A high-energy, extended-cavity femtosecond BiBO optical parametric oscillator synchronously pumped by a 1.0-ps, 1030-nm Yb:YAG, thin-disk pump laser is presented. The oscillator operated near degeneracy in a noncollinear interaction geometry, producing signal wavelength tunability from 1.99 to 2.20 μm. The signal pulses have an average power exceeding 2 W, producing 455-fs pulses at 7.08 MHz with energies up to 350 nJ, showing increased potential for tunable sources of scalable ultrafast pulses in the infrared.
© 2017 Optical Society of America
In recent years, an increasing number of applications have required high-intensity pulses with high average powers in the infrared (IR), including high harmonic generation (HHG) , ultrafast waveguide inscription  and laser remote sensing. With HHG, the electron energies for production of extreme ultraviolet (UV) light scale with pump intensity and λ2, making high-intensity pulses in the IR an attractive source for HHG. Furthermore, high repetition rates are beneficial for improved performance in HHG applications including spectroscopy, microscopy, and frequency metrology [3–6]. For remote sensing applications there is a particular need for high-intensity pulses with high repetition rates to monitor atmospheric CO2 and H2O at eye-safe wavelengths near 2 μm [7,8]. To date there have been very few options for creating tunable high-energy IR pulses with repetition rates extending beyond a few hertz because of the reliance of commercially available Ti:sapphire lasers as high-repetition-rate, ultrafast pump sources. A lack of power scalability in these devices has been the main limitation in available power from optical parametric oscillators (OPO’s), which are typically limited to the lower mW range for pulse energies exceeding nJ levels.
Over the past decade, advancements in novel pump sources, such as thin-disk oscillators [9,10] and fiber-based chirped-pulse amplifiers [11,12], have overcome these limitations, producing high average powers and scalable high-energy pulses in the near-IR. Synchronously pumped ultrafast OPO’s that have employed such devices have seen a significant increase in pulse energy and average power [13,14]. Cavity dumping has been used to yield remarkably high pulse energies with a trade-off in average power . In this work, we present to the best of our knowledge the highest energy from a Watt-class synchronously pumped femtosecond OPO.
2. Experimental setup
The pump laser was a homebuilt Yb:YAG–based, thin-disk, mode-locked laser capable of producing 44 W of average power with pulse energies of 6.2 μJ and a cavity length of 21.2 m based on previous thin-disk designs [16,17]. A semiconductor saturable absorber mirror (SESAM) initiated soliton mode-locking, creating transform limited pulses of 1.0 ps . The 1030-nm centered pulses were used to pump a bismuth triborate (BiB3O6, BiBO) nonlinear crystal. A spot size of 570 μm ensured that the crystal would not be damaged at anticipated intensities of the signal and idler beams.
BiBO was chosen for transparency, broad phase matching, effective nonlinearity, and reliability as was seen in previous ultrafast OPO’s [19,20]. Growth defects in the crystal yielded unfavorable absorption [as shown in Fig. 1(a)] over the tunability of the OPO but was still suitable for generating high-energy pulses. A single-layer SiO2 antireflective (AR)coating provided less than 1% surface reflection across the bandwidth of the OPO and 8.5% reflection of the pump—a compromise between simplicity and performance when compared to multilayer dielectric coatings. The signal and idler wavelengths minimized absorption and allowed for broadband phase matching near degenerate operation. The BiBO crystal was cut for Type I interaction in the x−z plane (φ = 90°) with a pump angle of 8.65° between the extraordinary pump beam and the crystal axis. A small internal noncollinear angle of 0.2° increased the available bandwidth angle and prevented the angularly dispersed idler beam from resonating, which reduced the complications for a doubly resonant cavity [21,22]. A crystal length of 8.4 mm optimized the gain while having a phase-matching bandwidth capable of producing sub-100-fs pulses [as shown in Fig. 1(b)].
A schematic of the OPO is shown in Fig. 2. With the exception of the output coupler, all mirrors had reflectivity profiles >99.9% and were designed to have less than ± 100 fs2. The position of the M3 end mirror was adjusted using a micrometer for fine cavity tuning. The cavity length was manually tuned while carefully monitoring the repetition rate of the pumpto maintain long-term use, although automated stabilization could be used . The size of the beam at the crystal and the stability of the cavity were controlled by the curvature and position of the end mirrors and the mirrors closest to the crystal. The cavity length was extended to 21.2 m in order to achieve synchronous pumping using image relays by setting all other mirrors at a distance of 2f from each other . This was modeled in numerous cavity configurations and had the ability to arbitrarily extend the cavity length without disrupting the critical cavity parameters, making it a useful strategy for designing other cavities to make use of pump lasers with repetition rates in the low-MHz range. The crystal was actively cooled using a Peltier module (CP0.8-127-06L, Laird Technologies) to enable stable steady-state operation at higher powers resulting from absorption of the signal and idler beams. The unpolished crystal face was placed on a chemically etched indium foil for good thermal contact with a 6 × 12 × 12-mm aluminum block that was temperature controlled by the thermoelectric cooler.
3. Experimental results
The typical duration of the output pulses was 455 fs, measured with a Femtochrome FR-103MN scanning autocorrelator [shown in Fig. 3(a)]. The pulse duration showed no significant change for different pump powers or over the tuning bandwidth of the OPO. No dispersive elements were used to optimize the pulse duration but the spectral width suggests that pulses shorter than 170 fs could be obtained with external compression. The tuning range of the OPO was 1.99 to 2.20 μm with a spectral width of 30 nm. The spectrum was measured with a homebuilt spectrometer, calibrated with the second harmonic of the OPO output through a 1-mm BiBO crystal analyzed with an Ocean Optics HR4000 spectrometer. The spectrum could be tuned over the entire bandwidth with a 40-μm change in the cavity length.
The pulse repetition rate of 7.08 MHz and temporal stability were investigated using a radio-frequency (rf) spectrum analyzer connected to a Thorlabs DET100A Si photodetector and DET10D InGaAs photodetector for the pump and signal, respectively. The free-running pump showed significant temporal jitter that could be reduced with active stabilization. This noise appeared in the OPO rf spectrum as well, as is expected because of the lack of active control [shown in Fig. 4(a)]. Despite the instabilities, this system allowed forless than 2% root mean square (rms), peak-to-peak power fluctuations over the course of 4 h and control of the center wavelength of the OPO to within 5 nm. The beam quality of the OPO was measured to be M2 < 1.15 using a Spiricon Pyrocam III and showed minor astigmatism [as shown in Fig. 4(b)].
Signal and idler powers were measured simultaneously in this system and are shown in Fig. 3(c). The pump threshold of the OPO was 14.5 W. Reliable performance was obtained for signal (and idler) powers of 2.5 W (5.2 W) corresponding to 350-nJ (730-nJ) pulses with a slope efficiency of 14%. At maximum pump power, the OPO was unstable and the beam became deformed from local heating of the BiBO crystal. The maximum output powers obtained under these conditions were 3.0 W with 430 nJ pulses and 7.6 W with 1.1-μJ pulses from the signal and idler, respectively. The conversion efficiency, calculated by comparing the amplified signal beam and the measured power of the idler with the total pump power, was estimated to be 25%, decreasing near maximum power to 22%. With improvements to the thermal mitigation, signal pulse energies as high as 700 nJ could be obtained from this system by reducing the crystal length and improving the method of heat extraction. Additionally, system design improvements could improve the efficiency of the system to make better use of available pump power.
We have demonstrated sub-500-fs pulses from a synchronously pumped ultrafast OPO operating near 2.08 μm with pulse energies of 345 nJ with a repetition rate of 7.08 MHz. Limitations in performance were caused by local heating effects resulting from linear absorption of the high-average-power signal and idler beams. With improvements to the engineering of the system, microjoule pulse energies at power levels in the tens of watts will be achievable based on recent advancements to a number of pump sources, making ultrafast OPO’s a unique solution to providing scalable high-energy pulses with broad tunability and excellent beam quality.
This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944, the University of Rochester, and the New York State Energy Research and Development Authority.
This report was prepared as an account of work sponsored by an agency of the U.S. Government. Neither the U.S. Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the U.S. Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the U.S. Government or any agency thereof.
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