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Abstract
The ASE (amplified spontaneous emission) level in a laser system consisting of an oscillator and a regenerative amplifier is very important, for example, in the interaction of an intense laser pulse and a thin foil, so a lower ASE level is always required. In this paper, we propose a new method to achieve a lower ASE level, which can be obtained by spectral matching of the seed laser beam and the ASE in a CPA (chirped-pulse amplification) Ti:sapphire laser system. In this method, two baffles are used to control the seed pulse spectrum by blocking a portion of the seed beam in a grating stretcher and it was found that the spectral matching method can reduce the temporal contrast ratio (after the regenerative amplifier) by a factor of 10 in a few hundred picosecond scale. This kind of spectral matching method is simple and it can be easily employed for other CPA laser systems to enhance the contrast ratio.
© 2017 Optical Society of America
1. Introduction
High intensity femtosecond (fs) lasers (I>1018 W/cm2) have been developed for applications in relativistic laser-plasma researches such as the laser-plasma particle accelerations [1–4], radiation sources development [5–7], etc. Such an intense laser system with fs pulse width can be realized by the chirped-pulse amplification (CPA) technique, which includes the temporal pulse stretching, amplification, and compression processes [8, 9]. However, the amplifiers in the CPA laser system increase not only the main pulse energy, but also the spontaneously emitted pulse energy. Hence, typical intensity of the amplified spontaneous emission (> 1011 W/cm2) is high enough to destroy a target (for example, a thin foil) or to generate a pre-plasma before the main pulse arrive. Thus the preformed plasma can cause undesirable effects in the laser-plasma interaction. In the laser-induced proton acceleration, for example, the pre-plasma on the thin foil surface influences in the proton energy that is sensitive to the plasma properties, and also the shock wave by the ASE can deform the shape of the target surface, resulting in proton generation with high deflection angle [3, 4]. Hence, the temporal contrast ratio, which is defined by the intensity ratio of the main peak signal (Ip) to the ASE pedestal (IASE), must be enhanced somehow.
So far, several techniques have been developed to increase the temporal contrast ratio, which include saturable absorbers [10], cross-polarized wave (XPW) generation [11, 12], double CPA (DCPA) [13], optical-parametric CPA (OPCPA) [14], and a plasma mirror (PM) [15]. Even though most of the methods can produce high contrast ratios, they require high cost and intricate beam alignments in the laser system. Moreover, when the laser pulse experiences the saturable absorber or the plasma mirror, the output laser energy is significantly reduced. Further pass through the optical elements in some techniques produces an unwanted dispersion, which disturbs the optimization of the laser pulse duration.
In this paper, we propose a new and simple method to improve the contrast ratio by matching the spectrum of a seed laser beam with that of the ASE. In this scheme, two baffles are employed to control the seed beam spectrum in the pulse stretching stage and the modified spectrum is matched with the ASE spectrum of the regenerative amplifier (RA) without the seed pulse injection. In this way, we found that the temporal contrast ratio can be enhanced by one order of magnitude while the output laser energy and pulse duration are maintained. In this paper, we demonstrate that the proposed method really works and details of the experiment are described below.
2. Spectral matching for contrast ratio enhancement
Since titanium-doped sapphire crystals (Ti:S) have a broad emission band (600 nm to 1050 nm), they have been widely used as a gain medium for fs CPA laser systems. Despite the advantage of the broad spectrum, amplification of some frequency components in the amplifier can be insufficient by the spectral difference between the input laser beam (to the amplifier) and the spontaneous emission (from the amplifier) that is achieved by optical pumping without the seed laser pulse injection. Thus, inefficient amplification in frequency leads to not only gain narrowing, but high intensity ASE pedestal corresponding to an inferior contrast ratio. For effective amplification of the frequency components of the seed laser pulse, which can lead to a high contrast ratio, the spectral matching between the seed pulse and the ASE is important.
To demonstrate the proposed spectral matching method, we used the Ti:sapphire CPA laser system in our laboratory, which consists of an oscillator and a regenerative amplifier. The oscillator (Fusion 20, FemtoLasers Produktions GmbH) generates a broadband pulse with 3 nJ/pulse and 20 fs, and the seed laser pulse is stretched by a single grating. And then it is injected into the regenerative amplifier system (Spitfire Pro, Spectra-Physics) and the amplified laser pulse is compressed by a pulse compressor, leading to a laser pulse of 1.6 mJ/pulse and 55 fs in pulse duration [9].
Since each frequency component is temporally and even spatially separated by the grating in the pulse stretcher, positioning two baffles between the grating and the concave mirror allows selection of specific frequency components of the laser pulse, as shown in Fig. 1. The seed beam block was carefully adjusted not to influence other beam properties except for the contrast ratio. Figure 2(a) shows the spectra of the ASE and the input pulse of the regenerative amplifier with and without the baffles, respectively. Compared to the broad seed spectrum with 68 nm bandwidth at center wavelength of 797 nm, the ASE has a relatively small bandwidth of 38 nm at the center wavelength of 786.5 nm. Thus, the bandwidth of the seed pulse is varied from 30 nm to 50 nm at the ASE center wavelength to investigate how frequency components are amplified. The original output beam from the regenerative amplifier, of course, has the largest bandwidth as shown in Fig. 2(b), but it shows substantial gain narrowing because of the inconsistent spectra of the seed beam and the ASE. In addition, the result shows that less spectral narrowing effect occurs as the seed bandwidth approaches the ASE bandwidth, which implies that most of the seed beam frequencies are effectively amplified, leading to a higher contrast ratio.
Fig. 1 Scheme for controlling the spectrum of the seed laser pulse. The center wavelength and bandwidth of the seed beam can be readily varied by blocking a part of the laser beam before the concave mirror in the pulse stretcher.
Fig. 2 (a) Input spectra of the laser pulse into the regenerative amplifier and (b) output spectra for the input spectral bandwidths.
The intensity contrast measurement was done by using a third-order cross-correlator (Sequoia, Amplitude Technologies) with a high dynamic range of 1010. Typical ASE is in a few ns time scale due to the cavity length of the regenerative amplifier, but the measurements are performed from 400 ps before the main pulse arrival because of the temporal range limit of the cross-correlator. Figure 3 indicates the temporal intensity ratios for the seed pulses with different bandwidths, while the center wavelength of the seed pulse is maintained with that of the ASE. Here, it should be noted that we neglect several peaks including pre-pulses and artifacts in time delay from −40 ps to 0 ps as only the contrast ratio between the main pulse and the ASE pedestal is considered in this paper.
Fig. 3 Temporal intensity contrasts for different input spectral bandwidths with center wavelengths of the ASE.
Without the beam block in the stretcher, a seed pulse with an energy of 600 pJ was sent to the regenerative amplifier, and then the inverted contrast ratio (ICR), which is defined by the ratio of the ASE intensity (IASE) to the peak intensity (Ip), was 2 × 10−8 at −400 ps, while the enhanced contrast was measured as the seed beam bandwidth decreases. Even though the input energy is lower in the case of using the baffles (300 pJ for Δλ = 38 nm) and thus the contrast degradation is expected [16, 17], the contrast ratio was improved to 3 × 10−9 under the spectral matching condition Δλseed = ΔλASE = 38 nm. In addition, when the bandwidth of the seed beam was smaller than that of the ASE, the intensity contrast ratio was similar to the case of Δλ = 38 nm at the long time delay of −400 ps, but improvement is observed between the delay −150 ps and −20 ps, as shown in the inset of Fig. 3.
This contrast improvement by the spectral matching can be presented by low internal loss, leading to a high temporal contrast ratio. From the recent paper [18], the minimum ICR for regenerative amplifiers can be estimated by using the loss term (L) and the parameters of a gain material and a laser system as follows:
where KΔν, Kp, and KΔΩ are the spectral-, polarization-, and spatio-angular acceptance, respectively, and τoutput and Eseed stand for the compressed pulse duration and the seed pulse energy. The saturation intensity of the gain medium Is can be expressed as , where σe is the emission cross-section, σa is the absorption cross-section, and τr is the radiative lifetime, which considers the quenching effect and the whole transitions between the energy levels in the gain medium for spontaneous emission. Here, calculation of the ICR can be conducted by using the values obtained from the experiment and other literatures except for γ(G0, L), which is an ICR dependence parameter of the internal losses (L) and the small signal gain per pass (G0). Here, the small signal gain per pass G0 and the gain coefficient g0 have a relation of G0 = exp(g0d), where d is the length of gain medium [19]. To simplify Eq. (1), γ was assumed as unity, which means that the losses are too low to influence the contrast ratio under a certain gain. For direct comparison between the experimental and theoretical temporal contrasts, the parameters for a Ti:sapphire crystal are determined from the spectroscopic data in [18, 20]: KΔν = 0.166, Kp = 0.71, and Is = 240 kW/cm2. Since the anisotropic materials like the Ti:sapphire, in general, provide different polarization acceptance for each polarization, Kp is considered for p-polarization here, which is the primary amplification direction. Furthermore, the spatio-angular acceptance is represented as KΔΩ = λ2/4π by assuming that the RA amplifies the TEM00 mode only, and the input energy of 300 pJ and the compressed pulse duration of 55 fs are considered. The calculated ICR is indicated as a dashed line in Fig. 3 and it matches well with the contrast ratio measured by using the spectral matching method. We believe that the spectral matching can reduce the internal losses due to the high amplification efficiency for the seed beam and it leads to the minimum ICR.From the results of Fig. 3, we found that the spectral matching is more significant than the seed energy to enhance the contrast ratio. In order to verify the distinct difference of both parameters on the ICR, two different pulses with the same energy are injected to the amplifier: One is the original seed pulse (λ = 797 nm, Δλ = 68 nm) with 300 pJ energy and the other is the pulse considering the spectral matching (λ = 786.5 nm, Δλ = 38 nm). The result in Fig. 4(a) shows that when the seed pulse energy is the same, the contrast ratio depends on the degree of spectrum matching between the seed pulse and the ASE. According to Eq. (1), since γ(g0, L) is the only parameter affecting the ICR at the same seed energy, the loss in the laser system may be primarily from the mis-matching in spectrum. Further ICR measurement was performed by using the seed pulses with different C.W.L. (center wavelengths), but the same bandwidth.
Fig. 4 Comparison of the temporal intensity contrasts for the spectrum-matched pulse and the original pulse with the same energy (a), and the spectrum-matched pulse and seed pulses with different C.W.L. (center wavelengths) (b).
As shown in Fig. 4(b), the best ICR is still obtained when the difference of the center wavelengths of the seed beam and the ASE is zero (λseed – λASE = 0). Moreover, the ICR for the blue-shifted seed pulse (λseed = 781 nm) is slightly better than that for the red-shifted pulse (λseed = 790, 795 nm). This can be interpreted by the gain saturation where the longer wavelength component has more gain than the shorter one during the amplification. Therefore, the blue-shifted seed pulse, even with low energy, can be efficiently amplified in the frequency components of the ASE.
3. Conclusions
In conclusion, we demonstrated that the contrast ratio can be enhanced by matching the spectra between the seed pulse and the ASE. Compared to the original ICR of 2 × 10−8, the ICR with spectral matching was improved to 3 × 10−9. From the measured contrast, it is verified that the matching condition in spectrum is a more significant parameter than the seed pulse energy. The experimental results agree well with the calculated contrast ratio in terms of the laser parameters and thus, the internal losses, which influence the ICR, may be minimized by the appropriate spectral matching. This spectral matching method is very simple to employ in the present CPA laser system and no more optical component is required in the beam path unlike other methods to improve the temporal contrast. In the near future, we will utilize the spectral matching method in the 25 TW/30 fs Ti:sapphire laser system of our laboratory for generation of high contrast intense laser pulses.
Funding
National Research Foundation of Korea (NRF) (grant #: 2014M1A7A1A01030173 and 2017R1A2B3010765)
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