Breakage of the induced mode synchronization in a solid state laser and the anomalous effect of spontaneous noise level on the synchronization bandwidth

The process of optical pulse formation in a solid state laser in the regime of induced mode synchronization and the development of breakage upon frequency detuning are considered based on a numerical solution of the Maxwell-Bloch equation. It is established that breakage of the sequence and shape of laser pulses in the course of an increase in the frequency detuning begins with the appearance of a periodic modulation of the pulse sequence envelope by low-frequency relaxation oscillations. These oscillations transform into regular spikes, which follow the scenario of period doubling that leads to a chaotic regime. It is also established that an optimum level of spontaneous noise exists at which the synchronization bandwidth is maximum, while the duration of pulses is minimum at a fixed loss modulation amplitude. This scheme of synchronization breakage was qualitatively confirmed in experiments on a YAG:Nd laser (λ = 1.06 μm) with cavity loss modulation at an intermode beat frequency.     

Complete characterization of a self-mode-locked ti:sapphire laser in the vicinity of zero group-delay dispersion by frequency-resolved optical gating

The intensity and the phase of ultrashort pulses from a self-mode-locked Ti:sapphire laser operating in the vicinity of zero group-delay dispersion (GDD) have been completely characterized by the technique of frequency-resolved optical gating (FROG). For small values of negative GDD, the appearance of a dispersive wave in the pulse spectrum is manifested in the measured FROG trace, and pulse retrieval directly shows its association with a broad leading-edge pedestal. For positive GDD, we confirm previous experimental observations of picosecond pulses with large positive chirp and report a new operating regime in which the output pulses are of picosecond duration but are intensity modulated at 20 THz. The physical origin of this modulation is discussed by analogy with similar effects observed during pulse propagation in optical fibers, and the experimental results are compared with a model of intracavity four-wave mixing about the cavity zero GDD wavelength.     

Propagation effects in electro-optic sampling of terahertz pulses in GaAs

Freely propagating terahertz pulses are detected in the time domain by electro-optic sampling in bulk GaAs. We investigate the influence of dispersion of the near-infrared sampling pulse on the transients by varying the thickness of the GaAs crystal. Pronounced propagation effects are identified that originate from the frequency dependence of the phase-matching condition between the terahertz and the sampling pulse.     

Stimulated Raman Scattering and Self-phase Modulation of Ultrashort Light Pulses in Optical Fibres

Stimulated Raman scattering of picosecond pulses in optical fibre is investigated, with account taken of pulse walk-off by group velocity dispersion, pump depletion and chirp production by the intensity-dependent refractive index. An analytical solution of the basic equation is given by which the characteristic parameters of the Stokes and pump pulses, such as pulse half-width, intensity, mid-frequency and chirp, are calculated. It is shown that the chirp of the Stokes pulse does increase faster than that of the pump pulse with increasing fibre length and may surpass it by nearly three times after the pulse walk-off.     

Generation of a superstable Lorentzian pulse train with a high repetition frequency based on a Fabry-Perot resonator integrated with an electro-optic phase modulator

A simple technique for converting a continuous-wave laser beam into a stable Lorentzian pulse train with a high repetition frequency is demonstrated experimentally. We generated transform-limited pulses of up to 40 GHz, which were composed of higher-order sidebands produced by a Fabry-Perot resonator integrated with an electro-optic phase modulator (EOM). The rf power supplied to the EOM determines the pulse width in the pulse train. This approach enables the pulse width to be continuously tuned from 2.1 to 7.0 ps at the same repetition frequency without any wavelength shift. Furthermore, we experimentally evaluated the stability of the pulse train's amplitude and obtained stable bit-error-free operation at 9.95 GHz.     

Optical statistical timer

A new type of light pulses timer allowing a direct locking within the electric oscillation delivered by a clock is presented. The light pulses to be timed cross an electro-optic crystal excited by an oscillating electric field, which modifies the light polarization. A polarimetry at the crystal exit allows deducing the light pulse arrival time. Accuracy in the picosecond range has been experimentally demonstrated     

Measurement of time-frequency parameters of picosecond optical pulses

A method is proposed that permits accurate and reliable measurement of the train-average pulse duration as well as the value and sign of the frequency chirp of picosecond optical pulses in high-repetition-rate trains.     

Q-switched mode-locking of an erbium-doped fiber laser using cavity modulation frequency detuning

We present the results of an investigation regarding a Q-switched mode-locked fiber laser scheme based on a cavity modulation frequency detuning technique. The approach is based on undamped laser relaxation oscillations occurring due to frequency detuning in the fundamental cavity resonance frequency. Through a range of experiments with an erbium-doped, fiber-based, ring-cavity laser, this approach has been shown to be capable of generating high-quality Q-switched mode-locked pulses from an optical fiber-based laser. The maximum frequency detuning range for a stable Q-switched mode-locking operation has been observed to vary depending on the pump power used. We found that the highest pulse peak power was obtained at the frequency detuning threshold at which the operation changed from the mode-locking to the Q-switched mode-locking regime.     

Realization of wavelength conversion with hyperbolic secant femtosecond pulse in normal dispersion regime

Realization of wavelength conversion based on second-order femtosecond dark solitons with hyperbolic secant pulse is presented. This is achieved by introducing localized dispersion perturbation along the optical fiber. We demonstrate that an initial 30 fs second-order pulse decays to similar sub-pulses by applying perturbation using a step increment of β₂ from 6.3 to 15.75 ps² km^?1. This shows that the realization of a 1 × 2 channel wavelength converter for femtosecond pulses is possible. Recent research shows the possibility of realizing wavelength conversion generated from picosecond solitons neglecting nonlinear effects. However, employing the same method for femtosecond pulses fails due to the manifestation of nonlinear effects. In this paper, pulse deformation under different levels of perturbation was tested, and appropriate perturbation leading to similar sub-pulses is achieved.     

Routes to various breather solitons in whispering-gallery-mode resonators through the variational method

In this paper, the dynamics of the optical field in the whispering-gallery-mode resonator described by the Lugiato–Lefever equation is studied by means of the variational method. For some special cases, exact explicit family of analytical breather-type solitons, such as time-periodic breather and space-time periodic breather are obtained. It is found that the pumping parameter, the detuning parameter and the dispersion coefficient, influences the nature of the optical field in the whispering-gallery-mode resonators.     

Effect of intensity dependent higher-order dispersion on femtosecond pulse propagation in quantum well waveguides

We have studied theoretically the dispersion of ultrafast coherent pulses in GaAs/AlGaAs quantum well waveguide structures as a function of optical intensity. Semiconductor Bloch equations are used to obtain the polarization induced in the medium due to an incident Gaussian electromagnetic beam. The partial differential equation describing the pulse propagation in the presence of group velocity dispersion is used to analyze the role of higher-order dispersion on femtosecond pulse propagation in the waveguide. Due consideration has been given to the intensity dependent optical susceptibility of the medium. The results of the numerical analysis manifest significant influence of higher-order dispersion on femtosecond pulse propagation over short waveguide distance.     

Picosecond-pulse wavelength conversion based on cascaded second-harmonic generation-difference frequency generation in a periodically poled lithium niobate waveguide

The wavelength conversion of picosecond optical pulses based on the cascaded second-harmonic generation-difference-frequency generation process in a MgO-doped periodically poled lithium niobate waveguide is studied both experimentally and theoretically. In the experiments, the picosecond pulses are generated from a 40 GHz mode-locked fiber laser and two tunable filters, with which the lasing wavelength can be tuned from 1530 to 1570 nm, and the pulse width can be tuned from 2 to 7 ps. New-frequency pulses, i.e., converted pulses, are generated when the picosecond pulse train and a cw wave interact in the waveguide. The conversion characteristics are systematically investigated when the pulsed and cw waves are alternatively taken as the pump at the quasi-phase-matching wavelength of the device. In particular, the conversion dependences on input pulse width, average power, and pump wavelength are examined quantitatively. Based on the temporal and spectral characteristics of wavelength conversion, a comprehensive analysis on conversion efficiency is presented. The simulation results are in good agreement with the measured data.     

Tenth-order rational-harmonic frequency multiplication and detuning of optical pulse injection-locked erbium-doped fiber laser

The jitter and frequency-detuning dynamics of a 10-GHz rational-harmonic frequency-multiplied pulse train generated from an erbium-doped fiber laser (EDFL) is studied. The EDFL is self-feedback seeded and optically injection locked by a gain-switched laser diode (GSLD) with a pulse width and an average power of 17.6 ps and 0.2 mW, respectively, at a repetition frequency of 1 GHz. The repetition frequency of the optical pulse train can be tenth-order multiplied by a slight detuning of the repetition frequency of the GSLD to match the rational-harmonic injection-locked condition of the EDFL. As the repetition frequency is multiplied from 1 to 10 GHz, the peak power, the pulse width, and the frequency-detuning bandwidth of the injection-locked EDFL pulses decrease from 1.2 to 0.3 W, from 40 to 21 ps, and from 40 to 9 kHz, respectively. The timing jitter of the injection-locked EDFL repeated at 1 GHz remains unchanged (< 0.5 ps) within the detuning bandwidth, which inevitably increases to 1.2 ps after tenth-order multiplication.     

New approach for the design of an optical square pulse generator

What is believed to be a new approach for the design and analysis of a reconfigurable optical square pulse generator using the concept of temporal optical integration and the digital signal processing method is presented. The reconfigurable square pulse generator is synthesized using compact active semiconductor-based waveguide technology, and it consists simply of the cascade of a tunable microring resonator (or a tunable all-pole filter) and a tunable asymmetrical Mach-Zehnder interferometer (or a tunable all-zero filter). The reconfigurable generator can convert an input picosecond pulse (i.e., soliton or Gaussian pulse) into an optical square pulse. The pulse width of the generated square pulse can be adjusted by controlling the time delay of a variable delay element in the tunable all-zero filter. The reconfigurable generator can convert an input picosecond pulse train into return-to-zero (RZ) and non-return-to-zero (NRZ) signals with square pulse shapes. The repetition rates of the generated RZ and NRZ signals can be varied by adjusting the bit period of the input picosecond pulse train, the input pulse width, and the time delay of the variable delay element. The effect of the deviation of the parameter values on the generator performance is also studied.     

Gauss-Legendre quadrature method used to evaluate the spatio-temporal intensity of ultrashort pulses in the focal region of lenses

We analyze the spatio-temporal intensity of sub-20 femtosecond pulses with a carrier wavelength of 810 nm along the optical axis of low numerical aperture achromatic and apochromatic doublets designed in the IR region by using the scalar diffraction theory. The diffraction integral is solved by expanding the wave number around the carrier frequency of the pulse in a Taylor series up to third order, and then the integral over the frequencies is solved by using the Gauss-Legendre quadrature method. The numerical errors in this method are negligible by taking 96 nodes and the computational time is reduced by 95% compared to the integration method by rectangles. We will show that the third-order group velocity dispersion (GVD) is not negligible for 10 fs pulses at 810 nm propagating through the low numerical aperture doublets, and its effect is more important than the propagation time difference (PTD). This last effect, however, is also significant. For sub-20 femtosecond pulses, these two effects make the use of a pulse shaper necessary to correct for second and higher-order GVD terms and also the use of apochromatic optics to correct the PTD effect. The design of an apochromatic doublet is presented in this paper and the spatio-temporal intensity of the pulse at the focal region of this doublet is compared to that given by the achromatic doublet.     

Optical parametric chirped-pulse amplifier and spatiotemporal shaping for a petawatt laser

We present a degenerate noncollinear optical parametric chirped-pulse amplifier pumped by a high-energy, diode-pumped Nd:Glass regenerative amplifier delivering monomode pulses at 527 nm. The spatial mode shaping of the pump pulses is achieved with a diffractive laser cavity element, and temporal pulse shaping makes use of an electro-optic modulator and an arbitrary electrical waveform generator. Amplification at gain saturation achieves tailoring of the signal pulses. Numerical simulations with Miró software are presented and compared with experimental measurements.     

Influence of spectral broadening on femtosecond wavelength conversion based on four-wave mixing in silicon waveguides

Femtosecond wavelength conversion in the telecommunication bands via four-wave mixing in a 1.5 mm long silicon rib waveguide is theoretically investigated. Compared with picosecond pulses, the spectra are greatly broadened for the femtosecond pulses due to self-phase modulation and cross-phase modulation in the four-wave mixing process, and it is difficult to achieve a wavelength converter when the pump and signal pulse widths are close to or less than 100 fs in the telecommunication bands because of the spectral overlap. The influence of the spectral broadening on the conversion efficiency is also investigated. The conversion bandwidth of 220 nm and peak conversion efficiency of -8 dB are demonstrated by using 500 fs pulses with higher efficiency than the picosecond pulse-pumped efficiency when the repetition rate is 100 GHz.     

Optical Kerr switching technique for the production of a picosecond,multiwavelength CO2 laser pulse

A wavelength-independent method for optical gating, based on the optical Kerr effect, has been demonstrated. Using this method, we produced 100-ps, 10-kW, two-wavelength pulses (10.3 and 10.6 microm) with a signal-to-background ratio contrast of 10(5) by slicing a long CO2 pulse. The capability of gating consecutive pulses separated on a picosecond time scale with this method is also shown.     

Intracavity pumped nanosecond optical parametric oscillator emitting in the eye-safe range

We report on a 1.57-mum wavelength singly resonant optical parametric oscillator (OPO) using a noncritically phase-matched KTP crystal intracavity pumped by an electro-optically Q-switched Nd:YAG laser. A simple model for numerical calculation of the temporal profile of the laser and signal pulses developed inside the intracavity pumped OPO (IPOPO) resonator is described. Working conditions for single-pulse and multipulse parametric oscillation are established. A single signal pulse of 8.1-mJ output energy was obtained when the laser was pumped three times above the pulsed IPOPO threshold. A pulse duration of 5.9 ns was measured at 1.5 times above threshold for the IPOPO.     

Rapid optical coherence tomography and recording functional scattering changes from activated frog retina

Optical coherence tomography (OCT) has important potential advantages for fast functional neuroimaging. However, dynamic neuroimaging poses demanding requirements for fast and stable acquisition of optical scans. Optical phase modulators based on the electro-optic effect allow rapid phase modulation; however, applications to low-coherence tomography are limited by the optical dispersion of a broadband light source by the electro-optic crystal. We show that the optical dispersion can be theoretically estimated and experimentally compensated. With an electro-optic phase modulator-based, no-moving-parts OCT system, near-infrared scattering changes associated with neural activation were recorded from isolated frog retinas activated by visible light.