We report on the performance of an acousto-optically Q-switched Er:YVO4 laser resonantly pumped in the 4I15/2 → 4I13/2 transition. The laser output was π-polarized at 1603 nm. It operated with a 1-40 kHz pulse repetition rate (PRR) and delivered up to 1.15 W of average output power. A slope efficiency of 45% with respect to the absorbed pump power was demonstrated at the pulse repetition rates of 10 kHz and above. The best Q-switched operation with 470 μJ, 50 ns long pulses was achieved at the PRR of 1-2 kHz and pumping with a fiber-coupled laser diode. This Q-switched pulse energy is the highest reported for Er:YVO4 single crystal laser.
© 2013 OSA
Solid-state lasers operating in the eye-safe atmospheric transmission window around 1500-1700 nm are attractive sources for active imaging-, remote sensing- and range finding applications. Up until now the common and only practical approach was to use lamp- and diode-pumped Er-Yb-doped glass lasers and Er:YAG crystalline lasers. Recently studied resonantly pumped, low quantum defect (QD) eye-safe lasers based on Er:YVO4 crystals are very promising as compact and efficient laser sources in this wavelength region. Under resonant pumping (at 1528 and 1538 nm) Er:YVO4 lasers have demonstrated nearly QD-limited CW laser operation with slope efficiencies as high as 83 – 85% at cryogenic temperatures and 54 – 58% at room temperature [1–4].
The Q-switched operation of lasers based on Er-doped vanadates is much less studied, especially with resonant pumping into the 4I15/2 → 4I13/2 transitions of Er3+. In a few known publications on these lasers, co-doping with Yb3+ was used to achieve efficient pumping with diode lasers into 4F7/2 → 4F5/2 transition of Yb3+ (either at 967 nm or 980 nm) with the subsequent energy transfer to Er3+ ions [5–8].
To the best of our knowledge, the first Q-switched operation of Er:Yb:YVO4 laser was reported in Tolstik et al . Using a Co2+:MgAl2O4 saturable absorber, pulses with an average power of 81 mW and with durations of 150 ns have been achieved. Similar passive Q-switched operation yielded 44 μJ/256 ns pulses at 36 kHz pulse repetition frequency (PRF) was reported in . With a spinning disc as a shutter, Tsang et al demonstrated pulses with energies up to 90 μJ and durations of 110 nsec at a repetition rate of 19.2 kHz . Pulses with 200 μJ energies and ~60 ns duration have been achieved with actively Q-switching of the Er:Yb:YVO4 laser by a rotating prism at 1 Hz PRF .
However, in the Q-switched mode, it is difficult to provide a low QD and high laser efficiency with Er:Yb:YVO4 pumped into Yb3+-ions absorption band. Indeed, with increasing energy storage in the 4I13/2 manifold of erbium ions, a direct Yb3+ → Er3+ energy transfer slows down and the reverse process becomes to play a vital role. In addition, laser efficiency is limited by up-conversion from the 4I11/2 level of Er3+. Alternatively, the resonant pumping into the 4I15/2 → 4I13/2 transitions of Er3+ reduces these detrimental effects, especially in the Q-switched regime when the inversed population density can be very high. Contrary to YAG crystals, there is no overlap between the emission and excited state absorption bands originating from the 4I13/2 level in Er:YVO4 .
In this paper, we report on the Q-switched performance of the Er:YVO4 laser resonantly pumped into the 4I15/2 → 4I13/2 transition of Er3+ by either a fiber laser or a fiber-coupled laser diode. Q-switching was achieved with an acousto-optical element at PRF of 1 - 40 kHz.
For spectroscopic characterization, we used a 0.5%-doped Er3+:YVO4 single crystal (NEr = 6.05 x 1019 cm−3) grown by the Czochralski technique. Figure 1 shows π- and σ – polarized ground-state absorption spectra of Er3+:YVO4 for in the wavelength range of 1520 - 1540 nm. It can be seen that the absorption of Er:YVO4 around 1529 nm is approximately equal for both polarizations, whereas at 1538.6 nm absorption is stronger for σ-polarization than for π-polarization. Figure 1 also shows spectra of the pump sources used in laser experiments - an Er-fiber laser and a fiber-coupled laser diode. The emission cross-sections of the 4I13/2 →4I15/2 transitions in Er:YVO4 were determined in . For π-polarization, the maximum emission cross-section is σem ~0.5 x 10−20 cm2 at 300 K at 1603 nm wavelength.
For laser experiments, an anti-reflection (AR) coated 0.5% Er3+:YVO4 crystal (NEr = 6.05 x 1019 cm−3) with a cross-section of 3 x 7 mm and length of 10 mm was used. The c-axis of the crystal was oriented along the 7 mm direction and therefore either of two polarizations can be used for absorption and emission. The crystal was mounted between two water-cooled copper plates and kept at a temperature of + 15°C. The experimental setup is shown in Fig. 2.
As was mentioned above, two pump sources emitting in the 1529 - 1538 nm wavelength range were used for pumping Er3+:YVO4. The first one was a CW Er-fiber laser with a narrow emission bandwidth of the 0.3 nm FWHM, which was completely overlapped by the 1538 nm absorption band of Er:YVO4. This band has a large σ-cross-section and a relatively small π-cross-section (see Fig. 1). The unpolarized pump beam was focused in the crystal by a spherical lens with a focal distance of 100 mm providing cylindrical pumped volume with a diameter of ~300 μm (at e−2 level).
The second pump source was a CW fiber-coupled laser diode (LD) emitting at 1529 nm. The fiber core diameter was 105 μm, and the fiber had a numerical aperture (NA) of 0.15. The emission spectrum of the LD could be slightly tuned by varying the temperature of the cooling water. A relatively good spectral overlap with the 1529 nm absorption band was achieved when the cooling water was kept at 14°C and the LD operated at the maximum output power of 23 W (see Fig. 1). The unpolarized pump emission from the fiber was collimated by a lens with 30 mm focal length and focused into the crystal through a flat dichroic mirror (HR R > 99.5% at 1580-1650 nm, HT T > 95% at 1500-1540 nm) by a spherical lens with a 75 mm focal length. The excited volume inside the crystal had a conical shape with the diameter varying from ~250 μm in the center to ~530 μm at the crystal ends.
A laser cavity was formed by a dichroic mirror and a concave output coupler with 100 mm radius of curvature (RoC) and 90% reflection at 1603 nm. A cavity length was chosen to be of 110 mm, for pumping with the fiber laser, and 94 mm, for pumping with the LD.
For the fiber laser pump, a good pump-to-laser mode spatial matching was achieved in the gain medium. In the diode pumping case, the spatial mode matching was worse due to the conical shape of the pumped volume (the spatial overlap ratio was ~60%). An acousto-optic Q-switch element (AOE) with Brewster-cut ends was inserted into the laser cavity between the crystal and the output coupler.
In order to correctly evaluate the absorbed pump power, which could be affected by the pump saturation effect , we directly measured the incident and the transmitted pump powers as well as the laser output power simultaneously, see Fig. 2. The attenuation of pump by the AOE in both polarizations was measured separately and was taken into account in the derivation of the absorbed pump.
2.1 CW operation
First, the Er:YVO4 laser was run in a CW mode in order to test its efficiency and estimate loss introduced by the AOE. Figure 3 shows the π-polarized output power (1603 nm) versus the absorbed, Fig. 3(a), and the incident, Fig. 3(b), pump power with- and without the AOE inserted into the cavity. The laser was pumped at 1538.6 nm by the Er-fiber laser. A maximum CW output of 1.8 W was achieved with the absorbed pump power of 4.4 W resulting in a slope and an optical-to-optical efficiencies of 55% and ~13%, respectively. The relatively low optical-to-optical efficiency was caused by inefficient absorption of the π-polarized component of unpolarized emission coming from the Er-fiber laser at 1538.6 nm. The optical-to-optical efficiency can be improved by using either a higher doped Er:YVO4 crystal or by using only a σ-polarized pump source. It can be seen that insertion of the AOE into the cavity resulted in the reduction of the slope efficiency from 55% to 45%. The multimode output spectrum peaked at ~1603.4 nm and had the ~0.3 nm bandwidth.
2.2 Q-switched operation
Figure 4 shows the average output power of the Q-switched laser versus the incident and the absorbed pump power at various PRF ranging from 1 kHz to 10 kHz. A typical pulse trace taken at 5 kHz PRF is shown in Fig. 5(a). The PRF under 1 kHz was not investigated to avoid damaging the Er:YVO4 crystal by the high laser fluence approaching the damage threshold of the material. For the PRF of 10 kHz and higher, the average Q-switched output power is the same as in the CW mode. But it gradually decreases with the reduction of PRF from 10 kHz to 1 kHz. There are two features relevant to Fig. 4 that should be emphasized.
First, in Fig. 4(b) every set of experimental data points corresponding to a different PRF were taken with the same sequence of incident pump powers. One can see that for a particular value of incident pump, the absorbed pump quickly decreases with the PRF reduction from 10 kHz to 1 kHz. For example, out of the ~18 W of the incident pump power, ~3.3 W was absorbed at PRF = 5 kHz, but only 2.75 W at PRF = 1 kHz. It could be explained by a growing influence of the pump saturation effect .
Secondly, with longer time intervals between Q-switched pulses, the amplified spontaneous emission (ASE) begins to affect energy storage (4I13/2 population) reducing the effective lifetime of the upper 4I13/2 laser level. This is why for a given absorbed pump power, the laser output decreases with the decrease in PRF as well.
Figure 5(b) depicts the dependence of the Q-switched pulse energy and the pulse duration on the PRF at the fixed incident pump power of 18.2 W (the absorbed power depends on PRF, see Fig. 4). The maximum Q-switched pulse energies of 300 μJ with 65-70 ns pulsewidths have been achieved at the PRF = 1 kHz. To the best of our knowledge, this is the highest Q-switched pulse energy reported for a resonantly pumped Er:YVO4 laser. Pulse energies decrease to 60 μJ and pulse-width increases to 250 ns at the 20 kHz PRF.
We also investigated the performance of this laser in Q-switched mode with pumping by the fiber-coupled diode at 1529 nm. Figure 6(a) shows how the average output power in Q-switched regime depends on the incident pump power for 2 - 10 kHz PRF range. As before, data is presented for two cavities: with- and without AOE. This time the output power is plotted only as a function of the incident pump power because an unabsorbed portion of pump could not be accurately measured when AOE was inside the cavity due to vignetting of the pump beam on the AOM aperture.
It can be seen that, similarly to the previous case of pumping with Er-fiber laser, the average Q-switched output power is the same as in CW the mode.
With the reduction of PRF, the average Q-switched output power decreases as was observed in the previous case. It should be mentioned that the spectrum of the diode laser could not be stabilized over the range of diode’s currents. Its spectrum maintained a good match with the 1529 nm absorption band of Er:YVO4 only at the maximum pump power of ~23 W and it slightly shifted toward shorter wavelengths at lower output.
Figure 6(b) shows the dependence of the Q-switched pulse energy and the pulsewidth on the PRF at the incident pump power of 23 W. Maximum Q-switched pulse energies of 470 μJ at 2 kHz PRF with pulse durations of ~50 ns have been obtained.
In conclusion, we demonstrated an efficient AO Q-switched Er:YVO4 laser resonantly pumped into the 4I15/2 → 4I13/2 transition either by Er-fiber laser at 1538.6 nm or fiber-coupled laser diode at 1529 nm. The pulse repetition frequency (PRF) ranged from 1 to 40 kHz. With pumping by a CW Er-fiber laser, 300 mJ/60 ns Q-switched pulses were achieved at the 1 kHz repetition rate. Almost 45% slope efficiency with respect to the absorbed pump power has been obtained in the Q-switched mode for the pulse repetition rates of 10 kHz and above. With pumping by a CW fiber-coupled diode, 470 μJ/50 nsec Q-switched pulses have been obtained at the 2 kHz rate. To the best of our knowledge, these Q-switched pulse energies are the highest ones reported for Er:YVO4 lasers.
References and links
1. N. Ter-Gabrielyan, V. Fromzel, T. Lukasiewicz, W. Ryba-Romanowski, and M. Dubinskii, “High power resonantly diode-pumped σ-polarization Er3+:YVO4 laser at 1593.5 nm,” Laser Phys. Lett. 8(7), 529–534 (2011). [CrossRef]
2. N. Ter-Gabrielyan, V. Fromzel, T. Lukasiewicz, W. Ryba-Romanowski, and M. Dubinskii, “Nearly quantum-defect-limited efficiency, resonantly pumped, Er3+:YVO₄ laser at 1593.5 nm,” Opt. Lett. 36(7), 1218–1220 (2011). [CrossRef]
4. N. Ter-Gabrielyan, V. Fromzel, W. Ryba-Romanowski, T. Lukasiewicz, and M. Dubinskii, “Spectroscopic and laser properties of resonantly (in-band) pumped Er:YVO4 and Er:GdVO4 crystals: comparative study,” Opt. Mater. Express 2(8), 1040–1049 (2012). [CrossRef]
5. N. A. Tolstik, A. E. Troshin, S. V. Kurilchik, V. E. Kisel, N. V. Kuleshov, V. N. Matrosov, V. N. Matrosova, and M. I. Kupchenko, “Spectroscopy, continues-wave and Q-switched diode-pumped laser operation of Er,Yb:YVO4 crystals,” Appl. Phys. B 86(2), 275–278 (2007). [CrossRef]
6. Y. H. Tsang, C. J. Mercer, and D. J. Binks, “Record performancel from a passively Q-switched Yb:Er:YVO4 laser,” Solid State Lasers And Amplifiers III, Proc. SPIE 6998, 6998U (2008). [CrossRef]
7. Y. H. Tsang, C. J. Mercer, and D. J. Binks, “A mechanically Q-switched Yb:Er:YVO4 laser,” Solid State Lasers and Amplifiers III, Proc. of SPIE 6998, 9981 (2008).
8. Y. H. Tsang and D. J. Binks, “Record performance from a Q-switched Yb:Er:YVO4 laser,” Appl. Phys. B 96(1), 11–17 (2009). [CrossRef]
9. Y. Sato and T. Taira, “Saturation factors of pump absorption in solid-state lasers,” IEEE J. Quantum Electron. 40(3), 270–280 (2004). [CrossRef]