Showing papers on "Chirp published in 1987"

Dispersion cancellation using linearly chirped Bragg grating filters in optical waveguides.

Abstract: The use of a linearly chirped Bragg grating filter for dispersion cancellation in an optical-fiber link is discussed. Numerical and theoretical calculations are made, which show that, with the proper taper function, the filter can have a high reflectivity and a quasi-constant nonzero dispersion, proportional to the inverse of the chirp. The filter can compress dispersion-broadened pulses by factors of 2-5 or more, if many filters are cascaded. Its compactness and efficiency would make it suitable for on-line implementation.

The relation of line narrowing and chirp reduction resulting from the coupling of a semiconductor laser to passive resonator

Abstract: We present a theory of adiabatic chirp reduction and narrowing of the Lorentzian laser line that occurs when a laser is coupled to an external passive resonator. Chirp reduction and line narrowing are simply related. We show that the reduction in the Lorentzian line width is equal to the square of the reduction in adiabatic chirp. Both are strongly enhanced near resonances of the external cavity.

Computer simulation of high-bit-rate optical fiber transmission using single-frequency lasers

Abstract: Chirp-induced dispersion penalties in high-bit-rate optical fiber transmission are assessed using numerical integration of laser rate equations and a Fourier transform fiber dispersion routine. The roles of the imposed modulation waveform and laser design parameters are evaluated from computer generated eye diagrams and simple analytical observations. Consistent with experiment, we find device dependent optimum laser extinction ratios. In addition, we address the delicate balance between nonlinear chirp-induced dispersion penalties and the speed limitations imposed by linear current filtering on both the laser transmitter and the receiver. These considerations become increasingly important at higher bit rates such as 8 Gbit/s.

Design of high-power ultrashort pulse amplifiers by expansion and recompression

Abstract: A system that expands and recompresses an ultrashort pulse by means of diffraction gratings is described. The expansion can be large enough to broaden the pulse to several nanoseconds. In this manner, as the signal and pump pulses have similar lengths, high-efficiency amplifiers can be designed. The expansion avoids inefficient energy storage in high-gain amplifiers and nonlinear effects in the amplifier medium, and the amplified spontaneous emission is drastically reduced by working in a saturated amplifier regime.

Bayesian Spectrum and Chirp Analysis

Abstract: We seek optimal methods of estimating power spectrum and chirp (frequency change) rate for the case that one has incomplete noisy data on values y(t) of a time series. The Schuster periodogram turns out to be a “sufficient statistic” for the spectrum, a generalization playing the same role for chirped signals. However, the optimal processing is not a linear filtering operation like the Blackman-Tukey smoothing of the periodogram, but rather a nonlinear operation. While suppressing noise/side lobe artifacts, it achieves the same kind of improved resolution that the Burg method did for noiseless data.

Evolution of femtosecond pulses in single-mode fibers having higher-order nonlinearity and dispersion.

Abstract: Using computer simulations, we examine the effects of higher-order dispersive and nonlinear propagation processes on the spectral and time development of ultrashort, high-intensity pulses propagating in single-mode optical fibers having normal dispersion. Our results indicate that both the cubic-dispersion term and the shock term of the nonlinear Schrodinger equation contribute to asymmetry in the pulse power spectrum and cause highly nonlinear chirp.

High-speed picosecond optical pulse compression from gain-switched 1.3-µm distributed feedback-laser diode (DFB-LD) through highly dispersive single-mode fiber

Abstract: Picosecond optical pulse compression characteristics of chirped pulses from gain-switched distributed feedback-laser diodes (DFB-LD) transmitting through highly dispersive media are studied theoretically and experimentally. It is clarified theoretically that gain-switched chirped pulses can be compressed to about a 0.7-time bandwidth product by normal dispersion of the dispersive media and that the optimum dispersion value to obtain a minimum compressed pulse is proportional to the square of original pulsewidth. Through a dispersion, shifted single-mode fiber with -48-ps/nm normal dispersion at a 1.3-μm wavelength, gain-switched 30-ps (FWHM) pulses from a directly modulated 1.3-μm DFB-LD at a 4.4-GHz repetition rate have been successfully compressed to 6.4-ps optical pulses with a 0.86-time bandwidth product. Experimental results agree with the theoretical analysis.

Optical systems using volume holographic elements to provide arbitrary space-time characteristics, including frequency-and/or spatially-dependent delay lines, chirped pulse compressors, pulse hirpers, pulse shapers, and laser resonators

Abstract: Optical systems, using volume holographic elements (gratings) having geometries which tailor the spatio-temporal dispersion of the optical pulses for the system. The input optical pulse is characterized by a frequency variation across the temporal profile of the pulse. The various frequency components of this pulse are first dispersed by at least one grating which may be of the blazed reflection or holographic volume transmission type. The resultant dispersed light is then diffracted by a holographic volume grating which imparts the desired temporal dispersion characteristics to the pulse. The shape of the holographic element will vary according to the input pulse frequency profile as formed by varied chirping techniques. A grating stage may then be repeated, preferably with additional elements in mirror symmetry to the first or by retro-reflection, in order to recombine the spatially dispersed pulse components into an exiting pulse which may be of vastly compressed temporal profile. In optical dispersive delay lines, the grating geometry provides temporal dispersion which is a desired function of wavelength of the optical pulses.

Wigner distribution method equivalent to dechirp method for detecting a chirp signal

Abstract: It is shown that the Wigner distribution (WD) method of optimally detecting a chirp signal in white Gaussian noise [1] is equivalent mathematically to a dechirp method.

Frequency chirp and spectra due to self-phase modulation and stimulated Raman scattering influenced by pulse walk-off in optical fibers

Abstract: A numerical study is presented on the frequency chirp and spectra of light pulses affected by both self-phase modulation and stimulated Raman scattering in optical fibers. The asymmetric distortion of pump and Stokes spectra is attributed to walk-off between the pulses and found to be in good agreement with recent experimental results. The stationary-phase method is employed to explain the relation between the structure of the spectra and the form of the pulse chirp. An analytic expression describing the chirp of a depletable pump and of the first Stokes pulse is given for negligible walk-off. The dominant influence of self-phase modulation over the cross phase modulation on the buildup of the chirp in the case of large walk-off and severe pump depletion is shown.

Observation of large quadratic electro-optic effect in GaAs/AlGaAs multiple quantum wells

Abstract: We simultaneously measure the intensity modulation level and the optical spectrum of the output of a multiple quantum well modulator, and use these data to deduce the electro‐optic coefficients. The effect is quadratic, with a measured ‖s33‖=4.6×10−13 cm2/V2 at a wavelength 12 meV below the band gap. This is approximately 800 times the coefficient measured further from the band gap. We are able to achieve a fractional change in the refractive index of 3.7%. Despite the size of this effect, when we operate the device as an intensity modulator, we observe a linewidth enhancement factor of α=1.0, which means the chirp induced in the device’s output will be small.

Analysis of chirp power penalty in 1.55-µm DFB-LD high-speed optical fiber transmission systems

Abstract: In a 155-μm high-speed transmission system using a DFB-LD, the chirp effect in the LD is a factor limiting transmission span length In order to evaluate the chirp effect, we derive expressions for the chirp power penalty for two cases: the cases of the chirp occurring in both edges of the pulse and in the whole time of the pulse The calculations based on the results of the LD chirp measurement predict that the chirp effect will be significant above 2 Gbit/s even if the zero dispersion wavelength of fiber is shifted to the 155-μm band Transmission experiments performed at 12 Gbit/s and 24 Gbit/s verify this prediction

Bandwidth-limited picosecond pulse generation in an actively mode-locked GaAs laser with intracavity chirp compensation.

Abstract: A Gires–Tournois interferometer has been applied for intracavity chirp compensation of an actively mode-locked GaAs laser. Fourier-transform-limited pulses as short as 4.6 psec have been obtained at 790 nm.

Analytic study of pulse broadening in dispersive optical fibers

Abstract: It is demonstrated that for an optical pulse propagating along an optical fiber the rms pulse width varies parabolically with distance, irrespective of initial pulse form and frequency chirp variation. Furthermore, the result is true to arbitrary dispersive order and should prove a very useful tool in determining the information-carrying capability of long-distance optical-fiber transmission systems.

Method and apparatus for measuring ultrashort optical pulses

Abstract: A method and apparatus for measuring ultrashort optical pulses is described. Pulse waveforms to be measured are the type which vary at high speed in a time close to or less than the response time of an optical detector. Changes of optical wavelength or frequency are measured at various sections of that pulse. The light to be measured is split into two beams, and recombined after provided with different delay times, respectively. The combined light is caused to enter a doubling crystal to generate a second-harmonic light, and the second-harmonic light component, or both the second-harmonic and fundamental components, is measured. The intensity shape and chirping of the pulses is obtained from the changes in the intensity of these components as a function of delay time difference by Fourier analysis including iterative arithmetic operations.

Dispersion compensation for femtosecond pulses using high‐index prisms

Abstract: We have compensated the residual chirp of the pulses emerging from the thick output coupler of our femtosecond dye laser with a sequence of four high‐refraction index prisms. Using the same prisms, we show that it is possible to compensate the great amount of dispersion due to a femtosecond multistage amplifier leading to a compact and low‐loss compensation system.

A chirp-compensation technique using incident-angle changes of cavity mirrors in a femtosecond pulse laser

Abstract: A technique for chirp-compensation in a CPM laser is presented. By using the change of the incident angle to multilayer dielectric cavity mirrors, the intracavity second-order dispersion \phi''(\omega) is adjusted without any additional elements. It is confirmed that the optimum value of \phi''(\omega) = +2.1 \times 10^{-28} s2obtained when up-chirp was compensated and pulses as short as 55 fs were generated is reasonable, by comparison to analytic results of chirp behaviors. In addition, the effect of the third-order dispersion \phi'''(\omega) at the optimum value of \phi''(\omega) on pulses is evaluated.

Picosecond measurement of chirp in gain-switched, single-mode injection lasers

Abstract: Wavelength shifts caused by chirp in pulses generated from a gain‐switched InGaAsP distributed‐feedback laser have been measured with 0.5 ps resolution. The applicability of a simple, two‐term analytical expression to describe the chirp is evaluated. For the pulses from the gain‐switched laser, the power‐dependent ‘‘wavelength‐offset’’ term in the formula is the most important.

Relation between chirp and linewidth reduction in external Bragg reflector semiconductor lasers

Abstract: A 1.5‐μm single longitudinal mode semiconductor laser, formed by butt coupling an external Bragg reflector to a standard Fabry–Perot laser, is shown to have greatly reduced wavelength chirp and emission linewidth. The linewidth reduction is found to be proportional to the square of the chirp reduction, in agreement with a previous theoretical prediction. The linewidth and chirp reduction factor can be varied by changing the operating point of the laser. With a 3‐mm‐long Bragg reflector, a linewidth of 1 MHz at 5 mW has been achieved.

Analysis of frequency chirping of semiconductor lasers in the presence of optical feedback.

Abstract: The frequency chirping of a single-mode semiconductor laser in the presence of optical feedback is studied by rate-equation analysis. The model includes the amplitude–phase-coupling and spectral-hole-burning effects. A simple analytical formula is obtained in the small-signal regime that shows that the amplitude–phase-coupling effect also enhances the chirp-reduction ratio and that the maximum reduction of frequency chirping can be achieved at the same time as maximum line narrowing in the in-phase condition.

Frequency chirping In some types of external modulator

Abstract: The frequency chirping caused by direct laser modulation limits the transmission bandwidth of a single-mode fiber system, even when we employ a directly modulated dynamic-single-mode laser. To eliminate this problem, some types of external intensity modulator, such as those employing electroabsorption1,2 and electrooptic effects3 have been studied. We have pointed out that frequency chirping is caused by phase modulation due to a refractive-index change in a loss modulator.4

Observation of Low Chirp Modulation in Long Wavelength InGaAs/InAlAs Multiple-Quantum-Well Optical Modulators

Abstract: Modulated light spectra are measured in long-wavelength InGaAs/InAlAs multiple-quantum-well intensity modulators under 990 MHz, large signal modulation. The linewidth broadening factor, α, is determined from the relation between the intensity modulation index and the sideband strength, relative to the carrier. The minimum α value is estimated to be 0.60, which is less than that reported in a bulk Franz-Keldysh modulator.

Method and apparatus for the operation of a distributed feedback laser

Abstract: There are disclosed a method and apparatus for producing laser pulses at two wavelengths alternately by switching the oscillations of a distributed feedback semiconductor laser of the continuous grating type back and forth across the stop band. The necessary shaping of the gain or index of refraction characteristic along the path of the oscillations is accomplished in part by two top electrodes, one of which is longer than the other along that path, and by varying the current supplied to at least one of those electrodes in the appropriate sense. Applications for optical communication and for optical logic are disclosed, the preferred embodiment being a transmitter for an optical communication system with minimal chirp of the optical pulses, in which pulses at one of the wavelengths are modulated at a bit rate exceeding 1 Gigabit by direct-modulation control of the switching of the laser, and the pulses at the other of the two wavelengths are discarded.

Compression of nonlinearly chirped pulses using Gires-Tournois interferometers

Abstract: In optical pulse compression experiments in the femtosecond regime, cubic phase distortion can adversely affect the compression ratio. This distortion arises from both the cubic phase of the grating pair used in such experiments and also from pulse propagation in the fiber, when taking into account the cubic order dispersion and shock terms. We numerically analyze how the cubic distortion, which leads to nonlinearly chirped pulses, can be compensated by Gires–Tournois interferometers. We discuss design considerations and show how the use of such interferometers can lead to even shorter optical pulses by utilizing more of its available bandwidth.

2 Gbit/s optical CPFSK heterodyne transmission through 200 km single-mode fibre

Abstract: The amplitude distortion of a continuous phase FSK signal due to fibre chromatic dispersion is measured. We also report a 2 Gbit/s optical CPFSK heterodyne detection transmission experiment. Since there was no chirping degradation, it was possible to transmit the signal through a 200 km single-mode fibre.

Chirp and Doppler optical gauge

Abstract: A laser distance gauge measures the distance and shape of a target by alternately performing (a) a Doppler shift measurement during optical scanning of the target's surface and/or motion of the target and (b) a chirp measurement with a chirp frequency laser signal. To ascertain changes in radial distance from the gauge to the target, the Doppler shift information is integrated and the result is combined with absolute distance measurements made in the chirp mode. A single CW laser, which operates single frequency, serves for both types of measurement. A computer changes the gauge from Doppler mode to chirp mode when the Doppler shift reaches a predetermined amount. When a chirp measurement of absolute distance is completed it reverts to Doppler mode.

Imaging CO 2 Laser Radar With Chirp Pulse Compression

Abstract: This paper describes the design of a coherent imaging CO2 laser radar, based upon the use of a continuous wave, single mode CO2 laser and the "chirp" pulse compression technique. This experimental breadboard generates three independent images of a given scene, respectively representative of the range, radial velocity and reflectance (at 10,6 μ) for each of its resolution spots. It has been tested outside and some typical images are pre-sented here, demonstrating the high Doppler sensitivity and range accuracy of pulse com-pression techniques applied to coherent CO2 laser systems.© (1987) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Performance of Acoustic Charge Transport Chirp Filters

Abstract: The measured performance of acoustic charge transport (ACT) based linear FM dispersive delay line filters is presented and compared to modelled performance. The excellent agreement between the theoretically predicted device performance and the measured results shows clearly that anodization of the nondestructive sensing array is an effective means for achieving weighted ACT filter responses.