A novel four-beam (named laserI, laser II, laser III and laser IV, respectively), passively Q-switched, pulse-burst ceramic Nd:YAG laser under 2 × 2 micro-lens array pumping was demonstrated for the purpose of laser-induced plasma ignition (LIPI). Multiple-beam output together with pulse-burst mode in which both high repetition rate and high pulse energy can be realized simultaneously were obtained to greatly improve the performance of LIPI. The pulse-burst contained a maximum of 5 pulses, 3 pulses, 2 pulses and 3 pulses for laserI, laser II, laser III and laser IV, respectively, and the corresponding repetition rate of laser pulses in pulse-burst was 10.8 kHz, 7.2 kHz, 6.8 kHz and 5.2 kHz, respectively. The output energy for single laser pulse in pulse-burst was in the range of 0.12 mJ to 0.22 mJ.
© 2015 Optical Society of America
Laser-induced plasma ignition (LIPI) as one of the promising alternatives for an ignition source for different engines has attracted a wide range of interests recently [1–3 ]. LIPI technique is an innovative method, compared to traditional spark plug ignition, which possesses the merits of shorter ignition delay time, arbitrary positioning of the ignition plasma, no erosion effects, and increase of engine efficiency [4–7 ].
For the LIPI method, one of the major difficulties for actual applications, especially for some small engines such as automobiles is the size of the laser head due to the fact that the laser head should be inserted into the engines. It is well-known that passively Q-switched lasers have advantageous of simple fabrication, compact structure, and low cost [8–10 ]. Therefore, this kind of laser is the most promising candidate for the LIPI system. 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 [6,11 ], and high repetition rate laser with kHz produces single pulse energy only in nJ to tens of μJ scale [12,13 ]. In LIPI, 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 LIPI. However, the two laser characters of high repetition rate and high pulse energy restrict each other because of the tolerable thermal loading of laser media. Instead of continuous pulse mode, we have verified that pulse-burst mode can realize both high repetition rate and high pulse energy simultaneously in a short period . Therefore, this kind of laser is especially attractive to LIPI.
Compared to single-point LIPI, it is found that multiple-point LIPI significantly increases combustion pressure, accelerates the combustion process and shortens combustion time . But if multiple light beams originate from individual lasers, the multiple-point LIPI would be challenged in overcoming the large size of the laser system and the high cost . In 2014, a 2 × 2 arrayed and passively Q-switched laser under Dammann-arrayed pumping was reported by Wang . But it cannot be used for LIPI because the output energy was in nJ scale due to the low damage threshold of Dammann grating. In 2011, a three beam output laser with low repetition rate of 5 Hz was demonstrated by Taira . It was pumped individually by three fiber-coupled diode lasers and therefore suffered high cost.
In this paper, we first demonstrated a novel four-beam, pulse-burst, passively Q-switched laser for LIPI under 2 × 2 micro-lens array pumping. Compared with Nd:YAG single crystal, ceramic Nd:YAG has merits of lower cost, larger size and easier fabrication . Therefore, it was used as the gain medium. One fiber-coupled pulse diode laser was served as the pumping source. This laser had advantageous of compact, low cost and highly favorable for LIPI.
2. Experimental setup
The schematic of four-beam, passively Q-switched ceramic Nd:YAG laser is shown in Fig. 1 . A fiber coupled (core diameter of 400 μm and numerical aperture of 0.22.) 808 nm pulse diode laser was used as the pumping source which had a maximum repetition rate of 100 Hz and variable pulse width. L1 was a collimating lens with focal length of 21.3 mm. A 2 × 2 micro-lens array (MLA) was adopted to divide the pumping light into four beams. The micro-lenses in MLA were glued together. MLA was square and both length and width were 10 mm. The focal length for each of micro-lens in MLA was 30 mm. M1 was a plane mirror and high-reflection coated at 1064 nm. M2 was output coupler with different transmissions (T) of 10%, 15%, 20%, 25% and 30% at 1064 nm, respectively. The Nd:YAG ceramic was provided by Shanghai Institute of Ceramics, Chinese Academy of Sciences in China, and the Nd3+ doped concentration was 1.0 at%. The total dimensions were 10 × 10 × 6 mm3. The Nd:YAG ceramic was wrapped with indium foil and placed into water-cooled copper heat sink with microchannel structure. For the passively Q-switched operation, Cr4+:YAG saturable absorber with initial transmission of 80% at 1064 nm was used. The thickness of the Cr4+:YAG crystal was 1 mm and the diameter was 10 mm. The cavity length for this laser was about 50 mm. In order to identify output lasers, the four beams were named laserI, laser II, laser III and laser IV in clockwise direction, respectively.
3. Experimental results and discussions
3.1 Multiple-beam generation without Q-switching
The output beam of diode laser was split by MLA. The individual absorbed pump energy was measured firstly when the total absorbed pump energy increased and it is shown in Fig. 2 . The repetition rate f P and pumping width W P for the diode laser were set to be 10 Hz and 500 μs, respectively. It can be seen that the individual absorbed pump energy for the four lasers was not equal to each other [see Fig. 2 (a)] and the energy proportions for laserI, laser II, laser III and laser IV were about 31.1%,24.9%, 19.4% and 24.6% [see Fig. 2 (b)], respectively. The difference in absorbed pump energy proportion was mainly from the spatial unconformity of energy distribution of pumping laser and the misalignment in the fabrication of micro-lens array due to the glue process.
M2 with different transmissions of 10%, 15%, 20%, 25% and 30% was tested to find out the optimum output coupler transmission and the output energy as a function of absorbed pump energy at pump repetition rate f P of 10 Hz is depicted in Fig. 3(a) . At the absorbed pump energy of 13.63 mJ, the total output energy of the four-beam laser was 4.54 mJ, 4.74 mJ, 4.29 mJ, 4.01 mJ and 3.61 mJ, respectively, for the M2 with different transmissions of 10%, 15%, 20%, 25% and 30%. It was found that output coupler with transmission of 15% was optimal. The individual output energy for laserI, laser II, laser III and laser IV was measured at transmission of 15% and it was 1.49 mJ, 1.11 mJ, 0.82 mJ and 1.32 mJ, respectively, when the total absorbed pump energy was 13.63 mJ. The obtained results are shown in Fig. 3(b). The difference in output energy resulted from the followings. Firstly, the individual absorbed pump energy proportion was different. Secondly, mode matching degree between pumping laser and oscillating laser may not be the same for the four lasers, which was possibly coming from misalignment in the fabrication of micro-lens array due to the glue process.
Four-beam laser output beam profiles at the absorbed pump energy of 13.63 mJ were captured by a CCD camera and displayed in Fig. 4 . It can be seen that the beam profiles of laserI, laser II, laser III and laser IV were Gaussian distribution, hence excellent beam quality was obtained.
3.2 Multiple-beam, pulse-burst, passively Q-switched laser generation
Cr4+:YAG crystal with initial transmission of 80% was inserted into laser cavity to perform passive Q-switching operation. For the pulse-pumped passively Q-switched lasers, more than one laser pulses will be generated if the pump intensity is high enough. In case of multiple pulses produced in one pumping period, the pulses are called pulse-burst. In order to increase the cavity gain to produce pulse-burst laser, the pumping width W P of diode laser was changed to 500 μs.
The output energy of pulse-burst laser, pulse number in pulse-burst laser and single pulse energy at f P of 10 Hz as a function of total absorbed pump energy for four-beam, passively Q-swithced ceramic Nd:YAG laser is shown in Figs. 5(a), 5(b) and 5(c) , respectively. It is observed that the output energy of pulse-burst for passively Q-switched laserI, laser II, laser III and laser IV was 0.70 mJ, 0.45 mJ, 0.31 mJ and 0.55 mJ, respectively, at a total absorbed pump energy of 13.63 mJ. The energy fluctuation was less than 5%. The difference in output energy mainly resulted from the difference in absorbed pump energy and mode matching. From Fig. 5(b) we can see that the pulse-burst contains a maximum of 5 pulses, 3 pulses, 2 pulses and 3 pulses for laserI, laser II, laser III and laser IV, respectively. As shown in Fig. 5(c), the output energy for single laser pulse in pulse-burst was in the range of 0.12 mJ to 0.22 mJ. The repetition rate of laser pulses in pulse-burst is displayed in Fig. 6(a) . The repetition rate started from 4.5 kHz for 2 pulses and ended in 10.8 kHz for 5 pulses for laser I. For laser II, laser III and laser IV, it was in the range of 4.1 kHz to 6.8 kHz, 4.1 kHz to 5.2 kHz, 4.1 kHz to 7.2 kHz, respectively.
The pulse width as a function of total absorbed pump energy is shown in Fig. 6(b). For the passively Q-switched laser, the output pulse width is only determined by modulation depth of saturable absorber and cavity round trip time . Therefore, it was insensitive with increasing of the pump energy and can be found in Fig. 6(b). For laserI, laser III and laser IV, the pulse width was about 10.5 ns, and for laser II, it was about 11.5 ns. The slight increase of pulse width for laser II maybe due to inhomogeneous doping of Cr4+:YAG crystal. The oscilloscope trace for pulse-burst with pulse width of 10.5 ns displayed in Fig. 7 indicated the laser pulses had good stability.
In conclusion, a novel four-beam output ceramic Nd:YAG laser under 2 × 2 micro-lens array pumping together with pulse-burst mode in which both high repetition rate and high pulse energy can be realized simultaneously were demonstrated for LIPI. End-pumping configuration using 10 Hz pulse diode laser was adopted in the experiments. The Q-switched output energy for four-beam pulse-burst lasers of laserI, laser II, laser III and laser IV was 0.70 mJ, 0.45 mJ, 0.31 mJ and 0.55 mJ, respectively, at a total absorbed pump energy of 13.63 mJ. The difference in absorbed pump energy proportion and mode matching degree between pumping laser and oscillating laser contributed to the different output energy. The pulse-burst contained a maximum of 5 pulses, 3 pulses, 2 pulses and 3 pulses for laserI, laser II, laser III and laser IV, respectively, and the corresponding repetition rate of laser pulses in pulse-burst was 10.8 kHz, 7.2 kHz, 6.8 kHz and 5.2 kHz, respectively. The output energy for single laser pulse in pulse-burst was in the range of 0.12 mJ to 0.22 mJ. The pulse width was insensitive with the pump energy. For laserI, laser III and laser IV, the pulse width was about 10.5 ns, and for laser II, it was about 11.5 ns.
This work was supported by the National Key Scientific Instrument and Equipment Development Projects of China (Grant No. 2012YQ04016401), the Major Program of National Natural Science Foundation of China (Grant No. 50990301), the National Natural Science Foundation of China (Grant No. 61505041 and 61505042), the General Financial Grant from the China Postdoctoral Science Foundation (Grant No. 2014M560262 and 2013M531040), the Special Financial Grant from the China Postdoctoral Science Foundation (Grant No. 2015T80350 and No. 2014T70336), the Postdoctoral Fund of Heilongjiang Province (Grant No. LBH-Z14074 and LBH-Z13081), the Natural Science Foundation of Heilongjiang Province of China (Grant No. F2015011), and the Fundamental Research Funds for the Central Universities (Grant No. HIT. NSRIF. 2015044 and HIT. NSRIF. 2014045), the Special Financial Grant from the Heilongjiang Province Postdoctoral Foundation (Grant No. LBH-TZ0507).
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