Invited Paper
Multi-watt orange light generation by intracavity frequency doubling
in a dual-gain quantum dot semiconductor disk laser
J. Rautiainen*a, I. Krestnikovb, J. Nikkinena, O. G. Okhotnikova
Optoelectronics Research Centre, Tampere University of Technology, Korkeakoulunkatu 3, 33720
Tampere, Finland;
b
Innolume GmbH, Konrad-Adenauer-Allee 11, 44263 Dortmund, Germany
a
ABSTRACT
We demonstrate a frequency doubled dual-gain quantum dot semiconductor disk laser operating at 590 nm. The
reflective gain element, grown by molecular beam epitaxy, has active region composed of 39 layers of InGaAs StranskiKrastanov quantum dots. The gain mirrors produce individually 3 W and 4 W of output power while the laser with both
elements in a single cavity reveals 6 W at 1180 nm with beam quality factor of M2<1.2. The loss induced by the
nonlinear crystal is compensated by gain boosting in the dual-gain laser and 2.5 W of output power at 590 nm was
achieved after frequency conversion.
Keywords: Semiconductor disk laser, frequency conversion, quantum dot
1. INTRODUCTION
Optically pumped semiconductor disk lasers (OP-SDLs), also known as vertical external cavity surface emitting lasers
(VECSELs), have recently gained much interest among scientific and industrial communities due to the unique
properties that are unlikely achievable with VCSELs or in-plane semiconductor lasers1. Specifically the capability of
producing high optical power with nearly diffraction limited beam is a combination that makes the SDL desirable in
numerous applications. In addition, SDL concept allows wide spectral coverage ranging from visible wavelengths to the
mid-IR2,3. The optical pumping by low-cost high-power multimode diode laser provides high potential for power scaling
while preserving good beam quality4.
The emerging demand for laser projection displays has created a need for a laser that can produce visible radiation with
good beam quality in order to form a high-contrast image5. Though the direct generation of visible light from a
semiconductor disk laser is basically limited to red wavelengths2, the high-Q cavity of an SDL allows for efficient
intracavity frequency conversion of infrared emission in a nonlinear crystal to cover the RGB spectral range6. The
efficiency of frequency doubling can be high even with continuous wave operation making these sources attractive
option for a laser projection display. This technology producing orange-red emission can be used in medical
applications7 and laser guide stars for correcting the atmospheric distortions of images obtained with terrestrial
telescopes8.
Orange emission can be generated by frequency doubling of an 1180 nm radiation. There are few options available for
producing this wavelength from an SDL, each having certain constraints that limit their practical value. The InGaAsbased gain materials, widely used in SDLs operating at 940 - 1180 nm spectral range4, are usually grown monolithically
on top of a GaAs/AlGaAs distributed Bragg reflector (DBR). This material composition offers high refractive index
contrast resulting in high reflectivity for the signal wavelength. High power 1180 nm InGaAs SDL with frequency
doubling has been reported9. Further increase in the emission wavelength is, however, limited due to high strain in the
suitable structures.
*[email protected]; phone +358 40 198 1063; fax +358 3115 3400
Nonlinear Frequency Generation and Conversion: Materials, Devices, and Applications X,
edited by Konstantin L. Vodopyanov, Proc. of SPIE Vol. 7917, 791702 · © 2011 SPIE
CCC code: 0277-786X/11/$18 · doi: 10.1117/12.873166
Proc. of SPIE Vol. 7917 791702-1
The strain inflicted by the InGaAs can be moderated by adding nitrogen to the crystal lattice. The resulted InGaAsN
material composition is commonly referred as dilute nitride compound10. A few percent of nitrogen could reduce the
bandgap considerably, and consequently extends the available wavelength to 1.55 μm11.The adding of nitrogen could
increase the rate of nonradiative recombination that in turn can reduce the laser efficiency and thus making the epitaxial
growth of these structures to be a critical matter. A post growth thermal annealing should be usually applied to prevent
the degradation of the crystal quality12. One promising alternative for generating 1180 nm emission is to use quantum dot
(QD) based active media, which exhibit loosen requirement for the lattice matching13,14. The increased localization of
charge carriers induces unique properties such as low threshold, temperature insensitive operation, wide gain bandwidth,
significant tunable operation and short pulse generation15. On a downside, the reduced overlapping of the light with the
active material results in lower gain that can be compensated to certain extent by increasing the quantum dot density.
Here we report a quantum dot semiconductor disk laser frequency doubled to 590 nm in a dual-gain cavity. The gain
mirror has 39 layers of InGaAs QDs grown in Stranski-Krastanov growth mode. The dual-gain configuration shared the
pump induced thermal load among two active media and produces gain sufficient for efficient intracavity frequency
conversion.
2. GAIN MIRROR STRUCTURE
The semiconductor gain element designed for 1180 nm was fabricated with molecular beam epitaxy (MBE) on a GaAs
substrate. The DBR, consisting of 35 layer pairs of λ/4 thick GaAs/AlAs, was grown first on top of the substrate. On top
of the mirror is the active region with 39 layers of InGaAs Stranski-Krastanov quantum dot layers that were designed to
operate at the first exited state. The QD layers were arranged in 13 identical groups at the antinodes of the standing
wave. A 35 nm thick GaAs layer was positioned between each QD layer and the same material was also used as a pump
absorbing spacer layer between each group. An AlGaAs window layer was grown on top of the active region, which was
protected against oxidation with 42.6 nm thick GaAs cap layer. The room temperature reflectivity for beam propagating
normal to the surface and the photoluminescence spectrum, measured from the edge of the wafer, are presented in Fig. 1.
It can be observed that the reflectivity has a stopband bandwidth of ~120 nm and the photoluminescence peaks around
1170 nm. More detailed description of the gain mirror is described elsewhere14.
Photoluminescence intensity (a.u.)
1.0
Reflectivity
Photoluminescence
Reflectivity
0.8
0.6
0.4
0.2
0.0
1000
1050
1100
1150
1200
1250
1300
1350
Wavelength (nm)
Figure 1. Room temperature reflectivity and photoluminescence intensity spectrum of the gain mirror.
Proc. of SPIE Vol. 7917 791702-2
The as-grown wafer was cut into 2.5 mm×2.5 mm chips and two of the chips were water bonded to ~300 μm thick
transparent intracavity diamond heat spreaders16. The assemblies were then sandwiched between copper blocks with
indium foil that operates as a fitting layer. The top surfaces of the diamond heat spreaders were antireflection coated for
the laser wavelength with SiO2/TiO2 layers. The reflectivity for the pump wavelength was ~10 %, which takes into
account the reflectivity from both surfaces of the diamond.
3. GAIN MIRROR CHARACTERIZATION
First, the two gain mirrors were tested separately in linear cavity configurations, in which the gain mirror operates as one
cavity mirror and a spherical mirror as an output coupler. The gain mirrors #1 and #2 were pumped optically with 808
nm and 788 nm fiber-coupled diode lasers, respectively, to a spot diameter of 180 μm. The spherical mirror has a 75 mm
radius of curvature (RoC) and the distance to the gain mirror ensures the mode size to be equal to the pump spot at the
gain mirror. Mirrors, with output coupling (OC) in a range from 0.2 % to 0.8 %, were tested. The temperature of the gain
mirrors was set to 9 °C with a Peltier element. The output power as a function of the launched pump power is plotted in
Fig. 2 for the both gain mirrors. The maximum output power with the gain mirror #1 was more than 4 W, while the SDL
with gain mirror #2 produced 3 W. The beam quality factor M2 was measured for the both gain mirrors, for two
orthogonal directions, with the 0.7 % output coupler. It was found that the M2 value for the gain mirror #1 varied
between 1.1 and 1.2 at different powers. The laser with gain mirror #2 exhibited somewhat larger M2 factors of 1.2-1.4.
4.5
4.5
OC=0.2 %
OC=0.4 %
OC=0.7 %
OC=0.8 %
3.5
3.0
OC=0.2 %
OC=0.4 %
OC=0.7 %
OC=0.8 %
4.0
Output power (W)
Output power (W)
4.0
2.5
2.0
1.5
1.0
0.5
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0.0
0
5
10
15
20
25
30
35
0
Input pump power (W)
(a)
5
10
15
20
25
30
35
Input pump power (W)
(b)
Figure 2. (a) Output power of the disk laser #1 and (b) disk laser #2 with different output couplers.
The next step was to characterize the wavelength tunability of the device. The tuning was performed with the gain mirror
#1 in a V-type cavity, where the gain mirror and a plane mirror operated as end mirrors and a spherical mirror as a
folding mirror. The temperature and pump spot diameter were kept the same as in previous measurements. The tuning
was performed with a 1.5 mm thick birefringent filter, placed at the Brewster angle between the spherical and the plane
mirrors. The pump power was 20 W. When all the mirrors were highly reflective, 68.8 nm tuning range was achieved
with 80 mW of maximum output power at the center of the tuning range. The replacing of the highly reflective plane
mirror to a mirror with a transmission of 0.4 %, resulted in a smaller tuning range of 22 nm but with increased output
power of 270 mW, as shown in Fig. 3.
Proc. of SPIE Vol. 7917 791702-3
300
200
150
100
50
Output power (mW)
Intensity (10dB/div.)
250
0
1160
1165
1170
1175
1180
1185
Wavelength (nm)
Figure 3. Wavelength tuning observed with 0.4 % output coupler.
4. DUAL-GAIN LASER AND FREQUENCY DOUBLING
Next, the dual-gain configuration was characterized in a Z-cavity built with the gain mirror #1 and a curved output
coupler as cavity end reflectors. The gain mirror #2 and a highly reflective spherical mirror acted as folding mirrors. The
cavity maintains equal mode sizes on both gain mirrors. In this configuration, the 0.8 % output coupler was found to
enable the highest output power of 6 W, as shown in Fig. 4(a). Since the Z-cavity exhibits higher losses, the optimal
output coupling transmission was only slightly higher as compared to the single-gain setup. As expected, more total
pump power can be launched to the gain mirrors, before the thermal rollover appears resulting in higher output power.
Typical spectrum of the laser, shown in Fig. 4(b), reveals the spectral components induced by the etalon effect of the
heat spreaders. The beam quality factor was found to be less than 1.2 and was fairly independent of the output power.
Output power (W)
5
4
3
2
OC=0,4 %
OC=0,7 %
OC=0,8 %
OC=1.0 %
1
0
0
10
20
30
40
50
60
Intensity (10 dB/div.)
6
1160 1165 1170 1175 1180 1185 1190 1195
Total pump power (W)
(a)
Wavelength (nm)
(b)
Figure 4. (a) Output power characteristics of the dual-gain laser with different output couplers. (b) Output spectrum of the laser
measured at 6 W of output power with 0.8 % output coupler.
Proc. of SPIE Vol. 7917 791702-4
The cavity with nonlinear crystal for second harmonic generation, shown in Fig. 5, uses the dual-gain scheme. The
spherical output coupler was replaced here with a highly reflective mirror and one more cavity arm was assembled where
the nonlinear crystal was placed. The waist diameter at the location of the 4 mm long BBO crystal was ~180 μm. The
crystal was antireflection coated for both, the fundamental and frequency doubled wavelengths. The laser produced 2.5
W of power at the 590 nm wavelength, as shown in Fig. 6.
3.0
2.5
Intensity (10 dB/div.)
Output power at 590 nm (W)
Figure 5. Dual-gain cavity configuration for the second harmonic generation. All the mirrors are highly reflective for the fundamental
wavelength. 100 mm RoC end mirror has high reflectivity for the second harmonic wavelength of 590 nm. The output is taken after
100 mm RoC folding mirror which has a high transmission for the orange wavelength.
2.0
1.5
1.0
0.5
0.0
5
10
15
20
25
30
35
40
45
50
55
560
Total pump power (W)
(a)
570
580
590
600
610
620
Wavelength (nm)
(b)
Figure 6. (a) Output power at 590 nm as a function of total launched pump power. (b) Optical spectrum of the laser recorded at 2.4 W
of output power.
Proc. of SPIE Vol. 7917 791702-5
5. CONCLUSIONS
We have demonstrated a dual-gain quantum dot disk laser. The laser produced 6 W of power at 1180 nm with beam
quality factor M2 less than 1.2. The dual-gain laser with frequency doubling in a BBO crystal demonstrates 2.5 W of
power at orange wavelength of 590 nm. The multiple gain approach is particularly useful with quantum dot based gain
media, in order to enhance the total gain and compensate the losses induced by the intracavity frequency conversion
element.
6. ACKNOWLEDGEMENTS
This work has received funding from the European Community’s Seventh Framework Programme (FAST-DOT) under
grant agreement 224338, Walter Ahlström Foundation and HPY Research Foundation. The authors would like to thank
Edik Rafailov and Mantas Butkus from the University of Dundee for useful discussions and technical help.
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