3µm wavelength VECSELS are on the Horizon
Scientists from Germany and Scotland have developed a mature laser technology based on the group III-antimonide material in the 1.9 - 2.8μm wavelength range. Furthermore, the researchers have demonstrated that these Vertical External Cavity Surface Emitting Lasers (VECSELs) are high-power, tunable, and have a narrow linewidth.
This latest development was achieved by improvising the semiconductor design structure and Molecular Beam Epitaxy (MBE) fabrication steps. With an overall layer thickness of nearly 12μm, the researchers found the MBE growth of this long-wavelength structure very challenging. Apart from the thickness, controlling the specific material composition needed for the quantum wells in the active region also proved to be tough.
The research was conducted by physicists from the Fraunhofer Institute, Germany, the Institute of Photonics, Glasgow and LISA Laser Products, Germany. Funded by the Sixth Framework Program of the European Union within the project VERTIGO (Versatile Two Micron Light Source), the new category of long wavelength semiconductor laser is known as the Optically Pumped Semiconductor Disk Laser (OPSDL).
The latest advancement would be well suited to applications including light detection and ranging (LIDAR) for clear-air turbulence or gas detection, long-range free-space optical communications, and medical diagnostics.
In particular, these OPSDLs could prove particularly important in medical applications as the distinct absorption features of human tissue (which are mostly determined by water absorption) are at 2 and 2.9μm.
In the 1μm wavelength range, with laser structures based on gallium arsenide (GaAs), VECSELs with output powers of up to several tens of Watts have been realized. Most recently, these lasers have entered the commercial market with great success, either with their fundamental laser emission around 1μm or frequency doubled to cover the visible range. At wavelengths longer than 1μm, however, the output power and efficiency of VECSELs generally degrade quite significantly.
In order to reach high output powers, efficient heat removal from the active region is necessary ; small pieces of the grown heterostructure were bonded to a silicon carbide (SiC) or diamond transparent heat spreader using the liquid capillary bonding technique.
The bonded samples were then mounted into a special submount (see Figure 1). This mounted semiconductor structure was the basis for all subsequent laser-resonator and module developments.
Figure 1. Optically pumped semiconductor disk laser (OPSDL) structure with transparent heat spreader, mounted inside a submount.
By optimizing heterostructure design, MBE growth, and bonding technology, the semiconductor VECSEL structures are claimed to set new international standards for high-power, high-efficiency semiconductor disk lasers above 2μm wavelength.
An output power of 3W was achieved in CW operation at 2.0μm emission wavelength for a heat-sink temperature of 20°C, and up to 6W was obtained when the sample was thermoelectrically cooled to −15°C (see Figure 2).
Figure 2. Continuous-wave (CW) output power versus absorbed pump power of a 2.0μm OPSDL pumped at 980nm for different heat-sink temperatures.
In pulsed operation (200ns pulse length), over 21W of on-time output power at room temperature was measured. The optical quantum efficiency of the devices reached very high values of 45% at room temperature and 55% at −15°C heat-sink temperature.
The scientists achieved further power scaling of these 2.Xμm OPSDLs by distributing the heat load over more than one chip by using a ‘double-chip cavity’. In this way, more than 8W output power for a cavity using two 2.0μm gain elements at a heat-sink temperature of −5° was attained.
The external resonator of OPSDLs is flexible, allowing the use of different resonator systems with the same basic chip for different applications. One development strand was a tunable single-mode OPSDL for the 2μm wavelength range (see Figure 3).
Figure 3. Schematic of an OPSDL in a V-shaped cavity with a birefringent filter for wavelength selection and stabilization. VECSEL: Vertical external-cavity surface-emitting laser.
Using a birefringent filter inside a V-shaped laser cavity allowed the laser to operate in single longitudinal mode (see Figure 4).
Figure 4. Single-mode spectra of the tunable laser setup for one specific emission wavelength of 2.00μm. a.u.: Arbitrary units.
The emission wavelength of the laser within a window of 120nm around the central lasing wavelength of 1.98μm was tuned by simply rotating the birefringent filter. For a single-mode laser, the output power was still very high, with a maximum value above 500mW at the central frequency, dropping to around 100mW at the edges of the tuning range (see Figure 5).
Figure 5. Output power versus emission wavelength of the tunable OPSDL (black line) and the free-running laser without birefringent filter (red star).
The linewidth of the laser, measured with a scanning Fabry-Perot Interferometer (FPI), was less than 2.3MHz. Since this was the resolution limit of the FPI, the true linewidth of the disk laser could have been even smaller.
To extend the wavelength range, the researchers fabricated an OPSDL based on gallium antimonide (GaSb) that emits at 2.8μm.
Critical issues for the MBE growth of this semiconductor structure were the total layer thickness of nearly 12μm and the high indium content needed for the long-wavelength quantum wells. This required optimization of the growth conditions, together with a comprehensive optical analysis of the grown wafers, to ensure a close match between the design and the actual structure.
Up to 120mW output power in CW operation at a submount temperature of 20°C was achieved. Under pulsed excitation, more than 500mW peak output power was obtained (see Figure 6).
Figure 6. Output-power characteristics of a 2.8μm OPSDL in CW and pulsed operation at 20°C. The inset shows the emission spectrum in pulsed mode, revealing multimode emission filtered by the etalon modes of the silicon carbide intracavity heat spreader.
The results for the 2.8μm emitting GaSb-based disk laser demonstrate the potential for the wavelength range of OPSDLs to be significantly expanded toward 3μm and beyond, while still obtaining acceptable device performance at room temperature.
Using the setup of the laboratory experiments, the scientists produced compact and rugged laser modules, such as a prototype hermetically sealed laser module, including pump laser, monitor photodiode, and red pilot laser (see Figure 7).
Figure 7. Hermetically sealed OPSDL laser system, including pump laser,
monitor photodiode, and red pilotlaser.
The researchers say that all 2.Xμm OPSDL laser modules have proved their robustness and reliability in tests, with no re-adjustment required after either shipment or prolonged operation.
Semiconductor disk lasers have the advantage of the wavelength flexibility provided by the semiconductor gain medium. This contrasts with classical solid-state lasers covering this wavelength range (such as holmium- or erbium-doped lasers).
A specific emission wavelength can be obtained and optimized for a specific application. Furthermore, each laser chip can be tuned around its central emission wavelength. Compared with diode lasers covering this wavelength range, the beam quality at high output powers of the OPSDL is said to be far superior.
These flexible laser sources may be optimized for beam quality or output power, say the researchers, and are thus ideal both for seeding solid-state or fiber lasers and amplifiers. It is hoped that these long-wavelength OPSDLs will follow the commercial success of their 1μm counterparts.
In the future, research efforts on the project will focus on further optimizing the semiconductor structure and thermal management to improve the output power and wavelength coverage. It is also anticipated that new laser-resonator configurations, including optical elements will be demonstrated, in order to increase the functionality of application-specific laser modules.
This research was funded by the Sixth Framework Program of the European Union within the project VERTIGO (Versatile Two Micron Light Source), number 034692.
The authors of the paper are
Benno Rösener, Marcel Rattunde, Christian Manz, Joachim Wagner (Fraunhofer Institute for Applied Solid State Physics Freiburg, Germany)
John-Mark Hopkins, David Burns (Institute of Photonics, Glasgow, UK)
Karsten Scholle (LISA laser products OHG Freiburg, Germany)
The full article is available via the following link http://spie.org/x41467.xml?highlight=x2404&ArticleID=x41467
