High power intracavity second harmonic generation in
Vertical External Cavity Surface Emitting Lasers at 1060 nm
A J Maclean, A J Kemp, K S Kim1, J Y Kim1, T Kim1 and D Burns
Institute of Photonics, SUPA, University of Strathclyde, Glasgow, UK
1
Samsung Advanced Institute of Technology, Gyeonggi-Do, 449-712, Korea
a.maclean@strath.ac.uk
Abstract. We have demonstrated over 1W of laser power in the green at 530nm using an
intracavity frequency doubled VECSEL. A diamond heatspreader was used for thermal
management and along with a birefringent filter constrained the linewidth and wavelength
within the phasematching conditions of the non-linear crystal. 6nm of tuning in the green and
over 20nm in the IR was also demonstrated.
1. Introduction
There are many applications in fields such as display technology, medical instrumentation, microscopy
and reprographics where small, low cost, but high power and high beam quality light sources in the
visible part of the spectrum are required. Many, such as atomic spectroscopy, require lasers at very
specific wavelengths, which are seldom covered by doped insulator lasers. As yet there are few
devices that can combine all of these traits at a wavelength tailored for the application. A particularly
attractive candidate is the Optically Pumped Semiconductor Vertical External Cavity Surface Emitting
Laser (OPS-VECSEL). This is an exciting new type of semiconductor laser source which combines
the wavelength flexibility of the semiconductor material with the beam quality and thermal
management advantages of a solid state thin disk laser. The external cavity makes these devices ideal
for the introduction of non-linear crystals and frequency selective elements for second harmonic
generation (SHG), extending the traditional semiconductor wavelength range to cover the visible
region [1, 2].
The VECSEL is an epitaxially grown semiconductor device made up of a distributed Bragg
reflector (DBR) mirror grown on a suitable substrate, with a gain region and capping layer grown on
top of it. The gain region consists of quantum wells (QWs) separated by barrier regions where the
pump light is absorbed. The barriers also act as spacers so that the QWs are positioned at the
antinodes of the laser standing wave in the subcavity created between the DBR and the top surface;
this is known as resonant periodic gain (RPG) [3, 4]. The laser resonator is completed by the addition
of external mirror(s), and other intracavity elements can also be added as required (figure 1). The
VECSEL used in this work is a 1060nm structure using InGaAs QWs as documented in [5].
2. Thermal management
The waste heat from the pumping process deposited in the small pumped volume (typically 100μm
diameter x 2μm deep) causes both the centre wavelength of the QW gain and the laser wavelength,
Fibre coupled
pump diode
Heatspreader
Cap
Confinement layer
Quantum
Wells (QW)
Heat sink
Multi Quantum
Well gain region
Distributed Bragg
Reflector (DBR)
Pump
absorbing
barriers
Substrate
Intracavity IR
Birefringent
Filter (BRF)
Visible output
SHG crystal
Visible output
Figure 1 Configuration of laser cavity. Insert: Layer structure of VECSEL chip with heatspreader
(layer thicknesses not to scale).
which tracks the sub-cavity resonance, and also represents the peak of the RPG in these so called
optically resonant devices, to shift, but at different rates. This leads to the laser wavelength becoming
misaligned from the gain as the pump power is increased and the laser eventually rolls over and
switches off [3, 4]. Higher output powers can be achieved either by removing the substrate to
decrease the thermal impedance of the heat path from the pumped area to the heatsink [3, 6], or by
introducing a heatspreader – an intracavity window of high thermal conductivity – which increases the
area over which the heat is removed through the device to the heatsink [7]. For material systems at
long and short wavelengths, where the DBR thermal conductivity is typically much higher, the
heatspreader approach provides a less material dependent thermal management system to achieve high
power operation [8, 9]. A thermal model showing heat rise in the active region for 1W of deposited
heat in devices appropriate for each wavelength (figure 2) shows the advantage of a heatspreader over
the substrate thinning approach for wavelengths where GaAs/AlAs mirrors cannot be used. For this
reason, the heatspreader can be viewed as the more generic technique and so was used exclusively in
this work.
Max Temperature Rise (K)
150
100
As Grow n
Thinned
Heatspreader
50
0
500
1000
1500
2000
2500
Wavelength (nm)
Figure 2 Finite-element model of temperature rise in VECSEL for 1W of heat deposited in active
region for structures appropriate for each wavelength.
The choice of material for use as a heatspreader is an important consideration; the heatspreader
should have high thermal conductivity and be transparent at the pump and laser wavelengths. The first
demonstration of a heatspreader in a VECSEL used sapphire [7]; however the thermal conductivity of
sapphire is lower than the GaAs substrate used in semiconductor lasers around 1μm, and so, can, at the
best, half the thermally induced wavelength shift. Diamond is the obvious choice as it has the highest
thermal conductivity of any material, although silicon carbide is a good second.
3. Frequency control
To achieve efficient second harmonic generation in a non-linear crystal, the linewidth of the laser light
must be less than the phasematching bandwidth of the crystal and the wavelength has to match the
phasematching wavelength, which is a function of temperature. This means that for a given crystal
temperature, the laser wavelength has to be kept constant and the linewidth narrow. Since the free
running linewidth of the VECSEL was 2-4nm and a typical phasematching bandwidth would be
<1nm, it was necessary to take measures to narrow the spectrum of the laser and control the
operational wavelength to match the phasematching conditions.
A single plate quartz birefringent filter (BRF) was used to select the wavelength of operation (see
figure 1); the thickness of the filter defines the pass band, with thicker pieces of material having a
narrower spectral characteristic, but also a lower free spectral range (FSR). Figure 3 shows the laser
spectrum as measured with a plane-plane scanning Fabry-Pérot etalon and the calculated spectral
widths at full width half maximum (FWHM) and full width at 5% of the maximum intensity for the
free running laser and with 2, 4 and 6mm birefringent filters in the cavity.
1.6
Spectral Intensity (A.U.)
Spectral Intensity (A.U.)
3
2.5
2
1.5
1
0.5
1.4
1.2
1
0.8
0.6
0.4
0.2
0
0
0
10
20
30
40
0
50
5
10
2.2nm FWHM, 3.8nm 5%
(a) Free Running
20
25
30
35
1.9nm FWHM, 2.3nm 5%
(b) 2mm BRF
1.6
1.8
1.4
1.6
Spectral Intensity (A.U.)
Spectral Intensity (A.U.)
15
Wavelength Change (nm)
Wavelength Change (nm)
1.2
1
0.8
0.6
0.4
0.2
1.4
1.2
1
0.8
0.6
0.4
0.2
0
0
0
2
4
6
8
Wavelength Change (nm)
0.2nm FWHM, 2nm 5%
(c) 4mm BRF
10
12
0
2
4
6
8
10
12
14
16
Wavelength Change (nm)
0.2nm FWHM, 0.4nm 5%
(d) 6mm BRF
Figure 3 Spectra with different BRFs taken with a scanning Fabry-Pérot etalon. Spectral widths were
calibrated from the known FSR of the etalon. Red line shows voltage ramp applied to piezo mounted
mirror.
The diamond heatspreader on the intracavity face of the gain chip is specified to be plane and
parallel, this means that it also acts as an intracavity etalon. The spectral modulation introduced by
this etalon is obvious from the spectrum of the free running laser, and the measured FSR is consistent
with the thickness and refractive index of the diamond.
The BRF is used to pick out one mode of the diamond etalon, but includes those to either side to
varying degrees depending on the filter function. With the 6mm BRF, the adjacent etalon modes are
almost entirely suppressed. Since both the 4mm and 6mm BRFs produce equivalent spectral widths
when measured at FWHM, it was not clear which of these would give more efficient second harmonic
generation.
One issue that was apparent when the BRF was inserted into the cavity at Brewster’s angle was that
there was a higher than expected reflection from the Brewster surface, indicating that there must be an
additional birefringence in the cavity. This was found to result from the diamond, but rotation of the
VECSEL-diamond composite was found to significantly reduce this extra loss. Further investigation
into this issue will be published elsewhere.
4. Second Harmonic Generation
Having achieved a spectrally narrow laser output, a frequency doubling crystal (LBO) was placed in
the cavity in front of the plane mirror, where the cavity mode size could be controlled independently
of the mode size at the VECSEL chip. The cavity was aligned for a spot radius on the plane mirror of
43μm, giving approximately one Rayleigh range over the crystal length. Both the plane and the
curved mirrors nearest the crystal were coated to have high transmission at the second harmonic so
green light was generated in both directions. Using the 6mm BRF an output of 490mW in each
direction was measured, whereas using the 4mm BRF an output of 590mW in each direction was
achieved. This suggests that the spectrum was sufficiently constrained with the thinner filter without
introducing additional intracavity loss. Figure 4 shows spectra of the fundamental and second
harmonic taken simultaneously on different gratings of the same spectrometer.
3000
2500
Spectral Intensity (A.U.)
Spectral Intensity (A.U.)
2500
2000
1500
1000
500
0
1040.00
1050.00
1060.00
1070.00
Wavelength (nm)
Spectrum
(a)
Centroid
1080.00
2000
1500
1000
500
0
520.00
525.00
530.00
535.00
540.00
Wavelength (nm)
Spectrum
Centroid
(b)
Figure 4 Spectra of (a) fundamental and (b) second harmonic.
An experiment was also undertaken to tune the second harmonic by tuning the fundamental with
the BRF. A tuning range of over 20 nm was possible in the fundamental but only a part of this
corresponded to a sufficient match to the phasematching bandwidth for second harmonic generation.
In an unoptimised cavity arrangement a tuning range of 4nm was achieved in the green, which was
equivalent to the phasematching bandwidth of the LBO (figure 5a). Tuning is also possible by
changing the crystal temperature and therefore the phasematching wavelength. Second harmonic light
was generated over 6nm with the crystal temperature varied between 14°C and 36°C (figure 5b).
300
250
200
150
100
50
0
1045
1050
1055
1060
1065
Wavelength of Fundamental (nm)
Green Power
(a)
IR Power
1070
1075
Power in each direction (mW)
Power in each direction (mW)
300
250
200
150
100
Increasing Temperature
50
0
523
524
525
526
527
528
529
530
531
Wavelength of Second Harmonic (nm)
(b)
Figure 5 Tuning response of frequency doubled VECSEL (a) via fundamental tuning with the BRF
and (b) via crystal temperature between 14°C and 36°C.
5. Conclusions
Over 1W of green light at 530nm has been demonstrated using intracavity frequency doubling of a
VECSEL. Use of a diamond heatspreader allows efficient thermal management and also helps to
constrain the laser linewidth to within the phasematching bandwidth of the non-linear crystal. A highpower, high-brightness tunable visible laser source – where the wavelength can be engineered to suit
the application – will be useful in a number of fields where current laser technology is limited.
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