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We report on efficient repetitively Q-switched laser operation of a miniature Yb:LuPO4 crystal rod with emission wavelengths ranging from 1000 to 1010 nm, at different repetition rates over a range of 1−50 kHz. An average output power of 6.55 W was produced at 50 kHz of repetition rate, with a slope efficiency of 40%; in a low-repetition-rate operation at 2 kHz, the maximum pulsed output power generated was 1.43 W, with pulse energy, duration, and peak power being, respectively, 0.715 mJ, 12.5 ns, and 57.2 kW.
© 2017 Optical Society of America
1. Introduction
The crystal of lutetium orthophosphate (LuPO4), which possesses a tetragonal zircon structure belonging to space group I41/amd (point group 4/mmm), has long been known to be a suitable host medium for trivalent rare-earth active ions such as Nd3+, Yb3+, Er3+, Tm3+, and Ho3+. In the late 1980s, crystal field analysis was carried out for both Yb3+ and Tm3+ ions doped in the LuPO4 crystal [1]; preliminary unpolarized absorption and emission spectra of Yb:LuPO4 were also given in an early study of spectroscopic properties of Yb-doped laser crystals [2], while a detailed study was conducted lately on the spectroscopy of Er:LuPO4 crystal [3]. The first laser action of rare-earth active ions, with LuPO4 serving as host crystal, was demonstrated with Nd:LuPO4 in 1999, showing promising laser performance under conditions of longitudinally pumping by a diode laser [4].
In the subsequent period of more than one decade, during which a wide variety of laser materials appeared, little progress was made in developing new laser crystals with LuPO4 as host medium. Very probably, this situation was due to the difficulty in crystal growth of LuPO4. In fact, such rare-earth orthophosphates cannot be grown by the conventional Czochralski technique owing to their incongruently melting nature, and it proves to be extremely difficult to obtain large crystals of high optical quality [5].
Recently much effort has been made by our group in the growth of high-quality Yb:LuPO4 crystal for laser application. Plate-shaped crystals, with thicknesses in the range of 0.2−0.5 mm and transverse sizes amounting at most to 5 mm × 2 mm, have been obtained by use of the high-temperature solution method [6]. Polarized absorption and emission cross-sections were determined for the Yb:LuPO4 crystal over a wavelength range of 850−1100 nm [7]. Employing such an unpolished Yb:LuPO4 crystal plate, an efficient microchip laser, operating in continuous-wave (cw) as well as passively Q-switched modes, has been demonstrated, producing a cw output power of 1.45 W with a slope efficiency of 73%, while the maximum Q-switched average output power being 0.53 W [8]. These experimental results reveal the potential of Yb:LuPO4 crystal in applications for making miniature solid-state lasers.
It is interesting to notice that apart from plate-shaped crystals, another entirely different form, miniature columnar crystal of Yb:LuPO4, could also be grown along its crystallographic c axis in high-temperature solution. Furthermore, high-power laser action could be realized with tiny crystal rod, which would only need to be cut from such a columnar crystal and to be end-face polished. In our initial experiment on a miniature Yb:LuPO4 crystal rod laser, 5.3 W of cw output power was produced with an optical-to-optical efficiency of 40% [9].
Given the potential of such kind of tiny Yb:LuPO4 crystal rod in generating high-power cw laser operation, it is also interesting to explore its Q-switched laser performance. In this paper we report on the repetitive Q-switching laser properties of miniature Yb:LuPO4 crystal rod. Actively Q-switched by an acousto-optic modulator, the miniature rod laser could generate pulsed operation over a wide range of repetition rate from 1 to 50 kHz, with a maximum output power in excess of 6 W, while the emission wavelengths falling in a range of 1000−1010 nm, which proves to be shorter than those achieved from most pulsed, directly diode-pumped, quasi-three-level based, Yb-doped crystal lasers that have been developed thus far with a simple two-mirror resonator, in the absence of any intracavity wavelength selecting element.
2. Description of experiment
The miniature Yb:LuPO4 crystal rod utilized in the experiment was cut directly from a tiny columnar crystal, which was grown along its crystallographic c axis from spontaneous nucleation in high-temperature solution [9]. The rod sample, having a transverse dimension of about 0.8 mm, was 2 mm long, with two end-faces polished but not coated. The Yb-ion concentration of the Yb:LuPO4 crystal was 5 at. % (6.15 × 1020 cm−3).
The repetitive Q-switching laser performance of the Yb:LuPO4 crystal rod was studied by employing a simple plano-concave resonator, as illustrated schematically in Fig. 1. The plane mirror (M1), which acted as reflector for the cavity, was coated on its inside surface for high-reflectance at 1020−1200 nm (>99.9%) and for high-transmittance at 808−980 nm (>98%); while on its outside surface it was coated for antireflection (AR) at 800−1000 nm. As the output coupler (M2), a concave mirror was used, having a radius-of-curvature of 200 mm and a transmission (output coupling) of T = 30% at 1030 nm. An acousto-optic Q-switch (I-QS027-5C4G-U5-ST1, Gooch & Housego), which was of an interaction length of 50 mm, with AR coating on its end faces for 1.06 μm, was utilized to induce repetitively Q-switched laser action. It was driven at 27 MHz with an rf power of 50 W during its operation. Inside the cavity the Yb:LuPO4 crystal rod, which was fitted into a copper holder that was cooled by cycling water, was placed close to the plane reflector; while the acousto-optic Q-switch was positioned between the crystal rod and the output coupler. The physical cavity length was 216 mm. A fiber-coupled diode laser at 976 nm (bandwidth of less than 1 nm), with a fiber core diameter of 200 μm and NA of 0.22, was used to pump the laser. The pump radiation from the fiber end, which was first focused by an optical re-imaging unit (ORU), was delivered into the miniature crystal rod with a pump spot radius of about 100 μm.
Fig. 1 Schematic diagram of the experimental laser setup. AO: acousto-optic Q-switch; ORU: optical re-imaging unit.
3. Results and discussion
Stable repetitively Q-switched laser operation was achieved at different pulse repetition frequencies (PRFs) over the range from 50 kHz down to 1 kHz. The output coupling employed was chosen to be T = 30%, which proven to be the optimal in producing high average output power as well as in suppressing the occurrence of multi-pulse operation.
Figure 2 shows the average output power as a function of absorbed pump power (Pabs) measured when the laser was operated under different pulse repetition rates. For comparison, the results for cw operation are also plotted in the figure. In the measurement of cw output power, the inactive Q-switch was left inside the laser resonator. The amount of Pabs was determined from Pin, the corresponding incident pump power, by Pabs = ηpPin, here ηp being the unsaturated absorption of the laser crystal for the pump radiation. For the 2 mm long Yb:LuPO4 crystal rod used, the magnitude of ηp was measured to be 0.94. It was found that in all cases the laser output was unpolarized, as could be predicted since the oscillating laser beam propagated along the optic axis of the uniaxial laser crystal. The threshold for Q-switched laser action was reached at Pabs = 2.35 W, independent of the pulse repetition rate. In the case of PRF = 50 kHz, the average output power increased almost linearly with Pabs, with a slope efficiency determined to be 40%. One notes that when Pabs was increased above approximately 16 W, the Q-switched operation became less efficient, a behavior arising from the increasing thermally induced losses. Nevertheless, an average output power of 6.55 W was generated at Pabs = 20.4 W, which was the highest pump power applied in the experiment, the optical-to-optical efficiency was 32%. By a comparison between the case of PRF = 50 kHz and the cw operation, one can see their similarity in output characteristics in the operational region where the thermal losses were not significant (Pabs < ~16 W). As is known, in actively Q-switched operation the average output power could reach its corresponding cw level under conditions of sufficiently high pulse repetition rates [10].
One sees from Fig. 2 that in the case of PRF = 30 kHz, the output characteristics proven to be very close to those for PRF = 50 kHz, with a slightly lower output power of 6.21 W produced at Pabs = 20.4 W. With the repetition rate further reduced to PRF = 10 kHz, however, the Q-switched laser oscillation was found to be significantly less efficient; the maximum output power achievable was 3.92 W, generated at Pabs = 18.3 W. When decreasing the repetition rate to 5 kHz, the highest average output power that could be produced amounted only to 2.23 W. In the case of a still lower repetition rate, PRF = 2 kHz, an average output power of 1.43 W was measured at Pabs = 15.6 W. In excess of this pump level, multi-pulse operation began to set in. At this low repetition rate the initial gain accumulated could reach such a level that the pulse build-up time became shorter than the Q-switching time, which was determined by the transit time of the acoustic wave across the laser beam [10], and hence more than one laser pulse could be established. It was found that stable Q-switched operation could also be obtained when the repetition rate was reduced to PRF = 1 kHz, but the output power produced in this case was limited to only about 0.5 W, before the occurrence of multiple pulses.
In order to suppress or delay the occurrence of multi-pulse operation in an acousto-optically Q-switched laser, a higher output coupling can usually be employed, which can make the pulse build-up time to be lengthened effectively due to the reduction in the net gain, thus enabling greater pulse energy to be produced from the laser operating at low repetition rates. We attempted this in the experiment, with the output coupler replaced by another one having a higher transmission of T = 40%. It was found that although the laser threshold was raised only moderately (Pabs = 2.51 W, compared to 2.35 W for T = 30%), the average output power that could be generated at PRF = 1 kHz was significantly lower (by a factor of roughly 2) than achievable at the same pump level under conditions of T = 30%. The less efficient Q-switched laser action with the coupler of T = 40% might be attributed, among other reasons, to the insufficient gain reachable with the 2 mm long crystal rod. At the pumping wavelength of 976 nm, the absorption and emission cross-sections, σabs and σem, have values of 2.5 and 2.3 × 10−20 cm2 [7], leading to a value for the saturation intensity Isat = hν/(σabs + σem)τf of 5.1 kW/cm2 (τf = 0.83 ms [2]). The on-axis pump intensity of this level corresponds to a pump power of 0.8 W for the pump spot size of 100 μm. As a consequence, considerable absorption saturation of the pump power would occur in the crystal rod, limiting the growth of gain with pump power. A longer crystal rod would be more desirable for the case of T = 40%.
Figure 3 depicts the laser emission spectra for different PRFs, measured at Pabs = 15.6 W, at which the maximum output power was generated in the case of PRF = 2 kHz. The emission wavelengths of the Q-switched miniature Yb:LuPO4 crystal rod laser were found to fall in a range of about 1000−1010 nm, this turns out to be shorter than obtained with other diode-pumped actively Q-switched Yb-ion crystal lasers, such as Yb:GGG, Yb:YSGG, Yb:GAGG, Yb:Ca3La2(BO3)4, and Yb:GdCa4O(BO3)3, whose emission wavelengths ranging roughly from 1025 to 1040 nm [11–15]. Indeed, oscillation wavelengths shorter than 1000 nm could be achieved with Yb-ion crystal lasers, e.g., in a pulsed Yb:Sr5(PO4)3F laser at 985 nm that was end pumped by a 900-nm Cr:LiSAF laser [16]; or in an intracavity pumped Yb:KY(WO4)2 laser at 981 nm [17]. These lasers, however, suffer from additional complexity and prove to be difficult in generating oscillation, owing to their intrinsic three-level nature. One notes that when operated at lower PRFs, the laser tended to oscillate at shorter wavelengths, this was due to the increased thermal losses in connection with the somewhat higher thermal load in the case of lower PRF.
Fig. 3 Emission spectra of the Q-switched miniature Yb:LuPO4 crystal rod laser, measured at Pabs = 15.6 W at different pulse repetition rates.
Figure 4 shows the measured pulse duration (FWHM) versus Pabs for different PRFs ranging from 50 to 1 kHz. In the case of PRF = 50 kHz the pulse duration, measured when the laser was operated not far above threshold, turned out to be quite long (434 ns at Pabs = 3.4 W); it decreased initially very rapidly with increasing pump power, and would eventually reach a magnitude (42.1 ns) that remained roughly unchanged. Such a varying behavior has been predicted by a simple theoretical model [18], which gives a very similar variation of pulse width with the factor by which the initial inversion exceeds the threshold inversion. Evidently, this factor is proportional to the absorbed pump power. It is also seen that at a given pump power, the pulse duration could be shortened greatly by decreasing PRF. Clearly, this was due to the fact that the initial inversion, and hence the gain, accumulated during the pump period between two successive pulses, could be enhanced largely as the PRF was reduced.
Fig. 4 Variations of pulse duration with absorbed pump power, measured at different pulse repetition rates.
Figure 5 shows the profiles of the shortest laser pulses at repetition rates of 2 and 50 kHz. The measurement was made at Pabs = 15.6 W (for PRF = 2 kHz) and Pabs = 19.6 W (for PRF = 50 kHz), where the pulse duration was already independent of pump power. The shortest pulse duration was 12.5 and 42.1 ns, for PRF of 2 and 50 kHz, respectively. One sees, in particular for PRF = 2 kHz, that the pulse shape is not symmetric, showing a slow trailing edge, which suggests a lower than optimum decaying rate of internal laser intensity [18]. Increasing the output coupling for the Q-switched laser would result in a more symmetrical and shorter pulse, but in the price of decreasing the output power and pulse energy.
Figure 6 illustrates a pulse train that was recorded at Pabs = 20.4 W, the highest pump power applied, when the actively Q-switched Yb:LuPO4 crystal rod laser was operated at a repetition rate of 30 kHz. The pulse amplitude fluctuations were estimated to be less than 7%. Table 1 lists the primary parameters characterizing the repetitive Q-switching laser performance of the miniature Yb:LuPO4 crystal rod, including maximum average output power (Pmax); maximum pulse energy (Ep); shortest pulse duration (tp); highest peak power (Pp); slope efficiency (ηs); and center emission wavelength (λc).
Table 1. Parameters Characterizing the Repetitively Q-switched Yb:LuPO4 miniature rod Laser
4. Summary
In conclusion, efficient repetitively Q-switched laser operation was demonstrated with a miniature Yb:LuPO4 crystal rod at emission wavelengths of 1000−1010 nm over a wide repetition frequency range of 1−50 kHz. An average output power of 6.55 W was produced at repetition rate of 50 kHz with a slope efficiency of 40%; while in an operation at a low repetition rate of 2 kHz the pulsed output power could reach 1.43 W, with pulse energy, duration, and peak power being respectively 715 μJ, 12.5 ns, and 57.2 kW. Given the substantial transverse inhomogeneity of the crystal rod in achievement of laser performance, which was evidenced from the experiment, one can expect there exists some room for further improvement in crystal quality. Through an optimization of crystal rod length, Yb-ion concentration, as well as output coupling of the resonator, still higher average output power and greater pulse energy might be generated from such a repetitively Q-switched miniature Yb:LuPO4 crystal rod laser. Such compact pulsed laser devices emitting at wavelengths of 1000−1010 nm, which could be built from miniature Yb:LuPO4 crystal rods, may find some practical applications, e.g., in generating blue-green (500−505 nm) or ultraviolet (333−337 nm) coherent radiation by second- or third-harmonic generation technique. Laser radiation at these specific wavelengths is usually not available with traditional Yb or Nd solid-state lasers of the same simple type.
Funding
National Natural Science Foundation of China (11574170 and 11374170).
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