1 Introduction
High-power 808 nm laser diodes (LDs), especially for edge-emitting laser (EEL) diode bars, have been widely used for neodymium-doped yttrium aluminum garnet (Nd:YAG) pumping in continuous wave (CW) and quasi-continuous wave (QCW) modes[ Reference Koechner1]. Compared with EEL diodes, high-power vertical-cavity surface-emitting laser (VCSEL) chips are considered as an alternative pumping source due to better wavelength stability, higher operation temperature, higher reliability, higher repetition rate, longer lifetime and lower manufacturing cost[ Reference Seurin, Xu, Khalfin, Miglo, Wynn, Pradhan, Ghosh and D’Asaro2, Reference Seurin, Ghosh, Khalfin, Miglo, Xu, Wynn, Pradhan and D’Asaro3]. The VCSEL has a λ/4 distributed Bragg reflector (DBR)-based microcavity; thus, the wavelength is generally stabilized with a typical shift of around 0.07 nm/K[ Reference Xiong, Leeuwen, Watkins, Seurin, Xu, Miglo, Wang and Ghosh4], while the EEL has a wavelength shift of around 0.28 nm/K[ Reference Liu, Zhao, Xiong and Liu5] at 808 nm. Compared with the challenging catastrophic optical damage (COD) of the EEL[ Reference Yanson, Levi, Shamay, Tesler, Rappaport, Don, Karni, Schnitzer, Sicron and Shusterman6], there is no optical coating on the VCSEL facet and the optical power density is much lower than that of the EEL; thus, there is no COD risk for the VCSEL at high temperatures, over current inrush or other harsh conditions. The lifetime of the VCSEL is expected to be at least 50 times longer than that of the EEL[ Reference Seurin, Ghosh, Khalfin, Miglo, Xu, Wynn, Pradhan and D’Asaro3]. The only optical limit for the VCSEL is thermal rollover when the junction is over heated. The uniform distribution of thousands of mW-level emitters on a large VCSEL chip offers a better heat stress during pulsed conditions compared with the EEL with watt-level striped emitters so that a long lifetime and stability can be expected at high repetition rates up to several kHz. High-repetition-rate VCSEL-pumped laser amplifiers have been successfully used for space debris laser in the region of 1 kHz, demonstrating stable operation for years[ Reference Ma, Chen, Yang, Lu, Zhang and Du7]. The fabrication and packaging of the VCSEL follow standard wafer-level procedures, such as those in the light-emitting diode (LED) industry, and thus the manufacturing cost is lower than that of the EEL, which involves cleaving, facet coating and critical packaging procedures[ Reference Michalzik8]. A general structure comparison of the EEL and VCSEL is shown in Figure 1.
Edge-emitting laser (EEL) and VCSEL structures[ Reference Li, Li and Wei9]. Unlike optical coating on the facets of the EEL, the VCSEL has a COD-free DBR facet grown via epitaxy.
Regarding the LD pumps for inertial fusion energy lasers, Lawrence Livermore National Laboratory predicted that the possible future use of surface-emitting diodes may offer appreciable future cost reductions and increased reliability[ Reference Deri, Geske, Kanskar, Patterson, Kim, Hartmann, Leibreich, Deichsel, Ungar, Thiagarajan, Martinsen, Leisher, Stephens, Harrison, Ghosh, Rabot and Kohl10] in 2011. However, the high-power VCSEL was not ready for volume commercial use at that time due to lower power conversion efficiency (PCE), a complicated substrate removal process during chip fabrication and expensive chemical vapor deposition (CVD) diamond packaging[ Reference Seurin, Xu, Khalfin, Miglo, Wynn, Pradhan, Ghosh and D’Asaro11]. As a result of authors’ recent efforts, high-power single-junction 808 nm VCSEL chips with regular 100 μm thickness substrate and metallized AlN ceramic packaging are now able to offer 100 W CW output power per chip with a maximum of 44% PCE and 270 W QCW output power with a maximum of 50% PCE[ Reference Li, Li, Yuan, Wang, Wei and Chen12]. The fabrication and packaging of these high-power VCSEL chips are mature and the production yield is very high. The cost per watt is now becoming lower than that of EEL bars. Higher PCE can be expected with further improvements, and optical power per chip can be doubled or tripled with multi-junction epitaxy in the near future. The high-power VCSEL for solid-state pumping is now ready for commercial use.
Among all the diode-pumped Nd:YAG lasers, side-pumping is the most popular method to obtain high-power and high-energy output and is widely used for industrial, scientific and medical applications[ Reference Song, Bian, Yu, Gong, Zong, Zong, Bo and Peng13, Reference Fan, Qiu, Tang, Bai, Kang, Ge, Wang, Liu and Liu14]. A typical side-pumped Nd:YAG laser resonator is shown in Figure 2.
The typical cross-section of a side-pumped laser cavity contains laser diode arrays (LDAs), a reflector and a laser rod. For water-cooled pump modules, a glass flow tube is usually included between the reflector and laser rod, providing water flow for cooling, as shown in Figure 3. In the textbook Solid-State Laser Engineering, Koechner[ Reference Koechner1] summarized the basic scheme of a pump chamber with three directions of LD arrays, a laser rod and a diffuse reflector. In 2001, Fujikawa et al. [ Reference Fujikawa, Furuta and Yasui15] reported a side-pumped Nd:YAG laser with six-direction pumping, which was combined by two circles of three-direction pumping units with 60-degree rotation to avoid reciprocal emitting. In 2009, Grechin and Nikolaev[ Reference Grechin and Nikolaev16] described the scheme of a pump module design with nine directions and a mirror reflector. In 2016, Raja Ramanna Centre for Advanced Technology (RRCAT)[ Reference Sundar, Ranganathan, Hedaoo and Bindra17] of India reported a modular pump head designed with three-direction pumping discs around a Nd:YAG rod, and it was observed that the triangular-like beam profile by the single disc pumping (120 degrees) was greatly averaged out when two rotated discs pumped together from six directions (60 degrees). The most popular commercial pump modules delivered by Northrop Grumman Cutting Edge Optronics (NG CEO) are typically designed with three, five, seven and nine pumping directions with internal reflectors. The fluorescence light of an NG CEO RE22 pump module[ Reference Xu, Briggs, Doster, Feeler and Stephens18] is shown in Figure 4.
Resonator of a diode side-pumped Nd:YAG laser.
Conventional LD pump cavities. Odd-numbered LD arrays are aligned to avoid reciprocal damage and an internal reflector is introduced to improve absorption and efficiency.
Fluorescence distribution from an NG CEO REA22 pump module[ Reference Xu, Briggs, Doster, Feeler and Stephens18]. Pumping beam traces from seven directions can be clearly found and the beam profile has no circular symmetry.
In these conventional pumping geometries, odd-numbered LD arrays are adopted to avoid reciprocal damage between the opposite pump diodes because the EEL bars usually suffer from strong light injection with catastrophic amplification due to low reflectivity of the emitting facet, such as 10% typically, for example, in Ref. [Reference Epperlein19]. To increase the absorption of the pumping beam and improve the pumping distribution, an internal reflector is usually needed to collect the unabsorbed pumping beam and turn it back for further absorption. This internal reflector is either coated with gold as a mirror reflector or made directly from diffuse reflectance material with high reflectivity. This internal reflector is widely used in commercial pump modules by laser manufacturers including NG CEO, Rofin-Coherent, FOBA and many others.
In the past few years, a few researchers have reported several VCSEL-pumped Nd:YAG laser experiments. In 2012, Princeton Optronics demonstrated a VCSEL side-pumped Nd:YAG laser slab with face-to-face pumping geometry[ Reference Seurin, Xu, Miglo, Wang, Leeuwen, Xiong, Zou, Li, Wynn, Khalfin and Ghosh20]. Two VCSEL stacks collimated by cylindrical lenses were placed at the opposite side facets of the laser slab for pumping. In 2016, Wang et al. [ Reference Wang, Wei, Jiang, Wang, Qiao, Guo, Fan and Li21] reported a VCSEL-pumped Nd:YAG rod laser with seven rows of pumping arrays in a gold-coated reflective chamber. In 2022, a VCSEL-pumped rod laser with 652 W CW power and 52.6% optical efficiency was reported by Li et al. [ Reference Li, Yang, Zhang, Yang, Wang and Peng22] and the laser module was designed with an 8 mm Nd:YAG rod with pumping in five directions.
However, the unique characteristics of the VCSEL compared with EEL diode bars were not fully discovered and significant opportunities remain for the optical tuning of the pumping geometry. In this paper, we will demonstrate a novel pumping geometry and multiple optical tuning mechanisms for a VCSEL side-pumped Nd:YAG laser cavity.
2 Vertical-cavity surface-emitting laser characteristics for pumping
The wafer for the 808 nm VCSEL chip is usually prepared with a metalorganic chemical vapor deposition (MOCVD) system based on an n-type GaAs substrate. During epitaxy, the wafer was latticed with an n-DBR at the bottom with high reflectivity close to 100% and a p-DBR at the emitting facet with reflectivity of approximately 99%, because the feedback of the emitting facet must be maintained at a very high level to reach the lasing threshold due to the low gain inside of the VCSEL cavity. During the fabrication process of the VCSEL, selective wet oxidation is used to form current and optical confinement, and metallization is introduced for the electrodes both at the bottom and the facet. Gold coating is deposited on the top of the VCSEL chip for current injection and wire bonding purposes. The general structure of the VCSEL is shown in Figure 5. The overall surface of the bare die has a high reflectivity from the top DBR and gold coating in the rest area, as shown in Figure 6.
VCSEL resonator principle. The surface of the VCSEL chip is highly reflective and external photon injection will not cause any damage in the emitters.
VCSEL emitting facet photo from an actual chip taken by the authors.
Chip-on-submount (CoS) packaging is processed after fabrication of the bare die. The metallized AlN submount is the most popular material for die attachment and wire bonding. The metallization of the submount contains a thick layer of copper at the bottom and gold coating at the surface. The finished VCSEL chip in CoS package is shown in Figure 7(a). Compared with the typical laser bar packaging, as shown in Figure 7(b), the VCSEL package has a much larger emitting area rather than a linear emitting line from the laser bar between the clamping electrodes. Apparently the CoS VCSEL package has a very high-reflection (HR) surface due to the top DBR and gold coating both on the chip and on the submount, and this characteristic can be used as a reflector in the pumping design rather than the surface of a laser bar assembly.
(a) VCSEL in CoS package on a metallized AlN submount with a rectangular emitting area and (b) laser bar assembly with a linear emitting line between the clamping electrodes.
Single pass pumping and absorption simulation at different distances from the (a) laser bar and (b) VCSEL.
Compared with EEL bars, another major difference is the divergence and beam profile. The laser bar has a linear emitting line with fast axis divergence up to 35–40 degrees (full width at half maximum, FWHM) and slow axis divergence up to 7–10 degrees (FWHM), while the VCSEL has a uniform two-dimensional (2D) emitting area with a uniform divergence of around 15–20 degrees (FWHM) in all axes. Microlensing can be introduced to reduce the divergence of the VCSEL if needed[ Reference Chen, Zhang, Jiang, Meng and Chen23]. In the near-field distance for pumping applications, which is usually a few millimeters to the laser crystal, the beam distribution of the VCSEL is close to near flat top, although the far field distribution varies due to the different modes from the emitters. If the emitting width of the VCSEL is less than the laser rod diameter, most of the pump power can be evenly spread on the laser rod surface with very little optical loss. However, due to the larger divergence and Gaussian distribution in the fast axis, the laser bar pumping is either uneven at a very short distance or has a very high loss over a longer distance.
The single pass pumping and absorption distribution can be described by optical simulation, as shown in Figure 8. In this optical simulation, the distance (d) from the emitting facet to the center of the Nd:YAG is set at 5 and 10 mm. The Nd:YAG diameter is 6 mm. Considering a variable doping of Nd:YAG from 0.6% to 1.1% and a practical pumping wavelength from 804 to 809 nm, we suppose a reasonable absorption coefficient of 4 cm–1 on average during the simulation. The emitting width (w) of the VCSEL is set at 4.8 mm with a divergence of 18 degrees (FWHM). The divergence of the laser bar is set at 36 degrees (FWHM) in the fast axis. In the long axis direction of the laser rod, we suppose uniform distribution and identical pumping power density per centimeter for both the laser bar and VCSEL. Different absorption uniformity can be found from the cross-section of the Nd:YAG.
Absorption spectrum of the Nd:YAG with peak absorption at 808.5 nm. Absorption data were provided by Beijing Opto-Electronics Technology Co., Ltd. and the figure was drafted by the authors for reference only.
Pump diode alignment around the Nd:YAG rod with uniform wavelength per circle. Each circle is required to be with the same wavelength and same optical power.
The simulation shows clearly that the VCSEL pumping can offer a better uniformity while maintaining a lower loss for the single pass pumping absorption compared with laser bar pumping for short pumping distance.
3 Optical tuning of the vertical-cavity surface-emitting laser pump cavity
3.1 Optical tuning targets and general measures
Although the application requirements may vary, most of solid-state laser systems are pursuing two important performances: beam profile and efficiency. For side-pumped Nd:YAG rod lasers, a near flat top or Gaussian beam profile is usually expected for the amplifiers and oscillators. Optical-to-optical efficiency should be optimized so that the LD power can be used economically and also the side effects of the wasted heat can be minimized. In a well-designed pump cavity, these two optical tuning targets must be both taken into account.
One important measure is to ensure the entire circle of pump diodes has the same or similar optical power and spectrum characteristics in the cross-section plane of the Nd:YAG rod. As shown in Figure 9, due to different absorption levels at different wavelengths, Nd:YAG pumping is sensitive to the wavelength distribution from the LDs. For efficient absorption, the LD is usually designed at central wavelengths from 804 to 809 nm to match the highest peak of the absorptance. However, the wavelength deviation of the LD both in the cross-section and in the axis direction of the Nd:YAG will cause a uniformity problem. The central wavelength must match the Nd:YAG rod diameter and doping level so that a proper path length of the pumping beam can be expected. As the absorption coefficient is higher at 808.5 nm, the absorption is very strong near this peak wavelength and the path length of the absorption is very short compared with pumping wavelength far from 808.5 nm, such as 804 or 810 nm. For a larger diameter or higher doping, a pumping wavelength with lower absorptance is required, which is also called a wing pump. For a smaller diameter or lower doping, pumping at the peak absorptance is preferred.
In the long axis direction of the laser rod, the same wavelength from each circle is preferred as well. However, from a practical point of view, it is not always economical to pick up exactly the same central wavelength for pumping because the deviation of the central wavelength during LD wafer epitaxy is up to ±2.5 nm in modern industrial standards. The tolerance of the central wavelength exists not only from wafer to wafer but also from the center to the edge of the same wafer due to the uniformity limit of the lattice growth in MOCVD. Strict wavelength sorting apparently will increase the cost of the pump diodes. Of course, volume Bragg grating (VBG) or other wavelength stabilization methods can be introduced but it will increase the cost as well. In the case in which it is not economically possible to pick up exactly the same wavelength, a mixed wavelength between different circles is also acceptable, but the path length of the absorption must be considered to compensate between different circles to ensure the uniformity of the accumulated absorption in the entire laser rod. Please refer to Figure 10.
The intensity distribution of the pump diodes is also important. Just like the central wavelength deviation, the threshold current, slope efficiency and optical power of each LD are also not uniform from wafer to wafer and from the center to the edge of the same wafer. The LD’s optical power control is following the same rule as the central wavelength distribution, which attempts to keep the uniformity of each circle and find a proper compensation between different circles.
The central wavelength and optical power of the pump diodes can be controlled by LD chip sorting procedures in advance. Nevertheless, from the epitaxy point of view, most of LD chips have similar performance in adjacent areas from the same wafer. So, using the chips from an adjacent area for each circle is an easy solution.
The other measure is to ensure the wavelength of the LD is designed against specific conditions for pumping purposes. Besides the normal wavelength temperature coefficient of 0.07 nm/K for the VCSEL or 0.28 nm/K for the laser bar, the LD’s central wavelength also changes against operation current, pulse width, etc. At a higher temperature, higher current or longer pulse width, the central wavelength has red-shift due to the temperature rise in the junction. For example, an LD bar may have a wavelength of 808 nm at CW but 804 nm at QCW (200 μs, 20 Hz typically) conditions at the same 40 A operation current, and a VCSEL chip may have a wavelength of 808 nm under CW but 806.5 nm under QCW (200 μs, 20 Hz typically) conditions at the same 60 A operation current.
3.2 Symmetrical pump cavity with inter-reflective chamber
Since the VCSEL chip in a CoS package has an HR surface and the facet is not sensitive to external pumping beam injection, the cross-section of the pump cavity can be designed to have symmetrical geometry instead of odd-numbered pumping directions. The surface of the VCSEL chip can be used as a reflector to the unabsorbed pumping beam from the opposite VCSEL. In this inter-reflective chamber, this symmetrical alignment of the VCSEL chips can improve the absorption of the pumping beam. On the other hand, the internal reflector in the conventional pump cavity can be removed if the emitting width of the VCSEL is less than or close to the laser rod diameter. Among all the possible symmetrical pump cavity designs, a typical hexagonal cavity is shown in Figures 11 and 12.
Pump cavity comparison between odd-numbered and even-numbered directions.
Two VCSEL stacks from a hexagonal pump cavity with three rows of VCSEL arrays each. These two stacks will be combined as an inter-reflective pump chamber.
Several experiments were conducted with different pumping directions from three, four, five, six and eight directions and different Nd:YAG rods with diameters from 4 up to 10 mm. The doping level is 0.8%–1% for the Nd:YAG rods. Both conductively cooled pump modules and water-cooled pump modules were designed. For each pump module, the fluorescence was captured at the end of the Nd:YAG rod with a camera and the absorption beam profile was analyzed. After fluorescence measurement, output energy at 1064 nm was measured in the plano–plano resonator, as shown in Figure 13, and the optical-to-optical efficiency was then calculated. The final results are listed in Figure 14.
It is clearly shown that pump modules with even-numbered pumping directions have better circular symmetry compared with odd-numbered pumping directions. Smaller Nd:YAG diameters (e.g., 4 or 5 mm) yield a more uniform distribution in even-numbered pump modules than larger diameters. We attribute this phenomenon to the fact that the absorption path length is significantly longer than the small Nd:YAG diameter (e.g., 4 or 5 mm), allowing the reflected and overlapping pump beams from the inter-reflective chamber to enhance uniformity. In contrast, for larger diameters, the absorption path length becomes shorter than the Nd:YAG rod size, reducing the efficiency of inter-reflection in improving pump uniformity.
Pumping experiments with (a) a water-cooled pump module and (b) a conductively cooled pump module. The 1064 nm output power/energy is measured in a plano–plano resonator with an output coupler with T = 20% (R = 80%) and cavity length of approximately 250 mm.
Distribution profiles in various cavity setups. The optical efficiency in the profiles above is defined as optical-to-optical efficiency from 808 to 1064 nm. Please notice that (b) and (d) are conductively cooled pump modules without flow tubes, while the rest are water-cooled pump modules.
According to the 4 and 5 mm Nd:YAG pumping with and without a reflector in Figures 14(a)–14(f), due to the inter-reflection between the VCSEL chips, very similar optical-to-optical efficiency can be maintained even without an internal reflector. The removal of the internal reflector will simplify the laser design.
(a) Diffusive reflectors made from Spectralon[ 27] and (b) material reflectivity[ Reference Zhang, Yang, Jin, Liu and Hsu28]. The reflectivity at 808 nm is approximately 99%.
The optical-to-optical efficiency increases along with the Nd:YAG diameter due to better absorption during the first pass. However, when the pumping directions increases, the inner diameter of the pump cavity will be increased due to the dimension of the pump chips. Then the distance between the VCSEL surface and the Nd:YAG rod center is also increased, and thus pumping efficiency will be slightly reduced. Please notice that all the water-cooled and even-numbered pump modules in Figures 14(c), 14(f) and 14(h)–14(l) have a high optical-to-optical efficiency from 39% to 54%, which is comparable with conventional laser bar side-pumped Nd:YAG lasers. Further tuning can be made to reach even higher optical-to-optical efficiency. In this paper, we will focus our topics on the geometry and principles but not challenging cutting-edge parameters.
Please also note that most of the distribution profiles are not centered. We believe that the main reason is the unbalancing of the central wavelength and optical power intensity from different pumping directions, as discussed in Section 3.1; the other possible reason is the uniformity of the doping in the Nd:YAG rod as the deviation of the industrial standard at present is ±0.1%. We can diagnose these reasons by rotating the laser rod or changing the laser rod while monitoring the changes of the distribution profiles. If the profiles change along with rotation, then the problem lies with the Nd:YAG rod.
In general, we can conclude that this VCSEL-based even-numbered inter-reflective pump cavity without internal reflector provides a more symmetrical beam profile with a comparable optical-to-optical efficiency relative to conventional pump cavities.
3.3 Pumping beam diffusion
In the pump cavity, diffusion of the pumping beam is usually introduced to improve the uniformity of the absorption. In conventional side-pumped rod lasers, the internal reflector is usually designed with diffuse reflectance material, such as polytetrafluoroethylene (PTFE) compound (for example, Fluorilon-99W material from Avian Technologies, Spectralon material from Labsphere), ceramic[ Reference Sharma, Singh, Mukhopadhyay and Oak24– Reference Xia, Yu and Chen26] or other special coatings, as shown in Figure 15. In these pump cavities, the pumping beam transfers through the laser rod and then reaches the diffusive reflector for further reflection and absorption. However, most of the pumping energy is absorbed during the first pass; the effectiveness of the diffusive reflector is limited to the unabsorbed beam only. To improve the uniformity of the absorption, a laser crystal with low doping level is usually introduced so that the diffusive reflector can be more effective to restructure the pumping beam, but this will be contrary to the absorption efficiency of the first pass.
Different diffusion solutions in the pump cavity.
In the symmetrical pump cavity without an internal reflector with, for example, a hexagonal cavity as shown in Figure 16, the glass flow tube that is usually used for the laser rod water cooling can be processed with a frosted surface as a diffuser for the pumping beam, as shown in Figure 17. The frosted surface of the glass has two impacts on the pumping beam, namely diffusive transmission and diffusive reflection. Since the reflected beam will be collected again by the inter-reflective chamber, the efficiency of the absorption will be barely influenced. The diffusive transmission will improve the uniformity of the laser rod absorption during the first pass and further passes of the pumping beam, which is quite different from the conventional design.
A comparison experiment was prepared between a regular polished and a frosted glass tubes in a hexagonal pump cavity with two Nd:YAG rods of 5 and 6 mm diameter with 1% doping and the same VCSEL chips. From the cross-section, the distance from the VCSEL chip facet to the Nd:YAG rod center is 9.5 mm. The central wavelength of the VCSEL is sorted with a ±0.5 nm deviation. Both the polished and frosted glass flow tubes have an outer diameter of 10 mm and an inner diameter of 8 mm. The frosted surface is achieved using 120-mesh diamond sand. The beam profile images were captured and analyzed, as shown in Figure 18.
Apparently, the fluorescence distribution with the frosted flow tube was more uniform than that with the polished flow tube because of the diffusion of the pumping beam in the inter-reflective chamber. The brightness from the surface of the Nd:YAG rod was also reduced with the frosted flow tube. At the same time, no significant difference was observed in the optical-to-optical efficiency, which means this optical tuning measure did not cause efficiency loss.
VCSEL pump stack with a frosted flow tube and a Nd:YAG laser rod.
Distribution comparison between the polished tube and frosted tube in the pump cavity with 5 and 6 mm laser rods.
Besides the fluorescence distribution, we processed further tests for the pump module with a 6 mm Nd:YAG rod in the inter-reflective pump chamber including a frosted flow tube for diffusion. There were in total 48 VCSEL chips in this hexagonal pump chamber with eight chips from each direction. The total length of the Nd:YAG was 85 mm and the pumping length was around 55 mm. The optical power of each VCSEL chip was measured up to 270 W in QCW mode at 200 μs, 20 Hz. The pump module was cooled by water at a temperature of 25°C and a flow rate around 3.5 LPM. An HR mirror and an output coupler (OC) with 20% transmission (80% reflection) were well aligned in a plano–plano cavity with a cavity length of around 250 mm. Average output power at 1064 nm was measured with a laser power meter and the peak power was then calculated. Peak output power of 5867 W at 1064 nm was achieved at a pump power of 12,960 W at 808 nm, as shown in Figure 19. The pulse energy was up to 1.17 J at 1064 nm with pulse width of 200 μs. The maximum optical-to-optical efficiency was calculated as up to 45%[ Reference Li, Li, Yuan, Wang, Wei and Chen12]. The fluorescence distribution was described in Figure 18(d).
Pump power at 808 nm versus 1064 nm output power in a hexagonal pump cavity with a 6 mm Nd:YAG rod and a frosted flow tube[ Reference Li, Li, Yuan, Wang, Wei and Chen12].
To demonstrate the amplification performance of a VCSEL-pumped laser module, we conducted an experiment within a master oscillator power amplifier (MOPA) configuration. The seed laser was an electro-optically Q-switched VCSEL-pumped Nd:YAG laser with 6 mm diameter and 220 mJ, 10 Hz, 6–8 ns output energy. The amplifier was a VCSEL-pumped Nd:YAG laser module with eight directions of pumping and an 8 mm diameter Nd:YAG rod. The seed laser was injected into the center of the amplifier directly in a single pass configuration. The final output energy from the MOPA system was measured with 803 mJ output with 3.65 times amplification at a pump power of approximately 15,900 W, as shown in Figure 20.
In general, the pumping beam diffusion technique through a frosted flow tube provides a simple solution to improve the uniformity of the VCSEL-pumped laser modules while maintaining a high optical-to-optical efficiency. This will benefit further applications with VCSEL-pumped oscillators and amplifiers.
3.4 Centric and eccentric pumping
The pump cavity designs above are all in centric pumping geometry. If the VCSEL chips have a uniform optical power and spectrum in each pumping circle, the absorption in the Nd:YAG rod should be centrosymmetric. From Figure 14, we can establish that there are generally two kinds of beam profiles: the near flat top and the near Gaussian distribution. Some applications, such as seed lasers, may follow the Gaussian distribution to obtain the maximum fundamental output in the oscillators. However, the Gaussian beam has 86.4% of its energy in the central core (defined by the 1/e2 intensity level); the high peak intensity in the beam center may lead to optics damage, thermal lensing and danger of damage, while the rest of the energy in the wing may be lost because of the threshold behavior existing in many processes, for example laser pumping and material processing[ Reference Chang, Cramer, Danielson and Norby29]. Many scientific, industrial and medical applications require flat top spatial energy distribution, high uniformity in the plateau region and complete absence of hot spots[ Reference Chang, Cramer, Danielson and Norby29, Reference Yin, Xu, Liu, Wang, Zhao and Tang30]. Particularly for high-energy laser amplifiers, a near flat top beam profile is always the primary target of the laser system design, for which many efforts have been made previously[ Reference Zhao, Fan, Xiao, Huang, Bai, Ge and Zhang31, Reference Zhao, Xiao, Huang and Fan32].
MOPA experiment with a 220 mJ seed laser and a VCSEL-pumped amplifier with an 8 mm Nd:YAG rod.
For most cases with a small Nd:YAG rod diameter or low doping level, the VCSEL wavelength can be properly designed to match the absorption distance of the pumping beam so that uniform absorption in the cross-section of the laser rod can be expected. However, in most Nd:YAG rod pumping, strong concentration in the center is more popular because of the limit of the pumping wavelength options (804–809 nm, typically) and Nd:YAG doping options (0.6%–1.1%, typically). In these cases where the concentration is unexpected in the center of the laser rod, measures must be taken to improve the uniformity from near Gaussian to near flat top. One of the optional measures is the eccentric pumping design, as shown in Figure 21.
From the fluorescence image of the 6 mm Nd:YAG with the frosted flow tube in Figure 18(d), we can see that the absorption still comes with a slightly strong center that takes over about 2/5 of the diameter (2.4 mm in diameter accordingly). To reduce the intensity of this center area, we rotate the central axis of each VCSEL chip clockwise by 8 degrees. Considering the 9.5 mm distance from the chip facet center to the Nd:YAG rod center, the eccentric distance of the pumping beam is calculated as 9.5 mm × sin(8°) ≈ 1.3 mm, which is close to the edge of the current strong center. A pumping experiment was carried out and fluorescence images were captured and compared, as shown in Figure 22.
It is clearly shown that by introducing the eccentric pumping mechanism, the concentration in the center was reduced and the absorption at the edge of the Nd:YAG rod was enhanced. However, the optical-to-optical efficiency dropped from 45% to 39% due to the distortion of the inter-reflective chamber, which may cause spatial misplacement, especially for the first pass of pumping.
Centric and eccentric pumping schemes.
Distribution comparison between centric and eccentric pumping with the same 6 mm laser rod and frosted tube.
Distribution comparison between centric and eccentric pumping with an 8 mm Nd:YAG rod.
Nevertheless, in some cases where the Nd:YAG rod diameter is much larger than the path length of the pumping beam, for example, a diameter of more than 8 mm, eccentric pumping is a good way to change the beam profile from near Gaussian to near flat top distribution. By using the same pump chamber as shown in Figure 22, we replaced the 6 mm Nd:YAG rod with an 8 mm Nd:YAG rod for further eccentric pumping experiment. Fluorescence was captured in Figure 23(b). Compared with centric pumping from Figure 23(a), which is the same as that in Figure 14(j), eccentric pumping has highly improved the uniformity in the center of the 8 mm Nd:YAG rod, while optical-to-optical efficiency was reduced from 51% to 44%.
In general, the tuning of the beam profile through eccentric pumping provides an optional measure to improve the uniformity of the pumping if necessary.
4 Conclusion
Compared with EEL bars, high-power VCSELs are considered as an alternative pumping source due to better wavelength stability, higher operation temperature, better reliability, higher repetition rate, much longer lifetime and lower manufacturing cost. Based on unique optical characteristics of VCSEL chips, a novel symmetrical pump cavity with an inter-reflective chamber was invented for VCSEL side-pumped Nd:YAG rod lasers. Even-numbered pumping directions were introduced instead of odd-numbered pumping and the conventional internal reflector was removed. Several optical tuning measures were taken to improve the uniformity of the pumping distribution, including optical power and spectrum sorting of the diode chips, optical alignment in the cross-section and long axis of the laser rod, a diffusion mechanism in the pumping chamber using a frosted glass flow tube and an optional eccentric pumping geometry. A series of VCSEL pumping experiments were conducted and optical tuning measures were evaluated through distribution profiles and optical-to-optical efficiencies. A new ideology of the VCSEL side-pumped Nd:YAG laser cavity was then finalized, which may benefit further developments and applications in the future.
Acknowledgements
This work was supported by the National Key Research and Development Program of China (Grant No. 2024YFB2807702).
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