We present a new measurement method for measuring the spectral reflectance of a semiconductor disk laser gain element under optical pumping, providing valuable information on the spectral dependence of gain under close-to-normal operating conditions

Bellancourt

Borgentun

Paajaste

Rudin

Wang

Wojcik-Jedlinska

Borgentun, Carl

Bengtsson, Jörgen

Larsson, Anders

Chalmers Publication Library

Method for Measuring Reflectance of Semiconductor Disk 
Laser Gain Element Under Optical Pump Excitation 
 
Carl Borgentun, Jörgen Bengtsson, and Anders Larsson 
Photonics Laboratory, Department of Microtechnology and Nanoscience (MC2), Chalmers University of Technology, Göteborg, Sweden 
carl.borgentun@chalmers.se 
 
Abstract: We present a new measurement method for measuring the spectral reflectance of a 
semiconductor disk laser gain element under optical pumping, providing valuable information on 
the spectral dependence of gain under close-to-normal operating conditions. 
OCIS codes: (140.5960) Semiconductor lasers; (120.5700) Reflection 
 
1.  Introduction 
The optically pumped semiconductor disk laser (OP-SDL), or vertical-external-cavity surface-emitting laser 
(VECSEL), is a laser capable of generating a high-power beam of almost diffraction-limited quality [1,2]. The 
properties of the spatial modes of the OP-SDL emission are controlled by the external cavity but the power and 
spectral properties are to a large extent governed by the semiconductor layer structure of the gain element. It is 
therefore of utmost importance to be able to design the gain element such that the performance characteristics fit the 
preferred ones. A standard way to experimentally test the design is to perform a spectral reflectance measurement of 
the gain element in the absence of optical pumping, i.e., to simply measure the reflected optical power at a known 
incident power for several data points within a wavelength interval [3]. This is a straight-forward and simple 
measurement method but the retrieved information is restricted to the case of a cold cavity and little or no 
information is retrieved about the actual performance of the gain element under pump excitation. 
 We here present a measurement method that, as compared to the standard reflectance measurement method, 
reveals information on the spectral and power properties of the gain element under optical excitation and enables a 
direct comparison with simulation results. This comparison is very useful in the iterative evolution of gain element 
design and particularly advantageous for more advanced designs such as those optimized for wide tuning [4,5] or for 
short pulse generation [6], since the measurement conditions more closely resemble the operating conditions in the 
actual OP-SDL cavity. In these kinds of design, a certain spectral dependence of the gain is sought after to render 
the wanted behavior of the OP-SDL. 
2.  Setup 
The setup, schematically shown in Fig. 1, probes the reflectance of the gain element by employing a tunable laser 
with narrow bandwidth (e.g., a Ti-sapphire laser). The gain element is simultaneously being optically pumped, and 
due to stimulated emission from the active region the reflectance can exceed unity, i.e., produce gain. Thus the setup 
is capable of measuring the gain spectrum of a gain element. For practical purposes the setup needs to be calibrated 
to account for fluctuations in the output power of the probe laser and absorption in optical components. Therefore 
the probe beam is split by a wedged beam-splitter, and part of the beam is reflected onto a photo-detector used as 
Attenuation 
filter 
Sample  
on holder 
Chopper 
Detector: 
Pincident 
Detector: 
Preflected 
Beam-splitter 
T=50%
Lens 
Pump laser 
Ti-sapphire 
laser 
Spectrometer 
Fig. 1. Schematic overview of the measurement setup. When aligning the spots from the probe and 
pump lasers, the top detector is removed to let a lens and a CCD camera image the sample surface. 
Lock-in 
amplifier 
CCD camera 
Lens
OSA/ CLEO 2011
       JTuI94.pdf 
 
© Optical Society of America
(1) 
reference of the incident power. The other part of the beam is incident on the optically pumped gain element, 
reflected back to the beam-splitter, where the light is reflected onto a second photo-detector. Due to uncertainties in 
the optical properties of the beam-splitter and the detectors, a calibration is necessary with a measurement on a 
sample with a known reflectance, e.g. a reflectance standard. The reflectance of the gain element can then be 
written: 
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ሺߣሻ
௜ܲ௡௖௜ௗ௘௡௧ሺߣሻ
ڄ ௖ܲ௔௟௜௕௥௔௧௜௢௡,௜௡௖௜ௗ௘௡௧
ሺߣሻ
௖ܲ௔௟௜௕௥௔௧௜௢௡,௥௘௙௟௘௖௧௘ௗሺߣሻ
ڄ ܴ௖௔௟௜௕௥௔௧௜௢௡ሺߣሻ. 
 The alignment of the probe and pump laser spots can be conveniently performed by removing the detector 
measuring the reflected power, thereby unblocking a lens that images the sample surface on a CCD camera. 
Naturally the pump spot has to completely cover the spot from the probe laser, and it might be necessary to insert a 
focusing lens that reduces the size of the probe spot on the sample so that the pump intensity can be sufficiently 
increased. 
 To prevent saturation of the detectors, an attenuation filter is inserted in the beam. This also functions as a 
beam-splitter as the reflected light is guided into a spectrometer for wavelength control. 
3.  Preliminary results 
Initial measurements using the described setup have been performed on a gain element optimized for wide tuning 
(details on the structure can be found in [5]) and the calibration sample was a broadband dielectric mirror with a 
well-known reflectance. The preliminary measurement results, shown to the left in Fig. 2, reveal a significant 
response to pump excitation and agree well with the simulation results, shown to the right in Fig. 2. The measured 
pump intensities are approximate due to difficulties in accurately determining the pump spot size and the cause for 
differences is most likely approximations in the model and deviations in the fabricated epitaxial structure from the 
prescribed one. 
The authors would like to thank Hans Lindberg of Osram Opto Semiconductors for fruitful discussions. 
4.  References 
[1] B. Rudin, A. Rutz, M. Hoffmann, D. J. H. C. Maas, A. Bellancourt, E. Gini, T. Südmeyer, and U. Keller, “Highly efficient optically pumped 
vertical-emitting semiconductor laser with more than 20 W average output power in a fundamental transverse mode,” Opt Lett 33(22), 2179-
2721  (2008). 
[2] T. Wang, Y. Kaneda, J. M. Yarborough, J. Hader, J. V. Moloney, A. Chernikov, S. Chatterjee, S. W. Koch, B. Kunert, and W. Stolz, “High-
Power Optically Pumped Semiconductor Laser at 1040 nm,” IEEE Photon Technol Lett 22(9), 661-663  (2010). 
[3] A. Wojcik-Jedlinska, K. Pierscinski, A. Jasik, J. Muszalski, and M. Bugajski, “Optical characterisation of vertical-external-cavity surface-
emitting lasers (VECSELs),” Optica Applicata 37(4), 449-457  (2007). 
[4] J. Paajaste, S. Suomalainen, R. Koskinen, A. Härkönen, M. Guina, and M. Pessa, “High-power and broadly tunable GaSb-based optically 
pumped VECSELs emitting near 2 µm,” J Cryst Growth 311(7), 1917-1919 (2009). 
[5] C. Borgentun, J. Bengtsson, A. Larsson, F. Demaria, A. Hein, and P. Unger, “Optimization of a Broadband Gain Element for a Widely 
Tunable High-power Semiconductor Disk Laser,” IEEE Photon Technol Lett 22(13), 978-980  (2010). 
[6] A. Bellancourt, Y. Barbarin, D. J. H. C. Maas, M. Shafiei, M. Hoffmann, M. Golling, T. Südmeyer, and U. Keller, “Low saturation fluence 
antiresonant quantum dot SESAMs for MIXSEL integration,” Opt Express 17(12), 9704-9711  (2009). 
 
Fig. 2. Measured (left) and simulated (right) gain spectra for different absorbed pump intensities. 
OSA/ CLEO 2011
       JTuI94.pdf 
 


Method for measuring reflectance of semiconductor disk laser gain element under optical pump excitation

Bengtsson, J\uf6rgen

Chalmers Research

Carl Borgentun

Jörgen Bengtsson

Anders Larsson

Crossref

Method for Measuring Reflectance of Semiconductor Disk Laser Gain Element Under Optical Pump Excitation

Borgentun, Carl,

Bengtsson, Jörgen,

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Method for measuring reflectance of semiconductor disk laser gain element under optical pump excitation

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