Wave-mechanical uncertainty and speed limitations

Modern science is eliminating the old, unrealistic approach to experimental errors. The Heisenberg principle of uncertainty has effected a fundamental change in the scientific attitude toward experimental errors and toward the degree of exactness in stating physical laws. Quantum physics has brought about the realization that a measurement or a mere observation at the quantum level perturbs the observed system in such a way that corrections cannot be made for the ``loading' effect of the measuring instrument. There is something more basic and fundamental to our limitations in measurements than the lack of perfection in experimental planning and equipment. This article examines the types of uncertainty in measurements or observations. The interest then focuses on the fundamental importance of the relation between experimental uncertainty in measuring, observing, or in any way identifying a physical quantity, and the energy required to accomplish this in a certain allotted interval of time. This last relation has a direct bearing on the speed limitations of measurements.     

Direct optical frequency modulation and injection locking of resonant tunnel diode oscillator

Preliminary results indicate that a resonant tunnel diode (RTD) oscillator can be optically tuned, frequency modulated, and injection locked. Experiments were performed in which a 2.8 GHz RTD oscillator was frequency modulated from DC to 100 MHz and injection locked over a 150 kHz bandwidth with a laser diode via fibre optics.<>     

GaAs MESFET demodulates gigabit signal rates from GaAlAs injection laser

This letter describes the experimental results of an optical transmission link with modulation rates up to 4 GHz. The transmitter consists of an FET driven GaAlAs injection laser emitting at a 8500 Å wavelength. The receiver makes use of a GaAs MESFET chip which is compared with a standard silicon avalanche photodiode demodulator.     

A theoretical analysis for gigabit/second pulse code modulation of semiconductor lasers

Investigation of the rate equations of a semiconductor laser suggests that bit rates of 3-4 Gbit/s can be achieved. Delay, ringing transients, and charge-storage effects can be removed by adjusting the dc-bias current and the peak and width of the current pulse to values prescribed by simple analytical expressions. Also, simple approximate formulas for the light pulse maximum, width, delay, and integrated values are given.     

Direct frequency modulation of an ADPLL for bluetooth/GSM with injection pulling elimination

We propose and demonstrate a Gaussian frequency-shift keying and Gaussian minimum-shift keying (GMSK) pulse-shape filtering for wireless RF transmitters with an arbitrary reference frequency. The filter is software controlled to work in a multistandard radio. Spurs, which are due to the frequency injection pulling, in cases when the reference harmonics are close enough to the oscillating frequency, are avoided by means of retiming the reference clock by the RF oscillator. Baseband clock for the pulse-shape filtering is derived through a simple fractional-N division of the reference frequency. This saves area and power since it is no longer required to create a low-jitter clock for baseband symbol generation and modulating data. It is especially advantageous when the available reference frequency is not an integer multiple of the symbol rate. The presented transmitter is realized without any explicit analog filtering and is part of a commercial single-chip fully compliant Bluetooth radio fabricated in a digital 130-nm CMOS process. We demonstrate the software programming capability through an experimental GMSK modulation for the global system for mobile communication.     

Direct modulation of semiconductor lasers

Methods for direct modulation of semiconductor lasers are reviewed with the objective of indicating the advantages and limitations of each method. Techniques for producing amplitude, pulse, and frequency modulation of the optical wave are included. The modulation capabilities of present pulsed lasers are analyzed with special attention given to their operation at room temperature. In addition, several ways of producing analog position or width modulation of microwave-rate optical pulses are described, and the capabilities of optical frequency modulation by acoustic waves are reviewed. A new way of obtaining mode-locked optical pulses with a semiconductor laser is also suggested.     

Direct modulation of gas lasers

Direct modulation of gas lasers such as internal modulation, coupling modulation, and phase locking, has been investigated in a 6328 Å He-Ne laser containing a composite modulation element with small insertion loss. It was verified theoretically and experimentally concerning internal modulation that the bandwidth of gas lasers for small perturbation of resonator loss decreases with an increase of resonator loss contrary to the passive circuit case and depends on the dynamicQof the gas laser as an active tuned circuit rather than the resonatorQ. The measured bandwidth of 6328 Å He-Ne lasers was about 1 Mc/s. Coupling modulation has a flat frequency response above the bandwidth of gas lasers except in the vicinity of harmonics ofc/2L. Forced phase locking by the frequencyc/2Lis necessary to realize the stable low noise operation of gas lasers. The modulation method simultaneously using the composite modulation element as coupling modulation element and phase-locking element assures a signal transmission of good quality in optical communications.     

Direct modulation of long-cavity semiconductor lasers

The application of a high-speed signal via the drive current to a long-cavity semiconductor laser (cavity length >~1 cm) differs from the short-cavity case in that the variation of the optical field within one round-trip time is not negligible. We theoretically investigate the modulation response for a semiconductor laser of arbitrary length and relate it to experiments on the multifrequency waveguide grating router laser, a long-cavity laser. We find that the largest modulation bandwidth is achieved by having the modulated amplifier and the desired output as far apart as possible, the modulation bandwidth can be further increased by simple electronic precompensation, and because of fast nonlinearities the broadband intensity modulation of long-cavity lasers exhibits substantially less chirp than short-cavity lasers, the chirp essentially equal to that of the amplifiers. Finally, we demonstrate the direct modulation of three channels simultaneously in a multifrequency laser at 1.24 Gb/s each     

Bidirectional colorless gigabit WDM-passive optical networks by use of injection locking and optical carrier suppression

A new bidirectional wavelength division multiplexed-passive optical network architecture for colorless optical line terminal and optical network unit was proposed by the use of injection locking of Fabry–Perot laser diode and optical carrier suppression. Error free transmissions (bit error rate of 10^?9) of both 1.25-Gb/s downstream signal and upstream signal were experimentally demonstrated at three representative wavelengths (1530.364, 1540.14, and 1556.713 nm). Accordingly, both optical line terminal and optical network unit were colorlessly operated within 25-nm range. Also, there was little impact of leaked channel on adjacent main one.     

Acoustooptic modulation resonances of injection lasers

A new approach is presented that explains the modulation resonance behavior of AlGaAs injection lasers as a consequence of collisions between photons and phonons in the lasing medium. The theory is applied to relate the laser's refractive index step with the measured modulation resonance frequency. Symmetrically located resonance peaks observed on unmodulated laser spectra are also explained with the new theory, which is compared with the conventional rate equation approach. Experimental data for modulated and unmodulated laser spectra and numerical calculations are presented in support of this acoustooptic theory.     

Large-signal direct modulation of injection lasers

Large-signal direct modulation of injection lasers is described with the aid of nonlinear rate equations. For periodic modulation of arbitrary amplitude, an approximate large-signal oscillatory solution is described. Large spiking amplitudes of the modulated photon density are generated near the resonance frequency of the system.     

Ultrafast gain recovery and modulation limitations inself-assembled quantum-dot devices

Measurements of ultrafast gain recovery in self-assembled InAs quantum-dot (QD) amplifiers are explained by a comprehensive numerical model. The QD excited state carriers are found to act as a reservoir for the optically active ground state carriers resulting in an ultrafast gain recovery as long as the excited state is well populated. However, when pulses are injected into the device at high-repetition frequencies, the response of a QD amplifier is found to be limited by the wetting-layer dynamics     

基于直接调制和外调制的高速半导体激光光源

直接调制和外调制的半导体激光光源在现代光纤通信系统中有着重要的应用。首先介绍了应用于10 Gb/s接入网系统的直接调制AlGaInAs多量子阱DFB激光器。由于AlGaInAs量子阱的导带不连续性较大,因此基于该材料的半导体激光器具有良好的温度特性,其特征温度达到了88 K。同时,该直接调制激光器的3 dB小信号调制响应带宽超过15 GHz。随后介绍面向40 Gb/s干线传输系统的高速DFB激光器/EA调制器集成光源。该集成光源采用同一外延层集成方案,并采用Al2O3高速微波热沉进行了管芯级封装,在3 V反向偏压下获得大于13 dB的静态消光比,3 dB小信号调制带宽超过40 GHz。     

Direct sequence spread spectrum Walsh-QPSK modulation

We present Walsh-quadrature phase-shift keying (Walsh-QPSK) pseudonoise (PN) modulation schemes for both coherent and noncoherent direct-sequence code-division multiple-access (DS-CDMA) systems, wherein the PN spreading sequences for in-phase and quadrature data in a conventional QPSK PN modulation scheme are coded by Walsh sequences indexed by a special rule to reduce the envelope variation of the transmitted signal. The signal characteristics of the two schemes are analyzed when a rectangular-shaped PN chip pulse is used, and it is shown that the proposed coherent DS-CDMA system has a constant envelope even in the presence of a transmitted phase reference. We simulate the signals to obtain the envelope variations when a spectrally efficient shaped PN chip pulse is used, and compare the results with those of conventional QPSK and orthogonal QPSK (OQPSK) PN modulation schemes. The results show that both the noncoherent and coherent Walsh-QPSK schemes have smaller envelope variations than the conventional noncoherent QPSK and OQPSK PN modulation schemes, even though in the coherent Walsh-QPSK scheme the pilot channel is added to the signal channel     

Direct modulation of widely tunable twin-guide lasers

The possibility to directly modulate widely tunable lasers up to several gigahertz is desirable in telecom applications. We discuss the dynamic properties of the recently proposed widely tunable twin-guide laser concept. It has promising prospects with a maximum theoretical bandwidth above 20 GHz and the 3-dB bandwidth at 250 mA indicates that an actual bandwidth of 12 GHz should be possible. The current lasers were not designed for high-speed modulation, so only 1-GHz modulation can be reached at the moment.     

Applications of public gigabit networks

Applications likely to require gigabit-per-second communications, including supercomputer networking, remote visualization, and virtual reality, are presented. Specifically, dynamic radiation therapy planning, visualization and telepresence, and medical imaging applications are examined. Public gigabit network requirements, applications of broadband ISDN to gigabit networks, the needs of applications with B-ISDN, and distance hiding in public gigabit networks are discussed     

In search of gigabit applications

Applications in existing specialized, metacomputer environments and those yet to be developed for services such as telecommunications, distributed data, and image transfer that will require networks which can handle volumes of data on the order of gigabits per second are discussed. Specific examples are presented from the fields of computational science, data navigation, and collaborative environments and instrument control. It is concluded that distributed computing and collaborative environments that support the interaction of multiple computers, as well as the interaction of computers with humans, are the paradigms that will characterize gigabit applications