Kerr-lens Mode-locked Laser Research Articles

Sub-6 optical-cycle Kerr-lens mode-locked Tm:Lu2O3 and Tm:Sc2O3 combined gain media laser at 2.1 μm.

We present a combined gain media Kerr-lens mode-locked laser based on a Tm:Lu2O3 ceramic and a Tm:Sc2O3 single crystal. Pulses as short as 41 fs, corresponding to less than 6 optical cycles, were obtained with an average output power of 42 mW at a wavelength of 2.1 μm and a repetition rate of 93.3 MHz. Furthermore, a maximum average power of 316 mW with a pulse duration of 73 fs was achieved.

Kerr-lens mode-locked Yb:LuAG ceramic thin-disk laser.

A Kerr-lens mode-locked (KLM) thin-disk laser with Yb:LuAG ceramic was demonstrated. Yb:LuAG ceramic is an attractive material for high-power lasers due to its high thermal conductivity and large emission cross section. The highest output power of 17 W with a pulse duration of 130 fs was achieved. Moreover, the pulse duration of 88 fs was also obtained with a high-Q factor cavity. To the best of our knowledge, this is the first demonstration of a KLM thin-disk laser based on Yb:LuAG, including both ceramic and single crystal. The results show the usefulness of ceramic thin disks for high-power ultrashort pulse laser sources.

Diode-pumped sub-50-fs Kerr-lens mode-locked Yb:GdYCOB laser.

We present a sub-50-fs diode-pumped Kerr-lens mode-locked laser employing a novel "mixed" monoclinic Yb:Ca4(Gd,Y)O(BO3)3 (Yb:GdYCOB) crystal as a gain medium. Nearly Fourier-limited pulses as short as 43 fs at 1036.7 nm are generated with an average power of 84 mW corresponding to a pulse repetition rate of ∼70.8 MHz. A higher average power of 300 mW was achieved at the expense of the pulse duration (113 fs) corresponding to an optical-to-optical efficiency of 35.8% representing a record-high value for any Yb-doped borate crystal. Non-phase-matched self-frequency doubling is observed in the mode-locked regime with pronounced strong spectral fringes which originate from two delayed green replicas of the fundamental femtosecond pulses in the time domain.

Diffractive saturable loss mechanism in Kerr-lens mode-locked lasers: direct observation and simulation.

Passive mode-locking relies critically on a saturable loss mechanism to form ultrashort pulses. However, in Kerr-lens mode-locking (KLM), no actual absorption takes place, but rather losses appear due to diffraction, and actual light must escape the cavity. The Kerr-lens effect works to generate through diffraction an effective instantaneous saturable absorber that depends delicately on the interplay between the spatial and temporal profiles of the pulse. Despite the importance of KLM as a technique for generating ultrafast pulses and the fundamental role of diffraction losses in its operation, these losses have never been observed directly. Here, we measure the light that leaks out due to diffraction losses in a hard-aperture Kerr-lens mode-locked Ti:sapphire laser, and compare the measured results with a numerical theory that explicitly calculates the spatiotemporal behavior of the pulse.

Observation of harmonic beams inside a Kerr lens mode-locked thin-disk ring laser oscillator beyond a repetition rate of 10 MHz

We report the highest intra-cavity pulse energy of 80 μJ inside a Kerr lens mode-locked thin-disk ring laser oscillator at a repetition rate beyond 10 MHz enabling observation of the 2nd and 3rd harmonic beams directly generated by the Kerr medium. By adding a tight focusing setup to the ring cavity, we could observe krypton gas flame at an intra-cavity peak intensity of ∼28 TW/cm2 yielding a conversion efficiency of 10−5 for the 3rd harmonic beam. A further upgrade of the ring oscillator could lead to realizing a table-top source of vacuum ultraviolet pulses for application in photoelectron spectroscopy.

Two-octave-wide (3-12 µm) subharmonic produced in a minimally dispersive optical parametric oscillator cavity.

We report a subharmonic (frequency-divide-by-2) optical parametric oscillator (OPO) with a continuous wavelength span of 3 to 12 µm (-37dB level) that covers most of the molecular rovibrational "signature" region. The key to obtaining such a wide spectral span is the use of an OPO with a minimal dispersion-through the choice of intracavity elements, the use of all gold-coated mirrors, and a special "injector" mirror. The system delivers up to 245 mW of the average power with the conversion efficiency exceeding 20% from a 2.35 µm Kerr-lens mode-locked pump laser.

Kerr-lens mode-locked Cr:ZnS oscillator reaches the spectral span of an optical octave.

We report, to the best of our knowledge, the first super-octave femtosecond polycrystalline Cr:ZnS laser at the central wavelength 2.4 µm. The laser is based on a non-polarizing astigmatic X-folded resonator with normal incidence mounting of the gain element. The chromatic dispersion of the resonator is controlled with a set of dispersive mirrors within one third of an optical octave over 2.05-2.6 µm range. The resonator's optics is highly reflective in the range 1.8-2.9 µm. The components of the oscillator's output spectrum at the wavelengths 1.6 µm and 3.2 µm are detected at -60 dB with respect to the main peak. Average power of few-cycle Kerr-lens mode-locked laser is 1.4 W at the pulse repetition frequency 79 MHz. That corresponds to 22% conversion of cw radiation of Er-doped fiber laser, which we used for optical pumping of the Cr:ZnS oscillator.

Kerr-Lens Mode-Locked Tm Solid State Laser with Spectral Broadening Effect

Kerr-Lens Mode-Locked Tm Solid State Laser with Spectral Broadening Effect

Operation of femtosecond Kerr-lens mode-locked laser with all-normal dispersion at 2.4 μm (Invited)

Operation of femtosecond Kerr-lens mode-locked laser with all-normal dispersion at 2.4 μm (Invited)

High power diode pumped solid-state femtosecond laser systems

We demonstrate high power direct diode-pumped femtosecond laser based on a commercially available configuration and pumped by multi-diode laser modules delivering high brightness and average power. The multi-diode modules employ beam-combining scheme preserving linear polarization and focusing properties of individual laser diodes. Stable passive Kerr lens modelocked (KLM) laser operation was demonstrated, producing 30 fs pulses at ∼300 mW average output power. A simple approach to numerical modelling of KLM laser operation in cases when longitudinal pump beam exhibits strong astigmatism is also discussed.

Q -switching stability limits of Kerr-lens mode locking

$Q$-switching instability restricts pulse shortening in Kerr-lens mode-locked lasers (KLMLs). However, $Q$-switching suppression in KLMLs has not been discussed to date because of the difficulty of treating the Kerr-lens effect theoretically. We investigated parameter ranges for stable Kerr-lens mode locking (KLM) against $Q$ switching theoretically and experimentally. We found the parameters and ranges required to suppress $Q$ switching in hard- and soft-aperture KLMLs. In soft-aperture KLMLs, both intracavity power and spatial mode matching between a pump beam and a cavity mode were found to be critical. These findings were verified experimentally using an Yb:${\mathrm{Y}}_{2}{\mathrm{O}}_{3}$ KLML. Our results provide cavity design criteria for stable KLMLs against $Q$ switching.

Kerr lens effect induced by a vortex LG0m laser beam

For a long time, there was this idea that it is impossible to mode lock a laser on a pure high-order transverse mode. Recently, it has been demonstrated experimentally the generation of femtosecond pulses by a Kerr-Lens-Mode-Locking (KLM) laser operating on a vortex LG0m mode. At the best of our knowledge, there is no studies on lensing effect induced by a doughnut laser beam inside a Kerr medium which is fundamental for modelling accurately the behaviour of a KLM laser and optimising its parameters. We have shown that the doughnut Kerr lens can be view as a ring-lens, or eccentric lens, showing two combined lensing effects. The first one is equivalent to a pure converging lens, and the second one to a diverging axicon. Both lensing effects have been analytically modelled, and provide values for the best focus position which are in a good agreement with the focal position calculated numerically by using a diffraction integral.

Elastic tape behavior of a bi-directional Kerr-lens mode-locked dual-comb ring laser.

We experimentally investigate a fixed point of a bi-directional dual-comb ring laser and the behavior of dual-comb signals in different spectral regions. We show that the results are quite different from those obtained with traditional dual-comb spectroscopy. We explain the difference using the elastic tape formalism that we apply to a bi-directional ring laser. We also discuss how the results can aid efforts to synchronize two bi-directional laser systems to enable rapid and high-resolution multidimensional coherent spectroscopy with a compact apparatus.

Raman-assisted broadband mode-locked laser

The pulse duration that is available from femtosecond mode-locked lasers is limited by the emission bandwidth of the laser crystals used. Considerable efforts have been made to broaden the emission gain bandwidth in these lasers over the past five decades. To break through this limitation, intracavity spectral broadening is required. Here, we propose a new spectral broadening method inside the mode-locked cavity based on use of stimulated Raman scattering and demonstrate significant pulse shortening using this method. We configured Kerr-lens mode-locked lasers based on Yb:CaGdAlO4, Yb:KY(WO4)2 and Yb:Y2O3 materials and achieved significant spectral broadening that exceeds the emission bandwidth. The spectral broadening in the Yb:CaGdAlO4 oscillator shortens the pulse duration to 22 fs, which is a one-third of the duration of our unbroadened mode-locked pulse. The results presented here indicate that Raman-assisted spectral broadening can break the limitations of the emission gain bandwidth and shorten the duration of pulses from femtosecond mode-locked lasers.

Sub-100 fs pulse generation from a Kerr-lens mode-locked Yb:Lu2O3 ceramic thin-disk laser.

In this Letter, we demonstrate, to the best of our knowledge, the first Kerr-lens mode-locked (KLM) ceramic thin-disk laser. Ceramic laser gain media have many advantages for a high-power laser such as high fracture toughness, size scalability, low cost, and short time fabrication. By using the Yb:Lu2O3 ceramic thin disk, the pulse duration of 98fs with the output power of 3.7W was demonstrated. The shortest pulse duration was 86fs with the decreased output power of 0.36W. From our results, it was proven that a KLM thin-disk laser with ceramic gain media can become a new option for high-power, sub-100fs pulse laser sources.

Frequency Comb Stabilization of Ultrafast Lasers by Opto-Optical Modulation of Semiconductors

In this paper, we review the current state and discuss new developments in opto-optical modulation (OOM) of semiconductor elements for frequency comb self-referenced stabilization of ultrafast lasers. This method has been successfully used for carrier-envelope offset (CEO) frequency stabilization of diode-pumped solid-state lasers operating in 1-μm and 1.5-μm regimes, providing high feedback bandwidth and resulting in low noise performance. We compare the achieved results for Er- and Yb-based laser materials and in different regimes of repetition rates up to 1 GHz. In addition, we present the first semiconductor OOM for CEO stabilization in an ultrafast fiber laser. Moreover, we discuss requirements and design guidelines for OOM chips. In most demonstrations, semiconductor saturable absorber mirrors have been used for OOM, which in parallel were also responsible for pulse formation. By separating the OOM functionality from the pulse formation, we expect that it will enable low-noise CEO stabilization in other types of ultrafast lasers, such as, for example, high-power Kerr-lens mode-locked thin disk lasers.

The effect of excitation intensity variation and silver nanoparticle codoping on nonlinear optical properties of mixed tellurite and zinc oxide glass doped with Nd2O3 studied through ultrafast z-scan spectroscopy

The research on Nd3+ doped new solid-state laser hosts with specific thermo-mechanical and optical properties is very active. Nd3+ doped tellurite glasses are suitable for these applications. They have high linear and nonlinear refraction index, wide transmittance range. The TeO2-ZnO (TZO) glass considered in the present work combines all those features and the nonlinear optical properties can be used for the development of Kerr-lens mode-locked sub picosecond lasers. Recently the laser performance of Nd3+ doped TZO glass and was reported and laser slope efficiency of 21% was observed. We investigate how the intensity variation and the silver nanoparticles codoping affects the nonlinear optical properties of Nd3+ doped TZO glasses. Intensity dependent nonlinear refraction indices coefficients at 750, 800 and 850 nm were observed. The nonlinear optical features were obtained through ultrafast single beam z-scan technique with excitations at 750, 800 and 850 nm and are up to two orders of magnitude higher than those reported in the literature.

70 femtosecond Kerr-lens mode-locked multipass-cavity Alexandrite laser.

We report, to the best of our knowledge, the shortest femtosecond pulses generated from a Kerr-lens mode-locked (KLM) Alexandrite laser operating near 750nm. The Alexandrite gain medium was pumped with a continuous-wave (cw), 532nm laser, and the performance of both the short and extended resonators was investigated. The use of an extended cavity eliminated the multi-wavelength spectral instabilities observed during the cw operation of the short cavity. Furthermore, since the repetition rate of the Alexandrite laser was reduced from 107 to 5.6MHz, the resulting increase in the intracavity pulse energy provided enhanced Kerr nonlinearity and eliminated the Q-switching instabilities during mode-locked operation. The KLM MPC Alexandrite laser produced nearly transform-limited, 70fs pulses at a pulse repetition rate of 5.6MHz with only 1W of pump power. The time-bandwidth product was further measured to be 0.331.

Continuous-wave to pulse regimes for a family of passively mode-locked lasers with saturable nonlinearity

The transition dynamics from continuous-wave to pulse regimes of operation for a generic model of passively mode-locked lasers with saturable absorbers, characterized by an active medium with non-Kerr nonlinearity, are investigated analytically and numerically. The system is described by a complex Ginzburg–Landau equation with a general m:n saturable nonlinearity (i.e , where I is the field intensity and m and n are two positive numbers), coupled to a two-level gain equation. An analysis of stability of continuous waves, following the modulational instability approach, provides a global picture of the self-starting dynamics in the system. The analysis reveals two distinct routes depending on values of the couple (m, n), and on the dispersion regime: in the normal dispersion regime, when m = 2 and n is arbitrary, the self-starting requires positive values of the fast saturable absorber and nonlinearity coefficients, but negative values of these two parameters for the family with m = 0. However, when the spectral filter is negative, the laser can self-start for certain values of the input field and the nonlinearity saturation coefficient Γ. The present work provides a general map for the self-starting mechanisms of rare-earth doped figure-eight fiber lasers, as well as Kerr-lens mode-locked solid-state lasers.

Kerr-lens mode-locked bidirectional dual-comb ring laser for broadband dual-comb spectroscopy

Fourier-transform spectroscopy is an indispensable tool for analyzing chemical samples in scientific research as well as the chemical and pharmaceutical industries. Recently, its measurement speed, sensitivity, and precision have been shown to be significantly enhanced by using dual-frequency combs. Moreover, recent demonstrations of inducing nonlinear effects with ultrashort pulses have enriched the utility of dual-comb spectroscopy. However, wide acceptance of this technique is hindered by its requirement for two frequency combs and active stabilization of the combs. Here, we overcome this predicament with a Kerr-lens mode-locked bidirectional ring femtosecond-pulse laser that generates two broadband frequency combs with slightly different pulse repetition rates and a tunable yet highly stable rate difference. Since these combs are produced by one and the same laser cavity, their relative coherence stays passively stable without the need for active stabilization. To show its utility, we demonstrate broadband dual-comb spectroscopy with the single laser.