Showing papers on "Chirp published in 1990"

Parameter estimation of chirp signals

Abstract: The problem of the parameter estimation of chirp signals is addressed. Several closely related estimators are proposed whose main characteristics are simplicity, accuracy, and ease of online or offline implementation. For moderately high signal-to-noise ratios they are unbiased and attain the Cramer-Rao bound. Monte Carlo simulations verify the expected performance of the estimators. It should be easy to extend this approach to signals having polynomials of any degree in the exponent. All the derivations will be done under the assumption that the signal-to-noise ratio is sufficiently high. >

Programmable femtosecond pulse shaping by use of a multielement liquid-crystal phase modulator.

Abstract: We report programmable shaping of femtosecond optical pulses by use of a multielement liquid-crystal modulator to manipulate the phases of spatially dispersed optical frequency components. Our approach provides for continuously variable control of the optical phase and permits the pulse shape to be reconfigured on a millisecond time scale. We use the apparatus to demonstrate femtosecond pulse-position modulation as well as programmable compression of chirped femtosecond pulses.

Theoretical analysis of waveguide second-harmonic generation phase matched with uniform and chirped gratings

Abstract: Theoretical analysis of second-harmonic generation (SHG) phase matched with gratings in a channel waveguide is presented from the viewpoint of device design. The analysis includes cases where efficiency is high and residual phase mismatch is involved. Nonlinear couples mode equations, with general expressions for effective SHG coefficients and coupling coefficients, are derived, solved, and discussed for uniform and chirped gratings. It was found that very high efficiency with broadened phase matching bandwidth can be simultaneously obtained with chirped gratings. The results are summarized in graphic data with normalized parameters, and guidelines and criterion for optimum design are given. >

Application of chirped ultrashort pulses for generating large-amplitude ground-state vibrational coherence: a computer simulation

Abstract: Computer simulations are presented to show that frequency-chirped ultrashort pulses, when properly employed, may be more effective than their compressed analogs for generating large-amplitude vibrational coherence in molecular systems. A one-dimensional model, which was used previously to simulate impulsive optical excitation of CsI, is used [ Chem. Phys. Lett.158, 238 ( 1988)]. A negatively chirped pulse (for which the light is red-shifted with time) is devised to excite the population initially onto the excited surface. As this population propagates in the repulsive upper state, the later red-shifted portions of the pulse follow the change in the resonance conditions and effectively dump the population back down into excited vibrational states of the ground surface. These results are compared with the dynamics simulated with the compressed and positively chirped analogs, and the experimental realization of this scheme is discussed.

ABCD matrices for dispersive pulse propagation

Abstract: The use of the ABCD matrices for the calculation of pulse propagation in second-order dispersive media is developed rigorously. Their usefulness is demonstrated by obtaining the results for the propagation of a Gaussian pulse with an arbitrary linear chirp. By analogy with ray optics, the concept of a time ray is introduced to give further insight. The analogy with paraxial optics is carried further by extending the matrix method to a complete Hermite-Gaussian basis and thus to arbitrarily shaped and nonlinearly chirped pulses. Some practical applications of the matrix method are discussed. >

The static and dynamic characteristics of single and multiple phase-shifted DFB laser structures

Abstract: The influence of longitudinal mode spatial hole burning (LMSHB) on the performance of distributed feedback (DFB) laser structures is examined in detail. A comprehensive model has been used to interpret the experimental results and to construct a theoretical framework that was utilized to develop more advanced device designs. An increasing side mode intensity with output power, movement of the lasing mode relative to the stopband, and curvature of the light-current characteristic at low power can all be manifestations of the influence of LMSHB on the static device performance. The dynamic behavior can also be affected, with extended wavelength chirp and amplitude patterning effects on the timescale of the effective carrier recombination time being particularly important. >

A high-resolution chirp transform spectrometer for microwave measurements

Abstract: A microwave heterodyne spectrometer consists of a radiometer front-end and a real-time spectrum analyser back-end. Common spectrometer types are filterbanks, autocorrelators and acousto-optic spectrometers (AOS). A high-resolution chirp transform spectrometer using reflective array compressor (RAC) filters with a time bandwidth product of 6400 is presented. The spectrometer has 1600 channels within a 40 MHz input bandwidth and a linear dynamic range of 63 dB for a CW input. It seems to be very promising for future space applications.

Optical equalization to combat the effects of laser chirp and fiber dispersion

Abstract: Optical equalizers capable of combating the effects of laser chirp and fiber chromatic dispersion on high-speed long-haul fiber-optic communication links at 1.5 mu m are described. They consist of both reflective and transmissive cavity structures. The equalizers are adaptive in the sense that the position of their periodic frequency responses are optimally adjusted in real time. The equalizers are evaluated by using computer simulation routines based on the rate equations for intensity-modulated lasers. It is shown that, by using equalizers of the type proposed, the receiver data eyes remain open well beyond where they would close without equalization. >

Optical logic device

Abstract: An optical logic device based on the time-shift-keying architecture is described in which digital logic functions are realized by applying appropriate signal pulses to a nonlinear shift or "chirp" element whose output is supplied to a dispersive element capable of supporting soliton propagation. In an optical fiber realization of the optical logic device, two orthogonally polarized pulses are supplied to the combination of a moderately birefringent fiber acting as the nonlinear chirp element and a polarization maintaining fiber acting as the soliton dispersive delay element having a anomolous group velocity dispersion at the signal wavelengths of interest. A nonlinear frequency shift is created in one of the pulses in the former element through cross-phase modulation and, in turn, the frequency shift is translated into a temporal shift of the affected pulse in the latter element. These devices operate at switching energies approaching 1pJ.

Communication system and apparatus using chip signals

Abstract: There are provided a communication system and an apparatus for modulating a communication signal into a chirp signal and executing the communication. In the system, a signal to be transmitted is input, a predetermined analog pattern signal is added to the input signal and is pulse position modulated, the modulated signal is converted into the chirp signal, and the chirp signal is output onto a transmission path. The converted chirp signal can be also converted into an optical signal and the optical signal can be also output onto the transmission path. After the analog or digital input signal was pulse position modulated, by converting the modulated signal into the chirp signal, the analog signal can be also transmitted and received as a chirp signal. Thus, the communication reliability can be assured without being influenced by disturbance noises or irrespective of a limitation of a transmission electric power of the transmission path.

Ultralow chirp and high-speed 1.55 mu m multiquantum well lambda /4-shifted DFB lasers

Abstract: An ultra-low-chirp, high-speed, 1.55- mu m, multi-quantum-well (MQW), lambda /4-shifted, InGaAsP distributed-feedback (DFB) laser is demonstrated. A record chirp width of 0.4-0.5 nm (20-dB down full width) is achieved at 10-Gb/s direct modulation. This low-chirp single-longitudinal-mode operation is attained by an increase in the quantum size effect in the MQW active layer. >

Optical equalization of fiber chromatic dispersion in a 5-Gb/s transmission system

Abstract: A 5-Gb/s lightwave transmission experiment demonstrates the effectiveness of an all-pass-type fiber Fabry-Perot optical equalizer for eliminating an error floor caused by laser chirp and chromatic dispersion. >

The segmented chirp Z-transform and its application in spectrum analysis

Abstract: The chirp Z-transform (CZT) algorithm is known to be more flexible than the fast Fourier transform in calculating the frequency spectrum. However, its mathematical implementation is complex, and additional memory space is required. A modified CZT algorithm called the segmented chirp Z transform (SCZT) is introduced. SCZT has the ability to handle a very large amount of input data with very limited memory space and to limit its calculation to a portion of the frequency spectrum of interest with greatly increased dynamic range and frequency resolution. >

Chirp and self-phase modulation in induced-grating autocorrelation measurements of ultrashort pulses.

Abstract: We show that induced-grating/four-wave-mixing ultrashort-pulse autocorrelation techniques that use slowly responding media offer the same phase information as interferometric second-harmonic generation. We also show that autocorrelation traces from nearly all possible induced-grating/four-wave-mixing beam geometries provide this information, with all yielding the same theoretical result, an integral of a fourth-order electric-field coherence function. Such traces clearly reveal chirp and self-phase-modulation effects without high-frequency fringes. Experiments using a two-beam-coupling arrangement in photorefractive media illustrate these effects.

Generation and cancellation of second-order harmonic distortion in analog optical systems by interferometric FM-AM conversion

Abstract: The generation of strong harmonic distortion from interferometric FM-AM conversion in analog lightwave systems is demonstrated both theoretically and experimentally. The effect results from the combined effects of wavelength chirping of the laser coupled with the nonlinear transmission characteristics of a dispersive element in the optical beam path. It is shown that reflections as small as 2-10/sup -4/ will result in second-order distortion as large as -65 dBc. The effect can also be used to cancel any laser- or detector-induced harmonic distortion. In one example, the -36 dBc second-order distortion in a distributed-feedback (DFB) laser is improved to better than -70 dBc. >

Laser diode radar with extended range

Abstract: An optical radar system includes a laser diode and an external cavity formed by a partial reflector for reflecting a first portion of the laser beam back into the diode. A second portion of the beam is passed out of the external cavity for backscatter thereof from a target back into the diode. The emission thereby has a beat frequency related to the velocity of the target. A frequency chirp is introduced by mechanically oscillating the partial reflector longitudinally resulting in a modulation frequency in the emission corresponding to the range of the target. A photodetector and processor are used to determine the velocity and range.

Mode selection and spatial hole burning suppression of a chirped grating distributed feedback laser

Abstract: A new kind of distributed feedback (DFB) laser with a linearly chirped grating is proposed and investigated theoretically for the first time. The eigenmodes of the chirped grating DFB (CG‐DFB) and the field intensity distribution are calculated. With a suitable cavity length and chirp factor, the threshold gain difference to the next competence mode can be very large and will keep the single longitudinal mode stable. The chirp factor can be chosen to make the field intensity distribution quite uniform and at the same time keep the threshold gain difference large. This behavior effectively suppresses spatial hole burning and hence makes CG‐DFB lasers that are stable single mode at high power, and with a narrow linewidth.

Limits of chirped pulse compression with an unchirped Bragg grating filter

Abstract: It has been suggested that the large value of group velocity dispersion (GVD) found in a Bragg grating filter written in an optical waveguide could be used for pulse compression. Three expressions are derived relating the compression ratio to the value of GVD and to the bandwidth over which GVD is nearly constant. It is shown that the maximum compression ratio that can be achieved with an unchirped grating is severely limited by the small bandwidth of the high dispersion regions in the transmission spectrum of the filter.

New dynamic model for multimode chirp in DFB semiconductor lasers

Abstract: A new technique for modelling the dynamic spectral characteristics of DFB semiconductor lasers above threshold, which is based on the transmission-line laser model, is described. This includes the effects of index carrier dependence and longitudinal index variations. The time-domain responses and spectra of quarter-wave shifted grating devices are compared with unshifted devices under transient conditions.

Free-electron laser oscillator for simultaneous narrow spectral resolution and fast time resolution spectroscopy

Abstract: A technique for increasing the temporal resolution of the optical pulses white simultaneously increasing the spectral resolu-tion of the pulse train in a free-electron laser. The increase in temporal resolution is achieved by introducing a quadratic temporal phase variation (i.e. linear frequency chirp) across the optical pulses, and then propagating the outcoupled pulses through a dis-persive delay line such as a grating pair or prism pair in order to compress them to the transform limit of the broadened spec-trum. Essentially, the quadratic time dependence of the optical phase broadens the spectrum and yields Fourier components whose phase exhibits a quadratic frequency dependence. To operate a free-electron laser in such a configuration, electron micro-pulses with a linear time-dependent energy are produced in the RF linac by positioning the bunches slightly off the peak of the RF wave at phases where the field gradient is large. These electron micropulses are injected into the wiggler and lead to the for-mation of optical pulses with a corresponding linear time-dependent frequency.

Laser pumped atomic frequency standard with high frequency stability

Abstract: A laser emits a laser beam with a predetermined frequency, serving as pumping light, to a double resonance section. A VCO oscillates a predetermined frequency signal that is the origin of a microwave. An oscillator oscillates a modulation signal and a low frequency signal to be a reference signal for a sync detection. A synthesizer generates a microwave acquired by subjecting the predetermined frequency signal to multiplication and phase modulation in accordance with the predetermined frequency signal and the modulation signal, and applies the microwave to the double resonance section. A photosensor detects a component of the laser beam modulated by the double resonance section. A PSD detects the level and phase of an output signal of the photosensor with the reference signal. A sweep signal generator generates a sweep signal for the microwave. A switch has first and second nodes to be mutually switched from one to the other, and feeds a PSD output back to the VCO when switched to the first node, and supplies the sweep signal to the VCO to permit sweeping of the frequency of the microwave when switched to the second node. An integration processor detects the amount of shift of a microwave resonance frequency according to the PSD output and feeds a laser frequency compensation signal back to the laser, in a state where the switch is set to the second node.

Tunable, picosecond pulse generation using a compressed, modelocked laser diode source

Abstract: The generation of pulses shorter than 6 ps, tunable from 1.26 to 1.32 mu m, is discussed. The source of these pulses consists of a mode-locked external-cavity 12.3- mu m InGaAsP diode laser with grating feedback combined with a compressor which is a grating pair containing an internal telescope. The dispersion in the compressor can be tuned in both magnitude and sign to optimally compensate the chirp in the mode-locked external cavity laser at each wavelength. >

Noise and modulation performance of Fabry-Perot and DFB semiconductor lasers with arbitrary external optical feedback

Abstract: A unified study of the noise and modulation characteristics of Fabry-Perot and DFB semiconductor lasers with arbitrary external optical feedback and arbitrary external cavity length is presented. By performing a small-signal analysis and taking into account the nonlinear gain saturation effect, generalisations of previous analytical results and novel features are presented in the following topics: intensity and frequency noise, intensity and frequency modulation responses, chirp reduction, frequency and damping rate of relaxation oscillations.

Aperture synthesized radiometer using digital beamforming techniques

Abstract: A digital aperture synthesized radiometer for synthesizing the imaging an image scene. A plurality of antenna arrays receive radiation emitted or reflected from an scene, and an analog to digital coverter converts received radiation into digitized signals. A digital beamformer synthesizes the digitized signals to provide an image corresponding to the scene. The digital beamformer comprises individual digital beamformers which generate a set of fanbeam signals for each array. The beamformers provide for cross track imaging of the scene. A digital interferometer correlates corresponding pairs of fanbeam signals from the two sets of fanbeam signals to produce a chirp signal for each pair. A matched filter processes the chirp signals to transform each chirp signal into a corresponding image point of the scene. This provides for along track imaging of the scene. The beamformers include an amplitude weighting and data turning circuit to reduce fanbeam signal sidelobe levels and eliminate alternate mainlobes from the digitized radiation signals to reduce mainlobe widening. A fast Fourier transform circuit in the beamformers generally comprises a decimation-in-time algorithm implemented by means of a plurality of parallel and cascaded butterfly computation circuits. Image processing methods for achieving digital radiometry are also disclosed.

Pulse compression of an actively modelocked diode laser using linear dispersion in fiber

Abstract: Pulses from a tunable actively mode-locked InGaAsP laser compressed using linear dispersion in single-mode optical fiber are discussed. High-repetition-rate pulses as short as 4.1 ps have been achieved. Compression is optimized by wavelength tuning of the mode-locked laser. Time-resolved spectral measurements show the compensation of pulse chirp by fiber dispersion. >

Improved transmission performance resulting from the reduced chirp of a semiconductor laser coupled to an external high-Q resonator

Abstract: The improvement in the receiver sensitivity due to the reduced chirp is examined for NRZ and RZ intensity modulation, direct detection systems operating in the 1.55- mu m wavelength region with conventional single-mode optical fiber. The methodology involves (a) solving modified rate equations numerically for the optical power and phase of the external resonator laser in response to an injected current waveform, (b) modeling the signal transmission properties of single-mode optical fibers by convolution and modulus squared operations, and (c) using a truncated pulse train approximation to evaluate the probability of error in the presence of intersymbol interference, shot noise, avalanche photodiode multiplication noise, and preamplifier circuit noise. The numerical results indicate an almost threefold improvement in the achievable transmission distance for a bit rate of 4.8 Gb/s. >

High-precision fiber-optic position sensing using diode laser radar techniques

Abstract: The analysis, design, and testing of a high-precision linear position sensor using diode laser radar techniques and fiber-optic signal distribution is described. A frequency-chirped, intensity-modulated semiconductor diode laser is used as the transmitter. Each sensor head consists of two reflectors -one moving and one fixed -in a differential ranging mode to cancel apparent range changes caused by temperature induced fiber length variations. The returned (round trip delayed) chirps are direct detected by a photodiode and then are mixed with the original (undelayed) chirp to produce a sum of beat frequencies, each proportional to the range of a reflector. Several sensors heads, located at different fiber distances, can be optically multiplexed by a single laser transmitter using a reflective or transmissive network. The performance of the laser radar position sensor is anal,rzed by first calculating the return signal-to-noise ratio (SNR). A Cramer-Rao lower bound is derived to relate the SNR, chirp bandwidth, and chirp duration to the root-mean-square (RMS) range error. The theoretical optimum performance of the experimental sensor system is determined. An experimental system was built that achieved 58 pm RMS range error using a 1 ms chirp duration with a processing time of 50 ps.

Multiple-quantum nmr with frequency-modulated chirp pulses

Abstract: Frequency-modulated 'chirp' pulses for exciting multiple-quantum coherences over large bandwidths can be considered as an alternative to composite pulses to combat the effects of large offsets and tilted effective fields. Refocusing of the phase dispersion of double-quantum coherence can be combined with suitable detection sequences to yield pure absorption two-dimensional double-quantum spectra. The method of symmetrical excitation and detection by time-reversal may be applied to obtain t1-modulated longitudinal magnetization, which may then be converted into observable single-quantum coherence by a chirp echo sequence. Similar approaches can be used for many other NMR experiments involving coherence transfer.