1. Introduction

High-power, narrow-bandwidth lasers operating in 2-5 μm wavelength region have long been of interest for various applications, including high-resolution molecular spectroscopy, remote sensing, medical diagnosis, military countermeasures [1–5]. Among many typical mid-infrared lasers, optical parametric oscillator based on periodically poled Mg-doped LiNbO₃ (PPMgLN) is one of the best candidates for its unique combination of high power, good beam quality and wide wavelength tunability [6–9]. However, the emission bandwidth of free-running PPMgLN OPO is rather broad since the only wavelength restriction is momentum conservation fulfilled by quasi-phase-matched (QPM) crystal. The most common approaches of producing high-power, narrow-bandwidth mid-infrared lasers are injection-seeded OPO and difference frequency generation (DFG) [5, 10], but with a trade-off between the system complexity and laser performance. Many efforts have also been made to achieve narrow-bandwidth mid-infrared OPO laser by using additional wavelength selective elements such as etalon or blazed grating in the cavity [11–15]. However, extra elements inserted in the resonator generally inevitably induce additional cavity losses. PPMgLN OPO with bandwidths of less than 0.7 nm at 2.98 µm had been demonstrated by inserting an etalon into the cavity, and the central wavelength drift was observed when the incident pump power was increased [12]. Blazed grating has also been successfully used for the bandwidth control of a PPKTP-OPO and a spectral bandwidth of 2.6 nm has been achieved [14]. Volume Bragg gratings (VBG) are alternative wavelength selecting elements with excellent spectral selectivity and high damage threshold. VBG with operating wavelength of less than 2700 nm are commercially available and have been successfully used in near infrared OPO to narrow the laser bandwidth [16–22]. Jiro et al. reported a 30Hz tilted-PPMgLN OPO with maximum output energy of 61mJ and a bandwidth of less than 1.4 nm at 2.128 μm by using a VBG as the output coupler [16]. Peter et al. achieved a single-longitudinal-mode 1.55 µm laser output with a bandwidth of 1.65 MHz from a ring cavity PPLN OPO based on a variable-reflectivity VBG as an output coupler [17]. Since VBG in the preferred mid-infrared wavelength region around ~3 μm is not yet obtainable, it is generally impossible to directly generate narrow-bandwidth laser emission at this wavelength region with VBG as a wavelength selecting element. A possible way to demonstrate narrow-bandwidth PPLN-OPO is to pump the PPLN with a narrow linewidth 1064 nm laser and restrict the signal bandwidth with a commercial VBG, hence, the preferred mid-infrared wavelength idler laser will also be wavelength-narrowed through difference frequency relation between pump and signal laser.

In this paper, we have demonstrated a high-power, narrow-bandwidth mid-infrared PPMgLN OPO at 2.907 µm. By using a narrow-bandwidth, pulsed 1.064 μm Nd:YAG MOPA-configured laser as the pump and a VBG with transparency around 400-2700 nm to reduce the bandwidth of the signal laser, the spectral bandwidth of the idler laser operating at 2907.55 nm was narrowed to <0.7 nm for 51.7 W of average output power. A free-running PPMgLN OPO exhibits maximum average output power of 71.6 W at 2.907 µm with a bandwidth of 9.22 nm. To our best knowledge, this is the highest output power for mid-infrared PPMgLN OPO at this wavelength range.

2. Experimental setup

The experimental setup is shown in Fig. 1. The L-shaped cavity was composed of plane pump mirror M1, output cavity mirror M2, VBG, and PPMgLN. The PPMgLN (QPM period is 31.3 µm) has 50 mm length and 3mm × 5mm aperture. Both end surfaces of the PPMgLN were antireflection (AR) coated for the pump (1.064 μm), signal (1.6-1.8 μm), and the idler (2.6-3.1 μm). The PPMgLN was mounted in the thermoelectric controlled copper holder with 0.1°C accuracy and temperature range up to 200°C. The 45° flat mirror M1 was AR coated at 1.064 μm, high-reflection (HR) coated in the 1.6-1.8 μm and 2.6-3.1 μm regions. The flat coupling output mirror M2 was antireflection coated around 1.064 μm and 2.6-3.1 μm wavelength region, and coated with reflectivity of ~60% around 1.6-1.8 μm wavelength region. The filter M3 had a reflectivity larger than 99.5% at 1.064 μm and 1.6-1.8 μm, a transmission larger than 98% at 2.6-3.1 μm. A volume Bragg grating was used as a cavity mirror in order to obtain narrow-bandwidth signals. The VBG had a specified reflectance bandwidth of less than 0.08 nm (FWHM) at 1678.9 nm (at 22 °C), with a diffraction efficiency greater than 90%. The dimensions of the VBG were 6.5 mm × 6.5 mm (in cross section) × 29 mm (in thickness). The grating direction tilt was 1.5 degrees with respect to the surface in order to eliminate unwanted parasitic oscillation. In respect that the elevation of the VBG temperature may induce a slight drift of the central wavelength of the VBG, we also utilized the similar thermoelectric controlled copper holder as mentioned above to accurately control the temperature of the VBG. The VBG reflectivity peak can be tuned via temperature tuning from 1678.9 nm to 1681.7 nm. The PPMgLN OPO was pumped by a polarized 1064.44 nm Nd:YAG slab MOPA laser with maximum output power of 310 W at 20 kHz and a spectral bandwidth of 0.04 nm. The beam quality M² factors were 1.53 and 2.67 in the horizontal and vertical directions, respectively. The pulse width was about 60 ns. The pump laser was focused by coupling lenses into the PPMgLN crystal (5mol% MgO doped) with a spot size of 1.4 mm × 1.7 mm. The beam polarization was matched to the e→e + e interaction in PPMgLN, hence, maximal nonlinear coefficient d₃₃ (27.4 pm/V) was available and walk-off was avoidable.

 

Fig. 1 Experimental setup of the narrow-bandwidth mid-infrared PPMgLN OPO, a VBG with reflectance bandwidth <0.08 nm at 1678.9 nm was used as a cavity mirror to narrow the spectral bandwidth of signal laser.

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3. Results and discussion

To evaluate the performance of the free-running OPO for comparison, the VBG was replaced by an ordinary mirror (OM) with high reflectivity at 1.6-1.8 μm. Figure 2 shows the output power of the PPMgLN OPO as function of pump power. Maximum output power of 71.6 W at 2907.6 nm was obtained for free-running operation under 310 W of pump power, corresponding to a slope efficiency of 26.7%. No saturation was observed up to the maximum pump power, suggesting further power scalability by simply improving the pump power. When using VBG for spectral control and narrowing of the signal laser, the efficiency decreased slightly compared with that of the free-running operation mode. This may be attributed to a slight thermal effect in the bulk VBG and more stringent temperature matching requirements in the VBG-based PPMgLN OPO. Output power saturation arose above 230 W of pump power. It may be attributed to the phase mismatching caused by temperature gradient increase in nonlinear interaction region of the PPMgLN. A maximum output power of 51.7 W at 2907.55 nm was obtained under 295 W of pump power by setting the temperature of PPMgLN at 92.7 °C and VBG at 25 °C, corresponding to a slope efficiency of 22.5%. The improvement in conversiton efficiency compared with that of 16.1% we have obtained with an etalon [12] is due to the reduced resonator loss with a VBG.

 

Fig. 2 Mid-infrared wavelength idler laser output power versus pump power at free running operation without VBG (○) and wavelength-narrowed operation with VBG (△).

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Figure 3 shows the spectra of pump source and OPO signal and idler lasers. The spectra of pump and signal lasers were measured by an AQ6370C spectrum analyzer under spectral resolution of 0.02 nm with wavelengths of 1064.44 nm and 1679.11 nm and bandwidths of about 0.04 nm and 0.12 nm, respectively. The idler wavelength of 2907.55 nm with bandwidth of 0.68 nm as shown in Fig. 3(d) were measured by a Mcpherson 209 spectrum analyzer with theoretical resolution of 0.09 nm. The bandwidth of 2907.6 nm idler laser in the free-running OPO was measured to be about 9.22 nm as shown in Fig. 3(c). It is evident that VBG takes a critical role for narrowing the mid-infrared idler laser bandwidth even though the VBG just selects and restricts the signal laser oscillation at 1679.11 nm. The signal central wavelength in VBG-based OPO linearly depends on the temperature of VBG, and the slope was measured to be about 0.014 nm/K. The absorption of signal laser could result in a slight drift of the VBG reflectivity peak wavelength. By utilizing thermoelectric controlled copper holders to accurately adjust the temperature of the VBG (20 to 200 °C) and PPMgLN (92.6 to 95.5 °C), continuously idler laser tunable range of about 8 nm (2900 to 2908 nm) was achieved. Additionally, the output idler wavelength in free-running OPO was tuned from 3.0 to 2.6 μm by simply elevating the PPMgLN crystal temperature from 30 to 200 °C.

 

Fig. 3 The spectrum of (a) the pump laser; (b) the OPO signal laser (with VBG as a cavity mirror); (c) the OPO idler laser (with OM as a cavity mirror); (d) the OPO idler laser (with VBG as a cavity mirror).

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Beam quality of 2907.55 nm laser in the VBG-based OPO was evaluated by measuring the size of laser spot using the knife-edge method at different locations and then hyperbolic fitting the experimental data, Fig. 4 shows the beam intensity distribution. The beam quality M² factors were ~5 and ~7 in the horizontal and vertical directions, respectively.

 

Fig. 4 Near-field intensity distribution of 2.907 µm laser beam.

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4. Conclusion

In conclusion, a simple high power, narrow-bandwidth ~2907 nm mid-infrared OPO has been demonstrated. By using VBG at signal wavelength as a spectral narrowing element and narrow-spectrum pulsed 1.064 μm Nd:YAG MOPA laser as pump source, spectral bandwidth of <0.7 nm 2907.55 nm mid-infrared idler laser was measured at a maximum average output power of 51.7 W, corresponding to a slope efficiency of 22.5%. It manifests that high power, narrow-bandwidth mid-infrared laser can be generated with a compact OPO system through narrowing the bandwidth of the signal laser with commercially available VBG. Further study will focus on lifting the output power with multilevel optical parametric amplifiers.