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 OSA | Compact OPCPA system seeded by a Cr:ZnS laser for generating tunable femtosecond pulses in the MWIR Abstract . A compact mid-wavelength infrared (MWIR) optical parametric chirped pulse amplification (OPCPA) system generates sub-150?fs pulses at wavelengths from 5.4 to 6.8??m with ${\gt}{{400}}\;{\unicode{x00B5}{\rm J}}$ energy at a 1?kHz repetition rate. A femtosecond Cr:ZnS master oscillator emitting 40?fs pulses at 2.4??m seeds both a Ho:YLF regenerative amplifier and a two-stage OPCPA based on ${\rm{ZnGe}}{{\rm{P}}_2}$ crystals. The 2.05??m few-picosecond pump pulses from the Ho:YLF amplifier have an energy of 13.4?mJ. Seed pulses for the OPCPA are generated by soliton self-frequency shifting in a fluoride fiber and are tunable between 2.8 and 3.25??m with a sub-100?fs duration and few-nanojoule energy. The intense MWIR pulses hold strong potential for applications in ultrafast mid-infrared nonlinear optics and spectroscopy. Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License . Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Intense few-cycle coherent light pulses are essential for a broad range of applications in nonlinear optics and ultrafast spectroscopy. Nowdays optical parametric chirped pulse amplification (OPCPA) is a key technique for generating few-cycle infrared pulses with energies reaching the millijoule level at kilohertz repetition rates [ 1 , 2 ]. OPCPA systems for femtosecond pulse generation typically combine a high- performance few-picosecond pump source with a multi-stage parametric amplifier chain [ 1 ]. OPCPA requires the synchronization of pump and seed pulses. The most elegant solution is the use of a femtosecond master laser with an emission spectrum sufficiently broad to seed the pump and the OPCPA chain in parallel and, thus, synchronize passively [ 3 ]. Ti:sapphire oscillators deliver broadband pulses centered at a wavelength of 0.8??m, where the long-wavelength wing is used to seed 1.0??m pump lasers [ 4 , 5 ]. Due to the available nonlinear crystals, OPCPAs with a 1?m pump are limited to wavelengths up to 4??m via parametric downconversion when aiming at scalability of the pulse energy to the millijoule level [ 6 ]. It should be noted that using novel Li-compound crystals, a 1??m pumped OPCPA was demonstrated, providing 142?fs pulses at 9??m wavelength with a 14??J pulse energy [ 7 ]. Accessing the mid-wavelength infrared (MWIR) wavelength range beyond 4??m with high parametric conversion efficiency requires crystals with high second-order nonlinearity such as ${\rm{ZnGe}}{{\rm{P}}_2}$ (ZGP). Due to their short-wavelength absorption edge, ZGP and other nonlinear crystals for the MWIR have to be operated at wavelengths longer than 2??m for the pump and the signal seed source [ 8 ]. This prerequisite has triggered the development of high-performance pump sources around 2.0??m based on Ho-doped gain media, delivering pulses up to several tens of millijoule and few-picosecond duration [ 9 – 11 ]. Most OPCPA architectures for the mid-infrared rely on complex front-ends, where the seed for the pump and the OPCPA signal pulses is provided by a combination of nonlinear frequency conversion processes such as difference frequency generation (DFG), supercontinuum (SC) generation [ 9 , 10 , 12 – 16 ] and/or optical parametric downconversion (OPG) [ 13 ]. While these solutions make the generation of broad optical spectra in the mid-infrared possible, the cascaded nonlinear stages increase the complexity and reduce both stability and efficiency of OPCPA. Femtosecond mid-infrared laser oscillators based on transition-metal-doped II-VI chalcogenides hold potential for serving as a single master oscillator and, thus, simplifying MWIR and LWIR OPCPA. Promising candidates are lasers based on Cr:ZnSe and/or Cr:ZnS as the active medium [ 17 , 18 ]. Femtosecond pulse generation by mode-locking a Cr:ZnSe oscillator was first demonstrated in 2006 [ 17 ] but only recently stable sub-50?fs few-cycle pulses with a broad spectrum in the 2–3??m range have become available [ 18 , 19 ] and been applied for nonlinear frequency conversion [ 20 , 21 ]. Recently a mid-IR OPA was presented relying on a femtosecond Cr:ZnS oscillator and a 1?kHz Cr:ZnSe regenerative amplifier (RA) with subsequent signal generation (SC and OPG). The generated combined signal and idler pulse energy was 130??J and compression of the signal to ${\sim}{{300}}\;{\rm{fs}}$ was demonstrated [ 22 ]. Here we report a highly compact MWIR OPCPA system that incorporates a femtosecond Cr:ZnS oscillator to drive a picosecond Ho:YLF RA as a pump and, in parallel, generate seed pulses for a two-stage ZGP OPCPA. Soliton self-frequency shifting (SSFS) of the 2.4??m sub-50?fs output of the Cr:ZnS oscillator in a fluoride fiber provides tunable (signal) seed pulses as an initial condition for OPCPA. We demonstrate MWIR idler pulses tunable from 5.4 to 6.8??m with an unprecedented pulse energy of ${\gt}{{400}}\;{\unicode{x00B5}{\rm J}}$ at a 1?kHz repetition rate. Re-compression of the OPCPA idler output currently provides sub-150?fs pulses. The experimental setup is shown in Fig.? 1 (a). A mode-locked ${{\rm{Cr}}^{2 +}}{:}{\rm{ZnS}}$ laser (IPG Photonics) operating at 79?MHz is the master oscillator for the OPCPA system. Its output spectrum with maximum at 2.4??m covers a bandwidth of 300?nm (FWHM) [Fig.? 1 (b)], the pulse duration is 82?fs. Adding an external 3?mm thick ZnSe substrate, the residual chirp is almost compensated resulting in pulses as short as 30?fs with an energy of 12.5?nJ. The Cr:ZnS laser spectrum is divided by a dichroic mirror [Fig.? 1 (a)]. The part centered at a wavelength of 2.4??m has an energy of 10?nJ, a pulse duration of 41?fs [Fig.? 1 (c)], and serves as a seed for the signal of the MWIR OPCPA. ? Fig. 1. (a)?Setup of the MWIR OPCPA. The main parts are the front-end, including a femtosecond Cr:ZnS master oscillator and a fluoride fiber (ZBLAN), the 2.05??m Ho:YLF RA as pump, and the two optical parametric amplifier (OPA) stages based on ZGP crystals. TFP, thin-film polarizer; $\lambda /{{2}}$ , wave plate; DM, dichroic mirror; S, C bulk stretcher and compressor. (b)?Emission spectrum of the femtosecond Cr:ZnS oscillator (brown) and signal seed spectrum, i.e.,?after separation of the short-wavelength wing for the pump (blue). (c)?Collinear autocorrelation trace of the signal seed pulse. Download Full Size PPT Slide PDF The short-wavelength wing of the Cr:ZnS laser spectrum extends down to 1.9??m [Fig.? 1 (b)] and provides the seed for the picosecond 2??m pump channel of the OPCPA [Fig.? 2 (a)]. The (pump) seed pulses are stretched using two chirped volume Bragg-gratings (CVBG) with a reflection band spanning from 2046–2058?nm [green bar, Fig.? 2 (a)] and pre-amplified in a Tm:fiber amplifier (AdValue Photonics). The pulses of 5?nJ energy are then stretched to roughly 1?ns and fed into the RA. The RA designed as a ring-cavity [ 11 ] comprises the gain medium Ho:YLF, which is pumped by a 50?W continuous-wave Tm:fiber laser at 1.94??m (IPG Photonic), and a Pockels cell. The latter couples the seed pulses into the resonator and thereby reduces the pulse repetition rate to 1?kHz. By carefully mitigating bifurcation instabilities, the RA delivers a pulse energy of 14.5?mJ in the saturated regime. The amplified spectrum of the RA together with its seed spectrum is shown in Fig.? 2 (b). It is centered at 2051?nm with a FWHM of 2.7?nm. The corresponding Fourier-transform limited (FTL) pulse duration is 1.9?ps. A Treacy-type compressor containing dielectric-coated gratings (98.5% diffraction efficiency per grating) compensates for the dispersion of the pulses subsequent to the RA, resulting in duration of 2.4?ps (FWHM). The small satellites visible in the measured autocorrelation function (ACF) at 17?ps [Inset Fig.? 3 (c)] stem from the accumulated B-integral which is estimated to not exceed 1?rad. The compressed pulses of the RA exhibit a record energy of 13.4?mJ having an excellent long-term pulse stability with a remarkably pulse-to-pulse fluctuation of only 0.35%, see Fig.? 2 (c). The beam quality is nearly diffraction limited with an ${{\rm{M}}^2}$ of ${\lt} 1.3$ [Inset Fig.? 2 (c)]. The resulting peak power of 5.4?GW surpasses that from other recent 2??m RAs for picosecond pulses [ 9 – 11 , 25 ]. ? Fig. 2. Performance of the 2.0??m pump source. (a)?Separated short-wavelength part of the Cr:ZnS laser spectrum (green bar: CVBG reflection band). (b)?Seed (dark yellow) and emission (green) spectrum of the Ho:YLF RA. (c)?Long-term pulse stability measurement of the re-compressed Ho:YLF RA pulses at a 1?kHz repetition rate. Insets: autocorrelation trace, measured and simulated, and far-field intensity distribution. Download Full Size PPT Slide PDF ? Fig. 3. Generation of tunable MWIR signal pulses via SSFS. (b)–(e)?Simulation of the propagation of the 2.4??m input pulses [Figs.? 1 (b)?and 1 (c)] in a 2?m long ZBLAN fiber (parameters used: nonlinear refractive index, ${5.4}\; \times \;{{10}^{- 20}}\;{{\rm{m}}^2}/{\rm{W}}$ ; Raman frequency, 0.24 [ 23 ]; mode-field diameter, 8.4??m [ 24 ]. The additional shock time of 1.5?fs is calculated from the derivative of the effective mode area.). (e)?Temporal pulse evolution. (c)?Spectral pulse evolution. (d)?Temporal pulse shape at the output of the ZBLAN fiber. (b)?Simulated and measured spectrum of the SSFS soliton after 2?m propagation in the fiber. (a)?Measured collinear autocorrelation trace of the SSFS soliton at 2.99??m (inset: measured spectrum). Download Full Size PPT Slide PDF For providing spectrally tunable seed pulses at the signal wavelength of the MWIR OPCPA, the 2.4??m output of the Cr:ZnS oscillator [Fig.? 1 (b)] is shifted to longer wavelengths. This shift is performed in a highly nonlinear fluoride fiber via Raman-induced SSFS [ 20 , 24 ]. Utilizing this approach signal pulses are provided exhibiting a very low noise level which is beneficial compared to other signal pulse generation for OPA [ 26 ]. A ZBLAN ( ${\rm{Zr}}{{\rm{F}}_4}\;{\rm{Ba}}{{\rm{F}}_{2}}\;{\rm{La}}{{\rm{F}}_{3}}\;{\rm{Al}}{{\rm{F}}_3}$ NaF) step-index fiber is employed with a core diameter of 6.5??m and a numerical aperture of 0.23 (Le Verre Fluoré). The high-order dispersion parameters are derived from the data provided by the supplier and the fiber dispersion is calculated giving the zero-dispersion wavelength at 1.99??m and a GVD of ${-}{81.5}\;{{\rm{fs}}^2}/{\rm{cm}}$ at 2.4??m. The pulse propagation in the fiber is simulated applying the nonlinear Schr?dinger equation ( Fiberdesk software). Figure? 3 ?shows the simulated spectral [Figs.? 3 (b) and 3 (c)] and temporal evolution [Figs.? 3 (d) and 3 (e)] of the signal pulses along the fiber length up to 2?m. One observes a soliton buildup from the?SC generated in the first few millimeters of the fiber. With the available pulse energy of 6?nJ, solitons with center wavelengths up to 3.25??m are created. Taking into account the 2.05??m pump pulse duration of 3.0?ps, set for pumping the OPCPA, the temporal separation of the soliton from the SC has to be at least 6?ps to interact only with the soliton in the parametric amplification process. For this, a propagation length of 2?m in the fiber is sufficient [Fig.? 3 (d)] and chosen in the experiment. The pulses of the master oscillator dedicated for the signal channel [Figs.? 1 (b) and 1 (c)] with an energy up to 9?nJ are launched into the ZBLAN fiber using a parabolic mirror with ${\rm{f}} = {12.7}\;{\rm{mm}}$ . Assuming a coupling of 70%, the intensity at the fiber entrance corresponds to ${{860}}\;{\rm{GW}}/{{\rm{cm}}^2}$ . The measured transmitted spectra, shown in Fig.? 3 (b), confirm SSFS solitons up to 3.25??m. SSFS calculations and measurements are in good agreement [Fig.? 3 (b)] and suggest that the soliton contains 57% of the transmitted energy. The experiments give a spectral width (FWHM) of 147?nm, whereas the simulations predict a FWHM of 108?nm for the $\lambda = {3.25}\;{\rm{\unicode{x00B5}{\rm m}}}$ soliton and the other spectral parts differ slightly. We attribute these differences mainly to the uncertainty of the input parameters in the simulation. The spectral shift of the solitons depends on the input intensity; thus, the soliton wavelength can be tuned by changing the input pulse energy. It was varied between 4.6 and 9.0?nJ to generate the solitons between 2.95 and 3.25??m. This is shown in Fig.? 4 (a) for the OPCPA output spectra. ? Fig. 4. OPCPA output. (a)?Spectral intensity of the signal (left) and the corresponding idler pulses (right). The tunable seed of the signal pulses is generated via SSFS in the ZBLAN fiber (Fig.? 3 ). (b), (c)?ACFs of an uncompressed signal and a re-compressed idler pulse. Download Full Size PPT Slide PDF ZBLAN fibers exhibit an intrinsic birefringence stemming from the drawing process. To tune the input polarization properly, a $\lambda /{{2}}$ wave plate behind the Cr:ZnS laser output coupler is used. The polarization is chosen such that a clean SSFS soliton is created in the fiber. Though the fiber is not polarization maintaining, the soliton has been measured to be linearly polarized. The existence of the soliton at 2.99??m is confirmed by recording its autocorrelation (APE, Mini TPA/PD). The collinear ACF shown in Fig.? 3 (a), exhibits a pulse duration of 78?fs which is close to the FTL limit of the associated bandwidth of 145?nm [inset Fig.? 3 (a)]. The layout of the MWIR OPCPA is adapted from our previous setup [ 16 ]. The amplifier is composed of two stages, OPA 1 and OPA 2 [Fig.? 1 (a)], containing ZGP crystals cut for type-I phase matching (BAE systems). OPA 1 is designed in a non-collinear geometry, while the design of OPA 2 is collinear. The latter ensures the generation of the idler without angular dispersion. Both stages are seeded at the signal wavelength. For adapting the signal pulse duration to the pump in each of the amplification stages, the signal pulses are stretched in front of OPA 1 and OPA 2 using bulk material, a 40?mm long sapphire, and a 80?mm long ${\rm{Ca}}{{\rm{F}}_2}$ rod, respectively. The 0.8?ps long signal pulses entering OPA 1 are amplified to 15??J in a 2?mm thick ZGP crystal $(\theta = {{59}}^\circ$ ). To exploit the available 2.05??m pump energy of 11.5?mJ for OPA 2 without the risk of damage, a ZGP crystal ( $\theta = {55.4}^\circ$ ) with an aperture of ${{15}}\; \times \;{{15}}\;{\rm{mm}}^2$ (thickness: 1.9?mm) is applied, and the pump beam is expanded to a diameter of 8.5?mm. The generated combined signal and idler energy in the 1?kHz pulse train amounts to 1.2?mJ with an uncompressed signal pulse duration of 1.1?ps [Fig.? 4 (b)]. The long-term pulse stability is remarkable and measured with a power fluctuation rms of 2.1%, and the beam quality is analyzed with an ${{\rm{M}}^2}$ value of 1.6. The idler pulse energy of ${\gt}{{400}}\;{\unicode{x00B5}{\rm J}}$ surpasses the highest energy reported for tunable OPAs beyond 4??m so far by one order of magnitude [ 25 ]. The signal and idler OPCPA output spectra shown in Fig.? 4 (a) were measured with a scanning monochromator equipped with a HgCdTe detector (Horiba). The spectral tunability of the signal pulses is inherently transferred to the idler and extends from 5.4–6.8??m. The bandwidth of the idler spectra amounts to ${\gt}{4}\;{\rm{THz}}$ (FWHM) and supports a FTL duration of less than 80?fs. ${{\rm{CaF}}_2}$ prisms are used for almost lossless re-compression of the chirped idler pulses. The measured non-collinear ACF (SHG in ${{\rm{AgGaS}}_2}$ recorded with a Pyrocam detector) of the re-compressed pulses at $\lambda = {5.4}\;{\rm{\unicode{x00B5}{\rm m}}}$ is presented in Fig.? 4 (c), giving a pulse duration of 136?fs. Since our OPCPA system is not purged, water vapor absorption is the main origin for the structured idler spectra [Fig.? 4 (a)], resulting in a substantial phase modulation and a pulse duration longer than the FTL. To overcome this problem, purging of the system will be the next step in connection with implementing a spectral shaper to compensate for the third and higher order-dispersions. In conclusion, a robust and compact novel MWIR OPCPA system was presented. Compared to our running table-top MWIR OPCPA [ 16 ], the footprint of the front-end has been reduced by 30%, i.e.,?the whole system now covers an area of only ${4.2}\;{{\rm{m}}^2}$ . In contrast to existing systems relying mainly on cascaded nonlinearities for generating the seed and/or pump pulses, a femtosecond Cr:ZnS master oscillator provides the seed for the pump at 2.0??m and the signal channel directly. This approach avoids instabilities and degeneration stemming from multiple nonlinear stages (SC, DFG, or OPG) and enables passive synchronization. After splitting the Cr:ZnS master spectrum, the resulting sub-50?fs pulses centered at 2.4??m are coupled into a nonlinear fluoride fiber to generate tunable seed pulses at the signal wavelength of OPCPA via SSFS. The tuning range extends from 2.8 to 3.2??m. The two-stage OPCPA is based on ZGP crystals and delivers tunable sub-150?fs idler pulses between 5.4 and 6.8??m with an energy ${\gt} 400 \; {\unicode{x00B5}{\rm J}}$ , resulting in a peak power of 3?GW. Further pulse shortening is possible by eliminating the impact of water vapor on the optical phase and implementing a spectral pulse shaper. 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