Innovation in diode laser design and technology, assembly techniques and optical design are used to realize high-brightness pump modules for application in pumping of fiber lasers. In a first demonstration, monolithically grating-stabilized diode lasers with wavelength around 970 nm are integrated into prototype modules that deliver 500 W of continuous wave TE-polarized optical power at a conversion efficiency > 55% within a spectral width of 1.2 nm (95% power) in a narrow beam, suitable for low-loss coupling into a 200-mu m core fiber. An especially simple opto-mechanical configuration is developed, without need for external volume Bragg gratings.

Spatial-hole-burning as a limit to the continuous-wave (CW) output power of GaAs-based diode lasers is experimentally studied. For 90 μm stripe lasers with 6 mm resonator length and 0.8% front facet reflectivity, spontaneous emission (SE) intensity data show that the carrier density in the device center rises rapidly at the rear facet with bias and falls at the front, consistent with simulation. At the front, the carrier density at the edge of the laser stripe also rises rapidly with bias (lateral carrier accumulation, LCA), consistent with previous observations of increased local current flow. Devices with 20% front facet reflectivity for a flat longitudinal optical field profile show smaller variation in the local carrier density. Weak variation is seen in the carrier density outside the stripe; hence, current spreading is not a power limit. SE wavelength data show higher temperatures at the front with a twofold higher increase in temperature for 0.8% than for 20% front facet. The increased front temperature likely triggers lateral spatial-hole-burning and LCA in this region, limiting power. Finally, pulsed threshold current is more strongly temperature dependent for devices with 0.8% than 20% front facets, attributed to the higher rear facet carrier density. The temperature dependence of slope in pulsed is comparable for both devices at low bias but is more rapid for 0.8% at 20 A, likely due to non-clamping at the back. The temperature dependence of slope for CW is strong with 0.8% facets, likely due to the high temperature and LCA at the front but reduced for 20% facets.

The performance characteristics of two stack modules (emitting near 780 nm) each consisting of 24 wide-aperture (1200 μm) diode laser chips is presented and the results are discussed. The stack modules are constructed using diode lasers from two different epitaxial design iterations. Compared to the first iteration, the second iteration was optimized for higher conversion efficiency and optical in-pulse power (lower losses), without compromising the beam characteristics. The stack modules make use of an established (field-proven) FBH design that utilizes innovative edge-cooling of both sides of the diode stack with large-channel (micro-channel free), water-cooled, thermally-expansion-matched heatsinks. We investigate here their performance up to high duty cycles and results for pulse width up to 10 ms at high duty cycle (50 %) operation is presented. Test of the completed modules show that the iteration 2 (power-optimized) chips deliver about 15 % more optical power without compromising the beam propagation ratio. Specifically, the stack module with first iteration chips delivers approx. 1.4 kW whereas the stack module with the optimized chips delivers approx. 1.6 kW. For the stack module that uses the first chip iteration a fiber coupling to a 1 mm core fiber was demonstrated with approx. 90 % coupling efficiency and loss channels are discussed. Finally, very high duty cycle operation (50 %) is demonstrated for the first time, using an iteration 1 stack module.

Asymmetric photon density (recombination-rate) along the high-power diode laser cavity leads to longitudinal-spatialhole- burning (LSHB), which limits maximum output power. Here, we summarize recent investigations on the impact of LSHB on current (longitudinal) and carrier (lateral and longitudinal) density distribution and hence total-recombination for continuous-wave (CW) operations. Custom diode lasers with 90 μm stripe and 3000-6000 μm resonator have been fabricated with segmented p-side contact to measure local current density and backside metallization window to measure relative carrier density (via spontaneous intensity) and infer temperature (via wavelength). Also, 98% back facet reflectivity and 0.8% and 20% front facet reflectivities have been used to vary the photon density profile and hence severity of hole-burning. We present data showing that current crowds at the front facet due to the high recombinationrate, which becomes more severe as the bias and resonator length increase. The current crowding effect is reduced using higher front facet reflectivity. Longitudinal one-dimensional simulation is broadly consistent with experiments at low bias; however, the current crowding effect is substantially stronger in the experiment than simulation at high bias. Further, spatially-resolved-spontaneous-emission measurements of intensity and wavelength demonstrate that the longitudinal carrier density is also non-uniform with a higher carrier density at the back facet for 0.8% front facet reflectivity, even at low bias, while it is flat for devices with 20%. At high bias, temperature increases at the front facet, leading to lateral carrier accumulation at the stripe edges, higher current and carrier density, which is not included in the simulation.

A diode laser module emitting 1.4 kW optical in-pulse power near 780 nm optimized for high (≥ 10%) duty-cycle operation in a micro-channel free design is presented. With full collimation, a beam quality with a nearly symmetric M2 of 205 × 295 (vertical × horizontal direction) for a wide range of pulse widths is found.

Spatially resolved spontaneous emission intensity and spectrum were used to demonstrate the longitudinal-spatial-hole-burning (LSHB)-induced non-uniform carrier density along the resonator in high power lasers. The impact of high bias is analyzed. Spontaneous emission spectra are calculated to separate the effect of carrier density, and temperature.

A 780nm diode laser pump source is presented that uses passive side-cooling to enable Kilowatt-class output at >10% duty cycle (10ms, 10Hz). Beam quality is M2<360 (BPP<90mm-mrad) and initial reliability test extrapolates over GShot-class lifetime.

Pumping solid state lasers in laser communication terminals (LCT) requires the application of highly reliable, low-noise semiconductor lasers. Two design variants of pump lasers have been developed and tested. The first design consists of broad area laser arrays, spectrally stabilized by an external Bragg grating. Those lasers exhibit decent reliability and thus they are utilized in various space missions. The drawback of that design is hardly controllable intensity noise at certain operating conditions induced by optical feedback. To overcome this drawback, arrays of ridge waveguide lasers with monolithically integrated Bragg grating (DBR RW) have been optimized aimed at low noise performance and high reliability over an extended operating time. Life test results indicate that the reliability goal can be achieved by careful preselection of the devices.

A diode laser pump source is presented using passive side-cooling to enable >10% duty cycle (optimal cooling, long time constants). 6 kW output (1.4 MW/cm2/sr ex-fiber) is demonstrated at 940 nm (0.1…100 ms pulses), with 780…980 nm also available.

The latest generation of high-energy-class pulsed laser facilities, under construction or planned, such as EuPRAXIA, require reliable pump sources with high power (many kW), brightness (>1 MW/cm2/sr) and electro-optical conversion efficiency (>50%). These new facilities will be operated at high repetition rates (around 100 Hz) and only diode lasers are capable of delivering the necessary performance. Commercial (quasi-continuous wave, QCW) diode laser pulse-pump sources are, however, constructed as low-cost passively cooled stacked arrays that are limited either in brightness, efficiency or repetition rate. Commercial continuous wave diode laser pumps constructed using microchannel coolers (as used in high-value industrial machine tools) can fulfil all requirements, but are typically not preferred, due to their cost and complexity and the challenges of preventing cooler degradation. A custom solution is shown here to fill this gap, using advanced diode lasers in a novel passive side-cooling geometry to realize 100 … 200 Hz pump modules (10%–20% duty cycle) that emit peak power of 6 kW at wavelength = 940 nm. The latest performance of these modules is summarized and compared to literature. We show that a brightness >1 MW/cm2/sr can be efficiently delivered across a wide range of laser pulse conditions with 10% duty cycle (pulse width: 100 µs … 100 ms … cw, repetition rate up to 1 kHz). Furthermore, we describe how these pumps have been used to construct and reliably operate (>109 pulses without degradation) in high-energy-class regenerative and ring amplifiers at the Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie (MBI). We also show first results on 100 Hz pumping of cryogenically cooled solid-state Yb:YAG slab amplifiers, as anticipated for use in the EuPRAXIA laser, and note that peak temperature is disproportionately increased, indicating that improved cooling and more detailed studies are needed.

Laser light sources emitting in the deep ultraviolet wavelength range between 210 and 230 nm are of great interest for spectroscopic applications. Here, a compact DUV diode laser system emitting at a wavelength of 222.5 nm is presented. The system is based on frequency doubling of the laser radiation from a micro-integrated GaN external cavity diode laser module (mu ECDL) emitting at 445 nm. The mu ECDL has an optical pump power of 1.4 W with an emission bandwidth of 35 pm. Narrowband laser radiation in continouos wave operation with an output power of 160 mu W at 222.5 nm is generated in a single-pass frequency doubling stage with a beta-BaB2O4 crystal. The results are suitable to address applications such as spectroscopic investigations of biological samples. The presented concept of a compact and efficient deep ultraviolet laser light source enables the realization of portable systems for which a small footprint and a low power consumption is essential.

A system for shifted excitation resonance Raman spectroscopy (SERRDS) suitable for the application in medical practice for the in vivo detection of carotenoids in human skin is presented. This system comprises a miniaturized (150 mm x 27 mm x 12 mm) handheld probe and a wavelength-tunable diode laser-based 488 nm SHG light source. The diode laser provides two closely spaced excitation wavelengths. In parallel with the resonance excitation of carotenoids in the skin, SERRDS separates the fluorescence background from the Raman peaks. Inhomogeneities of human skin are averaged by the applied spot diameter of 3 mm. The implemented optics are designed for a detection of carotenoids over the whole excitation spot area. The system was calibrated using skin phantoms, resulting in a detection limit of 0.03 nmol g(-1) (beta-carotene per gram of skin/tissue) which is more than one order of magnitude below the average beta-carotene concentration in human skin.

Diode lasers pump sources for future high-energy-class laser systems based on Yb-doped solid state amplifiers must deliver high optical intensities, high conversion efficiency (ηE = > 50%) at high repetition rates (f = 100 Hz) and long pulse widths (τ = 0.5…2 ms). Over the last decade, a series of pump modules has been developed at the Ferdinand-BraunInstitut to address these needs. The latest modules use novel wide-aperture single emitter diode lasers in passively side cooled stacks, operate at τ = 1 ms, f = 100…200 Hz and deliver 5…6 kW optical output power from a fiber with 1.9 mm core diameter and NA of 0.22, for spatial brightness BΩ > 1 MW/cm2 sr. The performance to date and latest developments in these high brightness modules are summarized here with recent work focusing on extending operation to other pumping conditions, as needed for alternative solid state laser designs. Specifically, the electro-optic, spectral and beam propagation characteristics of the module and its components are studied as a function of τ for a fixed duty cycle DC = 10% for τ = 1...100 ms, and first data is shown for continuous wave operation. Clear potential is seen to fulfill more demanding specifications without design changes. For example, high power long-pulse operation is demonstrated, with a power of > 5 kW at τ = 100 ms. Higher brightness operation is also confirmed at DC = 10% and τ = 1 ms, with > 5 kW delivered in a beam with BΩ > 4 MW/cm2 sr.

Summary form only given. Carotenes in skin, e.g. β-carotene and lycopene, as indicators for the health status of animate beings can be used for non-invasive point-of-care diagnostics and healthcare [1].In this work, a tunable 488 nm diode laser based light source together with a miniaturized Raman probe suitable for SERDS is presented. The light source is a frequency doubled tuneable DFB diode laser as presented in Ref. [4]. The spectral distance of both excitation wavelengths was adapted to the width of Raman signals originating from carotenes (νFWHM = 15 cm-1). To perform SERDS measurements a miniaturized set-up was designed for the excitation and the detection of the Raman signals.

Summary form only given. Gas lasers such as the HeNe-laser at 633 nm or the Kr-laser at 647 nm are still widespread in use for applications that require visible, highly coherent radiation. Replacing the gas lasers with semiconductor lasers that provide a beam with similar output power, wavelength and coherence would significantly improve the degree of miniaturization and lead to an abundance of new fields of applications.Earlier works already demonstrated the successful manufacturing of single-mode, high-power semiconductor lasers at wavelengths beyond 650 nm [1, 2]. To extend this work below 650 nm and to improve the optical output power and coherence properties we developed distributed-Bragg-reflector ridge-waveguide lasers (DBRRWL) at 633 nm and 647 nm with linewidths close to 1 MHz [3, 4]. The wavelength of the DBR-RWL can be tuned by changing the current and the temperature. This in turn changes the output power, which is undesirable for most applications. Also due to the lower mirror reflectivities in comparison to gas lasers, the DBR-RWLs are more susceptible to external feedback from back reflections of optical elements.

In this paper a micro-integrated laser-amplifier for a wavelength of 1180 nm is presented. The modules can amplify laser emission from any source, which is coupled into the polarization-maintaining input fiber of the module, to an optical power > 1 W. Thereby, the spectral properties of the seed source are maintained. The output of the module is free space allowing easy access to the emitted beam. The footprint of the module is only 47 mm " 34 mm. The article discusses the utilized amplifiers, preceding bench top experiments and gives a detailed experimental characterization of the amplifier module.

We present a concept and the realization of a diode laser-based second harmonic generation (SHG) light source at 488 nm with the capability of a spectral tuning over 2 nm (83 cm $^{-1})$ and a stable SHG power at 30 mW. Wavelength tuning is carried out via a common heat sink temperature for the pump laser and the crystal for nonlinear frequency conversion. The compact diode laser module with a footprint of $25 \times 5$ mm2 has no movable parts. This enables the implementation as an excitation light source into portable sensor systems such as handheld devices for in situ spectroscopic investigations.

Several holographic and interferometric applications would benefit significantly from a diode laser based coherent light source near 633 nm. For this purpose a laser diode based on an AlGaAs/AlGaInP structure for emission in the red spectral range was developed. The laser chip features a ridge waveguide and a DBR surface grating at the rear side with a peak reflectivity at 633 nm. The laser was mounted in a butterfly-style package for temperature stabilization. The beam emitted by the laser diode was shaped with two cylindrical micro-lenses and passed through a custom-built CdMnTebased micro-optical isolator. The beam behind the isolator was coupled into a polarization maintaining (PM) single-mode fiber using an aspherical lens. The optical output power of the fiber was about 1.7 mW at 100 mA.

In this letter, we investigate experimentally the influence of injection current-induced beam astigmatism change in a distributed Bragg reflector tapered diode laser on launching of its nearly diffraction limited, simple astigmatic laser beam into a single-mode fiber (SMF). In particular, the optical power and the laser beam properties in front and behind the SMF are examined in dependence on the current injected into the taper section of the diode laser. The fiber coupling measurements are conducted at two fixed configurations of the applied optical system, each of which is optimized for a different taper current value. A maximum power of 3.5 W ex SMF at a coupling efficiency of 67% is reached.