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Abstract
We report an optical parametric amplifier (OPA), providing a maximum pulse energy of ∼200 µJ at 700–950 nm and a pulse duration of ∼1 ps. The OPA is driven by a ∼1 ps pulse with ∼2.5 mJ energy at 1 kHz, provided by a commercial thin-disk based laser. Using the output pulse of the OPA as pump, the thin-disk laser pulses at 1030 nm as Stokes, and the second harmonic (515 nm) as probe, we investigate the coherent anti-Stokes Raman scattering (CARS) of N2 and CO2 at various temperatures.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Coherent anti-Stokes Raman spectroscopy (CARS) is a powerful tool for both thermometry and concentration measurements in combustion diagnostics [1,2]. Originally, CARS was used with ns-lasers and requires three beams: the pump, Stokes and probe beam with at least two different wavelengths. For high pressure applications, the use of ultrashort pulses is of additional benefit, as the CARS signal is free from distortions from molecular collisions, if the excited Raman level is probed within a few ps [3]. However, too short pulses (∼100–500 fs) result in poor spectral resolution [4]. Hybrid fs/ps-CARS [5–12] uses a spectrally narrowed probe pulse to provide sufficient spectral resolution. Thus, pulse durations around 1 ps are very promising.
N2 CARS is a typical means of measuring the gas phase temperature. Lucht et al. [3] have shown that the gas phase temperature and concentration measurement can be achieved from the magnitude and delay of the initial Raman coherence induced in nitrogen using fs CARS with 45 fs pulses. Yet, in air fed reacting environment, there are contributions not only from N2 but also from other species like CO. Roy et al. [13] investigated the effects of polarization interference between N2 (∼2330 cm-1) and CO (∼2145 cm-1) on the coherence dephasing signal after impulsive excitation with femtosecond laser pulses.
Another example is combustion diagnostics in CO2-O2 atmospheres. Here, one of the major species CO2 (Fermi dyads at 1285 cm-1 and 1388 cm-1) is simultaneously excited with O2 (∼1556 cm-1), where the Raman transitions differs only slightly. The polarization beating pattern makes the evaluation of the temperature measurement difficult [14]. Therefore, in order to investigate the characteristics of Raman transitions of various gases (e.g. CO2, CO, N2, CH4 and H2), a widely tunable, narrow spectral bandwidth (picosecond pulse duration) laser system providing pump, Stokes and probe beam is beneficial.
To provide three beams with different wavelengths, an optical parametric amplifier (OPA) is typically used. An OPA uses a nonlinear optical crystal with a large second order susceptibility to transfer energy from a fixed frequency pulse to two output pulses with lower photon energy. While fs-OPAs are typically based on Ti:sapphire [15–17] and are commercially available off-the-shelf, there is a lack of systems, which provide ps output pulses. A prerequisite for this is a ps pulse laser system, typically based on Yb:doped gain media. These systems are stable and reliable and have a high potential for scalable average and peak power [18–24].
In this study, we present an OPA in combination with a commercial thin-disk based 1 kHz laser system, providing a pulse energy of ∼200 µJ at 700–950 nm and a pulse duration of ∼1 ps over the full tuning range. Due to the high pulse energy and an almost constant spectral bandwidth around ∼18 cm-1, this setup is well-suited as a ps-CARS system for moderate pressure gas-phase analysis. We prove the feasibility with CARS measurements on N2 and CO2 at different temperatures.
2. Experimental setup
The pump source for the OPA is a thin-disk amplifier (Dira 500-10, Trumpf Scientific Lasers GmbH + Co. KG, Germany), providing ∼1 ps pulses at 1030 nm with a maximum pulse energy of 60 mJ at a repetition rate of 1 kHz. The laser beam has a Gaussian shape with a diameter of 12.7 mm (1/e2). A scheme of the experimental setup of the OPA is shown in Fig. 1. For the experiments, the pulse energy of the pump source is set to 8.9 mJ. A beam splitter (BS1; 50:50) is used to split the pump laser pulses into two branches. The transmitted pulse is used to drive the OPA, while the reflected pulse is used as Stokes beam for the CARS experiments (see section 4). To obtain comparable spectral widths of the OPA output and the thin-disk pump laser source, an ultra-steep long-pass edge filter (LEF) is used in front of the OPA to cut off roughly half of the original spectrum. After the LEF filter, 2.5 mJ of the laser pulse energy at 1030 nm is used to drive the OPA.
Fig. 1. Scheme of the optical parametric amplifier system. BS1: Beam splitter with R:T = 50:50. LEF: ultra-steep long-pass edge filter. λ/2: half-wave plate. P: polarizing beam splitter. SHG1 and SHG2: BBO nonlinear crystals. Telescopes: fL1:fL2 = 5:1, fL8:fL9 = 1:3.33, fL12:fL13 = 1.5:1, fL10:fL11 = 5:3. D1-D2: motorized delay stage. DM1-DM5: dichroic mirrors.
Inside of the OPA, the beam size is reduced by a telescope (5:1), before the pulse is split by a half-wave plate and a polarizer for the use in two different stages. The transmitted pump pulses are frequency-doubled in a BBO-based second harmonic generation (SHG) stage to 515 nm which act as pumping pulses for the subsequent parametric amplification processes. The seed spectrum is generated by phase-matched optical parametric generation (OPG) in an OPG/OPA double pass setup. Between the first and second path, the spectrum is significantly narrowed via a folded 4-f shaper with an adjustable slit aperture in the Fourier plane. The grating of the shaper is rotatable to enable a wide tunability of the transmitted center wavelength. In the main OPA stage (OPA2), the reflected pulses by the polarizer (P) generate the main SHG pump beam in a BBO nonlinear crystal (SHG2). The seed pulse and the main SHG pump beam are combined by a dichroic mirror (DM3). After the double path OPG/OPA-stage, the seed pulse energies of µJ levels are amplified in the main OPA stage to above 100 µJ.
3. OPA performance
The initial spectrum of the Trumpf pump laser and the spectrum after the long-pass edge filter are shown in Fig. 2. The spectral widths before and after the edge filter are 1.74 nm and 0.79 nm, and the corresponding powers are 5.14 W and 2.50 W, respectively.
Fig. 2. Spectra before and after the long-pass edge filter. One ultra-steep long-pass edge filter is tilted to a certain angle which cuts off roughly half of the initial pump spectrum to narrow the OPA input spectrum and elongate the input pulse duration.
The pulse durations of the thin-disk pump laser and the OPA output signal are measured with a SHG-autocorrelator (Pulse check, APE Angewandte Physik & Elektronik GmbH, Germany). Figure 3 shows the corresponding results yielding pulse durations of 1020 fs for the pump and 885 fs at 900 nm as an example.
Fig. 3. Autocorrelation measurement of the thin-disk pump laser and the OPA output at 900 nm. Assuming a sech2 pulse shape, the pulse duration of the pump laser is 1020 fs and 885 fs for the OPA.
Over the tuning range of the OPA, the measured spectral width is shown in Fig. 4. Due to the combination of the robust seed generation via OPG together with an adjustable narrowband spectral shaper, the signal output spectrum of the OPA system is tunable over a wide range from 700 to 950 nm with an almost constant spectral bandwidth of 17 cm-1 to 20 cm-1 over the entire tuning range.
Fig. 4. Spectral width of the signal output of the OPA over the tuning range yielding a spectral bandwidth between 17 to 20 cm-1.
The OPA output energy is 140 - 290 µJ at 1 kHz over the full tuning range, as presented in Fig. 5. The energy stability of the 900 nm pulse is 1.1% (RMS, 3,600,000 pulses). The combination of the constant spectral bandwidth and high energy output makes this OPA system an attractive source for CARS-pump/probe investigations for concentration and thermometry measurements.
4. Ps-CARS measurements of N2 and CO2
CARS is basically a four-wave mixing process, in which the pump and Stokes pulses with frequencies ω1 and ω2 coherently excite ro-vibrational molecular levels, when the energy difference between ω1 and ω2 matches the Raman shift of the molecule. Then, the probe pulse at frequency ω3 gives rise to the CARS signal at frequency ω4 = (ω1-ω2+ω3).
In addition, the phase matching condition has to be fulfilled. This can be achieved in the so-called “BOXCARS” configuration [25], where the three beams have defined angles. In gases, phase-matching is also fulfilled when all beams are collinear. Usually, BOXCARS is preferred, as the CARS signal is spatially separated from the other beams. In this study, we nevertheless choose the collinear setup to avoid additional adjustment of the phase matching angle for the Raman transitions under investigation. In our collinear CARS setup (Fig. 6 (a)), the output of OPA acts as pump, the fundamental wavelength of 1030 nm as Stokes and the SHG at 515 nm as probe. The CARS setup with the OPA signal output beam acting as pump is shown in Fig. 6 (b). Using a 50/50 beam splitter (BS2), the reflected 1030 nm beam from the BS1 beam splitter is split into two beams, with one acting as the Stokes beam. The other beam acts as probe after frequency doubling to a wavelength of 515 nm by passing through a 1.5 mm BBO crystal. A short-pass filter (10SWF-1000-C, Newport) is used to filter out the remaining fundamental wavelength. A long-pass dichroic mirror DM6 (DMLP650, Thorlabs) is used to spatially overlap the pump and probe beams. The pulse energy of the probe is kept at ∼30 µJ. The Stokes beam is sent through a half-wave plate and a polarizing beam splitter to control the pulse energy, which is kept at ∼100 µJ. The beam size of the Stokes beam is reduced to one third (4.2 mm) of the original beam size by a telescope (T; 3:1). Furthermore, the temporal overlap between the three beams is adjusted with two motorized delay stages (D3 and D4) in the probe and Stokes beam paths. In this study, a probe pulse delay of ∼1-2 ps is chosen to avoid contributions from the nonresonant background. Pump, probe and Stokes beams are spatially overlapped by a dichroic mirror (DM7) and focused into a heated gas cell by an f = 150 mm focusing lens (L14). The gas cell is described in detail in previous work [26]. The generated CARS signal is collimated by a subsequent lens (L15) with a focal length of 200 mm and coupled into a spectrometer (Shamrock 500i with Marana CMOS camera, Oxford Instruments plc, UK). The spectrometer acquisition time is 100 ms.
Fig. 6. (a) Sketch of four-wave mixing process with the output of the OPA acting as pump, the fundamental wavelength at 1030 nm as Stokes and the second harmonic at 515 nm as probe pulses. (b) CARS experimental setup. BS1-2: 50:50 beam-splitter. T: telescope to reduce the beam size with proportion of 3:1. λ/2: half-wave plate. P: cubic polarizing beam splitter. Filter: short-pass filter with a cutoff wavelength of 1000 nm. D3-D4: motorized delay stage. L14-16: focusing lens with f = 150 mm, 200 mm and 300 mm, respectively. LDM: long-pass dichroic mirrors with 490 nm cut-on wavelength. ND: neutral density filter.
With an almost constant spectral bandwidth around ∼18 cm-1 across the whole tuning range, we are able to resolve different molecules with this ps-CARS setup. In the air-fed combustion scenarios, N2, O2 and CO2 are the main products, which are also widely used to investigate in-situ temperatures. In a proof-of-principle experiment, we will thus focus on the time resolved CARS signals of N2 and CO2. In this work, the gas cell is filled either with N2 or with CO2 at a constant pressure of 3 bar at room temperature, 250 °C and 500 °C, respectively.
N2 CARS thermometry is based on the population difference of the selected resonances of rovibrational transitions in the ground state of N2 molecules. However, at room temperature only the vibrational ground state υ=0 is populated, corresponding to a Raman shift of Δυ∼2330 cm-1. The first hot band of N2 (Δυ∼2300 cm-1 for υ’=2→υ’’=1) typically appears at temperatures of ∼1000 K [27,28]. In this work, we focus on the temperature determination with the fundamental and first hot band of N2.
At first, we tuned the OPA wavelength to 831.1 nm and adjusted the probe delay time at ∼1 ps to detect the CARS signal of N2 at a Raman shift of Δυ∼2330 cm-1 (υ’=1→υ’’=0) for a pressure of 5 bar at room temperature. With the constant spectral bandwidth of ∼18 cm-1, it is sufficient to resolve different vibrational levels of N2 (e.g. Δυ∼2300 cm-1 for υ’=2→υ’’=1). In order to detect the first hot band (υ’=2→υ’’=1), we tuned the OPA wavelength to 833.2 nm and heated the gas cell up to the maximum temperature of 720 °C. As shown in Fig. 7, the CARS signals of the fundamental and the first hot band of N2 are both recorded at a probe delay of ∼1 ps at 20 °C (black lines) and 720 °C (red lines). The dash dotted line and the dotted line are the recorded signals at the Raman shift of 2330 cm-1 and 2300 cm-1, respectively. The intensity ratio between the first hot band and the fundamental peak is ∼0.2 (Fig. 7), which fits well to recent literature values at ∼1000 K (∼ 727 °C) [29]. In a recent study, Lauriola et al. [30] describe a burst-mode N2 ps-CARS system for high-speed flame thermometry at 100 kHz based on a customized OPG/OPA. Using a ps-Stokes pulse (∼75 ps) and a narrowband 532 nm pump/probe pulse (∼107 ps), accurate temperature measurements are achieved from 1800 to 2225 K at atmospheric pressure. In contrast, the OPA used in our study provides pulse durations of ∼1 ps and aims at a compromise between short pulses and high spectral resolution for elevated pressures.
Fig. 7. The vibrational Raman spectrum of N2 at various temperatures. The probe delay time is set at ∼1 ps to avoid nonresonant signal. The gas cell was filled with N2 at a pressure of 5 bar.
Roy et al. [27] extracted temperatures from a comparison between the theoretical and experimental probe-delay scans of the N2 CARS. Thus, the probe time delay dependence of the CARS signal is of great interest, too. Here we show one example of the measured CARS signal intensity corresponding to the probe delay scan at a temperature of 20 °C (Fig. 8). The step size of the temporal delay scan is 100 fs. In fs-CARS, thermometry is based on the excitation of closely spaced molecular transitions of a defined molecular species, but the large spectral bandwidth of fs-pulses may also excite transitions of species with nearby Raman shifts. This could lead to beating patterns in the time dependent Raman decay, which was observed for N2-CO- and O2-CO2-mixtures [13,14], where the Raman shifts of the molecules in the mixture differ by ∼200 cm-1. Here, we show that ps-CARS may provide both sufficient spectral resolution of ∼30 cm-1 (Fig. 7) to avoid simultaneous excitation of different molecules and the capability to probe the Raman decay (Fig. 8).
In contrast to N2, CO2 is a linear symmetric triatomic molecule, which has four vibrational modes: the symmetric stretching mode (quantum number υ1), the doubly degenerate bending mode (υ2) and the antisymmetric stretching mode (υ3). In the doubly degenerate bending mode υ2, one needs an extra angular momentum quantum number l to specify the quantum state. Due to the unperturbed vibrational frequency υ1 ≈ 2υ2, this leads to the Fermi-resonance effects (so-called Fermi dyads), yielding perturbed levels. Due to the relative low energy of the first excited bending mode (0,11,0), it is already populated at room temperature, which enables measurements of lower temperatures compared to N2. Thus, gas phase temperature measurements of CO2 are of great interest. In this work, we mainly consider the vibrational transitions corresponding to Raman shifts of 1388 cm-1 and 1409 cm-1 [31] for thermometry.
In order to generate the corresponding CARS signals of CO2 at Raman shifts of 1388 cm-1 and 1409 cm-1, we tuned the OPA output beam to wavelengths of 901.7 nm and 899.9 nm, respectively. Then we measured the two peaks at room temperature, 250 °C and 500 °C with a pressure of 3 bar in the gas cell.
The spectra of CO2 at Raman shifts of 1388 cm-1 in solid straight line and 1409 cm-1 in dash dot line at the temperatures of 20 °C, 250 °C and 500 °C are shown in Fig. 9. To avoid contributions from the nonresonant signal, the spectra are recorded at a probe delay time of 2 ps. At room temperature we can hardly observe the peak for the Raman shift at 1409 cm-1 due to the strong signal strength at the Raman shift of 1388 cm-1. With increasing temperature the small peak of the CARS signal at 1409 cm-1 becomes stronger due to the higher population of the first excited bending mode (0,11,0) [26]. At 500 °C, the intensity ratio between the two peaks is ∼0.7 (green lines in Fig. 9), whereas we measured ∼0.6 in a previous study, using ultrabroadband excitation and a slightly larger probe pulse duration of ∼1.5 ps ([7], Fig. 3). We attribute this discrepancy to the large overlap of the CARS-peaks at 1388 and 1409 cm-1, whereas the peaks are clearly separated for N2 (Fig. 7). The energy splitting between the fundamental peak and the first hot band is only slightly different for CO2 and N2 (∼21 cm-1 vs. 30 cm-1). Yet, a recent study on self-phase modulation (SPM) induced by 60 fs pulses shows that this effect is considerably larger for CO2 than for N2 [32], but longer pulse durations should reduce this effect [33,34]. Thus, our results clearly show that the choice of the pulse duration around 1 ps is crucial, and further experimental and numerical investigations are required on the influence of the pulse energy, pulse duration and focusing conditions on the CARS spectrum, especially for CO2.
Fig. 9. CARS signals of CO2 with a pressure of 3 bar at different temperatures. Measured CARS signal strengths of CO2 at Raman shifts of 1409 cm-1 and 1388 cm-1 at temperatures of 20 °C, 200 °C and 500 °C are presented.
5. Summary
In summary, we demonstrate an OPA with a commercial thin-disk based 1 kHz laser as pump source, providing a pulse duration of ∼1 ps across the full tuning range 700–950 nm. With this OPA, we show a customized experimental setup for CARS measurements of gases at moderate pressure and high temperature. The ps-system provides sufficient spectral resolution to differentiate the fundamental and first hot bands in N2 and CO2. Nevertheless, it allows probing the decay of the Raman coherence for thermometry. Further investigations will focus on comparing the accuracy of thermometry and concentration measurements with the ps-OPA, considering possible side effects such as SPM.
Funding
Bundesministerium für Bildung und Forschung (03Z1H535); Thüringer Ministerium für Wirtschaft, Wissenschaft und Digitale Gesellschaft (2018 FGI 0045).
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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