OSA's Digital Library

Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 18, Iss. 16 — Aug. 2, 2010
  • pp: 17066–17074

Optics InfoBase > Optics Express > Volume 18 > Issue 16 > Page 17066

« Show journal navigation

Composite Yb:YAG/SiC-prism thin disk laser

G. A. Newburgh, A. Michael, and M. Dubinskii  »View Author Affiliations


Optics Express, Vol. 18, Issue 16, pp. 17066-17074 (2010)
http://dx.doi.org/10.1364/OE.18.017066


View Full Text Article

Acrobat PDF (1014 KB)


Advanced Search

Browse Journals / Lookup Meetings

Browse by Journal and Year


   


Lookup Conference Papers

Close Browse Journals / Lookup Meetings

Article Tools

Share
Citations

Abstract

We report the first demonstration of a Yb:YAG thin disk laser wherein the gain medium is intracavity face-cooled through bonding to an optical quality SiC prism. Due to the particular design of the composite bonded Yb:YAG/SiC-prism gain element, the laser beam impinges on all refractive index interfaces inside the laser cavity at Brewster’s angles. The laser beam undergoes total internal reflection (TIR) at the bottom of the Yb(10%):YAG thin disk layer in a V-bounce cavity configuration. Through the use of TIR and Brewster’s angles, no optical coatings, either anti-reflective (AR) or highly reflective (HR), are required inside the laser cavity. In this first demonstration, the 936.5-nm diode pumped laser performed with ~38% slope efficiency at 12 W of quasi-CW (Q-CW) output power at 1030 nm with a beam quality measured at M2 = 1.5. This demonstration opens up a viable path toward novel thin disk laser designs with efficient double-sided room-temperature heatsinking via materials with the thermal conductivity of copper on both sides of the disk.

© 2010 OSA

1. Introduction

Average power scaling of solid-state lasers with preserved high beam quality has been one of the most important technical activities during 50 years of laser development. As noted early in the development of solid state lasers [1

S. D. Sims, A. Stein, and C. Roth, “Rods pumped by flash lamps,” Appl. Opt. 6(3), 579–580 (1967). [CrossRef] [PubMed]

], when lasers based on laser rods were the only known design option, optical pumping of the externally cooled gain medium leads to thermal gradients which results in degradation of the beam quality through thermal lensing and stress induced birefringence. The conservation of good beam quality while power scaling remained a major challenge until the importance of laser architectures with significantly reduced gain volume-to-surface-area ratio compared to rod-based architecture was recognized. A solution to the birefringence problem was proposed in the form of the slab laser [2

J. Eichler, N. Hodgson, and H. Weber, “Output power and efficiencies of slab laser systems,” J. Appl. Phys. 66(10), 4608–4613 (1989). [CrossRef]

]. Calculations indicated that the slab planar geometry would allow for one-dimensional heat flow within the gain medium thereby reducing the thermally induced optical disturbances. Experiments confirmed this expectation through the demonstration of multi-hundred Watt laser output with good beam quality [3

T. S. Rutherford, W. M. Tulloch, S. Sinha, and R. L. Byer, “Yb:YAG and Nd:YAG edge-pumped slab lasers,” Opt. Lett. 26(13), 986–988 (2001). [CrossRef]

]. About 15 years ago, thin disk laser technology emerged as a response to the need for even more efficient cooling while maintaining the beam quality of heavily pumped solid-state gain medium [4

A. Giesen, H. Hügel, A. Voss, K. Wittig, U. Brauch, and H. Opower, “Scalable concept for diode-pumped high-power solid-state lasers,” Appl. Phys. B 58, 365–372 (1994).

]. The thin disk concept extended the idea of decreasing the gain volume-to-surface-area ratio to an extreme, essentially reducing the gain medium dimensions to a ‘thick film’. The steady trend towards decreasing the thickness of the gain medium through which heat must flow reflects the realization that the gain medium itself, due to its low thermal conductivity (compared to that of heatsinking materials), is the main bottleneck to power scaling. Since its inception, the thin disk laser architecture has proven to be very practical and has now matured to the point where the thin disk laser may operate either in a TEM00 mode at a multi-hundred Watt level or with an M2 value of ~20 at ~10 kW with many industrial applications [5

A. Giesen and J. Speiser, “Fifteen years of work on thin disk lasers: results and scaling laws,” IEEE J. Sel. Top. Quantum Electron. 13(3), 598–609 (2007). [CrossRef]

].

In its original concept, the thin disk laser was cooled from one side of the disk only - through its bonded contact with a copper heat sink, while leaving the other side open for pumping and lasing [4

A. Giesen, H. Hügel, A. Voss, K. Wittig, U. Brauch, and H. Opower, “Scalable concept for diode-pumped high-power solid-state lasers,” Appl. Phys. B 58, 365–372 (1994).

]. Since then numerous allusions of potential benefits of the double-sided thin disk cooling have emerged (e.g., [6

V. Hasson, and H.-P. Chou, “Cooling of High Power Laser Systems,” US Patent 6,667,999 (2003).

,7

Y. Tzuk, A. Tal, S. Goldring, Y. Glick, E. Lebiush, G. Kaufman, and R. Lavi, “Diamond cooling of high power diode pumped solid state lasers,” in Solid State Lasers XII, R. Scheps, ed., Proc. SPIE 4968, 106–114 (2003).

]). Expectations are that the double-sided cooling of the active layer should lead to more than doubled cooling efficiency thereby providing the architecture with much improved power scaling potential [8

C. Ding-Xiang, Y. Hai-Wu, Z. Wan-Guo, H. Shao-Bo, and W. Xiao-Feng, “Temperature-related performance of Yb3+:YAG disc lasers and optimum design for diamond cooling,” Chin. Phys. 15(12), 2963– (2006). [CrossRef]

–10

G. A. Newburgh, M. Dubinskii, and L. D. Merkle, “Silicon carbide face-cooled 4% ceramic Nd:YAG laser,” Electron. Lett. 43(5), 286–288 (2007). [CrossRef]

]. So far, truly double-sided cooling of the Yb:YAG thin disk gain medium inside the laser cavity has been realized only at cryogenic temperatures - by using sapphire as the heatsinking transparent material [11

S. Tokita, J. Kawanaka, M. Fujita, T. Kawashima, and Y. Izawa, “Sapphire-conductive end-cooling of high power cryogenic Yb: YAG lasers,” Appl. Phys. B 80(6), 635–638 (2005). [CrossRef]

]. The authors of [11

S. Tokita, J. Kawanaka, M. Fujita, T. Kawashima, and Y. Izawa, “Sapphire-conductive end-cooling of high power cryogenic Yb: YAG lasers,” Appl. Phys. B 80(6), 635–638 (2005). [CrossRef]

] have taken advantage of the fact that the thermal conductivity of sapphire at 77K approaches that of diamond, while the index of refraction does not differ much from that of YAG so that intracavity Fresnel loss is minimal [11

S. Tokita, J. Kawanaka, M. Fujita, T. Kawashima, and Y. Izawa, “Sapphire-conductive end-cooling of high power cryogenic Yb: YAG lasers,” Appl. Phys. B 80(6), 635–638 (2005). [CrossRef]

]. Significantly more practical (at least for industrial applications) room-temperature laser power scaling with double-sided intracavity thin disk cooling at room temperature has lately undergone a thorough theoretical treatment [8

C. Ding-Xiang, Y. Hai-Wu, Z. Wan-Guo, H. Shao-Bo, and W. Xiao-Feng, “Temperature-related performance of Yb3+:YAG disc lasers and optimum design for diamond cooling,” Chin. Phys. 15(12), 2963– (2006). [CrossRef]

]. Practical implementation is hindered by the fact that highly thermo-conductive optical quality materials (e.g., SiC or diamond) have very high index of refraction, resulting in a high Fresnel loss at the ‘heatsinking material/gain material’ interfaces. This loss is at least an order of magnitude higher than that of sapphire/YAG interface [11

S. Tokita, J. Kawanaka, M. Fujita, T. Kawashima, and Y. Izawa, “Sapphire-conductive end-cooling of high power cryogenic Yb: YAG lasers,” Appl. Phys. B 80(6), 635–638 (2005). [CrossRef]

]. Meanwhile, bonding AR coated surfaces has proven to be impractical due to the issue of thermal resistivity and significant reduction in bonding strength. So far no practical solution has been offered for double-sided thin disk cooling at room temperature.

This paper presents experimental results obtained with a novel thin disk laser design specifically intended for efficient double-sided room-temperature heatsinking of the active thin disk via materials with the thermal conductivity of copper on both sides. The key to this design is a composite gain element fabricated from a 400 µm thick Yb(10%):YAG bonded to optical quality SiC prism. The composite Yb:YAG/SiC-prism gain element is designed in such a way that absolutely no optical coatings, either anti-reflective (AR) or highly-reflective (HR), are required at any interface inside the laser cavity. Under diode-pumped laser operation, 12 W of quasi-CW (Q-CW) output power with a beam quality of M2 = 1.5 and with ~38% slope efficiency were demonstrated in this first proof-of-concept experiment. We believe that this first demonstration of the thin disk laser based on a composite intracavity Yb:YAG/SiC-prism gain element enables a viable pathway towards novel thin disk laser designs with efficient double-sided cooling.

2. Laser design

Silicon carbide (SiC) is an advanced quality, wide-bandgap (4H-polytype SiC: 3.26 eV), highly thermo-conductive transparent material [12

http://www.cree.com/products/

]. SiC’s thermal conductivity is about the same as that of copper [13

V. A. Dmitriev, and M. G. Spencer, “SiC fabrication technology: growth and doping,” in SiC Materials and Devices , Yoon-Soo Park, ed., (Academic Press, London, 1998), pp. 21–63.

], which is four times lower than that of diamond [14

E. V. Ivakin, A. V. Sukhadolau, V. G. Ralchenko, and A. V. Vlasov, “Laser-induced transient gratings application for measurement of thermal conductivity of CVD diamond,” Proc. SPIE 5121, 253–258 (2003). [CrossRef]

], but still ~50 times higher than that of YAG. Although the thermal conductivity of SiC is much lower than that of diamond, the overall thermal resistance of a stack constructed of very high and low thermal conductivity materials is primarily determined by the low conductivity material. Therefore, SiC performs much closer to diamond in heatsinking applications than the simple ratio of their thermal conductivities would indicate [10

G. A. Newburgh, M. Dubinskii, and L. D. Merkle, “Silicon carbide face-cooled 4% ceramic Nd:YAG laser,” Electron. Lett. 43(5), 286–288 (2007). [CrossRef]

]. This has been shown with highly transparent SiC implemented as efficient heatspreader when optically contacted to the surface of the gain medium, either doped dielectric [10

G. A. Newburgh, M. Dubinskii, and L. D. Merkle, “Silicon carbide face-cooled 4% ceramic Nd:YAG laser,” Electron. Lett. 43(5), 286–288 (2007). [CrossRef]

] or semiconductor [15

J. E. Hastie, J. M. Hopkins, S. Calvez, C. W. Jeon, D. Burns, R. Abram, E. Riis, A. I. Ferguson, and M. D. Dawson, “0.5-W single transverse-mode operation of an 850-nm diode-pumped surface-emitting semiconductor laser,” IEEE Photon. Technol. Lett. 15(7), 894–896 (2003). [CrossRef]

] laser material.

The application of a flat SiC window to cool one side of the thin disk medium while bonding the opposite side of the gain medium to a copper heat sink could be a fairly straightforward solution to efficient double-sided cooling, but in this design the thin disk pumping and lasing beams would impinge on both the SiC/air and the SiC/YAG interfaces. Considering the high index of refraction of SiC (n0 = 2.59 at 1 μm for 4H polytype SiC - as extrapolated from data source [16

P. T. B. Shaffer, “Refractive index, dispersion, and birefringence of silicon carbide polytypes,” Appl. Opt. 10(5), 1034–1036 (1971). [CrossRef] [PubMed]

]), the SiC/YAG interface would result in a single-pass loss figure of 3.3%. While it is possible (and easy) to AR coat the significantly more reflective (~20%) SiC/air interface, the AR coating on the SiC/YAG interface presents a problem due to the issue of thermal resistivity and significant reduction in bonding strength. Our previous experience [10

G. A. Newburgh, M. Dubinskii, and L. D. Merkle, “Silicon carbide face-cooled 4% ceramic Nd:YAG laser,” Electron. Lett. 43(5), 286–288 (2007). [CrossRef]

] led us to conclude that the inclusion of AR optical coatings between the SiC and YAG during bonding is impractical due to insufficient bonding strength in this case.

In order to resolve the issue of reflection at the boundary between dissimilar optical materials, we conceived an optical design using an optical quality transparent SiC heatsinking component bonded to a thin disk of Yb:YAG, which requires absolutely no optical coatings, either AR or HR, inside the laser cavity. The solution is based on a particular design of the composite bonded thin-disk-Yb:YAG/SiC-prism gain element, wherein the laser beam impinges upon all surfaces inside the laser cavity at a Brewster angle, while forming the V-cavity vertex inside the thin disk at a total internal reflection angle (see Fig. 1 ). In this case, the SiC section of the composite gain element is used as a heat sink. The new design was, in some ways, inspired by the grazing incidence slab design [17

A. J. Alcock and J. E. Bernard, “Diode-pumped grazing incidence slab lasers,” IEEE J. Sel. Top. Quantum Electron. 3(1), 3–8 (1997). [CrossRef]

] (further developed into what’s now known as a ‘bounce laser’ [18

A. Minassian and M. Damzen, “20 W bounce geometry diode-pumped Nd:YVO4 laser system at 1342 nm,” Opt. Commun. 230(1-3), 191–195 (2004). [CrossRef]

]) as a remote analog. The obvious difference of what we propose from the [17

A. J. Alcock and J. E. Bernard, “Diode-pumped grazing incidence slab lasers,” IEEE J. Sel. Top. Quantum Electron. 3(1), 3–8 (1997). [CrossRef]

,18

A. Minassian and M. Damzen, “20 W bounce geometry diode-pumped Nd:YVO4 laser system at 1342 nm,” Opt. Commun. 230(1-3), 191–195 (2004). [CrossRef]

] is that we: (i) construct a gain element as a composite of dissimilar materials rather than a monolithic slab, (ii) design for V-cavity longitudinal pumping, such that both the optical pump path and the gain path in the thin disk gain medium are significantly increased.

Fig. 1 Schematic design of the composite thin-disk-Yb:YAG/SiC-prism gain element. Cross section is not to scale, thin disk layer is shown much thicker in order to better illustrate the angles at which the beam is traversing all relevant surfaces. Red line indicates the propagation path of the intracavity laser beam inside the prism. Side view is shown as seen from the bottom of the prism.

A sketch of the composite thin-disk-Yb:YAG/SiC-prism gain element is shown in Fig. 1. The thickness of the Yb:YAG layer in Fig. 1 is exaggerated for clarity. The isosceles prism was fabricated from a 3.3 mm thick, 4H (hexagonal polytype) SiC, so that the base length is 40 mm, base to apex distance is 8.3 mm. The optical axis of the hexagonal SiC in the prism is oriented to be perpendicular to the plane of the drawing. Thus the Yb:YAG laser beam propagates inside the prism in the a-plane, which helps to avoid birefringence issues. The prism was bonded to a 400 µm thick 10% Yb:YAG wafer. The composite prism was designed so that the P-polarized light would impinge all interfaces at a Brewster angle. In order to meet this requirement the prism roof angle was cut at 13.7 degrees based on the refractive index of no = 2.6 for SiC [16

P. T. B. Shaffer, “Refractive index, dispersion, and birefringence of silicon carbide polytypes,” Appl. Opt. 10(5), 1034–1036 (1971). [CrossRef] [PubMed]

] and 1.82 for YAG at ~1 μm wavelength. This allows light propagating at 8 degrees to the prism base, as shown in Fig. 1, to impinge at the Air/SiC interface at the angle of incidence (AOI) 68°, the SiC/YAG interface at the AOI of 34° and to enter the YAG material at the AOI of 55°. The large AOI of 55° ensured total-internal reflectance at the air/YAG interface (i.e., no HR coatings required for a bounce) as well as provided for increased effective beam propagation path inside the Yb:YAG layer (about 700 µm versus 400 μm at normal incidence).

In our proof-of concept experiments the thin disk was only cooled from one side through the large surfaces of the SiC prism. The other side, which is eventually intended for bonding to a copper heat sink, was kept open - in order for us to be able to measure the temperature profile inside the gain medium by the thermal camera (see Section 3).

The Yb:YAG/SiC-prism face-cooled thin disk laser was tested in a setup shown in Fig. 2 . As seen from Fig. 2, the SiC-cooled gain element was placed in a V-cavity and lased in a stable cavity configuration. The laser cavity was formed by a 1 m ROC dichroic pump mirror M1 (AR at 940 nm, HR at 1030 nm) and a flat outcoupler (M2) placed 80 mm apart. For efficiency optimization we used several outcouplers with reflectance values ranging from 70% to 98%.

Fig. 2 Optical layout of the face cooled Yb:YAG/SiC-prism thin disk laser. Shown on the inset is the 936.5 nm pump beam profile (dia. 800 µm in the focal plane of the lens L2 - in air)

The thin disk laser was pumped by a fiber coupled (400 µm, 0.22NA) laser diode source collimated and then focused by a 50 and 70 mm lens pair, L1 and L2. The pump spot was measured to be 800 µm at the focal plane of the lens L2 and was essentially flat-top in its intensity profile (Fig. 2, inset).

The room temperature coefficient of thermal expansion (CTE) of hexagonal 4H polytype SiC is found to be nearly isotropic (3*10−6 K−1 [19

M. Stockmeier, R. Müller, S. A. Sakwe, P. J. Wellmann, and A. Magerl, “On the lattice parameters of silicon carbide,” J. Appl. Phys. 105(3), 033511 (2009). [CrossRef]

], ), or very lightly anisotropic (3.2(c)-3.3(a)*10−6 K−1 [20

Z. Li and R. C. Bradt, “Thermal expansion of the hexagonal (4H) polytype of SiC,” J. Appl. Phys. 60(2), 612–614 (1986). [CrossRef]

], ), but is significantly mismatched to the CTE of YAG (~6*10−6 K−1 [21

R. L. Aggarwal, D. J. Ripin, J. R. Ochoa, and T. Y. Fan, “Measurement of thermo-optic properties of Y3Al5O12, Lu3Al5O12, YAlO3,LiYF4, LiLuF4, BaY2F8, KGd(WO4)2, and KY(WO4)2 laser crystals in the 80–300 K temperature range,” J. Appl. Phys . 98, 103514–1 - 103514–14 (2005). [CrossRef]

], ). Even though the adhesion-free bonding used for assembling the Yb:YAG/SiC-prism composite gain element is known to possess the so-called “stick-and-slip” flexibility [22

H. Lee, H. E. Meissner, and O. R. Meissner, “Stress Relief of Adhesive-Free-Bond (AFB®) Laser Crystal Composites at Elevated and Cryogenic Temperatures,” in 19th Solid State and Diode Laser Technology Review Technical Digest , 2006, Paper: Laser-5.

], it can be expected that in case of significant local thermal overload the two components of the composite gain element may delaminate. For that reason, we tried to limit the thermal load in this proof-of-concept experiment before further power scaling. In order to reduce thermal load, the thin disk laser was operated Q-CW with a 10% duty cycle (pulse repetition frequency (PRF) 100 Hz, pulse duration 1 msec).

3. Laser performance

Experimental laser output pulse energy versus absorbed pump pulse energy dependence for different output coupler reflectivities is presented in Fig. 3 . As seen from figure, the best efficiency was achieved with the 85% reflectance outcoupler. Calculation of the power absorbed by the of Yb(10%):YAG layer was based on an effective absorption length of 1.4 mm (single V-pass) and the absorption coefficient of α = 8.3 cm−1 at the pump wavelength of 936.5 nm. The radiation emitted from the pump fiber end is not polarized, therefore due to Fresnel losses for S-polarized component of the pump beam at all interfaces the fraction of pump reaching the Yb:YAG layer was calculated to be ~0.70 of total power incident onthe prism. The issue of ~30% pump power loss can be easily eliminated by merely using a polarized pump source.

Fig. 3 Q-CW laser performance (output pulse energy at 1030 nm versus absorbed pump pulse energy at 936.5 nm) as a function of the output coupler reflectivity for the face cooled Yb:YAG/SiC-prism thin disk laser. Output coupler reflectivities are color coded as listed on the inset.

Following the beam through the prism, we calculate that the longitudinal axis of the beam elongates in the gain medium by a factor of 1.74. This translates to an elliptical pump beam profile of 800 µm x 1400 µm as it traverses the Yb:YAG layer. Lasing threshold was achieved for the incident Q-CW pump power of 22-29 W depending on the outcoupler reflectance values, which corresponds to a laser threshold for incident pump densities of 3 - 4 kW/cm2 depending on the outcoupler reflectivity values.

Comparison of this rather high pump threshold density with a calculated value of 1.5 kW/cm2 [4

A. Giesen, H. Hügel, A. Voss, K. Wittig, U. Brauch, and H. Opower, “Scalable concept for diode-pumped high-power solid-state lasers,” Appl. Phys. B 58, 365–372 (1994).

] points to a high internal loss within the cavity. Based on laser modeling we estimate the single-pass loss as ~3% and attribute this loss to the optical quality of the available SiC material. The calculated loss figure is also consistent with the independently measured passive loss value of ~0.02 cm−1 (away from Yb3+:YAG absorption resonances) for SiC in combination with a path length of ~14 mm through the SiC section of the composite prism gain element.

We completed beam profiling of the laser output under Q-CW conditions (PRF 50 Hz, pulse duration 1 msec and ~80W pump) by imaging beam cross section onto a Spiricon camera in the vicinity of the focal point of the focusing lens (Fig. 4 ). For purposes of beam quality measurement, the beam profile was recorded at different distances (z) from the lens. Using the M2 method as outlined in [23

A. E. Siegman, G. Nemes, and J. Serna, “How to (Maybe) Measure Laser Beam Quality,” in Diode Pumped Solid State Lasers:Applications and Issues , M. W. Dowley, ed., OSA TOPS Vol. 17 (Optical Society of America, Washington, D.C., 1998), pp. 184–199.

], and calculating the second moment of the beam profile in X and Y directions, an M2 value of 1.5 both in X and Y was derived (Fig. 4). The polarization contrast of the output 1030-nm beam was measured to be at least ~1000:1.

Fig. 4 (a) Beam profile measurement: M2 = 1.5 and (b) a sample of the output beam profile as measured by a Spiricon camera

4. FEA analysis and surface temperature distribution measurements

Before attempting to lase the composite thin-disk-Yb:YAG/SiC-prism gain element, a calculation of temperature distribution on the surface of the thin disk was made using a 3-dimensional Finite Element Analysis (FEA) of the thermally loaded gain element. For this study thermal conductivities of SiC and Yb:YAG were set to 400 [13

V. A. Dmitriev, and M. G. Spencer, “SiC fabrication technology: growth and doping,” in SiC Materials and Devices , Yoon-Soo Park, ed., (Academic Press, London, 1998), pp. 21–63.

] and 6.5 W/m*K [24

A. Tunnermann, H. Zellmer, W. Schone, A. Giesen, and K. Contag, “New Concepts for Diode-Pumped Solid-State Lasers,” in High-Power Diode Lasers, Topics Appl. Phys. 78 , R. Diehl, ed., (Springer Verlag, Berlin-Heidelberg, 2000), pp. 369–408.

] respectively. The broad surfaces of the prism gain element were constrained to 300 K as the boundary conditions used for FEA calculation. The heat deposition within the gain element was calculated by allowing an elliptical pump beam of dimension 0.8 X 1.4 mm2 to propagate through the Yb:YAG layer at an angle of incidence of 55°, which absorbed according to Beer’s law using the value, α = 8.3 cm−1. A conversion of absorbed power to heat was calculated using the heating parameter, or fractional thermal loading, of Yb:YAG as η = 0.11 [25

T. Y. Fan, “Heat Generation in Nd:YAG and Yb:YAG,” IEEE J. Quantum Electron. 29(6), 1457–1459 (1993). [CrossRef]

].

It is evident from Fig. 5 that a centrosymmetric hot zone exists in the operating Yb:YAG thin disk, where intensity of the pump beam is strongest, demonstrating a maximum temperature excursion of ~10 °C. The temperature distribution measures approximately 1.2 mm by 2.3 mm in diameter as defined by its 1/e value from peak.

Fig. 5 FEA calculation of the surface temperature of a the 400 µm Yb(10%):YAG as pumped by 7.7 W average incident pump power and face cooled by SiC. The Yb:YAG surface shows a maximum temperature excursion of ΔT = 10 °C.

In order to validate the FEA calculation of the SiC-prism cooling approach under real lasing conditions, an EZtherm thermal camera (temperature resolution of 0.1 °C) was used to observe the actual Yb:YAG thin disk surface temperature under Q-CW pumping (10% duty cycle, 7.7 W average power incident upon the Yb:YAG layer). The measurement results of the temperature distribution on the surface of the Yb:YAG gain element bonded to a SiC-prism are shown in Fig. 6 . The spatial resolution of the thermal image was increased to 45 µm by combining the external ZnSe lens of 2.5 inch focal length with the camera’s own germanium (Ge) lens. Calibration of the camera readings was made by using an emissivity value for YAG of ε = 0.32. This emissivity value was experimentally determined in our separate experiment by thermally imaging a heated YAG disk over a temperature range of 30°C to 50°C. This emissivity agrees well with the value of ε = 0.3 quoted in [26

A. Golubovic, S. Nikolic, R. Gajic, S. Duric, and A. Valcic, “The growth of Nd:YAG single crystals,” J. Serb. Chem. Soc. 67(4), 291–300 (2002). [CrossRef]

]. Comparison of measured temperature distribution profile (Fig. 6) with simulation results depicted in Fig. 5 indicates good agreement both in the absolute temperature values and dimensions of the temperature distribution.

Fig. 6 Temperature distribution on the surface of the Yb:YAG gain element (7.7 W average pump power) [left], and the corresponding false-color scale [right].

We conclude that under lasing conditions, a face cooled 400 µm thick Yb(10%)YAG/SiC-prism thin disk laser will heat at the rate of ~7 °C for every kW/cm2 of incident pump power based on the 7.7 W average incident pump power, 800 µm diameter beam (5*10−3 cm2) used to pump the Yb:YAG layer. The FEA model predicts that the heating rate will remain constant for power scaling of the laser as long as the incident pump intensity remains constant. The laser can be power scaled by elongating the pump spot along the prism base while using an unstable cavity configuration for efficient power extraction.

5. Conclusions

We have demonstrated an Yb(10%):YAG thin disk laser based on the composite bonded Yb:YAG/SiC-prism gain element, in which cooling of the gain layer is accomplished through highly thermally conductive optical quality SiC heat sink. The intracavity composite bonded thin-disk-Yb:YAG/SiC-prism gain element is designed in such a way that the laser beam impinges on all refractive index interfaces inside the laser cavity at Brewster’s angles, while making a V-bounce at the bottom of the Yb(10%):YAG thin disk layer due to TIR. With this special design no optical coatings, either anti-reflective or highly reflective, are required inside the laser cavity. A laser slope efficiency of 38% was demonstrated at 12 W of quasi-CW output power at 1030 nm in this first experiment and a beam quality of M2 = 1.5 in both x and y directions was achieved. The measured temperature distribution on the surface of the Yb(10%):YAG thin disk under lasing conditions was found to be in a good agreement with the 3-dimensional FEA analysis of the thermally loaded gain element. We demonstrated that the idea of the composite intracavity Yb:YAG/SiC-prism gain element can be used as a pathway towards thin disk laser designs with efficient double-sided room-temperature heatsinking via materials with the thermal conductivity of copper on both sides of the disk. An FEA simulation anchored by our direct comparison with the experiment shows that the temperature extremes can be reduced by at least a factor of 2.5 by using this sort of a thin disk double-sided cooling.

References and links

1.

S. D. Sims, A. Stein, and C. Roth, “Rods pumped by flash lamps,” Appl. Opt. 6(3), 579–580 (1967). [CrossRef] [PubMed]

2.

J. Eichler, N. Hodgson, and H. Weber, “Output power and efficiencies of slab laser systems,” J. Appl. Phys. 66(10), 4608–4613 (1989). [CrossRef]

3.

T. S. Rutherford, W. M. Tulloch, S. Sinha, and R. L. Byer, “Yb:YAG and Nd:YAG edge-pumped slab lasers,” Opt. Lett. 26(13), 986–988 (2001). [CrossRef]

4.

A. Giesen, H. Hügel, A. Voss, K. Wittig, U. Brauch, and H. Opower, “Scalable concept for diode-pumped high-power solid-state lasers,” Appl. Phys. B 58, 365–372 (1994).

5.

A. Giesen and J. Speiser, “Fifteen years of work on thin disk lasers: results and scaling laws,” IEEE J. Sel. Top. Quantum Electron. 13(3), 598–609 (2007). [CrossRef]

6.

V. Hasson, and H.-P. Chou, “Cooling of High Power Laser Systems,” US Patent 6,667,999 (2003).

7.

Y. Tzuk, A. Tal, S. Goldring, Y. Glick, E. Lebiush, G. Kaufman, and R. Lavi, “Diamond cooling of high power diode pumped solid state lasers,” in Solid State Lasers XII, R. Scheps, ed., Proc. SPIE 4968, 106–114 (2003).

8.

C. Ding-Xiang, Y. Hai-Wu, Z. Wan-Guo, H. Shao-Bo, and W. Xiao-Feng, “Temperature-related performance of Yb3+:YAG disc lasers and optimum design for diamond cooling,” Chin. Phys. 15(12), 2963– (2006). [CrossRef]

9.

Y. Tzuk, A. Tal, S. Goldring, Y. Glick, E. Lebiush, G. Kaufman, and R. Lavi, “Diamond cooling of high-power diode-pumped solid state lasers,” IEEE J. Quantum Electron. 40(3), 262–269 (2004). [CrossRef]

10.

G. A. Newburgh, M. Dubinskii, and L. D. Merkle, “Silicon carbide face-cooled 4% ceramic Nd:YAG laser,” Electron. Lett. 43(5), 286–288 (2007). [CrossRef]

11.

S. Tokita, J. Kawanaka, M. Fujita, T. Kawashima, and Y. Izawa, “Sapphire-conductive end-cooling of high power cryogenic Yb: YAG lasers,” Appl. Phys. B 80(6), 635–638 (2005). [CrossRef]

12.

http://www.cree.com/products/

13.

V. A. Dmitriev, and M. G. Spencer, “SiC fabrication technology: growth and doping,” in SiC Materials and Devices , Yoon-Soo Park, ed., (Academic Press, London, 1998), pp. 21–63.

14.

E. V. Ivakin, A. V. Sukhadolau, V. G. Ralchenko, and A. V. Vlasov, “Laser-induced transient gratings application for measurement of thermal conductivity of CVD diamond,” Proc. SPIE 5121, 253–258 (2003). [CrossRef]

15.

J. E. Hastie, J. M. Hopkins, S. Calvez, C. W. Jeon, D. Burns, R. Abram, E. Riis, A. I. Ferguson, and M. D. Dawson, “0.5-W single transverse-mode operation of an 850-nm diode-pumped surface-emitting semiconductor laser,” IEEE Photon. Technol. Lett. 15(7), 894–896 (2003). [CrossRef]

16.

P. T. B. Shaffer, “Refractive index, dispersion, and birefringence of silicon carbide polytypes,” Appl. Opt. 10(5), 1034–1036 (1971). [CrossRef] [PubMed]

17.

A. J. Alcock and J. E. Bernard, “Diode-pumped grazing incidence slab lasers,” IEEE J. Sel. Top. Quantum Electron. 3(1), 3–8 (1997). [CrossRef]

18.

A. Minassian and M. Damzen, “20 W bounce geometry diode-pumped Nd:YVO4 laser system at 1342 nm,” Opt. Commun. 230(1-3), 191–195 (2004). [CrossRef]

19.

M. Stockmeier, R. Müller, S. A. Sakwe, P. J. Wellmann, and A. Magerl, “On the lattice parameters of silicon carbide,” J. Appl. Phys. 105(3), 033511 (2009). [CrossRef]

20.

Z. Li and R. C. Bradt, “Thermal expansion of the hexagonal (4H) polytype of SiC,” J. Appl. Phys. 60(2), 612–614 (1986). [CrossRef]

21.

R. L. Aggarwal, D. J. Ripin, J. R. Ochoa, and T. Y. Fan, “Measurement of thermo-optic properties of Y3Al5O12, Lu3Al5O12, YAlO3,LiYF4, LiLuF4, BaY2F8, KGd(WO4)2, and KY(WO4)2 laser crystals in the 80–300 K temperature range,” J. Appl. Phys . 98, 103514–1 - 103514–14 (2005). [CrossRef]

22.

H. Lee, H. E. Meissner, and O. R. Meissner, “Stress Relief of Adhesive-Free-Bond (AFB®) Laser Crystal Composites at Elevated and Cryogenic Temperatures,” in 19th Solid State and Diode Laser Technology Review Technical Digest , 2006, Paper: Laser-5.

23.

A. E. Siegman, G. Nemes, and J. Serna, “How to (Maybe) Measure Laser Beam Quality,” in Diode Pumped Solid State Lasers:Applications and Issues , M. W. Dowley, ed., OSA TOPS Vol. 17 (Optical Society of America, Washington, D.C., 1998), pp. 184–199.

24.

A. Tunnermann, H. Zellmer, W. Schone, A. Giesen, and K. Contag, “New Concepts for Diode-Pumped Solid-State Lasers,” in High-Power Diode Lasers, Topics Appl. Phys. 78 , R. Diehl, ed., (Springer Verlag, Berlin-Heidelberg, 2000), pp. 369–408.

25.

T. Y. Fan, “Heat Generation in Nd:YAG and Yb:YAG,” IEEE J. Quantum Electron. 29(6), 1457–1459 (1993). [CrossRef]

26.

A. Golubovic, S. Nikolic, R. Gajic, S. Duric, and A. Valcic, “The growth of Nd:YAG single crystals,” J. Serb. Chem. Soc. 67(4), 291–300 (2002). [CrossRef]

OCIS Codes
(140.6810) Lasers and laser optics : Thermal effects

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: June 29, 2010
Revised Manuscript: July 16, 2010
Manuscript Accepted: July 18, 2010
Published: July 27, 2010

Citation
G. A. Newburgh, A. Michael, and M. Dubinskii, "Composite Yb:YAG/SiC-prism thin disk laser," Opt. Express 18, 17066-17074 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-16-17066


Sort:  Author  |  Year  |  Journal  |  Reset  

References

  1. S. D. Sims, A. Stein, and C. Roth, “Rods pumped by flash lamps,” Appl. Opt. 6(3), 579–580 (1967). [CrossRef] [PubMed]
  2. J. Eichler, N. Hodgson, and H. Weber, “Output power and efficiencies of slab laser systems,” J. Appl. Phys. 66(10), 4608–4613 (1989). [CrossRef]
  3. T. S. Rutherford, W. M. Tulloch, S. Sinha, and R. L. Byer, “Yb:YAG and Nd:YAG edge-pumped slab lasers,” Opt. Lett. 26(13), 986–988 (2001). [CrossRef]
  4. A. Giesen, H. Hügel, A. Voss, K. Wittig, U. Brauch, and H. Opower, “Scalable concept for diode-pumped high-power solid-state lasers,” Appl. Phys. B 58, 365–372 (1994).
  5. A. Giesen and J. Speiser, “Fifteen years of work on thin disk lasers: results and scaling laws,” IEEE J. Sel. Top. Quantum Electron. 13(3), 598–609 (2007). [CrossRef]
  6. V. Hasson, and H.-P. Chou, “Cooling of High Power Laser Systems,” US Patent 6,667,999 (2003).
  7. Y. Tzuk, A. Tal, S. Goldring, Y. Glick, E. Lebiush, G. Kaufman, and R. Lavi, “Diamond cooling of high power diode pumped solid state lasers,” in Solid State Lasers XII, R. Scheps, ed., Proc. SPIE 4968, 106–114 (2003).
  8. C. Ding-Xiang, Y. Hai-Wu, Z. Wan-Guo, H. Shao-Bo, and W. Xiao-Feng, “Temperature-related performance of Yb3+:YAG disc lasers and optimum design for diamond cooling,” Chin. Phys. 15(12), 2963– (2006). [CrossRef]
  9. Y. Tzuk, A. Tal, S. Goldring, Y. Glick, E. Lebiush, G. Kaufman, and R. Lavi, “Diamond cooling of high-power diode-pumped solid state lasers,” IEEE J. Quantum Electron. 40(3), 262–269 (2004). [CrossRef]
  10. G. A. Newburgh, M. Dubinskii, and L. D. Merkle, “Silicon carbide face-cooled 4% ceramic Nd:YAG laser,” Electron. Lett. 43(5), 286–288 (2007). [CrossRef]
  11. S. Tokita, J. Kawanaka, M. Fujita, T. Kawashima, and Y. Izawa, “Sapphire-conductive end-cooling of high power cryogenic Yb: YAG lasers,” Appl. Phys. B 80(6), 635–638 (2005). [CrossRef]
  12. http://www.cree.com/products/
  13. V. A. Dmitriev, and M. G. Spencer, “SiC fabrication technology: growth and doping,” in SiC Materials and Devices, Yoon-Soo Park, ed., (Academic Press, London, 1998), pp. 21–63.
  14. E. V. Ivakin, A. V. Sukhadolau, V. G. Ralchenko, and A. V. Vlasov, “Laser-induced transient gratings application for measurement of thermal conductivity of CVD diamond,” Proc. SPIE 5121, 253–258 (2003). [CrossRef]
  15. J. E. Hastie, J. M. Hopkins, S. Calvez, C. W. Jeon, D. Burns, R. Abram, E. Riis, A. I. Ferguson, and M. D. Dawson, “0.5-W single transverse-mode operation of an 850-nm diode-pumped surface-emitting semiconductor laser,” IEEE Photon. Technol. Lett. 15(7), 894–896 (2003). [CrossRef]
  16. P. T. B. Shaffer, “Refractive index, dispersion, and birefringence of silicon carbide polytypes,” Appl. Opt. 10(5), 1034–1036 (1971). [CrossRef] [PubMed]
  17. A. J. Alcock and J. E. Bernard, “Diode-pumped grazing incidence slab lasers,” IEEE J. Sel. Top. Quantum Electron. 3(1), 3–8 (1997). [CrossRef]
  18. A. Minassian and M. Damzen, “20 W bounce geometry diode-pumped Nd:YVO4 laser system at 1342 nm,” Opt. Commun. 230(1-3), 191–195 (2004). [CrossRef]
  19. M. Stockmeier, R. Müller, S. A. Sakwe, P. J. Wellmann, and A. Magerl, “On the lattice parameters of silicon carbide,” J. Appl. Phys. 105(3), 033511 (2009). [CrossRef]
  20. Z. Li and R. C. Bradt, “Thermal expansion of the hexagonal (4H) polytype of SiC,” J. Appl. Phys. 60(2), 612–614 (1986). [CrossRef]
  21. R. L. Aggarwal, D. J. Ripin, J. R. Ochoa, and T. Y. Fan, “Measurement of thermo-optic properties of Y3Al5O12, Lu3Al5O12, YAlO3,LiYF4, LiLuF4, BaY2F8, KGd(WO4)2, and KY(WO4)2 laser crystals in the 80–300 K temperature range,” J. Appl. Phys . 98, 103514–1 - 103514–14 (2005). [CrossRef]
  22. H. Lee, H. E. Meissner, and O. R. Meissner, “Stress Relief of Adhesive-Free-Bond (AFB®) Laser Crystal Composites at Elevated and Cryogenic Temperatures,” in 19th Solid State and Diode Laser Technology Review Technical Digest, 2006, Paper: Laser-5.
  23. A. E. Siegman, G. Nemes, and J. Serna, “How to (Maybe) Measure Laser Beam Quality,” in Diode Pumped Solid State Lasers:Applications and Issues, M. W. Dowley, ed., OSA TOPS Vol. 17 (Optical Society of America, Washington, D.C., 1998), pp. 184–199.
  24. A. Tunnermann, H. Zellmer, W. Schone, A. Giesen, and K. Contag, “New Concepts for Diode-Pumped Solid-State Lasers,” in High-Power Diode Lasers, Topics Appl. Phys.78, R. Diehl, ed., (Springer Verlag, Berlin-Heidelberg, 2000), pp. 369–408.
  25. T. Y. Fan, “Heat Generation in Nd:YAG and Yb:YAG,” IEEE J. Quantum Electron. 29(6), 1457–1459 (1993). [CrossRef]
  26. A. Golubovic, S. Nikolic, R. Gajic, S. Duric, and A. Valcic, “The growth of Nd:YAG single crystals,” J. Serb. Chem. Soc. 67(4), 291–300 (2002). [CrossRef]

Cited By

 Alert me when this paper is cited

OSA is able to provide readers links to articles that cite this paper by participating in CrossRef's Cited-By Linking service. CrossRef includes content from more than 3000 publishers and societies. In addition to listing OSA journal articles that cite this paper, citing articles from other participating publishers will also be listed.


« Previous Article  |  Next Article »

OSA is a member of CrossRef.

CrossCheck Deposited