Showing papers on "Chirp published in 1993"

77-fs pulse generation from a stretched-pulse mode-locked all-fiber ring laser

Abstract: By incorporating a section of large positive-dispersion fiber in an all-fiber erbium ring laser, we obtain high-energy pulses with spectral widths of 56 nm. The chirp on these pulses is highly linear and can be compensated for with dispersion in the output coupling fiber lead. The result is a fully self-starting source of 77-fs pulse with 90 pJ of energy and greater than 1 kW of peak power at a 45-MHz repetition rate.

Simple measurement of fiber dispersion and of chirp parameter of intensity modulated light emitter

Abstract: We report a novel method to measure two important parameters for optical fiber intensity modulated transmission systems: dispersion of optical fibers and chirp parameter of modulated light emitters. The method is easy, quick, and accurate for chirp parameter in the -10-to-10 range. >

Dynamic Response of the

Abstract: Large signal dynamic responses of temporal and spectral characteristics for gain-coupled distributed feedback (DFB) lasers are calculated by a computer model based on power matrix methods. Spontaneous emission, longitudinal ef- fects of spatial hole burning and carrier induced variations of the coupling coefficient have been taken into account. Both an- tireflection (AR) coated and cleaved facet devices are analyzed for the purely gain-coupled DFB. For partly gain-coupled DFB's, it is found that the dynamic behavior is dependent on the phase of the index and gain coupling. Low chirp operation can be achieved by having index and gain coupling in anti- phase.

Spectral narrowing in the propagation of chirped pulses in single-mode fibers.

Abstract: A theoretical study of the nonlinear propagation of picosecond chirped pulses in single-mode fibers is presented. We show that, under appropriate conditions, spectral narrowing—rather than broadening, as is generally believed—is induced, owing to the interplay of self-phase-modulation and dispersion. For downchirped pulses at a wavelength of 0.9 μm and a peak power as low as 0.1 W, substantial spectral narrowing occurs.

Compensation of higher-order frequency-dependent phase terms in chirped-pulse amplification systems.

Abstract: We have developed a system that provides monotonic tuning of the higher-order frequency-dependent phase of ultrashort laser pulses. This technique utilizes geometric aberrations that arise from adjustments to the relative alignment of the elements of an air-spaced doublet lens. In a system such as a diffraction-grating stretcher, the spectral components of the optical pulses are spatially dispersed, and lens aberrations introduce frequency-dependent phase shifts. A numerical model of a general chirped-pulsed amplification system has been developed and verified by comparison with experimental and analytical results. Numerical results indicating that higher-order phase terms can be compensated by a properly adjusted air-spaced doublet design within the pulse stretcher are presented.

Generation of 98 fs optical pulses directly from an erbium-doped fibre ring laser at 1.57 μm

Abstract: Optical pulses as short as 98 fs at 1-57 μm have been successivelly generated for the first time directly from a laser-diode pumped erbium-doped fibre laser with a nonlinear amplifying loop mirror. By using a low GVD polarisation sensitive isolator and reducing the fibre length in a nonlinear loop, 124 fs pulses were obtained. They were further shortened to 98 fs by installing an EDFA as an active pulse compressor and a positive GVD fibre for chirp compensation

Propagation of light pulses in a chirped-pulse-amplification laser

Abstract: The propagation and compression of a broadband pulse in a chirped-pulse-amplification (CPA) laser is studied. Gain narrowing and self-phase-modulation (SPM) are the main mechanisms that modulate the final compressed pulse temporally and spectrally. The effect of gain saturation is also investigated. Due to the large chirp of this type of laser pulse, the power gain of the amplifier can be treated as a function of instantaneous frequency to simplify the analysis. Experimental results from an Nd:glass CPA laser system are in good agreement with the theory. Both experimental and theoretical results show that SPM can play an important role in determining the final shape of the compressed pulse, even at relatively low values of the cumulative B-integral, B >

Effect of third-order dispersion on passive mode locking

Abstract: The effect of third-order dispersion on the width of mode-locked pulses is investigated analytically and numerically. The pulse width increases monotonically with increasing third-order dispersion as a consequence of the symmetric chirp introduced by it. The chirp broadens the bandwidth and lowers the gain. Computer simulations show the appearance of a resonant sideband that also taxes the gain. Reducing the filter bandwidth partially suppresses the sideband and narrows the pulse.

Suppression of noise and distortion in fiber-optic systems

Abstract: Noise and distortion due to scattering and reflection in a fiber-optic communications system are suppressed by modulating the optical frequency of (chirping) a laser light source to broaden the spectrum of the laser light output. The broader spectrum spreads the noise produced by an unchirped source over a broader band. A noise and distortion suppression system includes a chirp signal generator coupled to the signal path of an RF input signal carrying information to modulate the laser optical output. In some cases, the frequency of the chirp generating signal may result in second-order intermodulation products falling within the information band. In such cases, the RF input signal is predistorted to offset the expected distortion products.

Tomographic measurement of temperature change in phantoms of the human body by chirp radar-type microwave computed tomography

Abstract: The chirp radar-type microwave computed tomograph (CT) measures the temperature change in a human body noninvasively. The paper examines its feasibility. A chirp pulse signal between 1 and 2 GHz is radiated from the transmitting antenna to the phantom. The transmitted waves are detected by the receiving antenna, which is place on the opposite side of the object, and the beat signal between the incident wave and the transmitted wave is produced by the mixer. By spectral analysis of the beat signal, only those signals transmitted on the straight line between the transmitting antenna and the receiving antenna are discriminated from multipath signals. The microwave tomogram can therefore be reconstructed easily using the conventional algorithms for an X-ray CT image. The microwave CT can use the chirp signal to remove the influence of multipath signals caused by diffraction and reflection. The imaging of dielectric materials with complicated structures is thus possible. The experimental results using phantoms show that the spatial resolution of this microwave CT is about 10 mm and that a two-dimensional distribution of temperature change can be measured.

Mode-locking matter with light

Abstract: Theory and experiments are currently being developed in our laboratory to use tailored light fields to control the dynamics of molecular reactions. We have recently shown theoretically that a properly chirped light pulse can be used to overcome the natural tendency of wave packets to spread on anharmonic molecular potential energy surfaces. The optimal chirp rate can be readily obtained by solving a linear optimal control equation. In this paper we analyze a number of aspects crucial to the success of the experiments. We test the robustness of the optimal electric field with respect to variations in the field parameters (chirp rates, pulse duration, and carrier frequency). We also study the effects of finite temperature by using a thermal vibrational distribution as the initial in the dynamics

Simultaneous compensation of laser chirp, Kerr effect, and dispersion in 10-Gb/s long-haul transmission systems

Abstract: The effects of chirp and fiber nonlinearity in a directly modulated 10-Gb/s intensity-modulated direct-detection (IM-DD) fiber transmission system are investigated by simulation, and a simple and flexible technique for compensating these effects is discussed. Self-phase-modulation (SPM) in optical fiber can be equalized by an anomalous dispersion fiber, whereas pulse broadening caused by laser transient chirp can be compensated by normal dispersion. Using these characteristics, laser transient chirp, SPM, and fiber dispersion can be simultaneously compensated by equalizing fibers inserted within certain intervals. Optimum compensation is always realizable for such fixed equalizing fibers, since the magnitude of SPM can be controlled by changing the optical power in the fiber. Simulation suggests that this technique enables 10-Gb/s, 100-km fiber transmission by direct modulation. >

Ultra wideband radar signal processor for electronically scanned arrays

Abstract: A radar system that includes an ultra wideband radar signal processor for electronically scanned arrays that utilizes frequency offset generation (FOG) to achieve beam steering as compared with phase shift and time delay techniques of conventional radars. The device comprises a transmit antenna, a chirp generator connected to the transmit antenna and a first summing circuit, a receiver antenna connected to the first summing circuit, a Doppler de-ramping chirp circuit connected to a second summing circuit, the output of the second summing circuit connected to an amplitude and weighting circuit and the output of the amplitude circuit connected to a spectrum analyzer of a Fast Fourier Transform (FFT) circuit. The signal processing consists of mixing the target returns with the transmitted signal to obtain a video beat note signal. This video beat note signal is mixed with a Doppler de-ramping chirp waveform which is matched to the desired target velocity. The output is amplitude weighted and the FFT algorithm applied. To achieve beam steering for the detection of off boresight targets, a phased array with distributed receivers is required. Also, frequency offset generation must be incorporated into the Doppler de-ramping chirp generator.

Modeling high-speed optical transmission systems

Abstract: Computer simulation of a 2.5-Gb/s intensity-modulated/direct-detected optical system with high path dispersion is described. This simulation, based on single-mode laser diode rate equations, includes transmit and receiver circuit filtering, receiver circuit noise, avalanche photodiode noise, fiber dispersion, and laser chirp. The rate equations are sufficiently general to model mode-offset distributed feedback laser diodes. The system power penalty, due to laser chirp and fiber dispersion, is calculated using Gaussian quadrature numerical integration. By simulating a population of some 12800 laser diodes and correlating the performance of each laser in a transmission system with its spectral characteristics, it is possible to deduce laser specifications that will assure satisfactory operation in long-span links. These results are used to study a laser diode specification under consideration by CCITT, and, by studying the simulated laser line shapes, some modifications to the CCITT specification are considered. >

Wigner distribution of noisy signals

Abstract: The effect of noise on the Wigner distribution is analyzed. The mean and variance of the Wigner distribution of signals contaminated by additive noise are derived. It is shown that in the case of white noise the Wigner distribution calculated by definition calculated by definition cannot be used for estimation. For estimation, windowing in the time domain is not sufficient; smoothing in the time domain is necessary. Very simple expressions for variance and bias estimation are obtained. If the signal and noise are analytic, only one window is sufficient to make the variance finite. The results are demonstrated by numerical examples with linear and sinusoidal chirp pulses. >

Chirp excitation of ultrasonic probes and algorithm for filtering transit times in high-rangeability gas flow metering

Abstract: The signal processing used in an ultrasonic high-rangeability gas flow meter using times of flight is presented. The flow meter under discussion uses a combination of continuous wave and chirp signals to measure at low flow velocities, below 20 m/s, and chirp signals alone to measure high flow velocities, above 20 m/s. Because of the need for a pulse compression technique in the signal waveform design the technique of pulse compression and the choice of signal waveforms are discussed. The advantages and disadvantages of amplitude weighting vis-a-vis frequency domain manipulations of the waveforms are also discussed. To eliminate spurious times of flight, a special filtering technique is used, based on assessing the gradient of ascendingly ordered time series of time-of-flight measurements. A summary of user experience with high-rangeability gas flow meters in use on different offshore platforms and in refineries is given. Long-term tests that examined the accuracy of the high-rangeability flow meter are also described. >

Induced phase modulation of chirped continuum pulses studied with a femtosecond frequency-domain interferometer.

Abstract: The effect of rapid phase change on chirped continuum pulses is studied with a frequency-domain interferometer. Because of the chirp, temporal evolution of the optical Kerr response in CS2 is projected into difference phase spectra. The chirped continuum shows spectral shifts that are due to induced phase modulation even when the continuum has a flat spectrum.

Large time-bandwidth chirp pulse generator

Abstract: Low time-bandwidth product linear frequency modulated chirp pulses are repetitively generated as contiguous subpulses to form a pulse of extended duration with each subpulse respectively mixed with one of a plurality of stepped intermediated frequencies so that the bandwidth of the contiguous subpulses is increased to the frequency bandwidth of all of the stepped intermediate frequencies such that the contiguous signal formed has a linearly varying frequency over the increased bandwidth and increased pulse duration, providing a large time-bandwidth product linear frequency modulated chirp waveform particularly useful in radar systems.

High-contrast terawatt chirped-pulse-amplification laser that uses a 1-ps Nd:glass oscillator.

Abstract: We have developed a fiberless 1-TW all-Nd:glass chirped-pulse-amplification laser system that uses high-contrast 0.8–1.4-ps pulses produced directly from a Nd:glass feedback-controlled oscillator. Employing grating-only expansion and compression, the system produces clean (~107 contrast ratio) 1-J, 1–1.4-ps recompressed pulses without added pulse cleaning. Clean microjoule-energy pulses from the oscillator require less subsequent amplification than cw oscillator schemes, thereby minimizing gain–bandwidth narrowing and offering improved contrast with amplified stimulated emission background.

High‐frequency modulation and suppression of chirp in semiconductor lasers

Abstract: We propose a new method for modulating laser radiation by controlling simultaneously the pumping current and the optical gain in the active region. The latter can be independently varied by modulating the effective carrier temperature. The method allows to eliminate the relaxation oscillations and enhance the modulation frequency to 50 GHz. It also allows to suppress the wavelength chirping in optical communication systems operating at pulse repetition rates of 10 Gb/s.

Target velocity estimation with FM and PW echo ranging Doppler systems. II. Systems analysis

Abstract: The theoretical foundation is presented for velocity estimation with a pulsed wave (PW) Doppler system transmitting linear FM signals. The Doppler system possesses echo ranging capabilities and is evaluated in the context of Doppler ultrasound for blood velocity measurement. The FM excitation signal is formulated and the received signal is derived for a single moving particle. This signal is similar to the transmitted signal, but with modified parameters due to Doppler effect and range. The demodulated received signal is subsequently derived and analyzed. It is shown that, due to the Doppler effect, this is a linear sweep signal as well. The velocity and range information obtainable from one and two consecutively received signals are described. The latter case establishes the basis for an FM Doppler system for blood velocity measurements. >

Gain-switched GaAs vertical-cavity surface-emitting lasers

Abstract: The authors have gain-switched GaAs vertical-cavity surface-emitting lasers (VCSELs) using sinusiodal electrical modulation at rates between 1.5 and 8 GHz and devices with operating wavelengths between 820 and 860 nm. The shortest pulse obtained directly from such lasers was 24 ps. The time-bandwidth products were between 0.6 and 3, which is smaller than those observed for gain-switched, single-frequency, edge-emitting lasers. Some of the excess bandwidth is caused by linear chirp, which was compensated using linear dispersion in single-mode optical fiber. The shortest compressed pulse was 15 ps. The pulses contained significant nonlinear chirp, however, which reduced the expected compression factor for linear dispersion to a factor of 2. The timing jitter for gain-switched pulse trains was 4-6 ps, which is comparable to that observed for the edge-emitting lasers. Device design tradeoffs which affect the duration of the pulses from gain-switched VCSELs are discussed. >

Speckle resistant method and apparatus with chirped laser beam

Abstract: Laser beam speckle effects are mitigated by frequency chirping a single-mode laser beam, reflecting it off a speckling surface, and then integrating the reflected beam over at least one chirp period. The speckle reduction is comparable to the use of a multi-longitudinal mode beam having a bandwidth similar to the chirped frequency excursion, while leaving the beam compatible with nonlinear optical processes that are not compatible with multi-longitudinal mode beams. Where the nonlinear process is stimulated Brillouin scattering (SBS) phase conjugation, the beam can be chirped at a rate up to the order of ##EQU1## where ΔνB is the gain bandwidth and L is the effective SBS interaction length of the phase conjugator, and C is the speed of light. Other nonlinear processes with which the beam may be used include harmonic generation and Raman-based beam cleanup or wavelength conversion.

Passive mode locking with an active nonlinear directional coupler: positive group-velocity dispersion

Abstract: We show that the active nonlinear coupler laser proposed by us can produce picosecond pulse trains in dual-core fiber lasers with positive group-velocity dispersion. These pulses are accurately described by a linearly chirped soliton profile. The linear chirp should make it possible to obtain subpicosecond pulses widths through extracavity chirp compensation.

Second-order distortion improvements or degradations brought by erbium-doped fiber amplifiers in analog links using directly modulated lasers

Abstract: Within lightwave analog amplitude-modulated (AM) CATV systems using directly modulated lasers, erbium-doped fiber amplifiers (EDFA's) act upon the signal distortion because of the interaction between the laser chirp and the EDFA wavelength-dependent gain. This interaction is theoretically investigated in order to predict the EDFA-induced distortion. The relevant gain tilt characteristic for analog applications and the way to measure it are described. Expected and measured distortions at the EDFA output are in excellent agreement. Fiber amplifiers are found to decrease the signal distortion level when the gain tilt is negative, i.e., for wavelength above the gain maximum. >

Design and performance of externally modulated 1.5- mu m laser transmitter in the presence of chromatic dispersion

Abstract: Key laser and modulator characteristics that impact the use of externally modulated lasers in the presence of chromatic dispersion, excluding effects due to fiber nonlinearities, are reviewed. After a brief consideration of transmission performance with directly modulated 1.5- mu m DFB lasers which have limited application of up to 80 km at 2.5 Gb/s, the key design characteristics of externally modulated transmitters are discussed. Experimental results showing the effects of modulator chirp and laser linewidth at a 2.5-Gb/s transmission rate are presented. It is found that lasers with CW linewidth under 100 MHz have less than 2-dB dispersion penalty for 600 km of non-dispersion-shifted fiber. Lower dispersion penalties can be realized if the modulator chirp is tuned so as to narrow the transmitted pulses. Excellent modulator stability is demonstrated for 60 days of error- and degradation-free 2.5-Gb/s operation. >