Picosecond Pulse Generation Research Articles

Backward three-wave optical solitons

Backward symbiotic solitary waves in quadratic media with absorption losses are generated through the nonlinear non-degenerate three-wave interaction. We study these solitary waves in the particular case of a doubly backward quasi-phase matching configuration. The same mechanism responsible for nanosecond solitary wave morphogenesis in the c.w. pumped Brillouin-fiber-ring laser may act for picosecond pulse generation in a quadratic c.w. pumped optical parametric oscillator (OPO). The resonant condition is automatically satisfied in stimulated Brillouin backscattering when the fiber-ring laser contains a large number of longitudinal modes beneath the gain curve. However, in order to achieve quasi-phase matching between the three optical waves in the Χ(2) medium, a nonlinear susceptibility inversion grating of sub-µm period is required. Such a quadratic medium supports solitary waves that result from energy exchanges between dispersionless waves of different velocities. We show, by a stability analysis of the non-degenerate backward OPO in the QPM decay interaction between a c.w. pump and backward signal and idler waves that the inhomogeneous stationary solution exhibits a Hopf bifurcation with a single control parameter. Above OPO threshold, the nonlinear dynamics yields self-structuration of a backward symbiotic solitary wave, which is stable for a finite temporal walk-off (i.e. different group velocities) between signal and idler waves. We also study the dynamics of singly backward mirrorless OPO’s (BMOPO’s) pumped by an incoherent field, in line with the recent experimental demonstration of this OPO configuration. We show that this system is characterized, as a general rule, by the generation of a highly coherent backward field, despite the high degree of incoherence of the pump field. This remarkable property finds its origin in two distinct phase-locking mechanisms that originate respectively in the convection and the dispersion properties of the fields. In both cases we show that the incoherence of the pump is transferred to the co-moving field, which thus allows the backward field to evolve towards a highly coherent state. We propose realistic experimental conditions that may be implemented with currently available technology and in which backward coherent wave generation from incoherent excitation may be observed and studied.

Picosecond optical pulse generation from self-pulsating bisectional GaN-based blue-violet laser diodes

Self-pulsations of 407 nm emitting GaN-based blue-violet laser diodes with bisectional (BS) electrodes were demonstrated by applying a reverse bias (VSA) to the subelectrode of the saturable absorber (SA) section. By increasing the injected current to the main electrode of the gain section with a reverse bias of VSA=−12 V, the optical pulses shortened to 30 ps. The optical peak output power was as high as 2.4 W with a pulse width of 30 ps and a repetition frequency of 0.9 GHz. This is so far the shortest pulse width from a self-pulsating BS GaN-based laser diode achieved by applying a reverse bias to the SA section.

Application of the nonlinear optical properties of platinum nanoparticles for the mode locking of Nd:glass laser

We analyzed the saturated absorption of a platinum nanoparticle suspension using 1064-nm, 50-ps pulses at different intensities of laser radiation. The concurrence of reverse saturated absorption and saturated absorption at high intensities allowed for defining the saturated intensity for different suspensions of Pt nanoparticles (5×109 W cm−2). This suspension was used for the mode locking of laser radiation in a Nd:glass oscillator (λ=1054 nm). We achieved a stable generation of picosecond pulse trains with pulse duration of 5 ps using the mode locker based on the platinum nanoparticle suspension. The comparison of this nanoparticle-based mode locker with the conventional dye films showed the better stability of picosecond pulse generation and longer lifetime in the former case.

Timing Jitter Tolerant All-Optical TDM Demultiplexing Using a Saw-Tooth Pulse Shaper

We experimentally demonstrate the generation of picosecond saw-tooth optical pulses, which are shaped using a fiber Bragg grating to provide timing jitter tolerant optical time-division multiplexing data demultiplexing in a scheme based on cross-phase modulation in an optical fiber, with subsequent offset filtering. A power improvement of more than 5 dB is obtained compared to demultiplexing with more conventional shapes, such as Gaussian, with a similar pulsewidth.

Benchtop time-resolved magneto-optical Kerr magnetometer

We present here the construction and application of a compact benchtop time-resolved Kerr magnetometer to measure the magnetization precession in magnetic thin films and lithographically patterned elements. As opposed to very expensive femtosecond lasers this system is built upon a picosecond pulsed injection diode laser and electronic pulse and delay generators. The precession is triggered by the electronic pulses of controlled duration and shape, which is launched onto the sample by a microstrip line. We used polarized optical pulses synchronous to the electronic pulses to measure the magneto-optical Kerr rotation. The system is integrated in a conventional upright microscope configuration with separate illumination, imaging, and magneto-optical probe paths. The system offers high stability, relative ease of alignment, sample changing, and a long range of time delay. We demonstrate the measurements of time-resolved dynamics of a Permalloy microwire and microdot using this system, which showed dynamics at two different time scales.

Diode-pumped Nd:BaY 2F 8 picosecond laser

Picosecond pulse generation at 1050 nm by a diode-pumped Nd:BaY 2F 8 (Nd:BaYF) active crystal has been investigated under low-power pumping (440 mW absorbed). The sample was oriented along the optical axes and cut at Brewster angle for y-axis laser polarization to minimize birefringence effects. Stable passive mode-locking operation has been achieved by inverse saturable absorption with 80-mW output power and 3.7-ps pulses.

Generation of flat-top picosecond pulses by coherent pulse stacking in a multicrystal birefringent filter

This paper deals with the pulse-shaping properties of birefringent filters that feature an optical layout similar to a Solc fan filter. A simple computational model is given that explains the pulse-shaping process in the fan filter in two steps: First, the input pulse is split into several mutually delayed replicas due to the birefringence of the crystals. Second, these replicas interfere at the output polarizer of the filter and form the shaped output pulse. Fine-tuning of the phases of the replicas of the input pulse is permitted by tuning the temperature of the crystals. A birefringent pulse shaper containing ten birefringent crystals was investigated experimentally. The shape of the output pulses was measured by means of a special cross-correlation technique. Although a variety of pulse shapes can be generated with the described filter, it is particularly well suited for generation of flattop pulses featuring a 20-ps-long plateau and rising and falling edges shorter than 2 ps.

Generation of flat-top picosecond pulses by means of a two-stage birefringent filter

We describe the type of pulse shaper for production of flat-top pulses that is used at the Photo Injector Test Facility (PITZ) at DESY. This shaper consists of a two-stage birefringent filter. Using a simple model, this pulse shaper can be regarded as a system that splits the Gaussian input pulse into four mutually delayed replicas and stacks them in a phase-coherent way. Although a variety of different pulse shapes can be generated by this method, the system is particularly suited for the generation of flat-top output pulses with duration between 10 and 100 ps. When operating in combination with an actively modelocked Nd:YLF oscillator, it can shape picosecond pulses arranged in long trains, as required for photocathode lasers for the TESLA-type linear accelerators. Using trains of equally shaped picosecond pulses has an additional benefit: It permits measuring the shape of these pulses by cross-correlation in real-time using a so-called optical sampling system. Such a system is employed as a measurement tool for appropriate alignment of the individual components of the pulse shaper.

Experimental investigation of a passively mode-locked fiber laser based on a symmetrical NOLM with a highly twisted low-birefringence fiber

We experimentally investigate the passive mode-locking operation of a figure eight-fiber laser based on a symmetrical nonlinear optical loop mirror (NOLM) with a highly twisted low-birefringence fiber in the loop. NOLM switching is achieved by the polarization asymmetry between the counterpropagating beams in the loop. The most efficient switching is obtained when we have linear polarization for one of the beams and the circular polarization for the other. We used a quarter-wave retarder (QWR) in the NOLM loop to break the polarization symmetry. Through the QWR position, it is possible to adjust the transmission behavior from a maximum to a minimum at a low input power. With our configuration, it is possible to get self-starting mode-locking operation at a specific position of the QWR. This QWR position corresponds to a value close to the minimal transmission. The pulse repetition frequency was 0.8 MHz. The mode-locked laser ran in a stable operation for hours. We achieved a stable generation of picosecond pulses with milliwatts of an average output power.

Stable Q-switch mode-locking of Nd:YVO4 lasers with a semiconductor saturable absorber

This paper reports on a passively mode-locked and Q-switched Nd:YVO4 laser generating picosecond pulses with an average output power exceeding 7 W. In a first step Q-switch mode-locking was obtained by self Q-switching of a mode-locked oscillator with appropriate cavity design, pump power and output coupling. In a second system the Q-switching was actively controlled and stabilized by modulating the resonator internal losses with an acousto-optic modulator. In the Q-switch mode-locking operation the laser provided 12.8 ps long mode-locked pulses with a repetition rate of 80 MHz. The repetition rate of the Q-switch envelope was 185 kHz. The maximum pulse energy of a single ps pulse was 0.55 μJ which is 5.5 times the pulse energy measured for cw mode locking. The total energy of the pulses within the Q-switch envelope was 42 μJ.

Looking for shear waves in glass forming liquids

The ability of the generation of picosecond shear acoustic pulses in solids using ultrashort optical pulses have been employed to look for high frequency shear waves in glass forming liquids. As a transducer we used a canted iron thin film deposited on a glass substrate. The strong shear efficiency of generation provided by this kind of samples allowed the study of several glass forming liquids. As a first try, we have detected shear brillouin scattering in m-toludine at 100K, that is below the glass transition temperature, at a frequency of 9 GHz. Then, we observed shear waves propagating at room temperature in glycerol at frequencies below 50 GHz.

Long-Period Fiber-Grating-Based Filter for Generation of Picosecond and Subpicosecond Transform-Limited Flat-Top Pulses

We suggest and demonstrate an all-fiber pulse shaper for synthesizing transform-limited flat-top waveforms. The proposed scheme can be easily scaled down to the (sub-)picosecond regime while offering low polarization dependence and extremely low insertion loss. It is based on a cascade of two long-period fiber gratings. As a proof-of-concept experiment, we report the generation of transform-limited flat-top pulses with full-width at half- maximum duration of 2 ps, which is over two times faster than the shortest transform-limited flat-top pulse ever generated with an all-fiber filter.

10 GHz, 24 ps pulse generation using a single-stage dual-drive Mach-Zehnder modulator

This Letter reports on picosecond pulse generation at a repetition of 10 GHz by using a single-stage electro-optic LiNbO(3) Mach-Zehnder modulator, stressing the simplicity of its setup. It is analytically and experimentally proved that dual-arm modulation with in-phase sinusoidal signals having slightly different amplitudes generated a highly coherent optical comb with a great spectral flatness and a parabolic phase relationship in its spectrum. The generated comb was Fourier synthesized and shaped into an ultrashort pulse train with an optical bandpass filter and a dispersive fiber. The pulse source was highly stable, exhibiting an ultralow timing jitter of less than 130 fs.

Passively mode-locked Yb:YAG thin-disk laser with pulse energies exceeding 13 μJ by use of an active multipass geometry

We demonstrate the generation of high-energy picosecond pulses directly from a thin-disk laser oscillator by employing a self-imaging active multipass geometry. Stable single-pulse operation has been obtained with an average output power in excess of 50 W, excluding a cw background of 8%, at a repetition rate of 3.8 MHz. Self-starting passive mode locking was accomplished using a semiconductor saturable absorber mirror. The maximum pulse energy was 13.4 microJ at a pulse duration of 1.36 ps with a time-bandwidth product of 0.34. Single-pass external frequency doubling with a conversion efficiency of 60% yielded >28 W of average power at 515 nm.

Picosecond flat-top pulse generation by low-bandwidth electro-optic sinusoidal phase modulation

We report the first experimental demonstration to our knowledge of a microwave frequency upshifting system based on phase modulation. A sequence of flat-top optical and RF pulses at a repetition rate of 18.22 GHz, each with a FWHM time width of approximately 25 ps, is generated from a sinusoidal RF tone of only 3.680 GHz, in good agreement with our analytical and numerical calculations. A simple explanation of this technique based on Talbot effect theory is provided. The practical limitations and capabilities of the phase-modulation-based frequency upshifting approach for ultrabroadband RF waveform generation are also discussed.

Generation of Fiber Tunable High-power Picosecond Laser Pulse

A fiber tunable high-power picosecond laser system has been for the first time demonstrated. A gain switch semiconductor laser diode was introduced as seed source, and a multi-stage single mode Yb-doped fiber amplifier was combined with a single mode double-clad Yb-doped fiber amplifier to construct the amplification system. High stability and good beam quality pulses with 1MHz tunable repetition rate, 3W average power, 90ps pulse duration, and central wavelength tunable from 1053 nm to 1073 nm were generated. The completely fiber integrated approach has the potential to be scaled to significantly higher average powers.

High-Power Picosecond Pulse Generation Due to Mode-Locking With a Monolithic 10-mm-Long Four-Section DBR Laser at 920 nm

A 10-mm-long four-section distributed Bragg reflector laser with a double-quantum-well heterostructure at 920 nm was realized. A maximum optical pulse power of 3.6 W with a repetition rate of 4.1 GHz corresponding to the laser length was reached. The full-width at half-maximum of the pulses was 7 ps measured with an autocorrelator. A maximum pulse energy of 25 pJ was reached.

High-energy picosecond near-vacuum ultraviolet pulses generated by sum-frequency mixing of an amplified Ti:sapphire laser

We demonstrate high-energy picosecond near-vacuum ultraviolet laser pulse generation. Frequency quadrupling is achieved by noncollinear sum-frequency mixing of the fundamental and the third harmonic of a two-stage Ti:sapphire amplifier in beta-BaB(2)O(4) crystal. UV pulses with energies of approximately 10 mJ tunable from 195 to 210 nm at a 10 Hz repetition rate are obtained.

Incoherent frequency-to-time mapping: application to incoherent pulse shaping

After temporal amplitude modulation of a spectrally incoherent optical source the averaged intensity profile at the so-called temporal far-zone regime coalesces with a magnified replica of the spectral density function of the source. This has provided the basis for the generalization of the frequency-to-time mapping technique in the partially coherent case. Based on this fact, temporal intensity waveform generation is demonstrated by spectral filtering the incoherent source before the temporal modulation stage. We refer to this technique as full incoherent pulse shaping. Although only the average intensity of the output signal is properly shaped, intensity fluctuations between the different realizations of the output shaped waveform are shown to be small in the practical situation. Finally, we provide some computer simulations concerning arbitrary picosecond pulse generation from an amplified spontaneous emission source.

Parameter tunable GaInNAs saturable absorbers for mode locking of solid-state lasers

Passively mode-locked solid-state lasers require semiconductor saturable absorber mirrors (SESAMs) with a recovery time up to 10–30 times longer than the final pulse duration. GaInNAs SESAMs normally have a high defect concentration that supports a fast picosecond recovery time and have been used to passively mode-lock lasers at 1.3 μm. Post-growth annealing is a very useful technique to adapt the absorption edge of the GaInNAs absorber to the laser. However, larger absorption edge tuning reduces the defect concentration and increases the recovery time too much for picosecond pulse generation. We have developed a novel growth technique that allows wavelength tuning with post-growth annealing, but without any increase of the recovery time and without degrading the absorber performance. This was achieved by exposing the GaAs barrier to nitrogen from a plasma source for about 20 s before the GaInNAs quantum well was grown.