1. Introduction

Coherent mid-infrared (mid-IR) sources are essential for a variety of applications including spectroscopy, telemetry, imaging and biomedicine [1,2]. Optical parametric oscillators (OPOs) represent one of the most effective methods for realizing such sources, offering exceptionally broad spectral coverage from a single device, power scalability, and stability [3,4]. By exploiting quasi-phase-matched (QPM) nonlinear crystals such as MgO-doped periodically-poled LiNbO₃ (MgO:PPLN), stoichiometric LiTaO₃ (MgO:sPPLT), or KTiOPO₄ (PPKTP), in combination with solid-state and fiber pump lasers near ~1 μm, OPOs can now readily access spectral regions across ~1.4-4 µm in the mid-IR [5–7], with wavelength generation beyond ~4 μm limited by multi-phonon absorption in such oxide-based materials. In general, operation of OPOs in the most practical configuration is achieved in singly-resonant oscillator (SRO) scheme, with the optical cavity providing feedback at the near-IR signal wavelength and the mid-IR idler wave extracted in a single-pass through the crystal [4]. While this approach has enabled reliable operation of OPOs with high performance in various time-scales, an important shortfall remains the spatial quality of the non-resonant idler beam in the mid-IR, which can be a major impediment for important applications such as remote sensing, range finding and LIDAR. The attainment of high beam quality in the mid-IR idler beam is also of critical importance for applications relying on subsequent nonlinear frequency conversion steps, such as cascaded pumping of OPOs into the deep mid-IR at wavelength beyond ~4 µm. To realize such long-wave OPOs, non-oxide nonlinear materials such as ZnGeP₂ (ZGP) and orientation-patterned GaAs (OP-GaAs) with broader transparency in the mid-IR have to be exploited. However, strong two-photon absorption in these crystals precludes pumping below ~2 µm, thus requiring high-power mid-IR radiation with high beam quality at longer wavelengths [8,9]. With the scarcity of practical solid-state laser sources above ~2 μm, cascaded pumping in which the output beam from a primary OPO could be used as the input pump for a secondary OPO, offers a viable alternative for mid-IR generation beyond ~4 µm. Such cascaded OPO schemes have been previously realized in pulsed nanosecond regime providing broadband tunability in the long-wave mid-IR [10,11]. However, in this case, the attainment of the highest beam quality is not vital because of the large pump beam sizes and OPO cavities with high Fresnel numbers. Cascaded pumping of femtosecond OPOs has also been previously demonstrated [12,13], but in this case the short interaction lengths result in higher tolerance to pump beam quality. On the other hand, the need for the highest beam quality becomes increasingly critical in high-repetition-rate picosecond OPOs where low peak pump pulse intensities result in small nonlinear gain. Under this condition, for successful OPO operation, it is necessary to use highly focused pump beams (w₀~50-100 μm) over long crystal interaction lengths (~30-50 mm) in highly stable OPO cavities with low Fresnel numbers [14]. For cascaded pumping, and many other direct applications, it is thus important to develop OPO sources providing coherent radiation at high power and with the highest beam quality in the mid-IR.

A potential approach to improve the spatial quality of mid-IR output radiation from OPOs is to provide feedback for the long-wave idler wave in SRO configuration. However, the increasing material absorption in the mid-IR, combined with the much higher intracavity intensity of the circulating idler wave in such a scheme can potentially lead to crystal damage, presenting a major barrier to practical OPO operation. Together with the difficulty in fabrication of highly reflective dielectric multilayer mirror coatings and low-loss crystal coatings with high damage threshold in the mid-IR, these factors make the realization of idler-resonant OPOs challenging in practice. These challenges are further compounded in the high-repetition-rate picosecond regime, where low nonlinear gains and small pumping intensities demand the lowest round-trip cavity losses and optical coatings of highest quality for successful OPO operation. Moreover, the high intracavity average powers together with long interaction lengths can potentially lead to thermal effects in such OPOs, further degrading device performance and the output beam quality.

The concept of idler-resonant OPO has been previously explored in the pulsed nanosecond regime, including an idler-resonant OPO based on MgO:PPLN pumped with an elliptical beam at 1064 nm at 7 kHz, generating up to 16.7 W of average power at 3840 nm with M²<5.89 [15]. In the femtosecond regime, an idler-resonant OPO synchronously pumped by a Ti:sapphire laser at 104 MHz was studied, providing up to 68 mW average power at 2.3 μm by using dual grating design in MgO:PPLN [16]. Here, we report a picosecond OPO resonant in the idler wave, for the attainment of high beam quality in the mid-IR. Based on a 42-mm-long MgO:PPLN crystal and pumped by a picosecond Yb-fiber laser at 80 MHz, the OPO provides practical average powers of up to 3.5 W in excellent beam quality with M² ~1.1-1.8, with high passive power stability better than 0.31-0.46% rms over 1 hour across a broad tuning range of 2198-4028 nm.

2. Experimental setup

A schematic of the experimental setup is shown in Fig. 1. The pump source is a picosecond Yb-fiber laser (Fianium, FemtoPower FP1060-20), delivering up to 20 W of average power at ~80 MHz at 1064 nm. The pump pulses are of ~20 ps duration and have a double-peak spectrum with a full-width-half-maximum (FWHM) bandwidth of 1.38 nm, resulting in a time-bandwidth product of ΔτΔν∼7.6. The nonlinear gain crystal for the OPO is a 42-mm-long, 1-mm-thick MgO:PPLN with seven uniform gratings, ranging in period from Λ = 28.5 to Λ = 31.5 μm, in steps of 0.5 μm. The crystal faces are antireflection (AR)-coated for the pump (R<1% at 1064 nm) and signal (R<0.5% over 1400-2000 nm), with high transmission over the idler wavelength range (R<7% over 2500-4000 nm). The crystal is housed in an oven adjustable from room temperature to 200°C with a stability of ± 0.1°C. The pump beam is focused to a beam waist radius of w_0p∼60 μm at the center of the crystal, resulting in a focusing parameter of ξ∼0.91 [17]. The OPO cavity is a four-mirror standing-wave resonator, comprising two concave mirrors (M₁-M₂, r = 150 mm), a plane mirror (M₃), and a plane output coupler (OC) with variable transmission over the idler wavelength range. The mirrors, M₁-M₃, are highly reflecting for idler (R>99.9% over 2500-4000 nm) and highly transmitting for the pump (T∼92% at 1064 nm) and signal (T>80% over 1300-2000 nm), thus resulting in singly-resonant oscillation for the mid-IR idler wave. The idler output is extracted through the OC, while the signal and pump are transmitted in a single pass through the crystal and collected after M₂. The cavity design results in an idler beam waist radius of w_0i∼109 μm inside the crystal, while the total optical length of the cavity is ∼3.75 m, ensuring synchronization with the pump laser repetition rate at ~80 MHz.

 

Fig. 1 Experimental configuration of picosecond idler-resonant MgO:PPLN OPO. FI: Faraday isolator; λ/2: Half-wave plate; PBS: Polarizing beam-splitter; L: Lens; M: Mirrors; F: Filter; OC: Output coupler.

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3. Results and discussion

3.1. Output power and tuning range

The simultaneously measured signal and idler average power across the entire OPO tuning range are shown in Fig. 2. For a maximum available pump power of ∼11 W, we extracted an average idler power ranging from 3.5 W at 2198 nm to 300 mW at 4028 nm, as shown in Fig. 2(a). The OPO provided a maximum idler power of 3.5 W at 2198 nm with >400 mW over almost the entire tuning range, except for a drop in the idler power around 2800 nm due to the OH-absorption in MgO:PPLN. Also shown in the inset of Fig. 2(a) is the transmission profile of the OC in the idler wavelength range, varying over 90-37.8% across 2198-4018 nm. The corresponding signal power varied from 4 W at 1446 nm to 600 mW at 2062 nm, with a maximum of 4.3 W at 1561 nm, as shown in Fig. 2(b). It is to be noted that the data presented here are not corrected for any AR coating or transmission losses of the crystal and mirror coatings. Although there is a 7% additional AR-coating loss per round-trip, the extracted idler power still follows the same trend as the OC transmission, as is clear from Fig. 2(a), which implies that the extracted power can be further improved by minimizing the residual intracavity losses. Moreover, it indicates that the idler-resonant picosecond OPO is very robust and can afford high output coupling similar to the conventional signal-resonant configuration [18]. We further investigated wavelength tuning characteristics of the idler-resonant OPO by changing the temperature as well as the grating periods of the crystal, at low input pump power. Typical grating tuning curves at two different operating temperatures of 40°C and 200°C are presented in the inset of Fig. 2(b), where the solid points are the experimental data, while the dashed curves correspond to the theoretical calculations using the Sellmeier equations [19]. The signal wavelengths were measured using a spectrum analyzer with ~0.2 nm resolution, while the idler wavelengths were calculated from energy conservation. As can be seen, signal tuning across 1482-2062 nm together with idler tuning over 3982-2347 nm was readily achieved, with excellent agreement between the experimental data and theory. A typical signal spectrum at 1565 nm, together with the corresponding idler spectrum centered at 3322 nm, is shown in Fig. 3. The idler spectrum was measured using a mid-IR spectrum analyzer with a resolution of 2 GHz. As evident from Fig. 3, the signal and idler spectrum have FWHM bandwidths of ~0.68 nm and 9.5 nm, respectively.

 

Fig. 2 Extracted (a) idler, and (b) signal average power across the tuning range. Inset (a) transmission of the OC in the idler wavelength range, (b) grating tuning of the MgO:PPLN OPO at two different temperatures of 40 °C and 200 °C.

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Fig. 3 (a) Signal at 1565 nm, and (b) the corresponding idler spectra from the picosecond idler-resonant MgO:PPLN OPO.

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3.2. Group velocity mismatch effects

In addition to temperature and grating tuning, we also attempted cavity length tuning of the OPO at maximum input pump power. However, due to the large group delay dispersion (for example, −30450 fs² at 3300 nm), we did not observe wavelength tuning with cavity length. On the other hand, the group velocity mismatch (GVM) between the pump and the resonant mid-IR idler pulses (v_gp-v_gi) reaches a zero at ~3500 nm. Here, v_g is the group velocity of the respective pulse defined as v_g = c/[n-(dn/dλ)], where c is the velocity of light and n is the refractive index of the nonlinear crystal at the operating wavelength. The pump-idler GVM over the extracted idler wavelengths is plotted in Fig. 4(a). Hence, it is interesting to study the effect of cavity length detuning, and thereby the change of GVM sign on the output power, on either side of the zero-GVM crossing point. The results are shown in Figs. 4(b)–4(d), where the extracted idler power is plotted as a function of cavity-length tuning, for three different wavelengths of 3350 nm, 3550 nm, and 4000 nm, with the corresponding GVM values of +14, −14 and −89 fs/mm, respectively. It is to be noted that for all three wavelengths, the pump-idler temporal walk-off ($\left|{\tau }_{GVM}\right|<3.7 \text{ps}$) due to GVM for the 42-mm-long MgO:PPLN crystal is much smaller than the pump pulse duration of ~20 ps. In a typical signal-resonant MgO:PPLN OPO pumped at 1064 nm, the pump pulse always travels more slowly than the signal pulse, resulting in a positive detuning sensitivity of the output power, as observed previously [20]. A similar behavior is also observed in the green-pumped picosecond MgO:sPPLT OPO [21]. In the present idler-resonant MgO:PPLN OPO, a similar effect due to GVM is observed. While the absolute output power varies depending on the operating wavelength, given the long interaction length and low GVM, the OPO exhibits significant tolerance of 4-6 mm to cavity detuning. To the best of our knowledge, this is the first study on output power sensitivity to cavity delay tuning about the zero-GVM crossing point in a synchronously-pumped OPO.

 

Fig. 4 (a) Pump-idler GVM as a function of the idler wavelength. (b) Variation of the extracted idler power at 3350 nm, (c) 3550 nm, and (d) 4000 nm, as a function of the cavity detuning.

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3.3. Power scaling

The power scaling results for the idler-resonant MgO:PPLN picosecond OPO are presented in Fig. 5. At a fixed crystal temperature of 200°C and for a grating period of Λ = 30 µm, with an output coupling of 67.4%, we extracted as much as 1 W of idler average power at 3340 nm, together with 4.3 W of signal power at 1561 nm, for an input pump power of ~11 W, at corresponding conversion efficiency of 9.5% and 38.2%, respectively. We recorded a pump average power threshold of ~5 W for the OPO, and a maximum pump depletion of 63%. The linear fit to the data result in a slope efficiency of 17.5% and 67.5% for the idler and signal, respectively, as shown in Fig. 5(a). We further performed power scaling of the OPO close to degeneracy, at an idler wavelength of 2198 nm, with the results shown in the inset of Fig. 5(a). As can be seen, despite an output coupling as high as ~90%, the OPO operates efficiently, generating as much as 3.5 W of idler average power with a threshold of ~4.5 W, owing to increased parametric gain near degeneracy. In this case, a slope efficiency of ~55.7% for the idler and ~30% for the signal was estimated, while the maximum pump depletion was recorded to be ~60%. We also performed similar measurements near the longest mid-IR idler wavelength at 4000 nm using a grating period of Λ = 29 µm at a crystal temperature of 40 °C, with the results shown in Fig. 5(b). The idler output coupling in this case was ~38%. The OPO generated a maximum idler average power of 340 mW, together with 4.2 W of signal at 1450 nm, with corresponding slope efficiencies of 6.8% and 80%, at an overall conversion efficiency of ~41%. The OPO threshold in this case was ~6 W, with a maximum pump depletion of ∼63%.

 

Fig. 5 Power scaling measurement at idler wavelengths of (a) 3340 nm and (b) 4000 nm and their corresponding signal wavelengths. Inset (a) output power scaling at 2198 nm.

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3.4. Power stability and beam quality

We also performed measurements of OPO output power stability characterized the OPO by recording long-term idler average power fluctuations at three different wavelengths across the full mid-IR tuning range. The results are shown in Fig. 6. The passive power stability of the idler at 3340 nm, recorded over 1 hour while generating maximum output power, was better than 0.31%rms. The simultaneously measured stability for the corresponding signal at 1561 nm was 0.9%rms. The same measurement performed at the longest mid-IR idler wavelength of 4000 nm resulted in a stability of better than 0.36%rms over 1 hour. In this case, the simultaneously measured stability for the corresponding signal at 1450 nm was 0.97%rms. Close to degeneracy, at 2198 nm, the idler power stability was measured to be better than 0.46%rms over 1 hour. For comparison, the power stability for the Yb-fiber pump laser during these measurements was 0.15%rms over the same period of time.Finally, to evaluate the spatial quality of the extracted output from the idler-resonant OPO, we measured the M² quality factor for generated idler and signal beams at maximum power. This was performed by using a scanning beam profiler and measuring the diameter of the focused beams through the Rayleigh range. The results for an idler wavelength of 3340 nm are shown in Fig. 7(a), where beam quality factors of M_x² ~1.1 and M_y² ~1.02 are confirmed. The x and y directions in these measurements correspond to the vertical and horizontal cross-sections of the beam in the laboratory space. The astigmatism in the idler beam could be attributed to the asymmetry in the mode-matching of the resonant idler beam to the pump, due to the angle of incidence on the curved mirrors away from the normal, thermal lensing in the nonlinear crystal, and thermal gradients in the crystal. The results for the corresponding signal wavelength of 1561 nm are shown in Fig. 7(b), where beam quality factors of M_x² ~1.02 and M_y² ~1.2 are obtained. The recorded idler and signal beam profiles are also shown in the inset of Figs. 7(a) and 7(b), confirming TEM₀₀ spatial profile with single-peak Gaussian distribution and excellent circularity at maximum output power in both cases. The M² factor for the idler at 2510 nm, 3560 nm and 4000 nm, and for the corresponding signal wavelengths, are presented in Table 1. It is clear from the data that the idler-resonant OPO can provide output beam of high spatial quality across the entire mid-IR and near-IR tuning range. In order to further establish the improved beam quality in the idler-resonant OPO, we made similar measurements on the idler beam from a signal-resonant OPO under the same pumping conditions, for comparison. The results are shown in Fig. 8, where we recorded the idler beam quality to vary from M²<3.7 at 2774 nm to M²<2.1 at 4020 nm, with M²>2 across the range. This demonstrates the major improvements in mid-IR output beam quality in the idler-resonant configuration.

 

Fig. 6 (a) Long-term passive power stability at extracted wavelength of 2198 nm, 3340 nm and 4000 nm.

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Fig. 7 Beam-quality measurement of the generated idler at 3340 nm and the corresponding signal. Inset (b) idler and (c) signal beam profile at maximum power.

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Table 1. Idler and signal beam quality at different operating wavelengths.

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Fig. 8 Variation of the idler beam quality factor as a function of wavelength from a signal-resonant picosecond OPO.

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4. Conclusions

In conclusion, we have demonstrated a stable picosecond idler-resonant OPO at 80 MHz based on MgO:PPLN. Pumped at 1064 nm, the source generates continuously tunable idler wavelengths across 4028-2198 nm and from 1446 nm to 2062 nm in the signal. We also investigated the output power sensitivity to cavity delay tuning about the zero-GVM-crossing point. Using an OC with variable transmission of ~38-90% across the idler wavelengths, and at maximum available pump power of ~11 W, the OPO produces >400 mW of idler power together with >4 W of signal power over almost the entire tuning range. Operating at 3044 nm with output coupler of 67.4%, the source provides as much as 1 W of idler average power together with 4 W of the corresponding signal wavelengths of 1561 nm with overall maximum extracted efficiency of ~45.5%. The extracted idler and signal beams show passive power stability better than 0.31%rms and 0.9%rms, respectively, over 1 hour of measurement compared to ~0.15%rms power stability of the pump over the same measurement time. The high output stability is further indicative of the absence of thermal effects and practical operation of the OPO despite the presence of high mid-IR intracavity power due to idler resonance. The studies on the extracted idler beam quality prove that our idler-resonant source is capable of generating picosecond mid-IR wavelengths with high beam-quality, with M² values of 1.1 and 1.02 in horizontal and vertical direction, respectively. The tunability of the OPO is limited by the reflectivity of the mirrors at idler wavelengths and the crystal coating. The extracted idler power and the overall efficiency can be further improved across the tuning range by optimizing the output coupling, as well as by using higher pump powers, given the absence of thermal effects. With the demonstrated performance characteristics, the idler-resonant OPO is a viable source of stable, high-power, and high-beam-quality mid-IR radiation for cascaded frequency down-conversion to wavelengths >4 µm and a variety of direct applications.