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Abstract
We report on two different concepts to generate µJ-level mid-infrared laser pulses with sub-10 ps pulse duration via nonlinear parametric generation at high pulse repetition rates. Both schemes rely on the recent development of compact and efficient CPA-free, Ho:YLF-based 2-µm laser sources pumping the highly nonlinear crystal ZnGeP2 used for parametric amplification. The first concept comprises a simplified OPG/OPA scheme efficiently producing signal and idler radiation at fixed wavelengths of 3.0 and 6.5 µm, respectively. In the second concept, we demonstrate a wide spectral tunability of the picosecond, µJ pulses over the entire tuning range between 2.5 and 12 µm maintaining a constant bandwidth of sub-20 cm−1 by integrating a two-color pumping scheme and a spectral shaper into the setup. The presented compact and efficient mid-IR picosecond radiation sources offer a way towards low-cost mid-IR material processing and spectroscopic applications.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The mid-infrared (mid-IR) spectral range (3–15 µm) hosts a multitude of wavelength-specific absorption features of polymers and organic molecules. Nowadays, a variety of scientific, industrial, and medical applications rely on these molecular transitions, but only few exotic laser gain materials exist, which exhibit direct emission in this wavelength region [1]. Alternatively, nonlinear frequency conversion of near-IR laser sources into the mid-IR is commonly used as it offers the potential of broad spectral tunability, ultrashort pulses, and high pulse energies. Since oxide-based nonlinear optical crystals (NOC) are limited to wavelengths below 5 µm, non-oxide NOCs have gained interest within the recent years as their transmission windows reach deep into the mid-IR [2]. Among them, ZnGeP$_2$ (ZGP) is highly attractive due to its comparably high nonlinear figure of merit [3], but requires pump sources with emission wavelengths $>1.9$ µm due to the onset of strong linear and higher-order photon absorption [4]. Typically, commercial laser systems based on the well-established near-infrared laser sources are nonlinearly down-converted to $>1.9$ µm for pumping of non-oxide NOCs [5,6]. However, this results in cascaded nonlinear frequency conversion schemes, which are comparably inefficient, complex, spacious, and often cost-intensive. An alternative approach is the direct generation of 2 µm ultrashort pulses in holmium (Ho)-doped fibers and crystals as laser gain materials. Most commonly, sub-10 ps laser pulses are amplified via chirped-pulse amplification (CPA) in regenerative amplifiers based on Ho-doped crystals towards the mJ-level [7–10]. Nevertheless, this approach requires highly efficient gratings for temporal pulse stretching and compression, which increases the system’s complexity and costs. Furthermore, high-voltage driven Pockels cells in regenerative amplifiers typically limit the achievable pulse repetition rate. As alternative to the common Ho-based CPA regenerative amplifiers, we recently demonstrated a CPA-free master oscillator power amplifier (MOPA) system based on Ho-doped fibers and Ho:YLF crystals [11]. It provides sub-10 ps laser pulses at a wavelength of 2.05 µm with energies in the µJ range at tunable pulse repetition rates up to hundreds of kHz. This laser source benefits from its comparably compact size in a simplified amplification scheme with turn-key operation, which makes it perfectly suited as 2-µm pump source for nonlinear frequency conversion into the mid-IR spectral region. Several techniques are established for the nonlinear frequency down-conversion of 2-µm pump pulses in non-oxide NOCs such as difference-frequency generation, supercontinuum generation, or optical parametric oscillators and generators. Among these techniques, optical parametric generation (OPG) benefits from its simplicity regarding its optical design and low requirements to the pump radiation. Despite these advantages, until now only few radiation sources based on 2-µm pumped OPG have been reported [6,12], often featuring complicated pump schemes based on Ti:sapphire-driven OPA systems and operating at low repetition rates of 1 kHz and below.
Here, we report on the realization of two different concepts to generate ultrashort mid-IR laser pulses at high repetition rates by taking advantage of the recent development of CPA-free, 2 µm picosecond laser sources for pumping of highly nonlinear ZGP crystals. The first concept is a simplified 2-µm-pumped OPG/OPA architecture consisting of two ZGP crystals in series, efficiently generating MW-level signal and idler radiation at wavelengths of 3.0 and 6.5 µm, respectively. In the second concept, the ZGP-based OPG stage is replaced by a 1-µm-pumped double-pass, MgO:PPLN-based OPG/OPA setup including a widely tunable, home-built spectral shaper used as spectral filter. Followed by a 2-µm-pumped, ZGP-based booster amplifier, this second concept enables the generation of widely tunable, narrowband mid-IR pulses at microjoule pulse energy levels.
2. Experimental setups
The applied pump laser is a home-built, CPA-free MOPA based on Ho-doped fibers and Ho:YLF crystals [11]. For the experiments, the laser was operated at a maximum pulse energy of 50 µJ at a repetition rate of 100 kHz. The laser pulse duration was measured to be 4 ps full-width at half-maximum (FWHM) with a spectral bandwidth of 2 nm centered around 2.05 µm.
The first concept is an OPG/OPA tandem configuration based on two ZGP crystals as schematically depicted in Fig. 1 (OPG concept 1). The ZGP crystals are 4.9 mm long and anti-reflection coated for the pump and mid-IR spectral range from 2 to 8 µm wavelength. They are cut at 55° for type-I phase-matching. The first ZGP crystal is pumped by a focused beam ($f = 100$ mm) with 5 µJ of pulse energy (22.5 GW/cm2 of peak intensity) to generate mid-IR radiation via OPG. The internal phase-matching angle is set to 53° in order to generate signal and idler wavelengths at 3.0 and 6.5 µm, respectively. An off-axis parabola (OAP) mirror ($f = 50$ mm) is used for collimation. A long-pass filter (LPF) separates the residual pump from the down-converted radiation. An additional band-pass filter (NB-3060-060nm, Spectrogon) with transmission around 3 µm (60 nm FWHM bandwidth, 80 % peak transmittance) is inserted into the beam to seed the subsequent OPA stage with a narrowband pulse, whose spectrum at 3 µm is nearly unaffected by water vapor absorption. An OAP mirror with focal length of 100 mm is used to focus the spectrally filtered signal from the OPG stage into the ZGP crystal of the OPA stage. The second ZGP crystal is collinearly pumped by up to 45 µJ of pulse energy from the picosecond 2 µm MOPA pump source. Pump and signal pulses are temporally synchronized via a delay stage. Another OAP mirror ($f = 100$ mm) collimates the three waves. A set of two LPFs with cut-on wavelengths of 2.4 and 3.6 µm is finally applied to separate them for output characterization.
Fig. 1. Schematic setup of the ultrashort pulse mid-IR laser system with the two different OPG concepts. LPF: long-pass filter, BPF: band-pass filter, OAP: off-axis parabola mirror.
In the second presented concept we replaced the 2-µm-pumped OPG stage by a 1-µm-pumped double-pass OPG/OPA setup integrating a folded spectral shaper. The applied pump laser is a previous version of the CPA-free MOPA laser used in the first concept, providing 30 µJ, 5.4 ps pulses at 2.05 µm wavelength and 20 kHz pulse repetition rate. The OPG/OPA setup is schematically depicted in Fig. 1 (OPG concept 2). First, about 4 µJ of pulse energy at a wavelength of 1.025 µm is generated via second-harmonic generation in a type-I phase-matched BBO crystal. This radiation is used to pump a double-pass MgO:PPLN-based OPG/OPA stage. In the first pass, a broadband signal spectrally tunable between 1.3 and 1.8 µm is achieved via OPG. The center wavelength can be tuned by adjusting the applied poling period of the fan-out MgO:PPLN crystal structure. The residual pump radiation at 1.03 µm is separated by a LPF. The spectral bandwidth of the signal is filtered to sub-20 cm−1 by propagating it through a folded, 4-f spectral shaper using a slit aperture in the Fourier plane [13]. The imaging system uses a lens with a 100 mm focal length and a reflection grating with a 800 lines/mm grating constant. After spectral filtering, both the residual pump and spectrally shaped signal beams are back reflected for the second pass in the MgO:PPLN crystal, amplifying the signal via OPA. Pump and signal pulses are temporally synchronized via a delay stage. The generated narrowband idler wave between 2.5 and 4.1 µm is then used to seed the 2-µm-pumped, ZGP-based booster amplifier stage, similar to the one described above in concept 1. After the booster OPA stage, the amplified wave tunable between 2.5 and 4.1 µm and the generated new idler wave with corresponding wavelengths between 4.1 and 12 µm are separated by LPFs and characterized.
3. Experimental results – 2-µm-pumped OPG/OPA concept
Here, we present the experimental results applying the first OPG concept (for details see Fig. 1). First, we investigate the phase-matching dependent spectral behavior of the generated radiation emitted by the ZGP-based OPG. 5 µJ of pump pulse energy at 2.05 µm are focused to peak intensities of 22.5 GW/cm2 into the 4.9 mm long ZGP crystal. The generated signal and idler spectra are shown in Fig. 2(a,b) in dependence of the internal phase-matching angle $\theta$ measured with a Fourier-transform infrared spectrometer (ARCoptix, FT-MIR Rocket 1.5 µm–8.5 µm) with a spectral resolution of 4 cm−1. The atmospheric absorption lines [14], which modulate the measured spectra, are displayed in grey. The obtained combined pulse energies of signal and idler as well as the respective energy conversion efficiency reaching a maximum of 25% at $\theta$ around 58° are illustrated in Fig. 2(c). Nevertheless, at this phase-matching angle, the signal and idler spectra are merged to one, making further use of solely the signal wave as seed in a multi-stage setup more complicated.
Fig. 2. Generated OPG signal (a) and idler (b) spectrum of the 2-µm-pumped, ZGP-based OPG stage (OPG concept 1) as a function of the phase-matching angle. (c) Energy conversion efficiency and center wavelength of signal (blue) and idler wave (red) of the OPG radiation in dependence of the phase-matching angle. Note that the sensitivity range of the used FTIR spectrometer spans from 2 to 8.5 µm.
For the further application in our OPG/OPA approach the OPG phase-matching $\theta$ is set to about 53° in order to obtain a signal spectrum at 3.0 µm which is well distinguishable from the respective idler spectrum. Furthermore, the signal center wavelength is selected well above the strong water absorption lines at 2.5 to 2.8 µm, since they could induce spatial and temporal distortions of the generated radiation. Figure 3(a) shows the generated OPG pulse energy (signal plus idler wave) as a function of the applied 2-µm pump intensity in the first ZGP crystal at a phase matching angle of 53°. The threshold of the OPG is determined to about 10 GW/cm2 and at higher pump peak intensities the generated pulse energy is linearly increasing up to 2.1 µJ at a maximum pump intensity of 45 GW/cm2. It is worth noting that laser-induced damage of the AR-coated entrance facet of the ZGP crystal occurred at pump peak intensities above 45 GW/cm2. As consequence, we limited the pump peak intensity to a maximum of 22.5 GW/cm2 in all following experiments. Figure 3(b) shows the spectrum of the generated signal (gray area) centered at a wavelength of about 3 µm and used for seeding the following OPA stage. The corresponding pump intensity was 22.5 GW/cm2 resulting in a respective pump pulse energy of 5 µJ. This signal wave was spectrally filtered to increase the spectral power density and, at the same time, improve the spatial coherence of the generated OPG signal (blue line in Fig. 3(b)).
Fig. 3. (a) Combined mid-IR pulse energy of signal and idler in dependence of the pump pulse peak intensity at a fixed OPG phase matching angle of 53°. (b) Corresponding OPG signal spectrum at a pump level of 22.5 GW/cm2 (5 µJ) (gray area) and the spectrally filtered OPG spectrum (blue line) used to seed the OPA stage.
Subsequently, it was used to seed the following OPA stage. The OPA signal and idler pulse energies in dependence of the pump pulse energy are depicted in Fig. 4(a) (solid lines). At a maximum pump pulse energy of 45 µJ the signal and idler pulse energies are 7.7 and 2.5 µJ, respectively. Note that the highest pump pulse energy corresponds to a peak intensity in the focus of 6.6 GW/cm2 well below the laser-induced damage threshold for our ZGP crystals. Taking into account a pulse repetition frequency of 100 kHz the maximum signal and idler pulse energies correspond to a combined mid-IR output power of $>1$ W and an optical-to-optical conversion efficiency of 22 %. The external quantum efficiency (EQE) is presented in the same figure (dotted lines). The maximum EQEs for the signal and idler pulses are 33 and 24 % at pump pulse energies of 15 µJ, respectively. With increasing pump pulse energy the parametric amplification process starts to saturate, which manifests itself through a dropping EQE as well as decreasing slopes of the signal and idler pulse energies.
Fig. 4. (a) OPA signal (blue) and idler (red) pulse energy as well as the corresponding EQE in dependence of the pump pulse energy. (b) Long-term stability measurement of the signal and idler output pulse energy at maximum pulse energy. Inset: Corresponding far-field beam profiles of the signal (left) and idler (right).
At the highest pump pulse energy, we measured the long-term stability of the mid-IR laser system. The measurement yields a root-mean-square (rms) power noise of $<1.3\,\%$ for both the signal and idler radiation over the course of 30 min (see Fig. 4(b)). We additionally measured the pointing stability as it is of major importance for e.g. industrial applications aiming for high-precision micromachining. While the signal pointing fluctuations are below 95 µrad for both the horizontal and vertical directions, the idler pointing is stronger with 130 (horizontal) and 360 µrad (vertical). The latter corresponds to a relative standard deviation of almost 10 % with respect to the idler beam diameter. We attribute the higher idler pointing to a dynamic spatial distortion of the beam due to water vapor absorption of the moving air inside the setup. Spatial and temporal distortions of ultrashort pulses during propagation in the presence of atmospheric molecular absorption have been investigated in detail in [15]. It has been shown that the spatial distortions can be significantly reduced by blowing air (by e.g. using a fan) over the propagation path of the beam. In contrast, a complete mitigation in the case of rather broadband ultrashort pulses can only be achieved in an atmosphere with very low absolute humidity (e.g. in a vacuum chamber or in a dry nitrogen atmosphere).
The pulse duration at maximum pump pulse energy was measured via an autocorrelation (AC) based on two-photon absorption (APE GmbH, PulseCheck MIR). Figure 5 presents the measured AC traces of the signal (a) and idler (b) pulses. The signal pulse duration with 3.9 ps (sech$^2$-fitted) is close to the initial pump pulse duration of 4 ps. In contrast, the idler pulse duration is slightly longer with 4.8 ps. We attribute this to some amount of temporal pulse distortion due to the water absorption in the idler spectrum [15]. The insets of Figs. 5(a) and (b) show the corresponding optical spectra of the signal and the idler. While the signal pulse spectrum is unaffected by the water absorption, the idler spectrum features clear dips that fit well with the molecular absorption data from HITRAN [14]. The full-width at half-maximum spectral bandwidths of the signal and idler pulses are 139 (125) and 169 cm−1 (714 nm), respectively.
Fig. 5. Measured autocorrelation (AC) traces (black solid lines) of (a) the signal and (b) the idler as well as the sech$^2$-fit (red dashed lines). The Fourier-limited pulse duration $\Delta \tau _{\mathrm {FL}}$ was calculated based on the corresponding optical spectrum given in the insets. The shoulder in the AC intensity of the idler at around −10 ps is a measurement artifact due to rather low signal-to-noise ratio.
4. Experimental results – 1-µm-pumped OPG/OPA, 2-µm-pumped booster OPA
In our second OPG/OPA concept, we substitute the single pass ZGP-based OPG-stage of the first concept, with a more elaborated seed generation scheme. Hereby we use a combination of a 1-µm-pumped, double-pass OPG/OPA-stage and a home-built, widely flexible spectral filter generating the seed, which is then amplified in a final 2-µm-pumped, ZGP-based booster amplifier (see Fig. 1 for details). This layout change features various advantages compared to the first demonstrated approach, allowing for the realization of a widely tunable light source. First, by using a 1-µm-pumped OPG-stage for seed generation and a 2-µm-pumped power amplifier, the required spectral seed tunability is separated from the degeneracy point of the OPG stage. Figure 6(a) shows the obtained OPG spectra in dependence of the phase-matching conditions when pumping the fan-out MgO:PPLN-based OPG stage with 4 µJ at 1.03 µm. The phase-matching conditions are adjusted by changing the applied phase-matching period in the fan-out crystal via a transverse movement. It can be seen, that the spectral tunability of the center wavelength is limited, when approaching close to the degeneracy point at 2.05 µm. Furthermore the spectral bandwidth changes dramatically from around 50 cm−1 at 1.35 µm center wavelength to more than 1000 cm−1 bandwidth when optimized for spectra around the degeneracy point. By using a 1-µm-pumped OPG seed generation stage, only spectra from 1.37 up to 1.75 µm are required for the further amplification process in order to obtain full mid-IR tunability at the end of the system. After the first pass of the OPG stage, the spectra are filtered in a folded, grating-based, 4-f spectral shaper setup with a slit aperture in the Fourier plane. The center wavelength of the spectral filter can easily be tuned by rotating the integrated grating, while the constant slit aperture size ensures a rather constant transmission bandwidth. Figure 6(b) shows the obtained OPG spectra after spectral filtering. All spectra feature a spectral bandwidth below 20 cm−1. Afterwards, the spectrally filtered OPG beam is passed in a second path through the OPG/OPA stage and is amplified by the also redirected, residual pump beam at 1.03 µm of the first path. This double-pass architecture enables a rather constant output power after the OPG/OPA stage, which would not be possible with a single-pass OPG architecture (see e.g. Figure 2(c)). The hereby generated idler wave, which is tunable from 2.5 to 4.1 µm, is used as seeding signal wave in the 2-µm-pumped, ZGP-based booster amplifier stage. This final OPA stage is pumped with 22.5 µJ pulses at 2.05 µm center wavelength.
Fig. 6. (a) Generated spectra of 1-µm-pumped, PPLN-based OPG-stage for different phase-matching periods of the PPLN crystal. (b) OPG spectra after spectral filtering via a tunable, folded spectral shaper. Spectral tuning is obtained by adjusting the generated OPG spectrum (see Fig. 6(a)) and the spectral shaper. The higher background noise for the OPG spectrum centered at around 1.63 µm is a measurement artifact due to normalization in combination with comparably lower signal-to-noise ratio.
The generated pulse energies of the signal and idler pulses at the output of the complete OPG/OPA scheme as function of the center wavelength are shown in Fig. 7(a). The output wavelength can be tuned from 2.5 to 12 µm, covering the entire mid-IR part of the transparency window of the nonlinear amplification crystal ZGP (transparency from 0.74 to 12 µm [16]). The maximum output pulse energy of 3.3 µJ is achieved at 2.7 µm center wavelength. Furthermore, the output pulse energy reaches $>1$ µJ for center wavelengths of up to 8 µm. In the inset of Fig. 7(a) the collimated beam profile at 6.1 µm output wavelength is shown. The external, overall optical-to-optical quantum efficiency of the parametric amplification process is around 8 % and higher over most of the tuning range (see Fig. 7(b)).
Fig. 7. (a) Generated pulse energy of signal (blue) and idler (red) wave after the 2-µm-pumped, ZGP-based OPA as a function of the center wavelength. Inset: Collimated beam profile at 6.1 µm output wavelength measured with a pyroelectric array camera (PyroCam IIIHR). (b) EQE of the amplification process in the main amplifier stage.
In Fig. 8(a) the respective measured output spectra of signal and idler waves are depicted. All spectra obtain Gaussian-like shapes with a spectral bandwidth at FWHM of below 20 cm−1 over the full tuning range (see Fig. 8(b)). This narrow bandwidth confirms that the provided spectral bandwidths after the home-built spectral filter are transferred throughout the consecutive parametric amplification processes to the generated mid-IR radiation. Furthermore, this paves the way for further spectral narrowing by increasing the spectral resolution of the 4-f spectral shaper and simultaneously reducing the aperture size in the Fourier plane of the spectral filter’s setup. Further narrowing of the here presented spectra where inhibited by the spectral resolution of the applied spectral shaper setup.
Fig. 8. (a) Generated spectra and (b) respective spectral bandwidth after the ZGP-based main OPA stage. Spectral tuning is obtained by adjusting phase-matching conditions of the double-path OPG/OPA crystal, of the main OPA crystal as well as by altering the transmission window of the spectral filter. Due to the longer emission wavelength the detector of the above-mentioned FTIR spectrometer was replaced by a module featuring a wider spectral range of 2–12 µm, which comes at the expense of less sensitivity.
Figure 9 shows the measured pulse durations of the mid-IR output pulses at various center wavelengths. All values are measured to be around 5 ps, yielding a time-bandwidth product of around 1.5 up to 2.5, depending on the wavelength. This relatively high time-bandwidth product (compared to transform-limited pulses) is not unusual for OPG seeded systems, since pulses generated via the OPG process obtain typically pulse durations similar to the pump pulse duration (in our case 5.4 ps at 2.05 µm) and their pulse durations are rather independent of the generated spectral bandwidth. Therefore one could assume that the pulse durations are around 5 ps over the full tuning range, yielding output peak powers in the range of around 0.1 MW at 11 µm up to 0.6 MW at 2.7 µm.
Fig. 9. (a) Measured AC trace at 5.7 µm center wavelength (black dots) as well as the sech$^2$-fit function (red dotted line). (b) Measured pulse duration of the output pulses as a function of the respective center wavelength.
5. Conclusion
We demonstrated and compared two different concepts for the generation of spectrally narrowband, picosecond pulses obtaining µJ-level energies in the mid-IR spectral region. Both concepts are based on the first combination of newly emerging CPA-free Ho:YLF MOPA lasers with parametric generation and amplification processes in the highly nonlinear ZGP crystal.
The first concept uses a 2-µm-pumped, ZGP-based OPG/OPA scheme for the generation of 7.7 µJ pulses at 3.0 µm and 2.5 µJ pulses at 6.5 µm wavelength. Considering the 100 kHz repetition rate, the combined maximum signal and idler pulse energies correspond to a mid-IR output power of $>1$ W. Compared to former reported approaches of ZGP-based OPG/OPA schemes, which used Ti:sapphire pump lasers and featured total optical-to-optical conversion efficiencies below e.g. 2 % [8], our concept achieves a high total optical-to-optical conversion efficiency (pump to combined signal-plus-idler pulse energy) of 22 % due to the direct pumping at 2 µm wavelength. Moreover, our concept is relatively simple and compact, due to the CPA-free layout of the Ho:YLF pump laser as well as of the frequency conversion scheme, providing high pulse peak intensities of about 2 MW for the signal wave at 3.0 µm and 0.5 MW for the idler wave at 6.5 µm wavelength.
The second presented concept uses a more complex seed generation scheme, but achieves superior spectral tunability for OPG-seeded concepts of more than 2.2 optical octaves from 2.5 up to 12 µm wavelength. Meanwhile, the narrow spectral bandwidth of below 20 cm−1 is maintained over the full spectral tuning range. This tunability is achieved by substituting the 2-µm OPG seed generation of the first concept, by a 1-µm-pumped, double-pass OPG/OPA stage including an intermediate home-built spectral filter. Since the consecutive booster amplifier stage is still a 2-µm-pumped ZGP OPA, the overall optical-to-optical conversion efficiency reaches values up to 10 %. The system features pulse energies at µJ-energy level over a majority of the tuning range with a maximum of 3.3 µJ at 2.7 µm center wavelength.
We believe that the presented laser systems will pave the way towards low-cost industrial applications such as material processing as they are compact, efficient and easy to align due to a simplified CPA-free architecture. While the first concept offers very high conversion efficiencies at constant output wavelengths, the second concept provides a wide spectral tunability. The demonstrated mid-IR pulse energy as well as average output powers could further be scaled by more than one order of magnitude, using mJ-energy level 2-µm pump sources. Such high-energy laser systems have recently been demonstrated by adding to the here used CPA-free Ho:YLF MOPA amplification approach an additional two-crystal, Ho:YLF-based, single pass amplifier stage [17].
Funding
Bundesministerium für Bildung und Forschung (13N13974 NUKLEUS).
Disclosures
The authors declare no conflicts of interest.
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