We demonstrate the generation of widely tunable sub-20fs Gaussian-shaped laser pulses using a grating-based 4-f pulse shaper and a liquid crystal spatial light modulator. Our pump source is an Yb:KGW solitary mode-locked oscillator at 44MHz repetition rate which is coupled into a large mode area microstructured fiber to generate a broad spectrum from below 900nm to above 1150nm. These pulses are precompressed by a prism sequence and subsequently sent into the pulse shaper. We use the multiphoton intrapulse interference phase scan (MIIPS) for phase shaping and iterative amplitude optimization to achieve Gaussian-like tunable sub-20fs pulses with output powers of up to 142mW as well as nontunable pulses with 310mW output power as short as 11.5fs.
© 2011 OSA
Ultrafast nonlinear spectroscopy benefits from both spectrally as well as temporally tailored and well-shaped laser pulses. In the past scientists have focused mainly on one of these two aspects, namely on the generation of laser pulses with a pulse duration which is as short as possible. In the visible and the near infrared region this has led to the generation of pulses as short as 3.7 fs [1, 2], and even laser pulses with a single optical cycle have been realized [3–5]. Such sources enhance nonlinear effects due to the high peak intensities and give superior temporal resolution in ultrafast experiments. However, all those systems show multipeak or even worse spectra, which make quantitative analysis of the measured results in spectroscopy often quite complex.
In contrast, in the lower repetition rate regime (≤ 1MHz) several systems based on optical parametric amplifiers have been developed which emit tunable and well-shaped sub-20fs laser pulses [6–8]. Unfortunately, spectroscopy applications at these low repetition rates are often hampered by a loss in signal to noise ratio. In the higher repetition rate domain (> 10MHz) a lot of sources exist and are commercially available, emitting spectrally well-shaped Gaussian-or sech2-like laser pulses, but only with pulse durations larger than about 50fs.
To combine both mentioned aspects, namely ultrashort pulse durations with well-shaped and custom-tailored spectra, we developed a setup that is capable of generating laser pulses in the ultrafast regime, and which is as well able to produce nearly arbitrary spectral shapes over a broad range. By this means we are able to realize a tunable system with Gaussian-like spectral shapes and a pulse duration of sub-20fs with average powers beyond 100mW.
2. Experimental setup
As pump source for the experiments serves a high average power Yb:KGW solitary mode-locked oscillator  emitting 170fs laser pulses with an average power of 2.1W and a repetition rate of 44MHz, see Fig. 1(top). After a Faraday isolator 1.75W of power is coupled into 8cm of a large mode area (LMA) photonic crystal fiber (PCF) (LMA-8 from NKT Photonics), where strong spectral broadening mainly due to self phase modulation (SPM) occurs. The great advantage of the fiber in comparison to other highly nonlinear fibers is its large core diameter of 8.5μm, which warrants a very stable input coupling over many hours as well as normal dispersion in the relevant wavelength regime, which ensures a recompressible phase distribution . Input and output coupling is achieved using an aspheric lens with a focal length of 11.0 mm and a 10x microscope objective, respectively. The output spans over a spectral range from below 900nm to above 1150nm and contains 1.15W of average power and shows an intensity noise which was measured to be smaller than 0.5% rms over one hour. The output of the fiber is then sent through a prism sequence consisting of two equilateral SF10 prisms to precompress the laser pulses. After the prism sequence the pulse duration of the laser pulses is already as low as 25fs with an average power of 1.0W. However, the broad spectrum supports much lower pulse durations, so there is higher order dispersion, which cannot be completely compensated using only the prisms.
To get rid of arbitrary orders of chirp and to shape our spectrum in amplitude we send the broadband laser pulses into a 4-f pulse shaper [11, 12] consisting of two 300lines/mm gold coated gratings and two parabolic gold mirrors with a focal length of 444.5mm. In the Fourier-plane of the pulse shaper a liquid crystal spatial light modulator (SLM) (Jenoptik SLM-S640d) with two 640 pixel masks stacked behind each other is placed for independent amplitude and phase modulation. By this means the output electric field Eout(ω) = A(ω)·exp(i·ϕ(ω))·Ein(ω) can be controlled in a diverse fashion. Here Ein(ω) denotes the spectral electric field of the laser pulses sent into pulse shaper and A(ω) and ϕ(ω) correspond to the amplitude and phase applied to the pulse shaper, respectively.
The output of the pulse shaper is finally frequency doubled in a 10μm thin BBO crystal, and the corresponding nonlinear signal is measured with a spectrometer.
3. Methods and results
In the context of pulse shapers, different kind of algorithms have been developed to determine the phase which has to be applied so that Fourier-limited laser pulses are obtained. Most popular among them are evolutionary and genetic algorithms . In addition to those, deterministic algorithms have been developed, which need only one or very few iterations for convergence. Hence we chose to use the multiphoton intrapulse interference phase scan (MIIPS) [14, 15], known to need only a few iterations for full compensation of nearly arbitrary orders of dispersion. Of course, an external pulse characterization method such as SPIDER , which instantaneously delivers the phase, is appropriate as well. Though, using the MIIPS algorithm no additional equipment is required.
The basic idea of MIIPS is the fact that phase modulation affects the efficiency of nonlinear optical processes. Hence, in order to determine the phase that leads to Fourier-limited laser pulses, a nonlinear signal, in our case the second harmonic, is spectrally measured while scanning a series of phase functions with the pulse shaper. Typically sinusoidal reference phase functions ϕMIIPS(ω, δ) = α · sin[γ · ω−δ] are used, where the parameter δ is scanned from 0 to 4π. The best results we obtained for the amplitude α to be equal to 2.5rad and the parameter γ set to 20fs. The measured result is a MIIPS trace, where the intensity of the second harmonic is plotted over wavelength and the δ-parameter. From iteratively measured MIIPS traces it is possible to retrieve the spectral phase via analytical expressions, except for its zero order and its linear term, since they do not have any influence on the nonlinear signal.
The corresponding measured MIIPS traces are shown in Fig. 1(bottom), showing the 1st, 3rd, and 5th iteration step. While in the 1st MIIPS trace the MIIPS features are strongly tilted and bent, already in the 3rd MIIPS trace the pulse quality has remarkably improved. In the 5th iteration the MIIPS trace shows features which are nearly parallel lines seperated by π in the δ-direction, indicating that the laser pulses are already very close to the Fourier-Limit. Finally, up to 10 iterations could still slightly improve the pulse quality and the compression.
After phase shaping of the broadband laser pulses, we observe pulse durations as short as 11.5fs. However, the pulse spectrum, shown in Fig. 2(a), still shows the modulated shape typical for SPM-broadened spectra, since up to now the amplitude A(ω) applied to the pulse shaper was set equal to one. To obtain a desired spectral shape as well, we set the pulse shaper amplitude A(ω) equal to a desired amplitude Adesired(ω). Of course, this does not yet lead to the desired spectrum |Adesired(ω)|2. Therefore, we measure the output spectrum Iout(ω), and use it to correct the amplitude A(ω) in the following way:Fig. 2(a) depicting a broadband Gaussian-like shaped spectrum.
To finally completely characterize our laser pulses in amplitude and phase we can use the pulse shaper as well. Therefore we multiply our actual amplitude A(ω) with a cosine-like amplitude modulation:17]. By this means we have realized a highly stable Michelson interferometer, since it contains no moving parts and the delay between the double pulses can simply be tuned by ramping the parameter Δτ. If we now measure the second harmonic signal in dependence of the delay Δτ with the spectrometer, we directly obtain the equivalent to an interferometric FROG trace, which we term PS-IFROG (pulse shaper assisted interferometric FROG) and which can be used for full amplitude and phase retrieval as described in . Fig. 2(b) shows a measured PS-IFROG trace from the Gaussian-shaped spectrum depicted in Fig. 2(a). Fig. 2(c,d) show the corresponding interferometric autocorrelations (IAC) of the unshaped and Gaussian-shaped spectrum from Fig. 2(a) which can be obtained by summing up the PS-IFROG traces over all wavelength components, showing pulse durations of 11.5fs and 13fs, respectively. These pulse durations have been obtained by evaluation of the corresponding measured PS-IFROG traces. Therefore the unmodulated kernel of the traces was extracted via Fourier filtering and subsequently entered into an SHG-FROG retrieval algorithm (Femtosoft Technologies).
Note that the temporal sidewings of the Gaussian-like shaped spectrum were remarkably suppressed with respect to the unshaped spectrum as well.
4. Realization of a Gaussian-shaped tunable sub-20fs laser source
By incorporation of all the abilities we presented above, we are able to shape nearly arbitrary spectra in the range of 900nm up to 1150nm. Here we demonstrate this by realizing widely tunable Gaussian-shaped sub-20fs laser pulses. We first use the MIIPS algorithm to compensate the spectral phase. Then we shape the spectrum to a Gaussian-like shape with a bandwidth of about 85nm, with central wavelengths tunable between 950nm and 1100nm. The corresponding measured spectra and their measured average powers are shown in Fig. 3(left). Of course, by amplitude shaping the average power is reduced, however the obtained spectra still show average powers from 30mW up to 142mW.
These laser pulses can then directly be characterized using the above described PS-IFROG procedure. The corresponding measured IACs and the pulse durations are displayed in Fig. 3(right), all exhibiting a pulse duration in the range of 20fs or below. The pulse durations have been obtained once more by the FROG retrieval of the measured PS-IFROG traces.
Of course, the pulse shaper is not limited to Gaussian-like spectral shapes presented in this paper. For example also squared or double Gaussian spectra can be realized. However, one has to keep in mind that changes of the spectral amplitude always lead to temporal changes as well.
Due to its high repetition rate, its tunability, its ultrashort pulse duration in combination with well-shaped spectra and its high peak power this system is specifically well suited for ultrafast nonlinear spectroscopy. Applications are in particular in the field of nonlinear nano-plasmonics as well as nonlinear and ultrafast spectroscopy of carrier dynamics in solids.
In conclusion, we have demonstrated a setup capable of emitting laser pulses with nearly arbitrary spectral shapes in the ultrafast sub-20fs regime and in the spectral range from 900nm to 1150nm at a high repetition rate of 44MHz. This was realized by amplitude and phase shaping of laser pulses which were first spectrally broadened in a LMA-8 PCF. For phase shaping we used a MIIPS algorithm leading to pulse durations as short as 11.5 fs and an average power of 310mW. Via amplitude shaping a widely tunable Gaussian-like laser source has been realized, with central wavelengths tunable from 950nm up to 1100nm and average powers between 30mW and 142mW. This corresponds to 32kW and 151kW peak power, respectively. The pulses could furthermore be directly characterized using the described PS-IFROG procedure. In the future we plan to increase the average output power and the spectral range even further using an 7.4W oscillator  for pumping.
We would like to thank NKT Photonics for support with PCF and acknowledge financial support from BW-Stiftung, DFG (FOR730 and SPP1391), and BMBF ( 13N10146). This work was supported by the German Research Foundation (DFG) within the funding program Open Access Publishing.
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