Compact diode stack end pumped Nd:YAGamplifier using core doped ceramics

Compact diode stack end pumped Nd:YAGamplifier using core doped ceramics

Thomas Denis,1 Sven Hahn,1 Sandra Mebben,1 Ralf Wilhelm,1 Christian Kolleck,1,2

Jörg Neumann,1,2,* and Dietmar Kracht1,2

1Laser Zentrum Hannover e. V., Hollerithallee 8, D-30419 Hannover, Germany2Centre for Quantum-Engineering and Space-Time Research-Quest,

Welfengarten 1, D-30167 Hannover, Germany

*Corresponding author: [email protected]

Received 12 November 2009; accepted 8 January 2010;posted 13 January 2010 (Doc. ID 119877); published 2 February 2010

We report on a compact Nd:YAG amplifier emitting a maximum pulse energy of 14mJ. By amplifying apassivelyQ-switched oscillator (M2 < 1:2) a good beam quality ofM2

≈ 1:7was achieved. The amplifier isdiode pumped by an 8 bar diode stack of 800Wpower and a nonimaging optic. This optic homogenizes thepump light and transfers it into a 5mm diameter core doped rod with a centrally neodymium dopedregion of 3mmand a samarium doped YAG cladding. We show that this cladding reduces parasitic effectsin the laser rod compared to an undoped YAG cladding. Finally, we compare the compact amplifier withan amplifier, which is mode selectively pumped by a fiber coupled pump diode. © 2010 Optical Societyof America

OCIS codes: 140.3280, 140.3480, 140.3540, 080.4298.

1. Introduction

Compact high energy Nd:YAG amplifiers for pulsedlasers, which provide nearly diffraction limitedbeams after amplification, have many applicationsin outdoor, airborne, or space environments, e.g.,for range finding, LIDAR, laser altimetry, or remoteLIBS [1–3]. For most of these applications, stringentrequirements must be fulfilled, regarding efficiency,compactness, robustness, and reliability in a widetemperature range. These requirements can bemet by mode selective end pumping techniques usingfiber coupled diode lasers [2,4]. Unfortunately, fibercoupled diode lasers need complex beam shapingoptics. Hence, they have a large volume and mass.Additionally, the beam shaping optics, which is nec-essary for fiber coupling of the pump light, needs acritical optical alignment with respect to every singleemitter of the diode stack. Thereby, these optics are

conceptually sensitive to thermally and vibrationallyinduced misalignments.

Here, we present a concept for diode stack endpumped amplifiers to achieve more compact androbust systems. Additionally, only a temperaturestabilization of the diode stack within a few degreescentigrade is required in contrast to transversallypumped systems. The design is based on nonimagingtransfer optics and composite core doped ceramicswith a transversal dopant profile. Core doped ce-ramics were used before, but only in side pumpedamplifiers and end pumped oscillators [5–8]. Popula-tion inversion in core doped rods can be concentratedon a small volume, yielding a better spatial overlapbetween the seed laser beam and pumped volume,compared to homogeneously doped crystals of thesame diameter. Therefore, high extraction efficiencyis expected. Homogeneously doped crystals with asize equal to the core geometry of the ceramics wouldlead to major clipping effects [9].

The pump optics should have a high transfer effi-ciency and provide a pump light distribution similarto a system, which is mode selectively pumped by a

0003-6935/10/050811-06$15.00/0© 2010 Optical Society of America

10 February 2010 / Vol. 49, No. 5 / APPLIED OPTICS 811

fiber coupled pump diode. In this case, the beamquality after amplification remains nearly constantand efficient amplification takes place [2]. Pump op-tics designs for efficient transfer of the diode stack’sradiation into the laser rod are nonimaging optics[10,11]. This optical concept leads to pump lighthomogenization, but also to highly divergent pumplight at the end facet of the pump optics [12]. Effi-cient internal pump light guiding is then requiredin the laser rods, especially when the absorption coef-ficient in the rod decreases due to a temperature in-duced wavelength shift of the diode stack emission.Therefore the rod’s barrel surface is polished to allowinternal pump light guiding by total reflections.Unfortunately, a polished barrel surface limits themaximum gain due to parasitic effects. Recently, Yagiet al. proposed the suppression of these effects incored doped ceramics by using a Sm:YAG claddinginstead of an undoped YAG cladding [13]. Sm:YAGabsorbs the spontaneously emitted photons at awavelength of 1064nm and is transparent at thepump wavelength of 808nm. More recently, suppres-sion of parasitic effects in end pumped laser oscilla-tors was demonstrated [14].In this study, we first investigate the potential of a

Sm:YAG cladding for suppression of parasitic effectsin an end pumped configuration. Then we design thepump optics for transferring the diode stack’s radia-tion into the core doped rods. Finally, experimentalinvestigations on the compact amplifiers are pre-sented and compared to a setup using a fiber coupledpump diode.

2. Suppression of Parasitic Effects

The parasitic effects were investigated by measuringthe time resolved fluorescence of the laser rod, whenend pumped with a fiber coupled diode laser. Here, afiber coupled diode was used due to its well definedspatial emission characteristic at the fiber (Fig. 1).Both of the core doped ceramics (Baikowski Chimie)that we used had a polished barrel surface, a lengthof 25mm, and a diameter of 5mm with a central1 at:% neodymium doped region of 3mm (Fig. 2).Their cladding material consisted of undoped YAGor 4at:% doped Sm:YAG, respectively. The fiber co-upled diode laser (Dilas GmbH, N7F-806.2-1000Q-H207) emitted a maximum power of 900W in quasicontinuous wave (qcw) operating mode with a pumpduration of 200 μs and a repetition rate of 14Hz. Itsemitted spectral width was 2:5nm (FWHM), and itscenter wavelength could be tuned by varying thetemperature of the pump diode. The fiber diameterwas 800 μm. The pump radiation of this fiber was fo-

cused into the laser rod by two lenses, producing apump focus diameter of 1:33mm, adopted fromNeumann et al. [2]. The time resolved fluorescencealong the optical axis was detected by a photodiode,which was placed behind the dichroic mirror to sepa-rate pump light and fluorescence emission.

Figure 3 shows the results at increasing pump en-ergies for core doped ceramics with YAG andSm:YAG cladding. Purely spontaneously emittedfluorescence intensity IðtÞ is proportional to the po-pulation inversion NðtÞ. Therefore,

IðtÞ ¼ I0½1 − expð−t=τÞ� ð1Þ

is valid [9]. I0 is the emitted fluorescence intensity forcontinuously pumped lasers, t the time, and τ the life-time of the upper laser level (τ ≈ 230 μs). The fluores-cence intensity should increase during the pumppulse and decrease exponentially with τ after the

Fig. 1. Setup for measurement of the time resolved fluorescence.

Fig. 2. Geometry of the core doped ceramics.

Fig. 3. Representative time-resolved fluorescence intensity forselected pump energies (only a few measured curves are shown):(a) core doped ceramic with YAG cladding, (b) core doped ceramicwith Sm:YAG cladding.

812 APPLIED OPTICS / Vol. 49, No. 5 / 10 February 2010

end of the pump pulse. Therefore, parasitic effectscould be identified by a spiking phenomenon or aquasi-stationary state of the fluorescence duringthe pump pulse and its sudden decrease at the endof the pump pulse. This behavior was more pro-nounced for higher pump energies, indicating stron-ger parasitic effects. Obviously, Sm:YAG claddingsuppressed these effects, because the threshold forspiking was reached at higher pump energy of ap-proximately 97mJ instead of 24mJ for an undopedYAG cladding. Additionally, note that the YAGcurves at high pump energy look similar to relaxa-tion oscillations of a starting laser reaching a steadystate at the pump pulse’s end [Fig. 3(a)].The amplification in core doped ceramics was ana-

lyzed in more detail by setting up a master oscillatorpower amplifier system [Fig. 4(a)]. The master oscil-lator was a passively Q-switched Nd:YAG laser withpulse duration of 3ns, a nearly Gaussian beam(M2 < 1:2), and pulse energy of 2:3mJ [2]. Maximumefficiency of amplification was reached by varyingthe seed laser’s beam diameter. Hence, the spatialoverlap between seed laser and stored population in-version in the core doped laser rod was matched. Wemeasured a maximum amplified pulse energy of only4:5mJ for a YAG clad rod due to the strong parasiticeffects, which limited the maximum achievable gain[Fig. 4(b)]. Use of a Sm:YAG clad rod led to a maxi-mum pulse energy of 18mJ in the same configura-tion. In this case, parasitic effects did not limit thepulse energy up to pump energies of 150mJ. There-fore, we only used core doped ceramics with Sm:YAGcladding for the compact amplifier experiments.

3. Pump Optics Design

A compact, diode stack pumped amplifier requires apump optics, which has a high transfer efficiency andhomogenizes the radiation distribution. This pumpoptics was designed with the ray tracing programZEMAX (ZEMAX Development Corporation). Thediode stack (Dilas GmbH, N7Y-808.3-800Q-H156)consisted of eight fast axis collimated bars, eachemitting 100W in qcw operating mode. Its emissionarea was 13:5mm × 10mm. The diode stack’s radia-tion distribution was described by a manufacturermodel for ZEMAX. We used 1.5 million rays forthe ray tracing calculations and described the spec-tral distribution of the pump light by a Gaussian dis-tribution with 2:5nm width (FWHM).

The pump light was transferred into the rod bya special designed optical concentrator [Fig. 5(a)][11]. Its entrance facet was a little larger (14mm×11:6mm) than the diode stack’s emission area(13:5mm × 10mm) to obtain maximum incouplinginto the concentrator. The pump light is guided in-side the concentrator by total internal reflectionsat the wedge faces.

These reflections combined with the concentrator’sdiminution mixes the initially slow or fast axis emit-ted rays. Thereby, a less inhomogeneous pump lightdistribution is achieved at the concentrator’s exit fa-cet [inset Fig. 5(b)]. We used an exit facet of 1:2mm ×1:6mm to achieve many total reflections (approxi-mately 2–7) on a short concentrator length. The

Fig. 4. (a) MOPA setup in single path amplification geometry,(b) amplified pulse energy for optimized overlap between pumpvolume and seed laser beam.

Fig. 5. (Color online) (a) Schematic drawing of an optical concen-trator, (b) calculated transfer efficiency as a function of the concen-trator length. Dimensions of entrance and exit facet were keptconstant. Inset: spatial distribution of the transmitted light0:5mm behind the exit facet of the concentrator.


transfer efficiency at the exit facet could be scaleddue to the lower angles to the normal of the wedgesurface by increasing the length, which is shownin Fig. 5(b). As a compromise between length andtransfer efficiency, we purchased a concentrator witha length of 120mm.As mentioned before, we intend to concentrate the

population inversion on a smaller volume by usingcore doped rods instead of homogeneously dopedrods. This was investigated by comparing the absorp-tion in a 5mm diameter, homogeneously doped rodwith the core doped ceramics of Fig. 2. The distancebetween the laser rod and the concentrator’s exit fa-cet was 0:5mm, which reduces losses due to the rod’saperture. The pump light absorption in the laser rodswas calculated by segmenting it transversally into151 × 151 volume elements and longitudinally into100 elements, resulting in a spatial resolution of5 μm in the transversal and 250 μm in the longitudi-nal direction. The amount of absorbed pump powerwas approximately 100W larger in the core dopedrod than in the homogeneously doped rods (Fig. 6).Please note that, for the homogeneously doped rods,only the light absorbed in a cylinder of 3mm dia-meter was taken into account. Also outside of this vo-lume, pump light is absorbed, but this inversioncannot be used for amplification because a reason-able small seed beam diameter is needed to achievehigh spatial gain saturation and prevent beamclipping.The detailed pump light absorption in the core

doped rod is shown in Figs. 7(a) and 7(b) for a pumpwavelength of 805nm. Nearly all incident pump lightwas absorbed because of the rod’s length and thepump light guiding by total reflections. After about5mm, where the first total reflections take place,64% of the incident light is already absorbed. The re-maining part of the pump light was absorbed in thelast 20mm of the rod. The distribution of the ab-sorbed pump light along the optical axis changedwith pump wavelength, but the absorption inte-

grated along the optical axis remained nearlyconstant.

The calculated distribution was verified bymeasuring the emitted fluorescence parallel to theoptical axis (FPOA). The FPOA distribution is pro-portional to the absorbed pump light integratedalong the optical axis, if we neglect parasitic effects.Experimentally, the FPOAwasmeasured by imagingthe emitted fluorescence using a telecentric opticalmapping with a numerical aperture of 3 × 10−3. InFig. 8 the cross sections through the centroids of slowand fast axes of the measured FPOA are comparedwith the calculated absorption revealing only minordeviations in the fast axis direction, which mightresult from the unknown smile factor of the diodestack.

4. Laser Amplifier Results

In order to compare the results obtained with the op-tical concentrator described in the previous section,we built a double pass amplifier pumped by the fibercoupled diode mentioned in Section 2 [Fig. 9(a)]. The

Fig. 6. Numerical calculations of the pump light absorption in a3mm diameter cylindrical volume around the optical axis for coredoped and homogeneously doped rods versus pump wavelength.

Fig. 7. (Color online) Numerical calculated distributions of pumplight absorption at a pump wavelength of 805nm in the core dopedrods: (a) integrated absorption along the optical axis, (b) distribu-tion along the optical axis. All distances are measured from theend face of the ceramic at the pump side.


pump optics consisted of two lenses with focallengths of 30mm and 50mm. For the amplifier, weused core doped ceramics with Sm:YAG claddingwith the same dimensions as mentioned before,but with a dichroic mirror directly coated on oneend facet. This allows a more compact design ofthe double pass amplifier. In addition, we used thepreviously noted master oscillator. The laser pulseemitted by the oscillator passed through the ampli-fier stage in a slightly V-shaped configuration in theslow axis direction. The amplification was optimizedby varying the spatial overlap between the laserbeam and population inversion. This was achieved

by changing the seed laser’s diameter in the laserrod. The optimized result is shown in Fig. 10, wherethe scale of the pump energy takes into account thelosses of 33% due to the fiber coupling of the pumplight. This resulted in an optical to optical efficiencyof the fiber-pumped amplifier of about 12%. The am-plified beam had a beam quality of M2

≈ 1:5, andtherefore the beam quality of the master oscillatorwas nearly preserved.

When the pump optics were exchanged by the com-pact pump configuration, consisting of a diode stackand concentrator [Fig. 9(b)], the beam quality of theamplified beam remained good (M2

≈ 1:7). The mea-sured beam diameters show that the extreme asym-metric distribution of the diode stack radiation wasnot transferred to the amplified beam profile(Fig. 11). The focal points in fast and slow axes werenearly identical, which means that the optical con-centrator successfully improved the pump light dis-tribution. The maximum pulse energy of theamplifier was 14mJwhen pumped with a diode stackpump power of 160mJ (Fig. 12). This amount ofpump power would result in a pulse energy of ap-proximately 20mJ, when the amplifier is pumpedby a fiber coupled diode. However, this fiber basedpump setup needs a ten times larger volume, wherewe have not yet taken into account the additionalspace needed for fiber accommodation and pumpoptics. The reason for the lower output energy ofthe compact amplifier is due to the larger diameter

Fig. 8. Cross section through the centroid of measured FPOAdistribution and calculated pump light absorption at a pumpwavelength of 806:5nm: (a) fast axis, (b) slow axis.

Fig. 9. (a) Double pass amplifier setup in V-shaped configurationpumped by a fiber coupled diode, (b) compact amplifier pumped bydiode stack and concentrator.

Fig. 10. Amplifier slope when pumped by a fiber coupled diode.

Fig. 11. Measured beam caustic after a lens with focal length of300mm, when amplified with the compact amplifier.


of the pump volume. It is about 3mm in diameter,compared to the fiber coupled system with a dia-meter of about 2:3mm. Therefore, a larger beam dia-meter of the seed laser was required for a similaroverlap between stored inversion and the seed beam.This reduces the intensity of the seed beam, whichalso reduces the gain saturation in the amplifierand consequently the pulse energy at the output.However, in further experiments this could be opti-mized by using a smaller core. Finally, the tempera-ture stability of the compact amplifier was alsomeasured. The amplified pulse energy is nearly con-stant, when varying the diode stack’s temperature ina range of 12K (inset Fig. 12), which is comparable tofiber coupled pump schemes.The advantage of the compact amplifier stage,

compared to the fiber coupled pump scheme, is itscompact design. The whole amplifier setup was tentimes smaller in volume than the fiber coupled diodelaser used. Still, its beam quality was similar and itspulse energy of 14mJ acceptable. We also showedthat the amplifier required only a temperature sta-bilization of the diode stack within a few degreesKelvin. Therefore, the concept possesses a key factorfor applications in harsh environments. Further po-tential of miniaturization exists as well, regardingthe pump optics, where suitable curvatures in slowand fast axes on the entrance facet and the exit facetcould be applied.

5. Conclusion

In conclusion, we described a compact and robust endpumped amplifier using ceramic composite coredoped Nd:YAG. We demonstrated that a Sm:YAGcladding instead of a YAG cladding led to reductionof parasitic effects due to the absorption of Sm:YAGat a wavelength of 1064nm. The design of pump op-tics was described, which enabled a pump light ab-sorption, which is similar to bulky fiber coupled

pump configuration. The amplified beam had a pulseenergy of about 14mJ, and its beam quality was good(M2 < 1:7). This was achieved with a setup, which isabout 10 times smaller than a fiber coupled pumpdiode. Additionally, the diode stack required only te-mperature stability within a few degrees centigrade.

The authors thank the German Research Founda-tion (DFG) and the Cluster of Excellence Centre forQuantum Engineering and Space-Time Research(QUEST) for funding.

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Fig. 12. Amplifier slopes of the compact amplifier at differentdiode stack temperatures, i.e., pump wavelengths.


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Compact diode stack end pumped Nd:YAGamplifier using core doped ceramics