650
`
`IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 6, NO. 4, JULY/AUGUST 2000
`
`A 1-kW CW Thin Disc Laser
`
`Christian Stewen, Karsten Contag, Mikhail Larionov, Adolf Giesen, and Helmut Hügel
`
`Abstract—The thin disc laser is presented as an optimal laser
`design for the operation of a quasi-three-level laser active medium
`in the high power regime with high optical efficiency. Numerical
`calculations of the laser output power show that operation with
`an output power up to 1 kW with an optical efficiency of 50%
`and more is possible at room temperature utilizing 16 absorption
`passes. Scaling of the output power can be realized by scaling the
`pumped area using one or more discs. The experimental investiga-
`tions yield a maximum output power of 647 W at 51% optical ef-
`ficiency for one crystal and of 1070 W with 48% optical efficiency
`for four crystals at a temperature of the cooling water of 15 C.
`Index Terms—CW lasers, diode-pumped solid-state lasers, high-
`power lasers, Yb : YAG.
`
`I. INTRODUCTION
`
`F OR laser operation at high output power with at the same
`
`time high optical efficiency and good beam quality, a
`careful choice of the laser design is required, including the
`geometry and type of the laser active medium, the pump, and
`the resonator configuration. With the geometry of a thin disc for
`the laser active medium, the ratio of cooling surface to pumped
`volume is increased compared to rod lasers, which is a basic
`advantage to extract high output power from a small volume.
`For a good overlap between pump and resonator mode and a
`high pump power density, a quasi-end pumped geometry is
`chosen. The operation with good beam quality is possible only
`if the faces of the disc are cooled, so that the heat flux and the
`laser beam axis are collinear to each other. As a consequence,
`thermal lensing effects are dramatically reduced. They stem
`mainly from a bending of the crystal due to different expansions
`of its front and back side with additional aberrations resulting
`from a radial temperature gradient at the edge of the pumped
`area. Due to the small thickness of the crystal, this configu-
`ration leads to a low absorption efficiency since the effective
`absorbing length is only twice the thickness of the crystal if
`the unabsorbed pump radiation is reflected at the back side of
`the crystal. To increase the absorbing length, the unabsorbed
`pump power is reimaged onto the crystal several times, which
`is possible if diode lasers are used as pump source. This leads
`to the design of the thin disc laser [1], as shown in Fig. 1.
`The crystal, which is AR coated for the wavelength of the
`pump and laser radiation at the front side and HR coated for both
`wavelengths at the back side, is fixed with a layer of indium onto
`
`Fig. 1. Principal setup of the thin disc laser.
`
`a heat sink, which is typically a copper disc of 1 mm thickness.
`This disc is mounted on a support and cooled with water from
`the back side.
`Repeated passes of the pump radiation through the crystal
`increase not only the absorption efficiency but also the effective
`pump power density. Therefore, the thin disc design is suitable
`for quasi-three-level systems like Yb : YAG, which need high
`pump power densities to overcome transparency threshold and
`efficient cooling at the same time. Because of an additionally
`high Stokes efficiency and good thermomechanical properties
`of the laser host, Yb : YAG was chosen as presently the most
`promising laser material to scale output power of the thin disc
`laser up to the kilowatt range.
`For its promising properties, several groups have investigated
`Yb : YAG in the last ten years. After the first realization of
`a diode-pumped Yb : YAG laser by Fan at Lincoln Labo-
`ratory in 1990 [2], the operation at high output power was
`studied especially at Lawrence Livermore National Laboratory
`(LLNL), Hughes Research Laboratories (HRL), and Institut
`für Strahl-werkzeuge (IFSW) in collaboration with Deutsche
`Forschungszentrum für Luft- und Raumfahrt (DLR). At LLNL
`and HRL, the geometry of the laser active medium was a thin
`rod, which like the thin disc has a high ratio of cooling surface
`to active volume, but it exhibits strong thermal lensing. At
`HRL the Yb : YAG rod was side pumped, which allows a high
`transfer efficiency from the diode lasers to the rod, without
`any beam shaping. This has led to an early demonstration of
`high-power operation but with low optical efficiency and low
`beam quality; their highest published output power is 1040 W
`with approximately 22% optical efficiency [3]. At LLNL, the
`rod was end-pumped by a diode laser array using a lens duct
`with the highest output power of 1080 W using two rods [4].
`At IFSW and DLR, the thin disc laser has been developed [1].
`
`Manuscript received December 15, 1999; revised April 24, 2000. This work
`was supported in part by the BMB+F under Contracts 13N6364, 13N6365, and
`13N7300.
`C. Stewen, M. Larionov, A. Giesen, and H. Hügel are with the Institut für
`Strahl-werkzeuge, Universität Stuttgart, Germany (e-mail: giesen@ifsw.uni-
`stuttgart.de).
`K. Contag is with the Forschungsgesellschaft für Strahl-werkzeuge mbH,
`Stuttgart, Germany.
`Publisher Item Identifier S 1077-260X(00)07080-5.
`
`II. NUMERICAL STUDIES
`In order to determine the design and operation parameters of
`a thin disc laser that is expected to work at higher temperature, a
`numerical model is used. In particular, it allows the determina-
`tion of the scaling properties of this laser concept. For the cal-
`culation various properties of Yb : YAG are needed, which are
`ASML 1134
`1077–260X/00$10.00 © 2000 IEEE
`(cid:3) (cid:55)(cid:75)(cid:76)(cid:86)(cid:3)(cid:80)(cid:68)(cid:87)(cid:72)(cid:85)(cid:76)(cid:68)(cid:79)(cid:3)(cid:80)(cid:68)(cid:92)(cid:3)(cid:69)(cid:72)(cid:3)(cid:83)(cid:85)(cid:82)(cid:87)(cid:72)(cid:70)(cid:87)(cid:72)(cid:71)(cid:3)(cid:69)(cid:92)(cid:3)(cid:38)(cid:82)(cid:83)(cid:92)(cid:85)(cid:76)(cid:74)(cid:75)(cid:87)(cid:3)(cid:79)(cid:68)(cid:90)(cid:3)(cid:11)(cid:55)(cid:76)(cid:87)(cid:79)(cid:72)(cid:3)(cid:20)(cid:26)(cid:3)(cid:56)(cid:17)(cid:54)(cid:17)(cid:3)(cid:38)(cid:82)(cid:71)(cid:72)(cid:12)(cid:3)
`
`

`
`STEWEN et al.: 1 kW CW THIN DISC LASER
`
`651
`
`Fig. 2. Measured temperature dependence of the heat conductivity. Open
`symbols show literature data.
`
`Fig. 3. Calculated dependence of absorption efficiency 
`, and slope efficiency  on pump power P .
`
`
`, optical efficiency
`
`discussed in detail in [5], and all the material parameters used
`are summarized in [6]. As the temperature distribution inside
`the crystal has a strong effect on the output power, a detailed
`knowledge of the heat conductivity is important. Therefore, the
`temperature dependence of the heat conductivity has been mea-
`sured within the frame of a subcontract (Austrian Research Cen-
`ters Seibersdorf), as shown in Fig. 2. Based on these data, the
`following function represents the dependence of the heat con-
`on the doping concentration
`and the tempera-
`ductivity
`ture
`
`and the cooling medium is included, which is determined from
`finite-element calculations taking into account all mechanisms
`of heat transfer from the crystal to the cooling medium.
`Based on former experiments and calculations, typical pa-
`rameters of the thin disc laser for high-power operation are:
`1) laser active medium:
`m;
`a) crystal thickness:
`%;
`b) doping concentration:
`c) fraction of heat generation inside HR-coating:
`%.
`
`C;
`
`%;
`
`2) cooling:
`a) temperature of cooling medium:
`Kmm /W;
`b) heat resistance:
`3) resonator:
`a) number of resonator passes through the crystal:
`;
`b) resonator internal losses:
`4) pump configuration and pump source:
`a) number of pump beam passes through the crystal:
`;
`W;
`b) pump power:
`c) central wavelength of pump radiation:
`nm;
`d) full-width half-maximum of pump radiation:
`nm;
`e) numerical aperture of the pump source:
`f) diameter
`of
`homogenizing
`pump
`mm;
`g) ratio of pump source to pump spot diameter:
`1 : 1.15.
`
`;
`source:
`
`A. Basic Dependences
`Using these values and neglecting losses in the pump con-
`figuration and also not taking into account mechanical fracture
`limits, optical efficiencies as shown in Fig. 3 are calculated for
`varying pump power (the symbols inside this and all the other
`figures of this chapter represent discrete calculated data). Op-
`tical efficiency is defined here as the ratio between laser output
`power and pump radiation power entering the pump optics. With
`
`K
`
`K
`
`K
`
`(1)
`
`with the value of
`
`at 300 K
`
`K
`
`W mK
`
`(2)
`
`For undoped YAG, the heat conductivity is given by
`
`(3)
`
`Wm
`
`K
`
`K
`
`K
`
`The numerical model consists of three main steps: the cal-
`culation of the distribution of the absorbed pump radiation in-
`side the crystal by Monte Carlo ray-tracing of the pump photons
`through the optical system, the calculation of the temperature
`distribution inside the crystal, and the calculation of the tem-
`perature-dependent output power. These three steps are repeated
`iteratively until a steady state is reached. The numerical code is
`discussed in more detail in [6] and [8] and has been verified by
`comparison with experimental results at low pump power using
`eight [6] and 16 absorption passes [9].
`The generation of heat is assumed to result from two mech-
`anisms: the Stokes defect of 8.6% between the energy of the
`pump and the laser photon on the one hand and the absorp-
`tion of pump and laser radiation as well as fluorescence by the
`HR-coating of the crystal on the other hand. The heat load is cal-
`culated according to the distribution of the absorbed pump ra-
`diation. In the calculations a heat resistance between the crystal
`
`

`
`652
`
`IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 6, NO. 4, JULY/AUGUST 2000
`
`Fig. 4. Calculated optical efficiency as a function of crystal thickness at a
`pump power of 1000 W for different doping concentrations.
`
`, absorption efficiency
`Fig. 5. Calculated dependence of optical efficiency 
`, optimized crystal thickness d
`, and absorption length c
`d on doping
`
`concentration c
`.
`
`1000 W pump power, the output power is 540 W with 88% ab-
`sorption efficiency, 58% slope efficiency, and a maximum tem-
`perature of the crystal at the front side of 130 C. With higher
`pump power, the absorption efficiency is reduced due to higher
`temperature (11.5 C per 100 W of pump power) and ground
`state depletion. This leads to a maximum optical efficiency of
`54% at 1400 W pump power and a slight decrease of the optical
`efficiency.
`The optical efficiency can be raised by reducing the crystal
`temperature, either by lowering the temperature of the cooling
`medium or by reducing the heat resistance between crystal and
`cooling fluid. At a pump power of 1000 W, the reduction of the
`heat resistance from 12.7 to 10 Km m /W leads to an increase
`of optical efficiency from 54% to 56%, and the lowering of the
`temperature of the cooling medium from 15 C to 50 C yields
`an increase of the optical efficiency from 54% to 65%.
`The dependence of the optical efficiency on the doping con-
`centration and the crystal thickness at a pump power of 1000 W
`and for 16 absorption passes is shown in Fig. 4. By increasing
`the crystal thickness, the absorption efficiency and, therefore,
`the slope efficiency increases while the number of laser ions,
`which have to be pumped to transparency, also goes up and
`therefore the laser threshold increases. This means that an op-
`timum crystal thickness exists for the highest optical efficiency.
`The optimum crystal thickness decreases while the absorbing
`increases with doping concentration, as shown in
`length
`Fig. 5. Therefore, the absorption efficiency and the optical effi-
`ciency increase with increasing doping concentration. Changing
`the doping concentration from 4% to 20% increases the optical
`efficiency at 1000 W pump power from 50.5% to 56.2%. With
`higher doping concentration, however, a growing density of im-
`purity ions may occur leading to a reduced fluorescence life-
`time and an increased heat generation. These effects may com-
`pensate the increase in optical efficiency with increasing doping
`concentration. A decrease in fluorescence lifetime from 951 to,
`e.g., 800 s would decrease the optical efficiency from 53.7%
`to 50.4%, and an increase in heat generation from 8.6% to 10%
`would decrease the optical efficiency to 51.2% using a doping
`concentration of 8%. Due to these effects, an optimum doping
`concentration exists.
`
`versus crystal thickness d for
`Fig. 6. Calculated optical efficiency 
`different numbers of pump beam passes M at 1000 W pump power.
`
`Optical efficiencies of more than 50% can be achieved by
`using 16 pump beam passes. Fig. 6 shows how the efficiency is
`influenced by the number of pump beam passes and the crystal
`to
`, the op-
`thickness. With an increase from
`tical efficiency increases from 40% to 54%. Fig. 7 shows that the
`gain of optical efficiency is caused by the increased absorption
`efficiency due to a larger absorbing length, although the crystal
`thickness is reduced. Due to the higher absorption, the slope ef-
`ficiency is higher and simultaneously the reduced crystal thick-
`ness leads to a lower laser threshold. Both aspects contribute to
`a rise of the optical efficiency.
`
`B. Scaling of the Output Power
`Scaling of the output power up to multi-kilowatt power levels
`is realized by scaling the pumped area at constant pump power
`density. This can be done either by enlarging the pump spot
`radius using one disc or by increasing the number of discs at
`constant pump spot radius.
`Fig. 8 shows the dependence of the optical efficiency on pump
`power density for one disc and for different pump spot radii; the
`same data are plotted in Fig. 9 against pump power. The results
`
`

`
`STEWEN et al.: 1 kW CW THIN DISC LASER
`
`653
`
`Fig. 7. Calculated dependence of the optimized optical efficiency 
`and absorption efficiency 
`as well as the optimized crystal thickness d
`and the absorption length d N on the number of pump beam passes N .
`
`Fig. 8. Calculated dependence of optical efficiency 
`power density E
`for different pump spot radii.
`
`on the absorbed pump
`
`Fig. 9. Calculated dependence of optical efficiency 
`for different pump spot radii r .
`
`on pump power P
`
`show a slight decrease in optical efficiency with increased pump
`spot diameter. This is due to the radial heat flux inside the crystal
`and the crystal mount, the contribution of which to the cooling
`is reduced: the larger the pump spot, the higher the maximum
`
`Fig. 10. Principal setup of the pump geometry allowing 16 absorption passes
`using a parabolic mirror.
`
`crystal temperature (in the calculations this is included as an in-
`creased effective heat resistance). As a consequence, the absorp-
`tion efficiency and hence the optical efficiency will be lower.
`The other possibility of enlarging the pumped area is an in-
`creased number of discs. The numerical model shows that the
`optical efficiency remains constant, although the resonator in-
`ternal losses increase proportional to the number of discs.
`
`III. EXPERIMENTAL SETUP
`Two modules of stacked arrays, which are the first realiza-
`tions of such modules, built in 1997, one from Jenoptik Laser-
`diode GmbH and one from Dilas Diodenlaser GmbH, are used
`as pump sources for scaling the laser output power up to 1 kW.
`The electric-optical efficiencies of these modules, defined as the
`ratio between the output power with the specified beam param-
`eter product of 750 mm mrad and the consumed electric power
`of the diodes, are 22.5% and 29.3%, respectively. To homoge-
`nize the radiation of the stacked arrays, it is coupled into a silica
`rod with 5 mm diameter, 200 mm length, and a numerical aper-
`. The fraction of pump radiation lost in the
`ture of NA
`homogenizing rod is 6%. The realization of 16 absorption passes
`of the pump radiation has been done by two different pump con-
`figurations.
`The configuration using a parabolic mirror [9] for pumping
`one crystal is shown in Fig. 10. The collimated pump radiation
`hits segment 1 of the parabolic mirror and is focused onto the
`crystal, which is placed in the focal plane (this is shown in Sec-
`tion 1 of Fig. 10). The unabsorbed pump radiation is collimated
`again when it hits segment 2 of the parabolic mirror. Reflected
`by two folding mirrors, the radiation hits the parabolic mirror on
`segment 3 (shown in Section 2 of Fig. 10). Then the radiation
`is imaged onto the crystal again. Using two additional pairs of
`folding mirrors and the other segments of the parabolic mirror,
`eight absorption passes through the crystal are achieved. A final
`flat reflector reverses the path of the pump radiation, generating
`16 absorption passes. The device, which has been used in the
`experiments, is shown in Fig. 11. With this device the scaling of
`the output power by increasing the diameter of the pump spot
`has been realized.
`For the investigation of the scaling of output power using sev-
`eral discs, a configuration as shown in Fig. 12 is used. It in-
`tegrates four crystals and two pump sources. The crystal and
`the flat mirrors are positioned in a straight line. The imaging
`mirrors are positioned on the wall of a virtual cylinder around
`
`

`
`654
`
`IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 6, NO. 4, JULY/AUGUST 2000
`
`Fig. 11. Experimental pump configuration for one crystal.
`
`Fig. 13. Experimental pump configuration for up to four crystals.
`
`Fig. 12. Principal setup of the pump geometry for four crystals using two pump
`sources allowing 16 absorption passes.
`
`this as a center line. In Fig. 12, the propagation of the radiation
`from pump source 1 through all the mirrors and pumped discs is
`shown. The collimated radiation is focused onto the first crystal
`by using a toric mirror. The unabsorbed radiation is collimated
`by another toric mirror, reflected by a flat mirror, and then hits
`the next toric mirror. Using collimating mirrors in an optical
`distance of two times the focal length between two successive
`elements, the divergence of the pump radiation can be kept con-
`stant. Following the path of the radiation up to the final mirror,
`which is spherical, each crystal is pumped by two absorption
`passes. The final mirror reverses the path of the pump radiation
`again to generate another two absorption passes through each
`crystal, which gives a total of 16 absorption passes for the pump
`radiation. The pump configuration that has been realized for the
`experiments is shown in Fig. 13.
`
`IV. EXPERIMENTS
`A. Scaling of Output Power Using One Disc
`With one disc, scaling of the output power is investigated by
`scaling the pump spot diameter at constant pump power density.
`By means of an aperture at the end of the homogenizing rod, the
`
`in comparison of
`Fig. 14. Laser output power P and optical efficiency 
`experimental and calculated results for various radii r of the pump spot.
`
`radius of the pump spot can be varied. Fig. 14 shows the exper-
`imental results obtained with four different pump spot sizes in
`comparison to numerical results. In these calculations, the mea-
`sured losses of the optics of the pump configuration are taken
`into account, in contrast to the results reported in Section II. The
`resonator consists of the laser crystal as end mirror and an out-
`coupling mirror with 1 m radius of curvature and 2.2% trans-
`mission; its length is 40 cm. The thickness of the crystal was
`230 m, and the doping concentration 8%. The highest output
`power was 480 W with 47% optical efficiency with respect to
`the incident pump power. With the smallest pump spot, the op-
`tical efficiency was 49%. This slightly decreased efficiency with
`larger pump spots reflects the reduced contribution to cooling
`of the radial heat flux within the crystal. This results in a higher
`crystal temperature, which leads to a reduced absorption effi-
`ciency and an increased threshold and therefore to a reduced
`slope efficiency and a reduced optical efficiency, as shown in
`Fig. 14. The dependence of the measured optical efficiency on
`
`

`
`STEWEN et al.: 1 kW CW THIN DISC LASER
`
`655
`
`Fig. 15. Measured optical efficiency versus pump power density for various
`pump spot sizes.
`
`Fig. 16. Output power and optical efficiency for the highest output power using
`one single disc (resonator length: 9 cm).
`
`the pump power density for the various pump spot sizes is shown
`in Fig. 15, which has to be compared with the calculated data
`in Fig. 8. The experimental and numerical results agree well,
`except for the case of the largest pump spot, where the exper-
`imental laser threshold is much lower than the calculated one.
`This is due to the distribution of the pump radiation, which is not
`sufficiently homogenized; in the center the pump power density
`is 50% higher than the mean value. This higher pump power
`density leads to a lower laser threshold in the experiment than
`is expected from the calculation.
`The high optical efficiency at room-temperature operation re-
`ported herein was enabled by the doubled number of pump beam
`passes through the crystal compared to former publications, e.g.,
`[7], as expected from the calculations (Fig. 6). The difference
`in optical efficiency using eight or 16 passes was investigated
`with a crystal of 248 m thickness, which is close to the calcu-
`lated optimum thickness of 250 m for eight passes, according
`to Fig. 6. For this case, at a pump power of 800 W, the optical
`efficiency is 34%. The change to 16 absorption passes leads to a
`rise of the optical efficiency from 34% to 43%. According to the
`calculations, the efficiency should increase even further if the
`optimum crystal thickness of 200 m at 16 absorption passes
`were used.
`The experiments discussed so far were performed with an in-
`homogeneous pump profile due to an insufficient homogeniza-
`tion of the pump radiation. For a better homogenization a silica
`rod with a diameter of 3.5 mm is used. At a lower accepted beam
`quality of the pump radiation, the electric-optical efficiency of
`the pump modules can be increased to 29.7%, leading to a higher
`value of available pump power of 1260 W. Utilizing both ef-
`fects (higher homogeneity and higher pump power), the highest
`output power using one single disc to date is 647 W at an optical
`efficiency of 51%, (Fig. 16) (the crystal was 224 m thick and
`doped with 9%).
`
`B. Beam Quality Aspects
`The beam quality characterized by the times-diffraction-limit
`is determined by the resonator configuration and is
`factor
`measured with a coherent mode master. With the crystal under
`the pumping conditions as described in Fig. 16, the beam quality
`
`Fig. 17. Output power and times-diffraction-limit factor using various radii of
`curvature of the outcoupling mirror with a transmission of 3%.
`
`. By in-
`for the highest output power of 647 W was
`creasing the radius of curvature of the outcoupling mirror, the
`, as shown
`times-diffraction-limit factor is reduced to
`in Fig. 17. The maximum output power is reduced to 508 W with
`45% optical efficiency. It is remarkable that the output power
`only slightly decreases with better beam quality. The maximum
`usable pump power, however, which is indicated by a change of
`the slope efficiency, is smaller at larger radii of curvature. This
`can be attributed to higher diffraction losses for higher funda-
`mental mode diameters or to reaching the stability boundaries
`of the resonator.
`Further investigations to reduce the times-diffraction-limit
`factor were made with a different laser crystal with 8% doping
`concentration and 238 m thickness. The measured output
`power and times-diffraction-limit factors for various resonator
`configurations with a transmission of the outcoupling mirrors
`,
`of 3% are shown in Fig. 18. With larger resonator length
`, the times-diffraction-limit factor
`and radius of curvature
`, but with a slight decrease
`can be reduced down to
`in output power. For every resonator configuration, the beam
`quality is nearly independent of the pump power.
`The measured dependence of the times-diffraction-limit
`factor on the resonator configuration can be used to determine
`
`

`
`656
`
`IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 6, NO. 4, JULY/AUGUST 2000
`
`Fig. 18. Output power and times-diffraction-limit factor for various resonator
`configurations.
`
`Fig. 20. Ratio of polarized and unpolarized output power.
`
`Fig. 19. Calculated times-diffraction-limit factor to determine the curvature
`radius of the crystal including the thermal lens. The measured variation of M
`is indicated by the horizontal lines.
`
`the radius of curvature of the crystal including the thermal
`of the funda-
`lensing effect. For this purpose the radius
`mental mode on the crystal is calculated for the different
`resonator configurations using the program “WinLase.” The
`times-diffraction-limit factor is given by
`, where
`is the effective pump spot radius, which can be used by the
`is the radius of the fundamental mode. Due
`laser mode and
`is slightly smaller
`to reabsorption in the unpumped area,
`. With
`mm, which
`than the pump spot diameter
`-values are calculated
`is 86% of the pump spot diameter,
`and shown in Fig. 19 for the three resonator configurations
`presented in Fig. 18. As indicated by the intersecting points
`versus
`curves in Fig. 19 the radius of
`between these
`curvature of the crystal (geometrical deformation and thermal
`lensing) is almost the same for the three curves. As can be seen
`is in the range from
`m (threshold) to
`from Fig. 19,
`m (highest power). From this it is obvious that within the
`range of operation a thermal lens is built up with 0.067 dpt,
`respectively, 15 m focal length. Altogether, this leads for the
`discussed conditions to a defocusing of the disc in contrast to
`the extreme focusing of a laser rod.
`
`Fig. 21. Scaling of output power by increasing the number of discs inside the
`resonator. The diameter of each pump spot is 4.8 mm.
`
`C. Polarized Operation
`The integration of a Brewster window inside the resonator
`cm and
`m leads to polarized laser op-
`with
`eration. The ratio of polarized to unpolarized output power as
`of the polarization plane for
`a function of the rotation angle
`different pump power values is shown in Fig. 20. The depolar-
`ization losses increase with higher pump power due to higher
`strain inside the crystal and indicate a preference in the polar-
`ization plane.
`
`D. Scaling of Output Power Using Several Discs
`The measured output power and optical efficiency using up to
`four discs is summarized in Fig. 21. The resonator with only one
`crystal uses the crystal as end mirror. The radius of the outcou-
`pling mirror was 0.5 m, and the transmission was 3%. The inte-
`gration of two crystals necessitates a folding mirror between the
`crystals. The distance between crystal and mirror was 200 mm,
`and the radii of all mirrors were 0.5 m. The transmission of
`the outcoupling mirror was 6%. The resulting times diffraction
`limit factor was approximately 50. When using four crystals,
`the distance between crystals and folding mirrors was shortened
`to 100 mm. The times-diffraction-limit factor increased to ap-
`proximately 80. The highest output power with four crystals was
`
`

`
`STEWEN et al.: 1 kW CW THIN DISC LASER
`
`657
`
`1070 W with 48% optical efficiency. This is nearly the same ef-
`ficiency (47%) as using two crystals with a maximum output
`power of 529 W. The efficiency with only one crystal is slightly
`higher (51%), due to lower resonator internal losses.
`Compared to the scaling of the output power using one disc,
`the use of several discs enables slightly higher optical efficien-
`cies at the same beam quality.
`
`V. CONCLUSION
`The realization of 16 absorption passes of the pump radiation
`through the crystal enables the operation of the thin disc laser
`in the high-power regime with high efficiency at room-tempera-
`ture operation. The comparison with former high-power results
`C and 49% ef-
`using eight absorption passes (346 W at
`ficiency [7]) shows the advantage of the 16 passes: nearly the
`same optical efficiency at a considerably higher temperature of
`the cooling fluid (avoiding cryogenic techniques) is achieved.
`
`REFERENCES
`[1] 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, vol. 58, p. 365, 1994.
`[2] T. Y. Fan, “Diode pumped solid state lasers,” Lincoln Lab. J., vol. 3, p.
`413, 1990.
`[3] D. S. Sumida, A. A. Betin, H. Bruesselbach, R. W. Byren, S. Matthews,
`R. Reeder, and M. Mangir, “Diode-pumped Yb : YAG catches up with
`Nd : YAG,” Laser Focus World, vol. 35, no. 6, pp. 63–70, 1999.
`[4] E. C. Honea, R. J. Beach, S. C. Mitchell, P. V. Avizonis, R. S. Monroe,
`J. A. Skidmore, M. A. Emanuel, S. B. Sutton, D. G. Harris, and S. A.
`Payne, Dual-rod Yb : YAG laser for high-power and high-brightness ap-
`plications, in Advanced Solid State Laser Conference 2000. to be pub-
`lished.
`[5] H. W. Bruesselbach, D. S. Sumida, R. A. Reeder, and R. W. Byren,
`“Low-heat high-power scaling using InGaAs-diode-pumped Yb : YAG
`lasers,” IEEE J. Select. Topics Quantum Electron., vol. 3, p. 105, Feb.
`1997.
`[6] K. Contag, M. Karszewski, C. Stewen, A. Giesen, and H. Hügel, “Diode
`pumped thin disc Yb : YAG lasers: Theoretical modeling and experi-
`mental investigations,” Quantum Electron., vol. 29, no. 8, p. 697, 1999.
`[7] M. Karszewski, U. Brauch, K. Contag, S. Erhard, A. Giesen, I.
`Johannsen, C. Stewen, and A. Voss, “100 W TEM operation of
`Yb : YAG thin disc laser with high efficiency,” in OSA Trends in
`Optics and Photonics, Advanced Solid-State Lasers, W. A. Bosenberg
`and M. M. Frejer, Eds. Washington, DC: Opt. Soc. Amer., 1998,
`vol. 19, p. 125.
`[8] K. Contag, U. Brauch, S. Erhard, A. Giesen, I. Johannsen, M.
`Karszewski, C. Stewen, and A. Voss, “Simulations of the lasing proper-
`ties of a thin disk laser combining high output powers with good beam
`quality,” in Proc. SPIE: Modeling and Simulation of Higher-Power
`Laser Systems IV, vol. 2989, U. O. Farrukh and S. Basu, Eds., 1997,
`p. 23.
`[9] S. Erhard, A. Giesen, M. Karszewski, T. Rupp, C. Stewen, I. Johannsen,
`and K. Contag, “Novel pump design of Yb : YAG thin disc laser for op-
`eration at room temperature with improved efficiency,” in OSA Trends
`in Optics and Photonics, Advanced Solid State Lasers, vol. 26, M. M.
`Fejer, H. Injeyan, and U. Keller, Eds. Washington, DC, 1999, p. 38.
`[10] G. A. Slack and D. W. Oliver, “Thermal conductivity of garnets and
`phonon scattering by rare-earth ions,” Phys. Rev. B (Solid State), vol. 4,
`p. 592, 1971.
`[11] T. Taira, S. Kurimura, J. Saikawa, A. Ikesue, and K. Yoshida, “Highly
`trivalent neodymium ion doped YAG ceramic for microchip lasers,” in
`OSA Trends in Optics and Photonics, Advanced Solid State Lasers, vol.
`26, M. M. Fejer, H. Injeyan, and U. Keller, Eds. Washington, DC, 1999,
`p. 212.
`
`Christian Stewen was born in Dortmund, Germany,
`in 1966. He received the diploma in physics from
`the Rheinisch Westfälische Hochschule Aachen, Ger-
`many, in 1993. He currently is pursuing the doctoral
`degree from Universität Stuttgart, Germany.
`He is presently a Laser Physicist with the Laser De-
`velopment Group of the Institut für Strahl-werkzeuge
`(IFSW), Universität Stuttgart, Germany. Since Jan-
`uary 2000 he has been with Rofin-Sinar Laser GmbH,
`Germany.
`
`Karsten Contag was born in Bonn, Germany, in
`1968. He received the diploma in physics from the
`Technische Universität Berlin, Germany, in 1996.
`He is currently a Laser Physicist in a collab-
`oration between the Forschungsgesellschaft
`für
`Strahl-werkzeuge (FGSW) and the Institut
`für
`Strahl-werkzeuge (IFSW). His main field of research
`is the numerical simulation of the thin disc laser.
`
`Mikhail Larionov was born in Sankt-Petersburg,
`Russia, in 1974. He received the diploma in physics
`from the State Technical University, Sankt-Peters-
`burg, in 1996.
`The topic of the diploma thesis was fast semi-
`conductor switches for high-power applications.
`Currently he is a Laser Physicist with the Laser De-
`velopment Group of the Institut für Strahl-werkzeuge
`(IFSW), Universität Stuttgart, Germany. His main
`field of research is the development of the thin disc
`laser.
`
`Adolf Giesen was born in 1946. He received the
`diploma in physics and the Ph.D. degree from the
`University of Bonn, Germany.
`In 1982, he joined the DLR (German Aerospace
`Establishment) working on RF-excited CO -laser
`development
`and
`characterization
`of
`optical
`components under high-power
`irradiation. Since
`1986 he has been Head of the Laser Development
`and Laser Optics Department of the Institut für
`Strahl-werkzeuge (IFSW) at
`the University of
`Stuttgart. His work is concentrated on thin disc
`lasers, phase coupling of diode lasers, and characterization of laser beams and
`of optical components.
`
`Helmut Hügel received the Dipl.-Ing. and Dr.-Ing.
`degrees
`from Technical University München,
`Germany, in 1962 and 1970, respectively.
`He is Director of the Institut für Strahl-werkzeuge
`(IFSW) and fu