Kerr-lens Mode-locked Laser Research Articles

Femtosecond forsterite Kerr-lens mode-locked laser pumped synchronously by an Nd3+:YAG laser

A femtosecond laser (pulse duration 30 fs, centre of the emission spectrum at λ = 1260 nm) with a chromium-doped forsterite crystal was developed. This laser operated on the basis of Kerr-lens mode locking. Synchronous pumping with an Nd3+:YAG picosecond laser resulted in forced generation of femtosecond pulses characterised by an enhanced stability of the output power, which was 5–7 times better than the stability of the pump laser. The output radiation of the forsterite laser was converted efficiently to the second harmonic (central wavelength 630 nm) in a thin nonlinear BBO crystal. The average power of the second-harmonic radiation was 15 mW.

Astigmatism and gain guiding in Kerr-lens mode-locked lasers

The effects of astigmatism and the radial variation of the gain on Kerr-lens mode-locking (KLM) are studied with the help of an extended ABCD matrix formalism. A dramatic broadening of the allowed cavity parameter region is predicted that permits KLM in almost the whole stability region. The cavity design for KLM with an aperture differs significantly from the design for KLM without an aperture, because of the saturable diffraction loss.

Simple analytical method of cavity design for astigmatism-compensated Kerr-lens mode-locked ring lasers and its applications

An analytical method based on a renormalized q parameter for Gaussian-beam propagation and properly matched self-consistent complex q parameters is proposed to aid in the design of arbitrary-astigmatism-compensated Kerr-lens mode-locked ring cavity lasers. The q parameters throughout the ring cavity can be calculated by solution of an algebraically quadratic equation for an arbitrarily thick Kerr medium when the intracavity laser power is less than the self-trapping power. The Kerr-lens mode-locking strength at the curved mirror was calculated over the stable range. The results indicate that the astigmatism is best compensated so that the stable range of x and y directions have optimal overlapping. Although to maximize the hard-aperturing effect the curved mirror separation must be at the far edge of the stable range, pump and cavity field matching inside the Kerr medium is also necessary. In addition, the insertion of a vertical slit is more effective than insertion of a horizontal slit. A spiking phenomenon is found in the intracavity z scan curve, which can be observed only with thick Kerr materials and a high laser power.

Astigmatism in Gaussian-beam self-focusing and in resonators for Kerr-lens mode locking

An analytical treatment for the propagation of astigmatic Gaussian beams in Kerr media is presented. This method, which is valid to the first order in the beam power, allows us to calculate in closed form the nonlinear variations of the spot size and the radius of curvature. This formalism is applied to the calculation of the self-consistent Gaussian mode of an astigmatic resonator with a Kerr medium, such as those used for Kerr-lens mode-locked lasers. Theoretical predictions are confirmed by the experimental results obtained with a femtosecond Ti:sapphire laser. In particular, the design of an optimized resonator permits self-starting of the mode-locking mechanism.

High-Power and High-Repetition Rate Femtosecond Chromium-Doped Forsterite Laser.

We report the amplification of a Kerr-lens mode-locking cavity-dumped frequency-doubled chromium-doped forsterite laser in a six-pass bow-tie dye amplifier pumped by a diodepumped Q-switched frequency-doubled Nd: YLF laser. This system generates the energy of 0.6μJ per pulse and a pulse duration as short as 43 fs (FWHM) at the wavelength of 630 nm with a repetition rate of 4 kHz. The amplified pulses were used to generate a continuum in a fused silica plate.

Compact dispersion-compensating geometry for Kerr-lens mode-locked femtosecond lasers

We demonstrate a new technique for dispersion compensation in Kerr-lens mode-locked (KLM) lasers that uses a novel resonator design in conjunction with a prismatic end mirror. This approach achieves intracavity dispersion compensation without the need for prism pairs or dispersion-compensating mirrors. A pulse-repetition rate of 1 GHz, to our knowledge the highest repetition rate achieved in a KLM laser to data, is generated with a pulse durations of 111 fs. Pulse durations of 54 fs are achieved at a repetition rate of 385 MHz. We believe that this new dispersion-compensation technique will be useful for the development of compact femtosecond sources.

Kerr lens modelocked solid state laser in the red(639 nm)

The authors report Kerr lens modelocking of the Pr3+:YLF laser, operating at 639 nm, initiated through the use of a solid state saturable absorber, generating pulsewidths of 8 ps at repetition rates around 100 MHz.

Femtosecond pulses from an all-solid-state Kerr-lens mode-locked Cr:LiSAF laser

We report on an all-solid-state Kerr-lens mode-locked femtosecond laser. Using an intracavity-doubled mode-locked Nd:YLF laser with two Nd:YLF rods, we have produced 295 mW of light at 659 nm that pumps a Cr:LiSAF laser. Self-sustained Kerr-lens mode locking is achieved in the Cr:LiSAF laser with pulse durations as short as 90 fs.

Characteristics of a femtosecond transform-limited Kerr-lens mode-locked dye laser

Using an SF6 glass plate as the intracavity Kerr medium and a double-prism pair for dispersion compensation, we developed a femtosecond transform-limited passively mode-locked dye laser. Self-starting mode locking is achieved with a dilute intracavity TCVEBI saturable-absorber jet. Within a 50% power drop the tuning range is 577-606 nm. Pulse characteristics of the laser agree with theoretical predictions based on the Ginzburg-Landau equation.

Resonators for Kerr-lens mode-locked femtosecond Ti:sapphire lasers

Resonators for Kerr-lens mode-locked lasers are studied with the help of an analytical model, and a new design procedure is presented. The experiments performed with a femtosecond Ti:sapphire laser demonstrate that this model permits an effective resonator design and offers reliable guidelines for the final experimental optimization. With a suitable resonator configuration, self-starting of the Kerr-lens mode-locking regime has been achieved.

Analytical design of symmetrical Kerr-lens mode-locking laser cavities

Analytical expressions for the spot size and the Kerr-lens mode-locking (LM) strength can be represented as explicit functions of the position in the cavity, the laser power, and the stability parameter for a four-mirror figure-Z laser resonator. The results indicate that the KLM strength achieves its maximum value at the edge of the stability range. Self-amplitude modulation and group-velocity dispersion compensation can be established by a prism pair. Simultaneously obtaining a large pumping efficiency and KLM is possible for this cavity.

Measurements of the self-starting threshold of Kerr-lens mode-locking lasers

We measured the self-starting threshold of passive Kerr-lens mode-locking dye lasers and Ti:sapphire lasers by varying the concentration of the intracavity dilute-dye saturable absorbers that start the mode locking. From the threshold absorber concentration, we determined the strength of the intracavity pulse-broadening effects that counteract the pulse-shortening mechanisms in the self-starting stage. Experimental results agree well with theories based on phase diffusion and mode pulling.

Kerr-lens modelocking of solid-state lasers and unidirectional ring cavities

Detailed results for the project of Kerr lens modelocked lasers using ring resonators are presented, and the working conditions for unidirectional operation are outlined. Both the small and large signal regimes are explored, discussing important issues such as the modelocking threshold and saturation of the amplitude modulation mechanism, both for femtosecond and picosecond systems. Useful simple relations for laser design are derived. >

Theory of Kerr-lens mode locking: role of self-focusing and radially varying gain

The ABCD-matrix formalism is generalized for the description of self-focusing and radially varying pump power in a nonparabolic approximation. This extended formalism is used for the investigation of the operation and for the optimization of the saturable aperture loss, the power-dependent average gain, and the resonator magnification of Kerr-lens mode-locked lasers with as well as without an internal aperture. The calculated amplitude modulation parameters are compared with the necessary condition for self-starting and for the shortest possible pulse duration limit.

Effect of beam ellipticity on self-mode locking in lasers

In the cavity of a self-mode-locked (Kerr-lens mode-locked) laser, a noncircular beam experiences a nonlinear coupling between beam parameters in the x and y planes. This coupling produces a significant change in beam radii throughout the cavity and may result in more than one TEM00 cavity mode. The dramatic nature of this effect is demonstrated with the Ti:sapphire laser as an example.

Periodic pulse evolution in solitary lasers

The periodic variation of femtosecond pulse characteristics within a mode-locked Ti: sapphire laser is investigated. By measurement of the output pulse characteristics at the dispersive and the nondispersive ends of a femtosecond Kerr-lens mode-locked laser cavity, it has been demonstrated that both the laser bandwidth and the pulse duration are functions of position within the laser cavity. A qualitative argument is also presented that predicts near-bandwidth-limited pulses at the cavity extremes, and this prediction is confirmed by experiment.

Starting dynamic, self-starting condition and mode-locking threshold in passive, coupled-cavity or Kerr-lens mode-locked solid-state lasers

The starting dynamic of mode-locked solid-state lasers is analyzed taking into account the gain reduction by the total random radiation. A simple self-starting condition is derived, depending on the pump-rate, the gain cross section, the nonlinear coefficient, the gain bandwidth and the resonator length. It is shown that an additional slow saturable absorber can greatly improve the self-starting conditions. Experimental results are in good agreement with the predictions of the theory.

Broadband fast semiconductor saturable absorber

Kerr lens mode-locked (KLM) solid-state lasers are typically not self-starting. We address this problem by introducing a broadband semiconductor saturable absorber that could be used as a tunable, all-solid-state, passive starting mechanism. We extend the wavelength tunability of a semiconductor saturable absorber to more than 100 nm using a band-gap-engineered low-temperature molecular-beam-epitaxy (MBE)-grown bulk AlGaAs semiconductor saturable absorber in which the absorption edge of the saturable absorber has been artificially broadened by continuously reducing the Al concentration during the MBE growth. We demonstrate its tunability and its feasibility as a starting mechanism for KLM with a picosecond resonant passive mode-locked Ti:sapphire laser. The extension to femtosecond KLM lasers has been discussed previously.

Cavity design for optimum nonlinear absorption in Kerr-lens mode-locked solid-state lasers

Resonators with an internal Kerr-lens for mode locking representing self-focusing effects in the laser rod and an aperture acting analog as a fast saturable absorber are analyzed. The cavity design for optimum nonlinear absorption and the nonlinear coefficient of the aperture combined with the Kerr-lens are calculated.