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Optics & Laser Technology 34 (2002) 675677www.elsevier.com/locate/optlastec

940 nm low beam divergence tapered window laser arraysYi Qu a; b; , Baoxue Bo a; b, Xin Gao a, Guijun Hu b, Xingde Zhang a , Jiawei Shi b

aNational Key Lab. of High Power Semiconductor Lasers, Changchun University of Science and Technology, 7 Road Weixing, Changchun, 130022,Peoples Republic of China

bDepartment of Electronics Engineering, Jilin University, Changchun 130023, Peoples Republic of China

Received 3 September 2001; received in revised form 5 July 2002; accepted 26 July 2002

Abstract

The structures of 940 nm low vertical beam divergence semiconductor lasers grown by molecular beam epitaxy are presented. Thehigh power laser consists of an array of closely spaced tapered waveguides giving lower parallel beam divergence and window structure.The emission wavelength is 939 nm. The FWHM of the far eld pattern is 8 30 . The maximum continuous wave (CW) output powerof 30 W has been achieved. In the aging tests, the laser arrays have been operating for over 3000 h under the CW condition of 25 W.? 2002 Elsevier Science Ltd. All rights reserved.

Keywords: High power; Low beam divergence; Taper window laser array

1. Introduction

High power laser arrays lasing at 940 nm have been an in-

dispensable pumping source of Yb3+

YAG solid state lasers.Because the doubling wavelength of the solid state laser is515 nm close to that of Ar-ion laser (514 nm), it becomes possible to replace large volume Ar-ion laser. So it has beenfollowed more and more with interest in the laser world.High power laser arrays with low beam divergence are veryimportant for coupling output power into bers and for fo-cusing emitted power onto small areas. Various structures,such as large optical cavity [ 1] and other methods [ 2,3],have been proposed to expand the vertical optical mode sizeand to reduce the transverse beam divergence. In this letter,we report the laser structures with vertically integrated pas-sive waveguide (a low index layer) replacing bulk opticalconnement layers such as conventional large optical cav-ity structures to control the vertical beam divergence and toconne the electrical carriers. The tapered laser arrays had been proposed to achieve high output power [ 4,5]. We re- port here, the performance of a new design variant of ta- pered laser array having low parallel beam divergence withnonabsorbing facets. In the tapered laser conguration, thenarrow parallel section acts as a mode lter leading into thetaper section, which, since the output facet is anti-re ection

Corresponding author. Tel.: +86-431-5303282; fax: +86-431-5384517.

E-mail address: yiqu@public.cc.jl.cn (Y. Qu).

coated, operates principally as an amplier. So the deviceshave high output power.

Although the strained InGaAs quantum well active layeremployed in these devices resists the 1 0 0 dark line rapiddegradation mode of GaAs active layer semiconductorlasers, sudden failures and catastrophic facet damage dooccur at high power levels. A cure is provided through theuse of window structure with nonabsorbing facets whichis known [ 6 8] to be a very e ective approach for attain-ing strong suppression of facets degradation due to localheating around the facets. In this letter, we designed a newwindow laser array structure with nonabsorbing facets. Thedevice is highly resistant to catastrophic damage.

2. Design and fabrication

The low beam divergence 940 nm laser wafers are grown by molecular beam epitaxy (MBE). The laser structure isgrown on a GaAs (1 0 0) substrate tilted 4 towards the(1 1 1) A direction. It is composed of an Si-doped GaAs bu er layer, 0 :5 m thick; an Si-doped Al 0:4Ga0:6As lowercladding layer, 1 :5 m thick; a low index Al 0:6Ga 0:4Aslayer, 0 :15 m thick; an Al xGa1 xAs waveguide layer, x = 0 :6 0, 0:15 m thick; a GaAs spacer layer, 10 nmthick; an multiple-quantum-well (MQW) active regionin the center; a GaAs spacer layer, 10 nm thick; anAl xGa1 xAs waveguide layer, x = 0 0:6, 0:15 m thick;

0030-3992/02/$ - see front matter ? 2002 Elsevier Science Ltd. All rights reserved.PII: S0030-399 2(02 )0009 8-1

mailto:yiqu@public.cc.jl.cnmailto:yiqu@public.cc.jl.cn

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Fig. 1. Schematic diagram of high power low beam divergence 940 nmlaser structure.

Fig. 2. Schematic diagram of 940 nm taper array with window structurechip.

a low index Al 0:6Ga0:4As layer, 0 :15 m thick; a Be-dopedAl0:4Ga0:6As upper cladding layer, 1 :5 m thick;. and -

nally a p++ -doped GaAs capping layer, 20 nm thick. Theactive region contains three 7 nm InGaAs QWs with two12 nm GaAs barriers. The structure is shown in Fig. 1.

We have made a new design variant of tapered win-dow laser array having low parallel beam divergence. Thelaser array structure with nonabsorbing window structures isshown in Fig. 2. Each array incorporates 25 taper elements.Each element, which tapers outwards the output facet of thechipto a width of 40 m,ismadeupofa4 m wide, 200 mlong parallel section which leads into an 1800 m long sec-tion. The separation between the elements at the output endis 15 m. The active stripes within the array are separated

from each other at a centerline spacing of 55 m. In essence,the device operates with 4 m wide (laser) section pump-ing the fundamental mode of the tapered (amplier) section.Each of the laser elements are optically decoupled from oneanother at the output facets. In the tapered laser congura-tion, the narrow parallel section acts as a mode lter lead-ing into the taper section, which, since the output facet isanti-re ection coated, operates principally as an amplier.

Using quantum-well intermixing, we have fabricatednonabsorbing window structures on the tapered laser arrayto resist COMD in order to achieve a better result. Afterquantum-well intermixing process, optical absorption at thefacet is prevented by local bandgap widening in the vicinity

Fig. 3. Light-current characteristics under CW operation in the laser array.

of the facet. Suppression of the intermixing process can beachieved by protecting the sample surface with a 2 :0 m photoresist layer deposited before sputtering. To illustratethe spatial selectivity of the intermixing process, the sampleis rst coated with a layer of resist which was lithograph-ically patterned to dene intermixed (resist removed) andnonintermixed (resist remaining) regions of the sample. Athin lm (200 nm) of sputtered SiO 2 by plasma-enhancedchemical vapor deposition (PECVD) is sputtered on thesample. The samples are mounted carefully on a rapid

thermal annealer (RTA) and heated for 30 s in a N 2 envi-ronment (860 C). It can obtain the nonabsorbing windowstructures on the laser array facets.

Bars are cleaved to width of about 1 mm and their rear andfront nonabsorbing facets are coated with high re ectivitydielectric lm (Si =SiO 2) and low re ectivity dielectric lm(Al 2O3) by electron beam. The rear facets re ectivity is98% and front facets re ectivity is 2%.

3. Results

The devices are bonded p-side up onto copper heatsinksusing indium solder and mounted on a water-cooled stagewhich is held at 25 C for all experiments. The emissionwavelength of the window laser arrays is 939 nm at 30 W(CW). The linewidth (FWHM) of the laser array emissionis about 5 nm. Fig. 3 shows output-current characteristics of the laser array under DC operation. Continuous wave (CW)output power of 30 W has been achieved. The beam qualityfactor M 2 of individual tapered elements of the laser arrayswas measured with a beam analyzing system. M 2-values arevery similar at 30 W output power with an average of 3.1.The far-eld patterns are shown in Fig. 4. The FWHM are8 and 30 in the directions parallel and perpendicular to the

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Fig. 4. Far eld patterns of parallel and perpendicular to the junction plane at 30 W (CW).

Fig. 5. CW aging test result of the laser array at 25 C, 25 W.

junction plane, respectively. Fig. 5 shows the CW aging testresult of the 940 nm tapered window laser array under thecondition of 25 W. The laser array has been operating forover 3000 h without remarkable increase of Iop as shownin Fig. 5.

4. Summary

In summary, in this letter, we demonstrated 940 nm low beam divergence window laser arrays. The window laserarray consists of an array of closely spaced tapered wave-guides. The FWHM of the far eld pattern is 8 30 . Contin-uous wave (CW) output power of 30 W has been achieved.

References

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