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Abstract
In this paper, a methodology to produce a multi-beam sub-nanosecond laser is proposed. Laser pulses with a pulse energy of 0.14 mJ and a pulse width of 490 ps are generated in a YAG/Nd:YAG/Cr4+:YAG microchip laser at a repetition rate of 200 Hz. After amplification with a laser diode (LD) side-pumped Nd:YAG module, four laser beams are generated because of the thermally induced birefringence. With a double-pass LD side-pumped amplifier, the single pulse energy of the four laser beams is amplified to 5.23 mJ with a peak power of ∼10.67 MW, and air breakdown with four points is achieved with a 2 × 2 lens array.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In recent years, laser ignition has attracted a lot of interests for the advantages of shorter ignition delay time, no erosion effects and arbitrary positioning of the ignition plasma [1–6]. For large-bore engines, the flame has to travel a long distance and the loss in flame speed is severe, which seriously affects the engine stability [7–9]. Besides, it’s also difficult for the single-beam laser to ignite lean fluid mixtures inside the combustion chamber [10]. Multi-point laser ignition technique has been proposed with advantages of increasing the combustion pressure and shortening the comb combustion time effectively [11].
Several methodologies have been proposed to realize multi-point laser ignition. Using individual lasers is a direct approach to obtain multi-point ignition despite high cost and complexity [11]. Laser beam splitting techniques with beam splitting mirror, diffractive lens and spatial light modulator have also been developed and utilized in laser ignition [12–15]. In 2005, Weinrotter et al. used a beam splitter to realize two-point laser ignition, achieving a peak-pressure rise by 7% and a time reduction by ∼50% compared with the single point ignition [12]. Using a spatial light modulator, diffractive multi-beam patterns were generated by Kuang et al. in 2017 [15]. Successful two-point laser ignition was achieved at an air-fuel ratio up to 1.45, beyond the air-fuel ratio limit of a typical electrical spark ignition (λ∼1.2). However, it’s difficult to realize ignition with more points by laser beam splitting techniques, because the pulse energy of each single beam is limited by the initial beam.
Direct generation of multi-beam laser is an effective approach for the laser ignition system to further reduce cost and complexity [16–19]. In 2014, Ma et al. presented a four-beam pulse-burst ceramic Nd:YAG laser under a 2×2 micro-lens array pumping, yielding a maximum single pulse energy of ∼0.22 mJ [16]. By diffracting the pumping light with a Dammann grating, a 2×2 arrayed and passively Q-switched Nd:YVO4 laser was developed and a single pulse energy of 0.2 to 0.5 µJ was obtained at a repetition rate of 80 to 300 kHz [17]. The diffraction efficiency of first-order beams was 65.8%, which limited the overall efficiency of the laser system. With four fiber-coupled laser diodes (LD) pumping a Nd:YAG/Cr4+:YAG composite ceramic, Vasile et al. built a multi-beam laser, yielding a single pulse energy of 3.8 mJ and a pulse width of 0.9 ns at a repetition rate of 2 Hz [19]. Using the multi-beam laser as the ignition source, about 10% decrease in combustion time and a wider range of air-fuel ratio was achieved. When generating multiple beams from a single gain medium, the thermal effects of the gain medium can’t be ignored, with a tradeoff between the pulse energy and the repetition rate. Thus, there are few reports on high-repetition-rate high-pulse-energy multi-beam lasers. On the one hand, sub-nanosecond laser pulses can typically provide a high peak power, and the minimum pulse energy is about 2 mJ for a sub-nanosecond laser to realize laser ignition [20]. On the other hand, under a high repetition rate the plasma energy can be accumulated [21], and it is also necessary to realize reignition in a short time in the case of jet airplane turbines or rocket engines [22]. Thus, a high-repetition-rate high-pulse-energy sub-nanosecond multi-beam laser system is in demand for laser ignition.
The thermally induced birefringence phenomenon has been a problem seriously affecting the output parameters. In our previous work, we have analyzed the thermally induced birefringence phenomenon during the amplification process theoretically and experimentally [23]. In this paper, the thermally induced birefringence in a side-pumped Nd:YAG amplifier is employed to produce multi-beam sub-nanosecond pulsed laser. The master oscillator is a YAG/Nd:YAG/Cr4+:YAG microchip laser with a pulse width of ∼490 ps and a pulse energy of 0.14 mJ at 200 Hz. Four laser beams with a single pulse energy of 0.27 mJ are obtained due to the thermally induced birefringence of the side-pumped Nd:YAG module. The single pulse energy is further amplified to 5.23 mJ by using a double-pass side-pumped amplifier. By focusing the four laser beams with a 2 × 2 lens array, air breakdown with four points is achieved.
2. Experimental setup
The experimental setup of the multi-beam sub-nanosecond master oscillator power amplifier (MOPA) laser system is shown in Fig. 1. An 808 nm LD pumped passively Q-switched YAG/Nd:YAG/Cr4+:YAG microchip laser is utilized as the master oscillator. With a pair of aspherical lenses, the pump laser is re-imaged into the composite crystal with a beam waist diameter of ∼600 µm. The composite crystal is made up of three parts. An undoped YAG crystal with a length of 1.5 mm is used to reduce the thermal effects in the composite crystal. A Nd:YAG crystal with a length of 1.5 mm and a doped concentration of 1.1 at.% is used as the gain medium. A Cr4+:YAG crystal with a length of 1.5 mm and an initial transmission of 50% at 1064 nm is used as the Q switch. In order to obtain polarized output from the master oscillator, the Cr4+:YAG crystal is cut along the [110] direction [24–26]. A monolithic resonator is achieved by coating the surface of YAG with a high reflectivity (HR) at 1064 nm and the surface of Cr4+YAG with a transmission of 60%. The surface of YAG is also coated for high transmission (HT) at 808 nm.
Fig. 1. Experimental setup of the multi-beam sub-nanosecond MOPA laser system. (a) Fluorescence intensity distribution of LD side-pumped module I, (b) fluorescence intensity distribution of LD side-pumped module II, (c) laser beam profile of the o-light, (d) laser beam profile of the e-light.
By using a plane mirror M1 (45° HR coated at 1064 nm) the laser pulses are guided to the LD side-pumped Nd:YAG module I. The Nd:YAG rod has a dimension of Φ3 mm × 100 mm and a doping concentration of 0.8 at.%. The thermally induced birefringence of the Nd:YAG rod under high power pumping changes the polarization state of the input laser. A polarization beam splitter (PBS) is used to separate the ordinary light (o-light) and the extraordinary light (e-light). Considering the amplification efficiency and the pulse energy of the e-light, the beam diameter of the seed laser is set to be 3 mm while entering the LD side-pumped module I, and the LD side-pumped module I is placed 200 mm behind the composite crystal. Two Faraday isolators are used to prevent potential feedback to the master oscillator and the LD side-pumped module I. Two half-wave plates are utilized to compensate for the polarization state rotation induced by the Faraday isolator. The e-light is double-pass amplified by another LD side-pumped Nd:YAG module, by being reflected by the plane mirror M2 (HR coated at 1064 nm). After passing through the quarter-wave plate twice, the polarization direction of the e-light is rotated by 90°, and the e-light is outputted from the PBS II. The Nd:YAG rod has a dimension of Φ5 mm × 80 mm and a doping concentration of 0.8 at.%. In order to reduce the energy loss caused by the thermally induced birefringence and prevent the optical damage under high power density, the distance between the two LD side-pumped Nd:YAG modules is set to be 250 mm and the beam diameter of the e-light is about 3.5 mm while entering the LD side-pumped module II. The two LD side-pumped Nd:YAG modules are specially designed to provide homogeneous fluorescence intensity distributions in the gain mediums [27]. So, it can be assumed that the pump distributions are also homogeneous in the gain mediums. A water-cooling device is utilized to keep the temperature of the LD, the composite crystal and the two LD side-pumped modules at 25°C.
The system timing is controlled with a digital pulse generator (DG535, Stanford Research Systems Inc.). The pump repetition rate of the MOPA system is 200 Hz. A fast photodiode (ET-3500, Electro-Optics Technology Inc. rising time: 25 ps) and a digital oscilloscope (DPO7104, Tektronix Inc. bandwidth: 1 GHz) are used to measure the pulse characters. A laser beam analyzer (LBA-712PC-D, Spiricon Inc.) is used to measure the beam profiles.
3. Generation of the multi-beam laser
With a pump duration of 230 µs, the pulse energy, pulse width and beam quality factor of the microchip laser are 0.14 mJ, 490 ps and M2=1.20, respectively. The laser beam profile has a good symmetry and agrees with the Gaussian distribution, as shown in Fig. 2. The good symmetry of the seed laser is beneficial for the symmetry of the multi-beam laser.
Fig. 2. The laser beam profiles of the microchip laser. (a) In two dimensions, (b) in three dimensions.
After passing the LD side-pumped module I, the pulse energy gets amplified. Due to the polarization state changes caused by the thermally induced birefringence, the o-light and the e-light are separated out by the PBS I. To provide high amplification factor and enough thermal phase difference between the o-light and the e-light, the pump durations of the two LD side-pumped modules are both set at 500 µs. The pulse energy of the o-light and the e-light versus the pump energy of the LD side-pumped module I is shown in Fig. 3. Under the pump energy of 1100 mJ, the pulse energy of the o-light and the e-light reach 5.80 mJ and 1.09 mJ, respectively. The amplification factor reaches ∼49.2. Because the thermally induced birefringence mainly affects the boundary of the Nd:YAG rod and the beam profile of the seed laser is in the Gaussian distribution, the pulse energy of the e-light is lower compared to that of the o-light. The diameters of the seed laser and the gain medium are both about 3 mm in the LD side-pumped module I. If it’s in double-pass configuration, the laser beam would be more divergent because of the serious thermal lens effect and would be limited by the apertures of the gain medium and the Faraday isolator. Therefore, a single-pass configuration is adopted here.
The laser beam profiles of the o-light and the e-light under the maximum pump energy of the LD side-pumped module I are measured, as shown in Fig. 4. The ordinary beam profile presented a diamond distribution. The four extraordinary beams are almost the same with each other, which means the pulse energy of each single beam is similar to each other. The beam profile of the e-light is stable in this experiment, which can be explained by the results given by Gleason et al. [28]. The thermal load in the gain medium is affected by the pump repetition rate. For the Nd:YAG crystal, when the pump repetition rate is above ∼5 Hz, the thermal effects can be regarded as independent on time [28]. Because the four extraordinary beams are generated from the same beam, there is no jitter time between each extraordinary beam. The beam quality factors of the ordinary beam are measured to be Mx2 = 1.40 and My2 = 1.42 in the horizontal and vertical directions. Besides, the beam quality factors of the single extraordinary beam in the first quadrant are measured to be Mx2 = 1.66 and My2 = 1.73 in the horizontal and vertical directions.
Fig. 4. Laser beam profiles under the maximum pump energy. (a1) Laser beam profile of the o-light in two dimensions, (a2) laser beam profile of the o-light in three dimensions, (b1) laser beam profile of the e-light in two dimensions, (b2) laser beam profile of the e-light in three dimensions.
4. Amplification of the multi-beam laser
Although it can provide multiple beams by amplifying and splitting the ordinary beam, it can also cause optical components damage easily for the relatively concentrated energy distribution of the ordinary beam. So here we choose to amplify the extraordinary beams with relatively dispersed energy distribution to obtain multiple beams with high pulse energy. Because the size of the gain medium in LD side-pumped module II is bigger, the thermal lens effect is not comparable as that in LD side-pumped module I. Besides, the beam size in the LD side-pumped module II can be controlled by adjusting the position of LD side-pumped module II. The diameters of the seed laser and the gain medium are ∼3.5 mm and 5 mm, respectively. Therefore, the energy loss caused by the thermally induced birefringence and the limitation caused by the aperture of gain medium can be reduced. To get a high amplification factor, the amplification of the four extraordinary beams is carried out with a double-pass side-pumped amplification system, as shown in Fig. 1. The pulse energy of the four extraordinary beams versus the pump energy of the LD side-pumped module II is shown in Fig. 5. Under the maximum pump energy, the pulse energy of the four extraordinary beams is amplified to 20.90 mJ. The corresponding single beam energy reaches 5.23 mJ, which is similar to the maximum pulse energy of the o-light. The low optical-to-optical efficiency is caused by the poor mode matching between extraordinary beam and pumped area, the small signal gain regime and the long pump duration.
Fig. 5. Pulse energy of the four extraordinary beams versus the pump energy of the LD side-pumped module II.
Figure 6 shows single pulse profiles and pulse trains of the seed laser, the o-light and the e-light under the maximum output energy. Due to the low extraction efficiency and high amplification factor of the amplifiers, the amplification process in this experiment can be classified as the small-signal amplification. Thus, the pulse widths of the o-light and the e-light keep constant at 490 ps after the amplification and the pulse profiles show a good symmetry as shown in Fig. 6(a). The maximum peak power of the ordinary beam and a single extraordinary beam is calculated to be ∼11.43 MW and ∼10.67 MW. To show the good amplitude stability of the seed laser, the o-light and the e-light under the maximum output energy, 100 pulses are captured in the pulse trains, as shown in Fig. 6(b), Fig. 6(c) and Fig. 6(d). The corresponding coefficient of variation (CV, the ratio of the standard deviation to the mean) is calculated to be ∼3.3%, ∼3.1% and ∼2.8%, respectively. The good amplitude stability is beneficial for enhancing the operating stability in practical applications.
Fig. 6. Single pulse profiles and pulse trains of the multi-beam MOPA laser system. (a) Single pulse profiles of the seed laser, the o-light and the e-light, (b) temporal pulse train of the seed laser, (c) temporal pulse train of the o-light, (d) temporal pulse train of the e-light.
The laser beam profiles of the four extraordinary beams with maximum output energy are measured, as shown in Fig. 7. The four extraordinary beams maintain the uniformity of the energy distribution among each single beam after amplification. The peak intensity of each single beam is similar to each other. Besides, the beam quality factors of the single extraordinary beam in the first quadrant are measured to be Mx2 = 1.98 and My2 = 2.02 in the horizontal and vertical directions.
Fig. 7. Laser beam profiles of the four extraordinary beams with maximum output energy.
(a) In two dimensions, (b) in three dimensions.
When using a 2 × 2 lens array with a focal length of 9 mm, air breakdown with four points is induced by the extraordinary beams under the maximum output energy, as shown in Fig. 8. The breakdown threshold of each bream is about 4.0 mJ. Because the four extraordinary beams are close to each other, when the beam centers and the lens centers coincid with each other, the beam size is beyond the lens aperture. Therefore, the breakdown threshold is relatively high. The separation of the four beams could be realized by inserting two right-angle prisms after each beam [19], which would decrease the breakdown threshold effectively.
5. Conclusion and discussion
In this paper, a multi-beam sub-nanosecond laser system is presented by using the thermally induced birefringence of the side-pumped Nd:YAG module. By amplifying laser pulses generated from a YAG/Nd:YAG/Cr4+:YAG sub-nanosecond microchip laser, the polarization state gets changed because of the thermally induced birefringence and four extraordinary beams are separated out. The four extraordinary beams are further amplified with a double-pass side-pumped amplification system, yielding a single beam energy of 5.23 mJ and a pulse width of 490 ps at a repetition rate of 200 Hz. By focusing the four extraordinary beams with a 2 × 2 lens array, air breakdown with four points is induced.
The pulse energy can be further increased by improving parameters oscillation stage to provide seed laser with higher pulse energy. The pulse energy of the ordinary beam and each extraordinary beam has exceeded the minimum pulse energy (2 mJ) required for a sub-nanosecond laser to realize laser ignition [20], which means that each of them can operate as a single beam in the multi-point ignition system. This multi-beam laser system can also operate at a lower repetition rate which is required for laser ignition in reciprocating engines. The pump duration of the LD side-pumped module I needs modification to match the thermal phase difference between the ordinary and the extraordinary light [29]. Besides, the pump energy of the LD side-pumped module II also needs adjustment to obtain stable output energy at various repetition rates. Further efforts need to be made to improve the compactness and reliability of the MOPA system.
Funding
Natural Science Foundation of Tianjin City (18JCZDJC37900, 19JCZDJC38400); National Natural Science Foundation of China (61705165, 61775167, 61975150).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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