Showing papers on "Chirp published in 1991"

Choice of filters for the detection of gravitational waves from coalescing binaries.

Abstract: Coalescing binaries are one of the most promising candidates for the detection of gravitational waves with the advent of the new generation of laser interferometric gravitational-wave detectors. Signals from coalescing binaries will most probably not stand above the broadband noise of the detector. Their detection is possible by the use of special data analysis techniques such as matched filtering which takes advantage of the fact that the wave form can be fairly well predicted. The wave form of the coalescing binary signal is known very accurately. However, the parameters of the signal are not known priori and the signal needs to be correlated with several filters which are copies of the coalescing binary wave form for different values of the parameters. In this paper we present an algorithm to choose a lattice set of filters by a criterion that every signal of a certain minimal strength is picked up by at least one filter of the set. The wave form is characterized by three parameters: the time of arrival, the mass parameter, and the phase of the signal. We show that it is enough to have just two filters corresponding to the phase of the signal. Determination of the lattice for various values of the mass parameter involves a knowledge of the cross correlation function of two chirp wave forms with different values of the parameters. It is shown that for a considerable range of the mass parameter, the peak value of the correlation function, in a certain approximation, does not depend on the absolute values of the parameters but only on their difference. This leads to a very convenient way of constructing most of the lattice. The maximum possible distance up to which we can see is restricted by the threshold of the detector. There is a further limitation on this distance brought about by the fact that we can use only a finite number of filters. The number of filters which one can use depends on the available computing power. Hence, there is an empirical relation between computing power and the distance up to which we can see. In a restricted sense, the computing power decides the number of detectable events. Numerical experiments indicate that parallel processing is a promising new approach to on-line data analysis.

Dispersion penalty reduction using an optical modulator with adjustable chirp

Abstract: Using a unique Ti:LiNbO/sub 3/ modulator, the value of the modulation chirp parameter that minimizes the transmission power penalty caused by fiber chromatic dispersion was experimentally identified. System experiments at 5 Gb/s using nonreturn-to-zero (NRZ) amplitude-shift-keyed (ASK) transmission with direct detection reception are discussed, and the optimum values of the modulation chirp parameter versus distance for transmission at 1.5 mu m wavelength over fibre having zero dispersion at 1.3 mu m are identified. 5 Gb/s NRZ transmission was achieved through distances of 128, 192, and 256 km of conventional fiber while incurring dispersion penalties of -0.5, 0.1, and 1.1 dB respectively, by operating at the quantum chirp value. >

Nonlinear distortion generated by dispersive transmission of chirped intensity-modulated signals

Abstract: Transmission through dispersive and nonlinear optical fibers produces distortion in subcarrier intensity-modulated systems. Analytic expressions for second- and third-order distortion are derived using the time-domain form of the field envelope wave equation. The distortion predicted by these expressions agrees well with numerical simulation and reasonably well with experimental data. Significant composite second-order distortion is predicted in typical 1.55 mu m cable television systems. >

Narrow line semiconductor laser using fibre grating

Abstract: A narrow linewidth, low chirp, wavelength selectable, packaged alternative to current DFB structures has been developed which employs a fibre reflection grating incorporated in the fibre pigtail. The wavelength of the packaged device can be determined by the appropriate choice of fibre grating coupled to the antireflection coated facet of a 1.55μm BH laser. Linewidths of less than 50 kHz, sidemode suppression of more than 30 dB and chirp when modulated at 1.2 Gbit/s of <0.5 MHz have been demonstrated.

Generation of 20-TW pulses of picosecond duration using chirped-pulse amplification in a Nd:glass power chain.

Abstract: Pulses of 20-TW peak power have been generated at 1064 nm using the chirped-pulse-amplification technique with a 90-mm output-aperture Nd:silicate glass amplification line. This system delivers 60 J of energy in a chirped pulse of 600-psec duration, with a capacity to maintain a 3.5-nm output bandwidth. These chirped pulses are compressed to 1.2 psec with an energy of 24 J by using large holographic gold-coated 1740-lines/mm diffraction gratings.

Effect of gain dispersion on ultrashort pulse amplification in semiconductor laser amplifiers

Abstract: The effect of gain dispersion in semiconductor laser amplifiers is discussed by considering an amplifier model that includes both gain saturation and gain dispersion. The saturated and unsaturated amplification regimes corresponding to whether the pulse energy is comparable to or much smaller than the saturation energy, are discussed separately. In the unsaturated regime, gain-induced group-velocity dispersion is found to play an important role. The amplified pulse can be compressed or broadened depending on whether the input pulse is chirped or unchirped. The pulse spectrum remains largely unaffected in the linear regime. In the saturation regime, self-phase modulation leads to considerable changes in the pulse spectrum. >

Suppression of the pedestal in a chirped-pulse-amplification laser

Abstract: The pedestal (prepulse and postpulse) associated with a chirped-pulse-amplification (CPA) laser is studied. Four components have been identified that contribute to the pedestal. Pulses are spectrally shaped by gain narrowing in a frequency-matched, regenerative amplifier, while self-phase modulation is avoided. The intensity contrast is further improved through the use of a saturable absorber, resulting in Gaussian pulses of ~0.9-ps duration with an intensity contrast exceeding 105:1. Both experimental and numerical descriptions of these processes are presented. This investigation makes possible the study of high-intensity ultrashort laser–plasma interactions with a fiber–grating CPA system.

Fiber-reflection-induced impairments in lightwave AM-VSB CATV systems

Abstract: Impairments due to multiple fiber reflections are evaluated for amplitude-modulated (AM) vestigial-side-band (VSB) lightwave cable TV (CATV) systems using directly modulated distributed feedback (DFB) lasers or externally modulated diode-pumped YAG laser transmitters. For DFB lasers, the spectral broadening caused by laser chirp results in a Gaussian optical spectrum with a width near 5 GHz. Square-law detection of multipath signals from two -30-dB reflections results in an effective intensity noise near -150 dB/Hz. For externally modulated YAG lasers, the same multiple reflections result in interference that cannot be described by an effective RIN. Although the total interference power is comparable to that of the DFB lasers, the signal and interference are highly correlated. As a result of this correlation, multipath interference for YAG-based systems cannot be measured using the standard test procedure with unmodulated carriers. >

Propagation of ultrashort light pulses in fibers with saturable nonlinearity in the normal-dispersion region

Abstract: The propagation of ultrashort light pulses in fibers with saturable nonlinearity and positive group-velocity dispersion has been investigated. Using a variational approach, frequency chirp and the other main characteristics of pulse evolution are studied. The minimum duration of the output-pulse of a fiber-grating compressor is calculated.

Ring laser gyroscope enhanced resolution system

Abstract: Disclosed herein is a Ring Laser Gyroscope Enhanced Resolution System comprising, in a preferred embodiment, a sampling frequency modulation phase-locked loop (or "chirp") which is used in combination with the fast filter. Enhanced resolution is achieved by using the fast filter (preferably a moving average digital filter) to gather optically integrated rate data and accumulate this data. During the data gathering process, the accumulated data is sampled at a rate set by a frequency modulated ("chirper") phase-locked loop in order to remove undesirable data due to aliasing effects upon the digital filter. In this manner, an enhanced resolution of the gyroscope angular measurement is achieved.

Distortion due to gain tilt of erbium-doped fiber amplifiers

Abstract: It was found experimentally and theoretically that the second-order intermodulation product, namely composite second-order (CSO) distortion, is caused by coupling of the gain tilt of an erbium-doped fiber amplifier (EDFA) with laser chirp. When the signal light is not at the gain peak of the EDFA, gain tilt exists and EDFA cannot be used in an AM-FDM optical analog video transmission system because of CSO degradation. To counteract the gain tilt induced CSO, the authors demonstrated that codoping Al and setting the laser oscillation wavelength at the gain peak are useful. Er-Al-codoped fiber amplifiers have better linearity than Er-doped fiber amplifiers because the gain tilt of the former is smaller. >

Electrical predistortion to compensate for combined effect of laser chirp and fibre dispersion

Abstract: It is shown that laser chirp causes significant nonlinear distortions in the RF domain of analogue subcarrier systems in dispersive fibre links. The effect can be consideredly reduced using the electrical compensation circuits shown.

50-fs pulse generation directly from a colliding-pulse mode-locked Ti:sapphire laser using an antiresonant ring mirror.

Abstract: 50-fs pulses were directly generated from a colliding-pulse mode-locked Ti:sapphire laser. To achieve the colliding-pulse mode locking, a miniature antiresonant ring containing an organic saturable dye jet was employed as the end mirror for the linear cavity laser. Based on measured dispersion of intracavity elements, a prism pair was implemented to control the cavity dispersion. The generated pulses have no linear chirp but do exhibit parabolic instantaneous frequency owing to third-order dispersion introduced by the prism pair.

Second-harmonic generation of femtosecond pulses: observation of phase-mismatch effects.

Abstract: We observe new effects in frequency doubling of colliding-pulse, mode-locked dye-amplified pulses (300 fsec, 620 nm, up to 1 mJ of energy) due to phase mismatch. If the second-harmonic generation in the nonlinear crystal (30-mm KDP crystal) is phase matched, the output is a square pulse. In contrast, when a phase mismatch is introduced, the generated pulse contains two peaks. We observe that the time profile is affected by depletion and chirp of the fundamental. The experimental results agree well with our numerical calculations.

Rastered, uniformly separated wavelengths emitted from a two-dimensional vertical-cavity surface-emitting laser array

Abstract: We report the first two‐dimensional vertical‐cavity surface‐emitting laser (VCSEL) array with each laser emitting a unique, nonredundant wavelength. This was achieved by implementing spatially chirped layers in the VCSEL structure and obliquely aligning the array axes to the direction of the thickness chirp. We obtained 77 wavelengths from a 7×11 VCSEL array with all the lasers emitting single mode. Uniform wavelength separation between neighboring lasers is achieved with the direction of increasing wavelength rastering through the array. All the lasers exhibit nearly the same optical and electrical properties, in spite of the intentionally implemented difference in the emission wavelengths.

Nonuniformly coupled microstrip transversal filters for analog signal processing

Abstract: The Fourier transform relationship between frequency response and impedance profile for single nonuniform transmission lines is used to derive the time-domain step response of single and coupled nonuniform lines. The expression for the step response of a characteristically terminated nonuniformly coupled transmission line structure is shown to correspond to the characteristic impedance profile. By using this relationship, any arbitrary step response can be realizing by utilizing nonuniformly coupled strip or microstrip lines for possible applications as waveform-shaping networks and chirp filters. A numerical procedure to compute the step response of the nonuniform coupled line four-port is also formulated in terms of frequency-domain parameters of an equivalent cascaded uniform coupled line model with a large number of sections. Sinusoidal and chirp responses are presented as examples that are readily implemented using coupling microstrip structures. The step response of an experimental nonuniformly coupled microstrip structure is presented to validate the theoretical results. >

Ultralow chirping short optical pulse (16 ps) generation in gain‐coupled distributed feedback semiconductor lasers

Abstract: We describe optical short pulse generation in novel gain‐coupled distributed feedback (GC‐DFB) semiconductor lasers by using a simple electrically pumped gain switching method. An optical pulse as short as 16 ps has been obtained from a 130 ps current pulse. The most exciting result is that the lasers keep single longitudinal mode oscillation with very low wavelength chirping (∼0.12 nm) during the gain switching operation. From the optical pulse width and the amount of the wavelength chirping, we have estimated the α parameter of the GC‐DFB laser to be less than 1.6.

High-speed and ultralow-chirp 11.55 mu m multiquantum well lambda /4-shifted DFB lasers

Abstract: A high-speed ultralow-chirp 1.55 mu m multiple-quantum-well (MQW) lambda /4-shifted distributed feedback (DFB) laser is demonstrated by using low capacitance structure and by the optimization of an NQW structure in an active layer. A 3-dB bandwidth of 14 GHz as an NQW-DFB laser was achieved, and a chirp width of 3.4 AA (20 dB down full width) was also achieved at 10 Gb/s direct modulation. In addition, kL dependence of chirp width is identified by the adiabatic chirping width is identified by the adiabatic chirping due to the spatial hole burning along a laser axis. From the systematic investigation of the nonlinear gain coefficient of NQW lasers as well as bulk lasers, it is suggested that the physical origin of nonlinear damping can be explained by the spectral hole burning theory. >

High powers and subpicosecond pulses from an external-cavity surface-emitting InGaAs/InP multiple quantum well laser

Abstract: We demonstrate that surface‐emitting lasers operating in an external cavity can produce high average powers, high peak powers, and ultrashort pulses. By optical pumping of InGaAs/InP multiple quantum well samples in an external cavity, we generated 190 mW both in continuous and mode‐locked operation at 1.5 μm. Synchronous pumping at 100 MHz yielded 7.7 ps pulses with 15 mW average power. These were shortened to 1. 1 ps pulses with 64 W peak power by chirp compensation using diffraction gratings, and to 710 fs by negative group‐velocity dispersion in an optical fiber.

Effect of chirping-induced waveform distortion on the performance of direct detection receivers using traveling-wave semiconductor optical preamplifiers

Abstract: The influence of chirping-induced waveform distortion on the performance of multigigabit-per-second traveling-wave semiconductor optical amplifier (TWSOA)/p-i-n direct detection receivers is evaluated. The results are based on a novel method of evaluating the probability of error in the presence of the signal-spontaneous and spontaneous-spontaneous beat noise components. Laser chirping causes the dependence of the receiver sensitivity on the fiber dispersion coefficient*length product DL to be different for TWSOA/p-i-n and avalanche photodiode (APD) receivers. Compared to the APD receiver, the sensitivity of the TWSOA/p-i-n receiver degrades less quickly. So for cases of practical interest, the TWSOA/p-i-n receiver is more tolerant of chirping-induced waveform distortion. >

Strong plasma wave excitation by a ‘‘chirped’’ laser beat wave

Abstract: A modification of the plasma beat‐wave excitation scheme is proposed, aimed at a significant increase of the resulting plasma wave amplitude. The modification employs two laser beams with a down‐chirped beat frequency that follows, on the average, the amplitude‐dependent frequency of the intense plasma wave. Numerical and analytical calculations that support the proposed scheme are presented. Also, new analytical solutions for the constant beat‐frequency case are obtained. Finally, a plausible experimental setup is suggested, where one of two CO2 laser beams attains the necessary frequency chirp by passing through a diffraction grating pair.

Observation of low-chirp modulation in InGaAs-InAlAs multiple-quantum-well optical modulators under 30 GHz

Abstract: Modulated light spectra were measured in long-wavelength InGaAs-InAlAs multiple-quantum-well intensity modulators under 30-GHz large-signal modulations. The linewidth broadening factor alpha is determined from the relation between the intensity modulation index and the sideband strength relative to the carrier. The minimum alpha value is estimated to be 0.70 at 1.54 mu m, which is almost the same as the lowest value so far reported in a bulk Franz-Keldysh modulator. This is significantly lower than what is obtained from direct-intensity modulation of injection lasers, making this a useful device for application to high-bit-rate long-haul optical communication systems. >

Chirp mechanisms in soliton-dragging logic gates.

Abstract: We theoretically and experimentally study the chirping mechanism responsible for the time shifts in soliton-dragging logic gates. Cross-phase modulation during the first one or two walk-off lengths in a birefringent optical fiber causes most of the frequency shift that translates into a time shift after propagation in a soliton dispersive delay line. Introducing gain or loss during the pulse interaction asymmetrizes the pulse walk-off and, consequently, increases the temporal window over which soliton dragging can result in a time shift. Analytic formulas for the time shift provide the scaling laws as a function of various fiber parameters.

Experimental study of additive-pulse mode locking in an Nd:glass laser

Abstract: Self-starting additive-pulse mode locking (APM) has been investigated experimentally in a continuous-wave Nd:glass laser. Stress has been put on the study of the self-starting process and relaxation oscillation instabilities. An intensity threshold for self-starting APM is observed and related to the linewidth of the first beat note of the power spectrum of the free-running laser output. Under steady-state conditions, two distinct operating regimes are obtained: mode locking with repetitive self-Q switching and pure mode locking. Increasing the intracavity power gives rise to a strong chirp developing on the mode-locked pulses and to a simultaneous disappearance of self-Q switching. >

Dispersion Penalty Reduction using a Optical Modulator with Adjustable Chirp

Abstract: High bit-rate fiber transmission systems operating away from the zero-dispersion wavelength require transmitters with very low frequency chirp, such as those realized using lithium niobate waveguide electrooptic modulators [1], It has been proposed that waveguide Mach-Zehnder amplitude modulators may be operated in a perfectly chirpless mode and recently devices designed to operate chirp-free have been reported [2,3]. However, the lowest fiber dispersion penalty is generally not obtained for a frequency chirp parameter identically equal to zero [2], Indeed, it is expected that choosing a non-zero value for the chirp parameter, α, depending on the fiber dispersion coefficient and distance, so as to provide some amount of pulse compression, can be advantageous.

Frequency-chirped copropagating multiple-bit stimulated-echo storage and retrieval in Pr(3+):YAlO(3).

Abstract: Using a frequency-chirped ring dye laser, we have stored and recalled as much as 48 bits of information with the copropagating stimulated-echo geometry on the 3H4−1D2 transition in Pr3+:YAlO3. Since the copropagating excitation scheme has all the excitation pulses spatially overlapping, this technique will be useful to the practical implementation of stimulated-echo memory because there is no need for alignment of beams. Our experiments also demonstrate that frequency chirping of the excitation pulses dramatically improves not only pulse-shape reproduction by the echo pulse but also the shot-to-shot stability of the amplitude of the recalled data.

Evidence for chirp reversal and self-compression in a femtosecond optical parametric oscillator

Abstract: We report the generation of tunable light pulses with durations as short as 65 fs by pulsed synchronous pumping of a singly resonant optical parametric oscillator. The measured wavelength dependence of the pulse duration provides evidence for self-compression by means of chirp reversal of the idler pulses and normal dispersion of the optical parametric oscillator cavity elements.

A stepped chirp technique for range resolution enhancement

Abstract: A novel stepped frequency chirp waveform concept for increasing the range resolution of an existing pulse compression radar is presented. The stepped frequency chirp technique is ideally suited to obtaining high range resolution in a radar system that has a limited instantaneous bandwidth, but a large tunable bandwidth. To achieve high range resolution using these given bandwidths, two or more chirp waveforms are sequentially transmitted and received at selected carrier frequencies in the tunable bandwidth, and attached together using signal processing techniques to simulate a chirp signal covering up to the entire tunable bandwidth. In contrast to conventional or hybrid stepped frequency methods, the technique achieves both unambiguous range coverage and good range sidelobes without requiring a large number of frequency steps. >