Multiwavelength picosecond pulses generated from a self-seededFabry-Perot laser diode with a fiber external cavity using fiber Bragggratings

A novel scheme for generating multiwavelength short pulses using fiber Bragg gratings (FBGs) was demonstrated. In this scheme the external cavity length for all wavelengths selected by the gratings is intrinsically identical and the FBGs can be cascaded into the external cavity. Simultaneous generation of 1-GHz dual-wavelength short pulses with a wavelength separation of 1.6 or 18 nm was demonstrated. A sidemode suppression ratio (SMSR) better than 30 or 25 dB was achieved at both wavelengths in the former and latter cases respectively. Three-wavelength 1-GHz ps pulses with a SMSR better than 25 dB at all three wavelengths were also successfully generated     

Coherence analysis of noise-like pulses generated by an erbium-doped fiber mode-locked laser

The noise-like pulses(NLPs)with tunable fraction of the pedestal height in the whole intensity autocorrelation(AC)trace are numerically demonstrated in the designed erbium-doped fiber(EDF)mode-locked laser,which contains the saturable absorber(SA)with nonlinear polarization rotation(NPR),sinusoidal-shaped or Gaussian-shaped filter,two segments of EDFs,and two pieces of single-mode fibers(SMFs)with normal dispersion and anomalous dispersion,re-spectively.The pedestal ratio of the intensity AC trace can be tuned by changing the gain saturation energies of EDFs.The results show that when the net cavity dispersion is 1.06 ps2,the tuning range of the pedestal ratio for the NLPs can reach its maximum values,which are 0.51-0.89 and 0.58-0.88 for the sinusoidal-shaped and Gaussian-shaped fil-ters,respectively.In addition,an appropriate choice of filter bandwidth is also conducive to obtain a wide range of the tuning pedestal ratio for the intensity AC trace.     

Phase and amplitude fluctuations in a mode-locked laser

Comparisons are made of the phase and amplitude fluctuations of a laser when it operates single mode and when it is mode locked with the same total average power. Despite the much lower signal-to- (spontaneous emission) noise ratio of the mode-locked laser, the linewidth of each of the locked modes is the same as that of the single mode. The fluctuation of the total intensity of the mode-locked laser, and the linewidth enhancement factor due to intensity fluctuation as recently analyzed by Henry, are the same in both cases.     

Comparison between theory and experiment of nonlinear propagationfor a-few-cycle and ultrabroadband optical pulses in a fused-silicafiber

Wave-propagation equations, including effectively the second derivative in time under the condition of a small difference between the group and phase velocities and the first derivative in position ξ in the group velocity coordinate, are derived based on the slowly evolving wave approximation. These can describe ultrabroadband optical pulse propagation with not only self-phase modulation (SPM), but also induced-phase modulation (IPM) in the monocycle regime in a fiber. It is shown that linear dispersion effects can be rigorously included in the numerical calculations. Calculations including SPM in a single-mode fused-silica fiber with the Raman effect are performed and compared with experimental results. Also, calculations including IPM in the fused-silica fiber are compared with experimental results. The effects of each term in the calculations on spectra are analyzed and it is shown that inclusion of the Raman effect and the dispersion of the effective core area is important for obtaining better agreement with experiments. It is shown that inclusion of more than third-order dispersion terms is necessary for calculations of monocycle pulse propagation     

Effect of field amplitude taper on measured antenna gain and sidelobes

Amplitude taper of the field of a pattern measurement range produces gain and sidelobe values lower than the true values. A calculation is made using one-parameter distributions, for which the gain can be found in closed form, thereby avoiding the errors in previous calculations. For small tapers both gain errors and sidelobe level errors are linearly proportional to taper (dB). These results are useful in the design of pattern ranges, especially for very low sidelobe antennas, where the amplitude taper is more critical.     

Phase and amplitude uncertainties in heterodyne detection

The quantum limits on simultaneous phase and squared-amplitude measurements made via optical heterodyne detection on a single-mode radiation field are established. The analysis proceeds from a fully quantum mechanical treatment of heterodyning with ideal photon detectors. A high mean field uncertainty principle is proven for simultaneous phase and squared-amplitude observations under the condition that the signal and image band states are independent, and the image band has zero mean. Operator representations are developed which show that no such principle applies when arbitrary signal/image band dependence is permitted, although the mean observations are no longer functions of the signal field alone. A multimode two-photon coherent state illustrating this behavior at finite energy is exhibited. Potential applications for the resulting improved accuracy measurements are briefly described.     

色散管理光纤激光器中类噪声脉冲的实验研究

Based on an erbium.doped dispersion-managed fiber laser,the characteristics of noise-like pulses under different net cavity group velocity dispersions (GVDs) are experimentally investigated.Results show that the spec-tral bandwidth of noise-like pulse will increase as the net cavity GVD increases and attains maximum when the GVD is slightly positive.The effect of Raman scattering is enhanced due to the temporal width attains minimum.When the net cavity GVD increases continually and further into the positive region, the spectral width begins to decrease and the effect of Raman scattering is suppressed due to the positive dispersion.Our experimental results are in good agreement with the previous prediction of numerical simulation.     

Nonlinear interaction between optical pulses from high-repetition gain-switched DFB-LD in optical fiber

Interaction forces between propagating pulse pairs from a gain-switched distributed feedback laser diode (DFB-LD) were investigated. The authors discuss the experimental observation of a nonlinear interaction force between the fiber-compressed, gain-switched optical pulses. Optical pulses were generated by a gain-switched 1.55 mu m DFB-LD at a 10 GHz repetition rate, and compressed by compensating for the red-shifted chirping in the optical pulses using a polarization-holding fiber with a 18.7 ps/km/nm normal dispersion at a 1.55 mu m wavelength. The measured interaction force, which was greater than that theoretically predicted, was caused by the wings and/or pedestal associated with copropagating pulses.<>     

Phase and logarithm of amplitude of a signal plus noise

The mean and variance of the logarithm of the amplitude of a signal plus noise are discussed. Ward and Tough's (see ibid., vol.24, no.2, p.85-7, 1988) explanation for the very small bias of this mean at high signal-to-noise ratios is shown to be closely related to the capture effect in FM reception     

Phase encoding and decoding of short pulses at 10 Gb/s usingsuperstructured fiber Bragg gratings

A 7-chip, 160-Gchip/s phase-shift keyed code is assigned to short pulses after reflection from a superstructured fiber Bragg grating. The code is then recognized by a second grating, which is nominally identical to the encoder grating. Such an encoding/decoding action is required in code-division multiple-access systems and packet-switched networks     

Phase and amplitude storage of an acoustic signal in a Schottky-diode correlator

An experiment is described wherein a matrix of Schottky diodes, biased by a short voltage pulse, stores an s.a.w. signal and then delivers its correlation with a signal arriving later. Phase and amplitude storage are demonstrated.     

Phase difference between linear components of ellipticallypolarized waves from amplitude only measurements

An amplitude-only measurement setup can measure the axial ratio and tilt angle of an elliptically polarized wave. During design, both the amplitude and phase of the linear components are needed. The equipment measures the amplitudes directly. A nomograph, using the well-known relationships between polarization components determines the phase of the vertical component relative to the horizontal. The value of the nomograph goes beyond the stated objective, because it shows the relationships between the variables. The nomograph gives both the requirements for good circularity, and the sensitivity of the various parameters     

Cancellation of timing and amplitude jitter in symmetric linksusing highly dispersed pulses

We study the properties of symmetric dispersion compensation in optical links using highly dispersed pulse transmission. We show analytically that by splitting the dispersion compensation equally between the input and output of the link, complete cancellation of the timing and amplitude jitter can be obtained in systems where the power profile is symmetric about the center. We explain the dynamics of this cancellation and show, theoretically and experimentaily, that with practical system parameters, symmetric compensation may lead to a considerable improvement in performance     

Narrow-band infrared picosecond pulses tunable between 1.2 and 1.4 µm generated by a traveling-wave dye laser

A novel type of picosecond light source is the traveling-wave dye laser. It consists of a generator and an amplifier cell, both pumped by single pulses. Using the fast switching dye No. 5 with a fluorescence quantum efficiency of5 times 10^{-4}, one obtains an energy conversion of the pump to the IR pulse of up to 10 percent. The spectral width of the IR pulses is 15 cm^-1over the tunability range of1.18-1.4 mum. At 1.33 μm we observe Gaussian shaped pulses of 4.4 ps duration for pump pulses of 4.5 ps generated by a mode-locked Nd:glass laser. The output pulses show a high stability with a jitter of less than 0.2 ps.     

Investigation of amplitude noise and timing jitter of supercontinuum spectrum-sliced pulses

We numerically investigate the amplitude-noise and the timing jitter of pulses obtained by slicing coherent supercontinuum spectra. Applications such as time-division-multiplexing/wavelength-division-multiplexing systems and all-optical data regeneration are addressed. System parameters for optimizing the quality of sliced pulses are also discussed. We show that supercontinuum generation is a suitable method for generating amplitude-stable pulses and that no timing jitter is associated to the sliced pulses.     

Degenerate-cross-phase modulation of femtosecond laser pulses in a birefringent single-mode optical fiber

Degenerate-cross-phase modulation has been observed for femtosecond laser pulses propagating through a short birefringent single-mode optical fiber. The difference in the spectral broadening of the two output polarization components of a single laser pulse was attributed to the combination of self-phase modulation and degenerate-cross-phase modulation processes in the optical fiber. Theoretical simulations based on the solutions of the two coupled nonlinear Schrodinger equations are in good agreement with the measured results.     

Phase and amplitude phase restoration in synthetic aperture radarimaging

Methods for addressing two types of multiplicative noise in synthetic aperture radar (SAR) imaging are presented. The authors consider a multiplicative noise with a real phase (i.e. the SAR signal's phase is contaminated but its amplitude is uncorrupted) that possesses unknown functional characteristics with respect to the radar signal's temporal frequencies. A perturbation solution for phase reconstruction from amplitude is developed from a wave equation governing the SAR signal and a Riccati equation that relates the amplitude and phase functions of the SAR signal. This solution is converted into a noniterative analytical solution in terms of the moments and powers of the log amplitude function. Next, the authors consider a multiplicative noise with a complex phase (i.e. both the amplitude and phase of the SAR signal are contaminated) that varies linearly with respect to the radar signal's temporal frequencies. The two wave equations governing the SAR signal at two temporal frequencies of the radar signal are combined to derive a method to reconstruct the complex phase error function.     

Line spectrum of series of time-correlated pulses having random shape and amplitude

The most general case is examined of series of pulses having a line spectrum besides a continuous one, characterized by the following conditions. begin{enumerate} item Pulses are distributed in time according to an arbitrary distribution functionP(x), xbeing the time interval between two consecutive pulses. item The time intervalsxrelative to different pairs of pulses are uncorrelated. item Pulse shape is random and no correlation exists between shapes of different pulses. However, an arbitrary correlation can exist between a pulse amplitude and the time interval separating this pulse from the preceding one. end{enumerate} It is shown that under these conditions a line component can be present only in the spectrum of series of pulses whose distribution functionP(x)has the form:P(x) = sum_{m} b_{m} cdot delta (x-x_{m}). A simple expression giving the intensity of the lines is derived by means of an integration method in the complex plane, which can be applied to similar calculations in more general cases. As an application it is shown that the power spectrum of series of pulses which have a distribution functionP(x)of the said form, and are moreover characterized by a random pulse position modulation, can be easily obtained from the derived general results.     

Gain saturation of a CW-pumped erbium-doped fiber amplifier for nanosecond pulses

We report on the gain saturation property of an erbium-doped fiber amplifier for high peak 100 ns pulses. Under CW pumping conditions, the 980 nm pump power dependence of the gain saturation is precisely measured. Pump power dependence is discussed by assuming that the pulse width is much shorter than the recovery time of the erbium population inversion.