Showing papers on "Chirp published in 1985"

Surface-wave devices for signal processing

Abstract: 1. Introductory Survey. 2. Acoustic Waves in Elastic Solids. 3. Electrical Excitation at a Plane Surface. 4. Analysis of Interdigital Transducers. 5. The Multi-Strip Coupler and its Applications. 6. Propagation Effects and Materials. 7. Delay Lines and Multi-Phase Transducers. 8. Bandpass Filters. 9. Chirp Filters and Their Applications. 10. Devices for Spread-Spectrum Communications. Appendices. References. Index.

Modulation induced transient chirping in single frequency lasers

Abstract: Wavelength excursions with magnitudes as large as 6 A are seen to occur in single frequency lasers (both C3and DFB) during a transition from one power level to another. The wavelength shifts briefly toward shorter wavelengths and then back to the equilibrium value during turn-on and toward longer wavelengths and back during turn-off. These excursions, which are well explained by a model in which the carrier density is temporarily driven out of equilibrium by a change in injection current, last for hundreds of picoseconds or about one half of the relaxation resonance period. This time dependent behavior gives rise to a dramatic degradation of lightwave system performance with increasing bit rate. Laser structures which heavily damp the relaxation resonance peak are seen to exhibit the least chirp and to perform best in high speed transmission systems.

On the optimality of the Wigner distribution for detection

Abstract: A variety of methods have been proposed for the detection of a signal, with unknown signal parameters, in a noisy environment. In this paper, the noise statistics are incorporated to reveal that certain processing of the Wigner distribution (WD) signal representation can lead to an optimal, and often easy to compute, detection scheme. For the special case of linear FM signals in complex white Gaussian noise, it is shown that the optimal detector is equivalent to integrating the WD along the line of instantaneous frequency. If the position and sweep rate of the linear chirp are unknown, then a Generalized Likelihood Ratio Test (GLRT) leads one to integrate the WD along all possible lines in the time-frequency plane and choose the largest integrated value for comparison to a threshold. Simulation examples of the WD detection scheme are given to demonstrate the utility of the proposed method. Finally, some comments concerning the detection of the general phase modulated signal are offered.

Shaping optical pulses by amplitude and phase masking

Abstract: Embodiments of the present invention control, manipulate and tailor the shape of input optical pulses to produce substantially transform-limited output pulses which can be shorter than the input pulses. In embodiments of a first aspect of the present invention an input optical pulse is chirped, the chirped pulse is then passed through an optical component that spatially disperses the frequency components of the chirped pulse and partially compensates the chirp, the spatially dispersed frequency components are then passed through spatial amplitude and/or phase masks that control and/or adjust the amplitude and/or phase of the frequency components, and, finally, the masked components are passed through the first or a second optical component that returns the masked, spatially dispersed frequency components substantially to the spatial distribution of the input pulse while substantially completing the compensation of the chirp to form an output pulse.

Chirp-free transmission over 82.5 km of single mode fibers at 2 Gbit/s with injection locked DFB semiconductor lasers

Abstract: The spectral and modulation characteristics of injection locked distributed feedback lasers operating at 1.5-μm wavelength have been investigated. The total elimination of excess chirp in directly modulated lasers by the injection locking technique has allowed us to perform transmission experiments over 82.5 km of single mode fibers at 2 Gbit/s without any measurable penalty related to dispersion. Device design parameters for efficient chirp suppression are also given.

Intracavity chirp compensation in a colliding pulse mode‐locked laser using thin‐film interferometers

Abstract: Dielectric multilayer mirrors designed as Gires–Tournois interferometers have been used in a colliding pulse mode‐locked dye laser to study the influence of intracavity group velocity dispersion on the pulse formation. Simple angle tuning of the interferometric mirrors which is equivalent to a change of the net cavity dispersion resulted in a shift of the laser wavelength between 627 and 614 nm. Within this range stable operation with pulse durations of 130 fs or less and average output powers between 30 and 80 mW was obtained. The shortest pulse of 80 fs duration was found at 626 nm.

Evolution of chirped pulses in nonlinear single-mode fibers.

Abstract: Computer studies of the evolution of pulses in nonlinear single-mode fibers are presented. In the case of anomalous dispersion, the time–bandwidth product of the initial pulse is varied by imposing a linear frequency chirp. The pulse broadening resulting from chirp greatly exceeds the broadening caused by the loss. In the case of normal dispersion the evolution of an initially unchirped pulse is shown to depend critically on the loss. This fact is explained by the position of the chirp's maxima and minima relative to the pulse-amplitude distribution.

Electronic counter measure system utilizing a digital RF memory

Abstract: An active radar jamming system using a digital radio frequency memory (DRFM) as the central subsystem in the generation and detection of specific signal characteristics. Associated systems control the operation, data processing, and loading of the DRFM. Signals generated by the jamming system are either repeated replicas of the received radar signal stored in the DRFM, or newly generated signals constructed from data loaded into the DRFM by the associated subsystems. Such associated subsystems generate appropriate data for providing carrier and noise RF signals. Systems are also provided for determining the presence of phase coding and chirp modulation in the received radar signal data contained in the DRFM, and for detecting the angle-of-arrival of the radar signal.

Dispersive effects in the reflection of femtosecond optical pulses from broadband dielectric mirrors

Abstract: The dispersive effects of high-reflectivity broadband mirrors on femtosecond optical pulses have been analyzed for three different multilayer structures. In each case the high-reflectivity zone can be divided into two different regions symmetrically located around the mirror central frequency: high-dispersion and low-dispersion regions. The calculated temporal behavior of the reflected pulse shows high distortion of the pulse profile, a frequency chirp, and a broadening as high as a factor of 5.6, due to a single reflection, within the high-dispersion region. The use of these types of mirror should therefore be strictly limited to their low-dispersion side.

Power spectrum of directly modulated single-mode semiconductor lasers: Chirp-induced fine structure

Abstract: The time-averaged power spectrum of a single-mode semiconductor laser under direct current modulation is calculated using the small-signal analysis of the stochastic rate equations. The general formalism includes the effect of frequency chirping owing to current modulation as well as the effect of phase diffusion related to spontaneous emission. Chirp-induced dynamic line broadening is shown to result from a superposition of the additional unresolved sidebands generated at multiples of the modulation frequency on both sides of the optical line. The effect of intensity modulation is to introduce an asymmetry such that the low-frequency sideband has a relatively higher amplitude. The theory explains reasonably well the reported experimental observations. At the same time it provides physical insight into the dynamic process of frequency chirping and its relation to the frequency modulation. In particular, the frequency and the decay rate of transient relaxation oscillations govern the interplay between simultaneously occurring intensity and frequency modulations.

Frequency chirping of external modulation and its reduction

Abstract: A general formula is given for frequency chirping of external intensity loss modulation which is caused by phase modulation due to an accompanied refractive index change. It can be reduced by optimising the optical modulation waveform, and a related ultimate bandwidth of such small chirping single-mode fibre transmission will be presented.

Advanced sounding: 1. The FMCW alternative

Abstract: HF radar employing linear frequency-modulated continuous wave or “chirp” modulation provides an alternative to the more conventional pulsed amplitude modulation technique in advanced ionosphere sounding. An analysis is presented which shows that the phases of the discrete Fourier components of the receiver output can be used in the same fashion as the phases of pulsed, fixed frequency ionospheric echoes to evaluate the standard digital parameters, including group path by the method of stationary phase. This can be used to overcome the group path/Doppler velocity ambiguity of linear frequency modulation. A description is given of a sounding structure from which unambiguous group range, angle of arrival, O/X mode, and Doppler velocity can be evaluated. A simple error analysis is also given.

Nonlinearly enhanced chirp pulse compression in single-mode fibers

Abstract: The possibility of combining linear-frequency chirp and fiber nonlinearity to obtain high compression ratios of optical pulses is assessed.

Signal analysis receiver with acousto-optic delay lines

Abstract: The system makes use of an interferometric Bragg cell for the time delays needed by the multiple antenna inputs in an angle-of-arrival (AOA) measurement system, for accurate measurements over wide bandwidths. A signal Bragg cell is modulated with a selected band of the EW signal environment, using a separate transducer coupled to each quadrant antenna element. A reference Bragg cell is modulated with a chirp signal. Coherent light from a laser is split and directed to both the signal cell and the reference cell. The output light beams from the reference Bragg cell and signal Bragg cell are combined and focused onto photo detectors having a one-dimensional array for each quadrant antenna element. The reference and signals are heterodyned at the photo detectors, which results in a channelized spectral output with each channel at the same intermediate frequency (IF). The outputs of the photodetectors are used by a phase comparison unit, cued by a frequency measurement receiver to the correct detectors, to provide the AOA measurement.

A rate equation analysis for the frequency chirp to modulated power ratio of a semiconductor diode laser

Abstract: An expression for the frequency chirp to modulated power ratio (CPR) is derived from a rate equation analysis of the small-signal, injection current modulation in a semiconductor diode laser. The model includes the effect of lateral carrier diffusion across the active region of the laser diode. The modulation frequency dependence of the CPR is flat from dc to a few hundred megahertz, beyond which it is proportional to the modulation frequency.

Chirp laser stabilization system

Abstract: A chirp laser system includes an intracavity electro-optical crystal which is modulated at a relatively high frequency, for example 250 kilohertz, to sweep the frequency of the transmitter laser above and below its nominal center frequency. One of the mirrors associated with the laser transmitter is adjustable to vary the length of the laser cavity and to thereby shift the frequency of the transmitter laser. When the frequency of the transmitter laser is shifted, the output amplitude of the laser changes to some extent. With the center frequency of the laser at a maximum gain point, the amplitude of the output signals during the positive and negative frequency excursions of the transmitter laser will be substantially symmetrical. However, if thermal or other effects cause the frequency of the laser to shift away from the maximum gain point, the output during positive and negative excursions will be asymmetrical, for example, with the gain dropping off during an upward frequency excursion, and perhaps increasing or not dropping off nearly as much during a low frequency excursion. A small part of the output from the transmitter laser is sampled, and the amplitude during positive and negative frequency shift excursions is compared, using a square wave derived from the chirp modulating driver. Where the detected signals for the positive and negative frequency excursions are substantially equal, no change in the position of the mirror is accomplished; however, when the excursions result in a difference in the amplitude of the detected signals, the position of the piezoelectrically adjustable mirror is shifted to restore the nominal center frequency of the laser to its maximum amplitude point on the gain curve.

Reduction of laser chirp in 1.5 μm DFB lasers by modulation pulse shaping

Abstract: The dynamic linewidth of 1.5 μm ridge waveguide DFB lasers is shown to be reduced by shaping the pulse of the laser modulating waveform. Pulse shaping is performed by a second-order network designed to cancel the small-signal laser resonance. Results demonstrate a dynamic linewidth reduction from 1.4 A to 0.55 A FWHM for a 500 ps pulse.

Coupled-cavity semiconductor lasers under current modulation: Small-signal analysis

Abstract: The modulation characteristics such as the power response and the chirp (dynamic line broadening) of single-frequency coupled-cavity semiconductor lasers are analyzed theoretically. The model is based on the small-signal analysis of a set of generalized rate equations and neglects the lateral variations of the optical field and the carrier density in the two cavity sections. When the controller section is biased below threshold in a three-terminal device the modulation response is not significantly affected by the intercavity coupling. By contrast, new features arise when both sections are biased above threshold. It is found that the chirp can be significantly reduced by a proper combination of the bias levels and the modulation splitting between the two sections. The chirp reduction also depends on the strength of the intercavity coupling and its phase, and the best performance is achieved for the case of in-phase coupling. The calculated results are in qualitative agreement with the reported experimental data and are useful as a guide to optimize the performance of a coupled-cavity device.

Picosecond optical pulse compression from gain-switched 1.3 μm distributed-feedback laser diode through highly dispersive single-mode fibre

Abstract: The first demonstration to compress the linearly chirped optical pulses from a gain-switched distributed-feedback laser diode (DFB-LD) is described at 1.3 μm wavelength. By travelling through a highly dispersive single-mode fibre with 48 ps/nm normal dispersion, a 26 ps (FWHM) pulse having red-shift frequency chirping of 85 GHz (FWHM) is compressed to 8.3 ps with the time-bandwidth product of 0.71.

Frequency chirp reduction in a 2.2 Gbit/s directly modulated InGaAsP semiconductor laser by CW injection

Abstract: Longitudinal-mode linewidth broadening due to frequency chirping in a high-speed modulated InGaAsP laser transmitter is reduced by CW injection of a master laser with a matched longitudinal mode. Biased below threshold and modulated with RZ format 2.2 Gbit/s current pulses, the transmitter laser's frequency chirp can be reduced at the expense of a small pulsewidth increase and on/off ratio reduction.

Small-Aperture Focusing Chirp Transducers vs. Diffraction-Compensated Beam Compressors in Elastic SAW Convolvers

Abstract: Abstmct-High performance has been achieved in SAW convolvers using diffraction-corrected multistrip beam compressors. However, two different levels of metalization are required for optimum performance. A significant simplification of device fabrication was achieved with the design of convolvers using small-aperture chirp transducers, which require only one metalization layer. A design procedure is described that makes the transducers focusing to minimize diffraction loss. Experimental results are presented, which show that performance of this new design surpasses conventional designs in several respects. I. INTRODUCTION M ONOLITHIC acoustic surface wave convolvers on YZ-LiNb03 combine high-speed signal processing capability with ruggedness, small size, and relatively low cost of the device-thanks to the simplicity of its construction. However, the intrinsic nonlinearity of the substrate material polarization, which is used to generate the mixing product of the two input signals, is low and limits the efficiency of the device. High efficiency is crucial to achieve high dynamic range and economize input power. Since the introduction of beamwidth compression [l] to boost device efficiency, the elastic convolver has been developed to high performance in several laboratories [2], [3], [4] by optimization of its various components. The aim of such optimization was to increase the device efficiency by eliminating sources of loss in the acoustic path and providing good match at all electrical ports, while tailoring the device characteristics to minimize signal distortion and suppress spurious signals [5]. This means that the convolution efficiency vs. frequency should be reasonably flat and smooth in magnitude and linear in phase over the desired bandwidth. Notably reflections of acoustic waves in the device have to be suppressed to avoid selfconvolution of one input signal. The distributed output signal has to be collected from the integrating electrode in such a way as to avoid electromagnetic propagation loss or destructive interference of various signal components due to long-line effects. By these considerations one arrives at a standard design scheme as shown in Fig. 1. Interdigital transducers, which convert the input signals, typically are unapodized to avoid

Observation of negative differential resistance in CHIRP superlattices

Abstract: Fabrication of a CHIRP (coherent heterointerfaces for reflection and penetration) superlattice device is presented. Negative differential resistance (NDR) is observed at around 85 K. This NDR is found only in one bias polarity and is not found when the bias is reversed, which is characteristic of CHIRP superlattices.

Picosecond pulse analysis of gain-switched 1.55 µm InGaAsP laser

Abstract: Results of computer simulation of gain-switched pulses from InGaAsP lasers emitting at 1.55 μm are reported. Multimode rate equations are solved for short triangular current pulses of large amplitude superimposed on a dc bias close to threshold. Lasers of conventional length (200μm) and with short cavity (50 μm) are analyzed. The effect of wavelength chirp on time-resolved and the time-averaged multimode spectra is demonstrated. Optical pulses as short as 10 ps are predicted for short-cavity lasers with reflective coatings. While the number of longitudinal modes in gain-switched spectrum is efficiently reduced for short-cavity lasers, the dynamic linewidth of each mode remains of a similar magnitude (up to 140 GHz) as for standard-cavity lasers. It is expected that gain saturation effects could reduce the calculated values of dynamic linewidth.

The Study of a Reflex Oscillator Used to Generate High-Power Microwaves

Abstract: The operation of a high-power short-pulse microwave generator is being studied both experimentally and computationally. The device is a reflex triode which uses a real and a virtual cathode. The operating voltage is 500 kV with an electrical power input of 18 GW and a microwave output power of 200-300 MW at frequencies ranging from 6 to 9 GHz for up to 50 ns. Microwave power is extracted axially in an 85-mm-diameter waveguide with an overall efficiency of 1.5 percent. We will discuss the effects of electron beam quality on output efficiency. Temporal behavior of the output shows a frequency chirping with time which can be related to the voltage and current profiles. Comparing the microwave output at 900 shows azimuthal symmetry. Oscillator behavior when immersed in an external magnetic field, with field strength ranges from 0 to 1.2 kG, will be discussed. Also, computer simulations show that, although the virtual-cathode and reflexing-electron frequencies can be different, a frequency locking can occur, resulting in a purer frequency spectrum with a higher power. Simulations at 2 MeV show microwave output efficiencies of up to 4 percent with peak powers approaching 10 GW.

Fabrication and performance characteristics of InGaAsP ridge-guide distributed-feedback multiquantum-well lasers

Abstract: The fabrication and performance characteristics of InGaAsP ridge-guide distributed-feedback lasers with multiquantum-well active layers are reported The lasers are 320 μm long and have four active wells (300 A thick) The lasers have threshold current 100 mA at 30°C, external differential quantum efficiency 01 mW/mA/facet at 30°C and To 60 K and can be operated in the same DFB mode up to 70°C The measured frequency chirp is about a factor of 3 smaller than that for conventional double-heterostructure lasers The smaller chirp should allow larger repeater spacing for high-bit-rate long-distance fibre transmission systems applications

Parametric frequency modulation of picosecond light pulses in quadratically nonlinear crystals

Abstract: Theoretical and experimental investigations were made of chirping (linear frequency modulation of the carrier frequency in a pulse) of radiation from an optical parametric oscillator pumped by linearly chirped pulses. It is shown that such an oscillator is an efficient converter of a pump chirp into a linear chirp of the output pulses. The latter may be stronger than the pump chirp, which provides new opportunities for mastering the femtosecond range.

Parametric chirp and 20-fold compression of pulses from a quasi-cw picosecond optical parametric oscillator

Abstract: A linear chirp was observed for phase-conjugate picosecond pulses from a resonator-type optical parametric oscillator based on an LiNbO3 crystal pumped by the second harmonic of a YAG:Nd3+ laser with active mode locking and switching at repetition frequencies up to 10 kHz. A 20-fold compression of the parametric oscillator pulses to 2.5 psec was achieved.

Advanced sounding: 2. First results from an advanced chirp ionosonde

Abstract: A BR Communications “chirp” ionosonde has been modified to allow the measurement of the phase of ionospherically reflected echoes. The use of a dual-channel receiver and microprocessor-based control allows phase comparisons of temporally and spatially diverse echoes from which Doppler velocity, angle of arrival, and polarization mode can be evaluated, in addition to group range and amplitude. A test study of the ionosphere at Grahamstown is described.

Pulse Compression Acoustic Microscopy Using SAW Filters

Abstract: The basic principles of surface-acoustic.,wave pulse compression acoustic microscopy is described and demonstrated by re- sults obtained at 60 and 750 MHz. A theoretical analysis discusses the behavior of such a system in terms of the required signal processing and the attainable imaging resolution. advantageous. At sufficiently high intensities, the imaging of the microscope will be affected by harmonic genera- tion, sometimes beneficially (g) but more usually in the sense of a large increase in effective attenuation loss (lo). It is clear that we can choose to use extended coded pulses to reduce the intensity at a constant illumination energy and hence the incidence of harmonic effects. We will describe the principles of operation of a pulse- compression microscope, based on surface acoustic wave (SAW) generated chirp pulses and illustrate the advan- tages of such a system by means of results from test sam- ples. A range of micrographs obtained at 60 MHz from metal-metal and metal-ceramic bonds with a processing gain of 17 dB is presented. At higher frequencies, the fre- quency-dependent attenuation in the coupling liquid im- poses two fundamental limitations on the performance of the pulse compression microscope: 1) reduction in the processing gain and 2) reduction in the imaging resolu- tion. Analysis shows, however, that even at high frequen- cies there can be a considerable advantage in using pulse compression. Experimental results at 750 MHz with a chirp bandwidth of 150 MHz have been obtained. We will show that the effective attainable resolution with such a coherent broadband system is in accord with a simple in- tuitive concept, based on monochromatic excitation, at a suitable averaged frequency. The reduction in processing gain due to loss dispersion, which we will calculate, is of course related to the partic- ular choice of coded pulse. However, the loss of resolution is a phenomenon that depends primarily on the spectrum of the pulse; for a pulse of a given spectral width one would not expect any significant differences between the effective resolution using a chirp or a rectangular RF pulse.