- Neodymium Vanadate Lasers Get Even Better
Experimental work leads to improvements in both continuous and pulsed lasers.
Neodymium vanadate lasers have found broad acceptance in materials processing, displays and other applications as a result of their high gain, polarized output and high optical efficiency. Recently, scientists at Technische Universität Kaiserslautern and at Lumera Laser GmbH, both in Kaiserslautern, Germany, demonstrated techniques to enhance the pumping efficiency and the output power of these lasers, and to maintain a constant pulse duration from a Q-switched laser despite dramatic changes in pulse repetition frequency.
The pump-light absorption of Nd:YVO4 is polarization-dependent, with one linear polarization component absorbed nearly four times as quickly as the orthogonal component at the peak absorbed wavelength of 808 nm.
When the material is end-pumped with unpolarized light, this leads to an uneven absorption along the length of the crystal: The highly absorbed polarization is absorbed in the first few millimeters of Nd:YVO4, while the other component is absorbed over a greater depth into the crystal. (End pumping is the preferred technique with Nd:YVO4 because it maximizes the overlap between the pump light and the intracavity laser mode.)
Figure 1. Unpolarized 808-nm pump power in Nd:YVO4 (broken line) is absorbed less evenly than polarized pump power (solid line). With the unpolarized pump, half the light follows the rule “Get absorbed quickly,” while the other half follows a different rule, “Get absorbed slowly.” With the polarized pump, all the light follows the same rule. In both cases, what is plotted here is the absorption per unit length, normalized to the maximum unpolarized absorption. Images reprinted with permission of Optics Letters.
This uneven absorption of pump light causes thermal focusing and aberrations that, in practice, limit the pump power to a few tens of watts. One solution is to spread the thermal load over several laser rods, but this introduces complications and expense. Another solution is to pump with polarized light. In this case, all the light follows the same absorption rule and is absorbed gradually over the length of the crystal (Figure 1). As indicated in the figure, the thermal loading on the face of the crystal is reduced by nearly a factor of two with polarized pump light. However, pumping with polarized light often is difficult, especially with a fiber-coupled diode, because the fiber tends to scramble whatever polarization may have existed in the diode-laser output.
But there are spectral regions in which both polarization components of pump light follow the same absorption rule (Figure 2), and the scientists in Germany took advantage of that fact to obtain the gradual absorption indicated in Figure 1 for polarized pump light — but with an unpolarized pump.
Figure 2. There are several wavelengths around 820 nm and 888 nm for which the absorption of both polarizations is the same in Nd:YVO4. The scientists chose the 888-nm wavelength to pump their laser because it provided a lower quantum defect than the 820-nm absorption lines. The values plotted are for Nd:YVO4 with 1 percent atomic doping, and the “a” and “c” designations indicate the crystal axes for the two orthogonal pump-light polarizations.
They selected one of the longer equal-absorption wavelengths (888 nm) as their pump wavelength to minimize the quantum defect in their laser. Although the absorption is much weaker at 888 nm than at the 808-nm peak, this is an advantage in that it allows the thermal load to be distributed over a greater length of the laser rod. An equivalent effect is usually achieved by reducing the doping concentration in the laser rod, but there are practical limits on how far the concentration can be reduced without introducing deleterious doping inhomogeneities.
The researchers further reduced pump-absorption variation by reflecting the unabsorbed pump light for a second pass through the laser rod (Figure 3). They generated up to 60 W of output power from this laser in a clean, TEM00 (M2 = 1.05) beam, with 55 percent optical-to-optical pumping efficiency.
Figure 3. The Z-resonator had the Nd:YVO4 laser rod at its center and a pair of convex folding mirrors to compensate for thermal focusing in the rod. The folding mirrors transmitted the pump light, but mirror M3 reflected the unabsorbed pump light for a second pass through the rod.
Constant Q-switched pulse
Q-switched Nd:YVO4 lasers have been successful in industrial applications such as via drilling, micromachining and semiconductor processing; however, with conventional Q-switched lasers, the pulse duration, repetition frequency and energy are interdependent, making it difficult if not impossible to select the optimal combination of parameters for a particular application. A Q-switched laser whose pulse duration is independent of other laser parameters would offer advantages in many of these applications. The scientists have demonstrated such a laser by combining the technologies of Q-switching and cavity dumping, a technique that was known as “pulse transmission mode” Q-switching during the early days of ruby lasers.
In a conventional Q-switched laser, the resonator quality (Q) is spoiled, usually by blocking one of the mirrors with a modulator (the Q-switch). With nowhere to go, energy builds up in the population inversion until the modulator switches to its transmissive mode. Then a few spontaneous photons quickly stimulate the laser to emit all the stored energy in a giant, Q-switched pulse. The duration of this pulse is dependent on many parameters, including the resonator’s length and output coupling, the repetition rate and the pump power.
The principle of pulse-transmission-mode Q-switching is similar, with one subtle but important difference. The resonator Q is still spoiled while energy builds up in the population inversion, but now it is spoiled not by blocking one of the mirrors, but by converting that mirror to a transmitter. Once the energy stored in the population inversion has reached its peak value, the mirror is suddenly switched from its high-transmission mode to a high-reflectance mode.
Figure 4. The BBO crystal serves as a switchable quarter-wave plate. When high voltage is not applied to the crystal, it is not birefringent, and light from the upper folding mirror experiences a 90° polarization rotation as it passes twice through the fixed quarter-wave plate. In this case, the light is transmitted through the polarizer. When a voltage is applied to the BBO crystal, it becomes a quarter-wave plate: The light from the upper folding mirror experiences a full 180° rotation (back to its original polarization) and is reflected from the polarizer back to the upper folding mirror. In effect, the components in the white triangle act as a mirror that can be switched from maximum transmission to maximum reflection. (The passive quarter-wave plate is present so the normal condition of the switch is voltage-off rather than voltage-on.)
Energy pours out of the population inversion into the intracavity circulating power via stimulated emission but cannot escape from the resonator because both mirrors are maximum reflectors. Once the circulating power has reached a maximum value, the mirror switches to its high-transmission mode, and all the intracavity energy is dumped from the resonator in a pulse whose duration is equal to the round-trip transit time in the resonator. In other words, the pulse duration depends only on the resonator length, and not on any other parameters.
The scientists realized pulse-transmission mode Q-switching in a Z-resonator similar to the one in Figure 3 (Figure 4). They pumped the laser at 888 nm to obtain the advantages discussed above. And, as expected, they observed a Q-switched pulse duration that was independent of repetition frequency (Figure 5). The system provided an average power of 47 W at its optimum pulse repetition frequency of 50 kHz, although it still provided 40 W at 100 kHz.
Figure 5. The pulse duration from the pulse-transmission-mode Q-switched laser (bottom line) was constant (and approximately equal to the 5-ns round-trip transit time of the resonator) as the repetition frequency varied. For comparison, the scientists also plotted the pulse duration of the same laser in normal Q-switched operation with a 40 percent transmission output coupler (top line).
Because, as indicated in the figure, the pulses were much shorter than those from a normal Q-switched laser, the peak power of the pulse-transmission-mode pulses was correspondingly higher. The high peak power leads to efficient harmonic conversion, and the scientists generated up to 31 W of green second-harmonic power in an LBO crystal, with a conversion efficiency of 70 percent.
Optics Letters, Nov. 15, 2006, pp. 3297-3299 and 3303-3305.
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