Nanosecond optical parametric oscillators (ns-OPOs) are efficient tunable sources in the near- and mid-infrared (NIR and MIR), but a narrow bandwidth and a wide tunability often come at the expense of system complexity [1–4]. Quasi-phase-matching (QPM) [5] provides design flexibility for the nonlinear process [6] and offers the best route for all three-wave mixing geometries (co-, counter-, and backward-propagating) [7]. With sub-micrometer ferroelectric domain gratings in bulk periodically poled Rb-doped potassium titanyl phosphate (PPRKTP) it has been possible to demonstrate nanosecond backward-wave optical parametric oscillators (BWOPO) [8] and, more recently, efficient first-order backward second harmonic generation [9]. In a BWOPO, the two down-converted parametric waves travel in opposite directions (see QPM-condition in Fig. 1), creating a self-seeded parametric oscillation [10]. The BWOPO is fundamentally different from an OPO as it is cavity-free. It reduces the complexity to a minimum and, importantly, provides a possibility for a fast, broad-range, and mode-hop-free spectral tuning. In addition, BWOPOs are efficient, with typical conversion exceeding 45% [8], reduced back-conversion, and substantially higher temperature stability [10] compared to OPOs with co-propagating geometry. BWOPO’s narrow gain bandwidth [11], stemming from the phase-matching condition, intrinsically enables the generation of narrowband pulses [12].

 

Fig. 1. The BWOPO concept with energy and QPM phase-matching diagram. The incident, tunable pump (blue/violet) is split into two parametric waves propagating in opposite directions. When the pump wavelength is tuned, shown with a color gradient, the forward wave follows the pump, shown in colors from red to yellow, while the backward wave is frequency stable.

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Long-range resolved differential absorption (DIAL) spectroscopy of atmospheric greenhouse gases requires a narrowband, mJ-level pulses in the MIR spectral range. Nonlinear sources on air- and space-borne platforms, such as MERLIN [13], must be mechanically robust and withstand a low Earth orbit radiation environment. PPRKTP—the material used in BWOPOs—is resilient to the proton and gamma radiation background levels relevant for low Earth orbits [14,15]. Recently we measured close to transform-limited bandwidths in a ns-BWOPO pumped with a single-axial mode laser [16]. Combining BWOPOs with a tunable pump of high beam quality that delivers mJ pulses could result in a highly enticing seed source for DIAL systems for greenhouse gas monitoring [17]. The robustness and cavity-free arrangement of the BWOPO will especially benefit mobile DIAL platforms.

This work presents a robust and stable continuously and precisely pump-tunable ns-BWOPO. It was pumped with an injection-seeded, high-energy, Yb-based hybrid master-oscillator power amplifier (MOPA) system, generating 15 ns pulses [see Fig. 4(b)] at a 5 kHz repetition rate [18]. The Yb-MOPA pumped BWOPO generated a 1981.1 nm forward signal and 2145.6 nm backward idler, reaching a record efficiency of 70% and an average power of 5.65 W. We demonstrate continuous BWOPO signal tuning over 10 GHz by tuning the seed of the Yb-MOPA within the same frequency range. The tuning range is limited by the seed fiber laser tuning range and not by the BWOPO. Apart from the steering mirrors in the bulk amplifier, the system does not contain any movable mechanical elements. The system is also free from optical cavities, making it robust and easy to align.

Figure 1 shows an intuitive illustration of the tuning of the MIR waves in a BWOPO. As the QPM grating vector (${\textrm{K}_\textrm{G}} = 2\pi /\mathrm{\Lambda }$) has a similar magnitude to the pump wave vector, ${\textrm{k}_\textrm{p}}$, it forces one of the parametric waves to travel backward with respect to the pump, forming a self-aligned and self-seeded parametric oscillation. Like a usual OPO, the BWOPO has a well-defined oscillation threshold. It is worth mentioning that the close-to-threshold gain bandwidth for counter-propagating interactions entails a very narrow acceptance, supporting transform-limited pulses in the nanosecond regime [11].

The BWOPO geometry also gives rise to its peculiar tuning properties, as described in Eq. (1) [10]. In this work, the forward wave (${\textrm{k}_\textrm{f}}$) was the signal (1981.1 nm), while the backward (${\textrm{k}_\textrm{b}}$) was the idler (2145.6 nm). The forward ($\mathrm{\Delta }{\omega _f}$) and backward wave frequencies ($\mathrm{\Delta }{\omega _b})$ tune with the pump ($\mathrm{\Delta }{\omega _\textrm{p}}$) as:

Two uncoated PPRKTP crystals with the same grating period of 580 nm were investigated in the experiments. Their apertures were 1 × 3 mm², while the lengths were 7 and 17 mm, respectively. The crystals were mounted in a copper block that was kept at a constant temperature slightly above room temperature. The crystal alignment accuracy of ±1° was sufficient. The pump beam with M²= 1.2 was focused (f = 150 mm) to a radius (1/e²) of 140 µm. Thermopile power meters were used to measure the input pump, signal, idler, and depleted pump at the output. The linewidths of the forward and the backward waves were measured using a piezo-driven, fiber-coupled Fabry–Perot (FP) interferometer (free spectral range of 10 GHz, F > 150). We simultaneously monitored the backward and forward wave wavelengths with NIR- and MIR- wavemeters (HighFinesse LSA IR-II and -III) as the pump was tuned. A fiber-coupled optical spectrum analyzer (OSA) (Yokogawa AQ6375) was used to measure the MIR spectra to complement the wavemeter measurements. The MIR temporal envelopes were measured with a HgCdTe detector (VIGO PEM-10.6), while a Si-detector (THORLABS DET10A2) was used for the 1030 nm pump pulses. The pulses were displayed on an oscilloscope (Tektronix DPO 4104) with a 1 GHz bandwidth. Dielectric beam splitters separated the backward and forward waves from the residual pump, and spectral filters removed any residual pump radiation.

Figure 2 compares the output power and efficiency for the two BWOPO crystals. The threshold was 370 µJ for the 7 mm sample and 70 µJ for the 17 mm sample, respectively. The latter corresponds to a record-low threshold intensity of 19.2 MW/cm², compared to the earlier record of 83 MW/cm² [11], thanks to the high pump beam quality, optimized structuring, and longer crystal. Using the long-pulse Gaussian approximation from Ref. [11], one can estimate the effective nonlinearity to be 7 pm/V and 7.2 pm/V for the long and short crystals, respectively. PPRKTP for regular type-0 OPOs have effective nonlinearity close to 10 pm/V [19]. The crystals used here have marginally lower nonlinearity, most likely due to a non-optimal duty cycle. The crystals have a very similar quality of the QPM structure as evidenced by the effective nonlinear coefficients and from comparing the efficiencies above the threshold [see Fig. 2 (b)]. The combined parametric output (signal + idler) was 1.13 mJ with a 1.92 mJ pump and 0.68 mJ with a 0.96 mJ pump for the short and long crystals, respectively. The output energies reached the millijoule level for both crystals. It should also be noted that high average powers were obtained, 3 W and 2.65 W, for signal and idler, respectively. The maximum output energy in the short crystal was limited by the optical damage of the surface at an average pump power of 12.5 W (8.1 J/cm²), close to the previously reported nanosecond damage threshold of 10 J/cm² [20] that was measured at a 100 Hz repetition rate. For the longer crystal, we kept the peak intensities well below the damage threshold. The maximum conversion efficiency for the combined signal and idler for the 7 mm crystals was around 60%, while a record-high optical-to-optical conversion efficiency of >70% was obtained with the 17 mm long crystal. The pump depletion follows the efficiency, as shown for the 17 mm crystal (Fig. 2(b)). The error bars represent the measurement uncertainty. A beam profile image of the backward wave is shown as an inset in Fig. 2(b). The camera filter causes the interference fringes seen superimposed on the beam profile.

 

Fig. 2. (a) Output pulse energies and conversion efficiencies for 7 mm and 17 mm long crystals. (b) Conversion efficiency in the two BWOPO crystals as a function of the pump intensity to threshold intensity ratio. Inset, the beam profile of the backward wave at 100 mW output.

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The numerical solution of nonlinear coupled wave equations (see Ref. [11]) confirms the high conversion efficiency and the virtual absence of back-conversion. The result shown in Fig. 2(b) by a dotted line was obtained for the PPKTP structure with an effective nonlinearity of 7 pm/V, using a plane wave model. The theoretical efficiency values had to be corrected by multiplying them homogeneously by a factor of 0.75 in order to get the alignment with the experimental data in Fig. 2(b). Such a correction is expected for Gaussian beams where the pump depletion is spatially inhomogeneous [21].

We measured the signal (1981.1 nm) and idler (2145.6 nm) wavelengths with a resolution of 0.05 nm using the OSA, and they are in accordance with the calculation using the phase-matching condition. In Fig. 3, we show how the signal and idler tuned when the pump wavelength was modulated over a 10 GHz range. These traces were measured using wavemeters. The forward wave followed the pump tuning, as predicted in Eq. (1) while the wavelength of the backward wave remained within the resolution limit of the wavemeter (30 MHz). Note that with the current equipment, we were unable to measure changes in the MHz range, which would be required to follow the modulation in the backward wave. The seed laser of the Yb-MOPA system provides a fast (20 kHz) modulation, which could make pulse-to-pulse modulation of the MIR pulses possible. However, in this work, the tuning rate was limited to the 10 s integration time of the MIR wavemeter. Figure 3(a) features two sequentially acquired data sets. The MIR wavemeter was used for the forward and backward waves by connecting pre-aligned fibers to the wavemeter’s input. Figure 3(b) is generated from Fig. 3(a) by overlapping the time windows where the pump detuning protocol was identical in the sequential measurements.

 

Fig. 3. (a) Measured frequency tuning of the pump and the BWOPO forward and backward wave. (b) Zoomed in part of the tuning.

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A narrow linewidth of the light source is vital to attain accuracy and sensitivity in spectroscopic measurements. We investigated the MIR bandwidths using the scanning FP interferometer. Figure 4(a) displays the measured spectra of the forward and backward waves. The central frequency is shifted to separate the traces over a common axis. The bandwidths were 407 MHz and 311 MHz (FWHM), respectively. Figure 4(b) shows the temporal traces, which were 12 ns for the parametric outputs. At an average pump power of 10 W (energy of 2 mJ), the averaged pump pulse had an FWHM Fourier transform bandwidth of 70 MHz, while that of the BWOPO pulses was about 90 MHz.

 

Fig. 4. (a) Fabry–Perot measurements of the bandwidths of the BWOPO parametric waves with FWHM are given in the legend. (b) Single-shot temporal traces of the waves.

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Some of the broadening in the BWOPO spectra can be associated with amplitude modulation which is transferred from the pump to the parametric waves [7]. The undepleted pump contained amplitude modulation, most likely related to parasitic reflections originating in the fiber amplifier in the hybrid Yb-MOPA [18]. As can be seen in Fig. 4(b), the depleted pump pulse at 59% efficiency is very smooth and modulation-free. The amplitude modulation is transferred to the backward and forward waves. The somewhat broader BWOPO spectra observed in time-integrated FP measurements can also tentatively be attributed to small temperature variations in the PPRKTP crystal and/or possibly a contribution of a small noncollinearity from focusing the pump [16]. For instance, a temperature variation of ±0.1 K during the measurement would result in the variation of the central frequency of the BWOPO wave by about ±200 MHz [16]. The BWOPO bandwidth is still very narrow, of similar magnitude as reported in injection-seeded nanosecond singly-resonant OPOs [22].

In this particular system, the tuning is limited by the seed fiber laser used for the Yb-MOPA pump. Ultimately, the limitation would be the Yb:YAG power amplifier gain bandwidth [23,24]. That is a sufficient spectral range to cover several closely spaced Doppler-broadened absorption lines of greenhouse gases in the spectral region close to 2 µm [19]. The Yb-MOPA tuning range could be increased, e.g., by using an ECDL seed source [4]. A BWOPO can readily cover this tuning range with continuous mode-hop-free tuning. BWOPO temperature tuning can achieve precise wavelength targeting at 2 GHz/K in this spectral range [16].

In summary, we have experimentally demonstrated a cavity-free, continuously tunable MIR Yb-MOPA BWOPO system, delivering 12 ns, mJ level pulses at a repetition rate of 5 kHz. We measure a linear detuning, close to the predicted 1 to 1.001 Hz/Hz frequency translation from the pump to signal at 1981 nm, while the idler at 2145 nm should have three orders of magnitude lower tuning rate. The optical-to-optical efficiency exceeded 70%, comparable to state-of-the-art OPOs [25] and close to the theoretical limit for Gaussian beam-pumped parametric devices [21]. This tunable system is robust, does not contain mechanical moving parts, and does not require active adjustments once aligned. These features are attractive for spectroscopy, e.g., DIAL systems on mobile platforms.

The pump system used here provides fast tuning and can be used for pulse-by-pulse precise central frequency tuning, although over a limited tuning range of 10 GHz. A slower tuning over a frequency range of about 300 GHz could be achieved by temperature tuning the PPRKTP crystal with up to 150°C. Employing the refractive index from Refs. [26,27], we find that three QPM periods in PPRKTP of interest for greenhouse gas monitoring are 565 nm (CO₂, 2051.00 nm), 511 nm (CH₄, 2289.90 nm), and 586 nm (H₂O/HDO, 1982.78 nm/1982.45 nm) if the same pump system is used. A Yb-MOPA-BWOPO system with output energy at the mJ level can be combined with a simple single-stage OPA energy booster providing tens of mJ, e.g., by employing large aperture PPRKTP crystals [28]. For instance, an OPA realized with PPRKTP with a QPM period of 38.75 µm would have a sufficiently broad bandwidth in the range of 1980nm–2050 nm to cover the relevant greenhouse gas absorption lines [19].