Search

The information capacity of a wavelength-division multiplexed optical fiber communication system is estimated in a model incorporating the nonlinear propagation effects of cross-phase modulation (XPM) and four-wave mixing (FWM). Physical models for propagation are used to calculate statistical properties of the conditional probability distribution relating input and output in a single WDM channel. A rigorous lower bound to the information capacity is found for coherent communication, in which both the signal phase and amplitude encode information. An approximate lower bound is computed for incoherent communication, in which only the amplitude is used. The results provide physical insight into the manner in which nonlinearities degrade the information capacity.

The exponential growth in the rate at which information can be communicated through an optical fibre is a key element in the 'information revolution'. However, as for all exponential growth laws, physical limits must be considered. The nonlinear nature of the propagation of light in optical fibre has made these limits difficult to elucidate. Here we use a key simplification to investigate the theoretical limits to the information capacity of an optical fibre arising from these nonlinearities. The success of our approach lies in relating the nonlinear channel to a linear channel with multiplicative noise, for which we are able to obtain analytical results. In fundamental distinction to linear channels with additive noise, the capacity of a nonlinear channel does not grow indefinitely with increasing signal power, but has a maximal value. The ideas presented here may have broader implications for other nonlinear information channels, such as those involved in sensory transduction in neurobiology. These have been often examined using additive noise linear channel models but, as we show here, nonlinearities can change the picture qualitatively.

In this paper, we discuss theoretical and experimental aspects of nonlinear pulse propagation in long-period fiber gratings. We derive nonlinear coupled mode equations for co-propagating core and cladding modes in a long-period fiber grating and show that the intensity dependent index can be used to tune the mode coupling, thereby creating a mechanism for pulseshaping and all-optical switching. Extensive numerical simulations, which describe the nonlinear pulse dynamics, are compared with experiments. We discuss the application of these nonlinear effects to signal processing in all-optical fiber communications.

We report the time-resolved circular dichroism induced by nonthermal distributions of carriers in GaAs quantum wells, separating completely, for the first time, the effects of screening and phase space filling. Thermalization of the initial excitation is strongly inhibited when carriers are laterally confined into zero dimensions by a large perpendicular magnetic field.

We compare the use of free-space electro-optic sampling (FSEOS) with photoconducting antennas to detect terahertz (THz) radiation in the range of 0.1–3 THz. For the same average THz power and low-frequency modulation, signal-to-noise ratio and sensitivity are better with antenna detection at frequencies smaller than 3 THz. When the modulation frequency is increased to more than 1 MHz in FSEOS, both detection schemes have comparable performance. Using a singular-electric-field THz emitter, we demonstrate the feasibility of a THz imaging system using real-time delay scanning in FSEOS and only 20 mW of laser power.

We propose and demonstrate a cascaded distribution fabric for WDM Passive Optical Networks (WDM-PON) that delivers distinct data to potentially tens of thousands of subscribers using a single, centralized WDM source. This is possible by assigning a unique time slot to each wavelength, and replicating this ‘bit-interleaved’ WDM signal in cascaded stages of amplification and power splitting, before data is encoded for each WDM-PON by a single TDM modulator.

We describe the bit-error-rate (BER) performance of an eight-channel wavelength division multiplexing (WDM) local access system using a spectrum-sliced and delay-line-multiplexed light-emitting diode (LED) source. Error-free transmission is demonstrated with all eight channels transmitting at 50 Mb/s using 16 km of distribution fiber. The measured crosstalk penalty is 3 dB and the fiber dispersion penalty is 0.7 dB. The unique properties of the WDM source permit the use of a receiver having 200-MHz bandwidth to minimize crosstalk and dispersion effects at 1.55-/spl mu/m wavelength even for a source linewidth of 25 nm.

The broad bandwidth of femtosecond lasers make them potential sources for wavelength-division multiplexing (WDM) systems. The difficulty is in spectrally slicing out the channels and modulating them. A technique for doing both simultaneously was recently demonstrated. In this technique, known as chirped-pulse WDM (CPWDM), a femtosecond pulse is linearly chirped by passage through a dispersive delay line such as a single-mode fiber. Here we demonstrate the scaling of this concept to 155-MHz repetition rate and the use of a feed-forward technique to actively equalize the channels, increasing the number of channels by approximately 50%.

A LiNbO/sub 3/ phase-conjugator and Raman pumping in fiber are used in order to cancel all Kerr nonlinearities in long fiber spans. We demonstrate this effect in single and multichannel transmission at 10, 40, and 100 Gb/s.

We investigate the reduction of power dissipation in absorptive semiconductor optical modulators via reduction in photocurrent. We present experimental results for proton implantation of p‐i‐n‐multiple‐quantum‐well modulators to levels of 1×1012, 1×1013, and 1×1014 cm−2. We find that photocurrent can be significantly reduced with moderate reduction in modulation performance relative to the unimplanted device. Application to asymmetric self‐electro‐optic devices is demonstrated.

We have observed room‐temperature exciton blue shift with applied voltage in a 〈111〉 In0.1Ga0.9As‐GaAs p‐i‐n multiple quantum well modulator. We have also observed optically induced bistability in a symmetric self‐electro‐optic effect device circuit composed of these modulators. Very large (2.5:1) ratios of photocurrent were obtained with only 0–3 V applied bias.

Nonlinear pulse propagation in long-period fiber gratings is studied with a mode-locked Q -switched laser pulse approximately 80ps in duration at a wavelength of 1.05 microm . Optical switching, pulse reshaping, and optical limiting are found at intensities in the range of 1-20 GW/cm(2).

We demonstrate a 206-channel wavelength division multiplexed transmitter using a single femtosecond laser at 1.55 μm and a single modulator. The channel spacing is 0.3 nm, and the bit rate of each channel is 36.7 Mb/s.

We discuss saturable Bragg reflector modelocking at 1541 nm in a Cr4+:YAG laser pumped by a high power Nd:YVO4 laser, and compare with Kerr-lens modelocking.

We present a novel multifrequeney wavelength-division-multiplexed (WDM) source based on a femtosecond fiber laser. Pulses are linearly chirped and stretched in time by propagation through a fiber, and 32 WDM channels can be defined and modulated sequentially in time by a single modulator.

Direct measurement of carrier dynamics, using femtosecond light sources, has yielded a wealth of understanding within the field of condensed matter physics[1]. Also, recently, near-field optical probe techniques have enabled optical measurements with spatial resolution below the diffraction limit[2]. An understanding of many scientifically and technologically important issues require a knowledge of the local behavior of carriers within materials and microstructures. We report the development of the required instrumentation, and the first measurement of carrier dynamics with spatial resolution below the diffraction limit and temporal resolution below 60 fs.

The authors demonstrate a record 206-channel wavelength-division multiplexed transmitter, using a single femtosecond laser at 1.55 /spl mu/m and a single time-division multiplexed electroabsorption modulator. The channel spacing is /spl sim/37 GHz (0.3 nm), and the bit rate in each channel is 36.7 Mbit/s.

We demonstrate self-starting passive mode locking of a Cr (4+):YAG laser, using an intracavity nonlinear mirror as a saturable absorber. The pump source is a diode-pumped Nd:YVO(4) laser. Output pulses are centered at 1541 nm, with 26-nm spectral bandwidth and 110-fs pulse width. Output powers of 70 mW are obtained with 8 W of pump power. This mode locking technique is compared with Kerr-lens mode locking.

Capacitance-voltage (CV) profiling measurements on delta-doped n-type GaAs reveal extremely narrow peaks with a full-width at half-maximum of 40 A. Comparison of experimental with self-consistently calculated CV profiles demonstrates that Si impurities are localized on a length scale of a lattice constant in delta-doped GaAs. Diffusion and segregation are of minor importance. The basic theory of CV measurements on quantummechanical systems such as delta-doped semiconductors is developed and presented.

Angle-resolved ultraviolet photoemission spectroscopy (ARUPS) is a powerful and well established tool for the study of bulk and surface electron states in solids. We report the first demonstration of picosecond time-resolved ARUPS. The technique was used to study the dynamics of electrons photoexcited by a 50 picosecond visible laser pulse on a cleaved InP (110) surface.

We review recent experimental and theoretical studies of the excitonic optical Stark effect in semiconductor quantum wells probed with femtosecond optical pulses.

Ultraviolet photoemission spectroscopy provides a powerful tool for studying both the bulk and surface electronic structure of solids. While previous photoemission studies of solids have concentrated on the electronic structure of filled electronic levels, i.e. the valence bands of semiconductors and the filled conduction bands of metals, the extension of this technique to study the dynamic transient processes experienced by electrons excited into otherwise unoccupied states has only begun to be exploited [1]. In this paper we describe an experimental system which utilizes the generation of short pulsewidth coherent vacuum ultraviolet light to perform photoemission studies at solid surfaces and in the bulk. Furthermore, the nature of the short time duration of the light pulses, 10 picoseconds, provides the opportunity to study the transient dynamics of electrons which are excited into unoccupied levels in the solid.