1180 nm VECSEL with output power beyond20 W

S. Ranta, M. Tavast, T. Leinonen, N. Van Lieu, G. Fetzer andM. Guina

The highest power result for an optically-pumped single-chip verticalexternal-cavity surface-emitting laser with emission near 1180 nm isreported. The gain mirror was grown by molecular beam epitaxy andincorporated a strain compensated GaInAs/GaAs/GaAsP activeregion. An intra-cavity diamond heat spreader was attached to thegain mirror for thermal management. In free-running operation, thelaser emitted more than 20 W at a mount temperature of about12 °C. The output spectrum was centred between 1165–1190 nmdepending on the mount temperature and pump power. By using anintra-cavity birefringent filter, the full width at half-maximum line-width could be narrowed to ≤1 nm and at the same time achievedapproximately 14 W of output power near 1178 nm. Moreover, thelasing wavelength could be tuned over more than 40 nm.

Introduction: High-power lasers emitting at 1160–1200 nm [1–3] haverecently gained increased attention owing to their ability to generateyellow-orange radiation via second-harmonic generation (SHG).

Vertical external-cavity surface-emitting lasers (VECSELs) that useGaInAs/GaAs/GaAsP gain mirrors have proven to be suitable for highpower operation near 1000–1120 nm spectral range [4, 5]. However,the high lattice strain associated with the high indium (In) compositionof the GaInAs quantum wells (QWs) has hampered the use of thismaterial system for longer wavelengths (1160–1200 nm) that arerequired for generating yellow-orange radiation. The highest outputpower reported for an 1175 nm GaInAs/GaAs/GaAsP VECSEL gainmaterial grown by metal-organic vapour-phase epitaxy (MOVPE) is∼8 W [3]. Alternatively, we have developed GaInNAs/GaAs/GaAsNgain material by molecular beam epitaxy (MBE), where the QWs areless strained, which led to the demonstration of 11 W at ∼1180 nm [6].

In this Letter, we demonstrate that a MBE-grown GaInAs/GaAs/GaAsP gain chip is also capable of emitting high output powers at thechallenging wavelength of ∼1180 nm, which corresponds to theyellow spectral range after SHG.

Experimental setup: The VECSEL consisted of a gain mirror and acurved output coupler that a formed a straight I-cavity, as shown inFig. 1. The radius of curvature (RoC) of the coupler was 150 mm,and the spacing between the gain chip and the cavity end mirror was∼150 mm.

output

BRF

curvedmirror

150 mm

pump laseroptics

pump laser

pump laser

pump laseroptics

gain chip

Fig. 1 Illustration of I-cavity laser setup. The birefringent filter (BRF) wasused in the wavelength tuning experiment

The GaInAs/GaAs/GaAsP gain medium was grown by MBE andcomprised a total of 10 QWs with a target In composition of ∼37%(nominal) in order to reach the 1178 nm emission wavelength duringlaser operation. The gain mirror design was similar to the 1120 nm struc-ture described in [7] except for the increased amount of In in the QWsand the use of 2 nm thicker strain compensation layers. In comparison tothe structure reported in [7], we reduced the QW growth temperaturefrom 545 to 460 °C to avoid relaxation of the QWs; a lower growthtemperature increases the critical thickness of the GaInAs layer [8].

To remove the heat efficiently, the gain mirror was cut into 2.5× 2.5mm chips that were capillary bonded [9] from the epi-side to anintra-cavity diamond (3 × 3 × 0.3 mm) heat spreader. The bonded chipwas then attached to a copper mount. The mount was cooled with a30-per cent-by-volume ethanol-water solution that flew at a rate of1 l/min during laser operation. The diamond surfaces were specified tobe flat and parallel to each other but in reality we noticed that the

ELECTRONICS LETTERS 3rd January 2013 Vol. 49

surfaces formed a slight wedge (<1°); this was observed by inspectinglaser beam reflections from the two surfaces.

The VECSEL was pumped with two fibre-coupled 808 nm diodelaser pump modules that were capable of providing a combined incidentpump power of approximately 100 W when focused to a 540 µm (diam-eter) sized spot. The focused laser beams were adjusted with the aid of acamera to form a single spot on the gain mirror.

Results: To find the optimal laser configuration for high power oper-ation, the VECSEL output power was measured using various partiallyreflecting cavity end mirrors (95.5–98.5%; RoC = 150 mm). The coolantwas set to 5 °C for this experiment, and the mount temperature was mon-itored with a thermocouple, since the temperature varied with the pumplaser power. The resulting output power for each cavity output coupleragainst incident pump power is plotted in Fig. 2. The highest outputpowers and slope efficiencies were achieved with a 97% reflectiveoutput coupler resulting in ∼20.5 W of output power and ∼33% slopeefficiency with a mount temperature of 12.3 °C. The measurementwas repeated using the 97% reflective mirror at a coolant temperatureof −10 °C in order to achieve even higher output powers. This configur-ation produced 23 W and a mount temperature of ∼0 °C.

0 20

20

25

15

5

10

040

1170 1180 1190

–25

–75

–50

60incident pump power, W

l, nm

pow

er, d

Bm

slope efficiency

outp

ut p

ower

, W

80 100 120

0.18

R = 95.5%mount 8 – 95 ºC0.25

0.33

R = 98.5%mount 7.7 – 10.4 ºC

R = 97%mount 9.2 – 12.3 ºC

R = 97%mount – 4.7 – 0.2 ºC

Fig. 2 Output power characteristics for different reflectivities (R) of cavityend mirror and mount temperatures

Inset: Spectrum measured at indicated operating point

The increase in output power in comparison to that reported in [6] islikely due to a combination of two factors. First, there is no nitrogen inthe gain mirror that can cause non-radiative recombination and limit theperformance. Secondly, we used a redesigned mount, thereby improvingheat removal. The heat dissipation from the new mount was limited bythe cooling system flow capacity.

The VECSEL emission spectrum was recorded at various outputpower levels using a 97% reflective output coupler. For a mount temp-erature of 9–12 °C (coolant set to 5 °C), the centre of the emission wasobserved to redshift by ∼25 nm from 1165 to 1190 nm when theincident pump power was increased from 1.9 W (threshold) to 77 W(20.5 W output). For a mount temperature of −4 to 0 °C (coolant setto −10 °C) the corresponding redshift extended from 1160 to 1188nm corresponding to a variation of the output power from threshold to23.2 W. The wavelength shift was caused by the increased operatingtemperature with increasing pump power. The inset of Fig. 2 showsthe measured spectrum at the output power of 20.2 W (64.4 W pumppower) and mount temperature of −0.9 °C. The distinct peaks seen inthe output spectrum are a result of an etalon effect caused by the intra-cavity diamond; the peak spacing of ∼1 nm corresponds to the freespectral range of the diamond.

A 1.5 mm-thick uncoated BRF (quartz), positioned at Brewster’sangle inside the cavity, was used to tune the output wavelength and tonarrow the emission linewidth. The midpoint of the BRF was located∼10 mm away from the curved cavity end mirror. A 98.5% outputcoupler was chosen to compensate for losses caused by the BRF. Thecoolant temperature was 5 °C, and the measurement was performed at85.6 and 43.6 W pump powers. Corresponding mount temperaturesfor these pump powers were 11–13 and 9–10 °C for 85.6 and43.6 W, respectively.

The measured tuning curves are shown in Fig. 3. The highest outputpower of ∼14.2 W was reached at 85.6 W pump power at ∼1179 nm.The emission spectra had a full width at half-maximum (FWHM) line-width of ≤1 nm for this pump power. The tuning bandwidth was∼40 nm (FWHM). A slightly wider tuning bandwidth of 45 nm was

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achieved at 43.6 W pump power. The VECSEL emitted a maximum of∼7 W at ∼1170 nm in ≤0.6 nm wide spectrum for a pump power of 43.6W.

0

2

4

6

8

10

12

14ou

tput

pow

er, W

1140 1155

45 nm

40 nmincidentpump power

43.6 W

incidentpump power

85.6 W

1170l, nm

1185 1200 1215

Fig. 3 Wavelength tuning curves measured with 98.5% reflective cavity endmirror and 1.5 mm-thick BRF at 85.6 W (mount at 11–13 °C) and 43.6 W(mount at 9–10 °C) pump powers

Conclusion: We have achieved more than 20 W of output power from asingle-chip VECSEL emitting at around 1180 nm. The key element forthis demonstration was the MBE-grown GaInAs/GaAs/GaAsP gainchip. Emission line narrowed output was obtained over a 40 nmtuning range using an intra-cavity BRF.

Acknowledgments: This work was supported by Areté Associates,TEKES Brightlase (project 40048/12), the Graduate School ofTampere University of Technology, and the Jenny and Antti WihuriFoundation.

© The Institution of Engineering and Technology 201328 September 2012doi: 10.1049/el.2012.3450One or more of the Figures in this Letter are available in colour online.

ELECTRON

S. Ranta, M. Tavast, T. Leinonen and M. Guina (OptoelectronicsResearch Centre, Tampere University of Technology,Korkeakoulunkatu 3, FIN-33101 Tampere, Finland)

E-mail: sanna.ranta@tut.fi

N. Van Lieu and G. Fetzer (2 Areté Associates, 2500 Trade Center AveSuite A, Longmont, CO 80503, USA)

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