Thermal management in disk lasers: doped-dielectric and
semiconductor laser gain media in thin-disk and microchip
formats
Alan J. Kemp, Alexander J. Maclean, John-Mark Hopkins, Jennifer E. Hastie,
Stephane Calvez, Martin D. Dawson and David Burns
Institute of Photonics, SUPA, University of Strathclyde, Glasgow, Scotland.
alan.kemp@strath.ac.uk
Abstract. Finite element and analytical modelling of thermal effects in doped-dielectric and
semiconductor disk lasers is used to assess advanced approaches to thermal management. The
prospective utility of high thermal conductivity materials such as diamond, particularly to
improve the spectral coverage in semiconductor disk lasers and to scale the output power of
quasi-monolithic microchip lasers is illustrated. The potential of materials with strong pump
absorption to improve the practicality of doped-dielectric thin-disk lasers, particularly for
mobile applications, is also outlined.
1. Introduction
The use of disk-like laser gain media has enabled the development of highly efficient kilowatt-class
diode-pumped bulk solid-state lasers (see e.g. [1]); the almost simultaneous, but largely separate,
development of semiconductor disk lasers (also known has vertical-external-cavity surface-emitting
lasers (VECSELs)) [2] has shown that optically pumped semiconductor lasers can be power-scaled
whilst retaining both beam quality and the inherent wavelength flexibility of semiconductor lasers –
difficult in traditional semiconductor laser geometries [3, 4]. These two classes of disk laser – dopeddielectric and semiconductor (see figure 1) – share many advantages and development challenges.
Taken together, they offer the potential for broad wavelength coverage, short pulse operation, and
tuneablity from a single laser geometry, merely by changing the gain medium. They are thus a flexible
and practical potential solution to the disparate engineering challenges set by today’s sophisticated
applications.
Disk lasers have three key advantages: the extraction of heat from a small volume through a large
area – simplifying thermal management; the absorption of pump light in a plane – improving overlap
with low brightness pumps; and the ability to exploit both semiconductor and doped-dielectric gain
media – extending wavelength coverage and functionality. In this paper, the prerequisites for practical
disk lasers will be discussed; finite-element thermo-optical calculation will be used to analyse
advanced approaches to thermal management. In section 2, the use of high thermal conductivity
materials will be analysed. Section 3 will discuss the use of high absorption materials to alleviate the
typically cumbersome pump recirculation requirements in doped-dielectric disk lasers. Finally, section
4 will deal with the potential for high-power quasi-monolithic microchip lasers using both dopeddielectric [5, 6] and semiconductor gain media [7].
Thin
Crystal
Simple
Cavity
Heat Sink
Multipass
Pump
(a)
(b)
Figure 1: Schematic diagrams of (a) a doped-dielectric thin-disk laser and (b) a VECSEL
Heatspreader strongly
modifies heat flow
Power
scaling
enabled
Heat resistance
of substrate and
DBR bypassed
Temperature Rise (K)
Temperature
rise reduced
Max Temperature Excursion (K)
z (mm)
2. Thermal management
The disk geometry is intrinsically advantageous from a thermal management perspective – it permits
aggressive cooling of a small volume through a large area and it promotes axial heat flow, minimizing
the detrimental effects of thermal lensing – however, pushing the boundaries of performance and
practicality often requires additional thermal management. In this section the use of high thermal
conductivity materials – such as diamond [8-10], silicon carbide [11], and sapphire [12-14] – to
improve the thermal management of disk lasers will be discussed.
250
Thin Device soldered
to Diamond & Copper
200
150
100
50
Diamond Heatspreader
0
0
0.02
0.04
0.06
0.08
0.1
DBR Thermal Conductivity (W/(mm.K))
r (mm)
(a)
(b)
Figure 2: (a) Finite element simulation of a VECSEL with a diamond heatspreader; (b) the variation of
the maximum temperature in a VECSEL with the substrate removed and one with a diamond
heatspreader as a function of the DBR thermal conductivity. (10W of pump in a 50μm pump spot
radius for a 980nm InGaAs VECSEL pumped at 808nm.)
Although there are a few reports of the use of diamond in laser systems (see e.g. [8, 9]), its
considerable potential to improve thermal management has yet to be widely exploited. One area where
it has had considerable impact is in semiconductor disk lasers (VECSELs) [15, 16]. These lasers are
particularly thermally sensitive: the gain comes from quantum wells located at the anti-nodes of the
intracavity field; as the temperature rises with increasing pump power, the spectral peak of the
quantum well gain and the period of this geometrical arrangement tune at different rates, eventually
causing the output power to roll-over. There are two widely used solutions: one is to bond a diamond
heatspreader to the top of the semiconductor wafer; the other is to remove the substrate on which the
VECSEL structure is grown and cool through the distributed Bragg reflector (DBR) mirror structure
on the rear (a thin device approach). The thermal impedance of the DBR limits the utility of the latter
approach to semiconductor materials systems with high thermal conductivity (see figure 2). By
contrast, the diamond heatspreader approach is far less dependent on the thermal conductivity of the
semiconductor materials used and has thus facilitated Watt level demonstrations at 670nm [17],
850nm [18], 980nm [7], 1060nm [19], 1320nm [9], 1550nm [15] and 2300nm [20].
Doped-dielectric disk lasers can also benefit from the use of high thermal conductivity materials.
The thin-disk geometry has the effect of making the conventional thermal lens nearly negligible. The
dominant thermal deformation is thus bowing of the thin laser material resulting in a convex end
mirror [21]. This can act both to reduce the beam quality and destabilize the resonator, but more
importantly, the associated stresses can lead to disk fracture. Liao and co-authors [22] demonstrated
that compressing the disk in a sapphire anvil could at least partially compensate for this effect. The
results of our finite-element analysis suggest that bonding to a high thermal conductivity material can
significantly reduce stress and temperature rise in doped dielectric disk lasers (see figure 3). The use
of diamond rather than sapphire should to further improvement. We have recently bonded diamond to
a disk of Nd:GdVO4 by means of liquid capillarity (the same method used to bond diamond
heatspreaders to VECSELs) and we are preparing to test the thermal management and laser
performance characteristics of this composite.
(a)
(b)
(c)
(d)
Nd:YVO4
YVO4
Nd:YVO4
Nd:YVO4
Sapphire
Sapphire
Nd:YVO4
43
-703
3731
98
98
Maximum Temperature Rise (K)
43
148
41
Radius of Curvature of the Deformation (Disk Bowing) (mm)
-34199
-2024
-62704
Radius of Curvature of the Thermal Lens (km)
2.4
1.6
2.0
Maximum Stress (MPa)
136
285
86
Maximum Stress in Doped Section (MPa)
136
139
86
(e)
Sapphire
Nd:YVO4
Sapphire
92
-2296
1.2
126
5
Figure 3: Summary of the finite element analysis results for a range of possible composite thin-disk
geometries. 500W of absorbed pump power is assumed in a pump spot of 3.3mm radius. The doped
section is assumed to be of 1atm% Nd:YVO4 and to be 122μm thick. The disk diameter is assumed to
be 7.5mm, while the total thickness of the undoped sections is taken as 1mm. A constant temperature
boundary condition is assumed between disk and heat sink (the bottom element in the diagrams).
3. Practical pumping arrangements
The thermal management potential of disk lasers is predicated on the use of thin gain media; thus the
advantages are usually bought at the expense of pump complexity – the pump light must be
recirculated to achieve efficient absorption. In high power Yb:YAG thin disk lasers, 16 or more pump
passes [1] can be required, making the laser cumbersome for mobile applications. One potential
solution is to use a composite disk (usually Yb:YAG and undoped YAG) to permit side pumping
across the diameter of the disk [23]. An alternative is to take advantage of materials with strong pump
absorption such as the Nd:GdVO4 or highly doped Yb:KYW. This approach is motivated by the
following analysis. Using some simplifying assumptions (four-level medium, plane-waves, uniform
heatload, 1-D heatflow), the maximum temperature rise in the disk can be approximated as:
Tmax


  o
2
  
 r2


2
   2  P   L  h  c    r
out

 p  p  e 
 o
  t
1


  2  k H 

(1)
where η is the fraction of the pump power converted to heat, Λ is the single pass passive loss, Λo is the
output coupler transmission, r is the radius of the pump mode, laser mode and crystal (assumed to be
equal), Pout is the output power, λL is the laser wavelength, λP is the pump wavelength, σe is the
stimulated emission cross-section, τ is the upper level lifetime, k is the thermal conductivity, t is the
disk thickness, and H is heat transfer coefficient between the disk and heatsink. This equation implies
that the disk thickness should be reduced as far as possible and a material of high thermal conductivity
selected. However, to treat the disk thickness as an independent parameter is misleading. In a thin-disk
context, the disk thickness is set by the requirement for efficient pump absorption. A typical
benchmark is 95% pump absorption [1], which, since practicality must limit complexity, should be
achieved in N passes. If the finite spectral width of the pump and the absorption are ignored for
simplicity, equation 1 becomes:
Tmax


  o
2

 
 r2


2
   2  P   L  h  c    r
out

 p  p  e 
 o
 
3
1


  2  N  k   H 

(2)
where  is the pump absorption coefficient. There are two clear strategies to reduce the temperature
excursion and hence also the stress. One can increase the spot size r, but this can be done only in so far
as the threshold (controlled by the second term in the middle set of parentheses) does not become
excessive. Thus one would like a material with a large σe.τ product. From a materials perspective,
one would certainly want a large thermal conductivity, but selecting such a material will only be
worthwhile if it also has good pump absorption. Comparing equations 1 and 2, it is clear that the
appropriate thermal figure of merit for a four level thin-disk material is not the thermal conductivity,
but the product of the thermal conductivity and the pump absorption coefficient: k.α.
Much like spot size scaling being limited by the consequent rise in threshold, maximising the k.α
product is only helpful in controlling the temperature when the heat transfer coefficient, H, is
sufficiently high. Thus, the nature and quality of the disk to heat sink interface will be crucial.
Nonetheless, high pump absorption materials will be key to rationalizing high power thin disk lasers
such that the thermal management advantages can be maintained whilst the practicality is improved by
reducing the number of pump passes. The bonding of very thin samples of strongly absorbing gain
media to high conductivity materials will be important, not only to improve thermal management, but
also be ease handling and mounting.
4. Microchip formats and managed thermal lensing
Another area where the use of high thermal conductivity ancillary materials and strongly absorbing
gain materials can combine is the field of microchip lasers. These are monolithic lasers made from a
plane parallel slice of laser gain material onto which dielectric laser mirrors are directly coated [5, 6].
When this slice of gain material is longitudinally pumped by a diode laser, the thermal lens stabilizes
the cavity and a high quality output beam is generated from a package much more convenient and
robust than a conventional diode-pumped solid-state laser. However, these lasers typically only
produce a few hundred milliwatts of output power before the thermal effects get too strong and either
the crystal cracks or the mode quality deteriorates. Our simulation indicate (see figure 4) that the use
of diamond to assist in managing the thermal effects in these devices should enable scaling to higher
output powers, perhaps in the region of 10W. As mentioned earlier, we have recently succeeding in
bonding diamond to Nd:GdVO4, the first step in testing this hypothesis.
Temperature rise within the
cavity on axis (K)
Max. Temperature Rise
= 386K
TEM00 Mode Radius (FEA) = 26m
10W
400
350
300
250
200
150
100
50
0
Pump
Absorbed
808nm
Max. Temperature Rise
= 78K
TEM00 Mode Radius (FEA) = 41m
200m
0
0.2
0.4
0.6
Distance through device (mm)
200
200m Nd:GdVO4
200
200m Diamond
Dielectric Mirror
Max. Temperature Rise
= 93K
TEM00 Mode Radius (FEA) = 30m
Devices cooled on a 3mm inner diameter annulus on both
large face. Gaussian pump profile, single pass pumping
50
120
100
40
80
30
60
20
40
10
20
0
0
50
100
150
200
250
Pump Spot Radius ((m)
m)
(a)
0
300
4
30
Diamond
3.5
25
3
20
2.5
2
15
1.5
10
1
5
0.5
0
0
50
100
150
200
250
Max Temperature Rise (K)
140
Sapphire
Optimum Thickness (mm)
60
Max Temperature Rise (K)
Optimum Thickness (mm)
Figure 4: Finite element modelling of the potential for improved thermal management in microchip
lasers using diamond.
0
300
Pump Spot Radius ((m)
m)
(b)
Figure 5: Optimum heatspreader thickness and maximum temperature rise as a function of pump spot
radius for a 980nm microchip VECSEL. The optimum heatspreader thickness is assumed to that
which ensures the laser and pump mode radii are equal. The pump power is assumed to be 5W.
Microchip lasers can also be fabricated using VECSEL gain media, bringing the advantage of
wavelength versatility [7, 24, 25]. Here, a dielectric mirror coating is applied to the outside surface of
the heatspreader to form a quasi-monolithic laser. In such lasers, it is important to match the pump and
laser mode sizes. If the laser mode is too small, multi-transverse mode operation will lead to lower
beam quality; too large and re-absorption losses will occur in the unpumped regions of the
semiconductor gain medium. Thus, the thermal lens strength must be appropriately managed. We have
used finite element simulations to model the thermal lensing effects in microchip VECSELs with
sapphire and diamond heatspreaders (see figure 5). Diamond is predicted to ensure lower temperature
rises and mode-matching at reasonable thicknesses. These predictions are in accord with experimental
demonstrations where microchip VECSELs using diamond have typically delivered higher beam
quality [7, 25, 26].
5. Conclusions
The intracavity use of high thermal conductivity ancillary materials is set to have significant impact on
the thermal management of diode-pumped solid-state lasers, both doped-dielectric and semiconductor.
In this paper, we have shown that in semiconductor disk lasers such materials can bypass the higher
thermal impedance of the semiconductor materials required in the visible and mid-infrared – an
important step towards realising a high power wavelength engineerable laser technology. Finite
element models of high power doped-dielectric disk lasers show that materials such as sapphire and
diamond can reduce bowing of the disk. Bonding to such materials will also render practical the use of
very thin slices of strongly pump absorbing materials; the analytical model presented in this paper
indicates that this will be a route to realising more practical disk lasers for mobile applications.
Materials like diamond will also have an impact on the power scaling of quasi-monolithic microchip
lasers, with the simulations presented here suggesting the scaling to multi-Watt power levels with
good beam quality is possible. By integrating high performance thermal management, disk lasers have
the potential to become enabling sources where applications require both performance and practicality.
Acknowledgements
AJK and JEH gratefully acknowledge personal research fellowships from the Royal Society of
Edinburgh and the Royal Academy of Engineering respectively.
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