Gbit/sec DSP-based single channel PAM4 at 56Gbaud that fits into QSFP is experimentally demonstrated. The DSP enables use of mature 25G optoelectronics for 2-10km datacenter intra-connections, and 8Tbit/sec over 80km interconnections between data centers.

Imaging through the atmosphere: practical instrumentation-based theory and verification of aerosol modulation transfer function

Recent experimental measurements of overall atmospheric modulation transfer function (MTF) indicate a significant difference between overall atmospheric and turbulence MTF’s except often at midday, when turbulence is strong. We suggest a physical explanation for these results that essentially relates to a practical instrumentation-based atmospheric aerosol MTF that is a modification of the classical aerosol MTF theory. Based on radiative transfer theory, this practical approach takes into account the effect of finite field of view, finite dynamic sensitivity, and finite spatial bandwidth of every existing imaging system. These generally limit the scattering angles of received light to values far less than the diffraction limit for aerosols, thereby decreasing blur radius and increasing spatial frequency bandwidth. This can explain the broadening of the aerosol MTF from that theoretically expected. We discuss the asymptote value that the measured aerosol MTF approaches at high spatial frequencies, which is significantly higher than the theoretical prediction of turbid medium transmittance. An important conclusion that we derive is that the aerosol MTF is often the dominant part in the actual overall atmospheric MTF. In addition, there seems to be an inescapable trade-off between image resolution and image irradiance. The system designer must choose between imaging of faint and bright objects at the expense of image quality or imaging of either faint or bright objects with improved image quality. The concepts here are basic to all long-range imaging through the atmosphere.

Optical switching speed requirements for terabit/second packet over WDM networks

Optical dynamics requirement for packet-over-WDM networks is analyzed. Optimization among optical switching speed, global resource availability, and local queuing considerations is performed, yielding multiterabit/second throughput capability by employing submicrosecond switching technology.

Nonlinear Effects Mitigation in Coherent Optical OFDM System in Presence of High Peak Power

The nonlinear effects of Mach-Zehnder modulator (MZM) and self phase modulation (SPM), in optical orthogonal frequency division multiplexing (OFDM) under the presence of high peak power is studied. A full coherent optical communication system is presented and analyzed. Standard method to reduce the peak to average power ratio (PAPR) values combined with improved technique to mitigate the nonlinear effect, by means of optimized digital pre-distortion, is analyzed and a full performance analysis is presented.

A novel self-heterodyne method for combined temporal and spectral high-resolution measurement of wavelength transients in tunable lasers

This letter presents a new experimental optical delayed self-heterodyne method to characterize tunable laser source instantaneous wavelength transients at accuracy of electronic spectrum.

CAUSES OF ATMOSPHERIC BLUR : COMMENT ON ATMOSPHERIC SCATTERING EFFECT ON SPATIAL RESOLUTION OF IMAGING SYSTEMS

A paper by Ben Dor [J. Opt. Soc. Am. A14, 1329 (1997)] concludes that the blur we measured in our experiments was not atmospherically scattered light and that our theoretical model is incorrect because it violates the rules of linearity. Their work is based in part on “lack of raw data” in one of our experimental papers [J. Opt. Soc. Am A12, 970 (1995)]. We present here the raw data measured in the experiments in question, which show clearly the measured atmospherically scattered light. Similar raw data has also been published elsewhere regarding other experiments. We also clarify some rules of linear systems that justify our conceptual approach, which is shown to be similar to that of turbulence modulation transfer function. A review of several dozen experiments and analyses by other investigators all over the world that directly contradict the Ben Dor et al. results and conclusions is presented. The well-known significance of aerosol blur in imaging through the atmosphere from satellites is discussed, and pictorial examples of satellite imagery are shown for different atmospheric optical depths. It is noted that atmospheric point-spread-function analyses in the remote-sensing literature generally neglect turbulence blur altogether and deal with aerosol blur only, which is often called the adjacency effect, and that such phenomena are well supported by many different types of experiments and many different Monte Carlo simulations for many different aerosol and instrumentation parameter situations. The Monte Carlo simulation results of Ben Dor et al. are shown also to contradict everyday reality such as the solar aureole. This wealth of literature by others strongly contradicts the results of Ben Dor et al. and confirms our conclusion that forward scatter of light by aerosols is indeed a significant source of blur in imaging through the atmosphere, especially if atmospheric optical depth is on the order of unity or more. This can be confirmed, too, by any observer looking through binoculars at the moon and surrounding moonlight even on a clear night. A broad system engineering approach involving both aerosol and turbulence blur is called for.