Composite Yb:YAG/SiC-prism thin disk laser

G. A. Newburgh*, A. Michael, and M. Dubinskii

US Army Research Laboratory, RDRL-SEE-O, 2800 Powder Mill Rd., Adelphi, MD 20783,USA *gnewburgh@arl.army.mil

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 Optical Society of America

OCIS codes: 140.3580 Lasers, solid-state; 140.3480 Lasers, diode-pumped; (140.6810)

Thermal effects

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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], 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]. 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]. 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]. 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].

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]. Since then numerous allusions of potential benefits of the double-

sided thin disk cooling have emerged (e.g., [6,7]). 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 [8–10]. 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

#130914 - $15.00 USD Received 29 Jun 2010; revised 16 Jul 2010; accepted 18 Jul 2010; published 27 Jul 2010(C) 2010 OSA 2 August 2010 / Vol. 18, No. 16 / OPTICS EXPRESS 17067

material [11]. The authors of [11] 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].

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]. 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]. 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]. SiC’s thermal conductivity is about the

same as that of copper [13], which is four times lower than that of diamond [14], 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]. 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] or semiconductor [15] 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]), 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] 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

#130914 - $15.00 USD Received 29 Jun 2010; revised 16 Jul 2010; accepted 18 Jul 2010; published 27 Jul 2010(C) 2010 OSA 2 August 2010 / Vol. 18, No. 16 / OPTICS EXPRESS 17068

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] (further developed into what’s

now known as a ‘bounce laser’ [18]) as a remote analog. The obvious difference of what we

propose from the [17,18] 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.

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] 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).

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.

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

#130914 - $15.00 USD Received 29 Jun 2010; revised 16 Jul 2010; accepted 18 Jul 2010; published 27 Jul 2010(C) 2010 OSA 2 August 2010 / Vol. 18, No. 16 / OPTICS EXPRESS 17069

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%.

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).

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 room temperature coefficient of thermal expansion (CTE) of hexagonal 4H polytype

SiC is found to be nearly isotropic (3*10−6

K−1

[19], ), or very lightly anisotropic (3.2(c)-

3.3(a)*10−6

K−1

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

K−1

[21],

). 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], 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 on

L1 L2 M1

M2

#130914 - $15.00 USD Received 29 Jun 2010; revised 16 Jul 2010; accepted 18 Jul 2010; published 27 Jul 2010(C) 2010 OSA 2 August 2010 / Vol. 18, No. 16 / OPTICS EXPRESS 17070

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.

the prism. The issue of ~30% pump power loss can be easily eliminated by merely using a

polarized pump source.

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] 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], 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.

#130914 - $15.00 USD Received 29 Jun 2010; revised 16 Jul 2010; accepted 18 Jul 2010; published 27 Jul 2010(C) 2010 OSA 2 August 2010 / Vol. 18, No. 16 / OPTICS EXPRESS 17071

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] and 6.5 W/m*K [24]

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].

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.

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].

Comparison of measured temperature distribution profile (Fig. 6) with simulation results

#130914 - $15.00 USD Received 29 Jun 2010; revised 16 Jul 2010; accepted 18 Jul 2010; published 27 Jul 2010(C) 2010 OSA 2 August 2010 / Vol. 18, No. 16 / OPTICS EXPRESS 17072

depicted in Fig. 5 indicates good agreement both in the absolute temperature values and

dimensions of the temperature distribution.

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 ∆T = 10 °C.

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.

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].

#130914 - $15.00 USD Received 29 Jun 2010; revised 16 Jul 2010; accepted 18 Jul 2010; published 27 Jul 2010(C) 2010 OSA 2 August 2010 / Vol. 18, No. 16 / OPTICS EXPRESS 17073

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.

#130914 - $15.00 USD Received 29 Jun 2010; revised 16 Jul 2010; accepted 18 Jul 2010; published 27 Jul 2010(C) 2010 OSA 2 August 2010 / Vol. 18, No. 16 / OPTICS EXPRESS 17074