With increasing pulse energy in ultrafast lasers, temporal contrast has become an important topic. In strong-field physics, where a highly energetic pulse interacts with matter, prepulses can lead to ionization and modify the experimental conditions. It is therefore usually required that the pulse contrast exceeds ten orders of magnitude. Common techniques to improve the temporal contrast are cross-polarized wave generation (XPW) [1], nonlinear ellipse rotation (NER) [2], and the use of plasma mirrors [3,4]. Even though these techniques are well established, XPW is limited to low pulse energies and average powers to avoid damaging the utilized crystal. The contrast enhancement achievable with NER is limited by the quality of the quarter-wave plates [2]. For ultrafast lasers with short pulse durations and wide spectral ranges, achromatic wave plates have to be used, but these also show a limit in the polarization preservation. Plasma mirrors only eliminate prepulses and require continuous replacement of the mirror substrate and by that limit the pulse repetition rate.

We present a novel approach that is simple to implement and almost peak-power maintaining with an efficiency of the order of 30% while being virtually unlimited in terms of contrast enhancement. The technique does not show any of the restrictions that nonlinear ellipse rotation or cross-polarized wave generation show. We demonstrate the functionality of the method theoretically by simulation and experimentally by a proof-of-principle experiment.

The novel pulse-cleaning technique is based on self-phase modulation (SPM) to broaden the spectrum of an incident low-contrast pulse. Since SPM is a peak-power/intensity-dependent effect, only the main pulse experiences significant spectral broadening and the lower intensity parts of the signal remain unaffected. The contrast improvement is achieved by suppressing the low-intensity parts of the signal with a spectral filter that only transmits the wavelength-shifted part of the main pulse. Due to the spectral hardcut in the compressor of the common chirped-pulse-amplification (CPA) system, the intensity in the spectral region of the filtered light is initially equal to zero and thus, the achievable contrast is virtually unlimited.

A simulation has been carried out to demonstrate the principle of this pulse-cleaning method. In Fig. 1, spectral broadening due to SPM of pulses with a central wavelength of 1030 nm has been numerically simulated. The spectrum of pulses was generated as a Gaussian spectrum with spectral modulations to introduce prepulses and postpulses and a hardcut of a compressor at 1020 nm and 1042 nm has been included. The input pulses have a pulse energy of 1 mJ and a pulse duration of 203 fs. With the numeric tool Fiberdesk [5], the propagation of these pulses through a 0.6 m long hollow-core fiber with a core diameter of 250 µm and a nonlinearity of $n_2=9.8\times {10}^{-24}{\mathrm{m}}^2{\mathrm{W}}^{-1}$, which corresponds to a gas pressure of 1 bar argon [6], has been simulated. Afterwards, a super-Gaussian filter of the order of eight has been used to filter the spectrum so that there is no overlap with the incident spectrum confined by the compressor’s hardcut. The filtered spectrum contains approximately 40% of the initial pulse energy and supports a pulse duration of 95 fs, which shows that this method is peak-power preserving. It has to be mentioned here that dispersion as well as the losses in the hollow-core fiber have been neglected in this simulation.

 

Fig. 1. Simulation of the SPM-based contrast-enhancement technique. For the simulation, a Gaussian spectrum with sinusoidal modulations was used. The shaded area in the input spectrum indicates the spectral hardcut of the compressor. As one can see in the first row, the modulated Gaussian spectrum generates a short pulse with strong prepulses and postpulses in the time domain. Due to self-phase modulation, the spectrum broadens while the pulse shape and contrast remain unchanged. By applying a super-Gaussian filter of the order of eight, only one sidelobe of the spectrum remains, which corresponds to a very short pulse with strongly improved contrast.

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For a given laser, only a suitable nonlinear medium has to be found to apply this contrast-enhancement technique. For high average powers and millijoule-class pulse energies, hollow-core fibers already showed great performance [7]. For pulse energies in the microjoule regime, SPM in solid-core fibers can be used [8,9]. Even towards high-energy joule-class laser pulses, self-phase modulation in bulk media is promising [10] as well as SPM in multipass cells [11]. The presented technique is also applicable to a wide spectral range, i.e., only limited by the availability of a transparent nonlinear medium. Additionally, the spectral shift of the signal enables, for example, use of the short-wavelength part of a broadened and contrast-enhanced part of a Nd-laser signal to seed an Yb-based system or vice versa. In the same way, an ytterbium laser can be adapted to seed a titanium–sapphire laser system with a high-contrast and broadband signal. It is also possible to apply the technique twice on the same signal to further improve the contrast and preserve the central wavelength of the pulse.

The fairly simple setup of our first proof-of-principle experiment is shown in Fig. 2. A low-contrast signal from an ytterbim-based ultrafast fiber chirped-pulse-amplification system (Yb:FCPA) [12,13] is transmitted through a noble-gas-filled hollow-core fiber (HCF) and spectrally filtered afterwards. The filtered signal is then analyzed by a third-order cross correlator (Amplitude Technologies Sequoia) that offers a dynamic range of up to ten orders of magnitude. The used filters are dielectric long-pass filters with the filter edge located at 1050 nm. Two identical filters, each with a contrast of up to six orders of magnitude and a transmission of more than 95% above 1060 nm, are utilized. In the experiment, pulses from an ultrafast Yb:FCPA system delivering 1.9 mJ pulse energy, 290 fs pulse duration, and a repetition rate of 1 kHz were used. The repetition rate in this experiment was limited by the acquisition speed of the cross correlator. The pulses emitted by the Yb:FCPA system are broadened in the HCF filled with argon at an absolute pressure of 1.2 bar. In Fig. 3, the spectra at different stages of the pulse cleaning are shown. The blue curve shows the spectrum of the Yb:FCPA pulses and is centered at 1030 nm with a spectral hardcut of the compressor located at 1042 nm, i.e., virtually no light is emitted from the laser above 1042 nm. The Yb:FCPA pulses are spectrally broadened (yellow curve) and filtered (red curve). As one can see, the filtered spectrum is centered at 1060 nm and has an energy content of approximately 30% of the HCF output pulses. It is also visible that it has no spectral overlap with the light of the Yb:FCPA system.

 

Fig. 2. Experimental setup used for the contrast enhancement. The light coming from the ultrafast fiber CPA system is coupled into a hollow-core fiber filled with argon. Afterwards, it is spectrally filtered by two dielectric filters.

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Fig. 3. Spectra of the incident, the SPM broadened, and the filtered pulses. It is clearly visible that the incident light from the Yb:FCPA system and the filtered light have no spectral overlap.

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The contrast measurements are shown in Fig. 4, proving that the contrast of the filtered signal exceeds ${10}^{-9}$. Compared to the contrast of the incoming pulses, which is of the order of ${10}^{-2}$, we have achieved a contrast enhancement of at least seven orders of magnitude. Note that the measured contrast is limited by the dynamic range of the cross correlator. Since we used two filters that provide six orders of magnitude contrast each, we can expect the contrast to be even higher. The postpulses visible in the filtered contrast measurement in Fig. 4 originate from internal reflections in the spectral filters and can be avoided by using wedged filter substrates. In Fig. 5, the auto-correlation measurement of the pulses is shown. The incident pulses have a duration of approximately 290 fs (FWHM), and the prepulses and postpulses are also visible in the auto-correlation trace. The filtered pulses have a duration of 177 fs (FWHM) without any further compression after the transmission through the gas-filled hollow-core fiber.

 

Fig. 4. Contrast measurement of the Yb:FCPA and the filtered pulses. The filtered pulses have a temporal contrast of at least ${10}^{-9}$, and the postpulses originate from internal reflections in the filters and can be avoided by using wedged filters.

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Fig. 5. Auto-correlation measurement of the pulses directly from the Yb:FCPA system and after the SPM broadening and spectral filtering. The input pulses have a duration of 290 fs and are shortened to 177 fs by the filter.

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In an additional experiment, a chirped-mirror compressor with a group delay dispersion (GDD) of $-4200{\mathrm{fs}}^2$ has been included in the setup. In Fig. 6, an auto-correlation measurement of the compressed pulses is shown. These have a duration of 95 fs (FWHM) after the filter and compressor. Thus it is shown that the spectrally broadened and filtered pulse can afterwards be compressed and the incident peak power can, in principle, be restored.

 

Fig. 6. Auto-correlation measurement of the filtered and compressed pulses. The pulse duration after a compressor with a GDD of $-4200{\mathrm{fs}}^2$ is 95 fs.

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In conclusion, we have demonstrated a novel technique for temporal contrast enhancement based on self-phase modulation and spectral filtering. The method is simple to implement and allows highly efficient pulse cleaning with the contrast enhancement only being limited by the filter characteristics. Due to its simplicity and the scalability of SPM, it is applicable for a wide range of different laser parameters, in terms of pulse duration, pulse energy, and wavelength. The efficiency of the method can be further increased if the long- and short-wavelength parts of the SPM-broadened spectrum are used.