Showing papers on "Chirp published in 1986"

Effect of nonlinear gain reduction on semiconductor laser wavelength chirping

Abstract: A simple wavelength chirping formula is presented which includes small nonlinearities in the net gain such as spectral hole burning. The two dominant terms are a laser‐structure‐independent derivative or ‘‘transient’’ chirp and a structure‐dependent ‘‘adiabatic’’ chirp, each with distinctly different lightwave system consequences. The two chirp contributions are indirectly related through their mutual association with relaxation oscillations. Time‐resolved spectral measurements on a number of different laser structures support the results.

Analysis of picosecond pulse shape synthesis by spectral masking in a grating pulse compressor

Abstract: This paper analyzes the synthesis of arbitrarily shaped optical pulses by spectral filtering in a fiber-and-grating pulse compressor. Spectral filtering of phase and amplitude is achieved by masking the spatially dispersed frequency components within the compressor. We show that the spectral filter is the convolution of the mask with the beam's transverse intensity profile. We discuss the effect of diffraction from the features of the physical mask, and show how finite spatial resolution limits the range of attainable temporal profiles. The fundamental limitation on spectral resolution is derived. Spectra and temporal pulse shapes corresponding to a variety of physical masks are calculated and are found to be in excellent agreement with experiments done with compressed pulses from a mode-locked Nd: YAG laser.

The effect of pulse walkoff on stimulated Raman scattering in fibers

Abstract: We have experimentally investigated stimulated Raman scattering in single-mode fibers in the regime of large Raman Stokes pulse walkoff from the pump pulse by the effect of group-velocity dispersion. Measurements are made with 36 ps duration pulses at 532 nm produced from a frequency-doubled and harmonically mode-locked Nd:YAG laser. We find that for 20 percent conversion, the Raman output is produced about two walkoff lengths into the fiber as a pulse of approximately the same duration as the input pulse. We also find that the Raman pulse is produced with a strong frequency chirp.

Compression of femtosecond optical pulses with dielectric multilayer interferometers

Abstract: The group velocity dispersion of a multilayer thin film Gires-Tournois interferometer used for reflection of ultrashort optical pulses can be continuously tuned from positive to negative values at an extremely low loss in pulse energy. Thus, this device can be applied for compression of femtosecond pulses independent of the sign of the frequency chirp by simple angle tuning of the interferometer. This has been demonstrated with up-chirped 210 fs pulses which have been compressed to an almost transform-limited duration of 115 fs.

Effect of chirping on solution propagation in single-mode optical fibers.

Abstract: The evolution of a linearly chirped pulse into a soliton is studied. Even under severe chirping, a soliton can be generated if the initial amplitude is high enough. However, the resulting soliton will always be broader than in the unchirped case.

Waveform design and doppler sensitivity analysis for nonlinear FM chirp pulses

Abstract: The use of pulse compression to obtain simultaneous long-range detection and good range resolution is described The types of modulation that can be used to obtain pulse compression are outlined with particular emphasis on their performance under Doppler shift It is shown that nonlinear frequency-modulated (FM) signals are capable of providing low range-sidelobes while being compressed using a matched filter A design method for nonlinear FM signals based on window functions is outlined Simulation results for pulse compression of nonliner FM signals based on four different window functions with Doppler shift are presented The results are used to define the effects of Doppler shift on the pulse compression An analysis is presented, and interpreted pictorially, that explains the effects of Doppler shift on the pulse compression The analysis is also extended to explain the better Doppler performance of hybrid FM pulse compression systems

Effect of frequency chirping on the performance of optical communication systems.

Abstract: The effect of frequency chirping on the performance of optical communication systems is analyzed theoretically using a simple but realistic model for the chirped pulses emitted by a directly modulated semiconductor laser. The effect of pulse-edge sharpness is investigated by considering the propagation of super-Gaussian chirped pulses in a dispersive single-mode fiber. The results are presented using the parameter values appropriate for a 1.55-μm optical communication system employing conventional silica fibers and are in qualitative agreement with the reported experimental observations.

Small signal analysis of frequency chirping in injection-locked semiconductor lasers

Abstract: A theoretical and experimental investigation of frequency chirping in injection-locked semiconductor lasers under a condition of small signal modulation is presented. It shows that a substantial reduction of frequency chirping can be obtained which, however, is effective only up to modulation frequencies of the order of the locking bandwidth and is strongly dependent on the frequency detuning between master and slave.

Phase phenomena in parametric amplifiers and generators of ultrashort light pulses

Abstract: This article reviews recent theoretical and experimental investigations in the development of femtosecond lasers based on optical parametric oscillators. Phenomena responsible for producing tunable high-power femtosecond pulses in media with quadratic susceptibility (parametric chirp, chirp reversal in real time, parametric self-compression of pulses with strong energy exchange) are discussed.

Coherent transient optical pulse-shape storage/recall using frequency-swept excitation pulses.

Abstract: We demonstrate that frequency-chirped laser excitation pulses may be employed to generate coherent transient signals possessing the same temporal profile as a particular excitation pulse. In comparison with the short, fixed-frequency pulses used in previous studies of this effect, chirped pulses can be temporally longer, of lower intensity, and hence easier to generate. The experiment was performed on the 555.6-nm transition of vapor-phase atomic ytterbium, and pulse-shape information was stored in a coherence between excited-state Zeeman levels. A simple theoretical analysis of our results is presented.

Subpicosecond pulses from a mode-locked semiconductor laser

Abstract: A GaAs laser was passively mode locked using a GaAs/ GaAlAs multiple quantum well saturable absorber. The laser output pulses were found to be frequency chirped due to the group velocity dispersion in the laser diode. An external grating compressor was used to remove the chirp and to produce subpicosecond pulses. Pulses as short as 0.83 ps have been obtained.

Method for generating orthogonal sweep signals

Abstract: A first sweep signal consists of a plurality of waveforms having non-invariant fundamental periods exhibiting a monotonic frequency progression of at least one octave over the duration of the sweep. Each waveform may consist of an ordered set of asymmetrical pulses distributed within the period of the waveform in a prescribed sequence as a function of time. A second sweep signal is orthogonalized with respect to the first by time-reversing the distribution-sequence of the pulse set.

Propagation characteristics of frequency-chirped super-Gaussian optical pulses

Abstract: It is demonstrated that the rms width of optical pulses propagating along an optical fiber varies parabolically with distance, irrespective of initial pulse form or frequency chirp. This result is used to analyze the propagation characteristics of super-Gaussian optical pulses with respect to pulse broadening, pulse compression, and information- carrying capability. Comparison with the corresponding properties for Gaussian pulses clearly shows the optimum character of the latter.

Heterodyne laser instantaneous frequency measurement system

Abstract: A heterodyne laser instantaneous frequency measurement system is disclosed. The system utilizes heterodyning of a pulsed laser beam with a continuous wave laser beam to form a beat signal. The beat signal is processed by a controller or computer which determines both the average frequency of the laser pulse and any changes or chirp of the frequency during the pulse.

Positive and negative chirping of laser pulses shorter than 100 fsec in a saturable absorber.

Abstract: We present a calculation of the chirp generated in laser pulses shorter than 100 fsec on propagation through a saturable absorber (DODCI in ethylene glycol) The calculation takes into account the absorber saturation and the solvent nonlinear refractive index At pulse energies greater than 10 nJ the chirp tends to be predominantly positive, and it increases rapidly as the pulse duration becomes shorter than 50 fsec At pulse energies in the 1–7-nJ range the chirp is mostly negative for pulses longer than 30 fsec

Experimental studies of photon-echo pulse compression

Abstract: Photon echoes generated by long (i.e., 400–800-nsec) frequency-chirped optical pulses are found to possess (after compensating for avoidable material relaxation) up to 25% of the energy of the first excitation pulse and to have a duration determined by the total chirp bandwidth of the excitation pulses. In our experiment, echoes 30 times shorter than the first excitation pulse were observed.

50× optical fiber pulse compression at 1.319 μm

Abstract: We report a 50‐fold fiber‐grating pulse compression in the near‐infrared region. 100 ps (FWHM) pulses from a mode‐locked neodymium:yttrium aluminum garnet laser operated at 1.319 μm have been compressed into 2 ps pulses by using a 2‐km dispersion‐shifted fiber and a grating pair separated by 2.41 m in a double‐pass configuration. The novel feature of this demonstration is the use of dispersion‐shifted fiber (zero‐dispersion wavelength at 1.59 μm) for producing a linear chirp over most of the pulse and, thus, almost all the input power appears in the compressed pulse.

Linear programming design of IIR digital phase networks

Abstract: This paper demonstrates that the IIR digital phase network can be designed by linear programming. The phase network is used not only to compensate the phase distortion of the signal by the filter, but it is used also in various other problems in communication area. In particular, the network with chirp characteristics is well known. There are IIR and FIR digital phase networks, but it is difficult to design completely flat amplitude characteristics for the latter. The design problem for the IIR digital phase filter has been considered as a nonlinear programming problem. By reducing this problem to that of linear programming, the optimal solution can be obtained, if it exists, without any particular consideration for the convergence of iteration or the initial values. This paper also discusses the order of the digital phase network so designed, and it is shown that the order is determined from the given design specifications. Finally, several design examples are illustrated, indicating that the proposed method is effective.

Pulse-shape effects on frequency chirping in single-frequency semiconductor lasers under current modulation

Abstract: Directly modulated semiconductor lasers exhibit dynamic frequency shifts (chirping) due to gain-induced variations of the refractive index. Using the small-signal analysis of the single-mode rate equations, the effect of current-pulse shape on frequency chirping is analyzed, and the results are compared for the cases of sinusoidal and square-wave modulations. The chirp is generally larger for the square-wave case. However, its magnitude depends on the pulse rise and fall times, decreasing for a pulse with slower turn-on and turn-off characteristics. Chirp analysis presented here includes the effect of power-dependent gain changes arising from the processes such as spectral hole-burning.

Femtosecond-pulse laser chirp compensated by cavity-mirror dispersion.

Abstract: The duration of pulses generated from a simple colliding-pulse mode-locked cw dye laser is measured as a function of cavity-mirror dispersion. The optimum amount of mirror dispersion of ϕ(ω) ≅ + 1.8 × 10−28 sec2 and a suitable mirror coating for upchirp compensation are identified. The adjustment of mirror dispersion only, without additional dispersive elements, generates continuous trains of pulses as short as 50 fsec.

Picosecond stimulated Raman generation, pump pulse fragmentation, and fragment compression in single-mode optical fibers

Abstract: Pump pulse depletion has been examined for high peak power (∼ 100 kW) 90 ps pulses from a Q -switched and mode-locked Nd:YAG laser in 4 m of single-mode optical fiber. Fragmentation of the fundamental and low Stokes Raman orders has been observed for this cascading process. Pulse compression using an interferometric technique has been carried out, obtaining compression ratios of up to times five, on these fragments which exhibited a negative frequency chirp.

Power-dependent enhancement in repeater spacing for dispersion-limited optical communication systems

Abstract: We have found numerically an intensity-dependent increase in the repeater spacing for dispersion-limited lightwave transmission systems. The enhancement is due to the combination of nonlinearity and anomalous dispersion occurring in conventional fibres at 1.55 ?m. For moderate powers we find about a factor of two enhancement in the repeater distance compared to the linear case. We also investigate the effects of laser frequency chirp.

Semiconductor laser device

Abstract: PURPOSE:To stabilize negative feedback of a wind band even if frequency of a semiconductor laser and temperature of an etalon are varied by so negative feeding back to sweeping means of a sweep type Fabry-Perot etalon that the output signal of a photodetector attains a special value. CONSTITUTION:A semiconductor laser 3 outputs a light having an oscillating frequency set by a sweep signal 1, and a light reflected by a beam splitter 4 enters a sweep type Fabry-Perot etalon 5. Since the micro voltage output of an oscillator 8 is applied to the etalon 5 and swept by frequency fm at the Miller interval of the etalon 5, its resonance characteristic laterally moves in frequency fm at the transmission peak. The transmitted light of the etalon 5 is detected by a photodetector 6, its output electric signal is synchronously detected with frequency 2fm by a controller 7 to become a quadratic differentiation waveform signal, and the controller 7 drives sweeping means 52 to attain a zero point of this waveform. Thus, even if the oscillation frequency of the laser 3 and the temperature of the etalon 5 are varied, a wide band negative feedback becomes stable to reduce a spectral width.

Analog radar signal design and digital signal processing —a Heisenberg nilpotent Lie group approach

Abstract: The notions of analog and digital radar auto- and cross-ambiguity functions are on the borderline with mathematics, physics, and electrical engineering. This paper presents the solutions of two problems of analog radar signal design: the synthesis problem (posed in 1953) and the invariance problem for ambiguity surfaces over the symplectic time-frequency plane. Both solutions are achieved via harmonic analysis on the differential principal fiber bundle over the two-dimensional polarized (resp. isotropic) cross-section with structure group isomorphic to the one-dimensional center of the simply connected real Heisenberg nilpotent Lie group. In this way, the linear oscillator representation of the three-dimensional real metaplectic group gives rise to a procedure for generating the energy-preserving linear automorphisms of any given radar ambiguity surface over the time-frequency plane by means of chirp waveforms (linear frequency modulated signals).