By using a new saturable absorber, Co2+:LaMgAl11O19 (Co:LMA) crystal, the passive Q-switched operation of a flash lamp pumped 1.54 μm Er:glass laser is realized. With different transmissions of the output mirror, the properties of static output and dynamic output of the laser are studied. The experimental results show that, when transmission of the output coupler is 15%, the static output curve has higher slope efficiency and higher energy conversion efficiency and meanwhile the Q-switched output pulses with higher single-pulse energy and narrower pulse width can be obtained.
©2007 Optical Society of America
The laser radiation within 1.5–1.6 μm wavelength range, because of its eye-safe characteristic, high atmospheric transmission capability and the availability of suitable detection devices, has enormous potentials in many applications such as communications, laser warning, laser radar and ranging, remote-sensing of aerosols. How can we get laser at the wavelength? Optical parametric oscillators (OPOs), such as the noncritical-phase-matching intracavity KTP-OPO [1–3], provide an effective way to produce 1.57 μm laser, but the configuration of the laser cavity is relatively complicated and the high-damage-threshold of nonlinear crystals and the coatings on their surfaces are needed to resist the high power density of the oscillating light in cavity. If the Er-doped crystals are chosen as the active medium, the 1.54 μm laser  can be generated directly from a simple resonator with two mirrors. Er-doped lasers belong to the three-level system so that the pumping threshold is relatively high, but the two-mirror resonator makes the laser setup very compact and simple. Due to the widespread use in optical communication, the Er:Yb:glass lasers pumped by laser diode have attracted more and more interests [5–6]; and in the same way, the researches of the Er:glass lasers pumped by flash lamp have also made much progress because of the unique advantages of eye-safe.
In this paper, we introduced, a flash lamp pumped passive Q-switched Er:glass laser with Co:LMA as saturable absorber. In order to obtain the larger output pulse energy, we choose the higher doped Er ions concentration of 50% so that the larger energy can be stored in the active medium. It can be used in some applications of laser ranging. Some papers have reported this type passive Q-switching laser [7–9]. As a new saturable absorber, Co2+:LaMgAl11O17 crystal can be used as a passive Q-switch in the lasers with output wavelength ranging from 1.3 μm to 1.6 μm [10–11]. The growth and the physical properties of the crystal have been reported in Ref. . We also investigated the output characteristics of the Er:glass laser under the static state and the dynamic state , respectively.
2. Experimental setup
The experimental setup is shown schematically in Fig. 1. In the plane-plane cavity, the mirror M1 is coated with high-reflection coating at 1.54 μm and the mirror M2 is coated with partial transmission coating at 1.54 μm. Er:glass is a cylindrical rod with the size of Φ3 mm×40 mm and Er3+ ion concentration is of 50 at.%. The two ends of the rod are coated with high transmission coating at 1.54 μm. The saturable absorber Co:LMA with the initial transmission T 0=78% is placed closely to M2 and the surfaces of the crystal are not coated with any coating. The duration of the Xe flash lamp pulse is about 200 μs and the pump energy can be calculated by the formula: E=CU 2/2 (C and U are the capacity and voltage of the pump power supply, respectively). The Xe flash lamp with the size of Φ5 mm×50 mm and the Er:glass rod are placed in the two focal axis of the elliptic cylinder cavity, respectively. The whole cavity length is 14 cm. We measured the output energy using an EPM2000 energy meter (Molectron Corp. USA), output pulse shapes with a germanium detector (Judson Technologies, response range: 800 nm-1800 nm) and a TDS 3032B oscilloscope (Tectronix), respectively.
3. Results and discussion
3.1 Static state performance
A free running cavity is built by removing the Co:LMA crystal from the cavity in Fig. 1. The output mirrors with the transmission of 5%, 15% and 35% at 1.54 μm are used in the setup, respectively. The output energy as a function of pumping energy of flash lamp is shown in Fig. 2. The variation curves indicate that pumping threshold of the laser is above 20 J. Two reasons resulted in the relatively high pumping threshold. Firstly, the Er:glass laser is a three-level system, to satisfy the population reversion condition, more than half of the population at ground state level are needed to be extracted to the upper level, so the pumping threshold of the three level system is much higher. Secondly, the Er3+ ion concentration in the Er:glass is very dense in our experiment, and the high Er3+ ion concentration of 50 at.% will inevitably increase the pumping threshold . Figure 2 also shows that the output energy grows linearly with the increases of pumping energy. Comparing with the output curve with transmission of 5%, the output curves with transmissions of 15% and 35% have the relatively higher slope efficiency and pumping threshold. Among the three curves, the output curve with transmission of 15% has the highest energy conversion efficiency and output energy of 198 mJ obtained at the curve end under the pumping energy of 47.9 J. For a certain laser resonator, there is an optimum transmission that could make the laser generate the maximum output energy . In our experiment, we believe the transmission of 15% is mostly close to the optimum transmission among the three output couplers.
3.2 Dynamic state performance
A Co:LMA crystal with initial transmission of 78% at 1.54 μm is used as the saturable absorber. For optimization of the passive Q-switched performance the gain medium with large stimulated emission cross section is not helpful because the Q-switch works well only when it is saturated before gain medium (the second threshold condition). According to the analysis of the coupled rate equations, the criterion for good passive Q-switching is given by :
Where T 0 is the initial transmission of the saturable absorber, A/AS is the ratio of the effective area in the gain medium to that in the saturable absorber, R is the reflectivity of the output mirror, L is the nonsaturable intracavity round-trip dissipative optical loss, σgs is the ground-state absorption cross section of the saturable absorber, σ is the stimulated emission cross section of the gain medium, γ is the inversion reduction factor with a value between 0 and 2 as discussed in Ref. , and β=σes/σgs is the ratio of the excited-state absorption cross section to that of the ground-state in the saturable absorber. Comparing with σgs ≈ 1.2×10-19 cm2 of the Co:LMA at 1.54 μm, σ ≈ 0.8×10-20 cm2 of the Er:glass at 1.54 μm is much smaller and in order to match with formula (1), A/AS should be higher than 0.196 with the initial transmission of 78%. Considering a relatively large mode volume in the plane-plane cavity adopted in the setup shown in Fig. 1 and the second threshold condition is satisfied for A/AS=1≫0.196 in this plane-plane cavity.
Figure 3 shows the dependence between output energy and pumping energy. When the output coupler with T=15% is used, the output energy of the laser increases with the increase of pumping energy, and if the pumping energy is not higher than 43.2 J, the output pulses are all the Q-switched single-pulse. If the pumping energy increases once more, multi-pulses will generate. Under the pumping energy of 45.6 J, the first double-pulse output occurred and output energy reached to 92 mJ that was almost the double of the output energy of 47 mJ under pumping energy of 43.2 J. As T=35%, all of the Q-switched output are single-pulse but the energy conversion efficiency is lower and the pumping threshold are relatively higher than those of the laser with T=15%.
Figure 4 shows the variations of output pulse width (FWHM) with pumping energy at output transmission of 15% and 35%. We can see from the tendency of the curves that the pulse width decreases with the increase of pumping energy and the output pulse width for output transmission of 15% is narrower and almost a half of that with transmission of 35%. The variation of the output pulse width is induced by the change of ni/nt (ni and nt are the initial and threshold inversed population, respectively). For a Q-switched laser, we know that the increase of ni/nt will cause the laser to generate narrower output pulse width and, apparently, the increase of pumping energy will result in the increase of the ratio of ni/nt directly.
In the whole process of the experiment, except for the double-pulse that occurred when the transmission of output coupler is 15% and the pumping energy is 45.6 J, all the output pulses are single-pulse. This is mainly due to the relatively small ground-state absorption cross section and the high Er3+ ion concentration in Er:glass. The long particle lifetime on the upper level and the small ground-state absorption cross section are helpful to store energy and restrain the multi-pulse in a flash lamp pumping system and, additionally, the high Er3+ ion concentration raises the pumping threshold, so it is very difficult to produce multi-pulse during a flash period of the pumping lamp.
According to Fig. 3 and Fig. 4, we calculated the peak power of output pulse and the variation curves are shown in Fig. 5. When the pumping energy is 43.2 J and output transmission is 15%, we get the narrowest pulse width of 54 ns and highest peak power of 0.86 MW. Figure 6 is the oscilloscope trace of the output pulse, and its width is ∼ 55 ns. In our experiment, the amplitude stability of the Q-switched pulses is about 10% and the pulse width of the Q-switched pulses is relatively stable.
We demonstrated the passively Q-switched 1.54 μm Er:glass laser using the Co:LMA crystal as a saturable absorber. The experimental results indicated that the Co:LMA crystal has desirable Q-switching properties at 1.54 μm. The Q-switched 1.54 μm Er:glass laser discussed in our paper has the properties of narrow pulse width (∼ 55 ns.), high peak pulse power (0.86 MW) and single-pulse output. If we choose the optimum output coupler transmission and lower initial transmission T 0 of the saturable absorber Co:LMA, the lower pump threshold and narrower output pulse width can be expected. The potential of the laser can be expected if they are used in the laser rangefinder with the advantage of eye-safe radiation.
This work is supported by the National Natural Science Foundation of China (No. 90201017)
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