We demonstrate a successful recompression of 4.5 mJ, 30 fs femtosecond pulses from a Ti:Sapphire oscillator amplified in a ring multipass optical parametric chirped pulse amplifier using β-barium borate crystal pumped by a commercial frequency doubled Nd:YAG laser. Pulses with duration close to the Fourier transform limit were obtained.
© 2006 Optical Society of America
Non-collinear optical parametric chirped pulse amplifiers  have been developed intensively during the last decade [2–4]. This development was stimulated by a great demand for high peak power laser sources in the area of strong field physics and nonlinear optics. Most modern high peak power laser sources rely on the Ti:Sapphire crystal as an amplification medium. This technology is very mature and robust, yet because of the limited gain available in Ti:Sapphire such systems usually consist of multiple amplification stages, which makes them complicated and bulky. It has been recognized that parametric amplifiers which are of much simpler design can also deliver multi-terawatt powers . Very fast progress in this area during the last few years has been due to the discovery of a very large parametric amplification bandwidth in a non-collinear geometry especially in the β-barium borate crystal (BBO) [6, 7]. Non-collinear parametric amplifiers have higher gain per distance in the gain medium than laser amplifiers and thus the total amount of material inserted in the beam path is smaller than in the equivalent Ti:Sapphire amplifier systems. This makes it easier to handle both higher order phase distortions due the material dispersion and nonlinear phase distortions due to self-phase modulation.
When considering parametric amplification process, one should bear in mind its instantaneous nature. The nonlinear medium cannot store energy as can the medium in a standard laser amplifier. This means that the pump, signal and idler waves should not only be phase matched, but pump and seed pulses must also overlap in time.
One way to satisfy the latter condition is to use pump and seed pulses of similar duration. This approach has been realized in the femtosecond non-collinear parametric amplifier (NOPA) . Several femtosecond NOPA schemes have been thoroughly studied in the last few years and proved to be capable of extremely broadband operation. The NOPA arrangement is beyond of the scope of this paper and, besides, there are excellent reviews on this topic, for example .
Another way of making the seed and pump pulses overlap temporally is to use a picosecond pulsed laser as a pump source. In this approach, the seed pulse is stretched from the femtosecond to picosecond range, amplified in a nonlinear crystal and subsequently recompressed. Because of picosecond regime, both the stretcher and compressor can be quite compact [5, 9].
The third method of matching pulse durations uses a nanosecond pump. In this scheme the seed pulse is stretched to a sub-nanosecond range and amplified in a series of nonlinear crystals. In each crystal the seed pulse overlaps with a “fresh” part of the few nanoseconds-long pump pulse, which is achieved by delaying the pump pulse every time before it reaches the next crystal .
One should also mention the schemes which involve a combination of a parametric preamplifier and Ti:sapphire power booster stages . A comprehensive overview of the current progress in the Optical Parametric Chirped Pulse Amplifiers (OPCPA) field can be found in .
We have shown that the arrangement with several nonlinear crystals can be effectively replaced with a scheme in which a single crystal is passed several times by a seed pulse . The amplifier is constructed in such a way that the amplified pulse makes several passes through the nonlinear crystal during the time that the pump pulse is presented in the crystal. As with nanosecond pump pulses the maximum available gain is limited by a relatively low damage threshold due to the pump, the multipass arrangement is a viable method to achieve very high overall gain comparable to the one achieved in schemes with fs and ps pump sources where damage threshold limitation is less severe. We have demonstrated a multipass NOPCPA (Non-collinear OPCPA) seeded by pulses from a femtosecond Ti: Sapphire oscillator and pumped by a commercial Q-switched, frequency doubled Nd: YAG laser with a pulse duration of several nanoseconds. Amplification higher than 106 and pulse energy exceeding 1 mJ have been achieved with four passes through a single β-barium borate crystal.
In this paper we present new results obtained with this amplification scheme. In particular, we show that although the output pulse energy can be considerably increased it is limited to the level of several mJ by the onset of amplified spontaneous fluorescence. We also demonstrate that the output pulse can be recompressed close to its Fourier limit with a simple diffraction grating compressor.
The experimental setup shown in Fig. 1(a) is similar to the one described in  except for a few modifications. As a seed source we use a Kerr lens mode-locked Ti:Sapphire oscillator (Coherent Mira Seed) delivering a 78 MHz train of pulses with 30 nm (FWHM) wide spectrum centered at 810 nm. First, the pulses are stretched in a standard diffraction grating stretcher. There are several partially contradictory demands on the performance of the stretcher in such an amplifier. On the one hand, one would like to have the stretched pulse as long as possible in order to fill properly the time window defined by the pump pulse. The duration of the stretched pulse is limited by the size of the diffraction grating used in the stretcher and the diffraction angle (150 mm and 30 degrees in our case). On the other hand, the bandwidth of the stretcher should be wide enough to enable amplification of short pulses. The bandwidth is defined by the dispersion of the grating i.e. its groove density (1200 lines/mm in our setup), the diffraction angle and the radius of curvature of the concave mirror (2438 mm in our case). And finally, phase errors introduced by the stretcher should be kept low so that a simple compressor composed of two diffraction gratings can be used. We have used a 150 mm-long 1200 lines/mm diffraction grating together with a concave parabolic mirror (1219 mm focal length). This resulted in the bandwidth of the stretcher equal to 64 nm and the temporal delay between the extreme spectral components of the seed pulse of 0.77 ns. After the stretcher, pulses with a repetition rate of 10 Hz are selected by a Pockels cell.
The seed beam leaving the Pockels cell is collimated and injected into a semi-ring amplifier formed by eight high damage threshold broadband dielectric mirrors with a maximum reflectivity at 800 nm and a flat metallic mirror, Fig. 1(b). The amplified beam makes 4 passes through a nonlinear crystal (BBO).
A second harmonic of a pulsed (10Hz) single longitudinal mode Nd:YAG laser (Continuum Powerlite 8000) is used to pump our multipass NOPCPA. The pump beam is collimated and then sent to the nonlinear crystal. A combination of a thin film polarizer and a half-wave plate is used to control the energy of the pump. The seed and pump beam diameters are 900 μm and 1400 μm respectively. The size of the pump beam is chosen to keep the maximum pump intensity safely below the damage threshold for the BBO crystal. We use a seed beam with a smaller diameter in order to maintain a good spatial overlap of pump and seed beams for all 4 passes.
The temporal FWHM of the pump pulse is 8 ns with a timing jitter of +/-0.5ns. All timings are controlled by an electronic delay generator (Stanford Research DG535).
Vertically polarized pump and horizontally polarized seed beams are crossed at an angle of approximately 2.4° inside the nonlinear crystal to provide the largest possible gain bandwidth. Type I BBO crystal, (cut angle θ=34°, length 11 mm, uncoated, 1° wedge) is used as a nonlinear medium. The distance between the opposite cavity mirrors is kept small (≈20 cm) to maximize the temporal overlap between the pump pulse and four consecutive passes of the seed pulse.
The semi-ring amplifier layout is modified in comparison with the previously reported scheme  in which the amplified beam traversed the nonlinear crystal twice on each pass. In order to reduce the material dispersion introduced by the nonlinear crystal we use an additional flat metallic mirror placed close to the BBO crystal. After each pass, the amplified beam is directed to this mirror and then reflected back to one of the high damage threshold cavity mirrors. The arrangement is similar to that used in multipass Ti:sapphire amplifiers .
A high damage threshold 45° mirror mounted on a translational stage is used to intercept the amplified beam after a particular pass. This allows us to optimize the amplified pulse energy on each individual pass by tuning the amplifier mirrors independently. In doing so, we assume that the maximum gain also corresponds to the widest amplification bandwidth. The angles between the beam pump and individual amplified beams [see Fig. 1(b)] are optimized in order to maximize the energy of the whole system. After amplification, the beam is expanded by a 1:3 telescope and passed to a grating compressor which consisted of a pair of 1200 lines/mm gratings separated by ~1200 mm. Since the amount of the material inserted into the beam path and thus the corresponding material dispersion is very small, the incidence angle in the compressor is set the same as in the stretcher and the distance between gratings is scanned to obtain the shortest possible pulse.
The signal versus pump energy dependence is summarized in Fig. 2 together with the spectra of the amplified pulse for selected pump energies. Theoretical unsaturated gain curve assuming where Esingal - the energy of the signal (amplified pulse), and Ep - the energy of the pump, is shown by the dashed line. The spectrum of the amplified pulse is monitored at each point of the curve. With growing amplification the spectrum evolves from the femtosecond oscillator spectrum clipped at the edges by the stretcher [Fig. 2(a)] to one resembling a rectangle [Fig. 2(d)] as expected for a saturated amplifier. We have observed that at the pump energies close to the gain saturation a broadband radiation which we attribute to the Amplified Spontaneous Parametric Fluorescence (ASPF) constitutes a significant part of the output pulse energy. Its smooth spectrum extends well beyond the spectral window defined by the stretcher allowing one to estimate its amplitude. With the seed beam blocked, we register the ASPF spectrum and then from the spectrum of the combined signal and ASPF estimate the ASPF pulse energy. Since the gain saturation lowers the ASPF pulse energy the number obtained in this manner puts an upper limit on the energy of the ASPF pulse. At pump energies close to the saturation the signal grows linearly with respect to Ep while the ASPF still grows much faster - exponentially with respect to . For the highest pump energies available in our setup the ASPF constitutes as much as ~40% of the total output pulse energy. Since the ASPF with its duration of several nanoseconds is uncompressible one should keep its energy at low, usually arbitrarily defined, level. With the pump pulse energy of 160 mJ the amplifier delivers a 6mJ output pulse measured before the compressor (4.5 mJ after the compressor) with the ASPF level below 10% and fluctuations less than 3.5% RMS. With a modest sacrifice in the output pulse energy the ASPF level can be lowered significantly, for example, it is below 1% with the output pulse energy of approx. 3 mJ. The results discussed here indicate that it would be rather impossible to obtain considerably larger output energies in our design. In order to achieve that one must consider a setup consisting of at least two stages: a high gain amplifier and a power amplifier. Such a solution which we plan to test in the future should not only allow for better control of the spontaneous parametric fluorescence but also improve the efficiency of the amplifier which in the present setup is around 4%.
Figure 3 shows the beam profile of the amplified pulse taken at the output energy of 6 mJ. It was recorded with a standard CCD camera at a distance of approximately 3 m from the amplifier. As indicated by the horizontal and vertical cross sections also shown in the figure the beam profile is smooth and can be well approximated by a Gaussian function which is characteristic for our Ti:Sapphire femtosecond oscillator. We attribute a slight ellipticity of the amplified beam to the same feature of the seed beam. It is obvious that the output beam profile must depend on the details of the design. For example, a strong saturation in the amplifier will lead rather to a flat-top profile not a Gaussian one.
In order to characterize the temporal features of the amplified and recompressed pulse we measured its single-shot FROG trace [Fig. 4(a)]. The results of the FROG algorithm: the recovered pulse spectrum and the spectral phase are shown in Fig. 4(b) together with the experimental pulse spectrum. The agreement between the measured and recovered pulse spectrum is moderate, which suggests a rather limited level of confidence. Still, the shape of the spectral phase agrees quite well with the results of our numerical modeling for the stretcher-amplifier-compressor system. The results of the numerical modeling show that because the amount of material in the beam path is small, we can almost completely compensate the second and third order terms in the spectral phase and the major uncompensated phase distortion comes from the aberrations introduced by the stretcher. We have also found out that a parabolic mirror is a better choice for the stretcher than a spherical one with the same focal length. If we approximate the spectral amplitude of the amplified pulse by a rectangular function of an appropriate width with a flat phase its temporal profile is expected to have a form.
The temporal intensity of such a pulse is shown by the shaded area (red) in Fig. 4(c) together with the temporal intensity obtained from the FROG trace. The width of the former is approximately 29 fs (FWHM) while the latter is only slightly broader (FWHM=30 fs). The wings observed in the experimental pulse temporal profile we attribute to the aberrations introduced by the stretcher and some residual phase distortion due to the dispersion of the BBO crystal, which could not be compensated by a simple compressor. The output pulse temporal shape can, in principle, be improved by the application of a programmable phase control exercised by either a pixilated liquid crystal light modulator, a deformable mirror or some other means. However, with the limited spectral window available in our setup the improvement in the pulse shape might not be worth the increased complexity of the system.
To summarize, we have shown a recompression of a femtosecond laser pulse amplified in a multipass non-collinear optical parametric chirped pulse amplifier pumped by a long pulse second harmonic of Nd:YAG laser. The temporal FWHM of 30 fs achieved for 4.5 mJ pulse is close to the Fourier-transform limit level of 29 fs. The total amplification gain of 1.8×107 is reached. We have also established the limits of the output pulse energy imposed by the amplification of the spontaneous parametric fluorescence.
This work was supported by the KBN grant no 2P03B 02926.
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