1. Introduction

Widely tunable and narrow-bandwidth lasers in mid-IR region are particularly attractive for many important applications in atmospheric pollution monitoring, remote target detection and high-resolution molecular spectroscopy [1–5]. Mid-IR lasers using an OPO based on periodically poled magnesium-oxide doped lithium niobate (PPMgLN) have attracted considerable attention due to their high efficiency, wide tuning and mature pump sources [6–10]. In general, the narrow-bandwidth output of the idler from a pulsed OPO not only requires a high-quality narrow-banded pump source, but also has to use additional wavelength selective elements such as etalons or blazed gratings in the cavity to suppress the bandwidth of the oscillated signal [11, 12]. However, inevitable additional losses caused by the wavelength selective elements not only raise the threshold of OPO, but also reduce the conversion efficiency of the idler [12–14], for example, Peng et al. reported a PPMgLN-OPO with a tuning range of 400 nm, the idler bandwidth was about 0.6 nm by theoretical analysis, the idler optical-to-optical conversion efficiency was about 12.6% at 2.9 μm, and the signal optical-to-optical conversion efficiency was about 19% at 1.655 μm [12]. Reflecting volume Bragg gratings (RBGs) with an operating wavelength range of 350-2500 nm are excellent wavelength-selecting elements with low losses and high damage threshold, which have been widely used in laser devices to suppress the output bandwidth [15–18]. However, the tuning range of idler with a narrow-bandwidth is limited by the temperature insensitivity and operating temperature range of RBG [16–18]. Alternatively, the self-injection-seeding technique can feasibly control the bandwidth by selecting the longitudinal modes of the seeding in a widely tunable laser device [19, 20]. Recently, Rahm et al. reported a mid-IR output with a narrow bandwidth of 0.035 nm and a wide tunable range of about 250 nm in an optical parametric generator (OPG) system by this approach [21]. However, unlike an OPO system, this OPG system lacking of oscillating amplification produced 0.17 W mid-IR output, with a pump to idler conversion efficiency of about 8.5% [21]. Thus, a potentially practicable way of achieving widely tunable, narrow-bandwidth and efficient mid-infrared radiation is to apply this self-injection seeding technique in an OPO system pumped by a narrow-linewidth laser.

In this paper, we have designed a self-seeding PPMgLN-OPO system double-pass pumped by a single-longitudinal-mode pulsed Yb-fiber laser. Using a master oscillator with an etalon reflector and a self-seeding oscillator with another etalon reflector to restrict the bandwidth of the signal, the bandwidth of the idler is narrowed to about 0.35 nm, with an idler output power of 2.67 W and an optical-to-optical conversion efficiency of 17.4%. The idler wavelength is widely tunable via synergistically controlling the temperatures of PPMgLN crystal and one of etalons.

2. Experimental setup

A schematic diagram of the experimental setup is shown in Fig. 1. The PPMgLN-OPO is pumped by a commercial pulsed Yb-doped fiber laser (Advalue Photonics, AP-P-SF). The pump source provides a linearly polarized single-longitudinal-mode output with a pulse duration of 312 ns at a repetition rate of 75 kHz, the longitudinal mode property measured by a F-P scanning interferometer with a free spectral range of 1.5 GHz (Thorlabs, SA200-8B) is shown in Fig. 2(a), and the estimated linewidth of about 95.9 MHz is shown in the inset of Fig. 2(a). A half-wave plate is employed to adjust the polarization direction of the pump into the PPMgLN crystal, and then a free-space isolator is used to deter any remaining backward laser radiation from entering the fiber laser to assure adequate protection for the fiber pump source. To ensure an optimum overlap of the pump and oscillated signal beams, the pump beam is focused to a radius of about 100 μm at the center of the PPMgLN crystal by a double-convex lens with a focal length of about 300 mm. The singly resonant dual etalon-coupled cavities consist of three mirrors (M1, M2 and M3) and two etalons. The master oscillator cavity consists of the mirror M1 and etalon 1, and the self-seeding cavity consists of the mirror M1 and etalon 2. The plano-concave input mirror M1, with a curvature radius of 500 mm, is coated with antireflection (AR) coating for the pump wave (1.065 μm), and a high-reflection (HR) coating for the signal (1.5-1.75 μm) and the idler waves (2.8-3.1 μm). The 45° flat mirror M2 is AR coated for the pump and the signal waves, and HR coated for the idler wave. The crescent mirror M3, with a radius of curvature of 80 mm for the concave and the convex, is AR coated for the signal wave on both surfaces and HR coated for the pump wave on the concave, and M3 is used to reflect the first pass pump beam and then focus it into the PPMgLN for a second-pump. Etalon 1 with a free spectral range of 0.19 nm and etalon 2 with a free spectral range of 0.36 nm are coated on both surfaces to achieve a reflectivity of 36% for the signal wave. For adjusting and stabilizing the temperature, the etalon 2 is mounted into a homemade oven with an accuracy of more than ± 0.1 °Ϲ and a temperature range up to 100 °Ϲ. Wrapped in polytetrafluoroethylene and mounted into another homemade oven with a more than ± 0.1 °Ϲ accuracy and a temperature range up to 200 °Ϲ, the PPMgLN crystal is a wafer with MgO concentration of 5mol%, a uniform quasi-phase-matched (QPM) period of about 31.2 μm, and dimensions of 1 mm × 8 mm × 45 mm. Both end surfaces of the crystal are AR coated for the pump, the signal and the idler waves.

 

Fig. 1 Schematic diagram of experimental setup.

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Fig. 2 (a) The longitudinal mode property of the pump laser measured with an F-P scanning interferometer. The spectrum of idler (b) only with ordinary mirror, (c) only the master oscillator remained, (d) with dual etalon-coupled cavities.

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3. Experimental results

To obtain the bandwidth of the idler from a free-running OPO for comparison purposes, the etalon 1 was replaced by an ordinary mirror with HR coating for the signal wave. The spectrum of the idler, measured by a Spectrometer-SM301-EX spectrum analyzer when the pump power was 15.3 W, is shown in Fig. 2(b). The bandwidth of the idler was measured to be about 34 nm. It is evident that the bandwidth of the idler from a free-running OPO is wide. For the next test, only the master oscillator in which etalon 1 with a theoretically analyzed effective reflectivity of about 76% at the reflection peak was used as the cavity mirror remained and the pump power was 15.3 W, in this test, the spectrum of the idler measured by a Bristol-721A laser spectrum analyzer is shown in Fig. 2(c). It is evident that the bandwidth of the idler is still wide and also has several separate narrow sidebands which correspond to several separate narrow sidebands of the oscillated signal due to the gain modulation by the etalon 1 cavity mirror, which conforms with De Matos’s theory analysis [22]. To suppress the sidebands for further narrowing the idler bandwidth, etalon 2 mounted in a homemade oven was adopted to form a self-seeding cavity in combination with the mirror M1, and the temperature of etalon 2 was adjusted accurately to make sure one of reflection peaks matched the central wavelength of the oscillated signal from the master oscillator, the theoretically analyzed effective reflectivity of the dual etalons at the reflection peak is about 90%. Then, the measured spectrum of the idler is shown in Fig. 2(d), it is evident that the sidebands are suppressed, this is because the center's narrowband has a greater model competitive advantage due to the coupling of self-seeding dual etalon-coupled cavities. The measured bandwidth of the idler is narrowed to about 0.35 nm.

In the experiment, the spectrum of the idler was monitored by a Bristol-721A laser spectrum analyzer. When one of reflection peaks of the etalon 2 matched the central wavelength of the oscillated signal from the master oscillator, the narrow-bandwidth output of the idler was achieved. With the temperature of the PPMgLN crystal controlled at 62.52 °Ϲ, the output power was measured by a laser power meter (Ophir), the output power and conversion efficiency of the idler versus pump power are shown in Fig. 3. It can be seen that the threshold of OPO with dual etalon-coupled cavities is about 5.6 W, and it is increased compared to the free-running case, in which the threshold of OPO is only about 3.9 W, while, it is reduced compared to the single etalon-coupled cavity case, in which the threshold of OPO is about 7.3 W. When the pump power is 15.3 W, the maximum output power of the idler is 2.67 W, which corresponded to an optical-to-optical conversion efficiency of 17.4%. With the pump power fixed at 15.3 W, the idler output power, as shown in Fig. 4, was measured at different wavelengths varying from 2.85 to 3.05 μm by adjusting the temperature of the PPMgLN and etalon 2 accordingly, and calculated output power variation of the idler at different wavelengths is about ± 6.7%. The idler bandwidths at the wavelengths of 2.849 μm, 3.003 μm, 3.025 μm and 3.048 μm were about 0.357 nm, 0.374 nm, 0.377 nm and 0.361 nm, respectively. With the temperature of the PPMgLN crystal fixed at 62.52 °Ϲ and the pump power fixed at 15.3 W, the peak-to-peak power fluctuation of idler at the center wavelength of 3.031 μm was measured to be about 11.7% in 30 min, which is further deteriorated compared to about 4.9% of free-running. At the pump power of about 15.3 W, the beam sizes of the idler at different locations was measured by a beam profiling camera (Spiricon, PY-III-C-A), and the beam quality M² of the idler were measured to be 1.7 and 1.6 at the horizontal and vertical directions, respectively.

 

Fig. 3 The varying power and efficiency of the idler with different pump power.

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Fig. 4 The idler power at different wavelengths when the pump power was at 15.3 W.

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4. Conclusion

The idler output bandwidth of a singly resonant OPO is seriously affected by the bandwidth of the pump and the bandwidth of the oscillated signal. We have designed a self-seeding architechure consisting of dual etalon-coupled cavities in a singly resonant OPO system pumped by a single-longitudinal-mode laser. By this approach, the bandwidth of the oscillated signal was suppressed, and the bandwidth of the idler was narrowed efficiently, which gives a mid-IR output with an maximum optical-to-optical conversion efficiency of 17.4%, furthermore, a widely tunable range from 2.85 to 3.05 μm was achieved by synergistically adjusting the temperatures of the PPMgLN and one of etalons. The mid-infrared OPO reported here may sheds a bright future to develop a new generation of narrow-bandwidth, widely tunable and efficient parametric devices. Further study will be focus on the spectrum stability and continuous tuning of the idler with narrow bandwidth by changing and stabilizing the temperature of the etalon1.