Open Access
Add to BibSonomyAbstract
A novel miniaturized Cr4+:YAG passively Q-switched Nd:YAG pulse-burst laser under 808 nm diode-laser pulse-pumping was demonstrated for the purpose of laser-induced plasma ignition, in which pulse-burst mode can realize both high repetition rate and high pulse energy simultaneously in a short period. Side-pumping configuration and two different types of laser cavities were employed. The pumping pulse width was constant at 250 μs. For the plane-plane cavity, the output beam profile was flat-top Gaussian and the measured M2 value was 4.1 at the maximum incident pump energy of 600 mJ. The pulse-burst laser contained a maximum of 8 pulses, 7 pulses and 6 pulses for pulse-burst repetition rate of 10 Hz, 50 Hz and 100 Hz, respectively. The energy obtained was 15.5 mJ, 14.9 mJ and 13.9 mJ per pulse for pulse-burst repetition rate of 10 Hz, 50 Hz and 100 Hz, respectively. The maximum repetition rate of laser pulses in pulse-burst was 34.6 kHz for 8 pulses at the incident pump energy of 600 mJ and the single pulse width was 13.3 ns. The thermal lensing effect of Nd:YAG rod was investigated, and an plane-convex cavity was adopted to compensate the thermal lensing effect of Nd:YAG rod and improve the mode matching. For the plane-convex cavity, the output beam profile was quasi-Gaussian and the measured M2 value was 2.2 at the incident pump energy of 600 mJ. The output energy was 10.6 mJ per pulse for pulse-burst repetition rate of 100 Hz. The maximum repetition rate of laser pulses in pulse-burst was 27.4 kHz for 6 pulses at the incident pump energy of 600 mJ and the single pulse width was 14.2 ns. The experimental results showed that this pulse-burst laser can produce high repetition rate (>20 kHz) and high pulse energy (>10 mJ) simultaneously in a short period for both two different cavities.
© 2014 Optical Society of America
1. Introduction
In recent years, interests are concentrated on laser-induced plasma ignition of air-fuel mixtures in internal combustion engines. Compared with traditional spark plug ignition system, laser-induced plasma ignition technology is an innovative method and has many advantages, such as arbitrary positioning of the ignition plasma, higher possibility for multipoint ignition, shorter ignition delay time, no erosion effects, and increase of engine efficiency [1–3]. For reliable plasma generation and ignition of combustible fuels, light intensities in the order of 100 GW/cm2 are required at the focal point of ignition [4,5]. Among the various kinds of laser sources, the laser-diode-pumped solid-state laser (DPSSL) can easily achieve millijoule pulse energies and megawatt peak powers with Q-switching operation. Therefore, it is applicable to laser-induced plasma ignition after optical focusing.
For the laser-induced plasma ignition, a lot of efforts have done to reduce the size of the laser head because in many actual applications a small size of laser is required. It is well-known that passively Q-switched lasers have become more and more important for its unique characteristics, such as simple fabrication, compact structure, and low cost [6–10]. Hence, this type of laser is the most promising candidate for the laser-induced plasma ignition. The energy of plasma produced by laser breakdown can be accumulated when a high repetition rate laser is used. And high energy laser pulse can produce high energy plasma. Therefore a laser with high repetition rate and high pulse energy is beneficial to increase the success probability and reliability of laser-induced plasma ignition, especially for the high speed flow, for example, rocket engine. However, the two characters of high repetition rate and high pulse energy restrict each other because of the tolerable thermal loading of lasing mediums. Usually, for the small size passively Q-switched laser without amplifier, the single pulse energy is in the range of several millijoules with its frequency of only 10 Hz to tens of Hz [3,10,11], and high repetition rate (≥10 kHz) laser produces single pulse energy only in μJ scale [12]. Instead of continuous pulse mode, pulse-burst mode can realize both high repetition rate and high pulse energy simultaneously in a short period [13]. Therefore, this kind of laser would be advantageous to realize laser-induced plasma ignition.
In this paper, we demonstrated a miniaturized DPSSL pulse-burst laser, which is the pulse-pumped Cr4+:YAG passively Q-switched Nd:YAG laser. The thermal lensing effect of Nd:YAG rod was investigated, and an optimized plane-convex cavity was adopted to compensate the thermal lensing effect of Nd:YAG rod and improve the mode matching. The output performance of pulse-burst laser using two different cavities was studied.
2. Experimental setup
The schematic of passively Q-switched Nd:YAG laser is shown in Fig. 1. It was a side-pumped configuration using 808 nm pulse laser diode. Two Nd:YAG rod crystals, C1 and C2, (both were 1.0 at. % Nd3+ concentration doping level, Φ3 × 75 mm3 dimensions) served as the gain mediums. The end faces F1 and F2 of C1 were both antireflection coated at 1064 nm (< 0.2%). The rod crystal C2 was integrated with cavity end mirror, so the end face F1 of C2 was high-reflection coated at 1064 nm (> 99.8%), and F2 of C2 was antireflection coated at 1064 nm (< 0.2%). M0 was a plane-convex mirror with high-reflection coating at 1064 nm. M1 was a plane mirror and high-reflection coated at 1064 nm. M2 was output coupler with different transmissions (T) of 45%, 55%, and 70% at 1064 nm, respectively. Cr4+:YAG as saturable absorber has advantages of improved thermo-mechanical properties, large absorption cross section, and high damage threshold [14,15]. Therefore, Cr4+:YAG crystals with various initial transmissions at 1064 nm was used to realize passively Q-switched operation. M3 and M4 were coated 45° high-reflection at 1064 nm (> 99.8%). An expanded and collimated He-Ne laser beam propagated through the Nd:YAG rod from the left to the right in order to measure the thermal lensing effect. The beam profiles of the transmitted He-Ne laser and the 1064 nm Nd:YAG laser were separately imaged on a CCD camera. The distance between M1 and F1 was 5 mm, and that for F2 and M2 was 15 mm.
Fig. 1 Schematic of the passively Q-switched Nd:YAG laser and the setup for thermal lensing measurement.
The cross-section of the side-pumping configuration is shown in the dashed circle in Fig. 1. All the laser diode arrays (LDA) were arranged around the Nd:YAG rod in triplex rows and the angle between every two rows was 120°. The pumping LDA can generate the highest peak power of 2400 W and the maximum repetition rate was 100 Hz with a constant pulse width of 250 μs and maximum pump energy of 600 mJ. A glass flow tube provided the necessary cooling and focused the pump light into the laser rod. The temperature of the water was kept constant at 20°.
3. Experimental results and discussions
3.1 Pulse-burst realization
Firstly, in order to have a simple structure and small size, the rod crystal C2 was used. Therefore, the laser cavity was built up by M2 and F1 of C2. The transmission of M2 was 55% and the cavity length was 90 mm. The output energy without Q-switching as a function of incident pump energy at three different pump repetition rates of 10 Hz, 50 Hz and 100 Hz is shown in Fig. 2. We can see that the output energy increased linearly with increasing of the incident pump energy for the 10 Hz laser, and the maximum output energy was 230.0 mJ at the incident pump energy of 600 mJ. But for the 50 Hz and 100 Hz operations, the output energies fluctuated observably when the incident pump energy was higher than 380 mJ and 140 mJ, respectively. This is because of the thermal deformation on the end face F1 of C2 caused by thermal effect under high incident pump energy at 50 Hz and 100 Hz. So the laser cavity formed by M2 and F1 became unstable, and the output energy fluctuated. There was no meaning to increase the incident pump energy under this situation.
Fig. 2 The output energy without Q-switching as a function of incident pump energy at three different repetition rates.
In the following experiments, for the purpose of stable laser oscillation, C2 was replaced by C1 and the laser cavity was constituted by M1 and M2. The cavity length was 95 mm. It is more compact compared with the cavity length of 170 mm in Ref [11]. M2 with different transmissions of 45%, 55%, and 70% was tested to find out the optimum output coupler transmission, and the output energy without Q-switching as a function of incident pump energy at pump repetition rate of 10 Hz is depicted in Fig. 3. At the incident pump energy of 600 mJ, the output energy was 226.1 mJ, 222.1 mJ and 230.5 mJ, respectively, for the M2 with different transmissions of 45%, 55%, and 70%. There is a fact that, on the one hand the maximum output energy without Q-switching was generated when M2 with transmission of 70% was used, on the other hand the high output coupler transmission is advantage to obtain high pulse energy in the passively Q-switched lasers [16]. Therefore, transmission of 70% is the best choice among these three different values of 45%, 55%, and 70% for lasers with and without passively Q-switched operations, and M2 with transmission of 70% was chosen to serve as output coupler in the later experiments.
Fig. 3 The output energy without Q-switching as a function of incident pump energy with different transmissions of M2.
The output energy without Q-switching as a function of incident pump energy at three different pump repetition rates of 10 Hz, 50 Hz and 100 Hz is shown in Fig. 4. With increasing of the incident pump energy, the output energy increased. But for the same incident pump energy the output energy was lowered when the repetition rate got higher. For example, at the maximum incident pump energy of 600 mJ, the output energy was 230.5 mJ, 220.2 mJ and 187.4 mJ, respectively, for the repetition rates of 10 Hz, 50 Hz and 100 Hz. The thermal effect became significant at higher repetition rate contributed to this phenomenon.
Fig. 4 The output energy without Q-switching as a function of incident pump energy at three different repetition rates.
The output energy and pulse width of passively Q-switched Nd:YAG laser as a function of initial transmission of Cr4+:YAG crystal at pumping rate was 10 Hz is depicted in Fig. 5. The incident pump energy was controlled so that there was only one output laser pulse during every pumping period. This means there is no pulse-burst. In our investigation range of 14.0% to 53.0% of initial transmission of Cr4+:YAG crystal, the modulation depth of passive Q-switch increased with decreasing of the initial transmission. So the output energy magnified and the pulse width reduced when the initial transmission became smaller. The pulse energy increased to 31.9 mJ as the initial transmission of Cr4+:YAG decreased to 14.0%, and in this condition the pulse width shortened to 4.9 ns. Although high pulse energy and short pulse width could be obtained using Cr4+:YAG with small initial transmission, it had risks of optical damage on coating films F1 and F2 due to the high intracavity power density. Therefore, in the following investigations, Cr4+:YAG with initial transmission of 43.4% was selected.
Fig. 5 The output energy and pulse width of passively Q-switched laser as a function of initial transmission of Cr4+:YAG.
Figure 6 shows the output energy as a function of incident pump energy when Cr4+:YAG with initial transmission of 43.4% was used. Because for the pulse-pumped passively Q-switched lasers, the output pulse energy is constant until the following pulse is generated if the pump intensity is high enough, and the output energy for each pulse is almost equal. In case of more than one pulse produced in one pumping period, the pulses are called pulse-burst. From Fig. 6 we can see that the pulse-burst contains a maximum of 8 pulses, 7 pulses and 6 pulses for repetition rate of 10 Hz, 50 Hz and 100 Hz, respectively. The energy obtained was 15.5 mJ, 14.9 mJ and 13.9 mJ per pulse for repetition rate of 10 Hz, 50 Hz and 100 Hz, respectively, and the corresponding total energy was 124 mJ (sum of 8 pulses), 104.3 mJ (sum of 7 pulses) and 83.4 mJ (sum of 6 pulses) at the incident pump energy of 600 mJ. There was some energy difference among the three repetition rate of 10 Hz, 50 Hz and 100 Hz, and we thought the difference in modes oscillation and losses caused by thermal effect contributed to it. The repetition rate of laser pulses in pulse-burst and oscilloscope trace of pulse-burst is displayed in Fig. 7 and Fig. 8, respectively. The repetition rate started from 8.79 kHz for 2 pulses and ended in 34.6 kHz for 8 pulses with a jitter of less than 10%. The oscilloscope trace for pulse-burst with pulse width of 13.3 ns depicted in Fig. 8 indicated the laser pulses had good stability.
Fig. 7 The repetition rate of laser pulses in pulse-burst as a function of pulse number for Cr4+:YAG passively Q-switched laser.
Fig. 8 The oscilloscope trace for pulse-burst and single pulse of Cr4+:YAG passively Q-switched laser.
3.2 Compensation of the thermal lensing effect
In Fig. 2, Fig. 4 and Fig. 6, we found that thermal effect of laser crystal had non-ignorable effect on the laser output performance, especially at high incident pump energy. So a CCD camera was used to image the He-Ne laser beam profile passed through the Nd:YAG rod under different incident pump energy and repetition rate. Because the thermal lensing effect can be treated as a focusing lens, the diameter variation of the recorded He-Ne laser beam profile caused by different incident pump energy and repetition rate can be used to calculate the thermal focal length (ft) with the help of geometrical optics. The distance between F2 and the camera was 910 mm. The imaged beam profiles are shown in Fig. 9. We can find, for the repetition rate of 50 Hz and 100 Hz, the beam diameter was diminishing when the incident pump energy was increasing. But for the 10 Hz, there were no obvious changes, which indicated the thermal effect for this condition can be ignored. Based on Fig. 9, the calculated results of ft are exhibited in Fig. 10. The thermal effect became more serious when the incident pump energy increased. At the maximum incident pump energy of 600 mJ and a repetition rate of 100 Hz, ft was 1200 mm.
Fig. 9 The beam profile of He-Ne laser passed through the Nd:YAG rod under different incident pump energy and repetition rates.
Fig. 10 Thermal focal length as a function of incident pump energy under two different repetition rates.
In order to eliminate the influence of thermal lensing effect, a plane-convex cavity was adopted to compensate it and improve the mode matching. Therefore, M1 was replaced by plane-convex mirror M0. The radius of curvature R of M0 should be optimized. If the plane-convex cavity compensates the thermal lensing effect completely, the g1*g2 factor of plane-convex cavity should be as same as that of the empty plane-plane cavity. Based on this judgment, we could get the theoretical expression of optimum Ropt which is given in Eqs. (1) and (2)
where L1 is the distance between the center of Nd:YAG rod and M0, L2 is that for the center and M2. Because the empty plane-plane cavity (g1*g2 = 1) cannot realize a stable laser oscillation, a correction factor is needed. With this factor the plane-convex cavity can realize a stable laser oscillation (g1*g2<1) and a large volume of fundamental mode (g1*g2 close to 1), and its experimental value is 1.05. After calculation using Eqs. (1) and (2), Ropt = −1250 mm was chosen to compensate the thermal lensing effect with thermal focal length of 1200 mm at an incident pump energy of 600 mJ and a repetition rate of 100 Hz. The experimental results are shown in Fig. 11. With increasing of the incident pump energy, the output energy increased. But different from Fig. 4, for the same incident pump energy the output energy was growing up when the repetition rate got higher, and the output energy grew faster with incident pump energy for the 100 Hz situation. At the maximum incident pump energy of 600 mJ, for the repetition rates of 10 Hz, 50 Hz and 100 Hz, the output energy was 87.3 mJ, 114.5 mJ and 149.0 mJ, respectively. This is due to Ropt of −1250 mm, which was selected to compensate the minimum thermal focal length of 1200 mm generated by an incident pump energy of 600 mJ and a repetition rate of 100 Hz. Laser output beam profiles for the two different oscillators of plane-plane cavity and plane-convex cavity at the maximum incident pump energy of 600 mJ are displayed in Fig. 12. For the plane-plane cavity, it was a flat-top Gaussian beam, and for the plane-convex cavity, it was a quasi-Gaussian beam. The measured M2 values were 4.1 and 2.2 for these two different lasers, respectively. This difference was resulted from the compensation of thermal lensing effect. For the laser-induced plasma ignition, the light intensities in the order of 100 GW/cm2 are required at the focal point of ignition. Therefore the pulse energy is important. Another very momentous feature for this application is the beam quality. Laser with good spatial distribution can reduce the energy requirement dramatically.
Fig. 11 The output energy without Q-switching as a function of incident pump energy at three different repetition rates using plane-convex cavity.
Fig. 12 Laser beam profile at the incident pump energy of 600 mJ: (a) plane-plane cavity; (b) plane-convex cavity.
The output energy as a function of incident pump energy and the repetition rate of laser pulses in pulse-burst as a function of pulse number for the plane-convex cavity are shown in Figs. 13(a) and 13(b), respectively. The output energy was 10.6 mJ per pulse for repetition rate of 100 Hz, and the corresponding total energy was 63.5 mJ (sum of 6 pulses) at the incident pump energy of 600 mJ. Compared with the output performance of the plane-plane cavity, there was some decrease in the output energy for the plane-convex cavity. We attributed this phenomenon to high order mode oscillation for the plane-plane cavity because of no compensation for thermal lensing effect. The repetition rate of laser pulses in pulse-burst was 27.4 kHz for 6 pulses with a jitter of no more than 10%, and the pulse width was 14.2 ns.
Fig. 13 Output performance of plane-convex cavity: (a) output energy as a function of incident pump energy; (b) repetition rate of laser pulses in pulse-burst as a function of pulse number.
4. Conclusions
In conclusion, a miniaturized Cr4+:YAG passively Q-switched Nd:YAG pulse-burst laser under 808 nm diode-laser pulse-pumping was demonstrated for the purpose of laser-induced plasma ignition, in which pulse-burst mode can realize both high repetition rate and high pulse energy simultaneously in a short period. Side-pumping configuration and two different types of laser cavities were employed. The pumping pulse width was constant at 250 μs. For the plane-plane cavity, the output beam profile was flat-top Gaussian, and the measured M2 value was 4.1 at the maximum incident pump energy of 600 mJ. The pulse-burst laser contained a maximum of 8 pulses, 7 pulses and 6 pulses for pulse-burst repetition rate of 10 Hz, 50 Hz and 100 Hz, respectively. The energy obtained was 15.5 mJ, 14.9 mJ and 13.9 mJ per pulse for pulse-burst repetition rate of 10 Hz, 50 Hz and 100 Hz, respectively. The maximum repetition rate of laser pulses in pulse-burst was 34.6 kHz for 8 pulses at the incident pump energy of 600 mJ and the single pulse width was 13.3 ns. The thermal lensing effect of Nd:YAG rod was investigated by applying He-Ne laser and CCD camera. An optimized plane-convex cavity using a plane-convex mirror with a radius of curvature of −1250 mm was adopted to compensate the thermal lensing effect of Nd:YAG rod and improve the mode matching. For the plane-convex cavity, the output beam profile was quasi-Gaussian, and the measured M2 value was 2.2 at the incident pump energy of 600 mJ. The output energy was 10.6 mJ per pulse for pulse-burst repetition rate of 100 Hz. The maximum repetition rate of laser pulses in pulse-burst was 27.4 kHz for 6 pulses at the incident pump energy of 600 mJ and the single pulse width was 14.2 ns. We can see that this passively Q-switched pulse-burst laser can produce high repetition rate (>20 kHz) and high pulse energy (>10 mJ) simultaneously in a short period for both two different cavities. It can be used not only for laser-induced plasma ignition but also for many other applications, such as laser ranging.
Acknowledgments
This work was supported by the National Key Scientific Instrument and Equipment Development Projects of China (No. 2012YQ04016401), the Fundamental Research Funds for Central Universities (Grant No. HIT. NSRIF. 2015044, 2014045 and 201165), General Financial Grant from the China Postdoctoral Science Foundation (Grant No. 2014M560262, 2013M531040), Special Financial Grant from the China Postdoctoral Science Foundation (No. 2014T70336), and Postdoctoral Fellowship in Heilongjiang Province (No. LBH-Z13081).
References and links
1. T. X. Phuoc, “Laser-induced spark ignition fundamental and applications,” Opt. Lasers Eng. 44(5), 351–397 (2006). [CrossRef]
2. M. Weinrotter, H. Kopecek, and E. Wintner, “Laser ignition of engines,” Laser Phys. Lett. 15, 947–953 (2005).
3. N. Pavel, M. Tsunekane, and T. Taira, “Composite, all-ceramics, high-peak power Nd:YAG/Cr4+:YAG monolithic micro-laser with multiple-beam output for engine ignition,” Opt. Express 19(10), 9378–9384 (2011). [CrossRef] [PubMed]
4. M. Lackner, S. Charareh, F. Winter, K. Iskra, D. Rüdisser, T. Neger, H. Kopecek, and E. Wintner, “Investigation of the early stages in laser-induced ignition by Schlieren photography and laser-induced fluorescence spectroscopy,” Opt. Express 12(19), 4546–4557 (2004). [CrossRef] [PubMed]
5. I. G. Dors and C. G. Parigger, “Computational fluid-dynamic model of laser-induced breakdown in air,” Appl. Opt. 42(30), 5978–5985 (2003). [CrossRef] [PubMed]
6. Y. F. Ma, X. Yu, F. K. Tittel, R. P. Yan, X. D. Li, C. Wang, and J. H. Yu, “Output properties of diode-pumped passively Q-switched 1.06 μm Nd:GdVO4 laser using a [100]-cut Cr4+:YAG crystal,” Appl. Phys. B 107(2), 339–342 (2012). [CrossRef]
7. H. Sakai, H. Kan, and T. Taira, “>1 MW peak power single-mode high-brightness passively Q-switched Nd 3+:YAG microchip laser,” Opt. Express 16(24), 19891–19899 (2008). [CrossRef] [PubMed]
8. Y.-F. Chen and Y. P. Lan, “Comparison between c-cut and a-cut Nd:YVO4 lasers passively Q-switched with a Cr4+:YAG saturable absorber,” Appl. Phys. B 74(4-5), 415–418 (2002). [CrossRef]
9. J. An, S. Z. Zhao, Y. F. Li, G. Q. Li, K. J. Yang, D. C. Li, J. Wang, M. Li, and Z. G. Yu, “LD-pumped passively Q-switched Nd:LuVO4 laser with a GaAs output coupler,” Laser Phys. Lett. 6(5), 351–354 (2009). [CrossRef]
10. H. Kofler, J. Tauer, G. Tartar, K. Iskra, J. Klausner, G. Herdin, and E. Wintner, “An innovative solid-state laser for engine ignition,” Laser Phys. Lett. 4(4), 322–327 (2007). [CrossRef]
11. G. Kroupa, G. Franz, and E. Winkelhofer, “Novel miniaturized high-energy Nd:YAG laser for spark ignition in internal combustion engines,” Opt. Eng. 48(1), 014202 (2009). [CrossRef]
12. A. Agnesi, P. Dallocchio, F. Pirzio, and G. Reali, “Sub-nanosecond single-frequency 10-kHz diode-pumped MOPA laser,” Appl. Phys. B 98(4), 737–741 (2010). [CrossRef]
13. P. P. Wu and R. B. Miles, “High-energy pulse-burst laser system for megahertz-rate flow visualization,” Opt. Lett. 25(22), 1639–1641 (2000). [CrossRef] [PubMed]
14. Y. F. Ma, X. Yu, X. D. Li, R. W. Fan, and J. H. Yu, “Comparison on performance of passively Q-switched laser properties of continuous-grown composite GdVO4/Nd:GdVO4 and YVO4/Nd:YVO4 crystals under direct pumping,” Appl. Opt. 50(21), 3854–3859 (2011). [CrossRef] [PubMed]
15. J. Liu, C. H. Wang, S. H. Liu, W. M. Tian, L. Li, S. S. Liu, and M. Liu, “Characterization of passively Q-switched mode-locking diode-pumped Nd:GdVO4 laser with Cr4+:YAG saturable absorber,” J. Mod. Opt. 55(12), 1971–1980 (2008). [CrossRef]
16. G. J. Spühler, R. Paschotta, R. Fluck, B. Braun, M. Moser, G. Zhang, E. Gini, and U. Keller, “Experimentally confirmed design guidelines for passively Q-switched microchip lasers using semiconductor saturable absorbers,” J. Opt. Soc. Am. B 16(3), 376–388 (1999). [CrossRef]
