Performance Studies of High-Power Optical Parametric Oscillators Pumped by a Pulsed Fiber Laser

Abstract

High-power optical parametric oscillators (OPOs), as mature radiation sources in mid-infrared (MIR), degenerate gradually with wavelength increase, mainly above 3700 nm. Using a periodically poled magnesium-oxide-doped lithium niobate (MgO:PPLN) as the nonlinear crystal, we build a high-power signal-resonant OPO pumped by ytterbium-doped fiber laser (YDFL). To improve the OPO’s output power at ~3.8 μm, the parameters, such as the pump beam’s waist diameter and location, the curvature radius of the output coupler and the length of MgO:PPLN, are discussed in detail. When pump power is 79 ± 4 W with a repetition rate of 200 kHz, the OPO provides up to 8 ± 0.4 W average power in beam quality with M² factors of ~1.84 and ~1.69 in the two axes. Under the highest output power, the center wavelength of the idler beam is 3768.4 nm with a full-width at half-maximum (FWHM) bandwidth of ~18.6 nm. When the output power reaches ~6.3 W, its power stability is 1.6% root mean square (RMS) over 7 h. Further analysis of the factors affecting OPO’s performance and simple structure are critically essential for compact OPO prototypes with a capacity of high output power.

1. Introduction

Due to excellent penetration ability in atmospheric and strong molecular absorption, high-power mid-infrared (MIR) laser plays a major role in scientific and technological applications, such as remote sensing [1], mass spectrometry imaging [2], light detection and ranging (LIDAR) system [3], medical diagnostics [4] and environmental monitoring [5]. Compared with quantum-cascade lasers [6] and pumping metal-ion-doped crystal technology [7], optical parametric oscillators (OPOs) are more convenient for realizing high output power and broad wavelength tuning. However, their captivity of high output power is limited by the photorefractive damage threshold of the nonlinear crystals and the Manley–Rowe relations [8]. A frequently used nonlinear crystal is periodically poled lithium niobate (PPLN), which has a high effective nonlinear coefficient [9]. Its damage threshold can be significantly enhanced by doping magnesium-oxide (MgO) [10]. Periodically poled magnesium-oxide-doped lithium niobate (MgO:PPLN)-based OPO with tens of watts of MIR power [11,12] and hundreds of milli joules of MIR energy [13] have been demonstrated.

Compared to its excellent performance in the shorter MIR band, the MgO:PPLN-based OPO’s capacity for high output power at a longer wavelength range (>3700 nm) is much more challenging. This is due to the increasing absorption coefficient of MgO:PPLN with wavelength increase. Combined with intra-cavity difference frequency generation (DFG) through a monolithic aperiodically poled magnesium oxide doped lithium niobate (MgO:APLN), the OPO realized the highest idler laser power exceeding 4 W at ~3800 nm under the pump power of ~25 W from a linearly polarized, pulsed ytterbium-doped fiber laser (YDFL) [14,15]. The crystal without periodical poling limits the wavelength tunning at a narrow range of 3.76–3.82 μm because cascaded parametric conversion is realized at a specific temperature range. Using two optical parametric amplifiers (OPAs), the MIR laser at 3800 nm with a maximum output power of 4.69 W and beam quality M² factor of 1.82 was obtained [16]. Both the two amplification experiments require cavity mirrors, temperature controllers, and crystals. The complex cavity increases costs and reduces reliability, which limits the popularization of OPOs in the fields of engineering and scientific research. With the rapid development of high-power pump source radiating at ~1 μm, tens of watts power of OPOs can be easily realized. Using a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser with a power of 104 W, the output power of 16.7 W at ~3.8 μm can be obtained. The M² factors were 2.03 and 5.89 in the parallel and perpendicular directions, respectively [17]. Furthermore, the output power was able to reach 22.6 W by increasing the pump power to 150 W, and the M² factors in the two axes were 1.74 and 4.86, respectively [18]. However, there is a huge difference of the M² factors between the two axes because the MgO:PPLN was pumped by an elliptical beam. In addition to the hundred-watt pump power, the high output power in Refs. [17,18] can be attributed to the high optical–optical conversion efficiency. The conversion efficiency can be improved significantly by using pump-double-pass configuration because the pump beam participates in frequency conversion two times. Using the configuration and optimizing the output coupler curvature radius, pump beam waist radius, and cavity length, a miniaturized idler-resonant OPO could afford a maximum power of 5.84 W at 3.76 μm (conversion efficiency > 14.0%) with M² factors of 1.57 and 1.49 along the horizontal and vertical directions, respectively [19]. Furthermore, after optimizing the parameters including the pump repetition rate, pulse width, and waist diameter, a signal-resonant and pump-double-pass OPO generated 10.82 W at 3.754 μm with a conversion efficiency of 13.68% [20]. By employing semi-external-cavity-structured OPO with pump-double-pass, 9.23 W of output power was obtained at 3.82 μm with a neodymium-doped yttrium orthovanadate (Nd:YVO₄) laser power of 48.2 W [21]. It is noteworthy that the setups in Refs. [17,18,19,20,21] contain isolators for avoiding pump source damage resulting from unwanted backward pump beam. Isolators with high damage threshold enlarge OPO’s module and increase costs.

This paper reports that a linearly polarized pulsed YDFL is used to pump a MgO:PPLN-based OPO. Based on pump-single-pass, the OPO’s configuration can be simplified as much as possible to only contain the necessary components. Without the assistance of other nonlinear technologies, the compact OPO is able to generate high power. To improve its output power at ~3.8 μm, the influences, including pump beam waist diameter and location, curvature radii of output coupler and length of crystal, are investigated, respectively. Optimizing the above influences, we obtained a maximum average output power of 8.2 W when the pump power was 78.6 W with a repetition rate of 200 kHz. Under the maximum output power, the wavelength is 3768.4 nm with a full-width at half-maximum (FWHM) of ~18.6 nm. The M² factors of the output laser along the horizontal and vertical directions are ~1.84 and ~1.69, respectively. At the output power of ~6.3 W, its stability over 7 h is 1.6% root mean square (RMS). Simple structure and systematic analysis of OPO’s performance provide a reference for compact OPO prototypes pumped by high-power pulse fiber lasers.

2. Experimental Setup

The schematic diagram of OPO was almost similar to the setups described previously [17,18,19,20]. The pump beam was a linear-polarized YDFL delivering a maximum power of 207 ± 10 W. To enhance optical–optical conversion efficiency, the pump polarization was rotated to match the e → e + e interaction in the nonlinear crystal. According to the OPO cavity configuration, the theoretical waist radius of signal mode at the center of the crystal was estimated by the nonlinear optics software SNLO (Version 79) [22] (AS-Photonics, LLC., Albuquerque, NM, USA). As shown in Figure 1, the pump beam, passing through the fiber collimator, was focused by a plano-convex lens into the MgO:PPLN crystal to match the mode of the signal beam. The MgO:PPLN crystal, whose two end faces were optically polished and coated with an anti-reflection-coat (T > 99%) at 1.064 μm, 1.4 to 1.6 μm, and 3.4 to 4.2 μm, had dimensions of 2 × 1 mm² with different lengths. A single domain period of 29.45 μm was designed to match the phase of the pump, signal, and idler beam at 1.064 μm, 1.47 μm and 3.8 μm, respectively. Theoretically, the crystal temperature for phase-matching is ~75 °C. Two sapphire flakes were used to sandwich the crystal for even heating and were housed in an oven with a thermal stability of ±0.1 °C. The signal-resonant linear cavity, whose length was constant at 42 mm comprised one plate mirror, M1, and one concave mirror, M2. The two mirrors, M1 and 2, were highly transmitting (T > 99.5%) for the idler beam (3.0 to 4.2 μm) and pump beam (1.064 μm). In addition, the mirror, M1, was highly reflecting (R > 99.8%) for the signal beam (1.3 to 2.1 μm), while M2, as the output coupler for the idler beam, was reflecting (~80%) for the range of 1.3 to 2.1 μm. M3 was highly reflecting at 1.064 μm and 1.3 to 2.1 μm for 45° angle of incidence and was used to separate the output idler beam from the residual pump beam and leaking signal beam.

3. Experimental Results and Discussion

3.1. Characteristics of Pump Beam

Before constructing the OPO, some basic information about the pump beam was investigated. The spectrum was measured by an optical spectrum analyzer (OSA205C, Thorlabs, Inc., Newton, NJ, USA). Figure 2a shows that the center wavelength of the pump beam is 1064.6 nm with FWHM bandwidths of ~2.1 nm. To obtain the M² factors, we measured the beam diameters around the focus. The M² factors for two axes and beam-intensity profile are presented in Figure 2b. On the x-axis, the M² factor and diameter are ~1.25 and 0.75 mm, respectively. On the other axis, the M² factor and diameter are ~1.18 and 0.76 mm, respectively. Under the repetition rate of 200 kHz and the power of ~81 W, we measured the temporal profile and long-term power stability using a photoconductive detector (PCI-4TE-12-1x1, VIGO Photonics, Ozarow Mazowiecki, Poland) and a power meter (S425C, Thorlabs, Inc.), respectively. The measurement uncertainty of the power meter is ±5% at the wavelength range of 250 nm to 17 μm. Figure 2c,d shows that pump beam has a pulse width of ~150 ns and power stability of 0.36% RMS over 6 h.

3.2. Parameters Affecting Idler Power

We studied how the pump beam size in the MgO:PPLN crystal affected the optical–optical conversion efficiency. By incorporating a set of plano-convex lenses with varying focal-lengths, the pump beam waist was effectively reduced to 155 ± 3.1 μm, 172 ± 3.4 μm, 201 ± 4.0 μm and 230 ± 4.6 μm. The beam sizes are measured by the laser beam profiler with a beam size accuracy of ±2% (LBP2-IR2, Newport Corporation, Irvine, CA, USA, and Franklin, MA, USA). Additionally, the focus point of the beam was consistently positioned at the center of the crystal. At a fixed crystal length of 38 mm and the M2 with a curvature radius of −100 mm, the estimated waist size was ~150 μm. The output power and corresponding conversion efficiency are summarized in Figure 3. At the range of low pump power, the smaller pump beam has higher power density and better mode matching. It is reasonable that the smaller the pump beam waist, the higher the conversion efficiency. The conversion efficiency at ~3.8 μm is 11.1 ± 0.8% with a maximum output power of 7.5 ± 0.4 W. However, when the average pump power exceeds 75 ± 4 W, the crystal will break easily if the pump beam size is too small, as shown in the dash-square line. When the pump beam waist was 155 ± 3.1 μm, thermal guiding in the MgO:PPLN crystal caused a noticeable step increase in the idler power from 4.7 ± 0.2 to 6.8 ± 0.3 W, which was called bi-stability in references [23,24]. Specifically, the open line and filled lines represent the decrease and increase in pump power, respectively. At the range, there is a noticeable difference, which indicates the OPO has two stable states. For the sake of comparing different parameters, only a downward trend of the output power has been exhibited in Figure 3a. A series of slight bi-stability phenomena were also observed, such as the steps from 4.2 ± 0.2 to 5.1 ± 0.3 W (201 ± 4.0 μm), 4.0 ± 0.2 to 5.0 ± 0.3 W and 6.1 ± 0.3 to 7.8 ± 0.4 W (172 ± 3.4 μm). Expressly, the bi-stability could be restrained effectively by expanding the pump beam waist.

To find the most appropriate pump beam waist location, the whole linear OPO cavity was moved along the beam propagation. The size of the pump beam waist, crystal length, and curvature radius of M2 were 172 ± 3.4 μm, 38 mm, and –100 mm, respectively. The estimated waist of ~145 μm can be obtained by the SNLO. The distance from the front face of the crystal to the waist varied as −5 mm, 6 mm, 16 mm and 21 mm. Figure 4 shows the output power and conversion efficiency as functions of the pump power for different waist locations. As shown by the dash-dot line in Figure 4, the output power and conversion efficiency are always the lowest when the waist lies outside the MgO:PPLN crystal. Due to the absorption of optical power, the thermal guiding leads to a surge in the output power, i.e., bi-stability (the dash-square line in Figure 4). It becomes apparent when the pump power exceeds 57 ± 3 W, indicating a better overlap between the pump and signal beam is found [22]. Compared to the above locations, the ideal waist location is around the center of the MgO:PPLN because the output power is hardly influenced by thermal guiding and increases gradually as the pump power grows.

The curvature radius of the output coupler M2 also influenced the OPO output power. To investigate this aspect, three output couplers with different curvature radii, including −100 mm, −200 mm and −300 mm (corresponding to the theoretical waist sizes of 145.0 μm, 183.6 μm and 205.7 μm), were employed individually. To protect MgO:PPLN crystal against breakage, the pump beam waist in the center of the crystal was adjusted to 201 ± 4.0 μm. The crystal length was 38 mm. As shown in Figure 5a, the output power of the OPO decreases as the curvature radius increases in the low pump power range, while the trend is reversed when pump power exceeds 50 ± 3 W. The phenomenon might be attributed to the mismatch between the actual pump beam waist and required values in the low-power state. The thermal guiding arising from high optical power expanded the signal beam size, which was more suitable for the OPO with a small curvature radius.

The length of the crystal was a crucial factor for the OPO output power. The pump beam with a waist of 172 ± 3.4 μm lay at the center of the MgO:PPLN crystal. M2’s curvature radius was stabilized at −200 mm, while the crystal length varied by 38 mm, 30 mm and 20 mm. The estimated waist sizes were ~150 μm and the difference of beam waists caused by different crystal lengths could be ignored. The dependence of the idler power and optical–optical conversion efficiency on the pump power is depicted in Figure 6. Optical–optical conversion efficiency increases while the pump threshold decreases with the rise of crystal length, which is identical to the prediction in reference [8]. As the dash-dot line shows, there is no roll-off in idler power, which means higher output power and conversion efficiency may be obtained with pump power increase. Similar to Figure 3, Figure 4 and Figure 5, an inconspicuous step increase in the idler power is observed when pump power is raised from 66 ± 3 to 69 ± 4 W.

3.3. Characteristics of Idler Beam

Based on the comprehensive analysis of the parameters affecting output power, we built the OPO with the highest output power and conversion efficiency. The curvature radius of the output coupler, crystal length and cavity length were −100 mm, 38 mm and 42 mm, respectively. In the cold cavity, the estimated waist of ~150 μm can be obtained by SNLO. To allow careful mode-matching at high power, the pump beam waist was reduced to 172 ± 3.4 μm by a convex lens with a focal length of 150 mm. The pump beam with a repetition rate of 200 kHz and a pulse width of 150 ns was focused into the center of the MgO:PPLN crystal. To investigate the performance of the OPO, we measured the variation of the output power in the idler beam with the increasing pump power, as shown in Figure 7. As evident, the frequency conversion starts at a threshold pump power of 33 ± 1.6 W. With the pump power increasing gradually, the idler power increases accompanied by two implicit bi-stability at 1.0 ± 0.1 W and 4.3 ± 0.2 W. Until the crystal break, the pump power increases to 79 ± 4 W, and a maximum idler power of 8.0 ± 0.4 W is obtained. The dash-square line in Figure 7 shows the corresponding optical–optical conversion efficiency at different output power. The conversion efficiency has a maximum value of 10.6 ± 0.7% and tends to level off when the output power exceeds 7.5 ± 0.4 W.

When the temperature of the oven was 75.0 °C, the idler spectrum was measured at maximum output power using a Fourier Transform Optical Spectrum Analyzer (OSA205C, Thorlabs, Inc.) with a resolution of ~0.36 nm. As evident from Figure 8a, the idler spectrum centers at 3768.4 nm with a FWHM bandwidth of ~18.6 nm. Using an infrared photodetector (PCI-4TE-12-1x1, VIGO Photonics) to measure the waveform of the idler beam, the data show its pulse width is ~130 ns, as shown in Figure 8b.

To estimate the beam quality M² factors of the idler beam, we used a lens with a focal length of 500 mm to focus the beam and measured its diameter around the beam waist by 90/10 knife-edge method. According to the hyperbolic fitting of the data, as shown in Figure 9a, the M² factors of the idler beam along the horizontal and vertical directions are ~1.84 and ~1.69, respectively. We also performed measurements of the output power stability characterized by the OPO by recording long-term average power fluctuations at a wavelength of 3768.4 nm. The results are shown in Figure 9b. While generating an output power of ~6.3 W recorded over 7 h, the RMS was better than 1.6%, which mainly attributed to the reliable stabilization of the crystal temperature and the appropriate output coupling optics to emit the intracavity power of the signal beam.

4. Conclusions

In conclusion, we presented the experimental characterization of a high average-power, YDFL-pumped signal-resonant OPO module. Owing to the pump-single-pass scheme, the module can be compacted as far as possible. The output power of the OPO affected by pump beam size, waist location, curvature radius of output coupler and crystal length were investigated, respectively. The experimental data shows that a smaller pump size (a waist of ~150 μm) improves the performance of the OPO but increases the risk of crystal damage. In addition, the pump beam should be focused on the crystal’s center to improve the conversion efficiency and robustness of the OPO. The conversion efficiency is also influenced by the curvature radius of the output coupler and crystal length. An output coupler with a small curvature radius and a long crystal should be adopted to optimize the conversion efficiency. A comprehensive analysis of the factors affecting output power is essential to improve optical–optical efficiency for high-power OPOs working at long mid-infrared ranges. It should be noted that the bi-stability phenomenon is extraordinarily ordinary in high-power OPOs. Using MgO:PPLN as the nonlinear crystal, the OPO afforded a maximum MIR power of 8 ± 0.4 W at 3768.4 nm with a repetition rate of 200 kHz and pulse width of 130 ns when the pump power was 79 ± 4 W. The M² factors of the idler beam along the horizontal and vertical directions were ~1.84 and ~1.69, respectively. Thanks to excellent thermal control and suitable output coupling of the signal beam, a long-term idler power fluctuation of 1.6% RMS was achieved. The output power and conversion efficiency can be further improved by optimizing the repetition rate, pulse width, as well as output coupler transmissivity. In addition, all pump power can be utilized fully to pump two OPOs simultaneously using a non-polarizing beam splitter cube, which can improve correspondingly two times output power. A higher beam quality can be obtained by using an idler-resonant configuration [18,19,25] because its mode size is larger than the signal-resonant OPO. Based on the demonstrated performance characteristics, future work will be focused on higher output power at MIR wavelength and a more compact structure.