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Abstract
We report a compact cavity-dumped burst-mode Nd:YAG laser master-oscillator power-amplifier system with a flat-top intensity distribution across the output-beam section. Custom-designed gain profile-controlled diode side pumping modules providing flat-top and concave gain profiles were utilized to generate a uniform beam profile and suppress thermal lensing during amplification, respectively. Bursts with an energy of 2.0 J and duration of 1.6 ms were operated at 10 Hz. Within the bursts, single pulses with an energy of 12.7 mJ and pulse width of 3.3 ns were achieved at 100 kHz.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
High-power and high-repetition-rate nanosecond pulsed lasers are widely used coherent sources in various fields including material science and processing, laser machining, laser-induced fluorescence and plasma, and light detection and ranging (lidar) [1–10]. Although laser beams with Gaussian intensity profiles are standard, flat-top beams are required in various applications [11]. For laser heating in high-pressure high-temperature material processing, flat-top beams can overcome various complications associated with laser heating and be applied to challenging applications such as high-pressure melting of germanium [12]. In laser skin treatments, flat-top beams cause less bleeding, tissue splatter, and pain than Gaussian beams [13]. In laser-induced fluorescence imaging of gaseous flows, flat-top beams are preferred to maximize the efficiency and increase the fluctuation detection limits [14]. In ultraviolet (UV) laser applications, such as UV lidars, nanosecond Nd:YAG laser beams with flat-top intensity profiles are required for efficient pumping of optical parametric oscillators to generate UV lasers [15]. In the field of microscopy, flat-top beams can help uniformly illuminate the back aperture of the objective to reach the ideal numeric aperture of the objective, thereby improving the imaging resolution [16–18]. Additional applications, including sensing, spectroscopy, and nonlinear optics, also require stable flat-top laser beams [19,20].
Optical beam shapers have been commonly used in previous studies to convert Gaussian beams into flat-top beams [11,12,21–24]. One simple way of creating near-flat-top beams is to use spatial filters that transmit only the paraxial (central) beam [25]. However, this method results in a power loss and low efficiency. Under high power, the spatial filter can also be damaged and needs to be frequently replaced [26,27]. Refractive beam shapers, including the πShaper, are ideally the best type of converter for converting Gaussian beams to flat-top beams with high efficiency [28,29]. They are widely used in applications such as laser heating and material processing [12]. However, these beam shapers require an ideal Gaussian-distributed input beam, and they cannot effectively convert near-Gaussian or irregularly distributed beams into the required flat-top beam. Diffractive beam shapers have a higher tolerance to beam profiles and input power; however, they cause laser speckles. Moreover, although they are effective in converting both ideal Gaussian and near-Gaussian beams, they still cannot work well with complex irregular beams, which are common in high-power side-pumped master-oscillator power-amplifier (MOPA) lasers that suffer from uneven gain profiles and severe thermal-lensing effects [3].
The laser MOPA scheme powered by side-pumped solid-state gain modules is a widely used and effective solution for nanosecond pulses with high power and repetition rate, which are required for various applications [2,30–33]. While required or preferred by many of these applications, flat-top beams are difficult to achieve using beam shapers from such lasers because of the above-mentioned non-Gaussian profile of its initial beam. The side-pumping scheme supplies significantly higher power; however, unlike the end-pumping scheme, which typically provides a Gaussian distributed gain profile close to the TEM00 mode [34,35], it distributes power irregularly across the gain medium because the pumping light originates from multiple directions perpendicular to the oscillation direction. This irregular distribution of pumping power causes complex gain profiles across the section of the gain medium, which eventually results in irregular non-Gaussian output beams [3,34]. Slipchenko et al. imaged beam intensity profiles in a burst in their 2013 publication [3], and the typical Gaussian distributed profile deformed to a pentagonal shape and self-focus as thermal effects took into effect. The pentagonal shape deformation is considered to originate from the illumination pattern caused by the side-pumping diodes [3]. In 2020, we imaged the fluorescence profiles of some commercially available side-pumping modules, and pentagram-shaped distribution profiles were clearly observed under five-directional side pumping [36]. As a result, irregular thermal lensing and beam profiles can be caused by complex gain profiles, which render them unconvertible by regular flat-top beam shapers. Therefore, it becomes a key point to control and pre-design the gain profile in realizing a flat-top beam output from side-pumped high-power laser MOPA systems. Compared to beam shapers, directly generating flat-top or near flat-top beams by altering the gain distribution profile also provides advantages in terms of efficiency, setup complexity, and high-power tolerance.
Apart from obtaining flat-top beams with high efficiency and low system complexity, gain-profile control can, especially when operating with high power and high repetition rate, reduce the undesired thermal lensing effect while providing flat-top beams. While high-power output can be realized by the MOPA scheme, the thermal lensing effect leads to the deterioration of the laser beam profile as the beam passes through multiple amplifier stages [3,4,33,37–39]. Especially in combination with a high repetition rate and short pulse width, significantly short thermal focal lengths can cause a self-focal point within the optical path of the oscillator or amplifier and damage the optical components [3,39–42]. The common Gaussian-distributed gain profile and beam profile exacerbate the thermal lensing effect as the gain medium center absorbs more pumping power, and the beam center is amplified more rapidly than the surroundings. As a result, a part of the gain medium paraxial to the beam center generates and accumulates a significant amount of heat. The significant temperature gradient from the central axis to the surroundings leads to a refractive index gradient and mechanical deformation, which are the major causes of the thermal lensing effect [43–46]. A common approach to compensate for the thermal lensing effect is to place concave lenses or specially designed lens sets in the optical path to recollimate the beam [38,39,47,48]. However, for this approach to be effective, the thermal focusing needs to be equivalent to the focusing from an ideal convex lens with circular symmetry and a single focal length. This condition is relatively well satisfied under the end-pumping scheme, as long as the pumping beam has a regular Gaussian-distributed intensity across its section. However, as described above, without well-controlled gain distributions, side-pumped MOPA lasers can emit complex irregular beams, rendering them unsuitable for regular concave lenses [36,39]. To reduce the complex thermal lensing effect, the pumping power distribution across the gain medium must be well-managed. While providing flat-top beams, side-pumping gain modules with well-designed and featured gain distribution profiles can help resolve the thermal focusing and beam deterioration.
In addition to uniform gain profiles, concave gain profiles can help further reduce thermal focusing and maintain flat-top beams during the amplification processes. In 2017, Zhao et al. demonstrated a side-pumping module that provides a concave gain profile in a very thick Nd:YAG rod with a diameter of 15 mm [49]. Concave gain profiles are more naturally formed in gain media with larger diameters because the majority of the pumping energy is absorbed by the margins of the rod. However, gain media with a smaller diameter, such as 5 mm, are more commonly required in various applications. This becomes more difficult for gain media with smaller diameters because the unlimited increase in the doping concentration of the gain medium is prohibited by concentration quenching.
In this paper, we demonstrate a laser MOPA system with high energy, high repetition rate, and flat-top beam profile. Custom side-pumping laser modules were designed using our simulation program to significantly reduce the irregularity of gain distribution through the gain medium. Side-pumping modules providing flattened-Gaussian (super-Gaussian) and concave gain profiles were designed to spatially control the gain and thermal effect, thereby shaping the laser beam profile. The flat-top distributed gain and beam intensity profiles reduced the temperature gradient and suppressed the thermal lensing effect. The cavity dumping technique was employed to maintain a short pulse width in operations with a high repetition rate. Limited by the average power and heat dissipation, a trade-off between the pulse energy and repetition rate limits applications such as laser-induced fluorescence imaging, which requires both high energy and high repetition rate in a limited period of time [2,3,50–52]. In our laser, the burst-mode operation scheme was adopted to alternatively realize a high pulse energy at a high repetition rate within the burst duration. Cavity-dumped nanosecond pulses with a single pulse energy of 12.7 mJ and a repetition rate of 100 kHz are achieved within the burst duration of 1.6 ms. The overall average power reaches 20.4 W with a burst energy of 2.0 J.
2. Experimental setup
Figure 1 shows the optical layout of the diode side-pumped cavity-dumped burst-mode laser MOPA system, which contains a master oscillator and two dual-pass amplifier stages. A custom-designed diode side-pumping laser gain module (SPM1 in Fig. 1) serves the master oscillator and provides a flattened-Gaussian (super-Gaussian) distributed gain profile across the section of the gain medium. The master oscillator was electro-optical Q-switched by a Pockels cell (EO-PC-1064, Thorlabs) made of potassium dideuterium phosphate (KD*P) crystals and operated in the cavity dumping mode. M1 and M2 are highly reflective mirrors (1.06 µm with an incident angle of 0°). The absorbed pumping power is accumulated in the Nd:YAG gain medium when no voltage is applied to the Pockels cell because the oscillation is blocked by the polarizing beam splitter (PBS) cube (PBS1), where the light is reflected out after passing through the zero-order quarter waveplate (QWP1) twice. When a quarter-wave (λ/4) voltage of 3350 V was applied to the Pockels cell, high-power lasing began as the Q factor increased, but was restricted within the cavity by M1 and M2. The pulse duration of the λ/4 voltage was set to 25 ns, and after the removal of the voltage, the laser pulse was output with a theoretical pulse width of approximately one round-trip time of light within the cavity. The rising and falling times of the λ/4 voltage were ∼10 ns and the repetition rate was set to 100 kHz. The geometric cavity length was 182 mm.
Fig. 1. Layout of the diode side-pumped cavity-dumped burst-mode laser MOPA system. SPM: side pumping module; M: mirror; PBS: polarizing beam splitter; QWP: quarter waveplate; HWP: half waveplate; PKC: Pockels cell; FR: Faraday rotator.
A Faraday rotator (M714, Conoptics) and two PBS cubes (PBS2 and PBS3; PBS3 is rotated at 45° along the beam axis with respect to PBS2) constitute an optical isolator that prevents reflection of light from entering the master oscillator and suppresses self-oscillation between the master oscillator and amplifier stages. A zero-order half-wave plate (HWP) is placed at the output of the isolator to rotate the polarization orientation back to s-polarization. M3, M4, and M5 are highly reflective mirrors (1.06 µm with an incident angle of 45°). M6 and M7 are highly reflective mirrors (1.06 µm with an incident angle of 0°). The s-polarized beam is reflected by PBS4 into amplifier stage one and passes through the side-pumping gain module (SPM2) and zero-order quarter waveplate (QWP2) twice. SPM2 provides the same flattened Gaussian (super-Gaussian) distributed gain profile as SPM1 does. When returning to PBS4, the polarization orientation is changed to p-polarized by QWP2; therefore, the beam passes through PBS4 and enters amplifier stage two, which has a symmetrical optical layout compared with amplifier stage one. After passing through SPM3 and QWP3 twice, the beam is amplified, turned back to s-polarized, and then reflected by PBS4. The SPM3 provides a concave gain profile that suppresses the thermal lensing effect.
The Nd:YAG gain media utilized in both the master oscillator and amplifiers had a Nd3+ doping concentration of 0.8 at.% and a diameter of Φ5 mm. All PBS cubes (PBS25-1064-HP, Thorlabs) had a damage threshold of over 10 J/cm2 under the condition of 10 ns pulses at 10 Hz. Under the burst-mode scheme, all side-pumping gain modules operate at 10 Hz with a burst duration of 1.8 ms. Single nanosecond pulses are operated within each burst with a repetition rate of 100 kHz. The diode pumping controllers for SPM1, SPM2, and SPM3 were synchronized to start and stop pumping simultaneously. The Q-switch trigger is not synchronized with the pumping diode controllers because the Q-switched pulse cycle is significantly shorter than the burst duration, making synchronization unnecessary. Because our burst duration of 1.8 ms is not considered significantly long with respect to the temperature change, the transient wavelength shift is not compensated. The static wavelength shift during the stable operation of the laser was considered, and the chiller temperature (∼29 °C) was optimized to maximize the average output power through an experimental trial.
3. Gain profile-controlled side-pumping modules
A custom-designed finite element analysis (FEA) program [36] was used to simulate the distribution of the absorbed power across the section of the gain medium. Figure 2 shows the simulated absorbed energy distribution and experimentally measured fluorescence distribution across the section of the gain medium. The side-pumping modules used in the master oscillator (SPM1) and amplifier stage one (SPM2) were designed to provide a flattened Gaussian (super-Gaussian) distributed gain across the section of the gain medium. SPM1 and SPM 2 deliver the pumping light from five equiangular directions around the gain medium and have a peak optical pumping power of 2000 W. The side-pumping module used in amplifier stage two (SPM3) is designed to deliver a concave (ring-shaped) gain profile and pumps the gain medium from seven equiangular directions with a peak optical pumping power of 2800 W. Both designs of the side-pumping module are optimized to operate in burst mode with a maximum duty cycle of 25%. The error between different diode bars within each module was controlled at ±1 nm through diode bar selection during assembly.
Fig. 2. Simulated absorbed energy distribution and experimentally measured fluorescence intensity distribution across the section of the gain medium for side-pumping modules designed to provide a flattened-Gaussian (super-Gaussian) gain profile (SPM1 and SPM3) and a concave gain profile (SPM3).
Because the fluorescence is photographed when the side-pumping module works without lasing, whereas the simulation assumes that lasing is present, a small difference is observed between the simulated and measured results. Without the energy consumption by lasing, a higher degree of pumping saturation can be reached when photographing the fluorescence. As a result, a more uniform distribution is expected under the non-lasing fluorescent condition compared with the distribution under lasing conditions (and what shows in the simulation) [36].
Table 1 lists the detailed parameters of the side-pumping modules utilized in the laser MOPA system. In 2020, we demonstrated the continuous-wave (CW) laser operation of a flattened-Gaussian (super-Gaussian) module (SPM1, SPM2), as well as its performance as a single amplifier stage [36]. As laser modules with this flattened Gaussian gain profile effectively reduce the pentagonal thermal distortion of the Gaussian beam [36,53], laser modules with newer designs in featured gain profiles are required for flat-top output beams. A seven-directional side-pumping module with a concave gain profile (SPM3) was newly developed for generating flat-top (or near flat-top) laser beams while simultaneously reducing the effect of thermal focusing in high-power multi-stage laser MOPA systems.
Table 1. Characteristics of customed-designed side-pumping modules
4. Results and discussion
Figure 3 shows the average output power, burst energy, single-pulse energy, peak power, and optical-optical efficiency of the master oscillator as the pumping power increases. The lasing thresholds for the average optical pumping power and per-burst optical pumping energy were 16 W and 1.6 J, respectively. The average output power, burst energy, single-pulse energy, and peak power increased linearly with the optical pumping power, while the optical efficiency increased logarithmically with the optical pumping power. For the master oscillator, the average power, burst energy, single-pulse energy, peak power, and optical-optical efficiency reached their maximum of 8.7 W, 0.87 J, 4.8 mJ, 1.9 MW, and 22%, respectively, under an average optical pumping power of 40 W.
Fig. 3. Output laser performance versus the average optical pumping power and per-burst pumping energy for the master oscillator.
The burst duration was ∼1.8 ms and had a repetition rate of 10 Hz, as shown in Fig. 4(a). The Q-switched single pulses operate at a repetition rate of 100 kHz, as shown in Figs. 4(b) and 4(c), with a full width at half maximum (FWHM) pulse width of 2.6 ns.
Fig. 4. Output laser waveform profile of the master oscillator. (a) Global profile; (b) burst profile; (c) single pulse.
The master oscillator is operated with an average power of 7.4 W for the following amplification processes. After passing through the isolator, the injecting beam for amplifier stage one (SPM2) has an average power of ∼5.5 W. Figure 5(a) shows the output performance of the amplifier stage one versus the optical pumping power of SPM2. The average output power, burst energy, single pulse energy, and peak power increase exponentially with the optical pumping power of SPM2 and reach 9.7 W, 0.97 J, 6.1 mJ, and 1.8 MW, respectively. The energy extraction efficiency increases logarithmically with the optical pumping power of SPM2 and reaches 20% at the maximum optical pumping power of 40 W (4 J per burst).
Fig. 5. Output laser performance of the amplified laser. (a) Output of the amplifier stage one versus the average optical pumping power and per-burst pumping energy of the gain module of amplifier stage one (SPM2) with the injected average power of 5.5 W; (b) output of the amplifier stage two versus the average optical pumping power and per-burst pumping energy of the gain module of amplifier stage two (SPM3) with the injected average power of 9.7 W.
The laser beam amplified by SPM2 was then injected into amplifier stage two (SPM3). With SPM3 powered off, the beam with an average power of 9.7 W passes through amplifier stage two and the remaining power is 8.4 W, resulting in a passing loss of 13.4%. Figure 5(b) shows the output performance of amplifier stage two versus the optical pumping power of SPM3. The average output power, burst energy, single pulse energy, and peak power increase exponentially with the optical pumping power of SPM3 and reach the maximum of 20.4 W, 2.0 J, 12.7 mJ, and 3.9 MW, respectively, while the energy extraction efficiency increases linearly to logarithmically with the optical pumping power of SPM2 and reaches 27% at the maximum optical pumping power of 40 W (4 J per burst).
Figure 6 shows the waveform profile of the dual-stage dual-pass amplified laser at a maximum output power of 20.4 W. The burst duration, burst frequency, pulse repetition rate, and pulse width are ∼1.6, 10, 100, and 3.3 ns, respectively. Compared with the master oscillator output shown in Fig. 4, the consistency of the pulse energy (pulse stability) within a burst increases owing to the saturated amplification process. The coefficient of variation (CV) is utilized to evaluate the pulse energy consistency and is defined as
where s is the standard deviation of the pulse energies within the burst and m is the mean of the pulse energies within the burst. Here, the CV of the master oscillator burst shown in Fig. 4(b) is 0.149, and the CV of the amplified burst shown in Fig. 6(b) is 0.095. A lower CV indicates higher pulse energy consistency.Fig. 6. Waveform profile of the amplified laser at the maximum average power of 20.4 W. (a) Global profile; (b) burst profile; (c) single pulse.
Figure 7 shows the small-signal gain of the amplifier (stage 1 + stage 2) measured under an injected power of 0.012 W. With the amplification optical pumping power increasing from 0 to 80 W, the output power increases from 0.002 W to 0.4 W with a maximum magnification of 33.3. The small-signal gain increases logarithmically with the optical pumping power and reaches maximum of 15.2 dB.
Fig. 7. The small-signal amplification performance and small-signal gain versus the total average optical pumping power of both amplifiers (SPM2 + SPM3).
The cross-sectional intensity distribution of the amplified laser beam at the maximum power was measured using a CMOS camera, and the results are shown in Fig. 8. As a result of the gain profile control implemented in the oscillator and amplifier, the output beam exhibited a flat-top intensity distribution. In comparison, with a similar structure except for the gain profile-controlled modules, the laser MOPA system we reported in 2017 generates a typical Gaussian-distributed beam [37]. Table 2 shows a quantitative comparison of the intensity distributions of the flat-top beam, regular uncontrolled beam (from our 2017 work) [37], and theoretical bivariate Gaussian profile. The theoretical bivariate Gaussian distribution [54] is described as
Fig. 8. Beam cross-sectional intensity distribution profile of the amplified laser at the maximum average power of 20.4 W.
Table 2. Analyses of intensity distribution and comparison between flat-top and Gaussian profiles
With the boundary intensity defined as 10% of the maximum intensity, the flat-top beam has 80.9% of its spot area showing an intensity higher than half of the maximum intensity, whereas that for the uncontrolled beam and the ideal Gaussian profile is 38.9% and 30.1%, respectively. This comparison shows that, with the customized gain distribution, the output beam is significantly reshaped towards a flat-top profile, and the power is uniformly distributed across the beam.
The cross-sectional intensity distribution profile was fitted to the ideal super-Gaussian equation shown in Eq. (5) in two orthogonal directions to analyze the order of the super-Gaussian distribution.
Table 3 shows the coefficient of determination (goodness-of-fit) R2 and the root-mean-square error (RMSE) of the fitting with different orders.
Table 3. Analyses of super-Gaussian fitting of intensity distributions in two orthogonal directions
For the X direction, the curve optimally fits into a 6-order (n = 6) super-Gaussian distribution with the parameters a = 169.4, b = 490.9, d = 427.9, R2 = 0.954, and RMSE = 10.3. For the Y direction, the curve optimally fits into a 4-order (n = 4) super-Gaussian distribution with the parameters a = 169.0, b = 485.3, d = 409.0, R2 = 0.959, and RMSE = 10.2. The optimal fits for the X- and Y-directional curves are shown in Fig. 9.
The beam quality factors were measured to be Mx2 = 6.8 and My2 = 5.1 by a traveling 90/10 knife-edge method [55,56] in two orthogonal directions. The M2 factors were measured when the laser was operated at the maximum laser output power, and the beam caustic results are shown in Fig. 10.
Fig. 10. Beam quality factors (M2) measured in two orthogonal directions of the amplified laser at the maximum average power of 20.4 W.
This study achieved spatial control of the pumping power and gain, and in future works, temporal control of the gain may potentially be implemented for higher uniformity of the pulses in a burst. Dynamic wavelength shift compensation may also be implemented to support longer burst durations.
5. Conclusion
In this study, a cavity-dumped burst-mode laser MOPA system with a flat-top output beam is developed and demonstrated. A custom FEA simulation program was employed to design side-pumping laser modules with featured gain distribution profiles. A flat-Gaussian (super-Gaussian) gain profile is applied to the master oscillator and amplification stage to generate and maintain the flat-top beam. With increased thermal impact through the amplification process, a laser module with a concave gain profile is adopted in amplification stage two to further suppress the thermal lensing effect. The effectiveness of the gain profile control was verified by comparing the laser beam profile with that in our previous work without a customized gain profile. The progression of thermal focusing is slowed down by the flat-top distributed gain and beam profiles, and can be further compensated by concave lenses owing to the uniform delivery of pumping power through the gain medium. The average power and burst energy reach 20.4 W and 2.0 J, respectively. Within a burst duration of 1.6 ms, cavity-dumped pulses were operated at 100 kHz with a single pulse energy of 12.7 mJ and pulse width of 3.3 ns. This high-energy high-repetition-rate laser with a flat-top beam shows great potential in research and industrial fields, including material science, laser-induced fluorescence and plasma, and lidar. In future studies, laser modules with more featured gain distributions can be designed using our simulation program to realize customized laser beam profiles and to fulfill the requirements of various applications.
Funding
National Natural Science Foundation of China (61605032, 61705165, 61775167, 61975150).
Acknowledgments
We thank Wentao’s parents, Xuezhu Kou and Jianren Wu, for their auxiliary support during this study.
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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