Steffen Adler

Recent Advances in 2-μm GaSb-Based Semiconductor Disk Laser—Power Scaling, Narrow-Linewidth and Short-Pulse Operation

An overview of the recent progress in a 2-μm GaSb-based semiconductor disk laser will be presented in this paper. Significant advances could be recently achieved in scaling the output power as well as in narrow-linewidth and short-pulse operation. Moreover, we will discuss the limiting effect for the output power imposed by laser emission perpendicular to the beam propagation, i.e., lateral lasing, and show means to suppress this phenomenon adversely affecting the laser performance. By using several gain elements in one laser cavity, the demonstration of more than 10-W output power from a GaSb-based SDL will be reported. Furthermore, employing a high-stability setup optimized for single-frequency emission more than 1-W output power at a linewidth below 50 kHz will be shown. Further comprehensive stability analyses were conducted paving the path for future stability improvements. Moreover, electro-optic cavity dumping of disk lasers will be demonstrated to be an effective means for achieving <;3 ns pulses at peak powers exceeding 30 W.

Recent Advances in Power Scaling of GaSb-Based Semiconductor Disk Lasers

GaSb-based semiconductor disk lasers (SDLs) cover the application-rich 2-3-μm wavelength range. The output power of these lasers is mainly limited by the active region heating and resulting thermal rollover, caused by the waste heat deposited in SDL chip. We present recent advances achieved in 1) reducing the heat load on the SDL chip by reducing the quantum deficit, and 2) removing the waste heat more efficiently by combining front- and backside heat sinking. The latter step was based on extensive thermal simulations of the heat distribution and heat flow within SDL chip and submount, which are also presented. Combining both approaches, we could demonstrate 20 W of continuous wave output power from a GaSb-based single-chip SDL operating at 2 μm and a heat sink temperature of 0 °C. A comparative analysis of the similarities and differences to GaAs-based SDLs emitting around 1 μm is given.

2-µm high-brilliance micro-cavity VECSEL with >2W output power

This paper presents recent advances of 2-μm GaSb-based vertical external cavity surface emitting laser (VECSEL) with special emphasis on quantum deficit reduction and miniaturization. Operating the VECSEL in a 5-cm long cavity, we could demonstrate an increase in maximum cw output power from 4.2 W to 7.2 W at room temperature when barrier pumping a 2.0-μm emitting VECSEL at a pump wavelength of 1.5 μm instead of 980 nm. Furthermore, miniaturized VECSELs were realized by depositing a high-reflectivity (~97 %) coating on top of a 375-μm thick SiC heat spreader, which acts as output coupler of the micro cavity (μC) formed. This planar cavity is rendered stable by thermal lensing induced by the absorption of pump light. At the same time, thermal lensing influences the beam quality. We will report a detailed study of the influence of the thermal lens on the stability and beam diameter of the μC-VECSEL by using two different VECSEL structures optimized for 980 nm and 1.5 μm barrier pumping, respectively. Using different pump photon energies results in different amounts of heat generated at a given pump photon flux, and thus thermal lenses with different focal lengths. Using the low-quantum deficit pumping scheme we could achieve a factor-7 increase in output power in TEM00 emission from the μC-VECSEL compared to the 980 nm pumped device, as well as a maximum output power of 2.2 W. This 2-μm μC-VECSEL exhibits 110-nm tunable single-frequency emission at a 7-MHz linewidth at an output power of up to 90 mW. The linewidth of the μC-VECSEL is comparable to that of VCSELs, which typically emit output powers in the milli-Watt range.

Continuous-tunable single-frequency 2 μm GaSb-based thin device semiconductor disk laser

The (AlGaIn)(AsSb) materials system has been shown to be ideally suited to realize semiconductor disk laser (SDL) for the 1.9–2.8 μm wavelength range [1-3]. Using barrier pumping with commercial diode lasers, cw output power exceeding 17 W at 20°C heatsink temperature has been shown recently [4]. In order to achieve these high output powers, an intracavity heatspreader is used, which is bonded on top of the SDL-chip. While these high-power SDLs are well suited for medical therapy or material processing, many applications in this wavelength regime, such as high-resolution spectroscopy, long-range gas sensing, LIDAR, seeding of pulsed laser systems and quantum optical experiments, require continuously tunable narrow linewidth, single-frequency emission.

Compact 2.1 μm Q-switched Ho:YAG laser intra-cavity pumped by a 2 μm OPSDL

Q-switched lasers operating in the nominally eye-safe 2 μm wavelength region are important for applications such as material processing, medicine, LIDAR systems, and pumping of optical parametric oscillators based on ZnGeP2 or periodically poled GaAs. Many of these benefit from wavelengths above 2.05 μm, which are not accessible by the actually widely used Tm-lasers. The long upper laser level lifetime and large energy storage capacity makes Ho-doped crystals very attractive for these applications. In band pumping of singly doped Ho-crystals around 1950 nm enables high laser efficiencies and reduced heat generation inside the laser crystal due to the low quantum defect. Optically pumped semiconductor disk lasers (OPSDL) based on GaSb could be an attractive and compact pump source. These OPSDL can be pumped with cheap laser diodes around 976 nm or 1470 nm and built up in simple and robust setups. In the wavelength range around 2 μm cw output powers up to 17 W have been demonstrated at room temperature [1].

GaSb-based VECSEL for high-power applications and Ho-pumping

The (AlGaIn)(AsSb) material system has been shown to be ideally suited to realize VECSELs for the 2-3 μm wavelength range. In this report we will present results on increasing the output power of the SDL chips with special emphasis on the 2.8 μm emission wavelength by means of low quantum defect pumping. Further on we have investigated concepts for a VECSEL-pumped Q-switched Ho:YAG laser in order to convert the high cw-power of the VECSEL into pulses with a high peak power. Up to 3.3 mJ of pulse energy were achieved with a compact setup (corresponding to a peak power of 30 kW at 110 ns pulse length) combined with stable pulsing behavior.

Optimization of 2.5 μm VECSEL: influence of the QW active region

Using the (AlGaIn)(AsSb) material system, VECSELs covering the 2 – 3 μm wavelength range can be realized. The best laser performance of GaSb-based VECSELs was achieved so far at emission wavelengths around 2.0 μm with a slope efficiency of more than 30 %, a low threshold pump power density of 1.1 kW/cm2 at 20°C heatsink temperature and concomitant a high output power exceeding 7 W in CW operation (depending on the mounting technology). These parameters were degrading significantly for longer wavelength devices emitting around 2.5 μm and 2.8 μm. But for applications like the generation of MWIR light (3-8 μm) by pumping ZGP-OPOs, high-power VECSELs around 2.5 μm are required to suppress absorption losses, while for medical laser treatment, high-power operation near the water absorption peak at around 2.9 μm is desirable. We will present results of our ongoing research strand for further optimization of the semiconductor heterostructure design of ≥ 2.5 μm emitting GaSb-based VECSELs. By using a low quantum deficit design (i.e. optical pumping at around 1.5 μm) in combination with highly strained QWs (compressive strain 2.1 %) we were able to realize a 2.5 μm emitting VECSEL with a slope efficiency above 30 %, corresponding to an external quantum efficiency exceeding 50 %, and a low threshold pump power density of 0.8 kW/cm2. These values are as good as those for the best performing 2.0 μm VECSELs. With a frontside SiC heatspreader and operated in a standard linear cavity, over 7 W of CW output power were achieved for this 2.5 μm emitting VECSEL structure when operated at 20°C. Furthermore, we will compare laser structures with different emission wavelengths and discuss the role of the QW strain, band-offset and active region composition on laser performance.

Power scaling of narrow-linewidth 2μm GaSb-based semiconductor disk laser

Summary form only given. In recent years, optically pumped semiconductor disk lasers (SDLs) have attracted considerable interest, since they deliver simultaneously high output power and excellent beam quality. Recently, the realization of high-performance SDLs based on the (AlGaIn)(AsSb) material system with emission wavelengths ranging from 1.8 to 2.8 μm has been reported. Room-temperature output powers of up to 4.2 W have been demonstrated recently, making this laser source interesting for direct applications such as medical therapy or materials processing. Further applications in this wavelength regime such as high-resolution spectroscopy, long-range gas sensing, LIDAR and free-space optical data transmission via phase modulation require single-mode narrow-linewidth (kHz range) emission and output powers in the 0.1-1 W range. Higher output powers are a clear benefit since, e.g. in data transmission, fewer, or even no subsequent power amplifier stages will be required. A typical benchmark for amplifier-free airborne communications is 1 W at <;100 kHz linewidth. In this contribution we will present a 2-μm semiconductor disk laser that emits in free-running operation an output power of 1 W cw at a linewidth of 60 kHz, which is a ten-fold power increase compared to our previous reports and outperforms any other 2-μm kHz-linewidth laser demonstrated so far by a factor of 40 in output power. Using Pound-Drever Hall (PDH) stabilization significant improvements of the wavelength stability could be obtained where beat-note spectra for free-running and PDH-stabilized operation recorded at 1 W output power during a sampling time of 100 μs are shown. For further power scaling, we used an improved cavity setup, which allows larger on-chip mode diameters. This way we could demonstrate that 2 μm SDL are capable of delivering even more than 2 W cw in single-frequency operation at narrow linewidths below 2 MHz. The limitations for further power scaling of this single-frequency laser and possible solutions will be discussed in detail.

30W peak-power 3ns pulse-width operation of a 2μm electro-optically cavity-dumped VECSEL

In this paper electro-optic cavity dumping of a 2 μm semiconductor disk laser is reported. Using this approach, pulsed mode operation with 3 ns pulse length, 30 W of peak power and pulse repetition frequencies between 87 kHz and 1 MHz has been achieved. For cavity dumping, a birefringent polarizer prism was inserted into the V-shaped external cavity formed by high-reflectivity mirrors, in order to establish a linear polarization of the intra-cavity field. The polarization of the intra-cavity field could be rotated by 90° when applying a voltage to a Pockels cell placed inside the cavity close to the end mirror. Photons with rotated polarization undergo then total internal reflection inside the polarizing prism and are coupled out of the cavity sideways. Furthermore, a model based on standard laser rate equations has been developed, which excellently reproduces the measured pulse shape and temporal evolution of the intracavity power. Based on this model, we have studied the pulse length and shape as well as peak power and pulse energy as a function of cavity length in order to explore routes for further increasing peak output power and pulse energy.

Micro-cavity 2-μm GaSb-based semiconductor disk laser using high-reflectivity SiC heatspreader

An optically pumped GaSb-based semiconductor disk laser (SDL) emitting at 2.05 μm has been realized with a very short (380 μm long) laser cavity by high-reflectivity coating the intra-cavity SiC heatspreader, which then serves as the outcoupling mirror. Room-temperature output powers in excess of 750 mW have been demonstrated in multimode operation and still 100 mW in TEM00 emission, which is a more than 100× increase in output power compared to previous reports on GaSb-based micro-cavity (μC) SDLs. Mode-hop-free tunable single-frequency emission with linewidths <7 MHz has been achieved which makes this type of miniaturized SDLs attractive for sensing applications requiring small-size 2-μm laser sources.