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Abstract
We experimentally explored the design parameter space for periodically poled ${\rm Rb}:{{\rm KTiOPO}_4}$ (PPRKTP) structures in type-II optical parametric oscillators (OPOs) pumped at 1.064 µm and operating in the 1.8–2.5 µm range. This study identifies a set of Sellmeier expansions and thermooptic coefficients that can be used to accurately design the type-II PPRKTP structures in this spectral range. The performance of this OPO regime is investigated by employing different periodicities of PPRKTP, and the effective nonlinear coefficient is determined. Output at the mJ level with a bandwidth of better than 10 MHz is readily generated without optical damage. The temperature tuning rate of about 8 GHz/K of the central OPO frequency is substantially lower than in similar periodically poled lithium niobate devices and can be exploited for precise frequency tuning and easier thermal stabilization. The stimulated backward polariton scattering in the OPO cavities is also investigated and discussed.
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1. INTRODUCTION
Coherent and bright mid-IR sources near 2 µm attract attention due to numerous potential applications—among others, in molecular spectroscopy [1], lidars [2–4], and clinical medicine [5]. Molecular spectroscopy in this range relies on resonant overtones, where greenhouse gases (GHGs), such as carbon dioxide (${{\rm CO}_2}$), methane (${{\rm CH}_4}$), and water vapor (${{\rm H}_2}{\rm O}$, HDO), are of particular interest and contribute to the Earth’s radiation budget and global warming [1,3,6]. Monitoring these GHGs by differential absorption lidar (DIAL) using airborne and space-based platforms requires nanosecond transform-limited pulses with certain precision; as for ground-level ${{\rm CO}_2}$ distribution measurement with an expected 0.4% accuracy, the precision of the central frequency should be better than 1 MHz [6–8]. Additionally, DIAL systems require a source that detunes the radiated frequency between the selected absorption line and background, i.e., typically detuning of tens of GHz [6,8]. Wide tunability of optical parametric oscillators (OPOs) throughout near- and mid-infrared spectral regions [9,10] around 2 µm, relevant for GHG detection, makes them suitable for applications in lidar emitters. With judicious OPO design and seeded $Q$-switched laser pumping, OPOs can produce transform-limited, wavelength-agile nanosecond pulses suitable for DIAL sensing instruments [5,11]. Energy upscaling for long-range monitoring, as needed for airborne and space-borne platforms, can be implemented by employing the optical parametric amplifier stage [12]. Here we limit our discussion to systems pumped by $Q$-switched Nd:YAG lasers, operating at 1064 nm, which are robust, commercially available, and suitable for operation in the space environment encountered in the low Earth orbit.
For the generation of transform-limited nanosecond pulses in OPOs, it is often necessary to resonate both the signal and idler. Doubly resonant OPO (DRO) has a lower oscillation threshold and allows for precise signal and idler tuning by Vernier longitudinal mode selection. For instance, such a concept is used in the nested cavity OPO design [11], which has been proven in narrowband lidar applications. For such cavity designs, spatial walk-off is detrimental, making quasi-phase matching (QPM) [13–15] the preferable approach. Using birefringence phase matching and the spatial walk-off compensation [16] in DROs makes the overall design more complicated and, potentially, less stable.
Fig. 1. Schematic of the OPO setup. M1, M2, OPO cavity mirrors; M3, pump separator mirror; HWP, half-wave plate; TFP, thin film polarizer. The $x {-} y {-} z$ coordinate frame relates to the PPRKTP crystal axes.
Highly efficient QPM-based OPOs typically utilize the largest nonlinearity (${{d}_{33}}$—available only through QPM) along the polar axis of ferroelectrics such as lithium niobate (LN) [17] and $\rm Rb:KTiOPO_{4}$ (RKTP) [14] and type-0 QPM interactions. However, close to degeneracy, similar signal dispersion and idler dispersion leads to the large acceptance bandwidth in type-0 interactions [18]. Similar free spectral ranges for the signal and idler in doubly resonant cavities make narrowband OPO operation impossible without employing additional intracavity elements [18,19]. By using type-II QPM, polarization degeneracy can be removed even at wavelength degeneracy, enabling simple narrowband DRO cavity designs. That comes at the cost of lower effective nonlinearity leading to the need for higher pump intensities at the OPO threshold. To choose the optimum material for type-II OPO, we need to consider the figure of merit (FOM), ${\rm FOM} = d_{{\rm eff}}^2/{n_p}{n_s}{n_i}$, and the laser-induced damage threshold. Here, ${d_{{\rm eff}}},{n_p},{n_s},{n_i}$ are the effective nonlinearity and indices of refraction at the pump, signal, and idler wavelengths, respectively. In type-II OPO pumped at 1 µm and operating close to degeneracy, periodically poled RKTP (PPRKTP) should be preferable to PPLN owing to a large damage threshold (${10 - 12}\;{{\rm J/cm}^2}$ [20] and 50% larger FOM [21–24]). At the same time, the coherence length for 1 µm pumped type-II parametric interactions is several times larger in RKTP compared to LN, leading to larger periodicities of the QPM gratings and substantially easier fabrication of large-aperture nonlinear crystals. This opens up the possibility of easier pulse energy scaling directly in the OPO stage. Moreover, this material can be used to realize backward-wave OPOs for narrowband mid-infrared generation suitable for molecular sensing [25].
Normally, two parameters can be used for precise targeting of OPO wavelengths to coincide with the molecular absorption lines: the QPM period and crystal temperature. In type-0 OPOs, this works very well, and small inaccuracies in the refractive index dispersion leading to small inaccuracies in the QPM periodicity can be compensated for by adjusting the crystal operating temperature. Unfortunately, in type-II PPRKTP OPOs, such a strategy is not very promising because thermooptic coefficients of the ${{ n}_y}$ and ${{n}_z}$ indices are very similar, leading to a large temperature acceptance bandwidth. Therefore, the temperature tuning is slow, and precise targeting of OPO wavelengths needs to be accomplished by increased precision in designing QPM periodicity. The Sellmeier equations published so far give widely different QPM periods, making precise QPM design for a specific wavelength in type-II PPRKTP OPOs difficult.
In this work, we address this gap in the precise knowledge of material parameters in PPRKTP pertaining to type-II interaction by fabricating a series of QPM structures with different periodicities that, according to preliminary estimates, would cover the degeneracy region in OPOs pumped at 1064 nm. The experiments were performed with all structures under the same pumping and temperature-controlled conditions. The measurements allowed us to identify Sellmeier expansions and thermooptic coefficients that can be used for precise targeting of molecular absorption lines of GHGs. The measurements showed that the effective nonlinearity of the structures is close to those expected from the known values of the ${{d}_{24}}$ coefficient in KTP. We also discuss the role of stimulated backward polariton scattering in the OPO cavities resonating $z$-polarized signal waves. The findings of this work will be important for designing narrowband tunable sources for lidar emitters and for developing spontaneous parametric generation sources for quantum optical applications that employ type-II interactions in PPRKTP [26–28].
2. EXPERIMENT
Figure 1 depicts the experimental setup, where the pump is polarized along the crystallographic $y$ axis of the PPRKTP.
A half-wave plate and a thin-film polarizer were used for pump power control. The OPO is a simple 22 mm linear semi-hemispherical cavity defined by the concave mirror M1 [radius of curvature (ROC) = 150 mm] and plane mirror M2. The OPO cavity resonated at the signal wave, except for the operation close to degeneracy, where both the signal and idler were resonant (the measured reflectivity spectra of the cavity mirrors are shown in Fig. 6). Both cavity mirrors were highly transparent at the pump wavelength. A dichroic mirror, M3, separates the depleted pump from the generated parametric wavelengths. The pump is an injection-seeded $Q$-switched Nd:YAG laser (InnoLas Laser GmbH) delivering transform-limited 12 ns (FWHM) pulses up to 250 mJ at 1064 nm at a 100 Hz repetition rate. A Galilean telescope collimated the pump to a beam waist radius (${{1/e}^2}$) of 142 µm inside the OPO cavity. The pump was propagating along the $x$ axis of PPRKTP crystals. Three PPRKTP crystals were employed, each with two QPM gratings, similar to [29]. The crystals were fabricated by the standard electric field poling technique [30]. The transition between the two gratings was done by translating the PPRKTP crystal along the $y$ direction. The QPM grating periods, $\Lambda$, were 50, 55, 60, 70, 80, and 95 µm, with structure lengths of around 7.5 mm. All crystals were AR coated for the pertinent wavelengths. They were mounted on Cu blocks bonded to Peltier elements for fine temperature control with an accuracy of 0.1°C. An optical spectrum analyzer (OSA, Yokogawa, AQ6376) was used to measure the spectra of the OPO beams with a resolution of 0.1 nm. The orthogonal polarization between the signal and idler was confirmed using a wire grid polarizer with an extinction ratio of 1000:1 before the OSA. As expected, the OPO signal was $z$ polarized for shorter QPM periods before degeneracy, while it had $y$ polarization for longer QPM periods (see schematic notation in Fig. 1).
Figure 2(a) shows the total signal and idler output energy of the OPOs as measured for all QPM structures. A linear fit is used to extrapolate the oscillation threshold. The conversion efficiencies versus incident pump energy for the six gratings are displayed in Fig. 2(b). The lowest OPO threshold was obtained for PPRKTP crystals with QPM periodicities of 55 and 60 µm. The OPO operates close to degeneracy for these crystals, where the reflectivities of the cavity mirrors for the idler wave are substantial. Therefore, these OPOs should be treated as DROs. The difference in the thresholds for singly resonant OPOs (SROs) is due to different reflectivities of the mirrors for different signal wavelengths, slightly different lengths of the QPM gratings, and the quality of the QPM gratings. We used the pump fluence at the threshold to estimate effective nonlinearity in SROs by employing theoretical expressions from [9]. The details are provided in Table 1. The bounds in the ${{d}_{{\rm eff}}}$ values are estimated from the uncertainties in the measurements of power (2.5%), cavity length ($\pm {0.2}\;{\rm mm}$), mode size ($\pm {5}\;{\unicode{x00B5}{\rm m}}$), crystal length ($\pm {0.05}\;{\rm mm}$), wavelength ($\pm {0.1}\;{\rm nm}$), QPM grating length [${\pm}\Lambda$ (one domain period)], and pulse length ($\pm {0.25}\;{\rm ns}$). The full bound is the quadrature sum of the seven considered independent error origins. The largest ${{d}_{{\rm eff}}}$ value of 2.38 pm/V is close to the expected value of 2.32 pm/V for the first order QPM [21]. We did not attempt to estimate the nonlinearities in DRO cavities due to substantial fluctuations in the threshold pump fluence related to the phase drift in unstabilized DROs. This estimated nonlinear coefficient is similar to that for PPLN of 2.8 pm/V [24]. However, higher refractive indices in PPLN result in lower nonlinear drive and require type-II nanosecond OPO operation close to the optical damage threshold.
Fig. 2. Total output energy (a) and total conversion efficiency as a function of incident pump energy (b).
Table 1. OPO Wavelengths and Evaluation of Effective Nonlinear Coefficients
The temporal profile of the incident pump, depleted pump, signal, and idler in $\Lambda = {60}\;\unicode{x00B5}{\rm m}$ grating at 25°C are shown in Fig. 3(a). The depleted pump was measured after the OPO with a Si detector with a rise time of smaller than 1 ns using a 4 GHz oscilloscope. The OPO was misaligned after the temporal measurement to give a correct impression of the depletion. The idler and signal are shown in the same figure, measured by a HgCdTe photo electromagnetic (PEM) detector (Vigo Photonics) after the wire-grid polarizer. The traces have been recorded at the maximum pump energy of 10 mJ, showing the depletion and cascading processes. The OPO is operating in the DRO regime very close to wavelength degeneracy. A pyroelectric camera was used to record the beam profile of the idler and signal output separated by polarization at a distance of ${\sim}{300}\;{\rm mm}$ along the optical axis. The close to Gaussian spatial transverse electromagnetic mode profiles of signal and idler output at 60 µm grating are shown in Figs. 3(b) and 3(c). Beam factors ${\rm M}_x^2$ of 2.72 and ${\rm M}_y^2$ of 1.76 were measured.
Fig. 3. Temporal pulses of the incident pump (black), depleted pump (red), signal (blue), and idler (magenta) (a), and beam profile of signal (b) and idler (c) in grating with 60 µm period.
The measured spectra in OPOs with PPRKTP with periodicities of 50, 55, 60, 70, 80, and 95 µm are shown in Fig. 4. As expected, the spectra of the type-II OPO pumped by a transform-limited nanosecond pulse are narrow, even close to degeneracy. Here the spectral width gives a value of 0.1 nm, which is limited by the resolution of the OSA.
Fig. 4. Measured spectra of the OPO with type-II PPRKTP with QPM periodicity of 50 µm (a), 55 µm (b), 60 µm (c), 70 µm (d), 80 µm (e), and 95 µm (f).
When pumped at high intensity employing type-II PPRKTP with QPM periods of 50 and 55 µm, we observed a shifted OPO signal generated together with the OPO wavelengths. The spectra with a shift of 8 THz signal are shown in Fig. 5.
Fig. 5. OPO spectrum of both $y$ and $z$ polarizations (top row), $z$-polarized (middle row), and $y$-polarized (bottom row) in the 50 µm grating (a), and that in the 55 µm periodicity QPM grating (b). The $z$-polarized signal gives rise to the Stokes at 8 THz separation.
KTP is known to have two strong IR and Raman active A1 symmetry phonon modes at 8 and 21 THz, which effectively couple to the electromagnetic radiation polarized along the crystallographic $z$ axis, giving rise to a stimulated phonon–polariton scattering (SPS) [31]. It has been shown that the periodic poling structure of PPKTP suppresses the forward SPS [32]. Due to large polariton wave vectors involved in the backward SPS and large polariton absorption, the periodic poling on micrometer scale does not affect the backward SPS [33]. Close to the phonon resonance frequency, the 8 THz shift in this OPO regime is an indication of the counterpropagating SPS process, corresponding to the backward propagating $z$-polarized Stokes.
In PPRKTP OPOs with periodicities of 50 and 55 µm, both the OPO signal and the Stokes that are $z$ polarized would be resonated in the cavity, as can be seen in Fig. 6. This leads to the higher SPS efficiency; thus, SPS was easily observed. In our experiment, SPS Stokes was not observed in PPRKTP with a QPM period longer than 55 µm. In the OPO with 60 µm periodicity crystal, the SPS Stokes is at ${\sim}{2250}\;{\rm nm}$, as shown with a green dashed line in Fig. 6. This Stokes cannot be resonated in this cavity due to the low M2 mirror reflectivity; thus, the SPS was not observed. For periodicities larger than 60 µm, the wavelengths of the resonated $z$-polarized OPO output, which are possible to be coupled to the strongest A1 symmetry phonon modes, are even longer and out of the high reflectivity range of the cavity mirror. These observations have important consequences for choosing a type-II PPRKTP OPO configuration for GHG sensing applications. Suppression of backward SPS requires a judicious choice of cavity mirror reflectivity spectra and, in SRO configurations, the resonating of the $y$-polarized parametric wave.
Fig. 6. Reflectivities of cavity mirrors M1 and M2. Dashed lines denote the observed and expected positions of $z$-polarized SPS Stokes for OPOs employing PPRKTP with different periodicities: 50 µm (yellow), 55 µm (purple), and 60 µm (green).
Fig. 7. Measured 1064 nm pumped OPO wavelengths as a function of QPM periods (data points) for $y$- (a) and $z$-polarized OPO output at 25°C (b), Lines: calculations for different choices of Sellmeier expansions; deviation between experimental and calculated wavelength for $y$- (c) and $z$-polarized output (d). Simulation1: ${n_y}$ and ${n_z}$ from [22]; simulation2: ${{n}_y}$ from [22] and ${{n}_z}$ from [35]; simulation3: ${{n}_y}$ from [34] and ${{n}_z}$ from [36]; simulation4: ${{n}_y}$ from [34] and ${{n}_z}$ from [35]; simulation5: ${{n}_y}$ from [34] and ${{n}_z}$ from [22].
3. FITTING SELLMEIER EXPANSIONS AND THERMOOOPTIC COEFFICIENTS
The phase-mismatch $\Delta{k}$ for type-II OPO satisfies $\Delta {k} =\def\LDeqbreak{} {2}\pi ({{n}_p}/{\lambda _p} - {{n}_s}/{\lambda _s} - {{n}_i}/{\lambda _i}) - {2}\pi { /}\Lambda$, in which $\Lambda$ is the poling period of the crystal, and ${{n}_j}$ and ${\lambda _j}$ (${j} = {p},\;{s},\;{ i}$) are the refractive index and wavelength for the pump, signal, and idler, respectively. Predicting quasi-phase-matched wavelengths requires high accuracy of Sellmeier expansions. For type-II QPM devices, we need a set of two accurate expansions. There are many published Sellmeier expansions for ${{n}_y}$ and ${{n}_z}$ indices in KTP. Most of them are not accurate enough for designing QPM devices. Before proceeding with new fits, we decided to perform an exercise, a sort of meta-analysis of already published data, with the hope of finding the combination of already existing equations that would provide a good enough fit to our experimental data so that the QPM period for type-II PPRKTP OPOs could be predicted within a practical temperature tuning range.
The measured OPO wavelengths with QPM periods for $y$- and $z$-polarized waves at 25°C are shown by data points in Figs. 7(a) and 7(b), together with theoretical lines using Sellmeier expansions and thermooptic coefficients from different references [22,34–36]. The wavelength measurement error bars are within the size of the data markers. The deviations between experimentally measured wavelengths and the calculated results for $y$- and $z$-polarized waves are shown in Figs. 7(c) and 7(d). From Fig. 7, the best fitting combination at room temperature is simulation3, which uses the refractive index of $z$ polarization from Fradkin [36] and $y$ polarization from König [34]. Although the original dispersion relation in [34] was verified only for the 0.8–1.6 µm region, it gave the best fit to our data in the range of 1.8–2.5 µm as well. The modest deviation of simulation3 implies a good fit, within the accuracy of less than 0.12%. Simulation5 gives a similar good fitting deviation of 0.14%, with the refractive index of $z$ polarization from Kato [22] instead. This accuracy is within the typical fabrication tolerances, and fine targeting of the OPO wavelength, if needed, can be achieved by temperature tuning. For instance, at the wavelength of 2.5 µm, the expected error would be within 3.5 nm or 168 GHz, which could be covered by changing the crystal temperature by 21°C (see below).
We measured the OPO wavelength tuning for all PPRKTP crystals, which are shown as data points in Fig. 8(a). The deviations between experimentally measured and calculated $y/z$-polarized wavelengths in the temperature range of 20°–70°C with a step of 5°C, using different thermooptic coefficients [22,37] combined with Sellmeier expansion fit from the set of simulation3 and simulation5 are shown in Figs. 8(b)–8(e). In this whole temperature range, the best fitting is Fig. 8(b) using thermooptic coefficients from [37] and the Sellmeier expansion fit from the set of simulation5 ($z$ polarization from Kato [22] and $y$ polarization from König [34]) with a deviation of less than 0.2%.
Fig. 8. Measured wavelength at temperature 20°–70°C with a step of 5°C for different periodicities (a) and deviations between experimentally measured and different calculations of $y/z$-polarized wavelengths as function of temperature (b)–(e). (b) Results using the thermooptic coefficients from [37] combined with ${{n}_y}$ from [34] and ${{n}_z}$ from [22]; (c) results using the thermooptic coefficients from [22] combined with ${{n}_y}$ from [34] and ${{n}_z}$ from [22]; (d) results using the thermooptic coefficients from [22] combined with ${{n}_y}$ from [34] and ${{n}_z}$ from [36]; (e) results using the thermooptic coefficients from [37] combined with ${{n}_y}$ from [34] and ${{n}_z}$ from [36].
Fig. 9. Measured signal wavelength as function of temperature (data points) for different periodicities: 50 µm (a), 55 µm (b), 60 µm (c), 70 µm (d), 80 µm (e), and 95 µm (f). Lines: calculations for different periodicities. Red: results using the thermooptic coefficients from [37] combined with Sellmeier expansions (ny from [34] and ${n_z}$ from [36]); blue: SNLO results using KTP-F (${{n}_y}$ and ${{n}_z}$ from unpublished data, and the thermooptic coefficients from [38]); green: SNLO results using KTP-H (${{n}_y}$ and ${{n}_z}$ from [39] and the thermooptic coefficient from [38]); purple: SNLO results using KTP-K (${{n}_y}$, ${{n}_z}$, and the thermooptic coefficients from [22]).
This best fitting of signal wavelength for all PPRKTP crystals has been calculated and shown in the red solid line closely aligned with the measurement data in Fig. 9 without any additional fitting parameters. We also include calculation results using a widely used software package, SNLO [40]. The Sellmeier expansions used in the SNLO package do not allow precise prediction of the QPM wavelength for a type-II PPRKTP OPO. The temperature tuning rate is the same one of the SNLO datasets as in our best fit. That dataset uses the same thermooptic coefficients from [22]. In general, a type-II PPRKTP OPO in this spectral range has a relatively slow tuning rate of about 8 GHz/K. For comparison, the type-II PPLN tuning rate is about 44 GHz/K in this spectral range [23,40]. A slower temperature tuning rate in type-II PPRKTP OPOs is beneficial for precisely targeting GHG absorption lines and for the overall thermal stability of the emitter.
4. DISCUSSION AND CONCLUSION
Narrowband OPOs, especially those employing nested cavity design [11] and used for GHG sensing, would need to cover on and off wavelengths for particular molecular species in the single device using fast tuning. Cluster hopping could be one such tuning mechanism if the gain bandwidth of the nonlinear crystal is sufficient. Table 2 lists the most important GHG species and their on- and off-wavelengths in the 2 µm spectral range based on the HITRAN database [41]. The on–off frequency difference is a few tens of GHz, at most. The parametric gain bandwidth of about 300 GHz cm in PPRKTP in this spectral range is similar to that in PPLN. Therefore using a single QPM period, the OPO could be tuned between on- and off-line wavelengths.
Table 2. Spectral Lines of Multiple GHGs
Fig. 10. Temperature tuning (black circles) and pump tuning rates (red circles) for type-II PPRKTP OPO $y$- and $z$-polarized outputs at wavelengths corresponding to specified GHG. The data labels denote the QPM period and the type-II PPRKTP OPO output polarization.
Table 3. Type-II OPO Designs for Multiple Greenhouse Gas Monitoring
In the type-II OPO, the same target wavelength can be generated in either $z$ or $y$ polarization using different periodicities. The choice of polarization will affect OPO tuning rate accomplished by tuning either the pump frequency or crystal temperature. Moreover, the polarization of the resonant wave can impose additional constraints, e.g., due to the emergence of the backward SPS process as discussed above. The calculated temperature and pump tuning rates at the target GHG molecular lines are shown in Fig. 10 for $y$- and $z$-polarized type-II PPRKTP OPO outputs.
As shown in Fig. 10, the $y$-polarized OPO output has a similar temperature tuning rate, although with the sign opposite to the $z$-polarized output. However, the $y$-polarized wave will have a substantially faster pump tuning rate compared to the $z$-polarized wave at the same target wavelength. If pump tuning is an option, for instance, by tuning the pump between 1030–1051 nm, the $y$-polarized OPO output could be tuned to cover ${{\rm H}_2}{\rm O}$ and ${{\rm CO}_2}$ absorption lines using the same QPM structure. On the other hand, the $z$-polarized output would tune about six times slower, and that could be exploited for increased output wavelength stabilization with respect to the pump frequency fluctuations.
A complete comparison of the type-II PPKTP and type-II PPLN OPO designs for multiple GHG-${{\rm CH}_4}$, ${{\rm H}_2}{\rm O}$, and ${{\rm CO}_2}$ monitoring is shown in Table 3. The temperature and pump tuning rates were calculated at 25°C and 1064 nm.
In conclusion, we investigated the design parameter space for type-II PPRTKP OPOs pumped at 1 µm and operating in the 1.8–2.5 µm range. Relative insensitivity to the temperature tuning in such OPOs requires precise design of the periodicity of the QPM structures. Here we found that combining Sellmeier expansions from [22,34] and thermotropic coefficients from [22] gives a sufficiently accurate prediction for PPRKTP structure design. Slow temperature tuning of about 8 GHz/K, which is about five times slower compared to PPLN type-II OPOs, makes PPRKTP OPO wavelengths less sensitive to environmental fluctuations. We verified that the effective nonlinearity in type-II PPRTKP is consistent with the expectations from previously reported values in a single-domain KTP. This then substantiates our claim in the Introduction that the FOM in PPRKTP type-II OPOs is higher than that in type-II PPLN OPOs in this spectral range. We have shown that mJ-level narrowband output can be readily generated in type-II PPRKTP OPOs using transform-limited ns pulse pumping without reaching optical damage. Currently, narrowband PPLN type-II OPOs used in lidar emitters provide two orders of magnitude lower energies [6]. As a result, in DIAL instruments for long-range GHG sensing where emitter energies of tens of mJ are required, the number of amplification stages could be reduced.
Funding
European Research Council (821868).
Acknowledgment
The authors acknowledge the continuous support and collaboration from InnoLas Laser GmbH.
Disclosures
The authors declare no conflicts of interest.
Data availability
No data were generated or analyzed in the presented research.
REFERENCES
1. F. K. Tittel, D. Richter, and A. Fried, Mid-Infrared Laser Applications in Spectroscopy, I. T. Sorokina and K. L. Vodopyanov, eds. (Springer, 2003), pp. 458–529.
2. R. T. H. Collis, “Lidar,” Appl. Opt. 9, 1782–1788 (1970). [CrossRef]
3. T. F. Refaat, U. N. Singh, J. Yu, M. Petros, S. Ismail, M. J. Kavaya, and K. J. Davis, “Evaluation of an airborne triple-pulsed 2 µm IPDA lidar for simultaneous and independent atmospheric water vapor and carbon dioxide measurements,” Appl. Opt. 54, 1387–1398 (2015). [CrossRef]
4. V. Wulfmeyer, M. Randal, A. Brewer, and R. M. Hardesty, “2 µm Doppler lidar transmitter with high frequency stability and low chirp,” Opt. Lett. 25, 1228–1230 (2000). [CrossRef]
5. P. V. Ketan and H. S. Prashant, “Thulium laser enucleation of the prostate is a safe and a highly effective modality for the treatment of benign prostatic hyperplasia—our experience of 236 patients,” Urol. Ann. 8, 76–80 (2016). [CrossRef]
6. J. Hamperl, J. F. Geus, K. M. Mølster, et al., “High energy parametric laser source and frequency-comb-based wavelength reference for CO2 and water vapor DIAL in the 2 µm region: design and pre-development experimentations,” Atmosphere 12, 402 (2021). [CrossRef]
7. D. M. Hammerling, A. M. Michalak, and S. R. Kawa, “Mapping of CO2 at high spatiotemporal resolution using satellite observations: global distributions from OCO-2,” J. Geophys. Res. 117, D06306 (2012). [CrossRef]
8. A. Fix, R. Matthey, A. Amediek, G. Ehret, F. Gruet, C. Kiemle, V. Klein, G. Mileti, J. P. do Carmo, M. Quatrevalet, J. Pereira do Carmo, and M. Quatrevalet, “Investigations on frequency and energy references for a space-borne integrated path differential absorption lidar,” Proc. SPIE 10563, 105630A (2017). [CrossRef]
9. S. Brosnan and R. Byer, “Optical parametric oscillator threshold and linewidth studies,” IEEE J. Quantum Electron. 15, 415–431 (1979). [CrossRef]
10. J. A. Giordmaine and R. C. Miller, “Tunable coherent parametric oscillation in LiNbO3 at optical frequencies,” Phys. Rev. Lett. 14, 973–976 (1965). [CrossRef]
11. D. Mammez, E. Cadiou, J.-B. Dherbecourt, M. Raybaut, J.-M. Melkonian, A. Godard, G. Gorju, J. Pelon, and M. Lefebvre, “Multispecies transmitter for DIAL sensing of atmospheric water vapour, methane and carbon dioxide in the 2 µm region,” Proc. SPIE 9645, 964507 (2015). [CrossRef]
12. G. Arisholm, Ø. Nordseth, and G. Rustad, “Optical parametric master oscillator and power amplifier for efficient conversion of high-energy pulses with high beam quality,” Opt. Express 12, 4189–4197 (2004). [CrossRef]
13. L. E. Myers, R. C. Eckardt, M. M. Fejer, R. L. Byer, W. R. Bosenberg, and J. W. Pierce, “Quasi-phase-matched optical parametric oscillators in bulk periodically poled LiNbO3,” J. Opt. Soc. Am. B 12, 2102–2116 (1995). [CrossRef]
14. J. Hellström, V. Pasiskevicius, H. Karlsson, and F. Laurell, “High-power optical parametric oscillation in large-aperture periodically poled KTiOPO4,” Opt. Lett. 25, 174–176 (2000). [CrossRef]
15. J. A. Armstrong, N. Bloembergen, J. Ducuing, and P. S. Pershan, “Interactions between light waves in a nonlinear dielectric,” Phys. Rev. 127, 1918–1939 (1962). [CrossRef]
16. G. Arisholm, E. Lippert, G. Rustad, and K. Stenersen, “Efficient conversion from 1 to 2 µm by a KTP-based ring optical parametric oscillator,” Opt. Lett. 27, 1336–1338 (2002). [CrossRef]
17. W. R. Bosenberg, A. Drobshoff, J. I. Alexander, L. E. Myers, and R. L. Byer, “93% pump depletion, 3.5-W continuous-wave, singly resonant optical parametric oscillator,” Opt. Lett. 21, 1336–1338 (1996). [CrossRef]
18. M. Henriksson, L. Sjöqvist, V. Pasiskevicius, and F. Laurell, “Narrow linewidth 2 µm optical parametric oscillation in periodically poled LiNbO3 with volume Bragg grating outcoupler,” Appl. Phys. B 86, 497–501 (2007). [CrossRef]
19. B. Jacobsson, M. Tiihonen, V. Pasiskevicius, and F. Laurell, “Narrowband bulk Bragg grating optical parametric oscillator,” Opt. Lett. 30, 2281–2283 (2005). [CrossRef]
20. R. S. Coetzee, N. Thilmann, A. Zukauskas, C. Canalias, and V. Pasiskevicius, “Nanosecond laser induced damage thresholds in KTiOPO4 and Rb:KTiOPO4 at 1 µm and 2 µm,” Opt. Mater. Express 5, 2090–2095 (2015). [CrossRef]
21. H. Vanherzeele and J. D. Bierlein, “Magnitude of the nonlinear-optical coefficients of KTiOPO4,” Opt. Lett. 17, 982–984 (1992). [CrossRef]
22. K. Kato and E. Takaoka, “Sellmeier and thermo-optic dispersion formulas for KTP,” Appl. Opt. 41, 5040–5044 (2002). [CrossRef]
23. G. J. Edwards and M. Lawrence, “A temperature-dependent dispersion equation for congruently grown lithium niobate,” Opt. Quant. Electron. 16, 373–375 (1984). [CrossRef]
24. I. Shoji, T. Kondo, A. Kitamoto, M. Shirane, and R. Ito, “Absolute scale of second-order nonlinear-optical coefficients,” J. Opt. Soc. Am. B 14, 2268–2294 (1997). [CrossRef]
25. R. S. Coetzee, A. Zukauskas, C. Canalias, and V. Pasiskevicius, “Low-threshold, mid-infrared backward-wave parametric oscillator with periodically poled Rb:KTP,” APL Photon. 3, 071302 (2018). [CrossRef]
26. X. Y. Pan, C. Kurtsiefer, A. Ling, and J. A. Grieve, “Temperature insensitive type II quasi-phasematched spontaneous parametric downconversion,” Appl. Phys. Lett. 119, 021107 (2021). [CrossRef]
27. R. B. Jin, R. Shimizu, K. Wakui, M. Fujiwara, T. Yamashita, S. Miki, H. Terai, Z. Wang, and M. Sasaki, “Pulsed Sagnac polarization-entangled photon source with a PPKTP crystal at telecom wavelength,” in Conference on Lasers and Electro-Optics Europe (2014).
28. T. Paterek, A. Fedrizzi, S. Gröblacher, T. Jennewein, M. Żukowski, M. Aspelmeyer, A. Zeilinger, S. Groeblacher, T. Jennewein, M. Zukowski, M. Aspelmeyer, and A. Zeilinger, “Experimental test of nonlocal realistic theories without the rotational symmetry assumption,” Phys. Rev. Lett. 99, 210406 (2007). [CrossRef]
29. L. E. Myers, R. C. Eckardt, M. M. Fejer, R. L. Byer, and W. R. Bosenberg, “Multigrating quasi-phase-matched optical parametric oscillator in periodically poled LiNbO3,” Opt. Lett. 21, 591–593 (1996). [CrossRef]
30. A. Zukauskas, N. Thilmann, V. Pasiskevicius, F. Laurell, and C. Canalias, “5 mm thick periodically poled Rb-doped KTP for high energy optical parametric frequency conversion,” Opt. Mater. Express 1, 201–206 (2011). [CrossRef]
31. V. Pasiskevicius, C. Canalias, and F. Laurell, “Highly efficient stimulated Raman scattering of picosecond pulses in KTiOPO4,” Appl. Phys. Lett. 88, 041110 (2006). [CrossRef]
32. H. Jang, G. Strömqvist, V. Pasiskevicius, and C. Canalias, “Control of forward stimulated polariton scattering in periodically poled KTP crystals,” Opt. Express 21, 27277–27283 (2013). [CrossRef]
33. H. Jang, A.-L. Viotti, G. Strömqvist, A. Zukauskas, C. Canalias, and V. Pasiskevicius, “Counter-propagating parametric interaction with phonon-polaritons in periodically poled KTiOPO4,” Opt. Express 25, 2677–2686 (2016). [CrossRef]
34. F. König and F. N. C. Wong, “Extended phase matching of second-harmonic generation in periodically poled KTiOPO4 with zero group-velocity mismatch,” Appl. Phys. Lett. 84, 1644–1646 (2004). [CrossRef]
35. M. Katz, D. Eger, M. B. Oron, and A. Hardy, “Refractive dispersion curve measurement of KTiOPO4 using periodically segmented waveguides and periodically poled crystals,” J. Appl. Phys. 90, 53–58 (2001). [CrossRef]
36. K. Fradkin, A. Arie, A. Skliar, and G. Rosenman, “Tunable midinfrared source by difference frequency generation in bulk periodically poled KTiOPO4,” Appl. Phys. Lett. 74, 914–916 (1999). [CrossRef]
37. S. Emanueli and A. Arie, “Temperature-dependent dispersion equations for KTiOPO4 and KTiOAsO4,” Appl. Opt. 42, 6661–6665 (2003). [CrossRef]
38. W. Wiechmann, S. Kubota, T. Fukui, and H. Masuda, “Refractive-index temperature derivatives of potassium titanyl phosphate,” Opt. Lett. 18, 1208–1210 (1993). [CrossRef]
39. H. Vanherzeele, J. D. Bierlein, and F. C. Zumsteg, “Index of refraction measurements and parametric generation in hydrothermally grown KTiOPO4,” Appl. Opt. 27, 3314–3316 (1988). [CrossRef]
40. https://as-photonics.com/products/snlo/.
41. L. S. Rothman, I. E. Gordon, A. Barbe, et al., “The HITRAN 2008 molecular spectroscopic database,” J. Quant. Spectrosc. Radiat. Transfer 110, 533–572 (2009). [CrossRef]
