An actively Q-switched Er:YAG laser generating pulses at 2.94 μm has been developed and investigated. For a single Er:YAG generator at 3 Hz repetition rate, pulses of 91.2 ns duration and 137 mJ energy have been obtained. It corresponds to pulse train with high-peak power of ~ 1.5 MW. For 10 Hz repetition rate 30 mJ of output energy in single pulse has been achieved. These results, according to our knowledge, are the best world-wide achievements.
©2004 Optical Society of America
A considerable interest in the mid-infrared Er:YAG lasers has been observed in recent years. This interest is shown particularly by specialists from different branches of medicine dealing with laser therapy, mainly surgery and microsurgery.
The laser radiation wavelength of 2.94 μm corresponds to a strong absorption peak in soft and hard biological tissues (~ 10 000 cm-1) which contain a significant amount of water [1,2]. When the absorption is so high, the density of the energy absorbed in a tissue makes ablation the dominant mechanism of the tissue destruction. The extent of thermal damage to surrounding tissue can be reduced if the laser pulse duration is shortened . Therefore, Q-switching would be preferred to the free running mode of laser operation. Then the extent of the tissue thermal damage is kept to a minimum (so as not to delay the healing process).
The great majority of literature reports about Q-switched Er:YAG lasers describe laser operation with the repetition rate of 1 Hz. Depending on Q-modulation technique, the following energies of generated pulses have been obtained:
Literature reports concerning Er:YAG lasers generating higher output energy (up to 100 mJ) present the generation of single pulses, without repetition rate defined.
Generation of giant pulses in Er:YAG laser is difficult for several reasons, mainly for the sake of active medium characteristics. The lifetime of the upper laser level is short (~ 100 μs) which increases requirements for pumping rate and makes energy storage in an active medium difficult. The gain obtained is small not only for the sake of short upper laser level lifetime but also due to low value of emission cross section of erbium dopant at 2.94 μm transition (~ 10-20 cm2). Moreover, the lifetime of the lower laser level is very long and therefore the laser, during pulse generation, behaves like three-level quantum system. Additionally, high absorption of laser radiation (in the range of 3 μm) in laser set-up elements (mirrors, polarizes, crystals, lenses) causes high losses in a laser cavity and, as a result of this, the rapid drop of pulse energy or just makes the laser generation impossible.
In the present paper we report on experiments on Q-switching of erbium laser with the aid of an electro-optic cell. The aim of the experiments conducted was to develop a laser set-up delivering high energy pulses in the range of 3 μm that could be used in surgical applications.
2. Er:YAG laser set-up
The general layout of the Er:YAG laser is shown in Fig. 1. The system operated at 2.94 μm. A ϕ4×100 mm Er3+-doped YAG crystal was used as an active medium. In order to reduce thermal lensing, both end-faces of the laser rod were concave with the radius of curvature of 5 m. Its both sides were AR-coated. The laser rod was pumped by a single xenon flashlamp, with a typical pump energy of up to 100 J and the repetition rate of 1–12 Hz, housed in a single diffuse (ceramics) cavity LMI 1610. The 39.5 cm-long optical cavity was formed by two flat dielectric mirrors M1, M2 with reflectivities of 85 and 100%, respectively.
The laser was Q-switched electrooptically, employing a Pockels cell (CS 1045 SN 1292) made of a LiNbO3 crystal with dimensions of 20×10×5 mm and with faces cut at Brewster’s angle (ΘB = 65°). The Brewster faces of the LiNbO3 crystal acted as partial polarizers situated on both sides of the crystal. An electric field was applied transversely to the direction of light propagation, and a DC voltage (UPC = 1.35 kV) was applied to the LiNbO3 crystal. This crystal with electrodes and a housing, on which the electronic switching circuit was mounted, was placed inside the optical cavity of Er:YAG laser between the laser rod and the high reflector (5 cm from mirror M2). The laser rod and the flashlamp are cooled with circulating distilled water.
The flashlamp was supplied by means of home-made power supply system PPM-6kW. It allowed us to control smoothly the time of the current pulse applied to the lamp, the supply voltage as well as the abrupt change of pulse repetition rate (ranging from 1 to 12 Hz). The pump pulse duration was determined at the level of 320 μs. Its further increase did not cause the rise of energy generated, but only the increase in heat load of the laser rod.
Both the course of the voltage applied to the Q-switch, used as trigger signal, and the laser pulse signal were monitored simultaneously with an oscilloscope (Tektronix TPS 3052) with high-voltage probe (Tektronix TekP6139A). The pulse width was always measured at FWHM. The pulse energy and the pulse width were measured with an energy meter (Laser Precision Rj-7100) and a pyroelectric detector (Molectron P3-01), respectively.
3. Experimental results
In Er:YAG laser set-up presented in Fig. 1 generation of nanosecond pulses at the repetition rate of 3 Hz has been achieved. The measurement of the output energy versus pump energy for two different values of the voltage applied to the Pockels cell (UPC) has been done. The results obtained are depicted in Fig. 2 and Fig. 3. A maximum output energy of 137 mJ was achieved in a single pulse with a pulse width of 91.2 ns at a pump energy of 59 J. Within the range of the power densities used in this study we did not observe any damage on LiNbO3 crystal, either on the optical surfaces, or in the bulk. The obtaining of such good results was possible thanks to accurate selection of an active Q-switch construction as well as the optimization of pumping process and Q-switch control.
Depending on the pump energy (gain in a laser medium) and the control voltage of Pockels cell single- or multi-pulse laser generation was possible (Fig. 4). The multi-pulse generation resulted from strong oscillations of Q-switch transmission occurring at the moment of losses switching. Additionally, in case of inadequate selection of the control voltage, the laser generation is less effective. In order to eliminate this situation, the measurement of dynamics of Pockels cell switching were carried out (Fig. 5). After switching the control voltage UPC, the transmission of the Q-switch initially rises and after about 50 μs begins to drop. The transmission course is characterized by oscillations with 1.25 MHz frequency lasting about 100 μs. These oscillations are the result of piezoelectric effect in LiNbO3 crystal. The parameters optimization of the Q-modulator supply and the pump energy allowed us to determine the optimal control voltage of Pockels cell UPC = 1.35 kV for pump energy of up to 60 J. The losses caused by this voltage were sufficient to prevent untimely lasing. For this value of the control voltage and for pump energy above 42 J the single-pulse laser generation was possible.
Along with the increase in pump energy, the shortening of generated pulses (to 170 ns for pump energy of 50 J and 92 ns for pump energy of 59 J) was observed. Laser pulses appeared after shorter time corresponding to the Q-switch process for higher values of pump energy. In our case the laser pulses appeared after the time of 750 – 440 ns (Fig. 6).
Additionally, the influence of repetition rate on output energy of generated laser pulses in free-running mode has been studied. The results obtained are shown in Fig. 7. Regardless of the value of the energy supplied to the laser rod in a single flashlamp pulse, along with the repetition rate increase, the drop of pulse energy is observed. For pump energy ranging from 49 J to 56 J and repetition rate of 12 Hz, the output energy drops to about 20% of that received at 3 Hz repetition rate. For the pump energy of 60 J and the repetition rate of 12 Hz we did not achieve the laser action. The analogous character of energy changes versus repetition rate was observed in case of Q-switch operation.
Over a quadruple drop of the output energy can be attributed to the strong thermal lensing within the rod and, therefore it reveals the necessity of its compensation. Thermal lesning is caused by the large temperature gradient set up during the pump pulse and can be reduced by the selective pumping of the upper laser level. By means of Xe-filled flashlamps, most of the pump light is absorbed by states well above the upper laser level and hence non-radiative decay of these levels causes the heating of the crystal.
The laser rods examined were characterized by thermal lens with the focal length of 35 cm for average pump power of 600 W. For comparison, a typical focal length of thermal lens for Nd:YAG rod is over 100 cm . The shorter focal length of this lens (just like in our experiment) entails the significant pulse energy decrease along with the repetition rate increase.
In the Q-switched Er:YAG laser set-up working at 10 Hz repetition rate, single pulses with energy of 30 mJ were achieved, which corresponds with the average power of 300 mW. This level of output energy is sufficient for a number of applications in medicine. For comparison, Fig. 8 shows Er:YAG laser interaction with tissue for the case of free-running mode and Q-switch mode. As a tissue model the gelatine was used. As can be easily observed, this amount of energy (30 mJ) is released in a time still short enough to avoid thermal side effects by laser ablation of a biological tissue.
In conclusion, we have produced Q-switch pulses in Er:YAG laser. A Pockels cell based on LiNbO3 crystal has been used as an active modulator. Pulses as short as 91.2 ns with the energy of 137 mJ (at 3 Hz repetition rate) have been generated. For the repetition rate of 10 Hz, the pulses of 30 mJ energy have been achieved. This drop of energy was caused by thermal lensing effect and the increase in diffractive resonator losses. However, the level of pulse energy obtained is sufficient to ablate most of biological tissues. The laser developed can be successfully implicated in microsurgery, laryngology or ophthalmology.
This research has been supported by Polish State Committee for Scientific Research (project 4T 11B 028 24). The authors wish to thank E. Burdziakowska for her help in preparing the manuscript.
References and links
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