High performance 2.2m optically-pumped vertical externalcavity surface-emitting laser
J-M Hopkins, R D Preston, A J Maclean, S Calvez, H Sun, J Ng1, M Steer2, M
Hopkinson1, and D Burns.
Institute of Photonics, University of Strathclyde, Wolfson Centre, 106 Rottenrow,
Glasgow, G4 0NW (UK)
1
University of Sheffield, Department of Electronic & Electrical Engineering
Mappin Street, Sheffield, S1 3JD (UK)
2
EPSRC National Centre for III-V Technologies, Mappin Street, Sheffield, S1 3JD
(UK)
johnmark.hopkins@strath.ac.uk
Abstract. We report the operation of an optically-pumped vertical-external-cavity surfaceemitting laser (OP-VECSEL) oscillating at wavelengths up to 2.2μm and at output powers
greater than 200mW. This versatile platform provides a broad gain bandwidth which may be
tuned and/or controlled by the addition of elements into the external cavity. Moreover, the
nature of the semiconductor structure permits precise engineering of the operating wavelength
when compared with traditional solid state crystalline lasers, and the thin (few microns) pump
absorption region, coupled with the external cavity control, permits the mode conversion of a
low brightness pump into a high quality single spatial mode TEM00 output.
1. Introduction
Laser sources covering the 2-2.5m atmospheric transmission window have a large number of
potential applications in communications, sensing, spectroscopy and security. Typically, the
bandwidth, range and sensitivity requirements for many of these applications demand a laser having
diffraction limited, single mode performance. Until recently, however, compact, wavelength versatile,
light sources in this range with these qualities have been few; where available they are generally
restricted to only a few milliwatts [1]. The recent emergence of the optically-pumped semiconductor
vertical-external-cavity surface-emitting lasers (OPS-VECSELS) [2] has provided the ability to obtain
high-power performance from semiconductor lasers with high spatial mode quality output at a large
range of wavelengths. Figure 1 shows schematically the typical configuration of a VECSEL cavity.
Figure 1 Schematic of a generic optically-pumped vertical external-cavity surface-emitting laser.
Critically, the use of intracavity heat spreaders has permitted the demonstration of high (Watt level)
performance in material systems and wavelengths away from the commonly used InGaAs at 980nm
[3]. In this paper, we report on the demonstration of a VECSEL device based on a quaternary
antimonide (InGaAsSb/AlGaAsSb) active region which is capillary bonded [4] to a diamond
heatspreading window for optimal thermal management and therefore efficient performance [5].
2. Semiconductor design and growth
The semiconductor VECSEL structure shown in figure 2 incorporates a carefully designed multi
quantum well InGaAsSb/AlGaAsSb gain region containing 14, 7.5nm-thick In0.29Ga0.71As0.19Sb0.81
quantum wells embedded within a thin (<1.2m) Al0.35Ga0.65As0.03Sb0.97 ‘barrier’ layer.
Figure 2 Schematic of the epitaxial semiconductor structure
The wells are arranged in groups within the active region to overlap with the antinodes of the
standing wave intracavity field in order to optimize the pump absorption and signal extraction. This
active region is grown on a high reflectivity multilayer 21.5 pair AlAsSb/GaAs Bragg mirror grown on
a GaSb substrate. When pumped by a high power diode laser, this entire device then forms an ‘active’
mirror in an external laser resonator, which permits the cavity mode size to be optimized. The laser
structure was grown by solid source MBE at around 500°C with the growth rates for the GaSb and
AlAsSb DBR layers were are 0.62 and 0.76 monolayers per second respectively. Room-temperature
reflectivity and photoluminescence (PL) plots for the sample are shown in figure 3.
1
0.9
Intensity / (arb units)
0.8
0.7
0.6
0.5
0.4
0.3
0.2
PL M3208
Reflectivity M3208
0.1
0
1800
1900
2000
2100
2200
2300
2400
2500
Wavelength / (nm)
Figure 3 Reflectivity spectrum (blue) and room temperature PL emission (red) from the VECSEL
structure
3. Experimental set up
The VECSEL sample was first capillary bonded to a 4mm diameter, 250m thick, plane-parallel,
natural diamond heatspreader window. The bonded sample was then compression mounted between
125m Indium foil sheets in a 1” diameter brass water-cooled heatsink. Pump light was provided by a
100m core fibre-coupled diode laser (Lissotschenko Mikrooptik GmbH - LIMO) capable of
delivering up to 14W at 980nm. The pump light was collimated and re-focussed onto the VECSEL
using two short focal length (f=14.5mm, f=11mm) coupling lenses in a custom lens-tube assembly.
The laser cavity configuration was typically the 3-mirror type shown in figure 4.
Figure 4 Typical 3-mirror cavity configuration of the 2.2m VECSEL
The output coupler was nominally a 98% reflectivity flat substrate with wedged rear surface
however separate measurements have shown the reflectivity to be closer to 96.5% at 2200nm.
4. Results
With the water bath temperature at 10ºC the laser operated up to 80mW output power for an input
pump power of 5.5W. The power transfer and typical free running power spectrum is shown in figure
5.
(b )
(a)
In ten sity (arb u n its)
O u tp u t P o w er (m W )
100
80
60
40
20
0
0
2000
4000
6000
8000
10000
2125
2130
P u m p P o w er (m W )
2135
2140
2145
2150
W avelen g th (n m )
Figure 5 (a). Power transfer at 10ºC (b). Typical free-running power spectrum for 5.5W incident pump
The laser operated with a circularly symmetric TEM00 mode throughout. With careful cavity
alignment, the laser could be operated in single longitudinal mode up to 50mW. The transition to
single longitudinal mode operation was accompanied by a characteristic small jump in output power
and an obvious improvement in power stability [6], confirming the single spatial mode operation.
Figure 6 shows the power transfer when the water bath was reduced in temperature to -10ºC. The laser
operated at output powers up to 210mW at an input pump power of 10W.
O u tp u t P o w er (m W )
250
200
150
100
50
0
0
2000
4000
6000
8000
10000
P u m p p o w er (m W )
Figure 6 Power transfer at -10ºC
When the output coupler mirror was replaced with an HR mirror and a 2mm quartz birefringent filter
was inserted in the long arm of the cavity, the laser could be tuned over 100nm from 2083nm to
2192nm with output powers up to 5mW. Figure 7 shows the tuning spectrum obtained and the
underlying FP etalon substructure caused by the diamond window.
Intensity (arb. units)
Tuning spectrum of 3-mirror VECSEL with BRF and HR
2080
2100
2120
2140
2160
2180
2200
Wavelength (nm)
Figure 7 Tuning spectra, blue diamonds – positions of maximum output power with BRF angle, coloured
spectra – individual FP filtered spectral features at each BRF position.
The etalon effect caused by the diamond could easily be removed by using a wedged diamond
window with an AR coating. The laser would then tune smoothly across this broad gain spectrum. The
addition of a separate angle-tuned etalon and feedback electronics would enable stable, tunable singlefrequency performance to be achieved across this wavelength range [7]. This would allow the remote
sensing of some important atmospheric gasses such as N2O, CH4 and NH3.
5. Conclusions
We have shown high power operation of a 2.1 to 2.2m VECSEL with high quality single spatial
output mode. We believe the performance of such antimonide VECSELs is appropriate to meet the
challenging demands of many applications in the 2-2.5m spectral region. Future work will
concentrate on enhanced thermal performance, greater efficiency, higher Watt-level output power and
longer wavelength operation as well as the demonstration of continuous, stable, mode-hop free tuning
over large wavelength ranges in the 2-2.5m region. The demonstration of such versatile lasers will
rapidly open up new avenues of deployment in this source weak region.
References
[1] A. Ouvrard, A. Garnache, L. Cerutti, F. Genty, and D. Romanini, IEEE Photonics Technol. Lett.
17, 2020 (2005).
[2] M. Kuznetsov, F. Hakimi, R. Sprague, and A. Mooradian, IEEE Photonics Technol. Lett. 9,
1063 (1997).
Tropper AC, Hoogland S, Prog. Quantum Electron., 30 (1), 1-43 (2006).
[3] J.-M. Hopkins, S. Calvez, A. J. Kemp, J. E. Hastie, S. A. Smith, A. J. Maclean, D. Burns and M.
D. Dawson., phys. stat. sol. (c) 3 (3), 380–385 (2006).
[4] Z. L. Liau, Appl. Phys. Lett. 77 (5), 651–653 (2000).
[5] Kemp AJ, Valentine GJ, Hopkins JM, Hastie JE, Smith SA, Calvez S, Dawson MD and Burns
D., IEEE Jn. Quantum electron., 41 (2): 148-155 (2005)
[6] J.-M. Hopkins, A.J. Maclean, and D. Burns, N. Schulz, M. Rattunde, C. Manz, K. Köhler and J.
Wagner, paper CThM3, presented at the Conference on Lasers and Electro-Optics, Long
Beach, (2006).
[7] Abram RH, Gardner KS, Riis E, Ferguson AI, Opt. Express 12 (22) 5434-5439 (2004).