A novel CPA-system with a spatially dispersive Yb:KYW-based regenerative amplifier is presented. Three prisms inside the amplifier divide the seed pulses from an Yb fiber laser system into spatially separated spectral components that pass through different volumes of the active medium and are amplified independently. This concept allows overcoming the effect of gain-narrowing. The spatially dispersive amplifier delivers laser pulses with energies of up to 30 µJ at a repetition rate of 100 kHz. Successful compression of the amplified pulses down to 171 fs is demonstrated.
©2009 Optical Society of America
The last decades witnessed a remarkable progress in the field of ultrafast laser technology. Ultrashort laser pulse sources have become more compact, reliable, and commercially available from many industrial suppliers. Applications of ultrashort laser pulses are rapidly growing and demands on characteristics of modern laser systems become more challenging. This paper investigates a new concept for the improvement of existing Yb-based ultrashort laser pulse generating systems, which are designed for applications in the field of material processing.
The majority of femtosecond lasers used so far for material processing is represented by Ti:sapphire-based chirped pulse amplification (CPA) systems . This kind of lasers has some disadvantages, which prevent their large scale application in industry. The Ti:sapphire-based systems require optical pumping with green light, which is usually generated by frequency doubling of the output of Yb- or Nd-based lasers. Because this frequency doubling is a nonlinear process, the available Ti:sapphire-based CPA-systems have relatively low efficiency. Moreover, they are adjustment sensitive and quite expensive.
Novel femtosecond CPA-systems are based on ytterbium tungstates (e.g. Yb:KYW) or other Yb-doped materials and can be directly pumped by InGaAs laser diodes. These systems are more efficient, smaller, cheaper, and simpler to optimize. Therefore, the Yb-based CPA-systems are much more promising for industrial applications.
The standard Yb:KYW-based CPA-systems generate laser pulses with energies in the microjoule range and durations limited to nearly 300 fs [2,3]. Although these pulse durations are sufficient for many applications, even shorter pulse durations are required for further improvements in material processing precision, quality, and speed. In order to achieve pulse durations shorter than 200 fs, the effect of gain-narrowing  during amplification has to be overcome. This can be realized by different spectral broadening techniques, like spectral filtering , nonlinear spectral broadening [6,7] and/or combination of multiple gain spectra . Another promising approach is based on the concept of spatially dispersive amplification [9–11], which, until now, has not been used in the Yb-based CPA-systems.
This paper presents a first realization of an Yb:KYW-based CPA-system with a spatially dispersive regenerative amplifier. Three intracavity prisms are used to divide the seed pulses from an Yb fiber laser into spatially separated spectral components. These spectral components pass through different volumes of the active medium and are amplified independently from each other. The pump intensity experienced by each spatially separated spectral component determines the amplified pulse spectrum. Thus, simply by pumping a sufficiently large area of the active medium, one can generate amplified pulse spectra, which support sub-200-fs pulse durations.
2. Experimental setup
An Yb-based seed system, consisting of a mode-locked fiber oscillator, a pre-amplifier, and a fiber stretcher, provides seed pulses with an energy of 3.5 nJ at a wavelength of 1033 nm. The seed pulses have a duration of around 28 ps and can be compressed to 164 fs. The seed spectrum has a full width at half maximum (FWHM) of 13.2 nm and corresponds to a Fourier-transform-limited (FTL) pulse duration of 150 fs.
The setup of the regenerative amplifier is shown in Fig. 1. The standing-wave resonator of the regenerative amplifier is formed by two flat end-mirrors. An optical switch composed of a thin film polarizer (TFP), a quarter-waveplate, and a BBO-based Pockels-cell is used to inject the seed pulses into the regenerative amplifier and to eject the amplified pulses. A concave mirror with 350 mm radius of curvature produces a resonator mode diameter of 0.11 mm inside the active medium. To divide the seed pulses into spatially separated spectral components one SF10 and two SF6 prisms are put inside the amplifier resonator. The prisms are all adjusted at Brewster’s angle and put into the focus of the concave mirror. The angular dispersion of the prisms breaks up the resonator mode into different spectral components. Afterwards, the spectral components propagate in different directions, until they are collimated by the concave mirror. The spatially separated spectral components pass through the active medium and are reflected at the end mirror. On the way back, they are again recombined by the combination of the concave mirror and the prisms. Thus, the concave mirror and the prisms generate an elliptical laser beam with spatial wavelength dependence, a spatial chirp, inside the active medium. For a wavelength difference of 30 nm the three prisms produce a spatial separation of 0.91 mm and spectral components with a bandwidth of 3.3 nm overlap inside the active medium. Because no significant change of the emission cross-section of the active medium occurs for a wavelength difference of 3.3 nm no competition effects between overlapping spectral components have to be expected. These competition effects would lead to a modulation of the amplified pulse spectrum and might arise when only one or two prisms are used inside the resonator since the bandwidth of the overlapping spectral components inside the active medium would increase by a factor of two or three, respectively. On the other hand the use of more than three prisms would increase the ellipticity of the laser beam inside the active medium and complicate the resonator setup.
The active medium is an Yb:KYW crystal with 5 % doping concentration. Yb:KYW has three perpendicular optical axes Np, Nm, and Ng . The highest absorption cross-section occurs at a wavelength of 980.5 nm and polarization along the Nm-axis. The Np-axis has the largest emission bandwidth with a FWHM of 29 nm and an emission cross-section maximum of 1.27 pm2 at a wavelength of 1040.5 nm. The crystal is 1.6 mm long, 5 mm wide, and 2 mm high with faces parallel to the Np-Nm plane. The crystal faces are anti-reflection coated and form a wedge of 1.15° degrees, in order to prevent etalon effects. The crystal is mounted on a water cooled (18° C) copper heat sink, and is cooled from top and bottom sides. The resonator mode is polarized horizontally along the Np-axis of the Yb:KYW crystal.
The pump light at 981 nm is provided by a collimated single bar diode laser and is polarized along the Nm-axis of the Yb:KYW crystal. The pump setup has been arranged to generate a flat, elliptical spatial pump distribution inside the active medium, covering a spectral range of 26 nm. The regenerative amplifier is operated with a pump power of 20 W which corresponds to a pump intensity of approximately 16.5 kW/cm2.
3. Experimental results
Figure 2 shows the amplified pulse energy (before compression) versus resonator round trip number at 100 kHz repetition rate. The amplified pulses reach energies of up to 30 µJ after 63 resonator round-trips. The amplified pulse spectrum after 63 round-trips is centered at 1032 nm and has a FWHM of 13.4 nm (Fig. 3) whereas the seed pulses had 13.2 nm FWHM.
The evolution of the FWHM of the amplified pulse spectra with resonator round trips is shown in Fig. 4. After 28 round-trips the amplified pulse spectrum has a FWHM of 9.5 nm. With increasing round-trip number the FWHM of the amplified pulse spectrum increases and reaches a value of 13.4 nm after 63 round-trips. The corresponding FTL durations decrease with round-trip number after a certain point and reach a value of 167 fs for 63 round-trips (see Fig. 5). Comparing these values with those of the seed pulse spectrum (13.2 nm FWHM, 150 fs FTL duration, peak at 1033 nm) shows that the effect of gain-narrowing is almost fully compensated. The initial increase of the FTL durations in Fig. 5 is caused by a decrease of the spectral wings of the amplified pulses. This decrease results from the small amplification at the edges of the spatial pump distribution inside the active medium.
The broadening of the amplified pulse spectrum with resonator round trips can be explained by the following consideration. During the spatially dispersive amplification the spectral gain profile changes due to the saturation of the active medium. In case of a flat spatial pump distribution, the initial gain is determined by the emission cross-section of the active medium. As in standard amplifiers, this leads to gain-narrowing of the seed pulses in the beginning of amplification. When the amplification phase is long enough for the pulse to produce saturation inside the active medium the spectral components with high initial gain will saturate first, while the spectral components with small initial gain at the wings of the pulse spectrum will still experience the unsaturated gain. Thus, the amplified pulse spectra will broaden with further amplification, resulting in some sort of “gain-broadening”, until all spectral components have saturated the active medium.
Figure 6 shows the horizontal beam profile inside the active medium over wavelength for 63 resonator round-trips. The horizontal beam profile has one peak and is approximately gaussian for all wavelengths. As intended, the beam profile inside the active medium has a strong spatial chirp. The beam profile at the output coupler is shown in Fig. 7 and also indicates that all wavelengths are in the fundamental mode.
4. Pulse compression
To dechirp the amplified pulses a grating compressor with two 900 grooves/mm gratings and an efficiency of 30% is used. The compressed pulse duration is determined from the intensity autocorrelation width using the deconvolution factor derived from the optical spectrum.
The measured intensity autocorrelation function of the compressed amplified pulses after 63 round-trips is shown in Fig. 8. Although the effect of gain-narrowing is compensated and the amplified pulse spectrum has a FTL duration of 167 fs (228 fs FWHM of autocorrelation function) the compressed amplified pulses are considerably longer (640 fs FWHM of autocorrelation function) and split up in two pulses.
Further measurements show that the pulse separation (distance between main and side peaks in the intensity autocorrelation function) increases with resonator round-trips and pump power (see Fig. 9 and Fig. 10). The pulse breaking even occurs for amplified pulse energies below 9 µJ. Neither etalon effects nor self-phase-modulation can explain the observed pulse breaking effects.
The pulse breaking can only be explained by spectral phase modulations produced by thermal effects inside the active medium. The optical pumping of the active medium produces a non-homogeneous temperature distribution, generating a spatially varying refractive index inside the active medium. Due to the spatial separation of the spectral components inside the active medium, each component experiences a different refractive index, resulting in an additional spectral phase. A shift in the refractive index of 10-5 corresponds to a temperature change of 0.1° C and is large enough to produce an additional phase of more than 2π. To prevent the pulse breaking and to compress the amplified pulses, the additional spectral phase has to be determined and removed.
The measured pulse breaking effects can be caused by a triangular spectral phase according to
To demonstrate this, the following pulse spectrum with the triangular phase has been investigated
where a is the inverse spectral width and b is the triangular phase slope. Figure 11 shows the corresponding intensity autocorrelation for a=1 ps and different values of b. For b=0 (constant phase) the intensity autocorrelation has a single peak, corresponding to a single pulse. With increasing slope b the pulse splits in to two and the intensity autocorrelation develops two side peaks. For b≫a the two pulses arrive at times -b and b. It can be calculated that each pulse corresponds to one part of the initial pulse spectrum. The first pulse contains all frequencies above ω0 and the second pulse contains all frequencies below ω0. The effects of the triangular phase for a gaussian pulse spectrum are analyzed in reference 13.
Assuming that the additional spectral phase is approximately triangular, a compression method can be derived by the following consideration. As mentioned, the triangular phase centered at ω0 produces two pulses with temporal separation of 2b, where b is the slope of the triangle. Each pulse corresponds to the spectral frequencies below or above ω0. Thus, it should be possible to overlap the two pulses by retardation of the part of the spectrum that corresponds to the first pulse. This is done by putting a float glass plate of 1.8 mm thickness into the spatially dispersed beam inside the grating compressor, retarding the part of the spectrum with shorter wavelengths. The exact retardation can be adjusted by tilting the glass plate relative to the beam. After the pulses are temporally overlapped, the relative phase of the two pulses has to be adjusted. This is also done by slightly tilting the glass plate and thereby changing the retardation by a few microns. The results achieved with this phase compensation method are presented in Fig. 12 and Fig. 13. Introducing the 1.8 mm glass plate removes the pulse breaking and allows for the generation of single amplified pulses with durations of 171 fs. The compressed amplified pulse spectrum has a FTL duration of 163 fs. The dip in the middle of the compressed pulse spectrum is produced by the edge of the glass-plate. The compression has an effiency of 25% which results from additional 5% reflections losses at the uncoated glass plate. A better compression efficiency of more than 80% can be obtained by use of transmission gratings and an anti-reflection coated glass plate.
These results prove that there is an additional spectral phase, resulting from the thermal lens inside the active medium, which prevents pulse compression. The wings in the intensity autocorrelation of the compressed pulses contain 36% of the amplified pulse energy and occur because the additional spectral phase is of course not exactly triangular and cannot be fully removed with the presented method. To achieve a better compression of the amplified pulses a commercially available spatial light modulator (SLM)  can be used. Because the thermal lens is independent of the seed pulse parameters, the SLM can compensate the additional spectral phase before amplification. It may also be possible to reduce the additional spectral phase and the thermal lens by use of cryogenic cooling to a level at which no pulse breaking occurs.
Up to this point all measurements have been made at a repetition rate of 100 kHz. By decreasing the repetition rate the pulse energy can be further increased. At 20 kHz repetition rate a maximum pulse energy of 120 µJ has been achieved. The corresponding pulse spectrum has a FWHM of 11.4 nm and by removing the triangular phase with the 1.8 mm glass-plate the pulses are compressed to 186 fs.
A diode-pumped spatially dispersive Yb:KYW regenerative amplifier has been demonstrated. The spatially dispersive amplification fully removes the effect of gain-narrowing during amplification. The amplified pulse spectra reach FWHM values of 13.4 nm and are even broader than the seed pulses with a FWHM of 13.2 nm. Due to the spatially dispersed beam inside the active medium the thermal lens adds an additional spectral phase to the amplified pulses. This spectral phase must be removed in order to compress the amplified pulses.
At 100 kHz repetition rate an amplified pulse energy of 30 µJ (before compression) has been achieved and the pulses have been compressed to 171 fs. Reducing the repetition rate to 20 kHz, leads to an amplified pulse energy of 120 µJ (before compression) and compressed pulse durations of 186 fs.
Finally, it has to be mentioned that the spectral widths of the amplified pulses have been only limited by the spectral widths of the available seed pulses. Using seed pulses with a broader spectrum should allow to achieve even shorter amplified pulse durations.
This work was funded by the German Federal Ministry of Education and Research within the FULMINA project under contract no. 13N8722.
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