Mode-locked Oscillator Research Articles

Intracavity gain shaping in millijoule-level, high gain Ho:YLF regenerative amplifiers.

We demonstrate intracavity gain shaping inside a 2 μm Ho:YLF regenerative amplifier with a spectral bandwidth of 2.9 nm broadened to 5.4 nm, corresponding to Fourier-limited pulses of 1 ps duration. The intracavity gain shaping is achieved by using a simple etalon, which acts as a frequency-selective filter. The output of the regenerative amplifier is amplified by a single-pass amplifier, and we achieve total energy of 2.2 mJ and pulse duration of 2.4 ps at 1 kHz with pulse fluctuations <1%. The amplifier chain is seeded by a home-built mode-locked holmium-doped fiber oscillator.

High-power, 1-ps, all-Yb:YAG thin-disk regenerative amplifier.

We report a 100 W, 20 mJ, 1-ps, all-Yb:YAG thin-disk regenerative amplifier seeded by a microjoule-level Yb:YAG thin-disk Kerr-lens mode-locked oscillator. The regenerative amplifier is implemented in a chirped pulse amplification system and operates at an ambient temperature in air, delivering ultrastable output pulses at a 5 kHz repetition rate and with a root mean square power noise value of less than 0.5%. Second harmonic generation of the amplifier's output in a 1.5 mm-thick BBO crystal results in more than 70 W at 515 nm, making the system an attractive source for pumping optical parametric chirped pulse amplifiers in the visible and near-infrared spectral ranges.

A Neurodynamic Account of Musical Tonality

Science since antiquity has asked whether mathematical relationships among acoustic frequencies govern musical relationships. Psychophysics rejected frequency ratio theories, focusing on sensory phenomena predicted by linear analysis of sound. Cognitive psychologists have since focused on long-term exposure to the music of one’s culture and short-term sensitivity to statistical regularities. Today evidence is rapidly mounting that oscillatory neurodynamics is an important source of nonlinear auditory responses. This leads us to reevaluate the significance of frequency relationships in the perception of music. Here, we present a dynamical systems analysis of mode-locked neural oscillation that predicts cross-cultural invariances in music perception and cognition. We show that this theoretical framework combines with short- and long-term learning to explain the perception of Hindustani rāgas, not only by encultured Indian listeners but also by Western listeners unfamiliar with the style. These findings demonstrate that intrinsic neurodynamics contribute significantly to the perception of musical structure.

Multi-Watt mid-IR femtosecond polycrystalline Cr(2+):ZnS and Cr(2+):ZnSe laser amplifiers with the spectrum spanning 2.0-2.6 µm.

We demonstrate efficient amplification of few-optical-cycle mid-IR pulses in single-pass continuously pumped laser amplifiers based on polycrystalline Cr(2+):ZnS and Cr(2+):ZnSe. The 1.7 W output of a Kerr-lens mode-locked master oscillator at 2.4 µm central wavelength, 79 MHz repetition rate was amplified to 7.1 W and 2.7 W in Cr(2+):ZnS and Cr(2+):ZnSe, respectively. High peak power of the input pulses (0.5 MW) and high nonlinearity of the amplifiers' gain media resulted in a significant shortening of the output pulses and in spectral broadening. Transform-limited 40 fs pulses of the master oscillator were compressed to about 27-30 fs. The spectrum of the pulses was broadened from 136 nm to 450 nm (at -3 dB level); the span of the spectra exceeds 600 nm at -10 dB level.

A cavity mode size adjustable high average power Ti: sapphire femtosecond regenerative amplifier

High average power femtosecond lasers based on Ti:sapphire are widely used in strong-field physics and ultrafast dynamics.Continued advances include isolated attosecond pulse generation,few-cycle pulse generation,ultrafast spectroscopy,time-resolved photo-chemical reaction dynamics and laser micro-machining benefit greatly from use of such laser systems.The regenerative amplifiers are mostly utilized and have inherent advantages over multipass ones for applications in chirped pulse amplification.In this paper we describe a design,performance,and the characterizations of a novel linear cavity regenerative amplifier which has produced 4.8 W average power with 35 fs pulse durations at 1 kHz repetition rate. The main difficulty in designing and constructing a high average power Ti:sapphire regenerative cavity is thermal lensing effect.In order to generate amplified pulses with an output power exceeding 5 W at 1 kHz,a green pump power higher than 20 W is required.Meanwhile,the focal pump beam diameter on the surface of Ti:sapphire crystal should have sub-millimeter mode size to demonstrate large pump fluence,inducing a focal length of a thermal lens about 100 mm,i.e.,which is much less than the scale of the cavity length.For our experiments,a cavity mode size adjustable geometry is employed to counteract thermal lensing effect and to optimize the conversion efficiency of the amplifier.We first characterize the cavity stability by applying the well-known ABCD matrix formalism.The cavity consisting of R=900 mm concave mirror,an 2=800 mm lens and a plane mirror has two stability ranges with increasing the focal length of the thermal lens.In order to obtain a highest thermal tolerance,the optimal cavity parameters are resolved when two stability zones merge into one.After characterizing the cavity in detail,we calculate the cavity mode and the pump beam size at the position of the Ti:sapphire rod as a function of the thermal focal length.The optimal mode radius occurs at 312 m,corresponding to the intersection point of two curves.Stability curve exhibits a weak thermal sensitivity which is defined as the change of radius of cavity mode size per unit focal power change of thermal lens, keeping well below 10 m/D in a range of 2 D-4 D.The calculated results show that the active compensation for thermal lens focal length from 100 mm to could be achieved by adjusting the lens position,without changing the cavity. 20 fs,3 nJ pulses at a repetition rate of 82 MHz produced by a home-made Kerr-lens mode-locked oscillator are first sent to a Martinez stretcher by using a 1200 lines/mm holographic reflectance grating,which temporally stretches the laser pulses to 200 ps.The seed pulses out of the stretcher is then injected into the regenerative cavity depicted above. The 20 mJ pumping energy at 1 kHz is focused through the R=900 mm concave mirror into a 10 mm Brewster-cut Ti:sapphire rod,which is cooled to 250 K by thermoelectric elements.Condensation was avoided by placing the crystal into a small evacuated chamber.Mode matchings of pump and laser beam are found to be of critical importance for high energy extraction efficiency and high beam quality.In our experiments it is accomplished by fine adjusting the F=800 mm cavity lens and the pump beam size.The amplified power of 6.5 W at 1 kHz is obtained with minimum beam distortion,giving a 33.6% slope efficiency.The trapped pulse is built-up quickly and saturated after 8-round trips. The beam size of the amplified laser is expanded to 15 mm in diameter before compressor.A transmission efficiency of 73.8% is achieved through the grating-pair Treacy-type compressor,leading to a 4.8 mJ pulse energy.The grating has a groove density of 1500 lines/mm,and the compressed output spectrum has a full width at half maximum of 29 nm. The pulse duration measurement is performed by using an interferometric autocorrelation.As a result,a typical autocorrelation trace corresponding to a 35 fs pulse width is displayed,and agrees well with the 32 fs transform limit.The far-field beam profile after the compressor is round and Gaussian in both s and p planes,respectively.This scheme is also sufficiently reliable and robust so that no components of the laser system were damaged over a year of operation. In summary,the theoretical analysis and experimental results show that the regenerative cavity developed in this work exhibits a high conversion efficiency and an extraordinary thermal stability,and it is very suitable for high power and high efficient amplification of femtosecond Ti:sapphire pulses.

Noise-like femtosecond pulse in passively mode-locked Tm-doped NALM-based oscillator with small net anomalous dispersion

A passively mode-locked thulium-doped fiber laser (TDFL) based on a nonlinear amplifying loop mirror (NALM) is presented. By adjusting the polarization controllers, stable noise-like (NL) mode-locked femtosecond pulse operation is obtained at the 2 μm band. In the experimental period of 200 min, the output power fluctuation is less than 0.06 dB and the 3 dB spectral bandwidth variation is less than 0.02 nm, indicating that the pulsed TDFL possesses good long-term stability. To the best of our knowledge, this is the first 2 μm band NALM-based TDFL with small net anomalous dispersion for a NL femtosecond pulse. At the maximum pump power of 3.52 W, the emitting laser has a NL pulse width of 460 fs, the repetition rate of 9.1 MHz, and the NL pulse energy of 32.72 nJ.

High-power graphene mode-locked Tm/Ho co-doped fiber laser with evanescent field interaction.

Mid-infrared ultrafast fiber lasers are valuable for various applications, including chemical and biomedical sensing, material processing and military applications. Here, we report all-fiber high-power graphene mode-locked Tm/Ho co-doped fiber laser at long wavelength with evanescent field interaction. Ultrafast pulses up to 7.8 MHz are generated at a center wavelength of 1879.4 nm, with a pulse width of 4.7 ps. A graphene absorber integrated with a side-polished fiber can increase the damage threshold significantly. Harmonics mode-locking can be obtained till to the 21th harmonics at a pump power of above 500 mW. By using one stage amplifier in the anomalous dispersion regime, the laser can be amplified up to 450 mW and the narrowest pulse duration of 1.4 ps can be obtained simultaneously. Our work paves the way to graphene Tm/Ho co-doped mode-locked all-fiber master oscillator power amplifiers as potentially efficient and economic laser sources for high-power laser applications, such as special material processing and nonlinear optical studies.

Focus on microscopy and photonics

Focus on microscopy and photonics

Efficient picosecond traveling-wave Raman conversion in a SrWO4 crystal pumped by multi-Watt MOPA lasers at 1064 nm

Raman conversion with a 50-mm-long SrWO4 crystal in a single-pass, traveling-wave setup has been investigated in both purely steady-state and transient stimulated Raman scattering (SRS) regimes. For steady-state SRS experiment, we employed as a pump source a Q-Switched master oscillator power amplifier (MOPA) laser system at 1064 nm, delivering 325 μJ, 550-ps-long pulses with diffraction limited beam quality and high spectral purity. At 2-kHz repetition rate, we obtained up to 90 μJ pulse energy and 250 ps pulse duration at 1180 nm, with a conversion slope efficiency close to quantum limit. To approach the transient SRS regime, we pumped the same crystal with 16-ps-long pulses from a hybrid MOPA laser system based on a mode-locked Yb-fiber oscillator followed by a diode-pumped bulk Nd:YVO4 power amplifier. At the maximum incident pump average power of 3.75 W, we obtained 1.4 W at the first Stokes Raman-shifted wavelength of 1180 nm (37 % optical-to-optical conversion efficiency), with 15 ps pulse duration and 70 % conversion slope efficiency.

Above 100 nJ solitons from a passively mode-locked thulium-doped fiber oscillator

AsbtractWe report on stable nanosecond pulses with a non-symmetrical shape and high energy from a passively mode-locked thulium (Tm) fiber oscillator in the purely anomalous dispersion regime. The laser generated wave-breaking-free solitons with pulse widths increased from ~460 ps to ~3.15 ns by boosting the pump strength. The solitons yielded an output average power of ~1.1 W, a repetition rate of 10.63 MHz, and a pulse energy up to ~103 nJ at the maximum available pump power. By removing the free-space components, the pulse energy was further scaled to ~268 nJ at a 11.2 MHz repetition rate. The corresponding polarization-locked vector solitons (PLVS) were also observed.

Few-cycle pulses from a graphene mode-locked all-fiber laser

We combine a graphene mode-locked oscillator with an external compressor and achieve ∼29 fs pulses with ∼52 mW average power. This is a simple, low-cost, and robust setup, entirely fiber based, with no free-space optics, for applications requiring high temporal resolution.

Generation of Broadband Spectrum from a Simple Nonlinear-Polarization-Evolution Mode-Locked Yb-Doped Fiber Oscillator**Supported by the National Basic Research Program of China under Grant No 2012CB821304, and the National Natural Science Foundation of China under Grant No 61378040.

We demonstrate a nonlinear polarization evolution mode-locked Yb-doped fiber oscillator with the broadband spectrum output operating in a dispersion-managed regime. Pumped by a 976 nm single-mode laser diode, stable mode-locked ultrashort pulses are emitted with an average power of 198 mW at a repetition rate of 124 MHz, corresponding to a pulse energy of 1.6 nJ. The output spectrum spans from 950 nm to 1150 nm so that the transform-limited pulse duration is as short as 23 fs. Due to the imperfect dispersion compensation, we compress the pulses to 32 fs in this experiment.

978 nm 1.24 ps ultra-short plused large mode area photonic crystal fiber laser

We report on a 980nm passively mode-locking Yb-doped large mode area photonic crystal fiber oscillator with semiconductor saturable absorber mirror (SESAM) and nonlinear polarization evolution (NPE) technique, simultaneously. The oscillator generates a maximum average output power of 497mW with a repetition rate of 87.37MHz. Because of the invisible filter effect of NPE mode-locked fiber laser, we achieved an ultra-short pulse width of 1.24ps. The output spectrum of the pulse is centered at 977.7nm with a full width half maximum (FWHM) of 1.90nm and has a characteristic steep spectral edges of dissipative soliton. In this paper, the pulse evolution process of 980nm mode-locking fiber laser is simulated and the experimental results are good agreement with the simulation results.

High average/peak power linearly polarized all-fiber picosecond MOPA seeded by mode-locked noise-like pulses

The characteristics of mode-locked noise-like pulses generated from a passively mode-locked fiber oscillator are experimentally investigated. By carefully adjusting the two polarization controllers, stable mode-locked noise-like pulse emission with a high radio frequency signal/noise ratio of >55 dB is successfully achieved, ensuring the safety and possibility of high power amplification. To investigate the amplification characteristics of such pulses, one all-fiber master oscillator power amplifier (MOPA) is built to boost the power and energy of such pulses. Amplified noise-like pulses with average output power of 423 W, repetition rate of 18.71 MHz, pulse energy of 22.61 μJ, pulse duration of 72.1 ps and peak power of 314 kW are obtained. Near diffraction-limited beam is also demonstrated with M2 factor measured at full power operation of ~1.2 in the X and Y directions. The polarization extinction ratio at output power of 183 W is measured to be ~13 dB. To the best of our knowledge, this is the first demonstration of high-power amplification of noise-like pulses and the highest peak power ever reported in all-fiber picosecond MOPAs. The temporal self-compression process of such pulses and high peak power when amplified make it an ideal pump source for generation of high-power supercontinuum. Other potential applications, such as material processing and optical coherent tomography, could also be foreseen.

260-megahertz, megawatt-level thin-disk oscillator.

A Kerr-lens mode-locked (KLM) Yb:YAG thin-disk oscillator delivering 215-fs pulses with 75-W average power and 1.4-MW peak power at a repetition rate of 260MHz is presented. Self-starting KLM is demonstrated at an output power of 68W. This is the highest repetition rate of any mode-locked thin-disk oscillators so far. Concepts for scaling the repetition rate up to 1GHz are discussed.

All-normal dispersion passively mode-locked Yb-doped fiber laser with Bi 2 Te 3 absorber

An all-normal dispersion passively mode-locked Yb-doped fiber laser with a Bi2Te3 absorber is presented. The modulation depth of this kind of saturable absorber was measured to be 8.4%. By incorporating a Bi2Te3/PVA film into a Yb-doped fiber laser oscillator, a mode-locked fiber laser oscillator was achieved. The repetition rate and the central wavelength are 25.6 MHz and 1052.7 nm, respectively. The 3-dB spectral width is 0.45 nm and the pulse duration is 417 ps. The results indicate that topological insulator Bi2Te3 possesses the potential for ultrafast fiber laser application.

All-optical Q-switching limiter for high-power gigahertz modelocked diode-pumped solid-state lasers.

Passively modelocked diode-pumped solid-state lasers (DPSSLs) with pulse repetition rates in the gigahertz regime suffer from an increased tendency for Q-switching instabilities. Low saturation fluence intracavity saturable absorbers - such as the semiconductor saturable absorber mirrors (SESAMs) - can solve this problem up to a certain average output power limited by the onset of SESAM damage. Here we present a passive stabilization mechanism, an all-optical Q-switching limiter, to reduce the impact of Q-switching instabilities and increase the potential output power of SESAM modelocked lasers in the gigahertz regime. With a proper cavity design a Kerr lens induced negative saturable absorber clamps the maximum fluence on the SESAM and therefore limits the onset of Q-switching instabilities. No critical cavity alignment is required because this Q-switching limiter acts well within the cavity stability regime. Using a proper cavity design, a high-power diode-pumped Yb:CALGO solid-state laser generated sub-100 fs pulses with an average output power of 4.1 W at a pulse repetition rate of 5 GHz. With a pulse duration of 96 fs we can achieve a peak power as high as 7.5 kW directly from the SESAM modelocked laser oscillator without any further external pulse amplification and/or pulse compression. We present a quantitative analysis of this Kerr lens induced Q-switching limiter and its impact on modelocked operation. Our work provides a route to compact high-power multi-gigahertz frequency combs based on SESAM modelocked diode-pumped solid-state lasers without any additional external amplification or pulse compression.

Hybrid mode-locked Er-fiber oscillator with a wide repetition rate stabilization range

We present a hybrid mode-locked Er-fiber oscillator whose repetition rate can be tuned from 101.3 to 103.0 MHz. By incorporating saturable absorption and nonlinear polarization evolution in a ring cavity, mode-locked states can be easily accessed with a 3 dB spectral range wider than 50 nm. In addition, by the combination of a computer-controlled optical delay line and a piezo actuator to change the length of the cavity, the repetition rate can be stabilized at any frequency in the tunable range for the long term. When phase locking to an Rb clock for 14 h continuously, the standard deviation of the fundamental repetition rate is 0.2 mHz with a gate time of 1 s. In addition, the feedback bandwidth can be extended to ∼400 Hz by stabilizing the 10th harmonic instead of the fundamental repetition rate. Furthermore, the linewidth of the comb modes is investigated by beating with a wavelength-tunable diode laser.

8.5 W mode-locked Yb:Lu1.5Y1.5Al5O12 laser with master oscillator power amplifiers.

We report on a diode-pumped passively mode-locked Yb:Lu(1.5)Y(1.5)Al(5)O(12) (Yb:LuYAG) laser for the first time to our knowledge. With the mixed crystal of Yb:LuYAG as gain medium, the mode-locked laser generated 2.2 W of average output power with a repetition rate of 83.9 MHz and pulse duration of 2.4 ps at the wavelength of 1030 nm. In order to obtain higher output power, the output from the mode-locked oscillator was further amplified to 8.5 W by two-stage single-pass amplifiers. The high-power picosecond laser is very useful for applications such as pumping of midinfrared optical parametric oscillators, material microprocessing, and UV light generation.

Highly-efficient 20 TW Ti:sapphire laser system using optimized diverging beams for laser wakefield acceleration experiments

We developed a highly efficient and compact Ti:sapphire multipass amplifier that uses divergent seed beams to generate 20 TW/40 fs laser pulses with a repetition rate of 10 Hz. The laser system consists of a mode-locked oscillator, a regenerative amplifier, and a multipass power amplifier. The thermal lensing effect is very important in this system, especially in the multipass amplifier, as it limits the conversion efficiency. In order to compensate the thermal lensing effect, we calculated the optimum divergence of the seed beam and used the result for the multipass amplifier, where the thermal focusing is taken into account. In this way, we achieved a very high conversion efficiency of 41%, which is close to the theoretical limit. The laser system was then used with a capillary gas cell to generate stable high-energy electron beams with electron energies of about 150 MeV and a beam divergence of 4 ± 1 mrad. In this paper, details of the laser system development and experimental results for electron generation are presented.