Eye-safe solid-state lasers that operate at 2 μm wavelength have many applications in medical, remote sensing and military technologies. With a 3-W CW laser-diode pumping, we obtained 760 mW 2.01 μm Tm:YAG laser under CW operation. The slope efficiency was 44% and the optical to optical efficiency reached 36%. In the acousto-optic Q-switched operation, laser pulses with the energy of 1.2mJ and 380 ns FWHM width have been achieved.
© 1999 Optical Society of America
Eye-safe, diode-pumped solid-state lasers operate near 2 μm spectral range at room temperature have many applications in medical, remote sensing and military technologies. In the last decade, much work has been done on the development of such laser devices. These include monolithic Tm:YAG lasers, [1–7] Tm-Ho:YAG lasers, [8–12] Tm:YVO4 lasers [13,14] and other Tm3+ or Tm3+-Ho3+ lasers. [15,16] But compared with Tm3+-Ho3+ co-doped systems, Tm3+ doped systems are advantageous in both CW and Q-switched operations at room temperature. The experimental results of diode-laser longitudinally pumped Tm:YAG and Tm-Ho:YAG lasers have shown that Tm:YAG crystals give superior CW performance than Tm3+-Ho3+ codoped YAG at room temperature. In Q-switched operation, the pulse energy of Tm:YAG lasers is also much larger than that of Tm-Ho:YAG lasers.  In Tm3+-Ho3+ codoped systems, Tm3+ ions are included for pump absorption and energy transfer by cross relaxation to the upper-state level of Ho3+ laser transition. At room temperature and high pumping intensities, Tm3+-Ho3+ cooperative upconversion and Ho3+→Tm3+ backtransfer processes will occur. The presence of such processes could not only restrict the effective energy storage lifetime of Ho3+ transitions to values significantly less than the intrinsic upper-state lifetime, but also give rise to instabilities during Q-switched operation. 
Because of the quasi-three-level properties of Tm3+ doped system, the design of diode-laser longitudinally pumped Tm:YAG lasers is significantly different from Nd3+ lasers operating at 1.06 μm. In four-level lasers, the gain element, in principle, can be made arbitrarily thick to absorb all the pump energy yet still have efficient energy extraction. But in quasi-three-level lasers, arbitrarily thick elements cannot be used because in regions where the pump intensity is low, population inversion will not be achieved. On the other hand, the thickness must be made sufficiently large to allow reasonably efficient pump absorption. [18,19] In addition, temperature plays an important role in quasi-three-level lasers. At room temperature, the thermal-population density on the lower laser level will highly increase the reabsorption losses in the laser cavity, and finally results in the increase of threshold and the decrease the slope efficiency. Under such considerations, we developed a diode-pumped Tm:YAG laser with a mirror coated on laser crystal to reflect back the unabsorbed pump radiation and a thermo-electric cooler to control the temperature of crystal, and realized highly efficient CW and Q-switched 2 μm laser performances.
2. Experiments and results
Fig. 1 shows the experimental setup. We used a 3-W CW laser diode (SDL-2482) to pump Tm:YAG crystal. The laser diode was fixed on a thermo-electric cooler and its emission was temperature tuned to 785 nm wavelength. With a spherical collimating lens L1 (f=5 mm), a cylindrical lens L2 (f=80 mm), and two spherical focusing-lens L3 (f=40 mm) and L4 (f=20 mm), the spot size of the diode laser beam in the Tm:YAG crystal was compressed to be about 250 μm × 70 μm. The coupling efficiency of pumping light was about 70%.
A Tm:YAG crystal doped with 5.2 at.% Tm3+ was designed to be 3 mm thick, with one side anti-reflection coated and the other side high-reflection coated for both 785 nm and 2.0 μm light. In order to reduce the reabsorption losses caused by the thermal population on the lower laser level, the Tm:YAG crystal was placed on a thermo-electric cooler and its temperature could be tuned from -25 °C to 20 °C. The laser cavity, as illustrated in Fig. 1, consisted of the flat highly reflecting mirror on the under-side of the Tm:YAG crystal, a highly reflecting concave mirror ( M ) and a flat output coupler. The concave mirror with a 20 cm radius of curvature was designed for high transmission at the wavelength of 785 nm and high reflection at the wavelength of 2.0 μm. The output coupler had a reflectivity of 3% for 2.0 μm laser. When the laser cavity length was about 30 cm, the diameter of the TEM00 laser mode in Tm:YAG crystal was calculated to be about 320 μm. It matched the diameter of the pump beam in the Tm:YAG crystal. One privilege of such a configuration is that the mirror coated on the Tm:YAG crystal can reflect back the unabsorbed pump light and therefore decrease the threshold power and increase the slope efficiency of the lasers. Although the installation of the spherical folded mirror under the angle of 45° can cause a little aberration, such configuration can produce large volume of laser-mode in Tm:YAG crystal and small diameter of laser beam at the place of the Q-switch. So it is beneficial for high-efficiency and Q-switched laser operation.
The laser output power was monitored with a pyroelectric power meter. The laser wavelength was measured with a 1-m monochrometer, with a calibrated accuracy of ±0.2 nm at 2.0 μm. In the Q-switched operation, we used a commercially available acousto-optic fused silica Q-switch. The laser pulse waveforms were monitored with a thermo-electric cooled Te-Ge-Hg detector and a 60 MHz oscilloscope.
Fig. 2 shows the output properties of the diode-pumped Tm:YAG disc laser under CW operations (without Q-switch). It can be found that the incident threshold power and the slope efficiency are strongly dependent on the temperature of the Tm:YAG crystal. At temperature of -20 °C, 760 mW, 2.013 μm laser was obtained with 0.26 W threshold power and 44% slope efficiency. The optical to optical efficiency was about 36%. When the temperature was increased to 0 °C and 20 °C, the threshold power increased to 0.36 W and 0.57 W, the slope efficiency decreased to 34% and 27%, respectively. In order to determine the temperature dependence of the threshold and the slope efficiency, we measured the threshold and slope efficiency at different temperatures. The results were shown in Fig. 3, which are roughly in accordance with the theoretical calculation. 
In the Q-switched operation, the laser pulse energy is related to the frequency repetition rate. Fig. 4 shows the experimental result. The maximum Q-switched energy of 1.21 mJ in a 380 ns FWHM pulse was obtained with a 120 Hz repetition rate at -20 °C. The incident pump power was 2.0 W. At the same pump power, the maximum Q-switched energy was only 0.38 mJ at room temperature (20 °C), and the related repetition rate was about 190 Hz.
As illustrated in Fig. 5, the Tm:YAG 2.0 μm laser is generated from the transition between the lowest Stark level of3F4 (5556 cm-1) and the higher Stark level of3H6 (588 cm-1). In such cases, the population density on the lower laser level is not presumed to be zero, but is assumed to have a small thermal population. According to the rate-equation of longitudinally pumped quasi-three-level system, the incident threshold power of Tm:YAG lasers can be expressed as: 
Where, vl is the frequency of 2 μm laser, ωp and ωl are the pump-beam waist and laser-beam waist, L is the round-trip loss of the laser cavity, T is the transmittance of output coupler, is the population density on the lower laser level, σ is the emission cross section, l is the length of the Tm:YAG crystal, τ is the fluorescence lifetime of the upper manifold, f=fa +fb , fa and fb , are the fractions of the total 3H6 and 3F4 population density residing in laser levels of a and b. a=1-exp(-αl) is the fraction of incident pump power absorbed in a crystal of length l with absorption coefficient α, ηq is the quantum efficiency, and h is the Planck’s constant.
Compared with Nd:YAG four-level laser, additional term 2 l appeared in equation (1). Because of strong temperature dependence of , the threshold power of Tm:YAG also depends on the temperature. We calculated the threshold power of the longitudinally pumped Tm:YAG lasers using equation (1) and found that it increases nearly 200% when the temperature increases from -20 °C to 20 °C.  Such result roughly agrees with the experimental results shown in Fig. 2 and Fig. 3.
As shown in Fig. 3, the slope efficiency of Tm:YAG laser is also related to the temperature of the crystal. In longitudinally pumped configuration, the slope efficiency of Tm:YAG lasers can be written as: 
Where, vl /vp is the quantum defect associated with that conversion, T/(T+L) is the fraction of laser photons lost from the cavity that are emitted through the output coupler. dS/dF is the efficiency that absorbed pump photons are converted to laser photons. It is expressed as: 
In this equation, parameters a, x, B, F, and S are defined as follows:
Where, r is the transverse radial coordinate, Φ is the total number of photons in the cavity. Taking the spectroscopic parameters of Tm:YAG crystal and the laser cavity parameters into above equations, we calculated the slope efficiency of Tm:YAG lasers, and found it decreases about 27% when the temperature increase from -20 °C to 20 °C.  But as shown in Fig. 3, the experimental result is larger than 27%. So we can conclude that in addition to the reabsorption-losses caused by the thermal population, other mechanisms arouse with the increase of the temperature. These mechanisms need further investigation.
From equation (1) and (2), we can also find that the threshold and the slope efficiency strongly depend on the absorption efficiency of the pump light. From the theoretical calculation, we can know that the threshold power will decrease and the slope efficiency increase 16% in two-pass pumping configuration than that in one-pass pumping configuration. But our experiment shows that they are not so much. Such results are caused by the mismatch between the laser mode and the reflected pump beam in the crystal.
In the Q-switched operation, we found that the repetition rate related to the maximum output energy increases from 120 Hz to 190 Hz when the temperature changes from -20 °C to 20 °C. We ascribe this to the shortening of the energy storage time at high temperature. For at high temperature, the fluorescence lifetime of Tm3+ will decrease because of strong interaction between Tm3+ ions. 
We have built a novel diode-pumped Tm:YAG laser and investigated its output properties at different temperatures. In the CW operation, we obtained 760 mW output power at wavelength of 2.013 μm when the incident pump power was 2.1 W. The slope efficiency was 44% and the optical to optical efficiency reached 36%. In the acousto-optic Q-switched operation, we obtained 1.21 mJ, 380 ns (FWHM) laser pulses when the repetition rate was 120 Hz. The estimate indicates that it should be possible to reach higher output power and Q-switched energy by improving the focusing system of the pump light.
References and links
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