Subpicosecond Pulse Generation Research Articles

Subpicosecond pulse generation at 134GHz using a quantum-dash-based Fabry-Perot laser emitting at 1.56μm

We demonstrate passive mode locking in one-section monolithic semiconductor laser diodes based on quantum-dash active layer at very high repetition rate in the 1.5μm window. Transform-limited pulses are generated at 134GHz with subpicosecond width, without any pulse compression scheme. A 50kHz linewidth of the radio-frequency spectrum is also demonstrated at 42GHz, the lowest value reported for any passively mode-locked semiconductor laser. We further show that the saturable absorption section in two-section devices has no significant impact on the mode-locking behavior.

Subpicosecond pulse generation at 134 GHz and low radiofrequency spectral linewidth in quantum dash-based Fabry-Perot lasers emitting at 1.5 µm

Passive modelocking in one-section monolithic semiconductor laser diodes based on a quantum dash active layer at very high repetition rate (>40 GHz), in the 1.5 µm window, is demonstrated. 800 fs pulse generation, without any pulse compression scheme, at 134 GHz, is reported. A 50 kHz linewidth of the radiofrequency (RF) spectrum at 42 GHz is also demonstrated, the lowest value reported for any semiconductor passively modelocked laser.

Ultra-short pulse operation of all-optical fiber passively mode-locked ytterbium laser

The generation regimes of an all-fiber passively mode-locked ytterbium laser with intra-cavity photonic crystal fiber have been studied with the aim to provide recipes for obtaining chirp-free sub-picosecond pulses directly from the cavity. Small-beam area photonic-crystal fiber is used for dispersion compensation of the intra-cavity normal dispersion of b-doped and single-mode fibers as well as for spectrum expanding due to enhanced nonlinearity. Regions of the gain and fiber parameters near the generation threshold were found in both cases of normal and anomalous net intra-cavity dispersion, which provide a stable generation of ultra-short sub-picosecond pulses directly from the cavity. Laser parameters of a transition to the multi-pulsed generation regimes were also found.

Pulse properties of external-cavity mode-locked semiconductor lasers

The performance of an external-cavity mode-locked semiconductor laser is investigated both theoretically and experimentally. The optimization analysis focuses on the regimes of stable mode locking and the generation of sub-picosecond optical pulses. We demonstrate stable output pulses down to one picosecond duration with more than 30 dB trailing pulse suppression. The limiting factors to the device performance are investigated on the basis of a fully-distributed time-domain model.We find that ultrafast gain dynamics effectively reduce the pulse-shaping strength and inhibit the generation of femtosecond optical pulses.

Picosecond and sub-picosecond flat-top pulse generation using uniform long-period fiber gratings

We propose a novel linear filtering scheme based on ultrafast all-optical differentiation for re-shaping of ultrashort pulses generated from a mode-locked laser into flat-top pulses. The technique is demonstrated using simple all-fiber optical filters, more specifically uniform long period fiber gratings (LPGs) operated in transmission. The large bandwidth typical for these fiber filters allows scaling the technique to the sub-picosecond regime. In the experiments reported here, 600-fs and 1.8-ps Gaussian-like optical pulses (@ 1535 nm) have been re-shaped into 1-ps and 3.2-ps flat-top pulses, respectively, using a single 9-cm long uniform LPG.

Sub-Picosecond Optical Pulse Generation with Repetition Rate of Several Tens of Gigahertz Using Electrooptic Standing-Wave Phase Grating with Periodic Domain Inversion

We propose and demonstrate a novel electrooptic spatiotemporal modulator for sub-picosecond optical pulse generation with a repetition rate of several tens of gigahertz. The pulse generation is directly accomplished with an electrooptic standing-wave phase grating with periodic domain inversion that operates over 10 GHz. This device produces a phase distribution that is like a sinusoidal standing wave in the beam cross section. The output optical beam undergoes Raman–Nath diffraction with diffracted beams whose intensities vary temporally. When we extract a certain diffracted beam, we can obtain a pulse train. A near-Fourier-transform-limit pulse train with a repetition rate of twice the modulation frequency can be obtained using the zero-order undiffracted beam. In the experiment, we found that our new device operates as theoretically predicted and that we can obtain a pulse train with a pulsewidth of about 1 ps from the fabricated device at the maximum modulation power. By improving the modulation efficiency of the device or by further increasing of the modulation frequency, a sub-picosecond optical pulse train with a repetition rate of several tens of gigahertz can be generated by the new device.

Cross-correlation frequency-resolved optical gating coherent anti-Stokes Raman scattering with frequency-converting photonic-crystal fibers

Photonic-crystal fibers provide a high efficiency of frequency upconversion of regeneratively amplified femtosecond pulses of a Cr: forsterite laser, permitting the generation of subpicosecond anti-Stokes pulses with a smooth temporal envelope and a linear positive chirp, ideally suited for femtosecond coherent nonlinear spectroscopy. These pulses from a photonic-crystal fiber were cross correlated in our experiments with the femtosecond second-harmonic output of the Cr: forsterite laser in toluene solution, used as a test object, in boxcars geometry to measure the spectra of coherent anti-Stokes Raman scattering (CARS) of toluene molecules (XFROG CARS).

Microstructure fibers as frequency-tunable sources of ultrashort chirped pulses for coherent nonlinear spectroscopy

Microstructure fibers are shown to allow the creation of new tunable sources for femtosecond nonlinear spectroscopy. These fibers provide a high efficiency of frequency upconversion of regeneratively amplified femtosecond pulses of a Cr:forsterite laser, permitting the generation of subpicosecond anti-Stokes pulses with a smooth temporal envelope and a linear positive chirp. These pulses from a microstructure fiber were used to measure the spectra of coherent anti-Stokes Raman scattering (CARS) of toluene solution by cross-correlating these pulses with the femtosecond second-harmonic output of the Cr:forsterite laser in boxcars geometry (XFROG CARS).

Spectroscopic and second harmonic generation properties of a new crystal: Yb-doped RbTiOPO4

Abstract RbTiOPO4 single crystals doped with a concentration of 1.96 × 1020 ions cm−3 of Yb3+ have been grown. These crystals exhibit a large splitting of the 2F5/2 sublevels of Yb3+ of about 70 nm, which is one of the largest crystal fields ever known for Yb-doped crystals. The larger compensation is ensured by a Nb-co-doping. As expected from the large splitting, a broadband fluorescence was observed around 1 μm, which is interesting for tunability and subpicosecond-pulse generation applications in the visible and in the infrared domains. The excited-state lifetime has a quite long value of 2.2 ms, which is favourable to get a high inversion of population under CW pumping. Measurements of second harmonic generation have shown a correlation between the fundamental wavelength of angular non-critical phase-matching and the concentration of Nb and Yb. The conversion efficiency is of the order of that of RTP.

Sub-picosecond pulse generation employing an SOA-based nonlinear polarization switch in a ring cavity

We demonstrate the generation of sub-picosecond optical pulses using a semiconductor optical amplifier (SOA) and a linear polarizer placed in a ring-laser configuration. Nonlinear polarization rotation in the SOA serves as the passive mode-locking mechanism. The ring cavity generates pulses with duration below 800 fs (FWHM) at a repetition rate of 14 MHz. The time -bandwidth product is 0.48. Simulation results in good agreement with the experimental results are presented.

Subpicosecond pulse generation using an electroabsorption modulator and a double-stage pulse compressor

We develop a practical subpicosecond pulse generator consisting of an electroabsorption modulator (EAM) and a double-stage pulse compressor. The pulse compressor is carefully designed so that the timing jitter and the pedestal level of compressed pulses are not increased during the pulse compression process. A 10-GHz 20-ps pulse from an EAM is compressed to a 400-fs pulse.

A mode-locked fibre laser using a Sagnac interferometer and nonlinear polarization rotation

We analyse the propagation in a nonlinear, birefringent optical fibre with twist. The results show that the polarization evolution is periodic and they are applied to the analysis of a Sagnac interferometer. We give a useful way to visualize the behaviour of the nonlinear optical loop mirror (NOLM) (as a function of birefringence, twist, length, and input polarization) in terms of the Poincaré sphere. We describe a new fibre laser configuration based on the NOLM with a symmetrical coupler, quarter-wave retarder, and highly twisted, birefringent fibre in the loop. We achieved stable generation of subpicosecond pulses with milliwatts of average output power.

X-ray diffraction experiments with femtosecond time resolution

Intense ultrashort laser pulses enable the generation of subpico-second X-ray pulses in the multi-kilovolt range of photon energies. These X-ray pulses have opened the door to ultrafast time-resolved X-ray spectroscopy. Here, we report on the use of time-resolved X-ray diffraction to study ultrafast laser-induced order-to-disorder transitions and large amplitude picosecond acoustic transients that accompany the phase transitions.

High-energy sub-picosecond pulse generation from 3 to 20 μm

We have produced sub-picosecond pulses in the wavelength range 3–20 μm with energies as high as 2 μJ, through difference-frequency mixing in the crystal GaSe. Input beams for the non-linear mixing process came from either a regenerative amplifier operating in two-color mode, due to the spectral shaping of its injection beam, or an optical parametric amplifier (OPA). The first method was straightforward to implement and provided easy access to the wavelength range 10–20 μm. The intermediate OPA stage, though technically more involved, allowed higher input beam irradiances and resulted in shorter duration MIR pulses, greater conversion efficiency and access to the MIR region between 3 and 10 μm.

Fiber laser mode locked by a Sagnac interferometer with nonlinear polarization rotation

We describe a new fiber laser configuration based on a nonlinear optical loop mirror with a symmetrical coupler, a quarter-wave retarder, and highly twisted, birefringent fiber in the loop. The nonlinear optical loop mirror configuration operates by nonlinear polarization rotation. We have achieved stable generation of subpicosecond pulses with milliwatts of average output power.

Nonlinear optical generation and detection of ultrashort electrical pulses in transmission lines

The nonlinear optical generation and detection of subpicosecond electrical pulses on coplanar transmission lines is demonstrated. The electrical pulses are generated by optical rectification of ultrashort optical pulses and detected by electro-optic sampling. Both processes are the result of a second-order nonlinear optical response that occurs in the same poled polymer medium. A bipolar temporal waveform with a FWHM duration of 180 fs for the positive lobe that was measured after a propagation distance of 125 mum was observed. Pulse broadening was minimized by careful attention to the device structure.

Direct generation of subpicosecond chirp-free pulses at 10 GHz from a nonpolarization maintaining actively mode-locked fiber ring laser

0.7-1.3 ps highly stable, transform-limited, sech/sup 2/-shaped pulses at 10 GHz in the 1536-1569 nm wavelength region from a nonpolarization maintaining (PM), single intensity modulator, actively and harmonically mode-locked fiber ring laser have been directly generated. Little degradation of long-term stability of the laser operation was observed due to the non-PM nature of the laser cavity.

Dynamics of generation of subpicosecond pulses in semiconductor injection lasers

A theory of the generation of subpicosecond pulses in semiconductor injection lasers taking into account multimode generation and a band structure of energy levels in doped semiconductors has been developed. Spatial-temporal dynamics of mode locking in a triple-section laser with a saturable absorber has been investigated. The analysis carried out has allowed us to determine the optimum conditions for the generation of ultimately short pulses. It has also permitted a study of the dependencies of parameters of the generated pulses on the pumping current, the reverse voltage, and a ratio of the lengths of the amplifying and absorbing sections.

Highly efficient subpicosecond pulse generation at 211 nm

We report the efficient generation of high-contrast subpicosecond fifth-harmonic pulses with a Nd:glass laser. Highly reproducible 340-µJ, 850-fs pulses at 211 nm were generated in 0.5-mm-thick β barium borate (BBO) crystal with a 33% energy conversion efficiency in the fifth-harmonic generation stage. By use of fifth-harmonic pulses, a two-photon absorption coefficient of β=2.45±0.15 cm/GW at 211 nm for the BBO crystal was measured.

High energy, sub-picosecond pulses from a Nd-doped double-clad fiber laser

We report on the generation of high energy sub-picosecond optical pulses at 1.065 μm wavelength from a Nd 3+-doped double-clad fiber laser. The laser is passively mode-locked by nonlinear polarization evolution and produces, without any extra-cavity pulse compression, pulses with an energy content of 4 nJ and a pulse duration of 940 fs at a repetition rate of 17 MHz.