1. Introduction

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 ⁴I_15/2 → ⁴I_13/2 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 ⁴F_7/2 → ⁴F_5/2 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 [5]. 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 [6]. 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 [7]. 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 [8].

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 ⁴I_13/2 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 ⁴I_11/2 level of Er³⁺. Alternatively, the resonant pumping into the ⁴I_15/2 → ⁴I_13/2 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 ⁴I_13/2 level in Er:YVO₄ [3].

In this paper, we report on the Q-switched performance of the Er:YVO₄ laser resonantly pumped into the ⁴I_15/2 → ⁴I_13/2 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.

2. Experiments

For spectroscopic characterization, we used a 0.5%-doped Er³⁺:YVO₄ single crystal (N_Er = 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 ⁴I_13/2 →⁴I_15/2 transitions in Er:YVO₄ were determined in [4]. For π-polarization, the maximum emission cross-section is σ_em ~0.5 x 10⁻²⁰ cm² at 300 K at 1603 nm wavelength.

 

Fig. 1 Er³⁺:YVO₄ absorption cross-sections of the ⁴I_15/2 → ⁴I_13/2 transitions for π- and σ-polarizations. The emitting spectra of the pump sources, Er-fiber laser and fiber-coupled laser diode, are also shown.

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For laser experiments, an anti-reflection (AR) coated 0.5% Er³⁺:YVO₄ crystal (N_Er = 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.

 

Fig. 2 Simplified optical layout of the Q-switched Er:YVO₄ laser.

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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 [9], 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.

 

Fig. 3 CW performance of the Er:YVO₄ laser pumped into the 1538.6 nm absorption line by the Er-fiber laser. Laser cavity: Z_CAV = 110 mm R_OC = 90%, R_CC = 100 mm. Acousto-Optical Element (AOE) was either inserted into- or removed from the cavity.

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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.

 

Fig. 4 The performance of the AO Q-switched Er:YVO₄ laser pumped into the 1538.6 nm absorption line by the Er-fiber laser. Laser cavity: R_OC = 90%, Z_CAV = 110mm. (a) Average output power versus the incident pump power; (b) - versus the absorbed pump power;

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Fig. 5 The performance of the AO Q-switched Er:YVO₄ laser pumped into the 1538.6 nm absorption line by the Er-fiber laser. Laser cavity: R_OC = 90%, Z_CAV = 110mm. (a) Typical pulse trace taken at 5 kHz PRF. (b) Pulse energies and pulsewidths when the CW incident pump power is 18 W.

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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 [9].

Secondly, with longer time intervals between Q-switched pulses, the amplified spontaneous emission (ASE) begins to affect energy storage (⁴I_13/2 population) reducing the effective lifetime of the upper ⁴I_13/2 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.

 

Fig. 6 The performance of the AO Q-switched Er:YVO₄ laser pumped into the 1529 nm absorption line by the 23 W fiber-coupled diode laser. Laser cavity: R_OC = 90%, Z_CAV = 110 mm. (a) Average output power of the AO Q-switched Er:YVO₄ laser versus the incident pump power measured at different PRF. For comparison, a CW operation of this laser with and without AOE inserted into the cavity is also shown; (b) Q-switched pulse energies and durations dependence on the PRF.

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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.

3. Conclusion

In conclusion, we demonstrated an efficient AO Q-switched Er:YVO₄ laser resonantly pumped into the ⁴I_15/2 → ⁴I_13/2 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.