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We report a multiple-gain-element Nd:YAG laser where the gain media (three pieces of slab crystal) are alternately bonded to two optical quality 4H-SiC wafers. Such composite gain configuration can efficiently remove waste heat from the gain medium, preventing thermal lensing and heat-induced birefringence/distortion under high power laser operation. Through near Brewster’s angles incidence designing and polarization discrimination, two orthogonally linearly polarized (P and S polarized) laser beams are generated simultaneously from different parts of the same system. Based on a T = 3% output coupler, this continuous wave laser produces maximum power of 5.34 W (0.83 W) with a slope efficiency of 21.1% (3.6%) for the S (P) polarized laser beam. At the 5-W level, the S polarized beam has a beam quality of M2~1.2. The wavelengths of these two perpendicularly polarized laser beams differ about 0.6 nm (1063.7 and 1064.3 nm). Polarized output behavior dependent on the output-coupler transmission is also studied, and it is found that increasing the transmission leads to steady growth of the P polarized laser beam; when a T = 1.3% output coupler is adopted, more than 99% of the output is the S polarized beam. The highest total output power is 6.75 W obtained with the T = 1.3% output coupler, corresponding to slope efficiency of 25.7%. This composite laser scheme, bonding multiple gain media with high-thermal-conductivity materials, opens a new avenue for high-power high-beam-quality solid-state lasers with multiple-polarization output beams.
© 2017 Optical Society of America
1. Introduction
Owing to quantum defects and absorption/scattering losses, high-power solid-state lasers (SSLs) are always accompanied with energy deficiency and thermal effects. In high power SSL systems, thermal-induced wavefront distortions and thermal lensing will degrade the beam quality and ultimately put a limit on their power scaling and laser conversion efficiency. Furthermore, thermal stress may induce the fracture of gain medium, and thermal birefringence will randomize the polarization state of output laser beam. Therefore, efficient heat removal from gain medium is a key issue in developing high-power high-beam-quality SSLs.
By using structure engineering of gain medium, e.g. thinning the gain medium to slab or disk shapes, conventional heat-conducting cooling technique has successfully scaled the average power of SSLs to over multi-kilowatts [1–4]. However, even for the disk laser, its beam quality will decrease to M2~20 when it is operated in the kW level [4]. Therefore, it is still a difficult task to achieve fundamental mode operation for high-power SSLs. In addition, traditional high-power SSLs have complicated and cumbersome cooling systems, which are either low efficient or not meet the application requirement where compact and light-weight systems are preferred.
Recently, researchers are paying attention to combining laser medium with high thermal conductive materials, e.g., diamond, silicon carbide (SiC) or sapphire, to improve the heat-removal capability of solid-state lasers [5–12]. By using SiC as the heat spreader, a composite Yb:YAG/SiC prism disk laser with 12 W output power (M2~1.5) [10] and a SiC-clad Nd:YVO4 laser with 83 W output power (M2~1.2 × 40) [11] have been reported. At the same time, using diamond as the heat spreader, output power from 25.7 W (M2~44 × 29) [9] to ~200 W (M2~7 × 35) [5] has been achieved. In all the previous researches, the high thermal-conductive materials have shown great potentials as efficient heat spreaders in high power SSLs. However, the low-conductivity problem of the gain medium has not been completely resolved, and so the issue of beam-quality degradation at high power levels remains. How to get high average power output with good beam quality is still an open question for these new-cooling-configuration SSLs.
In this paper, we design a composite gain structure through bonding three pieces of Nd:YAG crystal slabs with two pieces of 4H-SiC wafers for laser operation. The aim is to integrate high-conductive materials as true ingredients of gain media for efficient heat dissipation. Through dividing the total wasted heat in the gain media into several independent transporting channels, such laser gain structure can provide great advantage of high cooling efficiency and power scaling possibility. Through separate and efficient heat removing, the laser output power is scaled to over 6 W with beam quality M2 of ~1.2 and slope efficiency of 24.7% for the T = 3% output coupler, and the output power is just limited by the available pump power. When the T = 1.3% output coupler is used, the slope efficiency is improved to 25.7% and >99% of the output is the S polarized laser. This confirms that the hybrid/composite bonding gain-medium configuration with high-thermal conductivity and discrete-distributed heat spreader can be a good candidate for developing high-power SSLs. Based on polarization discrimination effect of light propagation at Brewster angles, two orthogonally linearly polarized laser beams are simultaneously generated from the same laser system, opening an avenue to tune laser polarizing characteristics through cavity configuration engineering.
2. Experiment and results
We chose crystalline 4H-SiC and Nd:YAG crystal to form the composite bonded gain medium. Crystalline 4H-SiC has refractive index of 2.59 at 1 μm and thermal conductivity of ~490 W/m·K. The great difference between the refractive index of SiC and that of Nd:YAG (1.82) can give a single-pass Fresnel reflection loss of ~3% for normal incidence. While anti-reflection (AR) optical coating between the SiC and YAG during bonding will greatly decrease bonding strength [10], we adopted the near Brewster angle propagation method to construct the laser system for reducing reflection loss. Provided that the thermal conductivity of SiC is over one order of magnitude higher than that of Nd:YAG, the waste heat generated inside Nd:YAG will be rapidly transported to the SiC wafer.
In our experiment, the 4H-SiC wafer used had a dimension of 35 × 35 mm2 and thickness of 0.5 mm, and the 1.0 at.% doped Nd:YAG crystal had a dimension of 15 × 15 mm2 and thickness of 1 mm. Three pieces of crystals and two pieces of SiC wafers were bonded alternately with the high temperature bonding method (Onyxoptics Co.) to form a sandwiched structure (see Fig. 1). In our proof-of-concept experiments the Nd:YAG crystals were cooled through transmitting their heat load to the SiC wafers bonded to them, so the middle crystal was cooled from both sides while the outer crystals were only cooled from one side that bonded to the SiC wafer. The outside periphery of both pieces of SiC wafers were weld to a turbulent water-cooled aluminum heat sink with indium foils.
Fig. 1 Experimental setup and design of the multi-element composite Nd:YAG/SiC bonded slab laser. P: parallel polarization; S: perpendicular polarization; L: lens; M: mirror; LD: laser diode. Blue arrows indicate the polarization directions of the output laser beams. Inset shows the schematic diagram of single light passing through the composite gain medium.
The experimental setup for the composite bonding laser system is shown in Fig. 1. The gain medium was pumped with a 50-W laser diode operating at ~805 nm (bandwidth of ~2.5 nm) with an output coupling fiber (400 μm cladding and a numerical aperture of 0.22). The pump laser beam was collimated and focused by two 50 mm lenses (L1 and L2) with a focusing spot diameter of ~600 μm. Owing to the specific configuration of this laser system, if the laser beam is incident on the air-Nd:YAG interfaces completely at the Brewster’s angle, then the incidence angle on the Nd:YAG-SiC interfaces will not be the Brewster’s type. There exists an optimized incidence angle to keep the total cavity loss (Fresnel reflection) as small as possible, which is found through tilting the two cavity mirrors and rotating the gain medium (mounted on a five-dimensional stage). By doing this, the monitored P and S polarized output power were maximized and the incidence angle (air-Nd:YAG) was measured to be ~66°, which is close to the Brewster’s angle (~61.2°).
The laser cavity was formed by two dichroic flat mirrors, the input mirror M1 (AR at 808 nm, HR at 1064 nm) and the output coupler mirror M2 (HR at 808 nm). Three kinds of output couplers (with transmission T = 1.3%, 3%, and 8.5% at 1064 nm) were adopted. The total cavity length is 45 mm. During laser oscillation, the P polarized laser beam transmits the gain medium near the Brewster angle so negligible reflection loss (~0.6%) occurs. On the contrary, the S polarized laser beam will experience ~34.8% reflection when it impinges on the interface between the Nd:YAG and air. This ~34.8% reflection acts as a reflection-type output coupler for the S polarized laser beam. Therefore, this laser system consists of two types of laser cavities, one for the P polarized laser beam and the other for the S polarized laser beam.
Experimental laser output powers versus absorbed pump power for both the P and S polarized laser beams with three kinds of output couplers are depicted in Fig. 2. The absorbed pump power was obtained by the following procedure. With no cavity mirrors included, we first measured the incident pump power, the reflected pump power by the air-Nd:YAG interface, and the residual pump power crossing the whole gain element at different power levels. Then, by subtracting the reflected power and the residual power from the incident pump, we got the absorbed pump power. Decreasing the output coupler’s transmission, the laser pump threshold drops off accordingly, with the lowest one being ~2.0 W obtained with the T = 1.3% coupler (the T = 8.5% coupler provides a threshold of ~2.9 W). This corresponds to a pump power density of ~0.71 kW/cm2. Such a laser threshold is comparatively lower than that of [10], confirming that such composite multiple-plates gain elements have acceptable intracavity loss (which is very important for further power scaling). With the T = 3% coupler, the maximum output power for the S polarized laser is 5.34 W (summed up the reflection output from both the upper crystal and the lower crystal), corresponding to a slope efficiency of 21.1% with respect to absorbed pump power of 28 W; the maximum output power for the P polarized laser beam is 0.83 W with a slope efficiency of 3.6%. Increasing the output coupler’s transmission leads to steady increasing of the P polarized laser power while decreasing the S polarized laser. When the output coupler transmission is increased from 1.3% to 8.5%, the proportion of the P polarized power in total output power rises from <1% (0.05 W) to 36.4% (2.15 W). With the T = 1.3% output coupler, most of the output is concentrated into the S polarized part (>99%), and the total output power is 6.75W corresponding to optical-to-optical slope efficiency of 25.7% (the highest one in our current laser system). The comparatively lower slope efficiency compared to that one-SiC-wafer based laser [12] can be attributed to the increased bonding-material-interfacial reflection and scattering loss, because in our current structure more interfaces are included (also more SiC absorption loss). Another reason is that the incidence angles on the air-Nd:YAG and SiC-Nd:YAG interfaces are not completely the Brewster type, which can be addressed by further optimizing the cavity design. Linear dependence of the output power on absorbed pump power indicates that further power scaling is only limited by the available pump power.
To characterize the temperature distribution across the crystal under laser operation, we measured the outside-most Nd:YAG’s temperatures (with the T = 3% coupler) at three different points (the central pumped region, the outmost edge region and the intermediate region between them) with a IR thermometer. At the maximum output level, the temperature of the central (intermediate) portion was 26.8°C (25.3°C) and that of the outmost edge region was 24°C (4 degree higher than that of the circulating cooling water). Small temperature gradient across the crystal (2.8°C) leads to negligible thermal distortion and thermal induced stress. This feature guarantees high beam quality when this structure is used for further power scaling.
Another feature of our composite bonded SSL is that it can produce two perpendicularly polarized laser beams (P and S polarized) simultaneously, with P light output transmitted from M2 while the S light output reflected from the outside crystals’ surfaces (see Fig. 1). The degree of polarization of the P and S laser (with the T = 3% coupler at maximum output power) was measured with the Glan-Taylor prism. Through rotating the prism, the measured transmitted power (through the prism) of these two laser beams (P output and S output, refer to Fig. 1) is shown in Fig. 3. It is clear both polarized laser beams (either P or S) show high degree of linear polarization. The P and S laser beams give polarization extinction ratios of 20.6 and 17.4 dB, respectively. Therefore, this laser system design gives some clues as how to achieve variable polarized laser beams through laser cavity engineering by using of Brewster angles.
For characterizing the output laser beam quality, we measured the beam profile of the S polarized beam with a CCD camera (BeamOn-HR, Duma Optronics Co.). To get the beam quality, the beam radius was recorded at different distances (z) away from the beam focused point (the beam waist) when the output power was kept at 5 W (with the T = 3% coupler), and the results are shown in Fig. 4. Measurement gives a beam radius of 168 and 160 μm for the x and y directions, respectively. The beam quality factor M2 was then fitted, giving values of 1.22 and 1.24 for the x and y direction laser beams, respectively. This confirms that this laser system is operated in the fundamental mode state. Measurement of the P polarized laser beam showed similar beam qualities.
Laser spectra recorded with a spectrometer (Ocean optics USB4000) for the S polarized and P polarized laser beams (under their respective maximum output levels with the T = 3% coupler) are shown in Fig. 5. It is clear that the spectrum of these two different polarized laser beams possesses slight spectral shift, with the P polarized beam (1064.3 nm) red-shifted about 0.6 nm compared to the S polarized beam (1063.7 nm). Their spectral widths show a similar value of 0.7 nm. The spectral features of these two polarized beams under other power levels were also measured and showed the same difference. This kind of wavelength shift for different polarized laser beams are mainly due to gain discrimination for different polarized laser oscillations.
3. Conclusions
We have reported a Nd:YAG laser based on sandwich-bonded multiple-crystal-slabs and SiC-wafers, where high thermally conductive SiC wafers act as separated/distributed heat spreaders. The composite gain element is such designed that the laser beams impinge on the Nd:YAG outside surfaces at near Brewster angles, leading to no optical coatings are required. By making use of polarization discrimination, two perpendicularly polarized (P and S polarized) laser output beams are simultaneously generated from the same laser cavity. For the S polarized laser beam, maximum output power of 6.7 W is produced with a slope efficiency of 25.5% (with the T = 1.3% coupler); for the P polarized laser beam, maximum output power of 2.15 W is produced with a slope efficiency of 8.2% (with the T = 8.5% coupler). Changing the output coupler’s transmission gives rise to change of the power ratio between the S and P polarized laser beams. At the 5-W level, the measured beam qualities for the S polarized beam in x and y directions are M2 = 1.22 and 1.24, respectively. Owing to gain discrimination, these two different polarized laser beams also show a wavelength shift of 0.6 nm around 1064 nm.
Output power of this multi-gain-slab composite bonded laser system is currently limited by the available pump power, thus higher-power (e.g., >100 W) linearly polarized output with high beam quality is anticipated. Further thinning (to micron-meter levels) and increasing the number of the gain element and the high-conductivity wafer (A periodic alternately bonded gain-disk/heat-removal-plate/…/gain-disk element) will lead to a highly integrated and high-conductive gain structure, and thereby open a new way for developing next generation of high power SSLs with ideal beam qualities.
Funding
National Natural Science Foundation of China (Grant Nos. 61275136, 61675129, 61405101 61138006, and 11121504).
References and links
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