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We report on a high power Nd:YAG spinning disk laser. The eight cm diameter disk generated 200 W CW output with 323 W of absorbed pump in a near diffraction-limited beam. The power conversion efficiency was 64%. The pulsed result, 5 ms pulses at 10 Hz PRF, was nearly identical to the CW result indicating good thermal management. Rotated at 1200-1800 RPM with He impingement cooling the disk temperature increased by only 17 °C reaching a maximum temperature of ~31 °C. The thermal dissipation per unit of output power was 0.61 watt of heat generated per watt of laser output, which is below the typical range of 0.8-1.1 for 808 nm diode pumped Nd:YAG lasers.
© 2016 Optical Society of America
1. Introduction
Thermal management in solid-state lasers (SSLs) remains a continuing challenge in the development of high-energy laser systems. Thermal loading limits both the extractable power and the brightness of the laser. Various solid-state laser designs including thin disks, large mode-area fibers, cryogenically cooled gain media, zig-zag and end-pumped slabs, planar waveguides, master-oscillator power amplifiers, and heat capacity approaches represent some of the most advanced methods of thermal management; all of these methods are designed to mitigate thermal effects and enhance brightness [1–5].
Spinning disk lasers (SDLs) or other moving gain media implementations are a relatively simple method of thermal control in SSLs [6–9]. In a SDL, the mechanical rotation of the disk allows for the efficient removal of waste heat, thereby reducing the steady-state thermal load in the extraction region. In principle, very high output powers can be obtained prior to reaching the thermal limits of the gain material, while minimizing thermal aberrations and maintaining good beam quality. Spinning large, precision fabricated, crystalline or ceramic gain disks are likely to provide the most thermally and mechanical stable motion when compared to other moving media approaches such as back and forth or multi-axis motion.
SDLs, also called rotating disk lasers, have been demonstrated in Nd:YAG, Nd:glass Yb:YAG, and CrZnSe. For a Nd:YAG SDL the highest reported power was 30.8 W with an optical efficiency of 32.4% [8] For an Yb:YAG SDL the highest average power reported was 142 W [6]. The Yb:YAG SDL was diode pumed and Q-switched at 25-100 kHz with an AOM ; the optical efficiency of the laser was 39%. Recently, Mirov et. al. have reported a novel design for a Cr:ZnSe spinning disk laser in which the pump radiation is doubly passed through the gain disk on opposing sides [9]. Continuous wave output was demonstrated by pumping at 1.9 μm using a 100 W Th fiber laser. Maximum output powers of 57 W at 2.45 μm and 20 W at 2.94 μm with optical-to-optical efficiencies of 57% and 20%, respectively, were reported. A small power rollover was observed at higher pump powers for both wavelengths but this was not attributed to a heating of the gain media but to self-focusing of the pump beam in the atmosphere.
2. High power Nd:YAG spinning disk laser
We present work on a diode pumped Nd:YAG spinning disk laser. An eight cm diameter Nd:YAG disk with a gain length/thickness of 4 mm was end pumped with the output of a laser diode array (LDA) at 808 nm. The output of the LDA was fiber coupled to a 400-µm core silica fiber and focused to a ~2.2 mm pump spot on the disk. The separation between the pump mirror and disk was ~6 cm. The pump radiation was singly passed through the gain medium. To optimize power and beam-quality a 3-m radius of curvature concave high reflector was used with a cavity length of 66 cm. The output coupler was a flat 90% reflector at 1064 nm. The disk was rotated at 1200 to 1800 RPM on a hydrostatic air bearing and was held in a cooled (T ~14 °C) aluminum housing. Two centimeters of the disk, measured vertically, are above the cooled housing. Helium impingement is used to actively cool the disk as it spins between the Al plates. The gap between the disk and the plates is kept small (15-25 μm) to facilitate heat removal [10]. The disk was a single crystal manufactured by VLOC and doped with ~1.1% Nd. The surfaces were broad band AR coated at 808 nm. Power meters were set up to record the Nd:YAG laser power, the reflected and transmitted pump power; these power measurements facilitated an accurate determination of the laser efficiency, the heat generated in the gain medium, and the fraction of heat generated per watt of laser output power.
3. Results
3.1 Rotating vs. Non-rotating SDL
We first compared the power and thermal performance of a stationary disk vs. a rotating disk. In both cases, the cooling and the He flow rate were identical. Figure 1(a) shows the laser power as a function of absorbed power. The stationary disk has some heat capacity but the power quickly rolls over due to the limited heat capacity and poor heat removal. The beam from the stationary disk also displayed significant thermally induced aberrations. When the disk is rotated at 1200 RPM there is no power roll-over and a smooth climb in power is observed. Figure 1(b) plots the disk surface temperature, monitored by a thermal camera, as a function of absorbed power. The stationary disk temperature rises rapidly and at 18 W of absorbed power, it is near 100 °C. In marked contrast, the temperature of 1200 RPM disk stays below 20 °C up to 110 W of absorbed pump. Clearly, the rotating gain element displays greatly enhanced thermal control.
Fig. 1 a. Comparison of laser output power vs. absorbed power for a stationary vs. rotated disk. 1b. Comparison of disk surface temperature for stationary vs. rotated disk.
3.2 CW and pulsed laser power
The CW laser power was measured up to the maximum power delivered by the LDA (~390 W). The laser displayed a low threshold value of ~390 W cm−2. The SDL produced a maximum of 200 W CW in a TEM00 mode, which was limited by the available pump power. The same disk laser in pulsed operation (5 ms pulses at 10 Hz) produced 184.3 W of peak power; the pulsed power result is lower due to the limitations of the pulsed power supply. The optical to optical efficiencies for the pulsed and CW cases were 63.3 and 63.6%, respectively, indicative of good thermal management. Figure 2(a) shows the CW and pulsed laser power results. Figure 2(b) plots the fraction of the pump power absorbed versus the launched pump power. The fraction of the pump absorbed increases steadily to a maximum of 87% as the pump diodes chirp into the Nd absorbance band. At the maximum absorbance the pump centroid is near 807.88 nm. The maximum pump absorbance could be increased to ~95% by doubly passing the pump radiation through the disk.
Fig. 2 a. Nd:YAG laser power for CW and pulsed operation. 2b. Fraction of pump power absorbed vs. launched pump power. The inset shows the LDA emission spectrum at the highest pump power.
3.3 Beam quality
To measure the M2 values a 90:10 beam splitter was used and the 10% beam line passed through a 20 cm focal length positive lens to a 99:1 beam splitter and finally imaged on a high resolution CCD. The beam metrics including the 1/e2 width were recorded as a function of distance, Z, from the lens and then fit to the expression:
where ω0, M2, and ZR are the beam waist, the M squared value and the Rayleigh range, respectively. Figure 3(a) shows a plot of the 1/e2 beam width vs. Z at 156 W of output power, which is ~25 times threshold. The M2 was measured at three different laser power levels of 16, 100 and 156 W. There was no significant dependence of the M2 value with pump power. The M2 value for the vertical and horizontal axes were 1.3 and 1.2, respectively.Fig. 3 A plot of Z vs 1/e2 beam waist for a laser output power of 156 W. The inset figure shows the beam profile. The plot asymmetry is due to the minimum lens to CCD array distance that is achievable.
Some dependence of the M2 value on disk rotation rate was observed with lower rotation rates yielding higher M2 values. The disk was rotated on an air bearing and smoother motion with less laser jitter is evident at the higher rotation rates. In addition, the tangential component of the thermal gradient in the extraction region is more pronounced at lower rotation rates due to the longer dwell time in the pumped zone. Rotation rates less than ~600 RPM had higher M2 values. The data in Fig. 3 was collected at a rotation rate of 1500 RPM.
3.4 Thermal properties
An IR thermal camera monitored the pump spot temperature. Figure 4 shows two images from the thermal camera. At higher pump powers a thermal ring appears at the location of the pump spot as the heat is averaged around the disk circumference. To maximize heat removal the gap between the disk spinner housing and the disk was kept as small as possible with a spacing of 15-25 μm. In addition, the He flow rate was set slightly above 1 LPM for improved heat removal. At no-load the disk temperature is only a few degrees warmer than the chiller water. The temperature rises linearly with the absorbed power to a maximum of ~31 °C for 321 W of pump radiation absorbed. A linear fit to the CW data yielded: T (°C) = 0.0528 × Pabs + 14.2. The temperature rise per absorbed watt is only 0.053 degrees indicating good thermal management. In earlier work on a Nd:YAG SDL a value of 0.138 °C/watt was reported [7]. However, in that experiment the distance from the cooled aluminum housing to the disk was ~50 µm, considerably larger than in the present case.
Fig. 4 Steady state thermal image of the disk at different pump powers (L): 112 W absorbed, Plaser = 62.4 W; (R): 269 W absorbed, Plaser = 156 W. The disk was rotated at 1200 RPM. The He flow rate was 1.05 liter per minute.
A good metric for the thermal performance of a solid state laser is the thermal dissipation per unit of output power [3]. The quantum-limited minimum value,, is given by:
where is the fraction of power that goes into quantum defect heating: . For Nd:YAG at 300 K and pumped at 808 nm = 0.31. Figure 5(a) plots the experimentally determined χ value vs. laser power. Near threshold, χ is large as most of the pump light is converted to heat. As the laser is pumped to many times above threshold the χ asymptotes to a minimum of 0.61 watt of heat generated per watt of laser output. In comparison, the typical range for χ for a diode pumped Nd:YAG laser is 0.8-1.1 [4] and indicates excellent thermal performance for the spinning disk laser. Another metric for thermal performance is the temp rise of the gain medium per watt of heat load as displayed in Fig. 5(b). For the SDL this value was 0.16 °C/W.Fig. 5 a. χ value versus laser power. The theoretical lower limit is indicated by the dashed line. 5b. Disk surface temperature versus heat load in watts.
4. Summary and Conclusions
We presented results on an eight cm diameter Nd:YAG spinning disk laser. A 200 W CW output was measured and was limited by the available pump power. The CW and pulsed results were nearly identical with no observed power degradation. The laser’s thermal dissipation per unit of output power, χ, was 0.61 which is lower than the typical range of 0.8-1.1 for diode pumped Nd:YAG lasers. The beam was TEM00 and was near diffraction limited up to 156 W of output power.
To our knowledge the recorded maximum CW output power of 200 W is the highest reported to date for a Nd:YAG spinning disk laser. For these experiments, the pump radiation was singly passed through the disk with ~87% of the pump light absorbed. We estimate that by employing a double pass pump configuration the maximum CW output would have increased to ~226 W. Further, with aggressive heat removal powers near 500 W CW should be possible with ~730 W pumping for this relatively small form-factor disk.
The spinning disk gain media displays improved thermal performance compared to other solid-state laser gain geometries. The potential of the technology coupled with advances in ceramic materials and cryogenics has not been fully exploited. CW powers in the kW region are certainly possible for materials emitting near 1 μm wavelength. Recent work with SDLs employing mid-IR emitting materials also has demonstrated the utility and high efficiencies obtainable by this method [9].
Acknowledgments
The authors would like to thank Dr. Harold C. Miller for initial work on the experimental laser device and for useful discussion on spinning disk laser technology.
References and links
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