Room-temperature operation of continuous-wave and Q-switched Tm,Ho:YLF lasers are reported. A 3W CW laser-diode in an end-pumped geometry is used to generate 393mW of 2µm laser output, representing a 14% optical to optical efficiency. In order to achieve single frequency operation, two intra-cavity solid etalons are used. Single frequency output power of 113mW is obtained, and the threshold power is only 250mW. The single frequency laser can be used as a seed laser for either a larger oscillator or an amplifier. In the acousto-optic Q-switched operation, laser pulses with the energy of 45µJ and 142ns FWHM width have been achieved for the pump power of 1.7W. We give the analytical formulas of the threshold pump power and slope efficiency to theoretically analyze the results obtained in the experiments, in which the energy transfer up-conversion and ground state re-absorption are taken into account.
©2005 Optical Society of America
Eye-safe, diode-pumped all-solid-state lasers operating in the spectral region near 2µm are regarded as promising sources for use in Doppler wind sensing, differential absorption radar (DIAL) water vapor profiling, and low altitude wind shear detection. Since the first demonstration of rare-earth laser operation in YAG obtained at cryogenic temperature, considerable progress has been recorded [1–10]. In the last decade, room-temperature, diode-pumped 2µm laser (continuous and Q-switched) have been integrated in systems for ground-based or airborne radar measurements [6, 10]. The Tm-Ho co-doped materials, operating on the Ho transition, are usually preferred for high-output energy application, many different hosts and transitions have been reported to laser [11, 12]. Unlike the single Tm-doped materials used in the low output energy lasers, Tm-Ho co-doped materials present a high-emission cross section that makes them suitable for efficient energy extraction under damage threshold.
The energy transfer dynamics and the extent of up-conversion losses are often more influential than the threshold pumping level in determining the laser performance. While the presence of Tm reduces the Stokes losses, the resonant Tm-Ho transfer time in YLF is at lease 1µs. Although this is faster than observed in Tm,Ho:YAG, it is longer than the typical pulse buildup time and consequently only that fraction of the excitation energy stored in the Ho 5I7 upper laser level is accessible when Q switching. For a given pump power one can access a greater fraction of the excitation energy in a YLF host which, in conjunction with reportedly reduced up-conversion losses in YLF, would favor Tm,Ho:YLF over co-doped YAG. Furthermore, YLF is chosen as a host crystal because of its long pump integration time, excellent optical damage resistance, lack of thermal induced birefringence, and linearly polarized output. We have conducted some experiments and reported the characteristics of Tm,Ho:YLF microchip lasers [13–16].
In this paper, we detailed study the performances of room-temperature continuous-wave and Q-switched Tm,Ho:YLF lasers pumped by a 3W laser diode, and give the analytical formulas of the threshold pump power and slope efficiency to theoretically analyze the results obtained in the experiment. The theoretical analysis agrees with the experimental results.
2. Experiments and results
The experimental setup is shown in Fig. 1. The pump laser is Coherent Inc S-79-3000-200-H/L 3W laser diode temperature tuned to 792nm. Its emission is collected with an 8mm focal length collimating lens followed by a cylindrical lens with focal length of 100mm to reshape. This laser beam then is focused onto the Tm,Ho:YLF crystal using a lens with a 50mm focal length. With this arrangement the pumping beam can be focused to a spot size of approximately 100×100µm2 at the entrance face of the laser crystal. The total transmission efficiency of the beam-reshaping system is about 91% at 792nm. The Tm,Ho:YLF laser crystal from II–VI corporation has do-pant concentrations of 6 at.% Tm and 0.4 at.% Ho with dimension of 5×5×2.5 mm3. The crystal is oriented with the c axis parallel to the polarization direction of the pump beam to utilize the higher π spectrum absorption. Because both the pump and laser cross sections are considerably enhanced in the π polarization, we should specify the preferred orientation of the laser element in any optical arrange.
A plane-concave resonator is employed to make the system very simple and compact. The near hemispherical resonant is formed between the planar crystal front face and the output coupler. A dichromatic coating on the front face is high transmitting at 792nm but is totally reflecting at 2µm. The other face is only polished and uncoated at both pump and output wavelengths. To efficiently remove the heat generated with pump power from the crystal, the crystal is wrapped with indium foils and held in brass heat sink. Temperature of the heat sink is held at a constant 293K with a thermoelectric cooler.
The laser output was monitored with a pyroelectric power meter. The laser wavelength was measured with a 0.75m monochrometer, with a calibrated accuracy of ±0.2nm at 2µm. The single frequency operation of the Tm,Ho:YLF laser was measured with a diagnostic air-gap scanning Fabry-Perot interferometer and a PbS detector. In the Q-switched operation, we used a QSGSU-6Q acoustic-optic modulator (The 26th Electronics Institute, Chinese Ministry of Information Industry). The laser pulse width and waveforms were observed with a room temperature mercury cadmium telluride photoconductive detector and a TDS3032B digital oscilloscope (Tektronix Inc., USA).
We performed several experiments with various output coupler transmissions: 1.26%, 2%, 2.97%, 4.74%, 6% and 10% at 2µm. The radius of curvature of these output couplers is 10cm. Fig. 2 shows the results of the output power as a function of the incident pump power at room temperature. The best performance was achieved with the 2% output coupler. The highest output power of 393mW was obtained under the pump power of 2.8 W with the 2% output coupler which corresponds to a slope efficiency of 14%. Slope efficiency can reach a maximum at a relatively low value of the transmission of the output coupler and then decreases thereafter. With the traditional four level laser system, the slope efficiency increases asymptotically towards a maximum value as the transmission value of the output coupler increases. However, in Tm-Ho laser systems, as the transmission value of the output coupler increases, higher population densities in the Ho 5I7 manifold are required to achieve threshold. Higher population densities in the Ho 5I7 manifold lead to increased up-conversion and therefore decrease slope efficiency. Thus a decrease in the slope efficiency at higher transmission values of the output couplers can imply larger up-conversion rates. The best performance was achieved with the 2% mirror, so that only the results obtained with this mirror will be presented in the following. The output spectrum of the free-running laser was measured on a monochrometer to be center on 2.067µm and to be linearly polarized parallel to the laser crystal c axis. Fig. 3 shows the output properties of the diode-pumped multimode Tm,Ho:YLF laser under CW operations (without Q-switch). It can be found that the threshold pump power and the slope efficiency are strongly dependent on the temperature of Tm,Ho:YLF crystal. The threshold pump power increases but the slope efficiency decreases when the crystal temperature is increased. The results are in accordance with the theoretical calculation [15, 16].
In the acousto-optic Q-switched operation, we placed the acousto-optic fused silica Q-switch in the laser cavity. The passive loss introduced into the cavity by the Q-switch crystal reduced the CW output power at the pump power of 1.7W from 270mW to 180mW. Fig. 6 shows the measured Q-switched pulse width as a function of the pulse repetition frequency at the incident pump power of 1.7W. When pulse repetition frequency was changed from 80 Hz to 10 kHz, the resulting laser pulse width increased almost linearly from 138ns to 226ns. Fig. 7 shows the single pulse energy as a function of the pulse repetition frequency when the incident pump power is 1.7W. It is noted from Fig. 7 that the largest pulse energy of 45µJ can be achieved at the lower pulse repetition frequency of 1 kHz, the pulse energy decreases from 45µJ to 15µJ with the increase of the pulse repetition frequency from 1 kHz to 10 kHz, and at sufficiently low pulse repetition frequency the pulse energy will saturate since the pumping time becomes large compared to the lifetime. Fig. 8 shows the shape of a single Q-switched pulse at the pulse repetition frequency of 1 kHz. The full-width at half-maximum (FWHM) was 142ns. Fig. 9 shows a consecutive pulse train of laser output operating at 1 kHz. The pulse-to-pulse amplitude fluctuation of Q-switched pulse train was measured to be less than ±5%.
3. Theoretical analysis and Discussion
Figure 10 shows an energy level diagram for Tm,Ho:YLF laser. When the 792nm pump light from the diode laser is absorbed into the 3H4 manifold, the 3F4 manifold is efficiently populated through the well-known two-for-one cross-relaxation process. A fraction of the energy stored in the Tm 3F4 manifold is then transferred to the Ho 5I7 manifold. As the populations of 3F4 and 5I7 grow, the long lifetimes of the levels are reduced by up-conversion process. The laser emission at 2.06µm is due to a transition between the lowest Stark level in the 5I7 manifold and a high level in the 5I8 ground-state manifold. 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.
We assume that the transverse modes of the pump and laser modes are TEM00 Gaussian beams with negligible diffraction in the gain medium. Then the normalized pump distribution is given by:
Where, ωp is the pump-beam radius, r is the transverse radial coordinate, ηα=1-exp(-αl) is the fraction of incident pump power absorbed in a crystal of length l with absorption coefficient α, and the normalized photon density in free space is given by:
Where, ωl is the laser beam radius, Leff=lc+ (n-1)l is the optical path length of the laser cavity with length lc, and n is the refractive index of the laser crystal. According to the rate equations of longitudinally pumped quasi-three-level system, the threshold pump power of Tm,Ho:YLF lasers can be expressed as [15, 16]:
In Eq. (3) parameters Pth0 and β are defined as follows:
Here, Pth 0 is the threshold pump power without up-conversion effect. Equation (3) indicates that up-conversion processes increase the threshold pump power by the factor (1+β). The parameter β is a measure of the magnitude of the effect of up-conversion. The influence of up-conversion effect depends on the threshold pump power Pth 0. In addition, the dependence of the β parameter on the spatial variation of both the pump beam and the cavity field can be explicitly expressed in Eq. (5). Because Tm,Ho:YLF laser is the quasi-three-energy level system, additional term flNHol appears in Eq. (4). Because of strong temperature dependence of fl, the threshold pump power of Tm,Ho:YLF laser depends on the temperature. The slope efficiency of Tm,Ho:YLF lasers can be written as:
In Eq. (4–6):
Where, νl is the frequency of 2µm laser, νp is the frequency of pump beam, Q is up-conversion coefficient, δ=L+T is the round-trip loss, L is the intrinsic cavity loss, T is the transmittance of the output coupler, σ is the emission cross section, τ is the fluorescence life of the coupling upper manifold, f=fu+fl, fu and fl are the fractions of the total 5I7 and 5I8 population density residing in laser upper level and lower level, fHo represents the fractional population of Ho ions in the coupling upper level, ηp is the quantum efficiency, and h is the Planck’s constant.
As show in Fig. 3, the slope efficiency of Tm,Ho:YLF laser is also related to the temperature of the crystal. From Eq. (6), we can find that the slope efficiency is inverse relation with the threshold pump power, so the slope efficiency also depends on the temperature of crystal. The slope efficiency will decrease and the threshold power will increase when the crystal temperature increases, and this is in good agreement between experimental results and theoretical analysis. At the same time, we also can find from Eq. (4–6) that the threshold pump power and the slope efficiency depend on the match between the laser mode and the pump beam in the crystal.
In the Q-switched operation, From Fig. 7, we find that the repetition frequency related to the maximum output energy is about 500Hz. At frequencies up to 500Hz, the population inversion between pulses can reach a maximum and the pulse energy is constant. However, beyond this the time between Q-switch pulses is less than the effective upper laser lifetime and the attainable inversion is reduced. The transition region occurs at Q-switch frequencies commensurate with an effective upper laser level lifetime of 2ms, a factor of 7 less than the 14ms fluorescence lifetime because of up-conversion effect.
We have built room-temperature continuous-wave and Q-switched Tm,Ho:YLF lasers and investigated their output performances. In the CW operation, we obtained 393mW multimode output power, representing a 14% optical to optical efficiency. In order to achieve single frequency operation, two intra-cavity solid etalons are used. Single frequency output power of 113mW is obtained, and the single frequency threshold power is only 250mW. For scaling to higher output powers, we would envisage using the source described here as a seed laser for either a larger oscillator or an amplifier. In the acousto-optic Q-switched operation, laser pulses with the energy of 45µJ and 142ns FWHM width have been achieved for the incident pump power of 1.7W. The experimental results are theoretically analyzed with the analytical formulas of the threshold pump power and slope efficiency in which the energy transfer up-conversion and ground state re-absorption are taken into account. We find the influences of the up-conversion effect and ground state re-absorption can be decreased by lowering the temperature of crystal.
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