experimental realization of monolithic diffractive broad-area polymeric waveguide dye lasers
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IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 43, NO. 7, JULY 2007 545
Experimental Realization of Monolithic DiffractiveBroad-Area Polymeric Waveguide Dye Lasers
Alexander Büttner and Uwe D. Zeitner
Abstract—We discuss the first experimental realization of apolymeric waveguide dye laser with intracavity diffractive el-ements. Due to a special technology used the diffractive phasestructures are directly integrated into the waveguide layer. Thus,there is no need for additional external optics or extensive align-ment effort. The elements are used to support a Gaussian-likefundamental mode while at the same time suppress the unde-sired higher order transverse laser modes. The technology hasthe potential to be used also for improving the beam quality ofsemiconductor broad area lasers.
Index Terms—Diffractive optics, laser resonators, polymeric dyelasers, waveguide lasers.
ACOMMON method to increase the output power of awaveguide laser while preserving a long lifetime is to
increase the resonator width to obtain a broad-area laser.However, in this case several transverse modes can oscillatesimultaneously yielding a poor beam quality. The demand tomaintain both a good beam quality and a high output powerhas lead e.g., to the development of the tapered lasers and am-plifiers  where the round-trip losses of higher order modesare increased by introducing a modal filter (e.g., single-modewaveguide), and to the development of the -distributed feed-back (DFB) laser  where higher order modes experiencelarger Fresnel losses than the fundamental mode.
Another method is to apply special diffractive phase ele-ments. By introducing diffractive elements into the resonatorboth the shape of the fundamental mode can be altered andespecially the modal discrimination can be increased, whichmeans a substantial reduction of the number of oscillatingtransverse modes. The idea of changing the shape of transverseeigenmodes in stable optical resonators by using asphericalgraded-phase mirrors was proposed by Bèlanger and Parè and experimentally demonstrated in a CO laser . Furtheron, Leger et al. applied the idea of diffractive mirror resonatorsto solid-state lasers , . Even in the field of semiconductorlasers external diffractive mirror resonators have been appliedto improve the transverse modal behavior of diode laser arrays and later on to achieve a super-Gaussian fundamental modewith large modal discrimination in a broad-area semiconductorlaser .
Manuscript received December 20, 2007; revised March 23, 2007.A. Büttner is with the Vistec Semiconductor Systems GmbH, 35578 Wetzlar,
Germany.U. D. Zeitner is with the Fraunhofer Institut fuer Angewandte Optik und Fein-
mechanik, 07745 Jena, Germany (e-mail: [email protected]).Digital Object Identifier 10.1109/JQE.2007.899471
Fig. 1. Top view of the monolithic diffractive broad-area laser emitting aGaussian fundamental mode U(y; z ) (L—resonator length, W—resonatorwidth).
However, particularly in the field of semiconductor lasers,the use of external diffractive mirrors and additional intracavitydiffractive elements makes the alignment of the experimentalsetup much more complicated, and requires a precise and ex-pensive adjustment. These difficulties could be avoided by usingthe precise fabrication techniques of integrated optics to inte-grate the diffractive structures directly into the active waveguideregion . The diffractive structures could be etched preciselyinto the cladding as a part of the fabrication process. Thus, thereis no need for additional external optics and alignment effort forsuch diffractive broad-area lasers.
The monolithic diffractive broad-area laser is displayedschematically in Fig. 1. The underlying principle is, that thevarying waveguide thickness changes the effective refractiveindex of the propagating fields and in this way the etchedstructures act as diffractive phase elements. The fabricationprocess has already been successfully demonstrated in and  where lens-like structures have been etched into theresonator to reduce the divergence of the laser beam.
In , we proposed a design for a diffractive broad-arealaser with a Gaussian fundamental mode and a strong suppres-sion of higher order modes. In this paper we want to provethe design concepts developed in  by discussing experi-mental results. Because of reasons of technological availabilitythe broad-area lasers have been fabricated not yet as semicon-ductor diode lasers, but as distributed Bragg reflection (DBR) polymeric waveguide dye lasers.
Up to now, for polymeric waveguide dye lasers, the in-vestigation of transverse mode distribution and intracavitybeamshaping has not been a field of interest at all. Therefore,one topic of this article is to demonstrate the unique possibilitiesof doing experiments with polymeric waveguide dye lasers. Webegin with a short repetition of the design concept in Section II.In Section III, we describe the fabrication technology of the
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546 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 43, NO. 7, JULY 2007
polymeric dye lasers, which is quite different from the fabrica-tion of diode lasers and discuss the calculated modal behavior.Experimental results are presented and discussed in Section IV.
II. BASIC DESIGN PROCEDURE
In this Section we recall the design concept of the monolithicdiffractive broad-area laser which is described in much moredetail in .
The aim of the design is to achieve a stable broad-area laserwith a Gaussian fundamental mode
in the -direction at the position of the output mirror atas displayed in Fig. 1 ( means the beam waist). In generalbroad-area lasers are designed to subtend only the fundamentalwaveguide mode in the -direction, which is specified bythe layer geometry of the waveguide. In this case, the design taskfor can be separated from the field andis considered to propagate like a field within a homogeneous,dielectric medium having the refractive index , where
denotes the local effective refractive index of the waveguidemode at the current position.
The beam waist is chosen to be becausein this case the fundamental mode experiences only negligibleloss at the aperture of the mirrors. Our design considerationsare restricted to plane mirrors having both the same aperturesize and a resonator length equal to the length of the ac-tive waveguide region. To establish the Gaussian distribution asfundamental mode of the resonator, the phase of the distribution
needs to be conjugated at the plane back mirrorlocated at . The condition is satisfied by introducing awell-suited diffractive phase element (structure 2 in Fig. 1) di-rectly in front of the back mirror. The special shape of the phaseelement near the outcoupling mirror (structure 1 in Fig. 1) ischosen in order to maximize the round-trip losses of the higherorder modes while preserving a low-loss fundamental mode.
The relation between the extension of the diffractive struc-ture and the resulting phase difference is given by
, where is the vacuum wavelength, isthe refractive index difference between the diffractive structureand the surrounding medium and is the desired phasedifference at position . In the case of waveguides, is similarto the effective refractive index of the waveguide mode andthe achievable index difference is typically much smallerthan in free-space.
Consequently the maximum extension of the diffrac-tive waveguide structures needs to be much larger than in thecase of free-space diffractive phase elements. Thus, diffractionof the fields while propagation through the structured regionsand especially at shape discontinuities does influence the ampli-tude and phase of the fundamental mode and increase the round-trip losses tremendously. Therefore, the choice of the shape ofthe diffractive structures needs to be done with care. In the caseof our design, the structure near the outcoupling mirror waschosen to act as a positive lens which focuses the fundamental
Fig. 2. Calculated field amplitude distribution inside the diffractive waveguidelaser resonator.
Fig. 3. Layout of the monolithic diffractive broad-area polymeric dye DBRlaser.
mode distribution having the energy concentrated near the res-onator axis (see. Fig. 2). In this way, the phaseof the fundamental mode is even smooth in the case of smallindex differences and the corresponding phase element near theback mirror is smooth as well near the optical axis . Outsidethe lens-like region, a blazed grating-like phase structure hasbeen introduced (see Fig. 1) to deflect the energy of the higherorder modes which is even located away from the resonator axis,out of the excited resonator region. In this way the losses of thehigher order modes can be increased additionally.
III. FABRICATION OF THE LASERS
A. Layout and Fabrication Process
The design concept has been realized using dye-doped, poly-meric waveguide lasers. Since the fabrication of high-qualityfacets is not practicable in the case of the chosen polymericwaveguide configuration, we had to use Bragg gratings as out-coupling and back mirrors. Both, the macroscopic diffractivestructures and nanometer-sized Bragg gratings have been fab-ricated in a single process by electron beam lithography. Thee-beam system used is a Leica LION LV1 machine operatingwith an acceleration voltage of 20 kV.
BÜTTNER AND ZEITNER: BROAD-AREA POLYMERIC WAVEGUIDE DYE LASERS 547
TABLE IPARAMETERS OF THE DIFFRACTIVE DBR-WAVEGUIDE LASERS
The layout of the resonator is shown in Fig. 3. Correspondingparameters and material data are listed in Table I. As activemedium we used PMMA doped with the well-known dyeRhodamine 6G. The thickness of the active layer (about 1 m)was chosen rather large to achieve a high gain but too small toallow more than the fundamental waveguide mode to be guidedin the vertical -direction. The Rhodamine 6G concentrationof 6 mM ( Rhodamine 6 G/liter PMMA) turnedout to deliver the highest gain value (see Section IV). Thewaveguide fabrication was done by spin-coating a solutionof R6G:PMMA:MAA (MAA—Methyl Aceto-Acetate) ontothe thermally oxidized silicon wafer. After applying a properheating treatment to remove the solvent the wafer has beensputtered with a thin layer of gold in order to avoid chargingduring exposure. Afterwards the variable dose electron beamexposure of the desired pattern has been performed. For thisstep we used the property of PMMA’s dissolution rate in asubsequent development process to be sensitive to the appliedelectron dose. Thus, after removing the gold layer the exposedR6G:PMMA was developed to obtain the surface profile ontop of the waveguides. The electron dose needs to be locallyvariable because the ideal depths of the subwavelength gratings( nm) and the macroscopic diffractive structures( nm) are different but the development time toachieve these depths for all waveguide regions needs to beequal for technological reasons. The structure depths have beenoptimized in several fabrication runs to minimize resonatorlosses. In the case of smaller structure depths the extensionof the diffractive structures would need to be even largerthan m and the influence of diffraction onthe round-trip losses and the shape of the fundamental modedistribution would increase. In the case of deeper structures, thesurface roughness of the structures would be too large and thegrating quality turned out to be too bad to achieve the desiredGaussian-like fundamental mode.
In the fabrication process, several patterned and unpatternedwaveguide lasers have been placed on one substrate to allowa direct comparison of the experimental results. An overviewimage of several waveguide lasers is shown in Fig. 4(a). Fig. 4(b)and (c) are microscope images of the patterned regions near theoutcoupling mirror and the back mirror. A SEM picture of the
Fig. 4. Images of the fabricated waveguide lasers. (a) Top view. (b)–(c) Mi-croscope images of the diffractive phase elements. (d) SEM image of the Bragggrating in the back mirror.
first-order Bragg grating section in the back mirror region isdisplayed in Fig. 4(d).
B. Design of the DBR Mirrors
The calculation of the length, period, and the reflectivity ofthe mirrors has been done according to ,  using the pa-rameters displayed in Table I. In the outcoupling region we useda second-order Bragg-grating (power reflectivity )to be able to couple out laser emission perpendicular to thewafer surface and not at the facets of the waveguide. In theback coupling region we used a first-order grating to obtain acalculated reflectivity of . The length of the Bragggrating was chosen to be 200 and 300 m in the outcouplingand back coupling regions, respectively. The grating periods of
nm and nm in the back-coupling and out-coupling regions, respectively, is adapted to achieve laser emis-sion at the wavelength nm. At this wavelength thehighest gain value can be achieved in the current waveguide con-figuration (see Section IV). In the case of the first order gratingthe width of the grating lines was chosen to be , in thecase of the second order grating . A small deviation of thelinewidth from these theoretical values will cause a small vari-ation of the coupling constant and thus a change of the mirrorreflectivity but does not affect the transversal mode shape. It
548 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 43, NO. 7, JULY 2007
Fig. 5. (a) Calculated round-trip losses of the fundamental mode V and thesecond modeV (assuming 100% reflectivity within the mirror aperture) for dif-ferent structure depths with respect to the ideal depth �d = 197 nm (viz effec-tive refractive indexes �n ) and constant structure extension �z = 133 �m.(b) Calculated fundamental mode intensity distributions for different structuredepths �d.
is therefore of minor concern for the presented experiments aslong as the devices are showing laser operation.
C. Calculation of the Modal Behavior
Keeping the right structure depth at the fabrication of thelasers seems to be very important to obtain the desired fun-damental mode distribution and transverse modal behavior.To determine fabrication tolerances of the structure depth forthe given resonator configuration we calculated the losses ofthe fundamental and second order mode for different nonidealstructure depths resulting in varying effective refractive indexeswhile keeping the extension of the structures in propagationdirection constant to be m. To calculate the modalbehavior we used a Fox–Li analysis . In the calculations,we ignored gain effects, but considered diffraction while prop-agating the fields through the patterned regions. Furthermore,the reflectivity of the mirrors is assumed to be 100% for themode calculation because this does not affect the transversalamplitude distribution of the modes. Thus, the round trip lossescalculated here include only absorption at mirror apertures dueto diffraction effects. The analysis method is discussed in detailin . The results of the calculations are shown in Fig. 5.In Fig. 5(a), the round-trip losses of the fundamental mode
and the second order mode are plotted for a range ofstructure depths with respect to the ideal depth nm.In the case of the ideal depth the fundamental mode loss is
Fig. 6. Sketch of the measurement setup for the characterization of the modalbehavior of the lasers. In the top region the whole measurement setup is shown(PD: Polarization rotator; CCD: Camera; W: waveguide laser; 1: outcouplingmirror; 2: excited region; 3: back mirror), the bottom region shows the orienta-tion of the waveguide inside the setup.
only , and the loss of the second order modeis considerably larger. Thus, a good suppression
of higher order modes should be possible for this resonatorconfiguration. Although and increases for differentstructure depths, the strong suppression of higher order modeswill be preserved. In the case of the unpatterned laser of thesame geometry, the difference in the energy round-trip lossesof the fundamental mode and the second order mode is onlyabout 3%. Therefore, multi-mode operation will be obtainedat a much smaller gain level than in the patterned resonator.In Fig. 5(b) the intensity distribution of the fundamental modeis displayed for the ideal depth nm and the caseswhere the depth is only 39 nm too flat or 33 nm too deep.In the case of the optimal depth nm the shape ofthe fundamental mode is nearly Gaussian with a beam qualityfactor . As it can be seen the relatively smallchanges in the structure depth have a strong influence on theshape of the fundamental mode distribution and therefore thetolerances in the fabrication of the structure depth need to bevery tight to only a few nanometers. On the other hand themeasured fundamental mode shape could be used to determinethe actual structure depth.
A. Measurement Setup
The experimental setup for the characterization of the lasersis displayed in Fig. 6. As excitation source we used a frequency-doubled Nd:YAG laser (Quanta Ray LAB 130, Spectra Physics)at emission wavelength 532 nm, pulse duration 6 ns, repetitionrate 10 Hz, and output energy 200 mJ. The pulsed Nd:YAGlaser has been chosen to allow an effective excitation of theused dye Rhodamine 6G. The excitation area is defined by avariable rectangular slit which is imaged onto the waveguide.The output beam is linearly polarized. The polarization rotatoris used to align the linearly polarized excitation beam parallelto the -direction. In this case the gain of the transverse electric(TE) modes in the propagation direction ( -direction) reaches itsmaximum and the gain for the transverse magnetic (TM) modes
BÜTTNER AND ZEITNER: BROAD-AREA POLYMERIC WAVEGUIDE DYE LASERS 549
Fig. 7. Measured fluorescence of the excited 6-mM Rhodamine 6G:PMMAwaveguide and spectral distribution of the lasers normalized to maximumintensity.
is much smaller and thus only the TE mode will oscillate. Sinceour design is optimized for the effective index of the TE funda-mental mode the shapes of the TM fundamental mode would bedifferent and falsify our experimental results.
The waveguide is tilted by 45 with respect to the optical axisof the excitation beam (similar to the setup in ). Thus, a partof the laser mode is coupled out of the waveguide by the seconddiffraction order of the outcoupling grating and can be imagedonto a CCD camera for characterization. This configuration ef-ficiently separates the laser mode from fluorescent light outsidethe aperture of the lasers. Due to the use of Bragg gratings forthe resonator mirrors and outcoupling of modes the difficult re-alization of high quality laser facets for the polymer waveguideswas not necessary.
B. Determination of Gain Coefficient and Spectral Distribution
The fluorescence spectra and the laser spectra have been mea-sured using a fiber spectrometer having a spectral resolution of1 nm. A typical fluorescence spectrum and laser spectrum is dis-played in Fig. 7. The fluorescence spectrum has been measuredat the laser threshold excitation intensity W/cmin a waveguide region between the lasers. Considering the reso-lution of the spectrometer the spectra of various lasers agree wellwith the target emission wavelength of nm. Patternedand unpatterned lasers obey the same spectral distributions. Asexpected in the case of the chosen excitation configuration, thelaser modes are TE-polarized. The spectral width of the laserpeak in Fig. 7 is mainly caused by the resolution of the spec-trometer of about 3 nm.
The dye concentration was chosen to be 6 mM Rhodamine6G:PMMA since in this case a maximum gain coefficient wasmeasured. To determine the gain coefficient we followed themethod described in . After the fabrication of the lasers thegain coefficient was determined at the laser threshold excitationintensity to be cm in an unstructuredregion of the waveguide. The excitation width was 260 m, thelargest excitation length 3.5 mm. Similar values of the small-signal gain coefficient of R6G:PMMA waveguides have beenreported by Ulrich and Weber in .
Fig. 8. Measured near-field intensity distributions of the broad-area laser(a) without and (b) with diffractive phase elements (1: outcoupling mirror;2: excitation region). The pumping intensity was I = 1:4I ; the thresholdintensity for the patterned laser was I � 0:7 � 10 W/cm .
C. Comparison of Unpatterned and Patterned Lasers
The different transverse modal behavior of unpatterned andpatterned lasers can be illustrated clearly by exciting both pat-terned and unpatterned lasers under exactly the same conditionsand measuring the near-field distributions in the outcouplingregion. The results of this basic experiment are displayed inFig. 8. In the upper parts of Fig. 8(a) and (b) the same regionon the wafer is shown. The difference is that the wafer has beenmoved upwards relative to the pumping area (2) to excite ei-ther an unpatterned [Fig. 8(a)] or a patterned laser [Fig. 8(b)].In the bottom parts of the Fig. 8(a) and (b) the correspondingoutcoupling regions (1) and the cross sections of the measurednear-field distributions are plotted in detail. The pumping inten-sity was chosen to be , the threshold intensity forthe patterned laser was again W/cm and thelength of the excited area was 3 mm.
It can be seen that although the width of the excitation areawas larger than the width of the laser mirror the intensity of thepatterned laser is concentrated only around the resonator axisand in the case of the unpatterned laser the output intensity isspread over the whole width of the mirror. The reason for thisbehavior may be that in the case of the unpatterned laser sev-eral transverse modes can oscillate simultaneously. To confirmour assumption we tried to separate the fundamental mode ofthe unpatterned laser by reducing the width of the pumping areato a small pumping stripe around the resonator axis. The cor-responding near-field distribution is shown in Fig. 9(a). It canbe seen that the near-field changed to a Gaussian-like distribu-tion. If we enlarge the width of the pumping area, a second mode
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Fig. 9. Measured near-field distributions for varying excitation widths and thesame excitation intensity in the case of a laser without diffractive phase ele-ments. In (a), the excitation width was smaller than in (b). In both cases thewidth was smaller than the aperture width of the resonator. At the top of (a) and(b), the near-field distributions in the outcoupling region is displayed, the bottomshows the corresponding cross sections.
Fig. 10. Comparison of the measured (straight lines) and calculated (dashedlines) near-field intensity distributions of different patterned lasers. In the calcu-lations the ideal structure depth of the diffractive elements has been considered.(I : excitation intensity; I : laser threshold excitation intensity).
which has near-zero intensity at the resonator axis [see Fig. 9(b)]can oscillate in addition. In contrast to the unpatterned resonatorthe shape of the intensity distribution of the patterned laser doesnot change when the width of the pumping area and the excita-tion intensity is increased. This demonstrates that the suppres-sion of higher order modes has been improved by integratingphase elements into the resonator.
To support our assumption we compared the calculated fun-damental mode distributions of the patterned resonator with themeasured near-field intensity distributions. In Fig. 10, the mea-sured and the calculated distributions are shown for two dif-ferent patterned resonators fabricated on the same wafer. In thefabrication process a set of lasers with varying structure depthhas been realized. The displayed measured near-field distribu-tions have been obtained from devices having the optimal struc-
Fig. 11. Comparison of the measured (straight lines) and calculated (dashedlines) near-field intensity distributions of one patterned laser at different excita-tion intensities (I : excitation intensity; I : laser threshold excitation inten-sity). In comparison to the lasers in Fig. 10, the real structure depth has beenfabricated too flat. For the calculation of the fundamental mode we supposedthe structure depth to be 40 nm below the ideal depth.
ture depth within the measurement accuracy of about 3%. Thus,for the calculations we also supposed the optimal structure depthof nm. The shapes of the measured and calculateddistributions agree especially in the case of the laser shown inFig. 10(b) in the center region. However, at the margins the mea-sured intensity distributions do not vanish. Reasons for this be-havior may be that in the Fox–Li analysis a so-called empty res-onator model has been supposed without considering the gainsaturation in regions of high intensities in comparison to regionsof low intensities . Other reasons may be the occurrence ofamplified spontaneous emission  and straylight. Imperfec-tions in the waveguide and the patterned regions may also be thereason for the statistical, speckle-like intensity variations.
As mentioned in Section III (Fig. 5) the matching of the struc-ture depth is very important to obtain the Gaussian-like fun-damental mode shape. We want to support our theoretical as-sumption by another experiment. One wafer with waveguidelasers has been fabricated to have a reduced depth of (157 5)nm of the diffractive structures which are thus about 40 nm tooflat. The measured fundamental mode of this laser is shown inFig. 11(a) at threshold and Fig. 11(b) well above threshold. Thecorresponding calculated fundamental mode intensity distribu-tions are shown as well. For the calculations we used the mea-sured depth value nm which is 40 nm below theoptimal depth. As can be seen, the theoretical and experimentalresults agree very well. Especially in the case of a high excita-tion intensity [Fig. 11(b)] the curves agree in the center and at
BÜTTNER AND ZEITNER: BROAD-AREA POLYMERIC WAVEGUIDE DYE LASERS 551
the margins as well. The reason for the decreasing signal under-ground may be that in the case of higher excitation intensitiesthe relation between the coherent laser mode intensity and inco-herent back ground radiation changes. The observed asymmetryin the measured intensity pattern are mainly caused by unde-sired imperfections of the waveguide surface. Because there isno upper cladding layer also small surface modulations do havea measurable effect on the lateral field distributions.
In this paper, we have discussed the experimental realizationof the first monolithic diffractive broad-area waveguide laser.The patterned lasers have been fabricated using variable doseelectron beam lithography. We demonstrated the strong sup-pression of higher order modes by using diffractive phase el-ements etched into the cladding layer of the waveguide. In thepast diffractive elements inside of resonators have been appliedto shape the transverse modal distributions in free-space diffrac-tive mirror resonators. We have shown that the underlying prin-ciples even works in the case of nanosecond-pulsed polymericdye lasers and can also be used in combination with the ex-tended reflection in Bragg resonator mirrors. Although effectsof the active medium have been ignored in the calculations, thetheoretical results agree with the experimental ones. Thus, theFox–Li analysis can be applied to describe even the transversemodal behavior of relatively high-gain pulsed waveguide dyeDBR lasers.
The next step is the experimental realization of monolithicdiffractive broad-area semiconductor lasers.
The authors would like to thank the group of E.B. Kley(Department of Applied Physics, Friedrich–Schiller UniversityJena, Germany) and especially D. Schelle for the lithographicexposure of the microstructures for the waveguide lasers,S. Nolte (Department of Applied Physics, Friedrich–SchillerUniversity Jena) for the possibility to use the laser laboratory,M. Will for laboratory assistance, and D. Radtke for principalmaterial investigations.
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Alexander Büttner received the diploma and Ph.D. degree in physics from theFriedrich–Schiller University, Jena, Germany, in 2001 and 2005, respectively.
From 1999 to 2005, he was with the Fraunhofer Institute of Applied Op-tics and Precision Engineering, Jena, Germany. Since June 2005, he is with theVistec Semiconductor Systems GmbH, Wetzlar, Germany. His research includesthe design of microoptical systems, especially for LED beamshaping and laserresonators, the fabrication of microoptical elements by laser beam writing andthe development of optical systems for wafer inspection.
Uwe D. Zeitner received the diploma and Ph.D. degree in physics from theFriedrich–Schiller University, Jena, Germany, in 1995 and 1999, respectively,where he worked in the field of fabrication of microoptical elements and systemsby e-beam lithography.
Since 1999, he has been with the Fraunhofer Institute of Applied Optics andPrecision Engineering, Jena, Germany. His field of research is optical design,microlithographic fabrication techniques, and application of microoptical ele-ments. He is particularly interested the development of laser resonators withintracavity diffractive optical elements.