1. Introduction

High-power eye-safe laser sources have paramount applications in laser radar, remote sensing, and range finders. The current methods for efficient laser generation in the eye-safe band include Erbium-doped fiber/crystalline lasers, Raman lasers, and optical parametric oscillators (OPOs) [1–4]. Despite their advantages, Erbium-doped fiber amplifiers (EDFAs) are not feasible for high-pulse-energy operation with nanosecond (ns) duration, which greatly restricts their applications. OPOs pumped by near-infrared solid-state lasers at 1.06 μm (e.g., Nd:YAG lasers) are ideal to achieve high-peak-power (> 1 MW) output and have been extensively investigated. The most popular OPO schemes are based on noncritically phase-matched (NCPM) KTP and KTA crystals [4–6], which have high laser damage threshold, large nonlinear gain, large acceptance angle, and no walk-off, and are applicable for multimode pumping. Their maximum single pulse energy output can reach hundreds of millijoules with 30% conversion efficiency [7].

Despite the significant achievements during the past decade, the solution to realize a wide-range tunability for eye-safe OPOs is still being investigated [8,9]. Given that the refractive indices for NCPM KTP and KTA crystals are insensitive neither to orientation nor to temperature, the traditional methods for OPO tuning using birefringence are inapplicable. Injection seeding can lock the OPO resonant frequency to a narrow-linewidth laser and allows frequency tuning within the PM bandwidth, which is effective in controlling the OPO frequency switching between “on” and “off” to gas resonances in differential absorption lidar (DIAL) [10,11]. However, the required frequency should coincide with the PM bandwidth; otherwise, injection seeding becomes invalid.

In this paper, we present a widely tunable singly resonant NCPM OPO in the eye-safe range using noncollinear PM in a specially designed three-mirror ring cavity. The slight tuning of the tilt angle of a cavity mirror resulted in a noncollinear angle between the pump and the resonant signal beams, which introduced another freedom to change the PM parameters for wavelength extension. Although noncollinear PM is different from NCPM, the discrepancy was ignored given that the noncollinear angle was very small and all the merits of NCPM still existed. A wide tuning range of 1572.9–1684.2 nm was achieved using a KTP crystal pumped by a 1.06-μm Nd:YAG laser at 3.1° noncollinear angle. The maximum signal power was 4.6 W (46 mJ per pulse) at 1572.9 nm for the collinear case, which corresponds to 33.3% conversion efficiency. Although the efficiency decreased with the degradation of nonlinear gain for noncollinear PM, it was higher than 11% over the tuning range. The proposed method is generally practical in wavelength extension for all kinds of OPOs, especially for solving the tuning problem of NCPM KTP/KTA OPOs.

2. Experimental setup

The proposed singly resonant ring-cavity NCPM KTP OPO is shown in Fig. 1. The pump laser was an injection-seeded pulsed Nd:YAG laser (Spectra-Physics Quanta-Ray Lab-190, 1064 nm, 300 mJ, 11 ns, 100 Hz, linewidth < 0.003 cm⁻¹). Prior to the incident to the OPO cavity, the pump beam (10 mm in diameter) first passed through a Teflon diaphragm (6 mm in diameter) and was then collimated by a telescope composed of a convex lens (L1, f = 100 mm) and a concave lens (L2, f = −35 mm) to reduce the beam size into around 2.3 mm. The cavity was in the form of an isosceles right triangle, composed of three plane-parallel BK7 mirrors M1, M2 and M3. M1 was high-transmission (HT, T>98.5%) coated for pump wavelength (1.06 μm) and partial-transmission (PT, T≈20%) coated for resonant signal wavelength (1.5–1.7 μm) at an incident angle of 22.5° for both the pump in-coupling mirror and the OPO signal output coupler. M2 and M3 were both coated for HR (R>99.5%) at the signal wavelength and anti-reflection (AR, R<2%) at the pump wavelength, but were designed to work at different incident angles of 22.5° and 45°. M4 was identical to M2 and was used to change the signal propagation direction to facilitate measurements. The nonlinear crystal KTP was 5 × 5 × 20 mm³ in size and cut along the x-axis (θ = 90°, φ = 0°) for NCPM, with AR (R<0.2%) coatings at both pump and signal wavelengths. The KTP crystal was mounted on a water-cooled aluminum heatsink to remove the heat generated by the absorption of the interacting waves. The reflection angle of M3 could be tuned in the horizontal plane, changing the resonant path away from the pump beam, and enabling noncollinear PM as shown in the insets of Fig. 1. The wavelength tuning and PM details are discussed in the succeeding sections.

 

Fig. 1 Experimental setup of the NCPM KTP OPO pumped by a Nd:YAG laser. The insets demonstrate the details of three interacting waves in noncollinear PM configuration.

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3. Wavelength tuning method

Figure 2 demonstrates the optical arrangement of the OPO cavity as well as the noncollinear tuning method. The pump beam, nonlinear crystal, and cavity mirrors M1 and M2 were all stationary throughout the experiment. Initially, the pump and signal beams are collinear in an isosceles-right-triangle cavity with a hypotenuse length of l. If a circle is drawn based on the diameter connected by the images of the crystal center (Point P) in M1 and M2, that is, P₁ and P₂, respectively, the circle should be tangent to M3. The diameter of the circle d equals the cavity length L, that is, $d=L=(1+\sqrt{2})l$. If M3 moves away by slightly tuning its deflection angle and position in such a manner that the incident point of M3 keeps along the circle, a new resonant cavity is formed, and the resonant signal beam becomes noncollinear to the pump beam. However, their intersection is fixed at the crystal center P, assuring sufficient gain during nonlinear interaction.

 

Fig. 2 Arrangement of the OPO cavity. Wavelength tuning by noncollinear PM is enabled by moving M3 along the circle.

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According to the geometry of noncollinear PM shown in Fig. 1, the law of cosines gives the relation between the k-vector magnitude (k_p, k_s, k_i) and internal noncollinear angle ψ

The biaxial crystal KTP acts similar to a positive uniaxial crystal when the propagation direction is close to the x-axis in the x–z plane. Thus, for type-II PM, the pump and signal waves should be ordinary waves (fast wave, polarizing along the y-axis) while the idler wave should be extraordinary wave (slow wave, polarizing in the x–z plane). On the other hand, the effective nonlinear coefficient for type-I PM in KTP is zero, which does not make sense. Incorporating Eq. (1) and the energy conservation law, the wavelength tuning curve (signal wavelength versus the orientation of the resonant signal k-vector) can be obtained based on the Fresnel equation and the refractive indices for the interacting waves calculated from the Sellmeier equations [4,12] as shown in Fig. 3. An internal noncollinear PM angle of 9.6° (or external angle of 16.7°) can produce a tuning range over 700 nm, while the collinear PM enabled by rotating the crystal can merely produce around 16 nm within the same angle tuning range. Although collinear OPO with a KTP crystal cutting at a much smaller angle (θ) can provide a rather wide tuning range as the slope of the PM curve becomes larger [4,5], obvious side effects will appear, including serious walk-off, small acceptance angle, and increased signal linewidth [13,14]. Sometimes, additional mode-selecting elements are required to maintain a stable and narrow-linewidth operation [15]. However, the noncollinear PM method is more efficient in wavelength extension and the merits of NCPM remain because of the very small noncollinear angle. Tuning a cavity mirror is also easier and more agile than rotating a bulk crystal, exhibiting greater practicability for various applications.

 

Fig. 3 Angle tuning range of KTP OPO for noncollinear (a) and collinear (b) PM. The PM angle represents the orientation of the resonant signal k-vector for both cases.

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4. Experimental results and discussion

The OPO cavity was aligned for collinear interaction at the beginning. The hypotenuse length of the isosceles-right-triangle cavity was 40 mm; thus, the total cavity length was around 96 mm. The output signal wavelength was 1572.9 nm with a linewidth of 0.3 nm measured by an Agilent 86142B optical spectrum analyzer (OSA), coincident with all the other reports in similar case [4–6]. The idler wave at 3.29 μm was not observed because of the considerable absorption in the BK7 mirrors and the KTP crystal, in addition to the lack of special coatings in this range. With a tunable attenuator comprising a quartz half-wave plate and a Brewster plate to adjust the input pump power, the signal output characteristics were measured with an Ophir 30(150)A-HE-17 power meter as shown in Fig. 4. The OPO began to oscillate when the incident pump power was around 5 W (50 mJ per pulse) and went almost linearly with the increase of pump power. The maximum signal output power was 4.6 W (46 mJ per pulse) when the pump power was 13.8 W (138 mJ per pulse, 400 MW/cm² in power density), which corresponds to 33.3% conversion efficiency and 51.8% slope efficiency. The photon conversion efficiency almost reached 50%, which is believed to be the highest for external singly resonant OPOs in the eye-safe range. The high efficiency should be ascribed to both the good beam quality and the high spectrum intensity of the narrow-linewidth pump laser.

 

Fig. 4 Output characteristics of the signal wave at 1572.9 nm.

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The OPO wavelength tuning was realized by simply driving the adjuster on the mirror mount for M3, which could change its deflection angle in the x–z plane of the KTP crystal and rebuild a signal resonator noncollinear to the pump beam. Although theoretically, the location of M3 might deviate slightly from the ideal condition as shown in Fig. 2, the influence was found inconspicuous with a very small noncollinear angle and a KTP crystal that was long enough. Figure 5 shows the signal tuning curve as well as the corresponding output power at the same input pump power of 13.8 W. When the external noncollinear angle (ψ_ext) was tuned from 0 to 5.8°, the signal wavelength varied from 1572.9 nm to 1684.2 nm. Specifically, when the signal path inside the crystal was tuned from θ = 90° to 86.9°, the signal wavelength was tuned by 111.3 nm. The experimental results supported the theories very well. For comparison, the traditional angle tuning by rotating the crystal could only produce a tuning range of less than 2 nm when the internal PM angle was varied by 3.1°, or the tuning range was less than 6 nm if the working temperature was varied from 20 to 100 °C. As the nonlinear gain decreased with the increase of the noncollinear angle and idler walk-off [16], the single-pass loss increased because of the degrading coating performance of all the optical elements and the signal output power gradually decreased with the increase of the noncollinear angle. The minimum signal output power was 1.53 W at 1684.2 nm. Further wavelength extension was still possible by increasing the noncollinear angle. However, the signal power would decline significantly because of the interference of the resonant signal beam with the crystal holder. The above results were all collected using a fixed pump size of 2.3 mm. Tighter focusing would yield a lower threshold and leave more space for angle tuning, but would also exhibit higher risk of laser damage to the KTP crystal. Moreover, the resonant signal beam would depart from the pump beam, which might lower the net gain at large noncollinear angles to the detriment of the wavelength extension. Conversely, if the pump size is too large, the conversion efficiency would be affected with reduced pump intensity and the mechanical parts would block the signal beam path more easily while tuning M3.

 

Fig. 5 Signal wavelength and the corresponding output power versus external noncollinear angle. The symbols and the curve represent the experimental and theoretical results, respectively.

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According to the principles of nonlinear optics [17], the bandwidth of the signal wave is inversely proportional to $\left|v_{gs}^{-1}-v_{gi}^{-1}\right|$ for collinear PM, where ν_gs and ν_gi are the group velocities corresponding to the signal and idler wavelengths, respectively. For noncollinear PM, it provides another freedom Ω (Ω is the angle between k_s and k_i), and the bandwidth becomes inversely proportional to $\left|v_{gs}^{-1}-v_{gi}^{-1}\cos \Omega \right|$. Although the introduction of Ω could theoretically produce a smaller bandwidth during wavelength tuning, the measured signal spectra did not show consistent variation because Ω was very small. The signal linewidth was kept at around 0.3 nm, which is demonstrated by typical spectra covering the whole tuning range in Fig. 6.

 

Fig. 6 Output signal spectra at 1572.9 nm, 1578.6 nm, 1599.3 nm, 1624.3 nm, 1646.0 nm, 1662.4 nm, and 1684.2 nm.

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The temporal behavior of the signal and depleted pump pulses was also investigated. Figure 7(a) shows the pump pulse evolution at the maximum input of 13.8 W under different noncollinear angles, which were detected by a fast-response InGaAs photodiode (Thorlabs DET08C). The pump depletion reached 69.1% under the case of collinear interaction, which gave the maximum conversion efficiency at 1572.9 nm and dropped to 28.1% at 1684.2 nm. A typical signal pulse is shown in Fig. 7(b). The pulse width (full wave half maximum, FWHM) was 6.5 ns; thus, the corresponding peak power at 1572.9 nm was around 7.1 MW. The variation of the signal pulse width was found inconspicuous during the wavelength tuning.

 

Fig. 7 Temporal pulse shapes of the depleted pump pulse (a) and the output signal pulse (b).

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The signal beam quality was measured using the knife-edge method, giving the M² factors around 5.0 along the two orthogonal directions (y and z-axes) for collinear PM. During wavelength tuning, the beam exhibited gradual degradation in the x–z plane due to the expansion of interacting area induced by the noncollinear angle between the pump and the resonant signal waves. The measured M² factors were 5.3 and 6.8 along the y-axis and in the x–z plane, respectively, at the wavelength of 1684.2 nm. The root-mean-square fluctuation of the average output power was around 2% in half an hour monitored by the power meter, and the peak-to-peak instability was less than 10% recorded by the photodiode. The power fluctuations (both average and peak-to-peak) characteristics were found to be independent of the wavelength tuning.

5. Conclusions

We demonstrated a widely tunable NCPM KTP OPO in the eye-safe range using noncollinear PM. The tuning range covered from 1572.9 nm to 1684.2 nm with an external noncollinear angle of less than 6° (less than 3.1° for the internal angle), which was realized by simply tilting one of the ring-cavity mirrors. Compared with the traditional angle tuning method by rotating the nonlinear crystal orientation, which could only give a tuning range of less than 2 nm when the internal PM angle is tuned from 90° to 86.9°, the noncollinear angle tuning method exhibited a significant advantage in wavelength extension. Moreover, tuning a cavity mirror was easier and more agile than rotating a bulk crystal. The conversion efficiency for collinear PM reached 33.3% with the signal output power of 4.6 W at 1572.9 nm, which can be attributed to the good beam quality and the high spectrum intensity of the narrow-linewidth Nd:YAG pump laser. The efficient and widely tunable OPO is a good candidate for applications in lidar, spectroscopy, and pumping chromium doped ZnS/ZnSe lasers.