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Abstract
We present a tunable, high-energy optical parametric chirped pulse amplification system with a front-end based on a femtosecond Cr:ZnS laser. By taking advantage of the broad emission spectrum of the femtosecond Cr:ZnS master oscillator, we are able to directly seed the holmium-based pump around 2 µm. At the same time, the signal pulses for the parametric process are generated via Raman self-frequency shifting of the red end of the spectrum centered at 2.4 µm. The solitons, generated in a fluoride fiber, are tunable over the wavelength range between 2.8 and 3.2 µm. The optical parametric amplifier operates at a 1 kHz repetition rate, and consists of two stages equipped with ZGP as nonlinear crystal. The generated idler pulses are tunable between 5.4 and 6.8 µm with a pulse energy of up to 400 µJ. Dispersion management using bulk material stretching and compression in combination with precise phase shaping prior to amplification enables idler pulses of a sub-100 fs duration, translating into a peak power as high as 4 GW.
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1. Introduction
Research on light matter interaction requires very strong fields of few-cycle laser pulses to generate extreme nonlinear effects. For the latter, high-order harmonic generation (HHG) is an important example [1,2]. At mid-infrared (mid-IR) wavelengths, intense few-cycle pulses are of high interest because their long oscillation periods allow an efficient acceleration of electrons in the light field, i.e., an enhancement of ponderomotive energy [3,4]. Moreover, ultrashort mid-IR pulses are an important tool of nonlinear vibrational spectroscopy in the femosecond time domain, including 2D-IR methods [5,6]. Nowadays optical parametric chirped pulse amplification (OPCPA) is a key technique of few-cycle pulse generation in the mid-IR with pulse energies reaching the millijoule level at kilohertz repetition rates [7,8]. High energy OPCPA systems for femtosecond pulse generation typically utilize a high-performance few-picosecond pump source in a chain of optical parametric amplifiers (OPA). Pump pulse durations of a few picoseconds are advantageous because the risk of optical damage in the nonlinear crystal is reduced [9] and the temporal walk-off between pump and signal gets neglectable in relation to the few-ps pulse duration [10]. Furthermore, large stretching and compression ratios are avoided with short pump pulses.
In the near infrared, Ti:sapphire laser oscillators, centered around 800 nm, deliver pulse durations in the femtosecond range, provide in parallel the pump and the signal seed for OPCPA, and thus, are synchronized passively [11]. In contrast, mid-IR OPCPA architectures often rely on rather complex front-ends. The signal is typically created by a combination of cascaded nonlinear effects such as supercontinua (SC), difference frequency generation (DFG) and/or OPG/OPA [12–18]. Although such complex solutions can lead to the generation of broad optical spectra in the mid-IR, they suffer from low stability and efficiency. Moreover, they hamper easy tunability.
Transition metal-doped II-VI chalcogenide crystals as a gain element in femtosecond mid-IR laser oscillators are a promising option for a master oscillator in OPCPA front-ends and, thus, simplifying such front-end architectures. Promising candidates are Cr:ZnS and/or Cr:ZnSe lasers. These active materials, sometimes referred to as the “Ti:sapphire of the mid-IR”, explore unique physical, spectral and nonlinear properties [19,20]. The first Cr:ZnSe laser, delivering femtosecond pulses was demonstrated in 2006 [19]. In recent years, reliable Cr:ZnS / Cr:ZnSe lasers became available. They can provide pulses with sub-50 fs duration, covering the spectral range between 2 and 3 µm [20,21]. Such sources have been successfully utilized for different applications, e.g., nonlinear frequency conversion [22–26]. Recently a Cr:ZnSe regenerative amplifier seeded by a femtosecond Cr:ZnS oscillator was presented. This source drives a mid-IR OPA at 1 kHz repetition rate with the signal generated via SC and OPG. The duration of the compressed signal pulses centered at 3.5 µm was ∼300 fs and the combined signal and idler pulse energy amounted to 130 µJ [27]. In 2019 a milestone for Fe-based chalcogenides was presented, a Fe:ZnSe amplifier system at 4.4 µm delivering 3.5 mJ, 150 fs pulses at 10 Hz repetition rate [28]. At longer wavelengths post-compression of few-ps CO2 laser pulses was demonstrated successfully, leading to sub-500 fs pulses at 9.2 µm with 279 mJ energy at a repetition rate of 1 Hz [29].
Besides a suitable seed source, nonlinear crystals with a high second-order nonlinearity and suitable transparency window are a prerequisite for power scalable parametric amplifier systems at wavelengths beyond 4 µm. The non-oxide nonlinear crystal ZnGeP2 (ZGP) transmitting up to 10 µm is a promising candidate for this application due to its large nonlinear coefficient. However, it needs to be pumped above 2 µm [30,31].
Here we present a table-top and robust midwave-infrared (MWIR) OPCPA front-end based on a femtosecond Cr:ZnS master oscillator. It allows for direct seeding of the Ho-based pump at 2 µm and provides the seed for the OPCPA signal generation. Tunable signal pulses are provided by solitons Raman self-frequency shifting (SSFS) in a highly nonlinear fluoride fiber. Compared to our previous setup [32], we are now able to phase shape the MWIR pulses up to the fourth order. At the same time, we operate the complete parametric setup as well as the pulse characterization in a practically water free nitrogen atmosphere. With these additional measures, it is now, for the first time, possible to generate clean idler pulses with a sub-100-fs duration and 4 GW peak power.
2. Experimental
2.1 Setup
The OPCPA front-end is based on a Cr:ZnS oscillator operating at a repetition rate of 79 MHz (IPG Photonics) and a nonlinear fluoride fiber. The oscillator provides 12.5 nJ pulses as short as 30 fs with a spectrum spanning from 1.9 µm to 2.6 µm (30dB-level). This spectrum is sufficiently broad to seed the pump and the signal arm simultaneously. For this purpose, a dichroic mirror splits the spectrum at 2.1 µm, the spectral part below 2.1 µm is used for the 2-µm pump, and the long wavelength part centered at 2.4 µm for the signal generation. Subsequently, the pulses centered at 2.4 µm are launched into a ZBLAN fiber, which shifts the central wavelength by Raman-induced SSFS to larger values. The entire setup, shown in Fig. 1, comprises the front-end, a high-energy 2-µm pump, an acousto-optic programmable dispersive filter (AOPDF) and two OPA stages.
Fig. 1. Setup of the MWIR OPCPA. It comprises the front-end including a fs Cr:ZnS master oscillator and a fluoride fiber (ZBLAN), the 2.05 µm Ho:YLF regenerative amplifier as pump and the two optical parametric amplifier (OPA) stages based on ZGP crystals. AOPDF, acousto-optic programmable dispersive filter; DM, dichroic mirror; S, bulk stretcher and C, prism compressor.
2.2 High performance pump
The seed for the 2-µm pump is stretched in a chirp-volume-Bragg-grating to about 1 ns. These pulses with 60 pJ energy are subsequently amplified in a Tm-doped fiber pre-amplifier. The amplified pulses centered at 2050 nm with 10 nm bandwidth and 2 nJ energy are coupled into a regenerative amplifier (RA) with the gain crystal Ho:YLF, at a 1 kHz repetition rate. Pumped by a 50 W continuous-wave Tm-doped fiber laser, the RA emits pulses centered at 2.05 µm wavelength with 13.5 mJ energy. Subsequently, pulse compression in a Treacy-type grating compressor compensates for the residual chirp, which enables for a pulse duration of 3.0 ps. The resulting peak power of 5.4 GW surpasses the highest reported results from other picosecond 2-µm RAs [15,16,33,34].
3. Results and discussion
3.1 OPA signal generation
The pulses from the Cr:ZnS oscillator separated for the signal generation have an energy of 10 nJ and a duration of 46 fs. They are launched into a ZrF4-BaF2-LaF3-AlF3-NaF (ZBLAN) fiber (Le Verre Fluoré) with a core diameter of 6.5 µm and a high nonlinear refractive index n2. The coupling is performed using a parabolic gold mirror having a focal length of 12.7 mm. Its calculated group velocity dispersion (GVD) is shown in Fig. 2, which exhibits a zero-dispersion wavelength (ZDW) at 1.99 µm and hence negative second order dispersion for the coupled pulses.
Fig. 2. Group velocity dispersion (GVD) of the ZBLAN fiber with 6.5 µm core diameter, computed using material parameters provided by the manufacturer (ZDW: zero dispersion wavelength, vertical dashed line: laser wavelength).
Fig. 3. Simulation and experimental demonstration of the spectral evolution behind the 2 m long fluoride fiber for different coupling energies (E: pulse energy of the soliton). The spectrum on top shows the input spectrum.
A perfect compensation of negative dispersion and self-phase modulation inside the fiber enables formation of a soliton pulse, maintaining its temporal shape over the whole traveling distance and having a Fourier-transform limited pulse duration. The solitons created inside the fiber self-frequency shift to longer wavelengths, due to the corresponding fractional Raman gain which depends on the nonlinear refractive index. The evolution of the soliton inside the employed fiber is simulated using the non-linear Schrödinger equation. The simulations rely on the computed dispersion (Fig. 2) assuming a non-linear refractive index of ${n_2}\; = \; 5.4 \times {10^{ - 20}}\; m^2/W$, [35], a fractional Raman contribution of 0.24 [35], a mode field diameter of 8.4 µm [36], and an additional shock time of 1.5 fs. The shock time has been calculated from the derivative of the effective refractive index of the fiber and the mode field diameter at the center frequency corresponding to the wavelength of 2.4 µm. The evolution of the experimentally achieved spectra for different pulse energies is shown in Fig. 3. It has been measured with a scanning spectrometer (Wavescan USB, APE) and demonstrates the transformation of the launched input pulses in dependence of the input energy to solitons with different central wavelengths. The simulation and experiment agree, as shown in Fig. 3 after a 2 m propagation in the ZBLAN fiber. The soliton wavelength can be tuned between 2.8 and 3.2 µm by increasing the launched pulse energy from 4.6 to 9.0 nJ, see Fig. 3. This is achieved experimentally by slightly modifying the distance between the focussing parabolic mirror and the fiber tip and hence the coupling efficiency into the fiber is changed. Soliton formation was verified by recording the autocorrelation trace at 3.0 µm (Fig. 3, spectrum (red line)) using a TPA autocorrelator (APE GmbH). It shows an almost Fourier-transform limited (FTL) pulse duration of 78 fs [32]. The fiber length of 2 m was chosen to realize a sufficient temporal separation of the Raman soliton from the slower propagating SC part in order to ensure its exclusive interaction with the 3.0 ps long 2-µm pump pulse in the OPA crystals. To estimate the timing jitter of the Raman soliton generation, we use the power stability measurement for SC spectral components at λ > 3.6 µm in Ref. 23, which showed a root-mean-square (rms) value of 0.4%. The calculation is performed using the moment method to analyze the pulse propagation [37,38] and resulted in a 1.6% timing jitter for a pulse stability of 0.4% (rms).
This SSFS approach provides signal pulses at a very low noise level which is beneficial compared to the typical OPA signal pulse generation via other nonlinear processes [39]. The signal energy is at the few-nJ level, i.e., much higher than in typical DFG front-ends, which enormously decreases the risk for parametric fluorescence. The latter is often a problem in high-energy OPA systems. Furthermore, this front-end simplifies the OPCPA architecture immensely and reduces the number of cascaded nonlinearities in the generation of a tunable signal wavelength.
3.2 Optical parametric amplification
Efficient parametric amplification requires the time overlap between the pump and the signal pulse. The duration of both pulses need to match on the relevant time scale. However, the initial signal pulse duration of ∼80 fs is much shorter than the pump pulse duration of 3.0 ps and, thus, the signal pulse duration is stretched behind the ZBLAN fiber to ∼1 ps in 40 mm long uncoated Al2O3. The negatively stretched signal pulses pass through an acousto-optic programmable dispersive filter (AOPDF, Dazzler Fastlite) [40] for additional phase shaping. Second, third and fourth order dispersion control prior to the parametric amplification stages is hereby enabled. The phase shaping element causes a reduction to roughly 20% of the pulse energy, depending on the amount of applied dispersion.
Signal and idler pulses are amplified in two OPA stages based on 2 mm thick ZGP crystals (BAE systems). The OPA design is adapted from our former system [32]. The first stage in non-collinear configuration ensures a broad spectral width of the signal pulses. The energy loss induced by the AOPDF for different dispersion values and by the tunability of the signal pulses is compensated by the high gain in the first OPA stage. The latter also applies to the variation of the lauched pulse energy in the ZBLAN fiber between 4.6 and 9.0 nJ, i.e., no reduction of the amplified signal pulse energy was observed. The signal pulses are further stretched in an uncoated 80 mm long bulk CaF2 and the mode waist is enlarged to 7 mm to allow for pumping with the maximum energy of 11.5 mJ. The following collinear OPA stage is seeded with 5 µJ and creates the desired idler pulses without angular dispersion and a nearly Gaussian intensity profile.
After the second parametric stage, we achieve uncompressed signal pulses of a ∼1 ps duration and >650 µJ energy. The corresponding idler energy varies from 401 µJ to 305 µJ in the achieved tuning range between 5.4 and 6.8 µm. The idler spectra together with its bandwidths are shown in Fig. 4 (recorded with a scanning monochromator Horiba, iHR320). The spectral bandwidth increases from 3.6 THz to 4.7 THz (FWHM) with tuning to longer wavelengths, all supporting a sub-100 fs FTL pulse duration. The OPA stages and the pulse characterization units are purged with nitrogen. However, the stronger water vapor absorption lines beyond 6.0 µm are still imprinted on the recorded spectra.
Fig. 4. Spectral intensity (normalized) of the tunable, high-energy idler pulses of the MWIR OPCPA (λ: center wavelength of the presented spectra; Δν: bandwidth (FWHM)).
The idler pulses exhibiting positive GDD are re-compressed using a CaF2 prism pair without coating. The pulses are characterized for the idler at 5.5 µm performing a second-harmonic generation (SHG) frequency-resolved optical gating (FROG) measurement (Fig. 5). The scanning monochromator (Horiba) is employed to record the SHG spectra and the corresponding FROG trace. In the case of the 5.5 µm pulses, 46 mm of CaF2 bulk material was introduced using a tunable prism pair. Additionally, 3000 fs2 GDD and 600 × 103 fs3 TOD was introduced by the AOPDF to enable the shortest pulse duration. The pulses are analyzed with the FROG code from Femtosoft Technologies. The analysis confirms a flat phase in spectral and temporal domain (Fig. 5(c) and 5(d)) and supports a pulse duration of 99 fs (± 5 fs) only slightly longer than the FTL duration of 91 fs in this case. The measured and retrieved FROG traces are shown in Fig. 5(a) and 5(b), respectively, where the retrieval is based on a 256 × 256 grid, slightly larger than the measured one and show an error of 0.67%. Also the directly measured idler spectrum agrees well with the retrieved spectrum (Fig. 5(c)).
Fig. 5. Characterization of the idler pulse at 5.5 µm. (a) Measured SHG FROG trace and (b) retrieved FROG trace presented at a logarithmic scale with 0.67% error. (c) Corresponding retrieved spectrum, and (d) retrieved pulse in the time domain (τ: pulse duration (FWHM)).
Spectral tuning of the idler pulses requires adaptation of the dispersion management for achieving the shortest pulses. The CaF2 prism pair is translated such, that it introduces different amounts of GDD to adjust the pulse duration. For precise phase compensation, GDD and TOD of the pulses is fine-tuned by the AOPDF. For the different idler wavelengths, this method ensures the tunability of the system without losing the ability to compress the pulse duration almost to the Fourier limit. The shape of the measured beam intensity is nearly Gaussian with an M2 of 1.6 and the long-term pulse stability amounts to an rms-value of 1.2%, both measurements are presented in Fig. 6. For tunable fs OPA’s beyond 4 µm, the measured idler pulse energy of up to 400 µJ exceeds that of the previously demonstrated systems by more than a factor of 10 [33].
Fig. 6. Long term pulse stability measurement of the idler pulses (center wavelength: 5.4 µm) at 1 kHz repetition rate. Inset: Far-field intensity distribution.
4. Summary
In summary, we have demonstrated the generation of high-energy sub-100 fs idler pulses tunable between 5.4 and 6.8 µm via OPCPA. The front-end is based on a Cr:ZnS oscillator and enables the direct seeding of the pump and the signal generation with a single, passive nonlinear frequency conversion stage. The generated signal is soliton self-frequency shifted inside a ZBLAN fiber to wavelengths between 2.8 and 3.2 µm by changing the launched pulse energy. With a demonstrated idler pulse energy of up to 400 µJ the present system remarkably surpasses the reported energies so far for tunable mid-IR OPA systems [33]. The compression of the idler pulses to sub-100 fs duration became possible through indirect phase shaping using an AOPDF in the signal beam path in front of the OPA and purging of the latter with nitrogen. The presented novel approach for MWIR OPCPA is simple, robust, compact in size and passively synchronized. The low noise signal with an energy in the nJ range, dramatically decreases the risk for parametric flourescence and enables straight forward energy scalability. The tunable high-energy femtosecond idler pulses cover part of the fingerprint frequency region and hold strong potential for nonlinear ultrafast vibrational spectroscopy. With further optimization on the signal generation, an even broader coverage of the MWIR to LWIR is feasible.
Funding
Deutsche Forschungsgemeinschaft (GR2116/5-1); Horizon 2020 Framework Programme (871124 LaserlabEurope).
Acknowledgements
Expert technical support of Dennis Ueberschär is gratefully acknowledged.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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