Resonantly pumped 2.118 μm Ho : YAP laser Q-switched by a Cr²⁺: ZnS as a saturable absorber

A laser in the range of 2 μm is at the atmosphere window, which can be applied to laser ranging [1] and photoelectrical countermeasures [2]. In addition, 2 μm lasers can be used as pump sources for mid-IR optical parametric oscillators (OPOs) to generate laser output in the range of 3–12 μm. Passive Q-switched (PQS) is a technology widely used in lasers to generate short pulses with high peak power that takes advantage of the nonlinear characteristic of a saturable absorber (SA) to control the loss of resonator. PQS technology has better prospects for development than active Q-switched technology for its advantages of simplicity, low cost, compactness, and reliability [3].

Currently, the most widely used crystals for generating a 2 μm Q-switched laser are Tm³⁺ [4] -doped crystal, Tm³⁺, Ho³⁺-co-doped [5] crystal, and Ho³⁺-doped [6] crystal. Compared to the Tm³⁺-doped laser and the Tm³⁺, Ho³⁺-co-doped laser, the Ho³⁺-doped laser has the advantages of low quantum loss, low up conversion efficiency, and it can achieve a high power output of 2 μm at room temperature, which is attracting wide attention from researchers.

YAlO₃ (YAP) crystal, as a typical representative of aluminate crystal, belongs to the orthogonal crystal system distortion perovskite structure, which is a negative biaxial crystal. The lattice constant is a  =  0.5307 nm, b  =  0.7355 nm, c  =  0.5176 nm [7]. It has a high thermal conductivity (0.11 W cm⁻¹ K⁻¹). Ho : YAP crystals have a larger absorption cross section at many wavelengths and a wider line width of the absorption peak. For E $\parallel$ a polarization, Ho : YAP crystals have strong absorption in the 1931 nm, 1970 nm, and 2045 nm vicinity, and the effective absorption cross sections are 3.5308  ×  10⁻²¹ cm⁻², 5.2942  ×  10⁻²¹ cm⁻², 9.1805  ×  10⁻²¹ cm⁻², 6.3052  ×  10⁻²¹ cm⁻², and 4.0902  ×  10⁻²¹ cm⁻². For E$\parallel$b polarization, Ho : YAP crystals have strong absorption in 1884 nm, 1923 nm, 1946 nm, 1984 nm, 2023 nm, and 2069 nm, and the absorption coefficients are 6.1953  ×  10⁻²¹ cm⁻², 7.7263  ×  10⁻²¹ cm⁻², 6.1719  ×  10⁻²¹ cm⁻², 5.7762  ×  10⁻²¹ cm⁻², 3.3942  ×  10⁻²¹ cm⁻², and 3.1506  ×  10⁻²¹ cm⁻². At the wavelength of 2118 nm, for E $\parallel$ a, E $\parallel$ b, E $\parallel$ c polarization, the emission cross sections of Ho : YAP crystals are 3.2576  ×  10⁻²¹ cm⁻², 2.6814  ×  10⁻²¹ cm⁻², and 5.3384  ×  10⁻²¹ cm⁻² [8]. The upper lifetime of the Ho : YAP is 4.8 ms. At present, the continuous and actively Q-switched characteristics of Ho : YAP laser have been demonstrated [9]; however, the PQS Ho : YAP laser is still unreported.

In this paper, we first report a comprehensive investigation of the continuous wave (CW) and PQS laser actions in a Ho : YAP laser pumped by a 1.94 μm laser. In CW operation, the maximum output power of 6.4 W at 2118.7 nm with a slope efficiency of 25.3% was obtained. Meanwhile, under PQS operation, a maximum average output power of 6.1 W at 2111.7 nm was obtained at the output coupler transmittance of T  =  30%. Furthermore, the minimum pulse width was 93.6 ns at the repetition frequency 7.5 kHz. The output beam was close to the fundamental transverse electromagnetic mode (TEM00).

The PQS Ho : YAP laser experimental configuration is shown in figure 1. The laser resonator was composed of the cavity mirror M1–M3. The plane mirrors M1, M2 were anti-reflection (AR) coated at ~1.94 μm and high-reflection (HR) coated at ~2.1 μm. The output coupler M3 was plane-concave with a radius curvature of 300 mm. The distances of M1–M2, and M2–M3 were 95 mm and 100 mm orderly, with a total cavity length of 195 mm. A diode-pumped Tm : YAP laser with an emission wavelength of 1.94 μm was utilized as a pump source with a maximum output power of 32.1 W. The pump beam was focused into the Ho : YAP crystal with a spot diameter of 0.4 mm. The Ho : YAP crystal with the Ho³⁺ (0.3 at%) doped concentration had a cross section of 3 mm  ×  5 mm and a length of 50 mm. Both end faces of the crystal were AR coated for the laser wavelengths around 2.1 μm and the pump wavelength around 1.94 μm. The Ho : YAP crystals were mounted in a separate water-cooled heat sink maintained at 18 °C. For PQS operation, a Cr²⁺ : ZnS SA with a dimension of 9 mm  ×  9 mm was inserted inside the laser cavity. The faces of the Cr²⁺ : ZnS crystal were AR coated near 2 μm leading to an initial transmission of T₀  =  92%. The SA was placed in the resonator, 20 mm away from the output coupler M3. It was mounted in a copper heat sink, which was cooled by water. The beam radius of the Ho : YAP laser inside the resonator was calculated by using the ABCD matrix, in which the |A  +  D|/2 value of the resonator was lower than 0.14. In the middle of the Ho : YAP crystal, the radius of the TEM₀₀ mode was about 400 μm, and the radius of the TEM₀₀ mode at the position Cr²⁺ : ZnS SA was about 450 μm. The output power of the Ho : YAP laser in the experiment was measured using a Coherent PM30 power meter. The laser pulse was recorded by a Tektronix MSO3034 digital oscilloscope (300 MHz bandwidth, 2.5 G samples s⁻¹) and a fast InGaAs PIN photodiode.

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The average output power of the Ho : YAP laser as a function of the total incident pump power is shown in figure 2 with different output coupler transmittances of 30%, 50%, and 60%. In the CW operation, with a total incident pump power of 32.1 W, the maximum average output powers of 6.4 W, 6.3 W, and 5.6 W were obtained for T  =  30%, T  =  50%, and T  =  60% respectively, corresponding to the slope efficiencies of 25.3%, 27.2%, and 25.0% (figure 2(a)). We investigated the output characteristics of the PQS Ho : YAP laser after inserting the Cr²⁺ : ZnS SA into the resonant cavity. Figure 2(b) shows the average output power for the output coupler transmittance of T  =  30%, T  =  50%, and T  =  60% versus the total incident pump power. For T  =  30%, we achieved a 6.1 W average output power under the total incident pump power of 32.1 W, corresponding to a slope efficiency of 25.1%. For T  =  50% and T  =  60%, output powers of 5.7 W and 4.9 W under the total incident pump power of 32.1 W were achieved, which indicated slope efficiencies of 27.0% and 24.9%. Compared with the CW operation, the threshold of the PQS mode was higher due to the increase in resonator loss after inserting the Cr²⁺ : ZnS SA.

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The CW laser spectrum was measured with a Spectrum Analyzer (Bristol Instruments 721) at the total incident pump power of 32.1 W, as shown in figure 3. The CW outputs with three different output couplers had the same central wavelength of 2118.7 nm. For the PQS mode, the central wavelength was measured by a WDM1-3 grating monochromator and a Tektronix MSO3034 digital oscilloscope (300 MHz bandwidth, 2.5 G samples s⁻¹) with a fast InGaAs PIN photodiode. Compared with the CW operation, the PQS outputs generated an obvious blue shift of the emission central wavelength of 2111.7 nm with three different output couplers, when the total incident pump power was 32.1 W. In the PQS mode, the losses in the resonator increased drastically due to the insertion of the Cr²⁺ : ZnS SA, leading to a higher population inversion density while the laser pulse was being constructed; thus, the gain coefficient changing was related to the emission cross section and emitting wavelength [10].

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The variation in the pulse width and repetition rate versus the total incident pump power at different output coupler transmittances is shown in figure 4. For three different output couplers with transmittances of T  =  30%, T  = 50%, and T  =  60%, the pulse width decreased from 153 ns to 93.6 ns, from 219.7 ns to 105 ns, and from 256.4 ns to 119.6 ns, respectively, with increasing the pump power. For a resonant with the certain output coupler, the pulse width became narrower with increasing the total pump power. When the total incident pump power increased, the population stored in the upper-level manifold became larger under the low Q value of the resonator. When the Cr²⁺ : ZnS SA was bleached, the resonator was under a high Q value, and the photons in the resonator appeared to rapidly increase in a shorter time, which generated a narrower pulse width. The pulse duration is determined by the intracavity photon life, which is positively correlated with the cavity length, and has an inverse relationship with the cavity loss. In order to obtain a shorter pulse duration, we can reduce the length of the cavity, the small signal transmission of the SA, and the transmittance of the output coupler.

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Figure 4(b) shows the pulse repetition rate versus the total incident pump power for three different output coupler transmittances. As can be seen in figure 4(b), the pulse repetition rate almost linearly increased with increasing the total incident pump power [11, 12]. The pulse repetition rate increased from 0.3 to 7.5 kHz, from 0.1 to 7.1 kHz, and from 0.4 to 6.8 kHz for the T  =  30%, T  = 50%, and T  =  60% output couplers, respectively. Under the condition of the same output coupler transmittance, the pulse repetition rate increased with increasing the total incident pump power. Under the same total incident pump power, the laser with the higher output coupler transmittance had the lower pulse repetition rate. The Q-switching stability can be improved by selecting an output coupler with a higher transmittance. This is because the cavity loss increases with the increase in the transmittance of the output coupler, so that the laser modes number decreases, resulting in weak mode competition and improving the Q-switching stability.

For the output coupler transmittance of T  =  30%, the pulse train with a repetition frequency of 7.5 kHz and the single-pulse profile with a duration of 93.6 ns are shown in figure 5. It can be seen from figure 5 that the output pulses show excellent stability. A pulse-to-pulse fluctuation ratio of 7.7% was obtained. Figure 6 shows the pulse energy versus the total incident pump power. For the Ho : YAP laser with different output couplers of T  =  30 %, T  =  50 %, and T  =  60%, the pulse energy increased from 0.39 to 0.83 mJ, from 0.62 to 0.81 mJ, and from 0.53 to 0.78 mJ with the increase in the total incident pump power, respectively.

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The laser beam quality was measured by the traveling knife-edge method along the x-axis and y-axis. By fitting a Gaussian function to these data, the M² factor was calculated to be 1.4 in the x-axis and 1.6 in the y-axis (see figure 7). It can be seen from figure 7 that the output beam was close to the fundamental transverse electromagnetic mode. A low spatial-resolution laser beam profile (inset in figure 7) at the highest pump power was observed by a pyroelectric camera.

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In conclusion, we first realized that the Tm : YAP laser pumped the PQS Ho : YAP laser with a Cr²⁺ : ZnS SA. The influence of the difference of output couplers transmittance on the laser performances was comparatively analyzed. For the different output couplers with transmittances of T  =  30%, T  = 50%, and T  =  60%, the central wavelength of the CW laser and PQS laser remained unchanged when the transmittance of the output couplers changed, which were 2118.7 nm and 2111.7 nm, respectively. A maximum CW output power of 6.4 W was obtained at a total incident pump power of 32.1 W for the output coupling transmittance of 30%, corresponding to a slope efficiency of 25.3%. Meanwhile, with a Cr²⁺ : ZnS SA, the maximum average output power of 6.1 W was obtained, corresponding to a pulse repetition frequency of 7.5 kHz and a pulse width of 93.6 ns. Furthermore, the $M_{x}^{2}$ factor of 1.4 on the x-axis and the $M_{y}^{2}$ factor of 1.6 on the y-axis were obtained.

This work is supported by the National Natural Science Foundation of China (No 61308009, No 61405047, No 50990301), the China Postdoctoral Science Foundation funded project (No 2013M540288), Fundamental Research funds for the Central Universities (Grant No HIT. NSRIF. 2014044, Grant No HIT. NSRIF. 2015042), and the Science Fund for Outstanding Youths of Heilongjiang Province (JQ201310).